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

Abstract

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.