Thienylquinonoidal Porphyrins and Hexaphyrins with Singlet Diradical Ground States

5 years ago

Thienylquinonoidal Porphyrins and Hexaphyrins with Singlet Diradical Ground States

Atsuhiro Osuka, Daiki Shimizu, Dongho Kim, Ko Furukawa, Jun Oh Kim, Koji Naoda

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 (ΔES-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 ΔES-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 λmax=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. Radical state: Oligothienylquinonoid-substituted porphyrins Pn and hexaphyrins Hn (n=0–2) were synthesized. P0 has been shown to be a closed-shell quinonoid, whereas P1 has been shown to be a diradical that is sensitive toward air and moisture. H0 and H1 have been shown to be quinonoids, whereas H2 has been characterized as a remarkably stable diradical with singlet ground state owing to the effective spin delocalization over the whole conjugated circuit.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701355