Strong Enhancement ofNonlinear Optical PropertiesThrough SupramolecularChiralityThierry Verbiest,* Sven Van Elshocht, Martti Kauranen,Louis Hellemans, Johan Snauwaert, Colin Nuckolls,Thomas J. Katz, Andre´ PersoonsA new approach to second-order nonlinear optical (NLO) materials is reported,in which chirality and supramolecular organization play key roles. Langmuir-Blodgett films of a chiral helicene are composed of supramolecular arrays of themolecules. The chiral supramolecular organization makes the second-order NLOsusceptibility about 30 times larger for the nonracemic material than for theracemic material with the same chemical structure. The susceptibility of thenonracemic films is a respectable 50 picometers per volt, even though thehelicene structure lacks features commonly associated with high nonlinearity.Susceptibility components that are allowed only by chirality dominate thesecond-order NLO response.Second-order NLO effects are usually ob-served only from noncentrosymmetric mate-rials (1). Commonly, such materials are con-structed by incorporating donor-acceptor–substituted molecules that have a nonvanish-ing molecular hyperpolarizability (2) intononcentrosymmetric structures such as poledpolymer films, Langmuir-Blodgett (LB)films, self-assembled films, or crystals (3, 4).The nonlinearity of such materials has beenimproved by optimizing the microscopic re-sponse of the molecules (5) or by improvingtheir alignment or orientation in the macro-scopic structure (6).Another way to achieve noncentrosymme-try is to use chiral molecules. Such moleculesare necessarily noncentrosymmetric, andtheir second-order NLO response is thereforenonzero (7). Even such highly symmetricmacroscopic assemblies as isotropic solutionsof a single enantiomer (a single mirror-imageform) of a chiral molecule are noncentrosym-metric and can be used for second-order non-linear optics, as shown by sum-frequencygeneration in sugar solutions (8, 9). The in-trinsic value of the nonlinear susceptibility ofsuch materials can be quite high (;0.4 pm/V). However, the process is not phase match-able, and its overall efficiency is thereforelow. A more efficient approach has been touse chirality indirectly, to ensure molecularcrystallization in a noncentrosymmetricgroup (10). Recently, nonlinear optics hasalso been used as a sensitive tool to studychiral surfaces (11, 12).We show that high nonlinearity can beachieved by assembling the molecules of anenantiomerically pure helicene (13) into su-pramolecular arrays. The NLO response oftheir films is much higher than that of filmsof the corresponding racemic (50/50) mixtureof the enantiomers, even though the constit-uent molecules in both films have the samechemical structure.The molecules we studied were those of thetetrasubstituted helicenebisquinone shown inFig. 1 (13, 14). In bulk samples, the nonrace-mic, but not the racemic, form of the materialspontaneously organizes (15) into long fibersclearly visible under an optical microscope.These fibers comprise columnar stacks of heli-cene molecules (15). Similar columnar stacksself-assemble in appropriate solvents, such asn-dodecane, when the concentrations are great-er than ;1 mM, and when they assemble, thecircular dichroisms (CD) of the solutions in-crease significantly (13).We prepared LB films (16) of the heli-cene by spreading a dilute chloroform solu-tion (2 3 10–4M) onto the pure water sub-phase of an LB trough. After the solvent hadbeen evaporated at 20°C, the films wereslowly compressed to a surface pressure of 20mN/m. After stabilizing for 30 min, the filmswere deposited by horizontal dipping ontohydrophobic glass [for second-harmonic gen-eration (SHG) measurements], fused quartz[for ultraviolet (UV)–visible absorption andatomic force microscopy (AFM) measure-ments], or silicon (for AFM measurements).The optical quality of the films was excellent.Although 60 is the largest number of layersdeposited to date, there is no indication thatthe quality of films with more layers wouldbe lower. Optical microscopy detected nofibers or other macroscopic features in thefilms. This means that in the LB films of eventhe nonracemic material, any supramolecularorganization extends only to submicrometerlengths.The samples were irradiated at a 45° angleof incidence with a fundamental beam from aNd–yttrium-aluminum-garnet (Nd:YAG) la-ser (1064 nm, 50 Hz, 8 ns), and the SHGsignals (532 nm) were detected in the trans-mitted direction. Half- and quarter wave-plates were used to control the polarization ofthe irradiating beam, and the second-harmon-ic light was resolved into p- (in the plane ofincidence) and s- (out of the plane of inci-dence) polarized components.The SHG signals measured arise from thequadratic response of the material to the fun-damental beam. This response is representedby the NLO polarization (1)Pi(2v) 5Oj,kxijkEj(v)Ek(v)where ijk refer to the cartesian coordinates,Ej(v) and Ek(v) are components of the elec-tric-field amplitude at the fundamental fre-quency v, Pi(2v) is a component of the non-linear source polarization at the second-har-monic frequency 2v, and xijkis a componentof the second-order susceptibility tensor. Forsufficiently thin samples, the polarizationleads to the amplitude of the second-harmon-ic field E(2v) growing linearly with thickness(1). Consequently, the intensity of the sec-ond-harmonic field, which is proportional tothe square of the amplitude, should increasequadratically with both the thickness of thefilm and the value of the susceptibility.The films of the nonracemic helicene gen-erated strong SHG signals whose intensityincreased quadratically as a function of thenumber of deposited layers (Fig. 2A), whichconfirms the good structural and optical qual-ity of the films. The strongest SHG signalfrom a one-layer nonracemic film was ap-proximately 1000 times as intense as thatfrom a similar racemic film, corresponding toa ;30-fold enhancement in the value of thesusceptibility. This enhancement is extraordi-nary, because the individual molecules inboth films have the same chemical structures.For the nonracemic sample, the SHG sig-nal was strongest when the incident beamwas p-polarized and the SHG beam was s-polarized (p-in–s-out signal), whereas, for theracemic sample, the signal was strongestwhen both beams were p-polarized (p-in–p-out signal). For isotropic surfaces and thinfilms (17), the p-in–s-out signal is due to thecomponents of the second-order susceptibili-T.