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 K_x and K_y, k_x ^r, and k_y ^nr and k_z ^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 T_x and T_y 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.