A R T I C L E S
Chakrabarti et al.
unequivocal conclusion that charge separation and charge
recombination processes must be taking place via the pendant
aromatic ring in both 2DBA and 1DBA.
Information). The plot provides reasonable values for the
reorganization energy ranging from 0.70 to 0.79 eV and ∆rG
values close to the values obtained from the charge-transfer
emission fit.
There is strong evidence that charge recombination in
DPMN[8cy]DCV takes place directly, through space, between
the two chromophores, which is facilitated by the electrostati-
cally enforced proximity of the two chromophores in the charge-
separated state of this species (see Figure 7c). Thus, the distance
between the two centroids in the charge-separated state of
DPMN[8cy]DCV, based on a model system (Figure 7c), is only
4.4 Å,19 which is sufficiently small to promote strong through-
space interchromophore coupling in this species.26 The distances
between the pendant group and DPMN and DCV chromophores
in the charge-separated state of 1DBA′ are between 3.4 and
2.7 Å, depending on the twist angle of the pendant phenyl ring
(see previous section). These distances are significantly smaller
than the aforementioned value computed for the charge-
separated state of DPMN[8cy]DCV. Thus, the finding that the
strength of the electronic coupling for charge-transfer fluores-
cence is substantially larger for 2DBA, compared to that for
DPMN[8cy]DCV, is understandable.
Conclusion
The electron transfer in U-shaped molecules 1DBA and
2DBA containing two different pendant groups in the cleft
between the donor and acceptor group was studied. 2DBA shows
charge-transfer emission in nonpolar and weakly polar solvents.
The magnitudes of the electronic coupling for photoinduced
charge separation in 1DBA and 2DBA were found to be 147
and 274 cm-1, respectively. The origin of this difference lies
in the electronic nature of the pendant aromatic group, since
charge separation occurs by tunneling through the pendant
group, rather than through the bridge. The charge-transfer
fluorescence for 2DBA in nonpolar solvents was used to
determine the electronic coupling for charge recombination,
|VCR|, the magnitude of which is ∼500 cm-1, much larger than
that for charge separation. This difference can be explained by
changes in the geometry of the molecule in the relevant states;
because of electrostatic effects, the DPMN and DCV chro-
mophores are about 1 Å closer to the pendant group in the CS
state than in the locally excited state. Consequently the through-
pendant-group electronic coupling is stronger in the CS states
which controls the charge-transfer fluorescence processsthan
in the locally excited stateswhich controls the CS process. The
magnitude of |VCR| for 2DBA is almost 2 orders of magnitude
greater than that in DMN-12-DCV, having the same length
bridge as for the former molecule, but lacking a pendant group.
This result unequivocally demonstrates the operation of the
through-pendant-group mechanism of electron transfer in the
pendant-containing U-shaped systems of the type 1DBA and
2DBA. Our observation of the modulation of the strength of
electronic coupling in the U-shaped system 2DBA, brought
about by electrostatically driven changes in molecular geometry,
suggests an intriguing approach to the generation of long-lived
charge-separated species: build a U-shaped system possessing
A fit of the rate constant data as a function of temperature to
eq 2 was used to extract values for the solvent reorganization
energy (see Table 4) for 1DBA and 2DBA. The solvent
reorganization energy values of 2DBA are higher than those
for 1DBA in all the solvents. The differences between their
solvent reorganization energy values are highest for the most
polar solvent acetonitrile and least for p-xylene. Since the
pendant groups in 1DBA and 2DBA have comparable sizes,
the difference is likely caused by differences in the polarities
of the pendant groups in these molecules, the electronegative
oxygen atom making the methoxyphenyl pendant group in
2DBA more polar than ethylphenyl group in 1DBA. The charge-
transfer emission fit was also used to determine the solvent
reorganization energy for charge recombination in 2DBA (Table
4). The values obtained from charge-transfer emission spectra
fitting is somewhat smaller than the values obtained from the
kinetic rate data and correlates with more negative values of
∆rG obtained from charge-transfer emission fit (Table 3).
The ∆rG values for 1DBA were obtained from the kinetic fit
of the experimental data by the molecular solvation model
whereas fitting to the charge-transfer emission was used to
calculate ∆rG values of 2DBA experimentally in different
solvents. The magnitude of ∆rG is least negative in p-xylene
and is most negative in the polar solvent acetonitrile. The ∆rG
for 2DBA cannot be determined from a kinetic fit as ∆rG is
too negative (from charge-transfer emission fitting); however,
the estimated free energy obtained from the molecular solvation
model for 2DBA is somewhat lower than the free energy of
1DBA. This finding indicates that there is some error associated
with the fitting. To estimate the error we have used the contour
plot of reorganization energy values as a function of different
free energy values in the fit in mesitylene (see the Supporting
a doubly positively charged acceptor, D-B-A2+ (e.g., A2+
)
viologen). Photoinduced electron transfer should generate D+-
B-A+. Repulsive electrostatic interactions should drive the
singly positively charged chromophores further apart, thereby
weakening the electronic coupling for charge recombination.
Such an effect has been observed and explained in terms of
this mechanism.27
Acknowledgment. We acknowledge financial support from
the Australian Research Council and the U.S. National Science
Foundation (CHE-041545).
(26) Paddon-Row, M. N.; Jordan, K. D. Through-Bond and Through-Space
Interactions in Unsaturated Hydrocarbons: Their Implications for Chemical
Reactivity and Long-Range Electron Transfer. In Modern Models of
Bonding and Delocalization; Liebman, J. F., Greenberg, A., Eds.; VCH
Publishers: New York, 1988; Vol. 6, p 115.
(27) (a) Jolliffe, K. A.; Bell, T. D. M.; Ghiggino, K. P.; Langford, S. J.; Paddon-
Row, M. N. Angew. Chem., Int. Ed. 1998, 37, 916. (b) Bell, T. D. M.;
Jolliffe, K. A.; Ghiggino, K. P.; Oliver, A. M.; Shephard, M. J.; Langford,
S. J.; Paddon-Row, M. N. J. Am. Chem. Soc. 2000, 122, 10661.
Supporting Information Available: Synthesis, characteriza-
tion, and rate constant data in different solvents of 1DBA and
2DBA and complete reference 20. This material is available
JA067266B
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