Tuning the Energy Levels of Photochromic Diarylethene Compounds

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Tuning the Energy Levels of Photochromic Diarylethene Compounds for Opto-Electronic Switch Devices Fredrik L. E. Jakobsson,† Philippe Marsal,‡,§ Slawomir Braun,# Mats Fahlman,# Magnus Berggren,† Je´roˆme Cornil,‡ and Xavier Crispin*,† Department of Science and Technology, ITN, Linko¨ping UniVersity, Campus Norrko¨ping S-601 74 Norrko¨ping, Sweden, Department of Physics, Chemistry and Biology, IFM, Linko¨ping UniVersity, S-581 83 Linko¨ping, Sweden, Laboratory for Chemistry of NoVel Materials, UniVersity of Mons-Hainaut, B-7000 Mons, Belgium, and CNRS, Aix-Marseille UniVersite´, CINAM-UPR3118, Campus de Luminy, Marseille cedex 09, France ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: September 3, 2009

Diarylethene molecules are photochromics (PCs) currently investigated for use in optical write/electrical read memory applications. The impact of the photoisomerization of PCs on the device behavior is analyzed with charge transport models. These results indicate that good electrical current switching can be achieved in a device when the PCs are combined with an organic semiconductor (in multilayered structures or blends). The frontier energy levels and dipole moment of a series of diarylethene compounds have been calculated using density functional theory. A good agreement is found between the calculated electronic structure and the measured ultraviolet photoelectron spectra. Shifts in the frontier energy levels and dipole moment are generated through two different approaches for chemical modification: (i) by changing the chemical nature of the aryl rings or (ii) by adding substituents on the ethylene bridge. The frontier energy levels can be tuned by more than 2 eV via such chemical modifications. We find that, for this family of photochromic compounds, the photoinduced current switch effect in diodes is mainly due to the modulation in the frontier energy levels rather than the changes in the amplitude of the dipole moment. 1. Introduction Molecular photochromics (PCs) undergo a reversible photoisomerization between two isomers upon irradiation of light at different wavelengths. The changes in the electronic structure result in different photophysical properties for the two isomers, such as the electron affinity, ionization potential, optical absorption, and fluorescence spectra, as well as the dielectric constant. Photochromism has been used in commercial products such as in ophthalmologic lenses and has been investigated for both optical data storage1-3 and optical switching.4 For optical data storage applications, the molecular PCs of one pixel are switched into a specific isomer state upon irradiation with light of a specific wavelength. Since the difference in optical absorption of the two isomers is significantly different, an optical readout approach can be used to sense the state of the PCs. However, optical readout is normally destructive since the state of the molecules is altered when probing with light. An alternative approach for readout is to instead measure the electrical properties of the PCs in a twoterminal device. This has been used in molecular junctions, in which the different electronic coupling between the PCs (while switched in between the two states) and the metallic contacts results in a switching of the conductance.5-8 However, fabrication of molecular junctions requires advanced techniques and precise control of the processing environment and is typically very expensive. On the other hand, neat films of molecular PCs, * Corresponding author. E-mail: [email protected]. † Department of Science and Technology, ITN, Linko¨ping University. ‡ University of Mons-Hainaut. § Aix-Marseille Universite´. # Department of Physics, Chemistry and Biology, IFM, Linko¨ping University.

Figure 1. Switchable blocking layer (a) and switchable charge traps (b). Full lines show the injection of holes (h+) across the Layer of PCs in (a) and the repeated trapping of holes in (b) when PC is in the ON state. Dotted lines show the blocking of hole injection in (a) and the trap-free transport of holes in (b) when PC is in the OFF state.

which could allow for cheap processing, show only moderate electric current ON/OFF switching ratios.9 An approach to achieve good current switching as well as an easy and low cost manufacturing process is to combine the PCs with organic semiconductors. This can be realized either in a multilayer device10-15 or in a solid state blend, in which the PC is dispersed in the organic semiconductor.16-22 In the multilayer devices, the layer of PCs acts as a dynamic charge carrier blocking layer. This is illustrated in Figure 1a, where the layer of PCs blocks or favors hole injection from the metal electrode depending on its state.11 The low energy gap isomer (ON state) has its highest occupied molecular orbital (HOMO)

10.1021/jp9043573 CCC: $40.75  2009 American Chemical Society Published on Web 09/28/2009

