Visible Photosensitization of trans-Styrylpyridine Coordinated to fac-[Re(CO)3(dcbH2)]+: New Insights

Article abstract of DOI:10.1021/acs.inorgchem.8b01304

A strategic methodology has been developed to effectively synthesize the fac-[Re(CO)₃(dcbH₂)(trans-stpy)]⁺ complex, where dcbH₂ = 2,2′-bipyridine-4,4′-dicarboxylic acid and trans-stpy = trans-4-styrylpyridine, which has been designed to efficiently absorb visible light. The complex exhibits outstanding trans-to-cis photoisomerization with 436 nm irradiation (φ_trans→cis = 0.50 ± 0.03), in contrast to the photochemical behavior previously reported in the literature (Faustino, L. A.; et al. Inorg. Chem. 2018, 57, 2933-2941). The main emphasis here is to address the synthetic strategy for obtaining the actual complex, its characterization, and an accurate description of its photochemical and photophysical behavior, which reveal new insights into this complex.

Full text of DOI:10.1021/acs.inorgchem.8b01304

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Visible Photosensitization of trans-Styrylpyridine Coordinated to
+
fac-[Re(CO) (dcbH )] : New Insights
3
2
Lais S. Matos, Ronaldo C. Amaral, and Neyde Y. Murakami Iha*
Laboratory of Photochemistry and Energy Conversion, Departamento de Química Fundamental, Instituto de Química, Universidade
de Sao Paulo, Avenida Prof. Lineu Prestes 748, 05508-000 Sao Paulo, Sao Paulo, Brazil
̃ ̃ ̃
*
S Supporting Information
ABSTRACT: A strategic methodology has been developed to
+
effectively synthesize the fac-[Re(CO) (dcbH )(trans-stpy)]
3
2
complex, where dcbH = 2,2′-bipyridine-4,4′-dicarboxylic acid
2
and trans-stpy = trans-4-styrylpyridine, which has been
designed to efficiently absorb visible light. The complex
exhibits outstanding trans-to-cis photoisomerization with 436
nm irradiation (Φ
= 0.50 ± 0.03), in contrast to the
trans→cis
photochemical behavior previously reported in the literature
(
Faustino, L. A.; et al. Inorg. Chem. 2018, 57, 2933−2941).
The main emphasis here is to address the synthetic strategy
for obtaining the actual complex, its characterization, and an
accurate description of its photochemical and photophysical behavior, which reveal new insights into this complex.
+
INTRODUCTION
fac-[Re(CO) (dmcb)(trans-stpy)] and fac-[Re(CO) (dmcb)-
(
3
3
■
+
trans-stpyCN)] [dmcb = 4,4′-dimethoxycarbonyl-2,2′-bipyr-
idine, trans-stpy = trans-4-styrylpyridine, trans-stpyCN = trans-
-(4-cyano)styrylpyridine], which are capable of initiating
photoisomerization in a lower-energy spectral region.
Here, we report a strategy for the synthesis of fac-
The photoisomerization of rhenium(I) carbonyl complexes
with stilbene-like ligands has been investigated extensively,
2
−39
4
with a special focus on compounds that extend their
absorption to the visible to create photochemical and
photophysical properties suitable for applications in molecular
38
+
3
,22,33,34,37,40
[Re(CO)
3
(dcbH
2
)(trans-stpy)] , where dcbH
2
= 2,2′-bipyr-
devices.
These studies, and an understanding of
idine-4,4′-dicarboxylic acid, and its photochemical behavior in
their properties, allow modulation and control of key roles into
the development of photoinduced devices, such as photo-
sensitizers, photoswitches, and photoresponsive devi-
achieving sensitization at up to the 436 nm irradiation.
1
Recently, Faustino et al. reported a similar investigation but
without photoisomerization in the visible, giving low
efficiencies with 404 nm irradiation.
The main goal of this paper is to discuss a judicious
synthetic route and strategy for the synthesis of fac-[Re-
CO) (dcbH )(trans-stpy)] along with a step-by-step H
2
2,32,40−45
ces.
Photochemical processes of stilbenes themselves have also
attracted significant attention.
tion occurs, in general, by singlet excited states (S → S or S
0
4
6−54
trans-to-cis photoisomeriza-
+
1
0
1
(
55
3 2
→
S , π → π*) that promote a double-bond twist, typical of
n
5
NMR characterization procedure and a study of its true
photochemical behavior.
6−60
motion in molecular machines.
Similar photoactivation,
still preserving the main photochemical characteristic of
stilbenes, can be achieved at lower energies when it is
coordinated to rhenium(I) tricarbonylpolypyridyl complexes,
EXPERIMENTAL SECTION
■
+
Materials. All chemicals employed in the syntheses (reagent
grade) and solvents for photochemical and photophysical measure-
ments (Aldrich, HPLC grade) were used as supplied.
Syntheses of Ligands. trans-4-Styrylpyridine (trans-stpy). The
trans-stpy ligand was synthesized with modifications of a procedure
fac-[Re(CO) (NN)(trans-L)] , where NN = polypyridyl and L
3
2
2,40−45
=
stilbene-like ligands.
facilitates electron and/or energy transfer because of the high
Coordination in the complex
spin−orbit coupling and extends absorption into the
4
2,43
visible.
20,30,31,37,39
previously described.
