Interplay Between Electronic and Steric Effects in ... - ACS Publications

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Interplay Between Electronic and Steric Effects in Multiphotochromic Diarylethenes Aurelie Perrier,*,† Franc- ois Maurel,† and Denis Jacquemin*,‡ †

Laboratoire Interfaces, Traitements, Organisation et Dynamique des Systemes (ITODYS), CNRS UMR 7086, Universite Paris 7 - Paris Diderot, Bâtiment Lavoisier, 15 rue Jean Antoine de Baïf, 75205 Paris Cedex 13, France ‡ Laboratoire CEISAM - UMR CNRS 6230, Universite de Nantes, 2 Rue de la Houssiniere, BP 92208, 44322 Nantes Cedex 3, France

bS Supporting Information ABSTRACT: The structures and electronic features of five complex multiphotochromic molecules incorporating two or three diarylethene units are investigated with quantum mechanical approaches. Four out of the five systems only display partial photochromism, and it is shown that the interplay between steric and electronic effects might explain this outcome. For a spiro-bonded system (I), the doubly closed isomer is reachable because the two photochromes are essentially independent and undergo no specific geometric stress. For a tetrathiafilvalenebridged derivative (II), there is no steric hindrance, but promoting the electron toward the reactive orbital is not possible. In dimers sharing a central thiophene ring (III and IV), the absence of the closedclosed derivative can be understood by either the compactness of the molecule or by a combination of conformational and electronic (lack of photochromic orbital) factors. Eventually, the reactivity of the trimer, V, is related to the variations of the distances between reactive carbon atoms. This contribution therefore paves the way toward an atomic-scale description of elaborated coupled switches and gives hints for the design of more efficient multiaddressable structures, by proposing a new architecture.

’ INTRODUCTION Photochromic entities may undergo a light-induced reversible transformation between two isomers presenting distinct features.1 Obviously, when the properties of the two forms are significantly different, photochromes may act as main building blocks in on/off nanodevices. In other words, photochromes are molecular switches and can be used to store a bit of data (0/1). One of the most successful families of organic photochrome is based on diarylethenes (DA, sometimes referred to as dithienylethenes, see Scheme 1). DA have been originally proposed by Lehn2,3 and Irie4,5 20 years ago and, since, have been the focus of an impressive number of investigations. In DA, the most stable form is a poorly conjugated open (o) isomer in which the two thiophene rings are (anti)parallel with respect to one another and perpendicular with respect to the central moiety, typically a fivemembered cycle. This open isomer is colorless with a λmax centered at ca. 300 nm. The less stable closed (c) form is strongly conjugated as the thiophene rings and the central group are coplanar, opening the way toward an optimal delocalization of πelectrons. The closed isomer displays vivid colors with an absorption band typically located between 450 and 650 nm, depending on the chosen auxochroms. The open to closed conversion can be induced by UV irradiation, the back reaction generally requiring visible light. The synthesis and properties of r 2011 American Chemical Society

Scheme 1. Representation of a Typical DA in Its Open (Left) and Closed (Right) Forms

typical DA have been reviewed by several authors during the past decade, and we redirect the reader to these publications for general information and a complete bibliography.610 To break the 0/1 barrier, that is, to store more information inside a single molecule, one can design molecules encompassing several photochromes; e.g., a trimer can in principle store one byte, rather than one bit of data.11,12 Despite this exciting potential, only a few numbers of molecules incorporating more than a single photoswitchable unit have been synthesized up to now. If one selects systems exclusively based on two or more DA Received: February 7, 2011 Revised: March 17, 2011 Published: April 15, 2011 9193

dx.doi.org/10.1021/jp201229q | J. Phys. Chem. C 2011, 115, 9193–9203

The Journal of Physical Chemistry C

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Scheme 2. Representation of the Head-Bonded Dimeric Derivatives Investigated in All Their Possible Formsa

a

Note that the strings between brackets indicate the closed/open nature of the different DA included in each molecule.

