As previously stated, the vast majority of efficient organic chromophores are D-π-A (usually dubbed “push-pull”) moieties, i.e. organic compounds with an electron acceptor and an electron donor group endcapping a conjugated π-electron system. An intramolecular charge-transfer (ICT) transition from the polar ground state to the first, charge-separated excited state produces effective polarization along the π-conjugated vector; the classical “two-state model” correlates the molecular hyperpolarizability β, through a proportionality constant k, with the energy gap between the two states (ΔE_g), the transition dipole moment between the two states (µ_g), and the difference in dipole moment between the two states (Δµ_g) [10]:

Evolving from relatively simple “push-pull” chemical structures such as p-nitroaniline or 4-dimethylamino-4’-nitrostilbene (DANS), which are still used as benchmark compounds to evaluate NLO properties of other molecules, molecular engineering of the chemical backbone has yielded single chromophores with second-order NLO responses, usually quoted as µβ values (µ being the molecular dipole moment), as high as 18000 x 10^-48 esu (structures 1 and 2 in Fig. 1) [11].

Fig. (1) Selected examples of organic chromophores possessing ultrahigh hyperpolarizabilities.

These compounds afforded an optimal trade-off between the NLO response and transparency issues: the presence, unavoidable in large conjugated systems, of bathochromic shifts in the absorption spectra of the compounds leads to an erosion of the transparency window in the near-IR wavelength region, which is the operational region for many photonic applications; these compounds also show excellent thermal and photochemical stabilities. An improvement in transparency and stability has been recently achieved by using multidimensional ("octopolar") charge-transfer chromophores, such as 3 [12] in Fig. (1) [13]. In octopolar NLO compounds, non-centrosymmetric molecules with no dipole moment, however, a significant increase in second order NLO performance, when compared to chromophores designed after the “traditional” push-pull approach, has not been achieved to date.

A new paradigm for the optimization of molecular hyperpolarizabilities β has been recently proposed by the use of "twisted" D-π-A chromophores [14]. These molecular modules (compound 4 in Fig. 1) consist of a highly polar, charge-separated zwitterionic ground state, and of a less polar, quinonoid-like excited state, thus µβ values exhibited negative signs. Insightful studies, performed by optical, NMR and IR spectroscopies revealed how these unconventional chromophores possess a strong negative solvatochromism, and exhibit a strong tendency for centrosymmetric aggregation in concentrated solutions. Exceptional molecular hyperpolarizabilities (up to µβ = -488000 x 10^-48 esu at 1907 nm, measured with the EFISH technique in diluted solutions) have been reported for these systems and, when embedded as hosts in guest polymers subjected to poling, these chromophores produced devices with a very large electro-optic coefficient (r₃₃) of ca. 330 pm/V [15].

The exploitation of novel strategies aiming at the efficient transfer of the optimized molecular EO properties to the macroscopic level is an active area of research. Although several strategies (polymer poling, Langmuir Blodgett films, etc.) have been briefly commented and referenced above, selected novel approaches, based on the use of weak, noncovalent interactions able to stabilize nanoscopic, acentric ordering in the material will be commented. Dipolar organic compounds with high hyperpolarizability can in principle assemble in noncentrosymmetric (acentric) structures, but electrostatic interactions favor thermodynamically antiparallel arrangement (Fig. 2), thus losing orientation of the chromophores and a strong, macroscopic NLO effect.

Fig. (2) Thermodynamically favored antiparallel assembling by electrostatic interactions of dipolar compounds.

Würthner et al. recently proposed a supramolecular approach in which a receptor is employed to orient two merocyanine dyes in the desirable head-to-tail parallel fashion through multiple hydrogen bonding. The design makes use of the organic functionalization of the dyes with complementary recognition groups, as illustrated in Fig. (3) [16].

Fig. (3) Parallel assembling of dipolar compounds can be obtained by the use of a complementary recognition groups-driven process.

