Journal of Physical Chemistry B, Vol.101, No.13, 2372-2380, 1997
Structural-Changes for Pi-Radicals of Free-Base Tetraphenylbacteriochlorin - A Model for the Electron-Donor and Acceptor in Bacterial Reaction Centers
Resonance Raman spectra are reported for electrochemically generated radical cations and anions of tetraphenylbacteriochlorin free-base, H2TPBC, and also for the photogenerated triplet state T-1. Isotope substitution, including C-13 at the meso positions, and deuteration of the pyrrole or phenyl rings permit mode assignments and correlations with the ground state RR spectrum of H2TPBC, for which a normal coordinate analysis has been carried out. Both up- and down-shifts of skeletal mode frequencies are observed. These shifts are satisfactorily calculated for the radical cation and anion by altering the stretching force constants of bonds in the conjugated pi-system of bacteriochlorin. The force constant changes are in qualitative accord with expected bonding changes upon removing an electron from the HOMO or adding an electron to the LUMO. However the frequency changes in the T-1 state are much larger than expected from these bonding changes and are not reproduced by averaging the radical cation and anion force constants. Thus the T-1 molecular distortion is not simply related to the properties of the HOMO or LUMO.
Keywords:RESONANCE RAMAN-SPECTRA;PHOTOSYNTHETIC REACTION CENTERS;RHODOBACTER-SPHAEROIDES R-26;NEAR-INFRARED-EXCITATION;LIGHT-HARVESTING COMPLEX;FOURIER-TRANSFORM RAMAN;MUTANT REACTION CENTERS;BACTERIOCHLOROPHYLL-A;RHODOPSEUDOMONAS-VIRIDIS;EXCITED-STATES