Fluorescence Spectral Properties of Methyl Orange in Homogeneous Media

The methyl orange [C₁₄H₁₄N₃SO₃Na], 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^-1cm^-1) and 11% (Ɛ_max = 6682 M^-1cm^-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.