Ayele Teshome

Symmetrical and Nonsymmetrical Chromophores with Tröger’s Base Skeleton: Chiroptical, Linear, and Quadratic Nonlinear Optical Properties—A Joint Theoretical and Experimental Study

We report on the novel chiral push-pull chromophores derived from 6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (Trögers base skeleton). The synthesis of symmetrical chromophores featuring two identical acceptors, as well as the synthesis of unsymmetrical chromophores featuring only one acceptor is given. Symmetrical chromophores were prepared in the enantiomerically pure form and their chiroptical properties were investigated. Second-order nonlinear optical (NLO) properties of new chromophores were investigated with the aid of hyper-Rayleigh scattering (HRS). The detailed theoretical analysis of the second-order NLO properties of the chromophores was also undertaken. The joint theoretical and experimental studies of chromophores derived from Trögers base skeleton, in comparison with benchmark chromophores featuring a dimethylamino group as the donor, provided insight into the relationship between the structure of the new chromophores and their NLO properties.

Testing computational models of hyperpolarizability in a merocyanine dye using spectroscopic and DFT methods.

The structural and electronic properties of a highly solvatochromic merocyanine dye, 2-(3-cyano-5,5-dimethyl-4-(3-(1-octadecylpyridin-4(1H)-ylidene)prop-1-enyl)furan-2(5H)-ylidene)malononitrile (pyr3pi), have been investigated using UV-vis, NMR, hyper-Rayleigh scattering, and Raman spectroscopies and further interpreted using computational chemistry. Spectroscopic data indicate that pyr3pi exists in its zwitterionic form even in low polarity solvents with electronic absorption spectra showing a hypsochromic shift with an increase in solvent polarity and NMR experiments indicating an increasingly zwitterionic structure in chloroform as the temperature is lowered. Raman spectra in increasingly polar solvents show small variations of the structure that are consistent with a change toward a structure with more zwitterionic character. However, comparison of the calculated and experimental vibrational energies and intensities and comparison of NMR coupling constants with calculated bond order indicate that calculations underestimate the amount of charge separation seen in low polarity solvents. Although for this system density functional theory (DFT) calculations and the two-state model qualitatively reproduce negative solvatochromism, they fail to reproduce the trends in hyperpolarizability seen experimentally. This is attributed to solvent field DFT calculations underestimating the degree of charge separation in reaction fields representing low polarity solvents.

Synthesis and properties of zwitterionic chromophores containing substituents for shape control

The synthesis of a series of new chromophores for nonlinear optics are described. Some of the compounds contain “bulky” groups designed to reduce aggregation between the molecules. While this may not impact on the electro‐optic response of the compounds, both X‐ray crystallographic and uv‐vis spectroscopic studies indicate that such modifications will likely reduce aggregation once the chromophores are incorporated into polymer matrices.

Second-order nonlinear optical properties of zwitterionic chromophores

The molecular linear and second-order nonlinear optical (NLO) properties of a series of donor (D)-π-acceptor (A) merocyanine molecules have been studied in three solvents, dimethylformamide (DMF), tetrahydrofuran (THF), and chloroform (CHCl3). All the compounds have a cyanodicyanomethylidenedihydrofuran electron acceptor system with either a pyridinylidene or quinolinylidene donor group. In high polarity solvents the molecules with a quinolinylidene donor have larger first hyperpolarizabilities than those with a pyridinylidene donor, while the opposite is true in low polarity solvents. The molecules under investigation have an aromatizable donor unit, which leads to a high degree of charge separation in the ground-state; as a result they have a strong tendency to aggregate. To minimize these interactions arene-rich bulky groups have been introduced in a number of these compounds.

Optimizing the second-order nonlinear optical response in some indoline-based chromophores at the molecular and macroscopic levels

The effect of extending the conjugation length, the deployment of various substituents and configurational locking of the polyene backbone on the second-order nonlinear optical (NLO) response of a series of indoline based compounds has been investigated. The compounds were examined using Hyper-Rayleigh scattering (HRS) and relative second-harmonic generation (SHG) techniques with a femtosecond operating system with a 1300 nm fundamental wavelength. All of the compounds were found to have high molecular hyperpolarizabilities with β values of up to 1230 × 10-30 esu. At the macroscopic level-for poled polymer thin films-a strong second-order NLO signal has been detected and d33 values of up to 217 pm/V are found-a response of some ten times greater than that found for the well known azo dye Disperse Red 1.

Synthesis and Properties of Nonlinear Optical (NLO) Chromophores with Indoline Donors and Azo Linkers

A simple method for the preparation of a suite of new nonlinear optical chromophores is presented. Using this methodology we have been able to prepare chromophores with azo linkers and a variety of acceptor systems in high yields and without the need for chromatography. The compounds are highly solvatochromic, and the predicted values of their NLO response suggest they are candidates for further development.