Self-Assembly and the Emergence of Frenkel/Charge Transfer-Coupled Exciton State

The asymmetric PDIs were synthesized in one step from perylene dianhydride and corresponding amine derivatives, which were isolated by column chromatography (Scheme 1, and Experimental Section in Supporting Information). L-PDI-1 dissolved in THF shows sharp 0→0 (522 nm), 0→1 (486 nm), and 0→2 (455 nm) vibronic transitions typical of the monomeric species (Figure 1a and Figure S2), and monomer emission with vibrational structure (Figure 1b). This absorption band is associated with the dipole moment polarized along the long (N−N) axis of the PDI chromophore.¹⁹ To promote the self-assembly of L-PDI-1, H₂O was added to the THF solution with different volume fractions, and their Vis absorption and photoluminescence spectra (PL) were measured (Figures 1, S2, S3). The color of the solutions changed from yellow (monomer solution in THF) to red upon increasing the fraction of H₂O above 50 v/v% (Figure S3a), indicating the formation of aggregates at higher H₂O contents. The absorption spectrum of the aggregates shows the disappearance of the sharp 0→0 (522 nm), 0→1 (486 nm), and 0→2 (455 nm) vibronic transitions observed in THF, giving a broad absorption maximum near 500 nm and a shoulder peak around 550 nm (Figure 1a and Figure S2). This absorption showed a considerable hypochromism compared to the absorption spectrum of the monomer. The observed blue shifts and hypochromism in the absorption peak are commonly observed in π-stacked PDI chromophores, which have been assigned to H-aggregates according to Kasha's exciton coupling model.²⁰ The exciton coupling in our system is confirmed by a positive and negative Cotton effect in CD spectra (Figure S4). These data indicate that the PDI cores assume parallel-aligned orientation with a slight twist in the long-axis dipole moments induced by the chiral glutamate units.³ Consistent with the observed aggregate formation shown in the absorption spectra, a broad and red-shifted fluorescence, typical of excimer emission, was newly observed around 691 nm (Figure 1b and Figure S3c).

Depending on the solvent composition, these aggregates were dispersed in the THF/H₂O mixtures for several hrs to days (Figure S5a). The dispersions with a water content (V_H2O) of 40–80 v/v % gave precipitates after 2 days of preparation, which were facilely redispersed by ultrasonic irradiation using a bath-type ultrasonicator. The incubated samples at a water content of 40 vol % showed a new complex absorption peak at 570 nm, in addition to the absorptions at 488, 525 nm, which are close to 0→1 and 0→0 vibronic transitions of the monomeric PDI chromophore (Figure S5b). The change in the absorption spectrum is accompanied by the appearance of an intense red emission peak at 636 nm in addition to the monomeric emission peaks at 535 and 578 nm (Figure S5c). The aqueous mixture with 40 vol % water appears to be an intermediate state between the 30 vol % water (molecularly dispersed state) and the 50 vol % water (aggregated state) specimens. Upon aging the mixtures at higher water contents of 50–80 vol % (Figure 1a and Figure S5b), two prominent absorption peaks are observed at 471 nm and 570 nm with almost similar intensities, which are located at shorter and longer wavelengths compared to the peak wavelengths of the H-aggregate (500 nm) and the 0–0 absorption of the monomer (522 nm). In addition, significant absorption intensity was observed in the wavelength region between the observed absorption splitting to the higher and lower-energy level exciton bands. These unique absorption spectral changes cannot be represented by classical Kasha's exciton model (Scheme 1c), which is based on the simple Coulombic coupling and point-dipole approximation.²¹ The observed broad spectra with both the blue-shifted and red-shifted peaks in Figures 1a and S5b (V_H2O, 40–80 %) can be explained by considering the strong coupling between long-ranged Frenkel excitons (FE) and short-ranged charge transfer excitons (CTE, Scheme 1d, e).²² In FE, the electron in the excited state and the hole in the ground state reside on the same chromophore, whereas in CTE, the electron and hole reside on different chromophores (Scheme 1d). Such mixing occurs when the energy levels of FEs and CTEs are close, so the diabatic CT states interact strongly via indirect coupling through the diabatic Frenkel exciton.²² The sufficient absorption intensity between the 471 nm and 570 nm peaks indicates an extended CT state in which electrons and holes can be separated by a considerable distance.²² It is reasonable that such FE-CT mixing is promoted in aqueous mixtures with higher water content since the contribution of polar CT states increases with the effective dielectric constants of the medium. Similar broad absorption spectra can be found for self-assembled perylene chromophores.^{14d, 14g, 23}

