The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: An experimental and theoretical insight

Article abstract of DOI:10.1039/c8dt01120f

Two isomeric tricarbonyl rhenium(i) complexes, ReL1 and ReL2, that possess a 2-pyridyl-1,2,n-triazole (pyta) ligand (n = 4 and 3, respectively) connected to a 2-phenylbenzoxazole (PBO) moiety, were synthesized in good yields. The X-ray structures showed t

Full text of DOI:10.1039/c8dt01120f

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PAPER
The unsuspected influence of the pyridyl-triazole
ligand isomerism upon the electronic properties of
tricarbonyl rhenium complexes: an experimental
and theoretical insight†
Cite this: DOI: 10.1039/c8dt01120f
a
b,c
d
Jinhui Wang,‡ Béatrice Delavaux-Nicot,
Mariusz Wolff,
§
e
f
a
Sonia Mallet-Ladeira, Rémi Métivier,
Suzanne Fery-Forgues
Eric Benoist and
a
*
Two isomeric tricarbonyl rhenium(I) complexes, ReL1 and ReL2, that possess a 2-pyridyl-1,2,n-triazole
pyta) ligand (n = 4 and 3, respectively) connected to a 2-phenylbenzoxazole (PBO) moiety, were syn-
(
thesized in good yields. The X-ray structures showed that in ReL1 the PBO moiety and the pyta ligand
almost form a right angle hindering electron delocalization, while in ReL2 their nearly planar arrangement
favors the electron delocalization in the whole organic ligand. Therefore, the nature of the ligand signifi-
cantly influences the electron distribution in the two complexes, as indicated by the results of TD-DFT
calculations. An electrochemical study highlighted that, by comparison with ReL2, the smaller HOMO–
LUMO energy gap of ReL1 is in line with its lower first reduction potential. From a spectroscopic view-
point, both complexes emitted phosphorescence in organic solvents, with distinct color and intensity.
They also emitted in the solid state, but only ReL1 showed significant aggregation-induced phosphor-
escence emission (AIPE). This complete study sheds light on the crucial role of structural isomerism of
the triazole group, which has been unsuspected for a long time although it can govern the geometry and
electronic properties of rhenium complexes. It is shown for the first time that grafting a non-coordinated
π-conjugated fragment on the N(4) atom of a 1,2,4-triazole group can be of high value for the design of
efficient light-emitting materials based on rhenium complexes.
Received 23rd March 2018,
Accepted 29th May 2018
DOI: 10.1039/c8dt01120f
rsc.li/dalton
Introduction
Since the discovery of their photoluminescence properties in
1
1
974, the air and water stable tricarbonyl rhenium(I) com-
a
SPCMIB, CNRS UMR5068, Université Toulouse III Paul-Sabatier, 118 route de
plexes have been widely studied from a photophysical view-
2
Narbonne, 31062 Toulouse cedex 9, France. E-mail: sff@chimie.ups-tlse.fr
point and have proven to be valuable photoluminescent cellu-
b
Laboratoire de Chimie de Coordination, CNRS UPR 8241, 205 route de Narbonne,
3–6
lar imaging agents. In addition to allowing detection in the
3
1077 Toulouse Cedex 4, France
c
visible, they strongly absorb in the middle infrared where light
penetration in tissues is optimal, and thus they enable
Université de Toulouse UPS, INPT 31077 Toulouse Cedex 4, France
d
Institute of Chemistry, Department of Crystallography, University of Silesia,
7
,8
9
th Szkolna St., 40-006 Katowice, Poland
bimodal IR and luminescence bioimaging. Some of them
could also serve as new agents for the delivery of carbon mon-
oxide to biological targets under the control of light, in the
e
Service commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Université
Toulouse III Paul-Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 9, France
f
PPSM, ENS Cachan, CNRS, Université Paris-Saclay, 94235 Cachan, France
9
frame of phototherapy. These complexes are very versatile
†
Electronic supplementary information (ESI) available: Experimental details,
because they also possess three facial positions available for
including proton numbering for NMR; picture of crystals, molecular views, and
CIF data; additional computational, electrochemical and spectroscopic data. substitution by various organic or inorganic ligands, which
CCDC 155800 and 1555801. For ESI and crystallographic data in CIF or other
allow a wide control over their physical, spectroscopic and bio-
electronic format see DOI: 10.1039/c8dt01120f
Institute of Drug Discovery Technology, Ningbo University, Ningbo 315211,
China.
Institut für Anorganische Chemie, Johannes Kepler Universität Linz,
5,6
logical properties.
In this context, pyridyl-triazole (pyta)
‡
ligands are of special interest. They combine the coordination
abilities of pyridine and triazole rings, have strong σ-electron-
donating capability, and can be functionalized much more
§
Altenberger Straße 69, 4040 Linz, Austria.
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easily than the ubiquitous bipyridine-based ligands. Moreover,
the triazole ring has various isomeric forms, the 1,2,3-isomer
being by far the most popular since the rediscovery of Huisgen
1
0
“
click” reaction by the Sharpless group. Each isomer offers
distinct substitution patterns and coordination abilities poten-
1
1
tially useful to modulate the complex properties. Recently,
1
2
13
Lo et al. and Bertrand et al. have compared isomeric tri-
carbonyl Re(I) complexes in which a 2-pyridyl group and an
aromatic group are respectively borne by the C(4) and N(1)
atom of the 1,2,3-triazole fragment, and vice versa. Both teams
have shown interesting differences in the optical transitions of
the corresponding complexes. To our knowledge, other triazole
isomers have not been considered.
The aim of the present work was to compare the behavior
of two substituted tricarbonyl Re(I) complexes that differ by
the position of the third nitrogen atoms in their triazole ring.
Specifically, the ReL1 complex contains the 3-(2-pyridyl)-1,2,4-
triazole fragment, some derivatives of which have been
Fig. 1 Conditions: (i) Benzoic acid, polyphosphoric acid, 110 °C, 16 h;
(
ii) 10% Pd/C, H
methylmethanamine, pyridine-2-carbohydrazide, acetic acid, CH
0–120 °C, 16 h; (iv) [Re(CO) Cl], MeOH, 65 °C, 16 h. (v) HCl (6 N),
NaNO , NaN , 0 °C to r.t.; (vi) 2-ethynylpyridine, Cu(OAc) ·H O, NaAsc.,
2
, MeOH/CHCl
3
, 6 bars, r.t., 24 h; (iii) dimethoxy-N,N-di-
3
CN,
5
5
2
3
2
2
recently used for the preparation of luminescent iridium com- CH CN, r.t., 16 h.
3
1
4
plexes. The ReL2 complex includes the well-known 4-(2-
pyridyl)-1,2,3-triazole fragment. In order to improve the emis-
sion properties, the idea was to combine the pyta ligand with
an organic dye moiety. To this aim, a 2-phenylbenzoxazole
through a one-pot condensation reaction as described in the
1
6
(
PBO) moiety was chosen for its excellent stability and fluo-
literature. Unfortunately, up to now, all attempts to improve
the formation of the pyridyl-1,2,4-triazole scaffold failed. The
preparation of ligand L2 went through an azide derivative (3)
that was subsequently condensed with 2-ethynylpyridine via a
Copper(I) catalyzed alkyne–azide cycloaddition using classical
click reaction conditions. Although L2 required an additional
step of synthesis, its overall yield was better than L1 (40% vs.
