Spectral signatures of thieno[3,4-b]pyrazines: Theoretical interpretations and design of improved structures

•Thienopyrazines are studied with theoretical tools.•The protocol reproduces both geometries and absorption spectra.•New very low-gap structures are designed.•One can independently control charge-transfer strength and transition energy.

The structural and electronic properties of thieno[3,4-b]pyrazine derivatives have been investigated with a Time-Dependent Density Functional Theory approach accounting for both solvent and long-range Coulombic effects. Using the selected protocol we could accurately reproduce the experimental geometrical parameters as well as the measured optical signatures, the latter including both local and charge-transfer transitions. New structures presenting optimized properties are designed and it is shown that compact structures based on the thieno[3,4-b]pyrazine core might present easily tunable transition energies, opening the door to new very low band-gap oligomers and systems with strong charge-transfer features. We also demonstrate that the absorption energies and charge-transfer strengths can be optimized independently for thieno[3,4-b]pyrazines.

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