D-D-π-A-π-A-based quinoxaline dyes incorporating phenothiazine, phenoxazine and carbazole as electron donors: Synthesis, photophysical, electrochemical, and computational investigation

Three novel D-D-p-A-p-A types of metal-free organic dyes, QX-D1, QX-D2, and QX-D3, were designed and synthesized in excellent yields (94-99 %). An auxiliary acceptor of 2,3-diphenylquinoxaline was incorporated between two units of thiophene as p-spacers so that one ring is connected with cyanoacrylic acid as an anchoring electron acceptor, and the other one lies between the quinoxaline moiety and an electron donor to build the p-A-p-A part. The other D-D part of the dye skeleton was made using donors-based phenothiazine (PTZ), phenoxazine (POZ), and carbazole (CZ) containing a tolyl moiety as an auxiliary donor. For this purpose, an efficient multistep synthetic approach for the synthesis of the dyes was investigated. The chemical structures of all compounds were confirmed by spectral data (FTIR, H-1 NMR, C-13 NMR, and HRMS). The QX-D1, QX-D2, and QX-D3 dyes showed ICT bands in THF solution at 511, 542, and 526 nm, respectively and high molar absorptivity of 23434, 29825, and 23214 M-1 cm(-1), respectively. Among the donor, QX-D2-based POZ revealed bathochromic and hyperchromic shifts in the absorption due to the influence of the oxygen atom in POZ compared with the sulfur atom and non in PTZ and CZ, respectively. The cyclic voltammetry data revealed that the HOMO and LUMO energy levels for QX-D1, QX-D2, and QX-D3 dyes were found to be (-5.07,-2.86 eV), (-4.90,-2.72 eV), and (-5.13,-3.04 eV), respectively. Furthermore, the density functional theory (DFT) and time-dependent TD-DFT at the B3LYP/6-31G(d) level were investigated to shed light on the bridged effect on geometric and TD-BH and H/6-31G(d) for optoelectronic properties. The binding energies of the complex dye@TiO2 cluster have also been explored. A good agreement between theoretical and experimental data was obtained. Thus, the dyes successfully achieved the desired outcomes, qualifying them for later use in photovoltaic applications such as sensitizers in dye-sensitized solar cells (DSSCs).

Automated summary

Three new metal-free organic dyes with D-D-p-A-p-A structure were synthesized successfully and their chemical structures were confirmed. The synthesized dyes exhibited strong absorption bands, high molar absorptivity, and distinct red-shift and hyperchromic shifts for different donors. The HOMO and LUMO energy levels of the dyes were determined, and the bridged effect on the optoelectronic properties was investigated. The theoretical and experimental data showed good agreement. These dyes have great potential for use as sensitizers in dye-sensitized solar cells.