Ultrafast molecular logic gate based on optical switching between two long-lived radical ion pair states

Full text of DOI:10.1021/ja0041122

2440
J. Am. Chem. Soc. 2001, 123, 2440-2441
Ultrafast Molecular Logic Gate Based on Optical
Switching between Two Long-Lived Radical Ion Pair
States
Aaron S. Lukas, Patrick J. Bushard, and
Michael R. Wasielewski*
Department of Chemistry, Northwestern UniVersity
EVanston, Illinois 60208-3113
ReceiVed NoVember 29, 2000
Decreasing the size of electronic devices introduces complexi-
ties arising from quantum tunneling effects and the localization
of charge in discrete electronic orbitals. Single molecules specif-
1
-10
11-13
ically designed for applications as switches,
wires,
and
rectifiers1
4-17
can take advantage of these discrete electronic
Figure 1. Energy level diagram for 1.
configurations. Moreover, theoretical work strongly supports the
idea that reversible photoinduced electron-transfer reactions can
electronic couplings to produce an ultrafast molecular logic gate.
The long-lived output states of this gate can be read out optically
by using the spectroscopic signatures of the radical ions.
serve as the basis for a molecular switch.1
8-20
Previous work in
this laboratory has demonstrated that the electron transport
direction in a branched molecule having a single donor and two
acceptors can be switched by using femtosecond laser pulses.21
However, strong electronic coupling between the electronic states
of the two branches leads to relatively short, subnanosecond
lifetimes for the reduced acceptor on one of the branches. The
work presented here shows that a branched donor-acceptor array,
which employs a bridging group with a high-energy LUMO, can
be used to provide both optimized free energies of reaction and
*
To whom correspondence should be addressed. E-mail: wasielew@
chem.northwestern.edu.
1) de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. Nature 1993, 364,
(
4
2-44.
(
2) Gust, D.; Moore, T. A.; Moore, A. L. IEEE Eng. Med. Biol. 1994, 94,
5
8-66.
(
3) DelMedico, A.; Fielder, S. S.; Lever, A. B. P.; Pietro, W. J. Inorg.
Chem. 1995, 34, 1507-13.
4) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. F.
J. Am. Chem. Soc. 1996, 118, 3996-3997.
5) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J.
(
(
M.; McCoy, C. P.; Radmacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-
In 1 (NI-ANI-NMI-PI) the electron donor is 4-(N-piperidinyl)-
naphthalene-1,8-dicarboximide (ANI), whose ground-state elec-
tronic absorption is centered at 400 nm in toluene. The excited-
state energy of ANI is 2.80 eV, while those of NI, PI, and NMI
are all >3 eV. The one-electron oxidation potential of the ANI
electron donor is 1.2 V vs SCE, while the one-electron reduction
1
566.
(6) Willner, I. W. B. J. Mater. Chem. 1998, 8, 2543-2556.
(7) Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998,
1
20, 8486-8493.
(
8) Gobbi, L.; Seiler, P.; Diedrich, F. Angew. Chem., Int. Ed. Engl. 1996,
5, 5, 674-678.
9) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart,
J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391-
94.
10) Collier, C. P.; Matterstei, G.; Wong, E. W.; Luo, Y.; Beverly, K.;
3
(
potentials of the NI, PI, and NMI electron acceptors are -0.53,
3
22
-
0.79, and -1.41 V vs SCE, respectively. The free energies of
(
reaction for the formation of the various ion pair intermediates
Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289,
1
172-1175.
within 1 were calculated by using spectroscopic/electrochemical
(
11) Barigelletti, F.; Flamigni, L. Chem. Soc. ReV. 2000, 29, 1-12.
12) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature
23
methods developed earlier, Figure 1. These data show that
(
1
electron transfer from the *ANI excited state to NI is strongly
1
998, 396, 60-63.
(13) Tans, S. J.; Devoret, M. H.; Dai, H.; Thess, A.; Smalley, R. E.;
favored energetically. Selective photoexcitation of ANI within 1
with 420 nm, 130 fs laser pulses results exclusively in the reaction
Geerligs, L. J.; Dekker: C. Nature 1997, 386, 474-477.
