Taming 2,2′-biimidazole ligands in trivalent chromium complexes

Abstract

Complete or partial replacement of well-known five-membered chelating 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands with analogous didentate 2,2′-biimidazole (H₂biim) provides novel perspectives for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with cationic partners. The most simple homoleptic complex [Cr(H₂biim)₃]³⁺ and its stepwise deprotonated analogues are only poorly soluble in most solvents and their characterization is limited to some solid-state structures, in which the pseudo-octahedral [CrN₆] units are found to be intermolecularly connected via peripheral N–H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N–H vibrations drastically quench the targeted near infrared (NIR) Cr^III-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies. Restricting the number of bound 2,2′-biimidazole ligands to a single unit in the challenging heteroleptic [Cr(phen)₂(H_xbiim)]^(1+x)+ (x = 2–0) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these [CrN₆] chromophores in the solid state and in solution, (ii) the stepwise and controlled deprotonation of the bound 2,2′-biimidazole ligand and (iii) the implementation of Cr-centered phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores as sensitizers in promising ‘complex-as-ligand’ strategies.

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Introduction

The lack of first-order orbital momentum in pseudo-octahedral trivalent chromium complexes makes isotropic and spin-only [CrX₆] units ideal partners for programming, tuning and rationalizing magnetic coupling with neighbouring paramagnetic d-block^1–5 and f-block^6–10 cations. In this context, many of the chromium-containing heterometallic assemblies exploit the kinetically inert [Cr(CN)₆]³⁻ synthon, which behaves as a ‘complex-as-ligand’ connecting cationic metals via cyanide bridges in solid state materials.^1–10 The realization that, beyond magnetic properties, both lanthanide-based light downshifting^11,12 and light upconversion^13,14 can be boosted by close Cr(III) sensitizers/emitters resulted in some active search for novel synthetic strategies, leading to Cr^III–Ln^III molecular pairs (Ln(III) is a trivalent lanthanide cation) beyond serendipitous co-crystallization processes.¹⁵ One approach involves self-assembly processes with segmental multisite ligands where labile Cr^II precursors are selectively recognized by didentate binding units while Ln^III is caught by adjacent tridentate sites. The subsequent Cr^II to Cr^III oxidation provides kinetically inert heterometallic triple-stranded Cr^III–Ln^III and Cr^III–Ln^III–Cr^III helicates with remarkable photophysical properties,^16,17 among which is the programming of the first molecular-based energy transfer light-upconversion (ETU) process.¹³

A more versatile synthetic strategy for incorporating open-shell [CrX₆] chromophores as tuneable and operable sensitizers in multimetallic (supra)molecular architectures involves extending the ‘complex-as-ligand’ strategy, originally used for introducing [Cr(CN)₆]³⁻ into multimetallic coordination polymers.^1–10,18 Taking advantage of kinetically-controlled ligand exchange processes around inert Cr(III), a few heteroleptic six-coordinate complexes could be prepared, in which an oxalate (Scheme 1a),¹² a phenyl-carboxylate (Scheme 1b),¹⁹ a difluoride (Scheme 1c)²⁰ or an extended ethyne-bis(benzimidazole)pyridine (Scheme 1d)²¹ acted as a bridging unit between the Cr(III)-based complex-as-ligand unit and some adjacent d-block or f-block partners. However, the molecular aspects of their association processes in solution remain elusive and only solid-state crystal data support the physicochemical analyses. Considering the recent recognition that strong-field [CrN₆] chromophores are ideal for maximizing phosphorescence quantum yields, emission lifetimes and sensitization in ‘molecular rubies’,^22–24 there is clearly a need for the design of novel [CrN₆] analogues working as complex-as-ligand, but using more accessible and reliable bridging units.

Scheme 1 Kinetically-inert heteroleptic six-coordinate Cr^III complexes used as complex-as-ligand when preparing multimetallic assemblies.^12,18–21

With this in mind, the 2,2′-biimidazole ligand (H₂biim, Scheme 2a)²⁵ is famous for working as a versatile bridging ligand after its binding to a metallic cation (Scheme 2b). In its protonated form, it can either form hydrogen bonds for sensing anions (mode A in Scheme 2b)^26,27 or connect other cations in a linear way (mode B in Scheme 2b).²⁸ More often,²⁹ multimetallic assemblies are obtained after stepwise deprotonation of the bound 2,2′-biimidazole ligand to give bridging Hbiim⁻ (modes C and D in Scheme 2b)³⁰ and biim²⁻ scaffolds (modes E–I in Scheme 2b).^31–34 Surprisingly, the studies dealing with the complexation of inert Cr^III centers by potentially bridging 2,2′-biimidazole are undervalued, probably due to the synthetic difficulties associated with the synthesis of these poorly soluble and pH-sensitive complexes.^35–38 To the best of our knowledge, only the molecular structures of [Cr(H₂biim)₃]³⁺,³⁶ [Cr(H₂biim)₂(Hbiim)]²⁺ (ref. 37) and [Cr(Hbiim)₃]³⁶ have been characterized in the solid state following poorly reproducible and serendipitous crystallization from intricate mixtures of ligands and metals in the presence of various amounts of counter-anions and/or solvent molecules. A recent synthetic improvement, which used anhydrous THF under microwave heating, gave [Cr(H₂biim)₃](NO₃)₃ in 94% yield (Fig. 1).³⁵

Scheme 2 a) Successive acid–base equilibria of fully protonated 2,2′-biimidazole (H₄biim²⁺) with pK_a values measured in DMF : H₂O = 7 : 3 (I = 0.1 M).²⁵ (b) Different coordination modes encountered in the literature for H₂biim (A^26,27 and B²⁸), Hbiim⁻ (C and D)³⁰ and biim²⁻ (E–I).^31–34

Fig. 1 Synthesis of [Cr(H₂biim)₃]³⁺ (ref. 35) and [Cr(Hbiim)₃].³⁶ The molecular structures of the complexes are those found in the crystal structures of [Cr(H₂biim)₃](NO₃)₃ (CCDC-603707) and [Cr(Hbiim)₃]·C₆H₆·2H₂O (CCDC-603730).³⁶ Color code: C = grey, N = blue, H = white, and Cr = orange.

Beyond the detailed descriptions of (i) sophisticated hydrogen-bonding networks in which the latter homoleptic complexes are a part and (ii) some expected axial flattening produced by didentate five-membered chelating 2,2′-biimidazole bound to Cr^III,^36–38 no effort has been focused on the thermodynamic deprotonation processes and the associated control of the photophysical properties. Moreover, to the best of our knowledge, no attempt to prepare heteroleptic [L₂Cr(H₂biim)]³⁺ has been made, while related systems with inert 4d and 5d metal ions have been designed regularly to access the two crucial pK_a values for their use as complex-as-ligand (Table S1 in the ESI†).^25,39–47 In order to provide new perspectives for exploiting 2,2′-biimidazole as a bridging ligand between photophysically-active Cr^III and promising open-shell lanthanides, we report here on the molecular structures and photophysical properties of the accessible and isolable homoleptic complexes [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ in solution and in the solid state. The second part proposes the synthesis of the unprecedented heteroleptic [(phen)₂Cr(H₂biim)]³⁺ complex with its detailed acid–base and photophysical properties, which become accessible in the solid state and in solution.

