Electronic structure of pendant-group polymers: Molecular-ion states and dielectric properties of poly(2-vinyl pyridine)

A spectroscopic characterization of poly(2-vinyl pyridine) is presented for the purpose of utilizing this material as a test case to examine the validity of a new theory of the electronic structure of saturated-chain pendant-group polymers. In this theory, which we refer to as the molecular-ion model, low-energy electronic excitations of the polymer are regarded as molecular-ion (charge injection) or molecular-exciton (absorption of electromagnetic radiation) states associated with the pendant-group moieties. These states are modified in the polymer relative to the corresponding states in gas-phase molecular moieties by virtue of being coupled to static and dynamic polarization fluctuations characteristic of the polymer matrix. The static fluctuations localize photogenerated hole states (i.e., valence molecular cations), and provide the predominant cause of the width of their (inhomogeneous) distributions in energy, which are called the densities of valence states (DOVS). The dynamic fluctuations further broaden the DOVS. Moreover, the electronic component of these fluctuations generates a relaxation-energy shift of the DOVS in poly(2-vinyl pyridine) to 1.5 eV higher in energy than in 2-ethyl pyridine. Comparison of the $h\nu =40.8\mathrm{}$-eV photoemission spectra of poly(2-vinyl pyridine) with those of pyridine and 2-ethyl pyridine reveals that, except for the fluctuation-induced relaxation energy shift and broadening, the low-ionization-potential cation states of the polymer are identical to those of the corresponding model molecule. An analogous result is obtained for the 2600-Å exciton absorption band. The far-uv electronic contributions to the dielectric function of poly(2-vinyl pyridine) are obtained by keV-electron-energy-loss spectroscopy. The absorption coefficients calculated from this dielectric function agree within a few percent with those measured directly using synchrotron radiation. A complete description of the dielectric response of poly(2-vinyl pyridine) is proposed that incorporates contributions from the low-frequency torsional modes, the infrared molecular vibrational modes, and the C and N core-level excitations, as well as from the valence electrons (i.e., excitons). This description is utilized to evaluate the influence of polarization fluctuations on the DOVS within context of the molecular-ion model. The model predictions are shown to be in agreement with both the photoemission and the contact-charge-exchange spectra of poly(2-vinyl pyridine). Of equal importance, it predicts correctly the relative magnitudes of the intermolecular contributions to the relaxation energies both in polar poly(2-vinyl pyridine) and in nonpolar polystyrene: a result that earlier models of solvation in dielectric media fail to describe. We conclude, therefore, that the molecular-ion model provides a quantitative and unique description of injected charges in the two saturated-backbone pendant-group polymers poly(2-vinyl pyridine) and polystyrene.