Circular and Magnetic Circular Dichroism Studies of BacteriochlorophyllcAggregates:  T-Shaped and Antiparallel Dimers

Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra have been measured on (3 1 R )-bacteriochlorophyll (BChl) c aggregates in organic solvents. BChl c exists as a monomer in acetone with a Q y (0−0) transition at 661 nm. In a mixed solvent system of methanol and dichloromethane (CH 3 OH/CH 2 Cl 2 = 1/4000), another Q y (0−0) transition appeared at 680 nm with a nondispersion-type CD signal. The ratio of MCD intensity to its dipole strength ( B / D ) for the Q y (0−0) transition is half the value of its monomer species, indicating a dimerization of BChl c . The apparent inconsistency between the CD and MCD results has been reconciled by assuming the formation of BChl c dimer with a T-shaped conformation. In neat CCl 4, a dispersion-type CD signal was observed for the Q y (0−0) exciton components at 680 and 710 nm. MCD spectra of the exciton-type Q y (0−0) components suggest that two BChl c molecules are parallel with their macrocycle planes as judged by the mixing effect between the excited states of Q x (0−0) and Q y (0−0). The CD and MCD results are consistent with the fact that a BChl c dimer with an antiparallel conformation is dominant in the neat CCl 4 as reported previously. Corresponding Q x (0−0) transitions were determined around 650 nm by the simultaneous deconvolution of absorption, CD, and MCD spectra. It is demonstrated that the antiparallel BChl c dimer shows two exciton components for both Q x (0−0) and Q y (0−0) transitions. Finally, we evaluated solvent dependence of the stability of the BChl c dimer by two-dimensional exchange spectroscopy (EXSY) experiments.