Multistep photochemical charge separation in rod-like molecules based on aromatic imides and diimides

A series of intramolecular triads with linear, rod-like structures has been developed that undergo very efficient two-step electron transfer following direct excitation of a chromophore possessing a charge transfer (CT) excited state. The CT state of 4-aminonaphthalene-1,8-imide (ANI), produced by direct excitation of the chromophore, has about 70% of a negative charge transferred from the amine to the imide. Attachment of aniline (An) and p-methoxyaniline (MeOAn) donors to ANI by means of a piperazine bridge results in linear dyads, An-ANI and MeOAn-ANI, that undergo rapid electron transfer in about 10^-11 s to give a > 99% yield of the ion pairs, An⁺-ANI^- and MeOAn⁺-ANI^-, in which the charges are separated by 7.7 Å. The formation and decay of these ion pairs can be monitored directly by transient absorption spectroscopy. Further attachment of a 1,8:4,5-naphthalenediimide (NI) electron acceptor to the imide group of ANI using a 2,5-dimethylphenyl spacer results in triads An-ANI-NI and MeOAn-ANI-NI. Excitation of the CT state of ANI within these triads results in the same high yield charge separation step observed in the corresponding dyads followed by a subnanosecond charge shift reaction to yield the giant dipole states An⁺-ANI-NI^- and MeOAn⁺-ANI-NI^- in 72% and 92% yield, respectively, in toluene. The lifetime of MeOAn⁺-ANI-NI^- is 310 ns. These triad molecules make explicit use of a CT excited state to initiate a multistep electron transfer process. The excited singlet CT state and the two ion pair states are all spectroscopically distinct, and all states are unambiguously spectrally resolved by transient absorption measurements. In addition, the ion pair states An⁺-ANI^- and MeOAn⁺-ANI^- undergo radiative recombination, thereby allowing a more detailed analysis of the energetics of charge separation and the influence of the CT excited state on the rates of subsequent longer distance charge shift reactions in these molecules.