Ying-Hui Wang

Color-Switchable Electroluminescence of Carbon Dot Light-Emitting Diodes

Carbon-dot based light-emitting diodes (LEDs) with driving current controlled color change are reported. These devices consist of a carbon-dot emissive layer sandwiched between an organic hole transport layer and an organic or inorganic electron transport layer fabricated by a solution-based process. By tuning the device structure and the injecting current density (by changing the applied voltage), we can obtain multicolor emission of blue, cyan, magenta, and white from the same carbon dots. Such a switchable EL behavior with white emission has not been observed thus far in single emitting layer structured nanomaterial LEDs. This interesting current density-dependent emission is useful for the development of colorful LEDs. The pure blue and white emissions are obtained by tuning the electron transport layer materials and the thickness of electrode.

Identification of H-aggregate in a monolayer amphiphilic porphyrin–TiO2 nanoparticle heterostructure assembly and its influence on the photoinduced charge transfer

Abstract The spectral properties of the monolayer 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin molecule (TriHHP)–TiO 2 nanoparticle heterostructure assembly were investigated with absorption, fluorescence emission and excitation spectra, in comparison with those of CHCl 3 solution and monolayer films deposited from pH 3 and pH 10 water subphases. A new fluorescence emission band, arising from a blue-shifted Q-absorption band, was observed in the assembly or the monolayer film deposited from pH 3 water subphase. An H-aggregate type species from the protonated TriHHP molecules was assigned to cause this new emission band. According to the Frontier orbital theory, this H-aggregate is suggested in the configuration of face-to-face π–π stacking interaction among the protonated TriHHP molecules. In the heterostructure assembly, the efficient fluorescence emission quenching of TriHHP by the photoinduced charge transfer occurs under excitation in Soret band region of the unprotonated TriHHP molecules. No contribution to the photoinduced charge transfer.

Manipulating fluorescence characteristics of conjugated fluorescent molecules incorporated into three-dimensional poly(methyl methacrylate) opal photonic crystals

We compare the fluorescence characteristics of conjugated molecules (coumarin 6, C-6) in solution, aggregation state, and poly(methyl methacrylate) PMMA opal photonic crystals (PCs), and investigate the manipulation mechanism of PCs on C-6. We find that the PCs effectively inhibit the aggregate-induced fluorescence quenching of C-6 by breaking the intermolecular interaction and manipulate the fluorescence characteristics of C-6 by changing the photonic stop band (PSB). The emission traces of C-6 have been analyzed using the continuous rate distribution model. The control of fluorescence characteristics of C-6 in PCs depends on the mesh microporous structure and the PSB of PCs.

Investigation on Photophysical Properties of D–π–A–π–D‐Type Fluorenone‐Based Linear Conjugated Oligomers by Using Femtosecond Transient Absorption Spectroscopy

The effects of π‐spacer and electron donor groups on the photophysical behaviors of fluorenone‐based linear conjugated oligomers have been systemically investigated. Solvent‐dependent steady‐state measurements exhibit that the fluorene vinylene (FV) spacer and the electron‐donating ability of donor group are able to modulate the spectral features of oligomers and the fluorescence quantum yield could decrease with the increasing of the solvent polarity. Meanwhile, quantum chemical calculation simulates their absorption spectra, and analyzes their electron transition components simultaneously. The transient absorption measurements focus on the photoexcitation dynamics of these oligomers in the toluene solution, which show that an intramolecular charge transfer state exists in the relaxation process of excited states, and its generation process could accelerate with the introduction of FV spacer and the enhancement of donor strength.

Linear and Nonlinear Optical Properties of Novel Multi-branched Oligomers

We investigate the fluorene-vinylene unit dependent photo-physical properties of multi-branched truxene based oligomers (Tr-OFVn, n=1–4) employing steady-state absorption and emission spectroscopy, transient absorption spectroscopy, two-photon fluorescence, and z-scan technique. The results show that the increasing of fluorene-vinylene unit leads to a red-shift in the spectra of absorption and fluorescence, and shortens the excited state lifetime. Meanwhile, two-photon fluorescence efficiency and two-photon absorption cross section of truxene based oligomers gradually enhance in company with the extension of π-conjugated length. In addition, the values of two-photon absorption cross section modeled on the sum-over-state approach agree well with the experimental ones. The results indicate multi-branched truxene based oligomers bearing oligo(fluorene-vinylene) arms are promising organic materials for two-photon applications.

