DiiodoBodipy-Rhodamine Dyads: Preparation and Study of the Acid-Activatable Competing Intersystem Crossing and Energy Transfer Processes

Iodo-bodipy/rhodamine dyads with cyanuric chloride linker were prepared with the goal of achieving pH switching of the triplet excited state formation. The pH switching takes advantage of the acid-activated reversible cyclic lactam <-> opened amide transformation of the rhodamine unit and the fluorescence resonance energy transfer (FRET). The photophysical properties of the dyads were studied with steady-state and femtosecond/nanosecond time-resolved transient absorption spectroscopies, electrochemical methods, as well as TD-DFT calculations. Our results show that the model dyad is an efficient triplet state generator under neutral condition, when the rhodamine unit adopts the closed form. The triplet generation occurs at the iodo-bodipy moiety and the triplet state is long-lived, with a lifetime of 51.7 mu s. In the presence of the acid, the rhodamine unit adopts an opened amide form, and in this case, the efficient FRET occurs from iodo-bodipy to the rhodamine moiety. The FRET is much faster (tau(FRET) = 81 ps) than the intersystem crossing of iodo-bodipy (tau(ISC) = 178 ps), thus suppressing the triplet generation is assumed. However, we found that the additional energy transfer occurs at the longer timescale, which eventually converts the rhodamine-based S-1 state to the T-1 state localized on the iodo-bodipy unit.