MSc Thesis

Imagine trying to make plastics that conduct electricity without extra additives—lightweight, flexible “wires” you could print like ink. To do that, chemists design conjugated polymers whose electrons can move easily. A key knob to tune is the band gap: the smaller it is, the easier electrons flow and the more “intrinsically conducting” the material becomes.

This work asks a direct question: What really makes the band gap small? Common design rules say that combining electron-donating and electron-accepting building blocks, minimizing bond-length alternation (how much single and double bonds differ), and mixing aromatic and quinonoid units should shrink the band gap. We put those rules to the test using quantum chemical calculations (DFT) on carefully chosen donors (thiophene, pyrrole) and acceptors (thieno[3,4-b]pyrazine and its fluorinated variant, and quinoxaline), from single units up to short polymer chains. From these data we predicted ionization potentials, electron affinities, band gaps, and bandwidths—and compared them to experiments.

Some rules held, but others broke in surprising ways. As expected, fluorination made one acceptor stronger and lowered the band gap (good), but it also narrowed the valence bandwidth (less favorable for charge transport). Donor–acceptor copolymers (thiophene–ThP and pyrrole–ThP) did show smaller gaps than the individual polymers, yet did not deliver the wider conduction bands that design heuristics predict. Even more striking, the stronger donor (pyrrole) did not outperform the weaker one (thiophene) in reducing the gap or widening the bands—contradicting the simple donor–acceptor picture.

We also checked whether bond-length alternation tracks the band gap and found no consistent correlation. Instead, the results point to a subtler cause: geometrical mismatch between aromatic and quinoid repeat units can distort the chain, reshaping electronic structure in ways that lower the gap but do not necessarily enhance bandwidth. In short, molecular geometry and packing-like effects can override textbook rules.

Why it matters: Narrow-gap polymers are central to next-generation flexible electronics, solar cells, and sensors. This study shows that simple design rules are necessary but not sufficient; conformation, distortion, and electronic coupling must be optimized together. These insights help researchers avoid trial-and-error, focusing instead on donor–acceptor pairs and geometries that truly balance low band gaps with useful bandwidths for real devices.

For more information: 

https://tez.yok.gov.tr/UlusalTezMerkezi/tezSorguSonucYeni.jsp