Understanding the effects of substituents on the spectra of chlorins is essential for a wide variety of applications. Recent developments in synthetic methodology have made possible systematic studies of the properties of the chlorin macrocycle as a function of diverse types and patterns of substituents. In this paper, the spectral, vibrational and excited-state decay characteristics are examined for a set of synthetic chlorins. The chlorins bear substituents at the 5,10,15 (meso) positions or the 3,13 (β) positions (plus 10-mesityl in a series of compounds) and include 24 zinc chlorins, 18 free base (Fb) analogs and one Fb or zinc oxophorbine. The oxophorbine contains the keto-bearing isocyclic ring present in the natural photosynthetic pigments (e.g. chlorophyll a). The substituents cause no significant perturbation to the structure of the chlorin macrocycle, as evidenced by the vibrational properties investigated using resonance Raman spectroscopy. In contrast, the fluorescence properties are significantly altered due to the electronic effects of substituents. For example, the fluorescence wavelength maximum, quantum yield and lifetime for a zinc chlorin bearing 3,13-diacetyl and 10-mesityl groups (662 nm, 0.28, 6.0 ns) differ substantially from those of the parent unsubstituted chlorin (602 nm, 0.062, 1.7 ns). Each of these properties of the lowest singlet excited state can be progressively stepped between these two extremes by incorporating different substituents. These perturbations are associated with significant changes in the rate constants of the decay pathways of the lowest excited singlet state. In this regard, the zinc chlorins with the red-most fluorescence also have the greatest radiative decay rate constant and are expected to have the fastest nonradiative internal conversion to the ground state. Nonetheless, these complexes have the longest singlet excited-state lifetime. The Fb chlorins bearing the same substituents exhibit similar fluorescence properties. Such combinations of factors render the chlorins suitable for a range of applications that require tunable coverage of the solar spectrum, long-lived excited states and red-region fluorescence.

Introduction

Understanding the effects of substituents on the electronic and photophysical properties of organic chromophores has been of longstanding interest in the field of photochemistry. Although this topic has been exhaustively investigated for compounds such as simple arenes (e.g. benzene) that absorb in the ultraviolet region, substantially less is known about larger, structurally more complex chromophores. The chlorophylls represent a class of compounds of tremendous fundamental interest owing to their central role in photosynthesis. Prior studies that probed the effects of substituents on the photophysical properties of chlorophylls have typically relied on semisynthesis rather than de novo synthesis owing to the dearth of versatile de novo synthetic routes to chlorophylls and analogs thereof. The structures of chlorophylls a and b are shown in Chart 1. Examples of semisynthetic modifications of chlorophylls that result in alteration of the optical spectra include (1) reduction or deoxygenation of the keto group in the isocyclic ring; (2) hydrogenation or oxidation of the 3-vinyl group; (3) homologation of the 3-vinyl group; and (4) derivatization of the 7-formyl group in chlorophyll b (1–14). Such synthetic modifications of the naturally occurring chlorophylls have enabled identification of the effects of a number of specific groups on photophysical characteristics, but the presence of a full complement of β-substituents in chlorophylls and the complexity of the chemistry has precluded a comprehensive examination. In particular, no studies have heretofore been performed that span the range from a fully unsubstituted chlorin, to chlorins bearing one or a few substituents, to chlorins that contain the isocyclic ring characteristic of naturally occurring chlorophylls.

Details are in the caption following the image — **Figure Chart 1.**
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Structures, numbering system and transition-moment directions. A chlorin contains a reduced ring (D, top left). A phorbine is a chlorin that also contains an isocyclic ring (E, top middle). A 13¹-oxophorbine contains an oxo group at the 13¹-position, exemplified by chlorophylls a and b. Oxochlorins differ from 13¹-oxophorbines in the absence of the isocyclic ring and in the position of the keto group. The x- and y-axes of a chlorin (or 13¹-oxophorbine or oxochlorin) are both in plane. The x-axis contains the reduced ring. The y-axis contains the N atoms of the A and C rings, which contain the 3 and 13 substituents, respectively, and is the direction of the transition moment of the long-wavelength, Q_y, absorption band. In the free base chlorins, the N–H protons are positioned on rings A and C (along the y-axis).

We recently developed a de novo synthesis of chlorins (15,16) and have been extending this synthetic route to probe the effects of substituents at defined locations (17–25). Each synthetic chlorin prepared in this manner bears a geminal dimethyl group in the reduced, pyrroline ring, which stabilizes the molecule to adventitious dehydrogenation leading to the porphyrin. Such chlorins are stable to routine handling in the laboratory. The chlorins available via this general route range from the fully unsubstituted chlorin to chlorins bearing one to three substituents at a variety of positions. Recently, we also gained access to 13¹-oxophorbines, which are chlorins that contain the isocyclic ring spanning the 13- and 15-positions (22). Chlorophylls a and b also contain the 13¹-oxophorbine skeleton. Chart 1 compares the structures of the chlorophylls and synthetic chlorin species, including a synthetic 13¹-oxophorbine and a synthetic oxochlorin (18) and also provides relevant nomenclature. Oxochlorins lack an isocyclic ring and also differ from oxophorbines in the position of the keto group.

A series of chlorins bearing a diverse range of substituents at the 3- and 13-positions were found to provide fine red region, spectral tuning of the long-wavelength absorption band (21). The substituents (i.e. auxochromes) include acetyl, triisopropylsilyl (TIPS) -ethynyl and vinyl at the 3-position and acetyl and TIPS-ethynyl at the 13-position. These chlorins, in conjunction with others prepared previously (15,18,20,23–25) and additional analogs prepared here, afford systematic sets of compounds that make possible an in-depth study of the electronic effects of diverse substituents arrayed about the chlorin macrocycle. The ability to synthesize chlorins with precise control over the number and type of substituents complements work in the semisynthesis of chlorophylls, where the nature of the substituents can be modified but a full set of six β-pyrrole substituents in the 13¹-oxophorbine framework is typically maintained. In this regard, we note that there is a large body of data spanning many decades about chlorophyll chemistry (26–30); however, little systematic spectral data are available. A review of the literature on substituent effects on spectral data will be presented elsewhere and is not cited herein.

In this paper and the companion article that follows (31), we describe studies of 24 zinc chlorins and 18 free base (Fb) chlorins, as well as a zinc oxophorbine and a Fb oxophorbine. The structures of the zinc chlorins are shown in Chart 2; the Fb chlorins are analogs of the zinc chelates. The range of chlorins examined encompasses the fully unsubstituted benchmark chlorin, chlorins with one to three meso-substituents and chlorins with diverse substituents at the 3- and/or 13-positions either with or without a 10-mesityl group (Chart 2). The substituents employed herein include acetyl (A), TIPS-ethynyl (E), pentafluorophenyl (F), mesityl (M), oxo (O), phenyl (P), p-tolyl (T) and vinyl (V) groups; the position of substitution is indicated by superscripts (e.g. 5, 10 or 15 [meso] positions and 3, 13 or 17 [β] positions). The parent framework is a chlorin (C) or oxophorbine (OP) in a Fb or zinc (Zn) chelate form.

The present paper focuses on synthesis, vibrational properties, fluorescence characteristics (spectra, lifetimes and quantum yields) and excited-state decay properties of the compounds. The companion paper gives a detailed analysis of the static electronic properties, including optical absorption spectra, redox potentials and the characteristics of the frontier molecular orbitals (energies and electron-density distributions) as revealed by density functional theory calculations (31). The results presented in the two articles provide insights into the fundamental effects of diverse substituents on chlorin electronic attributes. In addition to gaining a fundamental understanding of substituent effects, the ability to tune the spectral and photophysical features of chlorophyll-like molecules in a rational manner is important for a wide variety of practical applications. Such applications range from artificial photosynthesis to photomedicine, where strong absorption in the red region of the spectrum is highly desirable.

Materials and methods

Synthesis. ¹H NMR (400 MHz) spectra were collected at room temperature in CDCl₃ unless noted otherwise. Absorption spectra were obtained in toluene at room temperature. Chlorins were analyzed by laser desorption mass spectrometry (LD-MS) in the absence of a matrix. Fast atom bombardment mass spectrometry (FAB-MS) data are reported for the molecule ion or protonated molecule ion. Silica gel (40 μm average particle size) was used for column chromatography. THF was freshly distilled from sodium/benzophenone as required. Toluene was distilled from CaH₂. Anhydrous DMF and triethylamine (TEA) were used as obtained from commercial sources. All other solvents were reagent grade and were used as received. All palladium-coupling reactions were carried out using standard Schlenk-line techniques.

