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

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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

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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

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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. (jun.li.2@asu.edu )

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

ontophollow

bridge

300

S

S

Z

a) b)

c)

-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

0 10 20 30 40 50 60 70Au Density of States (arb. unit)

-4

-2

0

2

4E

ner

gy (

eV)

HOMO

HOMO

LUMO LUMO

Open Conformer Closed Conformer

Au Fermi Level

β(E)β(E)