: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

A survey is provided of recent progress in the understanding of singlet fission, a spin-allowed process in which a singlet excited molecule shares its energy with a ground-state neighbor to produce two triplet excited molecules. It has been observed to occur in single-crystal, polycrystalline, and amorphous solids, on timescales from 80 fs to 25 ps, producing triplet yields as high as 200%. Photovoltaic devices using the effect have shown external quantum efficiencies in excess of 100%. Almost all the efficient materials are alternant hydrocarbons of the acene series or their simple derivatives, and it is argued that a wider structural variety would be desirable. The current state of the development of molecular structure design rules, based on first-principles theoretical considerations, is described along with initial examples of implementation.

Recent Advances in Singlet Fission

Note: This article was originally published with an incorrect version of the Acknowledgments, which appeared on p. 218 of the print version. The correct version of the Acknowledgments appeared on pp. 1–2. The corrected article is available below.

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Human Health Risk Assessment for Aluminium, Aluminium Oxide, and Aluminium Hydroxide

Distance distributions between paramagnetic centers in the range of 1.8 to 6 nm in membrane proteins and up to 10 nm in deuterated soluble proteins can be measured by the DEER technique. The number of paramagnetic centers and their relative orientation can be characterized. DEER does not require crystallization and is not limited with respect to the size of the protein or protein complex. Diamagnetic proteins are accessible by site-directed spin labeling. To characterize structure or structural changes, experimental protocols were optimized and techniques for artifact suppression were introduced. Data analysis programs were developed, and it was realized that interpretation of the distance distributions must take into account the conformational distribution of spin labels. First methods have appeared for deriving structural models from a small number of distance constraints. The present scope and limitations of the technique are illustrated.

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DEER distance measurements on proteins

The preferred conformations of peptides heavily based on the currently extensively exploited achiral and chiral α-amino acids with a quaternary α-carbon atom, as determined by conformational energy computations, crystal-state (x-ray diffraction) analyses, and solution (1H-NMR and spectroscopic) investigations, are reviewed. It is concluded that 310/α-helical structures and the fully extended (C5) conformation are preferentially adopted by peptide sequences characterized by this family of amino acids, depending upon overall bulkiness and nature (e.g., whether acyclic or C ↔ C cyclized) of their side chains. The intriguing relationship between α-carbon chirality and bend/helix handedness is also illustrated. γ-Bends and semiextended conformations are rarely observed. Formation of β-sheet structures is prevented. © 2002 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 60: 396–419, 2001

Control of peptide conformation by the Thorpe-Ingold effect (C alpha-tetrasubstitution).

Optical excitation of additively coloured CaO crystals generates 1T1u and 3T1u excited states and F+ defects, the latter being revealed by EPR experiments. Interaction of the excited 3T1u triplet state and the F+ centre is documented by the emissive character of the transient EPR signal detected just after a laser pulse. A quenching of the initial polarization, observed at high microwave power, is ascribed to microwave saturation of the F+ spin levels.

Time-resolved EPR study of spin polarization and dynamics of F+ centres in additively coloured CaO crystals

A selected set of terminally protected β-hexapeptides, each containing two nitroxide-based (3R,4R)-4-amino-1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid (POAC) residues combined with four (1S,2S)-2-aminocyclopentane-1-carboxylic acid (ACPC) residues, was synthesised by using solution methods and was fully characterised. The two POAC residues are separated in the sequences by different numbers of intervening ACPC residues. The conformational features of the doubly spin-labelled β-hexapeptides were examined in chloroform by FTIR absorption and continuous-wave electron paramagnetic resonance spectroscopic techniques. In particular, the biradical exchange coupling (J) between two POAC residues within each peptide indicates unambiguously that the secondary structure overwhelmingly adopted is the 12-helix. Taken together, these results support the view that POAC is an excellent β-amino acid for exploring this type of helical conformation in doubly labelled β-peptides.

Synthesis and Conformational Characterisation of Hexameric β‐Peptide Foldamers by Using Double POAC Spin Labelling and cw‐EPR

Compact electron donor-acceptor triads based on carbazole (Cz) and naphthalenediimide (NDI) were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). By variation of the molecular conformation and electron-donating ability of the carbazole moieties, the electronic coupling between the two units was tuned, and as a result charge-transfer (CT) absorption bands with different magnitudes were observed (e = 4000-18 000 M-1 cm-1). Interestingly, the triads with NDI attached at the 3-C position or with a phenyl spacer at the N position of the Cz moiety, thermally activated delayed fluorescence (TADF) was observed. Femtosecond transient absorption (fs-TA) spectroscopy indicated fast electron transfer (0.8-1.5 ps) from the Cz to NDI unit, followed by population of the triplet state (150-600 ps). Long-lived triplet states (up to τT = 45-50 μs) were observed for the triads. The solvent-polarity-dependent singlet-oxygen quantum yield (ΦΔ) is 0-26%. Time-resolved electron paramagnetic resonance (TREPR) spectral study of TADF molecules indicated the presence of the 3CT state for NDI-Cz-Ph (zero-field-splitting parameter D = 21 G) and an 3LE state for NDI-Ph-Cz (D = 586 G). The triads were used as triplet photosensitizers in triplet-triplet annihilation upconversion by excitation into the CT absorption band; the upconversion quantum yield was ΦUC = 8.2%, and there was a large anti-Stokes shift of 0.55 eV. Spatially confined photoexcitation is achieved with the upconversion using focusing laser beam excitation, and not the normally used collimated laser beam, i.e., the upconversion was only observed at the focal point of the laser beam. Photo-driven intermolecular electron transfer was demonstrated with reversible formation of the NDI-• radical anion in the presence of the sacrificial electron donor triethanolamine.

Spin-Orbit Charge-Transfer Intersystem Crossing of Compact Naphthalenediimide-Carbazole Electron-Donor-Acceptor Triads.

Photoswitching of triplet-triplet annihilation upconversion with photo-generated radical from hexaphenylbiimidazole

Triplet excited states of isomers α, β and γ of N-methyl-[70]fulleropyrrolidine in glassy toluene (T = 120 K) are investigated with Time Resolved EPR (TR-EPR) spectroscopy using pulsed laser photoexcitation. Both the zero field splitting (ZFS) parameters D, E and the anisotropic triplet population rates are measured. Results reveal a strong dependence of the triplet state wavefunction on the position (α, β or γ) of the pyrrolidine addend with respect to the pole of the [70]fullerene unit. The relevant molecular orbitals have been calculated giving support to this finding. The dipolar tensor principal axes are located at the molecular reference frame from the inspection of the EPR spectra in a frozen liquid crystal (E7). Time evolution of the triplet EPR signals is different among the isomers, suggesting different triplet lifetimes and/or spin relaxation properties. The spin-density distribution in the first triplet state for N-methyl-[70]fulleropyrrolidine is compared with that previously observed for pristine C70.

Antonio Toffoletti

Papers

Time-resolved EPR study of spin polarization and dynamics of F+ centres in additively coloured CaO crystals

Synthesis and Conformational Characterisation of Hexameric β‐Peptide Foldamers by Using Double POAC Spin Labelling and cw‐EPR

Spin-Orbit Charge-Transfer Intersystem Crossing of Compact Naphthalenediimide-Carbazole Electron-Donor-Acceptor Triads.

Photoswitching of triplet-triplet annihilation upconversion with photo-generated radical from hexaphenylbiimidazole

Time resolved EPR of [70]fullerene monoadducts in the photoexcited triplet state