Vibrational-Mode-Selective Modulation of Electronic Excitation.

Vibrational-mode-selective modulation of electronic excitation is conducted with a synchronized femtosecond (fs) visible (vis) pulse and a picosecond (ps) infrared (IR) pulse. The mechanism of modulation of vibrational and vibronic relaxation behavior of excited state is investigated with ultrafast vis/IR, IR/IR, and vis-IR/IR transient spectroscopy, optical gating experiments and theoretical calculations. An organic molecule, 4'-(N,N-dimethylamino)-3-methoxyflavone (DMA3MHF) is chosen as the model system. Upon 1608 cm-1 excitation, the skeleton stretching vibration of DMA3MHF is energized, which can significantly change the shape of the absorption, facilitate the radiative decay and promote emission from vibrational excited states. As results, a remarkable enhancement and a slight blueshift in fluorescence are observed. The mode-selective modulation of electronic excitation is not limited in luminescence or photophysics. It is expected to be widely applicable in tuning many photochemical processes.

Blue-Shifted and Broadened Fluorescence Enhancement by Visible and Mode-Selective Infrared Double Excitations.

Mode-selective vibrational excitations to modify the electronic states of fluorescein dianion in methanol solutions are carried out with a femtosecond visible pulse synchronized with a tunable high-power, narrow-band picosecond infrared (IR) pulse. In this work, simultaneous intensity enhancement, peak blueshift, and line width broadening of fluorescence are observed in the visible/IR double resonance experiments. Comprehensive investigations on the modulation mechanism with scanning the vibrational excitation frequencies, tuning the time delay between the two excitation pulses, theoretical calculations, and nonlinear and linear spectroscopic measurements suggest that the fluorescence intensity enhancement is caused by the increase of the Franck-Condon factor induced by the vibrational excitations at the electronic ground state. Various enhancement effects are observed as vibrations initially excited by the IR photons relax to populate the vibrational modes of lower frequencies. The peak blueshift and line width broadening are caused by both increasing the Franck-Condon factors among different subensembles because of IR pre-excitation and the long-lived processes induced by the initial IR excitation. The results demonstrate that the fluorescence from the visible/IR double resonance experiments is not a simple sum frequency effect, and vibrational relaxations can produce profound effects modifying luminescence.

Graphene Coated Dielectric Hierarchical Nanostructures for Highly Sensitive Broadband Infrared Sensing.

Broadband infrared (IR) absorption is sought after for wide range of applications. Graphene can support IR plasmonic waves tightly bound to its surface, leading to an intensified near-field. However, the excitation of graphene plasmonic waves usually relies on resonances. Thus, it is still difficult to directly obtain both high near-field intensity and high absorption rate in ultra-broad IR band. Herein, a novel method is proposed to directly realize high near-field intensity in broadband IR band by graphene coated manganous oxide microwires featured hierarchical nanostructures (HNSs-MnO@Gr MWs) both experimentally and theoretically. Both near-field intensity and IR absorption of HNSs-MnO@Gr MWs are enhanced by at least one order of magnitude compared to microwires with smooth surfaces. The results demonstrate that the HNSs-MnO@Gr MWs support vibrational sensing of small organic molecules, covering the whole fingerprint region and function group region. Compared with the graphene-flake-based enhancers, the signal enhancement factors reach a record high of 103 . Furthermore, just a single HNSs-MnO@Gr MW can be constructed to realize sensitively photoresponse with high responsivity (over 3000 V W-1 ) from near-IR to mid-IR. The graphene coated dielectric hierarchical micro/nanoplatform with enhanced near-field intensity is scalable and can harness for potential applications including spectroscopy, optoelectronics, and sensing.

Restriction of crossing conical intersections: the intrinsic mechanism of aggregation-induced emission.

Elucidating the mechanism of aggregation-induced emission (AIE) is a prerequisite for designing more AIE-gens. The diphenylethylene (DPE) featured molecules are one of the most important AIE-gens due to their propeller structure. Three representative DPE-featured AIE-gens, triphenylethylene, cis-stilbene, and trans-stilbene, are explored via ultrafast ultraviolet/infrared (UV/IR) spectroscopy and theoretical calculations. Both experimental and computational results suggest that readily crossing conical intersections (CIs) with flexible structural evolutions in solutions significantly reduces fluorescence, whereas crossing CIs is restricted because of high energy cost, and therefore no fast nonradiative decay can compete with spontaneous emission in solids. The mechanism also well explains the different emission quantum yields and interconversion ratios between cis-stilbene and trans-stilbene after photoexcitation.

