Second harmonic Generation with achiral, Straight-chain carbamic acid derivatives

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, an acentrically crystalline, achiral, straight-chain N-nitrophenyl carbamyl compound, means for directing the output radiation of the laser onto the acentrically crystalline, achiral, straight-chain N-nitrophenyl carbamyl compound, and output means for utilizing the second harmonic frequency. N-nitrophenyl carbamyl compounds, and acentric crystals thereof that are capable of being used in the aforementioned devices and method, are also described herein.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

This invention is concerned with materials for nonlinear optical devices for conversion of optical energy at one frequency to optical energy at another frequency.

2. Discussion of the Prior Art

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; 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 not among those conveniently obtainable is required. Nonlinear optical crystals have, therefore, frequently been employed to convert coherent laser light of a fundamental frequency into coherent light of its second harmonic, that is to say, coherent light with a frequency twice the fundamental frequency. This conversion is termed "second harmonic generation" (SHG).

Use of organic molecules in nonlinear optical devices has generated much interest recently because many 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 electron donor and acceptor groups bonded at opposite ends of the conjugated electronic system of the molecule. 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, 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 one crystal form 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 doubled, frequency. 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 non-centrosymmetric 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 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. Pat. Nos. relating to non-linear optical properties of organic materials include U.S. Pat. Nos. 4,807,968; 4,808,332; 4,810,338; 4,818,616; 4,818,802; 4,818,898; 4,818,899; 4,824,219; 4,826,950; 4,822,865; 4,828,758; 4,835,235; 4,839,536; 4,851,270; 4,855,078; 4,855,376; 4,861,129; 4,865,430; 4,867,538; 4,867,540; and 4,868,250.

SUMMARY OF THE INVENTION

This invention provides devices for and method of generating coherent second harmonic light radiation. The devices comprise, in combination, a laser source of coherent light radiation at a fixed fundamental frequency, a crystalline, achiral, straight-chain N-nitrophenyl carbamyl compound that is crystallized in a non-centrosymmetric configuration, means for directing the output radiation of the laser onto the crystalline, achiral, straight-chain N-nitrophenyl carbamyl compound, and output means for utilizing the second harmonic frequency. The non-centrosymmetric, crystalline, achiral, straight-chain N-nitrophenyl carbamyl compound is a derivative of an N-nitrophenyl carbamic acid such that the straight chain is linked to the carbamyl carbon atom by an oxygen atom or nitrogen atom.

This invention also provides achiral, straight-chain, acentrically crystalline, i.e., crystallized in a non-centrosymmetric configuration, N-nitrophenyl carbamyl compounds, that are useful in the aforementioned devices and methods.

This invention further provides crystalline, achiral, straight-chain N-nitrophenyl carbamyl compounds capable of crystallizing in a non-centrosymmetric configuration that are novel per se.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a diagrammatic representation of a suitable arrangement for demonstrating the second harmonic generating properties of carbamic acid derivatives of this invention.

DETAILED DESCRIPTION

Carbamic acid derivatives suitable for devices and method of this invention are characterized as N-nitrophenyl carbamyl compounds derived from straight-chain alcohols or amines, and which may be crystallized in at least one non-centrosymmetric, i.e., acentric, crystalline form. Certain of these N-nitrophenyl carbamyl compounds are novel per se. Crystalline N-nitrophenyl carbamyl compounds suitable for this invention are molecularly achiral species that have no center of symmetry on the crystalline unit cell level. As used herein, the term "straight-chain" means that the molecule has a polymethylene chain containing two or more catenated carbon atoms. The term "non-centrosymmetric" is synonymous with the term "acentric".

N-nitrophenyl carbamyl compounds that have been found to be useful for this invention can be defined by the following two general formulas which depict two major subclasses, namely the ureas (I) and the urethanes (II):

R.sup.1 HNCONHR.sup.2 (I) R.sup.1 HNCO.sub.2 R.sup.2 (II) where R.sup.1 represents a nitrophenyl group or a substituted nitrophenyl group, R.sup.2 represents a polymethylene group, preferably having 2 to 22 catenated carbon atoms, more preferably 4 to 12 catenated carbon atoms, terminated by R.sup.3, where R.sup.3 represents a member selected from the group consisting of hydrogen, halogen, unsubstituted ethinyl group, substituted ethinyl group, unsubstituted vinyl group, substituted vinyl group, hydroxy group, alkoxy group, aryloxy group, acyloxy group, alkanesulfonyloxy group, arenesulfonyloxy group, alkanesulfonyl group, arenesulfonyl group, aryloxysulfonyl group, alkylthio group, arylthio group, cyano group, unsubstituted carbonyl, substituted carbonyl group, unsubstituted aryl group, substituted aryl group, unsubstituted amino group, substituted amino group, acyl amino group, and heterocyclic group. The aforementioned terminal group R.sup.3 can contain up to 15 carbon atoms, preferably up to 10 carbon atoms. The nature of the substituents for R.sup.1, R.sup.2, and R.sup.3 is not critical so long as they do not alter the achiral, acentrically crystalline nature of the N-nitrophenyl carbamyl compound. With the exception of those urethane compounds described herein for which R.sup.3 is hydrogen, all of these compounds are believed to be novel and are considered a part of this invention. Typically, these novel compounds exhibit more favorable crystallization tendencies and yield less fibrous crystals, i.e., crystals of lower aspect ratio, than those for which R.sup.3 is hydrogen; such properties are considered to be advantageous when utilizing these compounds in a second harmonic generator.

Representative examples of N-nitrophenyl carbamyl compounds suitable for use in this invention include those where R.sup.1 represents a p-nitrophenyl group, a m-nitrophenyl group, or a substituted nitrophenyl group. Representative examples of N-nitrophenyl carbamyl compounds suitable for use in this invention include those where R.sup.2 represents a straight-chain alkyl group such as ethyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-octadecyl, or n-docosyl. Further representative examples include those urethane compounds for which R.sup.1 represents a nitrophenyl or substituted nitrophenyl group, preferably a p-nitrophenyl group, and R.sup.2 represents a terminally substituted straight-chain alkyl group such as 5-phenylpentyl, 10-undecenyl, 6-fluorohexyl, 6-chlorohexyl, 11-bromoundecyl, 11-iodoundecyl, oleyl (cis-9-octadecenyl), elaidyl (trans-9-octadecenyl), 11-fluoroundecyl, 12-bromododecyl, 11-(phenylethinyl)undecyl, 9-[(phenylethinyl)ethinyl]nonyl, 6-(butylethinyl)hexyl, 8-hydroxyoctyl, 16-bromohexadecyl, and erucyl (cis-13-docosenyl).