1
5
rescence efficiency, both in solution and in the solid state.
An originality of this design is that a large π-electron conju-
gated moiety not directly involved in Re complexation is linked
to a triazole nitrogen atom, on the pyta fragment. To the best
of our knowledge, this design is surprisingly unprecedented
for complexes that incorporate a pyta fragment, whose nitro-
gen atoms usually do not bear more than a phenyl group as an
aromatic substituent.
22%). As expected, the formation of the pyridyl-1,2,3-triazole
scaffold was thus easier than that of the other isomer. The
corresponding tricarbonyl rhenium(I) complexes ReL1 and
ReL2 were then easily prepared in good yields (ca. 66%) by
The synthesis and crystallographic characterization of com-
plexes ReL1 and ReL2 were reported, along with the electro-
chemical,
spectroscopic
and
photophysical
studies.
5
reacting the commercial precursor [Re(CO) Cl] with L1 or L2
Experimental data were supported by TD-DFT calculations. At
first sight, both complexes look very similar, but they
behave in a very different way. This study reveals how the
nature of the pyta isomer and the functionalization of this
ligand by a PBO moiety can be of prior importance, especially
regarding the impact on the photoluminescence properties.
Thus, it paves the way towards new families of strongly emis-
sive rhenium complexes substituted by various organic
fluorophores.
in refluxing methanol. All compounds were obtained with
good overall yields after purification by chromatography.
1
Ligands and complexes were unambiguously identified by H
1
3
and C NMR, elemental microanalysis and high resolution
mass spectrometry. Detailed synthetic procedures and charac-
terization data are given in the Experimental section. It is note-
worthy that in infrared spectroscopy, the characteristic ν(CO)
stretching bands of the complexes appeared at 2025, 1919,
1
884 cm− for ReL1, and at 2030, 1920 and 1903 cm for
1
−1
−
1
ReL2, with average values at 1942 and 1951 cm , respectively.
According to a recent work from Sarkar’s group, the position
of these bands can be correlated with the electron density on
the metal center, and hence with the overall donor ability of
Results and discussion
Synthesis
1
7
the ligand. In our case, ligand L1 would be a slightly better
electron donor than L2, very close to a bipyridyl ligand (νaverage
The synthetic pathways for the preparation of the tricarbonyl
rhenium complexes are shown in Fig. 1. First of all, 6-nitro-2-
phenylbenzoxazole (1) was obtained in good yield by classical
condensation of 2-amino-5-nitrophenol with benzoic acid in
the presence of polyphosphoric acid, and then it was catalyti-
of Re(bpy)(CO)
3
Cl: 1943 cm− ).
1
18
Crystal structures
cally reduced to afford the corresponding amino derivative (2). Fortunately, single crystals of complexes ReL1 and ReL2 suit-
From this compound, ligand L1 was obtained in modest yield able for X-ray crystallography analysis were successfully grown
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by slow evaporation of organic solvents. Full crystallographic the free rotation between the triazole group and the PBO
data, as well as data regarding hydrogen bonding and packing moiety allowed the latter to occupy two distinct positions. In
mode, are reported as ESI,† along with the geometric para- 68% of the cases (type I), the oxygen atom of the benzoxazole
meters of complexes such as bond lengths and bond angles group and the chlorine atom of the coordination sphere
(Tables S1–S4, Fig. S1 and S2†).
pointed opposite directions, and the benzoxazole group was
As illustrated in Fig. 2, both complexes showed many simi- twisted by 16.2(6)° with respect to the phenyl group. In the
larities. Regarding the rhenium environment, they present a remaining 32% (type II), the benzoxazole oxygen and the chlor-
distorted octahedral geometry. As expected, the Re(I) atom is ine atom pointed in the same direction, and the benzoxazole
coordinated to three carbonyl ligands arranged in a facial con- group was twisted by 21.7(1)° with respect to the phenyl group.
figuration, one chlorine atom, and two nitrogen atoms of the The angle between the benzoxazole group and the triazole
pyta moiety. Two carbonyl groups C(2)–O(2) and C(3)–O(3) group was 64.0(2)° for the first molecule, and 69.3(3)°/82.0(6)
along with pyridine nitrogen atom N(2) and triazole nitrogen for the second one (type I and type II, respectively). Complex
atom N(3) occupy the equatorial positions. The third carbonyl ReL2 crystallized in the monoclinic P2 /c space group with one
1
group and one chlorine atom occupy the axial positions and acetone molecule per molecule of complex. No disorder was
coordinate to the Re(I) atom linearly with an angle equal to observed. The main feature is that the whole organic ligand
1
77.0(2)° in ReL2, and to 176.1(2)°/174.8(2)° in ReL1 for con- was almost planar, the angle between the benzoxazole group
formational type I and II, respectively (vide infra). Examining and the triazole group being only 9.0(2)°.
now the pyta moiety, for both complexes, only a small twist
Therefore, substitution of the triazole fragment by the
angle of approximately 5° was observed between the triazolyl pyridyl and PBO groups in adjacent positions in ReL1 induces
and pyridyl components. All bond length and angle values are strong steric hindrance, which does not exist with the 1,3-sub-
in line with those reported for other rhenium complexes, stitution pattern of ReL2. The conjugated systems of the com-
either containing various azole ligands combined with a plexes are strongly impacted. The marked bending of the
1
9–21
2
-pyridyl group,
or based on 4-(2-pyridyl)-1,2,3-triazole organic ligand in ReL1 suggests that the π-electron systems of
2
2
ligands and developed by our team.
the pyta and PBO moieties have little interaction and should
However, significant differences were also revealed by the behave almost independently, while the planarity of these moi-
crystallographic study. Complex ReL1 crystallized in the mono- eties in ReL2 should promote strong interaction between
clinic P2 /n space group with two crystallographically distinct them. To the best of our knowledge, this is the first time that
1
molecules of complex and one acetonitrile molecule in the such a difference appears for two rhenium complexes only
asymmetric unit. Most importantly, in one of the molecules, differing by the isomerism of their pyta ligand.
Regarding now the molecular packing (Table S2, Fig. S1
and S2†), for ReL1 (type I), π-stacking interactions occur
between two parallel triazole rings with a centroid-to-centroid
distance of 3.6 Å. However, molecules were slipped laterally,
and no significant overlap of the PBO aromatic systems was
observed. The molecular arrangement was compact and well
structured by a network of intermolecular hydrogen bonding
interactions. For type II molecules, the network seemed to be
looser than in the former case. In contrast, ReL2 complexes
exhibited crossed arrangement, which is quite frequent for
1
5
PBO derivatives. Stacked molecules were roughly situated in
parallel planes, offset from one another and oriented in the
same direction. They exhibited partial overlap of their aromatic
systems. Slipped π–π stacking interactions take place between
the 2-phenyl ring of one molecule and the benzene ring of its
closest neighbor with a centroid-to-centroid distance of 3.7 Å,
and between the oxazole ring of one molecule and the triazole
ring of the other one, with a centroid-to-centroid distance of
3.6 Å. Therefore, π-stacking interactions in the solid state were
stronger in ReL2 than in ReL1.
