Chem. Mater. 1993,5, 1641-1644
1641
Diacetylene and Polydiacetylene Derivatives of 2-Methyl-4-nitroaniline for Second-Harmonic Generation M. S. Paley’ and D. 0. Frazier Space Science Laboratory, N A S A Marshall Space Flight Center, Huntsville, Alabama 35812
S . P. McManus, S. E. Zutaut, and M. Sanghadasa Materials Science Program, University of Alabama in Huntsville, Huntsville, Alabama 35895 Received May 11, 1993. Revised Manuscript Received August 18, 199P
A novel diacetylene derivative of 2-methyl-4-nitroaniline (MNA), a well-known material for second-harmonic generation (SHG), is synthesized. This monomer (diacetylene methylnitroaniline, DAMNA) is characterized by means of the Kurtz technique a t 1064 nm and is found to have an SHG powder efficiency 62.5% that of MNA itself. Thin crystalline films of DAMNA are grown onto quartz, Teflon, and Kapton substrates by means of vapor deposition in vacuum. These films are then polymerized in the solid state by exposure to long-wavelengthUV radiation to give crystalline polydiacetylene thin films (PDAMNA). The films are next characterized for SHG, using an unpolymerized DAMNA film as a reference. Interestingly, films grown onto Teflon exhibit greater orientation and significantly greater SHG than those grown onto quartz and Kapton. This result is promising in that it demonstrates the potential of PDAMNA as both a crystalline and polymeric material for SHG applications, in which highly oriented thin films are desired. Computational modeling (usingAM1) is also carried out on DAMNA and is compared to the experimental results.
Introduction
Organic compounds have received much attention as promising new materials for nonlinear optical applications.’ Of particular interest are second-order processes suchas second-harmonicgeneration (SHG),i.e., frequency doubling of laser radiation (typically, doubling infrared radiation into the visible).2 Considerable research has been done on finding organic materials which are efficient and practical for SHG. This is no trivial matter. Although the molecular features required for good hyperpolarizability are reasonably well understood and obtained, achieving bulk materials useful for SHG is still quite challenging. This is primarily because in order to be capable of SHG a material must be noncentrosymmetric.2 Many organic molecules which exhibit good SHG pack in centrosymmetric crystal structures, thereby rendering them useless as bulk materials. Additionally, most organic liquids, polymers, and glasses are randomly oriented (unless treated by special means) and hence also are incapable of SHG. The main approaches around these difficulties have been to design molecules with structural features that favor packing in acentric crystal structures and to electricallypole polymer films doped with molecules useful for SHG (referred to as dyes) in order to induce a~ymmetry.~ In recent years, the latter approach seems to be more widespread. This is probably because thin polymer films offer more flexibility in terms of the variety of materials that can be used, mechanical properties, and ease of device fabrication. However, poled polymer films do have drawbacks. Their SHG is limited by the amount of dye that can be placed in the polymer matrix (asopposed *Abstract published in Advance ACS Abstracts, October 1, 1993. (1)Nonlinear OpticalProperties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, FL, 1987. (2) Reference 1, Vol. 1. (3) Reference 1,Vol. 1, Chapter 4.
to pure material in a crystal); moreover, poled polymer films, when removed from the electric field, relax back to a random orientation and thus lose their capacity for SHG (which,clearly,is not a problem with crystalline materials). For these reasons we have been interested in a class of compounds known as polydiacetylenes (PDAs),which are both crystalline and p~lymeric.~ Polydiacetylenes have been investigated extensively as third-order nonlinear optical materials, but considerably less so as second-order materials.5 However, theoretical calculations, based on quantum mechanical modeling, indicate that certain PDAs could potentially be amongthe best second-order nonlinear optical materials known.6 They are highly conjugated, which can lead to large optical nonlinearities, and crystalline monomers yield crystalline polymers that can be tailored to give a wide variety of properties. Hence PDAs offer significant advantages; because they are polymeric, they can be formed as thin films, which are desirable for device fabrication, and because they are crystalline, they can be highly oriented, which is essential for optimizing their nonlinear optical properties. Furthermore, because PDA films are crystalline, they do not possess the drawbacks associated with poled polymer films. By choice of a diacetylene monomer that crystallizes in a noncentrosymmetric structure, poling is not required to make the resulting PDA film capable of SHG, thereby circumventing the problem of relaxation. Also because PDA films are pure materials, they should be capable of greater SHG than doped polymer films. The preparation of PDA thin films consists of synthesizing diacetylenemonomers, forming thin crystalline films of the monomers, and then polymerizing these films (in (4) Reference 1, Vol. 2. (5) Nonlinear Optical Properties of Polymers; Heeger, A. J., Ulrich, D. R., Eds.; North Holland Press: New York, 1988. Also ref 1, Vol. 2. (6) Yoshimura, T.J. Am. Phys. SOC.1989,40,6292.
