Charge-transfer biexciton annihilation in a donor–acceptor co-crystal yields high-energy long-lived charge carriers

Organic donor–acceptor (D–A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ⋯DADA⋯ π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D˙⁺–A˙⁻, between adjacent D–A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D–A co-crystal. We have co-crystallized a peri-xanthenoxanthene (PXX) donor with a N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI) acceptor to give an orthorhombic PXX–Ph4PDI ⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for S_n ← S₀ excitation of PXX and Ph4PDI. Using polarized, broadband, femtosecond pump–probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤ t ≤ 500 ps), the CT excitons decay with a t^−1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>8 ns) electron–hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis.

Femtosecond transient absorption microscopy of donor–acceptor single co-crystals shows that photogenerated charge transfer excitons in one-dimensional donor–acceptor π stacks annihilate to produce high-energy, long-lived electrons and holes.     

Multinuclear solid-state high-resolution and 13C -{27Al} double-resonance magic-angle spinning NMR studies on aluminum alkoxides

A combination of 27Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, 13C-1H CPMAS, and 13C-{27Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution 27Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The 27Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. 13C-1H CPMAS chemical shifts resolved the different carbon species (-CH3, =CH2, =CH-, and =C=) in the structures. 13C-{27Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.     

Improved methods for urinary atrazine mercapturate analysis--assessment of an enzyme-linked immunosorbent assay (ELISA) and a novel liquid chromatography-mass spectrometry (LC-MS) method utilizing online solid phase extraction (SPE)

Elimination of interfering substances in urine by solid phase extraction (SPE) prior to analysis resulted in 10-fold improvement in the sensitivity of atrazine mercapturate (AM) enzyme-linked immunosorbent assay (ELISA) compared to previous reports. Of the two tested SPE systems, Oasis^® HLB and MCX, the mixed-mode MCX gave good recoveries (82%) of AM in spiked samples measured by ELISA, whereas the reverse-phase HLB phase was not compatible with the immunochemical method. At relatively high concentrations of urinary AM (>20 ng mL⁻¹), sample dilution was effective enough for the elimination of interfering substances. The new liquid chromatography-mass spectrometry (LC-MS) method developed for AM utilizes online-SPE with Oasis^® HLB, column switching and a stable-isotope internal standard. The limit of quantification (0.05 ng mL⁻¹) indicates improved sensitivity compared with most previously published LC-MS methods for AM. Validation of all three methods, LC-MS, ELISA + SPE and ELISA + dilution with spiked urine samples showed good correlation between the known and measured concentrations with R² values of 0.996, 0.957 and 0.961, respectively. When a set (n = 70 plus 12 blind duplicates) of urine samples from farmers exposed to atrazine was analyzed, there was a good agreement (R² = 0.917) between the log normalized data obtained by ELISA + SPE and LC-MS. High correlation among the data obtained by the two tested methods and the LC-MS method by the Center of Disease Control and Prevention (CDC), together with low variability among the blind duplicates, suggests that both methods reported here would be suitable for the analysis of urinary AM as a biomarker for human exposure of atrazine.     

Fatty acid profiling of raw human plasma and whole blood using direct thermal desorption combined with gas chromatography-mass spectrometry

Gas chromatography (GC) has in recent times become an important tool for the fatty acid profiling of human blood and plasma. An at-line procedure used in the fatty acid profiling of whole/intact aquatic micro-organisms without any sample preparation was adapted for this work. A direct thermal desorption (DTD) interface was used to profile the fatty acid composition of human plasma and whole human blood of eight volunteers in a procedure omitting the usual lipid extraction steps that precede sample methylation in the traditional (off-line) protocols. Trimethylsulfonium hydroxide (TMSH) was used as reagent for thermally assisted methylation. In a fully automated manner, the liner of the GC injector is used as a sample-and-reaction container with the aid of the DTD interface. The fatty acid methyl ester (FAME) profiles obtained using this novel approach, were very identical to those obtained when the traditional off-line protocol was applied. FAME yields obtained in the at-line DTD method were found to be very similar for saturated fatty acids, but significantly higher for polyunsaturated fatty acids compared to off-line yields. As a result of the contribution of circulating cell membranes in blood, substantial differences were observed when the amount of FAMEs obtained in whole human blood and human plasma samples were compared after their analysis. Thanks to the fully automated operation of this novel procedure, large series of analyses can easily be performed.     

