Article pubs.acs.org/IC
Synthesis, Structures, and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I) Complexes Benjamin Hupp, Carl Schiller,§ Carsten Lenczyk, Marco Stanoppi,‡ Katharina Edkins,† Andreas Lorbach,‡ and Andreas Steffen* Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *
ABSTRACT: Herein, we report on the synthesis and structural characterization of a series of trigonal and tetrahedral cationic copper(I) complexes, bearing phosphine or N-heterocyclic carbene ligands as donors, with benzthiazol-2-pyridine (pybt) and benzthiazol-2-quinoline (qybt) acting as π-chromophores. The compounds are highly colored due to their 1MLCT (MLCT = metal-to-ligand charge transfer) states absorbing between ca. λabs = 400−500 nm, with 1ILCT (ILCT = intraligand charge transfer) states in the UV region. The relative shifts of the S0→S1 absorption correlate with the computed highest occupied molecular orbital−lowest unoccupied molecular orbital gaps, the qybt complexes generally being lower in energy than the pybt ones due to the larger conjugation of the quinoline-based ligand. The compounds exhibit, for CuI complexes, rare intense long-lived near-IR emission with λmax ranging from 593 to 757 nm, quantum yields of up to Φ = 0.11, and lifetimes τ of several microseconds in the solid state as well as in poly(methyl methacrylate) films. Although a bathochromic shift of the emission is observed with λmax ranging from 639 to 812 nm and the lifetimes are greatly increased at 77 K, no clear indication for thermally activated delayed fluorescence was found, leaving us to assign the emission to originate from a 3(Cu→ pybt/qybt)MLCT state. The red to near-IR emission is a result of incorporation of the sulfur into the chromophore ligand, as related nitrogen analogues emit in the green to orange region of the electromagnetic spectrum. The photophysical results and conclusions have further been corroborated with density functional theory (DFT)/time-dependent DFT calculations, confirming the nature of the excited states and also the trends of the redox potentials.
■
alternatives for NIR emission with long lifetimes.3,4,22−24 Because of the luminescence in TM compounds generally occurring from the triplet excited state T1, a beneficially large Stokes shift is achieved, which leads to less interference between the excitation and emission spectra (technically, the Stokes shift is defined as the energy difference between the absorption and emission S0↔S1, but it is often used to describe the energy difference between the transitions S0→S1 and T1→ S025). The potential of this strategy may be exemplified by two very recent developments. Highly phosphorescent iridium and ruthenium complexes of perylene bismides (PBIs) emitting between 700 and 1150 nm have been reported, with lifetimes on the microsecond time scale and for PBI-based triplet emitters unprecedented Φ of up to 0.11 in solution.26 A breakthrough with regard to using complexes of 3d elements as NIR emitters has been achieved by the groups of Heinze and Resch-Genger, who successfully designed the water-soluble and air-stable chromium(III) compound [Cr(ddpd)2](BF4)3 (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine), which emits from its metal-centered 2E state and shows
INTRODUCTION The development of luminescent materials emitting in the nearIR (NIR) region of the electromagnetic spectrum is of great interest for future applications in NIR organic light-emitting diodes (OLEDs), light-emitting electrochemical cells (LEECs), singlet oxygen sensing, night vision-readable displays, fiber optic telecommunication, or biological imaging within the transparency window of tissue.1−6 Apart from fighting the problem of the energy-gap law, which states that the rate constant for nonradiative decay knr that reduces the quantum yield Φ becomes larger with decreasing energy difference between the emitting excited state and the ground state,7 the classes of compounds that research with regard to NIR emission has been focused on have specific intrinsic limitations. For instance, organic NIR emitters usually fluoresce from their singlet excited state S1 with very short lifetimes on the nanosecond time scale, and they are often prone to photodegradation.8−14 In contrast, beneficial long-lived emission is observed in lanthanide complexes from metal-centered states; however, their energies are not tunable.1,2,5,6,15−21 Transition-metal (TM) complexes provide an immense flexibility to tune the photophysical properties for a given application and have also been considered as potential © 2017 American Chemical Society
Received: April 19, 2017 Published: July 25, 2017 8996
DOI: 10.1021/acs.inorgchem.7b00958 Inorg. Chem. 2017, 56, 8996−9008
