Z to E light-activated isomerization of α-pyridyl-N-arylnitrone ligands sensitized by rhenium(I) polypyridyl complexes

Article abstract of DOI:10.1016/j.ica.2020.120009

A series of rhenium(I) polypyridyl compounds, bearing photoisomerizable nitrones as ligands, was synthesized and characterized by several techniques. The photochemical and photophysical behaviors of the compounds were investigated. Upon irradiation, acetonitrile solutions of the nitrones, or their respective complexes, exhibit changes in absorption, emission, and FTIR spectra. FTIR revealed the formation of the respective anilide as the photoproducts of irradiation of the uncoordinated nitrones, while irradiation of the complexes resulted in Z → E due to the photosensitized isomerization of the coordinated ligand, also confirmed by HPLC-MS and ¹H NMR. The photoisomerization quantum yields are dependent on the nature of the nitrone substituent, which changes the energy of the ³IL_Z-NitX excited state, which is populated by photosensitization. ³MLCT becomes the lowest-lying excited state in the E-product and results in an increase in emission intensity. The changes in spectroscopic properties of the Z or E coordinated nitrones can be exploited for molecular devices such as photosensors.

Full text of DOI:10.1016/j.ica.2020.120009

InorganicaChimicaActa514(2021)120009
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Research paper
Z to E light-activated isomerization of α-pyridyl-N-arylnitrone ligands
sensitized by rhenium(I) polypyridyl complexes
Julia F. Mamud, Giovanna Biazolla, Caroline S. Marques, Giselle Cerchiaro, Thiago B. de Queiroz,
Artur F. Keppler, André S. Polo
Universidade Federal do ABC, Av. Dos Estados, 5001 Santo Andre, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of rhenium(I) polypyridyl compounds, bearing photoisomerizable nitrones as ligands, was synthesized
and characterized by several techniques. The photochemical and photophysical behaviors of the compounds
were investigated. Upon irradiation, acetonitrile solutions of the nitrones, or their respective complexes, exhibit
changes in absorption, emission, and FTIR spectra. FTIR revealed the formation of the respective anilide as the
photoproducts of irradiation of the uncoordinated nitrones, while irradiation of the complexes resulted in Z → E
due to the photosensitized isomerization of the coordinated ligand, also confirmed by HPLC-MS and 1H NMR.
The photoisomerization quantum yields are dependent on the nature of the nitrone substituent, which changes
the energy of the 3ILZ-NitX excited state, which is populated by photosensitization. 3MLCT becomes the lowest-
lying excited state in the E-product and results in an increase in emission intensity. The changes in spectroscopic
properties of the Z or E coordinated nitrones can be exploited for molecular devices such as photosensors.
Photosensitized isomerization of nitrones
Rhenium(I) polypyridyl compounds
Photosensors
1. Introduction
dynamics of the process have also been investigated by quantum me-
chanics approach [36] as well as time-resolved experiments [37–39].
The photophysical properties of rhenium polypyridyl compounds
were first described in the 1970s [1]. The characteristics of these
compounds have been exploited in the development of potential mo-
lecular devices, such as molecular switches [2–4], luminescent sensors
[5–9], ion sensors [10], and electroluminescent layers for OLEDs
[11–15]. These compounds are also valuable tools for studying push–-
pull systems and proton-coupled electron transfer systems [16] as well
as in practical applications as triggers for the electron injection process
[17,18], and non-linear optics [19].
New compounds are sought for the development of molecular devices
based on photoassisted isomerization.
The photoassisted isomerization was investigated for non-polar
double bonds until now. Besides the stilbene or azo groups, the use of
nitrone groups can access information on the isomerization process by
using a polarized, charged, and oxidized double bond of nitrones (C]
N+–O−), since it presents structural and electronic characteristics that
contribute to the comprehension of photosensitized isomerization pro-
cesses. The literature reports that the direct irradiation of Z-nitrone
results in its decomposition. On the other hand, by populating the 3IL
state by photosensitization, the only product detected is the E-isomer.
