Optimization of second-order polarizabilities applications to real molecules

5 Optimization of second-order polarizabilities applications to real molecules 

The theoretical considerations that were developed in Section 3 for one- and two-dimensional NLO-phores merely required the incorporation of donor and acceptor groups coupled to each other. No assumptions were made concerning the structural molecular equivalent of the LCAO couphng parameter, c, between donor and acceptor. In the one-dimensional molecules to be dealt with first, a single donor-acceptor pair is present. For this case, the consequences of the different extent of couphng on molecular polarizabilities were calculated on the basis of a single electronic transition with charge-transfer character (p. 139). The crucial parameters and AE g 

in principle, all that remains to do now is to translate these requirements into a real chemical structure. However, the chemist s role in the optimization is not merely that of a craftsman. Chemical systems are still much too complex for a thorough theoretical treatment, and chemical intuition and knowledge are required not just for the synthesis of chemical structures whose properties have already been anticipated. The ID model developed above is quite crude in the following respects. 

Despite these shortcomings it will become clear that in the one-dimensional NLO-phores treated in this section, which display a wide range of seemingly disparate chemical structures, the crude model works surprisingly well. Thus, as a consequence of the validity of the two-state model, their second-order polarizabilities in principle reduce to p-nitroanhine . The reader may even gain the impression that the efforts to improve on the hyperpolarizabilities of even the simplest and most easily accessible tt systems (like p-nitroaniline) have been futile. It is true that an efficiency-transparency trade-off exists At a given wavelength of absorption (related to AE) a maximum value for the second-order molecular polarizability per volume element exists which is not tremendously different from that of very basic unoptimized tt systems. However, for apphcations hke the electro-optical effect, a bathochromic shift of the UV-visible absorption is tolerable so that to strive for maximum hyperpolarizabilities is a viable quest. Furthermore, molecular structures with the same intrinsic second-order polarizabihties may differ substantially in their chemical stabihties and their abilities to be incorporated into ordered bulk structures. 

Therefore, we have quoted second-order polarizabihties in terms of )3o of p-nitroanihne in dioxane (A ax = 354 nm, )3o = 13.5 Cm V, T convention, relative to quartz du = 0.5 pnW at 1064 nm). p-Nitroaniline is a truly one-dimensional NLO-phore with one significant component as has been verified experimentally by depolarized EFISHG (Wortmann et al, 1993). If different standards and conventions are taken into account, the values measured by different groups are quite consistent. Note that the intrinsic )3o of p-nitroanihne depends on the solvent, even when normalized for the solvatochromic shift of the CT absorption. We have chosen the lowest intrinsic )3o it is higher by a factor of 1.6 in very polar solvents (see p. 183). Also note that )3 values from HRS measurements of molecules with several significant tensor elements will not allow a true comparison of 

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Macroscopic susceptibilities and molecnlar polarizabilities 155 Experimental determination of molecnlar second-order polarizabilities Optimization of second-order polarizabilities applications to real molecules a Systems and one-dimensional rr systems 168 Two-dimensional (2D) NLO-phores ID and 2D architectnre 196 Conclnsion 206 Acknowledgements 208 References 208... 

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