Linear vs bent carbazole isomers: Molecular-shape-dependent assessment of their linear and nonlinear optical properties across gaseous and solvated phases

The present study describes the effect of the molecular shape of carbazole isomers, characterized by linear and bent arrangements achieved through strategic modifications of terminal groups, on their nonlinear optical (NLO) properties. For this study, we compared two constitutional forms of carbazole: linear ( L-ICZ-1 to L-ICZ-4 ) and bent ( B-ICZ-1 to B-ICZ-4 ). Through systematic variation of the terminal substituents, we noticed clear shifts in their structures and in how efficiently the charge was transferred. Our study involves the calculations of static linear and third-order NLO polarizabilities (α & γ), aiming to analyze the suitability of the studied isomers for advanced NLO applications. Compounds L-ICZ-4 and B-ICZ-4 exhibit the best γ values of 923.9 × 10 −36 esu and 427.9 × 10 −36 esu, respectively, standing as the front-runners in our research. The ∼2.2 times γ enhancement in L-ICZ-4 over B-ICZ-4 underscores stronger charge transfer capability. To explore the investigated compounds' second-order NLO polarizability (β), Hyper-Rayleigh scattering (βHRS) calculations were carried out using the established theoretical formalism. B-ICZ-4 stands out with the highest β HRS value of 6167.8 au, accompanied by a prominent octupolar (J = 3) contribution of 13,586.0 au. The dynamic NLO responses of our most efficient compound ( L-ICZ-4) , across various laser wavelengths ranging from 1400 to 1907 nm, are computed. The peak γ-values were recorded at 1400 nm for both the DC-Kerr (electro-optic Kerr) effect γ (−ω; ω,0,0) and electric-field-induced second-harmonic generation (EFISHG) γ (−2ω; ω, ω,0) methods, with values of 12,157 × 10 −36 esu, 20720 × 10 −36 esu, respectively. To uncover more about NLO, we conducted a unit spherical representation (USR) analysis at a 1907 nm laser wavelength. The USR diagram of L-ICZ-4 for γ indicates superior charge transfer properties compared to its bent counterpart, B-ICZ-4 , reinforcing its high potential for NLO applications. We also calculated the α iso and γ-values of L-ICZ-4 and B-ICZ-4 using both implicit and explicit solvent models to better understand the environment-dependent variations in their NLO responses. Interestingly, the γ-values in explicit-methanol saw a reasonable boost, ranging from ∼1.0 to ∼1.1 times those in the gas phase. Thus, this work shows that conformational isomerism effectively tunes optical nonlinearity, guiding the rational design of advanced optoelectronic and photonic systems.