Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures

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Article

Reference

Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures

DOISTAU, Benjamin, et al.

Abstract

The CrIIIN6 chromophores are particularly appealing for low energy sensitization via energy transfer processes since they show extremely long excited state lifetime reaching millisecond range in the technologically crucial near-infrared domain. However, their properties were barely harnessed in multimetalic structures because of the lack of both monitoring methods and accessible synthetic pathways. We herein report a remedy to monitor and control the formation of CrIII-containing assemblies in solution via the design of a CrIIIN6 inert

“complex-as-ligand” that can be included into polymetalic architectures. As a proof of concept, these CrN6 building blocks were reacted in solution with ZnII or FeII to give extended trinuclear linear Cr-M-Cr assemblies, the structure of which could be addressed by NMR spectroscopy despite the presence of two slow relaxing CrIII paramagnetic centers. In addition to long CrIII excited state lifetimes and weak sensitivity to oxygen quenching, these polymetallic assemblies display controlled CrIII to MII energy transfers, which pave the way for use of the “complex-as-ligand” strategy [...]

DOISTAU, Benjamin, et al. Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures. Inorganic Chemistry, 2020, vol. 59, no. 2, p. 1424-1435

DOI : 10.1021/acs.inorgchem.9b03163

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Publication: Inorg. Chem. 2020, 59, 1091-1103. DOI: 10.1021/acs.inorgchem.9b03163

A Key Strategy for the Rational Incorporation of

Long-lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures

Benjamin Doistau,*^,a Juan-Ramόn Jiménez,^a Sebastiano Guerra,^a Céline Besnard,^b and Claude Piguet*^,a

a Department of Inorganic and Analytical Chemistry, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland

b Laboratory of Crystallography, University of Geneva, 24 quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland

KEYWORDS

chromium(III), complex-as-ligand, NIR-emitter, lifetime, assembly

ABSTRACT

The Cr^IIIN6 chromophores are particularly appealing for low energy sensitization via energy transfer processes since they show extremely long excited state lifetime reaching millisecond range in the technologically crucial near-infrared domain. However, their properties were barely harnessed in multimetalic structures because of the lack of both monitoring methods and

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accessible synthetic pathways. We herein report a remedy to monitor and control the formation of Cr^III-containing assemblies in solution via the design of a Cr^IIIN6 inert “complex-as-ligand”

that can be included into polymetalic architectures. As a proof of concept, these CrN6 building blocks were reacted in solution with ZnÎI or FeÎI to give extended trinuclear linear Cr-M-Cr assemblies, the structure of which could be addressed by NMR spectroscopy despite the presence of two slow relaxing CrÎII paramagnetic centers. In addition to long CrÎII excited state lifetimes and weak sensitivity to oxygen quenching, these polymetallic assemblies display controlled CrÎII to MÎI energy transfers, which pave the way for use of the “complex-as-ligand” strategy for introducing photophysically active CrÎII probes into light-converting polymetalic devices.

INTRODUCTION

The design of metallosupramolecular architectures containing several metallic centers raised increasing interest among coordination chemists who aim to combine the different and complementary properties produced by the pertinent association of sensitizers, activators, catalysts and/or magnetic centers in compact microscopic objects. Since the original description of polynuclear copper double-stranded helicates by Lehn,¹ and the later extension of this successful strategy to heterometallic d-f and f-f’ helicates,^2,3 the synthesis of polymetallic complexes is dominated by self-assembly processes conducted under thermodynamic control.

