Bond-length alternation nonlinearities

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... 

The bond length alternation approach describes only the position of the molecular nonlinearities on the BLA axis, but does not give an absolute magnitude of the curves in Fig. 18. Schemes as conjugation length expansion (see below) are believed to increase the magnitude of the curves in the bond length alternation picture. 

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. 

Some examples of intuition-based or modeling-based advances in nonlinear properties, specifically the first hyperpolarizability response p, have been developed in the past three decades. These include the bond-length alternation motif, the idea of octopolar molecular structures, the stronger responses of excited states and the use of purposely twisted -ir-electron molecules to modify the admixture of quinoid and aromatic structures. 

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... 

Gorman, C. B., and Marder, S. R., An investigation of the interrelationships between linear and nonlinear polarizabilities and bond-length alternation in conjugated organic molecules. Pro. Natl. Acad. Sci, USA, 90, 11297-11301 (1993). 

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. 

Linear C species may be represented simply as shown in Fig. 1-6. For an even number of carbons, the simplest electronic structure may be either a dicarbene-cumulene structure, or a diradical-polyyne . The corresponding cyclic structures will be nonlinear and strained, but formally possess closed-shell cumulene or polyyne structures. These differ by having all bond lengths equal, or alternating bond lengths, respectively. For it = odd, the linear structures may be of the dicarbene-cumulene or tetraradical-polyyne type. The cyclic isomers may be cumulene or carbene-polyyne . 

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