Near infrared electroluminescence in light-emitting electrochemical cells from the binuclear copper(I) complexes bearing π-extended benzimidazole and benzothiazole ligands

Abstract

The design of compounds that efficiently emit into the deep-red to near-infrared (NIR) region is highly challenging, yet they can play pivotal roles in optoelectronic devices for phototherapy, encryption and telecommunication technology. To date, examples of NIR-emissive earth-abundant Cu(I) complexes are still very rare in literature. Herein, a series of binuclear heteroleptic Cu(I) complexes, namely, Cu1–Cu4, is presented and thoroughly characterized by chemical and time-resolved optical spectroscopies as well as single-crystal X-ray diffraction analysis. The optical and electronic properties are further elucidated with time-dependent density functional theory (TD-DFT) computations that confirm the nature of the transitions and excited states involved. It is shown that the introduction of sulphur and nitrogen heteroatoms in the peripheral π-accepting coordinating scaffolds, such as (substituted) benzimidazoles and benzothiazole, along with the thiazolo[5,4-d]thiazole bridging unit, results in deep-red to NIR emissive complexes both in CH₂Cl₂ solution and in the solid state, with long-lived emission profiles centred at λ_em = 734–776 nm and 644–757 nm, respectively, attributable to an emissive excited state with admixed ³MLCT/³LLCT character. Finally, the derivatives Cu1 and Cu4 are tested as electroluminescent materials in light-emitting electrochemical cells (LECs). The former displays deep-red electroluminescence (EL), with λ_EL = 686–697 nm and an external quantum efficiency (EQE) up to 0.5%, while the latter remarkably achieves NIR EL with λ_EL > 770 nm in combination with high spectral stability. Overall, the presented results demonstrate that Cu(I) complexes represent a valid alternative to precious metals for NIR EL devices.

---

Introduction

Near-infrared (NIR) light-emitting devices, including both inorganic and organic systems, have attracted significant research attention owing to their promising applications in sensors, night-vision displays, biomedical devices, and optical telecommunications.¹ Among them, NIR organic light-emitting devices (OLEDs) offer several attractive advantages, such as low power consumption, compatibility with large-area fabrication, and mechanical flexibility. However, their practical implementation is still hindered by the requirement of complex multilayer device architectures and the instability of air-sensitive cathodes.

Solid-state light-emitting electrochemical cells (LECs)² provide a compelling alternative to conventional OLEDs by utilizing in situ electrochemical doping under electrical bias. The accumulation of mobile ions at the electrodes induces the formation of electrochemically doped layers, which effectively lower the carrier-injection barriers and promote balanced charge injection. As a result, LECs can employ air-stable metals as their cathode materials, thereby reducing encapsulation requirements. Moreover, efficient electroluminescence (EL) can be achieved in a simple single-layer architecture that is readily compatible with solution-processing techniques.

NIR LECs³ have been realized using a variety of emissive materials, including small molecules,^4–6 conjugated polymers,^7,8 ionic transition metal complexes (iTMCs),^9–12 and perovskites.¹³ Among these systems, iTMC-based NIR LECs generally exhibit superior device efficiencies, primarily owing to the phosphorescent nature of iTMC emitters, which enables efficient exciton harvesting. However, many reported NIR iTMCs rely on rare transition metals, such as ruthenium,^14,15 iridium,^16–19 osmium,²⁰ and platinum,²¹ whose high cost significantly limits their prospects for large-scale applications. Consequently, increasing research efforts have been devoted to developing NIR iTMC emitters incorporating earth-abundant transition metals as more sustainable alternatives. Among the emerging candidates, copper(I) complexes have attracted particular interest as promising emitters for LECs due to the earth-abundant nature and low cost of copper. In addition, appropriately designed Cu(I) complexes can exhibit efficient excited-state processes, such as metal-to-ligand charge-transfer (MLCT) emission or thermally activated delayed fluorescence (TADF), enabling effective exciton utilization in EL devices. These features make Cu-based complexes attractive alternatives to rare-metal iTMCs for developing cost-effective NIR LECs.

Transition metal complexes (TMCs) that display NIR EL are still very limited in the literature, and only rare examples deal with more abundant metals such as Cu(I) and Ag(I).^22–24 These complexes are particularly interesting because their d¹⁰ electronic configuration rules out the possibility of thermally accessible low-lying metal centered (MC) states that that would quickly deactivate non-radiatively.²⁵ The scarcity of described examples is mainly due to the three following reasons: (i) the smaller SOC constants of lighter metals compared with those of second- and third-row metals, e.g., ζ_Cu = 857 cm⁻¹; ζ_Ir = 3909 cm⁻¹; ζ_Pt = 4481 cm⁻¹, yield complexes featuring reduced singlet–triplet mixing (S–T) and slower radiative rate constants (k_r); (ii) the smaller energy gap between the ground and excited states (S₀–T₁) increases the non-radiative vibronic coupling, thus increasing the k_nr (energy gap law); and, finally, (iii) red to NIR emitters possess intrinsically smaller k_r due to the third-power dependency on emission energy, as in Einstein's theory of spontaneous emission.^26,27

A notable class of luminescent Cu(I) coordination compounds is represented by cationic [Cu^I(N^N)(P^P)]⁺ complexes, where N^N is a neutral polypyridyl bidentate ligand and P^P is a rigid bidentate diphosphine. The photophysical behaviour of these species, firstly investigated in the early 2000s by McMillin, Walton, et al., is highly dependent on the geometry and nature of the ligands as well as environmental parameters. Their emission typically arises from a triplet-manifold metal-to-ligand charge transfer excited state (³MLCT) which formally entails an oxidation of the metal center to d⁹ Cu(II) and preferentially adopts an equilibrium geometry closer to a square pyramidal arrangement owing to the large pseudo Jahn–Teller distortion effects. This leads to more efficient vibronic quenching of the lowest lying excited states (T₁–S₀) and exposes the metal to solvent-assisted exciplex deactivation processes. Employing a bulky, bidentate diphosphine prevents solvent quenching, and the P^P bridge can slightly alter the energy of the d¹⁰ orbitals, thus modifying the MLCT energy gap and allowing for some degree of tuning in the emission through the diphosphine. Nevertheless, the largest influence on the emission profile is exerted by the N^N ligand, allowing [Cu^I(N^N)(P^P)]⁺ complexes to be tuned to yield emissions ranging from the blue-green to the red-NIR regions of the visible spectrum.^28,29

