Tuning the photochemical ring-closing reaction efficiency in diarylethene-based photoswitches through engineering of internal charge transfer.

The photochemical quantum yield is one of the key features for a photoswitch and its tuning is challenging. In an attempt to tackle this issue within the popular diarylethene-based switches, we have explored the potential to use internal charge transfer (CT), a readily controllable parameter, for an effective modulation of the photocyclization quantum yield. For this, a homogeneous family of terarylenes, a sub-class of diarylethenes, with different CT characters, but the same photochromic core was designed and its photochromic properties were fully investigated. A clear correlation was found between the cyclization quantum yield and the CT character of the switch. More precisely, almost linear relationships were established between the ring-closing quantum yield and (i) the electron density variation accompanying the S0 → S1 transition and (ii) the percentage of LUMO on the reactive carbon atoms. Such a correlation was rationalized by a joint spectroscopic analysis and theoretical modelling of both ground and first excited states, introducing the concept of "early" or "late" photochromes. Encouragingly, such a potentally predictive model also seemed relevant when applied to some other diarylethene-based switches reported in the literature.

Terphenylthiazole-based self-assembled monolayers on cobalt with high conductance photo-switching ratio for spintronics.

Two new photo-switchable terphenylthiazole molecules are synthesized and self-assembled as monolayers on Au and on ferromagnetic Co electrodes. The electron transport properties probed by conductive atomic force microscopy in ultra-high vacuum reveal a larger conductance of the light-induced closed (c) form than for the open (o) form. We report an unprecedented conductance ratio of up to 380 between the closed and open forms on Co for the molecule with the anchoring group (thiol) on the side of the two N atoms of the thiazole unit. This result is rationalized by Density Functional Theory (DFT) calculations coupled to the Non-Equilibrium Green's function (NEGF) formalism. These calculations show that the high conductance in the closed form is due to a strong electronic coupling between the terphenylthiazole molecules and the Co electrode that manifests by a resonant transmission peak at the Fermi energy of the Co electrode with a large broadening. This behavior is not observed for the same molecules self-assembled on gold electrodes. These high conductance ratios make these Co-based molecular junctions attractive candidates to develop and study switchable molecular spintronic devices.

A Photo- and Redox-Driven Two-Directional Terthiazole-Based Switch: A Combined Experimental and Computational Investigation.

Terthiazoles with redox-active substituents like an N-methyl pyridinium group and ferrocene have been synthesized and their photo- and electro-chromic behaviors were investigated. The presence of two lateral N-methyl pyridinium substituents in the structure of terthiazole proved to be effective in inducing not only the reductive ring-closure of the terthiazole core but also its oxidative ring-opening reaction, leading to the first terarylene-based switch able to fully operate both photochemically and electrochemically. Moreover, the large increase in the redox potential between its open and closed form (700 mV) means that a part of the photon energy necessary to trigger the cyclization is stored in the form of chemical potential available for other works. Introduction of a second redox-active unit such as ferrocene onto the central thiazolyl moiety is found to inhibit the photochromism of the switch but not its redox switchability, which, instead, got improved for the ring-opening reaction via the redox properties of the ferrocenyl unit. The optical and redox properties of the switch in its different oxidation states are analyzed with the aid of DFT calculations in order to rationalize different switching processes.

Fluorescent Zr(IV) Metal-Organic Frameworks Based on an Excited-State Intramolecular Proton Transfer-Type Ligand.

We report here the preparation of a series of Zr(IV) metal-organic frameworks (MOFs) of the MIL-140 structure type incorporating a ligand exhibiting an intense excited-state intramolecular proton transfer (ESIPT) fluorescence. These solids were obtained by systematically varying the substitution rate of 4,4'-biphenyldicarboxylate by 2,2'-bipyridine-3,3'-diol-5,5'-dicarboxylate, and they were thoroughly characterized by complementary techniques, including high-resolution powder X-ray diffraction, solid-state NMR spectroscopy, nitrogen sorption experiments, and time-resolved fluorescence. We show that the incorporation of the ESIPT-type ligand induces an increase of the hydrophilicity, leading ultimately to a higher sensitivity toward hydrolysis, a phenomenon rarely observed in this structure type, which is considered as one of the most stable among the Zr carboxylate MOFs. Eventually, optimization of the amount of fluorescent ligand within the structure allowed combining a decent microposity ( SBET > 750 m2·g-1) and a high stability even in boiling water, together with a high fluorescence quantum yield (>30%).

