1
Conduction switching of photochromic molecules
Jun T. Li1, Gil Speyer2, and Otto F. Sankey1
1Department of Physics and Astronomy, 2Department of Electrical Engineering Arizona
State University, Tempe, AZ 85287-1504
ABSTRACT: In pursuing smaller electronic devices for advanced computer
electronics, many molecular switching devices have been proposed [1-4]. Most
switching proposals are based on chemical control over the molecular redox states.
The synthesis of functional groups in single molecules laid the foundation for
massive fabrication of so-called ‘mono-molecular’ devices [5]. Photochromic centres
provide a suitable functional group to control the physical and chemical properties
of a molecule [6]. Recent experiment work observes a dramatic conductance
enhancement of more than two orders of magnitude after the optical switching of
single dithienylethene derivatives on gold [7]. However, both light-induced
structure changes and the ensuing conduction switching are not fully understood.
Here, we report a first-principles theoretical study of dithienylethenes as mono-
molecular devices. We reveal that light-induced intra-molecular conformation
conversion drives the molecular orbital swapping between distinct conjugated
structures. The shuffling of single and double bonds induces a conduction
switching when the molecule is sandwiched between metal electrodes. We attribute
the observed unusual photochromic quenching of one conformer to the alignment
of the metal Fermi level between the molecular frontier orbitals. Simulation such
as these provides a concrete guide to improve the performance of dithienylethene
devices. This study provides the theoretical basis of using dithienylethene
2
molecules as a new solution to integrated optoelectronic devices in post-silicon
technology.
Dithienylethene derivatives are promising photochromic molecules. A skeleton
of dithienylethenes is given in Fig. 1. Upon UV irradiation of the molecules, the open
conformer transforms to the closed conformer (ring closure), while the closed
conformer undergoes a structural transition to the open conformer under visible light
(ring opening). Thermally stable photochromics have been observed in solution,
polymers, glasses and crystals [8-10]. Besides being incorporated in supramolecular
devices as a trigger in optical memory media, photo-optical switching devices, and
displays [11-17], this kind of molecule (similar to 1c) has been fabricated as a mono-
molecular device recently [7]. A conduction enhancement of more than two orders of
magnitude was observed between the closed and open conformers after the optical
switching. The ring opening reaction on gold was observed to occur with visible light as
in solution. However, the ring closure reaction was quenched on gold. This unexpected
quenching challenges our current understanding of photochromic reactions; the ring
closure reaction with UV light has a very high yield compared to the reverse. Earlier
absorption spectra studies of molecules in solution indicated that both ring-closure and
ring-opening reactions occurred on the picosecond time scale [6]. Recent ultrafast
transient absorption spectra studies of poly-1, 2-bis (2-methylthien-3-yl)
perfluorocyclopentene (its unit similar to 1a) and 1, 2-bis (5-phenyl-2-methylthien-3-
yl)-cyclopentene (similar to 1b) in solution recorded an ultrafast optical switching on a
sub-200 femtosecond timescale [18, 19]. The fast switching ability makes dithienylethene
wires an appealing candidate for electronic switches at the molecular scale.
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Due to the heavy computational requirement, theoretical work has been confined
to electronic structure studies of small model systems like 1a and 1b by ab initio
multiconfiguration self-consistent field (MCSCF) [20, 21] and density functional theory
(DFT) [19]. The calculations were limited to the study of the potential energy profile of
static conformations along a reaction coordinate. Here we report a theoretical study of
dithienylethene derivatives as mono-molecular optical switches. We systematically
address the nature of the photo-induced structural changes and ensuing conduction
switching phenomena. We propose that dithienylethene molecules manifest a new kind
of molecular switching mechanism. In this mechanism, the molecular switching is based
on a π-conjugation pathway swapping induced by the optically controlled
conformational changes. It differs from redox controlled molecular switches. The
optoelectron current surge is due to distinct electron tunnelling in the molecular
forbidden gap of a distinct π-conjugation pathway, differing from the traditional
semiconductor based optoelectronic mechanism in which the photocurrent depends on
the photoinduced excess carriers.