A strategy that has been found to bathochromically shift the emission profile is the introduction of sulphur heteroatoms in the N^N ligand, as their higher electron-accepting character and lower oxidation potential, reflected by their generally lower HOMO and LUMO energies, cause an overall decrease in the band gaps of transitions involving the π* MOs, leading to significantly longer emission wavelengths. This approach recently allowed the emission of Cu(I) in the NIR by employing simple pyridyl-benzothiazole ligands, obtaining complexes with emission in the range λ_em = 520–760 nm and PLQY values up to 10% in thin-film and powder form but still barely detectable in CH₂Cl₂ solution, as described by Steffen et al.³⁰ Costa et al. showed that employing more rigid diphosphines, such as Xantphos, enabled the preparation of Cu(I) and Ag(I) complexes which displayed red luminescence in the solid state and in proof-of-concept light-emitting devices with λ_el = 670 and 645 nm, respectively, but external quantum efficiency (EQE) values not higher than ca. 0.01%.^22,31 A bathochromic shift of the emission can also be induced by dinuclearization of the complex, with a general formula of [Cu^I(P^P)(N^N–N^N)Cu^I(P^P)]²⁺, e.g., by employing a bis-bidentate 2,5-dipyridyl-thiazolo[5,4-d]thiazole bridging ligand. This approach has been successfully employed by our research group to prepare a new family of electroactive binuclear thiazolo[5,4-d]thiazole (TzTz) Cu(I) complexes that displayed an λ_EL up to 780 nm in LECs, effectively being the first reported examples of a Cu(I)-based NIR device,²³ and later set a record for the most bathochromically shifted emission of a Cu(I) LEC.²⁴ Herein, we explore the effect of the introduction of π-accepting benzothiazole and benzimidazole scaffolds to achieve more stable NIR EL with binuclear Cu(I) complexes. These heteroatomic scaffolds have been chosen to stabilize the π* orbitals localized onto the coordinated bridging ligands in order to maintain longer wavelength emission while potentially improving charge transport properties compared to previously investigated counterparts (Scheme 1).

Scheme 1 Molecular structures of the two landmark Cu(I) NIR emitters previously published by our group (top) and the new compounds Cu1–Cu4 investigated in this work (bottom).

Results and discussion

Synthesis and X-ray structures

The selected ligands feature a π-extended thiazolo[5,4-d]thiazole (TzTz) core and two lateral 1-R-1H-benzo[d]imidazol-2-yl (R = H, Et and Ph for L1, L2 and L3, respectively) and benzo[d]thiazol-2-yl (L4) moieties acting as the bridging bis-bidentate (N^N–N^N) scaffold. The synthetic pathway employed for their synthesis is depicted in Scheme S1 of the supporting information (SI) alongside the corresponding ¹H, ¹³C{¹H}, ³¹P{¹H} NMR and high-resolution mass spectrometry (Fig. S1–S19 of the SI). For the ligands, the procedure followed a straightforward condensation reaction between dithiooxamide and the corresponding aryl-carboxaldehyde in refluxing DMF and yielded the desired product as pure powder after washing.^32,33

Single crystals suitable for X-ray diffractometric analysis were obtained for L3, and the ORTEP diagram is displayed in Fig. S20 of the SI (see Table S1 for the crystallographic refinement parameters). Ligand L2 was prepared starting from the L1 parental ligand via N-alkylation of the benzimidazolyl N–H group with iodoethane in hot DMF (Scheme S1). The synthesis of the binuclear Cu(I) complexes followed a one-pot two-step procedure (Scheme S2), where to a solution of DPEphos (2 equiv.) in CH₂Cl₂ kept at room temperature was subsequently added the tetrakis-acetonitrile copper(I) precursor (2 equiv.) as PF₆⁻ salt and the chromophoric ligand L1–L4 (1 equiv.), yielding the target complexes of general formula [Cu(P^P)(L1–L4)(P^P)](PF₆)₂ (Cu1–Cu4, respectively), as red solids in excellent yield (>90%) (see Scheme 1 for chemical structure). For the complexes, the ³¹P{¹H} spectra displayed one single resonance in the region at δ ca. −12.0 ppm associated to the chelating phosphine that indicates that one single species is present in CD₂Cl₂ solution. Despite the somewhat labile nature of these binuclear species in solution, enhanced by the high electron-deficient nature of the ligands, all complexes yielded crystals suitable for X-ray crystallographic analysis by vapor diffusion of Et₂O in a CH₂Cl₂ solution of the complex. The crystal structures and packing of all the complexes were established by means of single crystal X-ray diffractometric analysis, which also confirmed the binuclear nature of all the species with the general formula [Cu₂(L1–4)(DPEphos)₂]²⁺ (see Fig. 1 and Fig. S21, S22 of the SI). The corresponding data can be found in Tables S2–S5 of the SI. Some relevant geometric parameters for Cu1–Cu4 are listed in Table 1, along with those of the reference complex Cu-D3²³ displayed in Scheme 1. Also, a capped-sticks representation of the XRD structure of derivatives Cu1–Cu4 as well as Cu-D3 are shown in Fig. 2.

Fig. 1 ORTEP diagram of Cu1 shown at 50% probability. Hydrogen atoms (except the NH of the benzimidazole), solvent molecules and PF₆⁻ counter anions are omitted for clarity.

Table 1 Selected bond lengths [Å] and angles [°] around one metal center for the Cu(I) complexes Cu1–Cu4 with their tetrahedral geometric indexes τ₄³⁸ and τ₄′.³⁹ N_az = nitrogen of the thiazolo[4,5-d]thiazole moiety and N_py = nitrogen of the peripheral moiety

	M(1)–Naz	M(1)–Npy	M(1)–P(1)	M(1)–P(2)	τ4,M(1)	τ′4,M(1)
a Data corresponding to the reference Cu(I) complex Cu-D3 are taken from ref. 24.
Cu-D3^a	2.063(3)	2.096(3)	2.2419(11)	2.2403(10)	0.87	0.86
Cu1	2.014(2)	2.136(2)	2.2201(7)	2.2392(6)	0.84	0.80
Cu2	2.018(3)	2.195(3)	2.2170(11)	2.2520(12)	0.85	0.82
Cu3	2.050(2)	2.131(2)	2.2137(7)	2.2788(7)	0.76	0.73
Cu4	2.099(5)	2.119(5)	2.2121(19)	2.2617(19)	0.77	0.75

---

Fig. 2 Capped-stick XRD structures viewed from the side of the complexes Cu1 (a), Cu2 (b), Cu3 (c), and Cu4 (d) and (e) reference complex Cu-D3.²⁴ Hydrogen atoms (except the NH of the benzimidazole), solvent molecules and PF₆⁻ counter anions are omitted for clarity.

