Introduction

Delocalized diradicals with, in contrast to biradicals, non-negligible electron exchange integrals between the radical centers¹ possess fascinating optoelectronic and magnetic properties. In some but not all diradical ensembles, the triplet manifold can be thermally populated as a consequence of a small singlet-triplet gap. Diradicals, inherently reactive, need steric or electronic stabilization at the positions of highest spin density and/or increased delocalization through extension of the conjugated π-system.² Species with high diradical character y₀ (y₀=0: closed-shell molecule; y₀=<1, diradical; y₀=1, (triplet) biradical)³ form from closed-shell quinoidal/zwitterionic species through aromaticity gain (pro-aromaticity) if the number of Clar sextets is higher in their diradical than in their closed shell forms.⁴ Conjugated diradicals are of the Kekulé- (one or more closed shell resonance forms, e. g. ortho- and para-quinodimethane (QDM), Figure 1a) or non-Kekulé-type, void of any closed shell resonance structure.¹ Examples of Kekulé diradicals include Tchichibabin's hydrocarbon,⁵ Wu's zethrenes,⁶ Kubo's bisphenalenyls,⁷ and Haley's indenofluorenes,⁸ whereas some triangulenes⁹ (including Clar's hydrocarbon),¹⁰ 1,14 : 11,12-dibenzopentacene¹¹ and extended tribenzo[f,k,m]tetraphenes¹² represent non-Kekulé diradicals.¹³ Substitution and/or regioisomerism can tailor diradical character and singlet-triplet gaps (ΔE_ST). Haley et al. synthesized syn- and anti-IIDBT in which the position of the sulfur atom has a marginal but measurable effect on y₀ (y₀=0.61–0.66 and ΔE_ST=8.77–8.06 kcal mol⁻¹), as it leaves the conjugation pathway unchanged.¹⁶

We recently reported o-A and p-A (Figure 1b). p-A, a p-QDM, is closed-shell with y₀=0.26; o-A is an open-shell, aromatic diradical (y₀=0.98). The o-QDM fully integrates the π-electron system, but in the p-QDM p-A the quinone and azaacene are cross-conjugated.¹⁵ The difference in diradical character of p-A and o-A is qualitatively estimated by simply tallying the Clar sextets (CX) of the closed shell (cs) and diradical (open shell, os) resonance forms of both compounds: CX(p-A_cs)=1, CX(p-A_os)=5 and for CX(o-A_cs)=0, CX(o-A_os)=5 (see SI, Scheme S2). This simple analysis ascribes the increased diradical character of o-A to the lack of aromatic stabilization in its closed shell form (note that we only compare the two Kékule systems with respect to extent of diradical character). Besides o-A and p-A the third regioisomer, m-A, can be formulated as an os m-QDM. Alternatively, m-A could possess a trimethylenemethane (TMM), or a cs zwitterionic structure, found in oligo(N-annulated perylene)quinodimethanes,¹⁷ S-containing 2,6-naphthoquinodimethane¹⁸ or asymmetric diradicals with Blatter and phenoxyl moieties.¹⁹ Counting the CX of m-A, the m-QDM form scores with 5 CX, the zwitterionic with 3 CX and the TMM form with 1 CX, suggesting that m-A will be a m-QDM with a high y₀, rather than a zwitterion.

Results and Discussion

Synthesis of 1 a,b starts with the condensation of 2 a,b with diketone 3 into 4 a,b (Scheme 1). Suzuki coupling of 4 a,b with boronic acid 5 leads to 6 a,b in 95–99 %. Oxidation of 6 a,b with K₃Fe(CN)₆ furnishes diradicals 1 a,b (m-A) as deep brown and green solids.

The absorption spectra of 1 b/6 b are red-shifted with respect to those of 1 a/6 a, testament to the larger acene subunit, also found in the spectra of their direct precursors. Under ambient conditions (dilute toluene solution) the half-life (τ_1/2) of 1 a is 6 h and 9.6 h for 1 b (see Figures S2–3), less than that of isomeric o-A (t_1/2=19 h) or p-A (t_1/2=104 h).¹⁵ In the solid-state (Ar, −40 °C) 1 a,b are stable for at least one week. The absorption spectra of 1 b, p-A and o-A (Figure 2) are fairly similar to each other with that of 1 b resembling that of o-A more than that of p-A (note that we refrain our comparative discussion to these regioisomers to rule out any influence of the acene length). The UV/Vis spectra of 1 b in solvents of different polarity (Taft scale²⁰) display only small spectral shifts—the lack of solvatochromism suggests a negligible contribution of zwitterionic resonance forms (see Figure S4).

