Photooxidation of oxazolidine molecular switches: Uncovering an intramolecular ionization facilitated cyclization process

Article abstract of DOI:10.1039/c8cc00983j

Photooxidation of oxazoline (OXA) molecular switches through media-induced intramolecular ionization is reported. The formation of the photooxidation product occurs by attack of the flexible donor group (-CH₂CH₂OH) to the adjacent CC with ¹O₂ as the oxidant. The novel seven-membered ring sub-structure of the photooxidation product was inferred by HRMS, IR and ¹H NMR spectroscopy. Additionally, acid released from solvent photolysis was found to affect the product formation and efficiency of the photooxidation.

Full text of DOI:10.1039/c8cc00983j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Photooxidation of oxazolidine molecular switches: uncovering an
intramolecular ionization facilated cyclization process
Qiaonan Chen,ab Lan Sheng,* b Jiahui Duab, Guan Xiab and Sean Xiao-An Zhang*ab.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Photooxidation of oxazoline (OXA) molecular switches through a
media-induced intramolecular ionization is reported. The
formation of the photooxidation product occurs by attack of the
1
flexible donor group (-CH2CH2OH) to the adjacent C=C with O2 as
the oxidant. The novel seven-membered ring sub-structure of the
photooxidation product was inferred by HRMS, IR and 1H NMR.
Additionally, acid released from solvent photolysis was found to
affect the product formation and efficiency of the photooxidation.
Scheme 1 Photooxidation of spiropyrans and spirooxazines.
stability of OXAs in electro-photochromic processes,12 however, the
photooxidation mechanism of OXAs remains unexplored. Based on
11
our interest in OXAs,10b, we sought to better understand the
Spiropyrans, spirooxazines, and oxazines belong to an exciting class
of molecular switches capable of responding to multiple types of
stimuli, such as light,1 heat,2 electricity,3 force,4 and acid / base.5
Upon exposure to one of these stimuli, the molecular switches
undergo significant color change. This property is key to their
success of a wide number of important applications, including high-
density optical storage, optical switches and displays.6 However, the
limited photo-stability of these molecules when exposed to light for
a long periods of time reduces their utility in practical applications.
Therefore, it is important to clearly understand photo-degradation
pathways of these molecular switches to further improve their
stability. Among these classes of molecular switches, the
photooxidation of spiropyrans and spirooxazines have been
extensively studied. For example, spiropyrans are reported to
photo-degrade into indoline and aldehyde (Scheme 1a).7 While
spirooxazines also photo-degrade into similar by-products, indoline
and an oxazine, Malatesta and coworkers were able to identify a
key intermediate during the oxidation process (Scheme 1b).8
photooxidation mechanism. Herein we report the first investigation
into the photooxidation of OXAs and describe the formation of a
new seven-membered ring oxidation product.
Solutions of OXAs were found to be stable in the dark or mild
room light conditions. For example, OXA-1 in dimethyl sulfoxide
(DMSO) or ethanol (EtOH) was stable for up to one year in the dark
observed spectroscopically. The addition of acid9c or water11 to
these solutions of OXA-1 (i.e., in EtOH or DMSO) is known to
facilitate the conversion from a colorless ring-closed form (OXA-1,
Fig. 1a, middle) to a blue ring-open form (ROF-1, Fig. 1a, left). This
transition can be easily monitored using UV-Vis where the λmax for
OXA-1 is 331 nm and λmax for ROF-1 is 588 nm (Fig. 1b). However,
when the DMSO solutions were exposed to sunlight for 6 days, the
OXA-1 solutions gradually turned red (Fig. 1a, right), with maximum
absorption peak around 520 nm (Fig. 1b, red line). Yet, this red
color change was irreversible. We hypothesized that light-induced
oxidation of OXA-1 might be leading to the formation of the red
product OXA-1-R. With this in mind we sought to better understand
this light-induced reaction and confirm the structure of OXA-1-R.
Firstly, OXA-1-R was confirmed to have mass-to-charge ratio
(m/z) of 378.1815 (Fig. 1b inset). This indicates that OXA-1-R has
one fewer hydrogen than OXA-1 ([M + H]+ = 380.1969), if it
possesses a positive charge or two fewer hydrogens, if it is a neutral
molecule. Furthermore, functional groups on OXA-1-R were
compared to the ring-open form of OXA-1 via infrared spectroscopy
(IR) using the corresponding bromate form (ROF-Br-1), which is a
precursor for synthesizing OXA-1 only needing a step of basification
(ESI†). As shown in Fig. 1c, the vibration absorption of main
functional groups, such as -NO2 (vas=1517 cm-1, vs=1296 cm-1), C=C
(v=1610 cm-1), =C-H (v=3027 cm-1), -CH2 (vas=2919 cm-1) and -CH3
Oxazolidines (OXAs), are
a related and emerging class of
molecular switches, that have attracted much attention in recent
years with applications in nonlinear optical devices,9 ion detection,10
and ink-free water-jet rewritable paper.11 Despite these important
contributions, the photo-stability of oxazolidines has not been
extensively studied. Recently, Szaloki and co-workers described the
a.State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, 130012, P. R. China. Email:
seanzhang@jlu.edu.cn
b.College of Chemistry, Jilin University, Changchun, 130012, P. R. China. Email:
shenglan17@jlu.edu.cn
Electronic Supplementary Information (ESI) available.
