Comparing Linear and Non-Linear Optical Properties of Symmetrical and Unsymmetrical Xanthene Dyes: DFT and TD-DFT Approach

Unsymmetrical xanthene dyes are a hybrid structure of two symmetrical xanthene dyes. Therefore, are the structural, spectral, electronic, linear, and non-linear optical (NLO) properties of unsymmetrical xanthene dyes comparable to, distinct from, or better than those of their parent symmetrical xanthene dyes? This study provides a detailed analysis of unsymmetrical and symmetrical xanthene dyes using the Density Functional Theory (DFT) and Time-dependent (TD)-DFT methods. DFT study demonstrated that symmetrical xanthene dyes are energetically more stable and exhibit BLA values lower than unsymmetrical xanthene dyes. Moreover, symmetrical xanthene dyes show destabilized energy of HOMO, LUMO, and lower LUMO–HOMO energy gap than unsymmetrical xanthene dyes. Vertical excitation calculated from the TD-DFT method shows reasonable agreement with experimental absorption. However, unsymmetrical xanthene dyes with higher dipole moments and hyperpolarizability show superior NLO properties.