Solvent dynamics regulated electron transfer in S(2)-excited Sb and Ge tetraphenylporphyrins with an electron donor substituent at the meso-position.
Electron and energy transfer processes from higher excited states are attractive for efficient photoenergy utilization because energy dissipating internal conversion processes can be avoided. In the present study, charge separation processes in meso-substituted Sb- and Ge tetraphenylporphyrins (TPPs) were investigated by means of laser spectroscopy. For both Sb- and GeTPPs, S2- and S1-fluorescence emissions were confirmed. Sb- and GeTPPs containing a meso-substituent with a higher electron donor-ability exhibited charge separation from the S2 state, and this was confirmed by transient absorption spectroscopy and a fluorescence up-conversion method. Charge separation was faster for SbTPP with a donor at the meso position than for SbTPP bearing a donor as an axial ligand. The faster charge separation of the present TPPs was attributed to a larger electronic coupling due to a larger HOMO electron density at the meso-carbon of the porphyrin ring. In addition, the charge separation processes in the present meso-substituted TPPs occurred under the adiabatic condition. Contribution of the intramolecular exciplex to the relaxation process was also indicated for the S1 state of GeTPPs.