New Cu(i) halide complexes showing TADF combined with room temperature phosphorescence: The balance tuned by halogens

Article abstract of DOI:10.1039/d0dt00192a

A series of Cu(i) halide complexes derived from tris(2-pyridyl)phosphine (Py₃P), [Cu₂(Py₃P)₂X₂] (X = Cl, Br, I), have been synthesized by a straightforward reaction in solution or through a mechanochemical route. At room temperature, the solid complexes exhibit bright dual-mode photoluminescence (λ_max = 520-550 nm, τ = 14.5-20.0 μs, and Φ_PL ≈ 53%), expressed by thermally activated delayed fluorescence (TADF) combined with phosphorescence (PH), originating from ¹(M + X)LCT and ³(M + X)LCT excited states, respectively. Remarkably, the balance of these radiative processes at 300 K is regulated by halogen atom nature, switching from TADF-assisted phosphorescence to PH-admixed TADF. The emission of [Cu₂(Py₃P)₂Cl₂] at 300 K is largely contributed by PH (73%) admixed with the TADF fraction (27%) and [Cu₂(Py₃P)₂Br₂] also emits mainly PH (65%) admixed with the larger TADF fraction (35%). Meanwhile, for [Cu₂(Py₃P)₂I₂], the TADF channel becomes dominating (61%) and PH contribution drops to 39%. The photophysical study corroborated by (TD)DFT computations has revealed that this effect arises mainly from the narrowing of the ΔE(S₁ - T₁) gap of the [Cu₂(Py₃P)₂X₂] complexes in the order Cl (1500 cm^-1) > Br (1250 cm^-1) > I (1000 cm^-1) which facilitates the TADF pathway and suppresses PH in the same order.

Full text of DOI:10.1039/d0dt00192a

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ARTICLE
New Cu(I) halide complexes showing TADF combined with room
temperature phosphorescence: the balance tuned by halogens
Andrey Yu. Baranov,a Alexey S. Berezin,a Denis G. Samsonenko,a Anton S. Mazur,b Peter M.
Received 00th January 20xx,
Accepted 00th January 20xx
Tolstoy,b Viktor F. Plyusnin,c,d Ilya E. Kolesnikov,e Alexander V. Artem’ev*a
DOI: 10.1039/x0xx00000x
A series of Cu(I) halide complexes derived from tris(2-pyridyl)phosphine (Py3P), [Cu2(Py3P)2X2] (X = Cl, Br, I), has been
synthesized by straightforward reaction in solution or through a mechanochemical route. At room temperature, the solid
complexes exhibit bright dual-mode photoluminescence (λmax = 520–550 nm, τ = 14.5–20.0 µs, ΦPL ≈ 53%), expressed by
thermally activated delayed fluorescence (TADF) combined with phosphorescence (PH), originating form 1(M+X)LCT and
3(M+X)LCT excited states, respectively. Remarkably, the balance of these radiative processes at 300 K is regulated by halogen
atom nature, switching from TADF-assistant phosphorescence to PH-admixed TADF. The emission of [Cu2(Py3P)2Cl2] at 300
K is largely contributed by PH (73%) admixed with the TADF fraction (27%), [Cu2(Py3P)2Br2] also emits mainly PH (65%)
admixed with the bigger TADF fraction (35%). Meanwhile, for [Cu2(Py3P)2I2], TADF channel becomes dominating (61%), and
PH contribution drops to 39%. The photophysical study corroborated by (TD)DFT computations has revealed that this effect
arises mainly from a narrowing of the ΔE(S1−T1) gap of the [Cu2(Py3P)2X2] complexes in the order Cl (1500 cm−1) > Br (1250
cm−1) > I (1000 cm−1) that facilitates TADF pathway and suppresses PH in the same order.