TD-DFT calculations
Computational studies based on the time dependent density
functional theory (TD-DFT) method at the PBE1PBE/LANL2DZ/
Fig. 2 Top: Molecular view of the asymmetric units of complex ReL1.
The molecule on the left side may have two conformations (types I and
II). Bottom: Molecular view of complex ReL2. Thermal ellipsoids are
drawn at the 50% probability level. Solvent molecules and H atoms are
omitted for clarity.
6-31+G** level were performed considering the two complexes
in dichloromethane (DCM). At the S ground state level, calcu-
0
lations gave a very good estimation of a number of coordinat-
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ing bond lengths and angle values, which differed at the most 96% for ReL1. On the other hand, the first three lowest unoc-
by 0.05 Å and 3°, respectively, with respect to the corres- cupied orbitals (LUMOs) are predominantly based on the
ponding X-ray crystallographic data (Tables S3 and S4†). Upon organic moiety with π* character, with the difference that orbi-
excitation, the geometries of the first singlet excited state S
1
tals of ReL2 extend significantly over the whole organic ligand,
and first triplet excited state T kept octahedral conformation. while those of ReL1 are well centered either on the pyta moiety
1
However, in comparison with the ground-state, the Re–N (LUMO and LUMO+2) or on the PBO moiety (LUMO+1).
bonds in S state were shortened by about 0.05–0.06 Å, while Therefore, the nature of the triazole group and its functionali-
1
Re–CO bonds were elongated by ca. 0.04–0.06 Å, which indi- zation by a PBO moiety on a different position influence the
cates that the CO ligands tend to break away from the Re atom electron distribution over the complexes. It also has a strong
and that the chelate ligand is getting close to the Re atom in effect on the energy levels. The most striking feature is the
the excited state. This effect can be attributed to the electron decrease of the LUMO energy in ReL1 with respect to ReL2,
density transfer from the Re–CO bonding orbital to the π* while the respective energy levels of the LUMO+1, HOMO and
orbital of the organic ligand upon excitation. The same trend HOMO−1 orbitals are almost unchanged in both compounds
was observed for the lowest triplet state T of both complexes. (Fig. 4). In contrast, it is interesting to see that isomerism has
1
Other data are given in the ESI (Tables S5–S8†). An interesting a weak effect on the composition of orbitals involved in the T
1
point to note is that the electrostatic potentials appeared to be state, which are centered on the metal environment and on
more localized in definite areas in ReL1 than in ReL2 the pyta moiety, with no contribution of the PBO moiety for
(Fig. S3†).
both complexes (Fig. 5). The calculated absorption and emis-
The composition and energy levels of frontier molecular sion energies (Tables S10–S13†) will be discussed below in
orbitals (FMOs) are presented in Fig. 3 and Table S9.† As comparison with the experimental data.
expected for both complexes, the two highest occupied orbitals
(HOMO and HOMO−1) are almost totally centered on the
rhenium moiety. In contrast, the HOMO−2 is dominantly loca-
lized on the organic ligand, and more precisely on the PBO
moiety with a contribution around 75% for ReL2, and up to
Electrochemical study
The electrochemical behavior of the complexes was studied by
cyclic voltammetry (CV) and Osteryoung square wave voltam-
metry (OSWV) measurements in DCM at room temperature. In
the OSWV anodic part, complexes ReL1 and ReL2 are charac-
terized by two oxidation processes around 1.46 and 1.76 V
(
Table 1 and Fig. 6a). The former process can be assigned to
2
1,23
an irreversible Re(I) oxidation process
which is slightly
easier for complex ReL1 than for ReL2. It could be expected
that in comparison with the folded structure of the organic
moiety in compound ReL1, the planar ligand structure of ReL2
favors electron delocalization, thus rendering the rhenium(I)
less electron rich and slightly decreasing the ease of its oxi-
Fig. 3 Isodensity plots of selected frontier molecular orbitals involved
in the first electronic transitions of ReL1 and ReL2 in DCM, according to
TD-DFT calculations at the PBE1PBE/LANL2DZ/6-31+G** level of
theory. Blue and yellow colors show regions of positive and negative
spin density values, respectively.
Fig. 4 Molecular orbital diagrams of ReL1 (left) and ReL2 (right).
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1
Fig. 5 Spin density distribution for the lowest triplet state T of ReL1
2 2
and ReL2 in CH Cl , according to a calculation based on the optimized
triplet state with DFT method at the PBE1PBE/LanL2DZ level. Blue and
yellow colors show regions of positive and negative spin density values,
respectively.
Table 1 Selected electrochemical data of complexes ReL1 and ReL2
−
3
(
6.5 × 10 M). Values determined by OSWV on a Pt working electrode in
a,b
CH
2
Cl
2
+ 0.1 M n[Bu
4
N][BF
4
] at room temperature.
Ferrocene was
used as internal reference
Oxidation
Reduction
Compounds
E
2
E
1
E
1
E
2
ReL1
ReL2
+1.74
+1.79
+1.44
+1.48
−1.31
−1.60
−1.90
−1.89
a
OSWVs were obtained using a sweep width of 20 mV, a frequency of
b
2
0 Hz, and a step potential of 5 mV. Potential values in volts vs. SCE Fig. 6 OSWVs: anodic (a) and cathodic (b) scans of complexes ReL1
+
(
Fc /Fc is observed at 0.55 V ± 0.01 V vs. SCE).
2
(red) and ReL2 (blue) on a Pt working electrode in CH Cl2 + 0.1 M
n[Bu
4 4
N][BF ] at room temperature (frequency 20 Hz, amplitude 20 mV,
step potential 5 mV).
dation. The reversibility of this process was not improved with
a decrease or an increase in scan rate.
Considering now the cathodic part, both complexes pre- showed that this process becomes quasi-reversible around
sented a reduction process around −1.9 V (Fig. 6b) that is very 1 V s− only in the case of compound ReL1 (Fig. S6†). This
likely attributed to the reduction of the PBO moiety of the property is rather uncommon as this process is generally
ligand and confirms that the latter is not involved in the com- irreversible when implying the pyta moiety. It is noteworthy
1
2
4
plexation process. Indeed, a similar reduction process was that, in cyclic voltammetry, a 1/1 intensity ratio was clearly
detected for both ligands (Fig. S4†) and was also clearly visible observed between the first one-electron reduction process and
in cyclic voltammetry (Fig. S5†). Another reduction process the first one-electron oxidation process of compound ReL2
appeared at more anodic potential. In a first approach, it could (Fig. S7†). The use of a glassy carbon electrode allowed us to
be attributed to the reduction process of the substituted tri- highlight the same phenomenon for compound ReL1 (Fig. S8†).
azole ring whose potential value may substantially decrease by
complexation as observed in related compounds.
