The use of fluorescent probes in the characterization of lignin:: the distribution, by energy, of fluorophores in Eucalyptus grandis lignin

Four fluorescent probes (biphenyl, naphthalene, pyrene and phenanthrene) were used to map the energy distribution of the structural units present in lignin fragments from Eucalyptus grandis wood. This distribution shows that these fragments present two regions with a high concentration of chromophores, one between 418 and 385 kJ/mol, and the other below 322 kJ/mol. When this lignin was treated with NaBH4, the two more intense regions occurs between 418 and 346 kJ/mol, followed by a significant increase in the concentration of chromophores in almost all the studied energy range. Lifetime distributions present a bimodal pattern, with two typical peak lifetime values, the first of 1.36 +/- 0.17 ns, with a relative amplitude above 80%, and the second of 8.48 +/- 2.32 ns, for both species, with some fluctuation for different lambda (em). The synchronous spectra indicates, for this lignin, at least three broad spectral envelopes, with a large superposition of the emission maxima. The results indicate the existence of at least three most representative fluorophores, most probably due to biphenyl, coniferyl alcohol and stilbene structures, with varying substituents. The majority of the fluorescence complexity of this lignin seems to be associated with the inhomogeneous emission decay kinetics associated with ground state heterogeneity, due to the complex mixture of the different fluorophores, on which are superimposed different distributions of environments. (C) 2001 Elsevier Science B.V. All rights reserved.