Each Cu(I) ion adopts a distorted tetrahedral coordination geometry involving the chelating DPEphos (P^P) ligand and two nitrogen atoms of the 2,5-di(heteroaryl)thiazolo[5,4-d]thiazole (L1–L4). The bond lengths and angles relative to the tetrahedral Cu(I) centers are shorter than those reported for similar mononuclear Ag(I) complexes, expectedly,^22,34–36 and are comparable to those observed in similar Cu(I) complexes.^23,24,30,37 A selected list of geometrical parameters is summarized in Table 1. In compounds Cu1 and Cu4, each DPEphos employs one of its phenyl rings in weak intramolecular π–π stacking on the imidazole (d = 3.46–3.61 Å), while another phenyl is involved in a S–π interaction with the central TzTz core (d = 3.28–3.53 Å) (see Fig. 2a and d). Complex Cu2 does not display any noticeable intramolecular interaction (Fig. 2b), but a network of intermolecular π–π stacking interactions (d = 3.34 Å) is present at the supramolecular scale, involving the 1-ethyl-1H-benzo[d]imidazol-2-yl moiety of the coordinated ligand L2 of each neighbor complex. Complex Cu3 displays some intramolecular π–π stacking between the phenyls of DPEphos and two intramolecular C–H⋯π interactions between one of the phenyls of each phosphine and the adjacent phenyl substituent of the 1-phenyl-1H-benzo[d]imidazol-2-yl moiety (d = 3.186 Å) (Fig. 2c).

While the dihedral distortion between the TzTz core and the peripheral substituents is not as marked as in the bis-(t-butylphenyl)-functionalized complex Cu-D3 (Fig. 2e), the TzTz ligands in Cu1–Cu4 are not always planar. If we look at complex Cu3 from the side (Fig. 2c), we can see how the two peripheral benzimidazoles are oriented out of plane, likely due to the steric hindrance of the lateral N–Ph substituent. This can be observed, to a very small extent, on the N–Et derivative Cu2 as well (Fig. 2b). The τ₄ and τ′₄ values for these complexes are shown in Table 1 and reflect the considerations above. For complexes Cu1 and Cu2, the tetrahedral geometrical indexes are in the range of 0.80–0.85 and indicate a minor distortion from the ideal T_d symmetry. Whereas, for derivatives Cu3 and Cu4 that bear bulkier benzothiazole and N-phenyl benzimidazole, these values decrease to 0.73–0.77, highlighting a slightly more significant distortion compared to their congeners and approaching that of Ag(I) complexes.²²

Photophysical investigation

For the sake of comparison, the electronic absorption and emission spectra were acquired in dilute (3 × 10⁻⁶ M) air equilibrated CH₂Cl₂ for the four ligands L1–L4 (Fig. 3a and Table 2). A full photophysical characterization of these compounds was carried out, and the results are in nice agreement with similar data recently reported by us for pyridyl- and quinolyl-functionalized TzTz ligands.^23,24,40 All the data, including measurements in the solid state and in 77 K CH₂Cl₂ glassy matrix, can be found in the SI (Fig. S23, S24 and Table S6).

Fig. 3 (a) Electronic absorption (dashed lines) and emission spectra (solid lines) of the ligands L1 (blue), L2 (red), L3 (violet), and L4 (grey) in dilute (3 × 10⁻⁶ M) air-equilibrated CH₂Cl₂ at room temperature upon excitation at λ_exc = 360 nm. (b) Electronic absorption (dashed lines) and emission spectra (solid lines) of the complexes Cu1 (blue), Cu2 (red), Cu3 (violet), and Cu4 (grey) in dilute (2 × 10⁻⁵ M) air-equilibrated CH₂Cl₂ at room temperature upon excitation at λ_exc = 460 nm.

Table 2 Electronic absorption and photoluminescence data in a dilute air-equilibrated CH₂Cl₂ solution for the ligands L1–L4

	λmax, Abs(ε) [nm, (10^3 M^−1 cm^−1)]	λem [nm]	PLQY (%)	τobs [µs]	kr [10^8 s^−1]	knr [10^8 s^−1]
sh denotes a shoulder.
L1	Poorly soluble
L2	282 (9.32), 292 (9.12), 382sh (38.98), 403 (51.95), 425 (37.57)	442, 468, 499sh, 543sh	64	1.21 ns	5.27	2.97
L3	284 (14.01), 293sh (12.99), 383sh (35.83), 402 (42.16), 426sh (27.64)	443, 470, 503sh, 542sh	39	1.25 ns	3.12	4.88
L4	290 (10.34), 305sh (8.45), 365sh (14.52), 387 (27.71), 407 (38.28), 431 (28.94)	444, 471, 504sh	9	0.41 ns	2.21	22.4

---

The electronic absorption spectra for L1–L4 display, for all ligands, a weak band (ε = 0.1–1.4 × 10⁴ M⁻¹ cm⁻¹) at higher energies (λ_abs = 280–290 nm) that can be attributed to transitions with π–π* singlet-manifold locally excited (¹LE) character admixed with intramolecular charge transfer (¹ICT) processes from the central TzTz moiety towards the peripheral substituents. At lower energies, a much more intense (ε = 0.4–5.1 × 10⁴ M⁻¹ cm⁻¹) and structured band is observable at λ_abs = 380–430 nm and is mainly attributable to the ¹π–π* LE transitions localized onto the π-extended chromophoric thiazolo[5,4-d]thiazole scaffold. Upon coordination, these bands experience an expected bathochromic shift and intensity enhancement, reflecting the enlargement of the system, their admixing with ¹MLCT (¹dπ(Cu) → π_TzTz*) transitions, and further perturbation of the ligand-centered ¹LC (¹π_Ph → π_BzIm*) and ¹LLCT (¹π_P → π_TzTz*) transitions.