Ion yield improvement for static secondary ion mass spectrometry by use of polyatomic primary ions

Static secondary ion mass spectrometry (S-SIMS) is one of the potentially most powerful and versatile tools for the analysis of surface components at the monolayer level. Current improvements in detection limit (LOD) and molecular specificity rely on the optimisation of the desorption-ionisation (DI) process. As an alternative to monoatomic projectiles, polyatomic primary ion (P.I.) bombardment increases ion yields non-linearly. Common P.I. sources are Ga+ (liquid metal ion gun (LMIG), SF5+ (electron ionisation) and the newer Au(n)+, Bi(n)q+ (both LMIG) and C60+ (electron ionisation) sources. In this study the ion yield improvement obtained by using the newly developed ion sources is assessed. Two dyes (zwitterionic and/or thermolabile polar functionalities on a largely conjugated backbone) were analysed as a thin layer using Ga+, SF5+, C60+, Bi+, Bi3(2+) and Bi5(2+) projectiles under static conditions. The study aims at evaluating the improvement in LOD, useful and characteristic yield and molecular specificity. The corrected total ion count values for the different P.I. sources are compared for different instruments to obtain a rough estimate of the improvements. Furthermore, tentative ionisation and fragmentation schemes are provided to describe the generation of radical and adduct ions. Characteristic ion yields are discussed for the different P.I. sources. An overview of the general appearances of the mass spectra obtained with the different P.I. sources is given to stress the major improvement provided by polyatomic P.I.s in yielding information at higher m/z values.     

A sensitive and simple ultra-high-performance-liquid chromatography-tandem mass spectrometry based method for the quantification of D-amino acids in body fluids

D-Amino acids are increasingly being recognized as important signaling molecules in mammals, including humans. D-Serine and D-aspartate are believed to act as signaling molecules in the central nervous system. Interestingly, several other D-amino acids also occur in human plasma, but very little is currently known regarding their function and origin. Abnormal levels of D-amino acids have been implicated in the pathogenesis of different diseases, including schizophrenia and amyotrophic lateral sclerosis (ALS), indicating that D-amino acid levels hold potential as diagnostic markers. Research into the biological functions of D-amino acids is hindered, however, by the lack of sufficiently sensitive, high-throughput analytical methods. In particular, the interference of large amounts of L-amino acids in biological samples and the low concentrations of D-amino acids are challenging. In this paper, we compared 7 different chiral derivatization agents for the analysis of D-amino acids and show that the chiral reagent (S)-NIFE offers outstanding performance in terms of sensitivity and enantioselectivity. An UPLC-MS/MS based method for the quantification of D-amino acids human biological fluids was then developed using (S)-NIFE. Baseline separation (R(s)>2.45) was achieved for the isomers of all 19 chiral proteinogenic amino acids. The limit of detection was <1 nM for all amino acids except d-alanine (1.98 nM), d-methionine (1.18 nM) and d-asparagine (5.15 nM). For measurements in human plasma, cerebrospinal fluid and urine, the accuracy ranged between 85% and 107%. The intra-assay and inter-assay were both <16% RSD for these three different matrices. Importantly, the method does not suffer from spontaneous racemization during sample preparation and derivatization. Using the described method, D-amino acid levels in human cerebrospinal fluid, plasma and urine were measured.     

Intramolecular directional förster resonance energy transfer at the single-molecule level in a dendritic system

We report on the directional F?rster resonance energy transfer (FRET) process taking place in single molecules of a first (T1P4) and a second (T2P8) generation of a perylenemonoimide (P)-terrylenediimide (T)-based dendrimer in which the chromophores are separated by rigid polyphenylene arms. At low excitation powers, single-molecule detection and spectroscopy of T1P4 and T2P8 dendrimers point to a highly efficient directional FRET from P donors to the central T acceptor, optical excitation at 488 nm resulting in exclusively acceptor emission in the beginning of the detected fluorescence intensity. Donor emission is seen only upon the bleaching of the acceptor. High-resolution time-resolved single-molecule fluorescence data measured with a microchannel plate photomultiplier reveal, for T2P8, a broad range of FRET rates as a result of a broad range of distances and orientations experienced by the donor-acceptor dendrimers when immobilized in a polymer matrix. Single-molecule data from T2P8 on 488 nm excitation are indicative for the presence, after terrylenediimide bleaching, of a P-P excited dimer characterized by a broad emission spectrum peaking around 600 nm and by fluctuating fluorescence decay times. At high excitation powers, single T1P4 and T2P8 molecules display simultaneous emission from both donor and acceptor chromophores. The effect, called "exciton blockade", occurs due to the presence of multiple excitations in a single molecule.     

Syntheses of Pantolactone and Pantothenic Acid Derivatives as Potential Lipid Regulating Agents

A series of pantolactone and pantothenic acid derivatives (1–10) were synthesized to be tested for their potential as lipid regulating agents that act as Coenzyme‐A mimics. The syntheses were performed with moderate to high yields.