Solvent effects on photophysical properties of copper and zinc porphyrins

The photophysics of Zn(tetraphenylporphyrin,TPP), Zn(tetra-2,4,6-trimethylphenyl porphyrin, TMP), Zn (tetra-(o-dichlorophenyl) porphyrin, TPPCl8), Cu(tetraphenylporphyrin,TPP), Cu(tetra-2,4,6-trimethyl- phenyl porphyrin,TMP), and Cu(tetra-(o-dichlorophenyl) porphyrin, TPPCl8, TPPCl8) in several solvents have been investigated on steady state and time-resolved spectroscopy. The Cu(TPPCl8 ) is normal and shows no evidence of CT transition in the visible or near UV regions in nonpolar solvent. However, Cu(TPPCl8)shows a blue shift in the absorption spectrum and intramolecular CT bands at absorption spectra in polar solvent, which shows a fluorescence maximum emission at 650 nm and 8.4 ns lifetime. The reason can be attributed to two points. Firstly, the increase of solvent polarity can enlarge outer reorganisational energy, which is favorable to reduce the activation free energy of charger-transfer transition based on Marcus theory of electron transfer. Moreover, the internal heavy-atom effect on Cu(TPPCl8) is encouraging to stabilize the 2T1 state also, which increases the possibility of population to CT band from 2T1 state. This result is in accord with an earlier estimate of a 10 ns lifetime and CT absorption at 640 nm bands for the CT state of Cu (II) octethylporphyrins. Other possible reasons arousing unusual fluorescence like H-bonding, axial ligands, molecular aggregation are excluded.