Thienylquinonoidal Porphyrins and Hexaphyrins with Singlet Diradical Ground States

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

doi:10.1002/chem.201701355

Thienylquinonoidal Porphyrins and Hexaphyrins with Singlet Diradical Ground States

Chemistry. 2017 Jul 3;23(37):8969-8979. doi: 10.1002/chem.201701355. Epub 2017 Jun 13.

Authors

Koji Naoda¹, Daiki Shimizu¹, Jun Oh Kim², Ko Furukawa³, Dongho Kim², Atsuhiro Osuka¹

Affiliations

¹ Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan.
² Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul, 120-749, Korea.
³ Center for Instrumental Analysis, Niigata University, Nishi-ku, Niigata, 950-2181, Japan.

PMID: 28449348
DOI: 10.1002/chem.201701355

Abstract

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.

Keywords: diradical; hexaphyrin; porphyrin; quinonoid; singlet ground state.