Accepted Manuscript Novel photochromic diarylethenes bearing an imidazole moiety Valerii Z. Shirinian, Andrey G. Lvov, Ekaterina Yu. Bulich, Alexey V. Zakharov, Mikhail M. Krayushkin PII: DOI: Reference:
S0040-4039(15)01321-0 http://dx.doi.org/10.1016/j.tetlet.2015.08.028 TETL 46612
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2 May 2015 10 August 2015 12 August 2015
Please cite this article as: Shirinian, V.Z., Lvov, A.G., Bulich, E.Y., Zakharov, A.V., Krayushkin, M.M., Novel photochromic diarylethenes bearing an imidazole moiety, Tetrahedron Letters (2015), doi: http://dx.doi.org/ 10.1016/j.tetlet.2015.08.028
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Novel photochromic diarylethenes bearing an imidazole moiety Valerii Z. Shirinian, Andrey G. Lvov, Ekaterina Yu. Bulich, Alexey V. Zakharov, Mikhail M. Krayushkin
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Novel photochromic diarylethenes bearing an imidazole moiety Valerii Z. Shiriniana,, Andrey G. Lvova, Ekaterina Yu. Bulichb, Alexey V. Zakharov a, Mikhail M. Krayushkina a b
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp, Moscow 119991, Russian Federation Mendeleev University of Chemical Technology of Russia, 9 Miusskaya Sq., Moscow 125047, Russian Federation
A RT I C L E I N F O
A BS T RA C T
Article history: Received Received in revised form Accepted Available online
Novel photochromic diarylethenes based on a 2,3-diarylcyclopent-2-en-1-one core and containing imidazole residues as the aryl moiety were synthesized. The photochromic properties of the obtained compounds were investigated in acetonitrile and a comparative analysis with oxazole and thiophene containing diarylethenes was performed. It was found that introduction of imidazole derivatives as the aryl moiety led to a significant decrease in thermal stability and an increase in both photosensitivity (quantum yields) and the bathochromic shift of the absorption maxima of both isomers.
Keywords: Photochromism Diarylethene Imidazole Thermal stability Quantum yield
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Photochromism is the phenomenon of reversible isomerization of compounds under the action of light.1 Currently, photochromic compounds are used as the basis for various photocontrollable materials2 and molecular devices.3 One of the important properties, defining the field of practical application, is the thermal stability of the metastable photoinduced form. Photochromes can be divided into thermally stable compounds (P-type), possessing photoisomer half-lives ranging from months to years, and thermally unstable compounds (T-type) exhibiting stability ranging from seconds to hours. Diarylethenes with heterocyclic residues are one of the most promising classes of photochromes.4 Typically this field is concerned with the design, synthesis and investigation of thermally stable photochromic compounds (P-type) for the development of optical memories and molecular switches. The thermal stability of diarylethene is mainly dependent on the aromatic stabilization energy of the aryl moieties. Diarylethenes bearing heterocycles with low aromatic stabilization energies (thiophene, benzothiophene, furan) show high thermal stability (up to many years – P-type)5 and most synthesized diarylethenes belong to this group. Conversely, diarylethenes with low thermal stability (T-type) are poorly studied. The low thermal stability of diarylethenes can be achieved by the introduction of groups possessing high aromatic stabilization energies (benzene, pyrrole, indole and related rings),6 utilization of an aromatic ethene “bridge”,7 or other destabilizing factors.8 T-type photochromes have found wide use in diverse applications, including optical filters, ophthalmic lenses, protective eyeglasses, optical devices for use in camera systems or photographic apparatus, novelty items and toys, smart windows, vehicles, security markers, and cosmetic products.9 Photochromic chromenes, spiroxazines,10 and bisimidazoles11 are most attractive for practical applications, however, a common problem of these photochromic compounds is related to the significant change in geometry and polarization during photochromic switching, that in turn complicates the choice of suitable polymeric matrices. Diarylethenes are devoid of these limitations because switching is provided by hexatriene-cyclohexadiene systems, which simplifies their application in various polymers. As noted above, the main factor determining the thermal stability of the diarylethenes is the nature of the aryl residues and their aromatic stability. Recently we have evaluated the thermal stability of diarylethenes comprising of azole (oxazole, thiazole, imidazole) and thiophene rings by quantum chemical calculations using density functional theory (DFT).12 The calculated data revealed that introduction of an imidazole ring as the aryl moiety could contribute to a decrease in the thermal stability of the photoinduced isomer, while oxazole residues should give the opposite result. The synthesis of diarylethenes bearing oxazole derivatives and an investigation of their photochromic properties confirmed the theoretical results regarding the stability of the photoisomer. It should be noted that compared with thiophene analogs, azole diarylethenes are poorly understood, despite the improvement in photochromic properties in many cases. In particular, diarylethenes of the thiazole and oxazole series demonstrate good thermal stability12,13 and high photocyclization quantum yields.12,14 Photoactive compounds based on pyrazole15 and isoxazole16 rings with remarkable light-induced switching properties have been also developed. At the same time, photochromic diarylethenes with an imidazole ring as the aryl moiety have been poorly studied17 and only recently were the first examples described in a patent.18 Additionally in two publications close analogues have been reported by Irie and coworkers (imidazo[1,2-b]pyridine)19 and by Kawai and co-workers
