Theoretical investigations of the structures and electronic spectra of 2-dicyanovinyldithieno[2,3-b:3′,2′-d]thiophene (DCST) and 2-dicyanovinyldithieno[3,2-b:2′,3′-d]thiophene (DCTT)

The DFT-B3LYP method has been used for the ground-state (S0) geometry optimization of two novel compounds, 2-dicyanovinyldithieno[2,3-b:3′,2′-d]thiophene (DCST) and 2-dicyanovinyldithieno[3,2-b:2′,3′-d]thiophene (DCTT). The CASSCF approach has also been used to optimize the S0 and S1 electronic states of their more stable isomers, DCST 1 and DCTT 2. The absorption spectra of DCST and DCTT have been investigated at the TD-B3LYP/cc-pVTZ level, compared with the experimental results in the hexane solution. The studies indicate that the two notable absorption bands of DCST 1 occur at 387.83 and 265.04 nm, respectively, which agree very well with the experimental data of 404 and 270 nm, respectively; the two major absorption peaks of DCTT 2 appear at 403.42 and 257.84 nm, respectively, also in good agreement with the observed values of 427 and 256 nm, respectively. In addition, the emission wavelengths of DCST 1 and DCTT 2, under the S1 geometries, are 454.17 and 470.79 nm, respectively, showing the remarkable red-shift in comparison with the corresponding absorption wavelengths.