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Cite this: Chem. Commun., 2013, 49, 11427 Received 3rd September 2013, Accepted 14th October 2013 DOI: 10.1039/c3cc46736h

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New heterocyclic systems to afford microsecond green-light isomerisable azo dyes and their use as fast molecular photochromic switches† ´ s,a M. Cida ´lia R. Castro,b Paulo Coelho,c Jaume Garcia-Amoro b M. Manuela M. Raposo* and Dolores Velasco*a

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The use of benzothiazole as an electron-withdrawing group allows obtaining the fastest thermal isomerisation kinetics reported heretofore for neutral azo dyes (70 ls at 298 K). These green light activatable molecules are valuable candidates as molecular photoswitches since they tolerate thousands of working cycles with no sign of fatigue.

Photochromic molecular switches benefit from light to swap reversibly between two or more different states of the molecule, which differ in their maximum absorption wavelength or absorbance value.1 Indeed, photocontrolled switchable materials are of great interest in a wide range of areas from nanotechnology to biology or medicine.2–10 Light holds enormous potential for switching purposes since it can be switched back and forth easily, remotely and rapidly enabling clean and quick modulation of the properties of a highly localized area of the material with no direct contact. Moreover, in the case of photochromic switches, light allows both the activation of the probe and the monitoring of the operation to be performed simultaneously. All these features have led photoresponsive materials to be the top ones among those now being researched worldwide and, therefore, there is great interest towards the design and generation of new substances that respond to light in a desired way. Azobenzene is the most used chromophore for photoswitchable materials since it exhibits a reversible photoisomerisation process between its trans and cis isomers. Moreover, cis-to-trans conversion takes place also thermally in the dark. Indeed, the incorporation of azo dyes into different matrices, dispersed or a

`nics, Institut de Nanocie`ncia i Nanotecnologia (IN2UB), Grup de Materials Orga `nica, Universitat de Barcelona, Martı´ i Franque`s 1, Departament de Quı´mica Orga E-08028, Barcelona, Spain. E-mail: [email protected]; Fax: +34 93 339 78 78; Tel: +34 93 403 92 60 b Centro de Quı´mica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal. E-mail: [email protected]; Fax: +351 253 604382; Tel: +351 253 604381 c Centro de Quı´mica – Vila Real, Universidade de Tra´s-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal † Electronic supplementary information (ESI) available: Synthesis and full spectroscopic data for the novel azo dyes 5–7 and experimental procedures and details of the different techniques used. See DOI: 10.1039/c3cc46736h

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covalently linked, has led to the preparation of several photoresponsive organic or hybrid advanced materials with a wide range of properties and abilities.11–17 The overall performance of all these materials relies on our ability to control the speed of both trans-to-cis and cis-to-trans isomerisation steps. Nevertheless, the timescale of the thermal back reaction is the most important parameter to consider for applications and, therefore, it may be molecularly engineered. Generally, for efficient photochromic molecular switches to be made, it is essential that the azo-dye exhibits a fast thermal cis-to-trans back conversion, most preferably within the microsecond time scale. Azo dyes that combine a strong push–pull configuration with the ability to establish an azo-hydrazone tautomeric equilibrium are promising chromophores for this purpose, since they show very fast thermal cis-to-trans isomerisation kinetics at room temperature.18 Indeed, we have recently described the use of hydroxyazopyridinium salts for photoswitching applications; these compounds exhibit relaxation times as low as 33 ms at 298 K.19 However, the main drawback of the use of positively-charged azo compounds arises from their low solubility in the most common host systems required for technological applications, e.g. liquid crystals and polymeric matrices. Hence, it remains a challenge to increase the isomerisation rate of non-charged azo compounds sufficiently for their isomerisation to occur within the microsecond time scale. The strategy presented herein consists of using both thiazole and benzothiazole rings as electron-withdrawing moieties and p-conjugated heterocyclic systems (bithiophene, thienylpyrrole and bithienylpyrrole) as electron-donating groups (Scheme 1). Few studies concerning the isomerisation of such heterocyclic azo dyes have been reported so far.20–22 In any case, none of these systems has ever exceeded the hundred millisecond time scale. Here we report for the first time on the thermal cis-to-trans isomerisation kinetics of several neutral heterocyclic azo dyes (see Scheme 1) that exhibit thermal isomerisation kinetics down to the microsecond time scale at room temperature. Actually, these are the fastest thermally-isomerising noncharged azo dyes described heretofore. These molecules are able to tolerate thousands of cycles with no sign of fatigue. In addition, all the systems reported herein are photo-activated Chem. Commun., 2013, 49, 11427--11429

