Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published March 23, 1994 | public
Journal Article Open

Experimental Demonstration of the Dependence of the First Hyperpolarizability of Donor-Acceptor-Substituted Polyenes on the Ground-State Polarization and Bond Length Alternation


It has been suggested that optimizing the first hyperpolarizability, β, of donor-acceptor compounds requires a specific donor-acceptor strength for a given conjugated bridge. For donor-acceptor polyenes, β can be maximized when an optimal degree of mixing between neutral and charge-separated canonical resonance forms This degree of mixing is related to the donor-acceptor strength and a molecular parameter, bond length alternation (BLA), defined as the difference between the average carbon-carbon single and double bond lengths in the polymethine backbone. The degree of BLA arises from the linear combination, or mixing, of the two-limiting charge-transfer resonance forms of the molecule (Figure 1).4 For unsubstituted polyenes or chromophores with weak donors-acceptors, the neutral canonical form is the dominant contributor to the ground state, resulting in large positive BLA. As the donor-acceptor strength increases, the charge-separated resonance structure contributes more to the ground state, resulting in smaller BLA, until both resonance forms contribute equally and the ground-state structure possesses essentially zero BLA, analogous to a symmetrical cyanine. Increasing the ground-state polarization further results in the charge-separated canonical form dominating the ground-state structure, leading to negative BLA.

Additional Information

© 1994 American Chemical Society. Received November 23, 1993. The research described in this paper was performed in part by the Jet Propulsion Laboratory (JPL), California Institute of Technology, and was supported by the Advanced Research Projects Agency and the Ballistic Missiles Defense Initiative Organization, Innovative Science and Technology Office, through a contract with the National Aeronautics and Space Administration (NASA). Support from the National Science Foundation (Grant No. CHE-9106689) and the North Atlantic Treaty Organization is gratefully acknowledged. G.B. thanks the National Research Council and NASA for a Resident Research Associateship at JPL. The work by F.M. is carried out within the framework of the Belgium Prime Minister Office of Science Policy "Pole d'Attraction Interuniversitaire en Chimie Supramoleculaire et Catalyse". The authors would like to thank Dr. Brian M. Pierce for helpful discussions and Matthew J. Perry and Ging Lee for experimental assistance.

Attached Files

Supplemental Material - ja00085a052_si_001.pdf

Supplemental Material - ja2619.pdf


Files (4.9 MB)
Name Size Download all
1.5 MB Preview Download
3.5 MB Preview Download

Additional details

August 20, 2023
August 20, 2023