The molecular properties of zwitterionic, non-linear optical molecules and their evolution with molecular environment.
The concepts involved with the experimental techniques of Electric Field Induced Second Harmonic generation (EFISH), dipole moment measurement, and solvatochromism, are introduced with particular application to the properties of organic molecules. A number of organic chromophores are introduced, but emphasis is applied to the study of tetracyanoquinodimethane (TCNQ) derivatives which are expected to possess large dipole moments (µ) and large hyperpolarisabilities (β). Furthermore, the behaviour of and with respect to the local environment of the molecule is discussed where a novel evolution is predicted from theoretical calculations. The measurement of µ, is discussed with particular reference to the geometry of the local field factor and the size of the molecule. Consequently, the choice of local field is found to be critical when dipole moments are large, as is the case with the TCNQ derivatives. The EFISH experimental technique is introduced where the calibration of the experiment is discussed. The measurement of the molecular figure of merit, µβ at 1.064 µm and 1.907 µm in chloroform and dichloromethane is presented for the TCNQ derivatives, again paying attention to the geometry of the local field factor. µβ is found to be moderate for most of the compounds, but β is found to be unexpectedly small. This is partly due to the fact that µ is large. A novel evolution of the transition frequency with solvent polarity is found for three of the chromophores under study, where the solvatochromic shift reverses. Solvatochromism experiments are conducted with binary solvent mixtures to ascertain the position of the cyanine limit (β=0) with respect to reaction field. It is found that the materials reside close to the cyanine limit in chloroform and dichloromethane. This is attributed as a reason for the low β measurements. Comparisons of µ and β are also made with Sum-Over-State calculations. A better correlation is found for ellipsoidal local field factors.