Photochromic formazans: Raman spectra, X-ray crystal structures, and 13C magnetic resonance spectra of the orange and red isomers of 3-ethyl-1,5-diphenylformazan

Abstract

The X-ray crystal structures of the two isomers of a photochromic formazan have been determined. Red 3-ethyl-1,5-diphenylformazan belongs to the orthorhombic space group P2₁2₁2₁ and adopts a syn,s-trans-configuration. The orange, light-stable isomer, belongs to the monoclinic space group P2₁/c and adopts an anti,s-trans-configuration. Raman and ¹³C n.m.r. spectra for the orange isomer both in the solid state and in solution confirm that the solution species retains the structure established for the solid state. The Raman spectrum for the red isomer in the solid state shows the shifts to lower wavenumber for the azo group stretching vibration characteristics of the syn,s-trans-configuration.