Results and discussion

Preparation and structures of the homoleptic [Cr(Me₂biim)₃]³⁺, [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ complexes

2,2′-Biimidazole (H₂biim) was synthesized from ammonium acetate and glyoxal under aqueous conditions with moderate yield.⁴⁸ Its methyl derivative 1,1′-dimethyl-2,2′-bi-1H-imidazole (Me₂biim) could be obtained by deprotonation, followed by methylation with methyl iodide (Fig. 2, top).⁴⁹

Fig. 2 Synthesis of ligands 2,2′-biimidazole (H₂biim) and 1,1′-dimethyl-2,2′-bi-1H-imidazole (Me₂biim) and their homoleptic [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃], [Cr(Hbiim)₂(biim)]⁻ and [Cr(Me₂biim)₃]³⁺ complexes. The molecular structures of the metallic complexes are those found in the associated crystal structures. Color code: C = grey, N = blue, H = white, and Cr = orange. The counter-ions and hydrogen atoms (for Me₂biim ligands) are omitted for clarity.

The original literature synthesis of [Cr(H₂biim)₃](NO₃)₃ involved the reaction of CrCl₃·3THF with [Ag(H₂biim)]NO₃ in MeOH³⁶ or in THF³⁵ (Fig. 1) because the formation of highly insoluble AgCl drove the reaction to completion. To simplify the procedure, the intermediate [Ag(H₂biim)]NO₃ was not isolated in this work, but was formed in situ by mixing AgNO₃ and H₂biim in MeOH. Then CrCl₃·3THF was added into the resulting solution of [Ag(H₂biim)]NO₃. The purification method was identical to that used in ref. 35 and [Cr(H₂biim)₃](NO₃)₃ was isolated in good yield (Fig. 2). Recrystallization by vapor diffusion of Et₂O into a methanolic solution provided crystals suitable for X-ray diffraction, the structural resolution of which at 120 K confirmed the crystal structure previously reported for [Cr(H₂biim)₃](NO₃)₃ at 290 K (Fig. 2, Tables S2–S3 and Fig. S1†).³⁶ The synthesis of [Cr(Hbiim)₃] was first described by Gruia et al.,³⁶ where they used a stochiometric amount of NaOMe to deprotonate [Cr(H₂biim)₃]³⁺ in MeOH. However, the complete insolubility of the formed neutral [Cr(Hbiim)₃] complex did not allow any reproducible recrystallization techniques. To overcome this problem, a methanolic solution of [Cr(H₂biim)₃]³⁺ was treated in this work with vapor diffusion of an excess of volatile triethylamine, the limited pK_a of which (10.74)⁵⁰ prevented any double deprotonation of the bound Hbim ligand and finally gave crystals of [Cr(Hbiim)₃] with good yield (61%) and in a reproducible way (Fig. 2, Tables S4–S6 and Fig. S2†). Further deprotonation of [Cr(Hbiim)₃] could not be obtained by Gruia et al.,³⁶ but some partial reports of analogs of [Cr(biim)₃]³⁻, i.e. Ba_1.5[Co(biim)₃]⁴¹ and K₃[Ru(biim)₃]⁵¹ that were obtained using harsh basic conditions (aqueous NaOH 2 M/BaCl₂ and ^tBuOK 1.2 M in MeOH, respectively) have been noted. Consequently, [Cr(Hbiim)₃] was dissolved in an excess of aqueous NaOH (0.5 M) until a clear yellow solution was formed. The addition of a concentrated solution of PPh₄Cl immediately resulted in the formation of an orange precipitate. Recrystallization from MeOH by vapor diffusion of ^tBuOMe provided block-shaped crystals of [Cr(Hbiim)₂(biim)]PPh₄(CH₃OH) suitable for X-ray diffraction analysis in a good yield (66%) (Fig. 2, Tables S7–S9 and Fig. S3†). A detailed geometrical analysis of these homoleptic complexes (Appendix 2 in the ESI†) concludes that the [Cr(H_xbiim)₃]ⁿ⁺ units display standard pseudo-octahedral arrangements of the six bound nitrogen donor atoms, with a compression along the pseudo-C₃ axis due to the 79.6–80.4° ligand bite angles (Table A2-1 in Appendix 2†), which is characteristic of five-membered chelating polyaromatic ligands²³ as reported for related [Cr(bipy)₃]³⁺ (bipy = 2,2′-bipyridine, 79.1°)⁵² and [Cr(phen)₃]³⁺ (phen = 1,10-phenanthroline, 81.0°) complexes.⁵³ The Cr–N distances in [Cr(H_xbiim)₃]ⁿ⁺ do not vary drastically (2.028–2.037 Å, compared with averages of 2.042 Å for [Cr(bipy)₃]³⁺ and 2.051 Å for [Cr(phen)₃]³⁺) and do not show obvious correlations with the degree of deprotonation of the bound ligand. Due to the chelation of semi-rigid didentate polyaromatic ligands, some standard trigonal distortions characterize all these complexes (Table A2-1 in Appendix 2†).

The main difference between [Cr(bipy)₃]³⁺ and [Cr(phen)₃]³⁺ on one hand, and the family of [Cr(H_xbiim)₃]ⁿ⁺ complexes, is associated with the bound didentate 2,2′-biimidazole ligands which may act as N–H hydrogen-bond donors when they are protonated and as N⁻ hydrogen-bond acceptors when they are deprotonated. For the fully protonated [Cr(H₂biim)₃](NO₃)₃ complex, the weak N–H⋯O hydrogen bonds observed between the bound ligand and the nitrate counter-anion (Fig. A2-2†) are responsible for the circa 400 cm⁻¹ decreases of the N–H stretching frequency in the vibrational spectrum with respect to the free ligand (ν(NH) ≈ 3000 cm⁻¹ in Fig. S14 and S18-top, Table S29†). The formation of strong intermolecular N–H⋯N hydrogen bonds observed in the deprotonated complexes [Cr(Hbiim)₃] (Fig. A2-4†) and [Cr(Hbiim)₂(biim)]PPh₄ (Fig. A2-6†) further weakens the N–H bond force constants and stepwise shifts the associated stretching frequency toward ν(NH) ≈ 2400 cm⁻¹ (Fig. S15 and S18 center, Table S30†) and ν(NH) ≈ 2300 cm⁻¹ (Fig. S16 and S18 bottom, Table S31†), respectively. In this context, the replacement of hydrogen atoms with methyl groups to give [Cr(Me₂biim)₃](CF₃SO₃)₃via the Kane-Maguire synthetic strategy (Fig. 2) maintains the pseudo-octahedral structure of the [CrN₆] core (Tables S16–S17 and Fig. S7†),⁵⁴ but it limits intermolecular hydrogen bonds in the crystal structure (Fig. A2-10 in Appendix 2†).

