Non-linear optical material

Abstract: A non-linear optical material contains a benzalacetofuron derivative represented by the following formula (I) wherein R is CH₃, CH₃S, Br, CN or NO₂.

Description: BACKGROUND OF THE INVENTION This invention relates to a non-linear optical material which consists of a benzalacetofuron derivative. Non-linear optical materials are the materials exhibiting so-called non-linear optical effects, in which non-linear responses result from induced polarization of electrons by the electrical field created by the light incident on the materials. These optical effects are generally due to the second and higher order terms in the following equation of P = χ¹E + χ²E·E + χ³E·E·E + ---- + χ n E n wherein P is polarizability of a material, E is intensity of electrical field, and χ n is non-linear optical susceptibility of the n'th order. It has been known that, due to a phenomenon known as the second harmonic generation (SHG) obtained by the specific utilization of the secondary effect, an incident light is converted into a light wave corresponding to the second harmonic wave and having a frequency twice as high as the frequency of the incident light or the refractive index is changed by voltage, so that the phenomenon is very conveniently utilized for performing various optical processings including conversion of wavelengths, processing of signals and modulation of laser beams, which are extremely advantageous. Although inorganic crystals, such as KH₂PO₄ (KDP), LiNbO₃ or NH₄H₂PO₄ (ADP), have hitherto been used as the non-linear optical materials, they have disadvantages that single crystals having high optical purities are very expensive, that they are so deliquescent as to be inconvenient in handling, and that the non-linear sensitivities thereof are not so high. On the other hand, since the utility of organic materials was suggested in 1983 in the symposium in the American Chemical Society, organic crystals of urea, aniline base compounds or the like have been reported to be utilizable as non-linear optical materials. However, these organic compounds do not exhibit satisfactory non-linear optical effects, or the compounds which exhibit relatively high level of non-linear effect have a disadvantage that they have light absorptive terminal groups that are significantly shifted towards the long wavelength range to thus limit the wavelength range of the light waves which can be processed therethrough. SUMMARY OF THE INVENTION It is an object of the present invention to provide a non-linear optical material consisting of a benzalacetofuron derivative with the specific structure, having excellent transparency and superior transmission and exhibiting extremely high non-linear optical effects. According to the present invention, there is provided a non-linear optical material comprising a benzalacetofuron derivative represented by the following formula (I) wherein R is CH₃, CH₃S, Br, CN or NO₂. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a chart showing the light absorption spectrum of a known non-linear optical material and the light absorption spectrum of a purified product of 4-methyl benzalacetofuron which is employed in the non-linear optical material of the present invention. PREFERRED EMBODIMENTS OF THE INVENTION The present invention will be explained in detail hereinbelow. A benzalacetofuron derivative, employed as a non-linear optical material of the present invention, is represented by the following formula (I): wherein R is CH₃, CH₃S, Br, CN or NO₂. The benzalacetofuron derivative represented by the formula (I) is selected from the group consisting of 4-methyl benzalacetofuron, 4-methylthio benzalacetofuron, 4-bromo benzalacetofuron, 4-cyano benzalacetofuron, 4-nitro benzalacetofuron and mixtures thereof. In preparing the benzalacetofuron derivative represented by the formula (I), 2-acetofuron and each of 4-methyl benzaldehyde, 4-methylthio benzaldehyde, 4-bromo benzaldehyde, 4-cyano benzaldehyde or 4-nitro benzaldehyde may be subjected to dehydration condensation reaction in the presence of a basic catalyst or an acidic catalyst. Sodium hydroxide, potassium hydroxide or a variety of quaternary ammonium salts may be employed as the basic catalysts, while boron trifluoride, phosphorous oxychloride or boron trifluoride etherate may be employed as the acidic catalysts. More specifically, the benzalacetofuron derivative of the present invention may be obtained by reacting 2-acetofuron and the specific benzaldehyde in the temperature range of preferably 0 to 50°C for 30 minutes to 50 hours in the presence of the above mentioned catalysts, if necessary, in suitable solvents, e.g. alcohols such as methanol or ethanol. The reaction temperature higher than 50°C is not desirable since various secondary reactions are produced by heat, whereas the reaction temperature lower than 0°C is also not desirable since the reaction time is prolonged with concomitant economical disadvantages. The above specific benzalacetofuron