Article
Inorganic Chemistry unprecedented Φ of 0.11 and 0.14 in H2O and D2O, respectively.27 To bypass metal-centered d−d* states, which may lead to premature nonradiative decay from the T1 state, a lot of attention has been paid to coinage metal (Cu, Ag, Au) compounds with a d10 configuration.28−39 Copper(I) complexes, in particular, have seen a phoenix-like rise in the last eight years, mainly due to the exploitation of their often-found thermally activated delayed fluorescence (TADF) in visible light-emitting devices, ensuring short lifetimes and high Φ.28−31,35,40−44 In addition, the various possible coordination geometries in d10 TM compounds, ranging from tetrahedral and trigonal to linear arrangements around the metal atom, provides further possibilities to influence the excited-state properties.28,30−37,42,44−55 Efficient deep-red to NIR phosphorescent emitters of d10 coinage metals are mainly based on cluster-like compounds, of which the nuclearity, structure, and photophysical properties are difficult to control.30,56−64 Molecular, and thus potentially easily tunable, soluble, and even environment-responsive, copper(I) complexes emitting efficiently at low energies are still rare, motivating us to make a foray into this area. Our strategy is inspired by the observation that incorporation of heavier analogues of light main-group elements, such as nitrogen or oxygen, can lead to excited states, which are much lower in energy. This has been shown for siloles, phospholes, and thiophenes, contrasting significantly the higher-energy emission of their carbon, nitrogen, or oxygen congeners.65−73 Copper(I) complexes of 2-(2′-pyridine)imidazole-type ligands are well-known to emit in the green to orange region of the electromagnetic spectrum (Chart 1),54,74−78 and we have thus
luminescence on the microsecond time scale becomes very intense in the solid state with Φ of up to 0.11.
■
RESULTS AND DISCUSSION Synthesis and Structural Characterization. The reaction of [Cu(MeCN)4]PF6 with either 2-(2′-pyridine)benzthiazol (pybt, a) or 2-(2′-quinoline)benzthiazol (qybt, b) in the presence of the respective phosphine ligand in dichloromethane at room temperature gives full conversion to the highly colored tetrahedral copper(I) complexes 1a,b−5a,b (Scheme 1). The Scheme 1. Synthesis of Cationic Tetrahedral Copper(I) Phosphine Complexes 1−5 and Trigonal Copper(I) NHC Compound 6 Bearing pybt (a) or qybt (b) Ligandsa
Chart 1. Known Green to Orange Emissive CuI Complexes Bearing 2-(2′-Pyridine)imidazole-type Ligands54,74−78
a
DPEPhos = bis[(2-diphenylphosphino)phenyl] ether; dppe = 1,2bis(diphenylphosphino)ethane.
pale yellow trigonal compounds 6a,b were obtained from [CuCl(IDipp)] (IDipp = bis(2,6-di-isopropylphenyl)-imidazol2-ylidene) by halide abstraction with silver hexafluorophosphate and subsequent addition of pybt or qybt, respectively. Although 6a was isolated in pure form, the synthesis of 6b was always accompanied by the formation of significant amounts of [Cu(IDipp)2]PF6 and of [Cu(qybt)2]PF6 according to NMR spectroscopic and mass spectrometric studies, which we were not able to fully remove by extraction, washing, or column chromatography, leaving analytically impure samples of 6b unsuitable for photophysical studies. All complexes were fully characterized by multinuclear 1H, 13 C{1H}, 19F, and 31P{1H} NMR spectroscopic studies and either elemental analysis or high-resolution (HR) electrospray ionization (ESI) mass spectrometry (MS), as for some compounds no satisfactory elemental analysis could be obtained on our instruments due to the fluorinated anions, even though crystalline or even single-crystalline material was used. For those compounds used for photophysical studies, HR-ESI mass spectra were always measured to unambiguously
chosen those for replacement of one nitrogen for sulfur, expecting low-energy emission in the red to NIR region. Furthermore, we modified the conjugation of the chromophore ligand to further control the excited-state energy. Indeed, this strategy leads to emission with λmax(em) ranging from 593 to 812 nm depending on the aggregation state, and the long-lived 8997
DOI: 10.1021/acs.inorgchem.7b00958 Inorg. Chem. 2017, 56, 8996−9008
Article
Inorganic Chemistry exclude minor impurities. To provide high-purity samples for photophysical studies (vide infra), the products were also recrystallized several times. For compounds 1b, 2a, 3a, and 6a, single crystals suitable for X-ray diffraction were obtained and confirm their assumed molecular geometry (Figures 1 and 2 and Table 1). In addition, we obtained a single crystal of the intermediate [Cu(MeCN)2(pybt)]PF6, confirming the reaction sequence depicted in Scheme 1.