The syntheses of many of the chlorin compounds have been reported. The previously synthesized compounds include zinc chlorins ZnC (21), ZnC-M¹⁰ (21), ZnC-V³M¹⁰ (21), ZnC-E³M¹⁰ (21), ZnC-M¹⁰E¹³ (21), ZnC-M¹⁰A¹³ (21), ZnC-E³E¹³ (21), ZnC-E³A¹³ (21), ZnC-E³M¹⁰E¹³ (21), ZnC-E³M¹⁰A¹³ (21), ZnC-A³M¹⁰A¹³ (21), ZnC-T⁵M¹⁰ (15), ZnC-F⁵F¹⁰ (15), ZnC-T⁵ (23), ZnC-P¹⁰ (23), ZnC-T⁵M¹⁰P¹⁵ (20) and ZnC-T⁵M¹⁰A¹³ (22); Fb chlorins FbC (24), FbC-M¹⁰ (24), FbC-P¹⁰ (24), FbC-T⁵ (24), FbC-P¹⁵ (24), FbC-F⁵F¹⁰ (15), FbC-T⁵M¹⁰ (15), FbC-T⁵M¹⁰P¹⁵ (20) and FbC-T⁵M¹⁰A¹³ (22); oxophorbines ZnOP-T⁵M¹⁰ (22) and FbOP-T⁵M¹⁰ (22); and the oxochlorins ZnC-T⁵M¹⁰O¹⁷ (18) and FbC-T⁵M¹⁰O¹⁷ (18). The syntheses of the zinc chlorins ZnC-P¹⁵, ZnC-P³M¹⁰, ZnC-T⁵M¹⁰A¹⁵, ZnC-T⁵M¹⁰V¹⁵, ZnC-M¹⁰E¹⁵, ZnC-M¹⁰P¹³ and ZnC-P³M¹⁰P¹³ and Fb chlorins FbC-P³M¹⁰, FbC-M¹⁰P¹³, FbC-P³M¹⁰P¹³, FbC-M¹⁰E¹³, FbC-M¹⁰E¹⁵, FbC-T⁵M¹⁰A¹⁵, FbC-T⁵M¹⁰V¹⁵, FbC-A³M¹⁰A¹³ and FbC-E³M¹⁰E¹³ are described below. The chlorin precursors for these syntheses that are not mentioned above include ZnC-M¹⁰Br¹³ (21), ZnC-Br³M¹⁰ (21), ZnC-Br³M¹⁰Br¹³ (21), ZnC-Br³Br¹³ (21), FbC-T⁵M¹⁰Br¹⁵ (20), FbC-M¹⁰Br¹⁵ (24) and FbC-Br¹⁵ (24).

Demetalation. 3,13-Diacetyl-17,18-dihydro-10-mesityl-18,18-dimethylporphyrin (FbC-A³M¹⁰A¹³). A solution of ZnC-A³M¹⁰A¹³ (9.20 mg, 0.0152 mmol) in CH₂Cl₂ (1.0 mL) was treated dropwise with trifluoroacetic acid (TFA; 35.0 μL, 0.454 mmol) over a 2 min period. The solution was stirred at room temperature for 2 h. CH₂Cl₂ was added, and the organic layer was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄) and filtered. The filtrate was concentrated to dryness. The resulting solid was chromatographed (silica, hexanes/CH₂Cl₂ [1:9]) to afford a purple solid (6.8 mg, 83%): ¹H NMR (300 MHz) δ−1.27 (br s, 2H), 1.85 (s, 6H), 2.03 (s, 6H), 2.62 (s, 3H), 3.07 (s, 3H), 3.27 (s, 3H) 4.61 (s, 2H), 7.26 (s, 2H), 8.35 (d, J = 4.5 Hz, 1H), 8.82 (s, 1H), 8.87 (d, J = 4.5 Hz, 1H), 8.93 (s, 1H), 9.31 (s, 1H), 10.10 (s, 1H), 10.64 (s, 1H) LD-MS obsd. 543.4; FAB-MS obsd. 542.2655, calcd. 542.2682 (C₃₅H₃₄N₄O₂); λ_abs 432, 687 nm.

17,18-Dihydro-10-mesityl-18,18-dimethyl-13-[2-(triisopropylsilyl)ethynyl]porphyrin (FbC-M¹⁰E¹³). A solution of ZnC-M¹⁰E¹³ (6.20 mg, 0.00883 mmol) in CH₂Cl₂ (0.8 mL) was treated dropwise with TFA (20.4 μL, 0.265 mmol) over a 2 min period. The solution was stirred at room temperature for 3 h. CH₂Cl₂ was added, and the organic layer was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄) and filtered. The filtrate was concentrated to dryness. The resulting solid was chromatographed (silica, hexanes then hexanes/CH₂Cl₂ [2:1]) to afford a purple solid (4.6 mg, 82%): ¹H NMR δ−1.97 (br s, 1H), −1.68 (br s, 1H), 1.36–1.42 (m, 21H), 1.85 (s, 6H), 2.06 (s, 6H), 2.61 (s, 3H), 4.63 (s, 2H), 7.25 (s, 2H), 8.42 (d, J = 4.4 Hz, 1H), 8.64 (s, 1H), 8.84–8.86 (m, 2H), 8.90 (d, J = 4.4 Hz, 1H), 9.18 (d, J = 4.4 Hz, 1H), 9.26 (s, 1H), 9.73 (s, 1H), LD-MS obsd. 638.6; FAB-MS obsd. 638.3826, calcd. 638.3805 (C₄₂H₅₀N₄Si); λ_abs 401, 418, 654 nm.

17,18-Dihydro-10-mesityl-18,18-dimethyl-3,13-bis[2-(triisopropylsilyl)ethynyl]porphyrin (FbC-E³M¹⁰E¹³). A solution of ZnC-E³M¹⁰E¹³ (11.5 mg, 0.0131 mmol) in CH₂Cl₂ (0.8 mL) was treated dropwise with TFA (22.0 μL, 0.393 mmol) over a 2 min period. The solution was stirred at room temperature for 2 h. CH₂Cl₂ was added, and the organic layer was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄), filtered and concentrated to dryness. The resulting solid was chromatographed (silica, hexanes/CH₂Cl₂ [2:1]) to afford a purple solid (8.5 mg, 79%): ¹H NMR (300 MHz) δ−1.85 (br s, 1H), −1.49 (br s, 1H), 1.36–1.42 (m, 42H), 1.85 (s, 6H), 2.04 (s, 6H), 2.61 (s, 3H), 4.60 (s, 2H), 7.25 (s, 2H), 8.39 (d, J = 4.5 Hz, 1H), 8.61 (s, 1H), 8.74 (s, 1H), 8.84 (d, J = 4.5 Hz, 1H), 8.94 (s, 1H), 9.22 (s, 1H), 9.92 (s, 1H), LD-MS obsd. 819.1; FAB-MS obsd. 819.5220, calcd. 819.5217 ([M + H]⁺, M = C₅₃H₇₀N₄Si₂); λ_abs 427, 672 nm.

Suzuki coupling. 17,18-Dihydro-10-mesityl-18,18-dimethyl-13-phenylporphyrin (FbC-M¹⁰P¹³). A solution of ZnC-M¹⁰Br¹³ (11.6 mg, 0.0193 mmol) in CH₂Cl₂ (1.0 mL) was treated dropwise with TFA (44.0 μL, 0.579 mmol) over a 2 min period. The solution was stirred at room temperature for 4 h. CH₂Cl₂ was added, and the organic layer was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄) and filtered. The filtrate was concentrated to dryness. The resulting crude solid was used in the next step. Following a reported procedure (20), a mixture of the crude product, 4,4,5,5-tetramethyl-2-phenyl-[1,3,2]dioxaborolane (7.87 mg, 0.0386 mmol), anhydrous K₂CO₃ (40.0 mg, 0.289 mmol) and Pd(PPh₃)₄ (6.70 mg, 0.0152 mmol) was heated in toluene/DMF (3.0 mL, 2:1) using a Schlenk line. TLC analysis of the reaction mixture indicated incomplete consumption of the starting chlorin after 6 h; therefore, an additional amount of Pd(PPh₃)₄ (6.70 mg) and K₂CO₃ (40.0 mg) was added, and the reaction mixture was stirred for 14 h at 90°C. The reaction mixture was concentrated. The resulting residue was chromatographed (silica, hexanes then hexanes/CH₂Cl₂ [1:1]) to afford a green solid (6.2 mg, 60%): ¹H NMR (300 MHz) δ−1.99 (br s, 2H), 1.85 (s, 6H), 2.06 (s, 6H), 2.60 (s, 3H), 4.61 (s, 2H), 7.25 (s, 2H), 7.55–7.63 (m, 1H), 7.70–7.80 (m, 2H), 8.14–8.32 (m, 2H), 8.46 (d, J = 4.2 Hz, 1H), 8.65 (s, 1H), 8.89 (s, 1H), 8.91 (d, J=4.8 Hz, 1H), 8.93 (d, J=4.2 Hz, 1H), 9.13 (s, 1H), 9.21 (d, J = 4.8 Hz, 1H), 9.80 (s, 1H); LD-MS obsd. 534.7; FAB-MS obsd. 534.2806, calcd. 534.2783 (C₃₇H₃₄ N₄); λ_abs 413, 504, 594, 646 nm.

17,18-Dihydro-10-mesityl-18,18-dimethyl-3-phenylporphyrin (FbC-P³M¹⁰). A sample of ZnC-Br³M¹⁰ (5.0 mg, 0.0083 mmol) in CH₂Cl₂ (1.0 mL) was treated dropwise with TFA (19 μL, 0.0249 mmol) over a 2 min period. The solution was stirred at room temperature for 4 h. CH₂Cl₂ was added, and the organic layer was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄) and filtered. The filtrate was concentrated to dryness. The resulting crude solid was used in the next step. Following a reported procedure (20), a mixture of the crude product, 4,4,5,5-tetramethyl-2-phenyl-[1,3,2]dioxaborolane (3.4 mg, 0.016 mmol), anhydrous K₂CO₃ (9.2 mg, 066 mmol) and Pd(PPh₃)₄ (2.9 mg, 0.0025 mmol) was heated in toluene/DMF (1.5 mL, 2:1) using a Schlenk line. TLC analysis of the reaction mixture indicated incomplete consumption of the starting chlorin after 6 h; therefore, an additional amount of Pd(PPh₃)₄ (2.9 mg) and K₂CO₃ (9.2 mg) was added, and the reaction mixture was stirred for 14 h at 90°C. The reaction mixture was concentrated. The resulting residue was chromatographed (silica, hexanes then hexanes/CH₂Cl₂ [1:1]) to afford a green solid (2.5 mg, 56%): ¹H NMR δ−2.02 (br s, 1H), −1.74 (br s, 1H), 1.85 (s, 6H), 2.08 (s, 6H), 2.60 (s, 3H), 4.64 (s, 2H), 7.24 (s, 2H), 7.67–7.71 (m, 1H), 7.80–7.84 (m, 2H), 8.31–8.34 (m, 2H), 8.44 (d, J = 4.4 Hz, 1H), 8.63 (d, J =4.4 Hz, 1H), 8.76 (d, J = 4.4 Hz, 1H), 8.87 (d, J = 4.4 Hz, 1H), 8.90 (s, 1H), 8.97 (s, 1H+1H), 9.88 (s, 1H); LD-MS obsd. 534.5; FAB-MS obsd. 534.2799, calcd. 534.2783 (C₃₇H₃₄ N₄); λ_abs 412, 504, 594, 644 nm.

