Solid state molecules second harmonic generation

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

Second harmonic generation has been recognized as a powerful probe to study the electronic states at surfaces and interfaces [16]. Under the electric dipole approximation, second-order nonlinear processes are forbidden in centrosymmetric systems. This principle makes the phenomena surface-specific in many cases. Indeed, the capability of SHG spectroscopy to explore surface electronic states has been demonstrated on various systems, dye molecules at solid/liquid interfaces [17], organic molecules at liquid/air interfaces [18], semiconductor surface states [19], organic molecules at metal surfaces [20], and so on. 

Gangopadhyay, P. Radhakrishnan, T.P. Helieal super-stractures of a C2-symmetric molecule exhibiting strong second harmonic generation in the solid state. Angew. Chem. Int. Ed. 2001. 40 (13). 2451. 

The name characterizes the state of the matter which is hybrid with properties in between solid crystals and hquid state. This is an important and large chapter of chemistry. Some chiral molecules dispersed in a solvent can behave as liquid crystals and can show enhanced electrooptical properties including second-harmonic generation (2002ACIE3882, 2002CM773). 

Although second harmonic generation is not possible in centro-symmetric media, such as gases, the symmetry is broken at the plane surface of solid materials. Therefore efficient SHG has been found at the boundary between a solid and a gas or vapour. Molecules in a thin layer at solid state surfaces can be smdied with SHG or sum frequency mixing, even if the have centro-symmetry. This surface enhanced spectroscopy has been already successfully used for Raman spectroscopy (see Vol. 2, Chap. 3) [591]. 

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