Light-Controlled Release and Uptake of Zinc Ions in Solution by a Photochromic Terthiazole-Based Ligand.

We have synthesized and fully characterized a photochromic zinc complex with a terphenylthiazole-based ligand with a salen-like cavity. The solution stability of the complex was found to be greatly dependent on the state of the photochromic ligand and an interesting photo-triggered release and uptake of zinc ions was found as well as monitored by its fluorescence. The contrasting stability difference of the zinc complex between its two isomeric states was rationalized by DFT calculations.

A multifunctional photoswitch: 6π electrocyclization versus ESIPT and metalation.

A terthiazole-based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady-state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time-dependent photochemical outcome and displays efficient ESIPT-based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.

4,4'-Bithiazole-based tetraarylenes: new photochromes with unique photoreactive patterns.

Five 4,4'-bithiazole-based tetraarylenes were prepared and their photochromic behavior investigated. With their 1,3,5,7-octatetraene photoreactive backbone, they offer not only more available sites for further functionalizations, but also a novel design principle for the development of a new class of biphotochromes.

Fluorescence photoswitching and photoreversible two-way energy transfer in a photochrome-fluorophore dyad.

A thorough photophysical study of a photochrome-fluorophore dyad (3), combining a fluorescent laser dye (DCM-type, , Φ(1) = 0.27) and a photochromic diarylethene (2), obtained by click chemistry, is presented. In addition to photochromism, the open form (OF) of 2 exhibits fluorescence (Φ(-OF) = 0.016), whereas the closed form (CF) does not. Fluorescence is switched upon alternate UV/visible irradiation of 2. The emission band of 2-OF matches the absorption band of 1 (400-550 nm), whereas the emission band of 1 overlaps the absorption band of 2-CF (550-700 nm). Therefore, a photoreversible two-way excitation energy transfer (EET), controlled by the state of the photochromic moiety, is obtained in the dyad 3. Their efficiencies are quantified as Φ(EET)(OF→F) = 85% and as Φ(EET)(F→CF) ~ 100% from the comparison of emission and excitation spectra between 1, 2, and 3. These results are fully compatible with the shortening of fluorescence lifetimes (from τ(-OF) = 70 ps and 170 ps essentially to τ(-OF) < 10 ps) and to the values of Förster radii determined for 3 (R(0)(OF → F) = 29 Å and R(0)(F → CF) = 71 Å), evidencing a Förster-type resonance energy transfer mechanism (FRET). An important outcome of this two-way FRET is the possibility to quench 49% of the fluorescence in 3 at PSS upon UV irradiation, corresponding to the conversion extent of the photochromic reaction, which is different from 2 (α(CF) = 91%). This is a clear example of a situation where the presence of FRET between the photochromic unit and the fluorophore affects noticeably the photochromic properties of the dyad molecule 3.

Photochromic and reductive electrochemical switching of a dithiazolylethene with large redox modulation.

A new dipyridylthiazolylethene (1a) and its dicationic analogue (2a), with two N-methylated pyridyl rings, have been synthesized and structurally characterized. Due to the N-methylation of the pyridyl rings, 2 a displays not only very different photochromic properties, but also undergoes a reductive ring-closing reaction to generate its closed-ring isomer 2b. Careful electrochemical studies coupled with EPR spectroscopy show that this reductive ring-closing reaction takes place when 2a is two-electron reduced. DFT calculations suggest that such a ground-state electrocyclization is driven by a very large stabilization of the reduced closed-ring isomer 2b relative to the reduced open-ring isomer 2a. In addition, 2b exhibits two successive and reversible one-electron reductions at half-wave potentials of 0.04 and -0.14 V versus SCE and a redox modulation as large as 1 V is achieved when passing from 2a to 2b.