Influence of the methyl group at C=C bridging bond of stilbene on the longest wavelength maximum in ultraviolet absorption spectra

The compounds stilbenes XArCH=CHArY(XSBY) and 1,2-diphenylpropylenes XArC(Me)=CHArY(XSMBY) have bridging groups CH=CH and C(CH3)=CH, respectively, in which the C(CH3)=CH has a side-group CH3 at the carbon-carbon double bond. A series of XSMBY were synthesized, and their longest wavelength maximum (max) (nm) in ultraviolet absorption spectra were measured in this work. We investigated the change regularity of the (max) (cm(-1), (max)=1/(max)) of XSMBY and compared it with that of XSBY. The results indicate that (1) there is no good linear relationship between the (max) of XSMBY and that of XSBY. (2) Because of the influence of the side-group CH3, in case of the same couple of groups X and Y, the (max) of XSMBY is shorter than that of XSBY, that is, it has a blue shift. (3) The cross-interaction between the side-group CH3 and Y has an important effect on the (max) of XSMBY, while the cross-interaction between the side-group CH3 and X has a little effect on the (max) and can be ignored. (4) The specific cross-interaction between X and Y has important effect on the (max) of XSMBY, whereas it has no important effect on the (max) of XSBY.