The frontier orbital π-path is an electronic tunnelling channel from one R-group
to the other R-group (Fig. 1). This forms the basis of distinct transport patterns of open
and closed conformers. The frontier orbitals, the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO), distribute along the
same π-conjugation chains as shown in Fig. 1. The HOMO has the π bonding character
while the LUMO has π* anti-bonding character. The first-principles electronic structure
calculation presents different π-paths for open and closed conformers. The π-path of
the open conformer contains severe distortion along the hexatriene centre (clockwise
ring from 2 to 2’, left panel of Fig. 1) due to the nonplanar geometry caused by
repulsion between X-groups. The frontier orbitals become localized around the highly
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strained 2/2’ positions. In the closed conformer, the frontier orbital is more extended
and coplanar along the shown polyene chain in Fig. 1 instead of around the cyclo-
hexadiene ring (clockwise ring from 2 to 2’, right panel of Fig. 1). In the open
conformer, the sulphur’s lone pair interrupts the π-channel while in the closed
conformer, the sulphur’s lone pair is located at the termini of the π-channel.
The photochemical ring-closure reaction follows the Woodward-Hoffman rule in
symmetry-conserved conrotatory mode [22]. The conserved C2 symmetry rotation axis is
showed in Fig. 1. The shuffling of the single-double bonds during structural
transformations correlates 6 (π, π*) orbitals of hexatriene in the open conformer to 4 (π,
π*) plus 2 (σ, σ*) orbitals of cyclo-hexadiene in the closed conformers as showed in the
orbital correlation diagram of Fig. 2a. During the reaction, the two sets of orbitals,
including the frontier HOMO and LUMO orbitals, undertake an orbital swap between
the two distinct π-paths.
We simulate the ring closure reaction and orbital swapping for free molecules
(without gold attached) by a direct first-principles quantum molecular dynamics
calculation [23]. The photo-absorption is simulated by boosting an electron in the open
molecule from HOMO to LUMO, constituting the HOMO-LUMO excitation
configuration. In each time step, forces are obtained from the Hellmann-Feynman
theorem and electrons follow the Born-Oppenheimer surface (details are given in
Methods; dynamic movies of 1a are provided as supplementary information). The
physics in this approach is that the excitation of a HOMO electron to the LUMO leads
to a reduction of attraction from the χ3 (π-bonding) orbital and an increasing of
repulsion by the χ4 (π*-antibonding) orbital (seeing Fig. 2a). This breaks the initial
doubly occupied π-bonds in favour of a π*-antibond and the system races to find a new
balance in the bonding; the open conformer undergoes a structural change to stabilize
the corresponding excited state surface of the closed conformer. Fig. 2b records a
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reaction coordinate (the distance between carbon atoms at 2 and 2’ sites) evolution
during structural inter-conversion. After almost 100 fs (400×0.25 fs), the open molecule
distance falls from 3.4 Å to about 2 Å and subsequently drops into oscillations about the
single C bond length of 1.5 Å of the closed conformer. Accompanying the structural
transition, Fig. 2c shows clearly how the two sets of frontier π orbitals are naturally
correlated. The open conformation (t=0) has a HOMO-LUMO gap of 3.3 eV (the
HOMO-LUMO gap of closed conformation is 1.7 eV). The crossing of the HOMO and
LUMO at 100 fs is a convenient marker for the ultrafast frontier orbital swapping
between distinct π-paths, defining a characteristic switching time which represents the
non-radiant relaxation after photon absorption. Since the HOMO-LUMO gap changes
after switching, this is in accord with the experimentally observed sub-200 fs optical
switching [18, 19]. Other theoretical work indicates that the ring closure reaction is along a
largely downhill direction in the energy landscape [20]; suggesting that this reaction is
not reversed spontaneously. Thus the reverse reaction of optically exciting the closed
conformer to produce the open conformer is not as clearly understood. This reaction
may involve more complex excitations such as multiphoton processes, which leads to
breaking the σ bond from 2 to 2’ [24].
