PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C60 DYADS

Abstract Porphyrin-C₆₀ dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz_n-C₆₀) and free base (P-C₆₀) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C₆₀ The lifetime of Pzn-¹C₆₀ is -5 ps in toluene, whereas the singlet lifetime of an appropriate C₆₀ model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P_zn^bull;+-C₆₀^bull;-. In toluene, P-¹C₆₀ is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C₆₀ triplet. In the more polar benzonitrile, P-¹C₆₀ underoes photoinduced electron transfer to give P^•+-C₆₀^bull;-. The electron transfer rate constant is −2 × 10¹¹ s⁻¹.     

Treatment methods for the determination of delta2H and delta18O of hair keratin by continuous-flow isotope-ratio mass spectrometry

The structural proteins that comprise approximately 90% of animal hair have the potential to record environmentally and physiologically determined variation in delta2H and delta18O values of body water. Broad, systematic, geospatial variation in stable hydrogen and oxygen isotopes of environmental water and the capacity for rapid, precise measurement via methods such as high-temperature conversion elemental analyzer/isotope ratio mass spectrometry (TC/EA-IRMS) make these isotope systems particularly well suited for applications requiring the geolocation of hair samples. In order for such applications to be successful, however, methods must exist for the accurate determination of hair delta2H and delta18O values reflecting the primary products of biosynthesis. Here, we present the results of experiments designed to examine two potential inaccuracies affecting delta2H and delta18O measurements of hair: the contribution of non-biologic hydrogen and oxygen to samples in the form of sorbed molecular water, and the exchange of hydroxyl-bound hydrogen between hair keratin and ambient water vapor. We show that rapid sorption of molecular water from the atmosphere can have a substantial effect on measured delta2H and delta18O values of hair (comprising approximately 7.7% of the measured isotopic signal for H and up to approximately 10.6% for O), but that this contribution can be effectively removed through vacuum-drying of samples for 6 days. Hydrogen exchange between hair keratin and ambient vapor is also rapid (reaching equilibrium within 3-4 days), with 9-16% of the total hydrogen available for exchange at room temperature. Based on the results of these experiments, we outline a recommended sample treatment procedure for routine measurement of delta2H and delta18O in mammal hair.     

Perylenediimide--metal ion dyads for photo-induced electron transfer

A novel perylene diimide incorporating a tetradentate ligand was synthesized and the photophysical properties of its Cu(+), Cu(2+), and Fe(3+) complexes were investigated by fluorescence experiments.     

Solution structure of a DNA duplex containing a guanine-difluorotoluene pair: a wobble pair without hydrogen bonding?

The incorporation of synthetic nucleoside analogues into DNA duplexes provides a unique opportunity to probe both structure and function of nucleic acids. We used 1H and 19F NMR and molecular dynamics calculations to determine the solution structures of two similar DNA decamer duplexes, one containing a central G-T mismatched or "wobble" base pair, and one in which the thymine in this base pair is replaced by difluorotoluene (a thymine isostere) creating a G-F pair. Here, we show that the non-hydrogen-bonding G-F pair stacks relatively well into the helix and that the distortions caused by each non-Watson-Crick G-T or G-F base pair are quite localized to a three base pair site around the mismatch. A detailed structural analysis reveals that the absence of hydrogen bonding introduces more dynamic motion into the G-F pair relative to G-T and permits the G-F pair to exhibit stacking and conformational features characteristic of both a Watson-Crick base pair (on the guanine containing strand) and a wobble base pair (on the strand containing the difluorotoluene). We used these results to posit a rationale for recognition and repair of mismatch sites in DNA.     

A brief critique of the Adam–Gibbs entropy model

This paper critically discusses the entropy model proposed by Adam and Gibbs in 1965 for the dramatic temperature dependence of glass-forming liquids’ average relaxation time, which is one of the most influential models during the last four decades. We discuss the Adam–Gibbs model’s theoretical bases as well as its reported experimental model confirmations; in the process of doing this a number of problems with the model are identified.     

Facile Synthesis of Hexakis(4-Formylphenoxy)-Cyclotriphosphazene and Hexakis(4-Acetophenoxy)-Cyclotriphosphazene

Synthesis of hexakis(4-formylphenoxy)cyclotriphosphazene and similar compounds may be achieved utilizing a phase-transfer reaction. This one-pot procedure eliminates the need for rigorous drying of solvents, purification of reagents, and reflux conditions during the reaction.     

Silylium ion-catalyzed challenging Diels-Alder reactions: the danger of hidden proton catalysis with strong Lewis acids

The pronounced Lewis acidity of tricoordinate silicon cations brings about unusual reactivity in Lewis acid catalysis. The downside of catalysis with strong Lewis acids is, though, that these do have the potential to mediate the formation of protons by various mechanisms, and the thus released Br?nsted acid might even outcompete the Lewis acid as the true catalyst. That is an often ignored point. One way of eliminating a hidden proton-catalyzed pathway is to add a proton scavenger. The low-temperature Diels-Alder reactions catalyzed by our ferrocene-stabilized silicon cation are such a case where the possibility of proton catalysis must be meticulously examined. Addition of the common hindered base 2,6-di-tert-butylpyridine resulted, however, in slow decomposition along with formation of the corresponding pyridinium ion. Quantitative deprotonation of the silicon cation was observed with more basic (Mes)(3)P to yield the phosphonium ion. A deuterium-labeling experiment verified that the proton is abstracted from the ferrocene backbone. A reasonable mechanism of the proton formation is proposed on the basis of quantum-chemical calculations. This is, admittedly, a particular case but suggests that the use of proton scavengers must be carefully scrutinized, as proton formation might be provoked rather than prevented. Proton-catalyzed Diels-Alder reactions are not well-documented in the literature, and a representative survey employing TfOH is included here. The outcome of these catalyses is compared with our silylium ion-catalyzed Diels-Alder reactions, thereby clearly corroborating that hidden Br?nsted acid catalysis is not operating with our Lewis acid. Several simple-looking but challenging Diels-Alder reactions with exceptionally rare dienophile/enophile combinations are reported. Another indication is obtained from the chemoselectivity of the catalyses. The silylium ion-catalyzed Diels-Alder reaction is general with regard to the oxidation level of the α,β-unsaturated dienophile (carbonyl and carboxyl), whereas proton catalysis is limited to carbonyl compounds.