Tuning of quadratic nonlinear optical activity of chromophores with indolizine donor moiety and polyene π-bridge on the basis of electrochemical data

Abstract

Electrochemical data (CV peak potentials, DPV potentials, frontier orbitals energy values and energy gap ∆Eel) were used to characterize chromophores with isomeric indolizine donor moieties, different acceptors and polyene π-bridges of various length. A good correlation was found between the electrochemical ΔEel, optical ΔEopt and calculated ΔEqch (DFT) characteristics. Electrochemical data make it possible to determine the energies of frontier molecular orbitals independently, in contrast to ∆Eopt one, calculated from λmax (UV-vis spectra). This advantage allows us to understand the origin of the chromophore energy gap changes when the nature of the donor, acceptor, or bridge varies. The polyene π-bridges are shown to provide better charge transfer ability compared to the hetarene-containing bridges; they are responsible for chromophore energy characteristics variation with changes in building blocks. The effect of the bridge length and the donor/acceptor end fragments is discussed. Indolizine donors produce NLO-chromophores with high first hyperpolarizability, β. The smallest ΔEel = 0.91 eV and the highest first hyperpolarizability - 1007⋅10−30 esu are achieved for a chromophore with 1-methyl-2-(4-methoxyphenyl)indolizin-3-yl donor, 3-cyano-2-dicyanomethylene-5,5-dimethyl-2,5-dihydrofuran-4-yl acceptor and octatetraene π-bridge. The correlation between β and 1/(ΔEel)2 is nearly linear. Polyene π-bridges provide high sensitivity of redox potentials to the nature of the donor and acceptor compared to chromophores with hetarene-containing bridges.