Quantitative Experimental and Theoretical Analysis of Photoinduced Relaxation Processes in Self-Assembled Porphyrin Triads

摘要

The main non-radiative competitive relaxation processes (excitation energy transfer and photoinduced electron transfer) were quantitatively studied for self assembled triads based on Zn-octaethylporphyrin chemical dimer (the energy/electron donor) and dipyridyl substituted porphyrin extra-ligands (acceptors) in toluene using steady-state spectroscopy, time correlated single photon counting technique, and femtosecond pump-probe spectroscopy. It was found that the dimer fluorescence quenching and decay shortening (from ns to ps time scale) in triads reflect the manifestation of both indicated deactivation channels. The comparative role of the energy and electron transfer processes were experimentally tested upon temperature variation and solvent polarity changes (addition of acetone to toluene solutions) followed by calculations using Foerster and Marcus theories. In addition, for the porphyrin extra-ligand in the triads, a fluorescence decay time shortening (by ~ 1.3—1.6 times in toluene at 293 K) is observed which becomes stronger with increasing solvent polarity as well as temperature lowering (from 278 К down to 160 K). The possible reasons and mechanisms of the non-radiative deactivation of locally excited S. states in the triads are discussed taking into account a close lying charge-separated state. The experimental data obtained are analyzed using the reduced density matrix formalism.