Benzaldehyde (7.0 mL, 40 mmol,
Coordination also improves the versatility and variety of
synthetic tools that can be conveniently employed to design
new compounds with desired characteristics to tune excited-
state properties through changes in polypyridyl and photo-
Aldrich) and 4-picoline (4.0 mL, 41 mmol, Aldrich) were refluxed for
12 h in acetic anhydride (4.3 mL, 46 mmol, Merck). The reaction was
monitored by UV−vis spectroscopy and thin-layer chromatography
(
TLC). The solution was distilled and the residual oil was dropped in
22
water to yield a light-yellow precipitate. The crude product was
isomerizable ligands. As a result of a series of studies, the best
results reported so far achieved up to 404 nm sensitization, but
2
4,27,30−32,42,45
usually with a lower quantum yield.
Only
Received: May 13, 2018
recently, our group succeeded in synthesizing new complexes
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
recrystallized from ethanol by the slow addition of ultrapure water.
The solid was filtered off, washed with ultrapure water and ethyl ether
was dried under vacuum (10 mmHg, 60 °C) to obtain 3.15 g of the
1
pure product (5.72 mmol, 90% yield). H NMR (300 MHz, DMSO-
6
3
3
(
Synth), and dried under vacuum (10 mmHg, 60 °C) to obtain 3.73 g
d ): δ 9.21 (d, 2H, J = 5.8 Hz), 9.14 (s, 2H), 8.13 (d, J = 5.7 Hz).
1
of the pure product (20.6 mmol, 51% yield). H NMR (CDCl , 300
MHz): δ 8.57 (d, 2H, J = 5.8 Hz), 7.53 (d, 2H, J = 7.4 Hz), 7.44−
7
MHz): δ 8.53 (dd, 2H, J = 6.2 and 1.7 Hz), 7.62 (d, 2H, J = 7.0 Hz),
7
fac-[Re(CO) (dcmb)Cl]. The fac-[Re(CO) (dmcb)Cl] complex was
3
3
3
3
3
8,9,39
synthesized according to a procedure previously described.
The
3
1
.28 (m, 6H), 7.00 (d, 1H, J = 16.3 Hz). H NMR (CD CN, 300
[Re(CO) Cl] (2.01 g, 5.55 mmol, Aldrich) precursor and an excess of
3
5
3
3
dmcb (1.51 g, 5.55 mmol) were suspended in 70 mL of xylene
(Synth) and heated to reflux for 5 h. The reaction was followed by
UV−vis spectroscopy and TLC. After cooling to room temperature,
the resulting solid was separated by filtration and washed with xylene.
The crude product was recrystallized from dichloromethane (Synth)
by the slow addition of n-pentane (Synth). The product was dried
3
13
,40 (m, 6H), 7.16 (d, 1H, J = 18.0 Hz). C NMR (CD CN, 75
3
MHz): δ 150.12, 144.50, 136.06, 133.07, 128.77, 128.67, 126.93,
1
1
25.91, 120.76. Anal. Calcd for C H N· / H O: C, 85.31; H, 6.17;
13 11 10 2
N, 7.65. Found: C, 85.41; H, 5.90; N, 7.49.
Protonated trans-stpy (trans-Hstpy ). The protonation of trans-
stpy best describes the coordination effects and was obtained by
adding HCl (400 μL, 10 mol L , Synth) to a saturated solution of
trans-stpy (0.380 g, 2.33 mmol) to yield a pure yellow precipitate. The
reaction was monitored by UV−vis spectroscopy and TLC. The solid
was filtered off, washed with ultrapure water and acetonitrile (Synth),
+
under vacuum (10 mmHg, 60 °C) to obtain 2.20 g of the pure
−
1
1
product (5.29 mmol, 95% yield). H NMR (300 MHz, CD CN): δ
3
3
5
4
5
9
.19 (dd, J = 5.7 Hz, J = 0.8 Hz, 2H), 8.95 (dd, J = 1.7 Hz, J = 0.8
Hz, 2H), 8.07 (dd, J = 5.7 Hz, J = 1.7 Hz, 2H), 4.02 (s, 6H). Anal.
3
4
Calcd for C H ClN O Re: C, 35.33; H, 2.09; N, 4.85. Found: C,
1
7
11
2
7
and dried under vacuum (10 mmHg, 60 °C) to obtain 0.34 g of the
3
5.39; H, 2.09; N, 4.65.
1
product (1.8 mmol, 76% yield). H NMR (CD CN, 300 MHz): δ
+
3
fac-[Re(CO) (dcbH )(CD CN)] . A total of 0.24 g (0.43 mmol) of
3
2
3
3
3
3
8
.56 (d, J = 6.8 Hz, 2H), 8.00 (d, J = 6.8 Hz, 2H), 7.90 (d, J = 16.4
Hz, 1H), 7.70 (d, J = 7.8 Hz, J = 2.3 Hz, 2H), 7.46 (m, 3H), 7.23 (d,
J = 16.4 Hz, 2H). Anal. Calcd for C H ClN· / H O: C, 67.72; H,
fac-[Re(CO) (dcbH )Cl] was suspended in 35 mL of tetrahydrofuran
3
2
3
4
(THF; Synth), and 0.11 g (0.43 mmol) of silver trifluoromethanesul-
3
5
1
3
12
7
2
fonate (Agtfms; Aldrich) was added to the mixture under an argon
atmosphere. After 2 h, AgCl was removed by filtration. Then, the
solution was rotovapped until the formation of an oil, which was
solubilized in CD CN to obtain the fac-[Re(CO) (dcbH )-
5
.87; N, 6.08. Found: C, 67.74; H, 5.70; N, 5.81.
