Exploring the impact of terminal and central positions of polycyclic aromatic cores on aromaticity and NLO properties: a systematic computational analysis under various solvation methods
摘要
Polycyclic aromatic compounds (PACs) are setting a higher benchmark for next-generation optoelectronics and nonlinear optical (NLO) materials, owing to their unparalleled π-conjugation and aromatic stability. In this study, we designed two tailored molecular sets, P1-A to P3-A and P1-D to P3-D, by blending diverse polycyclic aromatic cores with donor or acceptor terminals. We quantified the aromaticity of our designed compounds by using the harmonic oscillator model aromaticity (HOMA) and the nucleus independent chemical shift (NICS) to assess the extent of aromatic character and distinguish between aromatic and non-aromatic behavior. Strikingly, the terminal rings in compounds P1-A and P1-D exhibit nearly ideal structural aromaticity (HOMA = ~ 0.98 to ~ 0.99), yet show positive NICS(1)zz values (1.7 to 2 ppm), revealing a pronounced disconnect between geometric and magnetic aromaticity. Building on the aromaticity findings, we proceed to explore the NLO responses of the studied compounds. This evaluation demonstrates how their electronic and structural characteristics modulate NLO behavior. Our density functional theory (DFT) calculations indicate that P3-A and P3-D emerge as standout candidates with the highest level of third-order NLO polarizability (γ) of 160 × 10− 36 esu and 124.1 × 10− 36 esu, outshining their counterparts. Furthermore, unit spherical representation (USR) analysis confirms the superior performance of compound P3-A, providing strong evidence of its remarkable NLO behavior. Solvent effects, analyzed through integral equation formalism polarizable continuum model (IEFPCM), conductor-like screening model (COSMO), and the explicit model, present a promising trend, with αiso values increasing by approximately ⁓1.1 to ⁓1.2 times in methanol compared to the gas phase. In dynamic NLO, both the electro-optic Pockels effect (EOPE, γ (-ω; ω,0,0)) and the second-harmonic generation (SHG) process γ (-2ω; ω, ω,0) show significantly enhanced values (537.9 × 10− 36 esu and 11599.2 × 10− 36 esu) of NLO at 543 nm. TD-DFT studies underscore the significant excitation properties of P3-A and P3-D, with solid oscillator strength values (0.9080 and 0.8696). Meanwhile, UV-Visible results demonstrate a noticeable red shift for P1-D at 356 nm. Hence, these PAC-based derivatives are promising candidates with the potential to advance next-generation NLO technologies.