关键词: 主客体作用, 葫芦[7]脲, 香豆素, 紫精, 荧光

Host-guest system is one kind of supramolecular systems. Tremendous efforts have been devoted to develop new host-guest systems extending their applications in many research fields, such as molecular machine, drug delivery, chemsensor, and tuned emission materials. The cucurbit[n]uril family is a series of synthetic macrocyclic compounds, which has shown specific self-assembly properties such as high affinity, good selectivity, and high chemostability compared with the traditional supramolecular hosts, such as cyclodextrins, crown ethers, calixarenes, and many others. In the present work, a coumarin with 7-N,N'-diethly substituent, an intramolecular charge transfer (ICT) based chromophore, was chosen as the fluorophore. The coumarin terminal-decorated polyamidoamine (PAMAM) dendrimers (Gm-2(m+1)C, m=0, 1), the coumarin and viologen-labled polyamidoamine dendrimers (EV-Gm-2(m+1)C, m=0, 1) and the coumarin and viologen model compounds (C-Model and EV) were synthesized, their structures were characterized by ¹H NMR, IR and MALDI-TOF mass spectrometry. EV and each terminal coumarin group of Gm-2(m+1)C and EV-Gm-2(m+1)C assembled with cucurbit[7]uril (CB[7]) forming 1:1 inclusion complex with association constants of 3.3×10⁶ mol^-1·L and 1.4×10⁵～1.6×10⁵ mol^-1·L. The ¹H NMR study indicates that the 7-N,N'-diethly moiety along with partial coumarin ring are encapsulated into the rigid hydrophobic cavity of CB[7] forming the inclusion complex of coumarin with CB[7]. The formation of the inclusion complex of coumarin with CB[7] restricts the rotation of the N,N'-diethyl moiety, which suppresses the conversion from the emissive ICT excited state of coumarin to the nonemissive twisted intramolecular charge transfer (TICT) one, resulting in a dramatic increase of fluorescence emission. The hydrophobic microenvironment of the CB[7] cavity is also favorable to the ICT emission of coumarin. The interaction of CB[7] with EV-Gm-2(m+1)C proceeds via a stepwise mechanism. CB[7] assembles with the viologen group first and starts to form inclusion complex with coumarin when the equivalent of CB[7] exceeds that of EV. The quantity of the inclusion complex of CB[7] with coumarin can be controlled by adjusting the adding amount of CB[7], consequently, regulating the fluorescence intensity of Gm-2(m+1)C and EV-Gm-2(m+1)C. This study provides a potential strategy for developing controllable light-emitting dendritic system with efficient synthesis.

Key words: host-guest interaction, cucurbit[7]uril, coumarin, viologen, fluorescence