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Scheme 1

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Chemical structure of the heterocyclic azo derivatives 1–7.

upon irradiation with visible light, which is very convenient for possible further use in biology. Azo dyes 1–4 have been prepared as reported elsewhere.22–24 Compounds 5–7 were synthesized in moderate to good yields (40–60%, Scheme S2, ESI†) via the diazotation of 4-nitroaniline, 2-aminobenzothiazole and 5-aminobenzothiazole with NaNO2 in HCl or H2SO4 at 0–5 1C and further coupling with bithienylpyrrole25 in acetonitrile at 0 1C or in a solution of acetic anhydride in ice water. The kinetics of the thermal cis-to-trans isomerisation process of the different azo derivatives in acetone was studied by means of the conventional time-resolved UV-vis spectroscopy. In ethanol, a population of cis isomers was generated in less than 5 ns through the nanosecond laser flash-photolysis technique (see Table 1 and ESI†). The thermal relaxation time of the cis isomers, t (t = 1/k), was determined by fitting a monoexponential function to the data. The comparison between azo dyes 1–4 allowed us to evaluate not only the effectiveness of both thiazole and benzothiazole rings as electron-withdrawing moieties but also to analyse the effect of the chemical architecture of the electron-donating thiophene- (Tph) and pyrrole-based (Pyr) assemblies (Tph–Tph and Tph–Pyr) on the kinetic behaviour of such azo compounds. As a whole, it has been observed that the thermal cis-to-trans isomerisation process for azo derivatives 1–7 is always faster in ethanol than in acetone owing to not only the higher polarity of the former solvent but also to its ability to establish hydrogenbonding with the azo molecules (see Table 1). Table 1 Wavelength of maximum absorption of the trans isomer, lmax, and thermal relaxation time for the cis isomer, t, in acetone and ethanol at 298 K and maximum absorption change at lmax, DAmax, proportion of the cis isomer at the photo-equilibrium (Yapp = (1 Aph/A0)  100) and thermal activation parameters, DHa and DSa, for the thermal cis-to-trans isomerisation of azo dyes 1–7 in ethanol

lmax tacetone lmax Yapp tEtOH Azo (nm) (ms) (nm) DAmax (%) (ms)

DHa DSa (kJ mol 1) (J K 1 mol 1)

1 2 3 4 5 6 7

52  37  26  31  22  23  —a

a

487 487 499 507 534 517 490

637 714 909 833 317 202 62 000

490 494 502 500 544 515 —a

0.06 0.08 0.04 0.02 0.06 0.06 —a

7.3 35 12 109 8.9 108 5.4 1.0 19 0.07 8.0 1.6 —a —a

Not determined.

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1 1 1 1 1 1

44 104 140 88 91 115 —a

     

4 1 1 2 1 1

Fig. 1 Photoswitching capability of azo dyes 2 (a) and 4 (b) in ethanol solution at 298 K (lobs = 500 nm, [AZO] = 20 mM): transient absorption change photoinduced by laser pulse irradiation with green light (lirrad = 532 nm, Continuum Surelite I-10 Q-switched Nd-YAG laser with 5 ns pulse width and ca. 10 mJ per pulse).

The chemical structure of the electron-donating dyad has no defined effect on the kinetic behaviour of the azo derivative. In this way, the Tph–Tph dyad is the most effective one for the thiazole-based systems (35 ms vs. 109 ms for azo dyes 1 and 2, respectively, Fig. 1a), whereas for the benzothiazole-based chromophores the Tph–Pyr dyad induces the lowest relaxation times for the thermal back reaction (1.0 ms for 4, Fig. 1b). The extension of the conjugation of the electron-withdrawing part of the molecule by replacing the thiazole ring by a benzothiazole one induces a notable improvement of the photoswitching capability of the molecular switch in ethanol (109 ms vs. 1.0 ms for 2 and 4, respectively). In the wake of the low relaxation time for the benzothiazolebased azo-dye 4 of only 1.0 ms, the possibility of further enhancing the speed of the molecular photochromic switch by using even more p-extended electron-donating heterocyclic bridges was evaluated. Accordingly, a Tph–Tph–Pyr triad was incorporated into compound 5 instead of the initial Tph–Pyr dyad used for compound 4. As hypothesised, the use of a highly p-conjugated system containing several electron rich heterocycles induced very fast thermal isomerisation kinetics for the benzothiazole-based azo dye 5. With a relaxation time of only 70 ms in ethanol at 298 K, this is the fastest thermally-isomerising non-charged azo compound reported heretofore (Fig. 2). Furthermore, the thermal back reaction of this azo dye can be accelerated up to only 18 ms just by increasing the temperature up to 343 K; this temperature is compatible with the most common processes performed in materials engineering. The comparison of compound 5 with compound 6, which bears a p-nitrophenyl group as an electron-withdrawing function, shows that the thermal isomerisation process is 23-fold faster in the former demonstrating thereby that benzothiazole acts as a much stronger electron acceptor than the nitro function in this type of azo compound. Moreover, the position by which the benzothiazole acceptor group is linked to the azo function is of crucial importance for low thermal relaxation times to be met. In this way, when the connection between the two moieties is performed through This journal is

