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We carried out transient absorption spectroscopy of thioflavin T (ThT) molecules in various solvents employing an asynchronous optical sampling (ASOPS) scheme with dual synchronized and frequency up-converted mode-lock lasers in the near UV (NUV) spectral region. We developed a pair of synchronized femtosecond lasers with tunable center wavelengths ranging from 380 to 430 nm and spectral bandwidths of 30 nm. As a proof-of-principle experiment, we measured interferometrically detected time and frequency-resolved pump–probe signals of ThT in various solvents to study the twisted intramolecular charge transfer process of photo-excited ThT molecules. Both single-color NUV-NUV and two-color NUV-near IR (NIR) pump–probe measurements reveal that the vibronic coupling strengths of two vibrational modes with frequencies of 214 and 526 cm−1 in the excited state of ThT are reduced when ThT is dissolved in a chlorine-containing solvent, e.g., chloroform. We confirm theoretically that these vibrational modes have relatively high electric dipole moments in the excited state. As a result, the intramolecular charge transfer process of ThT in chloroform, which is driven by the solvation process of surrounding polar solvent molecules, could occur less efficiently, which results in an increase in the fluorescence quantum yield. Here, we demonstrate that the NUV-NUV and NUV-NIR ASOPS-transient absorption could be useful techniques for studying ultrafast photochemical reactions in condensed phases. [ABSTRACT FROM AUTHOR]

The deactivation process of adenine excited near the band origin of the lowest ππ* state (1Lb) is investigated using picosecond (ps) time-resolved photoionization spectroscopy. The transients obtained with a ps pump pulse at the sharp vibronic bands, 36 105 cm-1 (D) and 36 248 cm-1 (E), in the resonant two-photon ionization spectrum exhibit a bi-exponential decay with two distinct time constants of τ1 ∼ 2 ps and τ2 > 100 ps, whereas the transients with the pump at other wavenumbers in this energy region show a single exponential decay with τ = 1-2 ps. We suggest that the τ1 represents the lifetimes of the 1nπ* energy levels near the D and E peaks, which are excited together by the ps pump pulse having a broad spectral bandwidth, and the τ2 shows the lifetimes of D and E peaks. The long lifetime of D level is attributed to a small barrier for internal conversion from the minimum of the 1Lb state to the 1nπ* state. On the other hand, the long lifetime of E level is ascribed to the nuclear configuration of adenine at this level, which is unfavorable to reach the seam of the conical intersection leading to nearly barrierless deactivation to the electronic ground state. This study shows that the ps time-resolved spectroscopy provides a powerful tool to study mode- and energy-specific deactivation processes occurring in a multi-dimensional potential energy surface. [ABSTRACT FROM AUTHOR]

ZnO nanoparticles with different shapes and sizes were prepared by changing coordinating ligands. Hexagonal cones presented UV and green (∼500 nm) emissions, which originated from excitons and defects, respectively. Oxygen vacancies were found to be major defects in the hexagonal cones. Blue emission at ∼440 nm was observed for hexagonal plates, and analyses of time-resolved photoluminescence spectra showed that two transitions were responsible for this blue emission, because transitions from Zni to the valence band (430 nm) and from Zni to VZn (480 nm) were distinguished by emission wavelengths and lifetimes. The visible emissions from defects were related to the roles of coordinating ligands. [ABSTRACT FROM AUTHOR]

An oxygen-vacancy rich, bismuth oxyiodide-based Z-scheme 3D hierarchical MoS2/BiOI/AgI ternary nanocomposite photocatalyst was fabricated using a simple precipitation process in ethylene glycol and water. The presence of oxygen-vacancies in BiOI and the two-dimensional nature of molybdenum disulfides in the composite prolongs the charge carrier lifetime through a Z-scheme system and enhances the performance of the photocatalyst for the degradation of rhodamine B. On the basis of efficient separation of photoexcited electron–hole pairs, a mechanism is proposed whereby MoS2 and oxygen vacancy states increase charge carrier lifetimes and improve the photocatalytic activity. The Z-scheme mechanism of the photocatalysis is consistent with the results of static and time-resolved photoluminescence, scavenging, and terephthalic acid photoluminescence experiments. Among the as-synthesized photocatalysts, the one containing 2 wt% of MoS2 in a composite of MoS2/BiOI/AgI exhibited the highest photocatalytic activity towards rhodamine B degradation, and its activity was 7 and 16 times higher than that of BiOI/AgI and BiOI, respectively. Degradation of phenol, the colorless model pollutant, was studied to confirm the visible-light photocatalytic performance of the MoS2/BiOI/AgI composite. This easily fabricated Z-scheme based MoS2/BiOI/AgI composite exhibits promising photocatalytic activity and will be useful for potential applications in energy and environmental areas. [ABSTRACT FROM AUTHOR]

The photophysical properties of CuInxGa1−xS2 (CIGS) thin films, prepared by solution-based coating methods, are investigated to understand the correlation between the optical properties of these films and the electrical characteristics of solar cells fabricated using these films. Photophysical properties, such as the depth-dependent band gap and carrier lifetime, turn out to be at play in determining the energy conversion efficiency of solar cells. A double grading of the band gap in CIGS films enhances solar cell efficiency, even when defect states disturb carrier collection by non-radiative decay. The combinational stacking of different density films leads to improved solar cell performance as well as efficient fabrication because a graded band gap and reduced shunt current increase carrier collection efficiency. The photodynamics of minority-carriers suggests that the suppression of defect states is a primary area of improvement in CIGS thin films prepared by solution-based methods. [ABSTRACT FROM AUTHOR]