excitons can be harvested to lowest S1 state followed by
emissive decay to the ground state. For compound to exhibit
TADF, it is essential that the energy gap between the lowest S1
Introduction
Nowadays, organic light-emitting diode technologies lie at
the heart of a number of high-tech applications, including
display and lighting devices. Designing high-performance
OLEDs, it is critical to utilize the modern emitting materials, in
which both singlet and triplet excitons are harvested and
converted into light.1,2 Basically, there are two principal
mechanisms, wherethrough an internal OLED quantum
efficiency of up to 100 % can be achieved.3 In the context of
triplet harvesting effect,4–9 all the generated excitons are
harvested into lowest T1 state that further radiatively relaxes to
the ground state (S0). This channel, however, invokes rare and
expensive phosphorescent materials such as Ir(III), Pt(II), and
Os(II) complexes, whose strong spin-orbital coupling (SOC)
facilitates intersystem crossings (ISC) and permits fast triplet
decay rates.3–9 A more advantageous is singlet harvesting
mechanism operated with thermally activated delayed
fluorescence (TADF) phenomenon,10–16 by virtue of that all the
and T1 states will be around 1500 cm–1 or smaller ensuring
reverse intersystem crossing (RISC) at near ambient
temperature.10–19 The radiative rate kr(S1 → S0) is also desirable
to be as fast as possible to achieve shorter TADF decay time20,21
that is required for avoiding saturation effects in OLEDs.1 Note
that among materials which satisfy these requirements, Cu(I)
complexes are considered now as most promising owing to low-
cost, emission color tunability, and enhanced quantum
performance coupled with small ΔE(S1–T1) energy gaps.10,12,14–
17,20–22
Therefore, following the singlet harvesting effect,
efficient OLEDs with superior characteristics have been
fabricated basing on TADF Cu(I) complexes.14,16,17,23,24
Meanwhile, given the fact that both singlet and triplet
harvesting mechanisms exploit mono-mode radiation only,3 in
the real devices, the excitons confined on nonradiative T1 or S1
states, respectively, can be subjected to quenching.25,26 To
prevent these waste processes, and thereby improve OLED
performance, an additional radiative channel should be
opened, like T1 → S0 decay (phosphorescence) alongside with
TADF. Such kind of dual-mode emission, being harnessed both
triplet and singlet manifolds for the excitons harvesting,
completely addresses the above concerns, providing also
breakthrough in the decay time reduction problem. Originally
proposed by Yersin,27 this concept has been exemplified by a
Cu(I) complex that experiences effective SOC of the lowest T1
state, whereby the efficient TADF laced with phosphorescence
(20% fraction) have been implemented at room temperature.
An extension of the concept has been further demonstrated by
a. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of
Sciences, 3, Akad. Lavrentiev Ave., Novosibirsk 630090, Russian Federation
E-mail: chemisufarm@yandex.ru (Alexander V. Artem’ev)
b.
Institute of Chemistry, St. Petersburg State University, Russian Federation
c. Voevodsky Institute of Chemical Kinetics and Combustion, Institutskaya str. 3,
630090 Novosibirsk, Russian Federation
d. Novosibirsk State University (National Research University), 2, Pirogova Str.,
Novosibirsk 630090, Russian Federation
e. Center for Optical and Laser Materials Research, Saint Petersburg State
University, Ulianovskaya, 5, Saint Petersburg 198504, Russian Federation
Electronic Supplementary Information (ESI) available: general information, copies of
the experimental spectra, computation details. CCDC 1960446–1960449. See
DOI: 10.1039/x0xx00000x
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Dalton Trans., 2016, 00, 1-3 | 1
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a few Cu(I) complexes28–32 as well as Pd(II) one33 and some and, upon contact with chlorinated solvents, easilVyiecwoAnrtvicelerOtsnlitnoe
DOI: 10.1039/D0DT00192A
polyaromatic dyes,34,35 showing TADF combined with PH at 300 [Cu2(Py3P)2Cl2] (1).
K. One can mention a [Cu4Cl4(P^N)2] complex that emits TADF
N
and fast room temperature phosphorescence in an equal
X
proportion.36 Making use of the torsion angle changing between
C
u
P
N
the two co-ligand planes for tuning TADT/PH balance, examples
N
MeCN
r.t., 5 h
N
+
CuX
of structurally-related carbene Cu(I) complexes were
reported.37 In 2016, Xu and co-workers have constructed a
series of Cu(TTPP)X (X = Cl, Br, I) complexes with a tridentate
TTPP phosphine ligand, which also exhibit the dual-mode
emission with PH fraction amounting to 6% (Cl), 11% (Br), and
39% (I).38 A yellow OLED fabricated on the iodide complex had
made the dual-mode concept come true: the external quantum
performance of the device exceeds 15%, and the exciton
annihilation is effectively suppressed.38 More recently, Cu2Br2
and Cu2I2 type complexes has been announced manifesting
TADF with PH percent of 8 and 25%, correspondingly.39
However, despite the growing attention, such dual-mode
emitters remain rare and hardly-available materials because a
special and careful design is needed to enable a small ΔE(S1–T1)
separation in league with strong SOC.
N
P
N
N
N
P
Cu
X
N
1
2
3
X = Cl ( ), Br ( ) and I ( )
~99%
Scheme 1. Synthesis of complexes 1–3.
The compounds 1–3 are off-white powders, very stable to air
and moisture. Unlike most of the Cu(I) bromide and especially
chloride complexes,40 compounds 1 and 2 are found to be
surprisingly stable to air-oxidation. The identities of samples 1–
3 have been verified by powder X-ray diffractometry (PXRD,
Figs. S1–3) and microanalysis data. Their FT-IR spectra are
nearly the same (Fig. S7) and consistent with the available
structural data. Notably, the complexes obtained are almost
insoluble in organic solvents, demonstrating an extremely
limited solubility in dichloromethane only (<0.1 mg/mL). This
Herein, we report
a series of dual-mode emitting
[Cu2(Py3P)2X2] complexes with the TADF and PH intensities
regulated by halogen atom nature, switching from PH-
prevailing (73%, X = Cl) to TADF-dominating (61%, X = I) through
middle point with TADF/PH = 35:65 (X = Br). The convenient
synthesis, detailed photophysical study supported by
theoretical considerations, and demonstration of a sensing
ability are presented.