As for complexes Cu1–Cu4, their photophysical behavior was firstly investigated in dilute (2 × 10⁻⁵ M) CH₂Cl₂ solution. The absorption and emission spectra are displayed in Fig. 3b, and the photophysical data are summarized in Table 3. It is important to note that all four complexes are labile in highly diluted CH₂Cl₂ solutions, in particular upon repeated exposure to UV light. This can be noticed by the appearance of blue emission compatible with that of the ligands, and, while dilute samples of Cu1–Cu3 remained stable during their photophysical study, extended irradiation of Cu4 led to enhanced degradation compared to its congeners. Regardless, even in the absence of light, dilute solutions of these complexes (10⁻⁵ M) cannot be stored for more than a day, and all photophysical studies were carried out on freshly prepared samples.

Table 3 Electronic absorption and photoluminescence data in a dilute degassed and air-equilibrated CH₂Cl₂ solution for the complexes Cu1–Cu4

	λmax, Abs(ε) [nm, (10^3 M^−1 cm^−1)]	λem [nm]		PLQY (%)		τobs [µs]	τave [µs]	τobs [µs]	τave [µs]	kr [10^5 s^−1]	knr [10^5 s^−1]
	Air	Air	Deg	Air	Deg	Air		Deg
sh denotes a shoulder.
Cu1	282sh (32.82), 420 (42.68), 433 (40.21), 495sh (7.66)	734	734	0.38	0.54	0.316		0.401 (98%)	3.36	1.61	28.1
								10.29 (2%)
Cu2	282sh (31.55), 423 (46.75), 444sh (41.18), 517sh (5.66)	776	734	0.21	0.55	0.110 (97%)	0.198	0.128 (90%)	9.72	0.57	9.72
						0.672 (3%)		10.73 (10%)
Cu3	282sh (30.95), 424 (45.68), 447sh (40.09), 515sh (6.90)	775	746	0.24	0.45	0.123 (99%)	0.152	0.125 (97%)	11.2	0.40	8.56
						0.729 (1%)		14.30 (3%)
Cu4	274 (33.51), 281sh (32.06), 396sh (26.11), 416 (38.67), 441 (32.91), 506sh (3.08)	764	764	0.09	0.14	0.336		0.549 (99%)	0.973	1.44	101
								11.7 (1%)

---

For all complexes, the electronic absorption spectra display an intense shoulder (ε = 3.1–3.3 × 10⁴ M⁻¹ cm⁻¹) at higher energies (λ_abs = 280–350 nm) attributed to ¹LLCT ¹π_Ph → π_BzIm* transitions involving the aromatic groups of the phosphine ligands and the benzimidazole (bim) and benzothiazole (btz) substituents. At lower energies, an intense (ε = 3.7–6.5 × 10⁴ M⁻¹ cm⁻¹) and structured band is observable at λ_abs = 380–450 nm and is mainly attributable to admixed ¹LC/¹MLCT transitions involving the π-extended TzTz ligand and Cu(I). The absorption shoulders at even lower energy, located at λ_abs = 475–550 nm, are much broader and less intense (ε = 0.3–0.7 × 10⁴ M⁻¹ cm⁻¹) and can be ascribed to a mixed transition with singlet-manifold metal-to-ligand charge transfer (¹MLCT) and ligand-to-ligand charge transfer (¹LLCT) with ¹dπ(Cu) → π_TzTz* and ¹π_P → π_TzTz* characters, respectively. Overall, the absorption profiles of the four complexes moderately reflect the increasing π-accepting ability of the substituted TzTz cores, along the series btz (Cu4) → bim (Cu1) → Et-bim (Cu2) → Ph-bim (Cu3), with their λ_abs,max being 416, 420, 423 and 424 nm, respectively.

Upon excitation in the lower-lying ¹MLCT band (λ_exc = 440–480 nm), the four complexes display photoluminescence in dilute degassed and air-equilibrated CH₂Cl₂ with the maximum in the deep-red to NIR region of the electromagnetic spectrum arising from an excited state with admixed ³MLCT/³LLCT character (Fig. 3b and Table 3). The broad and featureless emission profile (FWHM ≈ 3400 cm⁻¹ ≈ 210 nm) presents a λ_em,max in the range 734–775 nm, with complexes Cu2–Cu3 being the most bathochromically shifted. Adopting the more conservative CIE 17-21-004⁴¹ lower boundaries for NIR (>780 nm), half of the emission of the complexes in dilute CH₂Cl₂ falls in the NIR, with % NIR values of ca. 48%–51%. As expected from the energy gap law, the bathochromic shift in the emission is accompanied by a lowering of the photoluminescence quantum yield (PLQY) and a shortening of the excited state lifetime, and these effects can be seen particularly well by comparing the photophysical properties of complexes Cu1 and Cu4 in dilute degassed CH₂Cl₂ solution, which show λ_em,max, PLQY and τ_avg,deg values of 734 nm, 0.54%, and 3.36 µs and 764 nm, 0.14%, and 0.973 µs, respectively. This effect, caused by the increased vibronic coupling between the T₁ and S₀ states, is reflected by the estimated radiative (k_r) and non-radiative (k_nr) rate constants characterizing the emissive excited state (Table 3). These constants were calculated using the following equations (eqn (1) and (2)):

The photophysical data of Cu1–Cu4 in the solid state as neat powders and in CH₂Cl₂ glassy matrix at 77 K are summarized in Table 4, and the emission spectra are shown in Fig. 4. Upon lowering the temperature to 77 K, the CH₂Cl₂ samples of complexes Cu1–Cu4 display a more structured emission profile with a clear vibronic progression that is hypsochromically shifted by ca. 1570–2050 cm⁻¹, indicating a small rigidochromic effect due to the limited charge transfer nature of the emitting excited state at lower temperature. This spectral shift is accompanied by a substantial increase of the excited state lifetime up to τ_avg = 72–500 µs. These observations agree with the large triplet nature of the emissive excited state that can be described with confidence as largely ³LC in nature. In addition, the observed large hypsochromic shift recorded for glassy matrix samples compared to room temperature ones might allow us to rule out thermally-activated delayed fluorescence (TADF) processes.⁴²

Table 4 Photophysical data recorded for the complexes Cu1–Cu4 as neat powders under vacuum and in a CH₂Cl₂ glassy matrix at 77 K

Cmpd	λem [nm]		PLQY (%)	τave [µs]		kr [10^3 s^−1]	knr [10^3 s^−1]
	Powder	77 K (CH2Cl2)	Powder	Powder	77 K (CH2Cl2)	Powder
sh denotes a shoulder.
Cu1	644, 696	638, 697, 770	4	67.2	293	0.59	14.3
Cu2	644, 700	638, 700, 775	2	70.9	499	0.28	13.8
Cu3	650, 715	647, 712, 788	2	72.9	211	0.27	13.4
Cu4	757	682, 751, 834	1	0.89	71.9	11.2	1106

---

Fig. 4 Emission spectra of the complexes (a) Cu1 (blue) and Cu2 (red) and (b) Cu3 (violet) and Cu4 (grey) as neat powders under vacuum at 298 K (solid traces) and in CH₂Cl₂ glassy matrix at 77 K (dashed traces). The spectra were recorded upon excitation at λ_exc = 450 and 480 nm, respectively.

