Second-Order Nonlinear Optical Properties Of Thiophene Containing Chromophores with Extended Conjugation
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Introduction In an attempt to understand and optimize second-order optical nonlinearities of organic molecules, the first hyperpolarizability (f3) has been correlated with bond length alternation (BLA), defined as the difference in length between adjacent carbon-carbon bonds in a conjugated chain [1-3]. Theoretical calculations [4] predict that the second-order response P is maximized when IBLAI is -0.04 A. However, we believe that it will be difficult to obtain this with conventional chromophores that have strongly aromatic value needed to maximize P3 ground-state structures, e.g. those containing stilbene bridges. Thus, for molecules containing multiple phenyl groups, the aromatic stabilization in the neutral canonical resonance form will hinder charge-separation and consequently lead to large values of IBLAI and diminished nonlinearities (Figure 1). Much larger nonlinearities can often be achieved for a given donoracceptor combination when the conjugated bridge between the donor and acceptor possesses reduced aromatic stabilization energy in the neutral form, e.g. a polyene chain or thiophene 485 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
bridge [5-10]. Alternatively, large nonlinearities can be achieved if one uses a donor that is aromatic in its neutral canonical resonance form and an acceptor that gains aromaticity upon charge separation. In particular, Marder et al. have found that compounds containing the N,N' diethylthiobarbituric acid acceptor can have unprecedented nonlinearities [5]. The N,N' diethylthiobarbituric acid compounds with the largest nonlinearities also contain extended polyene chains, which, although they are excellent bridges for charge transport, compromise the high temperature stability of the materials required for many poled polymer applications [11,12]. Jen et al. have synthesized and characterized several thiophene compounds with a range of acceptors including nitro, dicyanovinyl and tricyanovinyl [6-10]. These results indicate that thiophene compounds have much greater nonlinearities than the analogous phenyl containing chromophores. In this paper, we attempt to simultaneously achieve large nonlinearities by both adding an extra thiophene to the conjugated chain and by using acceptors We present the syntheses, first which gain aromaticity upon charge-separation. hyperpolarizabilities (P) and electro-optic coefficients (r33) of several phenyl and thiophene containing compounds. -
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Figure 1. Limiting resonance structures of a donor-acceptor substituted stilbene. Left: the aromatic/neutral form; Right: the quinoidal/charge-separated form. The contribution of the charge-separated form is hindered, due to loss of aromaticity, leading to large IBLAI, that in turn diminishes 13.
Experimental The syntheses of compounds 6, 8 and 9 (Table 1) have been described previously [7, 8]. Scheme 1 illustrates the syntheses of the compounds with three linked thiophene rings, 7 and 10. These compounds were purified by chromatography on silica and recrystallized fr
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