Large molecular third-order optical nonlinearities in polarized carotenoids

Garito and co-workers have suggested a mechanism to dramatically increase the second hyperpolarizability, γ, in linear π-electron-conjugated molecules. Polarization is introduced that leads to a difference between the dipole moments of the molecule's ground state and excited state. Here a series of carotenoids was examined that had increasing intramolecular charge transfer (ICT) from the polyenic chain to the acceptor moiety in the ground state, and γ was measured for these compounds as a function of wavelength by third-harmonic generation. The compound with the greatest ICT exhibited a 35- fold enhancement of γ(max) (the γ measured at the peak of the three-photon resonance) relative to the symmetric molecule β-carotene, which itself has one of the largest third-order nonlinearities known. Stark spectroscopic measurements revealed the existence of a large difference dipole moment, Δμ, between the ground and excited state. Quantum-chemical calculations underline the importance of interactions involving states with large δμ.