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Fig. 1 (a) Structure and solution color variation of OXA-1 in DMSO
for switching and oxidation. (b) UV-Vis spectra of OXA-1, ROF-1 and
photooxidation product of OXA-1 (DMSO, 1.0×10-4 M). Inset：
HMRS for OXA-1-R. (c) Infrared spectra comparison between OXA-
1-R and ROF-Br-1.
(vas=2954 cm-1, vs=2861 cm-1) are still present in OXA-1-R. However
the vibration absorption of –OH group is not present, compared to
the IR spectrum of ROF-Br-1. In addition, by monitoring the UV-Vis
spectra of OXA-1 in DMSO exposed to sunlight, after 2 days, the
spectrum reveals both spectral signatures of the ring-open form
and photooxidation product with λmax at 588 nm and 520 nm,
respectively (Fig. 2a, upper, cyan line). Further, OXA-1-R was also
observed by MS and UV-Vis spectrum after directly exposing DMSO
solution of ROF-Br-1 to sunlight (Fig. 2a, below and Fig. S1, ESI†).
Fig. 2 UV-Vis spectra variation of OXA-1 (upper) and ROF-Br-1
(below) in DMSO solution (1.0×10-4 M) before and after irradiation
(a) with sunlight or (b) with a halogen tungsten lamp (Vis) for 8 hrs
and a high pressure mercury lamp (UV) for 2 hrs. The hump bands
for spectra of 2d and 4d in (a) (cyan lines) were separated into two
peaks (slight blue dash lines) respectively by XPS PEAK 4.1 software.
(c) 1H NMR spectra variation of ROF-Br-1 in DMSO-d6 with
irradiation time (irradiated by the high pressure mercury lamp).
These results indicated that the ring-open form is
a key
methylenes leading to loss of their NMR resolution. This conclusion
was further confirmed by 2D NMR spectra (Fig. S4 – S7, ESI†). These
results all accord with the proposed structure of OXA-1-R in Fig. 1a.
During this investigation, it was also determined that the
photooxidation of OXA-1 was greatly diminished in oxygen-free
condition, which indicated that oxygen plays an important role in
the photooxidation of OXA-1 (Fig. S8, ESI†). To determine if reactive
oxygen species (ROS) or molecular oxygen was involved in the
photooxidation process, a number of control experiments were
conducted. The experiments were conducted with ROF-Br-1
because it is more easily oxidized than OXA-1 and it is believed to
be the key intermediate. Vitamin C (VC), as a ROS inhibitor,13 was
used to confirm whether ROS is involved in the photooxidation
process. As shown in Fig. 3a, generation of OXA-1-R decreases with
intermediate. OXA-1 and ROF-Br-1 were then independently
irradiated with a high pressure mercury light (λmax = 365 nm, 500W)
or a halogen tungsten lamp (250W) with a L-420 optical filter to
filter UV light. In these experiments, the formation of OXA-1-R was
only observed with UV light, which suggests that visible light will
not cause photooxidation (Fig. 2b).
To further confirm the oxidation process and elucidate the
structure of OXA-1-R, 1H NMR spectroscopy was conducted. For the
1H NMR spectrum of ROF-Br-1 in DMSO-d6, the sample was
irradiated with a high pressure mercury lamp and monitored over a
period of time. 1H signals for photooxidation product OXA-1-R were
found to gradually increase with irradiation time. After irradiation
for 10 hrs, the ratio of OXA-1-R to ROF-Br-1 is 1 : 2.2, a value which
is calculated by integrating the area of their characteristic peaks (Fig.
