Photochemical Reversibility of Ring-Closing and Ring-Opening

Feb 11, 2009 - (47) The disrotatory rotation leads to bonding interaction of the terminal atoms within the HOMO, as shown in Chart 1 However, such rot...
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J. Phys. Chem. C 2009, 113, 3826–3834

Photochemical Reversibility of Ring-Closing and Ring-Opening Reactions in Diarylperfluorocyclopentenes Aleksandar Staykov and Kazunari Yoshizawa* Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Fukuoka 819-0395, Japan ReceiVed: July 28, 2008; ReVised Manuscript ReceiVed: December 4, 2008

Time dependent density functional theory (TDDFT) is used to study the important factors that control the photoisomerization of diarylperfluorocyclopentenes. The calculations are carried out for free molecules and for diarylperfluorocyclopentenes perturbed by gold atoms. Potential energy surfaces for the cyclization reaction are obtained for the ground state and for the excited states involved in the photoswitching. Analysis of the computed UV/vis spectra, the excitation energies, and the spatial distribution of the frontier orbitals of both unperturbed and perturbed molecules give an inside view of the ring opening and the ring closing. The bonding interaction in the unoccupied orbials is considered to be the driving force for the photochemical cyclization while the antibonding interaction significantly hinders the reaction. The obtained theoretical results are in good agreement with the experimental data and provide an explanation of the one-directional and bidirectional photoswitching of diarylperfluorocyclopentenes attached to gold surface. 1. Introduction Diarylethenes are a large class of organic π-conjugated compounds, which exist in two structural forms, open-ring form and closed-ring form, and can convert between each other after photoirradiation with different wavelengths.1 The two isomeric forms are characterized by different π-electron systems. The open-ring form is a diaryl alkene and the closed-ring form is an Rω-substituted polyene. These two types of π-electron systems are characterized with different HOMO-LUMO gaps, different electron density distribution, and different spatial distribution of the frontier orbitals. The change in the electronic structure results in different electronic properties of the isomers.1-5 Thus, by macroscopic action, such as photoirradiation, one can “switch” the properties of an angstrom-size system without destructing it or irreversibly changing its structure.1,6-10 One of the disadvantages of the stilbene, a simple diarylethene, is the thermal instability of the closed-ring form, dihydrophenanthrene, with a lifetime of several minutes.1 With the help of the experimental design this failing has been overcome for the diarylperfluorocyclopentenes, a subclass of the diarylethenes, which show unlimited stability of both isomeric forms.1 Diarylperfluorocyclopentenes can find application in many processes like optical data storage, optical control of molecular magnetism, host-guest interaction in fine organic synthesis, optical control of the electric conductance in nanowires, and optical control of interface electric conductivity.1,3,4,6-13 The advance of electronics pushes the size of electric circuit components toward smaller scales.14 Recently the limit of 35 nm was reached in the CPUs of the leading manufactures. If this tendency is prevented, the molecular scales will be reached soon and the molecular electronics will play a leading role in near future technologies. The advance of the miniaturization will increase the influence of the quantum effects on the electronic components and will rise problems from the competence of modern physics.14 The idea of molecular replacements for the basic components in the electric circuits was originally * To whom correspondence should be addressed. E-mail: kazunari@ ms.ifoc.kyushu-u.ac.jp. Phone: +81-92-802-2529. Fax: +81-92-802-2528.

proposed by Aviram and Rathner with their donor-σ bridgeacceptor diode.15 The mechanically controllable break junction16 (MCBJ) and the scanning tunneling microscopy17 (STM) allowed us to measure the actual I/V characteristics of single molecules and to fabricate molecular devices with desired properties.7,18,19 The development of the nonequilibrium Green’s function method20 (NEGF) and Landauer’s theory21 gave us a theoretical inside view on the processes that take place in the electron transport through molecular junctions. The computational results were in fairly good agreement with provided experimental data.22-27 With the help of the NEGF method many molecular devices were proposed with conductive, diode-like, and transistor-like properties.28 Although the electron transport through single molecules can be described mainly as a physical phenomenon, one of the factors that control it has a discrete chemical origin, which is the phase, the amplitude, and the spatial distribution of the frontier orbitals of the molecule.29,30 The orbital view concept29 gave us a powerful tool to predict the current through molecular systems weakly interacting with the electrodes and we have shown how to apply it on systems that are strongly perturbed by the junction environment.29 In the last years the electron transport through different organic molecules was studied experimentally and theoretically and the I/V characteristics of many molecular wires were published.2,6-12,18,19,22,25-28,31-34 However, the molecular conductance is not sufficient itself for the design of angstrom-size electronic devices. An important issue is the control of the electric current. Theoretical and experimental papers describing diode-like24,28,32,33 and transistor-like molecules34 were published and significant theoretical efforts were made to explain the experimentally observed negative differential resistance18,19,27 (NDR) in organic nanowires. The diarylethenes, particularly the diarylperfluorocyclopentenes, found application as a possible optical “switch” of the electric current.1,2,7-10,16,31 The first experimental work of a unimolecular electric switch was published by Dulic´ et al.7 They have reported 3 orders of magnitude increased resistance upon photoirradiation of diarylcyclopentene nanowire in a MCBJ. The switching cor-