,2′-Bipyridine-4,4′-dicarboxylic acid (dcbH ). The dcbH ligand
2
2
2
was synthesized according to a procedure previously described, with
61
3
3
2
slight modifications. A total of 3.02 g (16.4 mmol) of 3,3-dimethyl-
-butanol was slowly added to a solution of K Cr O (9.70 g, 33.0
+
1
3
(
=
CD CN)] complex. H NMR (300 MHz, CD CN): δ 9.32 (d, J
3
3
1
3
4
2
2
4
5.7 Hz, 2H), 9.14 (s, 2H), 8.29 (dd, J = 5.7 Hz, J = 1.6 Hz, 2H).
fac-[Re(CO) (dmcb)(trans-stpy)]PF . The fac-[Re(CO) (dmcb)-
mmol, Vetec) in 98% H SO (40.0 mL, Mallinckrodt) under magnetic
2
4
3
6
3
stirring at room temperature. The resultant orange slurry became dark
green, and the reaction was interrupted after 30 min. The mixture was
then poured into 100 mL of cold water, forming a light-yellow
precipitate. After filtration and drying, the solid was recrystallized to
(
trans-stpy)]PF complex was synthesized according to a procedure
6
previously described for fac-[Re(CO) (dmcb)(Etpy)]PF , with slight
3
6
39,63
modifications.
.3 mmol) was added in 25 mL of THF (Synth). The suspension was
maintained under stirring and an argon atmosphere for ∼1 h; then,
.34 g (1.3 mmol) of Agtfms (Aldrich) was added and maintained
The fac-[Re(CO) (dcmb)Cl] precursor (0.77 g,
3
1
3
+
isolate the desired compound free of Cr ions by dissolution in an
0% alkaline aqueous NaOH (Aldrich) solution, followed by slow
4
0
acidification (pH = 2) with a 10% aqueous HCl solution (Synth). The
solid was filtered, washed with ultrapure water and ethyl ether
under reflux and stirring for 4 h. The reaction was followed by UV−
vis spectroscopy and TLC. The precipitated AgCl was collected by
filtration, and the solution was transferred to another flask and
maintained under stirring and an argon atmosphere for ∼30 min;
then, 0.37 g (2.0 mmol) of trans-stpy was added. The mixture
remained under reflux for 12 h, and the reaction was monitored via
both absorption spectroscopy and TLC. At room temperature, 1.1 g
(
Synth), and then dried under vacuum (10 mmHg, 60 °C) to obtain
1
3
.51 g (14.4 mmol, 88% yield) of the pure dcbH product. H NMR
2
3
(
(
300 MHz, ND OD): δ 8.98 (d, J = 4.8 Hz, 2H), 8.61 (s, 2H), 7.69
4
3
4
13
dd, J = 5.1 Hz, J = 1.5 Hz, 2H). C NMR (75 MHz, ND OD): δ
4
1
72.59, 155.66, 149.81, 146.52, 123.53, 121.23. Anal. Calcd for
1
C H N O · / H O: C, 57.95; H, 3.44; N, 11.26. Found: C, 57.88; H,
12
8
2
4
4
2
(
6.5 mmol, Aldrich) of NH PF was added, and the mixture was
4 6
3
.45; N, 11.37.
,4′-Dimethoxycarbonyl-2,2′-bipyridine (dmcb). The dmcb
ligand was synthesized with modifications of a procedure previously
rotovapped until the formation of a red oil and precipitated by adding
4
50 mL of ethanol. The excess of the trans-stpy ligand was removed by
62
suspension of the solid in ethanol (Synth). The solid was filtered off,
washed with ultrapure water and ethyl ether (Synth), and dried under
vacuum (10 mmHg, 60 °C) to obtain 0.78 g (0.89 mmol, 67% yield)
described. A solution containing 3.11 g (12.5 mmol) of dcbH in 60
2
mL of methanol (Synth) was added to 10 mL of concentrated H SO
2
4
(
98%, Mallinckrodt). The system was placed in the oven of a
1
3
of the pure product. H NMR (300 MHz, CD CN): δ 9.40 (dd, J =
microwave apparatus, and then it was irradiated for 4 h (reflux mode,
3
5
4
5
5
(
.7 Hz, J = 0.7 Hz, 2H), 8.90 (dd, J = 1.6 Hz, J = 0.7 Hz, 2H), 8.22
dd, J = 5.7 Hz, J = 1.7 Hz, 2H), 8.10 (dd, J = 5.4 Hz, J = 1.4 Hz,
2H), 7.56 (dd, J = 7.5 Hz, J = 1.9 Hz, 2H), 7.45 (d, J = 16.4 Hz,
H), 7.37 (m, 5H), 7.05 (d, J = 16.4 Hz, 1H), 4.02 (s, 6H). Anal.
2
1
5 W) and monitored by UV−vis spectroscopy and TLC. A total of
3
4
3
3
00 mL of ultrapure water was added to the mixture, and the pH was
3
3
3
−
1
adjusted to 10 with 4 mol L aqueous NaOH (Aldrich). The solid
was filtered off, washed with ultrapure water and ethyl ether (Synth),
and dried under vacuum (10 mmHg, 60 °C) to obtain 3.31 g (12.2
mmol, 97% yield) of the pure dmcb product. H NMR (300 MHz,
CDCl ): δ 8.93 (s, 2H), 8.84 (d, J = 5.0 Hz, 2H), 7.88 (dd, J = 5.0
3
1
Calcd for C H F N O PRe: C, 41.48; H, 2.67; N, 4.84. Found: C,
30 23
6
3
7
1
41.50; H, 2.70; N, 4.59.