drawback, however, is well compensated by
a high
decomposition temperature of 1–3 (> 300 oC, see Fig. S8)
coupled with their property to sublime under high vacuum,
forming brightly emissive thin films. This important advantage,
along with remarkable luminescence of 1–3 (vide infra), makes
them promising candidates for OLEDs fabrication via vacuum
deposition technique.1
Results and discussions
Synthesis and structural study of 1–3
Single-crystal X-ray diffraction analysis (sc-XRD) has revealed
that complexes 1–3 are isostructural and display dimeric
structure with an inversion center at the middle of molecule
(Fig. 1). Two Cu atoms are bridged by two Py3P ligands so that
each metal is coordinated by P atom from the one ligand and by
two N atoms from another one. The halogen atom completes
coordination sphere of the metal represented by a distorted
tetrahedron (τ4 ≈ 0.87). The bond lengths and angles around
metal atoms (Table 1) are normal for such type complexes.41–45
Two remaining pyridine rings are non-coordinated ones. The
intramolecular Cu···Cu distances in 1–3 being 3.6941(3),
3.6810(7) and 3.7949(5) Å, respectively, obviously rule out the
metallophilic interactions (cf. twice vdw Cu radius being 2.80 Å).
In crystals (Fig. S4), the [Cu2(Py3P)2X2] molecules form 3D
supramolecular network by mean of weak C–H···X, C–H···N, C–
H···C contacts. It is pertinent to note that the structure of 1,
obtained by another way, was previously reported by Bakhoda
et al.45
It was reported that the diphenyl-2-pyridylphosphine and
bis(2-pyridyl)phenylphosphine, when interacted with Cu(I)
halides, gave various complexes and coordination polymers
showing outstanding emission characteristics.15 In this study,
aiming to assembly new potential emitters with interesting
properties, we have involved in this reaction tris(2-
pyridyl)phosphine, an interesting multifaced ligand. The
experiments have revealed that the treatment of Py3P with
CuCl, CuBr or CuI results in dimeric [Cu2(Py3P)2X2] complexes,
wherein two CuX units are bridged by two ligands in a P,N,N′-
manner (Scheme 1). The reaction is carried out at ambient
temperature in acetonitrile that appears to be the best solvent
among the tested ones (acetone, CH2Cl2, EtOH, THF, DMF). Note
that the alternation of the CuX/L ratio has no apparent effect on
the product structure, implying an enhanced stability of the
titled complexes. Under the optimal conditions (CuCl/L or
CuBr/L = 1:1, and the CuI/L = 1.2:1; stirring for 5 h), the isolated
yields of 1–3 reach 90–95%. We have also attempt to synthesize
fluoride analog of 1–3, [Cu2(Py3P)2F2], exploiting the reduction
of CuF2 with NaBH4 in presence of Py3P, as well as interaction of
Cu2O/Py3P system with HF, and reacting the CuF2 with an excess
of Py3P. Although the first two approaches are likely feasible,
the anticipated fluoride complex turn out to be very unstable
The structure of 1 has also been determined at room
temperature to disclose possible structural changes responsible
for thermochromic behavior of the luminescence (vide infra). It
emerges, however, that the geometry and packing of 1 are
roughly the same both at 130 and 296 K (see Table S1).
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0DT00192A
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ARTICLE
(b)
(a)
(c)
Figure 1. Molecular structures of 1–3 (a–c) determined by sc-XRD analysis. Symmetry code: A (1–x, 1–y, 1–z).
Table 1. Selected bond lengths (Å) and angles (°) for 1–3.a
Complex 1
As illustrated in Figure 2, progress of the reaction can be
successfully monitored under UV-lamp. While the intact
reaction mixture is nearly non-emissive, its grinding already
after several seconds results in appearance of a strong green
luminescence. Noteworthily, about 1 min is necessary for the
reaction completion. The PXRD analysis of the resulting mixture,
obtained after additional washing with MeCN, displays a typical
pattern for 1–3 with no peaks from the unconverted reactants
(Fig. S5). Again, the microanalyses and FT-IR data also confirm
the phase purity of the complexes synthesized. It is worth
noting that the mechanosynthesis developed is easily scalable,
allowing to prepare phase-pure 1–3 in the gram amounts.