The spectroscopic properties of the ligands H₂biim and Me₂biim and the chromium complexes [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ could be recorded in methanol or in acetonitrile. Unfortunately, the neutral complex [Cr(Hbiim)₃] is not soluble enough in common organic solvents to perform reliable measurements in solution. The electrospray ionization mass spectrometry (ESI-MS) spectra (Fig. S19–S38 and Tables S33–S35†) show complicated mixtures in the gas phase containing various amounts of intact 1 : 3 complexes with variable degrees of deprotonation ([Cr(H_xbim)₃]ⁿ⁺) together with (i) partial ligand dissociation to give 1 : 2 complexes ([Cr(H_xbim)₂]ⁿ⁺ + H₂biim) and (ii) metal reduction into Cr(II)-based systems. All peaks could be identified from high-resolution MS spectra with a special emphasis on the detection of the ‘full’ deprotonated [Cr(Hbiim)₂(biim)]⁻ anion (negative mode) for [Cr(Hbiim)₂(biim)]PPh₄ (Fig. S25 and Table S34†). It is concluded that these deprotonatable complexes suffer from the ionization process and exist as fragmented mixtures in the gas phase.

Photophysical properties of the homoleptic [Cr(Me₂biim)₃]³⁺, [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ complexes

The absorption (Fig. 3 and 4a) and emission (Fig. 4b and S39†) spectra of the ligands H₂biim and Me₂biim and the complexes [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ could be recorded in MeOH. Unfortunately, the neutral complex [Cr(Hbiim)₃] is not soluble enough to perform spectroscopy except for solid-state investigations (Fig. S40†). The UV parts of the absorption spectra are dominated by intraligand-centered π* ← π transitions (ILCT), which are split and red-shifted upon coordination to Cr³⁺ in the associated complexes. Additional LMCT (Ligand-to-Metal Charge Transfer) transitions are known to also contribute within the 330–400 nm range for these [CrN₆] chromophores (Fig. 3).^29,55–57 The spin-allowed, but parity forbidden ligand-field Cr(⁴T₂ ← ⁴A₂) transition (intensity 10 < ε < 100 M⁻¹ cm⁻¹, highlighted in Fig. 3b) can be detected as a minor contribution to the low-energy tail of the charge transfer bands.^58–63 Careful spectral deconvolutions are required (Fig. S41–S43 and Tables S36–S39†) to safely assign E(⁴T₂), i.e., the energy of the Cr(⁴T₂) excited level with respect to the ground state E(⁴A₂) = 0 (Table 1, column 2).

Fig. 3 UV-visible absorption spectra of (a) non-coordinated ligands H₂biim (purple) and Me₂biim (dark blue) and (b) complexes [Cr(H₂biim)₃](NO₃)₃ (orange), [Cr(Hbiim)₂(biim)]PPh₄ (green) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (red) in MeOH at 293 K.

Fig. 4 NIR (a) absorption spectra of [Cr(H₂biim)₃](NO₃)₃ (water) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (acetonitrile) and (b) emission (λ_exc = 330 nm) spectra of complexes [Cr(H₂biim)₃](NO₃)₃ (orange), [Cr(Hbiim)₂(biim)]PPh₄ (green) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (red) in MeOH at 293 K.

Table 1 Energy of intrashell d–d transitions (in MeOH solution for E(⁴T₂) and in the solid state for E(²T₁) and E(²E)), ligand-field strength Δ (eqn (1)) and Racah parameters B (eqn (2)) and C (eqn (3)) for [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄, [Cr(Me₂biim)₃](CF₃SO₃)₃, and [Cr(phen)₂(H₂biim)]³⁺ and for closely related pseudo-octahedral [Cr^IIIN₆] chromophores^a

Complex	E(^4T2)/cm^−1	E(^2T1)/cm^−1	E(^2E)/cm^−1	Δ/cm^−1	B/cm^−1	C/cm^−1	E(^4T2) − E(^2E)/cm^−1	Δ/B	C/B	Ref.
[Cr(H2biim)3]^3+	20 268	14 376	13 717	20 268	717	2845	6551	28.3	4.0	This work
[Cr(Hbiim)3]	—	14 263	13 670	—	—	—	—	—	—	This work
[Cr(Hbiim)2(biim)]^−	19 956	14 096	13 482	19 956	686	2828	6474	29.1	4.1	This work
[Cr(Me2biim)3]^3+	21 026	14 164	13 538	21 026	712	2779	7488	29.6	3.9	This work
[Cr(phen)2(H2biim)]^3+	23 202	14 381	13 723	23 202	766	2697	9479	30.3	3.5	This work
[Cr(bpy)3]^3+	23 400	14 450	13 800	23 400	765	2730	9600	30.6	3.6	60
[Cr(phen)3]^3+	22 075	14 451	13 736	22 075	779	2700	8339	28.3	3.5	53
[Cr(tpy)2]^3+	18 750	13 584	12 953	18 750	790	2512	5797	23.7	3.2	61
[Cr(ddpd)2]^3+	22 990	13 550	12 903	22 990	756	2419	10 087	30.4	3.2	23
[Cr(dqp)2]^3+	24 937	13 864	13 405	24 937	656	2791	11 532	38.0	4.3	24
[Cr(dpc)2]^+	19 200	—	9370	19 200	470	1880	9830	40.9	4.0	62
[Cr(CN)6]^3−	26 600	—	12 400	26 600	480	2800	14 200	55.4	5.8	63
[Cr(bik)3]^3+	23 094	14 771	14 044	23 094	804	2737	9050	28.7	3.4	57
[Cr(bim)3]^3+	21 008	14 859	14 104	21 008	781	2842	6904	26.9	3.6	57
[Cr(bie)3]^3+	20 747	14 837	14 124	20 747	754	2902	6623	27.5	3.8	57

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Interestingly, for pseudo-octahedral d³ complexes, E(⁴T₂) provides a straightforward estimation of the ligand-field splitting (eqn (1)),^64,65 which covers a narrow 19 956 ≤ Δ ≤ 21 026 cm⁻¹ range for [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₂(biim)]⁻ and [Cr(Me₂biim)₃]³⁺ in solution (Table 1).

