Synthesis, Structures and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I) Com-plexesBenjamin Hupp, Carl Schiller,§ Carsten Lenczyk, Marco Stanoppi,‡ Katharina Ed-kins,† Andreas Lorbach,‡ and Andreas Steffen*Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.Phosphorescence – NIR – copper – sulfur – TADF

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 -chromophores. The compounds are highly colored due to their 1MLCT states absorbing between ca abs = 400-500 nm, with ligand localized 1ILCT states in the UV region. The relative shifts of the S0→S1 absorption correlate with the computed HOMO-LUMO 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 Cu I com-plexes, rare intense long-lived near-IR emission with max ranging from 593-757 nm, quantum yields of up to = 0.11 and lifetimes of several microseconds in the solid state as well as in PMMA films. Although a bathochromic shift of the emission is observed with max ranging from 639-812 nm and the lifetimes are greatly increased at 77 K, no clear indication for thermally activated delayed fluorescence (TADF) has been 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 analogs emit in the green to orange region of the electromagnetic spectrum. The photophysical results and conclusions have further been corroborated with DFT/TD-DFT calculations, confirming the nature of the excited states and also the trends of the redox potentials.

INTRODUCTIONThe development of luminescent materials emit-

ting in the near-IR (NIR) region of the electromag-netic spectrum is of great interest for future appli-cations 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 prob-lem of the energy-gap law, which states that the rate constant for non-radiative decay knr that re-duces the quantum yield becomes larger with decreasing energy difference between the emit-ting excited state and the ground state,7 the classes of compounds that research with regard to NIR emission has been focused on have spe-cific intrinsic limitations. For instance, organic NIR emitters usually fluoresce from their singlet ex-cited state S1 with very short lifetimes on the nanosecond timescale, and they are often prone to photo-degradation.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 im-mense flexibility to tune the photophysical prop-erties for a given application, and have also been considered as potential alternatives for NIR emis-sion with long lifetimes.3, 4, 22-24 Due to the lumi-nescence 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 S0S1, but is often used to describe the energy difference between the transitions S0S1 and T1S025). 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 700-1150 nm have been reported, with lifetimes on the microsecond timescale and for PBI-based triplet emitters unprecedented of up to 0.11 in solution.26 A breakthrough with regard to using complexes of 3d elements as NIR emit-

ters has been achieved by the groups of Heinze and Resch-Genger, who successfully designed the water-soluble and air-stable chromium(III) com-pound [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 unprecedented of 0.11 and 0.14 in H2O and D2O, respectively.27

In order to bypass metal centered d-d* states, which may lead to premature non-radiative 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 par-ticular 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 de-vices, ensuring short lifetimes and high .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 possibili-ties to influence the excited state properties.28, 30-37, 42, 44-54 Efficient deep-red to NIR phosphorescent emitters of d10 coinage metals are mainly based on cluster-like compounds, of which the nuclear-ity, structure and photophysical properties are difficult to control.30, 55-63 Molecular, and thus po-tentially easily tunable, soluble and even environ-ment-responsive, copper(I) complexes emitting efficiently at low energies are still rare, motivat-ing 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, phospho-les and thiophenes, contrasting significantly the higher energy emission of their carbon, nitrogen or oxygen congeners.64-72 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),53, 73-77 and we have thus chosen those for replacement of one nitrogen for sulfur, expecting low energy emission in the red to NIR region. Furthermore, we have modified the conjugation of the chro-mophore ligand to further control the excited state energy. Indeed, this strategy leads to emis-sion with max(em) ranging from 593-812 nm de-pending on the aggregation state, and the long-lived luminescence on the s timescale becomes very intense in the solid state with of up to 0.11.

Chart 1. Known green to orange emissive CuI complexes bearing 2-(2’-pyridine)imida-zole-type ligands.53, 73-77

RESULTS AND DISCUSSIONSynthesis 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 dichlorometh-ane at room temperature gives full conversion to the highly colored tetrahedral copper(I) com-plexes 1a,b-5a,b (Scheme 1). The pale yellow trigonal compounds 6a,b have been obtained from [CuCl(IDipp)] (IDipp = bis(2,6-di-isopropy-lphenyl)-imidazol-2-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 im-pure samples of 6b unsuitable for photophysical studies.

Scheme 1. Synthesis of cationic tetrahedral copper(I) phosphine complexes 1-5 and trig-onal copper(I) NHC compounds 6 bearing pybt (a) or qybt (b) ligands. DPEPhos = bis[(2-diphenylphosphino)phenyl] ether; dppe = 1,2-bis(diphenylphosphino)ethane.

All complexes were fully characterized by multi-nuclear 1H, 13C{1H}, 19F and 31P{1H} NMR spectro-scopic studies and either elemental analysis or high resolution-(HR)-ESI mass spectrometry, as for some compounds no satisfactory elemental analysis could be obtained on our instruments due to the fluorinated anions, even though crys-talline or even single-crystalline material was used. For those compounds used for photophysi-cal studies, HR-ESI mass spectra were always measured to unambiguously exclude minor impu-rities. In order to provide high purity samples for photophysical studies (vide infra), the products were also recrystallized several times. For com-pounds 1b, 2a, 3a and 6a, single-crystals suit-able for X-ray diffraction were obtained and con-firm their assumed molecular geometry (Figures 1 and 2, 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.

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. Interest-ingly, the P1-Cu-P2 angles in the structurally re-lated PPh3 complexes A, B, E, F, I, J and L are significantly larger with 123-131°.73-77 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 com-plexes.73-77 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 NHC complex 6a is best compared with [Cu(pybim)(IDipp)] (pybim = 2-(2’-pyridyl)benzimid-azole, M, Chart 1), which is also distorted from an ideal Y-shaped geometry due to the asymmetry of the bidentate -chromophore ligand.53 In addition, in both complexes the IDipp

ligand is coplanar with the N,N`-coordinating lig-and. 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),53 but within in the typical range as found for other CuI NHC complexes.44, 49, 50, 78, 79 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.53 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 bit more towards a T-shape geometry as observed, for example, in [Cu(tfppy)(IDipp)] (tfppy = 2-(2,3,4,5-tetrafluorophenyl)pyridyl).79

Table 1. Selected interatomic distances (Å) and Angles (deg) for the cations in 1b, 2a, 3a and 6a.

1b 2a 3a 6aCu-N1 2.089(2

)2.070(2)

2.070(2)

2.021(2)

Cu-N2 2.094(2)

2.091(2)

2.089(3)

2.150(2)

Cu-P1 2.2468(6)

2.2320(7)

2.2497(13)

Cu-P2 2.2313(6)

2.2486(7)

2.2418(14)

Cu-C1 1.906(3)

P1-Cu-P2

117.06(2)

115.70(3)

115.16(5)

N1-Cu-N2

78.87(8)

80.13(9)

79.37(11)

79.20(9)

C1-Cu-N1

152.67(11)

C1-Cu-N2

128.92(11)

Photophysical and Electrochemical Stud-

ies. The 2-(2’-pyridine)benzthiazol (pybt) com-plexes 1a-6a exhibit allowed high energy absorp-tions between abs = 300-350 nm with extinction coefficients at the respective maxima of = 14-18∙103 M-1 cm-1 (Figure 3, top, and Table 2), which we assign to mainly IL(-*) states localized at the pybt ligand. In contrast to the pale yellow trigonal NHC complex 6a, the tetrahedral phosphine com-pounds 1a-4a show an additional very weakly allowed ( < 2500 M-1 cm-1) and broad low energy band around 400-475 nm, presumably as a result of Cu→pybt MLCT transitions, giving them their intense yellow to orange colors. We note that this band is most intense ( = 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 influ-ences the Franck-Condon factors of that transi-tion. The reason for the absence of this band in the trigonal NHC complex 6a is most likely due to symmetry restrictions, i.e. the metal orbitals un-dergoing d→* MLCT are lying in the C(NHC)-Cu-N2 plane (vide infra).