10.1021/jp8066712 CCC: $40.75  2009 American Chemical Society Published on Web 02/11/2009

Photoisomerization of Diarylperfluorocyclopentenes responded to a ring-opening photochemical reaction. They managed to reproduce successfully the experiment. Unfortunately, they failed to switch the nanowire back to its original closed-ring conformation. The same nanowire can be easily switched between both stable forms when it is not perturbed by the gold electrodes. Recently, Feringa8,10 and Irie6 succeeded in modifying experimentally the diarylcyclopentene nanowire in such a manner that the bidirectional, reversible photoswitching, with some limitations, was possible. The resulting reversible photoswitching device is a network of nanosized gold particles bridged by diarylperfluorocyclopentene nanowires in the study of Irie,6 and a self-assembling diarylcyclopentene monolayer on a gold surface with an organic top electrode in the study of Feringa.8 Both experimental groups, so far, could not design a unimolecular, reversible photoswitching of electrical conductance, which can be observed by the MCBJ technique. One can conclude that the observed reversible photoswitching is a statistical phenomenon and only a few of the molecules in the monolayer or in the gold nanonetwork really switch, while the majority of the molecules cannot overcome the potential barrier between the two stable forms. This conclusion indicates that the electrode environment perturbs the photochemical reaction and in some cases the result is a one-directional switching7 while in others it significantly hinders the bidirectional switching.6,8 While the group of Feringa mainly used in their experimental work nonfluorinated diarylcyclopentenes, the group of Irie investigated diarylperfluorocyclopentenes.6-8 The obtained experimental results are comparable and indicate that the fluorination does not significantly affect the photoisomerization.6-8 In our study we investigate diarylperfluorocyclopentenes, but the obtained results are applicable to both nonfluorinated and fluorinated systems. Dulic´ et al. have made efforts to explain theoretically the one-directional photoswitching of the diarylcyclopentene molecule in the environment of MCBJ with a semiempirical level of theory.7 They have calculated the potential energy surface (PES) of the ground state and several excited states for the free molecule and have aligned them to the Fermi level of the gold electrode. These calculations gave an interesting view of the photochemical reaction, but they are not sufficient for its full understanding because the semiempirical level of theory is not accurate enough to reproduce well the PESs, and the Au atoms from the electrodes will modify the PESs and may change the conclusions. Further theoretical studies of the photochemical reversibility of diarylethenes in the MCBJ environment were performed by using DFT and TDDFT calculations.35-37 The results obtained in the work of Perrier et al.35 provided interesting information about the interaction between the Au surface and the photochromic unit. Their study shows that as a result of that, interaction molecular levels localized on the metal-sulfur bond arise within the HOMO-LUMO gap of diarylethene. Electron excitations to those levels alter the UV/vis spectra of diarylethenes. The study of Perrier et al.35 is limited to a model photoswitching system, but the same approach can be applied on the experimentally observed one- and bidirectional photoswitching molecules.6-8 In our study we have performed such theoretical simulations, which led us to conclusions about the driving force of the photoreversibility of diarylethenes on Au surfaces. The photoswitching of diarylperfluorocyclopentenes attached to gold electrodes is not the only reversibility issue that needs a detailed theoretical investigation. The quantum yield for the ring-closing reaction of free diarylperfluorocyclopentenes is at

