Luminescence properties of 5-Bromoindole in PVA films at room temperature: Direct triplet state excitation

We studied spectral properties of 5-Bromoindole (5-BrI) in poly (vinyl alcohol) (PVA) film. The fluorescence of 5-BrI in PVA films is similar to indole in PVA film but strongly quenched compared to unsubstituted indole. The quantum yield of 5-BrI is only 0.0034 comparing to 0.26 of indole. In contrast, phosphorescence of 5-BrI in PVA is comparable to indole phosphorescence in magnitude and lifetime but its emission shifted towards longer wavelengths by 50 nm for a maximum at 530 nm. The phosphorescence of 5-BrI in PVA can be effectively excited at longer wavelengths (450-500 nm). The maximum of phosphorescence excitation spectrum of the 5-BrI in PVA is at about 480 nm. This direct excitation to the triplet state results in high positive phosphorescence anisotropy of 0.2. Although the S-0 -> T-1 absorption transition is only about 800 fold weaker from the main 280 nm UV absorption band yielding measurable trace of absorption for 5-BrI in PVA film in the visible/blue range. We believe this is a first report of singlet-triplet absorption (S-0 -> T-1) at room temperature, not involving external heavy atoms or oxygen presence. The comparison of phosphorescence of 5-BrI in PVA film to Rhodamine 6G in ethanol fluorescence allows us to estimate the phosphorescence quantum yield of 5-BrI in PVA to be similar to 0.02. We measured independently fluorescence and phosphorescence lifetimes and quantum yields as well as phosphorescence excitation spectrum across all wavelengths from UV to the visible range. These measurements were used to determine rate constants for all processes involved the in 5-BrI in PVA excited state deactivation.

Automated summary

The study revealed that the fluorescence intensity of 5-BrI in PVA film is low, while its phosphorescence shows similarities to indole in PVA, but with some differences. By comparing various spectra and quantum yields, we were able to determine the rate constants for the excited state deactivation process of 5-BrI in PVA film.