Δ = E(⁴T₂) − E(⁴A₂) (1)

As established for the programming of spin crossover processes while tuning the ligand-field strength,⁶⁶ the replacement of didentate 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), made of two connected 6-membered heterocyclic rings, with 2,2′-biimidazole (H₂biim), made of two connected five-membered heterocyclic rings, is accompanied by an increase of the trigonal distortion in their pseudo-octahedral complexes. This is measured by a stepwise increase in the θ angular distortion (θ[Cr(phen)₃]³⁺ = 47.5° < θ[Cr(bpy)₃]³⁺ = 63.6° < θ[Cr(Me₂biim)₃]³⁺ = 80.4° computed with eqn (A2-2) and gathered in Table A2-1, see Appendix 2†), which results in a concomitant stepwise reduction of the ligand-field strength Δ[Cr(bpy)₃]³⁺ ≈ Δ[Cr(phen)₃]³⁺ ≈ 22 500 cm⁻¹ > Δ[Cr(Me₂biim)₃]³⁺ ≈ 20 500 cm⁻¹ > Δ[Cr(tpy)₂]³⁺ = 18 750 cm⁻¹ (Table 1).

The interelectronic repulsion is estimated using the Racah parameters B (eqn (2)) and C (eqn (3)), which requires the energy of the lowest doublet levels E(²E) and E(²T₁) to be accessible (Fig. 5).^64,65

Fig. 5 Jablonski diagram of homoleptic [Cr(H₂biim)₃]³⁺ or [Cr(Me₂biim)₃]³⁺ complexes showing the antenna effect upon UV excitation at 330 nm. The modeling of the energies of the Cr levels is taken from ref. 64.

Assuming an O_h symmetry, the ground state absorption bands Cr(²E ← ⁴A₂) and Cr(²T₁ ← ⁴A₂) have very weak intensities (0.05 < ε < 1 M⁻¹ cm⁻¹) due to the breaking of both parity and spin conservation rules. These transitions could be detected in the NIR domain of the solid-state absorption spectra of the four investigated complexes (Fig. S40†), while related solution data could be recorded only for the most soluble [Cr(H₂biim)₃](NO₃)₃ (c ≥ 10⁻² M in water) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (c ≥ 10⁻² M in acetonitrile) complexes (Fig. 4a). Introducing E(⁴T₂), E(²E) and E(²T₁) into eqn (1)–(3) provides the Racah parameters B and C collected in Table 1. Interestingly, 686 ≤ B ≤ 717 cm⁻¹ found for [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ points to a global increase of the nephelauxetic effect when didentate 2,2′-bipyridine or 1,10-phenanthroline type ligands bound to Cr³⁺ (765 ≤ B ≤ 790 cm⁻¹) are replaced with 2,2′-biimidazole type ligands.

For absorption spectra recorded in solution (Fig. 4a), it is possible to calculate the radiative rate constant of the emissive levels ²E and ²T₁ using the Strickler–Berg eqn (4) ^67,68, which is derived from Einstein's relationship for spontaneous emission (Table 2, column 2 and Table S40†).⁶⁹

Table 2 Experimental Cr(²E) lifetimes (τ) and radiative (k_rad) and non-radiative (k_non-rad) relaxation rate constants and intrinsic quantum yields (ϕ^intrinsic_complex = k_rad/(k_rad + k_non-rad)) of chromium complexes

Complex	k rad/s^−1	τ ^293 K, Ar ^a/μs	k ^293 K, Arnon-rad ^a/10^3 s^−1	ϕ ^intrinsiccomplex 293 K, Ar	τ ^293 K, air ^b/μs	k ^293 K, airnon-rad ^b/10^3 s^−1	ϕ ^intrinsiccomplex 293 K, air	τ ^77 K ^c/μs	k ^77 Knon-rad/s^−1	ϕ ^intrinsiccomplex 77 K	Ref.
a Recorded in degassed solution at room temperature. b Recorded in air-equilibrated solution at room temperature. c Recorded in a frozen solvent mixture at 77 K. d In MeOH. e In MeOH/H2O. f In CH3CN. g In CH3CN/C2H5CN. h In H2O. i In H2O/DMSO. j In aq. HClO4/MeOH. k In aq. HCl.
[Cr(H2biim)3]^3+	75(4)	0.47(2)^d	2120(106)	3(1) × 10^−5	0.44(2)^d	2260(113)	3(1) × 10^−5	2950(150)^e	263(13)	2.0(6) × 10^−1	This work
[Cr(Hbiim)2(biim)]^−	—	0.178(9)^d	—	—	0.141(7)^d	—	—	2780(140)^e	—	—	This work
[Cr(Me2biim)3]^3+	73(4)	4.28(4)^d	234(12)	3(1) × 10^−4	3.31(9)^d	302(1)	2.4(8) × 10^−4	3005(150)^e	260(13)	2.2(7) × 10^−1	This work
[Cr(phen)2(H2biim)]^3+	65(3)	38(1)^f	26(7)	2.5(8) × 10^−3	14(1)^f	71(7)	9(3) × 10^−4	2940(140)^g		1.9(6) × 10^−1	This work
[Cr(bpy)3]^3+	182	74^h	13.3	1.4(4) × 10^−2	52^h	19	9(3) × 10^−3	5000^i	18	9(3) × 10^−1	54, 58 and 70–72
		39^d	25.5	7(2) × 10^−3	29^d	34	5(2) × 10^−3	6500^j	≈0	≈1
[Cr(phen)3]^3+	319	356^h	2.49	1.1(4) × 10^−1	74^h	13	2.4(7) × 10^−2	2100^g	157	6.7(2) × 10^−1	53, 54, 58 and 70–73
		34^d	29.1	1.1(3) × 10^−2	22^d	45	7(2) × 10^−3	5300^j	≈0	≈1
		224^f	4.15	7(2) × 10^−2	37^f	27	1.2(3) × 10^−2
		270^k	3.38	9(3) × 10^−2
[Cr(tpy)2]^3+	—	—	—	—	0.14^f	—	—	540^i	—	—	61, 74 and 75
								670^g	—
[Cr(dqp)2]^3+	30	1270^h	0.76	4(1) × 10^−2	83^h	12	2.5(8) × 10^−3	3070^i	296	9(3) × 10^−2	24
		2140^f	0.44	6(2) × 10^−2	31^f	32	9(3) × 10^−4		296	9(3) × 10^−2
[Cr(CN)6]^3−	97	—	—	—	0.12^i	8330	1.2(4) × 10^−5	3950^i	156	4(1) × 10^−1	72
[Cr(bik)3]^3+	31	209^f	4.75	7(2) × 10^−3	128^f	7.8	4(1) × 10^−3	8220^g	91	2.6(8) × 10^−1	57
[Cr(bim)3]^3+	37	131^f	7.60	5(2) × 10^−3	73^f	14	2.7(8) × 10^−3	6750^g	111	2.5(8) × 10^−1	57
[Cr(bie)3]^3+	60	7^f	143	4(1) × 10^−4	8^f	125	5(2) × 10^−4	5250^g	130	3(1) × 10^−1	57