17,18-Dihydro-10-mesityl-18,18-dimethyl-3,13-diphenylporphyrin (FbC-P³M¹⁰P¹³). A solution of ZnC-Br³M¹⁰Br¹³ (25.0 mg, 0.0367 mmol) in CH₂Cl₂ (1.0 mL) was treated dropwise with TFA (85.0 μL, 1.10 mmol) over a 2 min period. The solution was stirred at room temperature for 4 h. CH₂Cl₂ was added, and the organic layer was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄) and filtered. The filtrate was concentrated to dryness. The resulting crude solid was used in the next step. A mixture of the crude product, 4,4,5,5-tetramethyl-2-phenyl-[1,3,2]dioxaborolane (44.9 mg, 0.220 mmol), anhydrous K₂CO₃ (76.0 mg, 0.550 mmol) and Pd(PPh₃)₄ (12.7 mg, 0.0110 mmol) was heated in toluene/DMF (4.5 mL, 2:1) using a Schlenk line. TLC analysis of the reaction mixture indicated incomplete consumption of the starting chlorin after 6 h; therefore, an additional amount of Pd(PPh₃)₄ (12.7 mg) and K₂CO₃ (76.0 mg) was added and the reaction mixture was stirred for 14 h at 90°C. The reaction mixture was concentrated. The resulting residue was chromatographed (silica, hexanes then hexanes/CH₂Cl₂ [1:3]) to afford a green solid (16.6 mg, 74%): ¹H NMR (300 MHz) δ−1.83 (br s, 1H), −1.68 (br s, 1H), 1.90 (s, 6H), 2.07 (s, 6H), 2.60 (s, 3H), 4.60 (s, 2H), 7.25 (s, 2H), 7.57–7.63 (m, 1H), 7.66–7.76 (m, 3H), 7.80–7.85 (m, 2H), 8.13–8.17 (m, 2H), 8.31–8.34 (m, 2H), 8.44 (d, J = 4.5 Hz, 1H), 8.64 (s, 1H), 8.86 (d, J = 4.5 Hz, 1H), 8.90 (s, 1H), 8.97 (s, 1H), 9.11 (s, 1H), 9.88 (s, 1H); LD-MS obsd. 610.5; FAB-MS obsd. 610.3124, calcd. 610.3096 (C₄₃H₃₈N₄); λ_abs 418, 507, 652 nm.

Stille coupling. 15-Acetyl-17,18-dihydro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)porphyrin (FbC-T⁵M¹⁰A¹⁵). Following a procedure for replacement of a bromo group with an acetyl group on a chlorin (21), a mixture of FbC-T⁵M¹⁰Br¹⁵ (62.8 mg, 0.100 mmol), tributyl(1-ethoxyvinyl)tin (68 μL, 0.20 mmol) and (PPh₃)₂PdCl₂ (28.1 mg, 0.0400 mmol) was refluxed in THF (10 mL) for 24 h in a Schlenk flask. The reaction mixture was treated with 10% aqueous HCl (10 mL) at room temperature for 2 h. CH₂Cl₂ was added, and the organic layer was separated. The organic layer was washed with saturated aqueous NaHCO₃, dried (Na₂SO₄) and concentrated to dryness. Chromatography (silica, hexanes/CH₂Cl₂ [1:2] then hexanes/CH₂Cl₂ [1:3]) afforded a reddish purple solid (15.9 mg, 27%): ¹H NMR δ−1.53 to −1.47 (br, 1H), −1.13 to −1.07 (br, 1H), 1.84 (s, 6H), 2.03 (s, 6H), 2.60 (s, 3H), 2.67 (s, 3H), 3.25 (s, 3H), 4.49 (s, 2H), 7.20–7.23 (m, 2H), 7.49–7.54 (m, 2H), 7.98–8.03 (m, 2H), 8.28 (d, J = 4.3 Hz, 1H), 8.41 (d, J = 4.3 Hz, 1H), 8.56–8.60 (m, 1H), 8.64–8.67 (m, 1H), 8.74–8.82 (m, 2H), 8.80 (s, 1H); LD-MS obsd. 590.1; FAB-MS obsd. 590.3096, calcd. 590.3046 (C₄₀H₃₈N₄O); λ_abs 408, 646 nm.

17,18-Dihydro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)-15-vinylporphyrin (FbC-T⁵M¹⁰V¹⁵). Following a procedure for replacement of a bromo group with a vinyl group on a chlorin (21), a mixture of FbC-T⁵M¹⁰Br¹⁵ (62.8 mg, 0.100 mmol), Bu₃SnCH=CH₂ (100 μL, 0.293 mmol) and (PPh₃)₂PdCl₂ (10.5 mg, 0.0150 mmol) was refluxed in THF (10 mL) for 16 h in a Schlenk flask. The reaction mixture was concentrated and chromatographed (silica, hexanes/CH₂Cl₂ [1:1]) to afford a reddish purple solid (27.4 mg, 48%): ¹H NMR δ−1.76 to −1.70 (br, 1H), −1.40 to −1.34 (br, 1H), 1.84 (s, 6H), 2.04 (s, 6H), 2.60 (s, 3H), 2.66 (s, 3H), 4.48 (s, 2H), 5.94 (dd, J =11.2, 2.0 Hz, 1H), 6.21 (dd, J = 17.3, 2.0 Hz, 1H), 7.20–7.26 (m, 2H), 7.48–7.52 (m, 2H), 7.99–8.04 (m, 2H), 8.22 (dd, J =17.3, 11.2 Hz, 1H), 8.30 (d, J =4.4 Hz, 1H), 8.44 (d, J =4.4 Hz, 1H), 8.53–8.56 (m, 1H), 8.74–8.76 (m, 1H), 8.78–8.80 (m, 1H), 8.82 (s, 1H), 8.98–9.01 (m, 1H); LD-MS obsd. 574.2; FAB-MS obsd. 575.3131, calcd. 574.3096 (C₄₀H₃₈ N₄); λ_abs 419, 649 nm.

Sonogashira coupling. 17,18-Dihydro-18,18-dimethyl-10-mesityl-15-[2-(triisopropylsilyl)ethynyl]porphyrin (FbC-M¹⁰E¹⁵). Following a reported procedure for Sonogashira coupling with porphyrinic compounds (19,20,32), samples of FbC-M¹⁰Br¹⁵ (16 mg, 0.031 mmol), Pd₂(dba)₃ (4.3 mg, 0.0046 mmol, 15 mol%) and P(o-tol)₃ (12 mg, 0.040 mmol) were weighed in a Schlenk flask. The flask was pump-purged with argon three times. A degassed mixture of toluene/triethylamine (5:1, 13 mL) was added, and the resulting reaction mixture was treated with (triisopropylsilyl)acetylene (21 μL, 0.093 mmol) and stirred at 60°C for 16 h. The reaction mixture was diluted with CH₂Cl₂ (∼50 mL) and filtered. The filtrate was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂ [1:1]) afforded a trace amount of an unidentified colored byproduct (first fraction) and the title compound (second fraction, orange-brown). The latter was rechromatographed twice (first column: silica, hexanes/CH₂Cl₂ [3:2]; second column: silica, hexanes/CH₂Cl₂ [2:1]) to afford an orange-brown solid (10 mg, 50%): ¹H NMR δ−1.62 to −1.50 (br, 1H), −1.50 to −1.42 (br, 1H), 1.36–1.40 (m, 21H), 1.84 (s, 6H), 2.04 (s, 6H), 2.59 (s, 3H), 4.70 (s, 2H), 7.23 (s, 2H), 8.38 (d, J =4.8 Hz, 1H), 8.58 (d, J =4.8 Hz, 1H), 8.79 (s, 1H), 8.80 (d, J =4.8 Hz, 1H), 8.83 (d, J =4.8 Hz, 1H), 9.11 (d, J =4.8 Hz, 1H), 9.21 (d, J =4.8 Hz, 1H), 9.67 (s, 1H); LD-MS obsd. 638.2; FAB-MS obsd. 639.3885, calcd. 639.3883 ([M + H]⁺, M = [C₄₂H₅₀N₄Si]); λ_abs 413, 655 nm.

Metalation. Zn(II)-17,18-Dihydro-10-mesityl-18,18-dimethyl-13-phenylporphyrin (ZnC-M¹⁰P¹³). A solution of Zn(OAc)₂·2H₂O (31 mg, 0.14 mmol) in methanol (0.5 mL) was added to a solution of FbC-M¹⁰P¹³ (5.0 mg, 0.0093 mmol) in CHCl₃ (2.0 mL) with stirring at room temperature. After 16 h, the reaction mixture was concentrated and filtered through a pad of silica (hexanes then hexanes/CH₂Cl₂ [1:2]). The eluate was concentrated to afford a green solid (4.6 mg, 82%): ¹H NMR (300 MHz) δ 1.88 (s, 6H), 2.02 (s, 6H), 2.58 (s, 3H), 4.48 (s, 2H), 7.21 (s, 2H), 7.51–7.56 (m, 1H), 7.63–7.70 (m, 2H), 8.02–8.06 (m, 2H), 8.37 (d, J =4.2 Hz, 1H), 8.54 (s, 1H), 8.61 (s, 1H), 8.76 (d, J =4.2 Hz, 1H), 8.80–8.84 (m, 2H), 9.08 (d, J =4.2 Hz, 1H), 9.60 (s, 1H); LD-MS obsd. 596.3; FAB-MS obsd. 596.1895, calcd. 596.1918 (C₃₇H₃₂N₄Zn); λ_abs 408, 615 nm.

