Synthesis of Symmetrical and Nonsymmetrical Bisthienylcyclopentenes

Bis(thienyl)ethenes with α-methoxymethyl groups. Syntheses, spectroscopic Hammett plots, and stabilities in PMMA films

A series of bis(thienyl)ethenes (BTEs) possessing perfluorocyclopentene backbones and methoxymethyl groups (MOM) in the 2/2'-positions of the thiophenes was prepared and examined. The substitution pattern of the 5/5'-positions was varied, covering the range from electron-donating to electron-withdrawing. The substituent effects of the absorption wavelengths of the ring-opened and the ring-closed isomers, which are interconverted by reversible 6π-electrocyclizations and cycloreversions, are studied by means of the spectroscopic Hammett equation and the Hammett-Brown equation. Excellent correlations of these linear free energy relationships were found, when the σp values of the Hammett equation, which summarize inductive, mesomeric and field effects, were replaced to the Hammett-Brown σp+ and σp- values which also take direct conjugation into account. We studied solvent effects on the spectroscopic properties and embedded the BTEs into polymethylmethacrylate (PMMA) coatings to examine their fatigue resistance. By our studies, the spectroscopic properties of BTEs can be adjusted by variation of the substitution pattern to a desired excitation wavelength for switching processes.

Synthesis strategies for non-symmetric, photochromic diarylethenes

Diarylethenes (DAEs) represent an important class of photochromes with notable characteristics, like thermally irreversible photoisomerization and high fatigue resistance. Structural diversification of the DAE scaffold has enabled further refinement of photochromic properties and realization of new applications, ranging from advanced materials to tools for studying biological systems. In particular, methods for synthesizing non-symmetric DAE scaffolds, which are typically more challenging to synthesize than their symmetric counterparts, have grown over the past 20 years. These developments are surveyed in this review, with discussion of how access to these compounds has contributed to the improvement of photochromic properties and paved the way for exploring new applications of DAEs. First, non-symmetric DAE structures are classified and their uses and applications are overviewed. Subsequent sections discuss the main strategies that have been used to access non-symmetric DAEs with examples illustrating the impact of non-symmetric DAEs in the growing field of light-controlled molecular systems.

Diarylethene Dyes

This article introduces the general characteristics of the diarylethene class of photochromic dye and the structural features that make photochromism possible. It touches on the methodologies employed to synthesize these compounds as well as the influences that typical substitution patterns exert on photocoloration. A demonstration is then given of the great diversity pertaining to the potential applications in which researchers are seeking to exploit them as functional colorants.

Synthesis of amphiphilic asymmetrical dithienylethenes with aryl groups at the reactive carbons

A synthetic route for the preparation of amphiphilic asymmetrical dithienylethenes that incorporate methyl groups at the 4- and 4′-positions and an aryl group at one of the reactive carbons has been developed. The presence of a bulky aryl substituent ensures a relatively large change in molecular geometry upon photoisomerization, whereas the presence of methyl groups provide enhanced photostability. Notably, a substituent effect was systematically revealed en route to the preparation of the dithienylethene precursors. In particular, this formal substitution reaction was significantly inhibited due to steric hindrance, stemming from the presence of aryl and methyl groups at the alpha positions of the preformed thienyl carbanionic carbon, and an aryl group on the monosubstituted perfluorocyclopentene derivative.

Facile synthesis of photoactive diaryl(hetaryl)cyclopentenes by ionic hydrogenation

A facile synthetic approach to photoactive diarylethenes comprising a cyclopentene ring as an ethene bridge was developed based on reduction of 2,3-diaryl(hetaryl)cyclopent-2-en-1-ones through an ionic hydrogenation reaction.

Improving the fatigue resistance of diarylethene switches

When applying photochromic switches as functional units in light-responsive materials or devices, an often disregarded yet crucial property is their resistance to fatigue during photoisomerization. In the large family of diarylethene photoswitches, formation of an annulated isomer as a byproduct of the photochromic reaction turns out to prevent the desired high reversibility for many different derivatives. To overcome this general problem, we have synthesized and thoroughly investigated the fatigue behavior of a series of diarylethenes, varying the nature of the hetaryl moieties, the bridging units, and the substituents. By analysis of photokinetic data, a quantification of the tendency for byproduct formation in terms of quantum yields could be achieved, and a strong dependency on the electronic properties of the substituents was observed. In particular, substitution with 3,5-bis(trifluoromethyl)phenyl or 3,5-bis(pentafluorosulfanyl)phenyl groups strongly suppresses the byproduct formation and opens up a general strategy to construct highly fatigue-resistant diarylethene photochromic systems with a large structural flexibility.