Reverse intersystem crossing in rhodamines by near-infrared laser excitation

The population of the long-lived first excited triplet state (T1) of a fluorescence dye represents a major limitation in single-molecule spectroscopy. Reverse intersystem crossing (ReISC) is one of the processes that may prevent considerable loss of luminescence. In the present quantum chemical study we have analyzed rhodamine A in aqueous environment. The T2 ⇝ S1 and T3 ⇝ S2 ReISC channels are predicted to be viable. The rate constant computed for the former channel is ≈2 × 10(6) s(-1). Hence, an excitation with suitable wavelength to one of the triplets should help repopulate the optically bright singlet state S1.