Single crystals were grown by slow diffusion of acetonitrile into a dichloromethane solution under nitrogen atmosphere (Figure 3). In 1 a,b the packing is dominated by the π–π interaction of the azaacenes, without appreciable intermolecular radical-radical interactions. 1 b forms dimers in the solid-state with a stacking distance of 3.32 Å. These pairs are repeatedly shifted relative to each other and create a wave-like pattern (Figure 3b). In 1 a the thiophene moieties are disordered. A ‘quasi C₂-symmetry’ results, precluding analysis of the bond lengths of the bithienyl units (see Figure S7). 1 a is arranged in one-dimensional stacks with two alternating molecules rotated ~76° relative to each other (Figure 3d).

The (measured) bond length alternation (BLA) of 1 b is compared to o-A and p-A with their corresponding bithienophenoxyl cutouts (Figure 4). For p-A a single/double BLA was observed testament to the quinone-moieties. In o-A the BLA is less pronounced, supporting a diradical form.¹⁵ For 1 b the BLA-form is asymmetric due to the position of the sulfur atoms. On the side with the sulfur facing the azaacene the bonds a-h are on average elongated. The other side displays a pronounced bond length alternation (j–q, Table 1) The CO bond length of o-A (1.28 Å) indicates a C−O single bond, whereas the CO bond length of p-A (1.22 Å) suggests double bond character.¹⁵ The CO bond length of 1 b amounts to 1.25 Å, exactly in between those of its regioisomers, indicating a minor contribution of zwitterionic resonance structures, if packing effects could be ruled out. Comparing the CC bonds e/m, g/k and i (see Table 1) in 1 b and o-A, these bonds are longer CC bonds and belong to the ‘maxima’ in Figure 4. Overall, 1 b is structurally similar to o-A, supporting an open-shell form.

We have compared the IR spectra of 1 b, o-A and p-A (Figure 5, for 1 a see Figure S5). The intense C=O band at 1572 cm⁻¹ for p-A suggests a quinone. For 1 b this vibration band is less pronounced and for o-A it is almost absent, in line with the BLA.

1 a and 1 b (see Figure S6) exhibit half-wave reduction potentials (cyclic voltammetry) at −0.57 V and −0.55 V (vs. Fc⁺/Fc) respectively, assigned to the radical anion and dianion of the bithienophenoxyl segment. Both compounds show two additional reversible half waves attributed to the reduction of the azaacene backbone at −1.22 V/−1.70 V for 1 b and −1.41 V/−1.89 V for 1 a.

Even at −90 °C, the ¹H nuclear magnetic resonance (NMR) spectra of 1 a/1 b only display broad resonances at ~1.25 ppm (see Figure S8) from their tert-butyl and TIPS-groups, testament to the delocalization of the electron spins over the conjugated system. The electron paramagnetic resonance (EPR) spectrum of 1 b (g-value=2.0045, see Figure 6; S9 for 1 a) is structured with hyperfine coupling constants of a_H=5.8 G, 4.7 G, and 3.4 G. Variable temperature (VT) solid-state EPR of 1 a and 1 b revealed after fitting the VT-EPR (T∫∫I_EPR-T) data with the Bleaney–Bowers equation singlet-triplet gaps ΔE_ST of 0.13 kcal mol⁻¹ for 1 b and 0.25 kcal mol⁻¹ for 1 a. Both species exist in an open shell singlet ground state with a thermally accessible triplet state. Superconducting quantum interference device (SQUID) measurements (Figures S10–11) support this interpretation. The χT-T plot for 1 a,b show an increasing signal from 4 K until 300 K as expected for a singlet ground state. The χT-values of 1.35 cm³ K mol⁻¹ for 1 b (0.85 cm³ K mol⁻¹ for 1 a) are in accordance with the spin-only equation²¹ confirming the presence of two unpaired electrons.