The 2-(2’-quinoline)benzthiazol (qybt) con-geners 1b-5b generally show the same behavior, but the high and the low energy absorption bands are bathochromically shifted by ca 20-30 nm (Fig-ure 3, bottom, and Table 2). In addition, the ex-tinction coefficients are slightly higher presum-ably 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 -chromophore ligand results in energy lowering of the 1MLCT states, as related nitrogen analogs (A, C, J and K)76, 77 exhibit these absorptions hypsochromically shifted by ca. 30-50 nm.

Figure 3. Absorption spectra in dichloromethane at room temperature of 1-6 with benzthiazol-2-pyridine (a, top) or benzthiazol-2-quinoline (b, bottom) lig-ands.

In solution, the emission is very weak and not detectable for 1b, 5a and 6a, with their maxima ranging from em = 524-760 nm (see Supporting Information). The broad red to near-IR emission is much more intense, but significantly hyp-sochromically shifted in 1% doped PMMA films (see Supporting Information) and in the solid state (Figure 4 and Table 2), except for 1a. For PMMA films, we note that the specific interaction of the complexes with the polymer matrix ap-

pears to influence the emission maxima in such a way, that no clear trend is observed. In contrast, the solid state properties can nicely be rational-ized.

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.

The use of PMe3 ligands in 1a,b leads to very low emission energies at room temperature in the solid state with em = 752 and 724 nm, respec-tively, the origin of which may very well lie in a flattening distortion of the excited state as previ-ously described for many tetrahedral copper(I) complexes with ligands of little steric demand (vide infra).37, 80 This interpretation is supported by the observed higher emission energies of 2a,b-4a,b and 5a, with larger triarylphosphine and chelating dppe and DPEPhos ligands, respec-tively. Their rigidity also gives rise to = 0.05 and 0.06 for 3a and 4a, respectively, and even = 0.08 and 0.11 for 2b and 3b, respectively, which is unusually high for CuI complexes emit-ting in the red to near-IR. The luminescence de-cay times at room temperature are in the range of ca. = 1-9 s, indicative for 3MLCT states being involved.

All compounds show a bathochromic shift of the emission at 77 K, which is most pronounced for 1a,b, reaching values of 812 and 778 nm, respec-tively, and 3b. These changes suggest that an emission pathway via thermally activated delayed fluorescence (TADF) may be involved.29, 35, 43, 81 Indeed, the temperature-dependent lumines-cence decays of, e.g., [Cu(pybt)(DPEPhos)]PF6 (4a) show multiple emission with very long life-times at 77 K (Figure 5). However, the observa-

tion of three lifetimes is indicative for multiple emissive triplet states, presumably as a result of different conformers or specific intermolecular interactions in the solid state. The estimated en-ergy gap

Table 2. Photophysical data of 1-6 in dichloromethane, PMMA and solid state under argon.aThe respective maxima are given. bFor bi-exponential decays, the normalized pre-exponential factors are

given in brackets. cAt 297 K, doped with 1% of the respective copper(I) complex.

E(S1-T1) of 430 cm-1 from the observed bathochromic shift of the emission at 77 K is within the range for previously reported copper(I)-based TADF emitters.40-42, 44, 48, 50-52, 82-88 However, a deeper analysis is not justified due to the low quantum yields of most compounds, making the assignment of the pure triplet state lifetimes chal-lenging, in particular as the lifetime plateaus indi-cating 100% emission efficiency of the T1 state are not observed even at very low temperatures.

Interestingly, unlike for the absorption behavior, there is no clear trend between the two -chro-mophore ligands 2-(2’-pyridine)benzthiazol (pybt, a) and 2-(2’-quinoline)benzthiazol (qybt, b) with regard to their influence on the luminescence properties (Table 2). Whereas the larger conjuga-tion of qybt leads to a bathochromic shift for the phosphine complexes 2b-5b with regard to their pybt congeners, a slight blue-shift is observed for the PMe3 complex 1b. Also, the quantum yields are higher for 2b-4b than for 2a-4a, but in the case of 1 is lower with qybt.

The enormous influence of sulfur incorporation into the -chromophore ligands on the photophys-ical properties quickly becomes obvious by com-

paring the emission wavelengths of the previ-ously reported copper(I) complexes A-M (Chart 1, Table 3), which generally emit in the

Figure 5. Exemplary lifetime decays at room tem-perature (brown) and at 77 K (blue) of [Cu(pybt)(DPEPhos)]PF6 (4a) in the solid state.

CH2Cl2, 297 K PMMAc Solid, 297 K Solid, 77 Kabs /nm ( / 103 M-1 cm-1)a

em / nm a

em / nm a

em / nm a

/ sb em / nm a

/ sb

1a 312 (14), 325 (16), 339 (13), 402 (2.3)

524 757 752 0.1 (42), 0.8 (58)

< 0.01

812 n.d.

1b 292 (14), 349 (22), 365 (20), 439 (2.6)

- 733 724 0.3 (64) 0.6 (36)

< 0.01

778 8.1 (70), 12 (30)

2a 328 (12), 341 (11), 389 (2.3)

668 593 644 0.4 (38), 1.9 (62)

0.02 681 16 (98), 100 (2)

2b 294 (22), 354 (23), 368 (23), 417 (3.5)

718 626 644 3.6 0.08 685 41 (52), 108 (33), 222 (15)

3a 329 (18), 343 (16), 405 (2.7)

599 600 647 0.2 (12), 1.5 (47), 2.8 (41)

0.05 678 20 (92), 69 (8)

3b 295 (22), 354 (23), 368 (23), 430 (3.2)

738 681 655 0.5 (15), 2.4 (35), 5.6 (50)

0.11 718 37 (90), 87 (10)

4a 293 (21), 310 (19), 327 (16), 345 (8.8), 415 (2.4)

735 598 622 0.3 (21), 1.5 (38), 3.6 (41)

0.06 639 35 (62), 99 (32), 274 (6)

4b 287 (24), 353 (20), 368 (19), 457 (3.2)

730 625 672 0.3 (29), 1.4 (52), 3.5 (19)

0.03 700 36 (80), 135 (20)

5a 300 (17), 316 (19), 327 (19), 344 (13), 448 (4.1)

- 650 653 0.4 (77), 0.8 (23)

< 0.01

660 71 (86), 174 (14)

5b 295 (20.3), 354 (20.7), 365 (18.8), 486 (4.2)

760 665 727 n.d. < 0.01

749 n.d.

