Bond length alternation, nonlinear optics

Meyers, F., Bredas, J.L., Pierce. B.M.. Marder, S.R. Nonlinear optical properties of donor-acceptor polyenes Frequency-dependent calculations of the relationship among molecular polarizabilities, a, 13, and y, and bond-length alternation. Nonlinear Optic.s 14, 61-71 (1995)... 

Meyers F, Marder SR, Pierce BM, Bredas JL (1994) Electric field modulated nonlinear optical properties of donor-acceptor polyenes sum-over-states investigation of the relationship between molecular polarizabilities (a, p, and y ) and bond length alternation. J Am Chem Soc 116 10703-10714... 

Spectroscopy of CP g states is a subject of current interest, up to now mainly because these states are important in the optical nonlinear properties of these molecules [280,281]. It has recently been shown that in PPV, a CP frequently used in EL, the lowest g excited state is 0.5 eV above the u exciton [282], and indeed the fluorescence quantum yield of PPV is high. Calculations relating the extent of bond-length alternation (see Chapter 11, Sections IV. A and IV.B) to the energy ordering of excited states indeed predict the situation observed in PPV [283]. Similar experimental and theoretical studies are certainly forthcoming. 

F. Meyers, S. R. Marder, B. M. Pierce, and J. L. Bredas,/. Am. Chem. Soc., 116,10703 (1994). Electric Field Modulated Nonlinear Optical Properties of Donor-Acceptor Polyenes Sum-Over-States Investigations of the Relationship Between Molecular Polarizabilities (ot, and -y) and Bond Length Alternation. 

One of the most obvious approaches to defeating the putative optical nonlinearity-optical transparency trade-off follows from the theory of Marder and Perry and is illustrated in Fig. 1. When an amine donor group is connected to a thiophene bridging group, replacement of an alkyl amine with an aryl amine leads to an increase in bond length alternation and an accompanying blue shifting (increase in optical gap) of the interband electronic transition. Aryl to alkyl amine substitution leads, as expected from the theory of Marder and Perry, to an increase in molecular first hyperpolarizability (see Fig. 1), which is, as already noted, accompanied by an increase in optical transparency. This is in contrast to the more... 

The degree of bond length alternation determined from spectroscopy for the bR chromophore with the use of Fig. 28.33 allows prediction of the values of the molecular nonlinear optical responses /8 and y. Indeed, in the cyanine-like range u 0) /8 values are small and cross zero with negative slopes and y can have negative values. The positive contributions to y have been measured according to ECC applied to NLO responses. 

A diagnostic criterion for the efficiency of delocalization is the bond length alternation [123]. Furthermore, physical properties directly correlated to conjugation phenomena are optical transitions, redox potentials and nonlinear optical effects. 

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