Hence judiciously harnessing the reversible interactions which occur in labile complexes between metals ions^4,5 and polytopic ligands, often based on polypyridyl moieties,^6,7 afforded plethora of remarkable polymetallic architectures⁸ such as cages,^9,10 grids¹¹ or helices.¹² Alternative inert metallic centers cannot benefit from this strategy, but they are not excluded from their inclusions into designed polymetallic architectures according that the “complex-as-

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ligand” strategy can be exploited.^13,14 In this case, a heteroleptic inert metal complex which holds at least one available free binding unit is preformed and used as a receptor for further complexation reactions. This strategy was shown to be particularly useful and successful for the introduction of inert RuÎI chromophores into metallosupramolecular assemblies.^15-25 These complexes were found to be appealing because of their easy structure elucidation by NMR, their compatibility with post-functionalisation methods and their ³MLCT excited state, the lifetime and energy of which are suitable for water splitting^26-29 or charge separation processes.^30-34 However, the microsecond range excited state lifetime spanned by costly RuÎI is not optimal for the design of technologically-adapted energy converting devices. The much cheaper CrÎIIN6

chromophores, which display long near infra-red (NIR) emission lifetimes that can reach the millisecond range in solution at room temperature, might be better suited for collecting and redistributing light-excitation.^35-41 Despite those outstanding photophysical properties which led to the first implementation of linear up-conversion in a molecular chromium-containing compound,^42-45 CrÎIIN6 inert complexes have been rarely introduced into designed polymetallic architectures because of (i) their only partial robustness regarding cross-coupling reactions, (ii) the scarce synthetic methods available for the preparation of heteroleptic CrÎII complexes^38,46-51 and (iii) the lack of accessible simple quantitative characterisation method in solution since high- resolution NMR spectroscopy is hampered by the long electronic relaxation time of the Cr(⁴A2) paramagnetic ground state. Nonetheless, important efforts were focused on the synthesis of multimetallic CrÎII-containing complexes for implementing magnetic properties.⁵² A first strategy exploits solvo-thermal conditions with tremendous heating to overcome the kinetic barriers for ligand exchange and reorganization, thus leading to serendipitous architectures,^53-57 among which, Winpenny and coworkers isolated remarkable and aesthetically appealing rings and

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wheels.^58-61 Some advances were brought by Piguet and coworkers at the turn of the century, when they used CrÎI as labile precursors in multimetallic self-assembling processes occurring at ambient temperature, followed by oxidation to obtain inert CrÎII containing helices.^42,62-65 The alternative “complex-as-ligand” approach relies on the preparation of inert CrÎII complexes with bridging oxalate (ox^2-) or cyanide (CN^-) binding units to give [Cr(NN’)x(ox)3-x]^{3+ 66-69} or [Cr(NN’)x(CN)6-2x]³⁺ ^70-73 building blocks. Further complexation to additional metallic cations provides extended multimetallic structures in the solid state which do not usually survive solvation processes operating in solution.^74-83 A significant improvement along this line was revealed by Long and coworkers, who took advantage of an inert capped [Cr(tacn)(CN)3] (tacn = 1,4,7-triazacyclononane) moiety for the self-assembly and crystallization of cubic architectures.^84-87 An inspiration followed by Lusby and coworkers for the building of a CrÎII/CuÎI cube with a [CrÎII(pbd)3] building block (Hpbd=1-(4-pyridyl)butane-1,3-dione).⁸⁸ However, none of those self-assembled architectures allows (i) the controlled thermodynamic formation of specific entities in solution and (ii) the monitoring of successive equilibria with the help of NMR technics. The latter point represents a severe drawback which restricts studies to solid state and probably explains the lack of rational development for the preparation of CrÎII-containing polymetallic devices along the last decades. In order to overcome those limitations, we propose here the synthesis of the heteroleptic complex [Cr(phen)2(L)](SO3CF3)3 (noted as CrL in Scheme 1, where L = 2,6-bis(N-methyl-benzimidazol-2-yl)-4-((1,10-phenanthrolin-5- yl)ethynyl)pyridine) which combines (i) a [Cr(phen)3]³⁺ chromophore showing long-lived room- temperature NIR emission,⁵¹ (ii) a tridentate bis-benzimidazole(pyridine) (bbzpy) free binding unit which is famous for providing large binding affinity for a broad panel of d or f block metals ions^89-91 and (iii) an alkyne spacer which allows sufficient electronic communication for

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promoting intermetallic energy transfers,⁴⁷ but fixes distances large enough for limiting coupling between Cr^III electronic spin and ¹H and ¹³C nuclear spins so that interpretable NMR spectra can be recorded.

