Efficient NIR electrochemiluminescent dyes based on ruthenium(ii) complexes containing an N-heterocyclic carbene ligand

Article abstract of DOI:10.1039/d0cc07595g

Three new ruthenium(ii) complexes containing an N-heterocyclic carbene (NHC) ligand (RuNHC) have been successfully synthesized and proved to be efficient near-infrared (NIR) ECL (electrogenerated chemiluminescence) luminophores. In addition to the advanta

Full text of DOI:10.1039/d0cc07595g

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Efficient NIR electrochemiluminescent dyes
based on ruthenium(II) complexes containing
an N-heterocyclic carbene ligand†
Cite this: Chem. Commun., 2021,
57, 1254
Received 19th November 2020,
Accepted 22nd December 2020
ab
a
Yu-Yang Zhou,
Liang-Zhi Li,
*
Yang-Ming Ding,b Wei Zhao,
Jian-Hua Dong,b
Hong-Yuan Chena and Jing-Juan Xu
*
b
ac
DOI: 10.1039/d0cc07595g
rsc.li/chemcomm
Three new ruthenium(II) complexes containing an N-heterocyclic visible regions, near-infrared (NIR) luminophores have outstanding
carbene (NHC) ligand (RuNHC) have been successfully synthesized advantages in bioimaging fields owing to their excellent tissue
and proved to be efficient near-infrared (NIR) ECL (electrogenerated penetration and negligible autofluorescence.4 However,
chemiluminescence) luminophores. In addition to the advantages although both the kinds and numbers of ECL dyes have
of the lower-charge main motif (+1), the much lower oxidation achieved significant advances in the past few decades, most
potentials, and the longer metal to ligand charge transfer (MLCT) of them are still limited to the visible regions and reports of
absorption bands, most importantly, these RuNHC complexes show NIR luminophores are relatively rare to date.5 Most impor-
higher, or at least comparable, ECL efficiency compared with tantly, along with emerging cutting-edge research areas, such
Ru(bpy)32+ under the same experimental conditions; this demonstrates as single-particle/single-cell imaging combined with a novel
their great potential for applications in the NIR ECL imaging field in the ECL imaging platform,6,7 developing novel high-efficiency NIR
future.
ECL luminophores has become more significant than ever.
Among the various kinds of ECL luminophores explored in
Electrogenerated chemiluminescence (ECL, also named electro- recent years, novel metal complexes, a type of small molecular
chemiluminescence) is the phenomenon of light-emitting on the luminophore, have received significant attention owing to their
electrode through a series of chemical and electrochemical tunable colors, high quantum efficiency, excellent photo-
reactions. Since the first detailed studies of tri-2,20- stability and bioconjugation abilities. One typical representative
bipyridylruthenium(II) (also abbreviated as Ru(bpy)32+) by Bard is the ruthenium(II) complex with an octahedron motif com-
and co-workers in the 1970s,1 and owing to the advantages of an prising six Ru–N bonds (containing three N^N bidentate
2+
ultra-high sensitivity and being free from the interferences from ligands), such as Ru(bpy)3 and its derivatives. In order to
excitation light, the ECL phenomenon has been successfully meet the needs of downstream applications, incorporating
developed into the most wide-spread commercially clinical substituents or changing the degree of p conjugation of these
diagnostic technique to date. In ECL-related studies, both in N^N bidentate ligands are both extensively employed to tune
fundamental research and bioanalytical applications, exploring the photophysical properties of ruthenium(II) complexes,
novel excellent luminophores with various colors and a high including the emission colors and quantum efficiencies.8
efficiency is always a hot topic and has received significantly However, owing to the limitations of the inherent natures of
more attention during the past few decades.2,3 It is well known N^N ligands and Ru–N coordination bonds, the ruthenium(II)-
that compared with luminophores with an emission in the based NIR luminophores haven’t made a significant break-
through so far.