0891-4756/93/2805-1641$04.00/00 1993 American Chemical Society
1642 Chem. Mater., Vol. 5, No. 11, 1993
Paley et al.
the solid state) by irradiation with UV light or y-rays.' This procedure is by n o means trivial. The synthesis of diacetylene monomers can be quite difficult and time consuming, and many monomers will not lead t o PDA films useful for SHG. Many diacetylene monomers do not pack in acentric crystal structures, and, additionally, not all crystalline diacetylene monomers will polymerize readily t o give PDAs. Furthermore, even if a PDA film is obtained, its optical properties still may not be suitable for SHG. For these reasons we have investigated t h e use of computational modeling for predicting the chemical and optical properties of diacetylenes and their polymers as a means of screening potentially interesting compounds.8 Specifically, we have successfully used AM1 methods to
Parallel
Alignment
calculate t h e optical nonlinearities of some diacetylene monomers and to predict the polymerizability of these monomers in the solid statea9 Such theoretical screening is essential for saving labor and carrying out t h e search for interesting a n d useful PDAs for SHG in a pedagogically sound fashion. I n t h e present work, we have investigated the properties of a diacetylene derivative (DAMNA) of 2-methyl-4H '-H
kN
H
0
DAMNA
/
H
nitroaniline (MNA), a well-known crystalline material for SHG. The monomer is synthesized, thin films are grown by vapor deposition in vacuum onto a variety of substrates, these films are polymerized by UV light, and the resulting PDAMNA films are then characterized for SHG. Computational modeling is also carried out on this compound and compared t o t h e experimental results.
Methods and Results Computational Modeling. The rationale behind selecting DAMNA is that, ideally, it should exhibit SHG similar to that of MNA, but, unlike MNA; also have the ability to form thin crystalline polymer films. The reason for placing an OH group on one side of the molecule is to take advantage of hydrogen bonding in an attempt to obtain favorable crystal packing. The strong hydrogen-bonding interaction of neighboring OH groups should inhibit the diacetylenemoleculea from aligningantiparallel in the crystal, which would prevent SHG. The MNA moiety on the other side of the molecule will provide the source of S H G the CH2 spacers should increase flexibility and allow the MNA groups to interact much in the same way as they do in MNA itself. Hopefully, by preserving the MNA crystal structure as much as possible, the key aspects of that structure which make MNA such a good SHG material willalso be present in DAMNA and PDAMNA. Additionally, it is desired that the crystal structure be sufficiently unhindered sterically so that the crystalline monomer can polymerize readily. To support the above reasoning, semiempirical AM1 calculations were carried out on DAMNA. Calculations were first carried out on a single monomer to optimize the molecular geometry and to determine the static second-order molecular polarizability (le), a measure of the molecular efficiencyfor SHG.2 For the sake of comparison, the same calculation was also carried out on MNA itself. Interestingly, the value of j3 obtained for DAMNA, 5.6 X 10-90 esu, is even greater than that obtained for (7) Wegner, G. Z. Naturforsch. 1969, 246, 824. Also Solid State Polymerization; Sandman, D. J., Ed.; American Chemical Society: Washington, DC, 1987. (8) Jalali-Heravi, M.; Zutaut, S. E., McManus, S. P., McDonald, J. K. Macromolecules 1991,24, 1055. (9) Paley, M. S.; Frazier, D. 0.; Abeledeyem, H.; McManus, S. P.; Zutaut, S. E. J. Am. Chem. SOC.1992,114,3247.
