Abstract:
Devices for and method of generating coherent second harmonic light radiation. The devices comprise a laser source of coherent light radiation at a fixed fundamental frequency, a crystalline N-aryl carbamyl compound that crystallizes in a non-centrosymmetric configuration, means for directing the output radiation of the laser onto the crystalline chiral N-aryl carbamyl compound, and output means for utilizing the second harmonic frequency.

Description:
TECHNICAL FIELD 
     This invention is concerned with materials for nonlinear optical devices for the conversion of optical energy at one frequency to optical energy at another frequency. 
     BACKGROUND OF THE INVENTION 
     Laser techniques have been developed that make it convenient to obtain various fundamental frequencies of coherent laser light by utilizing solid, gas, and liquid media. Outstanding among these are solid-state lasers, because they are small, inexpensive, and require no maintenance; however their output is limited to the near-infrared region of the spectrum and is of low power. However, in many applications, laser light having frequencies above those conveniently obtainable is required. Nonlinear optical crystals have, therefore, frequently been employed to convert coherent laser light of a fundamental frequency into laser light of the second harmonic, that is to say, laser light with a frequency twice the fundamental frequency. This conversion is termed &#34;second-harmonic-generation&#34; (SHG). 
     In the prior art, monocrystalline forms of potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), barium sodium niobate (BaNaNbO 3 ), and lithium niobate (LiNbO 3 ) have been used for generating higher frequency harmonics. Monocrystalline KDP and ADP, while offering greater resistance to optical-irradiation-induced surface damage due to laser beam bombardment, do not exhibit large optical nonlinearities. This rendered these crystals unsuitable for higher harmonic frequency generation by conversion of light derived from low-power sources. In contrast, BaNaNbO 3  and LiNbO 3  show larger nonlinearities but, unfortunately, a low resistance to optical damage. In this regard, the term &#34;resistance to optical damage&#34; means the number of times (shots) the surface of a crystalline material can suffer bursts of laser radiation of a given power density in watts per unit area before the subject crystal shows signs of opacity. Thus, a crystal showing high resistance can sustain a larger number of shots than a crystal of low resistance for the same power density of the incident laser beams. These niobate materials are also mechanically and thermally fragile, and are marginal in SHG efficiency at power levels that are characteristic of laser diodes. 
     The possibility of using organic molecules in nonlinear optical devices has generated much interest recently because a large number of molecules are available for investigation. Some substituted aromatic molecules are known to exhibit large optical nonlinearities. The possibility of such an aromatic molecule having large optical nonlinearities is enhanced if the molecule has donor and acceptor groups bonded at opposite ends of the conjugated system of the molecule. The potential utility for very high frequency application of organic materials having large second-order and third-order nonlinearities is greater than that for conventional inorganic electro-optic materials because of the bandwidth limitations of inorganic materials. Furthermore, the properties of organic materials can be varied to optimize mechanical and thermo-oxidative stability and laser damage threshold. 
     U.S. Pat. No. 4,199,698 discloses that the nonlinear optical properties of 2-methyl-4-nitroaniline (MNA) make it a highly useful material in nonlinear devices that convert coherent optical radiation including a first frequency into coherent optical radiation including a second, typically higher, frequency. The nonlinear devices have means for introducing coherent radiation of a first frequency into the MNA and means for utilizing coherent radiation emitted from the MNA at a second frequency. 
     U.S. Pat. No. 4,431,263 discloses that diacetylenes and polymers formed from diacetylenic species, which are amenable to close geometric, steric, structural, and electronic control, provide nonlinear optic, waveguide, piezoelectric, and pyroelectric materials and devices. Diacetylenes which are crystallizable into crystals having a noncentrosymmetric unit cell may form single crystals or, if they do not, may possibly be elaborated into a polar thin film upon a substrate by the Langmuir-Blodgett technique. Such films often may be polymerized either thermally or by irradiation for use in nonlinear optical systems. Diacetylenes are covalently bonded to substrates through the employment of silane species and subsequently polymerized to yield nonlinear optic devices asserted to have high structural integrity in addition to high efficiencies and optical effects. 
     Other U.S. patents relating to non-linear optical properties of organic materials include U.S. Pat. Nos. 4,208,501; 4,376,899; and 4,579,915. 
     SUMMARY OF THE INVENTION 
     The present invention provides a laser generator of coherent second harmonic light radiation by utilizing certain chiral carbamic acid derivatives and a method of generating coherent second harmonic light radiation with such a device. 
     In general, second harmonic generators of this invention comprise, in combination, a laser source of coherent light radiation at a fixed fundamental frequency, an organic molecular crystalline compound selected from particular classes of chiral carbamic acid derivatives, e.g. chiral N-aryl carbamyl compounds, means for directing the output radiation of the laser onto the organic molecular crystalline carbamic acid derivatives, and output means for utilizing the second harmonic frequency. 
     In another aspect, all of the chiral carbamic acid derivatives described herein are novel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a diagrammatic representaton of a suitable arrangement for demonstrating the second harmonic generating properties of the carbamic acid derivatives of this invention. 
    
    
     DETAILED DESCRIPTION 
     The carbamic acid derivatives of this invention are characterized as N-aryl carbamyl compounds which are chiral, crystalline, and non-centrosymmetric. As used herein, the term &#34;chiral&#34; means that the molecule and its mirror image are not superposable. N-Aryl carbamyl compounds that have been found to exhibit second harmonic generation are crystalline in form. N-Aryl carbamyl compounds suitable for this invention are non-centrosymmetric species which have no center of symmetry on either the molecular or crystalline unit cell level. 
     Chiral N-aryl carbamyl compounds that have been found to be useful for this invention can be represented by the following general formula 
     
         R.sup.1 R.sup.2 NCOR.sup.5                                 (I) 
    
     which may be conveniently categorized into two major subclasses, namely the ureas (II) and the urethanes (III): 
     
         R.sup.1 R.sup.2 NCONR.sup.3 R.sup.4                        (II) 
    
