The photochemistry of 3-methyl-1,2-dihydronaphthalene (3-MDHN) and 4-methyl-1,2-dihydronaphthalene (4-MDHN) has been studied in the gas phase. Photolysis of 3-MDHN with 254-nm light produces 2-methyl-1,2-dihydronaphthalene (2-MDHN) as the major primary product. Naphthalene is also formed, apparently as a secondary photoproduct from 2-MDHN. Addition of butane to the photolysis mixture quenches the formation of 2-MDHN while producing a new photoproduct, 1-isopropenylbenzocyclobutene (IBCB). This product is also formed when light centered at 300 nm is used for the photolysis. Photolysis of 4-MDHN vapor with 254-nm light gives three products unique to the gas phase: 1-isopropenyl-2-vinylbenzene (IVB), 3-(o-tolyl)-1,2-butadiene (T12B), and 1-methyl-1,2-dihydronaphthalene (1-MDHN). An apparent alkyl shift product, 3-methyl-1,2-dihydronaphthalene (3-MDHN), and naphthalene are also formed, apparently as secondary photolysis products from 1-MDHN. In addition, several photoproducts common to both the solution and gas phase are detected: 2-(o-tolyl)-1,3-butadiene (T13B), 1-methylbenzobicyclo[3.1.0]hex-2-ene (1-MBBH), 1-methyl-1,4-dihydronaphthalene (1-M-1,4-DHN), 1-methyltetralin (1-MT), and 1-methylnaphthalene (1-MN). Again, the presence of butane during the 254-nm photolysis, or the use of longer wavelength light, gives rise to a new photoproduct, 1-methyl-1-vinylbenzocyclobutene (MVBCB). The fluorescence excitation spectrum for 4-MDHN confirms that 254-nm excitation into S2 leads to minimal population of the emissive vibrational levels of S1. Two pathways appear to dominate the photochemistry: retro [4 + 2] cycloaddition to give o-quinodimethane intermediates and sequential hydrogen shifts. These pathways derive from S2 and/or upper vibrational levels of S1 (S1vib) as indicated by the characteristic responses of their ultimate products to the presence of buffer gas. The benzocyclobutenes are unique; they are postulated to arise through a 2 + 2 closure of a vibrationally relaxed precursor o-quinodimethane or via a [1,3] sigmatropic shift in a uniquely populated set of S1vib levels.
