Malcolm D. E. Forbes

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck–Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated +H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.

Nitronyl Nitroxide Radicals as Organic Memory Elements with Both n‐ and p‐Type Properties

Organic molecules are being actively explored for use in logical devices, either as individual memory elements or as components embedded in small organic and polymeric materials. Conventional inorganic semiconductor devices are limited in terms of performance improvement owing to increased costs for device fabrication as well as physical limitations on minimum feature dimensions. Organic memory, however, is a possible substitute for both volatile and non-volatile memory devices. It has the advantages of facile tailoring through organic synthesis, simple device fabrication (even upon flexible substrates), and very low power consumption. Volatile organic memory is expected to be applied towards dynamic random access memory (DRAM), which typically requires a data refresh every few milliseconds, while non-volatile organic memory can be applied to read-only memory (ROM) and flash-type memory. Several types of organic and polymeric materials have been reported for this purpose, such as organic semiconductors, charge-transfer complexes (including redoxactive compounds), and metal-nanoparticle-dispersed thin films. Recently, a new type of organic memory has been added to this list, namely organic radical molecules (nitroxide radicals, NOC) that contain an unpaired electron that is capable of undergoing oxidation or reduction by applied bias voltages. In 1901, Piloty and Schwerin succeeded in the synthesis and isolation of porphyrexide, the first organic nitroxide. The most prominent member of this class of compounds is the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO). TEMPO and many other NO radicals belong to the category of persistent radicals. Since this pioneering work, the Nakahara group has reported the synthesis of a polymeric TEMPO radical derivative, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), for an organic radical battery. The research group of Nishide extended this work toward applications, such as radical batteries as cathode active materials, organic light-emitting diodes as holeinjection layers, and memory as p-type redox active materials. The TEMPO radical is easily oxidized to yield the corresponding oxoammonium salt, returning to the TEMPO radical by a p-type one-electron reduction. However, for the complete circuit of the organic semiconducting device using PTMA, an n-type redox active material as a partner to the p-type material is required. Some previously reported polymer-based organic radical memory devices required additional organic layers, such as an electron-accepting layer for n-type and even a metalparticle-dispersed dielectric layer for actuation of the organic memory device. While previous research into polymerbased organic radical memory has led to significant advances, for a complete organic radical memory circuit it is crucial to find new organic radical molecules that demonstrate switchability and present both pand n-type properties within the molecule. Also, new molecules can facilitate understanding of the origin of memory effects and whether that effect is induced by the organic radical alone or whether other environmental or chemical factors must be considered. Herein we report novel molecular radical memory behavior using a stable organic radical molecule. We have synthesized and characterized the nitronyl nitroxide (NN) radical molecule 2-(3’-tert-butyl-4’,5’-dimethoxymethoxybiphenyl-4-yl)-4,4,5,5-tetramethylimidazolidine-1-oxyl-3-oxide (NN-Ph-CatMOM2) [14] (see also the Supporting Information). The NN radical possesses one unpaired electron that is delocalized across the two equivalent N O groups (Scheme 1). Owing to delocalization, the oxidized and reduced states of the NN radicals were expected to be stabilized over a wide window of applied voltages, leading to a high switchability for the NN radical memory. The ability of the NN-Ph-CatMOM2 to act as both electron donor and acceptor was investigated by cyclic voltammetry (CV) and simultaneous electrochemical electron paramagnetic resonance (SEEPR) spectroscopy under an applied voltage. Cyclic voltammograms were [*] Dr. J. Lee, E. Lee, S. Kim, Dr. G. S. Bang, Prof. H. Lee NCRI, Center for Smart Molecular Memory Department of Chemistry, Sungkyunkwan University Suwon 440-746 (Republic of Korea) Fax: (+ 82)31-299-5934 E-mail: hyoyoung@skku.edu Prof. D. A. Shultz, Dr. R. D. Schmidt Department of Chemistry, North Carolina State University Raleigh, NC 27695-8204 (USA) Fax: (+ 1)919-515-8920 E-mail: david_shultz@ncsu.edu

Time‐Resolved (CW) Electron Paramagnetic Resonance Spectroscopy: An Overview of the Technique and Its Use in Organic Photochemistry

