Photoinduced Electrocyclic 1,5-Closure of the Cyclooctatetraene Radical Cation to a Bridged 1,4-Bishomobenzene (Bicyclo(3.3.0)octa-2,6-diene-4,8-diyl) Radical Cation

Cyclooctatetraene (1) is well-known to have a tub-shaped (D{sub 2d}) geometry with alternating double bonds, and its radical cation (1{sup {sm bullet}+}) evidently also possesses a nonaromatic and nonplanar structure. Here the authors describe a novel photoisomerization in which 1{sup {sm bullet}+} undergoes homoaromatization by 1,5-closure to yield the bridged 1,4-bishomobenzene (bicyclo(3.3.0)octa-2,6-diene-4,8-diyl) radical cation 2{sup {sm bullet}+} at temperatures as low as 80 K, a symmetry-allowed reaction in which the excited state of 1{sup {sm bullet}+} correlates adiabatically with the ground state of 2{sup {sm bullet}+}. This photoisomerization is responsible for the previously reported photochromic effect in this system.