We have demonstrated that the optical switching of conformations induces the
frontier orbital swapping between two distinct π-conjugation pathways. We now show
that the two molecular conformers have distinct electronic transport properties when the
molecule acts as a molecular wire. We construct a conceptual device, in which
dithienylethene molecules are sulphur-bonded between gold electrodes in a sandwich
junction. Fig. 3 gives a detailed profile of such a device. This configuration is to mimic
the often used approach of scanning tunnelling microscopy [25] and has been used in our
previous study of carotene [26]. In the low bias region, the molecular HOMO-LUMO gap
produces a forbidden region for the conducting electrons of the metal [26]. The tunnelling
current can be evaluated from the transmission function in the framework of Landauer
6
transport theory [27-29]. Previous practice of I-V curve calculations of sandwiched
molecular wires has revealed that the generic I-V characteristic is determined primarily
by the electronic structure of the molecule [30].
We use the Landauer transport theory to study the I-V characteristics of
sandwiched dithienylethene molecules [31]. The self-assembled monolayer (SAM) on
gold surfaces is not fully understood, and there remains some controversy about the Au-
thiolate adsorption sites [32-34]. Model calculation indicated that the current could change
by more than an order of magnitude for different contact configurations [35], although a
detailed consideration of the surface bonding between sulphur and the gold surface for
carotene has produced good agreement between theory and experiment [26]. To
compensate for our uncertainty about the interface between dithienylethenes and the
gold surface, we consider three typical contact sites: ontop, bridge and hollow sites, as
indicated in Fig. 3b. DFT calculations show that the hollow site is the energetically
preferred site. For a given site, the molecule can, in principle, orient along the conical
surface of Fig. 3c. We assume that the molecule is tilted as an alkane SAM. For a given
orientation at a given site, we choose configurations so that the molecule is
symmetrically bonded between two parallel gold slabs with the surface (vertical)-S-C
angle close to 1100 as possible, the chemical surface bonding requirement [25]. I-V
curves are calculated for the well-defined contacts at the three bonding sites and our
attention is drawn to a systematic comparison between the two dithienylethene
conformers. We expect that the first-principles calculation will reveal trends in the I-V
characteristics of the two dithienylethene conformers.
The I-V curves of 1a are given in Fig. 4. It is clear that the closed conformer
conducts much better than the open conformer in all the considered contacts. It is
striking that the conduction enhancement, defined as the ratio of the low bias resistance
of the open conformer to the closed conformer, is relatively similar, being 22, 39, and
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31 for ontop, bridge, and hollow sites, respectively, even though the I-V curves
themselves for the different sites may vary by an order of magnitude. From these
observation, we predict a 20-40 times conduction enhancement between the closed and
open conformers during optical switching (other molecules, e.g. 1b, give different
results). Such a significant change in the tunnelling current should be easily measurable
(measurement has been done for the molecule similar to 1c [7]).
The nature of the conduction enhancement is largely attributed to the distinct
frontier orbital of the conformers. However, the two conformations also produce very
distinct alignment of the metal Fermi level within the molecular HOMO-LUMO gap.
Fig. 5 shows a schematic of the difference in the alignment for 1a. Table 1 shows that
the metal Fermi level is near the HOMO of the closed conformer (at the lower edge of
the β(E) decay curve [26]), giving an enhancement in the tunnelling current. On the other
hand, the metal Fermi level is located much nearer the middle of the HOMO-LUMO
gap of the open conformer (near the branch point of the β(E) decay curve [26]), leading
to a reduction in the tunnelling current. The significant dependence of I-V curves on the
contact sites reveals the exponential sensitivity of the alignment of the metal Fermi level
within the HOMO-LUMO gap [26, 36], which is self-consistently determined by the
interaction between the surface bonding sulphur and the gold atoms in the contact. It has
been found that the thiolate head is in sp hybridization at the ontop site and in sp3
hybridization at the hollow site [34]. Different bonding alters the alignment, and produces
an order of magnitude change in the tunnelling current. The ontop contact of the closed
conformer is an example of near resonance with a resistance close to the quantum
conductance, 77 µS (12.9 kΩ).