Scheme 1. Cr^IIIN6 building block [Cr(phen)2(L)]³⁺ (CrL) which can be exploited in the complex- as-ligand strategy.

RESULTS AND DISCUSSION

Synthesis of the complex-as-ligand [Cr(phen)2(L)]³⁺ (CrL).

The first synthetic strategy that we attempted to exploit for connecting a preformed heteroleptic alkyne-functionalized CrÎIIN6 moiety with a bromo-bbzpy partner took advantage of pallado-catalysed Sonogashira cross-coupling reactions. While related post-modification of coordination complexes are commonly used for modifying RuÎIN6 or PtÎI chromophores,17,18,92-94

this strategy only failed in our hands because of the limited robustness of trivalent chromium complexes toward reductive and basic conditions. An alternative pathway considers the coordination of the bridging ligand L to the [Cr(phen)2(SO3CF3)2](SO3CF3) precursor (Scheme 3). The use of an unsymmetrical ligand should favour the selective coordination of only one binding unit of the ditopic ligand. The synthesis of L (Scheme 2) started with the reaction of chelidamic acid with PBr5 affording an intermediate double acyl bromide holding a bromine atom at the 4 position of the pyridine. Addition of 4-nitro-N-methylaniline in a double condensation reaction provided the diamide compound 1 with 76 % yield. Reduction of 1 with

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dithionite followed by cyclisation in acidic medium yielded 59 % of Br-bbzpy 2. 5-Ethynyl-1,10- phenanthroline and 2 were finally connected using a Sonogashira cross-coupling reaction to give the Janus-type ditopic ligand L with 83 % yield.

Scheme 2. Synthesis of the bridging ligand L.

The final coordination key step was performed following an adaptation⁵¹ of the original method developed by Kane-Maguire for the synthesis of heteroleptic tri-didentate CrN6

complexes (Scheme 3).^48,49 According to this strategy, the [Cr(phen)2(SO3CF3)2](SO3CF3) intermediate was generated by reacting [Cr(phen)2Cl2]Cl with triflic acid. To achieve further selective coordination of the phen moiety of L to Cr^III without competition with the tridentate bbzpy unit, we took advantage of (i) the bbzpy bulkiness and (ii) the slightly more basic properties of the latter tridentate unit (ΔpKa = pKa(bbzpy)- pKa(phen)  1). The addition of pyridine showing intermediate pKa thus scavenges the traces of residual triflic acid accompanying the [Cr(phen)2(SO3CF3)2](SO3CF3) precursor and induces the selective formation and precipitation of the protonated [Cr(phen)2(LH)](SO3CF3)4 complex. Final deprotonation of the bbzpy moiety by treatment with the more basic 2,6-lutidine provided the expected

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[Cr(phen)2(L)](SO3CF3)3 (CrL) complex with 63 % yield. It should be stressed here that the use of a stronger base results in hydrolysis of the complex.

Scheme 3. Synthesis of the [Cr(phen)2(L)](SO3CF3)3 (CrL) complex as ligand.