Merocyanine dyes such as 6 in Fig. (4) are amongst the simplest yet efficient chromophores for NLO applications [4]. Organic functionalization of such substructures afforded compound 5, containing a complementary and suitable binding niche for the supramolecular binding of 6. Head to tail interactions in such compounds is driven by six hydrogen interactions, highlighted in red. The complex 5-6 has been characterized and its thermodynamic strength quantified by NMR spectroscopy. By monitoring the downfield shifts of the NH protons in diacyl pyridine units of 5 during the titration of 5 with 6 and vice versa, it was possible to obtain the binding constant for the formation of 1:1 complex in chloroform (K_a>10⁶M^-1) and in dioxane [K_a=(9±3)x10³M^-1]. Solubility problems in less polar solvents hampered the determination of binding constants in such solvents. The UV/Vis absortion spectra of 5 shows a maximum at λ=396 nm and at λ=379 nm for 6. The spectrum of complex 5-6 indicates additive behavior with two maxima at 380 nm and 389 nm. Another indication of the simple additive behavior, with respect to the absorption properties of the two species in solution, is that the sum of integral absorptions of 5 and 6 (which are proportional to the square of the electric transition dipole moment) matches that of the 1:1 complex 5-6. However, extinction absorption coefficients revealed the presence of mutual chromophore interactions upon complexation, and the variation of the band structure could be associated to the polarization of 6 within the hydrogen bond network, with a weak exciton coupling observed between two chromophores closely positioned in the complex.

Fig. (4) Parallel orientation in solution of merocyanine NLO dyes. Specific hydrogen bonding interactions are highlighted in red.

EOA (Electro optic absorbance) spectra, which are absorbance spectra in presence of an electric field and using polarized light, were used to further characterize the electronic structure of the complex. EOA spectra were run for compound 5, 6 and complex 5-6 with light polarized parallel to the electric field and with light polarized perpendicular to electric field. An increased absorbance was observed for the complex when the light beam was polarized parallel to the direction of the electric field, whereas a decrease in absorbance was detected when the direction was orthogonal to the electric field. Furthermore, the effect was far more pronounced in the case of the complex 5-6. The enhancement of absorbance in equimolar mixture confirmed cooperative orientation of dyes in the presence of an electric field. Thus, the judicious use of specific and complementary noncovalent interactions has demonstrated unprecedented degree of dye alignment for this type of chromophore in solution.

The utilization of octopolar NLO chromophores, possessing an inherent push-pull structure but no net dipole moment, is a promising strategy to overcome, at the molecular level, the unavoidable problem of the thermodynamically favorable aggregation tendency of dipolar molecules in concentrated, condensed or bulk phases (see also Fig. 2). Verbiest, Serrano and Heinrich have recently reported the synthesis and characterization of C₃- symmetrical octopoles, shown in Fig. (5) [17]. All the compounds showed a broad absorption around 350 nm, testifying for an efficient intramolecular charge-transfer interaction, and a low fluorescence, which facilitates determination of NLO properties using hyper-Rayleigh scattering. Compounds 7b-d, bearing long alkoxy chains, are effective mesogens exhibiting discotic (columnar or nematic) liquid crystalline behavior with quite marked differences in their temperature range intervals.

Fig. (5) Octopolar NLO chromophores functionalized with alkyl chains of varying lengths in order to induce liquid crystalline discotic behavior.

The hyperpolarizability values were found to be high for all these molecules, and in accordance with previously reported similar octopolar moieties. NLO devices were constructed, in the case of the compounds exhibiting liquid crystalline behavior, such as 7c, by heating a small amount of solid 7c between two glass slides above the melting point, followed by rapid cooling to room temperature. The sample was then placed at the heating stage for measurements of second-harmonic intensity versus temperature, and, as expected, SHG rose drastically (one order of magnitude) in the temperature range associated with liquid crystalline behavior, clearly demonstrating an acentric supramolecular order of the material. This study represents the first example of a simple, efficient NLO device built with octopolar compounds, with responses which are comparable to those of conventional poled-polymer systems.

The groups of Marks and Pagani have recently reported on the realization of heteroaromatic organic chromophores that can be self-organized from the vapor phase into intrinsically acentric, micrometer-scale films [18], (with second-order susceptibilities up to 100 pm/V). The devices are thermally stable and conveniently built in a few hours. Supramolecular acentricity is achieved without electric field poling, and enforced by head-to-tail hydrogen-bonding engineered in the molecules. Films formed by compounds 8-9 (Fig. 6) were prepared with a combination of covalent chemistry to fix the first layer of chromophores onto glass supports, and of a vapor deposition process to build up the ordered multilayer structure. Compounds 8 and 9 have been engineered in such a way that a head-to-tail orientation can be achieved via hydrogen bonding between the pyridine atom (acceptor) and the carboxylic moiety (donor). The resulting films were transparent at the NLO measurement wavelenght, and of excellent quality, with the electrooptic response of films made using chromophore 8 being two orders of magnitude higher than that of previously reported hydrogen-bonded films.