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Fig. 2 Photoswitching capability and fatigue resistance of azo dye 5: (a) transient absorption change photo-induced by laser pulse irradiation with green light (lirrad = 532 nm) at 298 K (lobs = 545 nm) and photostability after 1000 (b) and 5000 (c) laser pulses (ca. 10 mJ per pulse).

Financial support for this research was obtained from the Ministerio de Ciencia e Innovacio´n (Spain, CTQ2012-36074). J.G.-A. is grateful for a post-doctoral grant from the Generalitat ˜o de Catalunya (2011 BP-A-00270). Thanks are also due to Fundaça ˆncia e Tecnologia (Portugal) for financial support to the para a Cie portuguese NMR network (PTNMR, Bruker Avance III 400-Univ. Minho), FCT and FEDER (European Fund for Regional Development)-COMPETE-QREN-EU for financial support to the research units CQ/UM [PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER022716)] and CQVR/UTAD and a PhD grant to M. C. R. Castro (SFRH/BD/78037/2011). The authors thank Prof. Santi Nonell for the use of the laser flash photolysis setup.

Notes and references position 2 of the benzothiazole ring a relaxation time of only 70 ms is registered in ethanol (see above). If the linkage takes place through position 5 instead, a dramatically slower isomerisation is detected (62 s for compound 7 in acetone, see Table 1). This differential behaviour arises from the more effective conjugation in the former system without modifying the aromaticity of the benzene ring of the benzothiazole moiety. In addition, the activation parameters (DHa and DSa) for the thermal back reaction of azo dyes 1–6 were determined in ethanol (see Table 1). All enthalpy of activation values ranged from 22 to 52 kJ mol 1, which are more than 50 kJ mol 1 lower than the ones found for non-push–pull azo dyes.26 This feature points out to the operation of the rotational mechanism for the thermal isomerisation of azo dyes 1–7, which is responsible for the fast isomerisation kinetics observed in this family of heterocyclic azo derivatives. Both the repeatability and photostability of the different photochromic molecular switches were checked by subjecting them to several pulsed green light (532 nm)–dark cycles. Fig. 2 evidences the high photostability exhibited by the benzothiazole-based azo-dye 5 in ethanol solution at 298 K: after 5000 cycles neither the photoinduced absorbance variation nor the relaxation time of the molecular switch were altered by its continuous work. A similar behaviour was registered for the different azo dyes analysed. Summarizing, benzothiazole-based azo dyes bearing a highly p-conjugated system containing several electron-rich heterocycles are the fastest non-charged thermally-isomerising azo derivatives reported so far. The short excited lifetime (down to 70 ms at 298 K) and high photostability of the reported azo dyes make them ideal photo-active molecules for photochromic switching applications at the molecular scale. Additionally, the absence of positive charges in the structure of the azo chromophore is of relevance for further incorporation of such photoactive molecules into engineered materials. Additional optimization of the presented molecular architecture can open new ways to reach non-charged azo derivatives showing thermal isomerisation kinetics beyond the microsecond time scale. So, the appropriate design of the photo-active azo molecule dictates its isomerisation behaviour and, therefore, the usefulness of the final material.