In powder form, the complexes show a broad and somewhat structured emission which is not noticeably affected by the presence of the quenching species dioxygen, although its removal causes a ca. 1% increase in the PLQY and a ca. 10% prolongation of the excited-state lifetimes. For this reason, measurements on the powder samples were carried out in quartz tubes held under dynamic vacuum (ca. 10 mbar), revealing PLQY values as high as 4% for Cu1 and lifetimes in the range of 67–73 µs for complexes Cu1–Cu3. Complex Cu4, the most NIR of the series, displays the lowest PLQY (1%) and lifetime (0.89 µs), in agreement with the energy gap law. It should be noted that, while these results may be low compared to other orange-red photoactive TMCs based on Ir(III) and Pt(II) complexes,^43–45 the lower energy of their NIR emission makes these performances comparable to those of the landmark complexes we recently published (Scheme 1).^23,24

Computational investigation

To further elucidate the photophysical behaviour of this family of NIR emitters, ground state geometries, absorption spectra and optimized excited states of both the singlet and triplet manifolds were computed by means of time-dependent density functional theory (TD-DFT) calculations.

The experimental and computed structures (Table S7) are in good agreement, both displaying a distorted tetrahedral geometry at the Cu centers with a slight lengthening of one of the Cu–N_TzTz distances. Upon geometry optimization of Cu2 and Cu3, a pyramidal distortion around the [Cu(N^N)(P^P)]⁺ tetrahedron occurs, yielding the formation of a π-stacking interaction between one of its phenyl rings of the P^P chelates and one of the two benzimidazole fragments, in a similar fashion to what was observed experimentally in the X-ray structures of derivatives Cu1 and Cu4 (cf. Fig. S25 vs. Fig. 1 and 2).

The computed energies of the molecular orbitals closer to the frontier region for all the ligands and complexes are listed in Table S9 along with the HOMO–LUMO energy gaps Δ_H–L. The computed absorption spectra for L1–L4 are in good agreement with the experimental data (cf. Fig. 3a and 5a). For all proligands, a first transition is computed around 420 nm for L1, L2 and L3 and 440 nm for L4 (Table S8), corresponding to their S₀ → S₁ transitions. According to the computed electron density difference maps (EDDMs), which are displayed in Fig. 6, this transition can be described as possessing singlet ¹LE (π–π*) character on the benzimidazole (bim) and benzothiazole (btz) substituents with some ¹ICT from the TzTz toward the btz and bim. The features of the absorption profile observed experimentally can be attributed with confidence to the vibronic progression involving the rigid π-conjugated scaffold. A second significant transition appears at much higher energy, between 290 to 310 nm, due to the S₀ → S₆ transition which is mainly a ¹ICT from the btz to the TzTz core. No other transition is present in this energy range with significant absorbing intensity.

Fig. 5 Electronic absorption transitions recorded in CH₂Cl₂ of a) ligands L1 (blue), L2 (red), L3 (violet), and L4 (grey) and b) complexes Cu1 (blue), Cu2 (red), Cu3 (violet), and Cu4 (grey).

Fig. 6 EDDMs between the ground and excited states for the S₁ (left) and S₆ (right) absorbing states of L1. Electronically depleted and enriched areas are shown in red and green, respectively.

As far as the complexes are concerned, the LUMO remains located on the TzTz ligand, whereas the HOMO and HOMO−1 are mainly located on the copper centers. The highest occupied orbital located on the ligand is the HOMO−2 for Cu1–Cu3 and the HOMO−5 for Cu4. The computed absorption spectra nicely agree with the experimental ones (cf. Fig. 3b and 5b), and the list of computed transitions is collected in Table S10 of the SI. The spectra are similar for the four complexes, and the THEODore analysis⁴⁶ of the computed states is detailed in Fig. S26. The computed spectra present a first band above λ_abs = 500 nm corresponding to a set of charge transfer states either from the copper cation or from the disphosphine ligand towards the central TzTz ligand that can be overall described as having admixed ¹MLCT and ¹LLCT character. This corresponds to the band shoulder observed experimentally at λ_abs = 480–550 nm (Fig. 3b). Moreover, a very intense band is calculated at λ_abs = ca. 445 nm, which corresponds to states with a major contribution having large LC character located on the TzTz ligand and is similar for the four complexes. As an example, this transition involves S₅ and S₆ in Cu1 and can be ascribed to a LC state slightly admixed with a MLCT manifold (Fig. 7). On the other hand, the absorbing S₇ state possesses an almost pure ¹LC character. These states are bathochromically shifted compared to the pure ligand due to the expected perturbation of the coordinated metal centre. These peaks correspond to the intense experimental absorption observed at λ_abs = ca. 410 nm. In good agreement with experimental data, the computed spectra present a weak absorption process in the range λ_abs = 300–400 nm dominated by ¹LLCT transitions from the P^P from the TzTz ligand.

Fig. 7 EDDMs between the ground and excited states for the S₁ (left), S₅ (center), and S₆ (right) absorbing states of Cu1. Electronically depleted and enriched areas are shown in red and green, respectively.

Electrochemical and electroluminescent properties of LECs

Based on their optical properties, complexes Cu1 and Cu4 were selected for further investigation and for the preparation of light emitting devices. Amongst the complexes of the series, Cu1 was the most emissive one, whereas Cu4 displays the most bathochromically shifted emission spectra. The electrochemical properties of all the investigated complexes were assessed by means of cyclic voltammetry in CH₂Cl₂/0.1 M TBAPF₆ solution. The data are listed in Table 5.