In CD spectra, monomeric L-PDI-1 in aqueous THF below the water content of 30 vol % showed no CD signals in the S₀–S₁ transition band (Figure S4). This indicates that the point chirality of the L-glutamide group does not provide a chiral environment to PDI cores. Meanwhile, exciton-coupled CD signals were observed for initially formed H-aggregates at higher water contents (Figure S4), with a |g_CD| value of ∼2.2×10⁻³ at 465 nm (Figure S6), indicating the rotational displacement of the PDI chromophores in the aggregates. Meanwhile, upon aging, more intense CD signals were obtained (Figure 2c, Figure S6, S8) with 10-fold enhancement of the |g_CD| value at 570 nm (∼2.1×10⁻²) (Figure S6). Mirror-imaged CD spectra were observed for L-PDI-1 and D-PDI-1, respectively. We found that the CD spectra of the J_CT state show linear dichroism (LD) components (Figures S7, S8). Still, the mirror-symmetry CD curves observed indicate that the contribution of LD is not so significant as to affect the conclusion of the CD spectra (Figure S8). The middle absorption peak around 528 nm also exists in the CD spectra, supporting that the moderately intense absorption component reflects the FE-CTE mixing in the chiral PDI assemblies. The observed changes in the spectral pattern and significantly enhanced CD intensity are consistent with the formation of the mixed FE/CTE state in the incubated samples, which implies a slightly slipped orientation of the perylene chromophores relative to the initial H-aggregates.²² In this paper, the molecular orientation obtained after aging is referred to as a J_CT state to account for the contribution of CT excitons.

The Transformation from Kinetically Formed H_agg to J_CT States

The transformation of the H_agg to the J_CT-state was then studied over time by monitoring changes in the absorption spectra (Figure 2a, water content, 70 vol %). The time dependence of the absorbance at 570 nm exhibited a sigmoidal transition (Figure 2b), indicative of an autocatalytic process.^11a The correlation between the temporal changes in Vis spectra and the aggregation morphology was investigated by scanning electron microscopy (SEM). Immediately after adding water to PDI-1 in THF, nanoparticles (NPs) with average diameters of 170 nm are observed (Figure 2d and transmission electron microscopy (TEM) images in Figure S9). Meanwhile, after 2 hrs, well-developed, one-dimensional helical superstructures were observed for the J_CT-state (Figure 2e). The helical superstructures eventually transformed into nanotubes, as confirmed by TEM images (Figure 2f). Due to the chiral glutamine group connected to the PDI chromophore, homochirality was observed for L or D-type compounds (Figure S10). The J_CT-state of L/D-PDI-1 showed right- and left-handed helices, respectively. The observed morphological changes to the regular helical superstructures are consistent with the changes in Vis and CD spectra (Figures 2a, c). The observed time-dependent spectral changes (i.e., from the H- to the J_CT-state) depend on the water content, as shown in Figures S11–S14. The molecular orientation change is more rapid at lower water concentrations, taking only ∼ 20 min for a 50 % water volume fraction (Figure S11). On the other hand, the mixtures with 60 % and 70 % water content took longer, 60 min and 120 min, respectively, for the spectral changes. Except for the 90 % water sample (Figure S16), all of these samples showed the temporal change from the H- to J_CT-state with morphological changes from NPs to helical nanofibers (Figure S15). These are typical molecular orientational changes from the kinetically trapped, metastable state to the thermodynamically stable state.^{14a, 14f, 24} The transformation from the kinetically formed aggregate H_agg to the thermodynamically stable J_CT state was further confirmed by changes in the Vis spectra measured at various incubation times at the measurement temperature when the thermally generated monomer solution was cooled to ambient temperature (Figure S17, S18). We then pursued characterizations of the THF/H₂O mixture with the water volume fraction of 70 %, where both the high fluorescence quantum yield and the CD intensity of the J_CT-state were observed (Table S2 and Figure S8).