switches, it is obvious that most available compounds present problematic limitations: (a) the reversibility of the processes is hindered by several parasite reactions more frequently than for single photochromes, so that fatigue resistance is an important issue;13,14 (b) switching on one part of the compound may induce a loss of photoreactivity of other fragments, so that only mixed closed/open forms can be obtained;11,12,1519 (c) the few fully operative systems generally demonstrate increased contrast (larger intensity for the typical visible band), but no emergent properties are detected.18,2024 On top of that, controlling all selected conversions remains difficult, as one wavelength might trigger several processes. Therefore, though the potential impact of multi-DA switches is large, several drawbacks have to be circumvented to pave the way toward more reliable molecular building blocks. For this reason, quantum mechanical simulations providing an atomistic-scale understanding of the electronic phenomena are important. In that framework, we have performed initial ab initio simulations on DA dimers and trimers in which a central bridge connects the different DA through their side thiophene rings.2530 Our works, performed with TimeDependent Density Functional Theory (TD-DFT),3133 have demonstrated that first insights into the full or partial electrocyclization can be obtained by going beyond the simplistic and unhelpful HOMOLUMO approximation. Indeed, the topology of the LUMO þ n (typically n = 1) molecular orbital implied in the first UV band of the closedopen hybrid may be useful to predict the photocyclization of the remaining open DA. Only orbitals presenting a so-called “photochromic shape”27,34 may induce ring closure. Two requirements have to be met to have a photochromic orbital: on one hand, a bonding character for the to-be-formed CC sigma bond is necessary, and this can be checked by inspecting the relative signs of the lobes of the π orbitals located on these two carbons in the open structure; and, on the other hand, nontrifling electron densities have to be found on at least one of these two

reactive atoms. The latter criterion is of course quite qualitative, and such topological analysis remains a first-order approach, completely inadequate when side products are formed with large yields.27 In the framework of TD-DFT simulations of coupled DA, the number of works is therefore quite limited though we also wish to point out the recent contribution by Staykov et al.35 that investigates a molecular wire capped by two DA. To complete these previous investigations, we tackle here five molecules containing two or three DA (see Scheme 2, Scheme 3, and Scheme 4) that, to the best of our knowledge, have never been investigated previously with quantum mechanical (or other theoretical) approaches. All five compounds have been synthesized and experimentally characterized by Irie and collaborators12,15,16,24 and are not connected through a simple bridge—as were the coupled-DA investigated up to now with TD-DFT. The first molecule, I, contains two DA spiro-bonded through their central unit,24 an exceptional architecture, whereas in most dimers, the DA are connected through bridges attached to the reactive thiophene rings. In fact, we are aware of only a few other organic or organo-metallic structures of that category.3640 For I, full photochromism can be achieved in solution; that is, it is possible to close the two DA, and this results in a doubling of the intensity of the characteristic visible band but no variation of its position on the wavelength scale.24 On the contrary, for the tetrathiafulvalene case, II, the fully closed form cannot be reached.36 III is the smallest DA dimer one could design, as one of the thiophene rings is common to two DA.15 For this system, like in the more extended (but similar) IV, only intermediate closed/open forms might be achieved. Nevertheless, for IV the absorption spectra of the co and oc structures differ significantly, so that this molecule is an actually operating multicolor structure,16 presenting three well-separated states. In other words, IV takes advantage of its asymmetry to circumvent the absence of the fully closed isomer. Eventually, we have also 9194

dx.doi.org/10.1021/jp201229q |J. Phys. Chem. C 2011, 115, 9193–9203


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Scheme 3. Shared-Thiophene Dimer Investigated Herein

investigated a large trimer (V, see Scheme 4) that upon irradiation with smartly selected wavelengths may exist in five—out of the eight theoretically possible—forms.12 Indeed, in V, it is possible to simultaneously close the two-side DA. However, closing two vicinal photochromes remains impossible, and this trimer can therefore be viewed not only as a refined extension of IV but also as one of the most successful multiaddressable molecules proposed up to now. The present contribution therefore aims at understanding these photochromic behaviors, with a specific focus on the interplay between steric stress (almost absent in “conventional” architectures) and electronic features.

’ METHOD All simulations have been achieved with the Gaussian09 program,41 applying default procedures, integration grids, algorithms, and parameters, except for tightened SCF (109 au) and internal forces (105 au) convergence thresholds. We have adopted the computational strategy recently applied to investigate the photochromic properties of a series of conventional diarylethene dimers.27 We refer the readers to this publication and references therein for discussions regarding the selection of DFT functionals and basis sets. The computational protocol consists of three successive stages42 and systematically (all steps) includes a modeling of bulk solvent effects (here n-hexane, as in

the experiments43) through the Polarizable Continuum Model (PCM):44 (1) the ground-state geometrical parameters have been determined at the PBE0/6-311G(d,p) level,45 via a forceminimization process; (2) the vibrational spectrum of each derivative has been determined analytically at the same level of theory, that is, PBE0/6-311G(d,p), and it has been checked that all structures correspond to true minima of the potential energy surface; (3) the first ten or fifteen low-lying excited states have been determined within the vertical TD-DFT approximation using the CAM-B3LYP/6-311þG(2d,p) level of approximation.46 In the following, the contour threshold selected to represent the molecular orbitals was systematically set to 0.030 au. Note that only CAM-B3LYP/6-311þG(2d,p) orbitals are shown here.

