Quest for singlet fission of organic sulfur-containing systems in the higher lying singlet excited state: application prospects of anti-Kasha's rule

In this study, we explore the possibilities of the deactivating pathways of organic thione containing systems through first-principles calculations. We particularly pay attention to the second lying singlet excited state, S₂, due to its large energy difference from the lowest lying S₁ state in the sulfur-containing systems. Several theoretical models including the previously synthesized thiones and the strategically designed molecules are investigated to search for the basic conjugation unit that exhibits the prospect of S₂ fission. Various molecular motifs and different substituents are combined to maneuver the relative alignment of the relevant low excited energy states. The results lead us to conclude that the thione derivatives, under rational and delicate molecular designs, may be engineered to possess a sufficiently high S₂-S₁ energy gap as high as 2 eV and that these systems may exhibit S₂ fission to triplet excitons in the red to near infrared region.

Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor–acceptor (D–A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor–acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S₀ and S₁ states. Mixing a certain degree of local excitation (LE) with CT in the S₁ state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.

The reorganization energies may be significantly reduced by molecular symmetry effect.

Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (3TF), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S₁ absorption at 383 nm. Upon photoexcitation, 3NTF exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm^-1. Upon methylation on the thiol group, 3MeNTF, lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for 3NTF, followed by a tautomer emission lifetime of 120 ps. In sharp contrast to 3NTF, both 3TF and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible nπ* contribution to the S₁' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For 3NTF, the extension of π-electron delocalization by the diethylamino electron-donating group endows the S₁' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.

A computational exploration of the 1D TiS2(en) nanostructure for lithium ion batteries

First-principles investigations on 1D TiS₂(en) are performed to evaluate its potential as an electrode for lithium ion batteries. The intercalation of lithium ions into Li_xTiS₂(en) follows the Rüdorff model and the lithium ions are predicted to diffuse along the one-dimensional axis of the TiS₂(en) nanostructure with a small diffusion barrier of 0.27 eV.

Exploiting racemism enhanced organic room-temperature phosphorescence to demonstrate Wallach's rule in the lighting chiral chromophores

The correlation between molecular packing structure and its room-temperature phosphorescence (RTP), hence rational promotion of the intensity, remains unclear. We herein present racemism enhanced RTP chiral chromophores by 2,2-bis-(diphenylphosphino)-1,1-napthalene (rac-BINAP) in comparison to its chiral counterparts. The result shows that rac-BINAP in crystal with denser density, consistent with a long standing Wallach's rule, exhibits deeper red RTP at 680 nm than that of the chiral counterparts. The cross packing between alternative R- and S- forms in rac-BINAP crystal significantly retards the bimolecular quenching pathway, triplet-triplet annihilation (TTA), and hence suppresses the non-radiative pathway, boosting the RTP intensity. The result extends the Wallach's rule to the fundamental difference in chiral-photophysics. In electroluminescence, rac-BINAP exhibits more balanced fluorescence versus phosphorescence intensity by comparison with that of photoluminescence, rendering a white-light emission. The result paves an avenue en route for white-light organic light emitting diodes via full exploitation of intrinsic fluorescence and phosphorescence.

Termination Effects of Pt/v-Ti n+1C nT2 MXene Surfaces for Oxygen Reduction Reaction Catalysis

Ideal catalysts for the oxygen reduction reaction (ORR) have been searched and researched for decades with the goal to overcome the overpotential problem in proton exchange membrane fuel cells. A recent experimental study reports the application of Pt nanoparticles on the newly discovered 2D material, MXene, with high stability and good performance in ORR. In this work, we simulate the Ti _n+1C _nT _x and the Pt-decorated Pt/v-Ti _n+1C _nT _x ( n = 1-3, T = O and/or F) surfaces by first-principles calculations. We focus on the termination effects of MXene, which may be an important factor to enhance the performance of ORR. The properties of different surfaces are clarified by exhaustive computational analyses on the geometries, charges, and their electronic structures. The free-energy diagrams as well as the volcano plots for ORR are also calculated. On the basis of our results, the F-terminated surfaces are predicted to show a better performance for ORR but with a lower stability than the O-terminated counterparts, and the underlying mechanisms are investigated in detail. This study provides a better understanding of the electronic effect induced by the terminators and may inspire realizations of practical MXene systems for ORR catalysis.

