Journal of the American Chemical Society
ARTICLE
in porphyrin-perylenediimide (PDI) showed the delocalized PDI
radical anion to broaden significantly with increased stacking
interactions.27 The fact that we do not observe charge recombi-
nation on the time scale of these experiments is also consistent
with that work, where delocalization of radical anions led to
longer-lived charge separation.
’ CONCLUSION
We have described the bolaamphiphilic self-assembly of a donor/
acceptor dyad, 1, into 1D nanotubes containing a supramolecular
heterojunction. The nanotubes form via extensive intermolecular
πÀπ interactions between the NDI and TPP chromphores.
Transient absorption spectra in TFE and MeOH reflect the
generation of charge-separated states. The transient spectra of
nanotubes of 1 formed in 10% MeOH/H2O are very different
from those in TFE and MeOH. Notably, nanotube structure
results in very different photophysics compared with nonspecific
or minimally aggregated forms of 1. This work demonstrates the
importance of controlling local nanostructure in modulating the
photophysical properties of optoelectronic materials.
The transient absorption spectra in 10% MeOH/H2O are
different than in either MeOH or TFE (Figure 6b). In these
spectra, the absorption bands at 485 and 570À680 nm that are
attributed to the NDI radical anion are absent, as is the low-energy
band of the NDI excited state at 620 nm. The stimulated emission
band at 400 nm appears to have merged with the Soret bleach, which
has shifted to 430 nm. Only the NDI excited-state bands at 460 and
495 nm are clearly evident. The decays at 460 and 495 nm were best
fit to triexponential decays and showed two major components
with lifetimes of ∼3À4 and ∼50 ps, and a minor component
(<10%) of ∼1.5 ns lifetime. The lifetime component of 50 ps
is nearly identical to the fluorescence lifetime (45 ps) obtained
from TCSPC experiments. The close correlation of the TCSPC
value and the 50 ps transient absorption value is consistent
with decay of the NDI excited state and may be attributed to
electron transfer from the porphyrin to the electronically
excited NDI.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, TEM
b
and AFM images, CD, NMR, MS, and UV spectra, fluorescence
data, and transient absorption spectra/decay profiles for bolaam-
phiphile 1. This material is available free of charge via the Internet
’ AUTHOR INFORMATION
The lack of an observable NDI radical anion signal at 485 or
570À680 nm can be rationalized using two alternative kinetic
scenarios. One interpretation would suggest that charge separa-
tion in 10% MeOH/H2O (∼45À50 ps) is slower than in either
100% MeOH or TFE, and the lifetime of the charge-separated
state is significantly shorter-lived. The ultrafast component of
3À4 ps would then reflect the rate of charge recombination.
However, previous picosecond fluorescence experiments indi-
cate that NDI nanotube assemblies exhibit long-lived NDI
excited states8 that often are accompanied by rapid migration
of the excited-state energy. Therefore, it is also quite possible that
the radical anion has similar mobility/delocalization, which
would then be expected to result in a longer-lived charge-
separated state. The extensive delocalization of the radical anion
within the nanotube πÀπ arrays would be expected to result in
extensive broadening of the radical anion bands in the transient
absorption spectra, consistent with the broad feature observed in
the range 550À650 nm in the transient spectra. Indeed, Wasie-
lewski and co-workers have demonstrated long-lived charge
separation in stacked porphyrinÀPDI assemblies where the
radical anion was delocalized over several units in the assembly.
With these observations in mind, the two time constants we
observe in the transient absorption experiments likely emerge
from different charge-separation events, one with a time constant
of 3À4 ps and another with 45À50 ps, the latter of which is
observed in the TCSPC experiments and the former too fast to
resolve on our TCSPC instrument. The third, longer-lived
component present in the decay represents charge recombina-
tion but is too long-lived (τ > 1.5 ns) to be fit accurately.
Although these kinetic scenarios cannot be unambiguously
distinguished, the observation of two time constants for electron
transfer is consistent with the nanotube structure. Our prior
solid-state NMR studies of an NDI nanotube identified very
different conformational dynamics in the interior and exterior
environments of the nanotubes, and these two regions could be
distinguished spectroscopically.8 Hence, the two time constants
for electron transfer may arise from the different positioning of
the two NDI chromophores with respect to the interior and
exterior nanotube surfaces.
Corresponding Author
dmodarelli@uakron.edu; parquett@chemistry.ohio-state.edu
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(CHE-1057884, CRC-0526864 and CHE-0216371). We ac-
knowledge the technical assistance and usage of the AFM core
facility at Davis Heart and Lung Research Institute, and Michael
Severance and Lynetta Mier in the Ohio State Center for Chemical
and Biophysical Dynamics for their assistance with the transient
absorption experiments.
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