’ RESULTS AND DISCUSSION Spiro-Bridged True Twins and TTF-Bonded False Twins. The experimental24 and theoretical characteristics of the spiro structure are summarized in Table 1. For I, we have considered only antiparallel orientations of the reactive thiophene rings during the geometry optimizations, as the parallel DA structures do not display any photochromic properties.6 Nevertheless, we have identified three conformers differing by the relative positions of the thiophene rings and the central unit. The details are reported in the Supporting Information (SI), and we only discuss 9195



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Scheme 4. Irie’s Trimer in Its Five Experimentally Detected Forms (See Ref 12)

the most stable conformation here. The obtained isomers belong to the S4 point group (no imaginary frequency), but, of course, for co that is C2. This is a noticeable fact as most single DA are actually C1.47 Examination of Table 1 clearly indicates that the two switches are almost independent. Indeed, closing one unit implies an energetic ground-state penalty of 8.55 kcal/mol (a value in the expected range, see refs 27 and 47), whereas closing the second unit implies an additional cost of 8.32 kcal/mol. This additive pattern hints that the state of one photochrome has no large impact on the second. The distance separating the two reactive carbon atoms (see SI and Table 1) is also standard for open and closed DA,27,47 and the former are comparable to the value measured in crystal structures.24 Therefore, there is no specific steric hindrance in I, and the photoswitching mechanism is guided by electronic, rather than geometric, features. Due to the S4 symmetry, the fully open and fully closed isomers present degenerated (E type) λmax, but more importantly, the simulated spectra as well as the evolutions upon irradiation completely agree with the experimental data (see Figure 1). Indeed, upon successive electrocyclizations, the usual emergence and increase of the visible band (471 nm with TD-DFT, 465 nm experimentally) is observed and accompanied by a similar raise of the UV band close to 300 nm. This latter effect is uncommon (one generally notes the opposite trend),20,27 and the fact that the selected method perfectly reproduces this phenomenom is certainly encouraging. Following our previous investigations, we will now investigate the topology of the molecular orbitals implied in the most important electronic transitions. In the oo isomer, the band at 300 nm corresponds to transitions implying the four frontier orbitals sketched in the SI. Their shapes are consistent with two independent photochromes, as, on the one hand, only the relative signs of the orbitals centered on the two DA vary between the HOMO and the HOMO-1 (LUMO and LUMOþ1) and, on the other hand, there is no electronic contribution from the

central spiro group. As can be seen, the virtual orbitals present the so-called photochromic shape (see Introduction). Consequently, the fact that experimental irradiation at 313 nm yields the closedopen blend is consistent with our predictions. For I(co), the visible transition corresponds to a HOMO to LUMO transition, and these two orbitals are centered on the closed unit (see Figure 2), as expected. The 305 and 301 nm wavelengths reported in Table 1 correspond to HOMO-2 to LUMO and HOMO-1 to LUMOþ1 electronic promotions, respectively. As the first implies exclusively the closed DA, only the second should induce electrocyclization of the open switch (the LUMOþ1 has a photochromic aspect). Eventually, the 471 nm absorptions of the cc isomer imply four orbitals (see SI) that present the typical topologies for closed DA,47 i.e., the occupied (virtual) centered on the double (single) bonds of the π-conjugated core, with no contribution from the central spiro moiety. This therefore confirms that, even in its most conjugated form, I behaves like two independent DA. For II, the two DA are bridged through a conjugated tetrathiafulvalene (TTF) moiety (see Scheme 2 and ref 36). Geometry optimizations reveal that this central unit is slightly bent. Nevertheless, the deviations are limited: D2 optimizations of II(oo) and II(cc) yield one imaginary frequency of 5.6i and 6.3i cm1, respectively (see SI for structure representations). The difference of G between the three isomers shows an almost additive pattern (see Table 1), as in I, whereas the distance between the reactive carbon atoms is also within the expected range. One can therefore state that the photoreactivity of II is guided by electronic, rather than geometric, features. Experiments revealed that irradiation with a 254 nm light induces the formation of II(co) that presents the typical visible absorption band at ca. 445 nm, but no II(cc) could be obtained. Theory is again in good agreement (see Table 1) and predicts almost no change of λmax but a doubling of the intensity in II(cc). In II(co), the 450 nm absorption is related to a primary (secondary) 9196



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Table 1. Relative Gibbs Free Energies (G in kcal.mol1), Distances Separating the Reactive Carbon Atoms (di in Å), and Relevant Vertical Transition Wavelengths (λ in nm and the Corresponding Oscillator Strengths, f) for the Molecules of Scheme 2 and Scheme 3a theory structure I