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3827 least twice as large as that of the corresponding ring-opening reaction.1 The photochemical reversibility of free diarylethenes and diarylperfluorocyclopentenes was investigated by Nakamura et al. with complete active space self-consistent field (CASSCF) and TDDFT methods.38-40 Their theoretical studies introduced an important view on the cyclization mechanism and provided interesting information about the experimentally observed quantum yields and photoactivities for different conformers. The aim of our work is to suggest a mechanism that explains the photoswitching of free diarylperfluorocyclopentenes to investigate the effect of the perturbation caused by the Au atoms from the electrodes on that mechanism and to study the effect of the spacer unit between the diarylperfluorocyclopentene and the electrodes, which determines the one-directional and bidirectional photoswitching. 2. Computational Methods We used quantum mechanical methods to investigate the photochemical reactions in diarylperfluorocyclopentenes. First we calculated the UV/vis spectra of the two stable isomers, the closed-ring and the open-ring forms. Then, a PES scan of the ground state along the reaction coordinatesthe bond breaking that causes the ring openingswas performed. Once we have the energy profile of the ground state, we scan the PES of the first and, if necessary, of the higher excited states. From the UV/vis spectra and from the calculated oscillator strengths (f), we can conclude along which excited state PES actually performs the photochemical reaction. The calculations were repeated for diarylperfluorocyclopentenes perturbed by Au atoms and the results of the PES scans were compared with those of the free molecules. Analysis of the spatial distribution of the frontier orbitals was used to explain the energy profile of the PESs. All calculations in this paper were performed with the Gaussian 03 program package.41 PES scans of the ground states were performed at the DFT B3LYP42-44 level of theory with the 6-31G* basis set45 for the C, S, F, and H atoms and the LANL2DZ basis set46 for the Au atoms. The geometries were optimized for every step of the surface scan and only the reaction coordinate was kept frozen. The PES of the first and the higher excited states were performed with the TDDFT B3LYP level of theory. The excited state scans were done for the same steps like the ground state scan. For the excited state scans we performed single-point energy calculations for the corresponding optimized geometries of the ground state. The reason for this approximation is the large size of the investigated molecules whose excited states cannot be optimized with the available numeric gradient methods. The ring-opening reaction coordinate is the most important geometry change in the diarylperfluorocyclopentene molecules for the ground state, as well as for the excited states, and the variation along it takes into account all important features that influence the photochemical reaction. The number of excited states included in the TDDFT calculations is 30. 3. Results and Discussion 3.1. Basic Diarylperfluorocyclopentene Unperturbed by Au Atoms. The investigated photochemical reactions are summarized in Figure 1. Our first aim is to investigate the photochemical reversibility of the basic photoswitching unit, 1,2-bis(3-methylthienyl)perfluorocyclopentene, shown in Figure 1, part A1, in its free form and the perturbed form by Au atoms because all other diarylperfluorocyclopentenes can be considered as its derivatives. The free form of the basic photoswitching

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Figure 1. Investigated diarylperfluorocyclopentenes: (A) basic photoswitching system, (B) one-directional photoswitching system, and (C) bidirectional photoswitching system. The free molecules are indicated by index “1” and the diarylperfluorocyclopentenes perturbed by Au atoms are indicated with index “2”. In the left column are given the closed-ring forms and in the right column are given the open-ring forms. The reactions for which no experimental data were reported are indicated with an asterisk.

unit is investigated experimentally by Irie.1 In the ground state the open-ring form exists in two conformations, parallel, when the two methyl groups are spatially situated on the same side of the plain determined by the cyclopentene ring, and antiparallel, when the methyl groups are situated on the opposite sides of the plain. The experimental results1 show that only the antiparallel form can take part in a photochemical cyclization while the parallel form is photochemically inactive, as shown in Figure 2. This means that at the moment of photoirradiation at most 50% of the open-form molecules can convert to the closed form. The actual experimental results1 for the ring closing of the basic photochemical system show a quantum yield of 0.21. The experimentally observed quantum yield1 for the reversed reaction is 0.13. The open form is twice more photochemically active than the closed form. First we explain the photochemical activity of the antiparallel open form compared to that of the parallel open form. The photoswitching of diarylethenes is a cyclization reaction for which the Woodward-Hoffmann rules47 are fulfilled. The frontier orbitals of the antiparallel open-form system, calculated at the B3LYP level of theory, are shown in Figure 3. A

Figure 2. Photochemical reversibility of the basic photoswitching unit.

hexatriene structure takes part in the cyclization reaction. For a system with (4n + 2) π-electrons the symmetry-allowed ground state rotation is disrotatory.47 The disrotatory rotation leads to

Photoisomerization of Diarylperfluorocyclopentenes

Figure 3. Frontier orbitals of the basic diarylperfluorocyclopentenes antiparallel form.

