Polyene bond-length alternation

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... 

V-UV Application First Excited State of Linear Polyenes. The first electronic absorption band of perfect linear aromatic polyenes (CH)X, or perfect polyacetylene shifts to the red (to lower energies) as the molecule becomes longer, and the bond length alternation (BLA) would be zero. This was discussed as the free-electron molecular orbital theory (FEMO) in Section 3.3. If this particle-in-a-box analysis were correct, then as x > oo, the energy-level difference between ground and first excited state would go to zero. This does not happen, however first, because BLA V 0, next, because these linear polyenes do not remain linear, but are distorted from planarity and linearity for x > 6. 

The alternation of the single and double bonds in the polyene chain is inverted in the VB and CT forms, see Fig. 3.21, so that the model molecule can also be described in terms of the bond length alternation (BLA). The BLA is defined as ... 

Figure 3.5. Schematic sketch of bond length alternation starting from a polyene structure (a), passing a pattern with similar bond lengths and partial charges at the ends (b), to the final charge separated form (c) with a zwitterionic pattern.

Bourhill, G., Bredas, J.L., Cheng, L.-T., Marder, S.R., Meyers, F., Perry, J.W., Tiemann, B.G. Experimental demonstration of the dependence of the first hyperpolarizability of donor-acceptor-substituted polyenes on the ground-state polarization and bond length alternation. J. Am. Chem. Soc. 116, 2619-2620(1994)... 

Luo et al. have investigated the influence of the solvent polarity on the NLO properties of a simple donor-acceptor polyene molecule (Scheme 4) at the ab initio level of theory applying the continuum model of solvent [107]. It has been shown that the evolution of the TPA cross section with respect to the bond length alternation (BLA) closely follows that of the static first-order hyperpolarizability, p. The TPA cross section is strongly dependent on the geometrical changes. Moreover, these authors have noticed that the solvent effect on the TPA cross section (for the CT excited state) in tlie typical donor-acceptor polyene molecule exhibiting the positive solvatochromism is smaller than the influence of the solvent on the values of p. 

Besides these qualitative differences, there also exist quantitative discrepancies between the Hiickel model for polyenes and the experimental observations. The Hiickel theory predicts an order of magnitude larger oscillator strength in the absorption to the lowest dipole allowed state [4]. The bond length alternation required to fit the optical gap in polyenes within a Hiickel model is twice the experimentally observed bond alternation. Thus, the Hiickel model is mainly of pedagogical interest and one needs to go beyond it for dealing accurately with realistic conjugated systems. 

In order to assess the accuracy of the dynamic SDMRG technique, we have performed a series of model calculations we computed the dynamic linear polarizabilities (0(0 )) and third-order polarizabilities (7(0 , 01,0 )) corresponding to the third harmonic generation (THG), for the Hubbard and J7—V polyene-like chains of up to 20 sites with and without dimerizations (note that the dimerization parameter S reflects the degree of carbon-carbon bond-length alternation along the polyene chain nonzero S leads to alternating t and V values while zero 5 means uniform t and V). 

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. 

Semiempirical calculations on the cyclic polyenes C4 ,+2H4m+2 that included electron correlation by a variety of methods predict bond-length alternation for large m [X. Li and J. Paldus, Int J. Quantum Chem., 60,513 (1996) and earlier papers].Hie fact that we can write two equivalent VB resonance structures for C4, + 2H4 + 2 such that a given bond is single in one structure and double in the other is no guarantee of equal bond lengths. 

---