The final CrL building block was characterised by mass spectrometry (characteristic signals for {[Cr(phen)2(L)](SO3CF3)2}⁺, {[Cr(phen)2(L)](SO3CF3)}²⁺, and [Cr(phen)2(L)]³⁺ at m/z 1251.14, 551.10, and 317.75, respectively; Figures S1–S2 in the Supporting information) and by elemental analysis (Appendix S1). As expected, the long Crbbzpy separation (9Å < d < 18Å) allowed the recording of high-resolution ¹H and ¹³C NMR spectra showing interpretable signals for the non-coordinated bbzpy binding unit (Figure 1b and Figures S3-S5). However, the ¹H and

13C signals of the bound phen units are too broad for being detected because of their too close proximity with the slow-relaxing paramagnetic CrÎII center. Interestingly, the ¹³C signals of the alkyne bridge could not be detected, which allowed to set the CrÎIIC/H limit distance for high- resolution NMR signal detection to circa 9–10 Å. This CrL complex represents a rare case for which resolved NMR spectra could be recorded. To the best of our knowledge, a single NMR spectrum of a CrÎII complex bearing long pendant chains was previously reported.⁹⁵

Self-assembly of cylinder-shaped dimetallic CrÎII-containing architectures (CrLZnLCr and CrLM; M = FeÎI, ZnÎI).

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The coordination of CrL to the closed-shell Zn²⁺ ion was chosen as a benchmark reaction for exploring the thermodynamic assemblies of heterometallic complexes in solution. After having replaced CF3SO3- with BF4-by anion metathesis to get [Cr(phen)2(L)](BF4)3 (Appendix S1), its

1H NMR titration with Zn(BF4)2 in acetonitrile indicates the presence of two successive thermodynamic equilibria, for which the incriminated trinuclear [Zn(CrL)2]⁸⁺ (CrLZnLCr) and dinuclear [Zn(CrL)]⁵⁺ (CrLZn) species do not rapidly exchange on the NMR time scale (Figure 1a; Figures S6–S17).

Figure 1. a) ¹H NMR titration of CrL with Zn(BF4)2 in CD3CN at 300K, highlighting the operation of the first equilibrium transforming CrL into CrLZnLCr (Zn^IItot/CrLtot = 0 to 0.5 eq.;

a)

H_Me H_bz

H_py

H_py H_Me

H_bz

b)

CrL CrLZnLCr CrLZn

H_Me H_bz

H_py

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0.5 to 4.0 eq.; dashed arrows). b) ¹H-NMR spectra of the three successive species: CrL (red trace), CrLZnLCr (green trace) and CrLZn (blue trace).

The ¹H NMR spectra of the three species (Figure 1b), show the expected global deshielding of the pyridine protons (Hpy) upon coordination to Zn²⁺. On the contrary, the signals of the benzimidazole protons are shifted upfield in CrLZnLCr because the complexation of two orthogonal benzimidazole rings around Zn^II provides specific magnetic anisotropies, which are diagnostic for the formation of diamagnetic pseudo-octahedral D2d-symmetrical [M(bbzpy)2]²⁺

entities (M = Fe^II or Zn^II).⁹⁶ Despite the expected formation of three stereoisomers for the CrLZnLCr complex (i.e. a pair of enantiomers (Λ-CrL)Zn(Λ-LCr)/(Δ-CrL)Zn(Δ-LCr) and the meso form (Δ-CrL)Zn(Λ-LCr)), only one set of signals was observed in the NMR spectra, probably because of the long CrCr intramolecular separation between the stereogenic centers produces too small chemical shift differences for being detected. In addition, taking into account the free rotation around the alkyne bridges connecting the [Cr(phen)3]³⁺ and [Zn(bbzpy)2]²⁺

moieties, the exact C1-symmetry of CrLZnLCr transforms into an apparent D2d point group, thus simplifying the NMR spectrum (Figure 1b).

Figure 2. Speciation curves of ¹H NMR titration of CrL upon addition of Zn(BF4)2 representing the molar fraction of chromium for CrL (red), CrLZnLCr (green) and CrLZn (blue).

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 1 2 3 4

xCr(i)

2+

tot tot

Zn CrL

CrL CrLZnLCr CrLZn

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Experimental data (points) and theoretical fit with 1:2 and 1:1 successive binding model (full traces) are represented.

