Isomer-selective dative bond O→M (M = Si, Ge) for designing new photochromic hemi-indigo systems

Abstract

New photochromic systems MY2X-HI (M = Si, Ge) were designed (at the DFT level of theory) by replacing the hydrogen atom in the nitrogen of the pyrrole ring of hemi-indigo, HI, with silyl- or germyl-substituents. The E isomers of MY2X-HI contain a dative bond O→M (pentacoordinated atom M) both in the ground S0 and in the excited S1 states. This leads to a significant difference in the absorption and fluorescence maxima (λfl - λabs) of the Z and E forms of hypervalent hemi-indigos; this difference is more than twice that of the parent HI. There is a trend to increasing separation of first π→π* absorption bands of the E and Z isomers of MY2X-HI with the increase in their relative stability.