Temperature and solvent-polarity dependence of the absorption and fluorescence spectra of a fixed-distance symmetric chlorophyll dimer

Temperature-dependent absorption and fluorescence spectra and line-narrowed fluorescence and excitation spectra are reported for a dimer consisting of two methylpyrochlorophyllide a molecules that share a vinyl group at the 2-position of each macrocycle. The data (obtained for three solvents of widely differing polarity) show that the dimer exists in two conformations (A and B) and that excited-state relaxation from A to B onsets near the glass transition temperature (T_g). Molecular modeling suggests tha the two conformations are related by "bicycling" of the two single bonds joined to the vinyl group linkage. At sufficiently low temperature, the solvent dynamics are rate limiting for the conformational relaxation. For a solvent of sufficiently high polarity (DMF), the excited state of B is shown to access a new radiationless decay channel for T ≳ T_g. A charge-transfer state is suggested to be important for this decay. The model presented is shown to provide a qualitative explanation for the frequency domain and recently obtained picosecond and fluorescence quantum yield room-temperature data.