Luminescence behaviour of 7-hydroxyflavone in aerosol OT reverse micelles: Excited-state proton transfer and red-edge excitation effects

The fluorescence emission properties of 7-hydroxyflavone (7HF) are examined in reverse micelles of aerosol-OT (AOT) in n-heptane. Excited-state proton transfer (ESPT) leading to dual-emission behaviour (lambda(max) = 396-417 nm and 545-550 nm which can be assigned to the normal and ESPT tautomer emission respectively) as well as red edge excitation shift (REES) of the normal fluorescence band are observed. Upon gradual addition of water to the 7HF-AOT-n-heptane solution, conspicuous enhancement of the ESPT tautomer emission intensity takes place together with a progressive red shift of the normal emission, that continues up to ( [H2O]/[AOT]) = W-0 approximate to 8-10, beyond which no significant changes occur. Interestingly, with increasing value of W-0, the changes observed in the magnitude of the REES effect, Delta lambda (Delta lambda is the difference in lambda(max) of the normal fluorescence as lambda(cxc) is shifted from midband (lambda = 310 nm) to red edge (lambda = 350 nm) of the absorption band) parallel to changes in the lambda(max) of normal emission, as well as that of the relative intensity I-T/I-N of the tautomer vs. normal emission bands. Even at high W-0 (e.g. W-0=36), these parameters do not reach the limiting values found in bulk water, indicating that 7HF is predominantly localized near the head groups of AOT, mostly in the bound water phase.