Rational Enhancement of Second-Order Nonlinearity: Bis-(4-methoxyphenyl)hetero-aryl-amino Donor-Based Chromophores: Design, Synthesis, and Electrooptic Activity

Abstract

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborator's, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.

A Joint Experimental and Theoretical Investigation on Nonlinear Optical (NLO) Properties of a New Class of Push–Pull Spirobifluorene Compounds

“…[1] Much effort have been aimed at identifying relationships between organic structure and nonlinear optical activities so that these systems can be made more efficient. [2][3][4][5][6] In particular, since the nonlinear optical properties of a compound stems from a nonlinear polarization induced by a beam of high intensity (typically a laser), it was soon clear that the ability of a compound to show second order NLO phenomena should be related to the degree of charge separation in the ground state. Until now, most of the organic molecules characterized by second order NLO properties contain electrondonating (D) and electron-withdrawing (A) groups linked through a conjugated π-spacer ("push-pull" systems).…”

Using phenoxazine and phenothiazine as electron donors for second-order nonlinear optical chromophore: Enhanced electro-optic activity

“…For example, compounds L1 and L2 showed hypsochromic shifts of À53 nm and À40 nm, respectively, from chloroform to acetone. Such a phenomenon was reported by Davies [36] and was ascribed to the back electron transfer from the acceptor side to the donor side in polar solvents which causing blue-shift from the absorption [37].…”

Theory-Guided Design of Organic Electro-Optic Materials and Devices

“…. Experimentally, the acentric order parameter can be defined using techniques such as attenuated total reflection (ATR) [83][84][85], two-slit interferometry (TSI) [86,87], Fabry-Perot interferometry (FPI) [88], or a Mach Zehnder interferometric technique (MZI) [89], which permit measurement of both of the two non-zero electro-optic tensor elements, r 33 and r 13 , for poled OEO materials. The acentric order parameter can be extracted either from the ratio of r 33 /r 13 or from r 33 alone if the elements of the molecular first hyperpolarizability tensor (e.g., β zzz ) are correctly estimated from a combination of quantum mechanical calculations and hyper-Rayleigh scattering (HRS) measurements [53,90,91] and/or electric field induced second harmonic generation (EFISH) measurements [16].…”