Abstract— Steady‐state and time‐resolved electron paramagnetic resonance (TREPR) experiments are described. Comparison of the TREPR continuous wave method to other time domain EPR techniques such as Fourier transform EPR (FT‐EPR) is made, and the advantages and disadvantages of each are presented. The role played by several mechanisms of chemically induced dynamic electron spin polarization (CIDEP) in the appearance of the spectra is explained. The advantages of using higher frequency spectrometers than the standard X‐band (9.5 GHz) are presented and discussed. Examples are presented that are relevant to organic photochemistry and electron donor‐acceptor chemistry. The use of TREPR to study polymer photodegradation, polymer chain dynamics, free radical initiator chemistry and biradical spin exchange interactions is described. Emphasis is placed on magnetic field effects studied by multiple frequency TREPR in these systems. Finally, several future directions in the field are discussed in terms of new developments in microwave and magnetic field technology.

Time resolved CIDNP study of electron transfer reactions in proteins and model compounds

Intramolecular electron transfer (IET) from tyrosine to tryptophan cation radicals is investigated using time resolved chemically induced dynamic nuclear polarization (CIDNP) spectroscopy in combination with laser flash photolysis. In both the tryptophan-tyrosine dipeptide and the denatured state of hen lysozyme in aqueous solution, the transformation TrpH+ → TyrO by IET leads to an increase in the tyrosine radical concentration, growth in the tyrosine CIDNP signal, fast decay of the tryptophan CIDNP, and inversion of the phase of the CIDNP of the photosensitizing dye, 2,2′-dipyridyl. IET effects are not observed for mixtures of the amino acid or for the native state of lysozyme. The steady state CIDNP effects seen for denatured lysozyme thus depend not only on the accessibility of the amino acid residues on the surface of the protein but also on the reactivity of the radical intermediates.

Time-Resolved Electron Paramagnetic Resonance Spectroscopy: History, Technique, and Application to Supramolecular and Macromolecular Chemistry

Abstract The experimental technique of time-resolved (direct detection) electron paramagnetic resonance (TREPR) spectroscopy, and its role in the elucidation of free-radical structure, dynamics, and reactivity within the field of “spin chemistry,” is presented and discussed. Significant detail regarding the construction and execution of the experiment, which requires only minor modification of a commercial electron paramagnetic resonance spectrometer, is provided for the first time. Special requirements for the resonator, sample geometry, light source, timing sequences, and all additional required equipment are explained. Chemically induced electron spin polarization (CIDEP) mechanisms (radical pair mechanism (RPM), triplet mechanism, spin–correlated radical pair (SCRP) mechanism, and radical-triplet (RT) pair mechanism), which are commonly observed in this experiment, are briefly described in terms of their physical origin and their unique spectral appearance. Finally, examples of the use of TREPR in the study of modern problems in supramolecular and macromolecular chemistry are presented. These examples, which include the effect of electrostatics on the behavior of micellized radical pairs and the role of RT pairs in the study of polymer chain dynamics in the dilute condition, are selected because they contain particularly clear examples of each CIDEP mechanism.

B12-Mediated, Long Wavelength Photopolymerization of Hydrogels

Medical hydrogel applications have expanded rapidly over the past decade. Implantation in patients by noninvasive injection is preferred, but this requires hydrogel solidification from a low viscosity solution to occur in vivo via an applied stimuli. Transdermal photo-cross-linking of acrylated biopolymers with photoinitiators and lights offers a mild, spatiotemporally controlled solidification trigger. However, the current short wavelength initiators limit curing depth and efficacy because they do not absorb within the optical window of tissue (600-900 nm). As a solution to the current wavelength limitations, we report the development of a red light responsive initiator capable of polymerizing a range of acrylated monomers. Photoactivation occurs within a range of skin type models containing high biochromophore concentrations.

Radical reactions with double memory of chirality (2MOC) for the enantiospecific synthesis of adjacent stereogenic quaternary centers in solution: cleavage and bonding faster than radical rotation.

