Higher-Order Stark Spectroscopy: Polarizability of Photosynthetic Pigments

Abstract

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). A quantitative analysis of the Stark effect spectrum provides information on molecular properties associated with the movement of charge such as the change in dipole moment (&), polarizability (Aa), and hyperpolarizability for an electronic or vibrational2 transition. For a uniaxially oriented system, the contributions from Ap and Aa can be readily distinguished as the former depends linearly on the field, while the latter depends quadratically on the field. However, for isotropic, immobilized samples (frozen glasses or polymer films), which are far easier to study and are often the only conditions under which samples can be studied, the contributions from Ap and Aa, as well as all other electrooptic parameters, depend on the same power of the field; consequently, the contributions can only be obtained by analyzing the Stark line shape. In the following we outline a new experimental method, called higher-order Stark spectroscopy, for obtaining more information than was previously possible about the electrooptic properties of molecules. We specifically apply this method to several photosynthetic pigments. By comparing the higher-order Stark spectra for several different types of chromophores in different environments, it is possible, even at a qualitative level. to obtain information on the relative contributions of Ap and Aa. We find directly that the first electronic excited state of the special pair primary electron donor P in photosynthetic reaction centers (RCs) has a very large polarizability, a conclusion which was previously suggested from conventional low-temperature Stark spectroscopy via a complex line shape analysi~.~ In a subsequent paper, we will show that a quantitative analysis of the higher-order Stark spectra can be used to obtain the diagonal elements of the polarizability tensor of P, and this information can be used