Optimization of aggregation-induced phosphorescence enhancement in mononuclear tricarbonyl rhenium(i) complexes: The influence of steric hindrance and isomerism

Article abstract of DOI:10.1039/c9dt02786f

In order to improve the remarkable performance of a mononuclear tricarbonyl rhenium(i) complex (ReL1) that exhibits rare aggregation-induced phosphorescence enhancement (AIPE) behavior, two new complexes (ReL3 and ReL4) were prepared and investigated. The

Full text of DOI:10.1039/c9dt02786f

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Optimization of aggregation-induced
phosphorescence enhancement in
Cite this: DOI: 10.1039/c9dt02786f
mononuclear tricarbonyl rhenium(I) complexes:
the influence of steric hindrance and isomerism†
f
Jinhui Wang,a,b,c Alexandre Poirot,a Béatrice Delavaux-Nicot, d,e Mariusz Wolff,
Sonia Mallet-Ladeira,g Jan Patrick Calupitan, h Clémence Allain, h Eric Benoista
a
and Suzanne Fery-Forgues
*
In order to improve the remarkable performance of a mononuclear tricarbonyl rhenium(I) complex (ReL1)
that exhibits rare aggregation-induced phosphorescence enhancement (AIPE) behavior, two new complexes
(ReL3 and ReL4) were prepared and investigated. They incorporate a 2-pyridyl-1,2,4-triazole (pyta) ligand
connected to a 2-phenylbenzoxazole (PBO) moiety. Complex ReL3 differs from ReL1 by the presence of a
bulky tert-butyl substituent, and ReL4 is an isomer where the PBO group is linked to the pyta ligand by its
phenyl group. Theoretical calculations were in congruence with electrochemical and spectroscopic pro-
perties in solutions. Both new compounds exhibited strong AIPE and much better solid-state emission
efficiency than ReL1, with photoluminescence quantum yields up to 55% for ReL4. Crystallographic data
indicate that this increase in emission efficiency is due to optimum packing that prevents quenching. This
work shows that minor structural changes may have major effects upon the solid-state spectroscopic pro-
perties and it provides a rational basis for accessing AIPE-active strongly emissive rhenium(I) complexes.
Received 5th July 2019,
Accepted 13th August 2019
DOI: 10.1039/c9dt02786f
rsc.li/dalton
chemosensors, bioprobes, stimuli-responsive nanomaterials,
and optoelectronic materials.1 Tricarbonyl rhenium(I) com-
Introduction
Transition metal complexes that take advantage of aggregation plexes have been known for a long time for their photo-
to trigger light emission have been the subject of intense luminescence properties in solution,2 and their stability in air
research in recent years for their potential applications as and water makes them appreciated cell imaging agents.3 In
most cases, the predominant emission process is phosphor-
escence, whose long lifetime may bring benefits in terms of
detection. The solid-state emission properties have also been
studied, mainly in view of applications in the fields of opto-
electronic and photonic devices.4 However, only rare examples
of aggregation induced phosphorescence enhancement (AIPE)
behavior have been reported, first concerning dinuclear5 and
tetranuclear complexes,6 and more recently, mononuclear
complexes.7 In a recent work, we showed that a mononuclear
tricarbonyl rhenium(I) complex (ReL1, Fig. 1) exhibits clear
AIPE behavior. In the solid state, this compound displays
yellow to red emission, so it could be particularly valuable for
preparing nanoparticles for bio-imaging, although the photo-
luminescence quantum yield is quite modest (0.065).8 The aim
of the present work is now to improve the solid-state spectro-
scopic properties of these complexes through the rational
design of the ligands.
aSPCMIB, CNRS UMR5068, Université de Toulouse III Paul Sabatier,
118 route de Narbonne, 31062 Toulouse cedex 9, France.
E-mail: sff@chimie.ups-tlse.fr
bInstitute of Drug Discovery Technology, Ningbo University, Ningbo 315211, China
cState Key Laboratory of Chemical Oncogenomics, Key Laboratory of Chemical
Biology, The Graduate School at Shenzhen, Tsinghua University, Shenzhen,
Guangdong 518055, PR China
dLaboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne,
BP 44099 F-31077 Toulouse Cedex 4, France
eLCC-CNRS, Université de Toulouse, CNRS, Toulouse, France
fInstitute of Chemistry, Department of Crystallography, University of Silesia,
9th Szkolna St., 40-006 Katowice, Poland
gService commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Université de
Toulouse III Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 9, France
hLaboratoire PPSM, CNRS UMR 8531, ENS Paris-Saclay, 61 avenue du Président
Wilson, F-91230 Cachan, France
†Electronic supplementary information (ESI) available: Experimental details,
including proton numbering for NMR; crystal packing, molecular views, and CIF
data; additional computational, electrochemical and spectroscopic data. CCDC
1922851 and 1922852. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c9dt02786f
To this end, two new compounds closely related to ReL1
were synthesized and studied. As a matter of fact, our previous
study has revealed that ReL1, which incorporates the
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Fig. 3 Synthesis of complex ReL4. Conditions and reagents:
(a) 4-Aminobenzoic acid, polyphosphoric acid, 220 °C,
4 h (65%);
(b) N,N-dimethyl-N’-picolinoylformohydrazonamide (3),8 acetic acid,
CH3CN, 90 °C, 24 h (56%); (c) [Re(CO)5Cl], MeOH, 65 °C, 16 h (92%).
Fig. 1 Chemical structures of the rhenium(I) complexes.
