Rose Bengal in Films: Electronic Structures, Dynamics, and Kinetics of Triplet States

Rose Bengal (RB), a classic xanthene dye containing iodine atoms, is well-known for its efficient triplet-state generation upon photoexcitation. In this work, RB was prepared in poly(vinyl alcohol) (PVA) films to investigate the electronic structures and the triplet state dynamics of the monomeric and dimeric RB. Transmission electron microscopic imaging revealed self-assembled ca. 2 nm RB dots forming a snowflake nanostructure in 10 wt % PVA films. Density functional theory (DFT) calculations indicated that RB adopted a parallelly displaced π–π stacking in dimeric configuration. Upon excitation of diluted RB in PVA (ca. 0.17 wt %) detected using step-scan Fourier-transform near-infrared (ssFTNIR) spectroscopy, the transient absorption at 8500–10,500 cm–1 was attributed to the T1–T3 transition of the monomeric RB with a lifetime of roughly 300 μs. As the RB concentration increased >1 wt %, another accelerated decay within ca. 30 μs appeared and was attributed to the relaxation of the triplet state of the RB dimer. The depopulations of monomeric and dimeric RB triplet states were both intensified via the redox reaction with the adjacent RB molecules upon increasing the concentration of RB. The corresponding electron transfer parameters (β) for the monomeric RB and dimeric RB were 1.3 and 6.3 nm–1, respectively, suggesting a potential depopulation of triplet RB by its adjacent unexcited RB molecules. Concomitantly, the triplet–triplet annihilation seems less important for the triplet dynamics of RB in films. These findings provide clues to deal with the electronic characteristics of organic dyes immobilized in solid matrices, serving as guidance for the application of advanced photofunctional materials.