From Molecular to Supramolecular Nonlinear ... - ACS Publications

Chapter 28

From Molecular to Supramolecular Nonlinear Optical Properties J.-M. Lehn

Downloaded by UNIV LAVAL on February 25, 2013 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch028

Collège de France, 11, Place Marcelin Berthelot, 75005 Paris and Université Louis Pasteur, 4, Rue Blaise Pascal, 67000 Strasbourg, France

The arrangement of photoactive molecular species into organized polymolecular architectures may lead to novel optical properties at the supramolecular level. Push-pull conjugated molecules have been synthesized, that contain diacetylenic and polyolefinic units. They display interesting absorption and emission properties as well as pronounced nonlinear optical (NLO) effects. Derivatives incorporating photosensitive and redox-active groups have been obtained. The observation of second harmonic generation from non-dipolar non-centrosymmetric aromatic charge transfer molecules may lead to dipole independent NLO materials. Supramolecular engineering has been performed by incorporation of push-pull molecular components into organized systems such as liquid crystals and Langmuir-Blodgett films. Organized materials may be generated by molecular recognition induced self-assembling of liquid crystalline phases or polymers and of ordered solid state structures. The molecular components involved may be designed so that the formation of the organized phases depends on interaction between complementary molecular subunits, thus leading to molecular recognition induced nonlinear optical properties at the supramolecular level.

Nonlinear optical (NLO) properties are usually considered to depend on the intrinsic features of the molecule and on the arrangement of a material. An intermediate level of complexity should also be taken into account, that of the formation of well-defined supermolecules, resulting from the association of two or more complementary components held together by a specific array of intermolecular interactions (1). Such intermolecular bonding may yield more or less pronounced NLO effects in a variety of supramolecular species (2). Thus, three levels of nonlinear optical properties may be distinguished: the molecule, the supermolecule and the material. The molecular and supramolecular levels involve respectively - intramolecular effects and structures, -

0097-6156/91/0455-0436S06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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intermolecular interactions and architectures; both aspects intervene in the material together with collective effects. Supramolecular chemistry (1), the chemistry of the intermolecular bond, involves both organization and function, depending in particular on molecular recognition events. It extends towards the design of molecular devices that operate on photons, electrons or ions and could form through self-assembling of their components (1,2). We have for instance studied photonic devices performing light conversion and photoinduced charge separation (1,4), electronic devices acting as molecular wires (1,5) and ionic devices for ion translocation through membranes by means of mobile carriers (1,6) or channel type structures (1,1,7). It has been pointed out earlier that supramolecular species of various types (organometallic and coordination compounds, supermolecules involving hydrogen bonding, charge transfer interactions, etc) may present specific N L O features (2). I herewith briefly describe the results obtained on the N L O properties of species related to our work on molecular devices and point out the role of processes directing supramolecular arrangements, in particular by recognition induced self-organization. More detailed descriptions and references may be found in the papers cited. The design of molecules, supermolecules and materials presenting N L O activity involves molecular and supramolecular engineering. At the molecular level, a high polarisability, described by large quadratic β and cubic γ hyperpolarisability coefficients, is sought for. At the supramolecular level, it is necessary to achieve a high degree of organization, such as found in molecular layers, films, liquid crystals, solid state, which may be induced by molecular recognition and inclusion complex formation. Both features are required for materials to display pronounced second order N L O effects; the structure must also be non-centrosymmetric, due either to the molecular components or to their arrangement in condensed phase. On the other hand, centrosymmetric species present third order but no second order N L O properties. In addition, bulk characteristics such as stability, preparation and processing, mechanical features will determine the practical usefulness of a given material (for general presentations see for instance réf. &-1Ω as well as the book cited in ref. 2). Push-Pull Polyconjugated Molecules Modification and functionalisation of natural polyenes, the carotenoids, is an efficient way for the molecular engineering of polyenic chains. Terminal bis-pyridinium carotenoids, termed caroviologens, represent an approach to electron conducting molecular wires (5). Fitting polyconjugated chains with an electron donor group on one end and an electron acceptor on the other end yields push-pull systems of type 1 that may be considered as polarized, unidirectional (oriented) molecular wires and also possess marked N L O properties. Indeed such compounds containing long conjugated chains, are expected to be highly polarizable, to possess high lying π orbitals and low lying π * orbitals, to display solvatochromism and charge separation excited states.