II

III

isomer

G

d1

d2

λ

oo

0.00

3.486

3.486

300 ( 2)

co

8.55

1.534

3.490

λ

f 0.16 ( 2) 0.26

465

0.83

0.38

305

2.31

465 300

1.57 3.08

301

0.17

1.535

1.535

471 ( 2) 307

0.26 ( 2) 0.82

oo

0.00

3.731

3.732

346

0.12

305

0.54

co

11.89

1.531

3.727

450

0.17

367

0.49

295

0.35

290

0.36

cc

24.11

1.531

1.531

449 362

0.33 1.23

oo

0.00

3.580

3.806

346

0.51

co

31.77

1.574

3.803

3.733

1.542

oo

0.00

4.107

3.606

co

15.41

1.532

3.643

oc

27.21

4.056

1.549

1.61

470

16.87

18.43

310

ε

305 cc

oc

IV

experiment d1 3.53 and 3.68

d2 3.53 and 3.68

ref. 24

36

0.1) and a dominant LUMOþ1 contribution, though of course the LUMOþ1 is present in a few minor absorption bands. II is therefore similar to previously examined cases,27,28 where the photochromic orbital remains very difficult to reach when a first DA has been closed. In summary, I behaves like two uncoupled DA, so that closing one DA has essentially no effect on the other. Such an outcome

seems typical of DA bridged with connectors free of π-electrons, such as the SiMe2 of Feringa’s group,21 the spirobifluorene dimer of Irie and co-workers,38 or the corresponding tetramer of Tian et al.37 This kind of architecture allows the formation of the cc forms but lacks emergent characteristics necessary for multistate photochromism. Nevertheless, it is of course useful to enhance the intensity of the “on” signal. On the contrary, II cannot be fully closed because the experimental wavelength does not allow to promote the electron toward an effective orbital, though there is absolutely no steric hindrance. This is also the case for another similar DA dimer designed by Wong et al.40 Shared-Thiophene Dimers. For III, XRD structures are available for both the oo and co isomers,15,48 and the relative positions of the thiophene rings are close to the photoactive antiparallel conformation. Subsequently, these experimental geometries have been used as starting points for our DFT optimizations.49 To determine the structure of III(oc), the optimal III(oo) has been modified by closing the second DA, 9197



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Figure 1. Simulated UV/vis absorption spectra for the three isomers of I in their most stable conformation. A broadening Gaussian presenting a full width at half-maximum (fwhm) of 0.30 eV has been used. The top panel is the experimental graph (taken from Kobatake, S.; Kuma, S.; Irie, M. J. Phys. Org. Chem. 2007, 20, 960967. Copyright, John Wiley and Sons, Inc.). Reproduced with permission. Figure 3. Topology of the four frontier orbitals of II(co). The closed DA is located at the top.

Figure 4. LUMO and LUMOþ1 for the three existing isomers of III. For the co (oc) the closed DA unit is located on the left (right) of the molecule.

Figure 2. Topology of the relevant frontier orbitals of I(co). The closed DA is located at the top.

and the resulting geometry has been used as a starting point for the optimization. The energetic, geometric, and spectral data calculated for III are summarized in Table 1. One notes that the changes in total energy associated with electrocyclizations are

larger than in I and II. Nevertheless, III(oc) is more stable than III(co), though the formation of the latter is strongly favored in the experiment (∼10:1 ratio).15 This confirms that the total Gibbs free energies computed for the ground-state compounds are not major discriminating parameters in coupled DA,27 as one should normally locate the transition states/conical intersections on the excited-state surface (a task beyond computational reach here). In III(oo), the XRD distances between the two reactive carbon atoms are smaller for the first DA (lhs in Scheme 3) than for the second DA (rhs in Scheme 3), and this fact is well reproduced by DFT (see Table 150). For III(co), solid-state 9198



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Figure 5. Representation of the predicted III(cc) with top (left) and side (right) views.

Figure 7. Relevant virtual orbitals for (top to bottom) IV(oo), IV(co), and IV(oc). For the co (oc) the closed DA unit is located on the left (right) of the molecule. Figure 6. Simulated UV/vis absorption spectra for the three isomers of IV in their most stable conformation. A broadening Gaussian presenting a fwhm of 0.45 eV has been used. The top panel is the experimental graph (taken from Higashiguchi, K.; Matsuda, K.; Irie, M. Angew. Chem., Int. Ed. 2003, 42, 35373540. Copyright Wiley-VCH Verlag GmbH & Co. KGaA). Reproduced with permission.

