Solvent Polarity Dependent Photophysics of a Fixed‐Distance, Symmetric Chlorophyll Dimer. A Model of the Special Pair in Photosynthetic Reaction Centers

The synthesis and properties of a fixed‐distance symmetric bis‐pyrochlorophyllide‐a molecule are described. The molecule, 1, consists of two methyl pyrochlorophyllide‐a (MePChlide‐a) moieties which share a common vinyl group at the 2‐position of each macrocycle. The two chlorophylls are trans to one another across the vinyl linkage. The dihedral angle between the plane of each macrocycle and the vinyl linkage is about 50°. The Q_y absorption band of this dimer occurs at 689 nm and is essentially independent of solvent. The band is red‐shifted by about 27 run relative to that of MePChlide‐a. The fluorescence maximum of 1 occurs at 722 nm. This is a red‐shift of 49 nm relative to methyl MePChlide‐a. The circular dichroism spectrum of 1 shows strong, nonconservative bands in the red region. The fluorescence quantum yield, fluorescence lifetimes, and picosecond transient absorbance kinetics all exhibit a very strong dependence on solvent polarity. The fluorescence quantum yields of 1 in toluene, butyronitrile, and N, N‐dimethyl formamide (DMF) are 0.18, 0.008, and 0.002, respectively. The bleach of the 689 nm band (following excitation with a 1.5‐ps, 610‐nm laser pulse) recovers with single exponential time constants of 2.51 ns, 100 ps, and 64 ps in toluene, butyronitrile, and DMF, respectively. Possible mechanisms for the polarity‐dependent increase of the nonradiative decay rate of 1 are considered.