Q7(c). UV spectra of cis- and trans-stilbene

Introduction. Both stilbenes (1,2-diphenylethene) show an intense π→π* band of the Ph–CH=CH–Ph chromophore. Its position and intensity depend on how well the two rings conjugate with the C=C, which depends on planarity.

Figure: Sketch of the main π→π* band: trans (solid) at longer λ and higher ε than cis (dashed).

1. trans-Stilbene is planar: both phenyl rings lie in the plane of the C=C and the whole 14π system is conjugated. The HOMO–LUMO gap is smaller, so λₘax is longer (about 294 nm), and the long, planar chromophore has a high ε.
2. cis-Stilbene: the two phenyl rings on the same side clash (ortho H atoms), so the rings twist out of the plane. Conjugation is reduced: λₘax is shorter (about 274 nm) and ε is markedly lower (hypsochromic and hypochromic shift).
3. The isomer with the band at longer wavelength and higher intensity is therefore trans; the steric twist in cis is an example of steric inhibition of resonance seen in UV.

Conclusion. trans-Stilbene: λₘax about 294 nm, high ε; cis-stilbene: about 274 nm, low ε, because steric crowding makes cis non-planar and reduces conjugation.

Application. The same planarity argument distinguishes cis and trans azobenzenes and cinnamic acids; the change in UV absorption is followed in photo-isomerisation studies (molecular switches).