Zn(II)-17,18-Dihydro-10-mesityl-18,18-dimethyl-3-phenylporphyrin (ZnC-P³M¹⁰). A solution of Zn(OAc)₂·2H₂O (12.3 mg, 0.0561 mmol) in methanol (0.5 mL) was added to a solution of FbC-P³M¹⁰ (2.0 mg, 0.0037 mmol) in CHCl₃ (2.0 mL) with stirring at room temperature for 16 h. Standard workup and filtration through a pad of silica (hexanes then hexanes/CH₂Cl₂ [1:1]) afforded a green solid (1.9 mg, 86%): ¹H NMR (300 MHz) δ 1.85 (s, 6H), 2.04 (s, 6H), 2.58 (s, 3H), 4.52 (s, 2H), 7.21 (s, 2H), 7.62–7.66 (m, 1H), 7.75–7.80 (m, 2H), 8.20–8.22 (m, 2H), 8.33 (d, J =4.5 Hz, 1H), 8.53 (d, J =4.5 Hz, 1H), 8.57 (s, 1H), 8.59–8.60 (m, 1H), 8.65 (s, 1H), 8.77 (d, J = 4.5 Hz, 1H), 8.78 (s, 1H), 9.66 (s, 1H); LD-MS obsd. 596.3; FAB-MS obsd. 596.1819, calcd. 596.1918 (C₃₇H₃₂N₄Zn); λ_abs 409, 613 nm.

Zn(II)-17,18-Dihydro-10-mesityl-18,18-dimethyl-3,13-diphenylporphyrin (ZnC-P³M¹⁰P¹³). A solution of Zn(OAc)₂·2H₂O (60.0 mg, 0.250 mmol) in methanol (1.0 mL) was added to a solution of FbC-P³M¹⁰P¹³ (10.2 mg, 0.0167 mmol) in CHCl₃ (4.0 mL) with stirring at room temperature for 16 h. Standard workup and filtration through a pad of silica (hexanes then hexanes/CH₂Cl₂ [1:1]) afforded a green solid (10.6 mg, 96%): ¹H NMR (300 MHz) δ 1.90 (s, 6H), 2.04 (s, 6H), 2.58 (s, 3H), 4.50 (s, 2H), 7.22 (s, 2H), 7.52–7.57 (m, 1H), 7.61–7.70 (m, 3H), 7.75–7.80 (m, 2H), 8.04–8.08 (m, 2H), 8.20–8.24 (m, 2H), 8.35 (d, J =4.2 Hz, 1H), 8.54 (s, 1H), 8.61 (s, 1H), 8.76–8.80 (m, 3H), 9.67 (s, 1H); LD-MS obsd. 672.4; FAB-MS obsd. 672.2244, calcd. 672.2231 (C₄₃H₃₆N₄Zn); λ_abs 414, 623 nm.

Zn(II)-15-Acetyl-17,18-dihydro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)porphyrin (ZnC-T⁵M¹⁰A¹⁵). A solution of FbC-T⁵M¹⁰A¹⁵ (11.8 mg, 0.0200 mmol) in CH₂Cl₂ (4 mL) was treated with Zn(OAc)₂·2H₂O (87.8 mg, 0.400 mmol) in methanol (0.8 mL) at room temperature for 8 h. Standard workup and chromatography (silica, CH₂Cl₂) afforded a bluish green solid (6.3 mg, 48%): ¹H NMR δ 1.84 (s, 6H), 1.98 (s, 6H), 2.57 (s, 3H), 2.65 (s, 3H), 3.14 (s, 3H), 4.32 (s, 2H), 7.19 (s, 2H), 7.47 (d, J =7.6 Hz, 2H), 7.92 (d, J =7.6 Hz, 2H), 8.17 (d, J =4.4 Hz, 1H), 8.29 (d, J =4.4 Hz, 1H), 8.42–8.45 (m, 2H), 8.48 (s, 1H), 8.57 (d, J =4.4 Hz, 1H), 8.62 (d, J =4.4 Hz, 1H); LD-MS obsd. 651.8; FAB-MS obsd. 652.2199, calcd. 652.2181 (C₄₀H₃₆N₄OZn); λ_abs 414, 613 nm.

Zn(II)-17,18-Dihydro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)-15-vinylporphyrin (ZnC-T⁵M¹⁰V¹⁵). A solution of FbC-T⁵M¹⁰V¹⁵ (11.5 mg, 0.0200 mmol) in CH₂Cl₂ (4 mL) was treated with Zn(OAc)₂·2H₂O (87.8 mg, 0.400 mmol) in methanol (0.8 mL) at room temperature for 8 h. Standard workup and chromatography (silica, CH₂Cl₂) afforded a bluish green solid (7.8 mg, 60%): ¹H NMR δ 1.85 (s, 6H), 2.00 (s, 6H), 2.57 (s, 3H), 2.65 (s, 3H), 4.41 (s, 2H), 5.79 (dd, J =17.5, 1.9 Hz, 1H), 6.14 (dd, J =11.0, 1.9 Hz, 1H), 7.19 (s, 2H), 7.47 (d, J =7.9 Hz, 2H), 7.95 (d, J =7.9 Hz, 2H), 8.13–8.17 (m, 1H), 8.20 (d, J =4.5 Hz, 1H), 8.34 (d, J =4.5 Hz, 1H), 8.46 (d, J =4.5 Hz, 1H), 8.53 (s, 1H), 8.61 (d, J =4.5 Hz, 1H), 8.65 (d, J =4.5 Hz, 1H), 8.88 (d, J =4.5 Hz, 1H); LD-MS obsd. 635.8; FAB-MS obsd. 636.2203, calcd. 636.2231 (C₄₀H₃₆N₄Zn); λ_abs 418, 615 nm.

Zn(II)-17,18-Dihydro-18,18-dimethyl-10-mesityl-15-[2-(triisopropylsilyl)ethynyl]porphyrin (ZnC-M¹⁰E¹⁵). A solution of FbC-M¹⁰E¹⁵ (8.1 mg, 0.012 mmol) in CH₂Cl₂/MeOH (2.5 mL, 3:2) was treated with Zn(OAc)₂·2H₂O (48 mg, 0.22 mmol). The resulting mixture was stirred at room temperature for 3 h, concentrated and chromatographed (silica, hexanes/CH₂Cl₂ [1:1]). The first fraction (traces of starting material) and the second fraction (unidentified byproduct) were discarded and the third fraction (blue) was rechromatographed under identical conditions to afford a green solid (4.3 mg, 51%): ¹H NMR δ 1.35–1.36 (m, 21H), 1.84 (s, 6H), 2.01 (s, 6H), 2.58 (s, 3H), 7.19 (s, 2H), 4.60 (s, 2H), 8.27 (d, J =4.4 Hz, 1H), 8.46 (d, J =4.4 Hz, 1H), 8.50 (s, 1H), 8.65 (d, J =4.4 Hz, 1H), 8.72 (d, J =4.4 Hz, 1H), 8.99 (d, J =4.4 Hz, 1H), 9.07 (d, J =4.4 Hz, 1H), 9.47 (s, 1H); LD-MS obsd. 700.2; FAB-MS obsd. 700.2927, calcd. 700.2940 (C₄₂H₄₈N₄SiZn); λ_abs 420, 618 nm.

Zn(II)-17,18-Dihydro-18,18-dimethyl-15-phenylporphyrin (ZnC-P¹⁵). A solution of FbC-P¹⁵ (9.0 mg, 0.022 mmol) in CH₂Cl₂/MeOH (5 mL, 3:2) was treated with Zn(OAc)₂·2H₂O (95 mg, 0.43 mmol) with stirring at room temperature for 1 h. Standard workup and chromatography (silica, hexanes/CH₂Cl₂ [1:1]) afforded a green solid (7 mg, 80%): ¹H NMR δ 1.95 (s, 6H), 4.17 (s, 2H), 7.68–7.73 (m, 3H), 7.88–7.91 (m, 2H), 8.25 (d, J =4.4 Hz, 1H), 8.65 (s, 1H), 8.77 (d, J =4.4 Hz, 1H), 8.80–8.82 (m, 2H), 8.94 (d, J =4.4 Hz, 1H), 9.04 (d, J =4.4 Hz, 1H), 9.46 (s, 1H), 9.52 (s, 1H); LD-MS obsd. 477.7; FAB-MS obsd. 478.1155, calcd. 478.1136 ([M + H]⁺, M = [C₂₈H₂₂N₄Zn]); λ_abs 403, 607 nm.

Absorption and fluorescence spectroscopy. Static absorption (Varian Cary 100) and fluorescence (Spex Fluorolog Tau 2) measurements were performed as described previously, typically for very dilute solutions of the compounds in toluene (33,34). Fluorescence lifetimes were obtained using a phase modulation technique (33). Argon-purged solutions with an absorbance of ≤0.10 at the Soret-band λ_exc were used for the fluorescence spectral and lifetime measurements. For fluorescence spectra, the excitation and detection monochromators typically had a band pass of 1.8 and 3.7 nm, respectively, and spectra were obtained using 0.2 nm data intervals. The emission spectra were corrected for detection-system spectral response. Fluorescence quantum yields were determined for argon-purged solutions of the chlorin relative to chlorophyll a in benzene (Φ_f = 0.325 [35]) and were corrected for solvent refractive index. Samples for fluorescence polarization studies were prepared by dissolving a small aliquot of a concentrated toluene solution of the compound into a heated toluene solution of polyvinylacetate (Mw 167 000) in a 1 cm path glass cuvette and allowing the toluene to evaporate at room temperature and the matrix to solidify. A database of absorption and fluorescence spectra for a number of chlorins has been compiled (36).

