Impact of Substituent Effect on the Electronic and Optical Properties of TPA-Based Dyes Adsorbed on ZnO Nanoparticles Using Quantum Chemical Methods

The substituent effect (SE) on donor−π–acceptor (D−π–A) organic dyes based on triphenylamine (TPA), both in isolated and ZnO-adsorbed (hybrid) forms, was investigated using a quantum chemical methods based on the charge of the substituent active region (cSAR). For this purpose, substituents with diverse electronic properties (N(CH3)2, NH2, OH, OCH3, H, F, Cl, CHO, CN, NO2) were introduced into the donor moiety or π-bridge of dye, and the method proved effective for analyzing SE and predicting trends relevant for DSSC dye performance. The results show that substituent influence depends strongly on both electronic character and position, modifying the electronic structure of the entire system. Electron-donating (ED) groups enhance charge transfer from donor to acceptor, whereas electron-withdrawing (EW) groups diminish it. Analysis of cSAR values for the acceptor (CAA) indicates that substituent effects are stronger when located on the donor than on the spacer. Additionally, smaller changes in spacer aromaticity promote more efficient electron transport between the donor and acceptor. The cSAR-based results are in good agreement with the optical data obtained for TPA-derived structures. Linear correlations reveal that substituent effects on the donor (q(Donor)) relate to HOMO energies, while effects on the acceptor (cSAR(CAA)) correspond to LUMO energies. The electron-injection driving force (ΔGinj) is correlated with the electronic structure of the CAA acceptor, with the strength of this relationship varying with the substitution position. Overall, these results emphasize the importance of understanding SE for the rational design of efficient DSSC sensitizers.