Effect of the Substituents of New Coumarin-imidazo[1,2-]heterocyclic-3-acrylate Derivatives on Nonlinear Optical Properties: a Combined Experimental-theoretical Approach

A series of new coumarin-imidazo[1,2-]heterocyclic-3-acrylate derivatives 7a-h were synthesized by the Heck reaction between the corresponding 3-(imidazo[1,2-]pyrimidines)-(2-yl)-2-chromen-2-ones 4a-e and methyl acrylate in 45-87% yields. The effect of the distinct substituents on third-order nonlinear optical properties was examined, experimentally measuring their nonlinear refractive indexes by the -scan technique. Density functional theory and time-dependent density functional theory were utilized with the B3LYP, CAM-B3LYP, PBE (PBEPBE), and M062X functionals on Gaussian09 software to calculate the vertical excitation, relaxation of the brightest excited states, conformation, HOMO-LUMO gaps, oscillator strength, polarizability, and hyperpolarizabilities of all derivatives. Although all acrylates showed a nonlinear response at a certain level of power, the compounds bearing a diethylamino electron-donating group exhibited higher nonlinear refractive index values (∼10 cm W), which is in agreement with the trend in the computational calculations of the first and second hyperpolarization. According to the structural analysis, the electron-withdrawing group (acrylate) is mainly responsible for the loss of coplanarity because of increasing the dihedral angle between the coumarin and imidazo[1,2-]heterocyclic moieties (to 39.1°). On the other hand, the unsubstituted compound 4a presented the greatest nonlinearity due to its almost coplanar structure ( ∼ 10 cm W), highlighting the importance of this feature.