---

Here c is the speed of light in vacuum (cm s⁻¹), n is the refractive index of the solvent, N_A is the Avogadro number (mol⁻¹), g_GS is the degeneracy of the ground state (g(⁴A₂) = 4), g_ES is the degeneracy of the excited state (g(²T₁) = 6 and g(²E) = 4), [small nu, Greek, tilde] is the barycenter of the transition in the wavenumber (cm⁻¹) and is the area under the absorption spectrum of each transition (M⁻¹ cm⁻², Fig. S44 and S45†). The radiative rate constants of the emissive Cr(²E) excited level estimated for [Cr(H₂biim)₃]³⁺ (k_rad = 75(4) s⁻¹) and [Cr(Me₂biim)₃] (k_rad = 73(4) s⁻¹) are smaller than those reported for [Cr(bpy)₃]³⁺ (k_rad = 182 s⁻¹) and [Cr(phen)₃]³⁺ (k_rad = 319 s⁻¹, Table 2),^58,69–73 but fall within the expected range for pseudo-octahedral [CrN₆] chromophores (Table S40†).^70–77 Upon room-temperature ligand-based excitation at 330 nm in solution, the complexes [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ show the expected downshifted NIR spin–flip Cr(²E → ⁴A₂) phosphorescence at 730–740 nm together with weak shoulders corresponding to Cr(²T₁ → ⁴A₂) within the 700–710 nm domain (Fig. 4b and 5). The latter dual emission disappears at 77 K (Fig. S39†) as a result of the depopulation of the high-energy Cr(²T₁) level, and a single band Cr(²E → ⁴A₂) contributes to phosphorescence. Very similar results are observed in the solid state upon 330 nm excitation for the four complexes [Cr(H₂biim)₃](NO₃)₃, [Cr(Hbiim)₃], [Cr(Hbiim)₂(biim)]PPh₄ and [Cr(Me₂biim)₃](CF₃SO₃)₃ (Fig. S40b†), which ultimately demonstrate (i) the expected negligible Stokes shifts affecting the spin–flip Cr(²T₁,²E ↔ ⁴A₂) transitions (Fig. S46†) and (ii) the efficient sensitization of the spin–flip phosphorescence by all the accessible ligand-based excited states (Fig. S47†).

Upon pulsed laser excitation at 355 nm at 77 K in frozen MeOH/H₂O solutions, the characteristic lifetime of the emissive Cr(²E) levels for [Cr(H₂biim)₃](NO₃)₃ (τ^{77 K} = 3.0(1) ms), [Cr(Hbiim)₂(biim)]PPh₄ (τ^{77 K} = 2.7(1) ms) and [Cr(Me₂biim)₃](CF₃SO₃)₃ (τ^{77 K} = 3.0(1) ms) tends toward the radiative lifetime τ_rad = 13.4(7) ms, in agreement with minor non-radiative vibrational quenching constants at this temperature for rigid triple-helical units, as similarly reported for [Cr(bpy)₃]³⁺ and [Cr(phen)₃]³⁺ (Table 2 column 8). At room temperature, the Cr(²E) lifetimes drastically drop below the microsecond range for [Cr(H₂biim)₃](NO₃)₃ and [Cr(Hbiim)₂(biim)]PPh₄, which possess high-energy N–H oscillators, while the decrease of the lifetime for the methylated analogue [Cr(Me₂biim)₃](CF₃SO₃)₃ (τ^{293 K, Ar} = 4.28(4) μs) is less dramatic and mirrors those reported for [Cr(bpy)₃]³⁺ and [Cr(phen)₃]³⁺ (Table 2 and Fig. S48–S56†). Consequently, the non-radiative rate constant k^{293 K, Ar}_non-rad = 1/τ^{293 K, Ar} − k_rad = 2.1(1) × 10⁶ s⁻¹ measured for [Cr(H₂biim)₃](NO₃)₃ in MeOH corresponds to the largest vibrational quenching process along the series of [Cr(N^∩N)₃]³⁺ chromophores collected in Table 2. Finally, the presence of ³O₂ in solution has only a minor effect on the lifetime, indicating that quenching by oxygen is not a major contributor to energy relaxation in these systems (τ^{293 K, air} in Table 2, column 6).

Acid–base properties of homoleptic [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ complexes

In order to extract the acid–base thermodynamic constants connecting [Cr(H₂biim)₃]³⁺ with its successive deprotonated forms [Cr(H₂biim)₂(Hbiim)]²⁺ (K_a1), [Cr(H₂biim)(Hbiim)₂]⁺ (K_a2), [Cr(Hbiim)₃] (K_a3) and [Cr(Hbiim)₂(biim)]⁻ (K_a4), we performed two successive pH-metric titrations of aqueous solutions of [Cr(H₂biim)₃](NO₃)₃ using NaOH as the titrant. An initial addition of 0.2 equivalents of HNO₃ ensured that the complex exists initially in its fully protonated form [Cr(H₂biim)₃]³⁺ (Fig. 6). The pH curve profile is different from the standard pH-metric titration expected for a weak polyacid with a strong base due to the concomitant precipitation of insoluble [Cr(Hbiim)₃] (Fig. 6). It is therefore not possible to extract the searched pK_a values and one can only estimate qualitatively pK_a1, pK_a2 < 7. Moreover, the preservation of [Cr(Hbiim)₃] as a precipitate in the presence of a large excess of base indicates its negligible deprotonation for pH ≤ 12 in water and pK_a4 > 12.

Fig. 6 Reproducible acid–base titrations (blue trace for titration 1 and red trace for titration 2) of a solution of [Cr(H₂biim)₃](NO₃)₃ + 0.2 eq. HNO₃ with NaOH (water, 293 K) highlighting the formation of an insoluble yellow precipitate.