     
         R.sup.1 R.sup.2 NCO.sub.2 R.sup.3                          (III) 
    
     where 
     R 1  represents an aryl group having no more than eighteen carbon atoms, 
     R 2  represents hydrogen or an alkyl group, preferably having one to six carbon atoms, 
     R 3  represents a carbon-containing chiral group, 
     R 4  represents hydrogen, an alkyl group, preferably having one to six carbon atoms, a chiral group, or R 3  and R 4  together, along with the nitrogen atom to which they are attached, form a catenated heterocyclic ring containing from 3 to 7 ring members, and 
     R 5  represents --NR 3  R 4  or --OR 3 . 
     R 1  may be substituted or unsubstituted. If R 1  is substituted, the substituents are preferably selected from the group consisting of nitro, alkoxy, acyloxy, acyl, alkoxycarbonyl, aryloxy, alkaryloxy, aralkoxy, alkylthio, arylthio, alkyl, haloalkyl, aryl, aralkyl, alkaryl, cyano, and halo. The preferred substituents are nitro, acyl, alkoxycarbonyl, and cyano, the most preferred substituent being the nitro group. 
     If R 2  is an alkyl group, it may be substituted or unsubstituted. The R 2  alkyl group may be substituted with any substituent that does not adversely affect the second harmonic generating properties of the urea or urethane molecule of this invention. 
     The chiral groups R 3  and R 4  can be derived from chiral amines and alcohols, which will be described hereinafter. 
     If R 3 , R 4 , and the nitrogen to which they are attached form a ring, the carbon atoms in the ring may be interrupted by atoms selected from nitrogen, oxygen, or sulfur. 
     Although the molecule R 1  R 2  NCOR 5  must be chiral, the sign of its optical activity is not critical, i.e. it does not matter whether the molecule rotates the plane of vibration of polarized light to the right or to the left. 
     Representative examples of N-aryl carbamyl compounds suitable for use in this invention include those where R 2  and R 4  represent hydrogen, R 3  represents (R)- or (S)-alpha-methylbenzyl or (R)- or (S)-1-(alpha-naphthyl)ethyl and R 1  represents a group selected from the group consisting of 2-nitrophenyl, 3-nitrophenyl, 4-nitrophenyl, 4-cyanophenyl, 2,4-dinitrophenyl, 2-fluoro-5-nitrophenyl, 2-cyano-4-nitrophenyl, 2,6-dichloro-4-nitrophenyl, 2-benzoyl-4-nitrophenyl, 4&#39;-nitrobiphenyl-4-yl, 4-(2-(4-nitrophenyl)ethenyl)phenyl, 4-nitro-1-naphthyl, 5-nitro-1-naphthyl, 1-nitro-2-naphthyl, 6-nitro-2-naphthyl, 8-nitro-2-naphthyl, 7-nitro-2-phenanthryl, and 7-nitro-2-fluorenyl. Additional representative examples of N-aryl carbamyl compounds suitable for use in this invention include those where R 2  and R 4  represent hydrogen, R 3  represents (R)- or (S)-1-hydroxy-2-propyl and R 1  represents a group selected from the group consisting of 2-nitrophenyl, 3-nitrophenyl, 4-nitrophenyl, 2-chloro-4-nitrophenyl, 2-methyl-4-nitrophenyl, 2-methoxy-4-nitrophenyl, 4&#39;-nitrobiphenyl-4-yl, 4-(2-(4-nitrophenyl)ethenyl)phenyl, 4-nitro-1-naphthyl, 5-nitro-1-naphthyl, and 8-nitro-2-naphthyl. 
     Still additional representative examples of N-aryl carbamyl compounds suitable for use in this invention include those where R 2  represents hydrogen, R 5  represents XY where X represents --NR 4  or --O-- and Y represents (R)- or (S)-1-(ethoxycarbonyl)ethyl, and R 1  represents 2-nitrophenyl, 3-nitrophenyl, 4-nitrophenyl, 4-cyanophenyl, 2-chloro-4-nitrophenyl, 2-methyl-4-nitrophenyl, 2-methoxy-4-nitrophenyl, 4&#39;-nitrobiphenyl-4-yl, 4-(2-(4-nitrophenyl)ethenyl)phenyl, 4-nitro-1-naphthyl, 5-nitro-1-naphthyl, and 8-nitro-2-naphthyl respectively. 
     Species of N-aryl carbamyl compounds that are suitable for use in this invention include the following: 
     N-(5-nitronaphth-1-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(1-nitronaphth-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(6-nitronaphth-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(8-nitronaphth-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(4&#39;-nitrobiphenyl-4-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(7-nitrofluoren-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(7-nitrophenanthren-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(7-nitro-9,10-dihydrophenanthren-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(4&#39;-nitrostilben-2-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(6-nitrodibenzofuran-3-yl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-ethyl-N-(4-nitrophenyl)-N&#39;-(R/S)alpha-methylbenzylurea, 
     N-(5-nitronaphth-1-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(1-nitronaphth-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(6-nitronaphth-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(8-nitronaphth-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(4&#39;-nitrobiphenyl-4-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(7-nitrofluoren-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(7-nitrophenanthren-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(7-nitro-9,10-dihydrophenanthren-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(4&#39;-nitrostilben-2-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(6-nitrodibenzofuran-3-yl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-ethyl-N-(4-nitrophenyl)-N&#39;-(R/S)-1-(alpha-naphthyl)ethylurea, 
     N-(5-nitronaphth-1-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(1-nitronaphth-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(6-nitronaphth-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(8-nitronaphth-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(4&#39;-nitrobiphenyl-4-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(7-nitrofluoren-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(7-nitrophenanthren-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(7-nitro-9,10-dihydrophenanthren-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(4&#39;-nitrostilben-2-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(6-nitrodibenzofuran-3-yl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-ethyl-N-(4-nitrophenyl)-(D/L)-1-(methoxycarbonyl)ethylurethane, 
     N-(5-nitronaphth-1-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(1-nitronaphth-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(6-nitronaphth-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(8-nitronaphth-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(4&#39;-nitrobiphenyl-4-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(7-nitrofluoren-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(7-nitrophenanthren-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(7-nitro-9,10-dihydrophenanthren-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(4&#39;-nitrostilben-2-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-(6-nitrodibenzofuran-3-yl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     N-ethyl-N-(4-nitrophenyl)-(D/L)-1-(ethoxycarbonyl)ethylurethane, 
     In the foregoing compounds, the term (R/S) means either (R) or (S) enantiomer, but does not mean a mixture of (R) and (S) enantiomers; the term (D/L) means either (D) or (L) enantiomer, but does not mean a mixture of (D) and (L) enantiomers. 
     Compounds of this invention can be conveniently prepared by well-known methods, such as by the reaction of a suitable aryl isocyanate with the appropriate chiral amine or alcohol to provide the resultant urea or urethane, respectively. 
     Chiral amines or alcohols that are preferred for the preparation of the ureas and urethanes of this invention are those compounds wherein the chiral group R 5  has a molecular weight range from about 50 to about 300 with a preferable range from about 70 to about that of the weight of R 1 . It is more preferred that the chiral group have a molecular weight ranging from about 70% to 90% of that of the weight of R 1 . Chiral groups having molecular weights above about 300 are generally less useful in the practice of this invention, as they dilute the active part of the molecule. Preferred classes of chiral groups are those for which at least one asymmetric carbon atom is adjacent to the carbon atom attached to the linking group X of R 5  (i.e. --NR 4  or --O--), and the most preferred class is that for which the carbon atom attached to the linking group X of R 5  is itself asymmetric. 
     The urea and urethane compounds of this invention are substantially transparent to electromagnetic radiation having wavelengths from about 0.5 to about 1.5 micrometers. Accordingly, they are useful in second harmonic generators wherein both incident radiation and emergent radiation range from about 0.5 micrometer to about 1.5 micrometers; many of the carbamyl compounds are useful to as low as about 0.4 micrometer, or even somewhat lower. 