To sum up, when comparing both complexes, the two main
characteristic features are a significantly lower electrochemical
1
2,21,23,25
Remarkably, the value of the latter reduction potential strongly gap for compound ReL1 than for compound ReL2, and a
differed by more than 300 mV between the two compounds, different electrochemical behavior regarding the first reduction
i.e. −1.30 V for complex ReL1 and −1.60 V for complex ReL2. process. The latter property is probably related to the different
These trends are well supported by the fact that the first calcu- nature of the two compounds. For compound ReL2, the LUMO
lated LUMO energy level implying the pyta (π*) moiety is lower and LUMO+1 energy levels are close: 0.07 eV (around 70 mV).
for ReL1 (−2.53 eV) than for ReL2 (−2.23 eV). Electrochemical Consequently, the first reduction potential detected at −1.60 V
2
6
HOMO–LUMO gap values (Egel
)
found for complexes ReL1 probably originates from the contribution of both the LUMO
and ReL2, i.e. 2.55 and 2.85 eV respectively, fit very well with and LUMO+1 energy levels involving respectively the π* (pyta)
the calculated gap values 2.71 and 3.01 eV, highlighting good and π*(pyta) + π*(PBO) orbitals. In contrast for ReL1, the
correlations with theoretical studies (see Table S14†).
energy difference between these levels is greater (0.41 eV) and
Interestingly, the thorough examination of the first allows an easier electrochemical assignment of this reduction
reduction process of the Re complexes at different scan rates process exclusively resulting from the π*(pyta) moiety.
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Spectroscopic properties in solution
The spectroscopic properties of the two compounds in solu-
tion have been studied in three organic solvents of various
polarity and proticity, i.e. DCM, acetonitrile and methanol.
However, emphasis is given to results obtained in DCM for the
sake of homogeneity with calculations and electrochemistry,
and because these results are representative of the main behav-
ior of our compounds. Solutions in DCM were perfectly stable
at least for 24 h, and no aggregation was detected by UV-vis
absorption spectroscopy in the investigated range of concen-
−
4
trations, i.e. below 2.6 × 10 M. It was checked by HPLC that
the samples contained no traces of free ligands. All measure-
ments were conducted in aerated solutions. Bubbling with
argon led to an increase of the emission band intensity lower
or equal to 15%, whatever the band considered. Table S15† col-
lects the data in the three organic solvents, Table 2 presents
selected data in DCM.
As illustrated in Fig. S9,† the experimental UV-vis absorp-
tion spectra of both complexes shown in Fig. 7 were in very
good agreement with calculated spectra. The assignment of
wavelengths was made on the basis of TD-DFT calculations
Fig. 7 Spectra of complex ReL1 (top) and ReL2 (bottom) in DCM.
Absorption spectra (black dotted lines); normalized excitation spectra
(Tables S10 and S11†). Solutions of ReL1 in DCM were bright
yellow. The experimental absorption spectrum clearly showed
a main band situated around 296 nm and a distinct low-inten-
(blue lines) recorded at 560 nm and 540 nm, respectively; normalized
emission spectra (red and green lines) upon excitation at 380 nm.
sity band peaking at 384 nm and tailing up to 450 nm (Fig. 7, Harmonic bands have been deleted for the sake of clarity. Complex con-
centration between 2.5 × 10− M and 1.5 × 10 M. Insets: Solutions illu-
5
−5
0 8 0
top). The main band can be attributed to the S → S and S →
S9 transitions with intra-ligand (IL) and metal-to-ligand charge
transfer (MLCT) character, calculated at around 292 nm, and
minated by a UV lamp at 365 nm.
to the weak S
0 6
→ S transition calculated at 300.9 nm. The
long-wavelength band, typically observed in rhenium(I) tricar- respectively. The absorption tail mainly arises from the weak
4
,6,27
bonyl diimine complexes [ReX(CO) (α,α′-diimine)],
clearly
S → S transition, expected at 347.4 nm, which in this case
0 2
3
results here from a S → S
0 2
transition (at 378.4 nm), which does not generate a distinct MLCT band.
involves the HOMO−1 and the LUMO and has strong MLCT
Upon excitation at 365 nm by a hand-held UV lamp, solu-
character.
tions of ReL1 emitted red light detectable by the naked eye
Solutions of complex ReL2 were pale yellow. The absorption (inset of Fig. 7, top). The corresponding emission spectrum
spectrum showed an intense band at 309 nm that smoothly showed an intense band centered at around 628 nm and easily
extended above 400 nm (Fig. 7, bottom). The molar absorption attributed to phosphorescence, which is expected at 645.1 nm
coefficient was a little higher than for ReL1 and this is in line for emission coming from the first triplet state (Table S13†). By
with better electron delocalization. Most likely, the main band comparison with the UV-absorption spectrum, the excitation
arises from the S → S , S → S and S → S transitions with spectrum showed strong contribution of the MLCT band,
0
6
0
5
0
3
MLCT and ligand-to-ligand charge transfer (ILCT) character, suggesting that excitation in this band favors phosphorescence
predicted to take place at 304.1 nm, 316.4 and 324.4 nm, emission. The phosphorescence quantum yield Φ was around
P
Table 2 Spectroscopic data of the complexes in dichloromethane and in the solid state (powder). Maximum absorption wavelength (λabs), molar
extinction coefficient (ε), excitation wavelength (λex), emission wavelength (λem), maximum wavelength of phosphorescence (λ ) and photo-
P
luminescence (λPL) emission, phosphorescence and photoluminescence quantum yields (Φ
P
and ΦPL, respectively), luminescence decay time (τ). The
concentration of complexes in solution was between to 2.5 × 10− M and 1.5 × 10
5
−5
M
Dichloromethane solution
Solid state
ε (M− cm−1
1
)
λ
P
(nm)
Φ
P
τ (ns)
λPL (nm)
Φ
PL
τ (ns)
λabs (nm)
a
b
ReL1
ReL2
296, 384
309
27 800, 3900
34 100
628
546
0.017
0.015
74.7c
584, 622
542
0.065
0.016
338
414
d
192
a
b
c
d
If not specified, excitation was performed at 380 nm.
90–640 nm.
λex = 330 nm, λem = 560–720 nm.
λem = 560–690 nm.
λem = 500–630 nm.
em
λ =
4
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0
.017. The emission decay was almost monoexponential with a comment on possible S
m
→ T
m
(m > 1) ISC. For the sake of sim-
long lifetime of 74.7 ns, totally in line with phosphorescence plicity, it was assumed that high-energy singlet states de-
emission (Fig. S10†). activate to the S via internal conversion (IC), and S sub-
Solutions of complex ReL2 emitted green light upon exci- sequently decays to the lowest triplet state T that is respon-
1
1
1
tation with the UV lamp (inset of Fig. 7, bottom). The emission sible for phosphorescence. The diagram takes into account the
spectrum was dominated by a strong band with maximum at observation that excitation to higher electronic excited states
5
46 nm, close to the value calculated for phosphorescence contributes less to phosphorescence emission than excitation
2
8
emission (553.2 nm). The corresponding excitation spectrum in the MLCT band, as already reported in the literature. An
was close to the absorption spectrum, except for a little band explanation is that S , S and T involve orbitals with close
2
1
1
around 370 nm, i.e. in the MLCT region. Like for the former electron distribution, and so ISC may be facilitated by the
complex, albeit to a lesser extent, it seems that excitation in small electron reorganization required. In contrast, high-
this region promotes phosphorescence emission. The phos- energy excited states, which involve different types of orbitals,
phorescence quantum yield was 0.015. The decay was monoex- may undergo easy radiationless pathways down to the ground
ponential with a lifetime value of 192 ns (Fig. S10†).