The spontaneous transformation from H to J_CT nanostructures is accelerated by sonication. The transformation process was completed within 2 minutes of ultrasonic irradiation (Figure S19, S20). As can be seen from the SEM images in Figure S19e and Figure S21, the nanofibrils evolve from the initial nanoparticles. Based on these observations, the mechanism of aggregation and the following transformation can be considered as follows: when water is injected into the THF solution, the hydrophobic nature of the PDI core leads to the formation of nanoparticles via solvophobic interactions and π–π interactions, which kinetically determine the molecular orientation.^23a As time progresses, the molecular arrangement within the H-aggregates changes, eventually giving rise to thermodynamically favorable crystalline nuclei of the J_CT-state. Subsequently, a supramolecular polymerization reaction occurs on these nuclei, forming one-dimensional chiral nanofibers.

The changes in molecular orientation during the transformation process were further confirmed by Fourier transform infrared (FT-IR) spectra (Figure S22). The as-prepared and incubated samples were drop-cast on gold-coated glass, and solvents were evaporated under vacuum and then observed in reflection mode. As discussed below, different FT-IR spectra were obtained for these cast samples, which would reflect the corresponding dispersion state. The N−H stretching vibration of H_agg observed at 3380 cm⁻¹ shifted to 3307 cm⁻¹ in the J_CT-states, indicating stronger intermolecular hydrogen bonding formed in helical J_CT-nanostructures.^{14g, 25} CH₂ stretching vibrations in J_CT-nanostructures were detected at 2919 and 2850 cm⁻¹, suggesting the crystalline packing of long alkyl chains.^{25b, 26} In contrast, H_agg exhibited CH₂ stretching vibrations at 2923 and 2,854 cm⁻¹ on the higher energy side, and these high wavenumber peaks imply that the alkyl chains lack crystalline order.²⁶ These observations confirm that the helical or tubular nanoassemblies (J_CT-state) have regular crystalline order,²⁷ which is higher than that in the kinetically formed NPs (H_agg). As discussed in the former section, the H_agg in kinetically formed NPs showed exciton splitting in the CD spectra (Figure S4), indicating that the chromophores have a specifically ordered arrangement under the influence of the chirality of the glutamate unit. On the other hand, the lack of crystalline ordering of alkyl chains in NPs may be attributed to the coexistence of intramolecular hydrogen-bonded species in H_agg that disrupt the packing of alkyl chains (Figure S23).

These temporal transformation processes found for PDI-1 are also observed for PDI-2 and PDI-3, which have linear alkyl chains of carbon numbers C10 and C3, respectively (Figure 3, Figures S24–S28). Similarly to the case of PDI-1, initially formed H-aggregates with a nanoparticle morphology exhibited bisignated CD spectra. In contrast, the CD spectral intensities are significantly amplified upon transforming into the thermodynamically stable J_CT-state. It is to be noted that compound PDI-2 has a saturated C10 hydrocarbon chain instead of a flexible oleyl chain as in PDI-1, and the solvophilicity (i.e., the degree of solvation) of such a saturated hydrocarbon in organic media is lower than that of the oleyl chains.²⁸ In PDI-3, the alkyl chain length of C3 is very short in attaining solvophilicity, but its behavior in water-THF mixtures is similar to PDI-1 and PDI-2. These observations indicate that the chiral L-glutamate long-chain units containing the flexible ether and amide groups serve as solvophilic groups in the water-THF mixtures and direct the formation of the J_CT-state and helical superstructures.

Structural Requirements for PDI to Show FE/CTE Coupling

In contrast, PDI-6, in which chiral long-chained glutamate units are symmetrically introduced to both ends of the PDI chromophore, only gives absorption spectra and exciton-coupled CD spectra characteristic to the kinetically trapped H-aggregates, even after incubation of 2 weeks or ultrasonication for 40 min (Figure S29). Thus, the symmetric introduction of the lipophilic L-glutamate moieties into the PDI chromophore stabilizes the kinetically formed H_agg. The transformation of the J_CT state observed for PDI-1, PDI-2, PDI-3 is also absent in those with branched alkyl chains - PDI-4, PDI-5, and PDI-7. These PDIs with branched alkyl chains only formed H_agg, even sonicated in aqueous THF (50 % v/v), and inhibited conversion to J_CT-states (Figure 3d–f, Figures S30–S33). These observations indicate that the asymmetric introduction of linear alkyl chains to PDI chromophore is essential to show the distinct self-assembly pathway (H_agg to J_CT-state).

