Modeling solvent effects on excitation energies for polyenes

Abstract

Excitation energies of polyenes in solution are about 0.3-0.4 eV lower than in the gas phase. Understanding the solvent effect is important to the design of low band gap conducting polymers. This thesis is to evaluate this solvent effect theoretically by comparing the first allowed vertical excitation energies of polyenes, oligothiophenes and oligopyrroles. Influences of theoretical levels and basis sets on the optimised geometries, the HOMO- LUMO gap, and the TDHF excitation energies are reviewed and compared with experimental data in the gas phase. To calculate excitation energies, six levels with Stevens-Basch-Krauss pseudopotentials in connection with polarized split valence basis set (CEP-31g* basis in Gaussian 03 package) are employed in this thesis, including the HOMO-LUMO gap, CIS, TDHF, TDDFT, CASSCF and CASPT2. Three methods to take solvent effects into account were tested: implicitly by using the polarized continuum model (PCM) method, explicitly by treating a solute-solvent cluster and the combination of both methods. In PCM, heptane is considered as the solvent. PCM can be applied at different theoretical levels. In the cluster model, four corresponding alkane molecules surrounding a solute molecule in a parallel orientation form the first solvation shell. Solvent effects are determined by whether a theoretical level can form an effectively bound cluster. The combination of both can yield a closest result to experimental data. Further, solvent effects of water are evaluated with PCM and clusters. TDHF and PCM are applied for larger systems like oligothiophenes and oligopyrroles.