The origin of lignin fluorescence

Spruce lignin exhibits fluorescence emission spectra that peaks at approximate to 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 approximate to 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-methyo[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. (C) 1999 Elsevier Science B.V. All rights reserved.