Cu1–P1A
Cu1–N1
Cu1–N2
Cu1–Cl1
Cl1–Cu1–N2
2.1749(6)
N2–Cu1–P1A
N2–Cu1–N1
N1–Cu1–P1A
Cl1–Cu1–P1A
Cl1–Cu1–N1
116.052(44)
93.048(59)
119.779(44)
118.464(19)
101.761(42)
2.0698(14)
2.0567(16)
2.3164(4)
103.686(43)
Complex 2
Cu1–P1A
Cu1–N1
Cu1–N2
Cu1–Br1
N2–Cu1–N1
2.1850(11)
2.0660(37)
2.0824(30)
2.4399(6)
93.50(13)
N2–Cu1–P1A
N1–Cu1–P1A
Br1–Cu1–P1A
Br1–Cu1–N2
Br1–Cu1–N1
120.24(9)
116.239(99)
118.451(33)
101.093(100)
103.205(102)
Complex 3
Cu1–P1A
Cu1–N1
Cu1–N2
Cu1–I1
I1–Cu1–P1A
2.1927(7)
N2–Cu1–P1A
N2–Cu1–N1
N1–Cu1–P1A
I1–Cu1–N2
I1–Cu1–N1
113.39(6)
2.0620(21)
2.0573(19)
2.6207(4)
95.703(78)
115.798(60)
105.629(59)
103.354(60)
119.788(23)
a Symmetry code: A (1–x, 1–y, 1–z).
Mechanochemical synthesis
Eventually, we have found that complexes 1–3 can be best
accessed through mechanochemical synthesis comprising the
manual grinding of the reactants in the presence of a small
amount of assistant solvent. As applied to Cu(I) complexes, this
approach was earlier proved as efficient and “green” route
benefiting highest yields, simplicity of workup, shorter reaction
times and waste minimization.41,46 In this work,
mechanosynthesis again highlights its advantages, appearing to
be helpful for the preparation of 1–3 too. The latter are
quantitatively formed by simple grinding an equimolar mixture
of Cu(I) halide with Py3P in the presence of several drops of
MeCN. Without the assisting solvent the reaction does not
occur, whereas other examined solvents (acetone, EtOH,
toluene, Et2O) provide no complete conversion of the reagents.
(a)
(b)
(c)
Figure 2. UV-Monitoring of the mechanochemical synthesis of 1
(down views; the top images made under day-light): (a) dry mixture
of CuCl with Py3P; (b) the mixture with a few drops of MeCN; (c)
ground mixture.
CP/MAS NMR spectral study
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To finally verify the purity of the complexes obtained by
To gain insights into the electronic structure aVniedwtAhrteicleoOringlinine
DOI: 10.1039/D0DT00192A
mechanochemical route, the solid-state NMR study has been of electronic transitions in 1−3, DFT and TDDFT computations
performed as exemplified by 1. Only one signal is visible at –8 have been carried out at BP86/TZP theory level. The optimized
ppm in the 31P{1H} CP/MAS NMR spectrum (Fig. 3a). The signal ground-state (S0) geometries of 1−3 are in good agreement with
is split into the quartet, indicating that all phosphorous atoms the available X-ray structures, in the same manner as calculated
are coordinated to copper with the spin-spin coupling constant and measured IR spectra (Fig. S9). Since all the complexes have
1
1J(31P,63Cu) = 1500 Hz and J(31P,65Cu) = 1600 Hz (both isotopes a close HOMO/LUMO distribution patterns (Figs. 4, S10, S11)
of copper are spin 3/2 nuclei; the natural abundance of 63Cu is and related UV-Vis absorption profiles, the further discussion is
ca. 70% and that of 65Cu is ca. 30%). The 15N{1H} CP/MAS NMR focused on 3 as a representative complex. The HOMO of 3 is
spectrum (Fig. 3b) has three signals of equal integral intensity: mainly contributed by metal d-orbitals mixed with halogen p-
the singlet at 332.3 ppm and two quartets at 283.0 ppm (82 Hz orbitals (Fig. 4), while the LUMO is exclusively contributed by
1
coupling) and at 277.5 ppm (85 Hz coupling). The J(15N,63Cu) π*-orbitals on the pyridine rings. Such pronounced spatial
and 1J(15N,65Cu) couplings are not resolved. The 13C{1Н} CP/MAS separation of the frontier orbitals implies that the lowest singlet
NMR spectrum (Fig. 3c) contains several signals in the region (S1) and triplet (T1) emissive states of 3 should be of charge
characteristic for aromatic and heteroaromatic carbons. The transfer character as it is specific for Cu(I)
1J(13C,31P) and 2J(13C,31P) couplings, which are expected to be in complexes.12,16,17,20,21,48 To clearly corroborate this assertion, as
the order of 10–20 Hz,47 are not resolved due to the broadening well as to estimate the structural reorganization of 3 upon
of the signals. Therefore, from the 15N{1H} and 13C{1Н} CP/MAS excitation, its S1 and T1 states have been computed. It appears
NMR data one could conclude that two out of three nitrogen that the order of HOMO/LUMO distribution of the excited
atoms of the Py3P ligand are bonded to the copper center, while states (Fig. S12) is similar to that of the S0 state (Fig. 4). The both
the third nitrogen atom is essentially free. Indeed, according to exited state geometries are close and not quite different from
the data shown in Table 1, nonequivalence of the local the ground state, meaning a slight structural changing upon S0
environment of nitrogen atoms in pyridine rings, which are → S1 excitation and S1 →T1 intersystem crossing. This could be
bonded to copper atoms, is observed. Such nonequivalence attributed to a rigidity of the molecule 3 that significantly
leads to different values of the chemical shift in the 15N{1H} and restricts distortions stemming by photoexcitation. The spin
13C{1Н} NMR spectra.
density for the T1 state of 3 is largely localized on the pyridine
rings, copper and iodine atoms (Fig. S13). This indicates that the
lowest triplet state has a metal-to-ligand and halide-to-ligand
charge transfer character, 3(M+X)LCT.