Thus these compounds are promising candidates to evaluate the pho-
tosensitization process promoted by the rhenium(I) moiety.
The lowest-lying excited state of rhenium(I) polypyridyl compounds
is usually the triplet metal-to-ligand charge transfer one, 3MLCT. The
energy of this excited state is often sufficient to sensitize intra- or in-
termolecular chemical reactions, such as photocyclization [20–22] or
trans-to-cis isomerization of azo or stilbene-like compounds [23–29].
Stilbene-like ligands are the most investigated group for the iso-
merization sensitized by rhenium(I) complexes. Modification on these
ligands, such as the use of electron-withdrawing, multiple double bonds
[24,30] or surfactant groups [31] was done to evaluate their effects on
the photosensitized isomerization. Besides the modifications on the
photoisomerizable ligands, the use of coordinated polypyridyl ligands
having different substituents on the photochemical process has also
been investigated [32–34], as well as binuclear compounds [35]. The
In this work, we report the synthesis, characterization, photo-
chemical and photophysical properties of a new class of rhenium(I)
compounds featuring nitrone-containing groups (C]N+–O−) as iso-
merizable ligands, Fig. 1. Photochemical and photophysical tools, as-
sociated with the rational design of nitrone-containing ligands, led to
the investigation of the influence of electron-withdrawing and donating
groups on the photosensitized Z- to E- isomerization process. These
investigations aim to evaluate their possible use in the development of
molecular devices.
E-mail address: andre.polo@ufabc.edu.br (A.S. Polo).
https://doi.org/10.1016/j.ica.2020.120009
Received 19 June 2020; Received in revised form 4 September 2020; Accepted 4 September 2020
Availableonline12September2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

J.F. Mamud, et al.
InorganicaChimicaActa514(2021)120009
Fig. 1. Structures of the investigated Re(I) complexes (The nitrone functional group is highlighted).
2. Experimental
experiments. The values reported for the photostationary state, PSS,
were determined by the absorbance difference of the initial and final
samples, divided by the initial value. Emission and excitation spectra
were recorded using a Cary Eclipse spectrofluorometer, as described
elsewhere [42,43,46,47]. For all experiments, quartz cuvettes with a
1.000-cm path length and HPLC-grade solvents were employed. The
HPLC-ESI-MS analyses were carried out in an Agilent Technologies
1290 Infinity equipment having a DAD (UV/Vis) detector coupled to an
Agilent Technologies Electrospray Mass Spectrometer (ESI-MS), model
6130 Quadrupole. The chromatographic profiles were collected by
using an Agilent Poroshell 120 EC-C18, 2,1 × 50 mm, 4 µm column at
40 °C. 10 μL of the sample was injected and it was applied a two-solvent
elution gradient (A = Ultrapure Water + 0.1% TFA and B = 90%
2.1. General methods
FTIR spectra were obtained on a Perkin–Elmer Spectrum One
spectrophotometer using an ATR accessory. 1H NMR spectra were re-
corded at 300.15 K on a Bruker Avance III spectrometer operating at a
frequency of 300 MHz or on a DRX-500 Bruker Avance spectrometer
operating at a frequency of 500 MHz. 1H chemical shifts are reported in
parts per million (ppm), with TMS at 0 ppm as absolute standard, and
residual acetonitrile as internal standard with the CH3 peak centered at
1.96 ppm [40]. Additional 1H and 13C NMR spectra were acquired in a
Varian VNMRS 500 MHz spectrometer operating at a resonance fre-
quency of 125.7 (13C) and 500 (1H) MHz, using a 5 mm liquid state 1H
indirect probe. Great care was taken in shimming the magnet in order
to obtain highly resolved spectra without need for spinning the samples.
