Thienylquinonoidal Porphyrins and Hexaphyrins with Singlet Diradical Ground States

To explore stable organic diradicaloids, meso-thienylquinonoid-substituted porphyrins Pn and hexaphyrins Hn, where “n” denotes the number of thienyl units in the meso-substituents, were synthesized. P0 was identified as a closed-shell quinonoid, whereas P1 was shown to possess significant diradical character with diradical character index (y) of 0.99 and quite small singlet-triplet energy gap (ΔE_S-T) of −0.13 kcal mol⁻¹. P1 was certainly stable, allowing its isolation, but decomposed gradually in solution. In the hexaphyrin series, it was shown that H0 and H1 were closed-shell quinonoids, but H2 was a highly stable diradicaloid with y=0.85 and ΔE_S-T of −3.72 kcal mol⁻¹. The high stability of H2 was ascribed to effective spin delocalization over the entire conjugated network. Characteristically, H2 displays an intense absorption band in NIR region at λ_max=1175 nm with molar absorption coefficient (ϵ) of 8.81×10⁴ mol⁻¹ L cm⁻¹, a narrow HOMO–LUMO gap of 0.69 eV, and nine reversible redox potential waves.