Polycyclic 1,2,5-chalcogenadiazole dyes: structural, optical, and redox properties in neutral and radical-ion states (chalcogen = S, Se)

Neutral M and radical-ion [M]^·– and [M]^·+ states of phenanthro- (1, 2)/dibenzoquinoxalino- (3, 4) fused 1,2,5-chalcogenadiazoles are studied; chalcogen = S (1, 3), Se (2, 4). Experimentally, 1–4 are characterized by simultaneous thermogravimetry – differential scanning calorimetry, spectroelectrochemistry and optical (UV–Vis–NIR/FL) spectroscopy, 2–4 by cyclic voltammetry (CV) for reduction and oxidation, and [M]^·– by electron paramagnetic resonance ([M]^·+ were not detected). Compounds 3, 4 and [M]^·– (in the form of [K(18-crown-6)]⁺ salts; M = 1, 3) are characterized by X-ray diffraction (all are planar), and [M]^·– (M = 1–4) by UV–Vis–NIR. Theoretically, M and [M]^·– (M = 1–4) are specified by density functional theory (DFT) calculations. As compared with the archetypal 2,1,3-benzothiadiazole (5), in 1–4 the π-extension and replacement of S by Se jointly lead to increase of DFT adiabatic electron affinity (0.8 → 2.1 eV), decrease of the absolute values of CV potentials (−1.5 → −0.5 V), broadening and bathochromic shifts of UV–Vis (∼310 → ∼420 nm) and FL (∼380 → ∼470 nm) bands. DFT adiabatic ionization energy of 1–4 of ∼7.9 eV is invariant to their structure (5: 8.7 eV). FL spectra of 1–4 exhibit small Stokes shifts; and those of 2 and 3, vibronic structures. The estimated excited-state lifetime τ₁ is ∼1 ns (ns) for 3 and 4, and ∼2 ns for 2, while long-time component τ₂ is ∼9 ns for 2 and ∼7 ns for 3. The findings suggest that 1–4 are promising organic π-dyes/non-fullerene electron acceptors for small-molecule optoelectronics.