Preparation and structures of the heteroleptic [Cr(phen)₂(H_xbiim)]^(1+x)+ (x = 2–0) complexes

In order to limit the number of bound deprotonatable ligands and the concomitant formation of insoluble neutral complexes, we followed the Kane-Maguire synthetic strategy for preparing inert heteroleptic [Cr(phen)₂(H₂biim)]³⁺ (Fig. 7).⁵⁴ Taking advantage of the trans influence,⁷⁸ the complexation of 2.0 eq. of 1,10-phenanthroline (phen) to CrCl₃ yields almost quantitatively cis-[Cr(phen)₂Cl₂]⁺ under reducing conditions for catalyzing Cr(III)/Cr(II) exchange processes and ligand-exchange dynamics.⁵³ The replacement of inert Cr–Cl bonds with labile Cr–OSO₂CF₃ bonds^79–81 was performed under soft conditions by using Ag(O₃SCF₃) instead of an excess of triflic acid.²⁴ The addition of H₂biim finally provided [Cr(phen)₂(H₂biim)](CF₃SO₃)₃ in moderate yield (37%, Fig. 7). Subsequent deprotonation with aqueous NaOH under stoichiometric conditions gave [Cr(phen)₂(Hbiim)](CF₃SO₃)₂, whereas the use of an excess of base yielded the doubly deprotonated complex [Cr(phen)₂(biim)](CF₃SO₃). X-ray quality monocrystals could be grown by slow diffusion of diethyl ether into solutions of the complexes either in acetonitrile to give [Cr(phen)₂(H₂biim)](CF₃SO₃)₃(H₂O)_0.25 (Fig. 7 and Tables S18–S20†) or in methanol to provide [Cr(phen)₂(biim)](CF₃SO₃)(CH₃OH)_1.5 (Fig. 7 and Tables S24–S26†). X-ray quality prisms of [Cr(phen)₂(Hbiim)](CF₃SO₃)₂(H₂O)_0.5 (Fig. 7 and Tables S21–S23†) were isolated from a cooled (4 °C) aqueous solution.

Fig. 7 Synthesis of the heteroleptic cis-[Cr(phen)₂Cl₂]⁺ (CCDC-1865022),⁵³ [Cr(phen)₂(H₂biim)]³⁺, [Cr(phen)₂(Hbiim)]²⁺ and [Cr(phen)₂(biim)]⁺ complexes. The molecular structures of the metallic complexes are those found in the associated crystal structures. Color code: C = grey, N = blue, H = white, and Cr = orange. The counter-ions and hydrogen atoms (for 1,10-phenanthroline ligands) are omitted for clarity.

The molecular structures of the three [Cr(phen)₂(H_xbiim)]^(1+x)+ cations (x = 2–0) display pseudo-octahedral [CrN₆] chromophores with bond lengths and trigonal distortions typical of chromium complexes bound by three didentate 5-membered chelate polyaromatic ligands (Appendix 3†), similar to the discussion in the previous section for the homoleptic analogues [Cr(H₂biim)₃]³⁺, [Cr(Hbiim)₃] and [Cr(Hbiim)₂(biim)]⁻ (see Appendix 2†). For the heteroleptic complexes, one notices that the Cr–N bond lengths are shorter for the bound biimidazole ligands compared to those of the bound phen ligands (d_Cr–N(biim) < d_Cr–N(phen), Table 3). The contraction of the d_Cr–N(biim) bond lengths upon stepwise deprotonation can be assigned to the increased basicity of the bound biimidazole ligand. The compensating longer d_Cr–N(phen) bond lengths result from the reduced charge borne by the central chromium metal. In terms of intermolecular hydrogen bonds, the bound protonated H₂biim ligand in [Cr(phen)₂(H₂biim)](CF₃SO₃)₃(H₂O)_0.25 acts as a NH donor for acceptor oxygen atoms of triflate counter-anions and interstitial water molecules. For the mono-deprotonated bound Hbiim⁻ ligand in [Cr(phen)₂(Hbiim)](CF₃SO₃)₂(H₂O)_0.5, intermolecular hydrogen bonds between two adjacent complexes through N–H⋯N bonds are observed (Fig. A3-3†), as previously described for [Cr(Hbiim)₃] (Fig. A2-4†). Finally, the totally deprotonated bound biim²⁻ ligand in [Cr(phen)₂(biim)](CF₃SO₃)(CH₃OH)_1.5 is not involved in hydrogen bonding.

Table 3 Comparison of the average Cr–N bond lengths (d̄_Cr–N)^a of complexes [Cr(phen)₂(H₂biim)](CF₃SO₃)₃(H₂O)_0.25, [Cr(phen)₂(Hbiim)](CF₃SO₃)₂(H₂O)_0.5 and [Cr(phen)₂(biim)](CF₃SO₃)(CH₃OH)_1.5 in their crystal structures

Complex	d̄ Cr–N/Å	d̄ Cr–N(phen)/Å	d̄ Cr–N(biim)/Å
a The standard deviations refer to deviations from the computed averages.
[Cr(phen)2(H2biim)](CF3SO3)3	2.04(2)	2.054(3)	2.024(7)
[Cr(phen)2(Hbiim)](CF3SO3)2	2.05(2)	2.062(6)	2.018(0)
[Cr(phen)2(biim)]CF3SO3	2.05(4)	2.07(1)	1.997(5)

---

In the IR spectra of the heteroleptic complexes, the O–H stretching vibrations of co-crystallized water or methanol molecules involved in hydrogen bonding (3500 ≤ ν_OH ≤ 2500 cm⁻¹) hinder a straightforward interpretation of N–H stretching bands associated with the bound H_xbiim ligands (x = 2–0; Fig. S57†). On the other hand, and as previously mentioned for the related homoleptic complexes, the ESI-MS spectra recorded in acetonitrile do not vary significantly with the degree of protonation of the bound 2,2′-biimidazole ligands for the different [Cr(phen)₂(H_xbiim)]^(1+x)+ cations (x = 2–0, Fig. S58†). High-resolution ESI-MS analyses confirm the formation of [Cr(phen)₂(Hbiim)]²⁺ as the major gas-phase cation, regardless of the degree of protonation of the bound 2,2′-biimidazole ligand in the selected complex (Table S41 and Fig. S58–62†).

Acid–base properties of the heteroleptic [Cr(phen)₂(H₂biim)]³⁺ complex

The successive deprotonation of bound 2,2′-biimidazole in [Cr(phen)₂(H₂biim)]³⁺ has been quantitatively studied by pH metric titrations in water at fixed ionic strength (0.1 M KNO₃, Fig. 8a). The two successive pH jumps (5 ≤ pH ≤ 8 and 9 ≤ pH ≤ 11) are accompanied by concomitant abrupt changes in colors from yellow ([Cr(phen)₂(H₂biim)]³⁺) to dark orange ([Cr(phen)₂(Hbiim)]²⁺) and finally to dark brown ([Cr(phen)₂(biim)]⁺, Fig. 8a). The associated occupancy factors experimentally obtained with eqn (5) (red circles in Fig. 8b) display a two-step binding isotherm typical of the anti-cooperative successive fixation of two protons according to equilibria (6)–(7) and modeled with eqn (8). The best fits for the two successive deprotonation steps of [Cr(phen)₂(H₂biim)]³⁺ (black trace in Fig. 8b) correspond to pK_a1 = 4.67(3) and pK_a2 = 8.59(11), which are approximately eight orders of magnitude more acidic than those measured for free 2,2′-biimidazole (pK_a1 = 12.31 and pK_a2 = 16.33),²⁵

Fig. 8 (a) Titration of 51 mg (52 μmol) of [Cr(phen)₂(H₂biim)](CF₃SO₃)₃ (10 mL aqueous KNO₃ 0.1 M, c = 5.2 mmol L⁻¹) with NaOH 0.1 N highlighting the color changes and (b) associated binding isotherm depicted as plots of experimental (red circles, eqn (5)) and fitted (dashed black trace, eqn (8)) occupancy factors as a function of log(|H⁺|).