We have used model 1a to illustrate the nature of the frontier orbital swapping and
the intrinsic molecular conductance. We extend the theoretical prediction to the more
complex 1b and 1c (1d is discussed later), in which the R-groups are replaced by
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phenyl- and thienyl-rings, respectively. Table 2 shows the conduction performance can
be improved by using different functional groups. The enhancement is improved
dramatically in 1c. The predicted 100 times enhancement for the energetically preferred
hollow-site contact can be directly compared to the recent measurement of the
conduction enhancement on this molecule [7]. This prediction agrees well in the low end
of the experimental enhancement range from 100 to 1000 times. This shows the
theoretical ability to identify optimal candidates for molecular device design. We
conclude that dithienylethene derivatives possess the desired properties of
optoelectronic switches.
The observed quenching of the ring closure reaction will hinder the application of
dithienylethene molecules as optoelectronic switches. Here we give an account for the
quenching mechanism in the presence of gold contacts. We find in the dynamic
simulation that the structural conversion is uniquely associated with the HOMO-LUMO
excitation. Other direct exciting, e.g. from HOMO to LUMO+1 or from HOMO-1 to
LUMO, etc. will not give the ring closure reaction. Thus the ring closure reaction
quenching is equivalent to the quenching of the HOMO-LUMO excitation. The
experiment gives a strong evidence of the photo absorption quenching for the open
conformer on gold [7]. The life time of the excitation must be longer than the
characteristic switching time (in the hundred fs). The key factor affecting the life time
of the excited states is the electron transfer between the molecule and nearby metals.
The quenching of the open conformer is consistent with our finding of the alignment of
the metal Fermi level. We find the HOMO of the open conformer to be buried deep
under the Fermi level in a region of high density of states, while the closed conformer
lies close to the Fermi level. A deep lying HOMO level at a high metal density of states
offers the opportunity for many more possible electron transfer events via a Marcus
model [37], thus reducing the life time of the hole, and quenching the transition. Our
finding for the alignment of the metal Fermi level for 1a is shown in Fig. 5 (the
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alignment trend is the same for 1c). The HOMO of the open conformer is buried more
than 1 eV beneath the metal Fermi level (at the fast increasing edge of the 3d band of
Au). In contrast the HOMO of the closed conformer is within the low density of states s
band near the Fermi level. Both LUMOs are located at a similar energy location within
the Au density of states and play a secondary role on the quenching. Our hypothesis is
that the open conformer quenching mechanism is the electron transfer between the
metal contact and the hole on the switching unit, a remedy to reduce the quenching is to
reduce the interaction between the switching π-paths and the metal surface states. We
propose model 1d to reduce the electron transfer. In 1d, we introduce alkane chains to
separate the switching unit from the gold electrode. Calculations indicate that the
electronic states associated with the switching unit in 1d has a sharper local density of
states than in 1a, indicating less entangling between switching π-orbitals and metal
surface states in 1d. This may reduce the quenching of the ring closure reaction since
the electron transfer between the molecule and gold electrodes strongly depends on the
overlap matrix elements between the molecular orbital and the metal states [37]. Table 2
shows that 1d still has the same switching and similar enhancement as 1a. This may
provide the means to turn the one-way switch into a reversible switch.
We conclude that the unique π-electron systems make dithienylethene molecules
novel mono-molecular optoelectronic switches. Our calculation shows that there exists
one to two orders of magnitude conduction enhancement during the ultrafast optical
switching process. Dithienylethene molecules may manifest the first conformational
change-induced mono-molecular optical switch. We provide examples to tailor the
performance. We address the nature of recent observed phenomena in dithienylethene
mono-molecular switches. This work provides the theoretical foundation for the design
of similar devices.
