J . Phys. Chem. 1986,90, 5703-5706
5703
Second-Harmonic Generation in Transition-Metal-Organic Compounds C. C. Frazier,* M. A. Harvey, M. P. Cockerham, H. M. Hand, Martin Marietta Laboratories, Baltimore, Maryland 21 227
E . A. Chauchard, and Chi H. Lee Electrical Engineering Department, University of Maryland, College Park, Maryland 20742 (Received: February 14, 1986; In Final Form: May 2, 1986)
The second-harmonicgeneration efficiencies of over 60 transition-metal-organic compounds in powder form were measured, using 1.06-pm light from a Nd:YAG laser. Most of the studied compounds were either group VI metal carbonyl arene, pyridyl, or chiral phosphine complexes. Four of the complexes doubled the laser fundamental as well as or better than ammonium dihydrogen phosphate (ADP). Our study shows that the same molecular features (e.g., conjugation and low-lying spectroscopic charge transfers) that contribute to second-order optical nonlinearity in organic compounds also enhance second-order effects in transition-metal-organic compounds.
Introduction Numerous studies of second-harmonic generation (SHG) in inorganic crystals and powders have been regularly reported for more than 20 years.'-5 Measurements of the SHG efficiencies of organic compounds have been reported sporadically for nearly as long, but systematic studies of organic materials and comprehensive theoretical explanations of their nonlinear behavior have only been forthcoming within the past 10 years.611 As data and theoretical understanding of inorganics and organics have accumulated, a corresponding body of information has not developed for the nonlinear optical behavior of transition-metalorganic compounds. As noted below, many of the concepts developed for organic molecules can also be applied to metal-containing organic systems and can be used to guide the screening of these materials for enhanced nonlinear optical performance. We have begun exploring nonlinear optical properties of transition-metal-organic compounds with a study of SHG in powders from well-known categories of metal-organic compounds. Initial results are summarized and discussed in the following sections. Second-harmonic generation in crystals is a second-order nonlinear process and is influenced by molecular structure and Crystalline arrangement. For individual molecules, the parameters of interest are the tensor terms, &jk, of the second-order molecular hyperpolarizability. The crystalline symmetry and arrangement of individual molecules with respect to principal crystalline axes determine how the &jk terms will interact in the unit cell and contribute to the macroscopic second-order nonlinearity of the crystal. Zyss has discussed concepts useful in designing organic crystals with large macroscopic nonlinearities.l* Structural properties, which have been shown to generate large 0values in organic molecules, have been identified by Oudar and can be summarized as follow^:'^ conjugation, charge transfer, and acentric crystal structure. (1) Kleinman, D. A.; Ashkin, A.; Boyd, G.D. Phys. Rev.1966,145,338. (2) Jerphagnon, J.; Kurtz, S . K.Phys. Rev. B 1970, 1, 1739. (3) Adhau, R. S.; Wallace, R. W. IEEE J . Quantum Electron. 1973, OE-9. 855. (4) Maker, P. D.; Terhune, R. W.; Nisenoff, M.; Savage, C. M. Phys. Rev. Lett. 1962, 8, 21. (5) Belt, R. F.; Gashurov, G.;Liu. U. S . Laser Focus. Oct. 1985. ( 6 ) Kurtz, S. K.; Perry, T. T. J . Appl. Phys. 1968, 39, 3798. (7) Halbout, J. M.; Blit, S.;Tang, C. L. IEEE J. Quantum Electron. 1981, QE-17, 513. ( 8 ) Twieg, R.; Azema, A.; Jain, K.; Cheng, Y . Y . Chem. Phys. Lett. 1982, 92, 208. (9) Shigorin, V. D. Proc. (Tr.) P. N . Lebedeu Phys. Ins?. [Acad. Sci. USSR](Engl. Transl.) 1982, 98, 71. (10) Meredith, G. R. In Nonlinear Optical Properties of Organic and Polymeric Materials, Williams, D. J., Ed.; American Chemical Society: Washington, 1983; Chapter 2. ( 1 1) Garito, A. F.; Singer, K. D. Laser Focus (Feb. 1982), 59. (12) Zyss, J. J . Non-Cryst. Solids 1982, 47 211. (13) Oudar, J. L. J . Chem. Phys. 1977, 67 446.
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0022-3654/86/2090-5703%01.50/0 , I
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Conjugation in a highly polarizable ?r-electron system enhances
0, with /!Iincreasing nonlinearly as conjugation increases.
