Rigidochromism of tetranuclear Cu(I)–pyrazolate macrocycles: steric crowding with trifluoromethyl groups

Abstract

Macrocyclic Cu(I)–pyrazolate tetramers (Cu₄pz₄) can fold into compact structures with luminescent Cu₄ cores whose emission wavelengths are sensitive to steric effects along the periphery of the macrocycle. Introducing CF₃ at the C4 position of 3,5-di-^tBu-pyrazolate increases steric crowding that modifies the conformational behavior of the Cu₄pz₄ complex, highlighted by a low-temperature martensitic transition. Variable-temperature analysis of solid-state luminescence reveal an unexpected blueshifting of emission with rising temperature.

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Solid-state luminescence continues to be a fascinating subject, rejuvenated in recent years by interests in materials that can improve solid-state lighting efficiency,¹ or respond to external stimuli for sensor or imaging applications.² Among the myriad categories of luminescent materials, Cu(I) complexes have a special appeal: Copper is an earth-abundant element and has yielded a variety of photoactive materials, many of which can be made by mixing simple salts with appropriately designed organic ligands.^3–6 Recent advances include Cu(I) emitters for thermally activated delayed fluorescence (TADF),^7–9 circularly polarized emission from chiral Cu clusters,^10,11 and efficient radioluminescence with application in X-ray imaging.¹²

Copper(I)–pyrazolate complexes are a class of luminescent clusters that have yielded many interesting examples of solid-state luminochromism. Trinuclear Cu(I)–pyrazolates (Cu₃pz₃) have been studied extensively,^6,13 but attention is being paid more recently to tetranuclear Cu₄pz₄ species which are also highly luminescent.^14–17 One important distinction is that Cu₃pz₃ structures are planar and prone to intermolecular stacking which strongly affects their emissive states, whereas Cu₄pz₄ complexes are saddle-shaped and their emission wavelengths (λ_em) are unaffected by neighbouring clusters.

We recently studied a series of Cu₄pz₄ complexes prepared from 3,5-di-^tBu-pyrazole and C4-substituted derivatives, whose solid-state emissions depend primarily on electronic transitions from triplet cluster-centred (³CC) excited states.¹⁶ A remarkable feature of these macrocyclic complexes is the strong impact of the C4 substituent on λ_em, which is steric in nature rather than electronic. For example, a tetranuclear complex made with 3,5-di-^tBu-pyrazole (Cu₄(H-pz)₄, 1) emits yellow light (λ_em 559 nm),¹⁴ whereas a complex made with 3,5-di-^tBu-4-methylpyrazole (Cu₄(Me-pz)₄, 2) emits deep blue light (λ_em 457 nm).¹⁶ The C4 methyl causes the flanking ^tBu units to adopt bisected geometries, enabling macrocycle 2 to fold into a compact, conformationally rigid structure with the four Cu atoms compressed into a close-packed rhombus (Fig. 1, lower right). This geometry limits the excited-state contraction of the Cu₄ cluster, thereby supporting deep-blue emission.¹⁶ Such long-range effects on rigidochromism motivated us to examine the influence of bulkier C4 substituents on the global conformation and photoluminescence (PL) of related Cu₄pz₄ species.

Fig. 1 Top, Cu₄pz₄ complexes 1–3 (planar view and saddle-shaped structure). Bottom, top view of 1 and 2 with vdW contours for CH₃ and endo-methyl units in ^tBu groups. The aspect ratio (d_L/d_S) of the Cu₄ rhombus is 1.43 for 1 and 1.60 for 2, X-ray data from Ref. 16.

In this paper we describe the synthesis, structure, and PL of Cu₄(CF₃-pz)₄ (3), a tetranuclear complex prepared from 3,5-di-^tBu-4-trifluoromethylpyrazole (4). The van der Waals volume of CF₃ in 3 is nearly twice that of CH₃ in 2 (39.2 vs. 21.0 ų) and thus expected to maintain neighbouring ^tBu units in bisected conformations. However, the CF₃ groups influence solid-state behaviour in unexpected ways, including a polymorphic shift at low temperature and a high-energy PL band whose intensity increases with temperature for powders and thin films.

