Abstract
The electric field effect on several of the bands in the phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde, isolated in a durene single crystal at 4.2 K, was measured. While the weak origin of the first triplet state at 4140 Å was sufficiently narrow (∼1 cm−1) to resolve the Stark splitting, the intense bands in the region previously assigned to the second triplet state near 4060 Å were broad (∼10 cm−1). A selective laser excitation technique was developed to determine the Stark splitting of these bands. It was found that the most intense peaks in the 4060 Å region have the same dipole moment difference vectors, within the measurement uncertainties, as the T1 origin. This result is unexpected since the first triplet state is known to be ππ* in character, while the second is thought to be nπ*. An explanation of these observations in terms of coupling of the T2 (nπ*) state to a set of nearby T1(ππ*) vibronic states is proposed.
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