Ab Initio Investigation of the Electronic Properties of Coupled

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Ab Initio Investigation of the Electronic Properties of Coupled Dithienylethenes Denis Jacquemin,*,† Eric A. Perp ete,‡ Franc- ois Maurel,‡ and Aur elie Perrier*,‡ †

Unite de Chimie Physique Th eorique et Structurale (UCPTS), Facult es Universitaires Notre-Dame de la Paix, rue de Bruxelles, emes (ITODYS), 61, B-5000 Namur, Belgium and ‡Laboratoire Interfaces, Traitements, Organisation et Dynamique des Syst CNRS UMR 7086, Universit e Paris 7, Paris Diderot, B^ atiment Lavoisier, 15 rue Jean Antoine de Baïf, 75205 Paris Cedex 13, France

ABSTRACT The design of more efficient photochromes does not stand as the only challenge in the field of molecular switches; more complex and refined structures, allowing one to pave the way toward multiply addressable structures, are highly sought after. In this framework, compounds containing several dithienylethenes have been recently proposed. In the present Letter, we investigate the spectral properties of photochromes made of two dithienylethene units linked through conjugated bridges to form meta and para switches. All combinations of closed and open forms have been modeled by using (time-dependent) density functional theory. With this ab initio tool, the coupling between the photochromic centers could be assessed. Such calculations are a step necessary to understand the parameters guiding the formation of the fully closed forms. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

n the realm of molecular switches, dithienylethenes (also referred to as diarylethenes, DA) occupy the driver seat because they possess all properties that efficient photochromes should satisfy.1 Indeed, DA possess two thermally stable forms, with very different properties, that can be converted from one to the other by irradiation at wellseparated wavelengths. These direct and reverse conversions from the strongly conjugated closed form to the less-conjugated open form can be achieved a huge number of times and present both large quantum yields and short response times. In the last decades, refined applications of DA have been proposed, for example, magnetic commutators,2 switches combining photochromism and electrochromism,3 and nanoslings relying on solid-state photochromism.4 To go even further, the design of multiply addressable structures is of interest. In that framework, compounds containing several molecular switches bonded through delocalized bridges have been proposed by several research teams to reach higher logic operations than the “on/off” effect. The first effective structures of the DA category which we are aware of are the hybrid DA/hydorazulene molecules synthesized in 1999 by Mrozek et al.5 Later, mixed DA/naphthopyran structures were designed and characterized by Frigoli, Mehl, and co-workers.6 Structures containing only DA bonded through several units have also been designed in the past decade.7-16 In Table 1, we briefly summarize the experimental results obtained for DA dimers. As can be seen, a major challenge of these systems lies in obtaining the fully closed forms. Indeed, such structures were impossible to obtain if the DA were linked by a simple covalent bond,7 a bis(phenylethynyl)anthracene group,9 triple bonds,10 or small oligothienoacenes,14 and the same applies for ring structures with DA bonded through

diethynylbenzene and ethynylene bridges.12 Derivatives with fully active photochromes have been obtained with unconjugated spacers,13 a single bridging phenyl ring,8,11 inorganic architectures,16-18 and a delocalizable oligothiophene chain.15 The parameters helpful for the design of bridges allowing doubly closed forms have not been clearly identified at this stage. In addition, despite these important pioneering works, the information related to such coupled DA systems remains limited. In particular, if full photochromism is possible, one of the major (experimental) difficulties is to identify the structures presenting mixed forms (e.g., one open DA, one close DA) once photochromes of the same family are selected. In fact, the UV/visible spectra obtained through HPLC for Feringa's molecular wire15 and the λmax listed in ref 11 are the only experimental electronic data of which we are aware for DA dimers coupled through a delocalized bridge. In this Letter, we investigate, with the help of time-dependent density functional theory (TD-DFT), the properties of DA meta- and para-coupled through a typical bridging unit (Scheme 1). The objective is to evaluate the coupling between the different units and to obtain insights into the possibilities of forming such structures. TD-DFT is now well-recognized as an efficient and reliable tool for investigating the electronic energies of excited states. While TD-DFT calculations of the spectral properties have been performed previously by us (see our most recent works19,20 and references therein) and by other groups,21,22 it appears that the coupled DA systems of Scheme 1 have not been modeled previously. In fact, the only