Compared with [Co^III(en)₂(H₂biim)]³⁺ (pK_a1 = 5.5 and pK_a2 = 9.9; ionic radius = 0.545 Å),⁴¹ the one order of magnitude lower pK_a measured for [Cr^III(phen)₂(H₂biim)]³⁺ (ionic radius = 0.615 Å) suggests that the {Cr^III(phen)₂}³⁺ scaffold (with respect to {Co^III(en)₂}³⁺) better stabilizes the negative charges brought by the bound deprotonated 2,2′-biimidazole ligand. Consequently, upon successive deprotonation, one can reasonably predict the appearance of low energy (phen)π* ← (H_xbiim)π (x = 2–0) intramolecular ligand-to-ligand charge transfer (LLCT) bands in the absorption spectra of [Cr^III(phen)₂(H_xbiim)]^(1+x)+ upon deprotonation. These transitions are confirmed by TD-DFT calculations (Fig. A4-2 to A4-10 and Tables A4-6 to A4-8 in Appendix 4†) and indeed observed in solution (Fig. 9).

Fig. 9 Absorption spectra of (a) [Cr(phen)₂(H₂biim)]³⁺ (orange trace), [Cr(phen)₂(Hbiim)]²⁺ (red trace) and [Cr(phen)₂(biim)]⁺ (black trace) in CH₃CN (c ≈ 10⁻⁵ mol L⁻¹) and (b) [Cr(phen)₂(H₂biim)]³⁺ in CH₃CN (c ≈ 10⁻² mol L⁻¹). LMCT = ligand to metal charge transfer and LLCT = ligand to ligand charge transfer.

Finally, taking advantage of the pH-dependence of the absorption spectra, the determination of the pK_a values in water at ionic strength close to zero (I ≈ 0) could be carried out with the help of spectrophotometry to give pK_a1 = 3.9(1) and pK_a2 = 7.8(1) (Fig. S63†), which are close to those determined at I = 0.1 M (KNO₃).

Photophysical properties of the heteroleptic [Cr(phen)₂(H_xbiim)]^(1+x)+ (x = 2–0) complexes

The three heteroleptic complexes exhibit similar and intense absorption bands in the UV range (below 350 nm, Fig. 9a), which can be assigned to intraligand π–π* transitions (ILCT) completed by variable amounts of interligand (LLCT), ligand to-metal (LMCT) and metal-to-ligand (MLCT) charge transfer bands (see Appendix 4† for TD-DFT calculations). The spectra differ in the visible domain (350–750 nm, highlighted in Fig. 9a) mainly due to a shift of the interligand (phen)π* ← (H_xbiim)π (x = 0–2, LLCT) transitions toward lower energies upon successive deprotonations (Fig. S64 and Fig. A4-4, A4-7 and A4-9†). This shift is responsible for the color change accompanying the stepwise deprotonation of [Cr(phen)₂(H₂biim)]³⁺ (Fig. 8a). According to theoretical TD-DFT calculations (Table 1, entries 1–3) and CASSCF(7,12)/FIC-NEVPT2 (Table A4-2†) and in line with the trend observed in the homoleptic complexes, the ligand-field strength Δ, reminiscent of Cr(⁴T₂ ← ⁴A₂) transitions in octahedral geometry, also decreases with stepwise deprotonation: [Cr(phen)₂(H₂biim)]³⁺ (412.9 nm, Δ ≈ 24 219 cm⁻¹) > [Cr(phen)₂(Hbiim)]²⁺ (434 nm, Δ ≈ 23 042 cm⁻¹) > [Cr(phen)₂(biim)]⁺ (457.8 nm, Δ ≈ 21 844 cm⁻¹). This reduction in the ligand-field strength is accompanied by (i) a decrease of the computed total spin density at the chromium center (Fig. A4-1†) which reflects the reduced total positive charge borne by this atom and (ii) a negligible change in the estimated Racah parameters B and C (Table A4-1†). One can thus predict that the stronger interactions with the deprotonated 2,2-biimidazole ligand (d̄_Cr–N(biim) is becoming shorter) in [Cr(phen)₂(H_xbiim)]^(1+x)+ (x = 2–0) are more than compensated by the removal of the bound 1,10-phenanthroline ligands (d̄_Cr–N(phen) is becoming longer) as exemplified in the molecular structures in the crystalline state (push–pull effect, Table 3).

The NIR absorption spectrum of [Cr(phen)₂(H₂biim)]³⁺ (Fig. 9b) exhibits two well-resolved absorption bands that are assigned to the spin–flip transitions Cr(²E ← ⁴A₂) and Cr(²T₁ ← ⁴A₂) assuming O_h symmetry and with ε = 0.27 and 0.19 M⁻¹ cm⁻¹, respectively. For the deprotonated derivatives [Cr(phen)₂(Hbiim)]²⁺ and [Cr(phen)₂(biim)]⁺, the larger residual interligand charge transfer bands mask these weak forbidden spin–flip transitions (Fig. S65†). Consequently, Racah parameters B = 766 cm⁻¹ and C = 2697 cm⁻¹ (eqn (2) and (3), Table 1, entry 4) together with the radiative rate constant of k_rad = 65(3) s⁻¹ (eqn (4), and Table 2, column 1), typical of [CrN₆] chromophores, could be estimated only for [Cr(phen)₂(H₂biim)]³⁺.

At room temperature in CH₃CN, only [Cr(phen)₂(H₂biim)]³⁺ is emissive and shows the typical dual Cr(²T₁ → ⁴A₂) and Cr(²E → ⁴A₂) emission observed for many Cr(III) complexes (Fig. 10a). In addition to [Cr(phen)₂(H₂biim)]³⁺, the NIR emission of the complex [Cr(phen)₂(Hbiim)]²⁺ can be detected at low temperature in frozen solvent mixtures (CH₃CN/C₂H₅CN 6 : 4 at 77 K, Fig. 10b), whereas [Cr(phen)₂(biim)]⁺ remains non-emissive. Compared with the emission band of [Cr(phen)₂(H₂biim)]³⁺ (λ_max = 732 nm; [small nu, Greek, tilde] = 13 661 cm⁻¹), the first deprotonated analog [Cr(phen)₂(Hbiim)]²⁺ shows a red-shifted Cr(²E → ⁴A₂) transition at λ_max = 750 nm; [small nu, Greek, tilde] = 13 333 cm⁻¹ (Fig. 10b). Additionally, the spectrum of [Cr(phen)₂(Hbiim)]²⁺ contains a band foot at 733 nm originating from a small amount of [Cr(phen)₂(H₂biim)]³⁺, which is inevitably present in solution due to the proton-transfer equilibrium (9):

Fig. 10 Emission spectra (λ_exc = 350 nm) of (a) [Cr(phen)₂(H₂biim)]³⁺ at 293 K in CH₃CN and (b) [Cr(phen)₂(H₂biim)]³⁺ (orange trace) and [Cr(phen)₂(Hbiim)]²⁺ (red trace) at 77 K in frozen CH₃CN/C₂H₅CN (6 : 4).

