Search

We report the synthesis and properties of two series of homologous donor–acceptor (D–A) chromophores in which N,N‐dimethylanilino (DMA) or N,N‐dihexylanilino (DHA) donors and dicyanovinyl acceptors are separated by up to four C≡C triple‐bond spacers or up to three C=C double‐bond spacers. The intramolecular charge‐transfer (CT) interactions of the new D–A oligoynes and the known all‐transD–A oligoenes were investigated by X‐ray crystallography, electrochemistry, UV/Vis spectroscopy, and theoretical calculations. In both series, the optical and electrochemical HOMO–LUMO gaps decrease with increasing spacer length. The HOMO–LUMO gaps for the D–A oligoynes and oligoenes with a given spacer length are nearly identical. The effect of the spacer length was found to level‐off for spacers with more than six carbon atoms. The third‐order optical nonlinearity of both series of molecules was determined by measuring the rotational averages of the third‐order polarizabilities γrotby degenerate four‐wave mixing.

The first- and second-order hyperpolarizabilities have been extensively studied to identify universal properties near the fundamental limit. Here, we employ the Monte Carlo method to study the fundamental limit of the second hyperpolarizability. As was found for the first hyperpolarizability, the largest values of the second hyperpolarizability approach the calculated fundamental limit. The character of transition moments and energies of the energy eigenstates are investigated near the second hyperpolarizability’s upper bounds using the missing state analysis, which assesses the role of each pair of states in their contribution. In agreement with the three-level ansatz, our results indicate that only three states (ground and two excited states) dominate when the second hyperpolarizability is near the limit.

The off-resonant hyperpolarizability is calculated by using the dipole-free sum-over-states expression from a randomly chosen set of energies and transition dipole moments that are forced to be consistent with the sum rules. The process is repeated so that the distribution of hyperpolarizabilities can be determined. We find this distribution to be a cycloidlike function. In contrast to variational techniques that when applied to the potential energy function yield an intrinsic hyperpolarizability less than 0.71, our Monte Carlo method yields values that approach unity. While many transition dipole moments are large when the calculated hyperpolarizability is near the fundamental limit, only two excited states dominate the hyperpolarizability—consistent with the three-level ansatz. We speculate on the character of the Hamiltonian that is needed to optimize the intrinsic hyperpolarizability.