6a 333 (14), 350 (10), 390 (0.7)

- 683 683 0.2 (37), 0.6 (63)

0.02 689 0.6 (61), 1.5 (39)

green to orange, with those of 2a,b-5a,b and 6a, which are drastically bathochromically shifted into the red to near-IR. For example, the direct nitro-gen congener of 3a, namely J, shows triplet emis-sion in the solid state that is blue-shifted by 133 nm to em = 514 nm,77 while the difference be-tween 3b and A in PMMA (681 vs. 570 nm) is also more than 100 nm.76 A similar effect is also found for the DPEPhos compounds 4a (em(solid) = 622 nm) and 4b (em(PMMA) = 625 nm), and the dppe compounds 5a (em(solid) = 653 nm) and 5b (em(PMMA) = 665 nm), when compared with K (em(solid) = 521 nm)77 and C (em(PMMA) = 579 nm),76 respectively. Even the emission of the NHC complex 6a of em = 683 nm in PMMA is greatly shifted to much lower energy compared to [Cu(IDipp)(pybz)] (M), which exhibits its maxi-mum at em = 550 nm.53

Table 3. Emission wavelengths of known CuI

complexes with 2-(2’-pyridine)imidazole-type ligands at 297 K.53, 73-77

Compound em / nm (medium)[Cu(Hqybz)(PPh3)2]BF4 (A)a 570 (PMMA), 622

(CH2Cl2)[Cu(qybz)(PPh3)2] (B)b 559 (PMMA), 596

(CH2Cl2)[Cu(Hqybz)(DPEPhos)]BF4 (C) a

579 (PMMA), 638 (CH2Cl2)

[Cu(qybz)(DPEPhos)] (D)b 564 (PMMA), 612 (CH2Cl2)

[Cu(pyin)(PPh3)2] (E)c 562 (PMMA)[Cu(quin)(PPh3)2] (F)d 608 (PMMA)[Cu(pyin)(DPEPhos)] (G)c 566(PMMA)[Cu(quin)(DPEPhos)] (H)d 610(PMMA)[Cu(qbo)(PPh3)2]PF6 (I)e 560, 570, 600 (solid)[Cu(Hpybz)(PPh3)2]ClO4 (J)f 514 (solid), 562

(CH2Cl2)[Cu(Hpybz)(dppe)]ClO4 (K)f 521 (solid), 602

(CH2Cl2)[Cu(pyim)(PPh3)2]BF4 (L)g 625 (CH2Cl2)[Cu(IDipp)(pybz)] (M)h 550 (PMMA), 605

(CH2Cl2)aHqybz = 2-(2’-quinoline)benzimidazole. bqybz = 2-(2’-quino-line)benzimidazolyl. cpyin = 2-(2’-pyridine)indolyl. dqyin = 2-(2’-quinoline)indolyl. eqbo = 2-(2’-quinoline)benzoxazole. fHpybz = 2-(2’-pyri-dine)benzimidazol. gpyim = 2-(2’-pyridine)imida-zolyl. hpybz = 2-(2’-pyridine)benzimidazolyl.

The cyclic voltammetry measurements show that the electrochemical behavior is dominated by irreversible one-electron reductions and irre-versible one-electron oxidations (Table 4 and Fig-ure 6). The only exception is [Cu(pyb)(DPEPhos)]PF6 (4a), which exhibits reversible one-

electron oxidation and reduction waves. In gen-eral, the first and second oxidation peak poten-tials Epa(1) and Epa(2), respectively, are greatly influenced by the -donor strength of the phos-phine ligands and by the P-Cu-P angle, while changing from pybt to qybt generally has only a negligible influence. Consequently, the lowest oxidation peak potentials are found for the dppe complexes 5a,b and the PMe3 compounds 1a,b. Despite the very strong -donor properties of the NHC, complex 6a shows a rather high oxidation potential, which may be due to the different coor-dination geometry compared to the tetrahedral phosphine compounds leading to a

Figure 6. Cyclic voltammograms (CV) of 1a,b-5a,b and 6a in CH2Cl2/0.1 M [n-Bu4N][PF6] relative to the Fc/Fc+ couple. Inset: reversible CV of 4a.

decreased stability of the resulting dication. These findings suggest that the HOMO is mainly metal centered.

In contrast to Epa, the first and second reduction peak potentials Epc(1) and Epc(2), respectively, are dominated by the -chromophore ligand, as the larger conjugation of the 2-(2’-quinoline)benzthia-zol (b) compared to the 2-(2’-pyridine)benzthiazol (a) ligand gives rise to a much lower potential by ca. 0.2 V. The phosphine ligands in, e.g., 1b, 3b and 4b barely change the reduction peak poten-tials. Thus, the LUMO is supposed to be located at the pybt and qybt ligand, respectively, which fur-ther supports the assignment of the observed lowest energy absorption bands being of MLCT character.

Table 4. Cyclic voltammetry data of 1a,b-5a,b and 6a in CH2Cl2/0.1 M [n-Bu4N][PF6] relative to the Fc/Fc+ couple.

Epa(1) / V Epa(2) / V

Epc(1) / V Epc(2) / V

1a 0.69 0.95 -2.43 -1b 0.79 1.08 -1.68 -2.162a 0.95 1.23 -1.77 -2.412b 1.02 - -1.58 -2.153a 0.87 1.20 -1.79 -2.403b 0.92 1.24 -1.58 -2.174a 0.86 1.27 -1.85 -2.374b 0.99 1.30 -1.68 -5a 0.68 - -1.83 -2.355b 0.30 0.68 -1.64 -2.186a 1.17 - -1.79 -2.39

Thermal and Optical Stability. For future appli-cations of copper(I) complexes as NIR emitters in devices, thermal and optical stability are impor-tant factors. We have thus studied the thermal decomposition and performed differential scan-ning calorimetry (DSC) measurements of 1a,b-5a,b and 6a (see also Supporting Information). Whereas 2a,b, 5a,b and 6a are stable in the solid state beyond 250 °C, low thermal stability is observed for 1a and 1b (Tdec = 136 and 171 °C), but much higher decomposition temperatures are found for 3a,b (205, 225 °C) and 4a,b (243, 206 °C). The DSC studies were carried out for 3h at 120 °C, showing no phase transformations or decomposition under these conditions except for 1a, which starts to decompose after ca. 20 min-utes. We have also studied the influence of pro-longed times of exposure to sunlight in dichloro-methane at low concentrations (ca. 10-6 M) by absorption spectroscopic studies (see Supporting Information). Complexes 1b, 3a, 4b and 6a proved to be stable over 50 hours, while com-pound 1a shows minimal decomposition within

that timeframe, and 3b and 4a undergo pho-tolytic transformations after 20 hrs. Interestingly, the PPh3 (2a,b) and dppe (5a,b) complexes are only moderately stable under photolytic condi-tions in solution and decompose < 4 hrs.

Theoretical Studies. In order to shed more light on the optical and electrochemical proper-ties of the tetrahedral phosphine and trigonal NHC copper(I) compounds described above, we performed DFT and TD-DFT studies for all of them (see Supporting Information), of which the discus-sion we focus on 1a,b and 6a,b as representative examples for the tetrahedral and trigonal com-plexes, respectively. Despite no photophysical data are available for 6b, we have included that compound in the theoretical discussion for com-parison.

