Environmental and complexation effects on the structures and spectroscopic signatures of organic pigments relevant to cultural heritage: the case of alizarin and alizarin–Mg(ii)/Al(iii) complexes

Abstract

An integrated computational approach allowed an unbiased analysis of optical and structural properties of alizarin-based pigments, which can be directly compared with experimental results. Madder lake pigments have been modeled by Mg(II)- and Al(III)-coordinated alizarin taking into account solvation and metal-linkage effects, responsible for colour modifications. Moreover, different environmental conditions have been analyzed for free alizarin, showing in all cases semi-quantitative agreement with experimental spectroscopic data (UV-VIS). Our results point out the ability of in silico approaches to unravel the subtle interplay of stereo-electronic, dynamic, and environmental effects in tuning the physico-chemical properties of pigments relevant to cultural heritage.

The SERS spectra of alizarin and its ionized species: The contribution of the molecular resonance to the spectral enhancement

“…Moreover, the higher spectral intensity observed with 785 nm excitation at pH 12 with respect to that at pH 6 can be explained as due to the long wavelength shift of the absorption of the di-anionic species which is stable at higher pH. This is even more true when you consider the further shift to longer wavelength observed for the adsorbed species (see [9] and references therein). Under the same experimental conditions the spectral intensity with the 514 nm excitation is practically the same at the two pH values and, besides, at pH 12, it is lower than that obtained with 785 nm excitation.…”

“…The calculation indicates for the anion that the molecular structure is planar, with an intramolecular hydrogen bond for the OH group in position 2. This form is deemed preferential owing to the SERS results [6] as it allows the molecule to have two anchor points (1,9) closest to the metal nanoparticle. In the first excited electronic state, the system shows a decrease in the C-OH bond distance and a more marked alternation in the bond lengths in the aromatic system.…”

“…1) can act as binding groups with respect to mordant metallic atoms end/or surfaces after a monoor di-deprotonation of the neutral molecule. It has been assumed [9] that the complex species present in alkaline solutions are of the type 1,2 (involved are the two OH groups in position 1,2) or of the type 1,9 where the OH group in position 1 and the C = O group in position 9 are involved. It has been reported [6] that the molecules adsorbed on the metal surface exhibit a SERS spectrum corresponding to that of the 1,9 anionic forms of alizarin in alkaline solution (pH 7-12).…”

Implementation of a graphical user interface for the virtual multifrequency spectrometer: The VMS‐Draw tool

“…Both the red color predictet for Alizarin-Al(III) complex (shown as a red circle in the lower panel of Figure 1) and the orange coloration issuing from an analogous simulation for the Alizarin-Mg(II) complex are in line with experiment. [50] Although RR is, in principle, a vibrational spectroscopy, the underlying theory and computational machinery are closer to those of the vibrationally resolved electronic spectra discussed in this section. [53][54][55] As a matter of fact, RR spectra resemble their non resonant counterparts, but with strongly enhanced intensities and additional modulation by vibronic interactions with one or more excited electronic states lead in resonance by the applied laser radiation.…”

“…[50] Let us start, for purposes of illustration, from a rough overall picture, based on the computation of vertical transitions (shown in the upper panel), which, together with an analysis of the composition of electronic transitions, allows to select the excited states to be examined in deeper detail. Once the electronic transitions of interest are selected, vibronic computations with a suitably chosen model (here Vertical Gradient, see Refs.…”