Direct observation of conformations of a high-mobility n-type low-bandgap copolymer in solutions and solid films.

The aggregation morphologies of conjugated polymers in solutions and solid films are important for their optoelectronic applications. Due to the amorphous state of the polymers, it remains a great challenge to determine their conformations in either liquids or solids. Herein, a ps/fs synchronized 2D IR technique is applied to investigate the molecular conformations of a high-mobility n-type low-bandgap copolymer, N2200, dissolved in CHCl3 and CCl4, and in solid films cast from both solutions by the vibrational cross-angle method. In CCl4, the polymer forms more aggregates and folds more and the backbone dihedral angle of C-C(NDI)/C-S(Thiophene) of its average conformation is about 10° more distorted than that in CHCl3 and the most stable conformation for a free molecule. Anti-intuitively, the solid films cast from both solutions have the same molecular conformation, and the conformation is similar to that of the polar CHCl3 rather than the conformation of the less polar CCl4. The results imply that the interaction between the polymer backbones is probably stronger than its interaction with CCl4, which can naturally guide the rearrangement of polymer chains during the evaporation of solvent molecules. This work also implies that the balance and competition between the polymer/polymer interaction and the polymer/solvent interaction seem to be the dominant factors responsible for what morphology can form in a solid film cast from solution. It is not always true that different molecular conformations must exist in solid films grown from different solutions with different polarity or different extents of aggregates with different conformations.

Double crossing conical intersections and anti-Vavilov fluorescence in tetraphenyl ethylene.

Conical intersections (CIs) provide effective fast nonradiative decay pathways for electronic excitation, which can significantly influence molecular photoluminescence properties. However, in many cases, crossing a CI does not have direct observables, making studies of CIs experimentally challenging. Herein, the theoretically predicted double CIs by cis-trans twisting and cyclization in tetraphenyl ethylene, a well-known aggregation-induced emission molecule, are investigated with excitation dependent ultrafast UV/IR spectroscopy and fluorescence. Both the fluorescence quantum yield and the efficiency of cyclization are found to be smaller with a shorter excitation wavelength. An abrupt change occurs at about 300-310 nm. The results imply that crossing the twisting CI has a larger barrier than the cyclization CI, and the cis-trans twisting motion is probably involved with large solvation reorganization.

Two-atomic-layered optoelectronic device enabled by charge separation on graphene/semiconductor interface.

The Fermi level of graphene on different substrates usually changes significantly due to the interface difference between graphene and two-dimensional semiconductors. This feature opens many possibilities of manipulating optoelectronic devices by constructing graphene heterostructures through interface modification. Herein, we report the fabrication and optoelectronic response of an unconventional heterojunction device based on a graphene-MoSe2 hybrid interface. Different from the traditional three or more layered structure where the semiconductor is sandwiched between two electrodes, this device contains only two atomic layers: the MoSe2 layer serving as the photon absorber and the graphene layer functioning as the charge acceptor and both electrodes. This structure looks like short-circuited but shows an obvious photoelectric response, which is aided by electron transfers from MoSe2 to graphene. The photocurrent generation is explored quantitatively with electronic dynamics of graphene aided with ultrafast measurements. The two-layered architecture simplifies the fabrication of atomic-thick optoelectronic devices, allowing the as-grown semiconductors to be directly used and eliminating the damage-prone transfer process.

Second-harmonic generation divergence-a method for domain size evaluation of 2D materials.

Single-atomic-layered materials are important for future electronics. They allow optoelectronic devices to be fabricated at the single-atomic layer level. A single-atomic-layered two-dimensional (2D) transition metal dichalcogenide (TMD) film is usually composed of randomly orientated single-crystalline domains, and the size distribution of the domains on a large-area film has a significant impact on the applications of the film, but the impact is difficult to characterize. We report an approach to evaluate the size of the single-crystalline domains by measuring the second-harmonic generation divergence caused by the domains of different orientations. Using this method, domain size mapping on an 8×8mm2 region of a continuous MoS2 film is achieved. This method provides a fast and efficient way of domain size characterization across a large area in a non-destructive and transfer-free manner for single-atomic-layered TMD films.