measurements and theory also agree quite well. This distance is known to partly govern the photoreactivity of DA; e.g., in crystals an interval larger than 4.2 Å prevents photochromism.51 Our calculations demonstrate that the CC distances in the open DA are almost unaffected by the closed/open nature of its neighboring unit, although both switches partake a ring. This geometric parameter is therefore not sufficient by itself to explain the absence of the doubly closed derivative of III in the experimental pot under a 334 nm irradiation,24 and the remaining open unit in hybrid closed/open and open/closed systems still presents a relatively small CC distance. Our TD-DFT estimates for all isomers are rather accurate with theoretical (experimental) λmax of 346 (342), 448 (456), and 449 (475) nm for oo, co, and oc, respectively. Simulations reveal that the intense UV band of the most stable oo form is dominated by the usual HOMO to LUMO transition with a small HOMO-3 to LUMOþ1 component. For both co and oc hybrid derivatives, the UV bands nearby 334 nm (the experimental excitation wavelength) imply complex orbital mixtures, but the LUMO and LUMOþ1 prevail for the virtual contribution. As only the unoccupied orbitals are essential for providing insight into the photochromic features (see above), we have plotted these orbitals in Figure 4. For the III(oo) isomer, it is clear that the LUMO and the LUMOþ1 present photochromic shapes for both DA, but with larger densities for the reactive carbon atom on the left-hand side. This is consistent with the presence of both co and oc experimentally, as well as with a larger share of the former. For both hybrid isomers, it is obvious

that at least one of the two first virtual orbitals of Figure 4 retains a photochromic clear-cut topology for the remaining open DA (e.g., LUMOþ1 for III(co) and III(oc)). Therefore, the question pertains: why is there no III(cc) though both the distances between the reactive carbon atoms and the orbital shapes of the co and oc isomers seem to hint a possible formation? The answer is, in fact, pretty straightforward: III(cc) would imply a dramatic steric stress. This assertion is illustrated in Figure 5 with the result of the optimization of the putative III(cc). In this structure, some intervals between the hydrogen atoms of the central methyl groups are smaller than the 2.0 Å, whereas the CC distances between the reactive carbon atoms are 1.61 and 1.58 Å, both being very large in comparison to the typical values obtained at the same level of theory (see Table 1). Large CC values are known to be detrimental for the (thermal) stability of the product.52 Therefore, one can conclude that III(cc) cannot be obtained because such a molecule would clearly be extremely unstable: once a first DA is closed, the room remaining is too limited to allow for a second electrocyclization. For IV(oo), an XRD structure has also been obtained by Higashiguchi and co-workers,16 and it has been used as the starting point for our simulations. Nevertheless, as the DA side bearing the phenyl group does not present a clean antiparallel conformation in the XRD data, we have also performed extra conformational calculations. These simulations started with other chemically reasonable positions for the different groups. For the doubly open isomer, none was more stable than the one optimized from the original XRD data, though four are very close (within 2.0 kcal.mol1, see SI for representations of these structures), including two presenting a reactive conformation for the phenyl-sided DA (conformers C and D in SI). The energetic and geometric data listed in Table 1 for the most stable IV(oo) conformer demonstrate no specific difference compared to III but for the larger CC distance associated to the 9199



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Table 2. Calculated and Measured Data for Va theory isomer

G

d1

d2

d3

ooo

0.00

3.641

4.804

3.619

coo

oco

ooc

coc

24.77

18.05

19.97

53.75

1.539

3.621

3.697

1.540

5.199

1.529

5.221

5.051

3.752

3.667

1.553

1.558

experiment λ

f

331

0.37

319

0.23

284

0.22

273

0.18

λ

ε

334(sh)

1.40

273

2.70

473

0.22

494

0.72

328

0.26

334

1.49

312

0.16

271 270

0.18 0.17

279

2.25

575

0.74

587

0.38

347

0.15

335

0.24

375

0.86

416

0.14

429

0.47

319

0.20

285

0.15

276

2.37

284 469

0.11 0.23

443

0.44

416

0.14

310

0.15

271

0.16

272

2.23

a

Experimental data are from ref 12. See caption of Table 1 for more details.

above-mentioned parallel orientation. The computed CC intervals are again in good agreement with experiment.16 The experimental and theoretical absorption spectra are compared in Figure 6, whereas the main bands are collated in Table 1. Both are consistent with a strong difference between the λmax of IV(co) and IV(oc) and almost nonoverlapping bands in the visible domain for the three isomers, two hallmarks of IV.16 The 333 nm band of IV(oo) corresponds almost exclusively to a HOMO to LUMO electron promotion, and the LUMO is clearly centered on the nonsubstituted photochrome (see Figure 7, conformer A of SI). This is consistent with the much larger quantum yield measured for the IV(oo) f IV(oc) (Φ = 0.14) than for IV(oo) f IV(co) (Φ = 0.03) when a 334 nm incoming beam is used.16 In conformers C and D (see SI), that present an antiparallel conformation for the DA on the left, the LUMO presents a similar topology (centered on the second DA), but it has been checked that it has a bonding character on the first DA. Indeed, to obtain IV(co), one has to use an adequate oo conformer, so that the C conformation was selected as the starting point for TD-DFT calculations. The computed λmax for the resulting molecule is at 585 nm, in perfect agreement with experiment (580 nm, see ref 16 and Table 1). The transition closest to the experimental irradiation wavelengths (313 or 334 nm) is the 329 nm transition that implies the LUMO and LUMOþ1. As can be seen in Figure 7, the former is mainly located on the closed DA (as expected), whereas the latter presents an antibonding character for the open DA and is thus not useful to induce photochromism. Therefore, no clear IV(co) f IV(cc) path is found with TD-DFT. For IV(oc), the previous analysis hints that the most stable conformer should be similar to the one of its IV(oo) father, and this latter has been selected as an initial point. In the IV(oc) hybrid, the visible band, of no specific interest for ring