Methods
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In general, the DFT optimised structure does not reproduce the single-double
bond alternation as accurately as Hartree-Fock for conjugated system [38]. We use the
Hartree-Fock relaxed molecule structures in a 6-31G basis within GAMESS [39] as the
initial structure of the photo-induced dynamic simulation and in the construction of
molecular devices (The dynamics simulation does not depend on the choice of the initial
structure; we also achieved the same dynamics on the well-relaxed structure by
Fireballs).
The main first-principles code used is Fireballs [40], a local atomic-orbital DFT
based method in the pseudo-potential local-density approximation (LDA). In this
method, the Hamiltonian is constructed to simulate the photochromic dynamics and to
serve as the input for the I-V curve calculation. We must emphasize that the main
theoretical results are not dependent on one method, such as Fireballs. The plane-wave
basis DFT method, VASP [41] and Hartree-Fock method GAMESS give the same
frontier orbital characteristics of Fig. 1. We also achieved qualitatively the same
dynamics and I-V curves using another DFT code, Siesta [42].
To simulate the photo-induced dynamics, we adopted the simple and direct
approach as proposed by Fedders [43] et al., and has been successfully used in the study
of light-induced phenomena of glassy systems [23]. In this approach, the photo
absorption is simulated by boosting an electron from the HOMO to LUMO. The
Hellmann-Feynman force is calculated by the excited electronic configuration. The
conformation is allowed to freely evolve according to the force field. Due to the ionic
kinetic energy transferred from the excited electronic energy, the structural coordinate
will be driven over the excited energy surface to a new relative minimum. We ignore
the spin polarization and the time dependence of the exchange-correlation potential [44].
We used the standard DFT-LDA for the perturbed system with the rationale from
quasiparticle studies. Hybertsen and Louie have proven that the quasiparticle
11
wavefunctions usually produce good overlap with the LDA results [45]. Here the
important point is the quality of the wavefunctions, which rationalizes the calculation of
the force field. Of course, DFT does not accurately reproduce the energy spectrum but
does well with trends, which is what we emphasize in this work.
The conduction calculations employ a Green’s function transport kernel [31, 46, 47],
which uses electronic structure calculations from the DFT-LDA program. The Green's
functions needed to compute the current-voltage curves are calculated from the
Hamiltonian and overlap matrix elements by solving the self-consistent (at zero bias)
Kohn-Sham equation of the supercell system in Fig. 3. Though the electronic structure
calculation is performed with gold slabs of finite thickness, we extend the Green's
functions to include semi-infinite contacts using a block recursion technique [46].
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Supplementary Information photon-induced orbital dynamics of 1a are provided.
Acknowledgements: This work was supported by the NSF (ECS 01101175 and DMR 9986706). We are
thankful for long term discussions with D. Gust, S. M. Lindsay, T. A. Moore, A. L. Moore, N. J. Tao, D.
Ferry, and Jin He.
Correspondence and requests for materials should be addressed to Jun Li. ([email protected] )
figure 1. The molecule contains C2 symmetry along the indicated rotational axis.
The sulfur bonded to H in the R groups is used to bond the molecule to the gold
electrodes for the conductance calculation after dehydrogenation. In many
dithienylethene derivatives X-sites are methyl groups. The dashed lines depict
the π-electron path of the conjugated systems.
figure 2. The photoinduced π-orbital flips of model 1a. a) The orbital correlation
diagram by the Woodward-Hoffman rule. Each orbital may be classified as S or
A according to its symmetry or anti-symmetry about the C2 rotation axis. The
dumbbell is used to show the p orbital polarity at relevant C atoms only. (χ1 χ2
χ3) and (σ π1 π2) are occupied and (χ4 χ5 χ6) and (σ* π3 π4) are unoccupied in
the ground state for hexatriene (in open conformer) and cyclo-hexadiene (in
closed conformer), respectively. The two sets of orbitals are connected
17
according to the orbital symmetry and the bonding pattern along the reaction
coordinate. b) The reaction coordinate in the ring-closure reaction from the
photoinduced dynamical simulation. c) The orbital dynamics by tracking
electronic eigenvalues. The crossing of the frontier orbitals at 100 fs shows the
correlation of orbitals during the structural conversion from the open conformer
to the closed conformer.