3-(2-pyridyl)-1,2,4-triazole fragment, has superior emission
properties compared to ReL2, built on the very popular
4-(2-pyridyl)-1,2,3-triazole fragment.8 The appended 2-phenyl-
benzoxazole (PBO) moiety contributes to the conjugated elec-
tron system of these complexes, and restrictions on its mole-
cular movements in the solid state are thought to be mainly
responsible for the AIPE effect. Moreover, for the vast majority
of PBO derivatives, there is little π–π stacking interaction
between aromatic groups, and hence an increased photo-
luminescence.9 In the first new complex (ReL3), the phenyl
ring of the PBO moiety was substituted in position 4 with a
tert-butyl group. The electronic structure is not expected to
change with respect to ReL1, but the increased steric
hindrance could influence the crystal packing mode, leading
to differences in the solid-state emission properties.10 In the
second complex (ReL4), the PBO moiety was connected to the
pyta group by its phenyl ring. The substitution pattern of PBO
is known to affect the emission properties in solution,11 so it
is also expected to influence luminescence in the solid-state.
The spectroscopic behavior of these two new compounds
shows how simple structural modifications of the starting
complex ReL1 may induce remarkable effects upon the solid-
Detailed procedures and characterizations are given in the
Experimental section. On the one hand, 2-amino-5-nitro-
phenol was reacted with 4-tert-butylbenzoic acid to give
6-nitro-4′-tert-butyl-PBO (1), which then underwent a reduction
process to afford the corresponding amine (2). On the other
hand, 4′-amino-PBO (4) was directly obtained from the reaction
of 2-aminophenol with 4-aminobenzoic acid in the presence of
polyphosphoric acid. These amino derivatives were condensed
with a hydrazonamide derivative (3) to give ligands L3 and L4
with an overall yield of 64% and 56%, respectively. These
ligands were then reacted with [Re(CO)5Cl] in refluxing metha-
nol to afford the corresponding tricarbonylrhenium(I) com-
plexes ReL3 and ReL4 in good yields (71% and 92%, respect-
ively). Ligands and complexes were unambiguously identified
1
by H and 13C NMR spectroscopy, high resolution mass spec-
trometry and elemental microanalysis. The infrared spectra
showed the characteristic ν(CO) stretching bands of the fac-
[Re(CO)3]+ unit at 2028, 1920, 1883 cm−1 for ReL3, and 2021,
1934, 1887 cm−1 for ReL4. The average values (1943 and
1947 cm−1, respectively) suggest that, in these complexes,
ligand L3 is a slightly better electron donor than L4, according
to Sarkar’s theory.12
state emission properties.
A thorough experimental and
theoretical study supports these findings.
Crystal structures
X-Ray quality crystals were obtained by diffusion of diethyl
ether in DMF. Selected crystallographic data are given in the
Experimental section, while bond lengths and angles are
reported in Tables S1 and S2.† In both new complexes, the
rhenium ion was coordinated to three carbonyl groups in a fac
configuration, one chlorine atom, and two nitrogen atoms of
the pyta ligand, with a moderate distortion of the octahedral
geometry (Fig. 4). A comparison of the bond lengths and
angles revealed that the structural modifications brought to
ReL3 and ReL4 have little influence on the geometry of the
coordination sphere with respect to ReL1. The benzoxazole
and phenyl ring were almost aligned in the same plane.
Complexes ReL3 and ReL4 retained the most characteristic
feature of ReL1, i.e. the marked bending between the PBO
moiety and the 1,2,4-triazole group. The angle value was
62.8(3)°/72.5(4)° for molecules A and B in the asymmetric unit
of ReL3, and even reached 83.3(8)° for ReL4 (see Fig. 5). It is
noteworthy that the PBO group only had one position with
respect to pyta in ReL3 and ReL4, while two very distinct
Results and discussion
Synthesis
Complexes ReL3 and ReL4 were obtained by following the syn-
thesis pathways summarized in Fig. 2 and 3, respectively.
Fig. 2 Synthesis of complex ReL3. Conditions and reagents: (a) 4-tert-
butylbenzoic acid, polyphosphoric acid, 120 °C, 16 h (75%); (b) 10%
Pd/C, H2, CH3OH/CH2Cl2, 6 bars, 24 h (82%); (c) N,N-dimethyl-N’-
picolinoylformohydrazonamide (3),8 acetic acid, CH3CN, 90 °C, 24 h
(64%); (d) [Re(CO)5Cl], MeOH, 65 °C, 16 h (71%).
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and minimize the neighboring interactions, the system adopts
a head to head arrangement between two adjacent molecules,
so that the rhenium centers and organic ligands constitute
distinct layers (Fig. 5 and Fig. S1†). This arrangement is remi-
niscent of ReL1, with the major difference that in the latter
complex the organic fragments are displayed in parallel planes
with interactions occurring between triazole rings, while in
ReL3 the organic moieties are arranged in different planes, so
that no overlap of the aromatic systems was detected.
Complex ReL4 crystallized in the orthorhombic P212121
space group. The molecules formed antiparallel dimers
(Fig. S1†). Certainly, switching the connection between the
PBO moiety and the pyta group resulted in reduced steric hin-
drance with respect to ReL1, so that a head-to-tail arrangement
is preferred. Dimers were themselves displayed in a herring-
bone manner. Only a tiny overlap was detected between the
benzoxazole group of one molecule and the pyridyl group of
the neighboring one (Fig. S2†). Consequently, π–π stacking
interactions are significantly minimized with respect to ReL1.
Fig. 4 Molecular views of complexes ReL3 (one molecule of the asym-
metric unit) and ReL4. Disorders are indicated by dashed lines and
hydrogen atoms are not represented for the sake of clarity.
Displacement ellipsoids are drawn at 50% probability.