In summary, both complexes share some common spectro- by small differences in their photophysical processes.
scopic features in solution. In particular, they exhibit phos- According to Table S12,† for ReL1, the S excited state is
phorescence in solution at room temperature, as commonly almost the only excited state involved in the emission process
state. The behavior of the complexes could thus be explained
2
1
–9
observed for tricarbonyl Re(I) complexes,
spin–orbit coupling promotes efficient intersystem crossing the most important intermediate in collecting the emission
ISC) between singlet and triplet manifolds. The most striking energy, while for ReL2, the S and S states that have signifi-
in which strong with a significant oscillator strength (0.1748), so it could be
(
3
4
difference is that the phosphorescence emission of ReL1 was cant f values could also play an active role.
red-shifted by 82 nm with respect to ReL2. Though moderate,
the phosphorescence quantum yields of these compounds
Spectroscopic properties in the solid state
compare well with those of many rhenium complexes used for
3
–5
Notably, the solid complex ReL1 strongly emitted yellow light,
while ReL2 emitted green light (Fig. 9, insets). The spectro-
scopic behavior of the two complexes in pristine powder form
was thus investigated using an integrating sphere. For ReL1,
the emission spectrum exhibited two maxima at 584 and
bio-imaging in the literature. It is also noteworthy that their
phosphorescence intensity was reduced with increasing the
polarity and proticity of the solvent (Table S15†). This effect
was moderate for ReL1 and particularly strong for ReL2, for
which emission was almost no detectable in methanol. It is
reminiscent of the total phosphorescence quenching observed
in the presence of water for Re(I) tricarbonyl complexes posses-
sing a pyta fragment, where the 1,2,3-triazole unit is functiona-
622 nm (Fig. 9). The long wavelength band is reminiscent of
that observed in solution at 628 nm. For ReL2, the spectrum
was centered on 542 nm, very close from that of solutions.
Long decay times of 338 ns and 414 ns were predominant
for ReL1 and ReL2, respectively (Table S16 and Fig. S11†), con-
firming that the nature of solid-state emission is mainly phos-
phorescence. The lifetimes have been markedly increased with
passing from solutions to the solid state, and this stabilization
effect is more pronounced for ReL1 than for ReL2. The photo-
luminescence quantum yields ΦPL of ReL1 and ReL2 were
7
lized by an aliphatic substituent.
To get a clear overview of the photophysical behavior in
solution, the energy diagrams are presented in Fig. 8. In the
absence of ultra-fast spectroscopy measurements, we cannot
Fig. 8 Simplified schematic energy level diagram describing the main
photophysical processes of complexes ReL1 and ReL2 in DCM solution,
in optimized ground state, first singlet excited state and first triplet
excited state geometry (from left to right for each complex). Radiative
transitions are in colored solid lines, internal conversion (IC) in grey
Fig. 9 Normalized emission spectra of complexes ReL1 (orange line)
and ReL2 (green line) in the solid state (pristine powders) upon exci-
tation at 380 nm. Inset: Picture of the complexes as powders under illu-
mination by a UV lamp, λex = 365 nm.
dotted lines, intersystem crossing (ISC) in orange dotted lines. S
m m
→ T
(
m > 1) intersystem crossing not represented.
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0
.065 and 0.016, respectively (Table 2). The comparison with escence at rather short wavelengths (around 522–528 nm) in
7
,22b
the corresponding Φ values in solution shows that the whole organic solvents.
emission intensity is the same for ReL2, yet multiplied by 3.8 emission maximum is around 538–543 nm.
After substitution by a phenyl group, the
P
2
0,22a
In the case of
for ReL1. The latter phenomenon can be related to an increase ReL2, grafting of the electron-withdrawing PBO group to the
2
9,30
of phosphorescence emission upon aggregation (AIPE).
1,2,3-triazole moiety moderately shifts the emission spectra till
For our compounds, the decrease of phosphorescence inten- 546 nm in solution. Above all, the presence of the PBO group
sity induced by water prevents us from making the popular induces an enhancement of the phosphorescence quantum
demonstration of AIPE behavior, which consists in adding yield by one order of magnitude. By this respect, complexes
water to an organic solution of dye to promote aggregation are thus a little sensitive to the nature of the substituent borne
and to visualize the associated emission enhancement. by the 1,2,3-triazole moiety, although it has been shown for
However, the comparison of the solution and solid-state other metal complexes that this moiety generally behaves as an
1
2,21,32
quantum yields speaks for itself. The different behavior of insulator.
both complexes may be easily understood. Indeed, the π–π No direct comparison can be made for ReL1 because this
stacking interactions that take place between the aromatic substitution pattern has not been investigated in the literature.
rings of ReL2 lead to intermolecular quenching, which is detri- However, it is obvious that ReL1 emits at much longer wave-
mental to the emission of light in the solid state. In contrast, lengths than pyta-based complexes possessing a 1,2,3-triazole
the bent conformation of ReL1 prevents π–π stacking, and thus moiety. Most importantly, our TD-DFT calculations pointed
2
9
promotes light emission. In parallel, the intramolecular out a smaller HOMO–LUMO energy gap in ReL1 than in ReL2,
rotations between the pyta and PBO groups are restricted like which is in line with the lower first electrochemical reduction
other possible molecular motions, reducing non emissive de- potential and the red-shift of the phosphorescence spectrum
activation pathways. These combined effects are mostly respon- observed for the former complex. This difference is certainly
30
sible for the observed AIPE effect. So, the peculiar conformation related to the intrinsic electron properties of the complexes,
of ReL1 plays a key role regarding the emission properties. due to the very nature of the triazole group. It is noteworthy
It must be kept in mind that the solid-state characteristics that the HOMO and LUMO of both ReL1 and ReL2 are exclu-
reported here only refer to the investigated powders. Indeed, sively centered around the metal and the pyta group, with little
other values could have been found with single crystals or with involvement of the PBO moiety. The comparison with other
powders prepared from recrystallization in other solvents, as it molecules would allow determining the exact role played by
is well known that the molecular arrangement and surface the substituent in the HOMO–LUMO energy levels.
3
1
defects play an essential role in solid-state properties.
Remarkably, the presence of the PBO group plays a crucial
role in the solid-state emission properties. Here, it led to com-
plexes that emit light in the solid state at least with the same
efficiency as in solution. At the moment, ReL1 is the first
mononuclear Re(I) complex reported to exhibit AIPE behav-
Conclusions
3
0
For a long time, various triazole groups have been prepared via ior, and this property is obviously due to its bent geometry
click chemistry or conventional synthesis to play the role of that results from the presence of PBO on the 1,2,4-triazole
linkers or to be part of the coordinating agent of transition group. The development of Re(I) complexes based on this
metal complexes, as it is the case for pyta ligands. However, framework is presently underway in our laboratory with the
except for some complexes, the importance of the nature of aim of improving the solid-state luminescence properties.
1
1
1
2,13
the triazole group has attracted little interest.