Compounds suitable for this invention can be conveniently prepared by well-known methods, such as by reaction of a suitable nitrophenyl isocyanate with an appropriate straight-chain amine or alcohol to provide the resultant urea or urethane, respectively.

Straight-chain amines or alcohols that are preferred for preparation of ureas and urethanes suitable for this invention are those compounds wherein the group R.sup.2 has a molecular weight ranging from about 25 to about 300.

Urea and urethane compounds suitable for this invention are substantially transparent to electromagnetic radiation having wavelengths from about 0.5 to about 2 micrometers. Accordingly, urea and urethane compounds of this invention are useful in second harmonic generators wherein both incident radiation and emergent radiation range from about 0.5 micrometer to about 2 micrometers; many of the carbamyl compounds can be used in devices and processes where one wavelength is as low as about 0.4 micrometer, or even somewhat lower.

Generally, urethanes are preferred in a situation where the SHG signal is to be near 0.4 micrometer, and are particularly preferred if the SHG 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, ureas are preferred in other situations because their melting points are generally higher than those of urethanes, thereby allowing ureas to be used at higher temperatures and, consequently, at higher levels of laser power.

Devices that are capable of generating coherent second harmonic light radiation with N-nitrophenyl 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; 3,858,124; 4,756,598; and 4,818,899; all of which are incorporated herein by reference for the purpose of describing devices that can incorporate the acentrically crystalline N-nitrophenyl carbamyl compounds described herein and exhibit efficient second harmonic generation by means of such incorporation.

Crystals of N-nitrophenyl carbamyl compounds exemplified herein were evaluated for SHG efficiency using the second harmonic generation (SHG) powder test described in Kurtz and Perry, J. Appl. Phys. 39, 3798 (1968). Each sample was crushed (not ground) and sieved. The sample was then mixed with an "index-matching 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, (i.e., windows) spaced 0.35.+-.0.02 mm apart. Particles having mean diameters sufficiently small so as to pass through a 180 micrometer screen but sufficiently large so as to be retained by a 75 micrometer screen were used. Each sample was mixed with a drop of "index-matching" fluid (Cargille Scientific Co., Cedar Grove, N.J., n=1.63 or n=1.58 fluids, or n=1.631 polyphenylether described in U.S. Pat. No. 3,034,700). Samples were not index-matched critically, so that the actual SHG efficiencies may be higher than those reported in the examples.

Referring now to the drawing, infrared radiation at 1064 nm from a Q-switched Nd-YAG laser 10 was weakly focused onto a cell 12 containing the prepared sample. In the device illustrated in the drawing, the means, e.g., a lens, for directing 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 cell 12 containing a sample of N-nitrophenyl carbamyl compound was integrated into laser 10 and is not shown as a separate component. Means for directing the filtered output radiation of the laser onto the sample of N-nitrophenyl carbamyl compound are well-known to one of ordinary skill in the art. An infrared blocking filter 16, placed behind the sample, allowed only second harmonic frequency radiation to pass through a 1/3 meter monochromator 18 tuned at 532 nm. Output of 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 high second order coefficient, its ready availability, and its wide acceptance in the literature. The urea standard was prepared in the same manner as the samples. 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 rather than polycrystalline masses. 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 divided by that of the urea standard measured under the same experimental conditions.

Either or both of two tests can be used to determine non-centrosymmetry or acentricity of crystalline compounds suitable for use in this invention: (1) a pyroelectric test and (2) a piezoelectric test. Alternatively, an absolute method, such as single-crystal x-ray structure determination can be used. In some cases, it is possible to recognize an acentric crystal morphology by classical goniometric microscopic techniques. These latter two methods, where practical, are preferred.

In the pyroelectric test, when an acentric crystal is heated or cooled, it develops electric charges and becomes positive at one end and negative at the other end of its polar axis. Therefore, if a crystal is found to develop electric charges when heated or cooled, it must be concluded that it is non-centrosymmetric.

In the piezoelectric test, when an acentric crystal is compressed or extended in particular directions, it develops electric charges. Conversely, when a potential difference is applied to such a crystal, it expands or contracts. Thus, in an oscillating electric field, the crystal will produce mechanical oscillations which may be detected by suitable transducers. Again, if a crystal is found to thus produce mechanical oscillations, it must be concluded that it is non-centrosymmetric.

Both pyroelectric and piezoelectric tests are well known in the art and are described, for example, in Bunn, C. W., Chemical Crystallography, Second ed., Clarendon, Oxford, England, 1961, pp. 321-322, and references therein. "Use of the Pyroelectric Effect To Determine the Absolute Orientation of the Polar Axis in Molecular Crystals," Patil, A. A., Curtin, D. A., and Paul, I. C., J. Am. Chem. Soc. 1985, 107, 726-727, and references therein, also describes a pyroelectric test.

For examples of this invention a modification of the method described by Bunn was employed. This modified method was suitable for detecting moderate to strong pyroelectric behavior. Small crystals of the test material, preferably ca. 0.1 mm in size, are placed in the bowl of a polished stainless steel teaspoon and any charge is removed by means of a 4 millicurie polonium radioactive (alpha particle) source (3M 210 Static Eliminator, 4 inch length, Minnesota Mining and Manufacturing Company, St. Paul, Minn.) held at 1 cm distance for 10 seconds. The bottom of the spoon's bowl is contacted with liquid nitrogen for 40 seconds, thereby creating an electric dipole within each crystal of a non-centrosymmetric material, which polarity is detected by quickly inverting the spoon (in air) and noting adherence of the crystals to the spoon.

Compounds prepared according to the following examples are listed in Tables I-VI and were examined to determine their melting points (m.p.), their sixteen strongest powder x-ray signals for lattice spacings (d-spacings) derived from diffraction angles together with their relative intensities, their pyroelectric test results, and their second harmonic generation efficiencies relative to that of urea. Solvents or mixtures of solvents used for crystallization of the particular compound are also indicated in the aforementioned tables.

The sieved samples were analyzed by x-ray diffraction to establish their powder diffraction patterns, in the exact crystalline state in which the SHG tests were conducted. 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 instrumental resolution or to varying degrees of sample orientation (i.e., non-randomness) on the flat plastic slide which carries the sample into the diffraction apparatus (Automatic Powder Diffractometer, Model No. APD 3600, Philips Electronics Instruments, Inc., Mahwah, N.J.); however, the d-spacings will remain essentially unaffected.

It is necessary to realize that relative line intensity changes may somewhat alter the choice of the sixteen strongest lines, but such changes are extremely unlikely to prevent recognition of identity between two different samples of a compound having the same crystal form.

