Unusual photophysical properties of coumarin-151

The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone ( coumarin-151: C151) in nonpolar (NP) solvents are drastically different tha n those in other solvents of moderate to higher polarities. Thus, the absor ption and fluorescence maxima are largely blue-shifted and the Stokes' shif ts unusually low in NP solvents compared to those in the other solvents. Th e fluorescence quantum yields (Phi (f)) in NP solvents are also exceptional ly lower. While in all other solvents the fluorescence decays of C151 are s ingle-exponential. in NP solvents the decays show non-single-exponential be havior. The average fluorescence lifetimes (tau (f)) in NP solvents are als o substantially shorter, Low Phi (f), and tau (f) values in NP solvents ind icate very fast nonradiative deactivation for the S-1 state of C151. Invest igating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that t he fast nonradiative deactivation in NP solvents is not due to an enhanceme nt in the intersystem crossing rate. It is inferred from the present result s 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 fli p-flop motion of the NH2 group introduces the fast deactivation channel for the SI state of the dye. In other solvents of moderate to higher polaritie s, there is an intramolecular charge transfer (ICT) from the 7-NH2 group to the benzopyrone moiety, resulting in a planar structure for the S-1 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 h igh Phi (f) and tau (f) values and large Stokes shifts for the dye compared to those in NP solvents.