(imidazolyl cation).20 The use of imidazole derivatives as the aryl moiety in diarylethene molecules could lead to not only an improvement of the photochromic characteristics, but may also be useful for biological and materials sciences due to the presence of the sp2-nitrogen atom. In this work we report the synthesis and photochromic properties of diarylethenes bearing an imidazole moiety. In addition, the switching properties including the thermal stability of the prepared compounds were compared with oxazole and thiophene derivatives. Recently, we developed a new type of photochromic diarylethene based on a cyclopent-2-en-1-one core which was highly amenable to chemical modification.21 A wide range of photochromic compounds were prepared and the spectroscopic and kinetic characteristics studied. This has been used to develop thermally stable photochromic compounds,22 photochromes with high photocyclization quantum yield,12 and hybrids possessing modulated fluorescence.23 Photochromic diarylethenes bearing an imidazole ring were synthesized by alkylation of ethyl 4-(2,5-dimethylthiophen-3-yl)3-oxobutanoate 1 by bromoketones 2a-d in the presence of metallic sodium in absolute benzene. Subsequent cyclization in aqueous ethanol led to photochromic diarylethenes 3a-d (2135%, over 2 steps) (Scheme 1). The structures of compounds 3ad were proven by 1H and 13C NMR spectroscopy, massspectrometry, IR spectroscopy and high resolution massspectrometry. The mechanism of the photochromic reaction of diarylethenes involves UV promoted conrotatory electrocyclization of the hexatriene system, and cycloreversion which is induced by visible light (Scheme 2).
Scheme 1. Synthetic protocol for preparation of the target diarylethenes. O
N Ph R 3a-d (form A)
N R 3a-d (form B)
Scheme 2. Photochromic reaction of diarylethenes 3a-d.
The photochromic properties of the diarylethenes 3a-d were studied in acetonitrile and are summarized in Table 1 along with the relevant data for photochromes 412 and 521 which differ from compounds 3 by the residues at position 3 of the ethene “bridge”. All compounds exhibited typical photochromic properties having absorption maxima of initial form A in the UV region (323-329 nm) and photoisomer B in the visible range of the spectra (554565 nm). A comparison with the data of 4 and 5 showed that introduction of an imidazole ring instead of the oxazole or thiophene moieties led to a bathochromic shift of the absorption maxima of both isomers. The extinction coefficients of the EtO2C
1. Na, benzene 2. KOH, EtOH/H2O
N R 2a-d
S Me N R 3a-d
a: R = CH3 (21%); b: R = n-C12H25 (22%); c: R = Ph (35%); d: R = CH2Ph (25%)
3 imidazole derivatives were substantially higher when compared with those of bis-thiophene-substituted diarylethene 5, whereas in regard to the oxazole diarylethene 4, only a slight decrease was observed.
Figure 1 shows the absorption spectra of a solution of 3a before and after irradiation with UV light. Existence of
Table 1. Spectroscopic and kinetic properties of the diarylethenes obtained. Structure entry Ar1
λmaxA, nm (ε, M-1 cm-1)a
λmaxB, nm (ε, M-1 cm-1)a
Ph N n-C12H25
Ph N CH2Ph
N Me N
6 a c
Absorption maxima (extinction coefficients) of open-ring (A) and closed-ring (B) isomers; b Quantum yields of photocyclization under irradiation with 313 nm; Quantum yields of cycloreversion under irradiation with 517 nm; d Half-lives of the ring-closed isomers in the dark at 293 K; e Not measured.
an isosbestic point near 350 nm clearly shows isomerization without the formation of side products. In addition, the photochromic properties of 3a remained when incorporated into a polymeric (polyvinyl acetate) film (Figure 1, inset).
Figure 2. 1H NMR spectra (300 MHz, CDCl3) of compound 3a after irradiation with UV-light (365 nm) (photostationary state was not reached).
Figure 1. Changes of an absorption spectrum of compound 3a under irradiation with UV-light (313 nm) in acetonitrile (c = 2.7 × 10-5 M). PVA film of 3a before (left in inset) and after (right) irradiation with UV-light.
The colored photoproduct was detected by 1H NMR spectroscopy. The spectrum of 3a after irradiation with UV-light (365 nm) at 293 K in CDCl3 is depictured in Figure 2. The loss of aromaticity of both heterocyclic rings after photocyclization led to an upfield shift of all methyl group signals (including the methyl fragment at position 1 of the imidazole ring) and a downfield shift of the thiophene ring hydrogen atom signal owing to the rigidity of the photoisomer and the magnetic anisotropic effect of the carbonyl group.