For all compounds, HOMO to HOMO-2 are mainly of Cu d character (Figure 7, blue), with some P or C(NHC) contributions, and the HOMO is well sepa-rated in energy from the lower lying occupied orbitals as a consequence of it being of * charac-ter with respect to the Cu-P and Cu-N bonds in the phosphine and NHC complexes, respectively (Figures 7 and 8). Below HOMO-2, the pybt/qybt orbital (green) and the benzothiazole orbitals (orange) are found. The LUMO (red) is mainly lo-calized at the thiazole and pyridine or quinoline moiety, and also well separated in energy from the higher lying unoccupied orbitals due to incor-poration of the sulfur.

We note that the HOMO-LUMO gaps of the qybt complexes 1b, 3b, 4b, and 6b are smaller by ca. 0.15-0.30 eV compared to their pybt analogs mainly due to a stabilization of the LUMO as a result of the larger conjugation of the quinoline-type chromophore ligand. In addition, the ex-tended conjugation also leads to the chro-mophore ligand orbital being more destabilized in the qybt

Figure 7. Frontier MO diagram showing the two lowest unoccupied (red) and highest occupied or-bitals (colored according to their main components) of 1a,b, 3a,b, 4a,b, and 6a,b and the respective HOMO-LUMO gap (D3-PBE0/def2-TZVP/ZORA).

Table 5. TD-DFT results for the cations of 1a,b and 6a,b in the gas phase (D3-PBE0/def2-TZVP/ZORA).

State

Energy /ev (nm)

Transitions (%)

f Nature State

Energy /ev (nm)

Transitions (%)

f Nature

1a

T1 2.39 (519)

H→L (95) - MLCT 1b

T1 2.17 (571)

H→L (96) - MLCT

S1 2.83 (438)

H→L (96) 0.114

MLCT S1 2.62 (473)

H→L (93) 0.086

MLCT

S6 4.04 (307)

H-4→L (51)H-3→L (29)

0.256

ILCT/ MLCT

S5 3.70 (335)

H-3→L (44)H→L+1 (19)

0.140

MLCT/ ILCT

S9 4.23 (293)

H-5→L (63)H-3→L (27)

0.223

ILCT/ MLCT

S6 3.86 (321)

H-4→L (65) 0.344

ILCT

6a

T1 2.43 (510)

H→L (95) - MLCT 6b

T1 2.18 (569)

H→L (95) - MLCT

S1 2.55 H→L (97) 0.00 MLCT S1 2.30 H→L (97) 0.00 MLCT

(485) 1 (539) 2S9 3.99

(310)H-6→L (80) 0.13

0ILCT S11 3.99

(331)H-7→L (38)H-8→L (30)

0.252

ILCT/ MLCT

S10 4.05 (307)

H-9→L (34)H-5→L (25)

0.251

ILCTS12 4.05

(330)H-10→L (49)H-4→L (27)

0.197

ILCT/ MLCT

Figure 8. HOMO (left) and LUMO (right) of 1a,b and 6a,b (D3-PBE0/def2-TZVP/ZORA).

complexes compared to 1a, 3a, 4a and 6a (Fig-ure 7). The HOMO-LUMO gap trend correlates nicely with the observed general trend of the ab-sorption spectra (Figure 3), as the lowest excited singlet state S1 is described as a HOMO→LUMO transition (Table 5). This 1MLCT state has signifi-cant oscillator strength f for the phosphine com-plexes due to the HOMO being of symmetry with respect to the ligand-based LUMO, giving compounds 1-5 their intense colors. In contrast, the low-energy HOMO→LUMO transition in the NHC complexes 6a,b is symmetry forbidden, and thus that band has literally no oscillator strength to be observed in the experimental absorption spectrum of 6a (Figure 3). The observed high en-ergy absorption bands in the phosphine com-plexes and 6a are indeed intra-ligand CT states with some MLCT admixture.

The lowest energy triplet excited state T1 is of 3MLCT nature for all compounds, originating from

a HOMO-LUMO transition, giving rise to the ob-served broad emission (Tables 2 and 5, and Figure 4). Furthermore, the optimized geometries of the T1 states of 1a and 1b indeed show a severe flat-tening distortion compared to the tetrahedral ground state geometry (Figure 9), which explains why their emission reaches so far out into the near-IR, while the bulkier phosphine ligands in 2a,b-5a,b lead to a more rigid structure and thus to less geometric changes in the relaxation process upon photoexcitation in PMMA or in the solid state (vide supra), although 5b shows an unexpected low energy emission maximum of 727 nm in the solid.

Figure 9. Geometries of the ground state S0 (blue) and triplet excited state T1 (orange) of [Cu(pybt)(PMe3)2]PF6 (1a) and [Cu(qybt)(PMe3)2]PF6 (1b) show-ing the flattening distortion upon relaxation (D3-PBE0/def2-TZVP/ZORA).

CONCLUSIONSWe have reported on the synthesis and photo-

physical investigation of a series of trigonal NHC and tetrahedral phosphine copper(I) compounds with 2-(2’-pyridine)benzthiazol (pybt) or 2-(2’-quinoline)benzthiazol (qybt) as -chromophore ligands. The incorporation of the sulfur into the conjugated -ligand system leads to red to near-IR emission of the CuI complexes with maxima between max = 593-757 nm, while their nitrogen analogues exhibit green to orange emission.53, 73-77 Although the luminescence is weak in solution, it becomes for near-IR emitting CuI complexes, which are very rare, unusually intense with quan-tum yields of up to = 0.11 and long-lived with several microseconds in the solid state. Our low temperature measurements show an increase in lifetime and a bathochromic shift of the emission of the phosphine complexes at 77 K compared to RT with max ranging from 639-812 nm, but no clear evidence for TADF could be obtained, leav-ing us to assign the emission to originate from a 3(Cu→pybt/qybt)MLCT state, which is supported

by our DFT and TD-DFT studies. The low-energy emission is presumably a result of a flattening distortion in the excited state, as a hypsochromic shift is observed when the very rigid chelating DPEPhos is used compared to those CuI com-plexes with less rigid chelating dppe or with mon-odentate PR3 (R = Me/Ph/o-tol) ligands. However, the specific P-Cu-P angles as well as the -donor strength of the ligands also play a role, as these factors determine the energy of the metal d or-bitals and inevitably the energy of the 3MLCT states. We are currently exploring ways to de-crease the non-radiative rate constant or to in-crease the radiative one in order to obtain higher quantum yields for future applications exploiting near-IR emitters.

