First-Principles Investigations of Novel Guanidine-Based Dyes

Abstract

In the pursuit of finding efficient D-π-A organic dyes as photosensitizers for dye-sensitized solar cells (DSSCs), first-principles calculations of guanidine-based dyes [A1-A18] were executed using density functional theory (DFT). The various electronic and optical properties of guanidine-based organic dyes with different D-π-A structural modifications were investigated. The structural modification of guanidine-based dyes largely affects the properties of molecules, such as excitation energies, the oscillator strength dipole moment, the transition dipole moment, and light-harvesting efficiencies. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is responsible for the reduction and injection of electrons. Modification of the guanidine subunit by different structural modifications gave a range of HOMO-LUMO energy gaps. Chemical and optical characteristics of the dyes indicated prominent charge transfer and light-harvesting efficiencies. The wide electronic absorption spectra of these guanidine-based dyes computed by TD-DFT-B3LYP with 6-31G, 6-311G, and cc-PVDZ basis sets have been observed in the visible region of spectra due to the presence of chromophore groups of dye molecules. Better anchorage of dyes to the surface of TiO₂ semiconductors helps in charge-transfer phenomena, and the results suggested that -COOH, -CN, and -NO₂ proved to be proficient anchoring groups, making dyes very encouraging candidates for DSSCs. Molecular electrostatic potential explained the electrostatic potential of organic dyes, and IR spectrum and conformational analyses ensured the suitability of organic dyes for the fabrication of DSSCs.

Figure 1

Working scheme of DSSCs from ref (8) reused with permission from Elsevier license number 5712010860265.

Figure 2

(a) Schematic molecular orbital energy levels of guanidine-based dyes [A1–A9(trans)]and CB of TiO₂ electrolyte used as the reference. (b) Schematic molecular orbital energy levels of guanidine-based dyes (A10–A18)and CB of TiO₂ electrolyte used as the reference.

Figure 3

TD-DFT-B3LYP calculated excitation spectra of representative guanidine-based dyes [A9(cis), A15, A16(cis), A16(trans), and A17].

Figure 4

(a) FMOs for main transitions of guanidine-based dyes (A1–A5) using the DFT-B3LYP method, (b) FMOs for main transitions of guanidine-based dyes [A6–A9(trans)] using the DFT-B3LYP method, (c) FMOs for main transitions of guanidine-based dyes (A10–A14) using the DFT-B3LYP method, and (d) FMOs for main transitions of guanidine-based dyes (A15–A18) using the DFT-B3LYP method.

Figure 5

Optimized geometric structure of the dye A9(cis)-Ti₉O₁₈ bounded complex.

Figure 6

FMOs of the dye A9(cis)-Ti₉O₁₈ bounded complex.

Figure 7

MEP plots of Ti₉O₁₈, dye A9(cis)-Ti₉O₁₈, and dye A9(cis).

Figure 8

IR spectrum of dye A9(cis).

Figure 9

(a) PES of dye A9(cis). (b) PES grid of dye A9(cis).