Electronic structure
Computational studies based on the time dependent density
functional theory (TD-DFT) method were performed consider-
ing the two complexes in dichloromethane (DCM). At the S0
ground state level, the energy-minimized structures were in
positions were detected in ReL1. The only disorder observed good agreement with the data obtained by X-ray diffraction
for ReL3 comes from the free rotation of the tert-butyl group, (Tables S3 and S4†). Differences can be related to packing
and from the permutation between the chlorine atom and one effects, which are present in the X-ray determined structures
CO group, which generates two isomers in almost identical and not taken into account by the calculation. The overall
proportions. For ReL4, no ligand disorder was observed, except results show that both complexes are very similar from an elec-
a N/O swap with a 75/25 ratio in the five-membered PBO ring.
tronic viewpoint (Tables S5 to S13†). The HOMO−2 and
Regarding the molecular arrangement, complex ReL3 crys- LUMO+1 are both centered on the PBO fragment, the
ˉ
tallized in the triclinic P1 space group with two molecules in HOMO−1 and HOMO are localized on the rhenium atom with
the asymmetric unit. Both the inorganic moiety and the tert- contribution of the carbonyl and chlorine ligands, and the
butyl group strongly structured the network. In particular, the LUMO is localized on the pyta group (Table S5, Fig. 6, Fig. S3
tert-butyl group was involved in several intermolecular short and S4†). It is worth noting that the MOs of ReL3 and ReL1 are
contacts with the oxygen atoms of the carbonyl groups, as well almost identical, indicating the very small contribution of the
as with the carbon atoms of the pyridyl and phenyl groups of tert-butyl group. For ReL4, slight differences compared with
neighboring molecules. Other short contacts were found ReL1 are found in the shape of PBO-centered orbitals, which
between the benzoxazole heteroatoms and the phenyl hydro- change according to the connection to the triazole group.
gen atoms of neighboring molecules. To cope with the steric From an energetic viewpoint, both compounds have almost
constraints due to the tert-butyl group and rhenium moiety, the same HOMO–LUMO gap. The first frontier orbitals have
Fig. 5 Crystal cells of complexes ReL3 and ReL4. Hydrogen atoms not represented for the sake of clarity.
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Fig. 7 Segmented cyclic voltamograms of ReL3 (orange line) and ReL4
(green line) on a Pt working electrode in CH2Cl2 + 0.1 M n[Bu4N][BF4] at
room temperature and at a scan rate of 0.2 V s−1 toward anodic
potentials.
reversible around 1 V s−1 for ReL3, as was the case for ReL1.
For ReL4, the first reduction potential was 30 to 40 mV lower
than for ReL3, and the rather uncommon quasi-reversibility of
this process implying the pyta moiety was observed at a lower
scan rate, i.e. 0.2 V s−1. Moreover, for both ReL3 and ReL4
complexes, a 1/1 intensity ratio was clearly evidenced between
the first one-electron reduction process and the first one-elec-
tron oxidation process (Fig. 7).
Fig. 6 Isodensity plots of selected frontier molecular orbitals involved
in the first electronic transitions of ReL3 and ReL4 in DCM, according to
TD-DFT calculations at the PBE1PBE/LANL2DZ/6-31+G** level of
theory.
very similar energy levels, with the exception of the LUMO+1
All these trends are well supported by the TD-DFT calcu-
lations, which predict that the Re-centered HOMO on the one
hand, and the pyta-centered LUMO on the other hand, are very
close in energy for ReL1, ReL3 and ReL4. The experimental
electrochemical HOMO–LUMO gap values (Eegl)16 found for the
ReL3 and ReL4 complexes (2.55 and 2.50 eV, respectively) also
fit very well with the calculated ones (2.70 and 2.65 eV)
(Table S15†). Finally, compared to ReL1, the inversion of the
PBO connection (ReL4) perturbed slightly more the electronic
properties in solution than the addition of a tert-Bu group on
the phenyl moiety (ReL3).
that is slightly lower in ReL4 than in ReL3 (Fig. 6 and Fig. S5†).
Electrochemical properties
The electrochemical behavior of the new complexes was
studied by cyclic voltammetry (CV) and Osteryoung square
wave voltammetry (OSWV) measurements in DCM at room
temperature. In the OSWV anodic part, the ReL3 and ReL4
complexes were characterized by two oxidation processes
around 1.44 and 1.74 V (Table S14, Fig. S7 to S12†). The
former process can be assigned to an irreversible Re(I) oxi-
dation process.13 The reversibility of this process was not
improved by
a decrease or an increase in scan rate.
UV-vis absorption spectra
Considering the OSWV cathodic part, both new complexes pre-
sented a reduction process beyond −1.8 V, as did ReL1.8 This Experimentally, both complexes have very similar molar
absorption coefficients and maximum absorption in various
process was most likely attributed to the reduction of the PBO
organic solvents (Table 1 and Table S17†). For instance, in
fragment, confirming that the latter is not involved in the com-
plexation process.14 Indeed, a similar reduction process was DCM solutions, the absorption spectra showed an intense
band around 305 nm and a distinct band of weak intensity
above 380 nm, in good agreement with calculations (Fig. 8,
detected for both ligands L3 and L4 (Table S14, Fig. S13 to
S16†) and was also clearly visible by CV. At more anodic poten-
tial around −1.3 V, another reduction process was observed for Tables S6 and S7†). The main contributor of the highest
energy band was identified as a transition between HOMO−2
and LUMO+1 (Table S5†) with strong intraligand charge trans-
the complexes. It can be attributed to the reduction process of
the substituted triazole ring whose potential value may sub-
stantially decrease by complexation as observed in related fer (ILCT) character. The lowest energy band results from an
compounds.13,15 Remarkably, the value of this first reduction
HOMO−1 → LUMO transition, and can thus be assigned to
metal to ligand charge transfer (MLCT).
process seems to be characteristic of this family of complexes
incorporating a 2-pyridyl-1,2,4-triazole ligand8 and could be
related to the bent arrangement of the pyta and PBO moieties.
Emission properties
In CV, the careful examination of the first reduction process at The photoluminescence spectra in organic solvents were
different scan rates showed that this process becomes quasi- recorded upon excitation in the MLCT band (380 nm).