     Generally, the urethanes are preferred in the situation where the SHG signal is to be near 0.4 micrometer, and especially if the signal is lower than that wavelength, because, other factors being equal, their region of transparency extends slightly further into the ultraviolet region than does that of the ureas. Conversely, the ureas are preferred in other situations because their melting points are generally higher than those of urethanes, which will permit them to be used under somewhat more severe conditions of temperature and, thus, of laser power level. 
     Devices that are capable of generating coherent second harmonic light radiation with the chiral N-aryl carbamyl compounds described herein are well known in the art. Representative examples of such devices are described in U.S. Pat. Nos. 3,395,329, 3,431,484, and 3,858,124, all of which are incorporated herein by reference for the purpose of describing devices which can incorporate the chiral N-aryl carbamyl compounds described herein and exhibit efficient second harmonic generation by means of such incorporation. 
     Crystals of the chiral N-aryl carbamyl compounds exemplified herein were evaluated for SHG efficiency using the second harmonic generation (SHG) powder test described in Kurtz et al., J. Appl. Phys. 39, 3798 (1968). Each sample was crushed (not ground) and sieved. This step was often time-consuming due to the property of piezoelectricity (shown by all non-centrosymmetric crystals) which acts to cause crystals to cling together and not pass through the sieve; often it was necessary to repeatedly anneal the crushed crystals during the sieving operation. The sample was then mixed with a fluid, i.e., a liquid, to minimize scattering, refraction, or phase-incoherence caused by differences in the index of refraction between the particles and the ambient atmosphere. The index-matched sample was placed between cell flats spaced 0.35±0.02 mm apart. Particles having mean diameters such as to pass through a 180 micrometer screen but be retained by a 75 micrometer screen were used. Each sample was mixed with a drop of index matching fluid (Cargille Scientific Co. n=1.63 or n=1.58 fluids or n=1.631 polyphenylether described in U.S. Pat. No. 3,034,700). The samples were not index-matched critically, so that the actual SHG efficiencies may be markedly higher than those reported in the examples. 
     Referring now to FIG. 1, infrared radiation at 1064 nm from a Q-switched Nd-YAG laser 10 was weakly focused onto the cell 12 containing the prepared sample. In the device illustrated in FIG. 1, the means, e.g. a lens, for directing the output radiation of the laser first through a filter 14 (Corning CS2-60 color filter used to block any radiation at 532 nm) and then onto the cell 12 containing the carbamyl compound sample was integrated into the laser 10 and is not shown as a separate component. Such means for directing the filtered output radiation of the laser onto the organic molecular crystalline compound sample are well-known to one of ordinary skill in the art. An infrared blocking filter 16 placed behind the sample allowed only the second harmonic frequency radiation to pass through a 1/3 meter monochromator 18 tuned at 532 nm. The output of the monochromator 18 was directed to a photomultiplier tube 20, and the resulting signal was processed by a boxcar averager 22 that averages over many laser pulses. Urea was the chosen standard because of its moderately high second order coefficient and its ready availability. It is important to recrystallize the urea to form transparent crystals of a size greater than about 200 micrometers in their smallest dimension, so that the crushed particles made from them will be single crystals. The urea standard was prepared in the same manner as the samples. The urea standard was index-matched reasonably well, with a mismatch of about 0.01. The reported efficiency of a sample is its SHG signal normalized to that of the urea standard measured under the same experimental conditions. 
     Compounds prepared according to the following examples are summarized in Tables I-XIII and were examined to determine their melting points (m.p.), their sixteen strongest X-ray signals for lattice spacings (d-spacings) derived from diffraction angles, together with their relative intensities, and their SHG efficiency relative to urea. The sieved SHG samples were X-ray analyzed to establish their powder diffraction patterns, in the exact crystalline state on which the SHG tests were made. Unlike the melting point elemental analysis, NMR, or mass spectra, these diffraction patterns are highly characteristic not only for a particular compound but also for its particular crystal form. It should be understood that relative line intensities may vary systematically due to varying degrees of sample orientation on the flat plastic slide which carries the sample into the Automatic Powder Diffractometer, Model No. APD 3600, Philips Electronics Instruments, Inc., Mahwah, N.J.; however, the d-spacing will remain essentially unaffected. The solvent or solvent mixtures used for crystallization of the particular compound are also indicated. The particular materials and the amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. 
     EXAMPLE 1 
     N-(4-nitrophenyl)-N&#39;-(R-alpha-methyl-benzyl)urea (m.p. 179.5°-180.5° C.) was prepared by reacting freshly-purified commercial 4-nitrophenyl isocyanate with commercial R(+)-alpha-methyl-benzylamine in dried chlorobenzene. 
     Spectroscopic grade chlorobenzene (180 ml) was freshly boiled to remove water, and 40 ml of it was kept warm in a 50 ml Erlenmeyer flask. In a second 50 ml Erlenmeyer flask was placed 8.21 g (0.050 mole) of fresh commercial 4-nitrophenyl isocyanate (Eastman Chemical Co.). Because its hydrolysis product present as an impurity, bis-(4-nitrophenyl)urea, was virtually insoluble in chlorobenzene, a milky yellow solution was produced when 40 ml of this solvent was added. To remove the impurity, the isocyanate solution was filtered directly into a 250 ml Erlenmeyer flask containing 100 ml of still-warm chlorobenzene, into which had been placed 6.06 g (0.050 mole) of R(+)-alpha-methylbenzylamine (0.050 mole), (98+%, Aldrich Chem. Co., Milwaukee, WI). 
     A funnel equipped with a Whatman #54 9 cm filter paper, folded into a cone, was used for filtration. The paper had been dried, weighed, and freshly dried in an oven just prior to use. The funnel delivered directly into the reaction flask, which had been provided with a magnetic stirrer and had been placed on a hot plate. The unused 40 ml warm chlorobenzene was used to wash all isocyanate residues into the reaction mixture. The filtered and washed insoluble yellow impurity, i.e. the aforementioned hydrolysis product, weighed 0.26 g upon drying and corresponded to a 3% impurity level. The reaction mixture became very warm and later deposited crystals copiously; the reaction mixture was then heated to reflux, at which point all crystals redissolved and no precipitate of bis-(4-nitrophenyl)urea was visible. The magnetic stirrer was withdrawn and the solution allowed to cool to bring about crystallization; the resulting crystals had a melting point (m.p.) of 179.5°-180.5° C. This urea compound, in crystals of nominal size of 100 micrometers, i.e. sieved to between 75 and 180 micrometers, showed a SHG efficiency of about 30 times that of the standard urea powder sample. Upon recrystallization from tetrahydrofuran/heptane solvent mixture, the crystals showed a SHG efficiency of about 37 times that of the standard urea powder sample. When, contrary to the established powder SHG test method, certain larger unsieved plate-like crystals (see Example 1A in Table I) were tested in a similar manner, SHG values from 250 to 290 times that of the urea standard were recorded; however the enhancement was due at least in part to sample orientation effects. 
     The compound prepared in this example was shown to be useful as a nonlinear optical device by means of the following experiment. A thin crystal having a width of 2 mm and a length of 5 mm was placed in and normal to the beam path of a Nd-YAG laser operating in the infrared mode (wavelength of 1.064 micrometers). The light beam emerging from the crystal was aligned along the axis of the beam path of the incident radiation and was visible green light (wavelength of 0.532 micrometer). 
     The procedure of Example 1 was further utilized to prepare the ureas set forth and described in Tables I, II, III, and IV. Table I sets forth relative SHG efficiencies of ureas that differ primarily in the aryl substituent. Table II sets forth relative SHG efficiencies of 4-nitrophenyl urea compounds having various chiral groups. Table III sets forth the X-ray powder patterns for the ureas of Tables I and II. Table IV sets forth the crystallization solvents for the ureas of Tables I and II. 
     
