Journal of Photochemistry and Photobiology C: Photochemistry Reviews
Azole-based diarylethenes as the next step towards advanced photochromic materials
Introduction
Photochromism is the phenomenon of the reversible isomerization of molecules under the action of light. Photochromic diarylethenes (DAE’s) with heterocyclic moieties show high thermal stability of photoinduced form, high resistance to fatigue, reversible switching in solution and in the solid state, and a large rearrangement of electronic distribution upon photoisomerization (Scheme 1) [1]. Due to these prominent photophysical properties, DAE’s are the subject of extensive research aiming, for instance, the development of unprecedented photon-controlled functional materials/devices and realization of novel light-controlled processes [2]. Various applications of DAE’s include the development of elements for organic electronics [3] and photocontrollable sensors [4], control of optical [5], magnetic [6], and fluorescence [7] properties of transition metal complexes, photoswitchable nanosystems.[8] DAE’s are actively used in such emerging areas as spatiotemporal control of biological systems[9] and chemical reactions [10] as well as in photopharmacology [11].
Since the discovery of this class of photochromes by M. Irie in 1988 [12], thiophene derivatives were primarily used as aromatic moieties, which provided high thermal stability of photoinduced isomers. Although during the first 15 years after the discovery, some works dealing with DAE’s featuring azole moieties (thiazole, oxazole, imidazole) appeared [13], crucial effects of these moieties on photochromic properties were not recognized. Only during the last 10–15 years, a number of works were published, where a great potential of azole derivatives has been revealed. Replacement of thiophenes by azoles in a number of cases significantly changed the properties of an initial open form A and a photoinduced form B due to particular electronic effects and non-covalent interactions (Scheme 1). On the other hand, due to their excellent photochemical properties, azole-containing DAE’s became frequently used for the development of functional molecules, materials, and devices. Finally, on the basis of azole derivatives, irreversible photoinduced transformations were discovered, which opened new perspectives for preparative organic photochemistry.
This paper is not a comprehensive review on azole-containing DAE’s [14]. Our goal is to bring together highlights on azole-based DAE’s and to demonstrate how these photochromes contributed to a significant expansion of the research field during the recent years.
Section snippets
Spectral properties
Azoles (thiazole, oxazole, imidazole) strongly differ both from the thiophene and from each other in their electronic properties. For this reason, the introduction of various heterocycles allows the modulation of spectral properties of DAE's. This can be demonstrated by a series of symmetric DAE’s with a perfluorocyclopentene bridge elaborated by M. Irie and coworkers (Fig. 1). In the series of DAE’s with normal (β-substituted thiophene and 4-substituted azoles) heterocycles (1 [15] 2 13c] 3 [16
Advanced molecules and materials
DAE's featuring azole moieties (first of all, thiazoles) along with thiophene derivatives are widely used for the development of functional photocontrollable materials and devices. In this chapter, we consider the most impressive and interesting results obtained in this area. The azole-based DAE’s have many advantages: easy synthesis and a good combination of photosensitivity (quantum yields), photo- and thermal stability.
Photoacid generators
Photoactive molecules that release acids under the action of light are of great interest for lithography, polymer design, and other applications [105]. Highly effective photocyclization of thiazole-based DAE's was used to develop a new family of neutral photoacid generators (PAG’s) 84 (Scheme 21) [106]. These DAE's contain a thiazole with a hydrogen atom at the reactive carbon and thiazole or benzothiophene as a bridge. The second aryl moiety (derivative of benzene, thiazole or thiophene) bears
Conclusions
In recent years, a number of works on photoactive DAE's appeared, where significant effects driven by the replacement of traditional thiophene by azole heterocycles (thiazole, oxazole, imidazole) were revealed. The introduction of azole moieties makes it possible to control the spectral properties of photochromes in a wide range, which was used for developing various multi-color materials. Additionally, such replacement allows to vary the thermal stability of photoinduced isomers of DAE's. The
Acknowledgments
Chapters 1,2,4,5 were supported by Russian Science Foundation (grant 14-50-00126). Chapter 3 was supported by the Fonds der Chemischen Industrie (Liebig Fellowship) and Deutsche Forschungsgemeinschaft (DFG Research Grant KH 279/3). MMK is thankful to Prof. Karsten Meyer (FAU Erlangen-Nürnberg) for his general support.
Andrey G. Lvov graduated in 2011 in technology of organic compounds from the D. I. Mendeleev University of Chemical Technology of Russia (Moscow). His PhD, obtained in 2014 under the direction of V. Z. Shirinian (N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences), was on the synthesis and study of azole-based photochromic diarylethenes. His main research interests are related to the photochromism and irreversible phototransformations of diarylethenes.
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Andrey G. Lvov graduated in 2011 in technology of organic compounds from the D. I. Mendeleev University of Chemical Technology of Russia (Moscow). His PhD, obtained in 2014 under the direction of V. Z. Shirinian (N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences), was on the synthesis and study of azole-based photochromic diarylethenes. His main research interests are related to the photochromism and irreversible phototransformations of diarylethenes.
Marat M. Khusniyarov graduated from the Novosibirsk State University (Russian Federation) in 2002. He obtained his PhD in the group of Prof. Jörg Sundermeyer, Philipps-Universität Marburg (Germany), in 2006. As a postdoc, he joined the group of Prof. Karl Wieghardt at the Max Planck Institute for Bioinorganic Chemistry (Germany). In 2009, he was awarded a prestigious Liebig Fellowship and established an independent junior research group at the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU, Germany). Here, he accomplished his Habilitation mentored by Prof. Karsten Meyer in 2016. Since 2016, he is teaching as a Privatdozent and continue to lead his independent research group at the FAU. His research interests include photoactive and magnetic materials, molecular switches and sensors, molecular magnetism, applied spectroscopy, and theoretical calculations.
Valerii Z. Shirinian. Currently he is leading researcher at the N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences with expertise in Organic Chemistry, Photochemistry and Organic Synthesis; he received his PhD in Organic Chemistry (1992) and Doctor of Sciences in Chemistry (2010). His research focused on design and syntheses of novel functional photoactive compounds and dyes (diarylethenes, spiropyrans, spirooxazines and merocyanines), as well as spectral properties study of photosensitive compounds for memories and switches.