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Table 1 Spectroscopic data of the complexes in organic solution and in the solid state. Maximum absorption wavelength (λabs), maximum phos-
phorescence and photoluminescence wavelengths (λP and λPL), phosphorescence and photoluminescence quantum yields (ΦP and ΦPL), τ main life-
time. For solutions, complex concentration ∼3.5 × 10−5 M, λex = 380 nm
Dichloromethanea
Water/acetonitrile. 95 : 5 v/vb
Solid state
Compounds
λabs [nm]
ε [M−1 cm−1
]
λP [nm]
ΦP
τ [ns]
λabs [nm]
λP [nm]
ΦP
λPL [nm]
ΦPL
τ [ns]
ReL1c
ReL3
ReL4
296, 384
305, 383
310, 388
27 800, 3900
33 400, 4300
29 600, 5100
628
627
632
0.017
0.017
0.012
74.7
100
80
—
310
303
—
582
574
—
0.090
0.090
584d
565
542
0.065
0.21
0.55
338
890
563
a Samples bubbled with Ar for 5 min before measurements. b Undegassed solutions. c Data from ref. 8. d This work.
Fig. 8 Experimental (black line) and simulated (red line) UV-vis absorption spectra of ReL3 (a) and ReL4 (b) in dichloromethane.
Bubbling with argon only led to a small increase of intensity. rescence microscopy, which confirms their luminescence pro-
The excitation spectra resembled the absorption spectra perties. We further noted that the AIPE effect appeared at
(Fig. S17†). The emission spectra in DCM displayed an lower complex concentration and lower proportion of water for
intense band centered on 627 nm for ReL3 and 632 nm for ReL4 than for ReL3, probably due to solubility differences
ReL4 (Table 1), with moderate quantum yields (0.017 and between the two compounds.
0.012) and lifetimes of 80 and 100 ns (Fig. S18 and
Regarding the position of the emission spectra, a two-step
Table S16†), respectively. In acetonitrile and methanol, the behavior was noticed, more pronounced for ReL4 than for
emission spectra were shifted to short wavelengths and ReL3. Indeed, the emission spectra first went from red to
their intensity was slightly decreased (Table S17†). These yellow-green with the water proportion passing from 0 to 80%,
spectroscopic properties are very close to those of ReL1. and then went back to orange-yellow for a higher proportion of
TD-DFT calculations predict that emission arising from the water. The initial shift to short wavelengths may be attributed
first triplet state should give bands peaking at 642.8 nm and to a negative solvatochromic effect, similar to that observed for
658.6 nm for ReL3 and ReL4 in DCM, respectively dissolved molecules with increasing the solvent polarity
(Table S9†). The experimental data agree quite well with (Table S17†). This effect could arise from the large proportion
these values, and this red emission is therefore attributed to of molecules situated at the surface of small aggregates, in
phosphorescence. It is noteworthy that the triplet states contact with the medium. In contrast, the subsequent shift to
responsible for this emission have very similar electron dis- long wavelengths could be assigned to a packing effect of bulk
tribution in both ReL3 and ReL4, with orbitals located on molecules, which become preponderant in large aggregates
the rhenium center and its coordination sphere, including and are insensitive to solvent polarity.
the pyta ligand (Fig. S5†).
The AIPE effect can be explained as follows. In solution,
To highlight the AIPE effect, the complexes were dissolved although limited by steric hindrance, movements of the PBO
in acetonitrile, and then the proportion of water was increased fragment with respect to the pyta group are possible, as well as
from 0 to 95%, while keeping the complexes at constant con- the free rotation between the phenyl and benzoxazole rings of
centration. Under these conditions, the phosphorescence PBO. These movements may lead to a waste of excitation
quantum yield was markedly increased about 9 times for both energy, hence to weak phosphorescence efficiency. The bimole-
complexes (Fig. 9). This effect was accompanied by the for- cular quenching of phosphorescence by oxygen, although moder-
mation of aggregates, which were detected by UV-vis absorp- ate in our case, acts in the same direction. In aggregates, mole-
tion spectroscopy in media containing 60% water or more cular movements are restricted and the access of oxygen is
(Fig. S20†). The largest aggregates were also observed by fluo- limited, resulting in the observed phosphorescence enhance-
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Fig. 9 Bottom: Emission spectra of complexes ReL3 and ReL4 at 5.2 × 10−5 M and 3.6 × 10−5 M, respectively, in acetonitrile solutions containing 0, 60,
70, 80, 90 and 95% water (from bottom to top), λex = 380 nm. Top: Corresponding samples illuminated by a hand-held UV lamp (365 nm).
ment, provided that the molecular arrangement is compatible
with the emission of light. It must be emphasized that the aggre-
gates formed in the water/acetonitrile mixtures are in contact
with water molecules, and their crystallinity is unknown.
To complete this study, a comparison was made with the
powders of ReL3 and ReL4, obtained by crystallization in
methanol at the end of the synthesis process. Obviously, the
spectroscopic properties of these microcrystalline compounds
differed from those of solutions and aggregates. Upon illumi-
nation by a hand-held UV lamp, ReL3 emitted golden yellow
light. Its solid-state emission spectra peaked at 565 nm
(Fig. 10) and the photoluminescence quantum yield was 0.21,
Fig. 10 Comparative emission spectra of pristine powders of the four
complexes: ReL1 (orange line), ReL2 (dashed green line), ReL3 (orange
line) and ReL4 (green line). The intensity is proportional to the quantum
yield. λex = 380 nm for ReL1 and ReL2, 355 nm for ReL3 and ReL4. Inset:
i.e. 14 folds higher than in DCM solutions. In contrast, ReL4
emitted bright yellow-green light under the UV lamp. The
emission band peaked at 542 nm, with quantum yield as high
as 0.55, almost 46 times higher than in DCM solutions.
