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

✍ Scribed by W. H. Laarhoven; R. J. F. Nivard; E. Havinga

• Book ID: 104586252
• Publisher: Elsevier Science
• Year: 2010
• Tongue: English
• Weight: 478 KB
• Volume: 79
• Category: Article
• ISSN: 0165-0513
• DOI: 10.1002/recl.19600791110

✦ Synopsis

Abstract

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, λ~max~ 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.