Irradiation of 6,7-benzobicyclo[3.2.0]hepta-3,6-dien-2-one (2) in a rigid glass at -196 degrees C leads to the formation of 3,4-benzotropone (1), which exhibits a characteristic:uv-vis absorption extending to 535 nm and dimerizes to give syn- and anti-[pi 8 + pi 10] dimers 7 and 8 upon thawing the glass. The rate constant of the thermal dimerization of 1 is 12 +/- 3 M-1 s(-1) at -78 degrees C. Photochemically, 1 is stable under matrix isolation, but undergoes [pi 10 + pi 10] dimerization in solution. Compound I in a CFCl4-CF2BrCF2Br-CHCl3 glass at -155 degrees C shows an IR band at 1506 cm(-1) which is assigned to a vibration primarily involving C=O stretching mode by comparing the spectrum with that of O-18-labeled 1. Under the same conditions tropone exhibits a C=O vibrational band at a significantly higher frequency (1553 cm(-1)), suggesting that the 3,4-benzo annulation makes the tropone ring more strongly polarized. The large contribution of a polar resonance form Ib is reflected in the high affinity of 1 for protonation: 1 is nearly half-protonated in a 0.1 M solution of CHCl2CO2H in EPA (a 5:5:2 mixture of ether, isopentane, and ethanol). Furthermore, the electronic absorption spectrum undergoes a characteristic blue shift when the medium is changed from hydrocarbons to alcohols, suggesting that 1 is more polarized in the ground state than in the excited state. The generation of 1 from 2 is thermally effected also and, upon heating a mixture of 2 and olefin such as maleic anhydride or ethyl vinyl ether in benzene at 220 degrees C, adducts of the latter to 1 are obtained. The photochemical behavior of 2 is dependent on the state of medium in which 2 is irradiated; the photolysis of 2 in rigid glass results almost exclusively in the valence isomerization providing 1, while in fluid solution predominantly in the di-pi-methane rearrangement affording 3,4-benzotricyclo[3.2.0.0(2,7)]hept-3-en-6 (19). Plausible causes of the medium-dependent photochemical behavior of 2 are discussed.