EXPERIMENTAL SECTIONGeneral Conditions. All starting materials

were purchased from commercial sources and were used without further purification, except for the ligands benzthiazol-2-pyridine (pybt) and ben-zthiazol-2-quinoline (qybt), which were prepared according to literature reports.89-91 The organic solvents for synthetic reactions and for photo-physical studies were of HPLC grade, further treated to remove trace water using an Innova-tive Technology Inc. Pure-Solv Solvent Purification System and deoxygenated by purging with Argon. All synthetic reactions were performed in an Inno-vative Technology Inc. glovebox or under an ar-gon atmosphere using standard Schlenk tech-niques. 1H, 11B, 13C{1H}, 19F, and 31P NMR spectra were measured either on a Bruker Avance 500 (1H, 500 MHz; 13C, 125 MHz; 19F, 470 MHz; 31P 202 MHz) or on a Bruker Advance 200 (1H, 200 MHz; 19F, 188 MHz) NMR spectrometer. HRMS were recorded on the Thermo Scientific Exactive Plus Orbitrap MS system with Electrospray Ionization (ESI). Elemental analyses were performed on an Elementar vario MICRO cube elemental analyzer. Differential scanning calorimetry (DSC) was per-formed with a DSC 204 F1 Phoenix (Netzsch) in the temperature range of 20−120 °C. Samples were heated with a heating rate of 25 K min−1 and kept at 120 °C for 3h with a constant gas flow of 40 mL min−1 N2. The temperature of decomposi-tion was determined with an SRS OptiMelt sys-tem. Samples were heated with a heating rate of 10 K min-1 up to 250 °C, decomposition starts at the given temperature (Tdec).

Photophysical measurements. UV-visible absorption spectra were obtained on an Agilent 1100 Series Diode Array spectrophotometer using standard 1 cm path length quartz cells. Excitation and emission spectra were recorded on an Edin-burgh Instrument FLSP920 spectrometer, equipped with a 450 W Xenon arc lamp, double monochromators for the excitation and emission pathways, and a red-sensitive photomultiplier (PMT-R928) and a near-IR PMT as detectors. The

excitation and emission spectra were corrected using the standard corrections supplied by the manufacturer for the spectral power of the excita-tion source and the sensitivity of the detector. The quantum yields were measured by use of an integrating sphere with an Edinburgh Instrument FLSP920 spectrometer. The luminescence life-times were measured either using a μF900 pulsed 60 W Xenon microsecond flashlamp, with a repeti-tion rate of 100 Hz, and a multichannel scaling module, or with a TCSPC module operating with pulsed laser diodes (pulse width ca. 200 ps, in-strument response function ca. 800 ps). The emission was collected at right angles to the exci-tation source with the emission wavelength se-lected using a double grated monochromator and detected by the respective PMT. Low temperature measurements were performed in an Oxford Opti-stat cryostat.

Cyclic voltammetry measurements. Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600 potentiostat. A standard three-electrode cell configuration was employed using a platinum disk working elec-trode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip, serving as the reference electrode. Formal redox potentials are referenced to the ferrocene/ferrocenium ([Cp2Fe]+/0) redox couple by using decamethylfer-rocene as an internal standard. Tetra-n-butylam-monium hexafluorophosphate ([n-Bu4N][PF6]) was employed as the supporting electrolyte. Compen-sation for resistive losses (iR drop) was employed for all measurements.

X-ray Crystallography. Crystals suitable for single-crystal X-ray diffraction were selected, coated in perfluoropolyether oil, and mounted on MiTeGen sample holders. Diffraction data were collected on a Nonius Kappa three circle diffrac-tometer utilizing graphite monochromated MoKα radiation (λ = 0.71073 Å) from a rotating anode tube run at 50 V and 30 mA. The diffractometer is equipped with a Bruker ApexII area detector and an open flow N2 Cryoflex II (Bruker) device, the measurement was performed at 100 K. For data reduction, the Bruker Apex2 software suite (Bruker AXS) was used. Subsequently, utilizing Olex292 or ShelX93 the structures were solved us-ing the Olex2.solve94 charge-flipping algorithm or ShelXT,95 and were subsequently refined with Olex2.refine94 using Gauss-Newton minimization or ShelXle. All non-hydrogen atom positions were located from the Fourier maps and refined aniso-tropically. Hydrogen atom positions were calcu-lated using a riding model in geometric positions and refined isotropically, where possible to deter-mine unambiguously.

Computational Details. Calculations (gas-phase) were performed with the ORCA 3.0.2 pro-gram suite.96 Geometry optimizations were car-ried out with the PBE097-103 functional as imple-mented in ORCA, and a frequency analysis ensur-

ing that the optimized structures correspond to global energy minima. The def2-TZVP104, 105 basis set was used for all atoms together with the auxil-iary basis set def2-TZVP/J in order to accelerate the computations within the framework of RI ap-proximation. Relativistic effects of the Cu(I) com-plexes were accounted for by employing the ZORA106 method, and van der Waals interactions have been considered by an empirical dispersion correction (Grimme-D3BJ).107, 108 TD-DFT calcula-tions for the first 50 singlet and triplet excited states were performed with the same functional. Representations of molecular orbitals were pro-duced with orca_plot as provided by ORCA 3.0.2 and with gOpenMol 3.00.109, 110

Synthesis. General procedure for the prepara-tion of the phosphine complexes 1-5. [Cu(NCMe)4]PF6 (200 mg, 0.54 mmol) and pybt (115 mg, 0.54 mmol) or qybt (142 mg, 0.54 mmol) were dissolved in 15 mL dichloromethane under rigorous stirring for 1 h at room tempera-ture, giving a yellow colored solution. The addi-tion of the appropriate phosphine ligand (in a molar ratio of 1:2 for monodentate phosphines and equimolar ratio for chelating phosphines) dissolved in 10 mL dichloromethane led to a color change to yellow-orange. After stirring the reac-tion mixture for 1 h, in situ NMR spectroscopic measurements confirmed full conversion to the desired products. The volatiles were removed in vacuo and the resulting solid was washed three times with n-hexane and diethylether. In order to remove traces of grease and to obtain high purity samples for photophysical studies, the crude product was recrystallized several times from dichloromethane using diethylether as the anti-solvent via gas diffusion. The following yields thus represent the amount of pure crystalline or sin-gle-crystalline material.

[Cu(pybt)(PMe3)2]PF6 (1a). Orange crystals (178 mg, 53%). 1H-NMR (500 MHz, CDCl3) = 8.92 (d, JHH = 5 Hz, 1H, pyridine), 8.15 (m, JHH = 6 Hz, 2H, pyridine), 8.07 (d, JHH = 8 Hz, 1H, benzothiazole), 8.02 (d, JHH = 8, 1H, benzothiazole), 7.76 (m, JHH = 7, JHH = 3 Hz, JHH = 5 Hz, 1H, pyridine), 7.71 (dd, JHH = 8 Hz, 1H, benzothiazole), 7.60 (dd, JHH = 8 Hz, 1H, benzothiazole), 1.16 (br. s, 18, PMe3) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 164.5 (s), 150.8 (s), 150.6 (s), 147.7 (s), 139.0 (s), 134.6 (s), 128.5 (s), 127.9 (s), 127.8 (s), 124.2 (s), 123.2 (s), 122.7 (s), 15.5 (t) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = -47.0 (br. s, 2P, PMe3), -144.2 (sep, 1JPF = 708 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -72.9 (d, 1JPF = 708 Hz, PF6) ppm. HRMS (ESI) m/z: [1a-PF6]+ calcd for C18H26CuN2P2S 427.0582, found 427.0589; [1a-PF6-PMe3]+ calcd for C15H17CuN2PS 351.0141, found 351.0142; [1a-PF6-pybt]+ calcd for C6H18CuP2 215.0174, found 215.0177. Tdec = 136 °C.