Remarkably,
To sum up, this work showed that triazole-based complexes
the present work revealed that the triazole isomerism impacts must be carefully designed depending on the envisaged appli-
the properties of the corresponding complexes, at least in the cations. The 1,2,4-triazole isomer deserves being more widely
particular case where the triazole group bears a PBO moiety. used, and substitution on the N(4) nitrogen atom by an aro-
For instance, with the 1,2,4-triazole isomer like in ReL1, steric matic fragment with extended π-electron conjugation is prob-
hindrance occurs due to the proximity of PBO with the pyridyl ably a distinct advantage, as far as solid-state luminescence pro-
group. The PBO moiety is then positioned out of plane and perties are desired. Using the synthetic strategy described
behaves more independently from an electronic point of view. above, various organic dyes could be combined with the triazole
Electron delocalization is better in ReL2 that incorporates a fragment to modulate the spectroscopic and photophysical be-
planar organic fragment. However, this difference between havior of the desired complexes. Following this approach
complexes does not account for all the experimental results. should provide precious data to better rationalize the design of
For example, intuitively, it might have been expected that ReL2 highly emissive and specific Re(I) luminescent materials.
emits at longer wavelengths than ReL1, and this is not the case.
To better understand what occurs, it is instructive to
compare ReL2 with closely related tricarbonyl Re(I) complexes
reported in the literature. Complexes that incorporate a pyta
Experimental section
group in which the 1,2,3-triazole moiety is substituted by an All purchased chemicals were of the highest purity commer-
alkyl substitutent on the N(1) nitrogen atom emit phosphor- cially available and used without further purification.
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Analytical grade solvents were used and not further purified 0–4 °C with stirring, then extra ethanol was added until the
unless specified. Reactions were monitored by analytical thin solid dissolved completely. Then, sodium nitrite (262 mg,
layer chromatography (TLC) on Kieselgel 60 F254 (Merck). 3.8 mmol) in water was added dropwise and the mixture was
Chromatography purification was conducted using silica gel or stirred at 0–4 °C for 45 min. Sodium azide (195 mg, 3 mmol)
neutral alumia obtained from Merck. NMR, mass and infrared was added slowly. The mixture was stirred for another 2 h and
spectra were obtained in the relevant ‘Services communs de extracted with diethyl ether (50 mL × 3). All the organic layers
3
l’Institut de Chimie de Toulouse, Université de Toulouse were combined, washed with saturated NaHCO aqueous solu-
1
13
III-Paul-Sabatier’. H- and C-NMR spectra were measured tion (50 mL × 2), dried over anhydrous MgSO , filtered and
4
with Bruker Avance DRX 500 MHz or Bruker Avance 300 MHz. concentrated to dry. The residue was dried under vacuum
Attributions of the signals were made using 2D NMR data without further purification to afford 405 mg of yellow solid,
1
(
COSY, HSQC and HMBC). For ligands and complexes, protons yield 86%. H NMR (300 MHz, CDCl ): δ (ppm) = 7.04 (dd, J =
3
and carbon atoms were numbered according to Fig. S12.† 2.1 Hz, J = 8.5 Hz, 1H, ArH), 7.27 (dd, J = 0.5 Hz, J = 2.1 Hz, 1H,
Electrospray mass spectra were obtained using a QTRAP ArH), 7.53–7.53 (m, 3H, ArH), 7.72 (dd, J = 0.5 Hz, J = 8.5 Hz,
Applied Biosystems spectrometer and high-resolution mass 1H, ArH), 8.21–8.24 (m, 2H, ArH). DCI MS (NH ): m/z calcd for
3
+
+
−1
spectra (HRMS) were recorded using an LCT Premier Waters
spectrometer. Desorption chemical ionization (DCI) mass
C
13
H
9
N
4
O [M + H] : 237.1; found: 237.0. IR: ν(N3) = 2113 cm
.
2-Phenyl-6-(3-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)benzoxazole
spectra (NH or CH ) were obtained on a DSQ II Thermofisher (L1). To a solution of pyridine-2-carbohydrazide (287 mg,
3
4
apparatus. Infrared spectra were obtained on
3
a Nexus 2.1 mmol) in distilled CH CN (5 mL), N,N-dimethylformamide
Thermonicolet apparatus with DTGS as the detector. The dimethyl acetal (250 mg, 2.1 mmol) was added dropwise under
microanalyses were performed with a PerkinElmer 2400 argon. The mixture was heated to 50 °C with stirring for 3 h
elemental analyzer in the ‘Service d’Analyse Chimique du and then 2 (400 mg, 1.9 mmol) in CH CN (15 mL) was added
3
Laboratoire de Chimie de Coordination de Toulouse’ (LCC, as well as acetic acid (3 mL). The mixture was heated to 120 °C
Toulouse). The compound purity was checked by HPLC, using for 16 h. After cooling to room temperature, the mixture was
an Acquity CSH C18 column and a water/acetonitrile gradient filtered and the precipitate was washed with diethyl ether. The
as the eluent. Detection was made with MS, absorption and residue was purified on column chromatography using MeOH/
fluorescence detectors.
DCM (1 : 10 v/v) as eluent to afford 200 mg of light yellow
1
6
solid, yield 31%. H NMR (500 MHz, DMSO-d ): δ (ppm) = 8.97
General procedure for the preparation of ligands
(
s, 1H, H5′), 8.35 (ddd, J = 4.9, 1.8, 0.9 Hz, 1H, H6″), 8.28–8.16
6
-Nitro-2-phenylbenzoxazole (1). The mixture of 2-amino-5- (m, 2H, Hb,f), 8.10 (dt, J = 7.9, 1.1 Hz, 1H, H ), 8.02 (d, J =
3″
nitrophenol (10 mmol, 1.54 g), benzoic acid (10 mmol, 1.22 g) 2.0 Hz, 1H, H
and polyphosphoric acid (24 g) was heated to 110 °C with stir- 8.4 Hz, 1H, H
7
), 7.97 (td, J = 7.8, 1.8 Hz, 1H, H4″), 7.88 (d, J =
4
), 7.71–7.60 (m, 3H, Hc,d,e), 7.41 (ddd, J = 8.6,
1
3
ring for 16 h. After reaction, iced water (100 mL) was added. The 5.2, 1.7 Hz, 2H, H5,5″).
C NMR (125 MHz, DMSO-d6):
), 149.5 (C6″), 147.1
), 137.8 (C4″), 133.0 (C ), 132.9 (C ),
were combined, washed with water, dried over anhydrous 129.9 (Cc,e), 127.9 (Cb,f), 126.5 (C ), 125.0 (C ), 124.4 (C ),
resulting mixture was neutralized with 10% NaOH solution δ (ppm) = 164.3(C
2 8
), 152.1 (C2″), 150.2 (C
and extracted with ethyl acetate (3 × 60 mL). All organic layers (C5′), 146.9 (C3′), 142.0 (C
6
9
d
a
5″
3″
MgSO
fied by column chromatography using ethyl acetate/petroleum
ether (v/v = 1/5) as eluent to obtain 1.91 g of yellow solid, yield (%) for C H N O: C 70.79, H 3.86, N 20.64; found: C 70.51,
4 5 4 7
, filtered and concentrated to dry. The residue was puri- 124.0 (C ), 120.1 (C ), 110.0 (C ). ESI-HRMS: m/z calcd for
+
+
20 14 5
C H N O : 340.1198 [M + H] , found 340.1195. Anal. calcd
2
0
13 5
1
8
0
0%. H NMR (300 MHz, CDCl
3
): δ (ppm) = 8.48 (dd, J = H 3.63, N 20.79.
