Excitation Energy Transfer and Quenching in a Heterodimer: Applications to the Carotenoid-phthalocyanine Dyads

The dynamics of a molecular heterodimer composed of a long-lived excitation donor and a short-lived acceptor (quencher) is examined. In order to consider various dynamical regimes without any restrictions on the system parameters, the energy transfer is modeled employing the hierarchical equations of motion, while the relaxation to the ground state is treated by assuming a phenomenological spontaneous nonradiative decay rate. Time scales of the resulting two-exponential evolution are investigated as functions of the energy gap and the resonance coupling between the monomeric constituents of the dimer. Relevance of the present analysis to the recent experimental findings on artificial carotenoid-phthalocyanine dyads is discussed. By examining the first two time scales of the reported time-resolved spectra, it is shown that upon the increase of carotenoid conjugation length its first excited state approaches the first excited state of phthalocyanine from above, thereby inducing a remarkable quenching. The proposed model also provides a unified treatment of quenching in the regimes previously distinguished as energy transfer and excitonic state formation.