                                           TABLE I__________________________________________________________________________Aryl group in            m.p..sup.2               Relative                       Powder                            PowerExamplechiral urea.sup.1            (°C.)               SHG efficiency.sup.3                       quality.sup.4                            (mW).sup.5__________________________________________________________________________1    4-NO.sub.2 C.sub.6 H.sub.4 --            180               37.     F. Good                            121A   4-NO.sub.2 C.sub.6 H.sub.4 --            -- --      --   --2    3-NO.sub.2 C.sub.6 H.sub.4 --            178               0.1     Fair 122A   3-NO.sub.2 C.sub.6 H.sub.4 --            -- --      --   --3    2-NO.sub.2 C.sub.6 H.sub.4 --            159               2.4     Good 114    2-Cl--C.sub.6 H.sub.4 --            163               0.1     Good 115    3-Cl--C.sub.6 H.sub.4 --            144               0.3     Good 106    4-Cl--C.sub.6 H.sub.4 --            190               0.01    Good 117    3,4-Cl.sub.2 C.sub.6 H.sub.3 --            172               0.04    Fair 108    4-C.sub.6 H.sub.5 --O--C.sub.6 H.sub.4 --            134               0.4     Good 129    4-CH.sub.3 O--C.sub.6 H.sub.4 --            180               0.02    Fair 1110   1-C.sub.10 H.sub.7 --            226               0.001   V. Poor                            1111   2-C.sub.10 H.sub.7 --            186               0.1     Fair 1112   3-CF.sub.3 C.sub.6 H.sub.4 --            125               0.002   F. Good                            1113   2-Cl--4-NO.sub.2 C.sub.6 H.sub.3 --            169               0.3     F. Good                            1214   2-CH.sub.3 O--4-NO.sub.2 C.sub.6 H.sub.3 --            184               0.006   F. Good                            1215   2-CH.sub.3 --4-NO.sub.2 C.sub.6 H.sub.3 --            169d               0.01    Good 1016   4-N.tbd.C--C.sub.6 H.sub.4 --            192               0.5     Good 1217   7-NO.sub.2 --2-fluorenyl            286d               0.005   Fair 10(7-NO.sub.2 --2-C.sub.11 H.sub.8 --)__________________________________________________________________________ .sup.1 The aryl group of the chiral urea is represented by R.sup.3 of formula II. In Examples 1 through 17, inclusive, the chiral group was derived from Ralpha-methylbenzylamine. .sup.2 d indicates that the compound melts with decomposition. .sup.3 The relative SHG efficiency is based on the SHG efficiency of urea which has been assigned a value of 1.0. .sup.4 Powder quality refers to the clarity, lack of polycrystallinity, and size distribution of the crystal fragments as judged by microscopic examination. As powder quality improves, the measured SHG efficiency improves. F represents &#34;fairly&#34;. V represents &#34;very&#34;. .sup.5 mW represents megawatts. 
    
