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A.-M. Pana et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 22–28
1H NMR (500 MHz, CD3OD), ı (ppm): 7.98 (s, 2H), 7.31 (d,
J = 9.0 Hz, 1H) 7.18 (t, J = 7.7 Hz, 1H), 6.91–6.77 (m, 2H), 2.83 (dd,
J = 8.6, 3.6 Hz, 2H), 1.84–1.61 (m, 1H).
13C NMR (100 MHz, CD3OD) ı (ppm): 191.3, 156.6, 135.8, 132.9,
130.0, 122.9, 118.7, 115.1, 28.4, 23.2.
IR (cm−1): 3366, 3061, 2953, 2915, 2838, 1649, 1599, 1577, 1549,
1452, 1335, 1280, 1169, 1158, 1094, 761, 749, 567.
3. Results and discussion
thesized in basic conditions, in a good yield.
HBC is a curcumin analogue with a great ability for colour
switching along pH modification. This ability can be attributed to
the several species involved (Scheme 2) in the interconversion of
the compound using different acidic/basic conditions [2], similar to
those registered for the flavylium system [25].
In order to highlight the stable species, the NMR experiments
were carried out at two extreme pH values (pH < 1 and pH > 13)
and neutral pH conditions.
In the basic condition the 1H NMR spectrum recorded imme-
diately after the deuterated base was added, proved the total
transformation of Ct form into the deprotonated base Ct2−. The
conversion into the unprotonated base Ct2− occurs practically
instantaneous from the moment the base was added. The spectrum
was recorded again, after 1 and 10 days; the features were the same,
The NMR spectrum of the HBC at pH < 1 was recorded imme-
diately and in time after the acidic conditions was created. In the
spectrum recorded immediately was detected the presence of sev-
eral species and the total conversion of Ct towards the cation AH+
was reached after 29 h (see Table 1).
Because the compound does not turn instantaneously into the
AH+ compound, the protonated sample in the above conditions was
rerun several times in order to observe the moment when it turns
completely into the xanthylium derivative. Thus, several spectra
were recorded: immediately, after 20 min, then every 2 min for
30 min, every 20 min for 3 h, 29 h, 36 h (1½ days), and then from
time to time for 14 days. In the Fig. 1b, which presents the evolu-
tion of the NMR spectra of HBC in time, we can notice that even
after 50 min the sample has present the signals corresponding to
the neutral form of the compound and that the conversion of Ct into
AH+ specie is complete only after about 29 h. The NMR analysis, run
after 15 days, confirms the stability of the cationic form AH+ in the
Fig. 1. The evolution of the 1H NMR spectra of HBC in acidic media, at 25 ◦C; the
signals marked with the red circle correspond to AH+; (a) at t = 0; (b) at t = 50 min
Based on the NMR spectra recorded at different moments of time
and temperature of 25 ◦C, the kinetics of the Ct conversion into the
protonated adduct AH+ can be described by the curve presented in
Fig. 2.
The conversion of HBC into related species (see Scheme 2) was
theoretically investigated in terms of energy released or absorbed
reflected by the heat of formation (ꢂH). According to semiempirical
molecular orbital calculations (AM1), the theoretical stabiliza-
tion for the AH+ cation species is ꢂH = 154.52 kcal/mol, relatively
high positive value compared to those computed for the Ct, and
Ct2− species (ꢂHCt = −48.56 kcal/mol, ꢂHCt2− = −65.49 kcal/mol,
respectively). In this light, the most stable geometry was assigned
for the Ct2− comparing with the relatively unstable geometry of
AH+. Moreover, the values suggest that the conversion between
these species is easier for e.g., Ct into Ct2− and for Ct into Cc species
than for Ct into AH+ species because the conformational flexibility
of these species is different. The main difference between the Ct and
AH+ species is the increasing rigidity of the AH+ system; the con-
formational rigidity of AH+ system increases the thermodynamic
instability.
The UV–vis spectrometry allowed in emphasizing the colour
shift behaviour of the compound depending on the pH increase.
Fig. 3 presents the superimposed UV–vis absorption spectra of the
compound in different pH solutions, ranging from 1.54 to 5. The
samples in acid pH are reddish and exhibit a broad band with a
maximum absorption at 498 nm, corresponding to the formation
of the AH+ cation. As the pH value increases, the solutions became
non-coloured, corresponding to the neutral form of the compound.
The broad absorption band of 498 nm is not detectable for the neu-
tral form, and it practically disappears as the pH increases to values
beyond 2.25. Moreover, as the band from 498 nm increases in inten-
sity and the pH value drops, the absorption band from 258 nm
decreases in intensity, proving that the xanthylium cation has been
formed and the reddish colour of the solution is the straight conse-
quence. The UV–vis spectra also sustain the information obtained
via NMR, because even in acid environment both species, the