| Abstrakt: |
Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δ &nubar; = &nubar;[SUBabs] - &nubar; [SUBfl]), fluorescence quantum yields (Φ[SUBf]), and fluorescence lifetimes (τ[SUBf]) show more or less linear correlation with the solvent polarity function Δf = {(∊ - 1)/(2∊ + 1) - (n[SUP2] - 1 )/(2n[SUP2]+ 1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φ[SUBf] and τ[SUBf] in nonpolar solvents are also strongly)temperature dependent. It is indicated that the excited singlet (S[SUB1]) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersystem crossing process is negligible for the present dye in all the solvents studied. Photophysical behavior of C120 in nonpolar solvent has been rationalized assuming that in these solvents the dye exists in a nonpolar structure, with its 7-NH[SUB2] group in a pyramidal configuration. In this structure, since the (7-NH[SUB2] group is bonded to the 1,2-benzopyrone moiety by a single bond, the former group can undergo a fast flip-flop motion, which in effect causes the fast nonradiative deexcitation of the dye excited state. In moderate to higher polarity solvents, it is indicated that the dye exists in an intramolecular charge-transfer structure, where the bond between 7-NH[SUB2] group and the 1,2-benzopyrone moiety attains substantial double bond character. In this structure, the flip-flop motion of the 7-NH[SUB2] group is highly restricted and... [ABSTRACT FROM AUTHOR] |