CHART 1

CHART 2

bonding interaction of the terminal atoms within the HOMO, as shown in Chart 1 However, such rotation will bring the methyl group attached to one of the terminal C atoms very close to the closing ring. This steric hindrance will block the ground state reaction. In the excited state an electron is promoted from the HOMO to LUMO. The Woodward-Hoffmann rules say that for a system with (4n + 2) π-electrons the symmetryallowed excited state rotation is conrotatory.47 The conrotatory rotation leads to bonding interaction of the terminal atoms within the LUMO, as shown in Chart 1. In the case of conrotatory rotation, the two methyl groups move away from the closing ring and do not disturb the reaction. The Woodward-Hoffmann rules and the steric factors determine high activation barrier for the thermally activated reaction and low activation barrier for the photochemical reaction of antiparallel diarylperfluorocyclopentenes. When we apply the Woodward-Hoffmann rules47 to the parallel open form, we can see that the disrotatory rotation in the ground state does not lead to any steric hindrance, as shown in Chart 2. However, in the excited state, the conrotatory rotation brings one of the methyl groups close to the closing ring, which blocks the cyclization reaction. The Woodward-Hoffmann rules and the steric hindrance determine low activation barrier for the thermally activated reaction and high activation barrier for the photochemical reaction of parallel, open-form diarylperfluorocyclopentenes. The frontier orbital analysis is a powerful tool, which allows us to easily explain complicated organic reactions involving photoexcitations. The second purpose of our study is to determine the reason for the higher photochemical activity of the open-ring form

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3829 compared with that of the closed-ring form. We have performed a PES scan along the reaction coordinatesthe distance between the two terminal C atoms that participate in the bond formation. Another parameter, the torsion angles, plays an essential role in the cyclization, but it is not independent from the distance between the terminal C atoms. The results of the PES scans for the ground state and for the first excited state are summarized in Figure 4A. As predicted by the Woodward-Hoffmann rules,47 the ground state is characterized by high potential barrier between both stable isomers. The activation energy required for the ring-opening reaction is 44.7 kcal/mol. The reversed reaction requires 57.3 kcal/mol. The ground state reaction is impossible at room temperature, which makes both isomers thermally stable. This is a necessary condition for the design of a photoswitching device. Generally, the PES of the first excited state is similar to that of the ground state, but it is characterized by a lower energy barrier.48 This is not the case for diarylperfluorocyclopentenes, which makes these compounds very suitable for the use of photochemistry. The activation energy for the ringopening reaction in the first exited state is 8.8 kcal/mol, which is significantly lower than that of the ground state and can be easily overcome at room temperature. For the ring-closing reaction the energy of the first excited state decreases with a decrease in the reaction coordinate and reaches a minimum corresponding to the closed-ring form after an insignificant potential barrier of 1.6 kcal/mol. The TDDFT calculations show that the first excited state is a result of an electron transition from the HOMO to LUMO, which explains the good application of the Woodward-Hoffmann rules47 for this system. Similar first excited state PES for diarylethenes were calculated by Nakamura et al.38 The driving force of the ring-closing reaction is the energy minimization of the first excited state, due to the bonding interaction of the terminal C atoms in the LUMO. Thus, the electron transfer to the LUMO is crucial for the cyclization of diarylperfluorocyclopentenes. In a recent work Dulic´ et al.50 discussed the thermal dependence of the photochemical reaction of diarylpentenes and diarylperfluorocyclopentenes. They have shown that while temperature does not affect the ring-closing reaction, the ring-opening reaction is blocked at a certain cutoff temperature, ca. 130 K. This result is a piece of evidence for a higher thermal barrier on the excited state PES for the ring opening, which is absent in the case of ring closing. Our theoretical calculations are in agreement with these experimental data. The type of photochemical reaction can determine the photoactivity of the two isomers. In general, there are three basic types of photochemical reactions: adiabatic photoreaction, diabatic photoreaction, and hot groundstate reaction, shown in Chart 3.48 In the adiabatic photoreaction, the global minimum of the excited stated coincides with the minimum for the product of the ground state. The PES of the excited state is flat and any potential barriers can be easily overcome. For adiabatic photoreactions, theoretically, it is possible that all excited reactant molecules can transform to the product. The diabatic photoreactions are characterized with the minimum of the excited state, which coincides with the transition state (TS) of the ground state PES. The result is radiationless transition to the TS and equal possibility for the excited molecules to proceed to the product or to return to the reactant minimum. Theoretically, less than hundred percent of the excited molecules can convert to the product of the reaction. The quantum yield of diabatic photoreactions strongly depends on the exact location of the excited state minimum along the reaction coordinate. Depending

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Figure 4. Potential energy surfaces of the basic diarylperfluorocyclopentene for the ground state (S0), the first excited state (S1), and a higher excited state that participates in the photoswitching (Sn): (A) unperturbed system and (B) perturbed system.