13C chemical shifts were referenced to TMS at 0 ppm with the CN
acetonitrile at 118.06 ppm as internal reference [40]. The absorption
spectra were recorded using an Agilent 8453 spectrophotometer
equipped with an Agilent 89090A Peltier temperature controller de-
scribed elsewhere [41]. Irradiation of samples was performed using a
Newport solar simulator model 96,000 with an AM1.5G filter [42,43]
or a Newport system comprising a lamp house equipped with a xenon
(Hg) 200 W lamp [35]. The light passes through a water filter, followed
by the selection of the 405-nm wavelength by an appropriate inter-
ference filter. The light intensity was determined by the method de-
scribed by Hatchard and Parker using tris-(oxalato)ferrate actinometry
[44,45]. The quantum yields reported are the average of three distinct
HPLC grade acetonitrile + 0.1% TFA) at a flow rate of 0.300 mL min−1
.
The elution gradient as a function of solvent B was: 10% − 95% over
20 min; 95%-100% over 5 min; 100% for 10 min resulting in a total
time of 35 min per analytical run. DAD detection was set to monitor
absorption changes at 290 and 340 nm. The ESI-MS analyses were
performed by using ionization in positive mode, capillary voltage
3.0 kV, capillary temperature 350 °C, ion energy 5.0 V, nitrogen gas
flow 12 L min−1, solvent heater 250 °C, 35 psig pressure in the nebu-
lizer, multiplier 1.0 and cone at 35 V. The mass spectrometer was
programmed for scan the mass range of 100 to 1500 Daltons. All sol-
vents for synthesis were purchased from Synth – Brazil. All reagents for
synthesis and all deuterated and HPLC solvents were purchased from
Sigma-Aldrich - USA and used as received. The (Z)-3,4-dimethyl-N-
(pyridin-4-ylmethylene) aniline oxide (Z-Nit-Me2), (Z)-N-(pyridin-4-yl-
methylene)aniline oxide (Z-Nit-H) and (Z)-4-chloro-N-(pyridin-4-
Scheme 1. Synthetic routes for preparation of the ligands and coordination compounds.
2

J.F. Mamud, et al.
InorganicaChimicaActa514(2021)120009
ylmethylene)aniline oxide (Z-Nit-Cl) were prepared and characterized
as described elsewhere [48].
The compounds were prepared as depicted in Scheme 1 and de-
scribed below.
2.1.1. Synthesis of fac-[Re(CO)3(Ph2-phen)Cl]
The fac-[Re(CO)3(Ph2-phen)Cl] complex was prepared using a slight
modification of the procedure previously described for similar com-
pounds [49–51]. To xylene (100 mL), 4,7-diphenyl-1,10-phenanthro-
line, Ph2-phen, (1.08 g − 3.25 mmol) was added. The mixture was
continuously stirred and heated to boiling, upon which the complete
dissolution of the ligand was observed. Next, Re(CO)5(Cl)
(1.2 g−3.25 mmol) was added, and the mixture was refluxed for 2.5 h.
After the solution had reached room temperature, the formation of a
solid was observed, which was filtered and dried in a desiccator,
yielding 1.69 g of product, corresponding to 86% yield. The spectro-
scopic data for the product were coincident with those reported [52].
Fig. 2. Absorption spectra of Re(Z-Nit-X) (solid lines) and Z-Nit-X (dashed
lines), X = Me2, H, or Cl in acetonitrile.
2.1.2. Synthesis of fac-[Re(CO)3(Ph2-phen)(tfms)]
The fac-[Re(CO)3(Ph2-phen)(tfms)] complex was prepared using a
slight modification of the procedure described in the literature for si-
milar compounds [49–51]. To dichloromethane (75 mL), fac-[Re
(CO)3(Ph2-phen)Cl] (1.40 g−2.2 mmol) was added, and the mixture
absorption in the lower-energy region (ca. 300–450 nm) exhibited
molar absorptivities higher than those typically observed for rhenium-
polypyridyl compounds [1,53]. The high molar absorptivities can be
ascribed to a red-shift of nitrone absorption after coordination to the
rhenium(I) center, resulting in a mixture of IL (π-π*Nit) and MLCT
(dπRe-π*Ph2-phen) transitions in this region. This behavior was pre-
viously observed for similar compounds with stilbene-like ligands
[30,54].
was continuously stirred under argon for
1
h. Then, tri-
fluoromethanesulfonic acid, Htfms, (2 mL–23 mmol) was added, fol-
lowed by stirring under argon for additional 2 h. The product was ob-
tained by adding diethyl ether until the precipitation of the complex.