Samples of ReL3 and ReL4 illuminated by
a hand-held UV lamp
Therefore, the solid-state emission spectra of ReL3 and ReL4
were blue-shifted by 16 and 32 nm, and their photo-
luminescence efficiency was increased by 3 and 8 folds,
respectively, with respect to ReL1. Decay measurements
required three components for an acceptable fit, as was the
case for ReL1 (Table S18†). The longest lifetime component
(890 ns and 563 ns for ReL3 and ReL4, respectively) was associ-
ated to a very high fraction of intensity. Noticeably, these
values are significantly longer than in ReL1 (338 ns), possibly
(365 nm).
state, as well as from distinct deactivation pathways for only
one type of emitter.17
Conclusions
indicating a stabilization of the corresponding excited state. It was shown here that the presence of the electron-donating
The short lifetime component could be attributed to fluo- tert-butyl group in ReL3 has very little influence on the geome-
rescent impurities despite the extensive purification of the try and behavior of this complex in organic solutions, with
compounds. Although associated to a very small fraction of respect to ReL1. This is reminiscent of other photo-responsive
intensity, the intermediate lifetime (185 and 93 ns for ReL3 compounds where tert-butyl groups placed at the periphery of
and ReL4, respectively), was intriguing because it was not the photoactive core preserve the electronic properties of the
detected in solutions. Work is underway to determine the mother molecule in solution.18 Reversing the position of the
origin of these various lifetimes, which may arise for example PBO fragment on the pyta group in ReL4 also has little effect,
from the presence of multiple emitting species in the solid as long as dissolved molecules are considered. However,
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remarkable differences between the complexes were observed water (100 mL) was added. The resulting mixture was neutral-
in the solid-state emission properties. The introduction of a ized with 10% NaOH solution and extracted with ethyl acetate
bulky substituent in a molecule is a strategy generally used to (3 × 60 mL). All organic layers were combined, washed with
prevent molecular π–π stacking, which is extremely detrimental water, dried over anhydrous MgSO4, filtered and concentrated
to the emission of light.19 The bulkiness and structuring effect to dryness. The residue was purified by column chromato-
of the tert-butyl group previously allowed us to improve the graphy on silica gel using ethyl acetate/petroleum ether 1 : 10
molecular arrangement, and hence the photoluminescence v/v, and 1 was obtained as a yellow solid (3.33 g, yield 75%). 1H
properties, of organic PBO and phenylnaphthoxazole deriva- NMR (300 MHz, CDCl3): δ (ppm) = 8.49 (d, J = 1.8 Hz, 1H), 8.33
tives.20 In the present case, the tert-butyl group of ReL3 pre- (dd, J = 2.2, 8.8 Hz, 1H), 8.24–8.19 (m, 2H), 7.84 (d, J = 8.8 Hz,
vented any π–π stacking interaction between the aromatic moi- 1H), 7.61–7.57 (m, 2H), 1.39 (s, 9H, t-Bu).
eties, and led to much better solid-state emission properties
2-(4-tert-Butylphenyl)-6-aminobenzoxazole (2). To a solution
with respect to ReL1. With regards to ReL4, attaching the PBO of 1 (1.72 g, 5.8 mmol) in MeOH/CH2Cl2 (1 : 2 v/v, 30 mL) was
group to pyta via the phenyl ring led to a new crystal network, added 10% Pd/C (30% w/w, 0.52 g) and the mixture was stirred
and strongly enhanced the photoluminescence efficiency, at room temperature for 24 h under 6 bars pressure of H2.
which is by far the best for this series of complexes. The solid- After reaction, the mixture was filtered twice to remove the
state emission properties of mononuclear rhenium(I) com- catalyst. The filtrate was concentrated to dryness and purified
plexes can therefore be significantly improved by introducing by column chromatography on silica gel using ethyl acetate/
tiny molecular changes. The original molecular framework of petroleum ether 1 : 3 v/v as the eluent, to afford 2 as a pale
ReL4 is prone to various chemical modifications, and with its white solid (1.27 g, yield 82%). 1H NMR (300 MHz, CDCl3):
superior optical properties, this complex should be the first δ (ppm) = 8.12–8.08 (m, 2H), 7.53–7.50 (m, 3H), 6.87 (dd, J =
member of a new series of highly emissive rhenium(I) lumi- 2.2, 0.5 Hz, 1H), 6.70 (dd, J = 8.4, 2.2 Hz, 1H), 1.36 (s, 9H,
nescent probes.
t-Bu). ESI-MS: m/z 267.1 ([M + H]+ calcd for C17H19N2O, 267.1).
N,N-Dimethyl-N′-picolinoylformohydrazonamide (3). A mixture
of pyridine-2-carbohydrazide (0.82 g, 6 mmol) and N,N-di-
methylformamide dimethyl acetal (1.06 mL, 8 mmol) in
CH2Cl2 (20 mL) was stirred at reflux for 2 h. After the con-
sumption of pyridine-2-carbohydrazide, the solvent was evap-
Experimental section
General methods
All purchased chemicals were of the highest purity commer- orated and the resulting precipitate was purified by column
cially available and used without further purification. chromatography using CH3OH/CH2Cl2 1 : 10 v/v as eluent to
Analytical grade solvents were used and not further purified afford 3 as a yellow solid (1.09 g, yield 95%). 1H NMR
unless specified. Reactions were monitored by analytical thin (300 MHz, CDCl3): δ (ppm) = 10.04 (s, 1H, NH), 8.49 (ddd, J =
layer chromatography (TLC) on Kieselgel 60 F254 (Merck). 2.6, 1.7, 0.9 Hz, 1H), 8.19 (dt, J = 7.8, 1.1 Hz, 1H), 7.96 (s, 1H,
Chromatography purification was conducted using silica gel or HCvN), 7.83 (td, J = 7.6, 1.7 Hz, 1H), 7.38 (ddd, J = 7.6, 4.8, 1.3
neutral alumina obtained from Merck. NMR, mass and infra- Hz, 1H), 2.93 (s, 6H, 2CH3). DCI/NH3-MS: m/z 193.1 ([M + H]+
red spectra were obtained in the relevant ‘Services communs calcd for C9H13N4O, 193.1).
