Photophysics of NH tautomers of 2-NO2-5, 10, 15, 20-tetraphenylporphyrin in solvents of different polarity

In the present work we study the spectral and photophysical properties of a relatively simple porphyrin system with an electron-withdrawing substituent, namely 2-nitro-5, 10, 15, 20-tetraphenylporphyrin (H2TPP-NO2). Similar β-nitro-substituted tetraaryl-porphyrins have been studied recently in [1–4], for which unusual fluorescence properties were reported. In particular, it has been shown that these compounds exhibit broad, structureless and significantly Stokes-shifted fluorescence spectra. The Stokes shift was found to increase with an increase of solvent polarity (dielectric constant). The authors [1–4] have interpreted this fluorescent state as having intramolecular chargetransfer (ICT) character of the [(P+) — (NO2)-] type. It has been even suggested [1, 3, 4] that the fluorescent state may be of the “Twisted Intramolecular Charge-Transfer” (TICT) origin, that corresponds to full electron transfer from the porphyrin ring to the NO2 group. The lifetimes of this unusual fluorescence state were found to be in the range of ∼1–3 ns [1–4].