CHART 3

on its location, right or left from the ground state TS, the quantum yield can be significantly shifted toward the product or the reactant, respectively. In the hot ground state reactions, a radiationless relaxation from the excited state to the reactant provides energy enough for the molecule to overcome the TS. From the PESs in Figure 4A, we can conclude that the ring closing is an adiabaitic photochemical reaction while the ring opening is a diabatic photochemical reaction. This conclusion explains the difference in the quantum yields for the bidirectional photoswitching of the free diarylperfluorocyclopentenes. Nevertheless, this conclusion is based on the type of photochemical reaction and it provides only a qualitative explanation for its quantum yields. As mentioned above, the exact location of the conical intersection can alter the results quantitatively. In Figure 5 are shown computed UV/vis spectra for the open-ring and the closed-ring forms. The wavelengths of the irradiating light are denoted with transparent red rectangles: 500-700 nm for the closed form and 300-400 nm for the open form. The HOMO-LUMO excitations take place in these intervals. The oscillator strength for the ring opening is 0.23 and for the ring closing it is 0.08, which shows that a relatively low percentage of the molecules are excited and participate in the photochemical reaction. It is worthy to denote the additional HOMO-1 to LUMO+1 excitation with oscillator strength of 0.11 for the open-ring form. Although it occurs at 247 nm wavelength and does not play an important role for the basic diarylperfluorocyclopentene system, it is important for our further discussion. The LUMO+1 has different faces at the terminal, ring-closing carbon atoms, and in contrast to the LUMO it will lead to an antibonding interaction. Along the reaction coordinate it will increase the energy of the excited state and will block the photochemical cyclization. 3.2. Basic Diarylperfluorocyclopentene Perturbed by Au Atoms. The use of diarylperfluorocyclopentenes as a switching unit in the electronic devices is connected with the attachment of the molecules to the electrode surfaces. With some minor exceptions,49 gold electrodes were used up to now.6-10 The

reported experimental results show a major disadvantagesa reduced quantum yield or complete blocking of the photoswitching reaction.6-10 The perturbation caused by the Au atoms to the photochemical reaction is significant, and its study will provide the experimentalists with useful molecular design evidence. Although there are no experimental results for the basic diarylperfluorocyclopentene system attached to gold electrodes, it is our first object of investigation, because we can derive general conclusions that are applicable to other diarylperfluorocyclopentenes. In Figure 4B PES scans for the basic diarylperfluorocyclopentene perturbed by one Au atom bound to each terminating S atom are summarized, as shown in Figure 1, part A2. The ground state PES is very similar to that of the unperturbed molecule. The activation energy for the ring opening is 30.1 kcal/mol and that for the ring closing is 42.9 kcal/mol. However, the PES of the first excited state is significantly changed. It is similar to the ground state PES and the barrier for the ring opening is 28.5 kcal/mol and that for the ring closing is 22.0 kcal/mol. These potential barriers cannot be easily overcome at room temperature and the photochemical reaction cannot pass through the first excited state. The TDDFT calculations show that the HOMO-LUMO excitations (the first excited state) take place in the IR region for the closed-ring isomer and in the VIS region for the open-ring isomer, which is different from the wavelength of the irradiating light. This means that the photochemical reaction proceeds through a higher excited state. The analysis of the frontier orbitals shows that the LUMO and LUMO+1 are localized on the Au-S bonds. These orbitals are important for the electron transport through the molecular junction, but they have no effect on the photochemical reaction. In addition, it was shown that the oscillator strength of these peaks is high only for relatively small metal clusters and they are a result of charge transfer from the diarylethene molecule to the Au cluster.35 When the number of Au atoms in the cluster is increased to 9, the absorbance is nearly zero.35 In the case of Au surface these peaks will not be observed in the UV/vis spectra. The HOMO and LUMO+2 are very similar to the HOMO and LUMO of the unperturbed system, and one can expect that the HOMO-LUMO+2 transition is responsible for the photoswitching in the environment of the electrodes. The HOMO-LUMO+2 transitions are in the irradiated wavelength interval. We performed a PES scan of the HOMO-LUMO+2 excited state and the results are shown in Figure 4B. The surface is denoted with Sn. That surface is similar to the first excited

Photoisomerization of Diarylperfluorocyclopentenes

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Figure 5. UV/vis spectra, energy diagrams, and spatial distribution of the frontier orbitals for the closed-form and the open-form of the unperturbed basic diarylperfluorocyclopentene (Figure 1, part A1). The oscillator strengths of the transitions are denoted with f.