Energy Levels of Photochromic Diarylethene Compounds close to the Fermi level of the metal contact. Therefore, holes can be easily injected from the metal electrode and transported into the organic layer. Switching the PC into its high energy gap isomer (OFF state) blocks the hole current since the HOMO of the PC is stabilized and the hole-injection barrier is increased. In the same way, the electron injection may be controlled and can be treated analogously to the hole transport modulation by considering the difference between the lowest unoccupied molecular orbital (LUMO) and the Fermi level of the electrode. In a single layer PC/semiconductor blend, the molecular PCs are dispersed or phase segregated in the organic semiconducting matrix and act as switchable charge carrier traps (see Figure 1b). Two different mechanisms (or a combination of both) have been proposed to explain modulation in the electric current. The change in the frontier orbital energies upon photoisomerization is accompanied by a modification of the energy mismatch between the electronic structure of the host material and the dispersed molecular PCs. This can lead to the appearance of traps, which then reduces the charge mobility. In a one-electron picture, the molecular PCs acts as traps for holes (electrons) if its HOMO is higher in energy (LUMO is lower in energy) than the energy of the HOMO (LUMO) of the host semiconductor.16,17 Moreover, if the dipole moment of the PC is large, then the frontier electronic levels of the semiconducting chains in its vicinity will be perturbed because of long-range charge-dipole interactions, which act as carrier traps. When the dipole moment of the PC molecule is changed, upon photoisomerization, the depth of the induced traps will be modulated and a switchable charge trap is achieved.18-23 In this mechanism, the PCs modify the distribution of transport levels, thus tuning the energetic disorder rather than the positional disorder in the blends. There are several classes of photochromic molecules, for example, spirooxazines, spiropyrans, fulgides, and diarylethenes.24 In this contribution, we focus on the diarylethene family, which is particularly promising for memory applications due to the good thermal stability and cycling endurance exceeding 104 switching cycles.25 The prototypical diarylethene structure in the open and closed forms is shown in Figure 2a and consists of two phenyl groups connected via a bridging ethylene unit. The open form is the thermodynamically most stable, with the aryl rings twisted out of the plane, thus resulting in a poor π-conjugation across the molecule and a large HOMO-LUMO gap. When the molecules are irradiated with UV light, a C-C bond is formed between the carbon atoms 4 and 4′, forcing the molecule into a more planar configuration. In this so-called closed form, the π-conjugation path is extended over the molecule and leads to a reduction of the HOMO-LUMO gap. Upon irradiation of visible light, the C-C bond is cleaved and the molecule reverts back to the original open form. Adding methyl groups on the 4,4′-carbon atoms and replacing the phenyl rings with heterocyclic rings increase the thermal stability of the closed form,25 thereby preventing thermally induced switch back to the open form. Such heterocyclic diarylethene structures are shown in Figure 2b with N, O, or S as heteroatoms. The latter can be positioned in the 5,5′, 6,6′, or 7,7′ positions; see Figure 2b. In order to design electronic memories or latched image sensors using molecular PCs combined with organic semiconductors, there is a need to understand and control the variation in the frontier orbital energies and electric dipole moments of the two photoisomers. In this paper, the impact of the photoisomerization on the charge transport and injection of the two types of devices shown in Figure 1 is first investigated using state-of-the-art charge transport models. The influence of the

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Figure 2. (a) Prototypical diphenylethene structure in the open (left) and closed (right) forms, denoted a and b, respectively; (b) prototypical diarylethene structures with heterocyclic aryl rings when the heteroatom is N, O, or S. The heteroatom can be positioned in the 5,5′ position (2a, 5a, 8a), in the 6,6′ position (3a, 6a or 9a), or in the 7,7′ position (4a, 5a, 10a).

chemical nature of the aromatic rings and of the bridging unit on the frontier orbital energies and dipole moment of diarylethenes is then examined theoretically. Small heterocyles are chosen here to derive useful trends, though larger conjugated groups are likely to be more relevant for applications due to their higher photoswitching efficiencies.25 Quantum-chemical calculations have been performed at the density functional theory (DFT) level. The calculated electronic structure of the PCs is validated by comparison with experimental ultraviolet photoelectron spectroscopy measurements for selected model compounds. 2. Experimental Section 2.1. Theoretical Methodology. The electronic structure of molecular PCs has been characterized using density functional theory (DFT) with a hybrid exchange-correlation potential including both Becke and Hartree-Fock exchange and Lee, Yang, and Parr correlation (B3LYP)26 and a 6-31G* splitvalence basis set, using the Gaussian03 package.27 Diffuse functions are not taken into account in order to avoid convergence difficulties encountered with delocalized systems.28 This approach is known to provide reliable trends when describing hole and electron transport in conjugated compounds.42 Each stationary point has been validated by a frequency calculation. 2.2. Experimental Measurements. The three diarylethene derivatives involved in the experimental work (2,3-bis(2,4,5trimethyl-3-thienyl)maleic anhydride [TT-maleic], 2,3-bis(2,4,5trimethyl-3-thienyl)maleimide [TT-maleimide], cis-1,2-dicyano1,2-bis(2,4,5-trimethyl-3-thienyl)ethene) [TT-cyano], see Figure 4) were acquired from TCI Europe NV. Thin solid films of the diarylethene molecules were prepared in situ under ultra high vacuum (UHV; 10-10 Torr) conditions by physical vapor deposition. The molecules were evaporated from resistively heated glass Knudsen cell and condensed onto silicon substrates. In order to minimize the sublimation rate of the adsorbed molecules, the substrates were cooled to -100 °C during the film growth and measurements. Ultraviolet photoelectron spectroscopy (UPS) measurements were carried out using the