As we know, the chemistry of N-heterocyclic carbene (NHC)
a State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry
is always an active research area and has a great impact on
and Chemical Engineering, Nanjing University, Nanjing 210023, China.
catalysis, materials sciences and medicine.9 Owing to its strong
E-mail: xujj@nju.edu.cn, zhouyuyang@mail.usts.edu.cn; Fax: +86-25-89687294;
Tel: +86-25-89687294
donor ability and chemical stability, NHC is always one of the
most important ligands in the architectures of organometallic
complexes. Although metal–NHC complexes, as catalysts and
as candidates for potential medicines, have been extensively
studied, exploring the ECL luminophores from metal–NHC
complexes is still very rare. In the past few years, inspired by
b School of Chemistry and Life Sciences, Suzhou University of Science and
Technology, Suzhou 215009, China
c College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou 450001, China
† Electronic supplementary information (ESI) available: Experimental details,
photophysical data, theoretical calculations, etc. See DOI: 10.1039/d0cc07595g
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the emerging studies about iridium(III)–NHC complexes for in this work. Meanwhile, the groups of –OMe and –CF3 have also
deep-blue organic light emitting diode (OLED) dopant applica- been employed to investigate the electronic effect of the substi-
tions, both Hogan’s10 and our group11 have independently tuent on the ECL properties. Eventually, three bis(4,7-diphenyl-
pioneered ECL studies of iridium(III) complexes comprising 1,10-phenoline) ruthenium(II) complexes comprising NHC ligands
NHC ligands. Unfortunately, incorporating NHC ligands into (abbreviated as RuNHC in this work) with different substituents
iridium(III) complexes increases the energy gap between the (shown in Fig. 1) were rationally designed and successfully
highest occupied molecular orbital (HOMO) and the lowest synthesized in this work. The detailed synthetic routes and the
unoccupied molecular orbital (LUMO) and induces the emis- chemical structure characterization data have also been listed in
sion wavelength of Ir–NHC complexes, giving a hypsochromic the experimental section in the ESI† (Fig. S1–S12).
shift rather than a bathochromic shift.12 Herein, no successful
attempt to develop NIR ECL luminophores based on metal– RuNHC complexes in acetonitrile solution are presented in
NHC complexes have been reported to date. Fig. 2 and the corresponding photographs of the RuNHC
The absorption and photoluminescent (PL) spectra of the
Very recently, the synthetic method used to obtain a complexes in acetonitrile solution (40 mM) are shown in
ruthenium(II) complex with the general formula of [Ru(bpy)2 Fig. S13 (ESI†). As shown in Fig. 2A, these novel RuNHC
(C^C*)]+ (bpy = 2,20-bipyridne, C* represents carbene from complexes have an intense intra-ligand absorption band in
imidazolium) has been revealed by the Strassner group.13,14 the UV regions below 350 nm (e is up to 105 Mꢀ1 cmꢀ1) and a
This novel coordination model and the comparatively lower moderate, broad and featureless MLCT (metal to ligand charge
charged main motif of the complex attracted our attention and transfer) transition in the visible light region from 380 to
we became very curious about their performance in the ECL 650 nm (e is ca. 2.5 ꢁ 104 Mꢀ1 cmꢀ1). Compared with similar
fields, especially for exploring NIR ECL luminophores for features observed in the absorption bands of these three NHC
cutting-edge research. Herein, taking into account the conjugation complexes, the corresponding absorption intensities have
effect and the electronic effect comprehensively, three slight variations along with the changing substituents that have
novel ruthenium(II) complexes with different substituents, the different electronic effects. It was found that the methoxy
coordination model of which is [Ru(DIP)2(C^C*)]+ (DIP = 4,7- group, as an electron-donating substituent, would increase
diphenyl-1,10-phenoline, C* is carbene from benzoimidazo- the absorption intensity of the RuNHC complexes, while the
lium), have been deliberately designed and successfully synthe- trifluoromethyl group as the electron-withdrawing substituent
sized in this work (shown in Fig. 1). It is important to mention has a converse effect on the absorption intensity. Notably,
2+
that the ECL performance of these novel kinds of ruthenium(II) compared with the traditional Ru(N^N)3 complexes reported
complexes have been revealed for the first time in this work, in the literature,15 the incorporation of the C^C* ligand in this
and provides a novel avenue that can be used to explore metal work would not only induce the MLCT band of the RuNHC
complex-based NIR luminophores.
complexes to undergo a significant bathochromic shift, but it
It is well known that the degree of conjugation of a ligand is
one of the most significant factors that tunes the emission
wavelength of the corresponding metal complexes. Herein, in
order to make the wavelength of RuNHC complexes bathochromic
shift intensely, 4,7-diphenyl-1,10-phenoline (DIP), with a large
degree of conjugation, was deliberately chosen as an N^N coordi-
nation ligand, while N-methyl-N0-phenyl-benzo[d]imidazolium
and its derivative were selected as the C^C* coordination ligands
Fig. 1 The synthetic routes and chemical structures of the RuNHC complexes
in this work: (i) CuI, L-proline, K2CO3, DMF; (ii) CH3I, THF; (iii) [Ru(p-
cymene)Cl2]2, Ag2O, CH2Cl2, N2; (iv) 4,7-diphenyl-1,10-phenoline(DIP), ethanol,
reflux; and (v) ethanol, NH4PF6.