Charge transfer, which requires the presence of an electrondonating substituent and an electron-withdrawing substituent on an aromatic ring or other conjugated system, augments molecular nonlinearity, substantially in some cases. Acentric crystal structure allows the generation of optical phenomena arising from even-order susceptibilities x ( n ) ; these effects are only manifested in unit cells lacking a center of inversion symmetry. The above requirements for a large 0value may also be satisfied by the structural characteristics of metal-organic compounds. In particular, transition-metal complexes with pendant aromatic or polyene ligands have relevant features similar to organic compounds. The aromatic or polyene moiety of the complex provides a conjugated system with which the metal center(s) actively interacts, extending the conjugation and perturbing the *-electron system. Nonlinearity may be further enhanced by the fact that many organometallic compounds have low-energy excited states with excited-state dipole moments significantly different from their respective ground-state dipole moments. Most of these excited states involve transfer of electron density between the central metal and one or more associated ligands and have large oscillator strengths. Charge transfers of this type should provide a substantial contribution to @ in the same fashion as organic molecular charge transfer^.'^ A simple two-level model has been used to express this term for organics. Specific examples of transitionmetal-organic systems which should have large PCT terms are indicated in the Discussion section.
3e2h2
W
OCT = 2m (W - (2hw)2)(un- (hw)2)f&ex be, is the difference between excited- and ground-state dipole moments, f is the oscillator strength of transition, h w is the fundamental photon energy, and W is the energy of transition. It should also be noted that transition-metal-organic compounds have important advantages in the range and mix of nonaromatic ligands that can be attached to metals. These ligands shift the occupied and unoccupied metal d orbitals that interact with the *-electron orbitals of the conjugated ligand system, thereby providing a mechanism for fine tuning molecular hyperpolarizabilities. Results and Discussion The organometallic complexes examined in this study were evaluated as crystalline powders as described in the Experimental (14) Chemla, D.; Oudar, J. L.; Zyss, J. L'echo des Recherches (English) 1981, 47.
0 1986 American Chemical Society
5704 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Frazier et al.
TABLE i: Evaluation of SHG in Arenecarbonylchromium(0) Complexes
SHG signal
complex Cr(m-anisidine)(CO), Cr (p-Anisidine)(CO)
(ADP = 1.0) 0.3 0
Cr [(-)-a-ethylphenethyl alcohol](CO), Cr [ (S)-(+)-0-acetylmandelic acid](CO), S-(+)-0-acetylmandelic acid Cr(L-(+)-mandelic acid)(CO), L-(+)-mandelic acid Cr(D-(-)-mandelic acid)(CO), D-(-)-mandelic acid
0.01 0 0.6 0
0.17 0 0.12
Cr(N-methylaniline)(CO), Cr(S)-(+)-(2-methylbutyl)benzene(CO), Cr[N-(4-methoxybenzylidene)-4-butylaniline](CO), Cr(Nopolbenzy1 ether)(CO), Cr(L-phenylanilineethyl ester hydrochloride)(CO), L-phenylalanine ethyl ester hydrochloride Cr(2-phenyl-1-propanol)(CO), Cr(styrene)(CO), Cr( 1,2,3,4-tetrahydronaphthalene)(CO), Cr( 1,2,3,5-tetraphenylbenzene)(CO), 1,2,3,5-tetraphenylbenzene
Cr (0-toluidine)(CO), Cr(p-xylene)(CO)2nmdpp
0
1.7 0 1.3 0.6 0.02 0 1.8 0
0 0 0 0.01
Section. For second-order nonlinear optical effects to occur, the crystalline unit cell of solids must be acentric. Due to the difficulty in controlling all the molecular forces that determine intermolecular arrangements within a unit cell, chemists have found that acentricity in a crystal is best ensured by introducing chirality in the individual molecules. Chirality enables a compound to exist as a pair of nonsuperimposable mirror images (enantiomers). Since enantiomers crystallize with acentric unit cells, several of the complexes prepared for this investigation were synthesized as pure enantiomers. The principal categories of organometallic complexes studied in our preliminary effort are described below. Arene Group VI (Group 6)18Metal Carbonyl Complexes. Compounds of this type have the general formula (arene)M(CO),Ls-,, where an arene is an aromatic ring structure, e.g., benzene, to which a variety of functional groups may be attached, M is a group VI (group 6 ) (Cr, Mo, W) transition metal, L is a ligand, usually a phosphine, and n = 0, 1, 2, or 3. These metal complexes have been synthesized in large numbers and with considerable structural diversity for more than 25 years, and much is known about their physical properties, crystallography, spectroscopy, bonding, and chemistry. Although the nonlinear optical behavior of these complexes has not been previously explored, there are several reasons to expect that arene metal carbonyl complexes should exhibit significant nonlinear responses. While there may be weak, low-energy ligand field (metal-centered) transitions in these complexes, the lowenergy absorption spectra of the tricarbonyl species are dominated by intense MLCT absorptions near 3 1 OOO cm-I ( e lo4 L mol-’ cm-I), which can be expected to contribute significantly to ~ ( 2 ) . The MLCT transition for most complexes is believed to be a M arene CT with some M T* CO C T character and should involve a substantial &ex.15 The arene of the (arene)M(CO),LP3 complexes also plays a part in enhancing ~ ( 2 ) .Arenes are an aromatic system having a number of delocalized electrons which, as previously noted, enhance nonlinearity for organics. The number of ?r electrons can be varied by modifying the size of the aromatic ring, and the detailed electronic features (orbital energy levels) can be manipulated by the choice of substituents attached to the ring. All of these factors can be expected to influence the nonlinear performance of the complexes. Table I presents the second-harmonic-generation efficiency of several arene chromium carbonyl complexes. The data are normalized such that the efficiency of a 0.5-mm cell with 100-+
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(15) Geoffroy, G.L., Wrighton, M.S . Organometallic Photochemistry; Academic: New York, 1979, Chapter 2.
