The synthesis of nine curcuminoids and their difluoroboron complexes is described, with seven of them containing a meso-phenyl ring. Dynamic (19)F NMR confirmed the fact that rotation of that meso-aryl fragment is restricted in the latter systems at room temperature and become allowed at higher temperature (>45 °C). The molecular structure of a meso-substituted derivative in the solid state showed that the phenyl ring lies in a highly twisted plane with respect to the mean curcuminoid plane. The photophysical properties of the nine compounds were investigated in solvents of different polarity. Meso-substitution with a phenyl ring has little influence on fluorescence emission properties in solution, radiative and nonradiative kinetic constants being similar for meso- and nonsubstituted compounds, which is in contrast to the case of BODIPY derivatives. However, introduction of an electron donor p-methoxy group at the meso-phenyl ring leads to small perturbation of the curcuminoid π-system fluorescence emission. We also report the influence of the meso-phenyl group on the emission properties of the aggregated solids.

Two-photon excitation of curcuminoid borondifluoride nanoparticles in water results in rather efficient luminescence in the near infrared region due to a high two-photon cross-section.

Transition-metal-catalyzed allylic alkylations, using a broad range of metal complexes, have been intensively studied because of their potential applications in the synthesis of new olefinic compounds in particular for total synthesis.[1] Soft nucleophiles are usually used in Pd-, [1] Mo-, [2] Ir-, [3] Ru-, [4] Rh-, [5] Pt-, [6] and even Fe-catalyzed [7] allylic substitutions. Ni, [8] Co, [9] and Cu [10] catalysts allow the use of hard nucleophiles such as alkylzinc or Grignard reagents, but limited functional group compatibility and/or poor regioselectivity can be observed if the system is not designed carefully. To avoid handling the air-and moisture-sensitive organomagnesium and organozinc reagents, straightforward procedures, which do not require organometallic reagents, are highly desirable and many have now been developed. [11] To the best of our knowledge, direct transition-metal-catalyzed alkyl-allyl cross-couplings using in situ generated catalytic organometallic reagents are still unknown. However, a few years ago, we reported a related Co-catalyzed coupling reaction of aryl halides with allylic acetates; [12] these reactions in the presence of an appropriate reducing reagent, gave allylaromatic compounds. Such allylic carboxylates, whilst less reactive than allyl halides, are much more environmentally friendly.Given our previous experience with the direct Cocatalyzed functionalization, including alkylation, [11c] of aryl halides [13] we were interested to take the chemistry further, and herein we report a new and general method for direct reductive cross-coupling of allylic acetates with alkyl halides using a CoBr 2 /Mn system with an acetonitrile/pyridine solvent mixture. The approach accommodates a variety of simple and functionalized alkyl halides and substituted allylic compounds and is experimentally straightforward. Indeed it uses off-theshelf reagents without any particular precautions against air and moisture. First, we investigated the use of the readily available yet poorly reactive ethyl 4-bromobutanoate with a simple allyl acetate as the electrophile. The major challenge here lies in promoting cross-coupling rather than the formation of reduction and homocoupling products.

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Controlling photoinduced intramolecular charge transfer at the molecular scale is key to the development of molecular devices for nanooptoelectronics. Here, we describe the design, synthesis, electronic characterization, and photophysical properties of two electron donor-acceptor molecular systems that consist of tolane and BF2-containing curcuminoid chromophoric subunits connected in a T-shaped arrangement. The two π-conjugated segments intersect at the electron acceptor dioxaborine core. From steady-state electronic absorption and fluorescence emission, we find that the photophysics of the dialkylamino-substituted analogue is governed by the occurrence of two closely lying excited states. From DFT calculations, we show that excitation in either of these two states results in a distinct shift of the electron density, whether it occurs along the curcuminoid or tolane moiety. Femtosecond transient absorption spectroscopy confirmed these findings. As a consequence, the nature of the emitting state and the photophysical properties are strongly dependent on solvent polarity. Moreover, these characteristics can also be switched by protonation or complexation at the nitrogen atom of the amino group. These features set new approaches toward the construction of a three-terminal molecular system in which the lateral branch would transduce a change of electronic state and ultimately control charge transport in a molecular-scale device.

Hemicurcuminoids are based on half of the π-conjugated backbone of curcuminoids. The synthesis of a series of such systems and their borondifluoride complexes is described. The electrochemical and photophysical properties of difluorodioxaborine species were investigated as a function of the nature of electron donor and acceptor groups appended at either terminal positions of the molecular backbone. The emissive character of these dipolar dyes was attributed to an intraligand charge transfer process, leading to fluorescence emission that is strongly dependent on solvent polarity. Quasi-quantitative quenching of fluorescence in high polarity solvents was attributed to photoinduced electron transfer. These dyes were shown to behave as versatile fluorophores. Indeed, they display efficient two-photon excited fluorescence emission leading to high two-photon brightness values. Furthermore, they form nanoparticles in water whose fluorescence emission quantum yield is less than that of the dye in solution, owing to aggregation-induced fluorescence quenching. When cos7 living cells were exposed to these weakly-emitting nanoparticles, one- and two-photon excited fluorescence spectra showed a strong emission within the cytoplasm that originated from the individual molecules. Dye uptake thus involved a disaggregation mechanism at the cell membrane which restored fluorescence emission. This off-on fluorescence switching allows a selective optical monitoring of those molecules that do enter the cell, which offers improved sensitivity and selectivity of detection for bioimaging purposes.

(50 mV per unit of s), which we relate to the twisted ground-state geometry of the meso aryl substituent. The correlation models established in this study may be useful to anticipate the optical and electrochemical properties of borondifluoride complexes of curcuminoids with good reliability.

An original molecular fluorophore displaying single keto (K*) excited-state intramolecular proton transfer (ESIPT) emission is presented in this article. Substitution at the 3 and 5 positions of the phenol ring of a 2-(2'-hydroxyphenylbenzoxazole) (HBO) dye by triethylsilyl-ethynyl fragments leads to a drastic enhancement of fluorescence in the solution-state as compared to unsubstituted analogues. This intense fluorescence emission is also retained in a protic solvent like ethanol and in the solid-state as embedded in a potassium bromide pellet or as 1% doped in a poly(methyl methacrylate) (PMMA) film. The experimental optical properties were confirmed by ab initio calculations. Random lasing (RL) studies performed on this ESIPT emitter show the presence of stimulated emission occurring above the threshold level of pumping energy density (ρth ≈ 300 μJ cm-2) in the PMMA matrix. To the best of our knowledge, this study constitutes the first observation of RL based on ESIPT fluorescence.