Molecular design principles towards exo-exclusive Diels–Alder reactions

The exo selective Diels–Alder reactions, reported as special cases, usually involve catalytic reaction conditions and specific cyclic structural motifs on the diene and/or the dienophile. Here we report a systematic computational investigation on the substituent effect for simple, linear dienes and dienophiles towards exo control in Diels–Alder reactions under thermal conditions. Through detailed characterization of reaction pathways for Diels–Alder cycloadditions between linear dienes and dienophiles with various substituents, we summarize a set of design principles aiming for an optimal and nearly-exclusive exo selectivity. These results shall lead to valuable guidelines and more versatile strategies in organic synthesis that are in accordance with the principles of green chemistry.

Exo-exclusive stereoselectivity for simple, terminal-substituted dienes and dienophiles may be achieved under thermal conditions through a delicate control of substituent identities.

A computational exploration of CO2 reduction via CO dimerization on mixed-valence copper oxide surface

The electron-localization function plots of OCCO adsorbed on O_v–CuO(111), O_v–Cu₄O₃(202) and Cu₂O(111) surfaces.

Room-temperature phosphorescence from small organic systems containing a thiocarbonyl moiety

Room-temperature phosphorescence facilitated by the thiocarbonyl group in dTPT3-based derivatives.

Single-molecule electric revolving door

This work proposes a new type of molecular machine, the single-molecule electric revolving door, which utilizes conductance dependence upon molecular conformation as well as destructive quantum interference. We perform electron transport simulations in the zero-bias limit using the Landauer formalism together with density functional theory. The simulations show that the open- and closed-door states, accompanied by significant conductance variation, can be operated by an external electric field. The large on-off conductance ratio (~10(5)) implies that the molecular machine can also serve as an effective switching device. The simultaneous control and detection of the door states can function at the nanosecond scale, thereby offering a new capability for molecular-scale devices.

A Natural Helical Crystal Lattice Model for Carbon Nanotubes

We propose a novel helical crystal lattice model for chiral and achiral carbon nanotubes (CNTs) where the unit cell and the helical crystal vector are defined in a unique and systematic manner for arbitrary CNTs. The small unit cell of this helical crystal lattice leads to a natural and convenient description of the electronic structure of chiral CNTs. Also, using this model, the degenerate frontier Bloch wave functions at the Γ point can be conveniently chosen by their symmetry properties. In particular, the contour of the Bloch wave functions at the Fermi level can be easily predicted for all metallic CNTs.

Improving the electrical conductivity of carbon nanotube networks: a first-principles study

We address the issue of the low electrical conductivity observed in carbon nanotube networks using first-principles calculations of the structure, stability, and ballistic transport of different nanotube junctions. We first study covalent linkers, using the nitrene-pyrazine case as a model for conductance-preserving [2 + 1] cycloadditions, and discuss the reasons for their poor performance. We then characterize the role of transition-metal adsorbates in improving mechanical coupling and electrical tunneling between the tubes. We show that the strong hybridization between the transition-metal d orbitals with the π orbitals of the nanotube can provide an excellent electrical bridge for nanotube-nanotube junctions. This effect is maximized in the case of nitrogen-doped nanotubes, thanks to the strong mechanical coupling between the tubes mediated by a single transition metal adatom. Our results suggest effective strategies to optimize the performance of carbon nanotube networks.