3
3
fac-[Re(CO) (dcbH )(trans-stpy)]PF . The complex was obtained
3
2
6
3
4
13
by hydrolysis by dissolving fac-[Re(CO) (dmcb)(trans-stpy)]PF
Hz, J = 1.5 Hz, 2H), 3.97 (s, 6H). C NMR (75 MHz, CDCl ): δ
3
6
3
(
0.15 g, 0.17 mmol) in 10.2 mL of methanol (Synth). Then, 1.7
1
65.82, 156.57, 150.35, 138.79, 123.46, 120.77, 52.98. Anal. Calcd for
−
1
mL of a 0.40 mol L K CO (Aldrich) aqueous solution was added,
C H N O : C, 61.73; H, 4.44; N, 10.44. Found: C, 61.61; H, 4.33;
2
3
14
12
2
4
and the solution was maintained under stirring for 30 min. The
reaction was monitored by TLC, and after 30 min, 92 μL (1.0 mmol)
N, 10.03.
Syntheses of Complexes. fac-[Re(CO) (dcbH )Cl]. The fac-
3
2
[
Re(CO) (dcbH )Cl] complex was synthesized according to a
of a 60 wt % HPF aqueous solution (Aldrich) was added. The
6
3
2
procedure previously described for fac-[Re(CO) (bpy)Cl], with slight
modifications. The [Re(CO) Cl] (2.30 g, 6.36 mmol) precursor
mixture was rotovapped, and the solid obtained was suspended in
ultrapure water, filtered off, washed with water and ethyl ether
3
8,9
5
and an excess of dcbH (1.71 g, 7.00 mmol) were suspended in 50 mL
(Synth), and dried under vacuum to obtain 0.11 g (0.13 mmol, 76%
2
+
of xylene (Synth) and heated to reflux for 8 h. The reaction was
monitored by UV−vis spectroscopy and TLC. After cooling to room
temperature, the resulting solid was separated by filtration and washed
with xylene. The crude product was recrystallized from methanol
yield) of the pure product. HRMS (ESI-TOF, CH
for C28
(200 MHz, CD
3
CN, [M] ). Calcd
1
H N O Re: m/z 696.0781. Found: m/z 696.0786. H NMR
19 3 7
3
CN): δ 9.29 (s, 2H), 9.27 (d, J = 6.0 Hz, 2H), 8.15
(d, J = 6.0 Hz, 2H), 8.08 (d, J = 6.4 Hz, 2H), 7.52 (m, 2H), 7.45 (d,
J = 16.4 Hz, 1H), 7.35 (m, 5H), 7.02 (d, J = 16.4 Hz, 1H), 2.38
3
3
3
3
3
(
Synth) by the slow addition of ethylic ether (Synth). The product
B
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthetic Methodology for Obtaining fac-[Re(CO) (dcbH )(trans-stpy)]PF
6
3
2
1
+
Table 1. H NMR Data for fac-[Re(CO) (dmcb)Cl], trans-stpy, and fac-[Re(CO) (dmcb)(trans-stpy)] in CD CN at 298 K
3
3
3
a
1
b
1
3
00 MHz H NMR. 200 MHz H NMR (Figure S4).
(
br). Anal. Calcd for C H F N O PRe: C, 40.01; H, 2.28; N, 5.00.
All irradiations were performed in a specially designed cuvette, as
37
2
8
19
6
3
7
Found: C, 40.31; H, 2.13; N, 4.85.
previously described. Light intensities were determined by
potassium tris(oxalate)ferrate(III) actinometry before and after each
photolysis.
General Procedures. UV−Vis Absorption. Electronic absorption
spectra were recorded on a Hewlett-Packard 8453 spectrophotometer
with 1.000- or 0.100-cm-optical-length quartz cuvettes.
Microwave Reaction. Microwave reaction was performed on a
CEM Discover Synthesis Unit (CEM Co.), with a continuously
focused microwave power delivery system in a glass vessel (100 mL)
under magnetic stirring.
Apparent trans-to-cis photoisomerization quantum yields
app
(Φtrans→cis) were obtained following absorbance changes by eq 1
(Tables S1−S5). The latter are “apparent” because both trans and cis
8,9,20−22,24,27,30
isomers absorb in the same region.
1
1
app
1
I t
(Atrans − Airr)NAVirr
0 irr
εtrans
H NMR. H NMR spectra were recorded on a Bruker Advance III
200 MHz), a Varian Unity Inova (300 MHz), or a Bruker AIII (800
MHz) spectrometer at 298 K using CD CN, CDCl , ND OD, or
Φ
=
trans→cis
b
(1)
In eq 1, tirr = irradiation time (s), I = light intensity (quanta s ), b =
0
(
3
3
4
−1
6
DMSO-d as the solvent, having 1.94 ppm (CD CN), 7.27 ppm
3
optical path length of the irradiated cuvette (cm), Virr = volume of the
(
CDCl ), 4.75 ppm (ND OD), and 0.00 ppm (TMS), respectively, as
3
4
irradiated solution, and N = Avogadro number.
A
the internal standard.
εcis (λ) was obtained using the ratio between the integrals of the
Mass Spectrometry. High-resolution mass spectroscopy spectra
were recorded on a Bruker Daltonics MAXIS 3G electrospray
ionization time-of-flight (ESI-TOF) spectrometer.
1
trans and cis H NMR signals in an irradiated solution (eq 2)
1
H NMR
Airr(λ) − A
trans
(λ) %
Atrans(λ) %cis
trans
CHN. Elemental analysis was perfomed using a PerkinElmer 2400
εcis(λ) = εtrans(λ)
1
H NMR
series II analyzer.