     
                       TABLE II______________________________________                       Rela-                       tiveEx-  Chiral group of        SHGam-  4-nitroaryl     m.p..sup.2                       effi- Powder Powerple  urea compound.sup.1                (°C.)                       ciency.sup.3                             quality.sup.4                                    (mW).sup.5______________________________________18 ##STR1##       159d   1.07  Poor   8.5(derived from L-2amino-1-propanol; L-alaninol)19 ##STR2##       210d   0.8   Fair   8.5(derived from L-proline)20 ##STR3##       147d   0.3   F. Good                                    8.3(derived from1-ephedrine)21 ##STR4##       224d   0.003 Poor   8.5(derived from L-alanine)22 ##STR5##       150.sup.                       0.3   Fair   10(derived from L-alanineethyl ester)23   Salpha-methyl benzyl                177    0.01  Poor   28.5______________________________________ .sup.1 The chiral group of the 4nitroaryl urea compound is represented by R.sup.3 of formula II. In Examples 18 through 21, inclusive, the aryl group was 4NO.sub.2 C.sub.6 H.sub.4. In Example 22, the aryl group was 2CH.sub.3 O4-NO.sub.2 C.sub.6 H.sub.3. In Example 23, the aryl group was 4NO.sub.21-C.sub.10 H.sub.6. .sup.2 d indicates that the compound melts with decomposition. .sup.3 The relative SHG efficiency is based on the SHG efficiency of urea which has been assigned a value of 1.0. .sup.4 Powder quality refers to the clarity, lack of polycrystallinity, and size distribution of the crystal fragments as judged by microscopic examination. As powder quality improves, the measured SHG efficiency improves. F represents &#34;fairly&#34;. V represents &#34;very&#34;. .sup.5 mW represents megawatts. 
    
     
                                           TABLE III__________________________________________________________________________X-ray powder pattern.sup.1Example D   I  D   I  D   I  D   I   D   I  D   I  D   I  D   I__________________________________________________________________________1     22.53     100        11.31            3  7.53                   6  6.24                          5   5.65                                  7  4.73                                         12 4.26                                                14 3.82                                                       4 3.76     9  3.70            10 3.55                   10 3.301                          8   3.254                                  8  3.219                                         4  3.041                                                10 2.789                                                       31A    6.89     71 5.97            16 5.16                   59 4.20                          38  4.13                                  9  3.64                                         100                                            3.43                                                40 3.35                                                       7 3.211     13 2.852            40 2.721                   12 2.542                          5   2.423                                  11 --  -- --  -- --  --2     12.32     90 12.12            100               6.12                   54 6.06                          90  5.64                                  17 5.46                                         31 4.43                                                20 4.07                                                       14 4.03     24 3.80            24 3.75                   35 3.72                          14  3.403                                  14 3.376                                         17 3.351                                                13 3.027                                                       142A    12.15     83 6.07            100               5.63                   41 5.43                          64  4.42                                  45 4.37                                         52 4.23                                                28 4.04                                                       25 3.76     77 3.69            32 3.52                   31 3.364                          78  3.306                                  37 3.016                                         53 2.764                                                23 2.700                                                       183     8.76     35 6.85            16 5.85                   88 5.66                          100 5.23                                  27 4.36                                         30 4.30                                                30 4.22                                                       37 4.04     15 4.00            18 3.67                   26 3.60                          42  3.493                                  24 3.299                                         31 2.927                                                20 2.824                                                       174     12.51     7  8.65            58 6.26                   18 5.55                          10  5.42                                  100                                     4.34                                         32 4.17                                                9  3.94                                                       45 3.82     4  3.315            7  3.030                   18 2.924                          12  2.891                                  30 2.823                                         5  2.778                                                9  2.712                                                       85     13.08     57 6.51            33 5.50                   5  5.25                          1   4.51                                  1  4.32                                         100                                            4.24                                                4  3.59                                                       10 3.386     3  3.240            15 2.910                   3  2.764                          8   2.729                                  1  2.587                                         16 2.422                                                4  2.318                                                       86     10.71     13 7.00            22 6.55                   8  5.72                          40  5.35                                  9  4.86                                         100                                            4.77                                                17 4.36                                                       44 3.90     18 3.85            8  3.56                   12 3.126                          10  2.906                                  9  2.855                                         9  2.773                                                10 2.697                                                       87     13.21     31 6.62            33 5.80                   95 5.43                          46  4.93                                  64 4.42                                         88 3.74                                                38 3.66                                                       26 3.55     28 3.438            100               3.313                   19 2.955                          37  2.902                                  45 2.819                                         26 2.711                                                22 2.647                                                       198     7.04     41 5.60            18 5.04                   26 4.72                          62  4.66                                  42 4.59                                         100                                            4.47                                                46 4.42                                                       59 4.38     42 4.27            20 4.12                   35 4.06                          20  4.01                                  19 3.96                                         34 3.86                                                22 3.324                                                       188A    16.60     34 5.84            30 5.53                   7  5.26                          15  5.10                                  100                                     4.55                                         37 4.45                                                36 4.32                                                       25 4.14     15 3.95            15 3.90                   26 3.77                          19  3.62                                  40 3.443                                         50 3.115                                                10 2.771                                                       109     10.80     27 6.98            9  6.46                   5  5.80                          18  5.41                                  30 4.90                                         100                                            4.74                                                7  4.38                                                       44 3.91     8  3.86            3  3.61                   6  3.52                          7   3.149                                  7  2.922                                         9  2.839                                                5  2.706                                                       610    8.71     81 6.31            68 5.25                   54 4.72                          36  4.60                                  14 4.38                                         65 4.19                                                44 3.97                                                       100 3.71     57 3.63            25 3.51                   38 3.259                          40  3.160                                  23 3.038                                         18 2.922                                                12 2.648                                                       1411    12.47     13 9.04            12 7.06                   6  6.21                          92  5.71                                  7  5.31                                         100                                            4.53                                                24 4.36                                                       5 4.14     11 3.88            7  3.71                   9  3.52                          5   3.243                                  4  3.183                                         4  3.014                                                5  2.982                                                       412    8.78     14 7.66            13 6.22                   58 5.38                          38  5.06                                  100                                     4.60                                         81 4.39                                                38 4.19                                                       10 3.70     22 3.60            9  3.486                   18 3.061                          9   2.931                                  22 2.684                                         7  2.551                                                11 2.327                                                       613    10.61     9  9.12            17 8.59                   11 6.64                          7   4.59                                  19 4.52                                         18 4.14                                                14 3.92                                                       33 3.88     17 3.83            14 3.440                   19 3.317                          100 3.290                                  13 3.233                                         9  3.841                                                8  2.769                                                       814    6.93     94 6.19            56 5.35                   100                      5.27                          62  4.76                                  82 4.57                                         78 4.14                                                58 4.05                                                       46 4.02     42 3.96            64 3.76                   33 3.69                          34  3.63                                  47 3.55                                         39 3.51                                                54 3.467                                                       6615    9.08     95 6.78            63 6.30                   31 5.71                          21  5.35                                  100                                     5.18                                         24 4.61                                                65 4.54                                                       35 4.20     20 3.82            50 3.68                   35 3.485                          32  3.386                                  48 3.183                                         33 2.998                                                40 2.775                                                       3016    10.58     75 6.94            17 5.84                   18 5.69                          84  4.83                                  74 4.74                                         32 4.33                                                100                                                   4.08                                                       21 3.88     54 3.84            45 3.59                   15 3.55                          13  3.345                                  14 3.168                                         18 3.120                                                16 2.687                                                       1817    9.06     19 7.03            4  5.23                   67 5.18                          100 5.04                                  25 5.00                                         27 4.36                                                6  4.04                                                       5 3.96     7  3.418            4  3.305                   6  3.259                          6   3.053                                  6  2.605                                         7  2.575                                                5  2.512                                                       318    18.19     23 9.01            15 6.12                   17 4.78                          39  4.57                                  68 4.38                                         61 4.07                                                16 3.92                                                       14 3.73     100        3.67            70 3.441                   34 3.274                          46  3.228                                  39 3.113                                         70 2.947                                                13 2.699                                                       1419    7.08     9  6.11            14 5.67                   9  5.34                          83  4.62                                  16 3.61                                         44 3.50                                                57 3.43                                                       45 3.429     44 3.327            100               3.287                   67 3.203                          29  3.077                                  15 2.920                                         16 2.762                                                21 2.647                                                       920    17.55     70 8.89            50 6.56                   53 5.83                          94  5.50                                  100                                     5.35                                         22 4.46                                                30 4.02                                                       39 3.93     36 3.81            99 3.68                   51 3.232                          71  3.139                                  64 3.037                                         33 2.982                                                19 2.922                                                       4321    6.28     12 5.99            27 5.59                   21 5.46                          35  5.32                                  41 4.46                                         13 4.38                                                42 4.34                                                       75 3.55     42 3.51            49 3.260                   100                      3.141                          25  3.102                                  14 2.775                                         14 2.748                                                11 2.717                                                       1222    11.50     31 7.44            64 6.74                   17 6.01                          15  4.57                                  75 4.21                                         14 4.15                                                14 3.78                                                       100 3.73     39 3.63            17 3.469                   19 3.427                          13  3.354                                  20 3.293                                         42 2.892                                                17 2.844                                                       1523    9.33     14 7.10            26 6.36                   100                      4.66                          16  4.30                                  19 4.07                                         27 3.80                                                74 3.64                                                       82 3.51     13 3.414            33 3.320                   95 3.257                          50  3.201                                  25 3.169                                         20 3.113                                                13 2.862                                                       12__________________________________________________________________________ .sup.1 D represents the lattice spacing in Angstrom units. I represents intensity in percentages relative to the strongest signal. 
    