[Cu(qybt)(PMe3)2]PF6 (1b). Red crystals (298 mg, 88%). 1H-NMR (500 MHz, CDCl3) = 8.65 (d, JHH =

9 Hz, 1H, quinoline), 8.37 (d, JHH = 9 Hz, 1H, quinoline), 8.30 (d, JHH = 8 Hz, 1H, quinoline), 8.21 (d, JHH = 8 Hz, 2H, benzothiazole), 8.12 (d, JHH = 9 Hz, 1H, quinoline), 8.02 (m, 1H, quinoline), 7.80 (m, 1H, quinoline), 7.73 (m, 1H, benzothiazole), 7.64 (m, 1H, benzothiazole), 1.17 (s, 18H, PMe3) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 164.9 (br.), 151.4 (br.), 148.6 (br.), 146.4 (s), 139.1 (s), 135.3 (s), 131.9 (s), 129.4 (s), 129.0 (s), 128.7 (s), 128.6 (s), 128.0 (s), 127.4 (s), 123.2 (s), 123.1 (s), 122.0 (s), 120.0 (s), 14.5 (s) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = -47.1 (br. s, 2, PMe3), -144.6 (sep, 1JPF = 708 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -72.9 (d, 1JPF = 708 Hz, PF6) ppm. HRMS (ESI) m/z: [1b-PF6]+ calcd for C22H28CuN2P2S 477.0739, found 477.0734; [1b-PF6-PMe3]+ calcd for C19H19CuN2PS 401.0297, found 401.0292; [1b-PF6-qybt]+ calcd for C6H18CuP2 215.0174, found 215.0172. Anal. Calcd for C22H28CuF6N2P3S: C, 42.41; H, 4.53; N, 4.50; S, 5.15. Found: C, 42.34; H, 4.53; N, 4.31; S, 4.88. Tdec = 171 °C.

[Cu(pybt)(PPh3)2]PF6 (2a). Yellow crystals (165 mg, 27%).

1H-NMR (500 MHz, CDCl3) = 8.30 (d, JHH = 5 Hz, 1H, pyridine), 8.14 (m, 2H, pyridine), 8.00 (d, JHH = 8 Hz, 1H, benzothiazole), 7.54 (ddd, JHH = 5 Hz, JHH = 2 Hz, 1H, pyridine), 7.48 (m, 2H, benzothia-zole), 7.36 (m, 6H PPh3, 1H benzothiazole), 7.19 (dd, JHH = 7 Hz, 12H, PPh3), 7.08 (d, JHH = 7 Hz, 12H, PPh3) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.1 (t, J = 4 Hz), 149.7 (s), 149.6 (s), 147.5 (m), 139.9 (s), 134.5 (s), 132.9 (t, J = 8 Hz), 131.6 (s), 131.5 (s), 131.3 (s), 130.5 (s), 129.0 (t, J = 5 Hz), 127.9 (s), 127.6 (s), 127.5 (s), 125.1 (s), 123.1 (s), 122.5 (s) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = 2.5 (br. s, 2, PPh3), -144.2 (sep, 1JPF = 711 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -73.5 (d, 1JPF = 711 Hz, PF6) ppm. HRMS (ESI) m/z: [2a-PF6]+ calcd for C48H38CuN2P2S 799.1521, found 799.1508; [2a-PF6-pybt]+ calcd for C36H30CuP2 587.1113, found 587.1106. Anal. Calcd for C48H38CuF6N2P3S: C, 60.98; H, 4.05; N, 2.96; S, 3.39. Found: C, 60.98; H, 3.98; N, 3.14; S, 3.04.

[Cu(qybt)(PPh3)2]PF6 (2b). Orange crystals (143 mg, 28%). 1H-NMR (500 MHz, CDCl3) = 8.69 (d, JHH = 7 Hz, 1H, quinoline), 8.27 (d, JHH = 8 Hz, 1H, quinoline), 8.03 (d, JHH = 8 Hz, 1H, quinoline), 7.98 (d, JHH = 8 Hz, 1H benzothiazole), 7.86 (br. s, 1H, quinoline), 7.58 (dd, JHH = 7 Hz, JHH = 8 Hz, 1H benzothiazole), 7.48 (m, 1H, quinoline), 7.35 (dd, JHH = 7 Hz, JHH = 8 Hz, 6H, PPh3), 7.30 (m, 1H quinoline, 1H benzothiazole), 7.14 (dd, JHH = 7 Hz, JHH = 8 Hz, 1H benzothiazole, 12H PPh3), 7.06 (d, JHH = 7 Hz, 12H, PPh3) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.9 (s), 150.4 (s), 148.2 (s), 145.8 (s), 140.9 (s), 135.4 (s), 133.2 (s), 131.7 (s), 131.3 (s), 130.6 (s), 129.8 (s), 129.3 (s), 129.2 (s), 127.7 (s), 127.6 (s), 123.2 (s), 123.1 (s), 121.1 (s) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = 1.8 (br. s, 2, PPh3), -144.1 (sep, 1JPF = 714 Hz,

1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -72.9 (d, 1JPF = 714 Hz, PF6) ppm. HRMS (ESI) m/z: [2b-PF6]+ calcd for C52H40CuN2P2S 849.1678, found 849.1687; [2b-PF6-qybt]+ calcd for C36H30CuP2 587.1113, found 587.1106. Anal. Calcd for C52H40CuF6N2P3S: C, 62.74; H, 4.05; N, 2.81; S, 3.22. Found: C, 63.49; H, 4.22; N, 3.12; S, 3.12.

[Cu(pybt){P(p-tol3)}2]PF6 (3a). Yellow crystals (400 mg, 72%). 1H-NMR (500 MHz, CDCl3) = 8.60 (br. s, 1H, pyridine), 8.15 (br. s, 2H, pyri-dine), 8.01 (br. d, JHH = 8, 1H, benzothiazole), 7.41 (br. m, 1H pyridine, 2H benzothiazole), 6.97 (br. s, 24H, P(p-C6H4CH3)3), 2.32 (s, 18H, P(p-C6H4CH3)3) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.3 (br.), 155.3 (br.), 150.0 (br.), 148.0 (s), 140.7 (br.), 140.0 (br.), 134.8 (s), 133.1 (br.), 129.8 (br.), 129.0 (br.), 127.6 (br.), 125.0 (s), 123.0 (br.), 122.9 (br.), 122.8 (br.), 21.5 (s) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = 0.8 (br. s, 2, PTol3), -144.2 (sep, 1JPF = 711 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -73.0 (d, 1JPF = 711 Hz, PF6) ppm. HRMS (ESI) m/z: [3a-PF6]+ calcd for C54H50-CuN2P2S 883.2460, found 883.2461; [3a-PF6-pybt]+ calcd for C42H42CuP2 671.2052, found 671.2055; [3a-PF6-PTol]+ calcd for C33H29CuN2PS 579.1080, found 579.1084. Calcd for C54H50CuF6N2P3S∙0.5 CH2Cl2: C, 61.06; H, 4.80; N, 2.61; S, 2.99. Found: C, 61.30; H, 4.43; N, 2.95; S, 3.05. Tdec = 205 °C.