Introducing K_a1 and K_a2 gives K_exch = 1.2(2) × 10⁻⁴, from which the ratio of the equilibrium concentration implies contamination of [Cr(phen)₂(Hbiim)]²⁺ by circa 1% with the more emissive (protonated) [Cr(phen)₂(H₂biim)]³⁺ complex (red trace in Fig. 10b).

Excited state lifetimes for the NIR emission arising from the Cr(²E) excited state upon excitation at 355 nm were recorded in solution at room temperature and at 77 K for [Cr(phen)₂(H₂biim)]³⁺, and only at 77 K for [Cr(phen)₂(Hbiim)]²⁺ (Fig. S67–S70†). In frozen solutions at 77 K, [Cr(phen)₂(H₂biim)]³⁺ displays a mono-exponential decay of 2.94 ms, which is typical of Cr(III)-polyimine complexes (Table 2). The emission decay curve of [Cr(phen)₂(Hbiim)]²⁺ could not be fit with a mono-exponential function. Since the complex exists as a 98 : 1 : 1 mixture of [Cr(phen)₂(Hbiim)]²⁺, [Cr(phen)₂(H₂biim)]³⁺ and [Cr(phen)₂(biim)]⁺ (eqn (9)), one expects multi-exponential decays weighted by the mole fractions and the quantum yields of each contributor. A rough bi-exponential fit (Fig. S70†) is compatible with the experimental decay curves showing a long component (1.77(9) ms), which is reminiscent of [Cr(phen)₂(H₂biim)]³⁺, and a short contribution (723(30) μs) which is tentatively assigned to [Cr(phen)₂(Hbiim)]²⁺, for which the smaller energy gap between the emissive doublet state level Cr(²E) and the silent LLCT band probably boosts the efficiency of non-radiative decay (Fig. 11 and Appendix 4†). At room temperature, only the protonated complex [Cr(phen)₂(H₂biim)]³⁺ is emissive and it exhibits a mono-exponential emission decay of 38 μs in deaerated CH₃CN. This excited state lifetime is orders of magnitude longer than those of the three isolated homoleptic parent complexes [Cr(H_xbiim)₃]ⁿ⁺ (Table 2) due to the replacement of two 2,2′-biimidazole ligands with two 1,10-phenanthroline units, which are devoid of high-energy N–H stretching vibrations. The lifetime of [Cr(phen)₂(H₂biim)]³⁺ is in the same range as that of [Cr(bpy)₃]³⁺; however, it is one order of magnitude shorter than that of [Cr(phen)₃]³⁺ (Table 2). The emission lifetime is reduced to 14 μs in aerated solution because of some additional quenching via energy transfer to the ³O₂ molecules present in solution.

Fig. 11 Jablonski diagram of heteroleptic [Cr^III(phen)₂(H_xbiim)]^(1+x)+ complexes showing the antenna effect upon UV excitation at 330 nm, the existence of low-energy ligand-to-ligand charge transfer excited levels and their potential unfavorable effect on the global quantum yields via back energy transfer (BET) processes.

Finally, the global luminescence quantum yield of the complex [Cr(phen)₂(H₂biim)]³⁺ upon ligand-based excitation at λ_exc = 450 nm was determined experimentally using the relative method (Fig. S71†). We found ϕ^global_complex = 2.8(3) × 10⁻³ in deaerated acetonitrile and ϕ^global_complex = 9.5(9) × 10⁻⁴ in the presence of dioxygen at room temperature. One further notes that the intrinsic Cr(III)-centered quantum yields calculated with the help of the emission lifetimes τ_tot(²E) and k_rad(²E) collected at room temperature (Table 2 and eqn (10)) amount to ϕ^intrinsic_complex = 2.5 × 10⁻³ in deaerated acetonitrile and ϕ^intrinsic_complex = 9.1 × 10⁻⁴ in aerated acetonitrile.

Since (i) ϕ^global_complex = η_sens·ϕ^intrinsic_complex and (ii) ϕ^global_complex ≃ ϕ^intrinsic_complex, one concludes that the ligand-to-metal sensitization process is close to being quantitative (η_sens ≈ 100%). These values are in the same range as those reported for [Cr(bpy)₃]³⁺ (ϕ^{global, no air}_complex = 1.7 × 10⁻³ in deaerated acetonitrile and ϕ^{global, air}_complex = 8.9 × 10⁻⁴ in aerated water),^70,71,79,80 but four times smaller than that of [Cr(phen)₃]³⁺ (ϕ^{global, no air}_complex = 1.2 × 10⁻² in deaerated water + 1 M HCl),^53,81 which makes [Cr(phen)₂(H₂biim)]³⁺ a moderate emitter for a Cr(III) complex.⁷²

Conclusions

In line with the only minor interest attracted by the homoleptic [Cr(H_xbiim)₃]ⁿ⁺ during the past few decades,^35–38 we confirm here that their low solubility in any common solvent upon stepwise deprotonation, together with their poorly attracting photophysical properties (a short phosphorescence lifetime and negligible intrinsic quantum yield), makes them poorly adapted to be involved in the complex-as-ligand strategy for programming the assemblies of polymetallic optically-active Cr-based complexes. The solution to the problem arises from the successful synthesis of the soluble heteroleptic [Cr(phen)₂(H_xbiim)]^(1+x)+ (x = 2–0) complexes via a modified Kane-Maguire strategy. The bound H₂biim ligand can be stepwise deprotonated in solution at pH compatible for further complexation processes with d-block or f-block cations. The deprotonation processes are accompanied by characteristic color changes resulting from the appearance of low-energy intramolecular interligand (phen)π* ← (H_xbiim)π (x = 2–0, LLCT) ligand-to-ligand charge transfer transitions confirmed by theoretical TD-DFT calculations. The latter reversible process makes the putative heterometallic dyads [Cr(phen)₂(biim)–M^z+]^(z+1)+ (M^z+ is an open-shell d- or f-block cation) reminiscent of protonated [Cr(phen)₂(H₂biim)]³⁺ in terms of photophysical properties, which paves the way for their use as sensitizers in luminescent polymetallic assemblies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Swiss National Science Foundation is gratefully acknowledged (grant 200020_207313).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2355642–2355650. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt01608d

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