Switchable conductance in functionalized carbon nanotubes via reversible sidewall bond cleavage

We propose several covalent functionalizations for carbon nanotubes that display switchable on/off conductance in metallic tubes. The switching action is achieved by reversible control of bond-cleavage chemistry in [1 + 2] cycloadditions via the sp(3) ⇌ sp(2) rehybridization that it induces; this leads to remarkable changes of conductance even at very low degrees of functionalization. Reversible bond-cleavage chemistry is achieved by identifying addends that provide optimal compensation between the bond-preserving through-space π orbital interactions with the tube against the bond-breaking strain energy of the cyclopropane moiety. Several strategies for real-time control, based on redox or hydrolysis reactions, cis-trans isomerization or excited-state proton transfer are proposed. Such designer functional groups would allow for the first time direct control of the electrical properties of metallic carbon nanotubes, with extensive applications in nanoscale devices.

Intravenous fentanyl is exhaled and the concentration fluctuates with time

Previous studies have reported that fentanyl is eliminated predominantly by hepatic biotransformation, and that some is eliminated unchanged in urine and stools. No reports have described the elimination of fentanyl via the lungs. In this study, exhaled gas samples from eight anaesthetized patients undergoing cardiac surgery were analysed using solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (GC-MS). Results confirmed that fentanyl was exhaled by patients after intravenous administration, that the concentration of exhaled fentanyl fluctuated with time and peak concentrations were reached approximately 15 - 20 min after intravenous fentanyl administration. Thus, in addition to hepatic biotrans formation and elimination via urine and faeces, fentanyl is also eliminated unchanged by the lungs. The potential risk to operating theatre personnel from long-term exposure to low levels of exhaled anaesthetic agents following intravenous administration to patients during surgery warrants further research.

Luminescent Platinum(II) Complexes Containing Isoquinolinyl Indazolate Ligands: Synthetic Reaction Pathway and Photophysical Properties

New Pt(II) dichloride complexes [Pt(1-iqdzH)Cl2] (2a) and [Pt(3-iqdzH)Cl2] (2b), in which idqzH = 1- or 3-isoquinolinyl indazole, were prepared by treatment of the corresponding indazoles with K2PtCl4 in aqueous HCl solution. Despite their nonemissive nature, these complexes could react with excess indazole, sodium picolinate, and 3-trifluoromethyl-5-(2-pyridyl) pyrazole [(fppz)H] to afford the respective a and b series of luminescent complexes [Pt(1-iqdz)(L/\X)] and [Pt(3-iqdz)(L/\X)], where L/\X = 1-iqdz (1a), 3-iqdz (1b), pic (3a, 3b), and fppz (4a, 4b). Single-crystal X-ray diffraction studies of 1b, 2a, and 3b revealed a planar molecular geometry without notable intermolecular Pt...Pt contact in the solid crystal, a result of the steric repulsion imposed by the bulky indazole fragments. For coordination complexes 1, 3, and 4, photoluminescence in degassed CH2Cl2 revealed high quantum efficiency and short radiative lifetimes in the range of several microseconds. As supported by the spectral feature, the associated radiation lifetimes, and a computational approach based on time-dependent density function theory (TD-DFT), the origin of the emission is attributed to a mixed 3MLCT/3pipi transition. The TD-DFT approach further confirmed that, except for the series 1 complexes, the HOMO of 3-iqdz complexes 3b and 4b is much less located at the central Pt(II) atom than the HOMO orbitals of the respective 1-iqdz complexes 3a and 4a, leading to a smaller degree of MLCT contribution. Consequently, there are a blue-shifted emission signal and an inferior emission quantum yield for the 3-iqdz derivatives. OLED devices with a multilayer configuration of ITO/NPB/CBP:3a/BCP/Alq3/LiF/Al were fabricated using a CBP layer doped with various concentrations of 3a, ranging from 6% to 100%, within the emitting layer. The best device performance was realized using a 6% doping concentration, for which the external quantum yield of 4.93%, luminous efficiency of 12.19 cd/A, and power efficiency of 6.12 lm W-1 were observed at 20 mA/cm2, while a maximum luminescence as high as 20296 cd/m2 was also realized at 16 V, showing good prospect for the fabrication of Pt(II) based OLEDs.