de l’Institut de Chimie de Toulouse, Université de Toulouse
2-(4-tert-Butylphenyl)-6-(3-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)
III-Paul-Sabatier’. 1H- and 13C-NMR spectra were measured benzoxazole (L3). To a mixture of 2 (0.38 g, 1.4 mmol) and 3
with Bruker Avance DRX 500 MHz or Bruker Avance 300 MHz. (0.27 g, 1.4 mmol) in CH3CN (10 mL) was added acetic acid
Attributions of the signals were made using 2D NMR data (1 mL). The resulting mixture was refluxed at 90 °C with stir-
(COSY, HSQC and HMBC). Protons and carbon atoms were ring for 24 h. After cooling to room temperature, the mixture
numbered according to Fig. S10.† App = Apparent; * = The was concentrated to dryness and purified by column chrom-
multiplicity of the signal is more complex as it is part of an AA′ atography using CH3OH/CH2Cl2 1 : 10 v/v as eluent to afford a
XX′ system. Electrospray mass spectra were obtained using a light yellow solid. Then, the crude product was washed twice
QTRAP Applied Biosystems spectrometer and high-resolution with methanol (2 × 10 mL) and dried in vacuum to afford L3
mass spectra (HRMS) were recorded using an LCT Premier as a white powder (0.35 g, yield 64%). 1H NMR (500 MHz,
Waters spectrometer. Desorption chemical ionization (DCI) DMSO-d6): δ (ppm) = 8.97 (s, 1H, H5′), 8.36 (ddd, J = 4.8, 1.8,
mass spectra (NH3 or CH4) were obtained on a DSQ II 0.9 Hz, 1H, H6″), 8.15 (app. d*, J = 8.8 Hz, 2H, Hb,f), 8.10 (dt,
Thermofisher apparatus. Infrared spectra were obtained on a J = 7.9, 1.1 Hz, 1H, H3″), 8.00 (dd, J = 2.0, 0.5 Hz, 1H, H7),
Nexus Thermonicolet apparatus with DTGS as the detector. The 7.99–7.97 (m, 1H, H4″), 7.86 (dd, J = 8.4, 0.5 Hz, 1H, H4), 7.66
microanalyses were performed with a PerkinElmer 2400 elemen- (app. d*, J = 8.8 Hz, 2H, Hc,e), 7.42 (ddd, J = 7.6, 4.8, 1.2 Hz,
tal analyzer in the ‘Service d’Analyse Chimique du Laboratoire 1H, H5″), 7.39 (dd, J = 8.4, 2.0 Hz, 1H, H5), 1.34 (s, 9H, t-Bu).
de Chimie de Coordination de Toulouse’ (LCC, Toulouse).
13C NMR (125 MHz, DMSO-d6): δ (ppm) = 164.4, 155.9, 152.1,
2-(4-tert-Butylphenyl)-6-nitrobenzoxazole (1). A mixture of 150.2, 149.5, 146.9 (C3′), 142.1, 137.8, 132.8, 127.8, 126.8,
2-amino-5-nitrophenol (2.31 g, 15 mmol), 4-tert-butylbenzoic 125.0, 124.4, 123.9, 123.8, 122.2, 120.0, 109.8, 35.4 (Cqt-Bu),
acid (2.67 g, 15 mmol) and polyphosphoric acid (24 g) was 31.3 (CH3). DCI/CH4-HRMS: m/z 396.1813 ([M + H]+: calcd for
heated to 120 °C with stirring for 16 h. After reaction, iced C24H22N5O, 396.1824).
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2-(4-Aminophenyl)benzoxazole (4). A mixture of 2-amino- 7.6, 1.3 Hz, 2H, Hb,f), 8.12 (td, J = 7.9, 1.5 Hz, 1H, H4″), 8.08
phenol (2.21 g, 0.02 mol), 4-aminobenzoic acid (2.75 g, (app. br d*, 2H, 9 Hz, Hc,e), 7.92–7.87 (m, 2H, H7,4), 7.76 (ddd,
0.02 mol) and polyphosphoric acid (40 g, 0.4 mol) was stirred J = 6.7, 5.5, 1.2 Hz, 1H, H5″), 7.54–7.47 (m, 2H, H5,6), 7.29 (dt,
at 220 °C for 4 h. Distilled water (200 mL) was added to the J = 8.1, 1.1 Hz, 1H, H3″). 13C NMR (125 MHz, DMSO-d6):
mixture and a saturated solution of NaHCO3 was slowly δ (ppm) = 198.3, 197.7, 189.5 (CO), 161.4, 155.0, 154.8, 150.9,
poured into the solution until no bubbling was detected. After 148.5 (CH5′), 144.5, 141.8, 141.1, 135.2, 129.8, 129.5, 128.8,
filtration, washing with water and drying, the solid obtained 128.6, 126.8, 125.8, 123.7, 120.7, 111.7. ESI/CH4-HRMS: m/z
was sublimated at 240 °C under vacuum. Compound 4 was 608.0502 ([M − Cl]+ calcd for C23H13N5O4185Re 608.0497), m/z
obtained as a pink solid (2.72 g, yield: 65%). 1H NMR 649.0767 ([M − Cl + CH3CN]+ calcd for C25H16N6O4185Re
(300 MHz, CDCl3): δ (ppm) = 8.12–8.01 (m, 2H), 7.76–7.65 (m, 649.0763); Anal. calcd (%) for C23H13N5O4ReCl: C 42.83,
1H), 7.58–7.48 (m, 1H), 7.33–7.27 (m, 2H), 6.82–6.70 (m, 2H). H 2.03, N 10.86; found: C 42.58, H 1.98, N 10.62. IR(ATR):
DCI/CH4-MS: m/z 211.09 ([M + H]+ calcd for C13H11N2O, ν(CO) = 2021, 1934, 1887 cm−1
.
211.09).
