High pressure luminescence studies of metalloporphyrins in polymeric media

Abstract

High pressure luminescence measurements have been made on free base tetraphenylporphyrin (H2–TPP), as well as on Zn–TPP, Mg–TPP, Cr–TPP, and Cu–octaethylporphyrin (Cu–OEP), in PMMA and PS. In the case of H2–TPP, Zn–TPP, and Mg–TPP the efficiency of fluorescence and phosphorescence (Zn–TPP) was pressure dependent as was the relative energy of the excited singlet and triplet states. The relative transfer rates have been calculated, and it was shown that the fluorescence efficiency is limited by intersystem crossing while the phosphorescence efficiency is limited by both radiationless rates S1→T1 and T1→S0. The rate S1→T1 scaled very well with the S1–T1 energy difference, and the rate T1→S0 with the T1–S0 energy difference. In the case of Cu–OEP and Cr–TPP only phosphorescence was observed from two states. At low pressure phosphorescence comes mainly from the high energy excited state (2T1), while at high pressures it comes from the low energy excited state (4T1). The excitation maximum was found to be strongly dependent on pressure. The large red shift of the excitation maximum causes changes in the feeding of 2T1 and 4T1 states, which result in an apparent increase of the radiative rates. Some experimental results on three phthalocyanines are also briefly presented and discussed.