Antiparallel
Alignment
Figure 1. Gas-phasestructures calculated by AM1for a DAMNA monomer pair. MNA, 2.7 X 10-90 esu. Thus we expect the DAMNA molecule to be very capable of SHG. The next step is to model the crystal packing of the diacetylene monomer in order to make a prediction about its polymerizability. Using the method we have described previously? AM1 calculations were carried out on two monomer molecules optimized in a structure anticipated to occur in a crystal. Although these calculations are for gas-phase molecules, previous results suggest relevance to monomer packing in a diacetylene crystal. Thus we determined the crystal packing parameters ( d ,the intermolecular distance and 6, the intermolecularangle) for a geometry-optimized monomer pair, treating both the cases of parallel and antiparallel alignment of the DAMNA molecules (see Figure 1). Values of d = 6.7 A, 6 = 47.6' were obtained for parallel alignment, and d = 6.5A, 6 = 43.6' for antiparallel alignment. When compared to calculated values for other known diacetylenes, these values fall well within the range expected for solid-state polymerization' to occur without difficulty. Hence both computed Structures would be expected to undergo polymerization to give PDAMNA. At present, our method is not yet able to predict reliably which alignment is favored. It is interesting to note that our results are contradictory to a previous claim that DAMNA does not polymerize.10 Synthesis and Properties. The synthesis of DAMNA has been reported previously using a slightly different procedure than the one used here.1° Interestingly, it was claimed that the method of Cadiot and Chodkiewiczll could not be used directly to synthesize this diacetylene. However, we found that the method worked straightforwardly. The procedure consists of essentially three steps: preparation of bromopropargyl alcohol from propargyl alcohol, preparation of N-propargyl MNA from 2-fluordhitrotoluene and propargylamine,and, f i i y , coupling of these two components by the method of Cadiot and Chodkiewicz to give the diacetylene (Scheme I). (10) Horner, C. J.; Carib, A. F. Makromol. Chem. 1981,182, 19. (11) Chodkiewicz, W.; Cadiot, P. C. R. Hebd. Seances Acad. Sci. 1968, 241,1055.
Derivatives of 2-Methyl-4-nitroaniline for SHG Scheme I
+
HCEC-CHZOH
Br2
BrCEC--CH,OH
CH
L
BrC-C-CHBOH
+
HC=C-CH2-NH
The bromination of propargyl alcohol and the preparation N-propargyl MNA have been discussed previously.oJ2J* The coupling of bromopropargyl alcohol with N-propargyl MNA proceeds readily in the dark; the diacetylene monomer is purified by column chromatography on silica gel using dichloromethane/ acetone as the eluent. DAMNA is a yellow powder resembling MNA and has a melting point of 160-162 O C . This compound is slightly light sensitive and thus should be stored in the dark. Vapor Growthand Polymerization. DAMNA can be grown in the form of thin Crystalline f i s on quartz glass disks by means by vapor deposition in vacuum. Using a source temperature of 120 O C and a substrate temperature of 30 O C , under a pressure of 20 pmHg, a thin film can be grown in 4-5 h. The yellow polycrystalline monomer f i can then be polymerizedby exposure to long wavelength (366 nm) UV radiation for 24-36 h. Again, this result is contradictory to a previousclaim.@The degree of polymerization under these conditions is about 50%. The resulting translucent orange-red polycrystalline PDAMNA thin film is then ready for characterization. Under a magnification of 500X using an optical microscope, it is seen that the films consist of a network of crystalline grains. By means of profilometry, the thickness is determined to be 1.0-2.0 pm, with a surface roughnessof about 0.3-0.4 pm. The films are consistently thicker around the edges (about 2.0 pm) than near the center (1.0-1.5 pm). The size of the crystalline grains is in the range 10-15 pm. When grown onto quartz glass, the needle-shaped grains of the polycrystalline PDAMNA thin films are randomly oriented (viewed under a magnification of 500X). To be better suited for SHG applications, more highly oriented films (ideally, monocrystalline)are desired. Unfortunately, because of the large lattice parameters of diacetylenes, epitaxial growth of large-area singlecrystal thin films is often not possible.14 However, recent results in the literature have suggestedthat growth onto ordered polymer surfaces might provide an alternative to epitaxial growth. Specifically, ordered Teflon surfaces have shown an excellent ability to induce orientation into crystalline films grown onto them.l6 Kapton, a polyimide, can also be formed into highly oriented surfaces and, in contrast to Teflon, is quite polar. Hence it was decided to investigate the effects of growing the films on these two different types of ordered polymer surfaces. The orientation in the substrates was characterized by means of waveguide mode analysis,essentiallyverifyingthat their refractive indices varied with direction. Interestingly, growth of PDAMNA onto oriented Teflon tape results in a f i i which does indeed show greater ordering (viewed at 600X and 1OOOX) than one grown onto quartz under the same conditions (specified above). The grains appear to exhibit some degree of alignment along one particular direction, possibly that (12) Organic Syntheses; Miller, S. I.; Ziegler, G. R.; Wieleaeck, R.; Wiley: New York, 1973; Collect. Vol. 5, p 921. (13) Garito, F.; Homer, C. J.; Kalyanaraman, P. S.; Deeai, K. N. Makromol. Chem. 1980,181,1605. (14) Thakur, M.; Meyler, S. Macromolecules 1985, 18, 2341. (15) Wittman, J. C.; Smith, P. Nature 1991, 352, 414.