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monochromatized HeI radiation (hν ) 21.2 eV) of a helium discharge lamp in a spectrometer of our own design and construction.29 The binding energy scale was calibrated using the position of the Fermi edge of a sputter-cleaned polycrystalline gold sample. All UPS spectra are plotted relative to the vacuum level. The value of ionization potential (IP) is measured as the onset of the HOMO edge versus vacuum level. 3. Results 3.1. Current-Voltage Characteristics of Photochromic Devices. Charge injection and transport models are used here to investigate the impact of the frontier orbital energy changes, upon photoisomerization, on the current-voltage characteristics for the two device concepts introduced in Figure 1. 3.1.1. PC/Semiconductor Multilayer DeWices. Charge transport through a blocking layer of PCs, sandwiched between a metal electrode and an organic semiconductor (Figure 1a), is governed by three processes: (i) charge transfer across the metal-PC interface, (ii) charge transport across the layer of PCs, and (iii) charge transfer across the PC-organic semiconductor interface. The charge carrier mobilities in a layer of PCs with all molecules in one or the other photoisomer form are expected to have similar order of magnitude. Indeed, if the molecular packing does not change significantly, the transport of charges occurs by hopping between localized frontier levels that are of different energy depending on the photoisomer; this is expected to slightly affect the transport properties since the efficiency of a charge transfer does not depend on the absolute values of the energy of the transport levels but rather on the difference between the energetic levels of two interacting molecules. A significant decrease in mobility is thus expected only for a blend of photoisomers. The injection processes (i) and (iii) are very sensitive to the injection barrier heights30,31 that can be significantly altered by varying the HOMO/LUMO levels of the PC molecule. Hence, the attractive devices are those for which the current is not limited by (ii) but rather by (i) and (iii). Since the nature of the two interfaces is different (metalorganic vs organic-organic), the rate of charge injection is not expected to be the same even for similar injection barrier heights.30-32 In the case of a metal-PC interface, the total energy barrier ΦB1 at flat band conditions is approximated by the difference between the metal work function and the HOMO (LUMO) of the PC in the case of hole- (electron-)injection. This implies that the energy level alignment at the interface obeys the Schottky-Mott limit and neglects the presence of an interface dipole33 or of any Fermi level pinning effect.34 The profile of the barrier is, however, modified by the attractive image potential created by the highly polarizable electronic density at the metal surface that lowers the effective injection barrier.31 Along the same line, the energy barrier at the PC-organic semiconductor interface ΦB2 is the difference between the energies of the HOMO (LUMO) levels of PC and those of the neighboring organic semiconductor for hole- (electron-)injection. In this case, no image potential is generated since the charge carrier concentration in the layer of PCs is too low.30 The impact of these injection barriers on the current density at a given voltage has been examined with the theoretical models developed by Arkhipov et al.30,31 These models rely on a charge injection into specific molecular electronic levels associated to a certain energetic disorder and thus appear to be appropriate to simulate the charge injection in a layer of PCs. We have implemented these models with the following parameters: relative permittivity εr ) 3, the width of the density of states σ

Figure 3. (a) Injected current density (J) across a metal-organic interface, as predicted by the model developed in ref 31. Model parameters are εr ) 3, σ ) 100 meV, average intersite distance a ) 1 nm, inverse localization distance R ) 10 Å-1, sample thickness t ) 100 nm and temperature T ) 300 K; (b) Injected current density across an organic-organic interface as predicted by the model developed in ref 30. The same model parameters were used; (c) Charge mobility as predicted by the models in refs 39 and 40 normalized to the mobility in the absence of traps. The relative trap concentration is the ratio between the density of traps divided by the density of hopping sites. Model parameters are the same, and the width of the Gaussian trap distribution is assumed to be the same as the intrinsic Gaussian distribution. In (a) and (b), the numbers in the graph indicate injection barrier heights, and in (c) they indicate the trap depths.

) 100 meV, the average intersite distance a ) 1 nm, and the inverse localization distance R ) 10 Å-1. The sample thickness is set equal to t ) 100 nm, and the temperature T ) 300 K. If the bottleneck of charge transport occurs at the metal-PC interface, we can use the model developed by Arkhipov to describe the charge injection from a metal to a disordered organic semiconductor.30 The influence of the energy barrier height at the metal-PC interface on the current density is illustrated in Figure 3a. When the bottleneck for charge transport lies at the PC-organic semiconductor interface, the model developed for the injection of charges at the interface between two disordered organic semiconducting layers is then appropriate to simulate the injection from the PC to the organic semiconductor.31 Figure 3b shows the corresponding current densityvoltage curve for various injection barrier heights, that is, various energy mismatches between the frontier electronic levels of the PC and of the organic semiconductor. Figure 3a,b can be exploited to provide guidelines to design efficient switch devices, that is, to choose the PC-organic semiconductor pairs that would give a significant current modulation upon photoisomerization of the molecular PCs. The photoisomerization is accompanied by a change in the energy of the frontier electronic levels and thus by a modification of injection barrier heights. Hence, the current-voltage charac-

