Luminescence behaviour of 7-hydroxyflavone: temperature-dependent effects

Abstract

On excitation at 365 nm, 7-hydroxyflavone (7HF) emits a blue fluorescence (λ max ≈ 450 nm) in ethanol—methanol (1:1 by volume ) glass at 77 K. This fluorescence shifts to longer wavelengths, leading to a green emission (λ max ≈ 537 nm) at room temperature (298 K). The blue and green fluorescence is due to selective excitation of the same ground state species, namely the conjugate anion of 7HF (7HFA), which is present at low concentration in neutral alcohol solution. A large change in the excited state dipole moment of 7HFA compared with the ground state takes place; this has been calculated to be 13.7 ± 1 D using the solvatochromic shift method. The large blue shift of the steady state emission maximum on lowering the temperature from 289 to 77 K is rationalized in terms of solvent relaxation around the excited fluorophore.