Table 5 Electrochemical data recorded for the binuclear Cu1 and Cu4 complexes in a CH₂Cl₂/0.1 M TBAPF₆ solution. Potential values are given against ferricenium/ferrocene (Fc⁺|Fc) couple

Complex	Ep,O1,i [V]	ER1,i^0 [V]	ER2,i^0 [V]	ΔEH–L [eV]^c
a Irreversible process. Only the peak potential, Ep, can be reported.b Reversible process, unless otherwise stated. The formal potential, E^0, was calculated as the average of the cathodic and anodic peaks of the process.c The electrochemical band gap, eΔEH–L, was calculated as the difference between the peak potential of the O1,i process and the peak potential or formal potential of the R1,i process.
Cu1	+0.99^a	−1.50^a	−1.91^a	∼2.50
Cu4	+0.92^a	−1.11^b	−1.65^b	∼2.03

---

In the positive-going scan, Cu1 and Cu4 showed an irreversible oxidation process, O_1,i (with i denoting the investigated compound), whose peak potentials E_p fall within the range +0.92 to +0.99 V vs. the ferricenium/ferrocene (Fc⁺|Fc) redox couple, used as the internal standard. This redox process can be confidently ascribed to the oxidation that is mainly centered on the metal. Moreover, once the potential scan was reverted, a cathodic peak appeared in the range 0–0.5 V which might be related to the reduction of species formed during the irreversible oxidation process, likely followed by a chemical reaction (EC process), as previously observed for similar complexes.²³ In contrast, in the negative-going scan, up to three reduction processes R_n,i (where n denotes the process number and i the investigated compound) occurred which are mainly irreversible for Cu1 and reversible for Cu4. Therefore, peak potential and standard potential are provided and fall within the range from –1.11 to –1.91 V vs. Fc⁺|Fc⁰. Furthermore, the electrochemical energy band gap (ΔE_H–L) for the HOMO–LUMO was estimated from the O₁ and R₁ processes, taking into account the respective potential values for the irreversible and/or reversible processes. Overall, these findings agree well with the optical properties (see above) and support the idea that these complexes are potentially suitable candidates in the field of solid-state electroluminescence devices.

To investigate the EL characteristics of the proposed complexes, LEC devices were fabricated and characterized. Experimental details are described in SI. For each complex, device performance was optimized by evaluating three emissive-layer thicknesses under the optimal bias condition (see device architecture and energy levels in Fig. S5). The EL data of the fabricated LECs are summarized in Table 6. The time-dependent EL spectra of the LECs based on Cu1 and Cu4 with different thicknesses are presented in Fig. S28a–c and S29a–c of SI. Only minor spectral variations were observed during operation, indicating that microcavity effects arising from shifts of the emission zone during device operation^47,48 are negligible for relatively thin LECs (<200 nm). The stabilized EL spectra of LECs employing Cu1 and Cu4 with different emissive-layer thicknesses are shown in Fig. 8a and b, respectively. Cu1 exhibits deep-red EL emission (λ_em,max ≈ 700 nm), whereas Cu4 displays NIR EL emission (λ_em,max ≈ 770 nm). In both cases, the EL spectra largely resemble the corresponding PL spectra, suggesting similar emission mechanisms.

Table 6 Summary of the EL characteristics of the LECs based on the complex Cu1 or Cu4 (80 wt%) and [BMIM⁺(PF₆)⁻] (20 wt%)

Complex	Concentration (mg mL^−1)^a	Spin speed (rpm)^b	Thickness (nm)	Bias (V)	ELmax (nm)^c	Lmax (µW cm^−2)^d	ηEQE, max (%)^e	ηP,max (mW W^−1)^f
a Solution concentration for spin coating.b Spin speed for spin coating.c Stabilized EL emission peak wavelength.d Maximal light output power.e Maximal external quantum efficiency.f Maximal power efficiency.
Cu1	30	2000	71	2.3	686	2.91	0.22	1.70
	30	1000	111	2.3	694	3.65	0.40	3.00
	30	750	120	2.4	697	3.22	0.49	3.69
Cu4	50	2000	144	2.5	772	0.36	0.085	0.53
	60	2000	175	3	773	0.76	0.090	0.46
	70	2000	213	2.5	776	0.21	0.063	0.39

---

Fig. 8 Stabilized EL spectra of LECs based on (a) Cu1 and (b) Cu4 with different emissive-layer thicknesses. The PL spectra are included for comparison.

The time-dependent current density, light output, and EQE of the LECs based on complexes Cu1 and Cu4 with different emissive-layer thicknesses are shown in Fig. 9a–c and Fig. S30a–c of the SI, respectively. All the devices exhibit similar temporal evolutions of their EL characteristics. Upon bias application, mobile ions in the emissive layer drift toward the electrodes, gradually forming electrochemically doped regions that facilitate charge injection. Consequently, both the current density and light output rapidly increase at the initial stage of device operation. After reaching their maxima, the current density and light output progressively decrease, which is attributed to material degradation during prolonged operation. Meanwhile, the formation of doped layers improves carrier balance in the device, resulting in a rapid rise in EQE shortly after bias application, followed by a gradual decline due to the same degradation processes.

Fig. 9 Time-dependent current density, light output, and EQE of the LECs based on Cu1 with emissive-layer thicknesses of (a) 71, (b) 111, and (c) 120 nm. The applied bias voltage is indicated in each panel.

For the Cu1-based devices, the peak EQE increases with increasing emissive-layer thickness (Table 6). A thicker emissive layer enlarges the separation between the emission zone and the electrochemically doped regions, thereby suppressing exciton quenching.⁴⁹ In addition, device thickness influences the optical microcavity effect and thus the light-extraction efficiency, which also contributes to EQE optimization.⁴⁸ Owing to the limited solubility of Cu1 in dichloromethane, further increases in emissive-layer thickness were not feasible. The thickest Cu1-based device (120 nm) therefore delivered the highest EQE of 0.49%.

The better solubility of Cu4 allowed the fabrication of thicker emissive layers. The optimized Cu4-based device with a thickness of 175 nm exhibited a maximum EQE of 0.09%. To date, only a limited number of NIR Cu(I) LECs with EL peak wavelengths exceeding 750 nm have been reported.^23,24 Compared with those, the Cu4-based devices show comparable EL peak wavelengths and peak EQEs. Notably, they exhibit a smaller thickness-dependent EL spectral shift arising from the microcavity effect, resulting in improved spectral stability.

