Synthesis, spectroscopic, SC-XRD/DFT and non-linear optical (NLO) properties of chromene derivatives

In the present study, we reported the efficient synthesis of novel, heterocyclic, coumarin-based pyrano-chromene derivatives, 2-amino-8-methyl-5-oxo-4-[2-(2-oxo-2H-chromen-3-ylmethoxy)-phenyl]-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (4a) and 2-amino-8-methyl-5-oxo-4-[2-(2-oxo-2H-chromen-3-ylmethoxy)-phenyl]-4H,5H-pyrano[3,2-c]chromene-3-carboxylic acid methyl ester (4b). The chemical structures of synthesized compounds were resolved by employing various spectroscopic techniques like UV-Vis, FT-IR, ¹H & ¹³C NMR, and single crystal X-ray diffraction (SC-XRD) analysis. The compounds; 4a and 4b, with appealing π-bonded skeleton were further analyzed in terms of their electronic and structural aspects using an integral approach of density functional theory (DFT) and time-dependent DFT (TD/DFT). The methodology: M06-2X/6-31G(d,p) level of theory was applied to compare their experimental data with theoretical outcomes using quantum chemical analysis. The frontier molecular orbitals (FMOs) study revealed that, 4a possesses a low band gap (5.168 eV) as compared to 4b (6.308 eV). Global reactivity parameters were associated with E _gap values as 4a, with the lowest band gap showed the smaller value of hardness (0.094 eV) and a larger value of softness (5.266 eV). The non-linear optical (NLO) insight exhibited that, the average polarizability 〈α〉 and second hyperpolarizability (γ _tot) were observed in 4a as 6.77005 × 10^-23 and 0.145 × 10⁴ esu, respectively. Overall, the computational studies suggest that the investigated compounds have distinct NLO properties.