Environmental effects on electronic absorption spectra using DFT: An organic and positively charged fused polycyclic chromophore as a case study

The benzo[1,2]quinolizino[3,4,5,6-def]phenanthridinium,-9-phenyl molecule (hereafter BQPT-ph(+)) has been considered as a representative benchmark for simple organic and positively charged systems in solution to test the environmental effects on computed UV-Visible spectra. In particular the effects, both at geometric and electronic levels, related to the inclusion of bulk solvent, using a polarizable continuum model (PCM), and the explicit inclusion of the counterion (here BF4-) will be discussed. Electronic transitions were computed at TD-DFT level using a hybrid exchange correlation functional (PBE0) and a double zeta valence basis set. Effect of inclusion of diffuse and polarization function on the computed electronic spectra will also be discussed. Finally, the importance of the vibronic structure to simulate the experimental band shape will be considered focusing on the first electronic transition, and computing ground and first excited states optimized structures together with harmonic frequencies at DFT and TD-DFT level, respectively. The good agreement obtained between computed and experimental spectra confirms the good quality of both the optimized geometries and the harmonic force fields. (C) 2009 Elsevier B.V. All rights reserved.