On the possibility of direct triplet state excitation of indole.

We studied the luminescence properties of indole in poly (vinyl alcohol) (PVA) film. The indole molecules are effectively immobilized in this polymer film and display both fluorescence and phosphorescence emission at room temperature. We noticed that the phosphorescence of indole in PVA film can be effectively excited at a longer wavelength than its typical singlet to triplet population route involving intersystem crossing. The maximum of the phosphorescence excitation is about 410 nm which corresponds to the energy of indole's triplet state. Interestingly, the phosphorescence anisotropy excited with the longer wavelength (405 nm) is positive and reaches a value of about 0.25 in contrast to the phosphorescence anisotropy excited within the indole singlet absorption spectrum (290 nm), which is negative. Very different temperature dependences have been observed for fluorescence and phosphorescence of indole in PVA film. While fluorescence depends minimally, the phosphorescence decreases with temperature dramatically. The fluorescence lifetime was measured to be a single component 4.78 ns while the intensity weighted average phosphorescence lifetime with 290 nm and 405 nm excitations were 6.57 and 5.62 ms, respectively. We believe that the possibility of the excitation of indole phosphorescence in the blue region of visible light and its high anisotropy opens a new avenue for future protein studies.