Energy Levels of Photochromic Diarylethene Compounds teristics of the device can evolve from one curve to the other upon light irradiation. It is clear from these plots that charge injection is extremely sensitive to the height of the energy barrier determined from the frontier orbital energies of PC. An increase in the injection barrier from 0.3 to 0.5 eV results in a decrease of the injected current density by more than 2 and 3 orders of magnitude for the metal-to-PC and PC-to-semiconductor injection, respectively. A fundamental difference between charge transport across an organic-organic interface and a metal-organic interface is that the image charge potential in the metal causes a barrier lowering, which strongly influences the field dependence of the charge injection. The low carrier concentrations and slow dielectric relaxation in the organic injecting layer of the organic-organic interface do not allow for the creation of an image potential. Therefore, the charge transport across that interface depends only weakly on the electric field as opposed to the metal-organic interface. The variation of the current density versus modulation of the barrier heights, for the two kinds of interfaces, is due to the interplay between the image potential and the disorder in the organic materials. When the HOMO (LUMO) of the photochromic layer is positioned in between the Fermi level of the metal and the valence (conduction) band of the semiconductor (see Figure 1a), the injection barrier from the metal to the semiconductor is split into two smaller barriers ΦB1 and ΦB2. Since the injected current strongly depends on the barrier height, charge transport across two small barriers is significantly more efficient than transport across a single barrier with globally the same height (ΦB1 + ΦB2). When holes are the majority carriers, the device is expected to have the largest ON/OFF current ratio when the HOMO of the molecular PCs in the open form is lower than the HOMO of the organic semiconductor and when the HOMO of the PC in the closed form lies in between the metal Fermi level and the HOMO of the semiconductor (i.e., HOMOPC,OFF < HOMOSC < HOMOPC,ON). 3.1.2. PC/Semiconductor Blend DeWices. Solution-processed organic semiconducting layers are characterized by an energetic disorder resulting from the random orientation of the molecular dipoles or quadrupole moments. This energetic disorder is commonly described by a Gaussian density of states with a width in the range of σ ) 0.07-0.14 eV.35-37 Charges are transported via hopping between neighboring localized states within this energy distribution. As a consequence, the effective trap depth of PCs in a semiconducting host matrix is generally lower than the difference in the frontier orbital energies of the two materials. According to Mandoc et al.,38 we can assume that the trap depth is lowered by σ2/2kBT, that is, about 0.1-0.4 eV for σ ranging from 0.07 to 0.14 eV. In a PC/semiconductor blend (Figure 1b), the impact of photoisomerization of the molecular PCs on the charge carrier mobility can be simulated with the theoretical model derived by Martens et al.39 and Coehoorn.40 We have implemented this model46 by assuming an energetic disorder of similar width for the density of states (σ ) 0.1 eV) of the organic semiconductor matrix and of the molecular PCs. The other model parameters are the same as in the previous models (εr ) 3, a ) 1 nm, R ) 10 Å-1, t ) 100 nm, T ) 300 K). Figure 3c displays the resulting charge carrier mobility as a function of the concentration of photochromic molecules acting as trap sites. The various curves correspond to different trap depths. Traps with a depth lower than 0.2 eV require a high concentration to induce a considerable change in the mobility. In contrast, a trap depth of 0.5 eV lowers the mobility by 3 orders of magnitude already for a relative trap concentration of 10-4 (ratio of trap sites versus total

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18399 transport sites). In order to optimize the ON/OFF current ratio in a PC/semiconductor blend device, one thus needs to consider not only the modification in trap depth due to the variation of the energy of the frontier electronic level upon photoisomerization but also the concentration of the molecular PCs. The concentration of photochromics in the organic blend can compensate to some extent for a small variation in the frontier orbital energy. 3.2. Electronic Structure Calculations. While the charge transport and injection models simulate the modification in the current-voltage characteristics for various charge injection barrier heights or trap depths, the choice of the photochromic molecule to be used in conjunction with a specific organic semiconductor requires the knowledge of the actual frontier energy levels of both compounds. As a result, the calculations of the electronic structure of families of molecular PCs add essential information to the description of photochromic switch devices. 3.2.1. Validation of the Quantum-Chemical Approach. UPS measurements have been performed on thin films of three commercially available diarylethene compounds in their open form: 2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride [TTmaleic], 2,3-bis(2,4,5-trimethyl-3-thienyl)maleimide [TT-maleimide], and cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene) [TT-cyano]. Results from these studies were then utilized to validate the DFT calculations of the PCs. The chemical structure of these molecules (Figure 4) is similar to molecules 26, 27, and 18 discussed hereafter. Figure 4 displays the calculated density of valence states (DOVS) and the experimental valence spectra of the diarylethene molecules obtained from UPS; the DOVS are simulated within Koopmans’ approximation (i.e, using eigenvalues as binding energies), without estimating cross sections and by matching the HOMO of the single molecule to the experimental spectrum in order to account for solid-state polarization effects.41 Since UPS experiments were performed on solid films, the UPS spectra exhibit inhomogeneous broadening. In order to account for this broadening into the calculated DOVS, a Gaussian peak with a full width at half-maximum (fwhm) of 0.28 eV has been associated to each of the eigen-energies of the molecular orbitals (shown as vertical lines). The energy scale of the simulated DOVS has also been scaled by a factor of 0.9 to account for correlation effects. There is a very good match between the experimental and the calculated spectra (Figure 4). The changes in the chemical structure of the diarylethene molecules are reflected by changes in the shape of the valence spectra and in the modification of their ionization potential. In the recorded UPS spectra, the contribution from the HOMO level overlaps with contributions from other deeper orbitals so that the ionization potential was estimated from the edge of HOMO peak. The measured values of the ionization potential for molecules TT-maleic, TTmaleimide, and TT-cyano are 6.43 eV, 6.14 eV, and 6.52 eV, respectively. The relative energies are very well-reproduced by the calculated gas-phase ionization potentials (5.98 eV, 5.75, and 6.03 eV, respectively, estimated as minus the HOMO energy within Koopmans’ approximation). This validates the use of our DFT approach to investigate the influence from the derivatization of the diarylethene backbone on the electronic properties. 3.2.2. DeriWatization of the Aryl Rings. The energy levels of the diarylethene compounds depend on the nature of the heteroatom in the aryl rings, the most common being nitrogen (N-H) in pyrrole, oxygen (O) in furan, or sulfur (S) in thiophene

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Figure 4. DFT-calculated eigen-energies of the molecular orbitals (vertical lines) and density of valence states (DOVS) curves (dashed lines) and experimental UPS spectra (full lines) of (a) cis-1,2-dicyano1,2-bis(2,4,5-trimethyl-3-thienyl)ethene), (b) 2,3-bis(2,4,5-trimethyl-3thienyl)maleimide, and (c) 2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride.