In general, self-assemblies of asymmetric molecules, such as amphiphilic molecules, adopt symmetric aggregation structures because they are stable on the basis of statistical thermodynamics.²⁹ For example, in the aqueous self-assembly of lipids, bilayer structures with the hydrophobic alkyl chain end facing each other at the middle of bilayers are formed. Although detailed information on the alignment of PDI molecules in the current water-THF mixtures is unavailable, it is natural to assume that they give entropically favorable symmetric molecular assembly structures with respect to the inversion operation.²⁹

To provide clues to understanding the differences in self-assembly behavior observed in linear and branched alkyl chain compounds, the substructures near the PDI chromophore were optimized by DFT calculations (Figure S34). Two alkyl chains linked to the glutamine residue bound to the PDI chromophore will align in these PDI assemblies. If the alkyl chains introduced on the opposite side of the PDI are linear, they will inevitably be oriented to one side of the PDI plane with a certain tilt angle (Figure S34a). The aligned linear chains in the PDI assemblies allow for slippage to displace the orientation of the PDI chromophore while maintaining proximity distance. This situation is consistent with the discussion by Spano et al., which states that the change in molecular orientation from the H-aggregate to the CT-promoted J structure can be caused by small lateral displacements between adjacent chromophores.^22b On the other hand, in compounds PDI-4, PDI-5, PDI-6, and PDI-7, the branched alkyl chains extend above and below the PDI ring plane (Figure S34b-d), and they provide a steric hindrance to prevent the proximity distance of PDI chromophores required for the short-range coupling via CT interactions. Although the steric hindrance by the branched units can be alleviated by rotational displacement of the PDI chromophores, they are still H-aggregates whose orientation is unsuitable for short-range CT coupling (Figure S35). To support the above reasoning, we designed and synthesized another PDI compound PDI-8, which contains an isobutyl group (Figure S36). In this case, as with the linear PDI compound, the structure was optimized with the isobutyl group oriented on one side of the PDI plane. However, the spontaneous transformation from H_agg to J_CT-states was inhibited. On the other hand, it occurred very slowly after adding nanocrystalline seeds (Figure S36c), indicating that the branched isobutyl group also provides resistance to molecular reorientation. These observations underline that linear alkyl chains are optimal for smooth molecular reorientation to J_CT-states. The following section discusses the acceleration of transformation by nano-seeds in detail.

Living Supramolecular Polymerization of J_CT-States

Interestingly, J_CT-nanostructures formed by the asymmetric, chiral PDIs showed seed-induced living supramolecular polymerization. The seed nanocrystals of L-PDI-1 were prepared by ultrasonication of the H-aggregate dispersion for 15 min. This ultrasound treatment led to the aggregate fragmentation into shorter twisted fibers (Figure S37). We note that this process did not alter the optical properties compared to the J_CT self-assemblies formed via spontaneous transformation (Figure S38). When 0.05 or 0.1 equiv. (molar ratio) of nanoseeds was added to the freshly prepared dispersion of L-PDI-1, an immediate change from H_agg to J_CT-nanostructures was observed at ambient temperature without lag time (Figure 4a, Figure S39). Moreover, the time required for complete conversion into the thermodynamically stable J_CT-nanostructures was shortened to 20 minutes or 6 minutes after the addition of the nano-seeds (5 mol % and 10 mol % respectively (Figure 4, Figure S39), which are much faster than the spontaneous transformation (typically 120 minutes).

Subsequently, we carried out a consecutive polymerization process (Figure S40), i.e., utilizing the supramolecular polymer obtained from the first polymerization cycle as the crystalline nano-seeds to promote 2^nd polymerization, and the same procedure was repeated to a fourth cycle. When 10 mol % of the nanocrystalline seed from the first cycle was added to the freshly prepared solution of L-DPI-1, polymerization began immediately, although a more extended period of approximately 20 minutes was required to complete the transformation (Figure 4). The 3^rd and 4^th cycles also showed an accelerated polymerization process compared with the spontaneous process but with slower polymerization rates. These results indicate essential features in supramolecular polymerization: (1) The nano-seeds act as nuclei for the polymerization process in a living manner, and (2) although the ends of the supramolecular polymer remain active, the transformation rate is diminished due to a reduction in the number of active ends after the last seeded process.^{11a, 14b} This phenomenon is evident in the elongation of nanofibers after each seeded process (Figure S37 and S41). The seed-induced living supramolecular polymerization processes were also observed for L-PDI-2 and L-PDI-3 (Figures S24d and S27d, respectively). Furthermore, hetero-seeded polymerization processes can also be achieved for L-DPI-1, L-DPI-2, and L-DPI-3 (Figure S42). These results demonstrate that nanocrystalline seeds promote the transformation from H-aggregate to J_CT-nanostructures compared to the spontaneous process, and J_CT-nanoassemblies show living supramolecular polymerization.