The solution photochemistry of bis(phenylpyrrolidinonyl)ketones (R,R)-1b and (S,S)-1b exhibited a remarkably high memory of chirality. Stereospecific decarbonylation to products (R,R)-3b and (S,S)-3b, respectively, occurred with an ee of ca. 80%. The reaction is thought to occur along the single state manifold by sequential Norrish type-I alpha-cleavage, decarbonylation, and radical-radical combination in a time scale that is comparable to that required for the radical intermediate to expose its other enantiotopic face by rotation about an axis perpendicular to that of the p orbital (ca. 3-7 ps). The absolute configuration of a key intermediate and that of ketone (R,R)-1b were determined by single-crystal X-ray diffraction and the ee values of the photochemical products with the help of chiral shift reagent (+)-Eu(tfc)(3) and chiral LC-MS/MS. On the basis of the ee and de values at 25 degrees C, it could be determined that ca. 70% of the bond forming events occur with double memory of chirality, ca. 21% occur after rotation of one radical to form the meso product (R,S)-3b, and only 9% occur after double rotation to form the opposite enantiomer. This report represents the first example of a doubly enantiospecific Norrish type-I and decarbonylation reaction in solution and illustrates potentially efficient ways to obtain compounds with adjacent stereogenic quaternary centers.

Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

A systematic investigation of the photochemistry and ensuing radical chemistry of three guest ketones encapsulated in randomly methylated beta-cyclodextrin (beta-CD) hosts is reported. Dibenzyl ketone (DBK), deoxybenzoin (DOB), and benzophenone (BP) triplet states are rapidly formed after photolysis at 308 nm. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy, steady-state NMR spectroscopy, and time-resolved chemically induced nuclear polarization (TR-CIDNP) experiments were performed on the ketone/CD complexes and on the ketones in free solution for comparison. The major reactivity pathways available from these excited states are either Norrish I alpha-cleavage or H-atom abstraction from the interior of the CD capsule or the solvent. The DOB triplet state undergoes both reactions, whereas the DBK triplet shows exclusively alpha-cleavage and the BP triplet shows exclusively H-atom abstraction. Radical pairs are observed in beta-CDs by TREPR, consisting of either DOB or BP ketyl radicals with sugar radicals from the CD interior. The TREPR spectra acquired in CDs are substantially broadened due to strong spin exchange. The electron spin polarization mechanism is mostly due to S-T(0) radical pair mechanism (RPM) in solution but changes to S-T(-) RPM in the CDs due to the large exchange interaction. The TR-CIDNP results confirm the reactivity patterns of all three ketones, and DOB shows strong nuclear spin polarization from a novel rearrangement product resulting from the alpha-cleavage reaction.

Simple modification of Varian E‐line microwave bridges for fast time‐resolved EPR spectroscopy

Modification of the popular Varian E‐line series of electron paramagnetic resonance (EPR) spectrometers for fast direct detection EPR spectroscopy is described. The overall time response and signal reproducibility of the spectrometer is improved by changing the values of two capacitors in the microwave bridge preamplifier circuit, with no alteration of instrument performance in normal field‐modulated experiments. The rise times are slew rate limited for large input signals. There is a propagation delay of 40 ns before any preamplifier response is observed. Signal‐to‐noise ratios for the direct detection EPR experiment with the modified and unmodified bridges are comparable. A test of the modified bridge using a biradical, formed within 20 ns by laser flash photolysis, shows a signal decay consistent with that measured by optical methods, in contrast to the unmodified circuit. Other factors influencing the time response of the experiment are discussed. While the preamplifier rise time is still the limiting factor, spectra as close as 70 ns to the laser flash can now be measured with adequate signal‐to‐noise ratios.

Spin relaxation in acyl radicals measured using spin correlated radical pair (SCRP) polarization in flexible biradicals

Time resolved electron paramagnetic resonance spectra and the decay kinetics of spin correlated radical pair (SCRP) polarization in an acyl-benzyl biradical were measured over a wide temperature range (180–274 K). The major mechanism of intersystem crossing in this biradical is the spin rotation induced relaxation of the acyl moiety, which is associated with the rotation of the carbonyl group about the neighbouring CC bond axis. This relaxation determines the decay rate of the polarization. The relaxation time is largely viscosity independent; it changes by a factor of less than two going from room temperature (60 ns) to 180 K (110 ns) in 2-propanol.