Speciation curves were extracted from the NMR titrations by integration (Figure 2), and fitted with a 1/2 and 1/1 successive binding model to give two cumulative binding formation constants for CrLZnLCr (log(_1,2^{Cr Zn Cr}^{L L} ) 12.8(2) ) and CrLZn (log(_1,1^{Cr Zn}^L ) 7.3(2) ). These values compare well with those reported for the formation of 1:2 ZnL2 and 1:1 ZnL complexes with tridentate ligands in acetonitrile.^97-99 The stoichiometry of the CrLZnLCr architecture was confirmed by mass spectrometry showing characteristic peaks at m/z = 246.81, 378.39, 484.06, 642.06, 905.72 and 1433.55 assigned to {[Zn(CrL)2](SO3CF3)n}^(8-n)+ ions (n = 0, 2-6; Figure S18–S20).

Similarly the ESI-MS of CrLZn shows peaks at m/z = 203.84, 438.36 and 733.01 corresponding to {[Zn(CrL)](SO3CF3)n}^(5-n)+ (n = 0, 2, 3; Figure S21–S22). Numerous crystallization attempts finally provided single crystals of [Zn(CrL)2](BF4)8 (CrLZnLCr) suitable for XRD studies (Figure 3; Appendix S2; Tables S1–S3). The CrLZnLCr complex crystallizes in the monoclinic P21/c space group with a cell volume of 17651 Å³. Interestingly, the alkyne bridges impose approximate co-planarity between the phen and bbzpy moieties, which is compatible with π- delocalization and prospective efficient intermetallic electron and/or energy transfers in related CrLMLCr units.⁴⁷ The average Zn–N bond distance of 2.1 Å is in fair agreement with 2.15 Å observed for similar monometallic complexes,¹⁰⁰ which demonstrates the very weak effect (if any) of the appended chromium chromophores on the central pseudo-octahedral [Zn(bbzpy)2]²⁺

unit. The large separation between the two stereogenic centers (intramolecular CrZn distances of 13.91 Å and 13.98 Å, and intramolecular CrCr distance of 27.53 Å) in CrLZnLCr is consistent with the absence of different sets of NMR signals observed for the diastereomeric mixture in solution. However, the three diastereoisomers ΛΛ, ΔΔ and ΔΛ contribute to the

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crystal structure since we observed on each terminal sites a mix of Δ and Λ configurations with occupancy factors around 50% for each [Cr(phen)3]³⁺ unit (Appendix S2).

Figure 3. Crystal structure of the [Zn(CrL)2](BF4)8 (CrLZnLCr) complex. Color code: Cr (yellow), Zn (sky blue), C (grey), N (blue). Hydrogens and counter anions are omitted for clarity.

Only the (Λ-CrL)Zn(Δ-LCr) stereoisomer is represented.

Because of the large separation between the paramagnetic Cr^III centers in CrL, CrLZnLCr and CrLZn, only the through-space dipolar mechanism is operative and the paramagnetic ¹H relaxation times, T1, H^para (corrected for the minor diamagnetic contribution) are affected by the chromium electronic spin according to 1T1, H^para 1 r⁶Cr-H, where rCr-H is the CrH distance.^101,102 Table 1. Longitudinal relaxation time of protons (T1, H^para) for the three complexes CrL, CrLZnLCr and CrLZn recorded at 300 K in CD3CN

Proton^a

para

T1, H/ ms at [δ / ppm]

CrL CrLZnLCr CrLZn

Hpy 8(1) [9.00] 11(1) [9.40] 8(1) [9.14]

Hbz

16(1) [7.77] 77(1) [7.60] 29(1) [8.06]

43(1) [7.61] 81(1) [7.33] 49(1) [7.86]

28(1)^a [7.37] 45(1) [7.12] 68(1)^a [7.62]

46(1) [6.65]

HMe 56(1) [4.31] 30(1) [4.44] 24(1) [4.47]

a signal accounting for two protons.