X-ray crystallography
2-(4-(3-(Pyridin-2-yl)-4H-1,2,4-triazol-4-yl)phenyl)benzoxazole
(L4). Reacting 5 (0.42 g, 2.0 mmol) and 3 (0.38 g, 2.0 mmol)
according to the procedure described for L3, L4 was obtained
as a white powder (0.29 g, yield 43%) after purification on
column chromatography (silica gel, ethyl acetate) and two
washings with methanol. 1H NMR (500 MHz, DMSO-d6): δ
(ppm) = 9.03 (s, 1H, H5′), 8.41 (ddd, J = 4.8, 1.8, 1.0 Hz, 1H,
H6″), 8.28 (app. d*, J = 8.5 Hz, 2H, Hb,f), 8.12 (dt, J = 7.8, 1.2
Hz, 1H, H3″), 8.02 (td, J = 7.6, 1.8 Hz, 1H, H4″), 7.86–7.81 (m,
2H, H4,7), 7.62 (app. d*, 2H, Hc,e d, J = 8.5 Hz), 7.49–7.43 (m,
3H, H5,6,5″). 13C NMR (125 MHz, DMSO-d6): δ (ppm) = 161.8,
151.8, 150.8, 149.5, 146.7, 146.5 (C3′), 141.9, 138.4, 138.0,
128.5, 127.3, 126.9, 126.3, 125.5, 125.1, 124.4, 120.5, 111.5.
DCI/CH4-HRMS: m/z 340.1194 ([M + H]+ calcd for C20H14N5O,
340.1198).
Crystal data were collected at 193 K on a Bruker AXS Quazar
APEX II diffractometer using a 30 W air-cooled microfocus
source (ImS) with focusing multilayer optics using MoKα radi-
ation (wavelength = 0.71073 Å). Phi- and omega-scans were
used. The structures were solved by direct methods (SHELXS
97) or using intrinsic phasing method (ShelXT).21,22 All non-
hydrogen atoms were refined anisotropically using the least-
square method on F2.22 Selected crystallographic data are col-
lected in Table 2.
Computational details
The GAUSSIAN09 program package23 was employed for all cal-
culations with the aid of the ChemCraft visualization
program.24 The ground state (S0), the first excited state (S1) and
the lowest triplet state (T1) geometries of compounds were
fully optimized with the restricted and unrestricted density
functional theory (R-DFT and U-DFT) method using the
Perdew–Burke–Ernzerhof PBE1PBE functional without sym-
General procedure for the preparation of tricarbonylrhenium(I)
complexes
A mixture of ligand and [Re(CO)5Cl] (1.15 eq.) in methanol was
stirred for 16 h at 65 °C. After consumption of the ligand, the
mixture was cooled to room temperature and filtered, the pre-
cipitate was purified by chromatography on silica gel using
ethylacetate as eluent to afford the desired product.
Table 2 Selected crystallographic data of complexes ReL3 and ReL4
ReL3. 130 mg (0.33 mmol) of L3 and 130 mg (0.36 mmol)
of [Re(CO)5Cl] afforded ReL3 as a yellow solid (164 mg, yield
71%). 1H NMR (500 MHz, DMSO-d6): δ (ppm) = 9.30 (s, 1H,
H5′), 9.10 (ddd, J = 2.2, 1.5, 0.8 Hz, 1H, H6″), 8.47 (br s, 1H, H7),
8.22 (app d*, J = 8.7 Hz, 2H, Hb,f), 8.13 (dd, J = 8.5, 0.5 Hz, 1H,
H4), 8.07 (td, J = 7.9, 1.6 Hz, 1H, H4″), 7.76–7.74 (m, 1H, H5″),
7.72 (app d*, J = 8.8 Hz, 2H, Hc,e), 7.24 (ddd, J = 2.1, 1.2,
0.9 Hz, 1H, H3″), 1.36 (s, 9H, t-Bu). 13C NMR (125 MHz, DMSO-
d6): δ (ppm) = 198.3, 197.7, 189.5 (CO), 165.4, 156.3, 155.1,
155.0, 150.7, 149.0 (C5′), 144.5, 144.3, 141.2, 129.1, 128.8,
128.1, 126.9, 124.6, 123.8, 123.5, 121.5, 115.1, 111.3, 35.4
(Cqt-Bu), 31.3 (CH3). DCI/CH4-HRMS: m/z 699.0806 ([M]+ calcd
for C27H21N5O4185ReCl, 699.0812), m/z 664.1116 ([M − Cl]+
calcd for C27H21N5O4185Re 664.1123); Anal. calcd (%) for
C27H21N5O4ReCl: C 46.25, H 3.02, N 9.99; found: C 45.97,
ReL3
ReL4
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C27H21ClN5O4Re
701.15
C23H13ClN5O4Re
645.04
Orthorhombic
P212121
Triclinic
ˉ
P1
12.1819(9)
13.8261(10)
20.0905(14)
71.646(4)
75.370(4)
85.803(4)
3107.5(4)
4
8.0086(3)
10.5904(4)
25.9678(9)
90
90
90
2202.44(14)
4
1.945
γ (°)
Volume (Å3)
Z
Density (calcd) (g cm−3
Crystal size (mm3)
Reflections collected
)
1.499
0.100 × 0.080 × 0.020 0.100 × 0.100 × 0.020
63 669
60 111
5251
0.0899
0/307
0.0281
0.0441
1.381 and −1.121
Independent reflections 9032
Rint
0.0857
336/816
0.0642
0.2480
H 3.53, N 10.08. IR(ATR): ν(CO) = 2028, 1920, 1883 cm−1
.
Restraints/parameters
Final R1 index I > 2σ(I)
wR2 (all data)
ReL4. 130 mg (0.38 mmol) of L4 and 158 mg (0.44 mmol)
of [Re(CO)5Cl] afforded ReL4 as a yellow solid (197 mg, yield
80%). 1H NMR (500 MHz, DMSO-d6): δ (ppm) = 9.33 (s, 1H,
H5′), 9.11 (ddd, J = 5.4, 1.5, 0.7 Hz, 1H, H6″), 8.55 (app. dd*, J =
Largest diff. peak and
1.007 and −2.619
hole (e Å−3
)
CCDC
1922851
1922852
Dalton Trans.