Neutral RuII-Based Emitting Materials: A Prototypical Study on Factors Governing Radiationless Transition in Phosphorescent Metal Complexes

In addition to the metal-centered dd transition that is widely accepted as a dominant radiationless decay channel, other factors may also play important roles in governing the loss of phosphorescence efficiency for heavy-transition-metal complexes. To conduct our investigation, we synthesized two dicarbonylruthenium complexes with formulas [Ru(CO)2(BQ)2] (1) and [Ru(CO)2(DBQ)2] (2), for which the cyclometalated ligands BQ and DBQ denote benzo[h]quinoline and dibenzo[f,h]quinoxaline, respectively. Replacing one CO ligand with a P donor ligand such as PPh2Me and PPhMe2 caused one cyclometalated ligand to undergo a 180 degrees rotation around the central metal atom, giving highly luminous metal complexes [Ru(CO)L(BQ)2] and [Ru(CO)L(DBQ)2], where L = PPh2Me and PPhMe2 (3-6), with emission peaks lambda(max) in the range of 571-656 nm measured in the fluid state at room temperature. It is notable that the S0-T1 energy gap for both 1 and 2 is much higher than that of 3-6, but the corresponding phosphorescent spectral intensity is much weaker. Using these cyclometalated Ru metal complexes as a prototype, our experimental results and theoretical analysis draw attention to the fact that, for complexes 1 and 2, the weaker spin-orbit coupling present within these molecules reduces the T1-S0 interaction, from which the thermally activated radiationless deactivation may take place. This, in combination with the much smaller 3MLCT contribution than that observed in 3-6, rationalizes the lack of room-temperature emission for complexes 1 and 2.

A new class of laser dyes, 2-oxa-bicyclo[3.3.0]octa-4,8-diene-3,6-diones, with unity fluorescence yield

A new class of highly fluorescent dyes, 4,8-diphenyl-2-oxa-bicyclo[3.3.0]octa-4,8-diene-3,6-diones (1a-c), have been synthesized, they all exhibit unity fluorescence quantum yield and short radiative lifetime (< 4 ns) in common organic solvents and have demonstrated remarkable amplified spontaneous emission with a gain efficiency of > 10.

A population-based study of BRCA1 and BRCA2 mutations in Jewish women with epithelial ovarian cancer

OBJECTIVE

To determine the frequency of BRCA1 mutations 185delAG and 5382insC and BRCA2 mutation 6174delT in Jewish women with ovarian cancer and in matched controls in a population-based study.

METHODS

Forty-eight Jewish women with epithelial ovarian cancer (32 invasive and 16 borderline) and 33 Jewish control subjects were obtained from a population-based, case-control study of ovarian cancer in eastern Massachusetts and New Hampshire. Mutational analysis on exons 2 and 20 of BRCA1 and exon 11 of BRCA2 was conducted on blood samples from patients and controls.

RESULTS

Fourteen (44%) of 32 Jewish patients with invasive epithelial ovarian cancer carried either a 185delAG mutation of BRCA1 (n = 8) or a 6174delT mutation on BRCA2 (n = 6). Neither of these mutations was identified in 16 women with borderline ovarian tumors or in 33 controls. No 5382insC mutation of BRCA1 was identified in any of the patients or control subjects in the series. Family history did not predict mutation status.

CONCLUSION

BRCA1 185delAG and BRCA2 6174delT mutations are frequent in Jewish women with invasive epithelial ovarian cancer, irrespective of family history. Genetic counseling might be warranted in women with invasive epithelial ovarian cancer based on Jewish ethnicity alone.

Basal hormone levels in women who use acetaminophen for menstrual pain

OBJECTIVE

To compare basal gonadotropin and estradiol levels between women using acetaminophen versus those using no or other types of analgesics.

