Enhanced Nonlinear Optical Response in Zwitterionic Molecules:  A Computational Study on the Role of Orbital Interactions through σ Bonds

Abstract

Ab initio coupled perturbated Hartree-Fock calculations, using 6-31G** basis sets, on a heterocyclic zwitterionic molecule with a σ-spacer between the donor and the acceptor ring shows a static first hyperpolarizability ( 0 ) of around 240 × 10 -30 esu. Substitution of electron withdrawing functional groups, such as NO 2 , on the acceptor ring of this molecule enhances the 0 value to around 3960 × 10 -30 esu. Studies on various such zwitterionic molecules demonstrate the importance of π-σ orbital mixing (through bond interaction) between the π-aromatic rings and the σ-spacers in enhancing the nonlinear optical (NLO) response. Analysis of the transitions reveals low oscillator strengths, large changes in the dipole moments, and very low energy charge transfers that take place in the excited states of the molecule, while in the ground state they are stabilized in a charge separated resonance form. This is mainly responsible for the large NLO response. MP2 calculations on small molecules with dominant through bond interactions show that the inclusion of electron correlation further enhances the 0 value. The molecules, which have a strong IR absorption, have potential applications in filters, polarizers, optical recording, etc.

Theoretical study on the second‐order nonlinear optical properties of nonconjugated D‐π‐A chromophores

“…Bhanuprakash and coworkers [18,19] have studied the NLO activity in some nonconjugated 3D triazine derivatives from both experiment and theoretical calculation. Moreover, Bhanuprakash and coworkers [20] systematically studied the role of orbital interactions through -bonds on enhanced NLO response in zwitterionic molecules. Jin et al [21] have synthesized novel crosslinkable polymers for secondorder nonlinear optics using palladium-catalyzed carbon-carbon coupling reaction.…”

Two‐Photon Solvatochromism II: Experimental and Theoretical Study of Solvent Effects on the Two‐Photon Absorption Spectrum of Reichardt’s Dye

“…The measurement of 2PA cross-sections is performed for three solvents (chloroform, dimethyl formamide, and dimethyl sulfoxide) using the Z-scan technique. [39,40,42,67,80,[84][85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100] The spectroscopic studies of RD are mainly focused on the behavior of its long-wavelength CT absorption band in a variety of media to obtain E T (30) and/or E T N values. To unravel the solvent dependence of the 2PA cross-section, the electronic structure of RD is determined using a hybrid quantum mechanics/molecular mechanics (QM/MM) approach, in which polarization between the solute and solvent is taken into account by using a self-consistent scheme in the solvent polarization.…”

Reinvestigation of electronic structure and electric properties of large betaine molecules: A combined long-range-corrected DFT and coupled-cluster study

“…The authors repeatedly indicated the possible connections between their findings and the nonlinear optical (NLO) properties for this class of compounds without making any calculations. It is well established, however, that the NLO properties, studied in detail for betaine dyes on purely theoretical basis during the last decade [2][3][4][5][6][7][8], depend crucially on the geometry of the so-called pushpull systems which, in turn, can be analyzed in terms of the bondlength alternation (BLA) parameter [9][10][11]. The results presented by Perpète et al may serve here as an excellent example that the electron correlation (EC) significantly affects the geometry and the wavefunction [12].…”