Molecular wire behavior in π-stacked donor-bridge-acceptor tertiary arylureas

Article abstract of DOI:10.1021/ja073219n

The synthesis, structure, and electronic properties of a series of tertiary polyarylureas possessing pyrene and dimethylaniline capping groups separated by zero-to-four phenylenediamine bridging groups are described. These molecules adopt folded π-stacked

Full text of DOI:10.1021/ja073219n

Published on Web 07/21/2007
Molecular Wire Behavior in π-Stacked Donor-Bridge-Acceptor Tertiary
Arylureas
Tarek A. Zeidan,† Qiang Wang,‡ Torsten Fiebig,*,‡ and Frederick D. Lewis*,†
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113, and Department of Chemistry,
Boston College, Chestnut Hill, Massachusetts 02467-3961
Received May 7, 2007; E-mail: lewis@chem.northwestern.edu; fiebig@bc.edu
We report here the dynamics of photoinduced charge separation
and charge recombination in a series of donor-bridge-acceptor (D-
B-A) oligo(arylureas) possessing a pyrene (Py) hole donor and
dimethylaniline (DMA) hole acceptor as capping groups separated
by zero-to-four phenyl bridging units (1-5, Figure 1a). These ureas
adopt folded face-to-face geometries with slightly splayed π-stacked
phenyl rings, similar to that reported by Yamaguchi et al. for a
urea having phenyl capping groups and three bridging units.1 The
efficiency and dynamics of charge separation and charge recom-
Figure 1. Structures of ureas 1-7 and AM1 optimized geometry of 3.
narrow Py-• 500 nm band increases from 8.1 ps for 2 to 20.7 ps
bination have been determined by means of femtosecond (fs)
for 5. The 600 and 380 nm bands attributed to DMA+• grow in on
broadband pump-probe spectroscopy. Formation of a charge-
the same time scale. Following these changes, the 380, 500, and
separated state is rapid and highly efficient, independent of the
600 nm bands of 2-5 decay with similar single exponential decay
constants. In the case of 6, hole injection into the bridge can occur
length of the bridge. Rate constants for charge separation and charge
recombination in ureas 3-5 are weakly dependent upon the number
but is not accompanied by the change in the 380 nm band shape
of bridging phenyls, thus providing the first example of wire-like
observed for 2-5. Charge recombination of 6 is best fit as a dual
exponential having a minor long-lived component that is not
observed for 1-5.
behavior in a synthetic π-stacked D-B-A system.
Ureas 1-5 and the reference molecules 6 and 7 (Figure 1b) were
prepared by methods analogous to those we have employed
The rise and decay times for ureas 1-6 are summarized in Table
1. The ultrafast exergonic charge separation and charge recombina-
tion for 1 is analogous to that for other donor-acceptor dyad
systems.3,6 The behavior of ureas 2-5, as illustrated for 5 in Figure
3, is different from that of 1. The single exponential rise of the
500 nm Py-• band and the concomitant formation of the 600 nm
band and the double maximum 380 nm band assigned to DMA+•
is attributed to bridge-mediated hole transport rather than single-
step superexchange, on the basis of its weak distance dependence.
The similar 500 nm rise times for 3-5 and the control molecule 6
suggests that hole injection from Py S1 into the adjacent bridging
phenyl is at least partially rate determining and that hole transport
to DMA in 3-5 occurs on the same time scale as hole injection.
The slower 500 nm rise for 2-6 versus 1 is consistent with the
higher oxidation potentials of the bridging phenyls7 versus the
reference urea 7.8 The simultaneous decay of the 380, 500, and
600 nm bands of 2-5 is assigned to bridge-mediated charge
recombination in which charge delocalization into the bridge (de-
trapping) is at least partially rate determining. Since charge
recombination is slower than hole transport, the efficiency of charge
separation is high for all bridge lengths.
Plots of the log of the rate constants for charge separation (kcs
) τr-1) and charge recombination (kcr ) τd-1) versus center-to-
center Py-DMA distance for 1-5 are shown in Figure 4. Linear
fits of the data for 3-5 provide values of the attenuation factors â
) 0.005 Å-1 for charge separation and 0.01 Å-1 for charge
recombination. The values of â are similar to those reported for
D-B-A systems having oligo(p-phenylenevinylene) linkers that
display molecular wire behavior.9 Faster kcs and kcr for 2 may reflect
the occurrence of tunneling at short distances. We attribute the wire-
like behavior of the oligo(phenylurea) bridges to hole delocalization.
ZINDO calculations show extensive delocalization of holes on two
adjacent bridging phenyls, in accord with the measured oxidation
potentials of the oligo(phenylureas)7 but hole localization on the