-8.15 ppm
1J(31P,63Cu) = 1500 Hz
1J(31P,65Cu) = 1600 Hz
a
30
20
10
0
-10
-20
-30
-40
31P{1H} chem. shift, ppm
b
332.3 ppm
283 ppm 277.5 ppm
1J(15N,63/65Cu) = 82 Hz 1J(15N,63/65Cu) = 85 Hz
Figure 4. Frontier HOMO (left) and LUMO of 3 derived from DFT
calculations.
350 340 330 320 310 300 290 280 270 260
15N{1H} chem. shift, ppm
The absorption spectrum of 3, computed by the TD-DFT
method, is plotted in Figure 5 along with experimental
absorption and excitation curves. It is clearly seen that the
measured spectra of 3 reasonable fit the simulated one. The
computations outline that the low-energy absorption region (>
350 nm) of 3 is dominated by transitions of 1,3(M+X)LCT type. In
higher-energy window (≈230–350 nm), such transitions are
overlapped with those of intraligand (IL) kind. The similar
patterns are observed for 1 and 2, the UV-Vis spectra of which
have also been modeled at the same theory level (Figs. S14,
S15). In general, such assignments are in line with the literature
data on Cu(I) complexes,12,16,17,20,21 and besides supported by
the experimental results (vide infra).
c
170
160
150
140
130
120
110
13C{1H} chem. shift, ppm
Figure 3. CP/MAS NMR spectra of 1 recorded at 25 C: (a) 31Р{1Н}
CP/MAS NMR spectrum; (b) 15N{1Н} CP/MAS NMR spectrum; (c)
13С{1Н} CP/MAS NMR spectrum.
Theoretical study
4 | J. Name., 2012, 00, 1-3
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ARTICLE
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
τ(300 K) [µs]
kr(300 K) [s–1]b
14.5
3.8·104
3.1·104
560
18.3View Artic2l0e .O0nline
DOI: 10.1039/D0DT00192A
2.9·104
2.6·104
540
2.6·104
2.5·104
530
Calculated
Absorption
Excitation
knr(300 K) [s–1]c
λmax(77 K) [nm]
ΦPL(77 K) [%]
τ(77 K) [µs]
kr(77 K) [s–1]b
knr(77 K) [s-1]c
∆E(S1–T1) [cm–1]d
τ(S) [ns]d
100
92
90
36
48
91
2.8·104
<1.0·103
1500
8
1.9·104
1.6·103
1250
29
1.0·104
1.1·103
1000
92
TADF/
27/73
35/65
61/39
/Phosphorescence (300K) [%]e
250
300
350
400
450
500
550
600
a
b
Wavelength, nm
Excitation wavelength λex = 360 nm; Radiative decay rate: kr =
ΦPL/τ; c Non-radiative decay rate: knr = τ−1 – kr; d Estimated from the
temperature dependence of the lifetimes; d Evaluated using Eqs. 1; e
Estimated from the radiative decay rate kr data using Eqs. 2.
Figure 5. Comparison of the simulated UV-Vis spectrum with
experimental solid-state absorption and excitation ones for 3.
Vertical bars reflect the positions and oscillator strengths of the
electronic transitions computed.
Since the emission and excitation spectra of the above
compounds have a very similar dynamics pattern upon
temperature variation (Figs. 6, S17, S18), further attention will
again be focused on 3. With the temperature decrease from 300
to 77 K, emission spectra of 3 show a gradual enchantment of
intensity accompanied by bandwidth reduction and a red-
shifting of the maxima (520 → 530 nm) (Fig. 6). Owing to these
issues, luminescence color of 3 turns from green at 300 K to
green-yellow at 77 K (Fig. S19).
The ΔE(S1−T1) gap values of 1−3, approximately estimated
from the computed S0 → S1 and S0 → T1 vertical transitions,
change as follows: 2651, 2510 and 1680 cm–1, respectively.
These values, although overestimated, correctly reflect the
experimental trend of the ΔE(S1−T1) magnitudes being 1500,
1250 and 1000 cm–1 for 1−3, accordingly (vide infra). Therefore,
both theoretical and experimental results suggest the
narrowing of the ΔE(S1−T1) gap in the order of 1 > 2 > 3.