The solid was collected by filtration and resulted in 0.84 g of product,
which corresponds to 50% yield. The spectroscopic data for the product
was in agreement with those reported [52].
The presence of electron-withdrawing or electron-donating sub-
stituents on the nitrones had little influence on their absorption
maxima. However, small changes were observed for the complexes,
which featured maxima in the lower-energy region of 345, 341, and
340 nm for ReNit-X, X = Me2, H, or Cl, respectively.
2.1.3. Synthesis of fac-[Re(CO)3(Ph2-phen)(Z-NitX)]PF6; X = Me2, H or
Cl (Re(Z-Nit-X))
The fac-[Re(CO)3(Ph2-phen)(Z-Nit-X)] (X = Me2, H or Cl) com-
plexes were prepared using a modification of the procedure described
in the literature for similar compounds using stilbene-like ligands
[34,49–51]. The precursor complex, fac-[Re(CO)3(Ph2-phen)(tfms)],
and the respective Z-nitrone ligand (3 equivalents) were added to me-
thanol (40 mL). The mixture was heated to reflux for 5 h, followed by
adding NH4PF6 (2 equivalents). After volume reduction, the product
was collected by filtration and washed with diethylether. fac-[Re
(CO)3(Ph2-phen)(Z-Nit-Me2)]PF6 yield 60% (Anal. calcd for
Irradiation of deaerated acetonitrile solutions of Z-Nit-X or Re(Z-
Nit-X) by AM1.5G simulated sunlight initially verified the behavior of
the prepared compounds. The absorption spectral changes in Z-Nit-X
upon irradiation are exemplified by Z-Nit-Me2, Fig. 3a, while those
observed for the complexes are exemplified by the changes in Re(Z-Nit-
Me2), Fig. 3b.
Absorption changes for the pyridyl-nitrone solution resulted in
isosbestic points at 232, 239, and 265 nm (Fig. 3a). The FTIR spectra of
the compounds revealed the difference in the products obtained
through direct irradiation of the pyridyl-nitrone or by irradiation of the
coordination compound, Fig. 4. The pyridyl-nitrone solution exhibited
a peak at 1555 cm−1 characteristic of the nitrone and, after irradiation,
a new intense peak at 1666 cm−1 typical of an amide carbonyl group,
Fig. 4a. The photochemistry of nitrones depends on the reaction con-
ditions and can result in its geometric isomerization or its inter-
conversion. This last reaction can lead the nitrone into oxazirane or the
respective anilide [55]. Direct irradiation of the nitrone results in the
identification of the anilide species, by its intense characteristic peak at
1666 cm−1 in the FTIR spectra. The expected strong IR absorption at
1300 cm−1 region [56] for the respective oxazirane is not observed in
the spectra, indicating a major presence of the anilide species.
Irradiation of the Re(Z-Nit-X) solution also leads to absorption
changes as a function of time. The FTIR of the photochemical products
exhibits the characteristic nitrone peak at 1555 cm−1, while it is not
observed the benzanilide IR peak, at 1666 cm−1, Fig. 4b.