closure of the second DA, corresponds to a HOMOLUMO transition, and the LUMO are centered on the closed DA (Figure 7). Irradiations at 313 or 334 nm (the experimental wavelengths)16 should have no impact, as the closest computed transitions (at 318 and 290 nm) present negligible oscillator strengths (0.01 and 0.04, respectively). Furthermore, the largest contributions in these two excited states are transitions toward the LUMOþ1 that do not present a bonding character for the to-beformed CC bond. In other words, to induce an IV(oc) f IV(cc) transformation, one would first need to modify the conformation (as for IV(oo), but the hybrid molecule is significantly more rigid) and next to photocyclize through a (probably) low intensity band. This is clearly a highly unlikely outcome, so that one can reasonably state that the IV(oc) f IV(cc) path is, at best, very inefficient. In short, for IV, the absence of the doubly closed form is due to the lack of trail that combines a large ratio of antiparallel conformers and transition yielding to virtual orbitals presenting the required topology. Indeed, one suffers from a (negative) interplay between electronic and conformational/ steric effects. For the record, a hypothetical IV(cc) structure could be optimized through a conventional force-minimization process (see SI). This isomer is quite crowded and presenting a long CC bond on the rhs (1.58 Å, a detrimental factor for thermal stability).52 Five-State Trimer. For the trimer sketched in Scheme 4, the XRD data are available for the fully open form,12 and as could be expected, the molecule is very compact, with numerous closecontact distances (5 Å), making impossible the electrocyclization of two vicinal photochromes. In addition, this work also confirmed27 the remarkable accuracy of the PCM-TD-CAM-B3LYP model for simulating the absorption spectra of multi-DA switches systems. Indeed, for the nearly 30 absorption bands investigated herein for which direct comparisons between theory and experiment are straightforward, we obtain a mean absolute error of 10 nm. Of course, it should be recalled that these TD-DFT simulations are vertical and static so that energy transfer phenomena or formation of side products cannot be accounted for, so we certainly do not state that the selected technique is flawless and could be used blindly.27,29 Nevertheless, relevant insights can be obtained when both electronic and geometric effects are carefully analyzed. As stated above, it is now clear that to obtain effective multistate systems one needs electron communication between the different DA units (to allow emergent features), asymmetry (for multiaddressing possibilities), and a limited steric stress. For this reason, we propose, as an outlook, four new dimeric molecules composed of two nearby (but not packed) asymmetric DA (see Scheme 5). These systems can be viewed as a blend between the symmetric sexithiophene-photochromic wire of Browne and Feringa22,53,54 and one of the multicolor dimers treated herein, III.16 The results collated in Table 3 show that (1) adding a phenyl group at R2 has a negligible effect on the spectral features; (2) the same group at R1 has a significant impact when the lhs DA is closed; (3) the differences between the cc and co forms are relatively limited; (4) the R1dR2dPh derivatives could be of interest as they present several UV peaks dominated by the LUMOþ1 and slightly larger wavelength shifts than its hydrogen-capped counterpart. In Figure 9, we have presented the computed absorption spectra for this system. Interestingly, we predict that the near-UV band undergoes a hypsochromic displacement when the rhs DA is closed, whereas the usual visible band close to 600 nm is obtained when the lhs photochrome is activated. If the spectra of the four forms differ, the λmax of VI(co) and VI(cc) are unfortunately too alike. The main orbital components are listed in Table 3, whereas the topology of the two first virtual orbitals can be found in Figure 10. For VI(oo), the LUMO

Figure 10. LUMO and LUMOþ1 of the four possible isomers of VI capped by phenyl groups.

related to the 411 nm band is located on the rhs DA, whereas the LUMOþ1 (that is dominant in a nonlisted 314 nm/f = 0.11 transition) is centered on the other DA. Both have a clear photochromic shape. In VI(co), the LUMO is centered on the closed moiety and the vicinal thiophene ring, but the LUMOþ1, possibly reachable though the 353 nm transition (see Table 3), could be useful to close the second DA. The same holds for VI(oc), but in that case, the LUMOþ1 band (at 306 nm) is well separated from the other. In short, this suggests a possible VI(oo) f VI(oc) f VI(cc) path that would be associated with an initial hypsochromic displacement, followed by a large bathochromic shift. Though, it is clearly beyond the scope of the present ab initio work to assess the synthesis of such derivatives, and though these “static” predictions may fail to predict correctly the photoreactivity,27 it is our hope that this contribution could stimulate further experimental developments and measures in the field of coupledphotochromic entities.

