The origin of lignin fluorescence

Spruce lignin exhibits fluorescence emission spectra that peaks at ≈360 nm on excitation at wavelengths ranging from 240 to 320 nm. This can be explained by non-radiative energy transfer from lignin chromophores, that are excited in the wavelength range 240–320 nm, to an acceptor that emits fluorescent light at ≈360 nm. Examinations of lignin samples and model compounds suggest that small amounts of phenylcoumarone structures in the lignin is a conceivable acceptor. Such structures and stilbene structures are formed from structural elements in lignin of the phenylcoumaran type on various treatments. The photophysical properties of models for phenylcoumarone structures [2-(3,4-dimethoxyphenyl)-7-methoxy-3-methylbenzo[b]furan, 2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxybenzo[b]furan] and stilbene structures (the E and Z forms of 2-hydroxy-3,3′,4′-trimethoxystilbene) have been examined and are discussed on the basis of crystal structure determinations.