Chem. Mater., Vol. 5, No. 11,1993 1643 of or perpendicular to the polymer chains of the Teflon. On the other hand, growth onto Kapton tape appears to exhibit no ordering; the grains are randomly oriented in the same manner as they are when grown onto quartz. The grain size and film thickness is about the same for all three substrates. Hence it appears that growing PDAMNA onto oriented Teflon substrates shows promise for obtaining films that will be superior for SHG. Characterization. Once a PDAMNA thin film has been produced, the final step is to characterize the film by actually measuring its SHG. Because of the transluscence of the films (due to their polycrystallinity), the most readily applicable SHG characterization method is the Kurtz technique, which can be applied to both powders and thin films.16 Essentially, this technique consists of focusing a laser beam at some fundamental frequency (typically 1064 nm) onto the sample and then collecting and measuring the resulting scattered second-harmonic signal. First, the DAMNA powder must be characterized by comparing ita SHG efficiency to that of some standard, in this case MNA itself. When this carried out, it is found that the diacetylene monomer has a powder efficiency 62.5% that of MNA for SHG at 1064 nm (Le., converting 1064 to 532 nm). This is quite good; it lends support to our conjectures about the nature of the crystal structure of DAMNA. To characterize the thin films, a reference film of DAMNA is grown (onto quartz) under the same conditions as the PDAMNA films. This film should have about the same thickness as the polymer films, allowing for a more direct comparison of SHG efficiencies. When a PDAMNA film grown onto quartz is measured against this reference film, it is found to be 66% as efficient for SHG at 1064 nm. The reason that the polymer film is less efficient than the monomer film is that the polymer exhibits slight absorption at 532 nm, whereas the monomer does not (PDAMNA becomes transparent above 650 nm). Despite the fact that some of the second-harmonic signal generated by the polymer is being absorbed, the PDAMNA film still shows relatively good SHG. Next, the films grown onto the ordered polymer substrates are characterized. Not surprisingly, the film grown onto Kapton shows about the same SHG as that grown onto quartz. This is to be anticipated since both the quartz and Kapton PDAMNA films exhibit no orientation of the crystalline grains. In contrast, the film grown onto Teflon should be expected to show greater SHG because,in this case, the grains are partially oriented. Indeed this is the case; the PDAMNA film grown onto Teflon exhibits SHG 8 times greater than that grown onto quartz under identical conditions. Because the grain size of the film grown onto Teflon is essentially the same as that grown onto quartz, we conclude that the increased SHG is due to the orientation by the Teflon substrate. This result is very encouraging; it demonstrates that by orienting the PDAMNA films we can improve their capacity for SHG considerably.
Discussion and Conclusions On the basis of the results obtained, it appears that PDAMNA thin films show potential as useful materials for SHG. The synthesis of DAMNA monomer can be carried out without great difficulty, and the monomer is almost as effective for SHG as is MNA itself. Crystalline thin films can be grown from the monomer by vapor deposition methods, and these films polymerize readily under UV light to give crystalline PDAMNA films. All of these results are consistent with the computational modeling that was carried out on DAMNA, thereby illustrating further the usefulness of our computational modeling methods. The PDAMNA films obtained exhibit good SHG at 1064 nm, although the polymer films are not quite as effective as the monomer because of slight absorption at 532 nm. At longer wavelengths this would not necessarily be the case (we are in the process of obtaining the equipment necessary to carry out SHG measurements at 1907 nm). (16) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798.