y₀ of 1 a,b was calculated by population analysis of natural orbitals (NOs) at the DFT/B3LYP/6-311G(d,p) level of theory (see Table 2 and SI). Spin-flip time-dependent density functional theory (SF-TDDFT) calculations suggest open-shell singlet ground states for 1 a and 1 b with small singlet-triplet energy gaps of 0.25 kcal/mol for 1 a and 0.62 kcal/mol for 1 b. In comparison to the experimental values (see Table 2) the calculated values are a bit higher for all three regio-isomers but show the same trend: The singlet-triplet gap is largest for the para-isomer, followed by the ortho-isomer, and is smallest for the meta-isomer [ΔE_ST(p-A)>ΔE_ST(o-A)>ΔE_ST(1 b)].

Figure 7 displays the spin-density distribution of the triplet state, nucleus-independent chemical shift (NICS) (1)_zz values,²² anisotropy of the induced current density (ACID) plot and electrostatic potential maps of 1 b. The spin density is distributed over the whole azaacene backbone with alternating α and ß coefficients (Figure 7) explaining the absence of NMR signals, due to the contribution of the m-QDM diradical resonance form. NICS(1)_zz calculations (B3LYP/6-311+G(d,p)) for the triplet state of 1 b indicate negative, aromatic chemical shifts for the acene backbone and the thiophene rings as expected for the diradical form. The m-QDM unit connecting the thiophenes is almost non-aromatic, which is also the case for o-A and contrasts with the slight antiaromaticity for p-A. The phenoxy groups are non-aromatic—a finding that all three regioisomers (1 b, o-A and p-A) have in common and is in accordance with literature and attributed to the large residing spin density on the oxygen atoms and the neighboring rings (Figure 7).²³ The ACID²⁴ plot of 1 b supports the antiaromaticity of the m-QDM unit even further while the electrostatic potential maps of the triplet state of 1 b dispel the contribution of a zwitterionic resonance form.

Organic diradicals promise unique physical properties, such as enhanced third-order nonlinear optical (NLO) responses and unusually large two-photon absorption (2PA) cross sections.²⁵ The 2PA cross-section (σ₂) of 1 a and 1 b (Z-scan technique, from 800 to 1700 nm, see SI, Figure 8) is a function of the excitation wavelength. Both 1 a and 1 b exhibit substantial σ₂ over a broad portion of the examined spectral range. The maximum value for 1 a is σ₂=7140±200 GM at 940 nm and σ₂=8870±150 GM at 1370 nm for 1 b. These results place 1 a,b among the highest reported 2PA cross sections for small organic chromophores (not limited to diradicals,²⁶ exceeding those of organic dyes designed for high nonlinear absorption ($$ mostly below 1000 GM²⁷ with some notable exceptions such as (twisted) metalated porphyrinic oligomers²⁸) while we did not at all try to create NLO materials (for a more thorough discussion, see SI, Section 12).^{29a, 27e, 29b} We attribute the stellar NLO-performance to the diradicals’ planarity and extensive π-conjugation.^{29a, 27e}

Conclusions

In conclusion, we prepared and investigated (meta−)bithieno- phenoxylazaacenes 1 a,b with m-QDM subunits und compared 1 b to its ortho- (o-A) and para-isomer (p-A). Meta-1 a,b show surprisingly small singlet-triplet gaps (ΔE_ST=0.13–0.25 kcal mol⁻¹) according to VT-EPR measurements. The conjugation pathway over the m-QDM unit furnishes non-Kekulé structures with electronically semi-integrated systems; 1 b resembles o-A in its spectroscopic data/properties more than p-A, with its y₀ of 0.88 and a vanishing S−T-gap. Surprisingly though o-A displays an even higher diradical character. Both 1 a and 1 b are exceptional 2PA materials that highlights the potential of quinoidal azaacence diradicals as a key structural element for high-performance NLO materials in the near-infrared region.

Supporting Information

The authors have cited additional references within the Supporting Information.³⁰

Conflict of Interests

The authors declare no conflict of interest.