Chunxiang Xu

Tuning solid-state fluorescence of pyrene derivatives via a cocrystal strategy

1-Acetyl-3-(4-methoxyphenyl)-5-(1-pyrenyl)-pyrazoline (AMPP) was synthesized and crystallized to yield three types of crystals with different host–guest structures: AMPP crystal (I), AMPP–phenol (II) and AMPP–(2-naphthol) cocrystals (III). Twisted π-conjugated structures and different stacking modes of AMPP molecules were found in the three crystals. In crystal I, pyrene fluorophores adopt a face-to-face π-stacked arrangement, while pyrene fluorophores in crystals II and III adopt the monomer arrangements because AMPPs were widely separated by means of entrapment of guest molecules in the two lattices. Investigation of the optical-properties of the three crystals reveals that they are closely related to the arrangements of the pyrene fluorophores. Face-to-face π–π stacking of the pyrene fluorophores in crystal I cause a broad band emission with a red shift of 40–50 nm relative to pyrene fluorescence in solution. However, the emission spectrum of crystal II is similar to that of crystal III; both of them exhibit two sharp bands with a small red shift of 20 nm relative to pyrene fluorescence in solution, which are induced by the monomer arrangements of pyrene fluorophores and overlaps of π-orbitals between benzene and pyrene fluorophores. These results demonstrate that using a cocrystal strategy to regulate packing modes of pyrene fluorophores is an effective way for exploring highly blue-emissive pyrene derivatives.

Control mechanism behind broad fluorescence from violet to orange in ZnO quantum dots

ZnO quantum dots with tunable size in the range of 2.0–7.8 nm were synthesized through a facile sol–gel route. Photoluminescence and photoluminescence excitation spectral examinations revealed that two types of transition mechanisms might occur under control during excitation regulation corresponding to the coexistence of deep and shallow levels in energy band structure. The broad-color emission from violet to orange could be tuned by adjusting the excitation energy and quantum dot size. Moreover, ZnO quantum dots with tunable and broad luminescence were demonstrated to be promising in the anti-fake labeling applications.

Role of zinc vacancies in driving ferromagnetism in undoped ZnO granular films

In this paper, X-ray diffraction, X-ray energy dispersive spectrometer, photoluminescence, electron paramagnetic resonance were performed to examine the evolution of intrinsic defect states during the annealing treatment for understanding the origin of ferromagnetism in undoped ZnO granular films deposited by the magnetron sputtering method. A strong link between the ferromagnetism and the zinc vacancies was established, indicating zinc vacancies may be more effective in modulating ferromagnetism than oxygen vacancies in ZnO. This novel observation is significant not only to get further insight into the ferromagnetic origin but also to tune the magnetic properties for spintronic materials.

Investigation of molecular arrangements and solid-state fluorescence properties of solvates and cocrystals of 1-acetyl-3-phenyl-5-(9-anthryl)-2-pyrazoline

The title compound (APAP) was crystallized under different conditions to afford five types of crystals: the guest-free crystal (I), APAP·chloroform solvate (II), the APAP·phenol cocrystal (III), the APAP·2,2-bis(4-hydroxyphenyl)propane cocrystal (IV) and monohydrate (V). Single-crystal X-ray analysis revealed that a racemic chain motif is formed by the heterochiral APAP molecules in the guest-free crystal (I), in which anthracene fluorophores adopt a face-to-face π-stacked arrangement. In contrast, by entrapment of different guest molecules, the homochiral chains are formed by one-handed enantiomers in crystals II–V, in which no effective π-overlap exists between anthracene fluorophores. The optical-physical properties of crystals I–IV and their structure–property relationship were investigated. Crystal I exhibits a broad emission band with only one peak centered at 458 nm. However, crystals II–IV exhibit similar fluorescence spectra, in which the vibrational features are in good agreement with that of anthracene monomer emission and the λem are 430, 429 and 425 nm. As a control experiment, we speculate that the π-stacked arrangement of anthracene fluorophores should be responsible for larger red-shifted emission, lower emission quantum yield and longer lifetime. This strategy based on regulating the packing modes of anthracene fluorophores to tune the fluorescence properties and the investigation of structure–property relationships should be useful for the development of organic solid materials.

Three-Photon Absorption in a Charge Transfer Compound Pumped with Picosecond and Nanosecond Lasers

A new intramolecular charge transfer compound with an absorption band in the short wavelength range has been synthesized. The measured three-photon absorption coefficient and the corresponding molecular cross-section when pumped by a nanosecond beam are γ=2.2×10-17 cm3/W2 and σ3=6.4×10-74 cm6/s2, respectively. When pumped by a picosecond laser beam, the three-photon absorption cross section is one order of magnitude lower. Three-photon-absorption-induced frequency-upconverted fluorescence emission has been observed by pumping with 8 ns 1.06 µm laser pulses. The emission intensity dependence of the incident intensity obeys the cubic law.