Abstract: Antimony (Sb) doping of SnO2 nanowires (NWs) was investigated for its optical and electrical effects. The low-temperature photoluminescence spectra of SnO2 NWs varied significantly with increasing Sb content, where the temperature-dependence of the visible emission at ca. 400nm was distinctive with Sb-doping, indicating different defect states, such as neutral and positively charged oxygen vacancies. Field effect transistors (FETs) with low-level Sb-doped SnO2 NW channels exhibited higher mobility, charge concentration, and faster response and recovery to UV light than intrinsic SnO2 NW FETs. [Copyright &y& Elsevier]

Mid-infrared (mid-IR) spectroscopy is an incisive tool for studying structures and dynamics of complicated molecules in condensed phases. Developing a compact and broadband mid-IR spectrometer has thus been a long-standing challenge. Here, we show that a highly coherent and broadband mid-IR frequency comb can be generated by using an intrapulse difference-frequency-generation with a train of pulses from a few-cycle pulse Ti:sapphire oscillator. By tightly focusing the oscillator output beam into a single-pass, fan-out-type periodically poled lithium niobate crystal and tilting the orientation of the crystal, we show that a mid-IR frequency comb with more than an octave spectral bandwidth from 1550 cm -1 (46 THz) to 3650 cm -1 (110 THz) and vanishing carrier-envelope-offset phase can be generated. Using two coherent mid-IR frequency combs with different repetition frequencies, we demonstrate that a broadband mid-IR dual-frequency comb spectroscopy of aromatic compounds or amino acids in solutions is feasible. We thus anticipate that researchers will find our mid-IR frequency combs useful for developing ultrafast and broadband linear and nonlinear IR spectroscopy of chemically reactive or biologically important molecules in condensed phases.

The photoluminescence of AgIn 5 S 8 nanoparticles was examined to clarify the emissive relaxation processes of defect states and to explain the highly efficient photoluminescence of defect states. The large Stokes shift of the defect emission was explained by strong electron-phonon coupling in the nanoparticles. Steady-state and time-resolved photoluminescence spectroscopy indicated two emissive defect states with characteristic emission energies and lifetimes. Change of the surface-to-volume ratio in the nanoparticles affected the relative contribution of the two states, implying that defect emission in higher energy was attributable to surface-related defects. The defect emission in lower energy was attributable to intrinsic defects, which were also present in bulk. The quantum yield of the surface defects was larger than that of the intrinsic defects, which accounted for the unusually high quantum yield of AgIn 5 S 8 nanoparticles, although the origin of emission was the defect states, not the exciton recombination found in typical semiconductor nanoparticles.

To more accurately assess the pathways of biological systems, a probe is needed that may respond selectively to adenosine triphosphate (ATP) for both in vitro and in vivo detection modes. We have developed a luminescence probe that can provide real-time information on the extent of ATP, ADP, and AMP by virtue of the luminescence and luminescence lifetime observed from a supramolecular polymer based on a C 3 symmetrical terpyridine complex with Tb 3+ (S1-Tb). The probe shows remarkable selective luminescence enhancement in the presence of ATP compared to other phosphate-displaying nucleotides including adenosine diphosphate (ADP), adenosine monophosphate (AMP), guanosine triphosphate (GTP), thymidine triphosphate (TTP), H 2 PO 4 - (Pi), and pyrophosphate (PPi). In addition, the time-resolved luminescence lifetime and luminescence spectrum of S1-Tb could facilitate the quantitative measurement of the exact amount of ATP and similarly ADP and AMP within living cells. The time-resolved luminescence lifetime of S1-Tb could also be used to quantitatively monitor the amount of ATP, ADP, and AMP in vitro following the enzymatic hydrolysis of ATP. The long luminescence lifetime, which was observed into the millisecond range, makes this S1-Tb-based probe particularly attractive for monitoring biological ATP levels in vivo, because any short lifetime background fluorescence arising from the complex molecular environment may be easily eliminated.

An oxygen-vacancy rich, bismuth oxyiodide-based Z-scheme 3D hierarchical MoS 2 /BiOI/AgI ternary nanocomposite photocatalyst was fabricated using a simple precipitation process in ethylene glycol and water. The presence of oxygen-vacancies in BiOI and the two-dimensional nature of molybdenum disulfides in the composite prolongs the charge carrier lifetime through a Z-scheme system and enhances the performance of the photocatalyst for the degradation of rhodamine B. On the basis of efficient separation of photoexcited electron-hole pairs, a mechanism is proposed whereby MoS 2 and oxygen vacancy states increase charge carrier lifetimes and improve the photocatalytic activity. The Z-scheme mechanism of the photocatalysis is consistent with the results of static and time-resolved photoluminescence, scavenging, and terephthalic acid photoluminescence experiments. Among the as-synthesized photocatalysts, the one containing 2 wt% of MoS 2 in a composite of MoS 2 /BiOI/AgI exhibited the highest photocatalytic activity towards rhodamine B degradation, and its activity was 7 and 16 times higher than that of BiOI/AgI and BiOI, respectively. Degradation of phenol, the colorless model pollutant, was studied to confirm the visible-light photocatalytic performance of the MoS 2 /BiOI/AgI composite. This easily fabricated Z-scheme based MoS 2 /BiOI/AgI composite exhibits promising photocatalytic activity and will be useful for potential applications in energy and environmental areas.