The Mechanism of Spectral Shift and Inhomogeneous Broadening of an Aromatic Chromophore in a Polymer Glass

Abstract

We have attempted to obtain microscopic-level understanding of the absorption band of the chromophore s-tetrazine in a glassy polymer matrix (atactic polypropylene) at low temperatures. Our investigation has focused on the bathochromic shift of the lowest-energy π ← n (1B3u← 1Ag) electronic transition of the chromophore in the polymer matrix as well as on the width of the inhomogeneously broadened absorption band. The absorption spectrum of s-tetrazine in atactic polypropylene was measured over a range of temperatures for comparison with modeling results. Information on the geometry and the electronic structure of s-tetrazine in the ground and excited states was obtained from ab initio calculations. We have generated several polymer microstructures with imbedded chromophore molecules and used classical NpT molecular dynamics simulations to obtain ground-state trajectories of the chromophore. The classical Franck—Condon principle was invoked to calculate the average solvent shift. The dominant dispersion contribution to the solvent shift was calculated both, using an empirical parametrization of pair-potentials for the excited state (Kettley et al. Chem. Phys. Lett. 1986, 126, 107–12) and using the semiempirical theory of Shalev et al. (J. Chem. Phys. 1991, 95, 3147–66). The very encouraging results obtained indicate that the concurrent use of ab initio calculations, semiempirical methods, and classical simulation techniques can provide valuable insights into the complex microscopic interactions in low-temperature amorphous materials. It is also anticipated that such computational investigations may become valuable supplements to line-narrowing and single-molecule spectroscopic investigations of amorphous polymers at low temperatures. © 1995, American Chemical Society. All rights reserved.