(2)
Photochemical Measurements. Photolyses of solutions at 313,
in which εtrans(λ) = molar absorptivity for the trans-isomer complex (L
3
34, 365, 404, or 436 nm were carried out as previously
1
1
14,30
mol−1 cm ), %
−1
H NMR
H NMR
described,
using 200 W or 500 W mercury (xenon) Oriel systems
and %cis
= percentages of trans- and cis-
trans
by selecting the wavelength with the appropriate interference filters.
isomers in the irradiated solution obtained by the distinct integrals in
C
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
+
Figure 1. H NMR spectra for fac-[Re(CO) (dcbH )(trans-stpy)] in CD CN at 298 K: (A) 200 MHz; (B) 800 MHz.
3
2
3
1
the H NMR spectrum, Atrans(λ) = absorbance of the trans-isomer
Low-temperature emission spectra were obtained in glassy
(propionitrile/butyronitrile, 4:5, v/v) at 77 K using a quartz tube
inside a Dewar flask filled with liquid nitrogen.
solution before irradiation, and A (λ) = absorbance of the irradiated
solution.
irr
true
True quantum yields (Φ
) were obtained by correcting
trans→cis
app
−1
Φ
to the molar absorptivity of the cis-isomer [ε (λ)/L mol
trans→cis
cis
−
1
RESULTS AND DISCUSSION
cm ] using eq 3 (Tables S1−S5) and just named as Φ
■
trans→cis
66−69
hereafter.
Initially, a conventional
synthetic procedure for synthesis
of the complex was explored by reacting the fac-[Re-
(CO) (dcbH )Cl] precursor with trans-stpy but without
significant product formation. The direct reaction of trans-
stpy coordination with the acid precursor fac-[Re-
1
(Airr − Atrans)NAV
irr
true
Φ
=
3
2
trans→cis
I t
(ε − εtrans)b
(3)
0
irr
cis
Photophysical Measurements. Emission experiments were
performed at room temperature (298 K) as previously de-
(
CO) (dcbH )Cl] posed a significant challenge because of
3 2
8
,21,28,30
neutralization between trans-stpy and the carboxylic acid group
scribed
by using an ISS photon-counting spectrofluorometer,
model PC1, with a 1.000 cm optical length quartz cuvette for a fluid
solution. Emission measurements of pure trans-solutions were
performed using a quartz flow cell (0.5 mL min ) in order to
avoid any interferences of residual cis-photoproducts formed during
the experiments.
dcbH . The desired product was obtained by using a strategy
to protect to the acid group starting with the ester precursor
2
−
1
fac-[Re(CO) (dmcb)Cl] to promote smooth replacement of
3
−
Cl by the trans-stpy ligand (Scheme 1, step 1). Although an
additional step is required, the methodology is advantageous
for being selective, fast (a hydrolysis reaction takes place in 30
min) with a good yield (78%).
Emission quantum yields of the complexes in degassed acetonitrile
64
were determined against fac-[Re(CO) (bpy)Cl] (ϕ = 0.0060) using
a procedure previously described.
3
65
D
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
Table 2. H NMR Data for fac-[Re(CO) (dcbH )(trans-
3
2
+
stpy)] in CD CN at 298 K
3
Figure 3. Changes in the absorption spectrum for fac-[Re-
+
(
CO) (dcbH )(trans-stpy)] in CH CN as a function of the
3
2
3
photolysis time, leading to a PS (λirr = 436 nm, Δt = 5 s, and T =
298 K).
Table 4. Isosbestic Points Reported for fac-
+
[
Re(CO) (R bpy)(trans-L)] in Absorption Changes upon
3 2
trans-to-cis Isomerization
compound
isosbestic point
ref
+
fac-[Re(CO) (dcbH )(trans-stpy)]
284
295
285
287
284
282
284
∼270
280
this work
3
2
+
fac-[Re(CO) (bpy)(trans-stpy)]
71
71
38
38
39
39
17
23
3
+
fac-[Re(CO) (Me bpy)(trans-stpy)]
3
2
+
fac-[Re(CO) (dmcb)(trans-stpy)]
3
+
fac-[Re(CO) (dmcb)(trans-stpyCN)]
3
+
fac-[Re(CO) (bpy)(trans-stpyCN)]
3
+
fac-[Re(CO) (Me bpy)(trans-stpyCN)]
3
2
+
fac-[Re(CO) (Me bpy)(trans-bpe)]
3
2
2
+
fac-[Re(CO) (bpy)(trans-bpeMe)]
3
signals to lower δ and dmcb hydrogen signals to higher δ
because of the magnetic anisotropy from the ring-current
7
0
effect. As is typically observed for fac-[Re(CO) (NN)(trans-
3
+
L)] complexes, trans-stpy deshields the hydrogen atoms of
+
Figure 2. Electronic spectra for fac-[Re(CO) (dcbH )(trans-stpy)]
dmcb, while dmcb shields the hydrogen atoms of trans-
3
2
11,14,21,22,26,38,39
(
black ), fac-[Re(CO) (dcbH )Cl] (red ), trans-stpy (blue ),
stpy.
The hydrogen coupling constant between
3
2
+
3
and trans-Hstpy (blue − −) in acetonitrile at 298 K.
H and H (J ≈ 16 Hz) is characteristic of trans-olefin
c
d
7
,11,17,55
isomers.