’ ASSOCIATED CONTENT

bS

Supporting Information. Description of the isomers obtained for I. Relevant frontier orbitals for I(oo) and I(cc). Representation of D2 and C1 II(oo) and II(cc). Conformers of IV(oo). Structure, λmax, and frontier orbitals of IV(cc). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; denis. [email protected].

’ ACKNOWLEDGMENT The authors thank Professor Higashiguchi (Kyoto University) for providing unpublished XRD data. DJ thanks Prof. E. Ishow (Universite de Nantes) for fruitful discussions on coupled DA 9202


The Journal of Physical Chemistry C switches. DJ is indebted to the Region des Pays de la Loire for financial support in the framework of a recrutement sur poste strategique. This research used resources of (1) the Interuniversity Scientific Computing Facility located at the University of Namur, Belgium, which is supported by the F.R.S.-FNRS under convention No. 2.4617.07, (2) the GENCI-CINES/IDRIS (Grant c2011085117) and, (3) the CCIPL (Centre de Calcul Intensif des Pays de Loire).

’ REFERENCES (1) D€urr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier: New York, 2003. (2) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. J. Chem. Soc., Chem. Commun. 1993, 1439–1442. (3) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. Chem.—Eur. J. 1995, 1, 275–284. (4) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803–808. (5) Irie, M.; Sayo, K. J. Phys. Chem. 1992, 96, 7671–7674. (6) Irie, M. Chem. Rev. 2000, 100, 1685–1716. (7) Tian, H.; Yang, S. Chem. Soc. Rev. 2004, 33, 85–97. (8) Matsuda, K.; Irie, M. J. Photochem. Photobiol. C: Photochem. Rev. 2004, 5, 169–182. (9) Yun, C.; You, J.; Kim, J.; Huh, J.; Kim, E. J. Photochem. Photobiol. C: Photochem. Rev. 2009, 10, 111–129. (10) Tsujioka, T.; Irie, M. J. Photochem. Photobiol. C: Photochem. Rev. 2010, 11, 1–14. (11) Kaieda, T.; Kobatake, S.; Miyasaka, H.; Murakami, M.; Iwai, N.; Nagata, Y.; Itaya, A.; Irie, M. J. Am. Chem. Soc. 2002, 124, 2015–2024. (12) Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie, M. J. Am. Chem. Soc. 2005, 127, 8922–8923. (13) Peters, A.; Branda, N. R. Adv. Mater. Opt. Electron. 2000, 10, 245–249. (14) Shilova, E. A.; Heynderickx, A.; Siri, O. J. Org. Chem. 2010, 75, 1855–1861. (15) Higashiguchi, K.; Matsuda, K.; Matsuo, M.; Yamada, T.; Irie, M. J. Photochem. Photobiol. A: Chem. 2002, 152, 141–146. (16) Higashiguchi, K.; Matsuda, K.; Irie, M. Angew. Chem., Int. Ed. Engl. 2003, 42, 3537–3540. (17) Jung, I.; Choi, H.; Kim, E.; Lee, C. H.; Kang, S. O.; Ko, J. Tetrahedron 2005, 61, 12256–12263. (18) Choi, H.; Jung, I.; Song, K. H.; Song, K.; Shin, D. S.; Kang, S. O.; Ko, J. Tetrahedron 2006, 62, 9059–9065. (19) Aubert, V.; Ishow, E.; Ibersiene, F.; Boucekkine, A.; Williams, J. A. G.; Toupet, L.; Metivier, R.; Nakatani, K.; Guerchais, V.; Le Bozec, H. New J. Chem. 2009, 33, 1320–1323. (20) Kobatake, S.; Irie, M. Tetrahedron 2003, 59, 8359–8364. (21) Areephong, J.; Browne, W. R.; Feringa, B. L. Org. Biomol. Chem. 2007, 5, 1170–1174. (22) Areephong, J.; Hurenkamp, J. H.; Milder, M. T. W.; Meetsma, A.; Herek, J. L.; Browne, W. R.; Feringa, B. L. Org. Lett. 2009, 11, 721–724. (23) Areephong, J.; Logtenberg, H.; Browne, W. R.; Feringa, B. L. Org. Lett. 2010, 12, 2132–2135. (24) Kobatake, S.; Kuma, S.; Irie, M. J. Phys. Org. Chem. 2007, 20, 960–967. (25) Jacquemin, D.; Perpete, E. A.; Maurel, F.; Perrier, A. J. Phys. Chem. Lett. 2010, 1, 434–438. (26) Jacquemin, D.; Michaux, C.; Perpete, E. A.; Maurel, F.; Perrier, A. Chem. Phys. Lett. 2010, 488, 193–197. (27) Jacquemin, D.; Perpete, E. A.; Maurel, F.; Perrier, A. J. Phys. Chem. C 2010, 114, 9489–9497. (28) Jacquemin, D.; Perpete, E. A.; Maurel, F.; Perrier, A. Phys. Chem. Chem. Phys. 2010, 12, 7994–8000. (29) Jacquemin, D.; Perpete, E. A.; Maurel, F.; Perrier, A. J. Phys. Chem. Lett. 2010, 1, 2104–2108.

