Fluorescence Spectral Properties of Methyl Orange in Homogeneous Media

The methyl orange [C 14 H 14 N 3 SO 3 Na], an azo dye exhibited strong emission and large Stokes shift in various solvents, and the largest shift (Δλ = 125.51nm or Δν = 9297cm -1 ) was obtained in the water. The UV-visible spectra of the dye showed the absorption in the range (33,333 – 20,000) cm -1 . We found that solvent effects on the absorption wavelength are consistent. The bathochromic shift in water and the hypsochromic shift in methanol observed in the absorption (43 nm) as well as in the fluorescence (42 nm) spectra predict the strong solute-solvent interaction. The fluorescence quantum yield (ɸ f ) was decreased from 24% in DMSO to 5% in water. The fluorescent properties of this dye are strongly solvent dependent, the wavelength of minimum fluorescence emission (λ em = 435.51nm) shifts to the red. The maximum and minimum calculated oscillator strength is 32% with (Ɛ max = 29011 M -1 cm -1 ) and 11% (Ɛ max = 6682 M -1 cm -1 ) in methanol and DMSO, respectively. Protonated solvents without exception give a shorter lifetime and lower quantum yield. The average excited-state lifetime of the dye was found maximum (τ av = 5.36 ns) in DMSO. Also, fluorescence lifetime was combined to deduce the radiative and non-radiative decay rate.