For calculation of the integrated-intensity ratio of the Soret (B) and Q_y absorption manifolds ( inline image ), the Soret region was typically integrated from the red of the origin maximum (∼450 nm) to ∼350 nm in order to encompass the B_x(0,0), B_y(0,0), B_x(1,0) and B_y(1,0) features. Similarly, integration of the Q_y manifold encompassed the (0,0) band and the (1,0) feature(s), but not any (2,0) contributions. For each compound, the integration range for both the B and Q_y manifolds was chosen to attempt to best capture the associated oscillator strength while minimizing contributions from absorption to higher-energy electronic states (e.g. the Q_x bands in the case of the Q_y manifold).

Resonance Raman spectroscopy. Resonance Raman (RR) spectra were acquired for both solution and solid film samples of all the zinc complexes. The solution measurements were made on samples dissolved in CH₂Cl₂; the sample cell was a sealed 4 mm i.d. NMR tube, which was spun to mitigate photodecomposition. The film measurements were made on samples deposited on a copper tip that was mounted in an evacuated chamber. All of the RR spectra were acquired at ambient temperature. The spectra of the solution samples were obtained using only Soret excitation because the intrinsic fluorescence from the Q_y state of the solution samples precludes obtaining good quality RR spectra. The spectra of the solid samples were obtained using both Soret and Q_y-state excitation. Q_y-state excitation RR spectra can be obtained for the films owing to quenching of the fluorescence emission (37–39).

The RR spectra were acquired with a triple spectrograph (Spex 1877) equipped with a holographically etched 1200 or 2400 groove/mm grating in the first or third stage. The excitation wavelengths were provided by the discrete outputs of a krypton ion (Coherent Innova 200-K3), an argon ion (Coherent Innova 400-15 UV) or a dye (Coherent CR-590) laser. The dye laser was pumped with an argon ion (Coherent Innova 90-5 UV) laser. The scattered light was collected in a 90° configuration using a 50 mm f/1.4 Canon camera lens. A UV-enhanced charge-coupled device (CCD) was used as the detector (Princeton Instruments LN/CCD equipped with an EEV 1152-UV chip). The data acquisition times ranged from ∼0.5 h (180 × 10 s frames) to ∼5 h (180 × 100 s frames or 60 × 300 s frames), depending on the sample and excitation wavelength. Cosmic spikes were removed prior to addition of the datasets. The laser power at the samples was ∼5–15 mW, depending on the excitation wavelength. The spectral resolution was ∼2.5 cm⁻¹. The frequencies were calibrated using the known frequencies of indene, fenchone and 50/50 toluene/acetonitrile.

Results and discussion

Synthetic chlorins

The zinc chlorins examined herein are shown in Chart 2. For 18 of the compounds, the Fb analogs were also investigated. Each chlorin bears a geminal dimethyl moiety in the reduced, pyrroline ring. The geminal dimethyl structural motif precludes adventitious dehydrogenation of the chlorin to give the corresponding porphyrin, a prevalent problem with many other types of synthetic chlorins. (Such dehydrogenation is believed to be less problematic with chlorophylls owing to the trans-configuration of the vicinal alkyl substituents [18-methyl, 17-propionate] in the reduced ring of chlorophyll, which would result in steric hindrance upon planarization caused by dehydrogenation.) Most of the chlorins have been prepared previously. Additional chlorins were prepared for the spectroscopic studies described herein and in the companion paper (31). Each new synthetic chlorin was characterized by absorption spectroscopy, ¹H NMR spectroscopy and mass spectrometry (LD-MS and FAB-MS).

Chlorins bearing a phenyl group at the 3- and/or 13-position were prepared by Suzuki coupling of the corresponding bromochlorins (Scheme 1). Suzuki coupling of ZnC-Br³M¹⁰ or ZnC-M¹⁰Br¹³ with 4,4,5,5-tetramethyl-2-phenyl-[1,3,2]dioxaborolane under conditions (30 mol% of Pd(PPh₃)₄ and K₂CO₃ in toluene/DMF [2:1]) for use with chlorins (20) gave an inseparable mixture composed of the desired product and the starting chlorin. Therefore, the Suzuki coupling reaction using the corresponding Fb chlorin was employed. Thus, demetalation of chlorin ZnC-Br³M¹⁰ with TFA in CH₂Cl₂ at room temperature afforded the crude Fb chlorin, which was subjected to Suzuki coupling with 4,4,5,5-tetramethyl-2-phenyl-[1,3,2]dioxaborolane in the presence of 30 mol% of Pd(PPh₃)₄ and K₂CO₃. In this manner, FbC-P³M¹⁰ was obtained in 56% overall yield. The synthesis of 13-phenylchlorin FbC-M¹⁰P¹³ or 3,13-diphenylchlorin FbC-P¹³M¹⁰P¹³ was carried out in the same manner. Thus, demetalation of ZnC-M¹⁰Br¹³ or ZnC-Br¹³M¹⁰Br¹³ followed by Suzuki coupling of the corresponding crude product with 4,4,5,5-tetramethyl-2-phenyl-[1,3,2]dioxaborolane afforded FbC-M¹⁰P¹³ or FbC-P³M¹⁰P¹³ in 60% or 74% yield, respectively. This method provides a facile and regioselective synthesis of chlorins bearing phenyl groups at the 3- and/or 13-positions.

Chlorins bearing an acetyl group or vinyl group at the 15-position were prepared by Stille coupling in the same manner as employed for preparing the 13-acetyl or 3-vinyl substituted chlorins (23). Thus, Pd-mediated coupling of 15-bromochlorin FbC-T⁵M¹⁰Br¹⁵ with tributyl(1-ethoxyvinyl)tin followed by acidic hydrolysis afforded the 15-acetylchlorin (FbC-T⁵M¹⁰A¹⁵). Pd-mediated coupling of the same 15-bromochlorin with tributylvinyltin afforded FbC-T⁵M¹⁰V¹⁵ (Scheme 2).

A chlorin bearing an ethynyl group at the 15-position was prepared by Sonogashira coupling. The Sonogashira reaction was carried out using a copper-free method that was developed for use with porphyrinic compounds (32). Thus, reaction of FbC-M¹⁰Br¹⁵ with triisopropylsilylacetylene in the presence of Pd₂(dba)₃ (15 mol%) and tri(o-tolyl)phosphine in toluene/triethylamine (5:1) afforded the corresponding 15-[(2-triisopropylsilyl)ethynyl]chlorin FbC-M¹⁰E¹⁵ in 50% yield (Scheme 3).

Three chlorins available as the zinc chelate were demetalated. Thus, exposure of ZnC-M¹⁰E¹³, ZnC-E³M¹⁰E¹³ or ZnC-A³M¹⁰A¹³ to TFA in CH₂Cl₂ at room temperature gave the Fb 13-TIPS-ethynylchlorin FbC-M¹⁰E¹³, 3,13-bis(TIPS-ethynyl)chlorin FbC-E³M¹⁰E¹³ or 3,13-diacetylchlorin FbC-A³M¹⁰A¹³ in 82%, 79% or 83% yield, respectively. Seven chlorins available as the Fb were metalated with zinc acetate at room temperature to give the zinc chelate. The zinc chlorins prepared in this manner include ZnC-P¹⁵ (80%), ZnC-P³M¹⁰ (86%), ZnC-M¹⁰P¹³ (82%), ZnC-M¹⁰E¹⁵ (51%), ZnC-P³M¹⁰P¹³ (96%), ZnC-T⁵M¹⁰V¹⁵ (60%) and ZnC-T⁵M¹⁰A¹⁵ (48%).

Resonance Raman spectra

The vibrational characteristics of all of the zinc complexes were investigated using RR spectroscopy. The high-frequency regions of the RR spectra of selected zinc complexes are shown in 1-3. The complexes for which RR spectra are shown include the parent unsubstituted ZnC, several β-vinyl-, ethynyl- and acetyl-substituted zinc chlorins and the zinc oxophorbine. Spectra acquired for solution and solid film samples using excitation in the near-UV Soret (B) absorption band (390–440 nm; cfFig. 4) are shown in 1, 2, respectively. Spectra of solid film samples acquired using excitation in the Q_y absorption band (600–670 nm) are shown in Fig. 3. As noted in the Materials and Methods, Q_y-excitation spectra of solution samples cannot be obtained owing to the large intrinsic fluorescence from these samples.

The Soret and Q_y-excitation RR spectra for all the complexes are extremely rich, as is typical of chlorins, reflecting the lowered symmetry (versus a porphyrin) that results from reduction of one of the pyrrole rings (40,41). The detailed interpretation of the RR spectra of the various zinc chlorins is beyond the scope of the current paper. Consequently, the features that are most relevant to the studies are reported herein.

One important feature of the RR spectra is that the vibrational characteristics of the solution and solid samples are generally similar to one another (cf1, 2). Only minor shifts are observed in the frequencies of ring skeletal modes in the solution versus solid, indicating that the basic structure of the chlorin macrocycle is similar in the two media. Likewise, only minor shifts are observed in the frequencies of modes that are characteristics of the substituent groups. These include the C=O stretching modes of the acetyl groups (∼1665 cm⁻¹) and the C≡C stretching modes of the ethynyl groups (∼2147 cm⁻¹).

A second important feature of the RR spectra is that intensity enhancement of the modes due to the substituents is quite weak in general. Indeed, no bands could be clearly identified that could be attributed to the C=C stretch of the vinyl group (expected near 1620 cm⁻¹) or the C=O stretch of the oxophorbine (expected near 1690 cm⁻¹) (40–42). Interestingly, the modes due to the C=O and C≡C stretches of the acetyl and ethynyl groups undergo the largest RR intensity enhancement with Soret excitation. These modes exhibit negligible intensity with Q_y excitation despite the fact that the substituents lie along the Q_y axis of the molecule (Chart 1). This pattern of RR intensity enhancement has been previously observed for chlorins and chlorophyll model complexes, but is not clearly understood (42).

