Ya-Wei Hao

Time-resolved fluorescence study of aggregation-induced emission enhancement by restriction of intramolecular charge transfer state.

Cyano-substituted oligo (alpha-phenylenevinylene)-1,4-bis(R-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (CNDPASDB) molecules are studied in solution and aggregate state by time-resolved fluorescence techniques. CNDPASDB exhibits a strong solvent polarity dependent characteristic of aggregation-induced emission (AIE). By time-dependent spectra, the gradual transition from local excited state to intramolecular charge transfer state with the increasing solvent polarity is clearly resolved. The transition time in high polarity solvent DMF is very fast, around 0.5 ps, resulting in a low fluorescence quantum yield. While in aggregate state, the intramolecular torsion is restricted and the local environment becomes less polar. Thus, the intramolecular charge transfer state is eliminated and efficient AIE occurs.

Hybrid‐State Dynamics of Gold Nanorods/Dye J‐Aggregates under Strong Coupling

The interactions often appear as two oppositecases, namely weak and strong couplings. In the weakcoupling regime, wavefunctions of molecules and SP modesof plasmons are considered to be unperturbed. The strongcoupling or coherent coupling occurs when resonant exciton–plasmon interactionsmodifymolecularwavefunctions andSPmodes, whereby the excitation energy is shared and oscillatesbetween the plasmonic and molecular systems (Rabi oscil-lations), leading to vacuum Rabi splitting of energy levels atthe resonance frequency. This is similar to behaviors ofpolaritons in an optical microcavity.

The Role of Trap-assisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots

Hybrid metal halide perovskites have been paid enormous attentions in photophysics research, whose excellent performances were attributed to their intriguing charge carriers proprieties. However, it still remains far from satisfaction in the comprehensive understanding of perovskite charge-transport properities, especially about trap-assisted recombination process. In this Letter, through time-resolved transient absorption (TA) and photoluminescence (PL) measurements, we provided a relative comprehensive investigation on the charge carriers recombination dynamics of CH3NH3PbBr3 (MAPbBr3) perovskite films and quantum dots (QDs), especially about trap-assisted recombination. It was found that the integral recombination mode of MAPbBr3 films was highly sensitive to the density distribution of generated charge carriers and trap states. Additional, Trap effects would be gradually weakened with elevated carrier densities. Furthermore, the trap-assisted recombination can be removed from MAPbBr3 QDs through its own surface passivation mechanism and this specialty may render the QDs as a new material in illuminating research. This work provides deeper physical insights into the dynamics processes of MAPbBr3 materials and paves a way toward more light-harvesting applications in future.

Role of Protein Dynamics in Guiding Electron-Transfer Pathways in Reaction Centers from Rhodobacter sphaeroides

The role of protein dynamics in guiding multistep electron transfer is explored in the photosynthetic reaction center of Rhodobacter sphaeroides . The energetics of the charge-separated intermediates, P(+)B(A)(-) and P(+)H(A)(-) (P is the initial electron donor bacteriochlorophyll pair and B(A) and H(A) are early bacteriochlorophyll and bacteriopheophytin acceptors, respectively), were systematically varied in a series of mutants. A fast phase of P(+)H(A)(-) recombination was resolved that is very sensitive to driving force. Either increasing or decreasing the relative free energy of P(+)H(A)(-) resulted in a more prominent fast recombination component, and thus a decreased yield forward electron transfer. The fast phase apparently represents P(+)H(A)(-) charge recombination via an activated state, probably P(+)B(A)(-) (B(A) is situated between P and H(A)). In wild type, this activated state is largely inaccessible, presumably due to dynamic stabilization of P(+)H(A)(-) within the first 100 ps. In mutants that change the energetics, the rate of decay via the activated state accelerates and that pathway becomes significant. The dynamic stabilization of the protein makes it possible to achieve a nearly optimum environment of H(A) in wild type on two different time scales and for two rather different reactions. On the picosecond time scale, the energetics is nearly, though not perfectly, optimized for transfer between the excited state of P and H(A). After dynamic stabilization of the state P(+)H(A)(-), the environment is optimized to avoid rapid recombination of the charge-separated state and instead carry out forward electron transfer to the quinone with very high yield on the hundreds of picosecond time scale. Thus, by employing protein dynamics, the reaction center is able to optimize multiple reactions, on very different time scales involving the same reaction intermediate.

Nanoporous TiO2/polyion thin-film-coated long-period grating sensors for the direct measurement of low-molecular-weight analytes.