Table 3. Spectral Parameters for fac-
The hydrolysis (Scheme 1, step 2) requires careful control of
the reaction conditions (solvent proportion, temperature, and
reaction time; see the Experimental Section), following the
+
[
Re(CO) (dcbH )(trans-stpy)] , fac-[Re(CO) (dcbH )Cl],
3
2
3
2
+
Uncoordinated trans-stpy, and trans-Hstpy in Acetonitrile
at 298 K
CO release, to avoid substitution of the coordinated trans-stpy
2
λmax, nm (ε, ×10 L mol cm−1
)
4
−1
a
ligand.
compound
+
The identity of the fac-[Re(CO) (dcbH )(trans-stpy)]
complex was confirmed by H NMR spectroscopy, in
3
2
fac-[Re(CO) (dcbH )
221 (3.2), 325 (3.9) sh, 330 (4.2), 410 (0.27)
3
2
1
+
(trans-stpy)]
particular, a characteristic broad acidic hydrogen signal (2.38
fac-[Re(CO) (dcbH )Cl]
211 (2.6), 244 (1.6), 310 (1.1), 332 (0.67) sh,
3
2
4
20 (0.31)
ppm) of carboxylic acid (dcbH ) and corroborated by the
2
trans-stpy
trans-Hstpy+
199 (2.4), 222 (1.3), 227 (1.3), 318 (1.6)
200 (4.3), 236 (1.2), 274 (0.6), 341 (2.7)
complete absence of the 4.02 ppm signal of methyl ester
hydrogen atoms (Figure 1A). To make sure of the identity and
a
1
sh = shoulder.
purity of the complex, an 800 MHz H NMR spectrum was
also collected (Figure 1B). Identification has also been
ascertained by high-resolution mass spectrometry data (Figure
S10).
+
Thus, fac-[Re(CO) (dcbH )(trans-stpy)] was prepared by
3
2
hydrolysis of the ester precursor fac-[Re(CO) (dmcb)(trans-
stpy)] , as shown in Scheme 1, step 2, and isolated as a PF
The hydrogen signals for trans-stpy (Ha,b,c,d,e,f) have
practically the same chemical shifts for both fac-[Re-
3
+
−
6
1
+
salt. Each step is discussed with product characterization by H
NMR spectroscopy as follows.
(CO) (dcbH )(trans-stpy)] (Table 2) and fac-[Re-
3
2
+
(CO) (dmcb)(trans-stpy)] (Table 1), corroborating that the
3
Table 1 summarizes 1H NMR data for fac-[Re-
hydrolysis reaction does not promote substitution at the
coordinated trans-stpy ligand.
(
(
CO) (dmcb)Cl], trans-stpy, and fac-[Re(CO) (dmcb)-
3
3
+
trans-stpy)] (Scheme 1, step 1). The coordination of trans-
stpy to fac-[Re(CO) (dmcb)Cl] shifts trans-stpy hydrogen
The absorption band for the actual complex fac-[Re-
+
(CO) (dcbH )(trans-stpy)] lies between 200 and 270 nm
3
3
2
E
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
+
Figure 4. H NMR spectra for a fac-[Re(CO) (dcbH )(trans-stpy)] (black) and PS solution (gray) in CD CN at 298 K: (A) 200 MHz; (B) 800
3
2
3
MHz.
and arises from a predominantly π → π* intraligand transition
quence of electron-withdrawing stabilization of π*dcbH2 orbitals
1
in the dcbH ligand (Figure 2). A related IL
transition is
2
dcbH
2
by the carboxylic group.
+
also observed for fac-[Re(CO) (dcbH )Cl)]. The intense band
Irradiation of fac-[Re(CO)
3
(dcbH
2
)(trans-stpy)] acetoni-
3
2
trile solutions at 313, 334, 404, or 436 nm leads to absorption
changes as a function of the photolysis time because of trans-
to-cis isomerization (Figure 3). Continuous irradiation results
in a photostationary state (PS), where the concentrations of
the cis- and trans-species remain constant.
These typical changes in the absorption spectrum are
observed with a characteristic clear and well-defined isosbestic
point at 284 nm (Table 4) without any variation in the MLCT
band.
from 270 to 380 nm arises from a π → π* trans-stpy transition
1
(
ILtrans‑stpy), which is red-shifted upon ligand coordination. A
+
19,20,22,27,29,32,41,42
similar shift is also observed for trans-Hstpy .
The lower-energy absorption band at 400−455 nm arises from
1
I
a metal-to-ligand charge-transfer transition ( MLCT; d
→
Re
π*dcbH2), compared to the spectrum of fac-[Re(CO) (dcbH )-
3
2
Cl] (Table 3).
1
The MLCT
absorption band exhibits a slight blue
Re→dcbH
2
Efficient trans-to-cis photoisomerization for the actual
complex also occurs at 404 and 436 nm, a region in which
noncoordinated trans-stpy does not absorb, demonstrating an
efficient sensitization of trans-stpy coordinated to the rhenium
polypyridyl complex by intramolecular energy transfer.
shift after trans-stpy coordination compared to its parent
complex, fac-[Re(CO) (dcbH )Cl]. A similar blue shift has
3
2
+
been previously described for fac-[Re(CO) (bpy)(py)] after
3
44
chloride displacement by pyridine.