[Cu(qybt){P(p-tol3)}2]PF6 (3b). Orange crystals (385 mg, 66%). 1H-NMR (500 MHz, CDCl3) = 8.69 (d, JHH = 8 Hz, 1H, quinoline), 8.26 (d, JHH = 8 Hz, 1H, quinoline), 8.02 (d, JHH = 8 Hz, 1H, quino-line), 7.99 (d, JHH = 8 Hz, 1H benzothiazole), 7.88 (d, JHH = 9 Hz, 1H, quinoline), 7.59 (t, JHH = 7 Hz, 1H, benzothiazole), 7.49 (t, JHH = 7 Hz, 1H, quino-line), 7.30 (m, 1H quinoline, 2H benzothiazole), 6.92 (s, 24H, P(p-C6H4CH3)3), 2.30 (s, 18H, P(p-C6H4CH3)3) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.8 (br.), 157.3 (br.), 150.4 (s), 148.1 (s), 145.8 (s), 140.7 (s), 135.3 (s), 133.2 (s), 131.2 (br.), 129.9 (br.), 129.7 (s), 129.2 (s), 128.9 (br.), 127.6 (s), 127.5 (br.), 123.3 (s), 123.0 (s), 121.0, 21.4 (s) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = 0.0 (br. s, 2, PTol3), -144.1 (sep, 1JPF = 711 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -73.9 (d, 1JPF = 711 Hz, PF6) ppm. HRMS (ESI) m/z: [3b-PF6]+ calcd for C58H52CuN2P2S 933.2617, found 933.2618; [3b-PF6-qybt]+ calcd for C42H42CuP2 671.2052, found 671.2054; [3b-PF6-PTol]+ calcd for C37H31CuN2PS 629.1236, found 629.1241. Anal. Calcd for C58H52CuF6N2P3S: C, 64.53; H, 4.86; N, 2.59; S, 2.97. Found: C, 64.38; H, 4.50; N, 2.61; S, 2.60. Tdec = 225 °C.

[Cu(pybt)(DPEPhos)]PF6 (4a). Orange crystals (425 mg, 82%). 1H-NMR (500 MHz, CDCl3) = 8.55 (d, JHH = 5 Hz, 1H, pyridine), 8.01 (m, 2H, pyridine), 7.94 (d, JHH = 8 Hz, 1H, benzothiazole), 7.53 (m, 1H, benzothiazole), 7.40 (dd, JHH = 8 Hz, JHH = 2 Hz, 1H, pyridine), 7.33-6.70 (m, 28H DPEPhos, 1H benzothiazole) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.0 (t, J = 4 Hz), 158.6 (t, J = 6 Hz), 150.2 (s), 150.1 (s), 147.6 (s), 139.4

(s), 134.5 (s), 134.4 (s), 133.8 (br. s), 133.2 (t, J = 8 Hz), 132.7 (t, J = 8 Hz), 132.2 (s), 130.8 (t, J = 17 Hz), 130.6 (s), 130.4 (t, J = 17 Hz), 130.1 (s), 129.2 (t, J = 6 Hz), 128.9 (t, J = 6 Hz), 128.0 (s), 127.5 (s), 127.4 (s), 125.3 (m), 124.5 (s), 124.3 (s), 124.2 (s), 124.1 (s), 122.8 (s), 122.7 (s), 120.4 (s) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = -11.0 (br. s, 2, DPEPhos), -144.2 (sep, 1JPF = 711 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -73.1 (d, 1JPF = 711 Hz, PF6) ppm. HRMS (ESI) m/z: [4a-PF6]+ calcd for C48H36CuN2OP2S 813.1314, found 813.1317; [4a-PF6-pybt]+ calcd for C36H28CuP2 601.0906, found 601.0908. Tdec = 243 °C.

[Cu(qybt)(DPEPhos)]PF6 (4b). Orange crystals (431 mg, 79%). 1H-NMR (500 MHz, CDCl3) = 8.57 (d, JHH = 5 Hz, 1H, quinoline), 8.12 (d, JHH = 8 Hz, 1H, quinoline), 8.02 (m, 1H quinolone, 1H benzothiazole), 7.92 (d, JHH = 8 Hz, 1H, benzothia-zole), 7.82 (d, JHH = 9 Hz, 1H, quinoline), 7.49 (m, 1H benzothiazole, 1H quinoline), 7.32 (m, 1H quinoline, 1H benzothiazole), 7.33-6.70 (m, 28H DPEPhos) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.6 (t, J = 4 Hz), 158.8 (t, J = 6 Hz), 150.3 (s), 147.8 (t, J = 2 Hz), 145.8 (s), 140.2 (s), 135.0 (s), 133.5 (t, J = 8 Hz), 132.5 (s), 132.4 (t, J = 8 Hz), 131.0 (s), 130.8 (dt, J = 17 Hz, J = 4 Hz), 130.5 (s), 130.0 (s), 129.6(s), 129.1 (s), 128.9 (t, J = 6 Hz), 128.8 (s), 128.7 (t, J = 6 Hz), 128.6 (s), 127.7 (s), 127.6 (s), 125.5 (m), 124.7 (s), 124.5 (s), 124.4 (s), 123.1 (s), 122.8 (s), 120.6 (s), 120.2 (m) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = -11.0 (br. s, 2, DPEPhos), -144.2 (sep, 1JPF = 711 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -73.0 (d, 1JPF = 711 Hz, PF6) ppm. HRMS (ESI) m/z: [4b-PF6]+ calcd for C52H38CuN2OP2S 863.1471, found 863.1469; [4b-PF6-qybt]+ calcd for C36H28CuP2 601.0906, found 601.0906. Calcd for C52H38CuF6N2OP3S: C, 61.87; H, 3.79; N, 2.78; S, 3.18. Found: C, 62.01; H, 3.61; N, 2.71; S, 2.81. Tdec = 206 °C.

[Cu(pybt)(dppe)]PF6 (5a). Orange crystals (423 mg, 95%). 1H-NMR (500 MHz, CDCl3) = 8.50 (br. s, JHH = 3 Hz, 1H, pyridine), 8.22 (d, JHH = 8 Hz, 1H, pyridine), 8.13 (ddd, JHH = 8 Hz, 1H, pyridine), 8.00 (d, JHH = 8, 1H, benzothiazole), 7.65 (dd, JHH = 5 Hz, 1H, pyridine), 7.49 (m, JHH = 9 Hz, JHH = 8 Hz, 1H, benzothiazole), 7.36 (m, 1H pyridine, 1H ben-zothiazole, 2H phenyl), 7.29 (br. s, 2H, phenyl), 7.22 (br. s, 6H, phenyl), 7.17 (m, 6H, phenyl), 7.10 (br. s, 4H, phenyl), 2.70 (br. s, 4H, P-CH2), 2.35 (m, 2H, P-CH2CH2) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 164.9 (s), 150.7 (s), 150.0 (s), 148.1 (s), 139.6 (s), 134.5 (s), 133.4 (s), 132.6 (s), 132.0 (t), 130.5 (s), 130.4 (br. s), 130.3 (br. s), 129.3 (s), 129.0 (s), 127.9 (s), 127.8 (s), 127.7 (s), 124.8 (s), 123.0 (s), 122.9 (s), 28.9 (t), 28.0 (s), 20.0 (t) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = -12.7 (br. s, 2P, dppe), -144.1 (qi, 1JPF = 713 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -72.9 (d, 1JPF = 713 Hz, PF6) ppm. HRMS (ESI) m/z: [5a-PF6]+ calcd for C38H32CuN2P2S 673.1052, found

673.1045. Anal. Calcd for C38H32CuF6N2P3S: C, 55.71; H, 3.94; N, 3.42; S, 3.91. Found: C, 55.54; H, 3.86; N, 3.43; S, 3.63.