C
41H30F6N4O4PRe: C, 50.56; H, 3.10; N, 5.75; found: C, 50.24; H, 3.42;
N, 5.60). 1H NMR (300 MHz, CD3CN, δ / ppm) 8.41 (dd, 2H), 8.09 (m,
2H), 8.16 (s, 1H), 7.55 (d, 1H) 2.31 (s, 6H), 7.46 (dd, 1H), 9.67 (d, 2H),
8.09 (d, 2H), 7.67 (m, 10H), 8.11 (m, 2H). fac-[Re(CO)3(Ph2-phen)(Z-
Nit-H)]PF6·H2O yield 34% (Anal. calcd for C39H28F6N4O5PRe: C, 48.60;
H, 2.93; N, 5.81; found: C, 48.89; H, 2.72; N, 5.78). 1H NMR (300 MHz,
CD3CN, δ / ppm) 8.45 (dd, 2H), 8.12 (m, 2H), 8.21 (s, 1H), 7.75 (dd,
2H), 7.55 (m, 3H), 9.68 (d, 2H), 8.09 (d, 2H), 7.68 (m, 10H), 8.12 (m,
2H). fac-[Re(CO)3(Ph2-phen)(Z-Nit-Cl)]PF6·H2O yield 51% (Anal. calcd
for C39H27ClF6N4O5PRe: C, 46.92; H, 2.73; N, 5.61; found: C, 47.09; H,
2.74; N, 5.72). 1H NMR (300 MHz, CD3CN, δ / ppm) 8.45 (dd, 2H), 8.10
(d, 2H), 8.19 (s, 1H), 7.75 (dd, 2H), 7.54 (dd, 2H), 9.67 (d, 2H), 8.09 (d,
2H), 7.67 (m, 10H), 8.12 (m, 2H).
3. Results and discussion
The absorption spectra of the compounds Z-Nit-X, X = Me2, H or
Cl, and the corresponding coordination compounds, Re(Z-Nit-X), in
acetonitrile are presented in Fig. 2. The nitrone compounds exhibited
absorption up to 375 nm, which can be ascribed to π-π* electronic
transitions.
The E isomer is the exclusive product when intermolecular triplet-
sensitized isomerization occurs [56]. This pathway avoids the forma-
tion of other products, such as the anilide observed after direct irra-
diation of the pyridyl-nitrone by FTIR (Fig. 4). The absence of different
isomers than the geometric ones, as it is registered by FTIR, is an in-
dication of the nitrone functional group stability under the
The absorption spectra of the complexes included intense absorp-
tion bands in the higher-energy region (200–300 nm) due to the pre-
sence of intraligand π-π* transitions of the Ph2-phen ligand [52]. The
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J.F. Mamud, et al.
InorganicaChimicaActa514(2021)120009
photosensitization promoted by the rhenium(I) polypyridyl moiety.
Photochemical experiments using 405 nm irradiation, where the
light is only absorbed by rhenium(I) moiety, were also conducted.
Changes in absorption spectra confirm the occurrence of the photo-
sensitized process (supporting information). Besides these changes,
HPLC-MS experiments were conducted aiming to identify and quantify
the products formed, Fig. 5.
The chromatographic profile of the non-irradiated sample exhibits a
major peak at the retention time of 1 min, Fig. 5A. The mass spectrum
of this sample reveals two peaks, the first one at m/z = 710 and a
second one at m/z = 829, Fig. 5C. The first peak (m/z = 710) is as-
cribed to a fragment of the Re(Z-Nit-Me2), as depicted in the inset of
Fig. 5C, probably due to the ionization process of ESI-MS, and the
second one is ascribed to the molecular ion of the compound Re(Z-Nit-
Me2). Both peaks exhibit the characteristic isotopic pattern of the
rhenium metallic center, indicating the presence of the metal center in
both fragments detected [57]. After irradiation of the sample at
405 nm, Fig. 5B, it is observed a new peak on the chromatographic
profile. Still, the mass spectra for both peaks exhibited in the chroma-
togram are similar and coincident to the ones of the non-irradiated
sample (Fig. 5A and C). Unfortunately, the low affinity of Re(Z-Nit-
Me2) to the stationary phase did not allow the separation and quanti-
fication of the Z and E isomers after irradiation.
On the other hand, this technique reinforces the absence of other
photoproducts. Possible different products should have a distinct in-
teraction with the stationary phase, resulting in the observation of other
peaks; Fig. 5b exhibits only two peaks having very close retention time.