The carrier balance in LECs based on complexes Cu1 and Cu4 may be influenced by the ligand environment of the complexes. To evaluate this effect, the EQE of the devices was analyzed using eqn (3), which describes the key factors governing the EQE of LECs as follows:

η_EQE = η_out × γ × η_S,T × η_QY (3)

In this equation, η_EQE represents the measured device EQE, η_out the optical outcoupling efficiency, γ the carrier-balance factor, η_S,T the efficiency of emissive exciton formation, and η_QY the solid-state PLQY of the emitter. For phosphorescent complexes, both singlet and triplet excitons can be effectively utilized, giving η_S,T = 100%. The optical outcoupling efficiency of LECs is typically estimated to be approximately 20%–30%.⁴⁸

Using the measured EQEs (Table 6) and solid-state PLQYs (Table 4) and assuming η_out = 25%, the carrier-balance factors (γ) were estimated to be 49% and 36% for the Cu1- and Cu4-based LECs, respectively. Although uncertainties may arise from the estimation of η_out, the calculated γ values suggest that the benzimidazole ligand in Cu1 provides slightly improved carrier balance compared with the benzothiazole ligand in Cu4.

Conclusion

A small family of four new binuclear Cu(I) complexes of general structure [Cu^I₂(N^N–N^N)(DPEphos)₂]²⁺ where N^N–N^N is a bis-bidentate benzimidazolyl- or benzothiazolyl-functionalized thiazolo[4,5-d]thiazole ligand was herein presented. The four complexes all displayed photoluminescence in dilute degassed and air-equilibrated CH₂Cl₂ in the deep-red to NIR regions of the visible spectrum, with complexes Cu2 and Cu3 reaching λ_em,max as high as 775 nm and PLQY as high as 0.55%, leading to a % NIR ratio of around 50%. Comparing these values with those of NIR emitters previously reported by our group such as Cu(I) complex Cu-D3²⁴ (λ_em,max = 790 nm and PLQY 0.04% in degassed CH₂Cl₂), it can be seen that indeed the addition of further heteroatoms can cause a bathochromic shift in the emission, although π-extension of the system still plays an important role. Despite their overall low PLQY values in solution and the hypsochromic shift that generally accompanies solid-state measurements, powder samples displayed λ_em,max as high as 757 nm (Cu4) and PLQY values as high as 4% (Cu1). The interesting deep-red to NIR emission displayed by these complexes prompted their use as electroactive materials in LEC devices. Remarkably, the complex Cu4 displayed NIR electroluminescence with maximal λ_EL above 770 nm, improved spectral stability compared with other electroactive Cu(I) complexes emitting in the same spectral region and previously reported in the literature, and device performances comparable to NIR emitters based on noble metals. Overall, these results confirm that dinuclear Cu(I) complexes may act as valuable alternatives to NIR-emitting Ir(III) and Pt(II) phosphors.

Conflicts of interest

There are no conflicts to declare.

Data availability

Detailed synthetic procedures, NMR and HR-ESI-MS data, crystallographic structures, additional absorption and emission data, additional computational data and electroluminescence performances are available as supplementary information (PDF).

See DOI: https://doi.org/10.1039/d6tc00899b.

CCDC 2402730–2402732 and 2412038 contain the supplementary crystallographic data for this paper.^50a–d

Acknowledgements

M.M. gratefully acknowledges the Université de Strasbourg and the CNRS for the financial support and the French Agence Nationale de Recherche (ANR) for funding the grants ANR-22-CE07-0049-02 “BoostOLED”, ANR-23-CE29-0019 “QUINOC”, and ANR-24-CE29-2108 “E-Polar”. V.G. gratefully acknowledges the Collège Doctoral of the Université de Strasbourg for co-funding his PhD fellowship. N. Gruber and C. Bailly of the Service de Radiocristallographie, Fédération de Chimie Le Bel–UAR2042, the Université de Strasbourg and the CNRS are kindly acknowledged for their help in solving the X-ray structures. C.G. thanks the HPC of Strasbourg for the computational time. This work was supported by the Higher Education Sprout Project of the National Yang Ming Chiao Tung University and the Ministry of Education (MOE), Taiwan.