(see structures in Figure 2). Figure 5a shows the evolution of the HOMO and LUMO levels of the unsubstituted diarylethenes as a function of the nature of the heterocycle. In both forms, the evolution of the frontier orbital energy levels of the diarylethenes with the position of the heteroatom is similar for the three series when compared to the corresponding isolated rings. The position of the heteroatom in the aryl units has a significant impact on the energy of the frontier levels, as illustrated in Figure 5a. The largest difference in the energy levels between the open and the closed forms is observed for 3, 6, and 9. With the heteroatoms in the 6,6′ positions, bond 2-1 exhibit a stronger quinoı¨dic character (d(2-1) ) 1.46, 1.40, and 1.47 Å, respectively for 5, 6, and 7) and the associated increase in electron delocalization leads, as expected, to a stronger destabilization of the HOMO and a stabilization of the LUMO. The closed forms of these structures are thus expected to be very reactive and unstable. The energy difference between the two forms is above 3 eV for 3, 6, and 9 while typical values in between 0.5 and 1.5 eV are obtained for the other compounds (Figure 5b). Accordingly, structures based on heteroatoms in the 6-6′ positions will not be further considered in this study.

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Figure 5. DFT-calculated (a) HOMO and LUMO energies of diarylethene structures when varying the position of the heteroatom in the aryl ring for dipyrrolyl-, difuryl-, dithienyl-, and diphenyl-ethene, in the open form (open squares) and closed form (filled squares). The energy levels of the corresponding isolated heterocycles are shown with open circles; (b) total energy difference between the open and the closed forms for the different photochromic molecules.

The large difference in the LUMO energies between the structures with the heteroatom in the 5,5′ positions (i.e., 2, 5 and 8) versus 7,7′ positions (i.e., 4, 7, 10) is associated with changes in the degree of delocalization of the orbitals; see Figure 6. In the closed form, the sp3 hybridized 4,4′ carbon atoms do not contribute to the conjugation. When the heteroatom is in the 5,5′ positions (e.g., 8b in Figure 6), the LUMO is delocalized over eight atoms. In contrast, when the heteroatom is in the 7,7′ position (e.g., 10b in Figure 6), the conjugation is limited by the presence of a node on the S atom so that the LUMO extends only over four atoms. As a result, the LUMO level is more stable in 8b than in 10b upon photoisomerization. This is in agreement with the findings of Uchida et al.43 showing that the absorption maximum is shifted toward shorter wavelengths for structures similar to 10b compared with structures like 8b. In order to verify that the changes in the frontier orbitals upon derivatization of the aryl ring are not restricted to the case of an ethylene bridge, we have also extended our calculations to a perfluoroethylene bridge. The behavior is very similar except for a general shift of the frontier orbital energies by 0.7 eV induced by the electron withdrawing character of the bridge (vide infra). This indicates that the general picture is not affected by the exact nature of the bridge. In order to rationalize the orientation and amplitude of the permanent dipole moment of the diarylethene molecules, we have defined three local dipoles (defined from the Mulliken

Energy Levels of Photochromic Diarylethene Compounds atomic charges on the group of atoms considered): (i) the dipole moment of the two aryl rings, PA, and (ii) the dipole moment of the bridging ethylene unit, PB. The dipolar contribution arising from the aryl rings and from the bridging unit is sketched in Figure 7b. Note that the charge at the 3,3′ positions contributes to both PB and PA so that the total dipole moment is not exactly the sum of the individual dipole moments. The dipole moment of the aryl rings depends on the nature of the heteroatom. The nitrogen in pyrrole is negatively charged while the oxygen atom in furan and the sulfur atom in thiophene are positively charged. The dipole moment of pyrrole-based compounds has an opposite sign as compared with furan-based and thiophene-based structures (Figure 7a). Moving the position of the heteroatom from the 5′,5′ positions to the 6,6′ positions and the 7,7′ positions introduces a progressive rotation of the local dipole of each aryl ring, as illustrated in Figure 7b. As a result, PA in 2, 5, and 8 points in a direction opposite to that prevailing for 4, 7, and 10, respectively. The contribution to the total dipole arising from PA is significantly reduced in 3, 6, and 9 since the local dipole moments of the two aryl rings point in opposite direction and thus tend to cancel each other. The change in the dipole moment upon photoisomerization mainly originates from changes in the PA contribution. The larger change in dipole moment between the closed and the open forms observed for 8 as compared with that for 10 correlates very well with the changes in the dipole moment of the corresponding isolated heterocycles when considering their vectorial summation. 3.2.3. Functionalization of the Bridging Unit. Among the various aryl groups that can be considered, Irie et al.25 have shown that thiophene-based compounds yield the most thermally stable molecules. In this section, dithienylethenes are substituted by attaching covalently donor or acceptor moieties on the ethylene bridge. Hereafter, the ethylene unit connecting the aryl rings, including the substituents, is referred to as the “bridging unit” while the rest of the molecule is the “heterocyclic unit”. Three different types of bridging unit with substituents on the 1 and 2 positions have been considered (see Figure 8): (i)

