Effect of substituent on intramolecular charge transfer and excited state dipole moments of amino coumarins

The absorption and fluorescence spectra of two coumarin derivatives viz. C-480 (I) and C-540 A (II) have been recorded in various polar and non-polar organic solvents. The displacement of absorption and fluorescence maxima due to solvent polarity has been used to estimate the ground state as well as the excited state dipole moments of these molecules. The molecular volumes and the Onsagaer cavity radii of die coumarin derivatives are obtained using van der Waals method of radii increments. In case of both the coumarin compounds, dipole moment in the excited state is found higher than the ground state dipole moment and indicates that the lowest excited singlet states for these molecules have a ICT electronic configuration Further, it is noted that the increase in temperature from 25 to 85 degrees C neither affects the position nor the intensity of the fluorescence maxima of either of the compounds I and II. This is a clear indication that the structural features of these compounds are such that they inhibit the rotation and torsional non-radiative decay process. TICT formation does not occur.