Wenjing Tian

Piezochromic Luminescence Based on the Molecular Aggregation of 9,10‐Bis((E)‐2‐(pyrid‐2‐yl)vinyl)anthracene

Luminescent materials sensitive to environmental stimuli are of great interest from a scientific viewpoint owing to their potential applications in fluorescent switches and optical devices. Pressure is one of the most common natural external stimuli, and thus piezochromic materials, which show color changes resulting from external pressure or mechanical grinding, can be used as pressure-sensing and optical-recording systems. However, whereas pH-, light-, and temperature-sensitive materials are relatively well understood, studies of piezochromic materials remain inadequate owing to the absence of an effective mechanism to explain the relationship between changes in molecular assembly or packing and the corresponding luminescence properties of the material. Until now, successful systems have generally made use of transformations between monomeric and dimeric/excimeric states through hydrogen-bonding interactions as the mechanism to cause changes in luminescence. It is well-known that in the condensed phase, the luminescence properties of a given molecular system usually undergo significant variation according to the molecular aggregation state or stacking mode, since intermolecular interactions invariably alter photophysical processes. Therefore, an understanding of and the ability to control the molecular aggregation state and the consequent intermolecular interactions are still very important for the development of piezochromic materials. Herein we report an effective mechanism of piezochromic luminescence on the basis of the molecular aggregation state of 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene (BP2VA). BP2VA exhibited spectacular luminescence characteristics: grinding and the exertion of external pressure on the powder led to a change in its photoluminescence color from green to red. Three crystal polymorphs of BP2VA with different stacking modes involving gradually enhanced p–p interactions in the three crystalline states provided further insight into the origin of luminescence changes under the external stimulae. BP2VA was synthesized in a straightforward manner by a one-step Witting–Horner reaction according to a previously reported method, and the purified material was characterized by spectroscopic methods (see the Supporting Information). BP2VA powder exhibited a strong green emission at lmax= 528 nm, in contrast to its weak orange emission at lmax= 583 nm as a solution in THF. The weak orange emission was ascribed to a conformational relaxation in solution, which was reflected by the corresponding photoluminescence (PL) spectrum of BP2VA at low temperature (77 K; see Figure S1 in the Supporting Information). Furthermore, the emission of BP2VA aggregation as a solution in THF/water was blueshifted to lmax= 570 nm (from the value lmax= 583 nm in THF; see Figure S2). Interestingly, after being ground, BP2VA powder showed a strong red shift with a yellow emission (lmax= 561 nm) under UV light with a wavelength of 365 nm, and after being heated above 160 8C, the ground powder recovered its initial green emission (lmax= 528 nm; Figure 1b,c). The interconversion of the two states with their distinct emission colors is completely reversible through grinding and heating. The red shift of 33 nm in fluorescence emission upon grinding and the recovery of the initial state upon heating is a significant piezochromic effect. To gain further understanding of the piezochromic effect, we investigated the influence of applied pressure on the luminescence of BP2VA. The powder was placed in the holes (diameter: 200 mm) of a T301 steel gasket, which was preindented to a thickness of 50 mm. A small ruby chip was inserted into the sample compartment for in situ pressure calibration according to the R1 ruby fluorescence method. A 4:1 mixture of methanol and ethanol was used as a pressuretransmitting medium (PTM). The hydrostatic pressure on the powder was determined by monitoring the widths and separation of the R1 and R2 lines. The photoluminescence measurements under high pressure were performed on a QuantaMaster 40 spectrometer in the reflection mode. The 405 nm line of a violet diode laser with a spot size of 20 mm and a power of 100 mW was used as the excitation source. The diamond anvil cell (DAC) containing the sample was put on a Nikon fluorescence microscope to focus the laser on the sample. The emission spectra were recorded with a monochromator equipped with a photomultiplier. All experiments were conducted at room temperature. [*] Y. J. Dong, Dr. B. Xu, J. B. Zhang, L. J. Wang, J. L. Chen, Dr. H. G. Lv, Dr. S. P. Wen, Dr. B. Li, Dr. L. Ye, Prof. Dr. W. J. Tian State Key Laboratory of Supramolecular Structure and Materials Jilin Unversity Qianjin Street No. 2699, Changchun 130012 (China) E-mail: wjtian@jlu.edu.cn X. Tan, Prof. Dr. B. Zou State Key Laboratory of Superhard Materials Jilin University (China) E-mail: zoubo@jlu.edu.cn [] These authors contributed equally. [**] This research was supported by the 973 program (2009CB623605), the NSFC (Grant No. 20874035, 21074045, 21073071), and the Project of Jilin Province (Grant No.20100704). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201204660. . Angewandte Communications

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We demonstrated SnO2 films prepared by sinter-less spin-coating processes as an electron selective contact for CH3NH3PbI3-based planar-heterojunction perovskite solar cells (PSCs). A modified sequential deposition method, in which the grain size of PbI2 precursors was controlled by an equivalent solvent vapor annealing (SVA) process, was used to prepare the perovskite layer on SnO2. With this SVA process, the remnant PbI2 nanocrystals can stably occur at the interface of CH3NH3PbI3/SnO2 to carry out a passivation effect. The photovoltaic performance of SnO2-based PSCs is dependent on both the SVA time and the thickness of the perovskite layer. The optimized PSC device achieves the best power conversion efficiency of up to 13% under the AM 1.5 simulated sunlight illumination, which is highly durable over 30 days of storage time with exposure to the ambient air environment.

Organic Polymorphs: One‐Compound‐Based Crystals with Molecular‐Conformation‐ and Packing‐Dependent Luminescent Properties

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Self-assembled graphene quantum dots induced by cytochrome c: a novel biosensor for trypsin with remarkable fluorescence enhancement

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Multi-stimuli responsive fluorescence switching: the reversible piezochromism and protonation effect of a divinylanthracene derivative

A novel divinylanthracene derivative 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) was synthesized and its two polymorphs with different crystal structures were obtained. The introduction of pyridine in BP4VA leads to multi-stimuli responsive fluorescence. An investigation of the photophysical and stimuli responsive luminescent properties of BP4VA, including the piezochromism and protonation effect, demonstrates that the piezochromic luminescence originates from changes in the molecular aggregation state. Additionally, protonation–deprotonation of the pyridine moieties in BP4VA has a significant effect on the frontier molecular orbitals, resulting in distinct green and red emissions under acid and base stimuli. This study on BP4VA provides a comprehensive insight into the mechanisms within this type of stimuli-responsive luminescent material, and suggests that BP4VA may be a potential candidate for applications in sensing, detection and display devices with remarkable color-changing properties.

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Efficient single-layer electroluminescent device based on a bipolar emitting boron-containing material

A novel multifunctional 1,6-bis(2-hydroxyphenyl)pyridine boron bis(4-n-butyl-phenyl)phenyleneamine compound in which the hole-transporting (HT), electron-transporting (ET), and emitting (EM) components are integrated into a single molecule was synthesized and used as an emitting material to fabricate an efficient single-layer electroluminescent device.