Abstract
We have been attempting to correlate hyperpolarizabilities with bond-length alternation (BLA), which is defined as the difference in the average length between adjacent carbon-carbon bonds in a polymethine ((CH)n) chain.(1, 2) Polyenes have alternating double and single bonds (bond length equal to 1.34 Å and 1.45 Å, respectively), and thus show a high degree of bond-length alternation (BLA=-0.11Á). In donor-acceptor polyenes, this parameter is related to the degree of ground-state polarization in the molecule. To better understand this correlation, it is illustrative to discuss the wave function of the ground state in terms of a linear combination of the two limiting charge-transfer resonance structures. For substituted polyenes with weak donors and acceptors, the neutral resonance form dominates the ground-state wavefunction, and the molecule has a high degree of (conventionally negative) bond-length alternation. With stronger donors and acceptors, the contribution to the ground state, of the charge-separated resonance form increases and simultaneously, BLA decreases in absolute value. When the two resonance structures contribute equally, as in a symmetrical cyanine, the molecule exhibits essentially no bond-length alternation. Finally, if the charge-separated form dominates the ground-state wave function, the molecule acquires a positive bond-length alternation. Neutral molecules with aromatic rings have a diminished contribution of the charge-separated form to the ground-state wave function, due to the energetic price associated with the loss of aromaticity in that form.(3) As a result, push-pull molecules with aromatic ground states tend to be more bond-length alternated for a given donor and acceptor pair than for a polyene of comparable length.
© 1993 Optical Society of America
PDF ArticleMore Like This
Fabienne Meyers, Chin-Ti Chen, David Beljonne, Jean-Luc Brédas, and Seth R. Marder
MC.3 Organic Thin Films for Photonic Applications (OTF) 1995
R. S. KUMAR, J. KUMAR, A. BLUMSTEIN, and S. K. TRIPATHY
THL2 Conference on Lasers and Electro-Optics (CLEO:S&I) 1989
J. W. Perry, S. R. Marder, L.-T. Cheng, G. Bourhill, Kamjou Mansour, C. B. Gorman, S. Gilmour, and B. G. Tiemann
FA7 Nonlinear Optics (NLO) 1992