2-Phenyl-6-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)benzoxazole
.5 Hz, J = 2.2 Hz, 1H, ArH), 8.33–8.26 (m, 3H, ArH), 7.83 (dd,
J = 0.5 Hz, J = 8.8 Hz, 1H, ArH), 7.65–7.53 (m, 3H, ArH). (L2). A mixture of 2-ethynylpyridine (103 mg, 1 mmol),
+
+
DCI-MS (CH
4
): m/z calcd for C13
H
9
N
2
O
3
: 241.0608 [M + H] ; 6-azido-2-phenylbenzoxazole (3) (260 mg, 1.1 mmol), copper(II)
found: 241.0607.
acetate monohydrate (60 mg, 0.3 mmol) and sodium ascorbate
6
-Amino-2-phenylbenzoxazole (2). To a solution of 1 (1.07 g, (100 mg, 0.5 mmol) in acetonitrile (20 mL) was stirred at room
4
.5 mmol) in MeOH/CHCl (1 : 2 v/v, 30 mL) was added 10% temperature for 16 h. The resulting mixture was evaporated to
3
Pd/C (0.33 g). The mixture was carried out under 6 bars remove the solvent and then dissolved in DCM (50 mL),
pressure of H and stirred at room temperature for 24 h. After washed with saturated Na EDTA solution (30 mL × 2), dried
2
2
reaction, the mixture was filtered twice to remove the catalyst. with anhydrous MgSO
4
and filtered. The filtrate was evapor-
The filtrate was concentrated to dry and purified by column ated and recrystallized with DCM and diethyl ether to afford
1
chromatography using DCM as eluent to obtain 0.82 g of pale 225 mg of pale solid, yield 66%. H NMR (500 MHz, DMSO-
1
white solid, yield 87%. H NMR (300 MHz, CDCl
8
3
): δ (ppm) =
d
6
): δ (ppm) = 9.44 (s, 1H, H5′), 8.69 (m, 1H, H6″), 8.53 (d, J =
), 8.26–8.24 (m, 2H, Hb,f), 8.16–8.12 (m, 2H,
Hz, 1H, ArH), 6.70 (dd, J = 2.1 Hz, J = 8.4 Hz, 1H, ArH). DCI MS H4,5), 8.05 (d, J = 8.6 Hz, 1H, H ), 7.99–7.96 (m, 1H, H ),
.20–8.16 (m, 2H, ArH), 7.54–7.48 (m, 4H, ArH), 6.88 (d, J = 2.2 2.1 Hz, 1H, H
7
3″
4″
+
+
13
(NH
3
): m/z calcd for C13
H
11
N
2
O : 211.1 [M + H] ; found: 211.0.
7.70–7.64 (m, 3H, Hc,d,e), 7.44–7.42 (m, 1H, H5″). C NMR
): δ (ppm) = 164.4 (C ), 150.9 (C2″), 150.2
6
-Azido-2-phenylbenzoxazole (3). To an aqueous solution of (125 MHz, DMSO-d
6
2
HCl (6 N, 20 mL), compound 2 (420 mg, 2 mmol) was added at (C ), 149.9 (C ), 148.8(C ), 142.1 (C ), 137.8 (C ), 134.5 (C ),
6
″
8
6
9
4″
4′
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1
1
32.9 (C
d
), 129.9 (Cc,e), 128.0 (Cb,f), 126.5 (C
a
), 123.5 (C5″), The structures were solved by direct methods (SHELXS 97) or
3
3,34
22.2(C ), 121.1 (C ), 120.3 (C ), 118.1 (C ), 104.2 (C ). using intrinsic phasing method (ShelXT).
All non-hydro-
5
′
3″
5
4
7
+
+
ESI-HRMS: m/z calcd for C20
found: 340.1197; m/z calcd for C20
14 5
H N O : 340.1198 [M + H] ; gen atoms were refined anisotropically using the least-square
+
2 34
H
13
N
5
ONa : 362.1018 method on F . Full crystallographic data are collected in
+
[M + Na] ; found 362.1015. Anal. calcd (%) for C H N O: Table S1.†
2
0
13 5
C 70.79, H 3.86, N 20.64; found: C 70.49, H 3.69, N 20.52.
Computational details
General procedure for the preparation of tricarbonyl rhenium(I)
complexes
35
The GAUSSIAN09 program package was employed for all cal-
culations (the geometry optimization, the ground-state and
A mixture of ligand and [Re(CO)
stirred overnight at 65 °C. After consumption of the ligand, the aid of the ChemCraft visualization program. The ground
mixture was cooled to room temperature and filtered, the pre- state (S ), the first excited state (S ) and the lowest triplet state
) geometries of compounds were fully optimized with the
restricted and unrestricted density functional theory (R-DFT
5
Cl] (1.15 eq.) in methanol was excited-state electronic structures, and optical spectra) with the
3
6
0
1
cipitate was purified by chromatography on silica gel using (T
1
ethylacetate as eluent to afford the desired product.
ReL1. 50 mg (0.15 mmol) of L1 and 58 mg (0.16 mmol) of and U-DFT) method using the Perdew–Burke–Ernzerhof
3
7
[
Re(CO)
5
Cl] afforded 65 mg of the complex ReL1 as a yellow PBE1PBE functional without symmetry constraints. In all cal-
1
solid. Yield: 67%. H NMR (500 MHz, DMSO-d ): δ (ppm) = culations, the “double-ζ” quality basis set LANL2DZ with Hay
6
9
.31 (s, 1H, H5′), 9.17–9.06 (m, 1H, H6″), 8.47 (s, 1H, H
7
), and Wadt’s relative effective core potential ECP (outer-core
3
8
8
.35–8.21 (m, 2H, Hb,f), 8.15 (d, J = 8.4 Hz, 1H, H ), 8.04 (td, [(5s25p6)] electrons and the (5d6) valence electrons) was
d
J = 8.0, 1.5 Hz, 1H, H ), 7.84–7.55 (m, 5H, Hc,e,4,5,5″), 7.21 (dd, employed for the Re atom. The 6-31+g** basis set for H, C, N,
4
″
1
3
39
J = 8.1, 1.1 Hz, 1H, H3″). C NMR (125 MHz, DMSO-d
δ (ppm) = 198.3, 197.7, 189.5 (CuO), 165.4 (C ), 155.1 (C3′), lations were performed using the optimized structural para-
55.0 (C ), 150.8 (C ), 148.9 (C ), 144.5 (C ), 144.2 (C ), 141.2 meters of compounds, to confirm that each optimized struc-
6
): O and Cl atoms was used. The vibrational frequencies calcu-
2
1
6
″
8
5′
2″
9
(
C
4″), 133.3 (C
5
), 130.0 (Cc,e), 129.3 (C
6
), 128.8 (C5″), 128.2 (Cb, ture represents a local minimum on the potential energy
), 111.4 (C ). surface and all eigenvalues are non-negative. The optimized
f
), 126.2 (C ), 124.7 (C
a
4
), 123.8 (C3″), 121.7 (C
d
7
1
85
+
ESI-HRMS: m/z calcd for C H N O Cl ReNa : 666.0083 Cartesian coordinates of compounds are included in the ESI
2
3
13 5 4
+
185
+
[M + Na] ; found 666.0096; m/z calcd for C23
H
13
N
5
O
4
Re : part (Tables S17 and S18†). On the basis of the optimized
+
6
08.0497 [M − Cl] ; found: 608.0510. Anal. calcd (%) for ground and excited state geometries, the absorption and emis-
C H N O ReCl: C 42.83, H 2.03, N 10.86; found: C 42.59, sion properties were calculated by the time dependent density
2
3
13 5 4
−
1
H 1.92, N 10.67. IR: ν(CuO) = 2025, 1919, 1884 cm
.
