Quantum chemical prediction and benchmark study for structural isomers of terpyridine: electronic structure...

Terpyridine isomers, distinguished by their unique structural arrangement, have gained considerable attention for their potential in functional material design. This benchmark study presents a series of structural isomers of terpyridine (TP-1 to TP-12), systematically classified into α, β, and γ groups. TP-5 emerges as the most stable isomer (0 kcal mol^-1), also exhibiting the highest degree of planarity, as indicated by the molecular planarity parameter (MPP) and span of deviation from planarity (SDP) indices. Electron affinity and ionization energy analysis further define the accepting characteristic of the isomers, with values ranging from 8.30 eV to 9.35 eV. Building on these findings, a series of donor-acceptor (D-A) molecules (TPY-1 to TPY-6) are designated employing 2,2':6,6″-terpyridine (TP-5) as the electron-accepting unit, coupled with acene- and triphenylamine-based donor groups. Quantum chemical calculations are employed to explore their electronic, structural, and non-linear optical (NLO) properties. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations highlight that TPY-6 exhibits exceptional NLO performance compared to parent compound TPY-1. It shows isotropic and anisotropic polarizability (α_iso = 74.93 × 10^-24 esu and α_aniso = 58.63 × 10^-24 esu), along with the highest second hyperpolarizability (〈γ〉 = 291.4 × 10^-36 esu), which is approximately 40.3 times greater than that of para-nitroaniline (p-NA), a well-known prototype reference molecule. Its higher oscillator strength (ƒ_o = 1.378) and lower excitation energy gap (ΔE = 3.712 eV) contribute to its enhanced 〈γ〉 value. The variation in NLO response is further explained through an analysis of frontier molecular orbitals (FMOs), which reveals a significant decrease in the orbital energy gap, from 6.48 eV to 5.10 eV, across the studied compounds. The maximum redshift exhibited by TPY-6 at 344 nm is indicated by the UV-visible spectrum in tetrahydrofuran (THF) solvent. These findings demonstrate that positional isomerism in terpyridine significantly influences the electronic and optical behavior, and terpyridine derivatives are promising candidates for NLO applications.