PHOTOCYCLOADDITION OF 9,10-DICHLOROANTHRACENE TO 2,5-DIMETHYL-2,4 HEXADIENE - THE SINGLET PATHWAY FOR [4-ADDUCT FORMATION IN BENZENE(2])

Irradiation of 9,10-dichloroanthracene (DCA) in the presence of 2,5-di methyl-2,4-hexadiene (DMHD) in benzene at 25 degrees C gives a single adduct (Ad) corresponding to [4 + 2] addition of DMHD to the 9,10 posi tions of DCA. Quantum yields for Ad formation are reported as a functi on of [DMHD], [DCA], and methyl iodide concentration, [MeI]. Fluoresce nce quantum yields and lifetimes of DCA and (DCA/DMHD) singlet exciple x as a function of [Mel] are also reported. Transient absorption measu rements reveal the time evolution of excited DCA singlet, (1)DCA, and triplet, (3)DCA, states and of the (DCA/DMHD) singlet exciplex, (1)( DCA . DMHD). They show that the dominant decay path from (1)(DCA . DM HD) gives (3)DCA*. Quenching of (1)DCA* and exciplex fluorescence and transient absorption by Mel are observed. The dependence of Ad quantu m yields on [DMHD] and the strict proportionality between product quan tum yields and exciplex fluorescence quantum yields at different [MeI] establish a singlet mechanism for adduct formation. Assuming that Ad is a primary photoproduct, the results are consistent with Yang's prop osal of stepwise collapse of polar exciplexes to seemingly forbidden c ycloadducts. A singlet biradical could be the intermediate, but inters ystem crossing of the singlet exciplex to a triplet biradical as the i nitial step cannot be ruled out. The loss of DCA is enhanced at very h igh [DMHD] but oxygen quenching experiments show that this enhancement does not involve formation of the triplet biradical by addition of (3 )DCA to DMHD.