derivative may be employed directly or may be purified by recrystallization. In the non-linear optical material of the present invention, the benzalacetofuron derivative is employed so that it exhibits extremely high non-linear optical effects as well as excellent transparency and superior transmission for the wavelength having not less than 400 nm and hence may be used for a variety of optical applications. EXAMPLES OF THE INVENTION The present invention will be explained in more detail with reference to Examples and Comparative Examples. Example 1 1.1 g (0.01 mol) of 2-acetofuron and 1.16 g (0.01 mol) of 4-methyl benzaldehyde were charged, along with 20 ml of ethanol, into a reaction vessel, and stirred at 25°C while a mixed solution of 1 g of a 40 wt% aqueous solution of sodium hydroxide and 10 ml of ethanol was added dropwise to the reaction system. After termination of adding the mixed solution dropwise, the reaction was practiced at 25°C for 24 hours. After 20 ml of a 0.5 N aqueous solution of hydrochloric acid was added to the reaction system to terminate the reaction, a precipitated solid was filtered, washed several times with distilled water and dried. The so-produced crude product was recrystallized with an ethanol solvent to obtain 1.91 g of purified 4-methyl benzalacetofuron melting at 111.0°C at a yield of 90 %. The light transmission spectrum is shown in Fig.1. Examples 2 to 5 The procedures similar to Example 1 were followed except that benzaldehyde as a starting material was changed to 4-methylthio benzaldehyde (example 2), 4-bromo benzaldehyde (example 3), 4-cyano benzaldehyde (example 4) or 4-nitro benzaldehyde (example 5). The following products of the Examples 2 to 5 were similarly synthesized in accordance with the Example 1. Each of the products was analyzed in accordance with the example 1. The following are the results. Product obtained in Example 2 4-methylthio benzalacetofuron melting point : 122.7°C Product obtained in Example 3 4-bromo benzalacetofuron melting point : 131.3°C Product obtained in Example 4 4-cyano benzalacetofuron melting point : 187.9°C Product obtained in Example 5 4-nitro benzalacetofuron melting point : 228.4°C Example 6 The measurement of the second harmonic generation (SHG) of the produced compounds in the Examples 1 to 5 were then performed. For measurement, each of the samples was granulated to the size of 50 to 150 µm in diameter and sandwiched between a pair of slide glasses. Each sample was irradiated with 15 nsec pulses by an Nd⁺-YAG laser fitted with a Q-switch (wavelength : 1064 nm) to detect the second harmonics emanated from the sample. An urea sample, granulated in the similar manner, was used as a standard sample, and the ratio of the SHG strength or intensity of the sample was found with respect to the SHG strength of the urea which was set to 1. This method of measurement is well-known among those engaged in the art and described in detail in, for example, Journal of Applied Physics, vol.36, No.8, pages 3798 to 3813, 1968. The SHG intensity ratio of respective benzalacetofuron derivatives of the present invention is shown in Table 1. Comparative Example 1 The light transmission spectrum of 2-methyl-4-nitro aniline, which is a well-known non-linear optical material, was measured and the result of the light transmission spectrum is shown in Fig. 1. Comparative Examples 2 to 4 4-ethyl benzalacetofuron (Comparative Example 2), 4-chloro benzalacetofuron (Comparative Example 3) and 4-iodo benzalacetofuron (Comparative Example 4) were similarly synthesized in accordance with Example 1. The SHG intensity ratio of the products in the Comparative Examples 2 to 4 were measured in accordance with the Example 6. The results of the SHG intensity ratio are shown in Table 1. The produced respective compounds in the Comparative Examples 2 to 4 did not exhibit non-linear optical activity. Obviously from Fig.1, the product of the Example 1 of the present invention exhibited transmission of about 100 % for the wavelength having not less than 400 nm and was substantially transparent. On the contrary, with respect to 2-methyl-4-nitro aniline, which is the known non-linear optical material, the visible ray having the wavelength of 500 nm or smaller was absorbed and was not transmitted.

Claims: A non-linear optical material comprising a benzalacetofuron derivative represented by the following formula (I) wherein R is CH₃, CH₃S, Br, CN or NO₂. The non-linear optical material according to claim 1 wherein said benzalacetofuron derivative is selected from the group consisting of 4-methyl benzalacetofuron, 4-methylthio benzalacetofuron, 4-bromo benzalacetofuron, 4-cyano benzalacetofuron, 4-nitro benzalacetofuron and mixtures thereof.

Inventors: GOTO YOSHITAKA, NAKAYAMA MASAHARU, ZHANG GONG JIAN, GOTO, YOSHITAKA, NAKAYAMA, MASAHARU, ZHANG, GONG JIAN
Applicants: NIPPON OILS & FATS CO LTD, NIPPON OIL AND FATS COMPANY, LIMITED
Application year: 1990
Main classification: C07D307
IPC codes: C07D307, G02F1