Considering these data, the manifestation of TADF can be a
priori imagined for complexes 1–3, which satisfy the key
requirements of this mechanism.20,48 Even the relatively large
ΔE(S1−T1) splitting for chloride complex 1 is still within thermal
activation barrier required for the T1 → S1 reverse intersystem
crossing (RISC).10,20,48
1.0
T, K
77
100
150
200
250
300
125
175
225
275
0.8
0.6
0.4
0.2
0.0
Photophysical study of [Cu2I2(Py3P)2] (3)
At ambient temperature, solid complexes 1–3 emit bright
photoluminescence (PL) displayed by a broad band, the
featureless shape of which is indicative of charge-transfer
nature of the emitting state. When passing from [Cu2(Py3P)2Cl2]
(1) to [Cu2(Py3P)2I2] (3), the emission maxima are smoothly blue-
shifted from 550 to 520 nm as a result of weakening ligand-field
strength of halogen atoms in Cl > Br > I order. The photophysical
characteristics of 1–3 at 300 and 77 K points are summarized in
Table 2. As seen from the data, the PL quantum yields (ΦPL) of
all compounds are about 53% at 300 K, and close to 100% at 77
K. The ambient temperature lifetimes are 14.5, 18.3 and 20.0,
upon cooling to 77 K, they increase up to 36, 48 and 91 µs for
1–3, preserving the monoexponential character.
450
500
550
600
650
700
Wavelength, nm
Figure 6. Temperature-dependent emission spectra of 3 (λex = 360
nm).
The pronounced red-shift of the emission, as well as
decreasing of the radiative rate of 3 by about 3 times from
kr(300 K) = 2.6·104 s–1 to kr(77 K) = 1.0·104 s–1 (Table 2), give
reason to assume the TADF manifestation.10,20,48 To support this
assumption, the temperature dependence of the lifetimes has
been recorded (Fig. 7) and fitted with Equation (1) intended for
TADF mechanism.10,20
Table 2. Photophysical data for polycrystalline complexes 1–3.a
∆퐸
휏(푇) = (3 + exp ―
) ( 3
+
휏
푆
1 exp ( ― ∆퐸ST))
ST
(Eq. 1)
(
)
푘
푇
휏
푘퐵푇
[Cu2(Py3P)2X2]
λmax(300 K) [nm]
ΦPL(300 K) [%]
1 (X = Cl) 2 (X = Br) 3 (X = I)
퐵
푇
휏
휏
푇
550
55
530
53
520
51
wherein and are the lifetime of S (prompt fluorescence)
푆
1
∆퐸
and T1 (phosphorescence) excited states, respectively,
is
ST
푘
the energy of the singlet–triplet splitting, and 퐵is the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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6
Dalton Transactions
Boltzmann constant. Fitting the dataset for 3 with this that of 3 (Fig. 7). The low-temperature plateau (puVrieewPAHrticrleegOinolinne)
DOI: 10.1039/D0DT00192A
equation provides the following results: ΔE(S1−T1) = 1000 cm−1, for 1 and 2 is attained already at ≈200 and 170 K,
and the fluorescence decay time of τ(S1) = 92 ns. The ΔE(S1−T1) correspondingly, whereas the fitting curve of 3 reaches the
magnitude determined for 3 is comparable with the common plateau at 125 K only (Fig. 7). It is more important that the room
values for Cu(I) complexes with TADF effect.
temperature point for 1 and 2 falls to the drop-down region of
As follows form the τobs/T dependence plot (Fig. 7), the room the τobs/T plot thus meaning a higher PH percentage in the total
temperature luminescence of 3 represents TADF (61%) mixed emission. Quantitatively, the PH and TADF relative
with phosphorescence (39%). Beginning with 100 K, when the contributions of 1–3 at the certain temperature have been
curve reaches the low-temperature plateau, the TADF is estimated following the temperature dependence of the
completely suppressed, and pure phosphorescence starts to radiative decay rates (Eq. 2) assuming equal quantum yields of
dominate. This behavior implies that the emission bands of 3 the PH and TADF emission (for details, see §7 in ESI).27,29
should be by contributed by TADF and phosphorescence (PH)
푘(푐표푚푏푖푛푒푑) = 푘(푇퐴퐷퐹) +푘(푃퐻)
(Eq 2)
components, the ratio of which is temperature-regulated.
Indeed, the Gaussian analysis of the emission bands of 3 at 77–
The simulated intensities of the TADF and PH fractions are
300 K range (for details, see §7 in ESI) reveals that they can be plotted against the temperature (Fig. 8). As follows from the
approximated by two Gauss functions, λ1 = 2.275 eV (545 nm) relationships, balance point temperatures (when the TADF/PH
and λ2 = 2.463 eV (503 nm), with the separation of ∆λ = ratio is 50:50) vary considerably to be ≈280 K for 3 and ≈350 K
190 meV (52 nm, 1530 cm–1). The time-resolved PL spectra for 1. At room temperature, the TADF fraction grows in the
recorded with the 50 µs delay support the presence of the order 1 (27%) > 2 (35%) > 3 (61%) as a consequence of increase
above components (Fig. S21). The ratio between the low-energy of RISC rate and reduction of T1 → S0 channel in the same order.
