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