Dar bant aralıklı verici alıcı tipteki iletken polimerlerin yoğunluk fonksiyonel teorisi kullanılarak incelenmesi

ABSTRACTA DENSITY FUNCTIONAL STUDY ON NARROWBAND GAP DONOR-ACCEPTOR TYPE CONDUCTING POLYMERSOZAN KARALTIM. S. in ChemistrySupervisor: Assoc. Prof. Dr. Ulrike SalznerAugust 2004The band gap is one of the most important factors for controlling the physicalproperties. The search for polymers having narrow band gaps is a current topic.Tuning of the band gap by structural modification is possible. There are someapproaches used for designing narrow band gap polymers. One of the approachesused for designing low band gap polymers is the donor acceptor concept where itis thought that regularly alternating conjugated donor and acceptor like moietiesin a conjugated chain will induce a small band gap and at the same time will leadto widening of the valence and conduction bands. Forcing the polymers to adoptunfavorable structures and decreasing the bond length alternation along theconjugated backbone are other two methods used for synthesizing narrow bandgap polymers.Yamashita et al. synthesized copolymers composed of benzobis(1,2,5-thiadiazole) and [1,2,5] thiadiazolo[3,4-b]thieno[3,4-e] pyrazine units as acceptorand thiophene and pyrrole units as donors. These synthesized copolymers havevery narrow band gaps. The success with designing these systems were attributedto the donor-acceptor concept.We intended to understand the reasons for narrow band gaps and to determinewhether donor acceptor concept is valid. Density functional theory (DFT)iiicalculations were performed for homo and co-oligomers (having 1:1 and 1:2acceptor to donor ratios) of thiophene (Th), pyrrole (Py), benzo[1,2-c;3,4-c']bis[1,2,5]-thidiazole (BBT) and [1,2,5] thiadiazolo[3,4-b]thieno[3,4-e]pyrazine (TTP) We estimated the band gaps of polymers by extrapolating theHOMO-LUMO gaps of the oligomers, using second degree polynomial fit, at theB3P86-30% /CEP-31g* level of theory.Theoretical analysis showed that the main reason for the band gap reduction is notthe donor-acceptor concept and the prediction of the band width widening is notvalid. Influence of the quinoid structures and reduce in the bond length alternationare the resons for the band gap reduction.Keywords: Density functional theory, conducting polymers, conjugated organicpolymers, band gap, band width, donor-acceptor conceptiv