(λ1 = 545 nm, PH) and high-energy (λ2 = 503 nm, TADF) bands at Therefore, the luminescence of 1 and 2 at 300 K is represented
the 170–310 K range is well described by Arrhenius equation, by a TADF-assisted phosphorescence, whilst for 3 it is a PH-
giving the activation energy of ∆EA = 1050 cm–1 (Fig. S22). Note assisted TADF. Put differently, changing the halide atoms in the
∆퐸
that this value, being essentially
gap, perfectly matches [Cu2(Py3P)2X2] series, the luminescence can be easily tuned from
ST
that evaluated from the τobs/T plot (1000 cm–1). Due to TADF/PH = 27/73 (X = Cl) to TADF/PH = 61/39 (X = I) passing
suppression of the TADF, intensity of the high-energy band through the case with TADF/PH = 35/65 (X = Br). Such
belonging to TADF channel becomes weak upon cooling and remarkable scenario, to our knowledge, is unprecedented for
reduces to zero at near 77 K. Conversely, contribution of the Cu(I) and related complexes emitting TADF distinctly mixed with
TADF band upon heating from 77 K increases up to 60% at 300 PH at 300 K. It should be underlined firstly that such compounds
K, arising from thermal population of the higher-lying short- are scarce amounting in total about 12 examples.27–39 Secondly,
lived S1 state from the T1 one that is lower in energy by 1000 there are no precedents when the contributions of TADF and PH
cm–1 only. As a consequence, the combined emission band of 3 into total dual-mode emission can be inverted by means of
shifts in blue region.
halogen atom changing.
100
100
1
2
3
80
60
40
20
0
80
Phos
TADF
1
2
60
3
40
20
0
100 150 200 250 300 350 400 450
100 150 200 250 300 350 400
Temperature, K
Temperature, K
Figure 7. Temperature dependences of the lifetimes observed (τobs
for 1–3 (λex = 360 nm).
)
Figure 8. Temperature dependence of the fractional contributions of
TADF and phosphorescence to total emission of 1–3.
The photophysical properties for 1–3: a comparative study
The phenomenon disclosed is likely contributed by two
factors: the difference in ΔE(S1−T1) values and spin-orbital
Significantly, the chloride and bromide complexes 1 and 2 coupling (SOC) of the above compounds. In fact, the ΔE(S1−T1)
exhibit the different τobs/T dependences distinguished also from gaps, estimated by fitting the datasets of Fig. 7 with Eq. 1, are
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
narrowing in the order: 1 (1500 cm−1) > 2 (1250 cm−1) > 3 (1000 seems to be a promising material for luminescent sensing of
View Article Online
cm−1) (Table 2). As consequence, the larger ΔE(S1−T1) in 1 alkalis in solutions and volatile amines inDaOirI.: 10.1039/D0DT00192A
suppresses reversible ISC (RISC) form T1 level to S1, thus
To elucidate behavior of 1 in HCl solution, its 13C and 31P{1H}
reducing TADF fraction to 27% at 300 K. The narrower ΔE(S1−T1) NMR spectra have been recorded (Fig. S24, S25). The 31P{1H}
gap of 2 leads to the increasing of the TADF/PH ratio (up to spectrum reveals a sharp singlet at –31.6 ppm {cf. with that of
≈35/65). Finally, in 3, endowed with narrowest ΔE(S1−T1) HCl solution of solid 1, –8.15 ppm}. In 13C NMR spectrum of
splitting, the TADF fraction at room temperature is the highest, acidic solution of 1 there are ten peaks in the region of 127.6–
amounting to 61%. Another factor accountable for the TADF/PH 145.4 ppm, which can be ascribed to a pair of two
balance is SOC, which due to heavy atom’s effect, should be nonequivalent pyridine rings (Fig. S24). Considering these data,
enhanced for bromide 2 and especially iodide 3. Indeed, as our it can be tentatively assumed that that the protonation of two
computations predict, SOC constants between different Si (i = non-coordinated Py rings of 1 is occurs upon dissolution in
1−14) and Tj (j = 1−15) states of 3 can reach 1000 cm−1 (Fig. S16). hydrochloric acid.
It should be remarked that the stronger SOC commonly
facilitates T1 → S0 transition and thus increases the PH
contribution and shortens the T1 state lifetimes.48 In violation of
this relationship, iodide 3 has longer T1 decay time rather than
chloride 1 and bromide 2 (Table 2). In our opinion, this
discrepancy can be attributed to the much higher contribution
of halide-centered orbitals in HOMO of 3, as compared to those
in 2, not to mention 1 (see Tables S2–4). The LUMO is closely
similar for three compounds. This leads to reduction in the
spatial overlap of HOMO and LUMO in going from 1 to 3. The
higher charge separation in 3 results in the smaller ΔE(S1−T1)
gap (against that of 2 and 1), while the radiative decay from the
CT states becomes slower.48 Concerning radiative rate
constants (kr = ΦPL/τ), they decrease in the order 1 > 2 > 3 at
both 77 and 300 K since the ΦPL values are close for all these
complexes (about 53% at 300 K and 94% at 77 K).
