The development of luminescent materials that are sensitive to both optical and electrical stimuli is of high interest for the fabrication of future information-processing devices. In this study we report about a novel fluorescent molecular switch (1) that can be light- and redox-controlled. This compound has been synthesized by covalent tethering of a perylenediimide fluorophore (PDI) to a dithienylethene (DTE) unit. The photochromic properties of the DTE group are preserved in the dyad, which can be reversibly converted between open (1o) and closed (1c) states upon irradiation. In addition, 1 displays electrochromicity, and its ring-opening process can be promoted quantitatively by electrochemical oxidation. Whereas the open-ring state of the switch is highly fluorescent, the emission of the PDI group in 1c is quenched by energy transfer to the DTE group. This allows large fluorescence modulation between the two states of 1, which can be operated either as an all-optical switch or by a combination of photo- and electrochemical stimuli.

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A•–/D•+ for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, kq, to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm–Weller formula rests on an incorrect mechanism. We have remeasured kq for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and kq, including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros–Boltzmann (SB) equation kq = klim/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A•–/D•+). The quantity klim represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A•–/D•+ vs A•– + D•+). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of −0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A•–/D•+ relative to the free A•– + D•+, which opposes the Coulombic stabilization of A•–/D•+. The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A•– + D•+). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of kq on ΔG.

The current manuscript shows the electrochemical studies performed to rationalize the mechanism and develop new green synthetic routes for the synthesis of substituted nitroaromatics based on the advantages of the electrochemical approach to the nucleophilic aromatic substitution reaction (such as (a) low cost and ready availability of reagents, (b) atom economy, (c) high yields, approaching 100%) and the use of Room Temperature Ionic Liquids (RTILs) as green alternative solvents to organic aprotic solvents. Four of the most popular RTILs (1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide ([BMIM]TFSI) and 1-butyl-3-methylimidazolium acetate ([BMIM]AcO) have been chosen since they have different properties in terms of solvation effects that can increase the regioselectivity of the reaction. The nucleophiles used to study the feasibility and viability of the reaction were the classical hydride, methoxide, ketones, cyanides and amines, whereas the nitroarenes selected were 4-nitrotoluene, 1,3-dinitrobenzene, 2,4-dinitroaniline, 1,3,5-trinitrobenzene, 1,3-dinitronaphthalene, 1-chloro-2,4,6-trinitrobenzene and 2,4,6-trinitroanisole. The electrocatalysis and regioselectivity effects of using RTILs are also investigated. The article concludes by analyzing the economic cost of performing this electrosynthesis in RTILs and organic solvent electrolyte systems, which contain 0.1 M of supporting electrolyte.

On the route to single (large) molecule unimolecular chemistry, the adsorption of a photochromic dithienylethene dye on Cu(111) at a submonolayer level has been studied by Ultra High Vacuum-Scanning Tunneling Microscopy at Low Temperature. This technique has shown that the observed adsorbed molecule's shape is compatible with an helical conformation but has also revealed a surrounding electronic corrugation due to the perturbed surface states. Careful examination of the standing wave pattern indicated that only a part of the molecule is indeed interacting with the metallic substrate. Geometric considerations were used to infer that the bridging ethene moiety could be responsible for the electronic scattering. Scanning Tunneling Spectroscopy has shown a substantial amount of charge transfer from the surface to the adsorbate. The hypothesis that this precise double bond is a reactive locus toward charge transfer processes is confirmed by the electrochemical results: this double bond is indeed reduced upon coulometric reduction on glassy carbon. Furthermore, the use of a copper cathode strongly facilitates the reduction since a +0.6 V shift was recorded.

The electrochemical oxidation of either open or closed metacyclophanene isomers, which is a positive-photochromic system due to open isomers being more stable than closed ones, quantitatively yields a stable fluorescent dihydropyrene intermediate, a well-known negative T-photochromic system where the closed form is more stable than the open one. This is one of the first examples in which two different molecular switching systems can be electrochemically triggered.

On the route to single (large) molecule unimolecular chemistry, the adsorption of a photochromic dithienylethene dye on Cu(111) at a submonolayer level has been studied by Ultra High Vacuum-Scanning Tunneling Microscopy at Low Temperature. This technique has shown that the observed adsorbed molecule's shape is compatible with an helical conformation but has also revealed a surrounding electronic corrugation due to the perturbed surface states. Careful examination of the standing wave pattern indicated that only a part of the molecule is indeed interacting with the metallic substrate. Geometric considerations were used to infer that the bridging ethene moiety could be responsible for the electronic scattering. Scanning Tunneling Spectroscopy has shown a substantial amount of charge transfer from the surface to the adsorbate. The hypothesis that this precise double bond is a reactive locus toward charge transfer processes is confirmed by the electrochemical results: this double bond is indeed reduced upon coulometric reduction on glassy carbon. Furthermore, the use of a copper cathode strongly facilitates the reduction since a +0.6 V shift was recorded.

The electrochemical oxidation of either open or closed metacyclophanene isomers, which is a positive-photochromic system due to open isomers being more stable than closed ones, quantitatively yields a stable fluorescent dihydropyrene intermediate, a well-known negative T-photochromic system where the closed form is more stable than the open one. This is one of the first examples in which two different molecular switching systems can be electrochemically triggered.