Charge-transfer molecules, nonlinearity

Figure 2. Origin of the nonlinearity of charge-transfer molecules for a two-level-like disubstituted aromatic molecule. The donor (D) and acceptor (A) cooperate to distort the linear response (dotted lines) of a nonsubstituted benzene ring (below).

The summation runs over repeated indices, /r, is the i-th component of the induced electric dipole moment and , are components of the applied electro-magnetic field. The coefficients aij, Pijic and Yijki are components of the linear polarizability, the first hyperpolarizability, and the second hyperpolarizability tensor, respectively. The first term on the right hand side of eq. (12) describes the linear response of the incident electric field, whereas the other terms describe the nonhnear response. The ft tensor is responsible for second order nonlinear optical effects such as second harmonic generation (SHG, frequency AotAAin, frequency mixing, optical rectification and the electro-optic effect. The ft tensor vanishes in a centrosymmetric envirorunent, so that most second-order nonlinear optical materials that have been studied so far consists of non-centrosyrmnetric, one-dimensional charge-transfer molecules. At the macroscopic level, observation of the nonlinear optical susceptibility requires that the molecular non-symmetry is preserved over the physical dimensions of the bulk stmcture. 

Table I lists a variety of organic nonlinear materials which have appeared in the literature their relative powder efficiencies, absorption cutoffs and /3 values (if available) are also provided. These materials are "typical" only in that they represent results from the few classes of organic compounds investigated to date, yet they are instructive in that one learns which molecular properties may be important. A few caveats are in order to avoid misinterpretation of the data in Table I. Except for compound 10 (19) all the powder efficiency and cutoff data are from our own measurements. Powder measurements were performed on ungraded samples using the Nd YAG output at 1.06/t as fundamental since powder efficiency is a function of particle size distribution and a variety of other factors (3) these values are only semiquantitative. The cutoff values are the wavelengths for which 10-4M solutions in ethanol (unless otherwise indicated) have no absorbance. The cutoff values will be similar to those found in crystal state except where intermolecular charge transfer is important in the crystal or the molecule is solvatochromic, this latter effect being quite common for cyanine dyes such as...

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

When located at opposite ends (or at conjugated positions) in a molecular system, a donor and an acceptor do more than simply add up their separate effects. A cooperative phenomenon shows up, involving the entire disubstituted molecule, known as charge transfer (C.T.). Such compounds are colored (from pale yellow to red, absorption from 3,000 to 5,000 A) and show high U.V. absorption oscillator strength. "Figure 2 helps understand the enhancement of optical nonlinearity in such a system. 

In the case of MAP, the concept of chirality was used so as to prevent centrosymmetry a chiral molecule cannot be superimposed on its image by a mirror or center of symmetry so that a crystal made only of left or right-handed molecules can accomodate neither of these symmetry elements. This use of the chirality concept ensures exclusion of a centrosymmetric structure. However as we shall see in the following, the departure of the actual structure from centrosymmetry may be only weak, resulting in limited nonlinear efficiencies. A prerequisite to the introduction of a chiral substituent in a molecule is that its location should avoid interfering with the charge-transfer process. 

Only little has been reported on second-order hyperpolarizabilities yin two-di-mensionally conjugated molecules. Planar systems as e.g. phthalocyanines have been studied for two photon absorption which is proportional to the imaginary part of the nonlinearity y. For planar molecules with a three-fold symmetry, the importance of charge transfer from the periphery to the center of the molecule in order to realize large nonlinearities ywas reported [65]. Off-resonant DFWM experiments revealed promising third-order nonlinearities in two-dimensional phenylethynyl substituted benzene derivatives [66]. Recently, the advantage of two-dimensional conjugation to increase the values of the first-order hyperpolarizability p has also been pointed out [67-69]. 

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