Jin-Young Choi, T. J. Kang Department of Chemistry, Daegu University Gyoung san, 712-714 Korea

1. Solvent effects on the solvatochromism of 7-aminocoumarins in neat and binary solvent mixtures: Correlation between the electronic transition energies and the solvent polarity parameters Jin-Young Choi, T. J. Kang Department of Chemistry, Daegu University Gyoung san, 712-714 Korea The change in the electronic absorption and emission energies of 7-aminocoumarin derivatives in binary solvent mixtures has been studied. The electronic transition energy along with the Stokes’ shift is correlated with the orientation polarizability of the solvent as well as the empirical solvent polarity parameters ET(30). It is observed that the emission peak shift traces polarity change very nicely in the binary solvent mixtures. The emission transition is more strongly depend on the solvent polarity than the absorption. From the dependence of the Sokes’shift of 7-aminocoumarins with the solvent polarity parameters and the ground state dipole moment obtained by the semi-empirical AM1 calculations, the excited state dipole moment was estimated. The fluorescence lifetime change of 7-aminocoumarins in binary solvent mixtures was measured and the results are explained in terms of molecular conformation and solvent polarity. The study indicates the empirical solvent polarity ET(30) is a good measure of microscopic solvent polarity and it probes in general the non-specific solvent interactions. Conclusions Abstract Introduction Theorical background Results and discussion Structure of 7-aminocoumarin dyes Solvent Polarity Parameters Solvatochromism and the estimation of the dipole moment Solvation is important for understanding the solvent effects on chemical and biochemical processes. Biological systems in which various physiological processes such as transportation, signaling, metabolism are controlled by solvation. Changes in local polarity by solute-solvent interactions in biological systems are related to malfunction or disease. A variety of solvent polarity parameters proposed. But, there is no such thing as a single polarity parameter in explaining a multitude of solvent effects. ① Δf(ε,n) : Orientation polarizability ① Lippert, Mataga Effect of general solvent polarity ② Bakhshiev, Kawski ② ET(30) : Empirical solvent polarity ●The ET(30) measures solvent polarity with the charge transfer absorption maxima of propidium phenol betaine known as Reichardt dye . ③ Ravi et al ③ Other empirical polarity parameters ●Z value : Kosower ● Y-scale and π* Correlation between ETN and Δf(ε,n) Correlation of Stokes’shift with Δf(ε,n) and ETN ETN was measured for 14 aprotic solvents and 5 protic solvents, and plotted versus Δf(ε,n). It is noted that there is a linear correlation between ETN and Δf(ε,n) for aprotic solvents, but large deviation is observed for protic solvents. Δf(ε,n) Fig(1)(R=0.95347)seems to give better correlation Than Fig(2)(R=0.9011). Solvatochromic shift of 7-aminocoumarin dye Estimation of the excited dipole moment (a) Absorption and (b) fluorescence Spectra of coumarin 102 were measured in dioxane/ water binary solvent system. The plot of the Stokes’ shift of coumarin 102 in benzene-methanol binary solvent as a function of ETN is compare to the plot of the Stokes’shift as a function of the Δf(ε,n). The shape of the cavity in 7-amonocoumarin dyes is not spherical but is very much elliptical. Thus whenever the cavity radius is required for calculations, it should be taken very carefully. Δf(ε,n) They show batochromic shift as solvent polarity increases. Comparison of emission peak shifts with the change of ETN *Calculated from the molecular volume assuming a spherical shape **Semi-empirical AM1 Calculation Solvent polarity and fluorescence lifetime change Plotting fluorescence lifetime against solvent polarity indicates characteristic curvilinear change for different coumarin fluorophores. • The solvent effect on the solvatochromism is correlated to the empirical polarity parameter ETN better than the bulk solvent parameter function. The ETN value measures microscopic solvent polarity and it seems to probe more or less non-specific solvent interactions. • The dipole moment 7-aminocoumarins increases by almost twice upon excitation. The cavity shape is assumed to be very much elliptical and the cavity radius is predicted to be much larger than the generally considered value for these 7-aminocoumarins. • Preferential solvation is taking place in binary solvent mixture since the change of peak shift seems to reflect the change of ETNvery closely. • The lifetime of coumarin 102 gradually increases with increasing solvent polarity, but the fluorescence lifetime of coumarin 481 rapidly decreases as the solvent polarity increases. This is attributed to the nonradiative decay process which involves the formation of twisted intramolecular charge transfer state. Fig(a)~(c), The empirical solvent polarity parameter plottedas a function of X , the mole fraction of the more polar component of binary solvent mixtures.(d)-(f)the wavenumbers of the emission peaks for coumarin 102 dissolved in three binary solvent mixtures plotted as a function of mole fraction of more polar solvent. The microscopic solvent polarity is probed very nicely by an empirical polarity measure of ETN. (a) and (d) : benzene/acetonitrile mixture, (b) and (e) : benzene/methanol mixture, (c) and (f) : dioxane/water