Photoinduced Charge Separation Involving an Unusual Double Electron Transfer Mechanism in a Donor−Bridge−Acceptor Molecule

A series of rodlike donor−bridge−acceptor (D−B−A) molecules was synthesized to study the role of bridge energy levels on electron transfer (ET) rates. In these compounds, a 4-aminonaphthalene-1,8-imide (ANI) electron donor is linked to a 1,8:4,5-naphthalenediimide acceptor (NI) via the 1,4 positions on a phenyl bridge. The phenyl bridge is substituted at the 2 and 5 positions with methyl or methoxy groups to yield ANI-diMe-NI and ANI-diMeO-NI. These molecules differ only in the energy levels of the bridge molecular orbitals. Other parameters affecting ET rates such as donor−acceptor distance, orientation, and driving force are constant between the two systems. The rate constants for charge separation (CS) and charge recombination (CR) within ANI-diMeO-NI in toluene are 32 and 1400 times larger, respectively, than the corresponding rate constants for ANI-diMe-NI. Solvents of higher polarity diminish these differences in rate constants, making them comparable to those observed for ANI-diMe-NI. The relative energies of the ion pair states suggest that it is possible for the reaction 1 *D−B−A → D−B + −A - to occur via a double electron-transfer process that is somewhat analogous to Dexter energy transfer. The lowest excited singlet state of the donor, 1 *ANI, possesses about 70% charge-transfer character, so that significant positive charge is localized on its amine nitrogen, whereas significant negative charge is localized on its naphthalene-1,8-imide ring. Electron transfer from the naphthalene-1,8-imide ring of 1 *ANI to NI is concomitant with electron transfer from the p -dimethoxybenzene bridge to the electron-deficient amine nitrogen atom in 1 *ANI. A series of reference molecules in which the p -dimethoxybenzene bridge moiety is attached only to ANI or NI alone is used to establish the structural and electronic requirements for this unusual charge separation mechanism.