PROPERTIES OF THE EXCITED-SINGLET STATES OF BACTERIOCHLOROPHYLL-A AND BACTERIOPHEOPHYTIN-A IN POLAR-SOLVENTS

Several approaches were used to characterize the lowest excited-singlet states of bacteriochlorophyll a (BChl-a) and bacteriopheophytin a (BPh-a). Fluorescence quantum yields for BChl-a measured in five polar solvents ranged from 11% in methanol to 20% in pyridine; BPh-a had a fluorescence yield of 8% in methanol and 10% in acetone. In each solvent, the emission spectrum did not show mirror symmetry with the absorption spectrum: the fluorescence spectrum was narrower, and its vibrational shoulder was less pronounced and occurred at shorter wavelengths than predicted from the absorption spectrum. The apparent temperature of the excited state, as calculated by the Stepanov relationship, was higher than the ambient temperature (296 K) by about 30 K. The anomalous fluorescence properties can be explained qualitatively by inhomogeneous broadening and vibronic mixing in the excited state. In spite of these anomalies, the fluorescence yields are close to the yields calculated on the basis of previously measured fluorescence lifetimes (Connolly, J. S.; Samuel, E. B.; Janzen, A. F. Photochem. Photobiol. 1982, 36, 565-574) and calculated radiative lifetimes, indicating that the overall vibrational partition functions are similar in the ground and excited states. Picosecond transient absorption spectra were measured with BChl-a in methanol, 1-propanol, and pyridine, using 605-nm excitation pulses with a width of about 0.8 ps. The spectra in the 780-nm region are characterized by a sharp negative trough that shifts to the red with time. The shifting probably reflects adjustments of the stimulated-emission spectrum in response to dielectric relaxation of the solvent around the excited molecule. The relaxation in pyridine is well described by a single exponential with a time constant of 2.7 +/- 0.1 ps. The shifting in the alcohols can be fit by biexponentials with time constants of 1.5 +/- 0.2 and 18 +/- 3 ps in methanol, and 1.1 +/- 0.4 and 82 +/- 25 ps in 1-propanol. By using an actinometric method to determine the number of molecules in the excited state, the absorption changes at long delay times were used to calculate the S1 --> n absorption spectrum of the thermally equilibrated excited state. The broad, featureless spectrum has an absorption cross section sigma-1 --> n of (1.0 +/- 0.2) x 10(-16) cm2 at 750 nm.