ELECTROCHROMISM OF CHLOROPHYLL-A MONOMER AND SPECIAL PAIR DIMER

The quadratic Stark effect was measured for chlorophyll monomer and dimer (Chl . ethanol)2 in isotropic glassy solutions at low temperatures. The change in the permanent dipole moment, DELTA-mu, of chlorophyll monomer on excitation in the red absorption maximum was found to be (1.02 +/- 0.09) D for both mono- and disolvate. It forms an angle delta = 20-degrees with the electronic transition moment Q(y). The electronic excitation of the dimer within its main absorption band at 703 nm is accompanied by the change in dipole moment delta-mu = (5.17 +/- 0.2) D, for which delta = 23-degrees. A remarkable contribution from polarizability change, DELTA-alpha, to Stark spectra of both monomer and dimer was found within the Q(y) band. In the monomer, DELTA-alpha is several times higher in the region of vibrational side bands, where it is related to the second electronic transition (Q(x)). The large increase in delta-mu for the dimer as compared to the monomer is ascribed to charge-transfer contribution to the dimer excited state, due to significant overlap of electronic densities in the constituent monomers.