Cu₄(CF₃-pz)₄ 3 can be formed in one step from compound 4, which in turn can be prepared from a 4-iodopyrazole precursor (Scheme 1). However, the insertion of a bulky CF₃ between two ^tBu units is synthetically challenging. After exploring several different methods, we found trifluoromethyl thianthrenium triflate (CF₃-TT⁺OTf⁻) developed by Ritter and coworkers to be an excellent CF₃ transfer agent under Cu-mediated cross-coupling conditions,¹⁸ producing 3,5-^tBu₂-4-CF₃-pz 4 in 88% overall yield after debenzylation (details in ESI†). Pyrazole 4 was then mixed with [Cu(CH₃CN)₄]BF₄ in MeOH to form Cu₄pz₄ complex 3, which precipitated as a colourless solid in 70% yield.

Scheme 1 Synthesis of Cu₄(CF₃-pz)₄ (3). (a) TT(CF₃)OTf (2 eq.), Cu⁰ (3 eq.), DMF, 60 °C. (b) 5% Pd/C (cat.), H₂ (1 atm), 1 : 1 EtOAc:MeOH, rt. (c) [Cu(CH₃CN)₄]BF₄ (1 eq.), Et₃N (1 eq.), MeOH, rt.

Cu₄(CF₃-pz)₄ 3 produces a brilliant green luminescence in the solid state with a quantum yield of 42% and decay lifetime of 27.6 μs at 300 K, indicating room-temperature phosphorescence (Fig. S1 and Table S1, ESI†). The PL spectrum of 3 at 295 K in powder form shows a peak λ_em centred at 519 nm, plus a shoulder at roughly 450 nm that is amplified and broadened in thin film samples (Fig. 2a). Excitation spectra corresponding with each emission band both show a broad peak at 280 nm (Fig. S2, ESI†), a signature of the S₀ → T₁ transition for ³CC states.¹⁵ DFT calculations of 3 confirm that the HOMO–LUMO transition is controlled through CC orbitals (Fig. 2b). The primary role of ³CC states in Cu₄pz₄ emission is remarkable, given its history as a secondary, low-energy pathway in other Cu(I) clusters.^3,19

Fig. 2 (a) PL spectra of Cu₄(CF₃-Pz)₄ 3 in powder and thin-film forms (λ_ex 270 nm, peak λ_em 519 nm); inset, luminescent powder using 254-nm excitation. (b) DFT analysis of Cu₄(Me-Pz)₄ 2 and Cu₄(CF₃-Pz)₄ 3 with HOMO and LUMO structures and energies; analysis of 2 is described in Ref. 16.

The green luminescence of 3 is contrary to our initial expectations, as the role of steric bulk at C4 in enforcing the conformational rigidity of Cu₄pz₄ has been shown with smaller substituents (R = Cl, Br, and CH₃), all having λ_em < 460 nm.¹⁶ We thus considered whether the CF₃ group was sufficiently large to (i) cause distortions in the pyrazole ring by creating torsional strain between neighbouring ^tBu groups, and (ii) direct transannular interactions between opposing pyrazolate ligands that prevent Cu atoms from adopting a close-packed geometry.

X-ray analysis of crystals grown from a toluene/CH₂Cl₂ solution of 3 confirms that the Cu₄(CF₃-pz)₄ macrocycle adopts a saddle-shaped conformation (Fig. 3a). Analysis at 150 K yields a triclinic unit cell containing two sets of four independent structures, each with a slightly different conformation but otherwise adopting the same folded geometry (Fig. 3b and c). However, whereas the Cu₄ core of Cu₄(Me-pz)₄ 2 is a planar, close-packed rhombus with a large aspect ratio (d_L/d_S = 1.60; Fig. 1),¹⁶ the Cu atoms of 3 form nonplanar quadrangles with low aspect ratios (1.03–1.16).