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18401 donors or acceptors are added directly on the ethylene bridge by substituting the two hydrogen atoms (the C2R2 structures 15-19), (ii) the ethylene bridge is replaced by a cyclopentene unit, with donors and acceptors added to the 3 sp3 hybridized carbon atoms (the C5R6 structures 20-25), (iii) the bridge is replaced by a conjugated accepting unit (maleic anhydride unit (26) or a maleimide unit (27)). The electron donating or withdrawing character of the substituents can be quantified by the Hammett coefficients44 (see Table in Figure 8). Note that the theoretical functionalization of the bridging unit considered in this study leads sometimes to compounds (such as structures 20, 24, 25) which are likely to be difficult to synthesize or unstable. However, the results are instructive to extract useful trends in the evolution of the electronic structure upon substitution. The energies of the HOMO and LUMO levels of structures 15-27 are shown in Figure 9a. The frontier orbitals are stabilized by electron withdrawing substituents and destabilized by electron donating substituents and follow quite well the trends expected from the Hammett coefficients.45 The highest-lying HOMO in the open form (-4.68 eV) is obtained following NH2substitution, characterized with the largest Hammett coefficient (0.66), on the C2R2 bridging unit (15). The lowest value (-6.86 eV) is calculated for the structure with CN-substitution, associated to a large negative Hammett coefficient, on the C5R6 bridging unit (24). More subtle variations can be understood by considering the nature of the substituents (inductive, I, and mesomeric, M; donating (+) versus withdrawing (-)) and the spatial distribution of the orbitals. F and CH3 act predominantly via inductive effects while NH2 (-I, +M), CN (-I, -M), and NO2 (-I, -M) act via both inductive and mesomeric effects. Figure 9 shows that the shift in energy of the HOMO and LUMO levels is symmetric in both forms when CH3 or F substituents are added (16, 17, 21, and 23). These substituents interact inductively by donating or withdrawing σ electrons and have thus a global effect on the π-electron system. Hence, the shift does not critically depend on the connected atom and has the same magnitude for both

Figure 6. Energy and shape of the HOMO and LUMO levels of structure 8 (left) and 10 (right) in the open and closed forms.

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Figure 7. DFT-calculated dipole moment of the various diarylethene structures and of the corresponding isolated heterocycles; (b) sketch of the vectorial summation of the local dipole moments for molecules 8, 9, and 10. A positive dipole moment is pointing in the direction of the z axis.

Figure 8. Functionalized dithienylethenes with substituents added directly to the bridge (C2R2, left), substituents added to a cyclopentene bridge (C5R6, middle), and maleic anhydride or maleimide bridging units (C4O2R, right). The table gives the Hammett coefficients, with positive and negative values indicating electron donating and withdrawing character, respectively.44

the HOMO and the LUMO levels. When substituents that act mesomerically are added to the sp3 hybridized carbon atoms on the C5R6 bridging units, the HOMO and LUMO are shifted by same amount (except for 25) due to the lack of conjugation between the substituents and the carbon atoms of the ethylene bridge (i.e., the interaction is purely inductive). The poor

Figure 9. (a) DFT-calculated energies of the HOMO and LUMO levels of compounds 15-26. Data for the C2R2, C5R6, and C4O2R structures are shown from left to right. Within each group, the structures are ordered with respect to the Hammett coefficient of the substituents in the bridging units. Arrows indicate structures with poor switching in the HOMO or LUMO levels; (b) shape of the HOMO and LUMO levels for the prototypical dithienylethene compound 8 together with the normalized sum of the square LCAO coefficients over the bridge atoms; (c) shape of the HOMO and LUMO levels for 25; note the weak localization of the LUMO level on the heterocycle unit of the molecule.

switching of the LUMO level in 25 is attributed mostly to the strong localization of the orbital on the bridging unit in both the open and the closed forms (Figure 9c) since the torsion angle between the aryl groups and the bridge varies only by 3° between the two forms. Accordingly, since the switching primarily affects the aryl parts of the molecule, there is a small energy difference between the LUMOs of the two forms. Effective mesomeric interactions require a direct coupling between the π-orbitals of the substituent and those of the conjugated backbone. For a given substituent, one can classify

