Demonstration of Intermolecular Triplet-Singlet FRET in Dye-Doped PVA Films at Room Temperature.

We report the first observation of triplet-singlet resonance energy transfer at room temperature without involving the intersystem crossing process. Triplet (T1) to singlet (S1) Förster resonance energy transfer (FRET) has been measured at room temperature with a long-wavelength direct donor's triplet-state excitation. The donor coumarin 106 (C106D) and acceptor rhodamine 101 (R101A) were embedded in thin poly(vinyl alcohol) (PVA) films. The direct excitation of the C106D triplet state was at 470 nm, well outside the absorption, which avoids the donor singlet-state excitation and its involvement in the FRET process. The intensity of C106D decreases in the presence of R101 and is accompanied by an increase of acceptor R101A emission. The observed FRET results in a red glow of the illuminated area lasting hundreds of milliseconds. The FRET measurements with direct triplet-state excitation can also be used to estimate the photophysical parameters of the donor triplet state.