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 8.81×10 ⁴  mol ^-1  L cm ^-1 , a narrow HOMO-LUMO gap of 0.69 eV, and nine reversible redox potential waves.
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