Energy Levels of Photochromic Diarylethene Compounds semiquantitatively the strength of the coupling to the frontiers orbitals in the open and closed forms by looking at the density probability of the frontier orbital wave function on the carbon atoms at the 1 and 2 positions where the substituents are anchored. It is here instructive to examine the normalized sum S of the squares of the LCAO (linear combination of atomic orbitals) coefficients on the bridge atoms for the prototypical molecule 8 (Figure 9b). The contributions of the carbon atoms of the bridge in the HOMO are similar in the open form and closed forms (S ) 0.098 and 0.081, respectively). The difference between the open and the closed form is significantly larger for the LUMO (S ) 0.151 versus 0.081, respectively). When an electron withdrawing substituent with a strong mesomeric character (such as NO2 or CN) is added directly onto the bridge, the LUMO in the open form will be pushed down more efficiently than in the closed form. As a result, the energy difference between the LUMO levels of the two forms almost vanishes in 18 (∆LUMO ) 0.22 eV) and 19 (∆LUMO ) 0.12 eV) compared with 8 substituted with hydrogen atoms (∆LUMO ) 0.91 eV). The same picture holds true for 26 (∆LUMO ) -0.01 eV) and 27 (∆LUMO ) -0.07 eV) where the functional group is conjugated with the backbone of the molecule. On the contrary, the energy difference between the HOMOs in the open and closed forms remains relatively constant upon introducing of withdrawing substituents due to the similar involvement of the 2pπ orbitals in the two forms (∆HOMO ) -0.77 eV for 18 vs ∆HOMO ) -0.88 eV for 8). A similar but weaker effect is observed for the HOMO when electron donating NH2 units are attached directly to the bridge (15). The dipole moments of the different structures are collected in Figure 10a. With electron donating substituents, such as positively charged NH2 in 15, PB points in the same direction as PA and yields a large dipole moment. When electron withdrawing units are added such as in 19 or 18, PB is instead antiparallel to PA and the two dipole contributions partially cancel each other. However, in the case of such efficient electron withdrawing units, the bridging unit is strongly negatively charged so that the dipole moment is dominated by PB. 4. Discussion 4.1. Comparison of the Calculated Frontier Energy Levels of the PCs. Figure 11 summarizes the calculated amplitude of the switch of the HOMO (∆HOMO) and LUMO (∆LUMO) levels for the diarylethenes under study. ∆HOMO and ∆LUMO are defined as the energy in the closed form minus the energy in the open form. All structures have ∆HOMO > 0.5 eV, except for compounds 7 and 15. In view of the large sensitivity of the injection current to the barrier height as well as of the strong dependence of the mobility values on trap depth (see Figure 3), a large current modulation is expected in switch diodes based on these diarylethenes. As far as electron injection and transport is concerned, some molecules do not show the desired change in the LUMO energy. Poor candidates for switching are molecules with electron-withdrawing substituents that interact mesomerically (18, 19, 26, and 27) as well as molecules with the frontier orbital localized around the bridging unit in both the closed and the open forms (25). Photochromic molecules with a high electron affinity, that is, low LUMO energy, are required to facilitate electron injection from the metal to the layer of PCs. However, the stabilization of the LUMO induced by the addition of electron-withdrawing substituents is accompanied by a decrease in ∆LUMO, thus diminishing the modulation of the current density upon photoisomerization. Among the thermally stable structures, com-

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Figure 10. (a) DFT-calculated dipole moment for the diarylethene compounds 15-26; (b) sketch of the vectorial sum of the local dipole moments for 15 and 19.

Figure 11. Difference between the HOMO/LUMO energies (∆HOMO/ ∆LUMO) in the open and those in the closed forms.

pound 21 has the largest frontier orbital switch with ∆HOMO ) 1.10 eV and ∆LUMO ) -1.34 eV. 4.2. Comparison of the Calculated Dipole Moment of the PCs. In PC/semiconductor blends, charge traps might originate from the variation of the dipole moment carried by the photochromic molecules upon photoisomerization. In order to achieve switchable dipolar charge traps, the difference in the dipole moment between the open and the closed forms must be maximized and the magnitude of the dipole moment of PC (in at least one of the form) must be larger than that of the

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Jakobsson et al. been used in conjunction with charge injection and charge trapping models to predict the current density in PC/semiconductor multilayers and blends. ON/OFF current ratios of several orders of magnitude are predicted when the PCs are used both as switchable injection barriers and as switchable charge traps. The dipole moment of the diarylethenes under study is either too small in magnitude or has a too small contrast for the different isomerisations to be of interest for applications relying on charge trapping to control the current. Thus, we conclude that the strongest contribution to the switchable charge trapping effect is due to the shift of the frontier energy levels for the diarylethene family of photochromic molecules. The diarylethene molecules are clearly promising candidates for optical write/electrical read memory applications.

Figure 12. Ratio of the dipole moment in the closed versus open form (PClosed/POpen) for all structures under study.

disordered host material. According to Nesˇpu˚rek et al.,23 dipoles of 5 and 10 D in a nonpolar matrix result in traps with a depth of 0.3 and 0.5 eV, respectively; these dipoles promote an electrostatic shift of the frontier orbitals of the organic semiconductors in the vicinity of the molecular PCs. Since disorder lowers the effective trap depth significantly (by 0.4 eV for a width σ ) 0.14 eV), a strong dipole moment (∼10 D) is required to generate sufficiently deep traps able to affect charge transport in organic materials. Figure 12 shows the ratio of the dipole moment in the closed form (PClosed) versus that in the open form (POpen) for the set of PCs investigated. Structures 4, 8, 15, 16, 17, 21, and 22 have a large difference in dipole moment between the open and the closed forms (i.e., PClosed/POpen > 2 or < 0.5). For 4 and 17, the dipole moment is larger in the open from (POpen of 1.84 and 2.21 D, respectively) while for 8, 15, 16, 21, and 22, the dipole moment is larger in the closed form (PClosed of 2.06 D, 4.01 D, 2.83 D, 2.85 D, and 2.89 D, respectively). However, for all structures, the dipole moment is lower than 5 D; this suggests that the traps would be only ∼0.3 eV deep and thus not effective enough in a disordered organic blend. On the other hand, 18, 19, 24, and 25 have a large dipole moment (6.93 D, 7.46 D, 7.95 D, and 7.62 D, respectively in the closed form) but a very small switching effect (PClosed/POpen of 1.02, 1.15, 1.01, and 1.08, respectively). Hence, none of the diarylethenes under study fulfill the above-mentioned requirements and are therefore suitable for dipolar trap-based applications. 5. Conclusion The electronic structure of diarylethene derivatives has been investigated by density functional theory (DFT) calculations. This theoretical approach has been validated for specific compounds by comparison with experimental ultraviolet photoelectron spectra. The evolution of the electronic properties upon chemical functionalization of the aryl rings and ethylene bridging unit has been scrutinized. The calculations indicate that the HOMO and LUMO energies in the open [closed] form can be tuned by as much as 2.2 eV [1.8 eV] and 3.8 eV [2.3 eV], respectively. The switching of the HOMO level is robust to substitution while the switching of the LUMO deteriorates when strong electron-withdrawing units are attached to the bridging ethylene unit. The shift of the levels ∆HOMO and ∆LUMO can be tuned in the range between 0.36-1.10 eV and 0.01-1.34 eV, respectively. The calculated molecular properties have also

