期刊
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
卷 19, 期 6, 页码 4758-4767出版社
ROYAL SOC CHEMISTRY
DOI: 10.1039/c6cp07720j
关键词
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资金
- Science and Engineering Research Board (SERB), New Delhi through the J.C. Bose fellowship
- Council of Scientific and Industrial Research (CSIR), New Delhi
A comprehensive investigation of the photophysics of a cofacially oriented bis-coumarin based on naphthalene, i.e., Cou-Nap, designed and synthesized to examine the influence of Pi-electronic communication between the two fluorophores, reveals exceptional excited-state properties. While the anticipated [2 + 2] photocycloaddition is not observed despite the fact that the two reactive coumarin units are at a distance of 3.8 angstrom,the fluorescence quantum yields and singlet lifetimes in different solvents are found to be remarkably higher when compared to those of the parent coumarin and a mono-coumarin model system, i.e., Cou-Dur. In addition to large solvent-induced Stokes shifts, Cou-Nap displays intriguing temperature-dependent emission in a nonpolar solvent such as cyclohexane. The observed photophysical properties are reconciled based on the so-called `phane effect' that is operative in cyclophanes. In the latter, an effective Pi-Pi interaction between the aromatic rings modifies the attributes of the chromophore in such a manner that the observed properties cannot be associated with the individual aromatic rings. The temperature-dependent emission is proposed to arise as a consequence of thermally activated ISC from the singlet-excited state to one of the higher energy triplet states. The results constitute, for the first time, the demonstration of modification of the excited-state properties of a fluorophore in a non-cyclophane system by `phane effect'.
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