Polyenes vs polyynes: Efficient pi-frame for nonlinear optical pathways

The electronic and vibrational hyperpolarizabilities of polyenes NH2-(HC=CH)(n)-NO2 and polyynes NH2-(C=C)(n)- NO2 have been investigated. As the chain length increases, the first hyperpolarizabilities increase drastically for polyenes, while they increase moderately for polyynes. Furthermore, in polyenes the vibrational first hyperpolarizabilities are almost the same as the electronic first hyperpolarizabilities, while in polyynes the former is much smaller than the latter. An analysis of the electron densities of HOMOs of polyenes and polyynes confirms that the polyene pi-frame is more efficient for long pathways of nonlinear optical molecules, while the polyyne pi-frame is slightly better for very short pathways. In both polyenes and polyynes, the IR intensities arise mainly from the terminal carbon sites, while the Raman intensities, from the central carbon sites. (C) 2000 American Institute of Physics. [S0021-9606(00)70901-9].