ENHANCED QUADRATIC HYPERPOLARIZABILITY OF DONOR-ACCEPTOR MOLECULES - EXPERIMENTAL AND ANALYTICAL INVESTIGATIONS

Intramolecular charge transfer (ICT) molecules with donor-acceptor (D/ A) end groups can exhibit large static quadratic hyperpolarizabilities beta(0). In a simplified description, such molecules can be considere d as a mixture of two limiting (neutral and charge separated) forms. I n a two-state approximation the relative weight of these resonance for ms governs the intramolecular charge transfer and the molecular geomet ry of the ground and excited states. The two-state two-form model we h ave developed can he viewed as a generalization of the bond length alt ernation formalism developed previously by Marder and coworkers for po lyenes. The influence of the size of the conjugation path on the quadr atic hyperpolarizabilities (determined by EFISH measurements) of a ser ies of polyenes are analyzed within this model.