We present novel nanoporous TiO(2)/polyion thin-film-coated long-period fiber grating (LPFG) sensors for the direct measurement of low-molecular-weight chemicals by monitoring the resonance wavelength shift. The hybrid overlay films are prepared by a simple layer-by-layer deposition approach, which is mainly based on the electrostatic interaction of TiO(2) nanoparticles and polyions. By the alternate immersion of LPFG into dispersions of TiO(2) nanoparticles and polyions, respectively, the so-formed TiO(2)/polyion thin film exhibits a unique nanoporous internal structure and has a relative higher refractive index than LPFG cladding. In particular, the porosity of the thin film reduces the diffusion coefficient and enhances the permeability retention of low-molecular-weight analytes within the porous film. The increases in the refractive index of the LPFG overlay results in a distinguished modulation of the resonance wavelength. Therefore, the detection sensitivity of LPFG sensors has been greatly improved, according to theoretical simulation. After the structure of the TiO(2)/polyion thin film was optimized, glucose solutions as an example with a low concentration of 10(-7) M was easily detected and monitored at room temperature.

Surface plasmon enhanced absorption dynamics of regioregular poly(3-hexylthiophene)

An increased absorption of regioregular poly(3-hexylthiophene) due to surface plasmons (SPs) has been studied by steady state fluorescence spectra measurements and femtosecond transient absorption (TA) technique. The SPs were excited by periodic Ag grating structures transferred from the photoresist gratings which were fabricated by two-beam laser interference. The steady state fluorescence measurements show a maximum enhanced fluorecence of about 16 times larger arising from the absorption enhancement. Further investigation by TA technique reveals that the yield of the polaron pairs on Ag gratings significantly increase, much bigger than that of the pure sample, which also confirms the enhanced absorption efficiency as resulting from the excited SPs.

Strong coupling in hybrid plasmon-modulated nanostructured cavities

The understanding of strong coupling between local restricted electromagnetic field of cavity and surface-plasmon (SP) on the metal surface lays the groundwork for many photonic applications. However, much of the fundamental photophysical properties underlying this performance such as how this strong coupling is induced by these two components, namely, the SP modes and cavity modes have remained unknown. Here, we present a study of a hybrid plasmon-modulated coupled system with Rabi splitting energy at 336 meV, and the coupled hybrid states are highly sensitive to the prosperity of SP mode which is induced by morphology of inlaid grating surfaces.

Surface Plasmon-Modulated Fluorescence on 2D Metallic Silver Gratings

The emitting properties of fluorophores can be tailored by surface plasmon polaritons (SPPs), which offers the potential to understand the light-matter interactions at the nanoscale, and lays the groundwork for many future applications. As one of the useful features in SPPs-surface plasmon-coupled emission (SPCE) based on the metal gratings; however, the study on its underlying modulation mechanism remains far from satisfactory. That is, how it is affected by the morphology of the grating surfaces. In this letter, we have addressed this question in the dye-coated corrugated silver gratings that possess different periods and depths. It is found that the properties of SPCE are strongly dependent on the dispersion relations of SPPs modulated by the grating periods.

Investigation of Polaron Pair Dynamics in Poly(3-Hexylthiophene) Film by Time Resolved Spectroscopy

Polaron pair dynamics in regioregular Poly (3-hexylthiophene) (RR-P3HT) film was studied by applying both femtosecond transient absorption and fluorescence up-conversion techniques. The comparison of the dynamics of the pure fluorescence, the ground state bleaching recovery and the polaron pair absorption reveals that the polaron pairs decay back to the ground state of certain morphological places with distinct absorption features, instead of the singlet excited state.

Evidence of concerted inversion for the photon-induced molecular switching of azobenzene using rotation-free azobenzene derivatives

The excited state relaxation of two types of azobenzene derivative excited at 350 nm in the short-wavelength (S2(π, π*)) band was studied by transient absorption spectroscopy. We compared the relaxation dynamics and the time-resolved anisotropic relaxation properties of these two molecules in liquid solution and in bulk film. The average relaxation life-time for Azo-2 in bulk film (13.2 ps) is much slower than that of Azo-2 (4.06 ps) and Azo-1(1.31 ps) in liquid solution. The time-resolved polarization anisotropy spectroscopy measurements of these samples also showed significant differences. The value of anisotropy for Azo-2 remained almost constant at about 0.2 in bulk film but decayed slowly from 0.3 in liquid solution. The value for Azo-1 in liquid solution decayed very fast from 0.4 to 0.2 within 6 ps. Our results gave new evidence for the existence of a converted inversion channel. We also concluded that the concerted inversion process and the conventional rotation process govern the relaxation for the photon-induced switches of azobenzene and its derivatives under short-wavelength band excitation.