Synthesis of spirocyclic γ-lactones by cascade Beckwith-Dowd ring expansion/cyclization

A range of spirocyclic γ-lactones have been prepared exploiting a Beckwith-Dowd ring expansion cascade involving 1-, 3-, 4-, and 5-carbon expansion of cyclopentanone and cyclohexanone followed by 5-exo-trig or 5-exo-dig cyclization. This radical cascade reaction can be achieved with various substrates to provide a broad range of γ-lactones spirofused to 6- to 10-membered cycloalkanones.     

Cascade Olefin Isomerization/Intramolecular Diels–Alder Reaction Catalyzed by N‐Heterocyclic Carbenes

The addition of an N‐heterocyclic carbene to the carbonyl group of an α,β,γ,δ‐unsaturated enol ester affords a hemiacetal azolium intermediate that enables a cascade olefin isomerization/Diels–Alder reaction, for which mechanistic studies implicate Lewis base catalysis. Preliminary studies into the utility of the products have been undertaken with reductive and oxidative cleavage, giving materials for potential use in complex‐target synthesis.     

Formation of a defect-free π-electron system in single β-phase polyfluorene chains

Single-molecule spectroscopy can help to uncover the underlying heterogeneity of conjugated polymers used in organic electronics, revealing the most effective molecules in an ensemble in terms of the transport of charge and excitation energy. We demonstrate that β-phase polyfluorene chains can form a near-perfect π-electron system, whereas conventional polymers exhibit chromophoric localization due to perturbation of the conjugation. Broad-band excitation spectroscopy demonstrates that only one absorbing and emitting unit is present on the polymer chain with an average length of ～500 repeat units, illustrating that the material effectively behaves as a molecular quantum wire with strong electronic coupling throughout the entire system.     

Perdeuterated Conjugated Polymers for Ultralow-Frequency Magnetic Resonance of OLEDs

The formation of excitons in OLEDs is spin dependent and can be controlled by electron-paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin- Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π-conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero-field feature and local hyperfine fields. The zero-field peak results from a quasistatic magnetic-field effect of the RF radiation for periods comparable to the carrier-pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero-field peak, we suggest that this result may constitute a fundamental low-field limit of magnetic resonance in carrier-pair-based systems. OLEDs offer an alternative solid-state platform to investigate the radical-pair mechanism of magnetic-field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.     

An enamine controlling group for rhodium-catalyzed intermolecular hydroacylation

An enamine-controlled hydroacylation of alkynes using a rhodium(I)/dppe catalyst system is described. The reaction is highly selective, forming the linear enaminone products as single regioisomers in all examples. In situ hydrolysis of the enamine functionality generated α-substituted 1,3-diketone products, and Lewis-acid mediated intramolecular conjugate addition of the hydroacylation products gave substituted hexahydroquinolones.     

Enantioselective N‐Heterocyclic Carbene Catalysis that Exploits Imine Umpolung

The catalytic umpolung of imines remains an underdeveloped approach to reaction discovery. Herein we report an enantioselective aza‐Stetter reaction that proceeds via imine umpolung using N‐heterocyclic carbene catalysis. The reaction proceeds with high levels of enantioselectivity (all ≥96:4 er) and good generality (21 examples). Mechanistic studies are reported and are consistent with turnover‐limiting addition of the NHC to the imine.     

Single molecule spectroscopy of oligofluorenes: how molecular length influences polymorphism

Polyfluorene represents a unique model to study the influence of intramolecular conformation on the electronic properties of chromophores with an extended π-conjugation. According to the degree of planarity between the adjacent repeat units the electronic and optical properties can change substantially. This peculiar spectroscopic behavior has been described by identifying different phases, namely the glassy, the γ- and the β-phase. Here, we present low-temperature single-molecule spectroscopy of a series of oligofluorenes differing in the number of monomeric units, in order to gain information on the influence of chain length on the polymorphism. By monitoring the energy of the 0-0 transition we have classified single molecules belonging to the different phases. We demonstrate that a large number of molecules start to form the β-phase only when more than 9 repeat units constitute the molecular chain. The implications for the control of morphology in polyfluorene thin films are discussed.     

Picosecond time resolved photoluminescence spectroscopy of a tetracene film on highly oriented pyrolytic graphite: dynamical relaxation, trap emission, and superradiance

A detailed time resolved investigation of the photoluminescence of a thin tetracene film deposited on highly oriented pyrolytic graphite is presented. In agreement with Lim et al. [Phys. Rev. Lett. 92, 107402 (2004)], we find strong evidence for superradiance: an increase of the relative intensity of the pure electronic transition with respect to the vibronic sideband and a concomitant decrease of the radiative lifetime from 10 to 1.83 ns upon cooling from 300 to 4 K. For lower temperatures, a redshift (approximately 200 cm(-1)) of the free exciton is observed. Previously, this shift was attributed to a structural phase transition. Our time resolved spectra reveal that the spectral shift is related to a dynamical relaxation process which occurs within the first 50 ps.