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metry constraints.25 In all calculations, the “double-ζ” quality area under the fluorescence curve and n the refraction index.
basis set LANL2DZ with Hay and Wadt’s relative effective core Subscripts s and x refer to the standard and to the sample of
potential ECP (outer-core [(5s25p6)] electrons and the (5d6) unknown quantum yield, respectively. Coumarin 153 (ΦF
=
valence electrons)26 was employed for the Re atom. The 6- 0.53) in ethanol was used as the standard.30 The absorbance of
31+g** basis set for H, C, N, O and Cl atoms was used.27 The the solutions was equal or below 0.055 at the excitation wave-
solvent effect (dichloromethane, ε = 9.08) was simulated using length. The error on the quantum yield values is estimated to
the Self-Consistent Reaction Field (SCRF) under the be about 10%.
Polarizable Continuum Model (PCM).28 The vibrational fre-
The solid state spectrum of ReL1, previously recorded on a
quencies calculations were performed using the optimized Xenius SAFAS spectrofluorometer, was corrected using a home-
structural parameters of compounds, to confirm that each opti- made correction curve. Solid state spectra and photoluminescence
mized structure represents a local minimum on the potential quantum yields of ReL3 and ReL4 were recorded on a Fluorolog
energy surface and all eigenvalues are non-negative. On the spectrofluorometer from HORIBA Scientific equipped with an
basis of the optimized ground and excited state geometries, the integrating sphere. Solid samples were deposited on a metal
absorption and emission properties were calculated by the time support and luminescence spectra were corrected. The absolute
dependent density functional theory (TD-DFT) method at the photoluminescence quantum yield values (ΦP) were determined
PBE1PBE/LANL2DZ/6-31+G** level. These methods have already by a method based on the one developed by de Mello et al.,31 as
shown good agreement with experimental studies for different described elsewhere.8 The error was estimated to be about 20%.
rhenium(I) complexes.29
Fluorescence decays curves in dilute DCM solutions (Abs at
λex < 0.1) were recorded by time-correlated single-photon
counting method (TCSPC). Solutions were first bubbled with
Electrochemistry
The electrochemical properties of the new compounds were Ar for 5 min before measurements. An oscillating femtosecond
determined by cyclic voltammetry (CV) and Osteryoung square titanium–sapphire laser (Tsunami, Spectra-Physics) pumped
wave voltammetry (OSWV) in DCM. The solutions used during by a double Nd:YVO4 laser (Millenia Xs, Spectra-Physics) was
the electrochemical studies were typically 1 × 10−3 M in ligand used for excitation. Harmonic generators were employed to
or 6.5 × 10−3 M in complex, and 0.1 M in supporting electro- tune to λex = 380 nm. Emitted photons were detected at 90°
lyte. The supporting electrolyte (nBu4N)(BF4) (Fluka, 99% through a monochromator by means of a Hamamatsu MC P
electrochemical grade) was used as received and simply 3809U photomultiplier connected to a SPC-630 TCSPC module
degassed under Ar. DCM was dried using an MB SPS-800 from Becker and Hickl. The instrumental response function
solvent purification system just prior to use. The measure- was recorded after each decay curve. The decay curves were
ments were carried out with an Autolab PGSTAT100 potentio- analyzed with reconvolution and global non-linear least-
stat controlled by GPES 4.09 software. Experiments were per- squares minimization method using the Globals software
formed at room temperature (r.t.) in a homemade airtight package developed at the Laboratory for Fluorescence
three-electrode cell connected to a vacuum/Ar line. The refer- Dynamics at the University of Illinois at Urbana-Champaign.
ence electrode consisted of a saturated calomel electrode (SCE)
In the solid state, decay curves were measured by an LP920-
separated from the solution by a bridge compartment. The K spectrometer equipped with a Xenon lamp (450 W pulse), a
counter electrode was a Pt wire of ca. 1 cm2 apparent surface. monochromator with 300 mm focus, and a photomultiplier
The working electrode was a Pt microdisk (0.5 mm diameter). (Hamamatsu R928). Quartz sample holders (0.2 mm) from Starna
Before each measurement, the solutions were degassed by were used. The sample was excited by an oscillating Nd:YAG
bubbling Ar and the working electrode was polished with a nanosecond laser (7–8 ns) with a repetition rate of 10 Hz at its
polishing machine (Presi P230). Under these experimental con- third harmonic generation (355 nm). The instrumental response
ditions, Fc+/Fc is observed at +0.55 0.01 V vs. SCE. OSWVs function was recorded with each set of decay curves. The data
were obtained using an amplitude of 20 mV, a frequency of 20 were analyzed with reconvolution using the L900 software.
Hz, and a step potential of 5 mV.
Spectroscopy and photophysics
Conflicts of interest
Dye solutions were prepared by gentle heating in a solvent,
sonication and filtration on paper filter prior to measurement. There are no conflicts to declare.
Spectroscopic measurements in solutions were conducted at
20 °C in a temperature-controlled cell. UV-visible absorption
spectra were recorded on a Hewlett Packard 8453 spectrometer.
Fluorescence spectra in solutions were measured with a
Acknowledgements
Xenius SAFAS spectrofluorometer using cells of 1 cm optical J. Wang thanks the Chinese Scholarship Council (CSC) for a PhD
pathway. All fluorescence spectra were corrected. The fluo- funding. The calculations were carried out in the Wroclaw Centre
rescence quantum yields in solution (ΦF) were determined for Networking and Supercomputing, Poland (http://www.wcss.pl).
using the classical formula: ΦFx = (As × Fx × nx2 × ΦFs)/(Ax × Fs × ns2) We are also grateful to Dr Stéphane Balayssac for his help
where A is the absorbance at the excitation wavelength, F the in interpreting NMR spectra, to Dr Alix Sournia-Saquet
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(LCC-CNRS) for electrochemical measurements and to
Dr Rémi Métivier for fruitful discussions. J. P. Calupitan and
C. Allain also thank the European Research Council for
funding (ERC StG-715757 MECHANO-FLUO to C. A.), and
thank Arnaud Brosseau (PPSM) for his help with the time-
resolved fluorescence measurements.
8 J. Wang, B. Delavaux-Nicot, M. Wolff, S. Mallet-Ladeira,
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