DESIGN

Observational study.

SETTING

Three IVF clinics in greater Boston.

PATIENT(S)

Three hundred eighty-six women accepted for their first IVF treatment who completed questionnaires recording medical history, including analgesic use for menstrual pain, and who had blood drawn during the menstrual phase of a cycle before treatment.

MAIN OUTCOME MEASURE(S)

Basal FSH, LH, and estradiol.

RESULT(S)

Basal hormone levels, especially LH, were lower for women who regularly used acetaminophen compared with women who used no medication or other types of analgesics. Lower hormone levels in acetaminophen users were consistently observed when women were subdivided by age, body mass index, smoking history, and degree of menstrual pain-features that might have influenced analgesic use or hormone levels.

CONCLUSION(S)

This study provides preliminary evidence that acetaminophen may lower gonadotropin and estradiol levels and offers a biologic basis for the epidemiologic observation that acetaminophen use may reduce the risk of ovarian cancer.

Ascorbic acid glycation: the reactions of L-threose in lens tissue

L-Threose is a significant degradation product of ascorbic acid at pH 7.0 in the presence of oxygen. When compared to several other ascorbate-derived degradation products, it had the greatest ability to glycate and crosslink lens proteins in vitro. To determine whether L-threose was formed in the lens, the sugars in a TCA-soluble extract from human lenses were reduced to polyols with NaBH4, acetylated and analysed by gas-liquid chromatography. The threitol levels measured were 3.4 +/- 0.8 micrograms per lens (n = 4). GC-MS measurements made after reduction with NaBD4 indicated that threitol, but little or no threose, was originally present in the human lens. Rat lenses were incubated with [1-13C]D-threose for 24 hr, and considerable D-threitol formation was seen by NMR spectroscopy. Analysis of the lenses after medium removal showed that only [1-13C]threitol was present within the lenses indicating a rapid reduction of threose within the lens, presumably by aldose reductase. Assays with human recombinant aldose reductase and with human lens cortical and nuclear extracts all exhibited sorbinil-inhibitable aldose reductase activity with L-threose as substrate. This was confirmed by incubating a preparation of [1-14C]L-tetrose (a mixture of 40% L-threose and 45% L-erythrose) with both the pure aldose reductase and crude lens extracts followed by the subsequent identification of the [1-14C]L-threitol formed by thin layer chromatography. L-Threose degrades very slowly in 0.1 M phosphate buffer at pH 7.0, but the addition of a four-fold excess of N alpha-acetyl-L-lysine accelerated the rate of disappearance of threose 30-fold, indicating a rapid glycation reaction. When [1-14C]L-tetrose was incubated with a complete bovine lens homogenate, a linear incorporation into protein was observed over a 24 hr period. Increasing levels of lens extract exhibited increasing incorporation into protein. These data confirm a rapid reactivity of L-threose with lens protein and argue that glycation would occur in vivo in spite of the presence of aldose reductase.

The degradation of L-threose at Maillard reaction conditions

L-Threose, a comparatively unstable aldose, is produced from L-ascorbic acid in the presence of oxygen and participates vigorously in Maillard reactions, even at comparatively mild conditions. In the present study, the degradation of L-threose at pH 7.0 alone, in the presence of N-alpha-acetyl-L-lysine, and at pH 2.0 alone at 37 degrees C was investigated by identification of some of the products produced in the reactions by means of GLC and GLC-MS. Among the compounds identified were 3-deoxy-tetros-2-ulose (1), the predicted alkaline rearrangement product derived from 1 (2,4-dihydroxybutyrate, the 4-carbon metasaccharinic acid), as well as glyceraldehyde. Isotopic tracer studies clearly show that the glyceraldehyde is produced by loss of C-1 from the starting L-threose molecule. The presence of N-acetyl lysine in incubation solutions appears to accelerate the production of 1, but the formation of glyceraldehyde appears to be independent of the lysine derivative.