The quantum-chemical calculations of the model heterocycles showed decreased thermal stability for compounds containing the imidazole ring in comparison to oxazole and thiophene. 12 Investigations of the thermal stability were carried out in acetonitrile in the dark at 293 K. The thermal bleaching process was determined as a first-order reaction (Figure 3), permitting the measurement of the photoisomers half-lives which were compared to those for diarylethenes 4 and 5 (Table 1). As expected, introduction of imidazole as the aryl moiety led to decrease of the photoisomer half-lives. Thus, imidazole derivatives with alkyl (methyl (3a) and n-dodecyl (3b)) groups gave the smallest τBA1/2 values of about 30 h. Photochromes bearing phenyl and benzyl groups showed higher stability, equal to 175.0 h (3c) and 61.1 h (3d). In comparison with 2,3diarylcyclopent-2-en-1-ones 4 and 5 (τBA1/2 = 3200 and 940 h), imidazole derivatives exhibited sufficiently smaller τBA1/2 values permitting the assignment as a new group of diarylethenes with low thermal stability.
Figure 3. Thermal cycloreversion kinetics of 4b in acetonitrile, c = 2 x 10−5 M at 293 K.
The relative thermal stability is controlled by the activation energy EA of thermal cycloreversion (Figure 4) and decreasing the latter leads to destabilization of photoisomer B. Previously, Irie and co-workers established guiding principles for the design of P-type diarylethenes, stating that an improvement of stability could be achieved by introduction of aryl groups with low aromatic stabilization energy.24 An inverse relationship between the aromatic stabilization energy and ground state energy differences of the photoisomers was postulated. This rule has been proved in several works,25 and herein we calculated the ground state energy differences of photoisomers A and B for diarylethenes 3a, 4 and 5 (Figure 4) based on DFT (B3LYP1 hybrid density functional method, basis set 6-31G(d)). Replacement of the oxazole ring (compound 4) by thiophene (5) and then by imidazole (3a) resulted in an increase of ΔE values, which correlated with the experimental data. The diarylethene with most unstable photoisomer 3a (τBA1/2=30.5h) had the highest energy difference and therefore, smallest energy barrier EA.
light energy in photochromic processes, and much attention has been paid to determine a relationship between this characteristic and the structure of diarylethenes in order to obtain photochromes possessing high efficiency of both photoreactions.26,27 The quantum yields of the photoreactions for diarylethenes 3 are represented at Table 1. In comparison with 4 and 5, imidazole-based diarylethenes exhibited higher yields for photocyclization; for example, 3a possessed ΦAB = 0.40, whereas for photochromes 4 and 5 this value was only 0.22 and 0.23, respectively. A possible explanation for this phenomenon, by analogy with known examples,14,28 is the possibility of a weak non-covalent bond between the sp2-nitrogen of the imidazole ring and a hydrogen atom of the ethene “bridge” methylene group, which can stabilize the molecule geometry. Moreover diarylethenes 3 have higher cycloreversion quantum yields (ΦBA = 0.24-0.25), than for 4 (0.17) and 5 (0.09). In conclusion we have synthesized novel photochromic diarylethenes bearing an imidazole ring as an aryl moiety and studied their photochromic properties. It was found that the introduction of an imidazole ring decreased the thermal stability of the photoinduced form and improved the photosensitivity (quantum yields) of both the cyclization and cycloreversion reactions. We consider that the introduction of the imidazole ring to the diarylethene unit is a valuable route toward T-type photochromes. Further efforts focused on the synthesis of new diarylethenes with low thermal stability, by means of introduction of a second and third imidazole ring to the diarylethene unit are in progress. Acknowledgments Financial support by Russian Foundation for Basic Research (RFBR grants 14-03-31871 and 15-03-05546) is gratefully acknowledged. References and notes 1. 2.
Figure 4. Energetic diagram of the thermal cycloreversion reaction and the calculated ΔE values for diarylethenes 3a, 4, 5.
Low thermal stability of the photoinduced form (less than few seconds) is required for practical applications. We have demonstrated that replacement of one thiophene ring in dithienylethenes by imidazole led to a significant decrease of this value (reduced from 940 h to 30 h). Moreover, as can be seen from Table 1, the substituent at the sp3-nitrogen atom of the imidazole ring exerted a significant influence on the thermal stability. A relatively electronegative substituent (Ph) led to an increase in the thermal stability of the photoinduced form, and a donor substituent (Me) conversely led to a decrease of this parameter. Further reduction of stability could be achieved by utilizing imidazole as the ethene “bridge” (so called terarylenes26) and both aryl moieties. In a continuation of this study, we intend to develop methods for the preparation of terarylethenes based on imidazole derivatives and study their photoswitching properties. Along with predetermined thermal stability, good photosensitivity is an important feature for practical applications. The quantum yields describe the efficiency of the utilization of
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Supplementary Material Supplementary data (experimental procedures and characterization data of synthesized compounds) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/jtetlet.2015.xxx.