A more serious challenge to such recognition of sample identity exists as d-spacings become longer, especially above 10 Angstroms, since very small systematic errors in diffraction angle become translated into increasingly larger errors in d-spacing. It is, therefore, especially important to recognize the second and higher orders of diffraction, which result in d-spacings that are precise integral fractions of the longest ones. Inasmuch as these shorter d-spacings may be measured accurately by comparison with those from accepted standard materials, it becomes possible not merely to detect the aforesaid errors, but even to make corrections for them.

With appropriate attention being given to making any such indicated corrections, it is a straightforward matter to compare data gathered on separate and dissimilar x-ray diffractometers, and thus to recognize materials falling within the scope of this invention.

This invention is further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.

The following abbreviations were used in the tables following each descriptive example:

______________________________________MeOH methanolEtOH ethanolPrOH n-propanolBuOH n-butanolBuAc n-butyl acetateEt.sub.2 O ethyl etherTHF tetrahydrofuranMe.sub.3 C.sub.5 H.sub.9 2,2,4-trimethylpentaneMeEt 2-methyoxyethanolaq. aqueous; mixed with waterin. indecisive______________________________________

EXAMPLE 1

The following example describes a method of synthesizing a urethane (or carbamate) compound of this invention.

Para-nitrophenyl isocyanate (9 g) was dissolved in anhydrous ether (100 mL). (Corrections must be made for any undissolved yellow solids, specifically bis(p-nitrophenyl)urea, which may be removed by filtration at this point see Example 61). To this solution was added 15 g 11-bromo-1-undecanol (Aldrich Chemical Co.) and 0.5 g dibutyltin dilaurate in ether (120 mL). After the resultant exotherm had subsided, the reaction mixture was warmed to 30.degree.-40.degree. C. for about 2 hours before the yellowish solid product was removed by filtration. The solid was recrystallized from ethanol to give extremely pale yellow crystals in excellent yield (>95%), m.p.=122.degree.-123.degree. C. This product, after crushing and sieving, is reported as Example 1 in Table I. SHG efficiency of this material was sixty-seven times that of the standard urea sample when measured by the method of Kurtz and Perry.

Compounds of Examples 2 through 60, inclusive, were prepared in substantially the same manner as was the compound of Example 1.

