Reduced fluorescence quenching of coumarin 102 at higher phenol mole fractions in cyclohexane–phenol and anisole–phenol solvent mixtures: role of competitive hydrogen bonding

Abstract

Recently, we demonstrated that the fluorescence of coumarin 102 (C102) drastically quenches upon hydrogen bonding (H-bonding) with phenol in a non-polar solvent cyclohexane due to H-bond driven photoinduced electron transfer (PET) (J. Phys. Chem. A, 2013, 117, 3945–3953). However, the work was limited to low concentrations of phenol only, where predominately a 1 : 1 C102–phenol complex exists. Herein, we report an unusual fluorescence modulation of C102 over mole fractions of the H-bond donor (phenol) in the cyclohexane–phenol mixtures covering a higher range of mole fractions. We found that the fluorescence quantum yield (ϕ_f) and fluorescence lifetime (τ_f) of C102 first diminish steadily with increase in the concentration of phenol, but after a particular mole fraction (X_PhOH = 0.013) both the quantities (ϕ_f and τ_f) increase upon further increase in the phenol content. The results can be attributed to the competitive nature of the C102–phenol and the phenol–phenol H-bonding. Since the PET quenching operates through the H-bond between the CO group of C102 and the HO– group of phenol, the H-bond may fail to attain an optimum geometry when the phenol is linked with other phenols via the H-bond. The variation of the C102–phenol H-bonding nature with the mole fraction of phenol was supplemented by FT-IR measurements. Similar unusual variation of the C102 fluorescence is observed in the phenol–anisole mixtures to a lesser extent, but is completely absent in the anisole–cyclohexane mixtures, where H-bonding is practically impossible.