On the Optical Absorption of Protonated Retinal Schiff-Base with Torsion about Some Bonds in the Polyene Chain

Abstract

We calculate the effect of torsion about one single or double bond on the optical absorption of isolated PRSB (protonated reginal Schiff-base), according to our LCAO–ASMO–SCF–CI method for π-electrons. We also calculate the wavelength λ_max of absorption maximum of PRSB interacting with negative point charge e, as a function of the angles of torsion about C₁₂–C₁₃, C₁₄–C₁₅, C₁₁=C₁₂ and C₁₃=C₁₄ bonds. It is found for isolated PRSB that a small torsion about one single (double) bond makes λ_max shift to shorter (longer) wavelengths except the case about C₁₄–C₁₅ bond, and that the λ_max is assigned to S₀–S₁ transition in either case. It is also found that, for any conformation of isolated PRSB with torsion about one bond, the oscillator strength of absorption maximum is weaker than that of PRSB in all-trans or cis form. The same characteristics are also found for [PRSB+e] system with small angles of torsion.