     
                       TABLE IV______________________________________Example  Solvent of crystallization______________________________________1      Tetrahydrofuran/heptane1A     n-Propanol2      Methyl t-butyl ether/cyclohexane (1/10)2A     Crystallized from reaction in chlorobenzene3      Chlorobenzene4      Chlorobenzene5      Chlorobenzene6      Crystallized from reaction in chlorobenzene7      Aqueous methanol8      Selective partial evaporation of ethanol/water (2/1)8A     Toluene/heptane9      Crystallized from reaction in chlorobenzene10     Precipitated from reaction in chlorobenzene11     Precipitated from reaction in chlorobenzene12     Cold chlorobenzene13     Crystallized from reaction in toluene14     Ethanol/water (7/9)15     Ethanol/water (1/1)16     Ethanol/water (2/3)17     Nitrobenzene18     Evaporation of acetone solution19     Crystallized from reaction in toluene20     Crystallized from reaction in toluene21     Crystallized from reaction in toluene22     Evaporation of ethanol solution23     Crystallized from reaction in chlorobenzene______________________________________ 
    
     EXAMPLE 24 
     4-Nitrophenyl isocyanate was freshly purified by filtration of a toluene solution followed by boiling off most of the toluene and evaporating of the remainder upon crystallization. It had a melting point of 55°-57° C. The 4-nitrophenyl isocyanate (1.674 g, 0.01 mole) was dissolved in ca. 15 ml toluene. To this solution was added 1.30 g (0.011 mole) ethyl L-lactate and 1 drop dry pyridine. A slight precipitate formed. After 24 hours, the solvent was filtered off, and the filtrate was allowed to evaporate to an oil, which, on standing, crystallized after one week. Because excess ethyl lactate had been used, the product was washed with three 40 ml portions of water and allowed to dry overnight. The crystalline residue, 2.49 g (88% yield), had a melting point of 93°-95° C. and was extremely pale yellow. The SHG efficiency was very high, about 40 times that of the standard urea sample. 
     The procedure of Example 24 was further utilized to prepare the urethanes set forth and described in Tables V, VI, VII, VIII, IX, XI, XII, and XIII. Table V sets forth relative SHG efficiency of 4-nitrophenyl urethane compounds that differ in the chiral group. Table VI sets forth the X-ray powder pattern for the urethane compounds of Table V. Table VII sets forth the crystallization solvents for the urethane compounds of Table V. Table VIII sets forth relative SHG efficiency of urethane compounds that differ in the aryl substitutent. Table IX sets forth the X-ray powder pattern for the urethane compounds of Table VIII. Table X sets forth the crystallization solvents for the urethane compounds of Table VIII. Table XI sets forth relative SHG efficiency of 3-nitrophenyl urethane compounds that differ in the chiral group. Table XII sets forth the X-ray powder pattern for the urethane compounds of Table XI. Table XIII sets forth the crystallization solvents for the urethane compounds of Table XI. 
     
