Six water-soluble spiropyran derivatives have been characterized with respect to the thermal and photoinduced reactions over a broad pH-interval. A comprehensive kinetic model was formulated including the spiro- and the merocyanine isomers, the respective protonated forms, and the hydrolysis products. The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory. The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction. Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

No abstract

The function of a parity generator/checker, which is an essential operation for detecting errors in data transmission, has been realized with multiphotochromic switches by taking advantage of a neuron-like fluorescence response and reversible light-induced transformations between the implicated isomers.

DNA origami has received enormous attention for its ability to program complex nanostructures with a few nanometer precision. Dynamic origami structures that change conformation in response to environmental cues or external signals hold great promises in sensing and actuation at the nanoscale. The reconfiguration mechanism of existing dynamic origami structures is mostly limited to single-stranded hinges and relies almost exclusively on DNA hybridization or strand displacement. Here, we show an alternative approach by demonstrating on-demand conformation changes with DNA-binding molecules, which intercalate between base pairs and unwind DNA double helices. The unwinding effect modulates the helicity mismatch in DNA origami, which significantly influences the internal stress and the global conformation of the origami structure. We demonstrate the switching of a polymerized origami nanoribbon between different twisting states and a well-constrained torsional deformation in a monomeric origami shaft. The structural transformation is shown to be reversible, and binding isotherms confirm the reconfiguration mechanism. This approach provides a rapid and reversible means to change DNA origami conformation, which can be used for dynamic and progressive control at the nanoscale.

Photochromic spiropyrans modified with fluorophores were investigated as molecular platforms for the achievement of fluorescence switching through modulation of energy transfer. The dyads were designed in such a way that energy transfer is only observed for the open forms of the photochrome (merocyanine and protonated merocyanine), whereas the closed spiropyran is inactive as an energy acceptor. This was made possible through a deliberate choice of fluorophores (4-amino-1,8-naphthalimide, dansyl, and perylene) that produce zero spectral overlap with the spiro form and considerable overlap for the merocyanine forms. From the Förster theory, energy transfer is predicted to be highly efficient and in some cases of 100% efficiency. The combined switching by photonic (light of λ>530 nm) and chemical (base) inputs enabled the creation of a sequential logic device, which is the basic element of a keypad lock. Furthermore, in combination with an anthracene-based acidochromic fluorescence switch, a reversible logic device was designed. This enables the unambiguous coding of different input combinations through multicolour fluorescence signalling. All devices can be conveniently reset to their initial states and repeatedly cycled.

The use of molecules as elements for information processing and storage is a fast developing research field.[1] Replacing the materials traditionally used for the abovementioned purposes with molecular entities implies a change of paradigm for miniaturization, power consumption, etc. The light-induced color change of photochromic molecules makes them ideal candidates for optically controlled functions, as the absorption of the two isomers in different spectral regions may denote 0 or 1 -the universal digital language. Several approaches to photochromic molecular memories have been reported, and one of the most pressing problems addressed in these studies is how the stored information is to be retrieved optically without concomitant loss of data. [2][3][4][5][6][7][8][9] This is referred to as a nondestructive readout process. Monitoring the absorption of either isomeric form in the readout process is not a feasible approach since the distribution between the two isomers is affected, with resulting loss of information. Here we describe how a pyridine-decorated photoswitch from the dithienylethene (DTE) family is used together with a porphyrin dimer (P 2 ) to constitute a supramolecular memory with non-destructive readout capability. Porphyrins have been used as fluorescent reporters in various other photochromic molecular architectures designed for similar purposes. [10][11][12][13][14] In the vast majority of these cases, the photoinduced isomerization process switches electron transfer reactions on and off, and the porphyrin emission intensity is concomitantly toggled between a low and a high state (quenched or not quenched). As described below, our approach has instead been to harness the isomerization-induced structural changes of P 2 , which in turn are reflected in the spectral properties of the dimer. As these changes are probed in a spectral region outside the photochromically active absorption bands of DTE, the state of the memory is preserved in the readout process. Photoresponsive constructs containing the DTE backbone have been extensively utilized for optically controlled molecular logic applications due to their excellent thermal stability and fatigue resistance. [15][16][17][18][19] Figure 1 shows the structures and the isomerization scheme of the DTE derivative used in this work. Several research groups have used the same pyridine-appended DTE derivative for, e.g., non-destructive readout purposes, although the functional principles used differ from the approach that we have taken. [3, 4,10,20] The open form, DTEo, is isomerized to the closed one using UV light. The photostationary distribution after exposure to 302 nm UV light (ca. 1.5 mW cm -2 ) is essentially 100% in the closed form DTEc, as judged by NMR measurements. This is also the case for the open form after the reverse isomerization when triggered by broadband visible light (λ > 450 nm, ca. 100 mW cm -2 ). Figure 1. Isomeric forms of the pyridine-appended DTE photoswitch and the DTE-P 2 complex (Ar=3,5-di(octyloxy)phenyl, R=Si(C 6 H 13 )...

On-command changes in the emission color of functional materials is a sought-after property in many contexts. Of particular interest are systems using light as the external trigger to induce the color changes. Here we report on a tri-component cocktail consisting of a fluorescent donor molecule and two photochromic acceptor molecules encapsulated in polymer micelles and we show that the color of the emitted fluorescence can be continuously changed from blue-to-green and from blue-to-red upon selective light-induced isomerization of the photochromic acceptors to the fluorescent forms. Interestingly, isomerization of both acceptors to different degrees allows for the generation of all emission colors within the redgreen-blue (RGB) color system. The function relies on orthogonally controlled FRET reactions between the blue emitting donor and the green and red emitting acceptors, respectively.

The photochromic fluorescence switching of a fulgimide derivative was used to implement the first molecule-based D (delay) flip-flop device, which works based on the principles of sequential logic. The device operates exclusively with photonic signals and can be conveniently switched in repeated cycles.