functional theory (TD-DFT) method at the PBE1PBE/LANL2DZ/
ReL2. 100 mg (0.30 mmol) of L2 and 123 mg (0.34 mmol) of 6-31+G** level. The solvent effect (DCM, ε = 9.08) was simu-
[
Re(CO) Cl] afford 130 mg of complex ReL2 as pale yellow lated using the Self-Consistent Reaction Field (SCRF) under
5
1
40
solid. Yield: 68%. H NMR (500 MHz, DMSO): δ (ppm) = the Polarizable Continuum Model (PCM). These methods
δ (ppm) = 10.04 (s, 1H, H5′), 9.05 (ddd, J = 5.1, 1.8, 0.8 Hz, 1H, have already shown good agreement with experimental studies
H ), 8.57 (dd, J = 2.1, 0.5 Hz, 1H, H ), 8.39 (td, J = 7.9, 1.5 Hz, for different rhenium(I) complexes.
4
1
6
″
7
1
1
H, H4″), 8.33–8.27 (m, 3H, Hb,f,3″), 8.15 (dd, J = 8.6, 0.5 Hz,
H, H ), 8.10 (dd, J = 8.6, 2.1 Hz, 1H, H ), 7.74–7.67 (m, 4H,
Electrochemistry
5
4
1
3
Hc,d,e,5″). C NMR (125 MHz, DMSO): δ (ppm) = 198.0, 197.1, Osteryoung square wave voltammetry (OSWV) and cyclic vol-
1
90.0 (CuO), 165.2 (C
2
), 153.7 (C6″), 150.8 (C2″), 149.6 (C
8
), tammetry (CV) measurements were made in DCM at a ligand
−
3
1
48.9 (C ), 143.4 (C ), 141.4 (C4″), 133.2 (C
6
9
d
), 133.1 (C4′), 130.0 concentration of 3.4 × 10 M and a complex concentration of
Cc,e), 128.1 (Cb,f), 127.3 (C ), 126.3 (C ), 125.4 (C ), 123.2 6.5 × 10− M. The supporting electrolyte n[Bu N][BF ] (Fluka,
3
(
(
5″
a
5′
4
4
C
3″), 121.5 (C
5
), 119.1 (C
4
+
7
), 105.5 (C ). ESI-HRMS: m/z calcd 99% electrochemical grade) was used as received and simply
1
85
+
for
66.0099; m/z calcd for C H N O
13 5 4
23 13 5 4
C H N O Cl ReNa : 666.0083 [M + Na] , found degassed under Ar. DCM was dried in an MB SPS-800 Solvent
1
85
+
+
6
Re : 608.0497 [M − Cl] , Purification System just prior to use. The measurements were
found 608.0513. Anal. calcd (%) for C23 ReCl: C 42.83, carried out with a potentiostat Autolab PGSTAT100 controlled
H 2.03, N 10.86; found: C 43.15, H 2.21, N 11.18. IR: ν(CuO)
2
3
13 5 4
H N O
=
by GPES 4.09 software. Experiments were performed at room
temperature (r.t.) in a homemade airtight three-electrode cell
connected to a vacuum/Ar line. The reference electrode con-
sisted of a saturated calomel electrode (SCE) separated from
030, 1920, 1903 cm−
1
.
2
X-ray crystallography
X-Ray quality crystals of ReL1 were obtained by diffusion crys- the solution by a bridge compartment. The counter electrode
2
3
tallization of CH CN and diethyl ether. Single crystals of ReL2 was a Pt wire of ca. 1 cm apparent surface. The working elec-
and L2 were slowly grown in acetone and DCM, respectively. trode was a Pt microdisk (0.5 mm diameter). OSWV experi-
Crystal data were collected on a Bruker AXS Quazar APEX II ments were carried out at room temperature using a sweep
diffractometer using a 30 W air-cooled microfocus source width of 20 mV, a frequency of 20 Hz, and a step potential of
(ImS) with focusing multilayer optics using MoKα radiation 5 mV. Before each measurement, the solutions were degassed
(wavelength = 0.71073 Å). Phi- and omega-scans were used. with Ar and the working electrode was polished with a polish-
Dalton Trans.
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Paper
ing machine (Presi P230). Ferrocene was used as internal refer- the data. The instrumental response function was recorded
ence (Fc + /Fc, 0.55 ± 0.01 V vs. SCE).
before each decay measurement with a full width at half-
maximum (fwhm) of ∼25 ps. The fluorescence data were ana-
lyzed using the Globals software package developed at the
Spectroscopy and photophysics
Dye solutions were prepared by gentle heating in a solvent, Laboratory for Fluorescence Dynamics at the University of
sonication and filtration on paper filter prior to measurement. Illinois at Urbana-Champaign, which includes reconvolution
Spectroscopic measurements in solutions were conducted at analysis and global non-linear least-squares minimization
2
0 °C in a temperature-controlled cell. UV-visible absorption method.
spectra were recorded on a Hewlett Packard 8453 spectrometer.
Fluorescence spectra in solutions were measured with a Cary
Eclipse spectrofluorometer and a Xenius SAFAS spectrofluo- Conflicts of interest
rometer using cells of 1 cm optical pathway. All fluorescence
There are no conflict of interest to declare.
spectra were corrected. The fluorescence quantum yields in
solution (Φ ) were determined using the classical formula:
ΦFx = (As × Fx × nx × Φ )/(A × Fs × n ) where A is the
F
2
2
s
Fs
x
Acknowledgements
absorbance at the excitation wavelength, F the area under the
fluorescence curve and n the refraction index. Subscripts s
and x refer to the standard and to the sample of unknown
quantum yield, respectively. Coumarin 153 in ethanol
J. Wang thanks the Chinese Scholarship Council (CSC) for a
Ph.D. funding. M. Wolff acknowledges financial support from
the Foundation for Polish Science (START 2014). Calculations
were carried out in Wroclaw Centre for Networking and
Supercomputing, Poland (http://www.wcss.pl). We thank
Dr A. Saquet and A. Moreau (LCC) for electrochemical
measurements. We thank the Integrated Screening Platform of
Toulouse (PICT IBiSa) for providing access to HPLC equipment
and Mrs I. Fabing for skilled help. We are also indebted
to Prof. F. Alary (University of Toulouse III) for a fruitful
discussion.
4
2
F
(Φ = 0.53) was used as the standard for excitation at
3
0
80 nm. The absorbance of the solutions was equal or below
.055 at the excitation wavelength. The error on the quantum
yield values is estimated to be about 10%.
Solid state photoluminescence quantum yields were
recorded on a SAFAS Xenius spectrofluorometer equipped with
a BaSO integrating sphere and a Hamamatsu R2658 detector.
4
Solid samples were deposited on a metal support and
luminescence spectra were corrected. The absolute photo-
P
luminescence quantum yield values (Φ ) were determined by a
4
3
method based on the one developed by De Mello et al. The
excitation source was scanned in order to evaluate the reflected
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