(
(
(
14) Waldeck, D. H.; Beratan, D. N. Science 1993, 261, 576-577.
1
-
+
NI- *ANI-NMI-PI f NI -ANI -NMI-PI in toluene, which occurs
with a time constant of τ ) 120 ps, as indicated by the formation
of the strong electronic transitions in the visible spectrum
15) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950-957.
16) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999,
86, 1550-1552.
2
(17) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour,
-
-1
-1
characteristic of NI at 480 (ꢀ ) 23 000 M cm ) and 605 nm
(7 000 M cm ). Hence, the first photon provides for selective
charge transport down one branch of the bifurcated molecular
array. The resulting NI -ANI -NMI-PI ion pair has a τ ) 160
ns lifetime in toluene.
J. M. Appl. Phys. Lett. 2000, 77, 1224-1226.
-1
-1 24
(
18) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277-283.
19) Hopfield, J. J.; Onuchic, J. N.; Beratan, D. N. Science 1988, 241, 817-
(
8
19.
-
+
(
20) Metzger, R. M. AdV. Chem. Ser. 1994, 240, 81-129.
(21) Lukas, A. S.; Miller, S. E.; Wasielewski, M. R. J. Phys. Chem. B
2
000, 104, 931-940.
-
-
+
Selective excitation of NI within the NI -ANI -NMI-PI ion
(22) Electrochemical data were obtained by cyclic voltammetry at a Pt
pair with a 480 nm, 130 fs laser pulse at t ) 2 ns following its
working electrode in butyronitrile containing 0.1 M tetra-n-butylammonium
perchlorate.
-
+
formation results in the production of *NI -ANI -NMI-PI. The
(23) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R. J.
4
.0 eV energy of this state is calculated by adding the 1.6 eV
Am. Chem. Soc. 1996, 118, 6767-6777.
24) Wiederrecht, G. P.; Niemczyk, M. P.; Svec, W. A.; Wasielewski, M.
R. J. Am. Chem. Soc. 1996, 118, 81-88.
-
energy of the lowest electronic transition of NI to the energy of
(
the ion pair.21 The intrinsic excited-state lifetime of *NI is 260
-
1
0.1021/ja0041122 CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/14/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 10, 2001 2441
Figure 2. One and two pump pulse transient absorption spectra of 1 in
toluene at the indicated times. Inset: Kinetics at 720 nm following
excitation of 1 with two sequential pump pulses in toluene.
Figure 3. Transient absorption kinetics of 1 in toluene following the
indicated conditions.
+
-
-
NI-ANI -NMI-PI state. The magnitude of the NI bleach directly
ps,25 which provides sufficient time for electron transfer to occur
from this excited doublet state to a nearby electron acceptor.
Switching the direction of charge transport to the second branch
-
monitors the concentration of *NI produced following absorption
of the 480 nm photon. Using the extinction coefficients for the
-
-1
-1 26
-
7
20 nm PI band (ꢀ ) 41 700 M cm ) and the 605 nm NI
-
+
of 1 is detected by the appearance of PI within NI-ANI -NMI-
-1
-1 25
band (ꢀ ) 7000 M cm ) and their absorbance changes in
-
-1
-1 26
PI , which has an absorption at 720 nm (ꢀ ) 41 700 M cm ).
Two pulse transient absorption spectra and kinetics show that this
state forms with τ ) 5 ps, Figure 2.