1644 Chem. Mater., Vol. 5, No. 11, 1993 T h e films grown onto oriented Teflon substrates show both greater ordering and greater SHG t h a n those grown onto quartz. This result is very promising; to obtain films that can be useful for device fabrication, very highly ordered crystalline structures are required. At present t h e PDAMNA thin films, even grown on Teflon, are still not oriented sufficiently, nor are they of t h e required optical quality for practical devices (such as waveguides). However, by varying t h e growth conditions (i.e., maintaining slower growth rates and smaller gaps between the source a n d substrate temperatures), and by modifying t h e substratesfurther (i.e.,investigatingotherpolymersurfaces, besides Teflon a n d Kapton), we hope to obtain superiorquality PDAMNA films that exhibit less polycrystallinity a n d greater ordering. Ultimately we plan t o investigate t h e use of microgravity for thin film growth to improve the crystalline (and thereby t h e optical) quality of our
Paley et al. B. DAMNA.l0J1 This diacetylene is slightly light sensitive, so the synthesis is best carried out in the dark. To a stirring
Unless otherwise specified, all reagents were purchased from Aldrich and used without further purification. Computations were carried out using the AM1 method in version 5.04 of the MOPAC program, available from QCPE, Department of Chemistry, Indiana University, Bloomington, IN (Program No. 455).18 lH NMR spectra were run in acetone-& on an IBM Bruker 200MHz Fourier transform spectrometer. Thin films were viewed under a Ziess Axioplan universal microscope using a Hamamatsu image-processingsystem,operated by David Donovan at Marshall Space Flight Center. Profilometry measurements were carried out by Curtis Banks at Marshall using a Tekron profilometer. Waveguide measurements were carried out by Paul Ashley at the Army Missile Command. SHG measurements were performed using a 1064-nm Nd:YAG laser by Mohan Sanghadasa in the Physics Department at the University of Alabama in Huntsville. Synthetic Methods. A. N-Propargyl MNA.13 The procedure used is essentially that of Garito et al. The only difference is that the puTification step by column chromatographyis omitted; the crude product is simply recrystallized from benzene. This gives pure N-propargyl-2-methyl-4-nitroaniline (N-propargyl MNA) in about 50% yield as a yellow solid, mp 122-124 OC, which is stored refrigerated in the dark. 1H NMR (ppm): 8.06, doublet of doublets, 1H (aromatic); 7.97, d, 1H (aromatic); 6.84, d, 1H (aromatic); 6.15, s (broad), 1H (NH), 4.20, doublet of doublets, 2H (CH2); 2.75, t, 1H (acetylenic); 2.26, s, 3H (CH3).