Due to these reasons, the chromatograms emphasize the absence of by-
photoproducts.
Fig. 3. Absorption changes upon irradiation of Z-Nit-Me2 (a)
(conc. = 5.8 × 10−5 mol L−1) or Re(Z-Nit-Me2) (b) (conc. = 3.2 × 10−5 mol
L−1) in acetonitrile. (Pirr = 100 mW cm−2; room temp; Δt = 15 s; red line
t = 300 s). (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
The photoisomerization of the Re(Z-Nit-Cl) was also verified by 1H
NMR, Fig. 6. The assignment of the resonance peaks, indicated in the
inset, was derived from the 1H–1H COSY experiments and comparative
analysis with the 1H spectrum of the ligand (Figs. S3–S5). In this ex-
periment, the sample was irradiated for 80 min at 405 nm and achieved
15% of isomerization, determined by absorption changes, Fig. S9 The
1H NMR of the irradiated sample exhibits new peaks at 9.96, 8.65, and
7.62 ppm, in comparison to the spectrum of Re(Z-Nit-Cl) and are as-
cribed to the signal of Ha’, Hc’ and Hd’ of the Re(E-Nit-Cl) isomer
formed. No further signals are observed in the spectrum of the irra-
diated sample, confirming the absence of by-products, as was indicated
by HPLC-MS. Using the integral of Ha and Ha’ the conversion of Z to E
isomer achieved around 35%. This difference in the photolysis per-
centage is due to the absorption of both isomers in the same absorption
region, whilst they are well separated in the 1H NMR spectrum.
The photosensitized isomerization quantum yields of the complexes
determined by the absorption changes at 370 nm were ΦRe(Z-Nit-Me2)
=
(12.7
0.2) × 10−3, ΦRe(Z-Nit-H) = (8.8
0.7) × 10−3, and ΦRe(Z-
Nit-Cl)= (7.5
0.2) × 10−3. Irradiation to their respective photosta-
tionary state, PSS, in which there is not observed any further absorption
changes, resulted in photolysis percentage for Re(Z-Nit-Me2) = 55%;
Re(Z-Nit-H) = 53% and Re(Z-Nit-Cl) = 48%. Correspondence between
Φ and PSS values was observed. After reaching their PSS, the absorption
spectra of the samples were monitored for one hour at 25, 35, 40, 45,
50, and 60 °C. No significant changes were observed, discarding the
thermal reversibility.
Changes in photophysical properties are also observed after the Z →
E photoisomerization of the coordinated pyridyl-nitrones. Initially, Re
(Z-Nit-X) exhibits low-intensity luminescence in acetonitrile solution
(Fig. 7). As the sensitized isomerization of the coordinated ligand
proceeds, there is an increase in the emission intensity of the samples
due to the presence of E-isomer in higher concentrations. It is observed
similar behavior to rhenium(I) compounds using stilbene or azo ligands
[24,32].
Fig. 4. FTIR of Z-Nit-Me2 (a) or Re(Z-Nit-Me2) (b) (black lines) and after 10-
min irradiation (red lines). (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
All three complexes have the same broad and non-structured
emission profile and maxima after photoisomerization, indicating that a
common lowest-lying excited state is achieved for all E- isomers. The
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J.F. Mamud, et al.
InorganicaChimicaActa514(2021)120009
Fig. 5. Chromatograms of Re(Z-Nit-Me2) before (A) and after (B) irradiation at 405 nm (λ = 340 nm) and the mass spectra of the samples before (C) and after (D)
irradiation at 405 nm. Fig. 5C inset: expansion of the mass spectrum peaks and the respective fragment proposed.
Fig. 7. Emission spectral changes as a function of irradiation time of Z-Nit-Me2
(
) and Re(Z-Nit-Me2) (▬) acetonitrile solutions. (λexc. = 350 nm;
v = 600 nm min−1; Δtirr = 15 s).
