Higher-Order Stark Spectroscopy: Polarizability of Photosynthetic Pigments

Stark spectra for nonoriented molecules with typical broad inhomogeneous line widths are obtained by applying an ac electric field and detecting the change in absorbance at the second harmonic of the field modulation frequency. The analysis of such data in terms of molecular parameters such as changes in dipole moment and polarizability is often difficult. We outline a new method, called higher-order Stark spectroscopy, in which the higher even-powered harmonics of the response to an ac field are measured. The method is applied to electronic states of photosynthetic pigments, including pure monomeric bacteriochlorophyll, the homoand heterodimer special pair primary electron donors in photosynthetic reaction centers, and the carotenoid spheroidene, both pure in an organic matrix and in the B800-850 antenna complex. It is shown that even at a qualitative level these systems divide into two groups: those where the change in dipole moment dominates the change in polarizability (pure bacteriochlorophyll, the heterodimer, and spheroidene in the antenna complex) and those where the polarizability dominates the change in dipole moment (pure spheroidene and the homodimer special pair).