TABLE I__________________________________________________________________________ ##STR1##Example Crystallization Melting Pyroelectric SHGnumber R solvent point (.degree.C.) test efficiency__________________________________________________________________________ 1 Br(CH.sub.2).sub.11 EtOH 123.sup. + 67 2 C.sub.2 H.sub.5 EtOH (hot) 128.sup.a + 23 3 n-C.sub.3 H.sub.7 n-PrOH 117.sup.a - 0.000 4 n-C.sub.4 H.sub.9 n-BuOH 96.sup.a + 85 5 n-C.sub.5 H.sub.11 EtOH (hot aq.) 93.sup.a + 46 6 n-C.sub.6 H.sub.13 EtOH 104.sup.a + 154; High.sup.b 7 n-C.sub.7 H.sub.15 EtOH 104.sup.a + 110 8 n-C.sub.8 H.sub.17 EtOH 111.sup.a + 62 9 n-C.sub.9 H.sub.19 EtOH 108.sup.a + 8010 n-C.sub.10 H.sub.21 EtOH 114.sup.a + 9911 n-C.sub.11 H.sub.23 EtOH 112.sup.a + 2012 n-C.sub.12 H.sub.25 EtOH 118.sup.a + 813 n-C.sub.13 H.sub.27 n-C.sub.7 H.sub.16 115.sup. + 7.614 CH.sub.3 MeOH 179.sup.a - 0.00115 HCC(CH.sub.2).sub.9 EtOH 114.sup. + 3216 n-C.sub.16 H.sub.33 EtOH 120.sup.a + 2.917 C.sub.6 H.sub.5 (CH.sub.2).sub.5 EtOH .sup. 74 + 918 n-C.sub.18 H.sub.37 CH.sub.2 Cl.sub.2 122.sup.a + 1419 n-C.sub.19 H.sub.39 EtOH 120.sup. + 0.520 Cl(CH.sub.2).sub.6 n-PrOH .sup. 89 + 2421 I(CH.sub.2).sub.11 EtOH 129.sup. + 1722 n-C.sub.22 H.sub.45 EtOH 124.sup. + 0.623 cis-n-C.sub.8 H.sub.17CHCH(CH.sub.2).sub.8 EtOH .sup. 93 + 0.524 Br(CH.sub.2).sub.12 EtOH 102.sup. + 4425 Br(CH.sub.2).sub.16 EtOH 111.sup. + 2326 n-C.sub.7 H.sub.15 EtOH 104.sup.a + 0.00727 n-C.sub.7 H.sub.15 n-BuAc 105.sup.a in. 0.428 n-C.sub.7 H.sub.15 EtOH 103.sup.a + 0.429 n-C.sub.7 H.sub.15 Et.sub.2 O 104.sup.a + 0.630 n-C.sub.9 H.sub.19 EtOH 107.sup.a + 1331 n-C.sub.11 H.sub.23 n-C.sub.7 H.sub.16 111.sup.a 1332 n-C.sub.16 H.sub.33 n-C.sub.7 H.sub.16 121.sup.a + 6033 n-C.sub.18 H.sub.37 THF 120.sup.a 534 Br(CH.sub.2).sub.11 EtOH 123.sup. 3335 Br(CH.sub.2).sub.11 Acetone 123.sup. 3736 n-C.sub.13 H.sub.27 EtOH 117.sup. + 7.137 CH.sub.2CH(CH.sub.2).sub.9 n-C.sub.7 H.sub.16 103.sup. + 0.838 Cl(CH.sub.2).sub.6 EtOH .sup. 89 + 1839 n-C.sub.5 H.sub.11 EtOH (cool aq.) .sup. 93.sup.a in. 0.0340 n-C.sub.22 H.sub.45 EtOH 124.sup. + 0.441 Br(CH.sub.2).sub.16 EtOH 110.sup. + 0.642 CH.sub.2CH(CH.sub.2).sub.9 EtOH 104.sup. + 0.843 cis-n-C.sub.8 H.sub.17 CHCH(CH.sub.2).sub.12 Me.sub.3 C.sub.5 H.sub.9 .sup. 80 + 3.644 C.sub.2 H.sub.5 EtOH (cool) 130.sup.a + 5.545 CH.sub.3 S(CH.sub.2).sub.2 EtOH (aq.) .sup. 75 + 1846 C.sub.6 H.sub.5 O(CH.sub.2).sub.2 EtOH (aq.) .sup. 98 + 1.347 NC(CH.sub.2).sub.2 EtOH (hot) 167.sup. in. 0.0248 n-C.sub.3 H.sub.7 EtOH (hot aq.) 117.sup.a - 0.00049 HO(CH.sub.2).sub.8 EtOH (aq.) 116.sup. + 1050 p-O.sub.2 NC.sub.6 H.sub.4 NHCO.sub.2(CH.sub.2).sub.8 EtOH 217.sup. + 0.00451 CH.sub.3 CO.sub.2 (CH.sub.2).sub.8 MeEt 128.sup. + 0.00252 n-C.sub.7 H.sub.15 EtOH (hot) 104.sup.a + Good.sup.b__________________________________________________________________________ .sup.a Melting points for these compounds have been reported previously i scientific literature. Because of higher material purity, the listed melting points are on average more than 2.degree. C. higher than the values previously reported in scientific literature. The melting points reported herein were measured on apparatus calibrated to the Internationa Temperature Scale of 1990. .sup.b Observing visually as a bright green luminosity when irradiated by a NdYAG laser emitting at 1.064.mu. wavelength. TABLE II__________________________________________________________________________ExampleNumber d i d i d i d i d i d i d i d i__________________________________________________________________________ 1 15.55 100 10.42 87 4.74 45 4.54 29 4.16 53 3.89 42 3.80 91 3.63 62 2 16.68 100 8.21 34 6.29 6 5.82 21 4.88 61 4.01 49 3.83 13 3.68 16 3 9.96 2.5 5.32 1.4 4.92 1.7 4.53 2.2 4.11 4.6 3.73 4.3 3.66 2.2 3.59 2.4 4 18.00 100 9.06 38 5.34 32 4.74 90 4.46 18 4.22 10 4.08 26 3.94 9 5 21.61 100 10.82 10 5.38 3 5.22 4 4.91 7 4.29 4 4.02 2 3.83 6 6 20.91 100 10.69 29 5.35 15 5.24 18 4.71 50 4.25 6 4.13 23 3.90 12 7 24.72 100 12.22 4 6.05 3 5.29 7 4.79 28 4.26 5 4.12 5 3.99 6 8 24.38 100 12.44 2.4 8.27 1.5 6.17 5 5.21 7 4.73 20 4.21 10 4.09 11 9 26.57 95 6.75 28 5.19 20 4.76 100 4.47 19 4.29 24 3.89 41 3.82 3310 27.74 100 6.94 24 5.15 6 4.71 19 4.60 21 4.25 10 3.94 14 3.57 2111 30.05 100 7.56 32 5.15 8 5.02 10 4.77 36 4.34 11 4.28 6 4.11 612 30.50 100 7.64 24 5.16 8 5.07 7 4.77 25 4.34 9 4.14 3 3.78 3013 33.54 100 8.39 26 5.57 7 5.10 18 4.75 93 4.36 23 4.15 13 3.95 814 8.32 3.0 8.23 2.8 7.10 3.1 6.40 11 4.66 10 4.09 2.5 3.57 5 3.482 1.215 28.48 60 7.10 24 5.08 32 4.86 53 4.71 34 4.64 34 4.21 22 3.76 5516 37.47 52 18.60 47 12.47 12 9.41 52 6.27 24 5.04 32 4.70 87 4.36 4517 26.63 100 5.00 31 4.78 51 4.53 26 4.37 33 4.31 30 4.06 30 3.97 2618 40.63 75 20.32 35 13.53 14 10.15 21 6.78 15 5.50 4 5.00 43 4.69 10019 42.99 100 21.35 34 14.33 45 10.80 30 7.18 25 5.37 8 5.00 7 4.72 3920 7.80 17 5.13 29 4.82 45 4.51 17 3.95 41 3.88 100 3.79 24 3.59 1221 16.62 87 16.37 100 10.94 56 8.18 11 6.49 14 4.27 5 4.01 16 3.86 522 46.58 40 23.21 26 15.58 18 4.93 47 4.66 94 4.40 52 4.02 9 3.89 1623 40.65 4 20.32 0.9 13.63 2.6 9.98 2.4 6.86 1.9 5.49 1.1 4.94 50 4.70 10024 33.43 5 16.71 100 11.14 31 6.69 7 4.89 4 4.73 22 4.35 9 4.30 1025 19.30 100 13.06 46 7.91 65 4.91 20 4.84 13 4.67 85 4.36 42 4.24 1826 23.77 100 5.13 12 5.00 12 4.87 9 4.74 1 4.22 12 3.98 11 3.86 1827 23.66 100 11.94 12 5.98 10 5.10 8 4.99 8 4.86 6 4.73 13 4.21 1028 24.32 100 12.15 14 5.12 18 5.00 19 4.74 26 4.22 14 3.98 15 3.85 1830 27.69 33 6.73 29 5.17 21 4.75 100 4.45 21 4.28 26 3.88 45 3.81 4032 37.67 100 18.81 12 9.45 9 6.28 3 5.06 14 4.72 36 4.37 19 3.87 1036 33.01 80 8.33 50 5.55 11 5.09 19 4.74 100 4.36 27 4.14 23 3.94 2937 30.15 100 7.45 5 5.15 3 4.91 2.5 4.71 4 4.27 2.6 3.88 1.6 3.69 638 5.14 46 4.83 72 4.53 27 4.22 15 3.97 57 3.89 100 3.82 74 3.69 2139 22.00 100 10.86 13 5.33 4 5.06 6 4.90 3.4 4.23 3.5 4.01 6 3.87 743 49.57 100 24.75 24 16.50 18 12.01 3 8.17 8 7.16 2 6.38 1 5.34 844 16.03 100 8.10 39 6.21 8 5.77 27 4.84 72 3.98 59 3.80 10 3.66 1345 19.05 100 9.43 20 5.14 76 4.68 54 3.77 5 3.63 8 3.56 19 3.307 3746 15.99 26 7.86 11 6.30 10 5.97 10 4.90 11 4.71 9 4.60 26 4.34 1547 9.18 21 7.10 31 6.16 20 4.39 100 3.65 19 3.62 16 3.423 61 3.319 1848 9.93 6 5.31 1.9 4.95 3 4.56 3 4.13 6 3.74 6 3.67 2.0 3.62 2.249 25.89 100 8.57 8 6.42 1.4 5.14 1.3 5.02 1.4 4.82 3 4.28 4 4.15 1.752 23.90 100 11.84 5 5.96 4 4.75 2.3 4.22 1.1 3.97 7 3.86 1.7 3.72 