Fig. (6) Hydrogen-bonded systems form self-organized films by a vapor deposition process.

As an alternative to hydrogen bonding, halogen bonding is a rather new tool in the supramolecular basket, and can be briefly defined as any noncovalent interaction involving halogens as electrophilic sites. Resnati, Metrangolo and Cariati have recently reported organic chromophores which undergo halogen-bonding mediated assembly in order to tune their NLO response. Structures 10 and 11 (Fig. 7) are compounds composed of aromatic tetrafluorosubstituted halogen-bond donor (the terminal iodine) and aromatic halogen bond acceptor (the dimethylaniline unit), endcapping a π-electron conjugated system. They self organize in the solid state with head to tail interactions because of halogen bonding between iodine atoms and nitrogen atoms. Compound 12 was synthesized and tested as a model compound, since it hasn’t a terminal iodine atom and cannot give halogen bonding, even though it possesses a DπA structure similar to that of 10 and 11.

Fig. (7) Halogen-bonded (XB) systems.

The UV-Vis spectra of 10, 11, 12 in CHCl₃ show maxima at 382, 406, 392 nm respectively. Calculations showed that the HOMO and LUMO, both π-orbitals, are extended throughout the molecules, confirming an extended conjugation. The transition is associated with a charge-transfer interaction from a more polar ground state to a less polar excited state [19].

In agreement with the two state model mentioned above, positive values of hyperpolarizability in CHCl₃ were observed (µβ_λ for 10, 11, 12 are +124, +192, +173 x 10^-48 esu, respectively). However, measurements of hyperpolarizability in N,N'-Dimethylformamide (DMF) revealed a positive value for 12 (+70 x 10^-48 esu), while a strongly negative value for 10 and 11 (-380 and -465 x 10^-48 esu, respectively). DMF is an halogen bond acceptor moiety, and can therefore compete with the NMe₂ unit. The UV-Vis spectra in DMF of the three compounds were superimposable to those in CHCl₃, showing that the nature of HOMO-LUMO transition doesn’t change by changing the solvent. Measurements of hyperpolarizability in CHCl₃ in presence of Lewis bases capable of interact with halogen bonding as DMF, piperidine, and quinuclidine (in large excess with respect to compound 11) gave values of -103, -118, -487 x 10^-48 esu respectively. The stronger the halogen-bonding acceptor Lewis base, the more negative hyperpolarizability becomes. In agreement with the computational data, the experimental data show how the electrooptic response of these relatively simple push-pull compounds can be tuned, and its sign inverted, by using of complementary molecules (bases) capable of interacting with the substrate via halogen bonds. The hyperpolarizability sign inversion can be explained by the simultaneous presence of two competitive electron-donor sites, acting either intermolecularly or intramolecularly.

Since second harmonic generation (SHG) is only detectable when the bulk medium is non-centrosymmetric; the breaking of inversion symmetry, unavoidable in chiral systems, ensures nonzero second-order nonlinear response. A fully-implemented realization of this concept can be found in the work of Verbiest, Katz et al. [20]. The helicene used (a chiral organic molecule) has been studied both as a single enantiomer and as a racemic mixture (Fig. 8A). Only the enantiomerically-pure form of the compound is able to organize into long fibers, visible under a simple, optical microscope; these fibers are composed as columnar stacks of molecules, as briefly indicated in Fig. (8B), stabilized by noncovalent interactions. This chiral, ordered superstructuring has been confirmed also in solution of apolar solvents, by the use of CD spectroscopy. For the second-harmonic generation studies, films were prepared using the Langmuir-Blodgett technique, revealing excellent optical qualities. From a comparison between the result obtained with racemic and nonracemic films, the nonlinearity seems to arise from the chiral tensor components; furthermore, although the chemical design does not fulfill an optimized "push-pull" structure, the NLO response is high (ca. 50 pm/V). This data suggests that further improvements, with these and other chiral molecular modules, can be achieved through improved molecular engineering and chiral nanostructuring.

Fig. (8) (A) Structure of the helicene molecule and its 3D rendering. (B) 3D rendering of the supramolecular fibers made by the assembly of the helicene molecules (long alkyl chains have been omitted for clarity).