1,1-Dicyano-2,2-diphenyl-1,2-dihydronaphthalene: photochromism and evidence for photochemically induced C[bond]CN bond cleavage.
The photoreactions of the 1,1-dicyano-2,2-diphenyl-1,2-dihydronaphthalene (DHN-1) have been studied by photochemical techniques under various conditions at room temperature. A transient species, T(C), with a major maximum at 545 nm in the UV-visible spectrum, detected in small yield in polar aprotic solvents and in large yield in trifluoroethanol (TFE) and hexafluoropropan-2-ol (HFP), is assigned as a carbocation (1-cyano-2,2-diphenylnaphthalenium), which is generated photochemically by elimination of CN(-). The decay of T(C) was found to be a first-order process in HFP, in which the longest lifetime of 0.4 s was observed; in TFE, the carbocation decays on the ms timescale, while the shortest lifetime of 1 micros was found in ethanol. The yield of T(C) increases strongly upon addition of water to alcohols or acetonitrile, and remains substantial in the presence of large amounts of water (1-20 M). On addition of water to TFE or HFP, the lifetime of the carbocation becomes much shorter. This is supported by pulse-induced charge formation due to the carbocation and release of CN(-); the conductivity decay is related to the lifetime of the carbocation under selected conditions. In addition, a major irreversible and a minor thermally reversible photoprocess were spectroscopically observed in solvents of low as well as high polarity. The former photoproduct, absorbing below 300 nm, is tentatively ascribed to benzobicyclohexenes A produced by phenyl-vinylmethane rearrangement. In the latter process, a longer-lived benzoquinodimethane derivative B, having a maximum at 400-430 nm, is formed; this is related to ring opening and closure and represents a new example of photochromic ten-electron electrocyclisation. The distinct differences between the photochromism of DHN and that of dihydroazulene (DHA) stem mainly from stereoelectronic effects. This study, however, has also revealed that photochemically induced bond heterolysis and photochemical/thermal electrocyclisation, representing two basic processes of photochromism, may occur in one molecular unit.