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keywords: coordination frameworks;luminescence;mechanochromism;phase transitions;polyrotaxanes
Reversible Phase Transformation and Mechanochromic Luminescence of ZnII-dipyridylamide-based Coordination Frameworks


A 1D double-zigzag framework, {[Zn(paps)2(H2O)2](ClO4)2}n (1; paps=N,N′-bis(pyridylcarbonyl)-4,4′-diaminodiphenyl thioether), was synthesized by the reaction of Zn(ClO4)2 with paps. However, a similar reaction, except that dry solvents were used, led to the formation of a novel 2D polyrotaxane framework, [Zn(paps)2(ClO4)2]n (2). This difference relies on the fact that water coordinates to the ZnII ion in 1, but ClO4 ion coordination is found in 2. Notably, the structures can be interconverted by heating and grinding in the presence of moisture, and such a structural transformation can also be proven experimentally by powder and single-crystal X-ray diffraction studies. The related N,N′-bis- (pyridylcarbonyl)-4,4′-diaminodiphenyl ether (papo) and N,N′-(methylenedi-para-phenylene)bispyridine-4-carboxamide (papc) ligands were reacted with ZnII ions as well. When a similar reaction was performed with dry solvents, except that papo was used instead of paps, the product mixture contained mononuclear [Zn(papo)(CH3OH)4](ClO4)2 (5) and the polyrotaxane [Zn(papo)2(ClO4)2]n (4). From the powder XRD data, grinding this mixture in the presence of moisture resulted in total conversion to the pure double-zigzag {[Zn(papo)2(H2O)2](ClO4)2}n (3) immediately. Upon heating 3, the polyrotaxane framework of 4 was recovered. The double-zigzag {[Zn(papc)2(H2O)2](ClO4)2}n (6) and polyrotaxane [Zn(papc)2(ClO4)2]n (7) were synthesized in a similar reaction. Although upon heating the double-zigzag 6 undergoes structural transformation to give the polyrotaxane 7, grinding solid 7 in the presence of moisture does not lead to the formation of 6. Significantly, the bright emissions for double-zigzag frameworks of 1 and 3 and weak ones for polyrotaxane frameworks of 2 and 4 also show interesting mechanochromic luminescence.The research was conducted at the NSRRC beamline 01C2.

B.-C. Tzeng, T.-Y. Chang, S.-L. Wei, and H.-S. Sheu

Chem.-Eur. J. 18 , 5105 (2012)