Insights into Structure–Property Relationships in Quinoline–Based D–π–A Systems: A Synthetic and Computational Approach

In this study, four quinoline–based chromophores (EP01–EP04) with distinct π–conjugated architectures were studied to understand how molecular connectivity influences their electronic structure and nonlinear optical properties. Experimental characterization was combined with DFT and TD–DFT calculations to relate molecular structure to UV–Vis absorption, frontier–orbital distribution, and calculated electronic response. EP01, which contains a vinylene bridge connecting quinoline to an alkoxy–substituted aromatic ring, exhibited the smallest HOMO–LUMO gap in the series (5.65 eV) and the most red–shifted experimental absorption in methanol (353 nm). In comparison, the principal absorption maximum of EP02 occurred at 265 nm. The results for EP03 and EP04 showed that band position and intensity depend not only on the HOMO–LUMO gap but also on the orbitals involved in each transition. The calculated solvent effect on the most intense transitions was limited, with shifts of 2–7 nm between toluene and water. EP01 also exhibited the largest calculated first and second hyperpolarizabilities, approximately 2.58 and 2.92 times those of EP02, respectively. Together, these findings show that connecting the aromatic fragments through a continuous π–pathway favors the optical response of these derivatives and provides structural guidance for designing new quinoline–based chromophores.