previously for the synthesis of capped oligo(arylureas).2,3 The
optimized gas-phase structure for urea 3 obtained from semi-
empirical AM1 computational analysis (Figure 1c) and the upfield
1
shift of protons on the phenylenediamine bridge in the H NMR
spectra support the conformational preference of these “pro-
tophanes” for folded geometries.4 The UV spectra of 2-6 display
a weak band at 375 nm and a stronger structured band at 350 nm,4
similar to those of other pyrene derivatives.3 The spectrum of 1
has a single broad long-wavelength band. ZINDO calculations
support the assignment of the long wavelength bands of 2-6 to
Py-localized transitions and the band of 1 to a DMA-to-Py charge-
transfer transition.5
Transient absorption spectra for ureas 1-6 in acetonitrile solution
were determined with a spectral range of 300-700 nm and
resolution of 5-7 nm using 347 nm excitation from a Ti-sapphire-
based system having a time resolution of ca. 100 fs at time intervals
between 0 and 1900 ps.3 Transient spectra for ureas 1, 5, and 6 are
shown in Figure 2. Transient spectra for 2-4 resemble those for
5.4 In the case of 1, a narrow band at 500 nm assigned to Py-• 3 is
formed with a rise time of 0.5 ps (Figure 2a). Also formed at short
times are a broad band around 600 nm and a band with two resolved
maxima centered at 380 nm. Both of these bands are assigned to
DMA+• based on their similarity to the absorption spectrum obtained
by means of spectroelectrochemistry of urea 7 (Figure 2a). The
decay of the 380 and 600 nm bands of 1 is best fit by a dual
exponential function.
In the cases of 2-6, a broad band around 500 nm and a narrower
band with a single maximum at 380 nm, both of which are assigned
to Py S1 (Figure 2b,c), are formed within several picoseconds via
internal conversion from S2.3 The appearance of the transient spectra
of 2-5 then change to resemble that of 1. The rise time of the
† Northwestern University.
‡ Boston College.
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J. AM. CHEM. SOC. 2007, 129, 9848-9849
10.1021/ja073219n CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Temporal evolution of the pump-probe spectra of ureas (a) 1, (b) 5, and (c) 6 in the time range of 5-1900 ps following 347 nm excitation in
acetonitrile. Early spectra are shown in blue/green colors and late spectra in orange/red colors. The absorption spectrum of 7+• acquired by spectroelectrochemistry
is shown as the red overlay in (a).
Table 1. Femtosecond Pump-Probe Transient Data for Ureas
1-6a
transport in poly(N-vinylcarbazole) derivatives; however, lower hole
mobilities are found for smaller aromatic pendant groups such as
naphthyl or phenyl. High hole mobility is associated with delocal-
ization over two or more π-stacked aromatic groups. Modification
of the ends of oligomers with donor and acceptor groups has been
suggested as a means of probing the distance dependence of hole
transport in self-stacking systems.11 This has been accomplished
for porphyrin-bridge-quinone systems assembled with zero-to-two
urea
τ, ps (380 nm)
τ, ps (500 nm)
1
2
3
4
5
6
12 (0.003); 85.4 (0.005)
57
91
101
0.5 (r); 12 (0.008); 90 (0.01)
8.1 (r); 61
19 (r); 103
20 (r); 119
21 (r); 126
107
79 (0.03); 1260 (0.004)
24 (r); 100 (0.03); 1620 (0.008)
1,8-diarylnaphthyl linkers (â ) 0.43 Å-1 12
and for duplex
)
a All components are decaying components, unless otherwise noted.
oligonucleotides modified with synthetic donor and acceptor groups
separated by base pair domains of varying length and base
sequence.13 Hole transport in long DNA A-tracts displays weak
distance dependence with estimated rates of 50 ps to 1 ns per base
pair, slower than in D-B-A systems having oligo(p-phenylene-
vinylene) bridges. The present results establish that wire-like
behavior can be achieved by means of electronic coupling in a
π-stacked bridge.
Amplitudes are in parentheses; r ) rising components.
Acknowledgment. The authors thank Thea Wilson and Ana
Ramirez for help with electrochemical experiments. Funding for
this research was provided by National Science Foundation grant
CHE-0400663 (F.D.L.) and by the Volkswagen Foundation (T.F.).
Supporting Information Available: Experimental details, including
synthesis and characterization, NMR and absorption spectra, AM1
optimized structures, absorption spectra and transient spectra for ureas
1-6. This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. Excited-state behavior of urea 5 (broken arrows indicate transient
assignments; s ) single peak; d ) double peaks).
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(phenylurea) bridges in either the forward (charge separation) or
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