Fig. 3 (a) X-ray crystal structure of 3 (Conformer I) at 150 K; thermal ellipsoids drawn at the 50% probability level with H atoms removed for clarity; see Fig. S5 (ESI†) for other conformers. (b) and (c) Triclinic unit cell of Cu₄(CF₃-Pz)₄ 3 at 150 K with conformers I–IV; Cu₄ quadrangle drawn in orange with d_L/d_S in parentheses. (d) Edge view of CF₃-pz showing select F⋯H distances and dihedral angle of protruding CH₃. (e) Top view of 3 (conformer I) with vdW contours for CF₃ groups and endo-methyl units in ^tBu groups.

A close inspection of the pyrazolate ligands in 3 reveals that the CF₃ is tightly wedged against adjacent ^tBu groups with nearest-neighbour F⋯H distances of 2.15–2.35 Å, much shorter than the sum of their vdW radii (2.6–2.7 Å).²⁰ Torsional strain is reduced by (i) bending ^tBu groups out of plane by up to 10° and (ii) rotating their methyl units 12–24° away from their ideal bisected conformations (ϕ = 60°), with one unit projected nearly normal to the pyrazole ring (Fig. 3d). These distortions reflect the sizable allylic strain imposed on the ^tBu units by CF₃.

^tBu methyl groups that project inward (endo) perturb the conformation of the Cu₄pz₄ macrocycle. To reduce transannular steric interactions, the pyrazolate ligands twist so that each face is positioned directly across an opposing ^tBu unit, resulting in the interdigitation of endo methyls (Fig. 3e). The twisting of pyrazolate rings causes the Cu₄ quadrangles to buckle with bend angles of 27.4–32.9° (Fig. 3a and Fig. S8, Table S2, ESI†), and creates a sizable gap in the Cu₄ core of 3 with d_S values of 3.65–3.90 Å. In comparison, the Cu₄ rhombus of 2 has a bend angle of 0° with d_S of 3.05 Å (Fig. 1 and Table S2, ESI†).

The λ_em peak at 519 nm for 3 (Fig. 2a) is in accord with other luminescent Cu₄pz₄ complexes with nonplanar Cu₄ cores.^14–16 We have noted previously that Cu atom mobility promotes excited-state contraction and can induce a redshift in Cu₄pz₄ emission.¹⁶ In the case of complex 3, the presence of several conformers in the unit cell at 150 K indicates that the Cu₄pz₄ macrocycle can adopt multiple low-energy structures, with DFT calculations of conformers I–IV suggesting ΔΔH₀ <1 kcal mol⁻¹ (Table S5, ESI†). Although all Cu₄pz₄ conformations are stabilized by the interdigitation of ^tBu groups, time-dependent (TD) DFT analysis of their ground (S₀) and first excited triplet (T₁) states reveals very similar degrees of excited-state contraction by the Cu₄ core (Fig. S9 and S10, ESI†), confirming the importance of Cu-atom close packing in the rigidochromism of Cu₄pz₄ complexes.¹⁶

Gradual warming of 3 between 150 and 200 K induces a martensitic transition from a triclinic (P1̄) to monoclinic lattice (P2₁/c; Fig. 4). X-ray analysis at 200 K shows a single conformer with some rotational disorder in the CF₃ and ^tBu groups (Fig. S6, ESI†). The distance between Cu₄ centroids along the [01−1] direction at 150 K (13.45 Å) decreases by 1.1% in the [010] direction at 200 K and the separation of lattice planes along [001] increases by 4%, along with minor changes in Euler angles (Table 1). The Cu₄ quadrangle at 200 K has a fixed aspect ratio of 1.07 and bend angle of 28°, and a macrocyclic conformation similar to those of I–IV (RMS deviations of 0.05–0.19 Å). Further analysis of the static disorder at 200 K suggests that CF₃ reorientation drives the librational exchange of its neighbouring ^tBu groups (Fig. S7, ESI†). In addition to the low-temperature polymorphic shift, reversible phase transitions were recorded by differential scanning calorimetry at 170 and 200 K (Fig. S11, ESI†).