In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES


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Two series 2 and 3 of donor-acceptor polyconjugated molecules bearing either a benzodithia (2) or a dimethylaminophenyl (3) group as donor and a variety of acceptor groups A, have been synthesized and their photophysical properties have been studied

01).

The electronic absorption, fluorescence and excitation spectra of these compounds indicate the presence of an internal charge transfer (ICT) excited state giving rise to a fluorescence band that displays strong solvatochromism. Both the emission wavelengths and the Stokes shifts increase with solvent polarity, in agreement with a large increase in dipole moment in the excited state. As the chain length increases the


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absorption undergoes both bathochromic and hyperchromic changes. The emission also shows a bathochromic shift. These effects level off with increasing length of the conjugation path, insertion of the triple bond leading to weaker conjugation. They indicate an increase in délocalisation with chain length and point to a long distance internal charge transfer in the excited state. Thus, the molecules of series 2 and 3 behave as highly polarizable molecular wires. A detailed investigation of the N L O properties of molecules of series 2 and 3 has been performed in the powder state (12) as well as in solution by the electric-fieldinduced-second-harmonic (EFISH) generation method (13-15). It has allowed the analysis of the influence of the molecular parameters on the N L O features (12-15). There is a sharp increase in quadratic hyperpolarisability β with the number of double bonds n. The variation of μβ(0) as a function of η is not exponential, the dependence being about μβ(0) - η for the molecules of series 2 (A=CHO). Similar trends hold for series 3 (A=CHO). These results are in agreement with earlier experimental data (giving a nearly quadratic dependence) (16) and with calculations yielding a dependence in η for a series of type 2 (A=CHO) (17) and in η or η for compounds of type 3 ( A = N 0 or CHO) (18)· For the longest compounds ever measured (eight conjugated double bonds), the static μβ(0) values are exceptionnally large, about 50 times that of 4-nitroaniline, and the effect does still not level off. The introduction of a triple bond causes a hypsochromic shift and a drop in μβ(0) with respect to the expected value, indicating a reduction in the electronic transmission ability. The following sequences of donor and acceptor strengths have been obtained: 2


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Front Molecular to Supramolecular Properties

2

2

4

6

2

5

7

2

Donor :

Acceptor :

The difference in donating (or accepting) power between donor (or acceptor) groups falls off as the chain length increases. The efficiency of the donor —» acceptor interaction, for different donor-acceptor pairs, appears to level out with the lengthening of the conjugation path (saturation effect). Preliminary data from measurements of symmetrical, polyene α,ω-dialdehydes, have yielded large third order hyperpolarisabilities γ(0), that increase also markedly with chain length (Puccetti, G.; Ledoux, I.; Zyss, J., unpublished data).


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Switchable Molecular Wires and NLO effects


The insertion of a photosensitive group or of a redox active unit into the push-pull system 1 yields switchable molecular wires and push-pull molecules that contain a photo-switch or a redox switch S, as represented in 4. Compounds of such type containing for instance electroactive ferrocene groups and photosensitive metal complexes, have been synthesized. Some of them are shown in series 5 (Marczinke, B. ; Przibilla, K.J.; Lehn, J.-M., unpublished data).

5

(CO) ReCI 3

The electronic and optical properties of such compounds may be expected to respond to external electrical or light stimuli. The N L O properties of a ferrocene containing molecule have been reported (19).

