Three Pt(II) complexes with a formula of [Pt(Lx) 2 ], x ¼ 1, 2 and 3, in which the pyrazole chelate (Lx)H represents 4-(2,6-diisopropylphenyl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (1), 5-(2,6-diisopropylphenyl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (2) and 4-(tert-butyl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (3), were synthesized, and their photophysical properties were examined. Single-crystal X-ray diffraction studies of 1 and 2 reveal a planar molecular geometry with both Lx chelates adopting a mutually trans configuration. Distinctive packing arrangements within the crystal lattices are observed, e.g., complex 2 shows structural repetitions that prohibit formation of intimate p-p stacking, while complex 1 exhibits an ordered alignment along the axis of the square planar core framework and, remarkably, shows mechanoluminescence and concentration dependent photoluminescence in the poly(methyl methacrylate) matrix (PMMA). In sharp contrast, derivatives 2 and 3 show only the luminescence derived from the monomeric and aggregated state, respectively, which is attributed to the imposed substituent effect. For application, organic light emitting diodes (OLEDs) were also fabricated using the aforementioned blue dopant 1, attaining the peak external quantum efficiency, current and power efficiencies of h ext ¼ 9.1%, h c ¼ 18.4 cd A À1 and h p ¼ 16.6 lm W À1 respectively. With the same device structure, devices based on 3 exhibit an extremely high efficiency orange emission (20.0%, 47.6 cd A À1 , 50.8 lm W À1 ) due to the occurrence of extensive p-pstacking. Upon combining blue-emitting phosphor 1 and orange-emitting phosphor 3, two white OLED devices are fabricated, one of which obtained by sequentially doping 1, 3 and then 1 into separated layers exhibits performance efficiencies of 11.3%, 28.3 cd A À1 , and 21.2 lm W À1 , highly stable chromaticity (CIE x ¼ 0.41-0.40 and CIE y ¼ 0.43-0.41), and an adequate color-rendering index (CRI) of 80 at brightness of 370 to 10 800 cd m À2 . The other device obtained by co-doping 1 and 3 into a single emissive layer shows improved efficiencies of 15.0%, 44.8 cd A À1 , and 46.2 lm W À1 , but with a lower CRI of 63.

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Four new colorimetric receptors (1-4) were synthesized and characterized. Upon addition of maleate to receptor in DMSO, the appearance of the solution of receptor 1 showed a color change from dark-blue to dark-red, which can be detected by the naked eye at parts per million. Similar experiments were repeated using receptors 2-4; the solution showed a distinct color change from blue to violet for receptor 2 and from blue-green to purple for both receptors 3 and 4, when they are formed as complexes with maleate. The striking color changes are thought to be due to the deprotonation of the thiourea moiety of the 4-nitronaphthyl chromophore. Whereas, in the addition of fumarate to receptors 1-4, the color of the solution changed from dark-blue to bright yellow for receptor and did not induce any color change for receptors 2-4. Thus, for a distinct color change, receptors 1-4 can act as optical chemosensors for recognition of maleate versus fumarate. Especially, only receptor 1 has a unique color change for the recognition of fumarate, accordingly it can be used for detection of the fumarate anion. In this research it was also found that the performance of the receptor is highly dependent on the substituent group on the phenyl ring; a stronger electron-withdrawing group resulted in a receptor with a higher binding constant with the maleate anion.

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