Investigation of adsorption behavior and energy transfer of cationic porphyrins on clay surface at low loading levels by picosecond time-resolved fluorescence measurement

Time-resolved fluorescence and steady-state spectroscopic measurements were performed with +4-charged cationic porphyrins adsorbed on an anionic-type clay (Sumecton SA; SSA) surface at a low molecular loading level (10 % vs. cation-exchange capacity of clay) corresponding to an occupied area of ca 50 nm2 per molecule. Absorption spectra indicated no interaction between transition moments of the porphyrins on the clay surface. An efficient energy-transfer process from donor to acceptor porphyrin was observed on the clay surface even under low porphyrin loading conditions. The efficiency of energy transfer obtained from the steady-state measurement was 65 %. Real-time behavior of the porphyrins was successfully captured during energy transfer. The rate constant of the energy transfer obtained from time-resolved fluorescence measurements was found to be 5.3 × 108 s−1. According to the efficiency and the rate constant, it is proposed that the adsorbed porphyrins did not have a uniform and fixed distribution.