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Eighteen electron-rich nitrogen incorporated donors with a butadiene π-spacer and a cyanoacrylic acid acceptor (A) as photosensitizers (D1–π–A to D18–π–A) for dye-sensitized solar cell (DSSC) applications have been designed for improving the photovoltaic performance. The significance of the nitrogen centres for revamping the donating strength (ΔVmA) of D–π–A is scrutinized using molecular electrostatic potential (MESP) analysis at the B3LYP/cc-pVDZ level of density functional theory (DFT). During the transformation of a donor (D) to D–π–A, a certain delocalization of electron density from D to π–A has occurred, and the change in the MESP minimum (ΔVmA) observed at the cyano region of D–π–A is related to the donating strength of D. Optical and photovoltaic properties are analyzed at the TD/CAM-B3LYP/cc-pVDZ/SMD//B3LYP/cc-pVDZ level. In D1–π–A to D18–π–A, ΔVmA is in the range −7.0 to −19.0 kcal mol−1 and the increase in donating strength is found to be proportional to the number of planar nitrogens in the donors. D12–π–A exhibited the most negative ΔVmA (−19.0 kcal mol−1), indicating the highest electron-donating strength of D12, whereas the least negative ΔVmA (−7.0 kcal mol−1) displayed by D7–π–A is correlated to the weak donating character of D7. By increasing the electron-donating strength of D in D–π–A, a red-shift in the absorption maximum (Δλmax) by 162 to 294 nm is observed. Further, the open-circuit voltage (eVoc) calculated for the D–π–A systems showed a strong linear relationship with ΔVmA. The LUMO (lowest unoccupied molecular orbital) energy of all the D–π–A systems (−1.79 to −2.79 eV) is observed above the conduction band (CB) energy of TiO2 (−4.0 eV), which ensured a desirable electron injection efficiency (ΔGinject) for them. The analysis of the adsorption energy (Eads) of the D–π–A systems on the TiO2 semiconductor (D–π–A/TiO2) showed that D12–π–A has the highest adsorption stability. Improving the adsorption stability is better for improving eVoc and the power conversion efficiency (PCE). The maximum absorption wavelength (λmax) of the D–π–A/TiO2 systems ranges from 513 to 703 nm and all of them display a red-shift with respect to the bare D–π–A systems. The study suggests D12 as the most efficient photosensitizer for DSSC applications. Further, it deepens the understanding of the structure–performance relationship of D–π–A systems as photosensitizers.

Donor–π–acceptor (D–π–A) systems typically used in dye-sensitized solar cells (DSSC) have been studied for assessing the donating strength of six donors (D1–D6) under the influence of substituents such as CH3, C5H11, isopropyl, t-butyl, OH, OCH3, OC2H5, NH2, N(CH3)2, PhCH3, and PhNH2 along with π-spacer butadiene and acceptor moiety cyanoacrylic acid. The substituent effect enhances electron donation from D to A through the π-spacer. The enhancement in electron density at A has been quantified in terms of the difference in the molecular electrostatic potential (MESP) minimum at the cyano nitrogen (ΔVmA) between π–A and D–π–A. For unsubstituted D–π–A systems, ΔVmA is in the range −0.1 to −5.7 kcal mol−1, whereas the substitution enhances the negative character of ΔVmA in the range −0.8 to −8.0 kcal mol−1. In alkyls and Ph–CH3 substituted D–π–A systems, ΔVmA lies in the range −0.8 to −6.7 kcal mol−1, whereas the N(CH3)2 substituted systems exhibit more negative ΔVmA (more enhanced donating strength) in the range −5.1 to −8.0 kcal mol−1. The more negative value of ΔVmA implies the greater electron-donating ability of the D−π−A system. Optical and photovoltaic parameters (ΔGreg, ΔGinject, eVOC) are analyzed at the TD-CAM-B3LYP/SMD/cc-pVDZ//B3LYP/cc-pVDZ level of DFT. An excellent linear correlation is observed in all six sets between ΔVmA and the absorption maximum (λmax) showing that λmax increases with enhanced donating strength. The higher absorption maximum obtained by N(CH3)2 substituted D–π–A systems lies in the range 430 nm to 490 nm, explaining the outstanding donating ability of N(CH3)2 compared to other substituents. The reduced highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) gap (from 3.14 to 2.17 eV) with enhanced donating strength confirms the influence of substituent effects in broadening the absorption maximum. Furthermore, in photovoltaic parameters, a strong influence of the substituent effect is observed. The N(CH3)2 substituted D1–π–A (D1–N(CH3)2) exhibits the highest eVOC (1.38 eV). The strong linear correlation observed for the ground state property ΔVmA and open-circuit voltage eVOC provides guidelines for developing an effective strategy for designing dye sensitizers for desirable photovoltaic applications.