In summary, the incorporation of substituents in the synthetic chlorins studied here results in no major changes in macrocycle structure as revealed by the skeletal modes. The effects on the structural/electronic properties as assayed by vibrational characteristics are consistent with those found previously for chlorophylls and a number of synthetic chlorins.

Electronic absorption and fluorescence spectra

Figure 4A shows absorption spectra for three zinc chlorins and the zinc oxophorbine in toluene. Figure 4B gives spectra for the Fb analogs. Figure 5A and B give the corresponding fluorescence spectra. The four zinc complexes are the unsubstituted chlorin ZnC (black spectra), chlorin ZnC-T⁵M¹⁰A¹⁵ that contains p-tolyl, mesityl and acetyl groups at three meso-positions (red), chlorin ZnC-A³M¹⁰A¹³ that contains acetyl groups at the 3- and 13-positions along with a 10-mesityl group (blue) and oxophorbine ZnOP-T⁵M¹⁰ (green). Each absorption spectrum in Fig. 4 is normalized to the maximum of the strong near-UV Soret band, B(0,0), which contains underlying x- and y-polarized components. The B_x(0,0) and B_y(0,0) components are somewhat better resolved but remain substantially overlapped for the Fb compounds (Fig. 4B). The long-wavelength absorption band, Q_y(0,0), of each compound lies in the range 600–690 nm, with one or more resolved and weaker Q_y(1,0) vibronic components roughly 900–1300 cm⁻¹ to higher energy. The weak Q_x features lie between the B and Q_y bands. Each fluorescence spectrum shown in Fig. 5 has approximate mirror symmetry to the Q_y absorption contour for that compound. The fluorescence is dominated by a strong Q_y(0,0) feature, with one or more weaker Q_y(0,1) components at lower energy derived (43) from Franck–Condon and Herzberg–Teller active vibrational modes.

Fluorescence-excitation polarization spectra were acquired for the four Fb compounds whose absorption and fluorescence spectra are shown in 4, 5, namely FbC, FbC-T⁵M¹⁰A¹⁵, FbOP-T⁵M¹⁰ and FbC-A³M¹⁰A¹³. The polarization spectra generally exhibit a small negative polarization in the vicinity of the Q_x(0,0) band (e.g.∼560 nm in spectra g and h in Fig. 4B), consistent with this absorption having a polarization perpendicular to the Q_y fluorescence. The polarization spectra show a small positive (0–0.05) polarization across the Soret region, in general agreement with spectra that have been obtained for chlorophyll a and other chlorins (44,45). These findings are consistent with substantially overlapped (or mixed) x- and y-polarized components. These results, like previous experimental and theoretical work (44–48), leave open the question of the relative positions of B_x(0,0) and B_y(0,0) bands, the possible contribution of other states and the extent to which the relative energies depend on substituent effects.

The substituents have a marked effect on the positions of the B and Q_y(0,0) absorption maxima and their relative peak intensities. These effects are described in detail in the companion paper (31), where key trends are analyzed in conjunction with redox properties and molecular-orbital characteristics. We are most interested here in the wavelengths (energies) of the Q_y absorption and fluorescence transitions, as well as the oscillator strength of the Q_y absorption. These properties are related (via the Einstein coefficients for absorption/emission and the energy-gap law for nonradiative decay) to the decay characteristics of the lowest excited singlet state, namely the Q_y state. For example, the Q_y absorption intensity is proportional to the radiative (spontaneous fluorescence) rate constant k_f.

The wavelengths of the Q_y(0,0) absorption and fluorescence bands and the relative intensities at the B and Q_y absorption maxima ( inline image ) are listed in Table 1. The peak-intensity ratio has been used extensively for characterizing chlorophyll pigments, particularly where measurements of molar absorption (extinction) coefficients are often not possible or cannot be obtained with the precision necessary to track other properties (49). This simple ratio is a useful first-order gauge of the variation of Q_y intensity among a series of complexes. Indeed, in our prior characterization of 3,13-substituted chlorins (21), we noted the discrepancy between the systematic trend of increasing Q_y absorption (relative to B absorption) with redshift and the lack of apparent trends on the basis of the measured molar absorption coefficients (determined with tiny quantities of material).

Table 1. Spectral properties of the chlorins and oxophorbines*.

Compound†	Q _y(0,0) absorbance (nm)	Q _y(0,0) emission (nm)	‡	§	Φ_f	τ _f (ns)	(k_f)⁻¹ (ns)
Zn chlorin
ZnC	602	602	3.4	4.1	0.062	1.7	27
ZnC-T ⁵	605	606	4.2	4.2	0.077	1.7	22
ZnC-P ¹⁰	605	606	4.1	4.7	0.065	1.6	24
ZnC-M ¹⁰	606	606	4.3	4.4	0.061	1.6	26
ZnC-P ¹⁵	607	607	3.6	4.1	0.068	1.7	25
ZnC-T ⁵ M ¹⁰	608	609	5.0	6.0	0.058	1.6	28
ZnC-T ⁵ M ¹⁰ P ¹⁵	613	614	5.1	5.1	0.066	1.6	24
ZnC-P ³ M ¹⁰	613	616	3.8	4.0	0.088	1.8	20
ZnC-T ⁵ M ¹⁰ A ¹⁵	613	617	6.2	5.9	0.051	2.4	47
ZnC-T ⁵ M ¹⁰ V ¹⁵	615	618	6.0	5.6	0.041	2.1	51
ZnC-F ⁵ F ¹⁰	615	617	4.7	5.1	0.047	1.2	26
ZnC-M ¹⁰ P ¹³	615	617	3.9	3.8	0.095	1.9	20
ZnC-M ¹⁰ E ¹⁵	618	619	4.5	4.5	0.090	2.2	24
ZnC-V ³ M ¹⁰	620	622	3.4	3.6	0.078	2.0	26
ZnC-P ³ M ¹⁰ P ¹³	623	627	3.0	3.3	0.13	2.2	17
ZnC-M ¹⁰ E ¹³	625	626	2.2	3.0	0.16	3.1	19
ZnC-E ³ M ¹⁰	627	628	2.7	3.7	0.12	2.3	19
ZnC-M ¹⁰ A ¹³	632	633	2.2	3.1	0.22	5.2	24
ZnC-T ⁵ M ¹⁰ A ¹³	634	638	2.9	4.0	0.23	5.3	23
ZnC-E ³ E ¹³	645	645	1.5	2.2	0.18	3.1	17
ZnC-E ³ M ¹⁰ E ¹³	646	646	1.6	2.5	0.24	4.3	18
ZnC-E ³ M ¹⁰ A ¹³	652	654	1.5	2.4	0.26	5.0	19
ZnC-E ³ A ¹³	655	657	1.2	2.0	0.22	4.1	19
ZnC-A ³ M ¹⁰ A ¹³	662	666	1.5	2.2	0.28	6	21
Zn oxochlorin∥
ZnC-T ⁵ M ¹⁰ O ¹⁷	609	609	5.4	–	0.04	0.9	22
Zn oxophorbine
ZnOP-T ⁵ M ¹⁰	643	644	2.2	4.0	0.26	6.1	24
Fb chlorin
FbC	633	633	2.4	5.2	0.20	9	46
FbC-T ⁵	636	636	2.9	8.1	0.22	10	45
FbC-M ¹⁰	637	637	2.7	7.2	0.22	10	45
FbC-P ¹⁰	636	636	2.8	7.8	0.20	9.7	48
FbC-P ¹⁵	638	638	2.7	6.8	0.17	9.4	55
FbC-T ⁵ M ¹⁰	640	640	3.4	9.2	0.22	10.7	50
FbC-P ³ M ¹⁰	643	644	2.7	6.6	0.21	8.7	42
FbC-F ⁵ F ¹⁰	645	645	3.2	6.7	0.18	7.9	45
FbC-M ¹⁰ P ¹³	645	647	2.6	6.1	0.22	8.8	40
FbC-T ⁵ M ¹⁰ P ¹⁵	645	645	3.9	9.5	0.27	11	42
FbC-T ⁵ M ¹⁰ A ¹⁵	646	650	4.6	8.5	0.11	6.0	56
FbC-T ⁵ M ¹⁰ V ¹⁵	649	652	5.9	10.0	0.12	12.8	103
FbC-P ³ M ¹⁰ P ¹³	652	654	2.6	5.3	0.24	8.2	34
FbC-M ¹⁰ E ¹³	654	654	1.6	5.0	0.29	9.1	31
FbC-M ¹⁰ E ¹⁵	655	655	3.8	7.5	0.24	11.6	48
FbC-T ⁵ M ¹⁰ A ¹³	661	662	2.5	6.3	0.26	10	38
FbC-E ³ M ¹⁰ E ¹³	672	672	1.2	3.8	0.33	8.7	26
FbC-A ³ M ¹⁰ A ¹³	687	689	1.4	3.8	0.24	7.5	31
Fb oxochlorin∥
FbC-T ⁵ M ¹⁰ O ¹⁷	643	643	8.6	–	0.13	8.9	682
Fb oxophorbine
FbOP-T ⁵ M ¹⁰	660	660	2.1	5.6	0.33	13.8	42
Reference
Chlorophyll a	666	671	1.3	1.6	0.325¶	6.3#	19

*All samples were measured at room temperature in toluene except for chlorophyll a, which was in benzene. †For nomenclature, see text and Chart 1., Chart 2.. Substituent abbreviations: acetyl (A), TIPS-ethynyl (E), pentafluorophenyl (F), mesityl (M), oxo (O), phenyl (P), p-tolyl (T) and vinyl (V) groups. ‡Ratio of the peak intensities of the B and Q_y(0,0) bands. §Ratio of the integrated intensities of the B (B_x plus B_y components) and Q_y absorption manifolds, which includes the (0,0) and (1,0) features. ∥Reference 18. ¶Reference 35. #Values of 6.4 ns and 6.05 ns for chlorophyll a in toluene were measured here and in reference 61, respectively.

