Use of gain spectroscopy to obtain sole and exclusive evidence for excited-state solute-solvent complexes

The Amplified Spontaneous Emission (ASE) from the non- rigidized and rigidized amino-coumarins C1 and C102 respectively in p-dioxane, were studied for various added quantities of water, using a picosecond laser at 355 nm for excitation. While the ASE spectrum of C102 remained unchanged, that of C1 displayed an additional long wavelength ASE band at 445 nm, along with the normal ASE at 415 nm, found in the absence of water and the relative intensities of the long wavelength ASE to the short wavelength ASE increased with increasing water content. Addition of polar solvents such as acetonitrile to p-dioxane solutions of C1 did not produce the additional longer wavelength ASE. These findings indicate that apparently the TICT excited states of C1 are involved in this long wavelength ASE with the H-bonding water molecules facilitating their stabilization. These generally non radiative TICT states of C1 apparently then complex to form bicimers, whose allowed spontaneous transitions get amplified through the gain of the medium. Apparently under conditions of weak excitation such as in conventional fluorescence studies, one does not have adequate numbers of C1 bicimers and/or gain to be able to detect their emission as a second band at long wavelengths in p-dioxane water solutions. ©2005 Copyright SPIE - The International Society for Optical Engineering.