The electronic spectra of thieno[3,2,-b]thiophene

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Abstract
The uv spectrum of thiophthene is investigated in the vapor and crystal phase. Two intense absorption systems in the region 36 000–40 000 cm−1 are assigned to two allowed electronic transitions to Bu excited states, while a very weak absorption observed in the crystal spectrum at 34 394 cm−1 is assigned to a single–triplet transition allowed by spin‐orbit interactions. Ab initio CI computations of the energies of the excited states and relevant singlet–triplet spin‐orbit matrix elements provide support for the assignment. The vibrational analysis of the first intense absorption system suggests that the molecule has a distorted geometry in the first excited electronic state. An explanation is offered of the very different vibrational structure of the vapor and crystal spectra.