Table 1 also lists the integrated intensity ratio of the Soret and Q_y manifolds ( inline image ), which will be useful here for comparisons where relative oscillator strengths are important, particularly for compounds that differ in metalation state (e.g. Fb versus zinc chelates). The integrated-intensity ratio better accounts for the fact that the B_x(0,0) and B_y(0,0) components of the Soret absorption are markedly split in the Fb chlorins, greatly lowering the intensity at the maximum compared with the zinc chelates. The extent of overlap of the B_x(0,0) and B_y(0,0) components also depends on the substituent pattern. Additionally, part of the oscillator strength in each manifold rests in vibronic components that contribute little to the peak intensity. This characteristic of the spectra applies to both Fb chlorins and zinc chelates. The ranges of integration and limitations in inline image are described in the Materials and Methods. Below, we obtain an additional measure of the Q_y oscillator strength via the radiative rate constant k_f derived from fluorescence quantum yields and lifetimes.

The absorption spectra of the four zinc chelates in Fig. 4A show that the Q_y(0,0) band redshifts and the inline image peak-intensity ratio and integrated-intensity ratio vary along the series ZnC (Q_y = 602 nm, = 3.4, = 4.1); ZnC-T⁵M¹⁰A¹⁵ (613 nm, 6.2, 5.9); ZnOP-T⁵M¹⁰ (643 nm, 2.2, 4.0); and ZnC-A³M¹⁰A¹³ (662 nm, 1.5, 2.2). The Fb analogs have Q_y(0,0) maxima at longer wavelengths than the zinc chelates but show the same redshift: FbC (Q_y = 633 nm, inline image = 2.4, = 5.2); FbC-T⁵M¹⁰A¹⁵ (646 nm, 4.6, 8.5); FbOP-T⁵M¹⁰ (660 nm, 2.1, 5.6); and FbC-A³M¹⁰A¹³ (687 nm, 1.4, 3.8). Thus, substituents at the 5,10,15 positions or 3,13 positions of a chlorin, or the formation of the oxophorbine, result in a redshift in the Q_y(0,0) absorption band. Fig. 5 shows accompanying redshifts in the Q_y(0,0) fluorescence maximum. These data in conjunction with those in Table 1 show that, in parallel with the redshift, the addition of 5,10,15 substituents generally results in a decrease in the relative peak (and integrated) intensity of the Q_y band, whereas the addition of 3,13 substituents or the formation of the oxophorbine is generally accompanied by an increase in the relative Q_y intensity. These observations and comparison of the specific effects of 3,13 versus 5,10,15 substituents on the absorption characteristics are analyzed in detail in the companion paper (31). For our purposes here, these spectral data provide insights into how the substituent pattern alters the relationship between the energy of the lowest excited singlet state and the radiative excited-state decay, which is related to the oscillator strength of the associated absorption transition. This relationship is discussed below.

Fluorescence quantum yields

The fluorescence quantum yields of 10 zinc chlorins containing one to three substituents at the 5,10,15-positions lie in the range 0.041–0.090, with an average of 0.069 (Table 1). This average value is comparable with Φ_f = 0.062 for the unsubstituted chlorin ZnC, indicating little effect of meso-substituents on the fluorescence intensity. On the other hand, six zinc chlorins having one substituent at the 3- or 13-positions (and a 10-mesityl group) have Φ_f in the range 0.078–0.22. The average of 0.13 is about two-fold that of the meso-substituted compounds. An even greater average fluorescence yield of 0.22 is obtained for a set of six zinc chlorins having both 3 and 13 substituents (with or without 10-mesityl), which have Φ_f ranging from 0.13 to 0.28. This latter set has fluorescence properties comparable with the zinc analog of chlorophyll a (Φ_f ∼0.23 [50]).

The general comparison above, along with closer inspection of the data in Table 1, indicate a trend of increasing fluorescence yield with increasing oscillator strength of the Q_y absorption band (relative to the Soret band). The most luminous zinc chlorin (ZnC-A³M¹⁰A¹³), and that with the most intense (and most redshifted) Q_y band, has acetyl groups at both the 3- and 13-positions (and 10-mesityl). The quantum yields of the 18 Fb chlorins show similar general trends, but with less variation as a function of substituent pattern (average Φ_f = 0.22 ± 0.06).

The quantum yields of the zinc oxophorbine ZnOP-T⁵M¹⁰ (Φ_f = 0.26) and Fb oxophorbine FbOP-T⁵M¹⁰ (Φ_f = 0.33) are each near or at the upper end of the range of values for the respective 3,13-substituted zinc and Fb chlorins. Thus, the oxophorbines have fluorescence yields comparable with those of chlorophyll a (∼0.33) (35) and its zinc analog. In comparison, the oxochlorins are much less fluorescent (Chart 1). For example, zinc oxochlorin ZnC-T⁵M¹⁰O¹⁷ has a quite low fluorescence quantum yield (Φ_f = 0.040) (18). The lower fluorescence yields for oxochlorins compared with oxophorbines correlate with the less intense (and less redshifted) Q_y band. All of the above comparisons and trends for chlorins, oxophorbines and oxochlorins are traced below to a common dependence of Φ_f and the Q_y-band intensity on the radiative probability.

Fluorescence lifetimes

The excited singlet-state lifetime of each compound was determined by fluorescence methods (Table 1). The lifetimes are in the approximate range of 1–6 ns for the zinc chlorins and 6–13 ns for the Fb analogs. The oxophorbines have lifetimes of 6.1 (zinc) and 13.8 ns (Fb) that are at the upper ends of the ranges for the zinc and Fb chlorins, respectively. Closer examination of the data reveals several systematic differences in fluorescence lifetime with substituent pattern. Consider first the zinc chlorins. The six zinc chlorins bearing one to three aryl (p-tolyl, mesityl and phenyl) rings at the 5,10,15 positions all have lifetimes of 1.6–1.7 ns. For example, ZnC-M¹⁰ and ZnC-T⁵M¹⁰P¹⁵ both have 1.6 ns lifetimes. The incorporation of an auxochrome (acetyl/ethynyl/vinyl) at the 15 position causes a small increase in the lifetime, with the effect of acetyl being greater than vinyl. In particular, the lifetime of ZnC-T⁵M¹⁰V¹⁵ and ZnC-T⁵M¹⁰A¹⁵ are 2.1 and 2.4 ns, respectively.

Substituents at the 3 and 13 positions give rise to longer lifetimes than 5,10,15 substituents. This effect is modest for aryl rings, but is quite pronounced for vinyl, ethynyl and acetyl groups. For example, the presence of a 13-acetyl group in ZnC-T⁵M¹⁰A¹³ (5.3 ns) more than doubles the fluorescence lifetime compared with a 15-acetyl group in the chlorin ZnC-T⁵M¹⁰A¹⁵ (2.4 ns) that has otherwise the same substituents. The chlorins containing 3,13-substituents (also generally bearing a 10-mesityl group) show a systematic variation in fluorescence lifetime with the type and number of substituent as follows: mono- and bis-aryl (1.8–2.2 ns) < mono- and bis-ethynyl (2.3–4.3 ns) < mixed ethynyl and acetyl (4.1–5.0 ns) < mono- and di-acetyl (5.2–6.0 ns). At the lower end of this range, the incorporation of a single 3 or 13-phenyl ring in ZnC-P³M¹⁰ and ZnC-M¹⁰P¹³ (1.8–1.9 ns) and two phenyl rings in ZnC-P³M¹⁰P¹³ (2.2 ns) increases the lifetime modestly compared with ZnC-M¹⁰ and ZnC-T⁵M¹⁰P¹⁵ (1.6 ns). At the top of the range, the enhanced effect of acetyl versus ethynyl groups is conveyed by the trend ZnC-E³M¹⁰E¹³ (4.3 ns) < ZnC-E³M¹⁰A¹³ (5.0 ns) < ZnC-A³M¹⁰A¹³ (6.0 ns).

Even longer lifetimes are observed for the Fb chlorins. However, there is a less pronounced variation with substituents compared with the zinc chelates, and some of the trends are reversed. For example, six compounds that have one to three (nonfluorinated) meso-aryl rings, including FbC-M¹⁰ and FbC-T⁵M¹⁰P¹⁵ have an average lifetime of 9.9 ns (range 9.2–11 ns). This value is longer than the average lifetime of 8.6 ns (range 8.2–8.8 ns) for three chlorins including FbC-P³M¹⁰P¹³ that have a 3-phenyl or 3,13-diphenyl groups (along with 10-mesityl). This trend is reversed compared with the zinc chelates, where 3,13 substituents give longer lifetimes than 5,10,15 groups. The effect of acetyl, ethynyl or vinyl groups is also less pronounced for the Fb chlorins. For example, the average lifetime is essentially the same for three Fb chlorins with 3,13 groups (8.4 ns), namely FbC-M¹⁰E¹³ (9.1 ns), FbC-E³M¹⁰E¹³ (8.7 ns) and ZnC-A³M¹⁰A¹³ (7.5 ns). Indeed, a 13-acetyl group appears to result in a slightly shorter lifetime than an ethynyl group, which is opposite (and less pronounced) of the effect observed for the zinc chelates. The direction of the effect of acetyl, ethynyl or vinyl groups at the 15-position of the Fb chlorins is less clear, with FbC-T⁵M¹⁰V¹⁵ (12.8 ns) having a lifetime modestly longer than analogs with aryl substituents, including FbC-T⁵M¹⁰P¹⁵ (11.0 ns), while the lifetime for FbC-T⁵M¹⁰A¹⁵ (6.0 ns) is unusually short by comparison. For both Fb and zinc chlorins, the addition of pentafluorophenyl rings at the 5,10-positions results in a decrease in excited-state lifetime. This effect parallels those observed previously for porphyrin analogs, consistent with the heavy atom effect on the rate of singlet to triplet interconversion (51).

