Simulation of Intermediate State Absorption Enhancement in Rare-Earth Ions by Polarization Modulated Femtosecond Laser Field

We extend the third perturbation theory to study the polarization control behavior of the intermediate state absorption in Nd^（3＋）ions. The results show that coherent interference can occur between the single-photon and three-photon excitation pathways, and depends on the central frequency of the femtosecond laser field. Moreover,single-photon and three-photon absorptions have different polarization control efficiencies, and the relative weight of three-photon absorption in the whole excitation processes can increase with increasing the laser intensity.Therefore, the enhancement or suppression of the intermediate state absorption can be realized and manipulated by properly designing the intensity and central frequency of the polarization modulated femtosecond laser field.This research can not only enrich theoretical research methods for the up-conversion luminescence manipulation of rare-earth ions, but also can provide a clear physical picture for understanding and controlling multi-photon absorption in a multiple energy level system.

Ultrafast Excited State Dynamics of Biliverdin Dimethyl Ester Coordinate with Zinc Ions

As one of the biological endogenous pigments,biliverdin(BV)and its dimethyl ester(BVE)have extremely weak uorescence in solution with quantum yield less than 0.01%.However,the situation reverses with the addition of zinc ions.The strength for uorescence of BVE-Zn^2+ complex is greatly enhanced and uorescence quantum yield can increase to5%.Herein,we studied ultrafast excited state dynamics of BVE-Zn^2+ complex in ethanol,npropanol,and DMSO solutions in order to reveal the mechanism of uorescence quantum yield enhancement.The results show that BVE can form a stable coordination complex with zinc with 1:1 stoichiometry in solution.BVE is structurally and energetically more stable in the complex.Using picosecond time-resolve uorescence and femtosecond transient absorption spectroscopy,we show that smaller non-radiative rate constant of BVE-Zn^2+ complex in DMSO is the key to increasing its uorescence quantum yield and the excited state decay mechanism is also revealed.These results provide valuable information about the uorescence property change after BVE binding to metal ions and may provide a guidance for the study of phytochromes or other uorescence proteins in which BV/BVE acts as chromophores.

Ultrafast proton transfer dynamics of 2-(2'-hydroxyphenyl)benzoxazole dye in different solvents

The excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzoxazole dye in different solvents is investigated using ultrafast femtosecond transient absorption spectroscopy combined with quantum chemical calculations.Conformational conversion from the syn-enol configuration to the keto configuration is proposed as the mechanism of excited-state intramolecular proton transfer.The duration of excited-state intramolecular proton transfer is measured to range from 50 fs to 200 fs in different solvents.This time is strongly dependent on the calculated energy gap between the N-S;and T-S;structures in the S;state.Along the proton transfer reaction coordinate,the vibrational relaxation process on the S;state potential surface is observed.The duration of the vibrational relaxation process is determined to be from8.7 ps to 35 ps dependent on the excess vibrational energy.

2D/2D covalent organic framework/CdS Z-scheme heterojunction for enhanced photocatalytic H_(2) evolution:Insights into interfacial charge transfer mechanism

Covalent organic frameworks(COFs)with high crystallinity and flexible designability have been consid-ered as promising candidates for photocatalytic hydrogen evolution.However,the existence of unpropi-tious exciton effects in COFs leads to poor charge separation,and thus results in low photocatalytic effi-ciency.Herein,to improve the photoelectron migration efficiency,we designed a 2D/2D organic/inorganic direct Z-scheme COF-based heterojunction(TpTAP/CdS),by the in-situ growing of CdS nanosheets on the COF copolymerized via 2,4,6-tris(4-aminophenyl)-1,3,5-triazine(TAP)and 1,3,5-triformylphloroglucinol(Tp).The femtosecond transient absorption(fs-TA)decay kinetics of TpTAP-COF and TpTAP/CdS further reveal the processes of shallow electron trapping and the recombination of the free photogenerated electron-hole pairs.In particular,the transient absorption traces for TpTAP-COF and TpTAP/CdS normal-ized to the photoinduced absorption peak can effectively verify the Z-scheme charge transfer between TpTAP-COF and CdS,which could enhance the charge mobility and separation,thus reducing the pho-tocorrosion of CdS.Additionally,ultraviolet photoelectron spectroscopy(UPS),in-situ X-ray photoelec-tron spectroscopy(XPS),transient photovoltage measurements,and electron spin resonance(ESR)spec-troscopy further confirm the establishment of the internal electric field(IEF).This work demonstrates the important role of COFs in the construction of 2D/2D organic/inorganic direct Z-scheme heterojunctions and offers a new avenue to explain the criticality of dynamics of the photogenerated carriers for the construction of Z-scheme heterojunctions.

Ultrafast Photoinduced Electron Transfer in a Photosensitizer Protein

Photosystem Ⅱ(PSⅡ)is a large membrane protein(∼700 kDa)complex,harboring P680+,the strongest oxidant known in biological systems,which is responsible for driving tyrosine oxidation and ultimately O_(2) generation.While the enhancement and expansion of PSⅡ functions through genetic engineering would be beneficial for driving challenging chemical reactions,this has proven difficult due to its enormous complexity.Here,we report a genetically encoded,27 kDa photosensitizer protein(PSP3)that recapitulates the initial photoinduced key properties of PSⅡ.Through the genetic incorporation of benzophenone-alanine(BpA)into a fluorescent protein coupled with femtosecond transient absorption measurement,we show that photoinduced electron transfer from residue Tyr203 to the PSP3 chromophore occurs very rapidly(∼1 ps),which is comparable with that of the first electron transfer step in PSII.Since PSP3 can be overexpressed in high yield in Escherichia coli and genetically engineered easily,it might facilitate challenging oxidation and reduction reactions in vitro and in vivo.

Dual excited state deactivation pathways in TPZ2:A centrosymmetric dye with both high fluorescence and triplet state quantum yield

In this letter, excited state dynamics of TPZ2, a centrosymmetric PRODAN dye, has been studied by using several time-resolved spectroscopy techniques. Fluorescence quantum yield of TPZ2 is found to be 0.50 in both acetonitrile and ethanol solution. The radiative decay rate of the excited state of TPZ2 is determined to be 2.0×10^8 s^-1. Meanwhile, highly efficient triplet state and singlet oxygen generation have been observed in TPZ2 and the intersystem crossing（ISC） rate is determined to be 2.0×10^8s^-1. The almost identical ISC and non-radiative decay rates indicate that ISC is the only non-radiative decay pathway in TPZ2. Thus, dual excited state（S1） deactivation mechanism（50/50, fluorescence/ISC） of TPZ2 is proposed.Because of this unique property, TPZ2 has the potential to be used as biocompatible imaging and photodynamic therapy agent in the same time.

Effect of thickness on the photophysics and charge carrier kinetics of graphitic carbon nitride nanoflakes

We investigated the thickness effect on the photophysics and charge carrier kinetics of graphitic carbon nitride nanoflakes （g-CNN） by using ultraviolet visible diffuse reflectance spectroscopy, atomic force microscopy, femtosecond transient absorption spectroscopy, and picosecond time-correlated single photon counting measurement. For the first time, we found that g-CNN displays a layer-dependent indirect bandgap and layer-dependent charge carrier kinetics.