suspension (at room temperature) of 1.0 g of N-propargyl MNA in a solution containing 0.04 g of CuC1, 10 mL 70% aqueous ethylamine, and 15 mL of ethanol, maintained under a nitrogen atmosphere, is added dropwise 0.8 g of bromopropargyl alcohol in 10 mL of ethanol over 1 h. At this point a small amount of hydroxylamine hydrochloride is added to the solution, which is stirred for an additional hour. The solvent is rotoevaporated, and the remaining solid is stirred vigorously for 30 min with 150 mL of 1% (v/v) acetone/dichloromethane to extract the crude product. The solution is filtered, and the filtrate is loaded onto a column packed with 40 g of silica gel, which has been washed with 1%acetone/dichloromethane. The column is then eluted with this same solvent; the first 200-300 mL of eluent contains impurities (which are yellow),and the next 250-300 mL contains the product (also yellow). The solvent is rotoevaporated, yielding about 0.80-0.90 g of pure DAMNA as a yellow solid, mp 160-162 "C. This compound is light sensitive and should be stored refrigerated in the dark. 1H NMR (ppm): 8.08, doublet of doublets, 1H (aromatic); 7.97, d, 1H (aromatic); 6.83, d, 1H (aromatic); 6.23, s (broad), 1H (NH); 4.50, t, 1H (OH); 4.33, d, 2H (CH2); 4.26, 9, 2H (CH2); 2.25, 8, 3H (CH3). Vapor Growth and P o l y m e r i z a t i ~ n Approximately .~ 50 mg of DAMNA was placed in the bottom of the vapor deposition apparatus. A thin crystalline film of the monomer was grown onto a circular quartz glass disk (about 1.5-2.0 cm in diameter) using a source temperature of 120OC and a substrate temperature of 30 "C, under apressure of 20 pmHg. The typical time required to grow a film was 4-5 h. The thin monomer film, which was lemon yellow in color, could then be removed from the apparatus and was ready for polymerization. Polymerization was carried out by exposing the monomer film to a 15-W 366-nm wavelength UV light for 24-36 h, resulting in a red-orange PDAMNA thin film. The degree of polymerization was obtained by measuring the weight change upon washing the film with acetone to remove the monomer. Under the specified polymerization conditions, the degree of polymerization of the PDAMNA film was determined to be 50 5 % . To grow films onto Teflon or Kapton, the disks were covered with the appropriate tape, and the above procedure was repeated. Waveguide mode analysis was used to characterize these substrates; the refractive indices of both substrates were found to be directionallydependent, thus confirmingthat they were indeed anisotropic (i.e., oriented). The PDAMNA films were then viewed under an optical microscope to examine the size and orientation of the grains, using magnifications of 125, 500, and 1250X. By means of a calibration slide, we could measure the dimensions of the grains, which ranged from 10 to 15 pm in length. Grown under the conditions specified above, the PDAMNA films consisted of networks of crystalline grains. In the cases of the quartz and Kapton substrates, the grains appeared to be randomly oriented, but in the case of the Teflon substrate, the grains showed a tendancy to be oriented along one direction. Thickness measurements were obtained using profilometry; the films were determined to be about 2.0 pm thick near the edges and only 1.0-1.5 pm thick in the center. C h a r a c t e r i z a t i ~ n . ~PDAMNA ~ J ~ ~ ~ films were characterized for SHG by means of the Kurtz technique.16 Details of this method are published e1se~here.l~First, the SHG powder efficiency of DAMNA is determined relative to that of MNA itself. When this was carried out, it was found that the diacetylene monomer powder has an efficiency 62.5 % that of MNA for SHG at 1064 nm. Next, a reference thin film of the monomer was grown using the conditions specified above. This film had approximately the same thickness as the polymer films, thus allowing for a more direct comparison of SHG efficiencies. The SHG of each PDAMNA film was then determined relative to that of the reference film. Also, blank SHG measurements were carried out on the quartz, Teflon, and Kapton substrates; as expected, none of these exhibited any detectable SHG.
(17) Debe, M. K.; Poirier, R. J.; Erikson, D. D.; Tommet, T. N.; Field, D. R.; White, K. M. Thin Solid Films 1990, 186, 257. (18) Dewar, M. J. S.; Zoebiech, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. SOC.1985,107,3902.
(19) Sanghadasa, M.; Barr, T. A.; Gregory, D. A,, manuscript in preparation. (20) Introduction to Nonlinear Optical Effects in Molecules and Polymers; Prasad, P. N., Williams, D. J., Eds.; Wiley: New York, 1991.
thin fi1ms.l' Future work in this area will concentrate on optimizing t h e PDAMNA thin films by t h e means mentioned above and on attempting t o understand t h e nature of the interaction between the substrates and t h e films. Once we have succeeded in growing films with better optical quality, we will perform more detailed SHG measurements ) and phase-matching experincluding x ( ~determination iments. Additionally, we plan to investigate the solutionshear growth technique of Thakur a n d Meyler for obtaining large-area single-crystal films.14 We also are interested in investigating other PDAs as that show promise for SHG, in particular, compounds that possess conjugation between the NLO moieties and t h e diacetylene backbone (such is not the case for DAMNA and PDAMNA). Last, we intend t o continue refining our computational modeling methods in order t o increase our predictive capabilities. On t h e basis of the results obtained so far, we are confident that PDAs show potential as both crystalline a n d polymeric materials for SHG applications.
Experimental Section
*