In summary, the fluorescence lifetime of the zinc chlorins can be tuned from 1.6 to 6.0 ns by using (nonfluorinated) aryl rings and acetyl, ethynyl or vinyl groups at appropriate positions, and the complexes with longest lifetimes have the most redshifted absorption bands. Even longer lifetimes (>10 ns) can be achieved with the Fb chlorins, whose longest-wavelength absorption features lie even farther to the red than the zinc chelates. The zinc and Fb oxophorbines have lifetimes at the upper ends of the ranges for the respective chlorins. Thus, by tailoring the substituent pattern and choosing the metalation state (zinc or Fb), considerable latitude in excited-state lifetime and position of the Q_y absorption band can be obtained.

Radiative rate

The fluorescence lifetime and quantum yield are related by the rate constants for radiative (fluorescence) decay (k_f) and internal conversion (k_ic) of the excited singlet state to the ground state and intersystem crossing (k_isc) to the triplet excited state by the formulas τ = 1/(k_f + k_ic + k_isc) and Φ_f = k_f/(k_f + k_ic + k_isc). These equations and the measured values of these quantities for each compound can be combined to obtain the radiative rate via the expression k_f = Φ_f/τ. The radiative lifetime (in nanoseconds), the inverse of k_f, is tabulated in Table 1.

Consider first the effect of meso-substituents alone. The value of k_f does not change appreciably (≤10%) upon the addition of up to three aryl groups at the 5,10,15 positions for either zinc or Fb chlorins. For these compounds, the Q_y maximum undergoes only a small net redshift: 602 nm for ZnC to 613 nm for ZnC-T⁵M¹⁰P¹⁵ and 633 nm for FbC to 645 nm for FbC-T⁵M¹⁰P¹⁵. The effect of a 15 acetyl, ethynyl or vinyl substituent is quite interesting. For both zinc and Fb chlorins, compared with compounds with otherwise the same meso-substituent, a 15-acetyl group gives a 5–6 nm redshift and a ∼25% average decrease in k_f; a 15-vinyl group gives a 7–9 nm redshift and ∼50% average decrease in k_f. A 15-ethynyl group gives a larger (12–18 nm) redshift and a relatively small change in k_f.

Consider next the effects of substituents at the 3,13 positions (generally in conjunction with a 10-mesityl group). The data show that in progressing from the unsubstituted parent zinc chlorin (ZnC; Q_y at 602 nm) to the zinc chlorin having the most redshifted Q_y band, namely with 3,13-diacetyl and 10-mesityl substituents (ZnC-A³M¹⁰A¹³, Q_y at 662 nm) there is a ∼30% increase in k_f. Similarly, there is a ∼50% increase in k_f for the Fb analog FbC-A³M¹⁰A¹³ (Q_y at 687 nm) compared with that of FbC (Q_y at 633 nm).

The radiative rate k_f for (spontaneous) fluorescence and the oscillator strength for (stimulated) absorption to the lowest excited singlet state (Q_y) are intimately related in the expressions for the Einstein coefficients. In particular, inline image , where the integration over the molar absorption (extinction) coefficient covers the wavenumber () range of the vibronic absorption transitions from the ground to lowest excited singlet state (i.e. Q_y(0,0) plus Q_y(1,0), etc.). This simple analysis ignores the fact that the absorption and emission transitions do not occur at the same wavelength and obey an approximate mirror-symmetry relationship. Fig. 6 indicates that there is a general correlation between k_f and inline image times the integrated absorption-intensity ratio (the inverse of the value listed in Table 1), which is a measure of the total Q_y intensity relative to that of the Soret. The trend spans the zinc and Fb chlorins and the associated oxophorbines. Similar relationships between k_f and the peak-intensity ratio inline image are seen individually for zinc chelates and Fb chlorins, but not when the two types of complexes are grouped together on the same plot. This latter relationship is so because the variation in Soret (B) peak intensity does not track the oscillator strength (integrated intensity) across both sets of compounds due to differences in the net contributions of the x- and y-polarized Soret components to the peak intensity (i.e. substantial splitting for Fb chlorins), as described above.

The k_f values derived from the fluorescence yield and lifetime data, like the inline image and relative-intensity ratios, indicate that the oscillator strength of the Q_y transition increases as it is shifted to the red by 3,13-substituents, for both zinc and Fb chlorins (Table 1). This effect is illustrated by the spectra in Fig. 4A for ZnC-A³M¹⁰A¹³ (blue, d) versus for ZnC (black, a) and in Fig. 4B for FbC-A³M¹⁰A¹³ (blue, h) versus for FbC (black, e). On the other hand, the 5,10,15 substituents tend to result in a decrease in Q_y intensity as the band redshifts, particularly as reflected in the inline image and relative-intensity ratios. This effect is illustrated by the spectra in Fig. 4A for ZnC-T⁵M¹⁰A¹⁵ (red, b) versus for ZnC (black, a) and in Fig. 4B for FbC-T⁵M¹⁰A¹⁵ (red, f) versus for FbC (black, e). Although this trend is generally not as evident in the k_f values, it is apparent for both zinc and Fb chlorins bearing three meso-substituents including 15-vinyl and 15-acetyl groups (Table 1).

Effects of substituents on excited-state decay pathways

The excited singlet-state decays by fluorescence, intersystem crossing and internal conversion. The internal conversion rate (k_ic) is expected to increase as the energy of the lowest excited singlet state decreases due to an improved Franck–Condon (vibrational overlap) factor, which defines the energy-gap law for nonradiative decay. The data in Table 1 show that the zinc chlorins with the lowest excited singlet states (the most redshifted Q_y bands) have the largest relative Q_y intensity and k_f values. These compounds generally have 3,13 substituents, particularly vinyl, ethynyl or acetyl groups. Together, an increase in k_ic and k_f would predict a decrease in the lifetime of the lowest excited singlet state. However, we find that the lifetime actually increases among this series of zinc chlorins (by a factor of ∼3.5), as does the fluorescence yield (Table 1). For example, τ increases from 1.7 ns for ZnC to 6.0 ns for ZnC-A³M¹⁰A¹³ and Φ_f increases from 0.062 to 0.28. These findings imply a commensurate decrease in the rate constant (k_isc) for intersystem crossing to the excited triplet state.

This trend in photodynamics of the synthetic chlorins examined herein follows the trends in the decay characteristics of the excited singlet state along the series porphyrin→chlorin→bacteriochlorin (52–60). The average and typical range are as follows using data for Fb and closed-shell metal chelates (e.g. Zn and Mg) (18,19,46,51–61):

•
porphyrins: Φ_f∼0.05 (0.03–0.16), Φ_isc∼0.85 (0.7–0.9) and Φ_ic∼0.10 (0.05–0.2);
•
chlorins: Φ_f∼0.20 (0.1–0.3), Φ_isc∼0.70 (0.6–0.8) and Φ_ic∼0.10 (0.1–0.2);
•
bacteriochlorins: Φ_f∼0.15 (0.1–0.3), Φ_isc∼0.50 (0.3–0.8) and Φ_ic∼0.35 (0.3–0.4).

Thus, the trends with progressive reduction of the parent macrocycle include (1) a decrease in k_isc and Φ_isc; (2) an increase in the Q_y wavelength and oscillator strength; and (3) an increase in k_ic and Φ_ic (consistent with the Franck–Condon effect described above). These parallels suggest that the peripheral substituents on chlorins, particularly 3,13 substituents, give rise to diminished spin-orbit coupling as is observed upon progressive reduction of the parent macrocycle.

Conclusions

The availability of new synthetic methods has opened the door to systematic studies of substituent effects in chlorin macrocycles. Previously, limited studies only could be carried out by modification of the intact chlorophyll ring. Understanding the effects of substituents on spectral properties of chlorins is of fundamental interest and may provide a foundation for the rational design of tailored red-absorbing pigments. The key observation reported herein is that changes in the inherent radiative and nonradiative rate constants positively reinforce each other in a manner that endows the new synthetic chlorins with photophysical properties that are well suited for a number of applications. In particular, the new architectures bearing 3 and 13 substituents combine a redshift in the long-wavelength absorption band, an increase in the intensity of the long-wavelength absorption band, a redshift in the wavelength of the fluorescence maximum, only a small decrease in fluorescence lifetimes for the Fb chlorins (to 8–10 ns) and an actual increase in the fluorescence lifetime for zinc chlorins (to 4–6 ns). These characteristics are obtained with no significant effects on the structure of the macrocycle as evidenced by the vibrational properties of the complexes. Applications of such chlorins may include artificial photosynthesis in model systems, solar energy conversion in molecular-based photovoltaic cells and medical applications ranging from flow-cytometric fluorescent labels to photodynamic therapy.

Acknowledgements— This research was supported by grants from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US Department of Energy to DH (DE-FG02-05ER15661), DFB (DE-FG02-05ER15660) and JSL (DE-FG02-96ER14632). Mass spectra were obtained at the Mass Spectrometry Laboratory for Biotechnology at North Carolina State University. Partial funding for the NCSU Facility was obtained from the North Carolina Biotechnology Center and the NSF.

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Citing Literature

Volume83, Issue5

September/October 2007

Pages 1110-1124

Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 1: Synthesis, Vibrational Properties and Excited-state Decay Characteristics

Abstract

Introduction

Materials and methods