Investigation of singlet-triplet transitions by phosphorescence excitation spectroscopy. X A simple α, β-unsaturated ketone

Abstract

X-ray diffraction techniques and Patterson methods have been used to determine the crystal structure of the simple conjugated enone, ketone I. The crystals studied belong to space group P1 with a = 9.365(6), b = 6.578(4), c = 8.321(5), α = 101.70(3), β = 101.55(3), and γ = 101.37(3). Least squares refinement of 836 counter-collected, unique reflections having | F |2>2.5σ(| F |2) led to a final value of the conventional R1 factor (based on F) of 5.8%. The structure consists of rather loosely packed, strain free bicyclic molecules, in which the enone chromophore is planar. Phosphorescence excitation spectroscopy was used to obtain the polarized singlet-singlet and singlet-triplet absorption spectra of oriented crystals. The important results are: (i) The ordering of excited states in this molecule is (n, π*)>1(n, π*)>3(π, π*)3. (ii) Polarization measurements and Franck-Condon considerations indicate that the difference in geometry between the 3(π, π*) state and the gound state is small. (iii) The 1(n, π*) state appears to be the exclusive source of intensity for the origin band of the Tπ,π*← S0 transition. (iv) The Tπ,π*← S0 absorption spectrum indicates that there is extensive vibronic mixing between the 3(π, π*) state and the nearly degenerate 3(n, π*) state. (v) Vibronic mixing is much less important in the emission spectrum. Consequently, the Tπ,π*⇄ S0 absorption and emission spectra are far from mirror-image symmetric, and the observed radiative lifetime of the triplet state is quite different from that computed from the T← S0 absorption. (vi) The polarization of the Tn,π*← S0 transition can be accounted for in terms of spin-orbit coupling between the 3(n, π*) and 1(π, π*) states. (vii) The complete absence of vibrational structure in the Tn,π*← S0 spectrum is attributed to extensive mixing between the 3(n, π*) state and degenerate vibronic levels of the lower lying 3(π, π*) state. (viii) The significant in-plane polarization in the Sn,π*← S0 transition is attributed to perturbations of the planar enone chromophore by out of plane hydrogen and methyl groups which introduce intensity that is in-plane polarized but perpendicular to the C=O axis. (ix) Much of the intensity in the Sn,π*← S0 transition is vibronically induced and polarized along the C=O axis.