Temperature Dependence of Optical Spectra of Bacteriochlorophyll a in Solution and in Low-Temperature Glasses

Absorption, fluorescence emission, and fluorescence excitation spectra of bacteriochlorophyll a [BChl a] are examined throughout the temperature range from 298 to 79 K in several glass-forming solvents. Changes in the absorption spectra that occur continuously throughout this range may reflect increased extent of coordination of the central Mg, changes in solvent dielectric, and/or altered hydrogen bonding. Fluorescence emission spectra exhibit a new feature that grows steadily, beginning at temperatures below about 250 K in solvents that are hydrogen-bond donors:  1-propanol and 2-propanol. The emerging fluorescence band, located about 300 cm-1 to the blue of the fluorescence band seen at higher temperatures, achieves nearly equal amplitude at 163 K and below. It is noteworthy that no corresponding feature appears in the absorption on the blue side of the Qy absorption band. The Kennard−Stepanov relation between absorption and fluorescence, which holds with somewhat elevated T* values in the high-tempera...