Exploring the Intrinsic Piezofluorochromic Mechanism of TPE-An by STS Technique
© Jin et al. 2015
Received: 1 July 2015
Accepted: 3 August 2015
Published: 3 September 2015
9,10-bis(4-(1,2,2-triphenylvinyl)styryl)anthracene (TPE-An) materials have attracted considerable attention in recent years because they have high luminescence efficiency and excellent piezofluorochromic properties, which have potential applications in organic light-emitting display (OLED) area. Scanning tunneling spectroscopy (STS) technique was used to study the piezofluorochromic mechanism of aggregation-induced emission (AIE) materials for the first time. Photoluminescence (PL) experiments revealed that the emission peak of TPE-An is observed to exhibit a red-shift with the increase of the grinding time. A theoretical calculation was carried out to find the relationship between the bandgap of TPE-An and the external force by combination of the classical tunneling theory and STS results. It is found that when the pressure variation on the surface of TPE-An film was increased to be over 4.38 × 104 Pa, the shrink of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap can arrive at 1.1 eV. It is concluded that the piezofluorochromic behaviors of TPE-An should originate from the shrinking effect of the bandgap under external force. Moreover, this research method may shed light on comprehending and adjusting the piezofluorochromic characters of other AIE materials.
The luminescent behaviors of organic materials usually turn worse due to the aggregation-caused quenching (ACQ) effect , which blocks off their further applications. Their luminescent efficiency cannot be effectively improved until a typical aggregation-induced emission (AIE) material (silole molecule) was found to successfully overcome the ACQ effect in 2001 . In AIE material family, tetraphenylethene (TPE) derivatives have gotten a rapid development in the last decade [3–8]. As an important member of AIE family, 9,10-bis(4-(1,2,2-triphenylvinyl)styryl)anthracene (TPE-An) is very particular because it possesses unique piezofluorochromic characters [8–21]. As a result, TPE-An has attracted considerable attention because it has potential applications in organic light-emitting display (OLED), photovoltaic cells, transistors, and solid-state storage [22–26]. In recent studies, the photoluminescence (PL) peak of TPE-An material was firstly found to have a significant red-shift after a series of grinding treatments , revealing that the external force may have much effect on the fluorescence property of TPE-An. But until now, the relationship among the external force, the bandgap, and the PL peak shift of TPE-An cannot be thoroughly understood, which makes it hard to control the piezofluorochromic performances. Therefore, it provides a new challenge to find the intrinsic piezofluorochromic mechanism of TPE-An.
Scanning tunneling spectroscopy (STS) technique is a very useful tool to measure surface electron’s local density of states (LDOS) [28–30], which can give the level position of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the energy-band diagram. Most of all, the working performance of TPE-An under the electrostatic force in STS measurements is very similar to that of TPE-An under the mechanical force. Therefore, STS should be an ideal tool to in situ study the piezofluorochromic property of TPE-An under the external force.
In this work, the piezofluorochromic mechanism of TPE-An film was investigated by STS technique for the first time. Combined STS results with energy-band theory, the energy-gap diagrams of TPE-An before and after deformation are compared to find out the relationships among the external force, the bandgap, and fluorescence property of TPE-An. The possible piezofluorochromic mechanism is also discussed here.
The Synthesis Method of TPE-An Film
PL spectra of TPE-An samples under different grinding conditions were measured in air by the Shimadzu RF-5301PC spectrometer. The excitation wavelength was 365 nm in PL measurements. FTIR spectrometer (Thermo Fisher Scientific Inc, Nicolet 6700) was used to obtain the room temperature mid-infrared (Mid-IR) spectra of TPE-An, and the measurement wavenumber was ranged from 4000 to 1000 cm−1. And scanning tunneling microscope (STM) experiment was carried out in an ultrahigh vacuum (UHV) Omicron system. STM tips were prepared by electrochemical etching of a polycrystalline tungsten wire. The base pressure of STM chamber was 3 × 10−9 mbar. STS measurement was controlled by 7265 DSP lock-in amplifier electronics. The Kelvin probe force microscopy (KPFM) experiment was performed in the atomic force microscopy (AFM) system (Dimension FastScan, Bruker Corporation).
Results and Discussion
A summarization table of Δd, ΔP, ΔE, and Δλ at different set currents by tunneling theory
1.38 × 104
2.90 × 104
4.05 × 104
4.38 × 104
ΔE g (eV)
In summary, STS technique was firstly applied to investigate the piezofluorochromic mechanism of TPE-An film. The relationship between the HOMO-LUMO gap variation and the tip-surface electrostatic force has been quantitatively obtained by combination of the tunneling theory and STS results. It is found that the shrink of the HOMO-LUMO gap arrives at 1.1 eV when the pressure increment on the surface of TPE-An film is over 4.38 × 104 Pa, which can result in a clear red-shift (59 nm) of PL peak. Moreover, the HOMO-LUMO gap TPE-An has exhibited the same shrinking tendency with the increase of external force in both STS and PL measurements. Therefore, it is suggested that the gap-shrinking effect of TPE-An under external force may be responsible for its piezofluorochromic behaviors. And this research method may give a helpful reference for comprehending and modulating the piezofluorochromic behaviors of other AIE materials.
The authors are very thankful for the support of the Program for New Century Excellent Talents in University (NCET-12-0573), the National Natural Science Foundation of China, the National Project for the Development of Key Scientific Apparatus (2013YQ12034506) of China, and the Natural Science Foundation of Guangdong Province (No. S2012010010519).
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