Polienlerin uyarılma enerjileri üzerinde çözücü etkilerinin modellenmesi

ABSTRACTMODELING SOLVENT EFFECTS ONEXCITATION ENERGIES FOR POLYENESYAN LIM.S. in ChemistrySupervisor: Assoc. Prof. Dr. Ulrike SalznerAugust 2006Excitation energies of polyenes in solution are about 0.3-0.4 eV lower than in thegas phase. Understanding the solvent effect is important to the design of low bandgap conducting polymers.This thesis is to evaluate this solvent effect theoretically by comparing the firstallowed vertical excitation energies of polyenes, oligothiophenes andivoligopyrroles.Influences of theoretical levels and basis sets on the optimised geometries, theHOMO- LUMO gap, and the TDHF excitation energies are reviewed andcompared with experimental data in the gas phase. To calculate excitation energies,six levels with Stevens-Basch-Krauss pseudopotentials in connection withpolarized split valence basis set (CEP-31g* basis in Gaussian 03 package) areemployed 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 byusing the polarized continuum model (PCM) method, explicitly by treating asolute-solvent cluster and the combination of both methods. In PCM, heptane isconsidered as the solvent. PCM can be applied at different theoretical levels. Inthe cluster model, four corresponding alkane molecules surrounding a solutemolecule in a parallel orientation form the first solvation shell. Solvent effects aredetermined 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. TDHFand PCM are applied for larger systems like oligothiophenes and oligopyrroles.Keywords:Solvent effects, excitation energies, polyenes, polarized continuum model,solute-solvent cluster modelv