Effect of hydrogen bonding interaction on the photophysics of α-amino-orcein

This paper reports for the first time a detailed spectroscopic investigation into the ground-and excited-state properties of alpha-amino-orcein (alpha-AO), one of the main components of the orcein dye, in solvents of different proticity andwater at different pHs. In order to gain insight into the nature of the involved transitions and excited state deactivation pathways, the studywas carried out bymeans of UV-Visible steady state and ultrafast spectroscopic techniques with the support of quantum mechanical calculations (DFT and TDDFT). The results highlight that the photophysical and photodynamic behaviour of alpha-AO are highly sensitive to the solvent proticity and pH. In particular, protic environment induces a red shift (55 nm) of the absorption spectrum together with a relevant decrease of the fluorescence quantum yield (from 0.19 in acetonitrile to 6.6 x 10(-3) in methanol) and radiative rate constant (two orders ofmagnitude). A notable red shift is also caused by increasing the pH leading the molecule from monocationic to neutral and then monoanionic form through two deprotonation steps (pKa= 3.539 +/- 0.006 and 11.180 +/- 0.006). Following deprotonation, themolecule assumes spectral and photophysical properties very similar to those retrieved in protic media. The observed behaviour has been rationalized through the occurrence of hydrogen bonding, likely involving to a greater extent the carbonyl oxygen of alpha-AO and the protic solvent, that favours the charge delocalization on the whole chromophore as well as fast non-radiative excited state deactivation. The ultrafast spectroscopic investigation revealed in fact the presence, in protic solvent, of a short living component (tens of picoseconds), assignable to solvent complexed S-1 state, alongside the long living component (few nanoseconds) observed in aprotic media and attributed to the solvent free S-1 state. The results achieved in this study for alpha-AO provides an important contribution to the interpretation of absorption and fluorescence features of orcein dye mixture in more complex systems (protein based substrates within the many aspects of the cultural heritage and biomedical field) where hydrogen bonds are expected to play a crucial role in mediating the interaction with the environment. (C) 2019 Published by Elsevier B.V.