Diversely halogenated spiropyrans - Useful synthetic building blocks for a versatile class of molecular switches

Abstract

Spiropyrans are dyes that can be reversibly switched to a highly colored merocyanine form by a number of stimuli such as light, mechanical force or temperature. To make use of these molecules, there is a requirement to functionalize them appropriately. Herein we report a library of spiropyrans bearing two (pseudo)halide functional groups on either half of the molecule. Such halide substituents are valuable, because they themselves may be used as reactive sites in cross-coupling reactions, for example. Different combinations of halides, for which different reactivities in cross-coupling reactions may be expected, will facilitate selective consecutive cross-coupling reactions and condensations. Data concerning the UVvis characteristics, the photostationary equilibria of the materials as well as the half-life of the merocyanine forms in solution are presented.

Substituent Effects Control Spiropyran–Merocyanine Equilibria and Mechanochromic Utility

“…[28] In general, the nitro group in 6-position plays a pivotal role in stabilizing the corresponding MC, its half-life time and finally the number of cycles during photo-induced isomerization. [16,17,29] If other functional groups are used instead of NO 2 , the relative stabilities and equilibria of all species change, leading to different property profiles. Obviously, substitution at carbon C 6 is most effective as it is para to the phenolate oxygen in the corresponding MC form (Figure 1).…”

A speedy route to sterically encumbered, benzene-fused derivatives of privileged, naturally occurring hexahydropyrrolo[1,2-b]isoquinoline

“…Indolenines 9 . Indolenines 9a , b [ 26 ], 9d–g [ 26 ], 9h [ 36 ], 9i [ 37 ], 9j [ 38 ], 9k [ 39 ], 9p [ 40 ] are known compounds and were prepared from the arylhydrazine hydrochlorides and respective aldehydes or ketones according to the literature protocols.…”

Light, Force, and Heat: A Multi-Stimuli Composite that Reveals its Violent Past

“…Self-reporting materials are capable of showing a measurable, often visible indication of the conditions they have been subjected to. Spiropyrans are molecular switches that undergo a reversible molecular transformation to a merocyanine (MC) form in response to different stimuli such as mechanical stress, − light, and heat and thereby showing a color change. , As the intensity of the color is linked to the amount of active molecules present, spiropyran should be an ideal self-reporting material. , Several self-assembly, self-reporting materials and optical sensors based on spiropyran have been successfully produced. − In most of such systems, the spiropyran is either covalently linked to a polymer chain, − or grafted on metallic surfaces. − Most known systems of spiropyran as a mechanical stress indicator require its covalent incorporation into the main chain or as cross-links of the polymer. − This results in long synthesis routes from commercially available starting materials to the final product, thus complicating the scale-up process. , There have been examples of studies on the mechanical sensitivity of spiropyran derivatives in crystal form, illustrating mechanochromic switching under high hydro static pressure. − This implies that spiropyran can be used as an additive in the polymer composite system and would provide a more cost-effective solution. − Photoinduced mechanical switching of spiropyran in a composite system has been demonstrated in poly­(ethyl methacrylate- co -methyl acrylate) (PEMMA) films . These films were produced by blending 3 wt % of spiropyran, and the stiffness of the material was altered by the photoinduced switching.…”

“…In the present work composites with merely 3.5–7.5 wt % T-ZnO have been fabricated for the accessibility of the broad green PL band. The open form that is merocyanine (Figure b) which has a strong absorption peak in the visible region (550–570 nm) closes to the spiropyran form under green light illumination. ,, The defects present in ZnO induce green emissions under UV irradiation ,, and this should accelerate the switching of the merocyanine form back to the spiropyran, thus shifting the photostationary equilibrium toward the spiropyran form. In addition, the tetrapodal structure of T-ZnO forms an interconnected 3D network thus increasing the overall mechanical stability of the polymer matrix. ,− The composite samples presented here were prepared with a commercially bought polythiourethane (PTU) kit.…”

“…Spiropyrans are molecular switches that undergo a reversible molecular transformation to a merocyanine (MC) form in response to different stimuli such as mechanical stress, 1−4 light, 5 and heat 6 and thereby showing a color change. 7,8 As the intensity of the color is linked to the amount of active molecules present, spiropyran should be an ideal selfreporting material. 9,10 Several self-assembly, self-reporting materials and optical sensors based on spiropyran have been successfully produced.…”