Charge separation in the excited state of electron donor-acceptor compounds containing the piperazine moiety

Three bridged electron donor?acceptor systems are investigated containing a 4-cyano-1-ethenylnaphthalene electron-acceptor and piperidine (compound 1), phenylpiperazine (2) and 4-methoxyphenylpiperazine (3) electron-donating groups. In the intramolecular charge-transfer states of 1 and 2, the extent of charge separation is similar, but in compound 3 the positive charge is shifted towards the more powerful arylamine donor site, which results in a significantly greater dipole moment. Optical absorption spectra of model radical cations demonstrate that the predominant charge localization on the trialkyl nitrogen in 2 and on the aryl nitrogen in 3 are a consequence of the bistable nature of the piperazine donors.