Polychromatic irradiation in heterogeneous media

Frosted glasses were used to deposit a homogeneous coat of laccaic acid and acid lac dye⁸. The red colorants were irradiated with a Xenon lamp with a cut-off filter till 300 nm, simulating the sunlight exposure. The reaction was followed measuring colour changes with a colorimeter⁹ and the photodegradation was characterized by µ-FTIR and as previously by HPLC-DAD, after removing the dyes with MeOH.

After 4250 hours of irradiation, which corresponds to 203 years in the museum display (Appendix III, section III.3.) laccaic acid A and lac dye did not faded totally (Figure 2.18.)

Figura 2.18. Laccaic acid A before and after irradiation, during 108 h, 203 h, 366 h, 500 h and 4250 h, from left to right.

HPLC-DAD analyses performed on laccaic acid A, allowed to see 30% decrease of the main peak (tr=19.8 min), while new others were appearing as the irradiation time increased (Table 2.4.). Lac dye has a similar behaviour since the two main peaks of laccaic acid A and laccaic acid B (t_r= 19.5 min) decrease 30% and 20%, respectively;

and new peaks appeared at tr= 18.8 min and tr= 15.4 min.

*Relative area %, calculated at 280 nm

8 The lac dye used is from Fluka, By HPLC-DAD, it were detected the two main laccaic acids present in lac dye: A and B.

9 The system CIELab, was developed to characterize the colour, which is than defined by the coordinates L* (measures the luminosity, varies between 0 (black) and 100 (white), a* and b* measure the hue (-60 (green) < a* < 60 (red) and -60 (blue) < a* < 60 (yellow)) [108].

The FTIR spectra revealed that the NH distension attributed to the secondary amide present in the laccaic acid A (Figure 2.19.), has almost disappeared, being the 3415 cm^-1 peak after the irradiation just a shoulder (Figure 2.20.) and the second peak at 3075 cm^-1 more intense. Probably a conversion of secondary amide into a primary amine could occur, since a new band at 3245cm^-1, characteristic of primary amines [109], is now more evident, which could mean the loss of the group C=O-CH3; and the ratio between the C=O amide I asymmetric distension (1620 cm^-1) and amide II NH bending and CO distension (1570 cm^-1), decreases after irradiation. Those could explain the transformation of the secondary amide into primary amine, which presents only the NH2 bending vibration at ~1600 cm^-1, in fact there is a shoulder appearing at that wavenumber, in the irradiated dye. In the fingerprint region it is possible to note at 1407 cm^-1 (C=O distension of aromatic ring) the decreasing intensity, being after irradiated, a shoulder.

O O

OH OH OH COOH

HOOC

HO

R

OH

Figure 2.19. Structure of laccaic acids A, B, C and E.

Figure 2.20. FTIR spectra from laccaic acid A before (black line) and after irradiation for 4250 hours (blue line).

Laccaic acid A R= CH2CH2NHCOCH3

Laccaic acid B R= CH2CH2OH Laccaic acid C R= CH2CHCOOH

NH2

Laccaic acid E R= CH2CH2NH2

2.1.4. Conclusions

Photophysical data obtained for the analysed chromophores, revealed the possibility of two kinds of substituents, sugar in carminic acid and amide in laccaic acid A, strongly modify in distinct ways, the chromophores excited behaviour. Laccaic acid A and carminic acid, even having a similar structure to the one of purpurin, predicting an absence of ESIPT, showed that in the excited state laccaic acid A is almost similar to alizarin and carminic acid is similar to purpurin. The differences obtained in the lakes should be related to the structures of the complexes formed with aluminium ion, since previous studies have already shown that carminic acid is not a planar molecule [79], neither laccaic acid A due to CH-CH linkage between the anthraquinone and the amide, although alizarin and purpurin are planar, which could influence the excited state properties.

Stoichiometry of Al³⁺:alizarin complex seems to be a mixture of two complexes, a 1:2 and 1:1 complex Al³⁺:alizarin, which could be explained by the existence of two tautomers identified by liquid and solid state NMR [101], reacting simultaneously.

Stoichiometry of purpurin: Al³⁺ complex seems to be 1:1, which regarding the molecule structure is suitable, as there are two hydroxyl groups near the carbonyl groups that could react with Al³⁺. Even though further studies using more and new techniques available should corroborate this study.

Important evidences about apparently similar red chromophores were discovered with this work. A small structural difference can produce a broad of different ground but mainly excited state behaviours, which does not allow creating one single paradigm of these ancient molecules, but a set of them.

This work confirmed the fading resistance of all the chromopheres studied, in homogeneous and heterogeneous media. The difference between the ΦR obtained in monochromatic irradiation in homogeneous and heterogeneous media is higher to alizarin red S (ten folder) than alizarin (same magnitude order), which could be explained by the sulfonate group interaction with gelatine.

It was confirmed that the photodegradation of alizarin and purpurin are not so intense as for their lakes. Alizarin lake irradiated allowed to notice the formation of something more polar, which could be due to the presence of free radicals that would attack the double bonds of the carbonyl groups. According to these results, in heterogeneous media purpurin is in fact more photoreactive than alizarin. In homogeneous, it was concluded that the mordant has not a protecting action on their photodegradation, like it has on wool [91]. Despite of the results for purpurin lake in proteinaceous gel were not conclusive, the absorbance intensity decreasing in the first

hours, while the gel was transparent, was faster than in homogeneous media, revealing its higher reactivity.

Laccaic acid A revealed to be a vey resistant dye to photodegradation, since after been subject to a polychromatic irradiation of 12455 MJ/m² , the colour did not totally fade. HPLC-DAD enabled identify new compounds formed as the irradiation was carried on. The most expressive peaks have a retention time at 18.8 min and 15.4 min.

Knowing that laccaic acid B is eluted at 19.5 min, and laccaic acid C and E are co-eluted ate 15.9 min (Figure 2.19.), probably the compounds co-eluted at 18.8 min is an amine with the methyl group changed or with some of the carboxylic groups deprotonated or could be due to the loss of carbonyl. The one eluted at 15.4 min, should have a structure similar to laccaic acid C or E. These hypotheses were also in agreement with what observe in the FTIR spectra.

“The real magic lies not in seeing new landscapes, but in having new eyes”

Marcel Proust

2.2. µ-spectrofluorimetry: a new non-invasive technique to analyze red