2c and Fig. S2, ESI†). Furthermore, ROF-Br-1 could be completely
converted with mainly generating OXA-1-R by increasing the
irradiation time further (Fig. S3, ESI†). The number of H atoms in
high field region for OXA-1-R was conserved, that is, 6H for N(CH3)2,
6H for C(CH3)2, and two 2H for NCH2CH2O. However, in low field
region the intergration revealed only eight H atoms, compared to
nine H atoms observed for ROF-Br-1. This result is consistent with
HRMS and IR data. Additionally, the weak broad peak around 5.1
ppm belonging to H for –OH and signals for the two H on trans C=C
bond (doublet with coupling constant J = 15.4 Hz) disappear in OXA-
1-R, only 4H with single peak could be observed at 6.68 ppm. It is
worth noting that a phenomenon of signal broadening instead of
splitting for methylene signals of -NCH2CH2O- (i.e. 8H, 9H) was
observed for OXA-1-R. This is most likely due to that the formed
seven-membered ring restricts free rotation of hydrogens on
added equivalent of VC from
0 to 100 detected by high-
performance liquid chromatography (HPLC). The effect of VC on the
photooxidation process can be clearly seen by plotting the ratio of
OXA-1-R to ROF-Br-1 (Fig. 3b). Combined these results support the
role of ROS in the photooxidation of OXA-1.
To uncover the specific type of ROS, 1,3-diphenylisobenzofuran
(DPBF) was used. DPBF only reacts with singlet oxygen (1O2) and
superoxide anion (O2.-) which leads to fluorescence quenching.14
Upon addition of DPBF to DMSO solutions with varying
concentrations of ROF-Br-1, we monitored the fluorescence over
time. As shown in Fig. 3c, the fluorescence quenching rate of DPBF
increases with the concentration of ROF-Br-1. In contrast, under the
same experimental condition, the fluorescence quenching rate of
DPBF with different concentrations of OXA-1 showed no obvious
change (Fig. S9, ESI†). Based on these experiments we believe 1O2
or O2.- is involved in the photooxidation process and generates from
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Fig. 3 (a) HPLC spectra of ROF-Br-1 in DMSO with different
equivalent of VC (a ROS inhibitor) after 2 hrs’ irradiation by high
pressure mercury lamp. (b) Plot of integral area ratio of OXA-1-R
and ROF-Br-1 as VC equivalent varies in a. (c) Normalized
luminescence intensity of DPBF in DMSO (λex = 405 nm, 3.2 × 10-5 M)
at 460nm with different concentration of ROF-Br-1 changed with
irradiation time by a 365 nm handheld UV lamp. (d) HPLC spectra
for ROF-Br-1 in DMSO with or without addition of MB, respectively,
after being irradiated by a halogen lamp for 20 mins.
Fig. 4 UV-Vis spectra of OXA-1 in different solvents (a) before and (b)
after being irradiated by sunlight for 7 days. Gradual intensity
variation of representative absorption bands around 331 nm, 588
nm and 520 nm for photooxidation of (c) OXA-1 and (d) ROF-Br-1 in
various solvents against irradiation days by sunlight. (e) pH change
for different solvents before (gray bar) and after (purple bar)
irradiation by a high pressure mercury lamp for 2 hrs. (f) Photos of
ROF-Br-1 added in the irradiated solvents.
the ring-open form. To verify whether O2.- could oxidize ROF-Br-1 to
OXA-1-R, ROF-Br-1 was directly added into solutions with different
concentrations of O2.- (prepared through KO2 and 18-crown-6). Only
closing ring of ROF-Br-1 (under low concentration of O2.-) or
formation of a new compound with λmax at 450 nm (under high
concentration of O2.-) was observed (Fig. S10, ESI†). This suggests
1O2, instead of O2.-, participates in the photooxidation of OXA-1 to
OXA-1-R. To further confirm this hypothesis, methylene blue (MB),
a visible light sensitizer known to produce 1O2, was added to the
solution of ROF-Br-1, and then irradiated by a halogen tungsten
lamp. A control experiment without the addition of MB was also
conduced. As expected, the formation of OXA-1-R from the solution
containing MB was substantially greater than the solution without
MB. The amount of OXA-1-R formed during these experiments was
detected by HPLC (Fig. 3d). These results further support that 1O2
participates in the photooxidation of OXA-1 to OXA-1-R.
MeOH > MeCN > EtOH (absorbance change at bands around 588
nm and 520 nm indicates non-decomposed oxidation) and EtOAc ≈
PhMe > DMF (absorbance change only at band around 331 nm
indicates decomposed oxidation). This suggests that solvent affects
both the oxidation type and the oxidation rate of OXA-1. In addition,
the oxidation rate of OXA-1 to OXA-1-R depends on the generation
speed of its ring-open form. This was further confirmed by the
results from photooxidation of ROF-Br-1 in the same solvents (Fig.
4d, and S13, S14, ESI†). Compared to OXA-1, ROF-Br-1 exhibits
faster oxidation rates in DCM, DMSO and MeOH (Fig. 4d versus Fig.
4c, absorbance change at bands around 588 nm and 520 nm). It’s
worth nothing that the photooxidation of OXA-1/ROF-Br-1 is more
in favour of generating OXA-1-R in DMSO than in other solvents.