Second-Harmonic Generation from Non-dipolar Non-centrosymmetric Aromatic Charge-Transfer Molecules A study of second-harmonic generation (SHG) from powders of l,3,5-triamino-2,4,6trinitro benzene type compounds showed that the parent molecule 6 had an activity of about 3 χ urea, whereas N-substituted derivatives were inactive (Ledoux, L; Zyss, J.; Lehn, J.-M.; Siegel, J.; Brienne, M.-J., unpublished data). Compound 6 may be considered as a generalized 4-nitroaniline of symmetry. It has a non-dipolar ground state and is non-centrosymmetric, whereas 4-nitroaniline is


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Properties

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dipolar. The observation of SHG for 6 is compatible with a crystal structure departing from a hexagonal, centrosymmetric lattice. The absence of a dipole moment due to the symmetric structure of 6 precludes any molecular nonlinear contribution of vectorial nature. The origin of the SHG will be addressed elsewhere.. The observation of SHG from a molecule deprived of vectorial features, opens new perspectives in molecular engineering towards quadratic N L O properties. Non-dipolar non-centrosymmetric molecular moieties could serve as building block for novel types of N L O materials in which the organization is not influenced by dipole-dipole interactions.


Supramolecular Engineering of Organized NLO Materials The realization of efficient S H G materials involves performing supramolecular engineering on the compounds presenting pronounced N L O properties obtained by molecular engineering. This may be achieved by introducing the molecules into organized phases such as molecular films, liquid crystals or solid state structures, by suitable derivatization or mixing with host substances. Well organized Langmuir-Blodgett (LB) films have been obtained from mixtures of a push-pull carotenoid and ω-tricosenoic acid as shown in 7. These mixed films exhibit a very good cohesion, with an area of about 25 Â per carotenoid molecule. They can easily be transferred onto solid substrates. Examination by UV-visible linear dichroism measurements confirms that the carotenoid chains are oriented perpendicularly to the surface of the substrate in card-packed aggregates, in which the polyenic chains interact via excitonic coupling, as indicated by the large hypsochromic shift of the π-π* transition (20). 2


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The same push-pull carotenoid can also be introduced into LB multilayers built from 1/1 mixtures with an amphiphilic cyclodextrin. The polyenic chains are again perpendicular to the substrate and some carotenoid molecules do not aggregate, but are isolated by the host substance (21). Liquid crystals may be obtained from suitably modified molecules presenting N L O properties. Thus, various push-pull stilbene 8 and diacetylene 9 derivatives bearing long chains R have been shown to display nematic and smectic mesophases (22).

Preliminary measurements on some compounds of type 8 and 9 gave SHG effects similar to those of urea and quartz respectively. Mesophases of columnar nature are formed by derivatives of the symmetrical triamino-trinitro benzene unit 6 of the type 10 (R = C j 2 H25 for instance) Brienne, M.-J.; Lehn, J.-M., unpublished data). OR

Such L B films or liquid crystals as well as polymeric structures may yield efficient N L O materials and also provide protection of the S H G active species and processability.


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Molecular Recognition Induced Self-organization and NLO Effects Molecular recognition processes rest on selective intermolecular interactions between complementary components. They may affect the properties of the system at the molecular, the supramolecular and the material levels by respectively 1) perturbing the electronic and optical properties of the components; 2) generating supramolecular species; 3) inducing organization in condensed phase. A l l three effects are of importance with respect to the N L O properties of the material and its constituents. The binding of two complementary components, respectively of donor D and acceptor A type, has two consequences: 1) it yields a push-pull supramolecular species, in which, 2) the interaction may be expected to modify the initial D, A features of the isolated units. Such a process is represented in scheme 11. Both factors should influence the SHG activity, leading to recognition dependent N L O properties or, conversely, to the expression of the recognition event through N L O features. Scheme 12 illustrates the case of hydrogen bonding association; other interactions (electrostatic, D-A,...) may also be envisaged.

11

Molecular recognition directed self-assembling of organized phases has been described recently in the formation 1) of mesophases by association of complementary molecular component, as in 13 (23); 2) of supramolecular liquid crystalline polymers of type 14 (24) and 3) of ordered solid state structures, such as that represented by 15 (25). In all these cases, the incorporation of N L O active groups may be expected to produce materials whose SHG properties would depend on molecular recognition induced self-organization.

Conclusion The results presented above illustrate how combining the design of N L O active molecules with the manipulation of selective intermolecular interactions may produce novel N L O materials. Bringing together two basic features of supramolecular chemistry -molecular recognition and self-organization- with the optical properties of the components, opens ways towards the design of supramolecular photonic devices.



