Effects of a Paracyclophane Linker on the Charge-Transfer Transition of 4-(Dimethylamino)-4‘-nitrostilbene

Resonance Raman intensities of the push−pull chromophore 4-(dimethylamino)-4‘-nitrostilbene (DANS) have been measured in dichloromethane, acetonitrile, and methanol at excitation wavelengths spanning its strong visible charge-transfer absorption band. The effects of inserting a paracyclophane moiety between the donor and the acceptor are investigated by performing similar measurements on a second molecule, PCP−DANS, which consists of p -dihexylaminostilbene and p -nitrostilbene moieties attached through their unsubstituted rings by a paracyclophane linker. The Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine reorganization energies along the Raman-active normal coordinates. Excitation to the nominal charge-transfer state generates a much greater change in geometry in the vicinity of the nitro acceptor group in DANS than in PCP−DANS, while the geometry changes around the dialkylamino donor end are similar. Comparison with other push−pull chromophores coupled with the results of ZINDO electronic structure calculations both suggest that the transition in DANS is well-described as an intramolecular charge-transfer transition, whereas the transition in PCP−DANS has less charge-transfer character.