Photophysical properties of tris-methoxycoumarin derivative of a cryptand

The paper reports the photophysical studies of a macrobicyclic cryptand where the three secondary amino nitrogen have been derivatized with methoxy coumarin in various solvents at room temperature and at 77 K. The system having (F–S)3–R configuration (where F is fluorophore, S is CH2 spacer and R is receptor unit) shows intramolecular PET from the receptor nitrogen atoms of the cryptand moiety to the coumarin units reducing the fluorescence quantum yield and lifetime of coumarin in the ligand. However, the fluorescence intensity is enhanced in presence of protons and Zn+2 ion in dry THF medium. Metal ions like Cu(II) and Co(II) quench the fluorescence. Presence of protons induces a greater enhancement of emission intensity implying all the nitrogen lone pairs responsible for PET are protonated. Another interesting feature of this ligand is that it exhibits room temperature phosphorescence (RTP) apart from exhibiting PET. The observation of RTP has not been reported earlier for any other coumarin derivative.