Rotationally resolved electronic spectroscopy of 2,3-bridged indole derivatives: Tetrahydrocarbazole

The rotationally resolved electronic spectrum of tetrahydrocarbazole seeded in a molecular beam has been taken. The lowest excited singlet state of tetrahydrocarbazole could be shown to have La character, in contrast to nearly all other indole derivatives, which have the Lb as lowest state. An upper limit for the energy difference of both states can be estimated to be 1000 cm1 with the Lb state being structurally and energetically close to the intersection of both states. Comparison of the rotational constants and the vibrational wavenumbers to the results of ab initio calculations shows, that the saturated six-ring is in a twisted configuration in both electronic states.