1.4__________________________________________________________________________Examplenumber D I D I D I D I D I D I D I D I__________________________________________________________________________ 1 3.61 43 3.52 24 3.475 50 3.333 49 3.194 24 2.595 26 2.355 23 2.227 23 2 3.54 52 3.331 4 3.175 30 1.141 27 2.661 7 2.491 5 2.460 8 2.185 3.4 3 3.450 100 3.200 1.5 3.150 1.4 2.949 1.3 2.849 1.1 2.777 3.7 2.644 1.3 2.000 1.1 4 3.68 42 3.57 93 3.496 9 3.334 60 3.065 19 2.980 20 2.411 10 2.027 6 5 3.63 6 3.56 5 3.500 5 3.356 10 3.204 1.5 3.058 3 2.432 1.5 2.162 1.1 6 3.84 6 3.62 16 3.55 26 3.421 41 3.345 16 3.057 9 2.545 5 2.133 6 7 3.91 6 3.73 13 3.66 19 3.498 13 3.413 2.4 3.252 15 2.984 3 2.452 1.2 8 3.78 2.7 3.71 8 3.57 4 3.50 2.3 3.323 12 3.069 3.4 2.459 3.3 1.887 1.3 9 3.64 37 3.51 86 3.431 40 3.351 12 3.203 38 3.170 26 2.981 16 2.241 910 3.52 5 3.448 19 3.298 3 3.232 9 3.071 3.1 2.299 7 2.131 4 1.840 511 3.76 50 3.60 12 3.420 38 3.201 11 3.127 5 3.002 7 2.500 5 2.143 512 3.61 6 3.437 21 3.358 3 3.224 5 3.148 3 3.023 4 2.508 2.0 2.156 2.413 3.84 5 3.70 14 3.63 26 3.55 7 3.341 17 3.186 8 2.181 4 2.061 414 3.312 8 3.216 100 3.150 3.1 2.626 2.2 2.588 1.9 2.491 1.5 2.200 1.4 2.095 2.215 3.61 100 3.56 71 3.52 53 3.464 36 3.434 22 3.367 32 3.148 36 3.043 2116 4.13 13 3.86 38 3.72 50 3.58 100 3.419 13 3.327 29 3.258 18 3.098 2217 3.84 33 3.75 33 3.68 61 3.54 38 3.429 58 3.308 37 3.195 29 3.078 3218 4.37 65 4.02 23 3.74 12 3.66 8 3.494 17 3.352 8 2.238 5 2.074 419 4.42 14 4.27 18 3.87 26 3.70 8 3.62 54 3.56 16 3.400 21 2.086 1020 3.437 30 3.409 9 3.352 13 3.236 37 2.918 17 2.645 9 2.394 8 2.340 2021 3.278 6 3.199 17 2.908 11 2.667 16 2.460 6 2.282 17 2.132 6 1.880 822 3.79 85 3.70 50 3.57 100 3.466 19 3.345 28 3.210 18 3.081 12 2.484 1023 4.31 36 3.60 78 2.804 3.0 -- -- -- -- -- -- -- -- -- --24 4.14 15 3.95 9 3.86 5 3.79 6 3.72 4 3.63 22 3.381 5 3.314 925 3.93 66 3.85 27 3.70 85 3.58 19 3.53 14 3.444 27 3.338 18 3.274 2626 3.77 12 3.73 13 3.65 17 3.54 21 3.442 23 3.371 15 3.341 20 3.188 1427 3.96 19 3.85 14 3.73 10 3.64 15 3.53 20 3.434 17 3.332 19 3.181 1228 3.77 14 3.73 19 3.64 20 3.53 21 3.436 26 3.373 13 3.337 18 3.183 1630 3.64 41 3.51 88 3.422 43 3.347 17 3.197 42 2.977 16 2.242 9 2.081 732 3.73 11 3.60 27 3.431 4 3.340 7 3.263 4 3.108 5 2.524 3 2.151 336 3.83 13 3.69 39 3.62 91 3.54 23 3.343 57 3.185 22 3.001 12 2.183 1037 3.66 5 3.53 1.4 3.436 0.5 3.360 1.2 3.323 1.0 3.214 1.4 3.114 1.0 2.441 0.738 3.60 40 3.487 80 3.425 23 3.361 36 3.251 83 3.199 21 2.926 15 2.342 2039 3.82 5 3.70 2.6 3.63 6 3.51 9 3.425 11 3.345 5 3.282 2.9 3.053 543 4.80 33 4.44 6 4.39 7 4.10 4 3.76 14 3.56 23 3.419 22 -- --44 3.55 24 3.52 39 3.156 35 3.124 26 2.650 11 2.479 9 2.450 12 2.204 545 3.244 5 3.113 38 3.081 23 2.689 8 2.410 4 2.378 4 2.333 5 1.864 1346 4.08 14 3.96 81 3.91 47 3.60 30 3.55 10 3.354 9 3.188 100 2.656 847 3.272 21 3.134 81 3.056 15 2.699 16 2.571 31 2.198 51 1.957 16 1.924 1448 3.476 100 3.390 3 3.212 1.5 3.165 1.3 2.960 1.8 2.857 1.2 2.786 4 2.656 1.349 3.85 8 3.81 6 3.65 13 3.379 1.5 3.265 3 3.140 1.7 2.328 1.4 2.079 1.752 3.64 3 3.53 3 3.435 2.0 3.394 2.2 3.330 2.5 3.239 1.2 3.181 1.4 2.382 1.1__________________________________________________________________________ TABLE III__________________________________________________________________________ ##STR2##Example Crystallization Melting Pyroelectric SHGnumber R A B C solvent point (.degree.C.) test efficiency__________________________________________________________________________53 Cl(CH.sub.2).sub.6 CH.sub.3 H NO.sub.2 EtOH 101 - 0.00054 Cl(CH.sub.2).sub.6 OCH.sub.3 H NO.sub.2 EtOH 91 - 0.00055 Cl(CH.sub.2).sub.6 OCH.sub.3 H NO.sub.2 Et.sub.2 O 91 - 0.00056 Br(CH.sub.2).sub.11 H NO.sub.2 H EtOH 78 + 0.0557 Br(CH.sub.2).sub.11 Cl H NO.sub.2 EtOH 73 + 0.0658 Br(CH.sub.2).sub.11 CH.sub.3 H NO.sub.2 EtOH 83 - 0.0259 Br(CH.sub.2).sub.11 OCH.sub.3 H NO.sub.2 EtOH 96 .sup. +.sup.a 0.3460 Cl(CH.sub.2).sub.6 Cl H NO.sub.2 EtOH 73 - 0.001 60A.sup.b Cl(CH.sub.2).sub.6 Cl H NO.sub.2 EtOH__________________________________________________________________________ .sup.a Very fine crystals were clinging together in balls which fell off the spoon; this was attributed to acentric polarity. .sup.b Example 60A is a single crystal taken from Example 60 and subjecte to singlecrystal xray structure analysis, from which data the xray powder diffraction pattern was computed. The resulting positions and relative intensities of the matching peaks are shown in Table IV below; no discrepant peaks were found. This cyrstal form was found to have a center of symmetry and thus was not acentric, confirming the result of the pyroelectric test performed for Example 60. TABLE IV__________________________________________________________________________Examplenumber D I D I D I D I D I D I D I D I__________________________________________________________________________53 9.91 7 8.44 11 5.79 15 5.17 34 4.49 19 4.04 78 4.02 65 3.87 10054 10.29 12 7.31 10 5.81 7 4.35 6 4.24 4 4.08 7 3.91 17 3.76 10056 19.32 40 9.51 14 4.86 25 4.51 33 4.27 23 4.17 42 4.02 29 3.98 1757 21.86 100 6.30 10 4.90 11 4.48 28 4.38 13 4.28 14 4.16 27 4.08 1158 12.40 11 8.59 7 6.58 23 6.34 19 4.41 10 4.08 6 3.96 100 3.54 759 18.03 21 11.87 54 7.08 14 4.94 34 4.48 100 4.40 15 4.07 39 4.00 1960 8.50 18 5.14 24 4.95 6 4.02 71 3.84 36 3.82 100 3.78 89 3.53 36 60A.sup.a 8.495 24 5.143 19 4.924 8 4.014 46 3.865 11 3.808 100 3.764 59 3.556 12 3.514 16__________________________________________________________________________Examplenumber D I D I D I D I D I D I D I D I__________________________________________________________________________53 3.82 74 3.57 15 3.321 45 3.248 29 3.234 22 2.991 31 2.886 13 2.293 1354 3.65 5 3.56 11 3.410 9 3.118 5 3.078 8 2.889 9 2.837 7 2.684 856 3.92 100 3.78 26 3.58 14 3.407 40 3.347 15 2.970 15 2.689 17 2.139 1757 3.75 16 3.53 40 3.381 28 3.200 12 3.118 12 3.061 12 3.034 14 2.800 1858 3.470 60 3.305 11* 3.283 15 3.253 18 3.091 7 2.598 23 2.481 7 2.287 559 3.82 49 3.71 15 3.61 17 3.53 15 3.436 81 3.278 15 3.193 18 2.698 1460 3.280 47 3.203 20 3.036 4 2.980 6 2.859 8 2.558 9 2.485 4 2.075 4 60A.sup.a 3.275 46 3.190 9 3.036 2 2.973 6 2.887 3 2.571 2 2.481 3 2.066 3 3.019 2 2.845 3 2.550 5 2.543 5__________________________________________________________________________ .sup.a Multiple computed lines are shown for Example 60A; it was judged that such closely spaced lines had not been resolved experimentally.

