9320 J. zyxwvu Am. Chem. zyxwvut SOC. 1994,116, 932G9323 Investigations of the Hyperpolarizability in Organic Molecules from Dipolar to Octopolar Systems zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM T. Verbiest,* K. zyxwvuts Clays,*a C. Samyn,* J. Wolff? D. Reinhoudt,' and A. Persoonst Contribution from the Chemistry Department, University of Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium, Chemistry Department, University of Heidelberg, Im Neuenheimer Feld 270, D- W-6900 Heidelberg, Germany, and Chemistry Department, University of Twente, zy P.O. Box 21 7, 7500 AE Enschede. The Netherlands Received January 18. 1994' Abstract: In this paper wereport the first systematicnonlinearoptical study of molecules with octopolarchargedistribution. First hyperpolarizabilities were determined by the Hyper-Rayleigh Scattering technique and depolarization of the harmonic scattered light was measured and linked to the symmetry of the scatterers. It is shown experimentally that octopolar molecules have nonlinear optical properties that compare favorably to those of their dipolar counterparts. The hyperpolarizability of octopolar molecules is shown to be solvent independent, in contrast to the hyperpolarizability of dipolar systems. Introduction The classicalmolecular requirementsfor second-order nonlinear optical (NLO) effects have led to the development of strongly elongated and conjugated molecules,asymmetrizedby interacting donor and acceptor groups. Such compounds are (highly) anisotropic dipolar structures like p-nitroaniline derivatives, substituted stilbenes, and push-pull polyenes. Although they often exhibit extremely large hyperpolarizabilities, some major drawbacks are associated with their dipolar character, e.g. a high tendency toward unfavorable aggregation, difficult non-cen- trosymmetric crystallization, and small off-diagonal tensor components. Recently, it was recognized that octopolar molecules can circumvent the aforementioned disadvantages, since these non- polar molecules combine excellent (second-order) NLO char- acteristics with a strict cancellation of all vectorial properties.' Hence, it seems worthwhile to start the search for new octopolar molecules and materials where the drawbacksof dipolar materials are absent. Since they cannot be oriented in an electric field, the hyperpolarizabilitycannot be measured by the traditional electric field induced second-harmonic generation (EFISHG) method.* As a consequence, octopolar molecules have not yet been thoroughly investigated experimentally. However, with the recently developed Hyper-Rayleigh Scattering (HRS) technique, the hyperpolarizability of nonpolar compounds is a c c e ~ s i b l e . ~ ~ ~ In this paper we report the first systematic study of the second- order NLO properties of a series of octopolar molecules with the HRS technique. Hyperpolarizabilities and absorption charac- teristics of octopoles are compared with those of their dipolar counterparts and the influence of solvent on the octopolar hyperpolarizability is discussed. Theoretical Treatment In order to discuss the properties of octopolar molecules, some knowledge about the irreducible components of tensors and the concept of multipolar groups is required. These issues were t University of Leuven. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA f University of Twente. *Abstract published in Advunce ACS Absrrucrs, September 15, 1994. (1) Zyss, J. Nonlinear Opt. 1991, I, 3-18. (2) Williams, D. J. Angew. Chem., Inr. Ed. Engl. 1984, 23, 690-703. (3) Clays, K.; Persoons, A. Phys. Rev. Leu. 1991, 66, 2980-2983. (4) Verbiest, T.; Clays, K.; Persoons, A,; Meyers, F.; Brtdas, J. L. Opt. University of Heidelberg. Lert. 1993, 18, 525-527. already extensively discussed by Jerphagnon and Zyss and we will only give a brief overview of these con~epts.5-~ Any tensor T(n) of rank n can be decomposed in a sum of irreducible tensorial components that subtend a weightJ ir- reducible representation of the three-dimensional rotation group SO3 and can be written as: where the superscript zyxw y distinguishesbetween linearly dependent components of rank J. This equation is called the reduction spectrum of the tensor T(n). In the special case of a fully symmetric tensor the reduction spectrum involves only one irreducible tensor of weight Jif n + J i s even, and none otherwise. This means that the hyperpolarizability @, which is a fully symmetric third-rank tensor under Kleinmann symmetry? will only have two irreducible components of weight J = 1 (vector or dipolar part) and weight J = 3 (septor or octopolar part). Each irreducible component has 2 J + 1 independent coefficients. Recently, Zyss has shown that for molecules belonging to a multipolar symmetry group of order J, all irreducible tensorial componentsof order strictly lower than Jcancel due to symmetry requirement^.^ The multipolar order of a group can be readily obtained from a simple manipulation of the character table of the symmetrygroup. Sinceoctopolarmolecules belong to anoctopolar space group ( J = 3), all dipolar quantities ( J = 1) vanish, and only octopolar @-contributions remain. Purely octopolar groups are for example the orthorhombic Dz, the hexagonal D3h, and the cubic T symmetry groups. Since molecules belonging to octopolar groups have a zero dipole moment, their first hyperpolarizability cannot be deter- mined with the EFISHG technique, since this technique relies upon the orientationof the molecules in an electricfield. However, the recently developed Hyper-Rayleigh Scattering (HRS) tech- nique provides a sensitive tool for the determination of @ of nonpolar molecules in solution. When an isotropicsample is illuminated by intense laser pulses of intensity I(w), the incoherent second-harmonicscattered light I(20) is called HRS. On a molecular scale HRS depends on the (5) Jerphagnon, J.; Chemla, D.; Bonneville, R. Adv. Phys. 1978,27,609- (6) Jerphagnon, J. Phys. Rev. B 1970, 2, 1091-1098. (7) Zyss, J. J. Chem. Phys. 1993, 98, 6583-6599. (8) Prasad, P. N.; Williams, D. J. Nonlinear Oprical EJfecrs in Molecules 650. und Polymers; John Wiley & Sons, Inc.: New York, 1991; p 26. OOO2-7863/94/1516-9320$04.50/0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0 1994 American Chemical Society