Photoluminescence of monolayer MoS2 modulated by water/O2/laser irradiation.

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding in situ mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS2. Relatively low power density (<4.08 × 105 W cm-2) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS2, whereas relatively high power density (∼1.02 × 106 W cm-2) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O2 and is more remarkable in pure O2. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS2. FET devices suggest that the adsorbed water produces a p-doping effect on MoS2, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed H2O on the surface, thus reducing the PL.

Structural analysis of transient reaction intermediate in formic acid dehydrogenation catalysis using two-dimensional IR spectroscopy.

The molecular structure of a catalytically active key intermediate is determined in solution by employing 2D IR spectroscopy measuring vibrational cross-angles. The formate intermediate (2) in the formic acid dehydrogenation reaction catalyzed by a phosphorus-nitrogen PN3P-Ru catalyst is elucidated. Our spectroscopic studies show that the complex features a formate ion directly attached to the Ru center as a ligand, and a proton added to the imine arm of the dearomatized PN3P* ligand. During the catalytic process, the imine arms are not only reversibly protonated and deprotonated, but also interacting with the protic substrate molecules, effectively serving as the local proton buffer to offer remarkable stability with a turnover number (TON) over one million.

Synthesis of Lactams via Ir-Catalyzed C-H Amidation Involving Ir-Nitrene Intermediates.

x-membered lactams were synthesized via either an amidation of sp3 C-H bonds or an electrophilic substitution of arenes via Ir-nitrene intermediates. With the employment of a readily available iridium catalyst in dichloromethane or hexafluoro-2-propanol, a wide range of lactams were synthesized in good to excellent yields with high selectivity.

Direct Observation of Aggregation-Induced Emission Mechanism.

The mechanism of aggregation-induced emission, which overcomes the common aggregation-caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward-Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Wide-Range Color-Tunable Organic Phosphorescence Materials for Printable and Writable Security Inks.

Organic materials with long-lived, color-tunable phosphorescence are potentially useful for optical recording, anti-counterfeiting, and bioimaging. Herein, we develop a series of novel host-guest organic phosphors allowing dynamic color tuning from the cyan (502 nm) to orange red (608 nm). Guest materials are employed to tune the phosphorescent color, while the host materials interact with the guest to activate the phosphorescence emission. These organic phosphors have an ultra-long lifetime of 0.7 s and a maximum phosphorescence efficiency of 18.2 %. Although color-tunable inks have already been developed using visible dyes, solution-processed security inks that are temperature dependent and display time-resolved printed images are unprecedented. This strategy can provide a crucial step towards the next-generation of security technologies for information handling.

Intermolecular energy flows between surface molecules on metal nanoparticles.

Three model systems are designed to investigate energy transport between molecules on metal nanoparticle surfaces. Energy is rapidly transferred from one carbon monoxide (CO) molecule to another CO molecule or an organic molecule on adjacent surface sites of 2 nm Pt particles within a few picoseconds. On the contrary, energy flow from a surface organic molecule to an adjacent CO molecule is significantly slower and, in fact, within experimental sensitivity and uncertainty the transfer is not observed. The energy transport on particle surfaces (about 2 km s-1) is almost ten times faster than inside a molecule (200 m s-1). The seemingly perplexing observations can be well explained by the combination of electron/vibration and vibration/vibration coupling mechanisms, which mediate molecular energy dynamics on metal nanoparticle surfaces: the strong electron/vibration coupling rapidly converts CO vibrational energy into heat that can be immediately sensed by nearby molecules; but the vibration/vibration coupling dissipates the vibrational excitation in the organic molecule as low-frequency intramolecular vibrations that may or may not couple to surface electronic motions.

A Pseudodearomatized PN3P*Ni-H Complex as a Ligand and σ-Nucleophilic Catalyst.

In contrast to the conventional strategy of modifying the reactivities and selectivities of the transition metal and organocatalysts by varying the steric and electronic properties of organic substituent groups, we hereby demonstrate a novel approach that the sigma (σ) nucleophilicity of the imine arm can be significantly enhanced in a pseudodearomatized PN3P* pincer ligand platform to reach unprecedented N-heterocyclic carbene-like reactivity. Accordingly, the imine arm of the PN3P*Ni-H pincer complex efficiently catalyzes the hydrosilylation of aldehydes, cycloaddition of carbon dioxide (CO2) to epoxides, and serves as a ligand in the Ru-catalyzed dehydrogenative acylation of amines with alcohols.