                                           TABLE V__________________________________________________________________________                     RelativeChiral group of 4-nitrophenyl                     SHGExampleurethane compound.sup.1               m.p. (°C.)                     efficiency.sup.2                           Powder quality.sup.3                                   Power (mW).sup.4__________________________________________________________________________24 ##STR6##      95-97 30    Good    10(From S(-) ethyl lactate)25   Nopyl           76   0.001 Fair    12(From 1-nopol)26 ##STR7##      103   0.1   F. Good 1327 ##STR8##      123   30    F. Good 12(From S(-) methyl lactate)28 ##STR9##       52   20    Poor    12(From D(-)-2-methyl-butanol)29   Menthyl        116   0.03  Good    10(From 1-menthol)30 ##STR10##      51   0.01  Good    1131 ##STR11##      57   0.007 Good     832 ##STR12##     116   0.02  Fair    10(from 1-2-octanol)33 ##STR13##      48   6     Fair    11(from S butyl lactate)34 ##STR14##     113   3.5   Fair    1035 ##STR15##     173   0.008         10__________________________________________________________________________ .sup.1 The chiral group is represented by R.sup.3 of formula II. The asymmetric carbon atom is represented by C*. .sup.2 The relative SHG efficiency is based on the SHG efficiency of urea which has been assigned a value of 1.0. .sup.3 Powder quality refers to the clarity, lack of polycrystallinity, and size distribution of the crystal fragments as judged by microscopic examination. As powder quality improves, the measured SHG efficiency improves. F represents &#34;fairly&#34;. V represents &#34;very&#34;. .sup.4 mW represents megawatts. 
    
     
                                           TABLE VI__________________________________________________________________________X-ray powder pattern.sup.1ExampleD   I  D   I  D   I D   I D   I D   I  D   I D   I__________________________________________________________________________24   9.82    47 5.71           42 5.58                  77                    5.45                        47                          4.88                              70                                4.77                                    27 4.24                                           98                                             4.03                                                 1003.80    80 3.70           26 3.66                  55                    3.473                        40                          3.295                              83                                3.169                                    44 3.133                                           61                                             3.056                                                 2025   21.61    100       6.47           11 5.88                  23                    5.48                        57                          5.03                              52                                4.91                                    28 4.78                                           42                                             4.56                                                 424.34    50 4.13           31 3.99                  20                    3.69                        21                          3.372                              16                                3.319                                    23 3.206                                           28                                             3.043                                                 3126   11.14    11 6.51           18 6.38                  10                    5.66                        23                          5.06                              11                                4.90                                    24 4.51                                           20                                             4.40                                                 404.35    78 4.20           48 4.00                  45                    3.92                        28                          3.90                              28                                3.65                                    99 3.340                                           27                                             3.293                                                 10027   11.45    9  7.37           4  6.81                  14                    5.68                        4 4.68                              13                                4.50                                    100                                       4.31                                           4 4.18                                                 73.84    24 3.65           9  3.56                  7 3.232                        31                          2.957                              7 2.661                                    4  2.496                                           5 2.343                                                 427A  11.57    39 6.85           19 5.78                  7 5.07                        7 4.71                              22                                4.51                                    100                                       4.20                                           7 3.85                                                 283.66    20 3.57           14 3.237                  43                    2.969                        7 2.906                              8 2.346                                    10 2.319                                           14                                             2.301                                                 928   20.21    100       10.15           12 5.76                  6 5.10                        60                          4.70                              6 4.17                                    9  4.03                                           6 3.78                                                 93.65    10 3.50           36 3.438                  19                    3.377                        15                          3.218                              21                                3.052                                    6  2.907                                           7 2.539                                                 629   17.63    100       8.62           70 8.14                  10                    5.88                        16                          5.16                              34                                4.84                                    20 4.66                                           10                                             4.56                                                 124.18    13 4.10           25 3.82                  10                    3.373                        20                          3.346                              40                                3.106                                    12 2.929                                           13                                             2.854                                                 2230   14.92    100       7.43           10 5.61                  6 5.30                        22                          4.93                              60                                4.50                                    15 4.19                                           37                                             4.10                                                 73.86    52 3.70           20 3.63                  37                    3.55                        46                          3.434                              27                                3.355                                    12 3.281                                           15                                             3.147                                                 631   8.33    10 7.03           13 6.12                  66                    5.08                        22                          4.80                              6 4.67                                    35 4.17                                           17                                             3.94                                                 1003.81    25 3.76           7  3.62                  14                    3.57                        15                          3.443                              31                                3.378                                    19 3.148                                           8 2.680                                                 2032   11.32    42 6.51           3  6.26                  5 4.65                        33                          4.45                              6 4.11                                    100                                       3.96                                           36                                             3.78                                                 363.60    3  3.54           3  3.497                  4 3.228                        12                          3.196                              10                                3.129                                    9  2.929                                           6 2.662                                                 333   14.30    77 5.91           35 5.68                  19                    5.25                        18                          4.83                              19                                4.57                                    100                                       4.36                                           44                                             4.24                                                 304.11    27 4.05           15 3.90                  34                    3.57                        24                          3.383                              26                                3.320                                    17 3.141                                           24                                             3.076                                                 2134   12.61    55 6.35           18 6.06                  14                    5.35                        20                          4.84                              15                                4.76                                    23 4.70                                           11                                             4.44                                                 154.23    26 4.15           100              3.90                  42                    3.83                        35                          3.64                              17                                3.180                                    80 3.148                                           69                                             3.110                                                 3035   4.58    12 4.34           100              4.28                  30                    4.18                        17                          3.87                              19                                3.81                                    87 3.76                                           14                                             3.57                                                 173.372    28 3.386           41 3.244                  16                    3.185                        45                          3.037                              17                                2.378                                    10 2.295                                           37                                             1.930                                                 11__________________________________________________________________________ .sup.1 D represents the lattice spacing in Angstrom units. I represents intensity in percentages relative to the strongest signal. 
    
