Effect of Molecular Planarity and Aromaticity on Third-order NLO Polarizability for Exploration of Fused Heterocyclic Carbazole Derivatives

The present study investigates fused heterocyclic donors to understand how they affect molecular geometries, molecular planarity, and aromaticity. Three series of chromophores, each with three compounds abbreviated as (PhT-EBT – DPP-EBT) with D-π-A architecture, were designed from reference compound CEBT by its structural modification with fused heterocyclic donors. The aromatic character of isolated donor fragments was quantified using the Harmonic Oscillator Model of Aromaticity (HOMA) and Nucleus-Independent Chemical Shift (NICS) indices to determine the extent of electron delocalization and distinguish between aromatic and non-aromatic behavior. Notably, fused rings in DPP exhibited ideal structural aromaticity (HOMA = 0.870 to 0.829) and high negative NICS(1)zz values (-23 ppm to -28 ppm), indicating pronounced correlation between geometric and magnetic aromaticity. Furthermore, the DPP-EBT compound expressed the most significant planarity revealed by the Molecular Planarity Parameter (MPP) and the Span of Deviation from Planarity (SDP). Extending the aromaticity and planarity insights, we further investigated the Nonlinear optical (NLO) properties of the studied compounds. Our DFT computations show the DPP-EBT derivative holds a large <γ> amplitude (462.8 × 10 -36 esu) as compared to its corresponding derivatives. For DPP-EBT, the α iso and α aniso are 49.79 × 10 -24 esu and 55.52 × 10 -24 esu, respectively. This investigation ensures that the aromatic character and molecular planarity synergistically modulate NLO response. Unit Spherical Representation (USR) further verified the outstanding NLO behavior of DPP-EBT and validated its great performance. Complementarily, charge transfer properties of these compounds were studied using Frontier Molecular Orbitals (FMOs), Molecular Electrostatic Potential (MEPs), and Electron Density Difference (EDD) maps. This structure-property study revealed that more efficient charge transfer occurs in planar and aromatic compounds, resulting in a high NLO response. Overall, the current investigation is expected to pique the interest of researchers in developing the compounds mentioned above for advanced NLO materials and optoelectronic applications.