Water-Mediated Ion Pairing: Occurrence and Relevance.

We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li(+) and Na(+) and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems.

Dehydrogenation of Formic Acid Catalyzed by a Ruthenium Complex with an N,N'-Diimine Ligand.

We report a ruthenium complex containing an N,N'-diimine ligand for the selective decomposition of formic acid to H2 and CO2 in water in the absence of any organic additives. A turnover frequency of 12 000 h-1 and a turnover number of 350 000 at 90 °C were achieved in the HCOOH/HCOONa aqueous solution. Efficient production of high-pressure H2 and CO2 (24.0 MPa (3480 psi)) was achieved through the decomposition of formic acid with no formation of CO. Mechanistic studies by NMR and DFT calculations indicate that there may be two competitive pathways for the key hydride transfer rate-determining step in the catalytic process.

Solvation structure around the Li(+) ion in succinonitrile-lithium salt plastic crystalline electrolytes.

Herein, we discuss the study of solvation dynamics of lithium-succinonitrile (SN) plastic crystalline electrolytes by ultrafast vibrational spectroscopy. The infrared absorption spectra indicated that the CN stretch of the Li(+) bound and unbound succinonitrile molecules in a same solution have distinct vibrational frequencies (2276 cm(-1)vs. 2253 cm(-1)). The frequency difference allowed us to measure the rotation decay times of solvent molecules bound and unbound to Li(+) ion. The Li(+) coordination number of the Li(+)-SN complex was found to be 2 in the plastic crystal phase (22 °C) and 2.5-3 in the liquid phase (80 °C), which is independent of the concentration (from 0.05 mol kg(-1) to 2 mol kg(-1)). The solvation structures along with DFT calculations of the Li(+)-SN complex have been discussed. In addition, the dissociation percentage of lithium salt was also determined. In 0.5 mol kg(-1) LiBF4-SN solutions at 80 °C, 60% ± 10% of the salt dissociates into Li(+), which is bound by 2 or 3 solvent molecules. In the 0.5 mol kg(-1) LiClO4-SN solutions at 80 °C, the salt dissociation ratio can be up to 90% ± 10%.

Nonresonant energy transfers independent on the phonon densities in polyatomic liquids.

Energy-gap-dependent vibrational-energy transfers among the nitrile stretches of KSCN/KS(13)CN/KS(13)C(15)N in D2O, DMF, and formamide liquid solutions at room temperature were measured by the vibrational-energy-exchange method. The energy transfers are slower with a larger energy donor/acceptor gap, independent of the calculated instantaneous normal mode ("phonons" in liquids) densities or the terahertz absorption spectra. The energy-gap dependences of the nonresonant energy transfers cannot be described by phonon compensation mechanisms with the assumption that phonons are the instantaneous normal modes of the liquids. Instead, the experimental energy-gap dependences can be quantitatively reproduced by the dephasing mechanism. A simple theoretical derivation shows that the fast molecular motions in liquids randomize the modulations on the energy donor and acceptor by phonons and diminish the phonon compensation efficiency on energy transfer. Estimations based on the theoretical derivations suggest that, for most nonresonant intermolecular vibrational-energy transfers in liquids with energy gaps smaller than the thermal energy, the dephasing mechanism dominates the energy-transfer process.

Electron-phonon interactions in MoS2 probed with ultrafast two-dimensional visible/far-infrared spectroscopy.

An ultrafast two-dimensional visible/far-IR spectroscopy based on the IR/THz air biased coherent detection method and scanning the excitation frequencies is developed. The method allows the responses in the far-IR region caused by various electronic excitations in molecular or material systems to be observed in real time. Using the technique, the relaxation dynamics of the photo-excited carriers and electron/phonon coupling in bulk MoS2 are investigated. It is found that the photo-generation of excited carriers occurs within two hundred fs and the relaxation of the carriers is tens of ps. The electron-phonon coupling between the excitations of electrons and the phonon mode E1u of MoS2 is also directly observed. The electron excitation shifts the frequency of the phonon mode 9 cm(-1) higher, resulting in an absorption peak at 391 cm(-1) and a bleaching peak at 382 cm(-1). The frequency shift diminishes with the relaxation of the carriers.