Optical properties of 3(nπ*) benzophenones in 4, 4′-dibromodiphenylether host crystals

Abstract

A variety of optical and zero-field optically-detected magnetic resonance (zf ODMR) experiments have been performed on the lowest (nπ*) triplet states of benzophenone (BP-h 10), perdeuterobenzophenone (BP-d 10), and 4,4′-dibromobenzophenone (4,4′-DBBP) in single crystals of 4,4′-dibromodiphenylether (DDE) at temperatures of 1·4 K and above. The data obtained include high resolution phosphorescence, phosphorescence excitation, zf ODMR, amplitude-modulated zf ODMR phosphorescence excitation, and single-sublevel phosphorescence spectra as well as the guest sublevel phosphorescence lifetimes, relative radiative and non-radiative decay constants, intersystem crossing rates, and steady-state populations. At short wavelengths, the guest triplets are also populated via two energy transfer routes (S 0 H → T 1 H ↝ T 1 G and S 0 H → S 1 H ↝ T 1 H ↝ T 1 G); hence, the relative radiative rate constants and intersystem crossing rates of the host 3(ππ*) state sublevels have also been measured. The results are analysed to determine the principal spin-orbit (SO) and spin-orbit plus vibronic (SOV) mechanisms responsible for the population and depopulation of the spin sublevels of the parent molecule and its derivatives. A localized model suffices for triplet BP-h 10 in which the z sublevel is most active. The S 1 ↝ T 1 intersystem crossing process is governed primarily by SOV terms involving 3(ππ*) [and possibly 1(ππ*)] states and vibrations of a 2 symmetry, the T 1 → S 0 radiative process by SO coupling with 1(ππ*) states, and the T 1 ↝ S 0 non-radiative process by SO coupling with the ground state. Deuterium substitution has only a small effect on the radiative and non-radiative properties of 3(nπ*) BP. In contrast, bromine substitution has a large (and selective) effect; this is a consequence of n and π spin delocalization. All ISC rates and decay constants of 4,4′-DBBP are larger than those of the parent molecule. This is attributed to the increased importance of SO coupling with 1(σπ*) and 1(nσ*) states having significant heavy-atom character. The reasons for the preferential enhancement of Kx and Ky, kx r, and ky nr and kz nr are discussed. New vibronic coupling routes influencing the radiative decay are also introduced by the substitution of bromine atoms; these involve totally symmetric vibrations, affect only the Tx and Ty sublevels, and produce a decrease in the relative intensity of the carbonyl-stretching bands in the emission spectra of these sublevels. Finally, it is shown that correlations exist between the optical and magnetic properties of 3(nπ*) benzophenone and between the changes in these properties which are produced by heavy-atom substitution.