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Literature cited

1. Lehn, J.-M. Angew. Chem. Int. Ed. Engl. 1988, 27, 89. 2. Lehn, J.-M. In Nonlinear Optical Properties of Organic Molecules and Crys Chemla, D.S.; Zyss, J., Eds.; Academic: New-York, 1987; Vol. 1, 215. 3. Lehn, J.-M. Angew. Chem. Int. Ed. Engl. 1990, 29, in press. 4. Lehn, J.-M. In Supramolecular Photochemistry; Balzani, V., Ed.; Reidel: Dordrecht, 1987; p. 29. 5. Arrhenius, T.S.; Blanchard-Desce, M.; Dvolaitzky, M.; Lehn, J.-M.; Malthête, J. Proc. Natl. Acad. Sci USA 1986, 83, 5355. 6.

Lehn, J.-M. In Physical Chemistry of Transmembrane Ion Motions; Spach, G., Ed.; Elsevier: Amsterdam, 1983; p. 181.

7. Jullien, L.; Lehn, J.-M. Tetrahedron Lett. 1988, 3803. 8. Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D.J., Ed.; A.C.S. Symp. Ser. 233, Washington, 1983.

9. Williams, D.J. Angew. Chem. Int. Ed. Engl. 1984, 23, 690. 10. Garito, A.F.; Teng, C.C.; Wong, K.Y.; Zammani'Khamiri, O. Mol. Cryst. Liq. Cryst. 1984, 106, 219.



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11. Slama-Schwok, Α.; Blanchard-Desce, M.; Lehn, J.-M. J. Phys. Chem. 1990, 94, 3894. 12. Blanchard-Desce, M.; Ledoux, I.; Lehn, J.-M.; Malthête, J.; Zyss, J.J.Chem. Soc., Chem. Commun. 1988, 737. 13. Blanchard-Desce, M.; Ledoux, I.; Lehn, J.-M.; Zyss, J. In Organic Materials for Nonlinear Optics, Hann, R.A.; Bloor, D., Eds; Royal Society of Chemistry: London, 1989; special publication N° 69, p. 170. 14. Barzoukas, M.; Blanchard-Desce, M.; Josse, D.; Lehn, J.-M.; Zyss, J. Inst. Phys. Conf. Ser. N° 103: Section 2.6, 1989, 239. 15. Barzoukas, M., Blanchard-Desce, M.; Josse, D.; Lehn, J.-M.; Zyss, J. Chem. Phys. 1989, 133, 323. 16. Dulcic, Α.; Flytzanis, C.; Tang, C.L.; Pépin, D.; Fétizon, M.; Hoppilliard, Y. J. Chem. Phys. 1981, 74, 1559. 17. Toussaint, J.M.; Meyers, F.; Brédas, J.L. In Conjugated Polymeric Materials. Opportunities in Elecronics, Optoelectronics and Molecular Electronics, B J.L.; Chance, R.R., Eds; NATO-ARW Series E; Kluwer: Dordrecht, 1990; Vol. 182, p. 207. 18. Morley, J.O.; Docherty, V. J.; Pugh, D. J. Chem. Soc., Perkin Trans II, 1987, 1351. 19. Green, M.L.H.; Marder, S.R.; Thompson, M.E.; Bandy, J.A.; Bloor, D.; Kolinsky, P.V.; Jones, R.J. Nature 1987, 330, 360. 20. Palacin, S.; Blanchard-Desce, M.; Lehn, J.-M.; Barraud, A. Thin Solid Films 1989, 178, 387. 21. Palacin, S. Thin Solid Films 1989, 178, 327.I 22. Fouquey, C.; Lehn, J.-M.; Malthête, J. J. Chem. Soc., Chem Commun. 1987, 1424. 23. Brienne, M.-J.; Gabard, J.; Lehn, J.M.; Stibor, I. J. Chem. Soc., Chem. Commun. 1989, 1868. 24. Fouquey, C.; Lehn, J.-M.; Levelut, A.-M. Adv. Mater. 1990, 2, 254. 25. Lehn, J.-M.; Mascal, M.; De Cian, Α.; Fischer, J. J. Chem. Soc., Chem. Commun. 1990, 479. RECEIVED August 2, 1990


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