200
L. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 199–203
solution. As shown in Fig. 1, the irradiation of 254 nm UV light
H
led the absorption spectra to exhibit regular changes: the absorp-
tion at kmax = 294 nm gradually decreased, while the absorption
at the region of 326–344 nm gradually increased; the absorption
band at kmax = 356 nm decreased along with little blue-shifted
(from 356 to 344 nm). At the same time, a new absorption at
kmax = 426 nm appeared and intensified with irradiation.
Above changes observed in the time-dependent absorption
spectra were similar to the experimental results of other SAs re-
ported in the literatures [26,27]. In theses literatures, such changes
were assigned to the transformation from enol-form to keto-form.
Herein, our interest focused on disclosing the actual photochemical
product through appropriate method. As we known, NMR tech-
niques can provide detailed and accurate structural information
of samples. Next, we would employ the time-dependent 1H NMR
to track the photochemical process.
O
H
H
C
N
O
O
O
C
N
N
C
OH
O
HO
OH
2
1
3
H
H
H
H
N
C
C
N
N
C
C
N
HO
HO
OH
OH
4
Scheme 1. The molecular structures of compounds 1, 2, 3 and 4.
SAs in chloroform solution. Following, we studied the time-depen-
dent absorption spectroscopy experiments of compounds 1, 2, 3
and 4 in other solvents such as benzene, acetonitrile, and cyclohex-
ane. However, no reactions were observed in these solvents. Thus,
it could be confirmed that the decomposition was attributed to the
solvent (chloroform). Finally, we proposed the possible decomposi-
tion mechanism.
The time-dependent 1H NMR experiments of compound 1 were
accomplished in CDCl3. Upon irradiation, considerable changes of
chemical shifts were observed (shown in Fig. 2). Specifically, the
chemical shifts of Ha1 and Hb1 gradually decreased until eventually
disappeared, but meanwhile two new chemical shifts (at d =
11.37 ppm and d = 9.66 ppm) appeared and intensified with the
irradiation. From the above results, it could be determined that
the compound 1 had been completely transformed into its photo-
products. The next task is to identify the detailed structure of the
photoproducts through the results of NMR.
Experimental
General
The new chemical shifts (at d = 11.37 ppm and d = 9.66 ppm)
appeared in the Fig. 2 would provide us with some useful informa-
tion, whereas it was found amazingly that the final spectrum
shown in Fig. 2 and the 1H NMR spectrum of the original reactant
1f were virtually identical except for the region from 7.70 to
7.30 ppm. Fig. 3 showed the comparison of them. The chemical
shifts on the region from 7.70 to 7.30 ppm in the above (1 after
irradiation) assigned to the phenyl in the reactant 1f and aniline.
However, in the below (1f), it merely assigned to the phenyl in the
reactant 1f. The two signals (at d = 11.37 ppm and d = 9.66 ppm)
appeared in Fig. 2 undoubtedly assigned to the characteristic
chemical shifts He and Hc of 1f, respectively. Therefore, it was pos-
sible that compound 1 had been decomposed into the reactants 1f
and aniline during the irradiation.
Low-resolution mass spectra were measured on an Agilent
7890A-5975C GC–MS using electron impact (EI) ionization at
70 eV. NMR spectra were obtained on a Bruker instrument. 1H
NMR spectra were recorded at AV-400. 13C NMR spectra were re-
corded at 100 MHz. High-resolution mass spectra were taken on
an Agilent 1200-6520 Q-TOF electrospray mass spectrometer.
The absorption spectra were performed on a Hitachi (Model U-
4100) UV–Vis–NIR spectrophotometer.
Preparation of 1–4
Compounds 1, 2, 3 and 4 were synthesized by the condensation
of the corresponding amines with salicylaldehyde, according to
standard procedures as previously reported [28]. The pure prod-
ucts were obtained after recrystallization twice from appropriate
solvent. The preparation details of compounds 1, 2, 3 and 4 and
the analysis data can be seen in the Supplementary files.
In order to obtain further evidence, the 13C NMR spectrum of
the final irradiation sample was measured. As shown in Fig. 4, a
chemical shift at d = 193.95 ppm was observed. Such 13C NMR sig-
nal was similar to the chemical shift of carbon in aldehyde. Com-
pared with the 13C NMR spectrum of the original reactant 1f, it
Time-dependent absorption spectroscopy and time-dependent NMR
experiments
The time-dependent absorption spectra were accomplished in
chloroform solution. The concentrations of 1–4 for absorption
spectral analysis are 6.06ꢀ10ꢁ5 M. A low-pressure mercury lamp
(16 W) was used as light source for irradiation reactions. The inter-
val time was 1 min.
The time-dependent 1H NMR experiments were accomplished
in deuterochloroform; 2-mg sample and 0.5 mL of deuterochloro-
form were added in quartz NMR tube, respectively, and then, the
tube was sealed with parafilm for irradiation reactions. A high-
pressure mercury lamp (500 W) was used as light source. The
interval time was 1 h.
Results and discussion
Photochemical experiments and analysis of compound 1
First of all, we studied the photochemical behavior of com-
Fig. 1. Time-dependent absorption spectroscopies of 1 in chloroform (kir = 254 nm).
pound 1 by the time-dependent absorption spectra in chloroform
Dash: before irradiation, solid: after irradiation. Interval time is 1 min.