Figure 6. UV/vis spectra, energy diagrams, and spatial distribution of the frontier orbitals for the closed-form and the open-form of the perturbed basic diarylperfluorocyclopentene (Figure 1, part A2). The oscillator strengths of the transitions are denoted with f.

state of the unperturbed molecule with an activation energy of 14.8 kcal/mol for the ring opening and 1.4 kcal/mol for the ring closing. These potential barriers can be overcome at room temperature, which makes the photochemical reaction possible. However, the oscillator strengths for the transitions to this PES are very low: 0.04 for the closed-ring form and 0.03 for the open-ring form. The low oscillator strengths, as well as the possibilities for radiationless relaxations, make the photochemical reaction statistically impossible. In Figure 6 the UV/vis spectra, the energy diagrams, and the frontier molecular orbitals are summarized. From the calculations of the basic diarylperfluorocyclopentene system, we can derive the following important conclusions: (1) The photochemical reaction in unperturbed diarylperfluorocyclopentenes is a result of the bonding interaction in the LUMO, which leads to minimization of the first excited state energy. (2) In the perturbed system there exists a higher excited PES similar to the first excited state of the unperturbed system, which is a result of an electron transition to an unoccupied orbital similar to the unperturbed LUMO. The photochemical reaction can proceed through this PES. (3) The oscillator strengths of

the transitions to this unoccupied orbital determine the statistical probability of the photochemical reaction. These conclusions will be used to explain the experimentally observed onedirectional and bidirectional photoswitching.6-10 3.3. One-Directional Photoswitching of Conductance. The modification of the diarylperfluorocyclopentene molecule by the addition of π-conjugated spacer units can alter the spectral properties without changing the photoswitching mechanism. Such an alternation of the UV/vis spectra can provide the investigated systems with improved usability. The first successful experiment for the conductance switch was performed by Dulic´ et al.7 They have added thiophene rings at the two ends of the basic diarylperfluorocyclopentene, as shown in Figure 1, part B1. The unperturbed system shows good photoreversible properties, but when it was attached to the Au electrodes, the ring-closing reaction was blocked. However, a one-directional switching was observed with an increased electrical resistance of 2-3 orders of magnitude. In Figure 7 the UV/vis spectra, the energy diagrams, and the frontier molecular orbitals of the perturbed system are summarized, shown in Figure 1, part B2. The wavelengths of the irradiating light are denoted with

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Figure 7. UV/vis spectra, energy diagrams, and spatial distribution of the frontier orbitals for the closed-form and the open-form of the onedirectional optical switch (Figure 1, part B2). The oscillator strengths of the transitions are denoted with f.

Figure 8. UV/vis spectra, energy diagrams, and spatial distribution of the frontier orbitals for the closed-form and the open-form of the bidirectional optical switch (Figure 1, part C2). The oscillator strengths of the transitions are denoted with f.

transparent red rectangles: 500-700 nm for the closed form and 300-400 nm for the open form. The frontier orbital analysis shows that the LUMO and LUMO+1 for both isomers are localized on the Au-S bond. The HOMO and LUMO+2 are similar to the HOMO and LUMO of the basic, unperturbed diarylperfluorocyclopentene system and they are not affected significantly by the thiophene rings. The LUMO+2 determines the bonding interaction of the open-ring form and an electron transition to this orbital will induce the photoswitching. The HOMO-LUMO+2 transition for the closed-ring form occurs at 599 nm, which is within the experimental irradiation. The peak is shifted by 80 nm to the longer wavelengths compared to the basic diarylperfluorocyclopentene. The significant difference from the perturbed basic diarylperfluorocyclopentene system is that the oscillator strength of the transition is 0.20. The high oscillator strength determines the high probability of electron transition to the LUMO+2 and the high probability for photochemical ring opening, which is in agreement with the experiment. The HOMO-LUMO+2 transition for the openring form occurs at 380 nm. The thiophene rings shift the peak with 50 nm. The oscillator strength of this transition is only 0.02, which makes the electron transition to the LUMO+2