Investigation on Excited-State Photophysical Characteristics of Low Bandgap Polymer APFO3

The excited state photophysics of low bandgap polymer APFO3 has been investigated in detail. The chemical calculations confirm that the intrachain charge transfer (ICT) may occur after photo-excitation and is mainly responsible for the first absorption band. The transient absorption results confirm that ICT indeed exists and competes with the vibrational relaxation at the same time, when APFO3 is in a monodisperse system. This ICT process would disappear due to the influence of interchain interaction when APFO3 is in the condensed phase, where the exciton decay would be dominant in the relaxation process after photoexcitation. The photoexcitation dynamics of APFO3 film blending with PC61 BM are presented, which shows that the exciton may be dissociated completely as the percentage of PC61 BM reaches ∼50%. Meanwhile, the photovoltaic performance based on blend heterojunction shows that the increase of photocurrent is little if the percentage of PC61 BM exceeds ∼50%. Overall, the present study has covered s...

Emission and energy transfer characteristics of coumarin 6 molecules doped in opal polymer photonic crystal

The emission of coumarin 6(C-6) doped in opal polymethylmethacrylate(PMMA) photonic crystal(PC) was effectively manipulated. Meanwhile the energy transfer(ET) of C-6 in PCs, which are infiltrated with sulforhodamine B(S-B), was influenced by the concentration of energy acceptor in solution, the size of PMMA microsphere(SM) and the photonic stop band(PSB). The results should be beneficial to people to further understand the potential application of PCs in optoelectronic fields.

Investigation on "Excimer" Formation Mechanism of Linear Oligofluorenes-Functionalized Anthracenes by Using Transient Absorption Spectroscopy.

We study the photophysical characters of two oligofluorenes‐functionalized anthracenes molecules with different fluorine‐vinylene (FV) units, which exhibits that “excimer” state appears in the solution after photoexcitation. The dynamic data shows that two mechanisms are responsible for the generation of “excimer”. The fast one is controlled by the arene‐arene interaction between molecules and the slow one is influenced by the diffusion motion of molecules. Increasing the number of FV units may suppress the DM‐dependent “excimer” and enhance the yield of intrinsic fluorescence, which finally improves the fluorescence property of molecules in solution.

Studying the emission complexity of conjugated molecules by manipulating the molecular aggregate state

We prepared quantum dots (QDs) based on a conjugated molecule (coumarin 6, C-6), which is considered to be a mesoscopic aggregate, to build a bridge linking the monodisperse state (solution state) and macroscopic aggregates (powder state). Their spectral features, including steady spectral data, dynamic features and fluorescence quantum yields, have been compared in detail. All dynamic data were modelled using the continuous rate distribution function. The final results showed that the fluorescence quantum yield of organic quantum dots based on C-6 gradually decreases as the size of the QDs increases, which may be attributed to enhancement of the rate of non-radiative relaxation, which originates from the aggregation. This is beneficial for further understanding the optical characteristics of conjugated molecules.

Studying of the photoluminescence of MEH-PPV-Au nanoparticles hybrid system

The photoluminescence (PL) characteristics of MEH-PPV hybrid films blended with Au nanoparticles are studied in detail. Due to the strong coupling effect between the surface plasmon of Au nanoparticles and the emission of MEH-PPV, the introduction of Au nanoparticles could enhance the PL intensity and modulate the shape of absorption and excitation spectra, according to our steady spectroscopy experiments. Besides, the relaxation dynamic experiments show that the non-radiative rate in the hybrid system decreases. The temperature and the excitation intensity (EI)-dependent PL spectral data indicate that hybrid MEH-PPV films have lower activation energy and higher thermal exchange rate. As a result, the thermal accumulation effect and the non-linearity of EI-induced PL enhancement are suppressed in hybrid films.