High pressure luminescence studies of indole and 5-methoxyindole in polymethylmethacrylate

Abstract

High pressure luminescence measurements have been made on indole and 5-methoxyindole in PMMA. At both 300 and 110 K the fluorescence peaks shifted to lower energy and the quantum yield decreased with increasing pressure. The peak shift at 110 K was identical with that at 300 K, and the quantum yield increased by a pressure independent factor over the temperature range studied (2.3 for indole and 1.8 for 5-methoxyindole). At 110 K pressure shifted the phosphorescence peaks to lower energy but less rapidly than it shifted the fluorescence peak. The observed phosphorescence lifetime dropped with increased pressure. The loss of fluorescence quantum yield is explained as due to an increase in the intersystem crossing rate. This intersystem crossing scaled quite well with the S1–T1 energy difference. Differences in behavior between indole and 5-methoxyindole may be related to differences in the energies of the triplet states involved.