Computational design of small organic dyes with strong visible absorption by controlled quinoidization of the thiophene unit

Computational design of small phenothiazine dyes for dye-sensitized solar cells by functionalizations affecting the thiophene unit

“…Computational screening of different dyes was performed in vacuum. We have shown previously that the results obtained for this model dye in vacuum carry over to real (larger) dyes in solution [23]. After identifying the best dye, we also computed the absorption spectra of larger dyes based on it (i.e., dyes with the alkyl chain on the phenothiazine and with the triphenylamine units present in the original dye [3]) in dichloromethane and in acetonitrile using the PCM (polarizable continuum model) with the integral equation formalism (IEFPCM), in which the solvent reaction field is self-consistent with the solute's electron density [36].…”

“…Atom color code for VMD visualization here and elsewhere: cyan C, yellow S, blue N, red O, gray H, purple F environment of the five-membered ring, but not the effect of of its (thiophene's) functionalization. Results relating to the role of BLA remained inconclusive in that functionalizations with CN groups did not result in significant changes in the BLA, even though they prompted a significant redshift of the spectrum [23,24].…”

“…We adopt the following notation to name the dyes. First, a number specifies the dye from which the molecule is derived (e.g., 1, 2, 71, or 72, where dyes 1 and 2 are those introduced above and 71 and 72 are analogues of 1 and 2, respectively, with the methine unit replaced by CN); this notation is consistent with our previous works [23,24]. Next comes a dot, after which we specify which hydrogen atom, of the methine (Bm^) or the thiophene (Bt^) unit, is functionalized, denoting functionalization at the position closest to the donor moiety as Bt1,f unctionalization at the position closest to the attachment group as Bt2,^and functionalization at both positions as Bt.^Then, after another dot, we specify the functional group added, i.e., one of CN, F, CH 3 , CF 3 (BF2^or BCN2^are used with Bt,^as two functional groups are attached in that case).…”

“…Given the significant degree of charge transfer in the transition responsible for the visible absorption peak, as well as the changes in the BLA studied here, the range-separated CAM-B3LYP functional was used [31]. Even though this functional is known to overestimate the excitation energies E exc and the band gap [32] somewhat, we verified in [23] that it provides a better accuracy for adsorption spectra of dyes of the type studied here than B3LYP [23]. We have previously observed that relative changes in spectra associated with changes in BLA may not be reproducible with standard hybrid functionals such as B3LYP [13,20,27,28], as they are sensitive to long-range interactions [32][33][34].…”

“…In [22], this effect was confirmed in a computational study of indolinecarbazole-based dyes. In [23], we studied the potential for controlled quinoidization through appropriate selection of functional groups and the conjugation order around the thiophene unit in small phenothiazine dyes, and predicted that it is possible to shift the absorption peak from 450 to 520 nm by inserting a fumaronitrile linker between the thiophene unit and the attachment acrylic group. Recently, we computed that the excitation energies of these dyes can be decreased by about 20 % by introducing electron-withdrawing CN units around the (quinoidized) thiophene [24].…”