Excited state dynamics of Rhodamine B and its excitonically coupled dimer: A computational and experimental approach

Rhodamine B (RhB) is widely used for its high fluorescent quantum yield and strong absorption in the visible range of the spectrum; however, its facile aggregation in aqueous solutions dramatically affects its photophysical properties. Surprisingly, the excited state dynamics of rhodamines are not fully understood despite the extensive literature around them. Here, we present a complete picture of the RhB and the most likely dimer formed in aqueous solution. We deconvolved the excited state dynamics of RhB and its non-covalent dimer using a combination of steady-state (SS) and time-resolved spectroscopic methods supported by density functional theory (DFT) and time-dependent DFT (TD-DFT). In particular, we have determined the dimerization equilibria constant in sodium perchlorate solutions and measured the dimer’s photo-induced charge separation and charge-recombination events. Additionally, we studied 12 DFT exchange–correlation functionals (XCF), and determined which are appropriate and which are not appropriate for studying and interpreting the excited state dynamics observed through femtosecond transient absorption (fsTA).