The First Hyperpolarizability of Organic Donor Acceptor Molecules

4 The First Hyperpolarizability of Organic Donor/Acceptor Molecules. - Much of the interest in organic materials for optoelectronics, that arose in the late 

An attempt at an ab initio calculation of the pNA hyperpolarizability by Sim et a/.38 using double zeta quality basis sets with diffuse polarizing functions within the finite field/HF/MP2 regime gave values which are even lower than those obtained by the semi-empirical methods. These calculations are at zero frequency but have been corrected to 1064 nm using the usual two state model. Bartlett and Sekino90,91 have discussed the relationship between hyperpolarizabilities calculated by ab initio methods and by SOS formalisms. They conclude that only the most powerful recent methods for correlated calculations can be expected to produce accuracy of about 10% for gas phase hyperpolarizabilities, while ab initio calculations interpreted in terms of the SOS expansion give very poor results when the equivalent number of states is small. 

There are still many uncertainties about the true values of the first hyperpolarizability of pNA. Assuming that the interpretation given by Willets et al. is correct the source of the large discrepancy has to be explained. One relevant factor is that the ab initio calculations clearly refer to the isolated molecule whereas the experimental results are obtained in solution. All the solvents used have some polar character and there are wide variations in the results obtained in different solvents. Whether the enhancement of the polar characteristics with the associated implicit change in the excitation frequencies would be sufficient to bring the ab initio result into closer agreement with experiment is not currently known. 

One final source of uncertainty in the experimental values which would affect both solution and solid state results is that the standard quartz reference value for the strength of an SHG signal does not seem to be very securely established in absolute terms. 

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