title: DiiodoBodipy-rhodamine dyads: preparation and study of the acid-activatable competing intersystem crossing and energy transfer processes.
abstract: 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 μs. 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 (τFRET = 81 ps) than the intersystem crossing of iodo-bodipy (τISC = 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 S1 state to the T1 state localized on the iodo-bodipy unit.
authors.affiliation: †State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Rd., Dalian 116024, P. R. China.
authors.forename: Kejing
authors.initials: K
authors.lastname: Xu
authors.affiliation: ‡Department of Chemistry, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States.
authors.forename: Yun
authors.initials: Y
authors.lastname: Xie
authors.affiliation: †State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Rd., Dalian 116024, P. R. China.
authors.forename: Xiaoneng
authors.initials: X
authors.lastname: Cui
authors.affiliation: †State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Rd., Dalian 116024, P. R. China.
authors.forename: Jianzhang
authors.initials: J
authors.lastname: Zhao
authors.affiliation: ‡Department of Chemistry, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States.
authors.forename: Ksenija D
authors.initials: KD
authors.lastname: Glusac