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1 Molecular Switches, ed. B. L. Feringa and W. R. Browne, Wiley-VCH, Weinheim, 2nd edn, 2011. ¨tzel, J. Frisch, 2 E. Orgiu, N. Crivillers, M. Herder, L. Grubert, M. Pa E. Pavlica, D. T. Duong, G. Bratina, A. Salleo, N. Koch, S. Hecht and P. Samorı`, Nat. Chem., 2012, 4, 675. 3 J. E. Green, J. W. Choi, A. Boukai, Y. Bunimovich, E. JohnstonHalperin, E. Delonno, Y. Luo, B. A. Sheriff, K. Xu, Y. S. Shin, H.-R. Tseng, J. F. Stoddart and J. R. Heath, Nature, 2007, 445, 414. 4 A. J. Kronemeijer, H. B. Akkerman, T. Kudernac, B. J. van Wees, B. L. Feringa, P. W. M. Blom and B. de Boer, Adv. Mater., 2008, 20, 1467. 5 I. Yildiz, S. Impellizzeri, E. Deniz, B. McCaughan, J. F. Callan and F. M. Raymo, J. Am. Chem. Soc., 2011, 133, 871. 6 F. M. Raymo, J. Phys. Chem. Lett., 2012, 3, 2379. ´s, M. Dı´az-Lobo, S. Nonell and D. Velasco, Angew. 7 J. Garcia-Amoro Chem., Int. Ed., 2012, 51, 12820. 8 A. A. Beharry and G. A. Woolley, Chem. Soc. Rev., 2011, 40, 4422. 9 P. Stawski, M. Sumser and D. Trauner, Angew. Chem., Int. Ed., 2012, 51, 5748. 10 A. Mourot, T. Fehrentz, Y. Le Freuvre, C. M. Smith, C. Herold, D. Dalkara, F. Nagy and D. Trauner, Nat. Methods, 2012, 9, 396. 11 H. Yu and T. Ikeda, Adv. Mater., 2011, 23, 2149. ´s, A. Pin ˜ol, H. Finkelmann and D. Velasco, Org. Lett., 12 J. Garcia-Amoro 2011, 13, 2282. ´s, E. Go ´mez, E. Valle ´s and D. Velasco, Chem. Commun., 13 J. Garcia-Amoro 2012, 48, 9080. 14 P. J. Coelho, C. M. Sousa, M. C. R. Castro, A. M. C. Fonseca and M. M. M. Raposo, Opt. Mater., 2013, 35, 1167. ´rrez, C. Tejedor and G. Cuniberti, Nat. 15 M. del Valle, R. Gutie Nanotechnol., 2007, 2, 176. ´pez, H. Finkelmann, P. Palffy-Muhoray and 16 M. Camacho-Lo M. Shelley, Nat. Mater., 2004, 3, 307. 17 M. Petr, B. Katzman, W. DiNatale and P. T. Hammond, Macromolecules, 2013, 46, 2823. ´s and D. Velasco, Beilstein J. Org. Chem., 2012, 18 J. Garcia-Amoro 8, 1003. ´s, S. Nonell and D. Velasco, Chem. Commun., 2011, 19 J. Garcia-Amoro 47, 4022. 20 P. J. Coelho, M. C. R. Castro, S. S. M. Fernandes, A. M. C. Fonseca and M. M. M. Raposo, Tetrahedron Lett., 2012, 53, 4502. 21 P. J. Coelho, M. C. R. Castro, A. M. C. Fonseca and M. M. M. Raposo, Dyes Pigm., 2011, 92, 745. 22 M. M. M. Raposo, A. M. C. Fonseca, M. C. R. Castro, M. Belsley, M. F. S. Cardoso, L. M. Carvalho and P. J. Coelho, Dyes Pigm., 2011, 91, 62–73. 23 M. M. M. Raposo, M. C. R. Castro, A. M. C. Fonseca, P. Schellenberg and M. Belsley, Tetrahedron, 2011, 67, 5189–5198. 24 M. M. M. Raposo, M. C. R. Castro, M. Belsley and A. M. C. Fonseca, Dyes Pigm., 2011, 91, 454–465. 25 C. Herbivo, A. Comel, G. Kirsch and M. M. M. Raposo, Tetrahedron, 2009, 65, 2079. ´s, M. Martı´nez, H. Finkelmann and D. Velasco, 26 J. Garcia-Amoro J. Phys. Chem. B, 2010, 114, 1287.

Chem. Commun., 2013, 49, 11427--11429

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New heterocyclic systems to afford microsecond green-light isomerisable azo dyes and their use as fast molecular photochromic switches.

The use of benzothiazole as an electron-withdrawing group allows obtaining the fastest thermal isomerisation kinetics reported heretofore for neutral ...
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