Photophysical studies of hydroxynaphthoquinone-Al3+ complexes in solution and on textiles

The formation of complexes upon addition of Al 3+ to juglone, 5-hydroxy-1,4-naphthoquinone ( 5HNQ ), and naphthazarin, 5,8-dihydroxy-1,4-naphthoquinone ( DHNQ ), was investigated in solution by UV–Vis spectrophotometry and fluorometric techniques (steady-state and time-resolved) in order to elucidate the photophysical behaviour of plant-derived naphthoquinones applied to protein- and cellulose-based fabrics, as found in historical artworks. The absorption and emission spectra of the resulting complexes in solution showed significant differences compared to those of the isolated molecules, including red shifts and enhanced vibronic resolution. Chelation-enhanced fluorescence (CHEF) was observed, with a marked increase in fluorescence quantum yields and lifetimes, together with higher radiative rate constants and reduced raditionless deactivation. The stoichiometry of the complexes was determined using the mole ratio (Yoe-Jones) and continuous variations (Job) methods. The study conducted on fabrics dyed with Alkanna tinctoria extract, with and without alum as a mordant, demonstrated that the photophysical behaviour on textiles is primarily governed by Al 3+ :DHNQ chelates. These species exhibit distinctive absorption features, setting them apart from those of other historical violet dyestuffs, thus enabling their non-invasive identification in ancient textiles. These findings provide valuable insights into the chemistry of naphthoquinones while significantly enhancing the application of UV–Vis spectroscopy to the diagnostics of cultural heritage.