Fig. 2 Absorption (A) and normalized PL (B) spectra of the bis(4,7-
diphenyl-1,10-phenoline)ruthenium(II) complexes with a C^C* cyclome-
talated carbene ligand (40 mM) in acetonitrile solution. lex = 515 nm, under
an argon-saturated atmosphere.
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also obviously increases the absorption intensity, which is pro- potential peaks between RuNHC-1 and RuNHC-3. However,
foundly important to bioimaging technologies that require lumino- RuNHC-2 with the –CF3 group substituent has a much higher
phores that possess long wavelength excitation.16 The summarized oxidation potential and lower first reduction potential compared
photophysical data have also been listed in Table S1 in the ESI.†
with RuNHC-1, in other words, RuNHC-2 has much larger energy
Fig. 2B shows the PL spectra of the RuNHC complexes in gaps between the HOMO and LUMO derived from the above
acetonitrile solution at room temperature. All of these three redox potential results, which are very consistent with the results
RuNHC complexes display an NIR emission centered at ca. 780 of the PL experiment and theoretical calculations (listed in the
or 820 nm under the excitation of 515 nm light. The PL spectra ESI,† Fig. S15 and Table S2) in the sequence.
of RuNHC-2 with a substituent of –CF3 has a distinct hypso-
Based on the above mentioned redox properties derived
chromic shift compared with that of RuNHC-1 and RuNHC-3. from CV techniques (including the electrochemical data of
Moreover, a shoulder peak at 616 nm has also been observed in co-reactants listed in Fig. S16 in the ESI†), ECL studies were
RuNHC-2, which may be related to the complicated compositions finally performed. In order to obtain the best experimental
of the excited states for these novel complexes. Furthermore, conditions for ECL generation, both the concentration and
2+
using Ru(bpy)3 as the ref. 17, the PL quantum efficiencies of kinds of co-reactant have been optimized for these novel
these RuNHC complexes have also been calculated and are RuNHC complexes. These experimental data have also been
listed in Table S1 in the ESI.† Accordingly, although these shown in Fig. S17 and S18 (ESI†). Accordingly, it was easily
RuNHC complexes still have very low PL quantum efficiencies observed that n-tripropylamine (TPA) is much better than
like most of the reported metal complex-based NIR 2-(dibutylamino)ethanol (DBAE) and triethylamine (TEA) for
luminescence,18 RuNHC-2 with a –CF3 substituent has a all of these three RuNHC complexes in the positive scanning
comparatively higher PL quantum efficiency compared with ECL model and the best ratio of [TPA]/[RuNHC] is 400. Herein,
RuNHC-1 and RuNHC-3, which may provide a novel avenue to the plots of CV and ECL versus potential have been presented in
further design NIR luminophores with a high quantum effi- Fig. 3A–C. Owing to the large ratio of [TPA]/[RuNHC], the
ciency through molecular structure modification with oxidation wave of RuNHC was intensely covered up and the
fluorine atoms.
oxidation wave recorded in Fig. 3 can be ascribed to TPA.20 As
Owing to the crucial roles of redox properties in the ECL shown in Fig. 3, along with the oxidation of TPA, these RuNHC
studies, cyclic voltammetry (CV) was initially employed to inves- complexes exhibited increasing ECL signals and the ECL inten-
tigate the electrochemical parameters of RuNHC complexes in sity reached a peak value at ca.1.4 V versus NHE.