Fig. 6. 1H NMR spectra of Re(Z-Nit-Cl) at t = 0 min and after 80 min of irra-
sensitizing this excited state responsible for the photoisomerization
process. However, due to the proximity of these states, a low-intensity
emission is observed from 3MLCTRe-Ph2-phen, even for the Z-isomers. It
deactivates this excited state, resulting in photochemical low quantum
yield values. After the isomerization, the new 3ILE-Nit-X is more energetic
than 3MLCTRe-Ph2-phen. Thus, the excitation of the E-isomer results in the
observed emission spectrum instead of the isomerization process. The
photochemical and photophysical data are summarized in Table 1.
The results of Φ and PSS suggest a dependence of the photochemical
behavior on the substituent on the pyridyl-nitrone. Lower Φ and PSS
values are observed for Re(Z-Nit-Cl), whereas a higher Φ and PSS are
observed for Re(Z-Nit-Me2). These results are in agreement with the
absorption spectra of the complexes; the lowest energy maximum was
observed for Re(Z-Nit-Me2) (345 nm), and Re(Z-Nit-H) and Re(Z-Nit-
Cl) had approximately the same energy (341 and 340 nm, respectively),
resulting in similar isomerization quantum yields. The absorption
diation at 405 nm. (initial conc. = 6.62 × 10−4 mol L−1).
excited state responsible for the observed luminescence was confirmed
by the emission spectrum of fac-[Re(CO)3(Ph2-phen)(py)]+, py = pyr-
idine, recorded under the same conditions. The normalized spectra
were superimposed, Fig. 8, confirming the nature of the lowest-lying
excited state as the 3MLCTRe-Ph2-phen one, as reported for fac-[Re
(CO)3(Ph2-phen)(py)]+ [58].
Considering the photosensitized isomerization process, the distinct
photophysical properties of the rhenium(I) compounds having the Z- or
E-pyridyl-nitrone isomer and emission characteristics of the E-isomer, a
qualitative energy diagram can be proposed, Fig. 9. In this diagram, for
the Z-isomer, 3MLCTRe-Ph2-phen is more energetic than 3ILZ-Nit-X, the
excited state responsible for the isomerization process. Thus, the energy
harvested by the metal complex can be transferred to 3ILZ-Nit-X,
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J.F. Mamud, et al.
InorganicaChimicaActa514(2021)120009
Table 1
Photochemical and photophysical data determined for the complexes in-
vestigated.
λmax,abs (nm) / ε(104
L
PSS (%) λmax,em
b
Compound
Z
E
a
mol−1 cm−1
)
Re(Z-Nit-Me2) 345 / 3.5
12.7
8.8
0.2 55
560
560
560
Re(Z-Nit-H)
Re(Z-Nit-Cl)
341 / 4.3
340 / 3.1
0.7
0.2
53
48
7.5
a. Absorption of the band having MLCT + IL mixture;
b. Emission of the E-isomer after irradiation of the respective Z-isomer.
and Re(Z-Nit-Cl), 3ILZ-Nit-X is destabilized in comparison to 3ILZ-Nit-Me2
,
making the 3ILZ-Nit-X and 3MLCTRe-Ph2-phen excited-states closer. In this
case, the competition between phosphorescence and photoisomeriza-
tion decreases the Φ and PSS values. Importantly, the energy of
3MLCTRe-Ph2-phen, which is responsible for sensitizing Z → E iso-
merization, is identical for all compounds. Thus, the difference in Φ can
be due to the substituents on the nitrone ligands.
Fig. 8. Emission spectra of Re(Z-Nit-X); X = Me2, H, or Cl after irradiation and
fac-[Re(CO)3(Ph2-phen)(py)]+ (dashed line) in acetonitrile. (λexc. = 350 nm;
v = 600 nm min−1).