     
                       TABLE VII______________________________________Example   Solvent of crystallization______________________________________24        Evaporation of ethanol solution25        Evaporation of solution in CH.sub.2 Cl.sub.2 /isooctane,     then washed with CF.sub.2 ClCFCl.sub.226        Toluene27        Partial (selective) evaporation of methyl     t-butyl ether/isooctane solution28        Cold CF.sub.2 ClCFCl.sub.229        Neat, then washed with CF.sub.2 ClCFCl.sub.230        Neat, then washed with CF.sub.2 ClCFCl.sub.231        Neat, then washed with CF.sub.2 ClCFCl.sub.232        Cold CF.sub.2 ClCFCl.sub.2 /C.sub.7 F.sub.16 (4/1)33        Partial (selective) evaporation of methyl     t-butyl ether/isooctane solution34        Ethanol35        Methanol/acetic acid (4/1)______________________________________ 
    
     
                       TABLE VIII______________________________________                        Rela-                        tive   Pow-Ex-                          SHG    deram-  Aryl group in    m.p.   effi-  qual-                                    Powerple  chiral urethane.sup.1                 (°C.)                        ciency.sup.2                               ity.sup.3                                    (mW).sup.4______________________________________36   2-Cl4-NO.sub.2C.sub.6 H.sub.3                 53     0.5    Fair 1037   2-CH.sub.3 4-NO.sub.2C.sub.6 H.sub.3                 71     0.001  Poor 1038   4-NCC.sub.6 H.sub.4                 53     0.2    Poor 10______________________________________ ##STR16## .sup.2 The relative SHG efficiency is based on the SHG efficiency of urea which has been assigned a value of 1.0. .sup.3 Powder quality refers to the clarity, lack of polycrystallinity, and size distribution of the crystal fragments as judged by microscopic examination. As powder quality improves, the measured SHG efficiency improves. F represents &#34;fairly&#34;. V represents &#34;very&#34;. .sup.4 mW represents megawatts. 
    
     
                                           TABLE IX__________________________________________________________________________X-ray powder pattern.sup.1ExampleD  I  D  I  D  I D   I  D   I D   I D   I  D   I__________________________________________________________________________36   14.62   100      7.38         24 5.24               7 5.18                     8  4.23                            12                              3.98                                  26                                    3.93                                        14 3.84                                               103.76   13 3.61         17 3.56               8 3.501                     7  3.378                            14                              3.319                                  11                                    3.223                                        11 3.188                                               1237   14.20   45 7.35         100            7.25               81                 7.13                     56 5.99                            39                              4.76                                  48                                    4.62                                        45 4.51                                               314.39   31 4.05         50 3.78               38                 3.400                     100                        3.348                            31                              3.291                                  45                                    3.257                                        43 3.081                                               4038   11.36   16 9.81         22 5.95               22                 5.82                     25 5.66                            18                              5.26                                  17                                    4.89                                        100                                           4.64                                               184.47   69 4.30         40 3.97               25                 3.85                     24 3.71                            55                              3.56                                  66                                    3.267                                        27 2.715                                               15__________________________________________________________________________ .sup.1 D represents the lattice spacing in Angstrom units. I represents intensity in percentages relative to the strongest signal. 
    
     
                       TABLE X______________________________________Example         Solvent of crystallization______________________________________36              Cold CF.sub.2 ClCFCl.sub.237              Cold CF.sub.2 ClCFCl.sub.238              Neat (Very slow)______________________________________ 
    
     
                       TABLE XI______________________________________Chiral group ofEx-  3-nitrophenyl        Relativeam-  urethane      m.p.   SHG     Powder Powerple  compound.sup.1              (°C.)                     efficiency.sup.2                             quality.sup.3                                    (mW).sup.4______________________________________39 ##STR17##    65     0.040.01                             GoodFair                                    121340 ##STR18##    44     0.5     Poor   1041   menthyl       83     0.03    Fair   13(l-menthol)______________________________________ .sup.1 The chiral group is represented by R.sup.3 of formula III. The asymmetric carbon atom is represented by C*. .sup.2 The relative SHG efficiency is based on the SHG efficiency of urea which has been assigned a value of 1.0. .sup.3 Powder quality refers to the clarity, lack of polycrystallinity, and size distribution of the crystal fragments as judged by microscopic examination. As powder quality improves, the measured SHG efficiency improves.  F represents &#34;fairly&#34;. V represents &#34;very&#34;. .sup.4 mW represents megawatts. 
    
     
                                           TABLE XII__________________________________________________________________________X-ray powder pattern.sup.1ExampleD  I  D  I D  I D  I D   I D   I  D   I D   I__________________________________________________________________________39   11.47   33 11.27         37           5.42              65                5.37                   42                     4.63                         25                           4.53                               23 4.29                                      33                                        3.89                                            303.84   100      3.73         35           3.66              28                3.62                   31                     3.58                         40                           3.56                               66 3.54                                      66                                        2.922                                            1840   15.10   80 5.56         40           5.45              35                5.22                   37                     4.99                         20                           4.86                               100                                  4.47                                      35                                        4.10                                            703.79   80 3.72         40           3.63              65                3.50                   65                     3.394                         35                           3.349                               20 3.217                                      25                                        3.169                                            3541   20.34   30 17.75         28           14.33              50                8.52                   40                     8.00                         43                           5.83                               44 5.79                                      37                                        4.74                                            1004.68   72 4.61         58           4.52              90                4.30                   36                     4.25                         36                           4.23                               30 4.06                                      39                                        3.85                                            37__________________________________________________________________________ .sup.1 D represents lattice spacing in Angstrom units. I represents intensity in percentage relative to the strongest signal. 
    
     
                       TABLE XIII______________________________________Example    Solvent of crystallization______________________________________39         Evaporation of solution in CF.sub.2 ClCFCl.sub.2 /      isooctane/methyl t-butyl ether40         Evaporation of solution in CH.sub.2 Cl.sub.2 /methyl      tertbutyl ether41         Evaporation of solution in cyclohexane,      washing with CF.sub.2 ClCFCl.sub.2______________________________________ 
    
     Compounds of this invention may employ chiral alpha-amino acid moieties or alpha-amino acid ester moieties, as shown in Examples 19, 21 and 22. However, these materials are regarded as least desirable because, unlike the other ureas, they are capable of ready racemization. It is well known that chiral amino acids and especially their N-acyl derivatives undergo racemization at higher tempeatures or in the presence of weak acids and bases. See, for example, P. A. Levene, J. Biol. Chem. 76 299 (1928); M. Bergmann and L. Zervas, Biochem. Zeitschrift 203 280 (1928); V. DuVigneaud, J. Biol. Chem. 96 511 (1932) and 98 295 (1932); and I. Sakieki, Nippon Kagaku Zasshi, 80 1035 (1959). 
     Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.