ARTICLE

(30) Perrier, A.; Maurel, F.; Ciofini, I.; Jacquemin, D. Chem. Phys. Lett. 2011, 502, 77–81. (31) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997–1000. (32) Casida, M. E. In Time-Dependent Density-Functional Response Theory for Molecules; Chong, D. P., Ed.; World Scientific: Singapore, 1995; Vol. 1, pp 155192. (33) Burke, K.; Werschnik, J.; Gross, E. K. U. J. Chem. Phys. 2005, 123, 062206. (34) Laurent, A. D.; Andre, J. M.; Perpete, E. A.; Jacquemin, D. J. Photochem. Photobiol. A: Chem. 2007, 192, 211–219. (35) Staykov, A.; Areephong, J.; Browne, W. R.; Feringa, B. L.; Yoshizawa, K. ACS Nano 2011, 5, 1165–1178. (36) Uchida, K.; Masuda, G.; Aoi, Y.; Nakayama, K.; Irie, M. Chem. Lett. 1999, 1071–1072. (37) Tian, H.; Chen, B.; Liu, P. H. Chem. Lett. 2001, 990–991. (38) Kobatake, S.; Kuma, S.; Irie, M. Bull. Chem. Soc. Jpn. 2004, 77, 945–951. (39) Yam, V. W. W.; Lee, J. K. W.; Ko, C. C.; Zhu, N. J. Am. Chem. Soc. 2009, 131, 912–913. (40) Wong, H. L.; Ko, C. C.; Lam, W. H.; Zhu, N.; Yam, V. W. W. Chem.—Eur. J. 2009, 15, 10005–10009. (41) Frisch, M. J. Gaussian 09, revision A.02, Gaussian Inc.: Wallingford CT, 2009. (42) Jacquemin, D.; Perpete, E. A.; Ciofini, I.; Adamo, C. Acc. Chem. Res. 2009, 42, 326–334. (43) Of course, with this aprotic/apolar solvent, the environmental effects are rather limited. Gas-phase test calculations have been peformed for II(oo) and II(cc). For the former, the CC distances in the gas phase are 3.725 Å instead of 3.731 Å in n-hexane. For the TD spectra of II(oo), the two first important vertical transitions are 347 nm (f = 0.08) and 302 nm (f = 0.36) in the gas phase but 346 nm (f = 0.12) and 305 nm (f = 0.54) in n-hexane. For the doubly closed structure, gas-phase simulations yield 444 nm (f = 0.26) and 360 nm (f = 0.94), whereas PCM provides 449 nm (f = 0.33) and 362 nm (f = 1.23). In short, bulk solvent effects seem to principally impact the oscillator strengths in the present case. (44) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999–3094. (45) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158–6170. (46) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393, 51–56. (47) Perrier, A.; Maurel, F.; Aubard, J. J. Photochem. Photobiol. A: Chem. 2007, 189, 167–176. (48) Higashiguchi, K. Private communication, 2010. (49) Note that, for the fully open form, two polymorphs have been identified by XRD (R and β in ref 15), but the same molecular minima is reached after the PCM-PBE0 force minimization process. (50) The distance ranges in Table 1 are due to the presence of several molecules of slightly different geometries and/or to thermal effects that do not allow one to define only one bond length. (51) Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem. Commun. 2002, 2804–2805. (52) Maurel, F.; Perrier, A.; Perpete, E. A.; Jacquemin, D. J. Photochem. Photobiol. A: Chem. 2008, 199, 211–223. (53) Milder, M. T. W.; Herek, J. L.; Areephong, J.; Feringa, B. L.; H Browne, W. R. J. Phys. Chem. A 2009, 113, 7717–7724. (54) Milder, M. T. W.; Areephong, J.; Feringa, B. L.; H Browne, W. R.; Herek, J. L. Chem. Phys. Lett. 2009, 479, 137–139.

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