1
The MLCT
transition of the actual complex is
1
Re→dcbH
2
The H NMR spectrum of the PS solution (Figure 4)
remarkably red-shifted relative to related fac-[Re(CO) (NN)-
exhibits two doublet signals with a characteristic coupling
3
+
3
7,11,17,55
(
trans-L)] complexes with NN = 2,2′-bipyridine and its
constant of cis-olefinic hydrogen atoms (J = 12.0 Hz)
ascribed to H (6.40 ppm) and H (6.89 ppm), consistent
3
−5,11,17−19,23,32,71
substituted analogues (R bpy),
as a conse-
2
c′
d′
F
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
a
Table 5. H NMR Data for the PS Solution
a
Asterisks indicate signals overlapped with trans-complex hydrogen signals.
with successful photoisomerization. These signals are shifted to
higher δ compared to the trans-complex because of planarity
4
loss around the py−CHCH−Ph group, which leads to a
1
strong shielding of the cis- H nucleus, in special H , H , H ,
b′
c′
d′
and H atoms (Table 5). It is noteworthy that all hydrogen
e′
atoms exhibit distinct δ values in comparison to the trans-
isomer complex because of the change of the chemical
environment caused by the olefinic bond twist, similar to other
8
,11,17,20−22,24,27,30,31,33,37−39,72
rhenium(I) complexes.
1
1
The assignments were confirmed by H− H correlation
spectroscopy (COSY; Figure S7).
The ratio of H NMR integrals for cis- and trans-isomers
1
+
Figure 5. Electronic spectra for fac-[Re(CO) (dcbH )(trans-stpy)]
3
2
+
results in a 53% cis-fraction in the PS solutions, the value used
to obtain molar absorptivity of the cis-isomer complex to
(
red ) and fac-[Re(CO) (dcbH )(cis-stpy)] (blue − −) in
3
2
acetonitrile at 298 K. ε (λ) values for the cis isomer were determined
cis
27
by eq 2.
generate the cis-complex spectrum (Figure 5).
+
Table 6. Quantum Yields for trans-to-cis Photoisomerization of fac-[Re(CO) (dcbH )(trans-stpy)] and fac-
3
2
+
a
[
Re(CO) (R bpy)(trans-L)] in Acetonitrile at 298 K
3 2
λ irradiation
compound
fac-[Re(CO) (dcbH )(trans-stpy)]
313 nm
334 nm
350 nm
365 nm
404 nm
436 nm
ref
+
0.49 ± 0.06
0.51 ± 0.03
0.54 ± 0.05
0.51 ± 0.04
0.35−0.43
.48 ± 0.03
0.31 ± 0.07
0.43 ± 0.03
0.39 ± 0.03
0.36 ± 0.03
0.55 ± 0.05
0.50 ± 0.03
no absorption
no absorption
no absorption
0.44 ± 0.05
no absorption
no absorption
0.56 ± 0.04
this work
25
3
2
+
b
fac-[Re(CO) (bpy)(trans-stpy)]
3
0
71
71
38
39
39
38
4
11
+
fac-[Re(CO) (Me bpy)(trans-stpy)]
3
2
+
fac-[Re(CO) (dmcb)(trans-stpy)]
0.54 ± 0.04
0.44 ± 0.02
0.43 ± 0.02
0.65 ± 0.05
0.55 ± 0.03
0.44 ± 0.01
0.47 ± 0.02
0.64 ± 0.04
0.53 ± 0.02
0.45 ± 0.02
0.48 ± 0.06
0.62 ± 0.05
0.66
3
+
fac-[Re(CO) (bpy)(trans-stpyCN)]
3
+
fac-[Re(CO) (Me bpy)(trans-stpyCN)]
3
2
+
fac-[Re(CO) (dmcb)(trans-stpyCN)]
3
+
fac-[Re(CO) (bpy)(trans-stpyNO )]
no absorption
3
2
+
c
fac-[Re(CO) (bpy)(trans-stpyODA)]
0.49
3
+
c
fac-[Re(CO) (bpy)(trans-stpyODO)]
0.43
5, 11
17
17
3
+
fac-[Re(CO) (bpy)(trans-bpe)]
0.21
0.23
no absorption
3
+
fac-[Re(CO) (Me bpy)(trans-bpe)]
3
2
a
c
b
trans-stpyODA = 4-[4′-(N-octadecylamide)styryl]pyridine; trans-stpyODO = N-(4-octadecyloxy-4′-styryl)pyridine-2-carbaldimine. Estimated.
In CH Cl .
2
2
G
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
(
CO) (dcbH )(trans-stpy)] are listed in Table 6, along with
3 2
values for other rhenium(I) bipyridyl complexes.
Emission spectra of trans and PS solutions are pictured in
Figure 6. Their maxima (∼574 nm) are similar for both
complexes with negligible quantum yields (∼0.0002) under
3
78−400 nm excitation. Usually, trans-stilbene rhenium(I)
complexes are practically nonemissive compared to their
emissive cis-isomers.
4
,6,8,9,14,16,17,20−22,24,26,30,38,42
The en-
hanced emission of cis-complexes, as reported previously,
arises from a change in the nature of the lowest-lying excited
22
state and results from the destabilization of ILcis‑stpy states,
while MLCT remains constant and becomes the lowest
Re→NN
9
,13,22,30
state.
In spite of the low emission of the fac-[Re(CO) (dcbH )-
3
2
+
3
(
cis-stpy)] complex, the lowest-lying excited state is MLCT.
+
This emission is lower than usual because it may be quenched
by the acid group and is enhanced upon the addition of an
external base (Figure S8).
Figure 6. Emission spectra for fac-[Re(CO) (dcbH )(trans-stpy)]
3
2
(
[
black ), PS acetonitrile solution (gray ), and standard fac-
Re(CO) (bpy)Cl)] (pink - - -). The inset shows amplified emission
3
3
spectra (λexc = 378 nm, 298 K) for the trans complex and PS solution
λirr = 436 nm and tirr = 10 min).
The identity of this MLCT emission is also demonstrated
73,74
(
by a rigidochromic effect,
which shows a hypsochromic
shift of the emission at 77 K (Figure S9).
