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

Abstract

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-temperature region, is seen to fail dramatically at lower temperatures as the short-wavelength fluorescence feature grows. The short-wavelength feature is interpreted as fluorescence resulting from an excited electronic state that is conformationally unrelaxed. At temperatures below 178 K evidence for additional spectroscopic features appears, especially in conjunction with measurements of emission spectra using different excitation wavelengths and of excitation spectra of fluorescence measured at different emission wavelengths. This is in the region of matrix glass formation, and the new BChl a components may reflect site inhomogeneity. Similar spectroscopic studies of BChl a in non-hydrogen-bonding solvents do not provide evidence of new blue-shifted fluorescence in the 298−79 K temperature range. They do, however, exhibit evidence of site inhomogeneity in the low-temperature glass matrices. Implications are discussed regarding the interpretation of low-temperature spectral features that have been reported for photosynthetic membranes and isolated pigment−proteins.