I

r 2009 American Chemical Society

Received Date: November 19, 2009 Accepted Date: December 8, 2009 Published on Web Date: December 15, 2009

434

DOI: 10.1021/jz900293g |J. Phys. Chem. Lett. 2010, 1, 434–438

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(12.7 kcal 3 mol-1 in terms of 6-311G(d,p) Gibbs free energies). These values are perfectly in the line with a previous DFT investigation.19 For the electronic absorption properties, we obtain a strong absorption in the UV domain (333 nm) for the open form and an extra peak in the visible part of the electromagnetic spectrum (614 nm) appearing for the closed DA. This is again consistent with previous TD-DFT investigations19,20 and experimental trends.1 In both cases, the λmax mainly corresponds to a HOMO-LUMO transition. For the closed isomer, both orbitals are partly delocalized on the triple bond and phenyl ring. The HOMO of the open isomer is mainly located on the lateral chain and the thiophene ring bonded to it, whereas the corresponding LUMO is centered on the DA core.

previous theoretical calculations of which we are aware for similar molecules are the TD-PBE0/6-31G(d,p) investigation of the open/open and open/closed structures of ref 14, the HOMO and LUMO of fully-closed star-shaped structures,23 and the determination of the frontier orbitals for thiophenebridged DA.15 The spectral characteristics computed for the single DA (I in Scheme 1) and meta (IIm)- and para (IIp)-linked DA pairs are listed in Table 2. In all cases, all open/closed combinations have been considered. For the reference system containing only one DA, it turned out that the open form is more stable than the closed form by 13.2 kcal 3 mol-1 in terms of 6-311þG(2d,p) internal energies Table 1. Synopsis of selected Experimental Measurements Performed for DA Coupled through an Organic Bridgea

Table 2. Vertical Transitions Obtained at the PCM-TD-PBE0/ 6-311þG(2d,p)//PCM-PBE0/6-311G(d,p) Levela

λmax (ε) bridge

cc

co

oo

ref

bis(phenylethynyl)anthracene

550

455 (50500)

9

covalent bond

505 (18180) 600

324 (18500) 300

7

588 (25000) 634 (44000)

569 (13500) 615

274 (42700) 435 (41000)

11

506 (21800)

506 (11000)

282 (48000)

13

584

320 (23000)

10

diethynylbenzene phenyl oligothiophene Si(Me)2 triple bonds

λ

ΔE

f

c

614 345

2.02 3.60

0.44 0.79

o

333

3.72

1.24

cc

625

1.98

0.73

351

3.53

0.78

structure

form

I

IIm 12

co

15 IIp

611

2.03

0.49

346

3.58

1.44

oo

338

3.67

2.01

cc

690 383

1.80 3.24

1.39 1.06

co

632

1.96

0.79

439

2.82

0.85

397

3.12

2.54

oo

a

We refer the reader to the appropriate references for an exact description of all structures and experimental conditions. Values in italics are estimated from graphs; cc, co, and oo stand for fully-closed, mixed closed/open, and fully-open forms, respectively. Wavelengths are in nm, and molar absorption coefficients are in L 3 cm-1 3 mol-1.

“Structure” refers to the compounds of Scheme 1, whereas “form” indicates the closed/open nature of the DA connected to the central unit. Wavelengths (λ) are in nm, transition energies (ΔE) are in eV, and oscillator strengths ( f ) are given. Only relevant transitions are listed. a

Scheme 1. Representation of the Compounds under Scrutiny (cc forms)


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Figure 1. Topology of HOMO and LUMO orbitals for IIm (top) and IIp (bottom) in their oo form, obtained at the PCM-PBE0/6-311þ G(2d,p) level using a contour threshold of 0.30 au.

