Substituent position-dependent photophysics in fluorinated porphycenes

Meso-fluorinated porphycenes reveal photophysical properties that vary significantly with the number and position of the substituents. 9,10-difluoro-2,7,12,17-tetra-tert-butylporphycene emits weakly, whereas two derivatives bearing the fluorines on the oppositemesopositions: 9,19- and 9,20- exhibit much higher fluorescence quantum yields and longer decay times. For weakly emitting porphycenes, fluorescence can be increased by placing the chromophore in a viscous solvent. The same effect is observed for two novel porphycenes in which a methyl group is placed next to a fluorine: 9-fluoro-2,7-di-tert-butyl-10,19-dimethylporphycene and 9,20-difluoro-2,7-tert-butyl-10,19-dimethylporphycene. These results can be explained by calculations that reveal, in the lowest excited singlet state, a highly nonplanar structure, from which rapid depopulation toS₀can occur. The energy required to attain such geometry is of the order of a few kcal/mol. The relative energies calculated for the three difluorosubstituted porphycenes correlate well with the experimentally obtained fluorescence quantum yields and lifetimes. Based on these results, we propose a model that postulates that the nonradiative deactivation channel in the lowest excited singlet state of porphycenes originates from geometry distortion due to the loss of aromaticity.