The synthesis and spectroscopic characterization of a series of new blue-emitting silafluorene–fluorene copolymers is described. The polymers are synthesized using kinetically controlled hydrosilylation copolymerization of 1,1-dihydridosilafluorene with a series of 9-substituted 2,7-diethynylfluorenes. The polymers contain a trans-only framework with molecular weights in the range of 13 000–20 000, as determined by gel permeation chromatography (GPC) using polystyrene standards, and by 1H NMR spectroscopy using dimethylphenylsilane as an end-capping marker group. The three stereoregular polymers synthesized include a 9,9-dihydridofluorene (PSF1), a 9,9-dimethyl-9H-fluorene (PSF2), and a 9,9′-spirobifluorene (PSF3) comonomer with the frameworks. These fluorenyl units are conjugated through the silicon center of the silafluorene moiety by bridging vinylene groups. Quantum yields of fluorescence range from 20 to 100% with PSF3 having the highest quantum efficiency. Polymers PSF1-3 emit in the blue region of the spectrum (∼475 nm), showing good color purity with little change in luminescence properties between the solution and solid-state phases. The polymers were tested for explosives detection properties by a fluorescence-quenching mechanism. Targeted explosives include laboratory prepared TNT, DNT, picric acid, RDX, HMX, PETN, TNG, and Tetryl, as well as production line PETN and C-4. All three polymers exhibit detection of explosive particulates with limits as low as 1 pg cm−2 for Tetryl. Polymer PSF1 simultaneously acts as a selective fluorescence “turn-on” sensor for nitrate ester explosives when irradiated with UV light. In the presence of nitrate ester-based explosives such as PETN, PSF1 initially exhibits fluorescence quenching, but continued exposure to UV-light (302 nm), promotes a photochemical reaction forming a luminescent green fluorenone copolymer. This is the first example of a single material acting as both a turn-off and turn-on selective fluorescent sensor for an explosive material.

Efficient blue-emitting silafluorene–fluorene-conjugated copolymers: selective turn-off/turn-on detection of explosives

C–H oxidation using graphite oxide

In a search for environmentally sensitive (solvatochromic) dyes with superior properties, we extended the electronic conjugation of one of the best solvatochromic dyes, Prodan, by substituting its naphthalene core with fluorene. The newly synthesized fluorene derivatives bearing strong electron-donor (dialkylamino) and -acceptor (carbonyl) groups at the 2 and 7 positions showed red-shifted absorption (close to 400 nm), twice as large of a absorption coefficient (43 000 M−1 cm−1), and a manifold larger two-photon absorption cross section (∼400 GM) compared to Prodan. Studies in solvents revealed much stronger fluorescence solvatochromism of the new dyes, which is connected with their twice as large transition dipole moment (14.0 D). Similarly to Prodan, they exhibit high fluorescence quantum yields, while their photostability is largely improved. Thus, substitution of the naphthalene core in Prodan with fluorene resulted in new fluorophores with superior spectroscopic and solvatochromic properties. We expe...

Fluorene Analogues of Prodan with Superior Fluorescence Brightness and Solvatochromism

A simple cobalt(II)/N-hydroxyphthalimide catalyst system has been identified for selective conversion of benzylic methylene groups in pharmaceutically relevant (hetero)arenes to the corresponding (hetero)aryl ketones. The radical reaction pathway tolerates electronically diverse benzylic C–H bonds, contrasting recent oxygenation reactions that are initiated by deprotonation of a benzylic C–H bond. The reactions proceed under practical reaction conditions (1 M substrate in BuOAc or EtOAc solvent, 12 h, 90–100 °C), and they tolerate common heterocycles, such as pyridines and imidazoles. A cobalt-free, electrochemical, NHPI-catalyzed oxygenation method overcomes challenges encountered with chelating substrates that inhibit the chemical reaction. The utility of the aerobic oxidation method is showcased in the multigram synthesis of a key intermediate towards a drug candidate (AMG 579) under process-relevant reaction conditions.

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Co/NHPI-mediated aerobic oxygenation of benzylic C–H bonds in pharmaceutically relevant molecules

A novel class of triazole-derivatized BODIPY compounds have been synthesized on solid-phase by employing mild reaction conditions based on the copper-catalyzed azide–alkyne cycloaddition. The resulting BODIPY-triazoles exhibited MegaStokes shifts (up to 160 nm) and remarkable environmentally sensitive quantum yield increments that asserted their potential as turn-on fluorescent sensors. Out of a library of 120 compounds, we identified BDC-9 as a fluorescent chemosensor with high sensitivity and remarkable species-selectivity towards human serum albumin. These results validate MegaStokes BODIPY dyes as new fluorophores for the development of environmentally sensitive fluorescent probes.

MegaStokes BODIPY-triazoles as environmentally sensitive turn-on fluorescent dyes

Various arylalkanes such as fluorenes, xanthene, anthrone, 9,10-dihydroanthracene, and diphenylmethane are effectively transformed into the corresponding aryl ketones, while amino and aldehyde functionalities remain intact, under air in the presence of cesium carbonate.

Facile and selective aerobic oxidation of arylalkanes to aryl ketones using cesium carbonate

A novel water-soluble solvatochromic molecule, 7-(dimethylamino)-2-fluorenesulfonate (2,7-DAFS), was prepared by a three-step reaction from 2-nitrofluorene in good overall yield. The pH and solvent effects on the UV-VIS absorption and fluorescence spectra of 2,7-DAFS have been studied. Protonation of the dimethylamino group switches the absorption from intramolecular charge-transfer (ICT) to π → π* transition. The ground state pKa value of 2,7-DAFS was determined as 4.51. The fluorescence spectrum of the excited basic form, *(DAFS), shows a structureless single band with a large Stokes shift, whereas that of the acidic form, *(+HDAFS), exhibits a structured band with a small Stokes shift. The emission intensities of the basic and acidic forms versus pH/Ho plots show stretched sigmoidal curves and indicate that (1) the rate of deprotonation of *(+HDAFS) is comparable to the fluorescence decay of the species, and (2) the efficient proton-induced quenching of *(DAFS) fluorescence occurs. The pKa* was estimated as −1.7 from the fluorescence titration curve. The fluorescence maximum of *(DAFS) is blue-shifted as the polarity of solvent decreases. However, no clear dependency of the emission intensity and spectral half width, and thus fluorescence quantum yield, on the solvent polarity was revealed. It appears that the fluorescence sensitivity of 2,7-DAFS is 15 ∼ 25 times greater than the sensitivity of a widely utilized fluorescent probe, 5-(dimethylamino)-1-naphthalenesulfonate. This higher sensitivity, together with the ease of derivatization, would provide the fluorene-based fluorescent molecules significant advantages for a variety of applications.

Kwanghee Koh Park

Papers

Facile and selective aerobic oxidation of arylalkanes to aryl ketones using cesium carbonate

Solvent and pH effects on the fluorescence of 7-(dimethylamino)-2-fluorenesulfonate.

Facile and Selective Aerobic Oxidation of Arylalkanes to Aryl Ketones Using Cesium Carbonate.