[Cu(qybt)(dppe)]PF6 (5b). Red crystals (385 mg, 81%). 1H-NMR (500 MHz, CDCl3) = 8.66 (d, JHH = 8 Hz, 1H, quinoline), 8.32 (d, JHH = 8 Hz, 1H, quinoline), 8.05 (d, JHH = 9 Hz, 1H, quinoline), 7.95 (d, JHH = 8, 1H, quinoline), 7.81 (d, JHH = 9 Hz, 1H, quinoline), 7.58 (dd, JHH = 7 Hz, 1H, quinoline), 7.51 (dd, JHH = 8 Hz, 1H, benzothiazole), 7.47 (dd, JHH = 8 Hz, 1H, quinoline), 7.35 (m, 1H), 7.30 (d, JHH = 7 Hz, 1H), 7.22 (m, 6H, phenyl), 7.10 (m, 14H, phenyl), 6.97 (m, 4H, phenyl), 2.86 (br. s, 2H, P-CH2), 2.77 (br. s, 2H, P-CH2), 2.65 (m, 1H, P-CH2CH2), 2.46 (m, 1H, P-CH2CH2) ppm. 13C{1H}-NMR (125 MHz, CDCl3) = 165.2 (s), 150.3 (s), 148.1 (s), 145.9 (s), 140.3 (s), 135.0 (s), 133.7 (s), 133.3 (s), 132.6 (s), 132.0 (t), 131.8 (t), 131.7 (s), 130.3 (s), 130.2 (s), 129.6 (s), 129.3 (s), 129.0 (m), 128.9 (m), 128.8 (s), 128.7 (s), 127.9 (s), 127.8 (s), 127.7 (s), 123.2 (s), 123.0 (s), 120.7 (s), 28.4 (t), 28.0 (s), 20.0 (t) ppm. 31P{1H}-NMR (202 MHz, CDCl3) = -13.3 (br. s, 2P, dppe), -144.1 (qi, 1JPF = 715 Hz, 1P, PF6) ppm. 19F{1H}-NMR (470 MHz, CDCl3) = -72.9 (d, 1JPF = 715 Hz, PF6) ppm. HRMS (ESI) m/z: [5b-PF6]+ calcd for C42H34CuN2P2S 723.1208, found 723.1196. Anal. Calcd for C42H34CuF6N2P3S: C, 58.03; H, 3.94; N, 3.22; S, 3.69. Found: C, 57.90; H, 4.05; N, 3.42; S, 3.45.Synthesis of the NHC complex 6a. [CuCl(IDipp)] (122 mg, 0.25 mmol) was suspended in 10 mL dichloromethane and AgPF6 (63 mg, 0.25 mmol) was added under rigorous stirring at room tem-perature. After stirring the reaction mixture for 3 h, pybt (53 mg, 0.25 mmol) dissolved in 10 mL dichloromethane was added dropwise. The reac-tion mixture was stirred overnight, and filtered over celite. The volatiles were removed in vacuo, and the remaining solid was washed with n-hex-ane and diethylether. In order to remove traces of grease and to obtain high purity samples for pho-tophysical studies, the crude product was recrys-tallized several times from chloroform using di-ethylether as the anti-solvent via gas diffusion. The following yield thus represents the amount of single-crystalline material.

[Cu(pybt)(IDipp)]PF6 (6a). Yellow crystals (164 mg, 81%). 1H-NMR (500 MHz, CD3CN) = 8.40-6.80 (br. m, 6H CHaryl, 2H imidazolium-CH, 4H ben-zothiazole, 4H pyridine), 2.63 (sept, JHH = 7 Hz, 4H, iPr CH), 1.20 (d, JHH = 7 Hz, iPr CH3), 1.14 (d, JHH = 7 Hz, iPr CH3) ppm. 31P{1H}-NMR (202 MHz, CD3CN) = -144.6 (qi, 1JPF = 715 Hz, PF6) ppm. 19F{1H}-NMR (470 MHz, CD3CN) = -72.9 (d, 1JPF = 715 Hz, PF6) ppm. 13C{1H}-NMR (125 MHz, CD3CN) = 182.6 (s), 147.3 (s), 147.1 (s), 146.3 (s), 139.1 (br.), 136.5(s), 133.2 (s), 131.4 (s), 128.7 (s), 128.1 (br.), 127.5 (s), 127.1 (s), 125.7 (s), 125.5 (s), 125.4 (s), 125.3 (s), 125.2 (s), 125.0 (br.), 124.7 (s), 124.6 (s), 79.1 (s), 29.4 (s), 24.3 (s), 24.1 (s) ppm. HRMS (ESI) m/z: [6a-PF6]+ calcd

for C39H44CuN4S 663.2577, found 663.2568; [4a-PF6-pybt]+ calcd for C36H28CuP2 601.0906, found 601.0908.

ASSOCIATED CONTENT Supporting Information. Further photophysical measurements, cif files, coordinates used for DFT and TD-DFT studies, and NMR and HRMS spectra. This material is available free of charge via the Inter-net at http://pubs.acs.org.

AUTHOR INFORMATIONCorresponding Author* E-mail: andreas.steffen@uni-wuerzburg.dePresent Addresses† School of Pharmacy, Queen’s University Belfast, 97 Lisburn Road, BT9 7BL Belfast, UK.‡ Fachbereich Chemie, Fach 707, Universität Kon-stanz, 78457 Konstanz, Germany.§ Institut für Anorganische und Angewandte Chemie, Universität Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany.Author ContributionsThe manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENT Funding of this work by the DFG (STE1834/4-1, GRK 2112) and the Bavarian State Ministry of Science, Research, and the Arts via the Collaborative Re-search Network “Solar Technologies go Hybrid” is gratefully acknowledged. We thank Dr. J. P. Sprenger for assistance with the DSC measurements. A.S. thanks also the Keck-Köppe-Foundation and the Dr.-Otto-Röhm-Memorial Foundation for funding, and Prof. T. B. Marder for his generous support.

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The substitution of light main group elements by sulfur in 2-(2’-pyridine)imidazole-type -chro-mophore ligands shifts the emission bathochromically by more than 100 nm from green/yellow to, for CuI complexes, rare intense long-lived near-IR emission with max ranging from 593-812 nm, quantum yields of up to = 0.11 and lifetimes of several microseconds in the solid state as well as in PMMA films.

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