Structural and electronic properties of 5,6-dihydroxyindole oligomers. Integrating experimental and theoretical approaches

Elucidation of the structural factors governing the absorption properties of 5,6-dihydroxyindole oligomers is an essential step towards an understanding of the unique optical properties of eumelanins. Herein, we report a systematic study of the electronic absorption properties of the main oligomer products of 5,6-dihydroxyindole so far isolated, namely the 2,2’-, 2,4’- and 2,7’-biindolyls; the 2,4’:2’,4”- and 2,4’:2’,7”-terindolyls, and tetramers from 2,4’- and 2,7’-biindolyls. At the dimer level, the 2,2’-biindolyl exhibits a bathochromically shifted chromophore followed by the 2,4’- and 2,7’-biindolyls, in that order, with shifts of up to 60 nm. Density functional theory (DFT) calculations indicated that 2,2’-biindolyl can adopt a planar arrangement. At the trimer level, a 15 nm difference was observed between 2,4’:2’,4”- and 2,4’:2’,7”-terindolyls which reflected the effect of positional isomerism at one of the terminal units. The 2,4’:2’,3’’:2’’,4’’’-quaterindolyl exhibited an almost featureless chromophore extending over the entire UV range without distinct absorptions, indicating a mixture of conformers with torsionally constrained interring bonds causing partially disrupted conjugation. It is concluded that (i) the absorption properties of 5,6-dihydroxyindole oligomers depend on both positional isomerism and length, although no simple relationship can be drawn; (ii) oligomer growth in the 2-4’ and/or 2-7’ modalities decreases only to a limited extent the HOMO-LUMO gap; (iii) the 2,3’ mode of coupling disrupts conjugation and limits bathochromic shift. These data indicate that the mode of coupling of the indole units is a crucial determinant of the electronic absorption properties of the oligomers and that this structural parameter must be carefully considered in predicting the structure and optical properties of eumelanin basic motifs.