New Electron Acceptor for the Control of Crystal Structures and the Generation of High Optical Nonlinearities
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INTRODUCTION Squaraine (SQ) dyes shown below (Figure 1), generally indicate sharp absorptions in 620 670nm and large molecular absorption coefficients ; more than 105cm-1 M-1. They are also known to have interesting semiconductive and photoconductive properties in the solid state.
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Figure 1. Chemical structure of squaraine (SQ) dye
Owing to the exciton interactions, they generally exhibit intense and panchromatic absorption (400-1000 nm) and have been shown to be useful for xerographic applications[1], organic solar cells[2] and ablative optical recordings[3]. After an investigation of SQ dyes on the relationships between molecular structures and photoconductivities, we have observed very sensitive and non-mutagenic SQ dyes containing Nbenzyl substituents[4]. On a base of the study for SQ chemistry, we have developed new nonlinear optical materials with high second order nonlinear optical properties[5 9]. Compared with inorganic materials, it is well recognized that organic nonlinear optical materials will be useful for substances with high nonlinear optical susceptibility ($(2)), high laser damage threshold, increased operating speed and improved processability. In this paper, a chiral electron acceptor of cyclobutenediono compounds (Compounds containing cyclebutenedione as an acceptor; CDs) is shown to create new highly active nonlinear optical materials with perfectly aligned polarization one dimensionally in crystals. RESULTS AND DISCUSSION High molecular hyperpolarizability In order to produce a bulk material with high second order nonlinear optical effect, constituent molecules must possess a high value and should be arranged non-centrosymmetrically. However, it is generally hard to control the molecular orientation in such a desired way. Most organic molecules with large dipole moments have a propensity to orient in centrosymmetric manner, due to electrostatic interaction between adjacent molecules. Through an investigation on SQ dyes, we have focused on the molecular structure, which has Donor-Acceptor-Donor constitution. One of the simplest CDs is squaric acid. Squaric acid is water soluble and has very strong acidity, like nitric acid. Squarate anion in water has a stable 2 delocalized electron system. It is suggested that CD substituent has a strong electron accepting property as nitro (NO2) substituent, which is expected to enhance values of an organic molecule. New materials containing CD were synthesized and identified in our laboratory. In order to compare with nitro compound, we have evaluated the optical properties, values of N,Ndimethylamino-p-nitroaniline (DMPNA) and CDs (materials 1 5). As shown in Table 1, materials 1 (DAD; (-)-1-(4-dimethyl aminophenyl)-2-(2-hydroxypropylamino)cyclobutene-3,4dione) and 2 (MACD) indicate higher absorption coefficients than nitro analogue (DMPNA). Figure 2 shows absorption spectra of DMPNA and DAD at the same concentration in C2H5OH solution. It shows that the absorption peak of DAD is higher and sharper than the absorption pe
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