title: Light-Enhanced Fluorescence of Multi-Level Cavitands Possessing Pyridazine Upper rim.
abstract: Completely different fluorescence behaviour of cavitands based on a same calix[4]resorcinarene 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.
authors.affiliation: Institute of Physics, University of Pecs, Ifjusag u. 6, Pecs, H-7624, Hungary. tzjanosi@gamma.ttk.pte.hu.|Szentagothai Research Centre, Spectroscopy Research Group, University of Pecs, Ifjusag u. 20, Pecs, H-7624, Hungary. tzjanosi@gamma.ttk.pte.hu.
authors.forename: Tibor Zoltan
authors.initials: TZ
authors.lastname: Janosi
authors.affiliation: Institute of Physics, University of Pecs, Ifjusag u. 6, Pecs, H-7624, Hungary.|Szentagothai Research Centre, Spectroscopy Research Group, University of Pecs, Ifjusag u. 20, Pecs, H-7624, Hungary.
authors.forename: Geza
authors.initials: G
authors.lastname: Makkai
authors.affiliation: Department of Inorganic Chemistry, University of Pecs, Ifjusag u. 6, Pecs, H-7624, Hungary.|MTA-PTE Research Group for Selective Chemical Syntheses, Ifjusag u. 6, Pecs, H-7624, Hungary.
authors.forename: Timea
authors.initials: T
authors.lastname: Kegl
authors.affiliation: Department of Organic Chemistry, Semmelweis University, Hogyes E. 7, Budapest, H-1092, Hungary.
authors.forename: Peter
authors.initials: P
authors.lastname: Matyus
authors.affiliation: Department of Inorganic Chemistry, University of Pecs, Ifjusag u. 6, Pecs, H-7624, Hungary.|MTA-PTE Research Group for Selective Chemical Syntheses, Ifjusag u. 6, Pecs, H-7624, Hungary.
authors.forename: Laszlo
authors.initials: L
authors.lastname: Kollar
authors.affiliation: Institute of Physics, University of Pecs, Ifjusag u. 6, Pecs, H-7624, Hungary.|Szentagothai Research Centre, Spectroscopy Research Group, University of Pecs, Ifjusag u. 20, Pecs, H-7624, Hungary.
authors.forename: Janos
authors.initials: J
authors.lastname: Erostyak
mesh_terms: D004988:Ethers, Cyclic / Q000737:chemistry* / Q000528:radiation effects; D005453:Fluorescence; D008027:Light; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D011724:Pyridazines / Q000737:chemistry*; D012118:Resorcinols / Q000737:chemistry* / Q000528:radiation effects; D013050:Spectrometry, Fluorescence
chemical_list: D004988:Ethers, Cyclic; D011724:Pyridazines; D012118:Resorcinols; C442939:cavitand; C062482:pyridazine
keywords: Conformational isomer; Fluorescence enhancement; Internal fluorescence quenching; Photochromism; Photoswitchable material; Triazole pyridazine cavitand