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.