Dar bant aralıklı iletken polimerlerin eldesinde kullanılan yaklaşımların teorik incelenmesi

ABSTRACTTHEORETICAL INVESTIGATION OF APPROACHES FOR OBTAININGNARROW BAND GAPS IN CONDUCTING POLYMERSSERDAR DURDAĞIM. S. in ChemistrySupervisor: Assoc. Prof. Dr. Ulrike SalznerAugust 2004Over the last few years, there has been a great deal of research interest indeveloping organic conjugated polymers with narrow energy band gaps. Narrowband gap polymers would be intrinsically conducting, and thus eliminating the needfor doping. There are several approaches for the construction of low band gapsystems. Copolymerization of aromatic and o-quinoid heterocycles, minimization ofbond length alternation, copolymerisation of donor and acceptor moieties might bemost important factors for the lowering the band gap.The main aim of this work is to determine the reasons for low band gaps and toanalyse the major effects, separately.iiiRecently a number of low band gap systems were synthesized. These systemsconsist of aromatic donors and quinoid acceptors. To analyse the behaviour ofdonor/acceptor systems, we performed theoretical studies for these systems. Wechose thieno [3,4-b] pyrazine (ThP), F-substituted thieno [3,4-b] pyrazine (F-ThP)and quinoxaline (Qx) as an acceptor and thiophene (Th) and pyrrole (Py) as donorunits.Monomer through octamer of ThP, monomer through tetramer of F-ThP andQx, were optimised by using density functional theory (DFT). All calculations wereperformed with the Gaussian 98 program. For ThP-Th and ThP-Py co-oligomers,energy levels were calculated for monomer through tetramer and, for F-ThP-Py,monomer through trimer data were used. Ionization Potentials (IPs), electronaffinities (EAs), band gaps, and band widths of polymers were obtained byextrapolation. IPs and EAs are taken as negative HOMO and LUMO energies.The order of the band gaps of the homo-polymers is found: F-PThP (1.1 eV)