References

1. G. Qian and Z. Y. Wang, Chem. Asian J., 2010, 5, 1006–1029.
2. Q. Pei, G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Science, 1995, 269, 1086–1088.
3. Y.-D. Lin, C.-W. Lu and H.-C. Su, Chem. Eur. J., 2023, 29, e202202985.
4. A. Pertegás, D. Tordera, J. J. Serrano-Pérez, E. Ortí and H. J. Bolink, J. Am. Chem. Soc., 2013, 135, 18008–18011.
5. M. Mone, S. Tang, Z. Genene, P. Murto, M. Jevric, X. Zou, J. Ràfols-Ribé, B. A. Abdulahi, J. Wang, W. Mammo, M. R. Andersson, L. Edman and E. Wang, Adv. Optical Mater., 2021, 9, 2001701.
6. Y. Chen, Y.-X. Wang, C.-W. Lu and H.-C. Su, J. Mater. Chem. C, 2022, 10, 11211–11219.
7. W. Xiong, S. Tang, P. Murto, W. Zhu, L. Edman and E. Wang, Adv. Optical Mater., 2019, 7, 1900280.
8. S. Tang, P. Murto, J. Wang, C. Larsen, M. R. Andersson, E. Wang and L. Edman, Adv. Opt. Mater., 2019, 7, 1900451.
9. S. Xun, J. Zhang, X. Li, D. Ma and Z. Y. Wang, Synth. Met., 2008, 158, 484–488.
10. P.-C. Huang, G. Krucaite, H.-C. Su and S. Grigalevicius, Phys. Chem. Chem. Phys., 2015, 17, 17253–17259.
11. B. N. Bideh, C. Roldán-Carmona, H. Shahroosvand and M. K. Nazeeruddin, J. Mater. Chem. C, 2016, 4, 9674–9679.
12. A. K. Pal, D. B. Cordes, A. M. Z. Slawin, C. Momblona, A. Pertegás, E. Ortí, H. J. Bolink and E. Zysman-Colman, RSC Adv., 2017, 7, 31833–31837.
13. Y.-H. Su, Y.-C. Ji, Y.-T. Huang, D. Luo, S.-W. Liu, Z.-P. Yang, C.-W. Lu, C.-H. Chang and H.-C. Su, J. Mater. Chem. C, 2022, 10, 18137–18146.
14. B. Pashaei, H. Shahroosvand, H. Douroudgari, S. Abaspour, M. Vahedpour and M. K. Nazeeruddin, Inorg. Chem., 2023, 62, 7622–7635.
15. B. Pashaei, H. Shahroosvand, M. Moharramnezhad, M. A. Kamyabi, H. Bakhshi, M. Pilkington and M. K. Nazeeruddin, Inorg. Chem., 2021, 60, 17040–17050.
16. Y.-D. Lin, P.-W. Hsiao, W.-Y. Chen, S.-Y. Wu, W.-M. Zhang, C.-W. Lu and H.-C. Su, Chem. Eng. J., 2023, 469, 144055.
17. J. E. Namanga, N. Gerlitzki, B. Mallick and A.-V. Mudring, J. Mater. Chem. C, 2017, 5, 3049–3055.
18. E. C. Constable, C. E. Housecroft, G. E. Schneider, J. A. Zampese, H. J. Bolink, A. Pertegás and C. Roldan-Carmona, Dalton Trans., 2014, 43, 4653–4667.
19. G.-Y. Chen, B.-R. Chang, T.-A. Shih, C.-H. Lin, C.-L. Lo, Y.-Z. Chen, Y.-X. Liu, Y.-R. Li, J.-T. Guo, C.-W. Lu, Z.-P. Yang and H.-C. Su, Chem. Eur. J., 2019, 25, 5489–5497.
20. A. R. Hosseini, C. Y. Koh, J. D. Slinker, S. Flores-Torres, H. D. Abruña and G. G. Malliaras, Chem. Mater., 2005, 17, 6114–6116.
21. L. M. Cinninger, L. D. Bastatas, Y. Shen, B. J. Holliday and J. D. Slinker, Dalton Trans., 2019, 48, 9684–9691.
22. S. Lipinski, L. M. Cavinato, T. Pickl, G. Biffi, A. Pöthig, P. B. Coto, J. Fernández-Cestau and R. D. Costa, Adv. Optical Mater., 2023, 11, 2203145.
23. A. Jouaiti, L. Ballerini, H. Shen, R. Viel, F. Polo, N. Kyritsakas, S. Haacke, Y. Huang, C. Lu, C. Gourlaouen, H. Su and M. Mauro, Angew. Chem., Int. Ed., 2023, 62, e202305569.
24. A. Jouaiti, L. Ballerini, W.-M. Zhang, F. Polo, C. Gourlaouen, H.-C. Su and M. Mauro, Adv. Optical Mater., 2025, 13, 2402666.
25. ed. H. Yersin and K. L. Bray, Transition metal and rare earth compounds: excited states, transitions, interactions I, Springer, Berlin, Heidelberg, 2001.
26. R. Englman and J. Jortner, Mol. Phys., 1970, 18, 145–164.
27. J. V. Caspar and T. J. Meyer, J. Phys. Chem., 1983, 87, 952–957.
28. V. Balzani and S. Campagna, Photochemistry and photophysics of coordination compounds, Springer, Berlin, 2007.
29. D. G. Cuttell, S.-M. Kuang, P. E. Fanwick, D. R. McMillin and R. A. Walton, J. Am. Chem. Soc., 2002, 124, 6–7.
30. B. Hupp, C. Schiller, C. Lenczyk, M. Stanoppi, K. Edkins, A. Lorbach and A. Steffen, Inorg. Chem., 2017, 56, 8996–9008.
31. E. Fresta, M. D. Weber, J. Fernandez-Cestau and R. D. Costa, Adv. Optical Mater., 2019, 7, 1900830.
32. D. Bevk, L. Marin, L. Lutsen, D. Vanderzande and W. Maes, RSC Adv., 2013, 3, 11418.
33. A. Dessì, M. Calamante, A. Mordini, L. Zani, M. Taddei and G. Reginato, RSC Adv., 2013, 4, 1322–1328.
34. E. Fresta, J. M. Carbonell-Vilar, J. Yu, D. Armentano, J. Cano, M. Viciano-Chumillas and R. D. Costa, Adv. Funct. Mater., 2019, 29, 1901797.
35. V. Giuso, J.-Y. Zhuang, C. Gourlaouen, C. Cebrián, H.-C. Su and M. Mauro, manuscript submitted.
36. V. Giuso, C. Gourlaouen, L. Arrico, L. Di Bari and M. Mauro, manuscript submitted.
37. K. F. Baranova, A. A. Titov, A. F. Smol’yakov, A. Y. Chernyadyev, O. A. Filippov and E. S. Shubina, Molecules, 2021, 26, 6869.
38. L. Yang, D. R. Powell and R. P. Houser, Dalton Trans., 2007, 955–964.
39. A. Okuniewski, D. Rosiak, J. Chojnacki and B. Becker, Polyhedron, 2015, 90, 47–57.
40. V. Giuso, C. Gourlaouen, C. Cebrián, A. Jouaiti and M. Mauro, manuscript in preparation.
41. International Commission on Illumination, CIE 17-21-004.
42. R. Czerwieniec, M. J. Leitl, H. H. H. Homeier and H. Yersin, Coord. Chem. Rev., 2016, 325, 2–28.
43. J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau and M. E. Thompson, Inorg. Chem., 2002, 41, 3055–3066.
44. C. D. Ertl, C. Momblona, A. Pertegás, J. M. Junquera-Hernández, M.-G. La-Placa, A. Prescimone, E. Ortí, C. E. Housecroft, E. C. Constable and H. J. Bolink, J. Am. Chem. Soc., 2017, 139, 3237–3248.
45. L. Ballerini, W.-M. Zhang, T. Groizard, C. Gourlaouen, F. Polo, A. Jouaiti, H.-C. Su and M. Mauro, J. Mater. Chem. C, 2024, 12, 12769–12783.
46. F. Plasser, J. Chem. Phys., 2020, 152, 084108.
47. H.-C. Su, ChemPlusChem, 2018, 83, 197–210.
48. R.-H. Yi, C.-L. Lo, D. Luo, C.-H. Lin, S.-W. Weng, C.-W. Lu, S.-W. Liu, C.-H. Chang and H.-C. Su, ACS Appl. Mater. Interfaces, 2020, 12, 14254–14264.
49. J.-S. Lu, J.-C. Kuo and H.-C. Su, Org. Electron., 2013, 14, 3379–3384.
50. (a) CCDC 2402730: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2ln7fy; (b) CCDC 2402731: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2ln7gz; (c) CCDC 2402732: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2ln7h0; (d) CCDC 2412038: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2ln7p6.

---

Footnote

† These authors equally contributed to the work.

---