Fig. 4 (a) Triclinic unit cell for 3 at 150 K (P1̄; a 10.75 b 33.09 c 33.10 Å; α 104.1 β 95.6 γ 95.6°; V 11278.5 ų). (b) monoclinic unit cell for 3 at 200 K (P2₁/c; a 10.81 b 26.11 c 20.43 Å; α 90 β 99.0 γ 90°; V 5695.8 ų). Both cells are viewed along the a axis.

Table 1 Centroid distances and angles for crystalline lattices of 3 at 150 and 200 K

Parameters	150 K	200 K^a
a All Cu4pz4 clusters have equivalent conformations at 200 K.
Centroid distances (Å)
1–2	13.53	13.28
2–3	13.36	13.28
2–2′	11.61	11.40
Euler angles (deg.)
1–2–2′	60.7	61.8
3–2–2′	114.8	116.8
1–2–3	162.6	159.0

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To determine whether low-temperature phase transitions might influence solid-state emission, variable-temperature PL studies were performed on powder and thin-film samples of 3 (Fig. 5). Both samples produce strong and well-defined emission bands at 77 K (λ_em 532–537 nm); minor peaks in the violet region (400–415 nm) are also observed. The main PL band broadens and blueshifts upon warming to 300 K (513–518 nm), accompanied by a notable increase of a secondary PL band in the blue region (420–480 nm).

Fig. 5 Variable-temperature PL spectra of 3 in the solid state. (a) Powder in borosilicate glass tube (λ_ex 300 nm); (b) thin film on quartz (λ_ex 270 nm).

The higher energy PL band is curious and may be related to the lowering of excited-state energies by the strongly electronegative CF₃ group (Fig. 2). Variable-temperature analysis of PL lifetimes indicates a modest decrease in τ at 450 nm and no changes at 520 or 532 nm with rising temperature, ruling out the possibility of TADF (Fig. S5, ESI†). We postulate that complex 3 in these samples adopts numerous conformations at domain interfaces or in amorphous regions. The distribution of states can increase from several conformers below 150 K to a multitude of conformations above 200 K, with an increasing number of close-packed Cu₄ clusters that support blue emission.¹⁶ While the distributions are under thermodynamic control, the solid-state conformations are kinetically stable on the microsecond timescale and support varying degrees of rigidochromism based on their ground-state structures.

In conclusion, introducing CF₃ between two ^tBu groups on a trisubstituted pyrazole generates steric crowding that impacts the conformational and luminescence behavior of the Cu₄pz₄ macrocycle. Whereas C4-CH₃ units drive neighbouring ^tBu groups into bisected rotamers that result in a compact Cu₄pz₄ conformation with nearly close-packed Cu atoms,¹⁶ C4-CF₃ units distort local geometries that produce competing steric effects and a gap in the Cu₄ core. Overall we find that the rigidochromism of Cu₄pz₄ is best reinforced by C4 substituents of intermediate size, to support conformations that minimize excited-state reorganization of the Cu₄ cluster.

S. K. R.: synthesis, PL studies, DFT analysis; M. Z.: x-ray crystallography; S. S.: PL training; L. V. S.: DFT supervision; A. W.: research design. S. K. R. and A. W. wrote the manuscript.

We thank the US National Science Foundation (CHE-2102639, CHE-2204206), Dept. of Energy (DE-SC0018239), NIH (P30-CA023168), and Yuichiro Watanabe for discussions. Research was also partly supported through computational resources provided by Information Technology at Purdue University.

Data availability

Additional data supporting this article have been included as part of the ESI.† Crystallographic X-ray data for compound 3 at 150 and 200 K has been deposited as CIF files into the Cambridge Structural Database (CCDC 2370077, 2370073†) and can be downloaded from https://www.ccdc.cam.ac.uk/data_request/cif. DFT and TD-DFT output files (Gaussian16) will be made available upon request. Figures for X-ray structures were generated using publicly available data (Mercury 2024.1.0, Build 401958) from the Cambridge Crystallographic Data Centre (CCDC) at https://www.ccdc.cam.ac.uk/solutions/software/free-mercury/.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis details and chemical characterization, photophysical and x-ray crystallographic data, computational DFT and TD-DFT analysis. CCDC 2370073 and 2370077. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc04259j

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