Influence of gap states on the nonresonant second hyperpolarizabilities of conjugated organic polymers
The presence of conjugation and substitution defects introduces 'gap states' in finite polyenes that are shown to influence the size and sign of the second molecular hyperpolarizability, γxxxx. Using a one-electron tight-binding model, we calculate the dependence of γxxxx on the defect-state occupancy and energy in finite polyenes. Defects can cause a significant decrease or enhancement of γxxxx by impeding charge delocalization or by creating partly filled bands (mimicking the one-band limit), respectively. Concomitant sign changes in γxxxx are predicted. Calculations of these effects are made for molecules in which a central atom's identity and electron occupancy are varied. The results suggest strategies for designing molecules that can be either photochemically or electrochemically switched between states with considerably different second hyperpolarizabilities.