EXAMPLE 61

The following example describes a method of synthesizing a urea compound (N-octadecyl-N'-p-nitrophenyl urea) suitable for this invention.

Because ostensibly pure p-nitrophenyl isocyanate may contain sizable proportions of its hydrolysis product, bis-(p-nitrophenyl)-urea, it is often necessary to repurify the p-nitrophenyl isocyanate by dissolving it in warm toluene (or in ether) and removing the urea by filtration. Repurified isocyanate crystallizes upon evaporation of toluene (or ether) from the tepid solution.

N-Octadecylamine (Aldrich Chemical Co., Milwaukee, Wis.)(1.35 g, 0.005 mole) and freshly repurified p-nitrophenyl isocyanate (Eastman Chemical Co., Rochester, N.Y.)(0.83 g, 0.005 mole) were dissolved in a total of 25 mL toluene and mixed in a 50 mL Erlenmeyer flask fitted with air condenser. The resulting mixture was heated for about two hours on a steam bath. A yellowish precipitate formed and was collected by filtration, washed with toluene and dried; the solid weighed 1.94 g (theor. 2.18 g) and had a melting point of 119.degree.-121.degree. C. The precipitate was dissolved in 40 mL boiling acetone and crystallized upon cooling. After filtration, washing with about 20 mL acetone, and drying, a white solid was recovered (1.29 g, m.p. 120.degree.-122.degree. C.). An additional 0.30 g, m.p. 120.degree.-121.degree. C., was recovered from the filtrate. This product, after crushing and sieving, is reported as Example 61 in Table II; its SHG efficiency was five times that of the standard urea sample when measured by the method of Kurtz and Perry.

Compounds of Examples 62 through 80, inclusive, were prepared in substantially the same manner as was the compound of Example 61, though often with anhydrous ether rather than toluene as the reaction solvent.

