Ultrafast Vibrational Cooling Inside of a Molecular Container.

Vibrational cooling of azulene encapsulated in a hemicarcerand molecular container was studied by pump-probe spectroscopy. Within 1.5 ps of excitation of azulene to the S1 state, rapid internal conversion through a conical intersection leads to the formation of a vibrationally hot (∼1080 K) ground state, the subsequent cooling of which can be monitored by tracking the evolution of the red-shifted hot band at the edge of the ground-state absorption. It was found that the cooling of the hot S0 state of azulene in the host-guest complex (hemicarceplex) is 2-4 times faster than that in common organic solvents. Such large acceleration points to a high density of matching vibrational modes and efficient mechanical coupling between the guest and the host. The experimental observations were fully corroborated by the results of molecular dynamics simulations.

Ultrabroadband mid-infrared noncollinear difference frequency generation in a silver thiogallate crystal.

We report the generation of ultrabroadband mid-infrared (mid-IR) pulses by noncollinear difference frequency mixing. The signal and the idler output beams of an optical parametric amplifier are combined in a silver thiogallate crystal (AgGaS2) to generate mid-infrared radiation. We show that a noncollinear geometry facilitates broadband phase matching. Spectral bandwidths up to 1750 cm-1 were obtained at an external noncollinear angle of 4.2 deg, which is more than three times broader than in a collinear geometry. The broadband spectrum is tunable in the range of 1500-4500 cm-1. Pulse energies up to 1 µJ were achieved. The broadband pulses were used in sum frequency generation in ZnSe and in vibrational absorption spectroscopy experiments of liquid samples.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Morphology-Independent Efficient Singlet Exciton Fission in Perylene Diimide Thin Films.

Perylene diimides are conjugated chromophores that are of considerable interest owing to their ability to transform a singlet excited state into two triplets by singlet fission. Although singlet fission has previously been reported for certain perylene diimide derivatives, there is some uncertainty about the rates and yield of the process in these materials. In this report, ultrafast transient absorption spectroscopy is used to demonstrate that singlet fission in perylene diimides can occur on a sub-picosecond timescale with quantum yields approaching the theoretical limit of 200 %.

Structure Evolution and Thermoelectric Properties of Carbonized Polydopamine Thin Films.

Carbonization of nature-inspired polydopamine can yield thin films with high electrical conductivity. Understanding of the structure of carbonized PDA (cPDA) is therefore highly desired. In this study, neutron diffraction, Raman spectroscopy, and other techniques indicate that cPDA samples are mainly amorphous with some short-range ordering and graphite-like structure that emerges with increasing heat treatment temperature. The electrical conductivity and the Seebeck coefficient show different trends with heat treatment temperature, while the thermal conductivity remains insensitive. The largest room-temperature ZT of 2 × 10-4 was obtained on samples heat-treated at 800 °C, which is higher than that of reduced graphene oxide.

Density of Trap States and Auger-mediated Electron Trapping in CdTe Quantum-Dot Solids.

Charge trapping is an ubiquitous process in colloidal quantum-dot solids and a major limitation to the efficiency of quantum dot based devices such as solar cells, LEDs, and thermoelectrics. Although empirical approaches led to a reduction of trapping and thereby efficiency enhancements, the exact chemical nature of the trapping mechanism remains largely unidentified. In this study, we determine the density of trap states in CdTe quantum-dot solids both experimentally, using a combination of electrochemical control of the Fermi level with ultrafast transient absorption and time-resolved photoluminescence spectroscopy, and theoretically, via density functional theory calculations. We find a high density of very efficient electron traps centered ∼0.42 eV above the valence band. Electrochemical filling of these traps increases the electron lifetime and the photoluminescence quantum yield by more than an order of magnitude. The trapping rate constant for holes is an order of magnitude lower that for electrons. These observations can be explained by Auger-mediated electron trapping. From density functional theory calculations we infer that the traps are formed by dicoordinated Te atoms at the quantum dot surface. The combination of our unique experimental determination of the density of trap states with the theoretical modeling of the quantum dot surface allows us to identify the trapping mechanism and chemical reaction at play during charge trapping in these quantum dots.

Cyclometalated red iridium(III) complexes containing carbazolyl-acetylacetonate ligands: efficiency enhancement in polymer LED devices.

The design, synthesis, photophysical and significantly improved electrooptical properties of a series of red emitting cyclometalated iridium(iii) complexes containing carbazolyl-acetylacetonate ligands are described.