Table 1. Selected Interatomic Distances (Å) and Angles (deg) for the Cations in 1b, 2a, 3a, and 6a Cu−N1 Cu−N2 Cu−P1 Cu−P2 Cu−C1 P1−Cu−P2 N1−Cu−N2 C1−Cu−N1 C1−Cu−N2
1b
2a
3a
2.089(2) 2.094(2) 2.2468(6) 2.2313(6)
2.070(2) 2.091(2) 2.2320(7) 2.2486(7)
2.070(2) 2.089(3) 2.2497(13) 2.2418(14)
117.06(2) 78.87(8)
115.70(3) 80.13(9)
115.16(5) 79.37(11)
6a 2.021(2) 2.150(2)
1.906(3) 79.20(9) 152.67(11) 128.92(11)
E, F, I, J, and L are significantly larger with 123−131°.74−78 The Cu−P bonds in 1a−3a of 2.2313(6)−2.2497(13) Å are within the typical range found for these types of complexes.74−78 We also note that the higher electron density at the copper(I) center in 1a caused by the strong σ-donation of PMe3 leads to longer Cu−N distances (2.089(2)/2.094(2) Å) compared to 2a and 3a (2.070(2)/2.091(2); 2.070(2)/ 2.089(3) Å). The trigonal N-heterocyclic carbene (NHC) complex 6a is best compared with [Cu(pybim)(IDipp)] (pybim = 2-(2′pyridyl)benzimidazole, M, Chart 1), which is also distorted from an ideal Y-shaped geometry due to the asymmetry of the bidentate π-chromophore ligand.54 In addition, in both complexes the IDipp ligand is coplanar with the N,N′coordinating ligand. The Cu−C(carbene) bond in 6a of 1.906(3) Å is slightly longer than in the pybim complex M (1.884(3) Å),54 but within in the typical range as found for other CuI NHC complexes.44,50,51,79,80 However, the neutral nature of the pybz ligand in cationic 6a leads to a longer Cu− N1 bond (2.021(2) vs 1.9227(18) Å) and a shorter Cu−N2 bond (2.150(2) vs 2.2907(18) Å) compared to neutral A bearing the negatively charged pybim ligand.54 The latter is presumably also responsible for the slightly larger C1−Cu−N1 angle of 154.24(8)° in M compared to 6a, distorting the geometry a bit more toward a T-shape geometry as observed, for example, in [Cu(tfppy)(IDipp)] (tfppy = 2-(2,3,4,5tetrafluorophenyl)pyridyl).80 Photophysical and Electrochemical Studies. The 2-(2′pyridine)benzthiazol (pybt) complexes 1a−6a exhibit allowed high-energy absorptions between λabs = 300−350 nm with extinction coefficients at the respective maxima of ε = (14−18) × 103 M−1 cm−1 (Figure 3, top, and Table 2), which we assign to mainly intraligand (IL)(π−π*) states localized at the pybt ligand. In contrast to the pale yellow trigonal NHC complex 6a, the tetrahedral phosphine compounds 1a−4a show an additional very weakly allowed (ε < 2500 M−1 cm−1) and broad low-energy band around 400−475 nm, presumably as a result of Cu→pybt metal-to-ligand charge transfer (MLCT) transitions, giving them their intense yellow to orange colors. We note that this band is most intense (ε = 4100 M−1 cm−1) for 5a and extends for this compound to 525 nm. The specific bite angle of the dppe ligand apparently leads to a destabilization of the Cu(d) orbitals involved in the MLCT, and also beneficially influences the Franck−Condon factors of that transition. The reason for the absence of this band in the trigonal NHC complex 6a is most likely due to symmetry restrictions; that is, the metal orbitals undergoing d→π* MLCT are lying in the C(NHC)−Cu−N2 plane (vide infra).