this work. CV curves and the corresponding summarized electro-
Furthermore, the ECL studies with a potential stepping
chemical data of the RuNHC complexes have been presented in mode have also been carried out in this work. The ECL intensity
Fig. S14 (ESI†) and Table 1, respectively. All CV characterization at various concentrations of TPA in the potential stepping
in this work was carried out in an argon-saturated anhydrous model have also been recorded and demonstrated in Fig. S19
acetonitrile solution with 0.1 M TBAPF6 as the supporting (ESI†). Along with the increase of the concentration of TPA, the
2+
electrolyte. Compared with that of Ru(bpy)3
and its ECL intensity was also enhanced accordingly and reached the
derivatives,10,19 these novel RuNHC complexes in this work peak value when the ratio of [TPA]/[RuNHC] was 400 for all of
displayed a much lower reversible oxidation wave with peak
potentials at 0.72, 0.83 and 0.73 V versus a normal hydrogen
electrode (NHE), respectively. Meanwhile, two reductive peaks of
these RuNHC complexes have also been recorded in the range
between ꢀ1.2 and ꢀ1.6 V versus NHE. Most of these reductive
curves are reversible except the second reduction peak of
RuNHC-1. Incorporating the electron-donating group of –OMe
into the coordination NHC ligand would obviously improve the
reversibility of the second reduction wave of RuNHC-3, while
there are almost no changes in the position of the reduction
Table 1 Electrochemical data from the RuNHC complexes presented in
this work
a
b
Complex
EoPx
ErPed
lmax of ECL
FECL
FECL
RuNHC-1
RuNHC-2
RuNHC-3
0.72c
0.83c
0.73c
ꢀ1.32c; ꢀ1.53d
ꢀ1.34c; ꢀ1.53c
ꢀ1.30c; ꢀ1.50c
808
784
802
132%
314%
126%
42%
100%
40%
Fig. 3 The ECL performance of RuNHC complexes in this work. (A–C) CV
and ECL intensity vs. potential plots of 0.1 mM RuNHC complexes in
acetonitrile solutions containing 0.1 M TBAPF6 and 40 mM TPA. The scan
rate is 50 mV sꢀ1. (D) Normalized ECL spectra of 0.1 mM RuNHC com-
plexes in acetonitrile solution containing 40 mM TPA and 0.1 M TBAPF6
under potential stepping conditions. The potential was set at 1.6 V versus
NHE.
a
2+
Potential scan experiment, refers to Ru(bpy)3 (FECL is defined as
b
100%), TPA as the co-reactant. Potential stepping experiment, refers
2+
to Ru(bpy)3 (FECL is defined as 100%), TPA as the co-reactant.
c
d
Reversible. Quasi-reversible.
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the three RuNHC complexes. Meanwhile, TPA also outper- acetonitrile solution exhibited an ECL efficiency that was three
2+
formed DBAE and TEA in the following optimization studies times higher compared with Ru(bpy)3 in positive potential
for the kinds of co-reactant in the potential stepping mode, as scanning mode under the same experimental conditions.
shown in Fig. S20 in the ESI.† Based on the optimized kinds In our opinion, these novel complexes, with excellent ECL
and concentrations of the co-reactant, the ECL spectra of these quantum efficiency and exhibiting the longest emission wave-
RuNHC complexes have also been recorded under the potential length of previously reported ruthenium(II) complexes, demon-
stepping mode. The spectra and wavelength data are shown in strated in this work could pave the way for the further
Fig. 3D and Table 1, respectively. Just as expected, these novel development of ECL cutting-edge research.
complexes exhibit NIR ECL emission centered at 808, 784 and
This work was supported by the National Natural Science
802 nm for RuNHC-1, RuNHC-2 and RuNHC-3, respectively. To Foundation of China (NSFC No. 22034003, 21505097), and the
the best of our knowledge, these novel complexes display the Excellent Youth Project of Suzhou University of Science and
longest ECL wavelength among the reported ruthenium(II) Technology, Qing Lan Project of Jiangsu province. The authors
complexes so far,5 which is very important to the development also thank the Supercomputing Center, CNIC, CAS, for calcula-
of ruthenium(II)-based NIR luminophores. The further tion support.
potential stepping ECL with BPO as a ‘‘reductive-oxidation’’
co-reactant and annihilation ECL were also performed and
characterized and are shown in Fig. S21 and S22 (ESI†),
respectively. However, their ECL intensities are much lower
than that of the ‘‘oxidation–reduction’’ ECL. In addition, these
Conflicts of interest
There are no conflicts to declare.
RuNHC complexes showed a good water solubility and the ECL
performances of RuNHC in aqueous ProCell solutions are
shown in Fig. S23 (ESI†).
Notes and references
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synthesized in this work. As expected, the incorporation of NHC
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)
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