TABLE V__________________________________________________________________________ ##STR3##Example Crystallization Melting Pyroelectric SHGnumber R solvent point (.degree.C.) test efficiency__________________________________________________________________________61 n-C.sub.18 H.sub.37 Acetone 122 + 562 HO.sub.2 C(CH.sub.2).sub.10 EtOH .sup. 184.sup.b - 0.0063 ##STR4## EtOH 172 - 0.00464 (CH.sub.3).sub.2 N(CH.sub.2).sub.3 EtOH 90 in. 0.0365 n-C.sub.5 H.sub.11 EtOH 131 in. 0.00166 n-C.sub.6 H.sub.13 n-C.sub.6 H.sub.13 NH.sub.2 112 + 0.967 n-C.sub.10 H.sub.21 EtOH 118 + 3868 n-C.sub.12 H.sub.25 EtOH 118 + 869 n-C.sub.14 H.sub.29 EtOH 121 + 1170 n-C.sub.16 H.sub.33 EtOH 120 + 671 trans-C.sub.8 H.sub. 17 CHCH(CH.sub.2).sub.8 EtOH 105 + 1.672 n-C.sub.18 H.sub.37 THF 123 + 0.973 n-C.sub.18 H.sub.37 CH.sub.2 Cl.sub.2 123 - 0.00074 n-C.sub.18 H.sub.37 CH.sub.2 Cl.sub.2 122 - 0.00275 n-C.sub.18 H.sub.37 THF 121 - 0.00076 n-C.sub.18 H.sub.37 Acetone 121 +77 n-C.sub.10 H.sub.21 EtOH 118 + 378 n-C.sub.18 H.sub.37 CH.sub.2 Cl.sub.2 122 + 1479 n-C.sub.5 H.sub.11 Acetone + Me.sub.3 C.sub.5 H.sub.9 133 + 0.00380 n-C.sub.12 H.sub.25.sup.a Toluene 98 + 0.004__________________________________________________________________________ .sup.a Aryl group is 2chloro-4-nitrophenyl .sup.b Melted with decomposition TABLE VI__________________________________________________________________________Examplenumber d i d i d i d i d i d i d i d i__________________________________________________________________________61 41.54 100 20.66 8 4.69 21 4.64 25 4.45 40 4.23 46 4.08 28 3.95 6662 19.26 42 18.68 23 4.97 19 4.60 20 4.17 25 4.10 23 3.97 70 3.89 2363 9.11 9 6.48 9 5.99 13 5.50 11 4.89 57 4.56 8 4.30 30 3.96 4364 11.75 21 9.66 32 5.50 14 5.23 86 4.80 25 4.62 18 4.28 37 4.16 1165 12.34 8 9.92 8 8.68 12 8.56 12 6.15 14 5.47 16 5.34 8 4.52 1166 22.92 100 11.26 12 5.53 8 4.77 5 4.58 13 4.21 24 3.92 24 3.74 1467 29.83 100 7.16 9 4.75 20 4.60 22 4.43 12 4.23 22 4.09 8 3.99 4168 7.95 15 6.11 8 4.78 4 4.65 50 4.49 100 4.20 20 4.01 85 3.82 1069 34.42 75 17.11 22 8.64 100 5.76 31 4.58 24 4.44 12 4.29 28 4.23 3670 38.46 100 19.23 8 12.84 4 9.63 10 6.39 5 4.68 3 4.56 5 4.43 471 40.66 100 20.30 13 13.55 15 10.19 20 6.77 12 4.69 41 4.46 63 4.23 8272 41.26 100 20.63 12 13.77 6 10.34 8 7.47 3 6.85 5 4.62 3 4.52 573 25.02 11 12.55 8 12.35 8 8.25 3 7.04 4 6.21 4 5.09 5 5.03 374 25.30 74 12.54 34 8.32 10 7.10 6 6.27 6 5.72 4 5.12 7 4.58 1876 40.79 100 20.31 12 13.61 14 10.23 18 6.80 14 4.65 21 4.44 33 4.26 4177 28.46 100 9.60 1.0 7.18 10 4.75 4 4.61 1.3 4.44 0.8 4.23 2.5 4.00 678 40.63 75 20.32 35 13.53 14 10.15 21 6.78 15 5.00 43 4.69 100 4.49 479 12.11 22 11.93 14 9.91 14 8.48 38 6.06 31 5.45 47 5.41 31 5.28 2580 21.15 42 10.51 12 4.67 13 4.35 11 4.28 8 4.07 100 3.83 10 3.63 12__________________________________________________________________________Examplenumber D I D I D I D I D I D I D I D I__________________________________________________________________________61 3.78 22 3.69 44 3.56 30 3.419 11 3.300 10 3.237 8 3.158 8 3.120 762 3.80 100 3.69 21 3.55 25 3.50 27 3.308 47 3.168 30 3.017 41 2.247 3063 3.91 26 3.82 21 3.490 6 3.394 6 3.360 7 3.190 100 3.138 9 2.962 1064 3.89 20 3.54 100 3.408 95 3.278 14 3.108 12 2.887 22 2.781 17 2.622 1265 4.25 27 3.76 31 3.72 100 3.65 8 3.326 9 3.320 11 3.294 8 3.095 766 3.65 26 3.496 16 3.406 7 3.352 5 3.228 10 3.081 6 2.951 5 2.092 1267 3.78 30 3.71 8 3.63 22 3.53 15 3.432 11 3.402 12 3.335 10 3.123 968 3.73 13 3.58 15 3.487 25 3.404 2 3.314 7 3.202 2 3.123 10 2.903 269 4.04 78 3.88 59 3.83 32 3.60 37 3.50 31 3.436 23 3.307 14 3.271 1570 4.28 4 4.10 12 4.03 5 3.96 8 3.91 5 3.67 4 3.59 5 3.53 371 3.91 66 3.77 27 3.66 49 3.52 22 3.397 24 3.247 14 3.169 9 3.111 1172 4.26 7 4.09 13 3.95 14 3.87 6 3.69 5 3.63 6 3.57 6 3.51 373 4.55 30 3.96 3 3.82 100 3.421 34 3.401 51 3.155 6 2.241 3 1.961 374 3.98 3 3.86 100 3.420 25 3.211 4 3.167 5 3.041 5 2.846 3 2.042 576 4.20 38 3.95 63 3.67 41 3.54 17 3.415 11 3.276 10 3.139 8 3.114 877 3.79 4 3.64 1.3 3.59 1.0 3.53 5 3.408 2.2 3.338 1.0 3.136 1.3 2.823 1.178 4.37 65 4.02 23 3.74 12 3.66 8 3.494 17 3.352 8 2.238 5 2.074 479 4.51 22 4.29 21 4.21 20 3.71 100 3.63 19 3.443 17 3.309 25 3.091 2280 3.58 18 3.471 17 3.375 7 3.264 38 3.160 13 2.937 8 2.166 12 2.104 8__________________________________________________________________________

Data in Tables I, III, and V clearly demonstrate a high qualitative correlation between SHG efficiency and moderate to strong pyroelectricity. The data also indicate a lower limit, below which the simple pyroelectric test used herein may be inaccurate. This limit may be reached in certain cases where the value of SHG efficiency is below one. Exact characterization of the crystal form must be effected by x-ray methods. Specifically, single-crystal x-ray analysis may be used to establish unambiguously both the chemical composition and crystal structure, including acentricity, of a material. For example, single-crystal x-ray structure analysis was done on the material of Example 60 and showed it to be centrosymmetric, in accordance with the pyroelectric and SHG results for this material. From data thus gathered, it is possible to compute with a high degree of accuracy the x-ray powder pattern of a "perfect" (i.e., randomly oriented) powder of that same crystal form. The results are shown in Table IV, Example 60A. This pattern can then be compared with the experimental x-ray powder pattern of the sample, to prove identity or non-identity. Such a comparison may be made between Examples 60 and 60A in Table IV.

Data in Tables I and II show the dramatic effects of the crystallizing technique and/or crystallization solvent on SHG efficiency. For example, the C.sub.7 urethanes crystallized from n-butyl acetate, ether, and especially ethanol, i.e., Examples 7 and 26-29, all showed different SHG Values. Their powder x-ray patterns differed markedly. From these results, one can conclude that orientation of the SHG active portion of the molecule is sensitive to solvents. The solvent can act either to enhance the desired parallel arrangement of the SHG active moiety or perhaps favor an unwanted antiparallel alignment; however, it is not the sole influence upon the alignment.

