A substructure-based topological quantum chemistry approach for the estimation of the ultraviolet absorption energy of some substituted linear conjugated compounds

Abstract

The linear conjugated compounds are a class of important organic compounds with good ultraviolet (UV) absorption, which are usually employed as the parent molecules of a lot of optical materials. The traditional topological quantum chemistry approach can accurately predict the UV absorption energies of unsubstituted linear conjugated alkenes. However, for the alkyl-substituted conjugated alkenes, this approach seems unsatisfactory. It is known that the π conjugated chain is a good conductor for the electron mobility, while the alkyl substituent is a bad one. As a result, their influences on the π electron transition will be different from each other. In order to distinguish the contribution of these two different structural parts, a substructure approach was proposed, in which a substituted linear conjugated molecule was divided into two substructures: the π conjugated chain part and the substituent part. Different methods were employed to characterize their structures. The relative frontier orbital energy gap of the conjugated chain (ΔREHL0) and the polarization energy of the substituents (PRE) were calculated by substructure-based topological quantum chemistry method. A good linear regression model was built between the UV absorption wavenumbers and two parameters ΔREHL0 and PRE for 23 typical conjugated alkenes. Based on this model, the UV absorption wavenumbers of other 70 alkenes were also predicted with high accuracy. This substructure-based topological quantum chemistry method was also extended to the investigation of the UV absorption of 89 olefinic aldehydes and ketones, and good results were also obtained.