Figure 1. Molecular structures of the intermediate [Cu(MeCN)2(qybt)]PF6 (left) and the cation in 1b (right) in the solid state obtained by single-crystal X-ray diffraction. H atoms omitted for clarity. Thermal ellipsoids drawn at 50% probability.
Figure 2. Molecular structures of the cations in 2a (top), 3a (middle), and 6a (bottom) in the solid state obtained by single-crystal X-ray diffraction. H atoms omitted for clarity. Thermal ellipsoids drawn at 50% probability.
The phosphine complexes 1b, 2a, and 3a exhibit the expected distorted tetrahedral coordination geometry around the copper ion, in which the bite angle of the 2-(2′pyridine)benzthiazol (pybt) and 2-(2′-quinoline)benzthiazol (qybt) ligands of ca. 80° allow for a larger angle than the optimal 109.5° of a tetrahedron between the phosphine ligands. Specifically, the strongly σ-donating PMe3 ligands in 1a lead to a slightly larger P1−Cu−P2 angle of 117.06(2)°, while the tri(aryl)phosphines give smaller angles of only 115.70(3) and 115.16(5)° for 2a and 3a, respectively. Interestingly, the P1− Cu−P2 angles in the structurally related PPh3 complexes A, B, 8998
DOI: 10.1021/acs.inorgchem.7b00958 Inorg. Chem. 2017, 56, 8996−9008
Article
Inorganic Chemistry
the 1MLCT states, as related nitrogen analogues (A, C, J, and K)77,78 exhibit these absorptions hypsochromically shifted by ca. 30−50 nm. In solution, the emission is very weak and not detectable for 1b, 5a, and 6a, with their maxima ranging from λem = 524−760 nm (see Supporting Information). The broad red to near-IR emission is much more intense, but significantly hypsochromically shifted in 1% doped poly(methyl methacrylate) (PMMA) films (see Supporting Information) and in the solid state (Figure 4 and Table 2), except for 1a. For PMMA films, we
Figure 3. Absorption spectra in dichloromethane at room temperature of 1−6 with benzthiazol-2-pyridine (a, top) or benzthiazol-2-quinoline (b, bottom) ligands.
The 2-(2′-quinoline)benzthiazol (qybt) congeners 1b−5b generally show the same behavior, but the high- and the lowenergy absorption bands are bathochromically shifted by ca. 20−30 nm (Figure 3, bottom, and Table 2). In addition, the extinction coefficients are slightly higher presumably due to the larger conjugation of the quinoline system b compared to the pyridine-based ligand a, resulting in a larger absorption cross section. We note that introduction of the sulfur atom into the conjugated π-chromophore ligand results in energy lowering of
Figure 4. Emission spectra in the solid state at room temperature (solid) and at 77 K (dotted) of 1−6 with benzthiazol-2-pyridine (a, top) or benzthiazol-2-quinoline (b, bottom) ligands.
Table 2. Photophysical Data of 1−6 in Dichloromethane, PMMA, and Solid State under Argon PMMAa
CH2Cl2, 297 K λabsb/nm (ε/1 × 103 M−1 cm−1) 339 365 389 368
(13), 402 (2.3) (20), 439 (2.6) (2.3) (23), 417 (3.5)
524 668 718
757 733 593 626
752 724 644 644
solid, 77 K Φ
λemb/ nm
0.1 (42), 0.8 (58) 0.3 (64) 0.6 (36) 0.4 (38), 1.9 (62) 3.6
