The electronic spectrum of benzvalene

A detailed investigation of the electronic spectrum of benzvalene tricyclo (3.1.0.0^2,6)hex-3-ene] in the vapor phase is reported. The spectrum appears to consist of two overlapping electronic transitions above 200 nm. The lower energy transition is broad and structureless, has an onset near 285 nm, and is assigned as ${}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1$. The second transition, showing forbidden vibronic structure based on two false origins near 226 nm, is assigned ${}^1\text{A}{}_2\text{←}{}^1\text{A}{}_1$. The ¹A₂ state presumably arises from a $\text{σπ}{}^1$ configuration, which indicates significant ring strain and/or σ-π interaction. A one-state assignment of the spectrum is also possible.