unlikely. Furthermore, the transition from the HOMO-1 to LUMO+3 is calculated at 325 nm, which is within the experimental irradiation wavelength. Its oscillator strength is 0.12. The LUMO+3 has antibonding interaction for the terminating, ring closing, C atoms and will increase the energy of the PES along the reaction coordinate. An electron transition to such an orbital will block the ring closing, just as observed experimentally. 3.4. Bidirectional Photoswitching of Conductance. The first reversible, bidirectional conductance switch is based on the basic diarylperfluorocyclopentene system with two benzene rings attached on the terminating C atoms, as shown in Figure 1, part C2. It was used in the photoswitching devices designed by Irie et al.6 and Feringa et al.8 So far, the photoswitching devices are based on monolayers8 or nanoparticle networks6 and the conductance switch is rather a slow, statistical process than a single molecule property. We have investigated the UV/vis spectra of the benzene-substituted diarylperfluorocyclopentene with TDDFT level of theory and have compared the results with those of the thiophene derivative. The results are summarized in Figure 8. Just as in the case of the thiophene derivative, the LUMO and LUMO+1 are localized on the Au-S bonds. The

Photoisomerization of Diarylperfluorocyclopentenes transitions from the HOMO to LUMO and LUMO+1 for the closed-ring form are in the IR spectrum. The LUMO+2 is very similar to the LUMO of the basic, unperturbed diarylperfluorocyclopentene, responsible for the photochemical reaction. The transition from HOMO to LUMO+2 is at 584 nm, which is within the wavelength of experimental irradiation. The oscillator strength is 0.30, which allows the ring-opening reaction. Possible radiationless relaxations can statistically slow down the ring opening in monolayers. The major difference in the properties of the perturbed thiophene derivative, shown in Figure 1, part B2, and the perturbed benzene derivative, shown in Figure 1, part C2, is the ring-closing reaction. For the open-ring thiophene derivative the oscillator strength of the HOMO-LUMO+2 transition was very low and an electron transition to the LUMO+3, an unoccupied orbital with antibonding interaction of the terminating C atoms, is responsible for the blocking of the cyclization. The oscillator strength of the HOMO-LUMO+2 transition for the open-ring benzene derivative is also 0.02. The computed wavelength is 354 nm. However, within the experimental irradiation wavelengths, we have not observed an electron transition to an unoccupied orbital characterized with antibonding interaction for the ring-closing carbon atoms. At 300 nm wavelength, we have calculated a transition to the LUMO+2 from a lower occupied orbital with oscillator strength of 0.37. This high probability electron transition to an unoccupied orbital with bonding interaction of the ring-closing C atoms determines the bidirectional photoswitching of the benzene derivative. 3.5. Calibration of the TDDFT Method for Calculation of UV/vis Spectra of Diarylethenes. The calculations of the UV/vis spectra of diarylethenes throughout this work were performed with the TDDFT method at the B3LYP level of theory with the 6-31G* basis set. The conclusions about the reversibility of the photochemical reaction based on the UV/ vis spectra require calibration of the theoretical method with experimentally available data. We have calculated the UV/vis spectra of the free nonsulfidated benzene-substituted diarylperfluorocyclopentene shown in Figure 1, part C1, in its closedring and open-ring forms. The experimental UV/vis spectra for these compounds are available in the literature.1 The experimentally observed peak, which corresponds to an excitation to the first excited state for the closed-ring isomer, is at 562 nm.1 Our theoretical prediction is at 596 nm. The difference of 34 nm corresponds to 0.14 eV photon energy or 3.3 kcal/mol. The theoretical prediction for the peak corresponding to an excitation to the first excited state for the open-ring form is at 343 nm. The experimentally observed peak is at about 280 nm.1 The shift corresponds to 0.36 eV photon energy or 8.3 kcal/mol. 4. Conclusions We have studied the photoswitching phenomena in diarylperfluorocyclopentene with the TDDFT level of theory. The aim of our work was to explain the existing experimental results and to offer a theoretical approach that will help the prediction of the photochemical reversibility. We have applied the Woodward-Hoffmann orbital rules to explain the photoactivity of the antiparallel isomers compared to the parallel ones. The calculated PESs help us to determine the types of photochemical reactions in both directions and to explain the different photoactivity of the open-ring and the closed-ring forms. With the help of the frontier orbitals analysis, we have determined the driving force of the photocyclizationsminimization of the PES energy, due to the bonding interaction of the ring-closing C atoms in the LUMO or a higher virtual orbital. We have

