Finding organic molecules with large nonlinear optical NLO polarisabilities is desirable for many applications. An important example is the imaging of biological cells using fluorescent protein chromophores.
These images based on two photon microscopy using fluorescent protein chromophores.
Understanding how the NLO properties are related to the structure of the molecules is a fundamental question that has attracted considerable attention. In particular the NLO polarisability of conjugated dyes has been correlated with the amount of bond length alternation.
Seth Olsen and I have just finished a paper Bond Alternation, Polarizability and Resonance Detuning in Methine Dyes which looks at these questions.
Even though the molecules of interest can have a complex chemical structure it is possible to establish structure-property relations.
This Figure shows how the bond length variations can be correlated with a single parameter which measures the deviation of the molecule from resonance. An important result in our paper is the definition of the reference bond lengths in an asymmetric dye.
The above Figure shows how the nonlinear polarisability beta varies with the deviations from resonance.