Light-Enhanced Fluorescence of Multi-Level Cavitands Possessing Pyridazine Upper rim
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Completely different fluorescence behaviour of cavitands based on a same calixresorcinarene compound was observed. While the fluorescence intensity of the parent compound, tetramethyl-cavitand (1) slowly faded as a result of UV-light exposure, the emission of the three-level cavitand with pyridazine moieties at the upper rim (5a) was enhanced by the excitation in the UV-region. The structure of fluorescence emission (characterized by excitation-emission matrices) and the absorption of 5a remained unaltered. The analysis of fluorescence decay curves reveals the presence of two separated components assigned to two individual emitting species. The measured significant increase of the average lifetime and quantum yield is the consequence of the UV-light induced transition between the different states of 5a. These observations can be explained by the structural difference between 5a and 1. As a counterpart of the naked cavitand (1) with methyl substituents at the upper rim only, 5a has three additional moieties benzene, triazole and pyridazine levels. Computational studies proved the existence of two conformational isomers of 5a. Upon ultraviolet light excitation a “dark” to “light” conformational transition occurs between the two isomers. This hypothesis was confirmed by anisotropy decay measurements.
KeywordsTriazole pyridazine cavitand Fluorescence enhancement Conformational isomer Internal fluorescence quenching Photochromism Photoswitchable material
The authors gratefully acknowledge the financial supports from the Hungarian National Development Agency. Project codes:
SROP-4.2.2.A-11/1/KONV-2012-0065, Synthesis of supramolecular systems, examination of their physicochemical properties and their utilisation for separation and sensor chemistry.
SROP-4.2.2.B-15/KONV-2015-0011, Supporting scientific training of talented youth at the University of Pecs.
T.Z. Janosi, G. Makkai and J. Erostyak acknowledge the grant SROP-4.2.2.D-15/1/Konv-2015-0015: Environmental industry related innovative trans- and interdisciplinary research team development in the University of Pecs knowledge base.
The authors thank Z. Jakab for his kind cooperation in the synthesis of the azide reagent.
The authors are grateful for the scientific consultation with Prof. Laszlo Ujj, Department of Physics, University of West Florida, Pensacola Florida.
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