(a)
(b)
(c)
Luminescent sensing of bases using a protonated form of 1
Figure 9. An enhancement of luminescence associated with
formation of complex 1 upon dropwise addition of NaOH solution on
a paper soaked in a HCl solution of 1 (a–c) Down views made under
UV-lamp, and the top images are on day-light.
The presence of the basic pyridine rings in complexes 1–3
makes them potential materials for luminescent sensing of
acids. Assuming that the protonation of these pyridine rings
should alter the emission characteristics of 1–3, we have
examined behavior of 1, a representative complex, toward acid
vapors. Contrary to our expectations, no apparent luminescent
response has been detected after exposing of 1 under AcOH,
HCOOH or HCl vapors. Meanwhile, during these experiments we
have found that solid 1 is easily soluble in aqueous hydrochloric
acid (25–35%) to form non-emissive solution. Its following
alkalization with NaOH quantitatively recovers 1 in phase-pure
form (Fig. S6). We believe that 1 in the acidic solution exists as
a protonated form, e.g. [1·2H]2+, that easily returns to the
parent complex upon action of bases. To our knowledge, such
reversible dissolution-precipitation upon pH solution varying is
unprecedented for the emissive Cu(I) complexes.
Further we have exploited this interesting finding to
demonstrate potential application of 1 for sensing of organic
and inorganic bases. For this purpose, filter paper has been
soaked in a solution of 1 (≈20 mg) in aqueous HCl followed by
drying in air. The pieces of paper obtained are non-emissive, but
their treatment with alkali solution immediately triggers a
bright luminescence appearance (Fig. 9). The similar effect,
although with a longer response time, also takes place upon
exposing the “test-papers” under volatile amine vapors
(exemplified by Et3N, Fig. S23). Thus, the protonated complex 1
Conclusions
In summary, a series of Cu(I) halide complexes presented
features efficient TADF combined with phosphorescence,
whose proportions are modulated by halogen atom nature.
These complexes, viz. [Cu2(Py3P)2X2] (X = Cl, Br, I), have been
synthesized in excellent yields by treatment of Py3P with Cu(I)
halides under classical solution condition. Mechanochemical
variant of the reaction has also proved to be efficient and
“greener” option for access to the above complexes. The solid
complexes at ambient temperature emit a green dual-mode
photoluminescence with PLQYs of 51–55% at 300 K and the
lifetime of 14.5–20.0 µs. When passing from the chloride to
iodide, the emission maxima are gradually blue-shifted from
550 to 520 nm (300 K) in the wake of ligand-field strength
reduction of the halogen atoms in the same order. The room
temperature emission of title compounds brings about TADF
combined with phosphorescence (PH), which is originated from
1(M+X)LCT and 3(M+X)LCT exited states, respectively. Below 100
K, the TADF is frozen out so that the pure PH is only observed.
Remarkably, that the contributions of TADF and PH into
total emission of title complexes has been found to be easily
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inverted through halogen atom (X) alteration. Indeed, while
DOI: 10.1039/D0DT00192A
the combined emission of chloride at 300 K is mostly dominated
by PH (73%), the bromide displays lower PH contribution (65%)
not to mention the iodide emitting 61% TADF and only 39% PH.
To our knowledge, a similar scenario has not been described
yet. According to our photophysical results and (TD)DFT
computations, this effect is contributed by efficient spin-orbit
coupling of these Cu(I) complexes alongside with a narrowing of
the ΔE(S1−T1) energy gap in the order Cl (1500 cm−1) > Br (1250
cm−1) > I (1000 cm−1) that facilitates TADF pathway and
suppresses PH in such order. From a fundamental standpoint,
the findings presented contribute to the chemistry and
photophysics of Cu(I) complexes, providing new scope for the
design of air- and photostable materials exhibiting either TADF-
assisted phosphorescence or PH-accompanied TADF. From a
practical side, the novel emitters can exploit both singlet and
triplet harvesting mechanisms for more efficient conversion of
power into light.
Acknowledgements
This work was supported by Russian Science Foundation (Project 18-
73-10086). A part of luminescent measurements was carried out in
the Centre for Optical and Laser Materials Research of Saint
Petersburg State University Research Park. Another part of
luminescent measurements was performed within the framework of
Russian Science Foundation grant (Project 18-13-00246).
29 A. Schinabeck, N. Rau, M. Klein, J. Sundermeyer, H. Yersin,
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Graphical abstract
View Article Online
A series of Cu(I) halide complexes showing thermally activated delayed fluDoOrIe: 1s0c.1e0n39c/De0D(TT0A0D19F2A)
combined with room temperature phosphorescence is announced.