A series of organic nonlinear optical chromophores containing different auxiliary electron-donating groups (alkoxy, benzyloxy and alkyl) and p-conjugated bridges (vinyl and azobenzene) have been synthesized and systematically characterized using UV-Vis spectroscopy, density functional theory calculation and thermogravimetric analysis. The microscopic molecular hyperpolarizability was obtained via the simple solvatochromism method, where the synthesized chromophores were dissolved in several solvents with different refractive index for absorption spectra measurement. It was found that the introduction of auxiliary groups obviously improved the microscopic molecular hyperpolarizability of all the synthesized chromophores, especially for the benzyloxy modified azobenzene based chromophore, up to sevenfold enhancement of molecular hyperpolarizability as well as good visible light transmittance and thermal stability up to 247°C were obtained, which should be mainly attributed to the enhanced charge transfer enabled by the benzyloxy modification that is confirmed by the electron density distribution in the highest occupied molecular orbital and lowest unoccupied molecular orbital.
A novel nonlinear optical (NLO) chromophore 2-[{2-[(4-diethylamino-styreneyl)-furyl-5]-vinyl}-3-cyano-5,5dimethyl-5H-furan-2-ylidene]malononitrile (EFFC) containing furan conjugating bridge was designed with tricyanofuran as electron acceptors and dialkylamine as donors. The molecular has been synthesized via two steps of aldol condensation reactions. In the first step, 5-methylfurfural (1 equiv.) and drops of acetic acid in THF was added dropwise to a solution of 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) (1 equiv.) and pyridine. After the addition, the mixture was stirred for 24 h at room temperature. The reaction mixture was filtrated and the insoluble residue was washed several times with ethanol. Secondly, the intermediate (1 equiv.), 4-(diethylamino)benzaldehyde (1 equiv.) and ten drops of piperidine were dissolved in MeCN. The solvent was removed after heated to reflux for 24 h. The product EFFC was purified by a silica gel chromatograph column and characterized by FT-IR, 1 H NMR and elemental analysis. The thermal decomposition temperature (T d) as high as 250 ℃ was determined by TGA testing. Furyl conjugating bridges lowered the T d compared with the non-furyl corresponding structure DCDHF-2-V. Density functional theory (DFT) B3LYP/6-31G method was used to optimize the structures and calculate the static hyperpolarizability (β 0). The β 0 value of EFFC was calculated as large as 6.5×10-28 esu. It was larger than the corresponding chromophores with dimethylamine donors. The chromophore was mixed into polysulfone (PSU) with the weight ratio of 18%. The mixture was dissolved in cyclohexanone and the films were spun onto ITO/glass substrates. The chromophores in the polymer matrix were aligned into a noncentrosymmetrical orientation by corona poling technique. The poling was performed in a wire-to-plane geometry under in-situ conditions. The discharging wire to plane distance was 1 cm. 12 kV of corona voltage was applied and kept for 15 min at 100 ℃. The film was cooled down to the room temperature at applied electric field maintaining a constant current. The second-order nonlinear coefficient r 33 value reached to 80 pm/V at 1064 nm was measured by second harmonic generation method.
In this work, novel organic non-linear optical chromophore containing azo-furan based conjugation bridge (EF-NFC) was designed and synthesized. Moreover, the optical and thermal properties of EFNFC were systematically compared with those of two different conjugation bridges of vinyl-furan (EFFC) and azo-benzene (EFNC) containing same electron donor and acceptor moieties, respectively. It was found that similar maximum absorption wavelength in DMSO solution was detected for EFNFC (573 nm) and EFNC (568 nm), which indicated an improved visible light transmittance compared to EFFC. Meanwhile, the second-order polarizability of EFNFC (4.479×10 -49 esu) was lower than that of EFNC (5.971×10-49 esu), but larger than that of EFFC (3.194×10 -49 esu) according to the solvatochromism test. All three chromophores exhibited good thermal stability as their 5% decomposition temperatures (T d ) were over 250 ℃, especially the highest T d of 275 ℃ was detected from the synthesized novel EFNFC chromophore.
In order to study the impact of different additional donors on molecular thermal decomposition temperature (T d ) value of the chromophores as well as the second polarization (μ g β) value. Three novel materials EFC-OH, EFC-OBu and EFCOBe were synthesized by attaching additional donors to the benzene ring based on the reported 2-[{4-(4-diethylamino)styreneyl}-3-cyano-5,5-dimethyl-5H-furan-2-ylidene]malononitrile (DCDHF-2-V). The three chromophores were characterized by 1 H NMR, 13 C NHR, FT-IR and elemental analysis. The T d value was determined by TGA testing, and the result showed that three molecules had a good thermal performance. EFC-OBe processing a T d of 278.6 ℃ showed the best among these molecules. The hyperpolarizability was measured and calculated by solvatochromism method. EFC-OBe still showed the best μ g β value of 4.7149×10 -44 esu. Comparing to the DCDHF-2-V without additional donors on T d value and μ g β value, additional donors had little effect on T d value while achieved an improvement of one order of magnitude in μ g β values, which indicates that the molecules can work in high temperature and performance better in optics.
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