This is might due to that DMSO is well-known to trigger ROS
sensitized by dyes.15 Because it is known that OXA-1 converts to the
ring-open form in the presence of acid, we suspect the different
oxidation rate of OXA-1 is due to their pH change after irradiation
by the high pressure mercury lamp, which is solvent dependent.
To evaluate the role of acid, pH was monitored for various
solvents without molecules before and after irradiation with a high
pressure mercury lamp for 2 hrs. As shown in Fig. 4e, the pH value
of all the solvents are around 7 before irradiation. After exposure to
light irradiation, the solutions became more acidic, with the
exception of DMF, probably due to producing an amine from its
photolysis. The pH values for each solvent are as follows: DCM
(2.22), DMSO (3.30), MeOH (3.75), EtOH (4.36), MeCN (5.00), EtOAc
(3.34), PhMe (4.37), DMF (11.3). When OXA-1 was added into the
irradiated acid solvents respectively, most of the solutions turned to
blue quickly as anticipated, except in EtOAc, PhMe (Fig. 4f). This is
mainly due to the low polarity of EtOAc and PhMe, which cannot
stabilize the zwitterion form of ROF-1. Hence, by decreasing the
Different oxidation rates of OXA-1 in DMSO and EtOH (Fig. S11,
ESI†) aroused our interest in invesꢀgaꢀng the solvent effect on the
photooxidation process of OXA-1. For these studies we monitored
the change in UV-Vis spectra of OXA-1 in eight solvents, including
DMSO, dimethylformamide (DMF), methanol (MeOH), EtOH,
acetonitrile (MeCN), ethyl acetate (EtOAc), dichloromethane (DCM)
and toluene (PhMe), while being exposed to sunlight. Before light
irradiation, OXA-1 exists mainly in its ring-closed form in these
solvents (Fig. 4a). However, after irradiation, the characteristic band
for OXA-1-R around 520 nm was generated only in DMSO, MeOH,
EtOH, MeCN and DCM, but not in EtOAc, PhMe and DMF. Of note,
in EtOAc, PhMe and DMF, OXA-1 degraded into other molecules
with low molecular mass, which was further confirmed by MS. (Fig.
4b and S12, ESI†). To gain a better understanding of the oxidation
process of OXA-1 in these solutions, we monitored the absorption
bands around 331 nm (OXA-1 or its generated degraded products),
588 nm (ROF-1) and 520 nm (OXA-1-R) over the course of 18 days.
From the absorbance change in Fig. 4c, we can easily get the
oxidation rate of OXA-1 in these solvents, that is, DCM > DMSO >
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formation of key intermediate ROF-1, the oxidation of OXA-1 to photooxidation were investigated in detail. It was discovered that
OXA-1-R in EtOAc, PhMe and DMF is suppressed. What's more, blue surrounding media of solvents releasing H+ from photolysis plays an
shades of the solutions indicate the ring-open form ratios are in the important role in product type and speed of the photooxidation.
following order: DCM ≈ DMSO > MeOH > EtOH > MeCN, which is in This work may not only enlighten people how to avoid
accordance with their irradiation-induced acidity, and similar to photooxidation of oxazolidines based materials, but also explained
oxidation order of OXA-1 to OXA-1-R. These results confirm that why UV light is so harmful to bio-systems for that all solvents tested
ring-opening, caused by the UV irradiated acidity of solvents, affects here suffered chemical bond breaking from UV photolysis.
the oxidation rate for OXA-1 to OXA-1-R, and also explains why
OXA-1 suffers different oxidation pathways in different solvents.
We thank the NSFC (No. 51603085, 21572079) for financial
support. We appreciate helpful discussion with Javier Read de
To evaluate the role of the tethered (-CH2CH2OH) group, we Alaniz (University of California, Santa Barbara).
investigated derivatives lacking the flexible electron donating group
(instead of with -CH3). This compound was found to be stable under Conflicts of interest
the same condition upon irradiation by a high pressure mercury
The authors declare no competing financial interest.
lamp (Fig. 5a). This indicates that the flexible electron donor (-
CH2CH2OH) is critical for the formation of the seven-membered ring.
Notes and references
From the above data, we propose the following photooxidation
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Fig. 5 (a) Photos for solutions of ROF-Br-1 and its contrast molecule
(1.0×10-4 M) before and after irradiation by a high pressure mercury
lamp for 2 hrs. (b) Proposed mechanism for the photooxidation of
OXA to OXA-R. (c) Calculated and experimental m/z results of
potential photooxidation products of OXAs by HRMS.
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Photooxidation of oxazolidine molecular switches through media-induced
intramolecular ionization forming flexible donating group facilitated cyclization with
1O2 as oxidant.