The photoassisted isomerization quantum yields determined for Re
(Z-Nit-X) are lower than those reported for stilbene-like ligands in si-
milar compounds using the same equatorial ligand, Ph2-phen
[32,34,52]. The lower quantum yield values are probably due to the
presence of the phosphorescence deactivation pathway, which is not
observed for compounds having stilbene-like ligands.
spectra of these compounds have contributions from both ILZ-Nit-X and
MLCTRe-Ph2-phen electronic transitions in this region. Thus, the different
maxima observed for this series of compounds are due to changes in ILZ-
energy, and such effects can be extended to 3ILZ-Nit-X , the excited-
Nit-X
4. Conclusions
state responsible for the isomerization process.
Changes in the energy of 3ILZ-Nit-X will alter the 3MLCTRe-Ph2-phen
deactivation pathways. After reaching the 3MLCTRe-Ph2-phen excited-
state, it can be deactivated by two pathways. The first one is through
phosphorescence, resulting in the low emission observed for the com-
plex having the Z-nitrone ligand. The second deactivation pathway is by
transferring energy to 3ILZ-Nit-X responsible for the isomerization pro-
cess. The energy-gap between the 3ILZ-Nit-X and 3MLCTRe-Ph2-phen has a
close relationship to these pathways. If the energy-gap is small, phos-
phorescence and photosensitization become more competitive, acti-
vating both pathways. On thv other hand, as the 3ILZ-Nit-X state became
more stabilized, the energy-gap increases, favoring the photosensitiza-
tion process. As previously discussed, Re(Z-Nit-Me2) has the more sta-
bilized 3ILZ-Nit-X excited-state among the compounds investigated.
Consequently, the energy transfer pathway to 3ILZ-Nit-Me2 is favored,
and its Φ and PSS are the highest values in the series. For Re(Z-Nit-H)
A
series of rhenium(I) polypyridyl compounds with photo-
isomerizable pyridyl-aryl-nitrone ligands were synthesized and char-
acterized. The lower-energy region of the absorption spectrum of the
complexes exhibited a mixture of ILZ-Nit-X and MLCTRe-Ph2phen transi-
tions. Due to the ILZ-Nit-X contribution to this absorption band, the use of
different substituents on the nitrones resulted in changes in the ab-
sorption maxima, which were ascribed to the nature of the substituent
on the nitrone. Irradiation of the complexes led to the Z → E nitrone
isomerization pathway, avoiding other products, as confirmed by
HPLC-MS and 1H NMR. This pathway is due to photosensitization
promoted by the Re(Ph2-phen) moiety. The isomerization quantum
yields are also dependent on the substituent on the nitrone. Thus, the
quantum yield can be modulated by changing the substituent on the
nitrone. The E isomer exhibits a more intense emission in comparison to
the Z isomer. Thus, the emission intensity increases as a function of
Fig. 9. Qualitative energy diagrams for the photoassisted isomerization of Re(Z-Nit-X) compounds and the emission of their products.
6

J.F. Mamud, et al.
InorganicaChimicaActa514(2021)120009
photolysis time because of the concentration of the E isomer increases.
Since nitrones are efficient, OH●− scavengers, the different emission
properties of the isomers and the possibility of promoting Z → E iso-
merization in situ make these compounds promising candidates for
molecular devices to detect this radical. Further investigation of the
chemistry and photochemistry of these compounds in the presence of
OH●− and the isomerization quantum yields of new compounds with
similar ligands will be carried out to explore the potential of these
compounds as photosensors.
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CRediT authorship contribution statement
Julia F. Mamud: Data curation, Investigation, Methodology,
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Declaration of Competing Interest
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The authors declare that they have no known competing financial
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The authors acknowledge Fundação de Amparo à Pesquisa no
Estado de São Paulo (FAPESP) (grants: 2018/14152-0, 2017/07289-7,
2016/21993-6, 2015/20007-5 and 2012/07717-5) and CNPq (grant
454971/2014-1) for financial support and the Multiuser Central
Facilities (CEMs) of the UFABC for the experimental support. This study
was financed in part by the Coordenação de Aperfeiçoamento de
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