Effect of Alkyl- vs Alkoxy-Arene Substituents on the Deactivation Processes and Fluorescence Quantum Yields of Exciplexes.

Decay processes of exciplexes of cyanoanthracenes with alkylbenzene donors were compared to those with alkoxybenzenes. For the three decay processes of exciplexes, the radiative rate constant () of alkoxy derivatives is slightly lower than those of alkylbenzenes at the same average exciplex energy. However, the corresponding deactivation rate constants, intersystem crossing () and nonradiative decay (), are considerably higher.

Observation of strongly forbidden solid effect dynamic nuclear polarization transitions via electron-electron double resonance detected NMR.

We present electron paramagnetic resonance experiments for which solid effect dynamic nuclear polarization transitions were observed indirectly via polarization loss on the electron. This use of indirect observation allows characterization of the dynamic nuclear polarization (DNP) process close to the electron. Frequency profiles of the electron-detected solid effect obtained using trityl radical showed intense saturation of the electron at the usual solid effect condition, which involves a single electron and nucleus.

Batch and flow photochemical benzannulations based on the reaction of ynamides and diazo ketones. Application to the synthesis of polycyclic aromatic and heteroaromatic compounds.

Highly substituted polycyclic aromatic and heteroaromatic compounds are produced via a two-stage tandem benzannulation/cyclization strategy. The initial benzannulation step proceeds via a pericyclic cascade mechanism triggered by thermal or photochemical Wolff rearrangement of a diazo ketone. The photochemical process can be performed using a continuous flow reactor which facilitates carrying out reactions on a large scale and minimizes the time required for photolysis.

High-field 13C dynamic nuclear polarization with a radical mixture.

We report direct (13)C dynamic nuclear polarization at 5 T under magic-angle spinning (MAS) at 82 K using a mixture of monoradicals with narrow EPR linewidths. We show the importance of optimizing both EPR linewidth and electron relaxation times by studying direct DNP of (13)C using SA-BDPA and trityl radical, and achieve (13)C enhancements above 600. This new approach may be best suited for dissolution DNP and for studies of (1)H depleted biological and other nonprotonated solids.

Water-soluble narrow-line radicals for dynamic nuclear polarization.

The synthesis of air-stable, highly water-soluble organic radicals containing a 1,3-bis(diphenylene)-2-phenylallyl (BDPA) core is reported. A sulfonated derivative, SA-BDPA, retains the narrow electron paramagnetic resonance linewidth (<30 MHz at 5 T) of the parent BDPA in highly concentrated glycerol/water solutions (40 mM), which enables its use as polarizing agent for solid effect dynamic nuclear polarization (SE DNP). A sensitivity enhancement of 110 was obtained in high-field magic-angle-spinning (MAS) NMR experiments.

Generation and characterization of 1,2-diaryl-1,1,2,2-tetramethyldisilane cation radicals.

Nanosecond laser flash photolysis methods were used to generate and spectrally characterize the cation radicals of 1,2-diaryl-1,1,2,2,-tetramethyldisilanes (Ar = p-X-Ph, X = H, CH(3), OCH(3)) in hexafluoroisopropanol (HFIP) at room temperature. The disilane cation radicals rapidly reacted with methanol, with bimolecular rate constants ranging from 0.63 to 2.

Bonded exciplex formation: electronic and stereoelectronic effects.

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes.

Bonded exciplexes. A new concept in photochemical reactions.

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5-0.8 eV.

Synthesis, optical, and electrochemical properties of a new family of dendritic oligothiophenes.

A new class of semi-flexible dendrimers with oligothiophene (OT) arms up to the third generation have been synthesized and investigated. The synthetic methods employed include a combination of palladium-catalyzed Stille cross-coupling reactions for oligothiophenes, Sonogashira cross-coupling reactions for building blocks, and carbodiimide-mediated esterification for building up the various dendrimers. The optical and electrochemical properties of this series of oligothiophenes-based dendrimers are shown to be strongly influenced by their morphologies as demonstrated by their pronounced solvatochromic and thermochromic responses under different environmental conditions.