13
17
18
SHG Efficiencies
Figure 1. Arene chromium carbonyl complexes with second-harmonic-
generation efficiencies larger than ADP. TABLE 11: Evaluation of SHC in Pyridyl Carbonyl Group VI Metal Complexes ~
complex W(C0)s(4-acetylpyridine) W(CO)s(4-benzoylpyridine) M0(C0)~(4-benzoylpyridine) Cr(C0)s(4-benzoylpyridine) 4-benzoyl pyridine W(C0)5(4-(4-~hlorobenzoyl)pyridine) W(CO)s(3-(4-~hlorobenzoyl)pyridine) W (CO),( 3-(4-~hlorobenzoyl)pyridine)~ 4-(4-chlorobenzoyl)pyridine 3-(4-chlorobenzoyl)pyridine W (C0)s(4-cyanopyridine)
4-cyanopyridine W( C0)5(2,4-dimethylaminopyridine) W(CO),(methylnicotinate)
methylnicotinate W(CO)s(ethyl 2-methyl nicotinate) Cr(C0)4(6-methoxyquinoline)z W(CO)s[5-phenyl-2-(4-pyridyI)oxazole] Cr(C0)J 5-phenyl-2-(4-pyridyl)oxazole] S-phenyl-2-(4-pyridyl)oxazole W(CO),(pyridine)(PPh,) Mo(C0)4(pyridine)(PPh,)
SHG signal (ADP = 1.0) 0 0.2
0 0 14 1 .o
0 0 11 0 0 0
0 0 0 0
0.04 0.06 0 0.07 0 0
pm-sized ADP (ammonium dihydrogen phosphate), a commonly used laser frequency doubler, is represented as a value of 1.O. The structures of the three complexes, which performed slightly better than ADP, are shown in Figure 1. Two of the molecules are chiral and were prepared as pure enantiomers. The third, styrenetricarbonylchromium(O), is not chiral, but, based on these data, must have an acentric unit cell. The second-harmonic-generation efficiencies of the arena from which the complexes were prepared are also included in Table I when the compounds existed as solids at room temperature. The chiral acetylmandelic acid and mandelic acids demonstrate SHG, whereas the corresponding chromium complexes do not. This result may indicate that electron-withdrawing substituents near the arene diminish SHG efficiency in arene metal carbonyl complexes. Conversion of L-phenylalanine ethyl ester hydrochloride into a chromium carbonyl complex raises its S H G efficiency from 0.02 to 0.6, demonstrating that the interaction of the metal atom can play a positive role in enhancing second-order susceptibilities. However, without detailed knowledge of the crystal structures of these compounds, the data currently available are too limited to permit firm conclusions regarding these issues. Pyridyl Group VI (Group 6) Metal Carbonyl Complexes. This series of complexes has the general structure M(CO)s_nL,(pyridyl group), where M is a group VI (group 6 ) transition metal (Cr, Mo, W), L is a ligand (frequently a phosphine), n is usually 0 or 1 , and pyridyl group refers to a substituted pyridine attached to the metal through the ring nitrogen atom. These pyridyl metal carbonyl complexes also have spectroscopic features that should provide nonlinear optical response, Le., low-lying pyridyl ligand orbitals that participate in MLCT transitions. Varying the substituents on the pyridine ring controls whether the MLCT will lie above or below the lowest energy ligand field band, e.g., the lowest energy transition for the pyridine complex is ligand field in origin, whereas for the 4-formylpyridine complex this transition is charge transfer in ~ h a r a c t e r . ’ ~ None of the pyridyl ligands examined to date have been chiral. However, some of these ligands are sufficiently asymmetric that the pure organics and, in some cases, the related metal complexes
The Journal of Physical Chemistry, Vol, 90, No. 22, 1986 5705
S H G in Transition-Metal-Organic Compounds TABLE III: Evaluation of SHG in (Chiral Phosphine) Carbonyl Transition-Metal Complexes SHG signal complex (ADP = 1.0)
TABLE IV: Evaluation of SHG for Miscellaneous Transition-Metal Complexes
~
Fe (CO) 4nmdpp M~(CO)~nmdpp W(CO),nmdpp nmdpp ligand = neomenthyldiphenylphosphine
0.01