Figure 2. Topology of frontier orbitals for the mixed open/closed systems. See Figure 1 for more details.

maximum at 614 nm. For a compound very similar to IIm(co), the experimental measurements also detected an absorption close to 600 nm.12 The visible peaks of these “mixed” molecules still essentially correspond to HOMO f LUMO transitions. The topologies of these frontier orbitals are presented in Figure 2. Obviously, they are completely asymmetric and located on the side of the closed switch, though in the LUMO of the para structure, a trifling contribution remains on one of the reactive carbon of the open DA. Of course, the second strong transition appearing at the limit of the ultraviolet domain could be effective for the ring closure of the second DA of IIm and IIp. However, it is not the case. Indeed, the larger orbital contribution in the 346 nm absorption of IIm is a HOMO-4 to LUMO transition, which is unhelpful for photochromism as the virtual orbital remains the LUMO that does not present the correct shape. While there are small contributions from LUMOþ1 and LUMOþ2 (which present the required topology, being partly located on the reactive carbon of the open DA) in this 346 nm peak, they remain limited, probably explaining why the second electrocyclization is unobserved experimentally for similar structures.9,12 At this stage, a comparison with the results obtained for a molecule in which the cc isomer was detected appears necessary.15 In fact, the frontier orbitals of the co structure reported in ref 15 are similar to the one of Figure 2; they are localized in the vicinity of the closed DA, with no sizable contribution at the opposite side of the molecule. However, by performing TD-DFT simulations for the co structure of Feringa and co-workers (the compound with terminal chlorine atoms, 2, in ref 15 was selected to reduce computational cost), we obtain a first transition above 600 nm corresponding to a HOMO to LUMO transition, as for IIm and IIp.

For the dimers, the relative energies of the co and cc structures compared to the oo reference are, respectively, 13.5 (13.5) and 27.4 (26.9) kcal 3 mol-1 for IIm (IIp). This indicates that the energy cost related to ring closure is nearly perfectly additive when structures with multiple DA are considered. For the records, we note that the para isomers are more stable than their meta counterparts by approximatively 1 kcal 3 mol-1, this value being nearly independent of the states of the two photochromic units. From the results listed in Table 2, it is obvious that the fully open (oo) structures show a first electronic transition at relatively small wavelength, though, compared to the open I, bathochromic shifts of þ5 nm (þ66 nm) and a strong intensity increase are predicted for IIm (IIp). These λmax correspond to an electron promotion from the HOMO to LUMO (see Figure 1) for both the meta and the para derivatives. The occupied orbitals are localized on the multiple bonds of the central conjugated unit of the molecules, whereas the LUMOs' densities are centered on single bonds and possess large contributions on the reactive carbons of the thiophene rings connected to the central bridging unit. For IIm(oo), electronic density on the two sides of the DA are also noticeable for the LUMO. Even if no significant components of the virtual orbitals are detected on the other reactive carbon atoms of the DA units, ring closure should not be prevented as the presence of density on one of the reactive carbons is sufficient for cyclized DA.24 This conclusion is also consistent with experimental measurements carried out for compounds using similar bridging moieties between the two DA.9,12 For the closed/open (co) structures, TD-DFT foresees λmax of 611 nm (632 nm) for IIm (IIp), which is almost unchanged compared to the reference I that presents a vertical absorption


