Unusual Photophysical Properties of Coumarin-151

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Abstract

The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone (coumarin-151; C151) in nonpolar (NP) solvents are drastically different than those in other solvents of moderate to higher polarities. Thus, the absorption and fluorescence maxima are largely blue-shifted and the Stokes' shifts unusually low in NP solvents compared to those in the other solvents. The fluorescence quantum yields (Φf) in NP solvents are also exceptionally lower. While in all other solvents the fluorescence decays of C151 are single-exponential, in NP solvents the decays show non-single-exponential behavior. The average fluorescence lifetimes (τf) in NP solvents are also substantially shorter. Low Φf and τf values in NP solvents indicate very fast nonradiative deactivation for the S1 state of C151. Investigating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that the fast nonradiative deactivation in NP solvents is not due to an enhancement in the intersystem crossing rate. It is inferred from the present results that in NP solvents the 7-NH2 group of C151 exists as a free substituent without participating in resonance with the benzopyrone moiety and the flip-flop motion of the NH2 group introduces the fast deactivation channel for the S1 state of the dye. In other solvents of moderate to higher polarities, there is an intramolecular charge transfer (ICT) from the 7-NH2 group to the benzopyrone moiety, resulting in a planar structure for the S1 state in which the flip-flop motion of the 7-NH2 group is restricted. Involvement of the ICT structure in moderate to higher polarity solvents causes very high Φf and τf values and large Stokes' shifts for the dye compared to those in NP solvents.

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