Acknowledgment. F.J., X.C., and M.B. gratefully acknowledge The Swedish Foundation for Strategic Research (OPEN), VINNOVA, EnergiMyndigheten, The Royal Swedish Academy of Sciences, and The Swedish Research Council for financial support of this project. S.B. and M.F. gratefully acknowledge financial support from the Swedish Research Council (project grant, Linneus center), the Carl Tryggers Foundation, the Knut and Alice Wallenberg Foundation, and the Swedish Foundation for Strategic Research. The work in Mons has been supported by the Belgian Federal Government Office of Science Policy (PAI 6/27) and the Belgian National Fund for Scientific Research (FRS-FNRS). J.C. is an FNRS Research Fellow. The authors acknowledge Professor A. Samat for helpful discussion. References and Notes (1) Irie, M. Jpn. J. Appl. Phys. 1989, 28, 215. (2) Myles, A. J.; Branda, N. R. AdV. Func. Mater. 2002, 12, 167. (3) Yao, B.; Wang, Y.; Menke, N.; Li, M.; Zheng, Y.; Ren, L.; Chen, G. Mol. Cryst. Liq. Cryst. 2005, 430, 210. (4) Huang, Y.; Liang, W.; Poon, J. K. S.; Xu, Y.; Lee, R. K.; Yariv, A. Appl. Phys. Lett. 2006, 88, 181102. (5) Dulic´, D.; van der Molen, S. J.; Kudernac, T.; Jonkman, H. T.; de Jong, J. J. D.; Bowden, T. N.; van Esch, J.; Feringa, B. L.; van Wees, B. J. Phys. ReV. Lett. 2003, 91, 207402. (6) Li, J.; Speyer, G.; Sankey, O. F. Phys. ReV. Lett. 2004, 93, 248302. (7) Speyer, G.; Li, J.; Sankey, O. F. Phys. Status Solidi B 2004, 24, 2326. (8) Kronemeijer, A. J.; Akkerman, H. B.; Kudernac, T.; van Wees, B. J.; Feringa, B. L.; Blom, P. W. M.; de Boer, B. AdV. Mater. 2008, 20, 1467. (9) Yassar, A.; Garnier, Y.; Jaafari, H.; Rebie`re-Galy, N.; Frigoli, M.; Moustrou, C.; Samat, A.; Guglielmetti, R. Appl. Phys. Lett. 2002, 80, 4297. (10) Tsujioka, T.; Hamada, Y.; Shibata, K.; Taniguchi, A.; Fuyuki, T. Appl. Phys. Lett. 2001, 78, 2282. (11) Tsujioka, T.; Iefuji, N.; Jiapaer, A.; Irie, M.; Nakamura, S. Appl. Phys. Lett. 2006, 89, 222102. (12) Tsujioka, T.; Kondo, H. Appl. Phys. Lett. 2003, 83, 937. (13) Tsujioka, T.; Masuda, K. Appl. Phys. Lett. 2003, 83, 4978. (14) Tsujioka, T.; Masui, K.; Otoshi, F. Appl. Phys. Lett. 2004, 85, 3128. (15) Tsujioka, T.; Shimitzu, M.; Ishihara, E. Appl. Phys. Lett. 2005, 87, 213506. (16) Andersson, P.; Robinson, N. D.; Berggren, M. AdV. Mater. 2005, 17, 1798. (17) Andersson, P.; Robinson, N. D.; Berggren, M. Synth. Met. 2005, 150, 217. (18) Nesˇpu˚rek, S.; Sworakowski, J.; Combellas, C.; Wang, G.; Weiter, M. Appl. Surf. Sci. 2004, 234, 395. (19) Nesˇpu˚rek, S.; Wang, G.; Toman, P.; Sworakowski, J.; Bartkowiak, W.; Iwamoto, M.; Combellas, C. Mol. Cryst. Liq. Cryst. 2005, 430, 127. (20) Toman, P.; Nesˇpu˚rek, S.; Weiter, M.; Vala, M.; Sworakowski, J.; Bartkowiak, W.; Mensˇ´ık, M. Polym. AdV. Technol. 2006, 17, 673. (21) Weiter, M.; Vala, M.; Salyk, O.; Zmesˇkal, O.; Nesˇpu˚rek, S.; Sworakowski, J. Mol. Cryst. Liq. Cryst. 2005, 430, 227. (22) Weiter, M.; Vala, M.; Zmesˇkal, O.; Nesˇpu˚rek, S.; Toman, P. Macromol. Symp. 2007, 247, 318. (23) Nesˇpu˚rek, S.; Sworakowski, J. Thin Solid Films 2001, 393, 168. (24) Crano, J. C. Guglielmetti, R. J. Organic Photochromic and Thermochromic Compounds; Kluwer Academic Publishers: New York, 2002.

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