1,3-Bis(2,4,6-trimethylphenyl)imidazolium chloride is reduced electrochemically and chemically to produce a nucleophilic carbene, namely 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene. The carbene was also shown to be compatible with and persistent in the ionic liquid tetradecyl(trihexyl)phosphonium chloride.

Phosphonium ionic liquids (PhosILs), most notably tetradecyl(trihexyl)phosphonium decanoate (PhosIL-C(9)H(1)9COO), are solvents for bases such as Grignard reagents, isocyanides, Wittig reagents (phosphoranes), and N-heterocyclic carbenes (NHCs). The stability of the organometallic species in PhosIL solution is anion dependent. Small bases, such as hydroxide, react with the phosphonium ions and promote C-H exchange as suggested by deuterium-labeling studies. A method to dry and purify the ionic liquids is described and this step is important for the successful use of basic reagents in PhosIL. NHCs have been generated in PhosIL, and these persistent solutions catalyze organic transformations such as the benzoin condensation and the Kumada-Corriu cross-coupling reaction. Phosphoranes were generated in PhosIL, and their reactivity with various organic reagents was also tested. Inter-ion contacts involving tetraalkylphosphonium ions have been assessed, and the crystal structure of [(n-C(4)H(90)(4)P][CH(3)CO(2).CH(3)CO(2)H] has been determined to aid the discussion. Decomposition of organometallic compounds may also proceed through electron-transfer processes that, inter alia, may lead to decomposition of the IL, and hence the electrochemistry of some representative phosphonium and imidazolium ions has been studied. A radical derived from the electrochemical reduction of an imidazolium ion has been characterized by electron paramagnetic resonance spectroscopy.

Either light or electricity can be used to trigger the reversible cyclization reactions of three bis(N‐methylpyridinium)dithienylethene derivatives that differ from each other by the presence of either thiophene rings or methyl groups at the two carbon atoms of the photoresponsive hexatriene system involved in forming the new C–C bond. All three derivatives undergo ring‐closing isomerization reactions when irradiated with UV light (365 nm) or when electrochemically reduced (–1.0 V). All three derivatives can also be ring‐opened by irradiating them with visible light (> 490 nm) or by electrochemically oxidizing them (+1.5 V). The presence of additional thiophene rings attached to the two C2 ring positions of the dithienylethene (DTE) backbone enhances the electrochromic behavior, while methyl groups in these positions results in improved photochromic performance. The nature of these groups also greatly affects the thermal properties of the compounds in their ground states. Replacing each methyl group at the C2 ring positions with a thiophene ring systematically lowers the activation energy of spontaneous ring‐opening by 8 kJ mol–1, which correlates with the enhanced efficiency of the oxidative ring‐opening reactions and with the limited photochromism of the thiophene‐functionalized derivative.

One-electron oxidation of N-heterocyclic carbenes (NHCs) has been carried out using oxidising agents such as tetracyanoethylene (TCNE) and ferrocenium [Cp(2)Fe](+); the formation of carbene radical cations is postulated.

Photoisomerization of a coordinating, photochromic dithienylethene bearing a pyridine and a methylpyridinium group was investigated as a means to reversibly modulate the luminescence from CdSe-ZnS core-shell quantum dots. Resonance energy transfer and electron transfer are both plausible quenching mechanisms based on an increase in the spectral donor-acceptor overlap and an anodic shift in the reduction potential accompanying the isomerization reaction of the dithienylethene photoswitch. Photochemical degradation of both the quantum dot and photochromic quencher was observed after repeated cycling between the two isomers, suggesting irreversible electron transfer from the quantum dot to the dithienylethene as the dominant luminescence quenching mechanism.

Photoswitchable luminescence in polymers is a versatile physical change useful in fluorescent sensor and optoelectronic device applications. It is particularly appealing for use in the latter application because the ªON/OFFº properties of the polymers can be used to store and process information, with high sensitivity and resolution.[1] Luminescence can be regulated by using photochromic compounds, [2] which are compounds that interconvert between two isomeric forms when stimulated by two different wavelengths of light. [3] Molecules with this behavior are some of the best contenders for use in erasable memory media, where each isomer of the photochromic compound can represent ª0º or ª1º of a digital code. [4] Photochromic compounds that affect the luminescence intensities of neighboring chromophores help circumvent one of the most serious problems associated with the use of photochromic compounds in memory media: the concomitant erasing of the stored information during a detection event that relies on measuring variations in absorption intensities.[5]Photoregulated emission is possible using photochromic compounds only when there is a discriminating interaction between the excited state of the fluorescent dye and the ground state of one of the photochromic isomers. This interaction can be through photoinduced energy or electron transfer processes. Photochromic phenoxynaphthacenequinone (PNQ) derivatives undergo thermally irreversible isomerization reactions (1a > 1b; see below) with a high degree of fatigue resistance.[6] We have recently described the first example of photoregulation of luminescence in a hydrogen-bonded porphyrin±quinone system by reversibly changing the redox properties of the electron acceptor in a donor±acceptor pair.[7]The quenching of luminescence can be regulated in this supramolecular complex by reversibly varying the ability of the photochromic PNQ to act as an electron acceptor species. Isomer 1b can be reduced at a significantly less negative potential than 1a and, thus, acts as a better electron acceptor to suppress the fluorescence read-out signal from the porphyrin through what we have attributed to a photoinduced electron transfer mechanism, although this mechanism has yet to be unequivocally proven.One approach to extend this photoregulation to polymeric systems is to evenly disperse both the fluorescent dye and the photochromic compound in the polymer film. This strategy has been shown to work, [1a] but is potentially plagued by problems such as phase separation, crystallization, and concentration gradients when the amounts of the additives become too great. We have reported that ring-opening metathesis polymerization (ROMP) of strained bicyclic olefins is an excellent polymerization method for photochromic compounds, due to its mild reaction conditions, functional group compatibility, and control over polymer chain length.[8] Polymers generated using this technique are less susceptible to the problems associated with polymers doped with the photochromic components. Here, we re...

Farbe bekennen! In einer Ringschlussreaktion ändert ein photochromes Bis(pyridinium)‐Dikation mit zentraler Dithienylethen‐Einheit seine Farbe von schwach Gelb (offenkettiges Isomer) nach Blau (cyclisches Isomer). Dieser Prozess kann durch Bestrahlung mit UV‐Licht oder durch elektrochemische Reduktion induziert werden (siehe Schema). Der elektrochemische Ringschluss liefert noch ein weiteres Produkt, das auf dem photochemischen Weg nicht entsteht.

This paper presents an overview of several examples of molecular systems in which the electronic changes that result from the photoinduced isomerization of photochromic phenoxynaphthacenequinone (PNQ) and dithienylethene (DTE) derivatives significantly influence the luminescence behavior of attached porphyrins.