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3833 shown that a high probability electron transition to such an orbital allows the photochemical reaction, while an electron transition to an unoccupied orbital characterized with antibonding interaction of the ring-closing C atoms blocks the photoswitching. The UV/vis spectra and the oscillator strengths play an essential role for the photochemistry of the diarylperfluorocyclopentenes. Different substitutuents can alter the optical properties of the compounds, which allow high flexibility for the experimental design. We believe that the suggested theoretical model will provide good support to the experimentalists in their search for photoswitching systems with better reversibility and usability. Acknowledgment. K.Y. acknowledges Grants-in-Aid (Nos. 18350088, 18066013, and 18GS0207) for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Culture, Sports, Science and Technology of Japan (MEXT), the Nanotechnology Support Project of MEXT, and the Joint Project of Chemical Synthesis Core Research Institutions of MEXT for their support of this work. Supporting Information Available: Atomic Cartesian coordinates for the optimized geometries of all investigated structures and complete ref 41 This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Irie, M. Chem. ReV. 2000, 100, 1685. (2) Kondo, M.; Tada, T.; Yoshizawa, K. Chem. Phys. Lett. 2005, 412, 55. (3) Dietz, F.; Tyutyulkov, N. Chem. Phys. 2001, 265, 165. (4) Dietz, F.; Tyutyulkov, N. Phys. Chem. Chem. Phys. 2001, 3, 4600. (5) Dietz, F.; Staykov, A.; Karabunarliev, S.; Tyutyulkov, N. J. Photochem. Photobiol. A: Chem. 2003, 155, 21. (6) Ikeda, M.; Tanifuji, N.; Yamaguchi, H.; Irie, M.; Matsuda, K. Chem. Commun. 2007, 13, 1355. (7) Dulic´, D.; van der Molen, S.; Kudernac, T.; Jonkman, H.; de Jong, J.; Bowden, T.; van Esch, J.; Feringa, B.; van Wees, B. Phys. ReV. Lett. 2003, 91, 207402. (8) Kronemeijer, A.; Akkerman, H.; Kudernac, T.; Wees, B.; Feringa, B.; Blom, P.; Boer, B. AdV. Mater. 2008, 201467. (9) Katsonis, N.; Kudernac, T.; Walko, M.; Wees, B.; van der Molen, S.; Feringa, B. AdV. Mater. 2006, 18, 1397. (10) Kudernac, T.; van der Molen, S.; Wees, B.; Feringa, B. Chem. Commun. 2006, 34, 3597. (11) Zhao, P.; Fang, C.; Xia, C.; Liu, D.; Xie, S. Chem. Phys. Lett. 2008, 453, 62. (12) Nakagawa, T.; Hasegawa, Y.; Kawai, T. J. Phys. Chem. A 2008, 112, 5096. (13) Tyutyulkov, N.; Staykov, A.; Mu¨llen, K.; Dietz, F. Langmuir 2002, 18, 10030. (14) Carroll, R.; Gorman, C. Angew. Chem., Int. Ed. 2002, 41, 4378. (15) Aviram, A.; Ratner, M. Chem. Phys. Lett. 1974, 29, 277. (16) Zhou, C.; Muller, C.; Deshpande, M.; Sleight, J.; Reed, M. Appl. Phys. Lett. 1995, 67, 1160. (17) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. ReV. Lett. 1983, 50, 120. (18) Tour, J. Molecular Electronics; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2003. (19) Chen, J.; Reed, M.; Rawlett, A.; Tour, J. Science 1999, 286, 1550. (20) Brandbyge, M.; Mozos, J.; Ordejon, P.; Taylor, J.; Stokbro, K. Phys. ReV. B 2002, 65, 165401. (21) Landauer, R. IBM J. Res. DeV. 1957, 1, 223. (22) Girard, Y.; Kondo, M.; Yoshizawa, K. Chem. Phys. 2006, 327, 77. (23) Seminario, J.; Zacarias, A.; Tour, J. J. Am. Chem. Soc. 2000, 122, 3015. (24) Stokbro, K.; Taylor, J.; Brandbyge, M. J. Am. Chem. Soc. 2003, 125, 3674. (25) Taylor, J.; Brandbyge, M.; Stokbro, K. Phys. ReV. B 2003, 68, 121101. (26) Taylor, J.; Guo, H.; Wang, J. Phys. ReV. B 2001, 63, 245407. (27) Taylor, J.; Brandbyge, M.; Stokbro, K. Phys. ReV. Lett. 2002, 89, 138301.

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