Explain the shift in the absorption wavelength in going from coumarin to 7-diethylaminocoumarin.
Answers
7-DCCA is a hydrophilic molecule, whereas 7-DCCAE is more hydrophobic in character. We have observed considerable differences in the absorption and emission spectral properties, fluorescence lifetime and rotational relaxation time of 7-DCCAE with 7-DCCA in reverse micelles. The photophysics of 7-DCCA in reverse micelles was reported by us earlier.25 In reverse micelles we have observed that the absorption and emission peak of 7-DCCAE (Table 1) is red shifted as compared to 7-DCCA.25 For both molecules we have observed REES in the reverse micelles and the extent of REES gradually decreases with increasing the w0 value, although the extent of REES for 7-DCCAE is less compared to 7-DCCA in the reverse micelles. This may be due to the presence of a carboxylic group in 7-DCCA. The fluorescence quantum yield value (ϕf) of 7-DCCA and 7-DCCAE gradually decreases with the gradual addition of water content inside the reverse micelles. This result clearly ascertains that both probe molecules face more polar environment with rising of the water content. The magnitude of ϕf of 7-DCCA is higher25 in respective reverse micelles as compared to 7-DCCAE. For both molecules the quantum yield values are different at different excitation wavelengths, this demonstrates the excitation wavelength dependent behaviour of both 7-DCCA and its ester molecule.