Effect of Alkyl Substitution on the Electronic Structure and Spectrum of Rhodamine

Abstract

The structures of eight kinds of rhodamine compounds,substituted by different alkyls at different positions,were optimized using the density functional theory (DFT) at the B3LYP level. On the basis of these optimized structures,the influence of the substituting group on the electronic structures and spectral properties as well as frontier molecular orbitals of rhodamine were analyzed using the singlet configuration interaction (CIS) method and time dependent density functional theory (TD-DFT). Results indicate that the frontier molecular orbitals are mainly distributed in the xanthene ring of rhodamine when only one H atom of the two N-ends in the rhodamine molecule is substituted by alkyls. The distribution of the highest occupied molecular orbital (HOMO) is the greatest in the main conjugate ring and the difference in the distribution ratio between the HOMO and the the lowest unoccupied molecular orbital (LUMO) is small. When the four H atoms of the two N-ends in the rhodamine molecules are substituted by methyl,the bandgap is found to be the narrowest and the red shift in the gas maximum absorption wavelength is the largest. When the four H atoms of the two N-ends in rhodamine molecules are substituted by ethyl,the gas fluorescence maximum emission wavelength is the longest.