figure 3. The device of model 1a. a) Side-view of the metal-molecule-metal
junction. The gold electrode is an infinite two dimensional lattice (3X3) made up
of eight ideal Au (111) layers in a supercell structure as in [26]. Testing
calculations indicate that the I-V curves of the (3X3) slab converged well to the
result of a (4X4) slab. b). Three typical contact sites; ontop, bridge and hollow.
The distance between gold surface and sulfur is 2.42[32], 2.07[33], and 1.9[34, 36] Å
for ontop, bridge and hollow sites, respectively. c) Molecular orientation. We
assume the dithienylethene can be inserted into a SAM alkane matrix tilted 300
as in the case of carotene [25, 26]. On a given contact site, the molecule possibly
directs along a conical surface oriented 300 about the surface normal.
figure 4, I-V curves of model 1a at a) ontop, b) bridge and c) hollow sites. Each
curve is averaged from 4 molecular orientations. The resistance is calculated
from the linear low bias region. The resistance of closed conformer at ontop site
shows the smallest orientation changes, less than 2%; the hollow site has the
largest dependence of 37%.
figure 5, The schematic alignment of the Au Fermi level between the HOMO-
LUMO gap of the open and closed conformers of model 1a. The semi-elliptical
β(E) curves indicate how the tunnelling decay rate is expected to change with
Fermi level alignment.
18
Table 1. Parameters of model 1a. Eg=HOMO-LUMO gap, τ=open to closed switching time, ∆E=HOMO with respect to Au Fermi level.
Open Closed Eg (eV) 3.326 2.225
τ (fs) 100 Ontop Bridge Hollow Ontop Bridge Hollow
∆E (eV) -1.168 -1.387 -1.414 -0.096 -0.131 -0.352
Table 2. Dithienylethene models with different R groups. Enhancement is defined as the low bias resistance ratio of the open conformer to the closed conformer. We assume that the conformation change would not affect the contact significantly. In estimating the enhancement, we compare a pair of conformers, which have similar orientation at the same contact. (o-b-h) means the ontop, bridge, and hollow sites.
Model 1b 1c 1d
Open Closed Open Closed Open Closed
Eg (eV) 3.152 1.697 3.015 1.642 3.089 2.171
τ (fs) 100 88 100
Enhancement(o-b-h) 37-29-47 57-53-109 10-11-16
1a, X=H, R=HS
1b, X=H, R=phenylthiol
1c, X=H, R=thienylthiol
1d, X=H, R=propanethiol
RX
RXS S
Open conformer Closed conformerUV.
VIS.
RX
RXS S
3.4Å1.5Å
C2
C2
2 2’34
53’ 4’
5’ 234
5 2’3’4’
5’
SH
SSH
SH
S S
S S
S S
S S
S S
S S
S S
S S
S S
S S
S S
S S
χ1, A
χ2, S
χ3, A
χ4, S
χ5, A
χ6, S
S, σ
A, π1
S, π2
A, π3
S, π4
A, σ*
a) Orbital correlation diagramb) Reaction coordinate
0 200 400 600 800 1000Time (×0.25fs)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Dis
tanc
e (Å
)
0 200 400 600 800 1000Time (×0.25fs)
-6
-4
-2
0
2
Eig
en v
alue
(eV
)χ3
χ4
π2
π3
c) Orbital dynamics
-30-20-10
0102030 Closed
Open
-30-20-10
0102030
Cur
rent
(µA
)
-1.0 -0.5 0.0 0.5 1.0Bias (V)
-30-20-10
0102030
a) Ontop
b) Bridge
c) Hollow
Rclosed
=18.9KΩR
open =423.7KΩ
Rclosed
=61.2KΩR
open =2392KΩ
Rclosed
=136.2KΩR
open =4167KΩ