Tuning the Band Gap of Low-Band-Gap Polyselenophenes and Polythiophenes: The Effect of the Heteroatom

Abstract

A series of new low-band-gap thieno-or selenolo-fused polyselenophenes (P5 and P6) and selenolo-fused polythiophene (P4) (as well as previously reported thieno-fused polythiophene, P3) was prepared systematically by electropolymerization (P4-P6) and by solid-state polymerization (P3, P5 and P6). The 2,5-dibrominated monomers (3Br 2 , 5Br 2 , and 6Br 2 ) undergo solid-state polymerization under slight heating and produce insoluble P3, P5, and P6 as black conducting powders. The spectroelectrochemically measured optical band gaps of P4-P6 films are 0.96, 0.72, and 0.76 eV, respectively. DFT calculations performed on P3-P6 provide excellent estimations of the experimental band gaps of these polymers. The band gap of the polyselenophenes (P5 and P6) is 0.2 eV lower than that of the corresponding polythiophenes (P3 and P4). We introduced a new scheme for band gap control in conjugated polymers by replacing the sulfur atom with a selenium atom in the main and/or peripheral ring, which leads to significant and predictable changes in the band gap of the polymers. This is due to the lower aromaticity of a selenophene ring compared to a thiophene ring. Thus, we have achieved band gap control in very low band gap (∼0.7-1.0 eV) polymers through the use of different combinations of selenium and sulfur atoms in the main and peripheral rings.

Structural and electronic properties of oligo- and polythiophenes modified by substituents

“…7 eV for the vinyl-bridged polymers. Apparently, the vinyl bridges reduce the aromaticity of the polymers by modifying the structure toward a quinoid form, leading to reduced band gaps, as the band gap of conjugated polymers depends (among other factors) on the degree of the quinoid or aromatic character of the backbone [20,46]. …”

“…Considering the significant potential of organic chemistry at synthesizing and manipulating compounds, there is definitely a demand for a better understanding of how the electronic structure of compounds such as PTp can be manipulated by using these tools. There have been already several studies addressing the electronic structure of thiophenes with electronic structure methods [13–20]. In these computational studies, typically oligothiophenes of varying size have been considered based on density functional theory (DFT), and the properties of polythiophenes have been derived by using scaling relations [21].…”

New poly(selenophene–thiophene) bearing π-conjugating spacers for polymer field-effect transistors and photovoltaic cells

“…45 Note that the The difference of E opt g between these polymers is mainly attributed to the varied chemical structure of π-conjugated spacers.…”

The role of relativity and dispersion controlled inter-chain interaction on the band gap of thiophene, selenophene, and tellurophene oligomers

“…24 Earlier, experimental and theoretical studies on polythiophene predicted a non-degenerate ground state with a band gap of 2.1 eV, while the band gap of selenophene polymer is 1.9 eV and this lowering is attributed to the increase of quinoid character in the polymer backbone. [25][26][27] Albeit, some theoretical predictions are available, estimation of experimental band gap of polytellurophene is still missing. The band gap of polytellurophene is expected to be lower than that of its S and Se counterpart and is reported as 1.48 eV when calculated at the B3LYP/lanl2dz level of theory using extrapolation method.…”