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However, the two next strongest absorptions among the six first excited states (around 400 nm, in reasonable agreement with experiment) are both characterized by an orbital pattern dominated by the large contribution of the LUMOþ1 level. This virtual orbital is localized on the open DA and presents the desired topology, hinting that constant irradiation at (relatively) small wavelengths may indeed lead to the fully closed form for that molecule. As the dynamics of the problem is also essential for multiple DA systems,10 this preliminary conclusion obtained from a static model should certainly be confirmed for a larger set of molecules and states. Nevertheless, this Letter indicates that the investigation of coupled photochromes might gain from theoretical studies not limited to the frontier orbitals usually modeled in theoretical investigations of DA. For the doubly closed derivatives, cc, we predict relatively small bathochromic displacements compared to the co case, with variations limited to þ14 nm (þ58 nm) for IIm (IIp). The available literature values for the co/cc shifts also clearly indicate small bathochromic displacements, þ19 nm for the phenyl-bridged DA of ref 11 and an estimated þ19 nm for the molecular wire 1 from the graph in ref 15. The λmax of IIp corresponds to a HOMO to LUMO transition. For IIm(cc), the 625 nm absorption is related to (almost) equal shares of HOMO-1 f LUMO and HOMO f LUMOþ1 contributions as the first two occupied and unoccupied orbitals are nearly degenerate (they only differ by the relative signs of the contributions centered on each photochrome). As can be seen in Figure 3 (for the meta case, only one set of the two degenerate orbitals is depicted), the occupied orbitals are mainly localized on the two dithienylethenes, whereas their virtual counterparts are completely delocalized, a typical feature of molecules absorbing at long wavelengths. The structure and electronic properties of dithienylethenes coupled through a delocalized bridge have been investigated with a theoretical spectroscopic approach, namely, (timedependent) density functional theory. The relative energies of the open and closed forms are nearly additive in nature, whereas the transition energies are not. Indeed, the closure of the first DA provokes a large bathochromic shift, whereas a more modest effect is predicted for the cyclization of the second photochrome. In all cases, para-coupled molecules absorbs at longer wavelengths than the corresponding meta derivatives. This investigation also points out the importance of going beyond the simple HOMO/LUMO picture to assess the possibility of the second ring closure. We are currently modeling the electronic features of coupled photochromes relying on a panel of bridges in order to help design more efficient multiaddressable molecular architectures.

Figure 3. Topology of frontier orbitals for the fully closed systems. See Figure 1 and the text for more details.

structures for the open and closed forms are indeed true minima by determining the vibrational spectrum. Then, the first 10 low-lying excited states have been determined within the vertical TD-DFT approximation using a 6-311þG(2d,p) basis set that yields converged values for DA.27 The bulk solvent effects have been included during both the force minimization and TD-DFT step by means of the polarizable continuum model (PCM).28 The solvent used is cyclohexane, to ensure the selection of an aprotic environment in which the performances of the PCM approach are optimal. Default PCM parameters have been applied throughout, but for the use of the UAKS (united atom Kohn-Sham) radii, which are known to be optimal for DFT calculations. All calculations have been performed with the hybrid PBE0 functional (sometimes named PBEh or PBE1PBE), which was designed from the pure PBE (Perdew-Burke-Ernzerhof) on purely theoretical considerations.29 While the choice of this functional can be justified25 by its consistency for evaluating vertical transitions, previous works on dithienylethenes have shown that this functional tends to undershoot the transition energies of the low-lying states of perfluoro DA presenting a significant charge-transfer character.20 This limitation is however nontroublesome as we are interested in the trends in a homologous series of DA, for which PBE0 is indeed efficient.20

METHOD Our computational procedure has been reviewed recently;25 therefore, we only briefly summarize the protocol applied here. We have used the Gaussian03 program throughout26 and selected default algorithms and parameters except when noted. In a first step, we have optimized, with the 6-311G(d,p) basis set, the ground-state geometry of all derivatives without imposing any symmetry constraints. For the model system I, it has been confirmed that the obtained


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: denis. [email protected] (D.J.); [email protected] (A.P.).

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ACKNOWLEDGMENT D.J. and E.A.P. thank the Belgian National

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Fund for Scientific Research for their research associate and senior research associate positions, respectively. The calculations have been partially performed at the Interuniversity Scientific Computing Facility (ISCF), installed at the Facult es Universitaires Notre-Dame de la Paix (Namur, Belgium), for which the authors gratefully acknowledge the financial support of the FNRS-FRFC and the “Loterie Nationale” for the Convention Number 2.4578.02 and of the FUNDP. The collaboration between the Belgian and French groups is supported by the Wallonie-Bruxelles International, the Fonds de la Recherche Scientfique, the Minist ere Franc-ais des Affaires etrangeres et europ eennes, the Minist ere de l'Enseignement sup erieur et de la Recherche in the framework of the Hubert Curien Partnership.

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Ab Initio Investigation of the Electronic Properties of Coupled

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