For a given solvent, the crystallizing temperature can also affect SHG efficiency of the solid product. For example, when the C.sub.5 urethane was crystallized from aqueous ethanol, two crops of crystals were obtained, i.e., Examples 5 and 39. Their powder x-ray patterns, while similar, are distinguished by relatively strong lines at 3.356.ANG. and 3.425.ANG., among others. The first crop of crystals was harvested as the hot mixed solvent was cooling to a warm temperature (above room temperature) while the other crop of crystals was harvested when the warm solvent had cooled to room temperature. The first crop of crystals had a SHG efficiency of 46, while the second crop had a SHG efficiency of 0.03. This factor of greater than 1000 in SHG efficiency leads one to suspect that the second crop of crystals were actually inactive but contaminated by a small amount of the first, which would account for the small SHG value and the pyroelectric results.

When the C.sub.2 urethane was crystallized from hot ethanol, two crops of crystals, i.e., Examples 2 and 44, were obtained. The first crop of crystals, which were fine needles, showed a SHG efficiency of 23.4, while the second crop of crystals, which were chunky and diamond-shaped, showed a SHG efficiency of 5.5. While crystal forms may sometimes be an indication of real differences in crystal structures, it is well known even for common table salt that the same internal structure of a material, as shown by x-ray powder patterns, can yield quite differently-shaped crystals when one or another facet's growth is retarded by solvents or impurities. In the case of Examples 2 and 44, the d-values for the powder x-ray lines, and their relative intensities, are in sufficiently close agreement as to be judged identical in crystal structure. It is in no way surprising that the SHG efficiencies differ by a factor of about four, since it is impossible to crush crystal forms so dissimilar in aspect ratio to identical coarse powders. Differences in average dimension or in aspect ratio can be expected to affect SHG efficiency measurements by perhaps as much as a factor of ten.

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.

Claims

1. A second harmonic generator comprising a laser source of coherent light radiation at a fixed fundamental frequency, an acentrically crystalline, achiral, organic compound, means for directing the output radiation of the laser onto said compound, and output means for utilizing the second harmonic frequency, said compound being a straight-chain N-nitrophenyl carbamyl compound crystallized in a non-centrosymmetric configuration, said crystalline compound being transparent to radiation at said fixed fundamental frequency and said second harmonic frequency, and said crystalline compound being a derivative of an N-nitrophenyl carbamic acid such that the straight chain is linked to the carbamyl carbon atom by an oxygen atom or nitrogen atom.

2. The second harmonic generator of claim 1, wherein said compound is selected from a group represented by the formulae:

R.sup.1 HNCONHR.sup.2 (I)

and

R.sup.1 HNCO.sub.2 R.sup.2 (II)

where R.sup.1 represents one member of a group consisting of a nitrophenyl and substituted nitrophenyl group, and R.sup.2 represents a group having two to twenty-two catenated methylene groups terminated by R.sup.3, where R.sup.3 represents a member selected from the group consisting of hydrogen, halogen, unsubstituted ethinyl group, substituted ethinyl group, unsubstituted vinyl group, substituted vinyl group, hydroxy group, alkoxy group, aryloxy group, acyloxy group, alkanesulfonyloxy group, arenesulfonyloxy group, alkanesulfonyl group, arenesulfonyl group, aryloxysulfonyl group, alkylthio group, arylthio group, cyano group, unsubstituted carbonyl, substituted carbonyl group, unsubstituted aryl group, substituted aryl group, unsubstituted amino group, substituted amino group, acyl amino group, and heterocyclic group.

3. The second harmonic generator of claim 1, wherein said compound is selected from a group represented by the formulae:

R.sup.1 HNCONHR.sup.2 (I)

and

R.sup.1 HNCO.sub.2 R.sup.2 (II)

where R.sup.1 represents one member of a group consisting of an unsubstituted nitrophenyl group and a substituted nitrophenyl group, and R.sup.2 represents a polymethylene group having two to twenty-two catenated methylene groups terminated by one member of a group consisting of a hydrogen atom, a halogen atom, an unsubstituted vinyl group, a substituted vinyl group, an unsubstituted ethinyl group, a substituted ethinyl group, an unsubstituted aryl group, a substituted aryl group, a hydroxy group, an ether group, an ester group, a thioether group, and a thiolester group.

4. The second harmonic generator of claim 1, wherein said laser is a Nd-YAG laser.

5. The second harmonic generator of claim 1, wherein said laser is a solid-state laser.

6. A process for converting a fixed fundamental frequency of coherent laser light into a second harmonic frequency which comprises passing said laser light through a nonlinear optical element comprising an acentrically crystalline, achiral, organic compound, said compound being a straight-chain N-nitrophenyl carbamyl compound crystallized in a non-centrosymmetric configuration, said crystalline compound being transparent to said fixed fundamental frequency and to said second harmonic frequency, and said crystalline compound being a derivative of an N-nitrophenyl carbamic acid such that the straight chain is linked to the carbamyl carbon atom by one member of a group consisting of an oxygen atom and nitrogen atom.

7. The process of claim 6, wherein said acentrically crystalline compound is selected from a group represented by the formulae:

R.sup.1 HNCONHR.sup.2 (I)

and

R.sup.1 HNCO.sub.2 R.sup.2 (II)

where R.sup.1 represents one member of a group consisting of a nitrophenyl and substituted nitrophenyl group, and R.sup.2 represents a group having two to twenty-two catenated methylene groups terminated by R.sup.3, where R.sup.3 represents a member selected from the group consisting of hydrogen, halogen, unsubstituted ethinyl group, substituted ethinyl group, unsubstituted vinyl group, substituted vinyl group, hydroxy group, alkoxy group, aryloxy group, acyloxy group, alkanesulfonyloxy group, arenesulfonyloxy group, alkanesulfonyl group, arenesulfonyl group, aryloxysulfonyl group, alkylthio group, arylthio group, cyano group, unsubstituted carbonyl, substituted carbonyl group, unsubstituted aryl group, substituted aryl group, unsubstituted amino group, substituted amino group, acyl amino group, and heterocyclic group.

8. The process of claim 6, wherein said acentrically crystalline compound is selected from a group of represented by the formulae:

R.sup.1 HNCONHR.sup.2 (I)

and

R.sup.1 HNCO.sub.2 R.sup.2 (II)

where R.sup.1 represents one member of a group consisting of an unsubstituted nitrophenyl group and a substituted nitrophenyl group, and R.sup.2 represents a polymethylene group having two to twenty-two catenated carbon atoms terminated by a hydrogen atom, a halogen atom, an unsubstituted vinyl group, a substituted vinyl group, an unsubstituted ethinyl group, a substituted ethinyl group, an unsubstituted aryl group, a substituted aryl group, a hydroxy group, an ether group, an ester group, a thioether group, and a thiolester group.