Ultraviolet absorption spectra of some sterically hindered p,p′‐dimethoxystilbenes

The ultraviolet absorption spectra of a series of methyl- and ethyl-substituted p,p′-dimethoxystilbenes were measured. The spectra could be described by assuming the presence of four overlapping absorption bands. Three of these bands, the positions of which (∼ 207, 230 and 285 mμ) are hardly affected by alkyl substitution, are tentatively interpreted as “benzene bands”; 4,4′-dimethoxydibenzyl is found to show comparable absorptions. The fourth band (at 307 mμ in the case of unsubstituted 4,4′-dimethoxystilbene) is assumed to correspond to an N V transition in the stilbene chromophore. With the substituted dimethoxystilbenes the oscillator strength of this band decreases in the following sequence: α-methyl- ; 2,2′-dimethyl- ; 2,2′-diethyl- ; α,2′-dimethyl- ; α,2-dimethyl- ; 2,2′,6,6′-tetramethyl- ; α,α′-dimethyl- ; α,2,2′-trimethyl- ; α,α′-diethyl- ; α,α′,2-trimethyl- and α,α′,2,2′-tetramethyl-4,4′-dimethoxystilbene. There is an increasing hypsochromic shift that in general runs parallel to the decrease in extinction coefficient, λmax gradually falling from 307 mμ to ∼ 230 mμ. The angles of twist (ψ) of the phenyl nuclei are estimated from the oscillator strengths of the long-wavelength stilbene absorption bands. The values obtained show qualitative agreement with the results of calculations by the molecular orbital method as performed by Suzuki and by Riezebos.