Irradiation of the PS solution of fac-[Re(CO) (dcbH )-
stpy)] at 255 nm does not lead to spectral changes arising
3
2
+
(
from cis-to-trans photoisomerization, and there was no
evidence for a back-reaction under these conditions (Figure 7).
A schematic energy diagram to explain the photochemical
and photophysical behavior is proposed in Figure 8.
The photochemical behavior proposed is in accordance with
the diagram proposed previously for the isomerization of
coordinated trans-bpe, followed by time-resolved IR measure-
ments that demonstrate that photoisomerization occurs
13
through a triplet mechanism.
It is noteworthy that the isomerization quantum yields for
Figure 7. Electronic absorption spectra of a PS solution with 255 nm
irradiation at 298 K. The inset shows absorption at 330 nm versus
irradiation time.
+
fac-[Re(CO) (dcbH )(trans-stpy)] do not depend on the
3
2
irradiation wavelength from 313 to 436 nm, displaying an
effective sensitization toward visible excitation through the
MLCT state.
3
The wavelength dependence reported previously by
1
Faustino et al. seems to be more appropriate for an
+
isomerization of trans-Hstpy . Also, it is noteworthy the
1
similarity of the H NMR spectra that they show (ref 1, Figure
, trans, p 2936) compare with the trans-Hstpy+ H NMR
1
4
(
Figure 9A) and its variation upon photoisomerization (Figure
S5). One of the possible rhenium complexes observed in their
1
1
H NMR spectra is a solvento complex. Here, we synthesized
+
1
fac-[Re(CO) (dcbH )(CD CN)] , and its H NMR spectrum
is shown in Figure 9B. Figure 9C is an overlap of trans-Hstpy
and fac-[Re(CO) (dcbH )(CD CN)] , which is very similar
3
2
3
+
+
3
2
3
to Figure 4, trans, of the previous work.
Moreover, there are other inconsistences in their data, such
Figure 8. Simplified energy diagram.
as MLCT band variations and the isosbestic point, that are not
+
observed for other fac-[Re(R bpy)(trans-L)] complexes and
2
1
1
1
The intraligand π → π* transition for stpy, ILstpy, occurs
H NMR assignments. For the latter, they report H NMR
1
mainly in the 270−380 nm range. The IL
transition is
cis‑stpy
spectral changes that are appropriate for the behavior of the
noncoordinated ligand with the absence of NN (H , H , and
1
blue-shifted compared to IL
because of the planarity
trans‑stpy
α
β
loss and a consequent π* destabilization.
H ) signal changes that appear for cis-complexes. In all
γ
The Φ values following irradiation at 313, 334, 365,
trans→cis
literature data, there are clearly distinct NN signals for both
4
04, or 436 nm are constant and independent of the excitation
11,17,20−22,24,27,28,30,31,33,37−39,71,72
trans- and cis-complexes.
energy within experimental error, confirming the same reaction
pathway with population of a lowest-lying IL excited state
3
stpy
CONCLUSION
and a wavelength-independent mechanism, previously reported
■
42
only for the bpe complexes. The average quantum yields,
The data reported in this paper are inconsistent with the
conclusions reached in a previously published paper. Here we
1
Φtrans→cis, for trans-to-cis photoisomerization of fac-[Re-
H
DOI: 10.1021/acs.inorgchem.8b01304
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
+
+
Figure 9. H NMR spectra (300 MHz) in CD CN at 298 K for (A) trans-Hstpy , (B) fac-[Re(CO) (dcbH )(CD CN)] , and (C) an overlap of A
3
3
2
3
and B.
report a step-by-step synthetic procedure for the proposed fac-
AUTHOR INFORMATION
■
+
1
[
Re(CO) (dcbH )(trans-stpy)] with H NMR assignments at
3 2
Corresponding Author
E-mail: neydeiha@iq.usp.br.
ORCID
each step. We also demonstrate behavior expected for the
*
actual complex with effective photosensitization up to 436 nm
3
and an important possible role as a MLCT sensitizer for
potential applications in solar energy conversion. The
distinction is important because of the promise of the
chromophore for applications in solar-light-induced devices.
Lais S. Matos: 0000-0001-9119-3984
Ronaldo C. Amaral: 0000-0002-1729-3511
Neyde Y. Murakami Iha: 0000-0002-3262-6484
Author Contributions
All authors have given approval to the final version of the
ASSOCIATED CONTENT
■
*
S
Supporting Information
manuscript.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01304.
Notes
The authors declare no competing financial interest.
1
+
ACKNOWLEDGMENTS
H NMR spectra for trans-stpy, trans-Hstpy and its
■
+
This work was supported by Fundaca
̧
o
o
̃
de Amparo a
de Aperfeicoamento de
̀
Pesquisa
irradiated solutions, fac-[Re(CO) (dmcb)Cl] , fac-[Re-
3
do Estado de Sao
̃
Paulo, Coordenaca
̧
̃
̧
+
(
[
CO) (dmcb)(stpy)] , and the PS solution for fac-
3
́
Pessoal de Nıvel Superior, and Conselho Nacional de
+
Re(CO) (dcbH )(stpy)] , COSY NMR for the PS
Desenvolvimento Cientıfi
thankful to Gizele Celante, Marcos Archilha, and Alcindo dos
Santos for 200 MHz H NMR spectra.
́
co e Tecnologico. The authors are
́
3
2
+
solution for fac-[Re(CO) (dcbH )(stpy)] , emission
3
2
1
+
spectral changes for fac-[Re(CO) (dcbH )(stpy)] as
3
2
dcbH is deprotonated, emission spectra for fac-[Re-
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2
■
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I
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