the spectrum shown in Figure 2, one obtains a 53% yield of the
+
-
+
NI-ANI -NMI-PI state. Thus, the formation of NI-ANI -NMI-
-
PI must be competitive with the τ ) 550 fs (∆G ) -1.2 eV)
charge recombination reaction: *NI -ANI -NMI-PI f NI- *ANI-
NMI-PI. Using this lifetime and the 53% yield of NI-ANI -NMI-
PI , the time constant for the initial charge shift reaction *NI -
ANI -NMI-PI f NI-ANI -NMI -PI is 490 fs. Thus, the time
constant for the subsequent thermal reaction NI-ANI -NMI -PI
f NI-ANI -NMI-PI is about 5 ps (∆G ) -0.5 eV), which
makes this reaction the rate determining step for the formation
There are two likely mechanisms for the switching process
observed within 1. The *NI -ANI -NMI-PI excited state either
-
+
1
-
+
+
+
-
directly populates the NI-ANI -NMI-PI ion pair via a super-
exchange mechanism mediated by NMI, or this process occurs
via sequential charge shift reactions in which the NI-ANI -NMI -
PI state is a real intermediate. The superexchange mechanism
depends on orbital overlap between the LUMO of NMI with both
the *NI excited state (NI LUMO+1) and the LUMO of PI, as
well as the vertical energy gap between the *NI excited state
and the NI-ANI -NMI -PI virtual state. Previous studies have
demonstrated that superexchange mediated electron transfer occurs
only when the LUMO of the bridging group lies above the excited
-
-
+
+
-
+
-
+
-
+
-
-
-
of PI .
-
In addition to facilitating electron transfer to produce the NI-
ANI -NMI-PI state, the NMI bridge also inhibits the return
charge shift reaction: NI-ANI -NMI-PI f NI -ANI -NMI-PI
∆G ) -0.35 eV). When PI is attached directly to the benzene
branch point this reaction occurs with τ ) 400 ps, while insertion
of the NMI bridging group extends this lifetime by about 2 orders
of magnitude to τ ) 25 ns without significantly slowing the
forward reaction rate, Figure 3. The much longer lifetime of the
electron on the thermodynamically uphill PI acceptor is likely
due to the fact that the charge shift reaction to return the electron
+
-
+
-
+
-
-
+
(
-
27
state of the donor, in this case *NI . When the vertical energy
gap is less than 200 mV, charge injection onto the bridge will
occur, resulting in a sequential charge shift to a low energy trap
21
site.1 For 1, in which ∆G ) -0.75 eV for the reaction *NI -
2
-
+
+
-
ANI -NMI-PI f NI-ANI -NMI -PI, charge shift to the NMI
bridge is energetically highly favorable, and precludes the
possibility of a single step, superexchange mediated process. Thus,
the observed τ ) 5 ps time constant for the formation of the NI-
+
-
to NI must be mediated by NI-ANI -NMI -PI. The large positive
G ) 0.5 eV for the reaction NI-ANI -NMI-PI f NI-ANI -
+
-
+
∆
+
-
ANI -NMI-PI ion pair is most likely a result of two sequential
steps: *NI -ANI -NMI-PI f NI-ANI -NMI -PI f NI-ANI -
NMI-PI .
-
NMI -PI results in a slow back electron transfer. Thus, the NMI
bridging group both facilitates the switching process and inhibits
back electron transfer. The results obtained on 1 show that the
discrete energy levels of bridge molecules can be used to optimize
electron flow within molecular architectures designed for potential
device applications such as ultrafast optoelectronic gates.
-
+
+
-
+
-
-
Unfortunately the reduced NMI intermediate absorbs at 420
-
1
-1 25
nm (ꢀ ) 23 500 M cm ) and is hidden beneath absorption
changes due both to ANI and NI. However, the magnitude of the
-
NI band bleach at 605 nm in the transient spectrum of 1
Acknowledgment. This work was supported by the National Science
Foundation (Grant CHE-9732840).
following absorption of the second photon at t ) 2 ns, Figure 2,
can be used as an internal standard to determine the yield of the
Supporting Information Available: Synthetic and spectroscopic
details (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(
25) Gosztola, D.; Niemczyk, M. P.; Svec, W. A.; Lukas, A. S.;
Wasielewski, M. R. J. Phys. Chem. A 2000, 104, 6545-6551.
26) Rak, S. F.; Jozefiak, T. H.; Miller, L. L. J. Org. Chem. 1990, 55,
794-4801.
27) McConnell, H. M. J. Chem. Phys. 1961, 35, 508-515.
(
4
(
JA0041122