We then applied the hetero-seeded polymerization process to the branch-chained compounds L-PDI-4, L-PDI-5, L-PDI-6, and L-PDI-7. When 10 mol % of nano-seeds obtained from L-PDI-3 was added to the freshly prepared samples of L-PDI-4, L-PDI-5, L-PDI-6, and L-PDI-7, no significant changes were observed, as shown in Figure S43. These results indicate that the branched chain-substituted PDIs are inert to the nanocrystalline seed of L-PDI-3 and do not show pathway complexity³⁰ because of the considerable difference in molecular structures and alignments. Thus, nanocrystal-induced living supramolecular polymerization occurs specifically for asymmetric, linear-alkyl chain-having PDI molecules that form J_CT-nanostructures.

Fluorescence and CPL Characteristics of J_CT-Assemblies

The significant exciton interactions in J_CT-nanostructures prompted us to study the relationship between molecular organization, fluorescence, and circularly polarized luminescence in PDI-1.^{12a, 31} PDI-1 is monomerically dissolved in pure THF (Figure 1a, b), which gave a high fluorescence quantum yield Φ_fl of 69.7 % and a lifetime of 5.2 ns at λ_em=535 nm. These values are consistent with the previous report for PDI chromophores.³² On the other hand, the fluorescence of π-stacked PDI chromophores is generally significantly quenched. Consequently, quantitative evaluation of radiative and nonradiative decay channels in molecular assemblies has rarely been performed. The obtained fluorescence Φ_fl and lifetimes for kinetically formed H_agg and thermally stable J_CT-state observed at water volume fractions of 50–80 % are summarized in Table S2.

The red-colored excimer fluorescence observed for H_agg at 691 nm under UV irradiation (365 nm) showed an emission lifetime τ_ave of 9.2 ns (Figure 5a, b, Table S2). Interestingly, the J_CT–state showed a 14 nm blue-shifted excimer emission at 677 nm with a shorter emission lifetime τ_ave of 4.5 ns.

In order to unveil the contribution of radiative and nonradiative decay processes and to understand the effect of mixed FE/CTE state on fluorescence characteristics, radiative and nonradiative decay time constants for each of H_agg and J_CT-state was determined by using Φ_fl=k_r×τ_ave and k_r=1/τ_ave–k_nr. We find that H_agg shows k_r of 1.30×10⁶ s⁻¹ and k_nr of 1.07×10⁸ s⁻¹, whereas J_CT-state shows k_r of 2.89×10⁷ s⁻¹ and k_nr of 1.93×10⁸ s⁻¹, respectively. Compared to H_agg′s nonradiative decay time constant k_nr, that of J_CT-state showed a 1.8-fold increase. The significantly increased fluorescence quantum yield Φ_fl observed for J_CT-state by 10 times is ascribed to a more than 22-fold increase in the radiative decay constant k_r. These results show for the first time that FE/CTE mixing in the self-assembly of PDI significantly increases the quantum yield and that this is due to a significant increase in the radiative decay constant k_r. The enhanced radiative decay constant indicates the presence of coherent excited domains that show efficient excitonic migration in the J_CT-state assembly.³³

Subsequently, the excited state chirality was investigated by CPL (Figure 5d). Mirror-imaged CPL signals were observed for freshly prepared H_agg, reflecting the exciton coupling between PDI cores aligned with rotational displacement. Notably, the g_lum value also exhibited a remarkable increase in the course of transformation from H_agg (∼5×10⁻³) to J_CT-nanostructures (∼1.3×10⁻²). These results indicate that controlled molecular orientation from kinetically trapped H_agg to thermodynamically stable J_CT-nanostructures in helical superstructures enhances chiroptical properties. This is the first example of supramolecular living polymerization for the chiral FEs/CTEs coupling systems, which gave thermodynamically stable chiral molecular self-assemblies with remarkably enhanced fluorescence, CD, and CPL spectral properties.