Pathways for photoinduced electron transfer in meso-nitro-phenyl-octaethylporphyrins and their chemical dimers

The photophysical properties of meso-nitro-phenyl-octaethylporphyrins and their dimers with electron-accepting NO2 groups in the para-, meta- and ortho-positions of the phenyl ring were studied. For the ortho-NO2 case in deaerated toluene at 295 K, strong fluorescence quenching is caused by the intramolecular electron transfer from the porphyrin S1 state in the absence of phenyl ring librations around the single C–C bond (`normalʹ region, non-adiabatic case). T1 state lifetime shortening for the same compounds is explained by thermally activated transitions to upper-lying charge-transfer states of the radical ion pair as well as by the rise of the intersystem crossing T1 ⤳ S0 rate constants caused by T1 states mixing with charge-transfer states.