Singlet—Triplet Transitions in p-Dihalogenated Benzenes

Abstract

The polarized absorption and also some emission spectra of single crystals of p‐dichlorobenzene (DCB) and p‐dibromobenzene (DBB) at 4.2°K are presented and vibrationally analyzed. The absorption to the lowest triplet states (T₁) is sharp and displays a pure exciton origin in both crystals; at 27 890 cm⁻¹ in DCB, and at 27 910 cm⁻¹ in DBB. The assignment of these origins is confirmed by emission at 4.2°K. Modes belonging to species a_g (all but one of the possible six are observed), b_2g, b_3g, b_1u, b_3u, and a_u are observed in the spectra, and the polarizations of the individual vibronic bands are presented. The bulk of the transition is polarized normal to the molecular plane. The identification of long‐axis polarization in the 0, 0 band and normal polarization in a b_2g vibronic level suggests an assignment ³B_2u for the spatial character of the lowest triplet states. The T₁→S₀ emission of DBB is very sharp and consists of an alternating intensity pattern in a long (∼20) sequence of the 278 b_2g ground‐state mode. From the analysis we conclude that the state T₁ is slightly distorted into a trans chair form, and that the upper state contains a double minimum along this mode. Crystal‐field effects on the spectra T₁←S₀ are discussed in relation to lattice structure—which is prominent in the spectrum of DBB but not DCB—and in relation to the polarization ratios, and intermolecular spin—orbit coupling. The origin bands of DBB and DCB consist of triplets at about 0, 0+4.3, and 0+10.3 cm⁻¹ and some possible reasons for this are discussed. The temperature dependence of the 0, 0 band of DBB is attributed to a subtle change in crystal structure with temperature. For completeness the S₁←S₀ spectra were also examined, and a comparison is made of the vibrational frequencies in the states S₀, S₁, and T₁. The mechanisms of spin—orbit coupling in dihalobenzenes is examined in detail with reference to the symmetry and polarization assignments. Also a brief theoretical justification for the appearance of u‐vibrational modes in the transition is given.