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With this in mind, the nuclear relaxation times collected in Table 1 for protons can be considered as direct estimations of their separation from the Cr^III atoms in solution, which allows direct comparisons between solid state and solution structures. For the three complexes, the protons assigned to the pyridine units (Hpy) display the shortest relaxation time (8–11 ms), which is consistent with their shortest average CrHpy distance estimated at 9.60 Å in the crystal structure. The relaxation time of the HMe protons are the longest for CrL (56 ms) and is significantly reduced for CrLZnLCr (30 ms) and CrLZn (24 ms); an observation in line with the W (trans-trans) to U (cis-cis) conformational change that shortens the CrHMe distance upon coordination of the tridentate bbzpy chelate to Zn^II.

Moreover, the promising complex-as-ligand CrL proved to be compatible with DOSY experiments, which give access to the size of the assemblies formed in solution. 2D ¹H-DOSY recorded in acetonitrile at 293 K gave diffusion coefficients DCrLZnLCr = 6.6(3) × 10^-10 m²s^-1 for CrLZnLCr, DCrLZn = 7.4(3) × 10^-10 m²s^-1 for CrLZn and DCrL = 8.4(3) × 10^-10 m²s^-1 for the complex-as-ligand CrL (Figures S23–S25). Considering that the three complexes correspond to rough cylinders with constant radius Θ but different lengths L, the diffusion coefficients can be modelled with the parametric eq. 1 (kB the Boltzmann constant, T the temperature, and η the viscosity of the medium ^{293 K}acetonitrile3.45 10 kg.m .s ^⁴ ^-1 ^-1).103-105,106

1 2

B ln 0.312 0.565 0.1

3π

k T L L L

D L   

 

       

              (1)

Assuming Θ ≈ 9 Å, which corresponds to the diameter of the [Cr(phen)3]³⁺ moiety,⁵¹ eq. 1 predicts LCrL = 20.5(5) Å, LCrLZn = 27.0(5) Å and LCrLZnLCr = 34.4(5) Å (Figure 4). The length obtained for the CrLZnLCr complex is in good agreement with the average size of 35.7(5) Å

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measured in its crystal structure, which validates this NMR methods for addressing the shapes and dimensions of Cr^III-containing architectures in solution.

Figure 4. Thermodynamic equilibrium between the CrL, CrLZnLCr and CrLZn species considered as cylinders of lengths L determined by DOSY experiment.

Related studies were conducted with open-shell Fe²⁺ instead of Zn²⁺. As expected, titrations of CrL with Fe(BF4)2 first showed the selective and quantitative formation of [Fe(CrL)2]⁸⁺

(CrLFeLCr) for Fe^II/CrLtot = 0.5 (Figure S26–S28). ¹H, ¹³C and HSQC NMR spectra (Figure S29–S31) recorded for CrLFeLCr mirror those previously discussed for CrLZnLCr, which implies a pure diamagnetic low-spin configuration for the central Fe^II cation, an observation corroborated by its deep blue colour, which is characteristic of MLCT transitions occurring in

= 27.0(5) Å

= 34.4(5) Å

= 20.5(5) Å

CrL

CrLZnLCr

CrLZn

θ= 9.0 Å

θ= 9.0 Å θ= 9.0 Å

DOSYCr

L _L

DOSYCr Zn Cr

L _{L L}

DOSYCr Zn

L _L

= 35.7(5) Å

Crystal structure Cr Zn Cr

L _{L L}

Cr Zn Cr

D L L = 6.6(3)×10^‐10m²∙s^‐1

DCrL= 8.4(3)×10^‐10m²∙s^‐1

Cr Zn

D L = 7.4(3)×10^‐10m²∙s^‐1

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low-spin Fe^IIN6 chromophores (Figure S32). Furthermore, the slightly more pronounced shielding of benzimidazole protons and the deshielding of pyridine ones in CrLFeLCr (Figure S33) compared with CrLZnLCr implies a larger magnetic anisotropy due to the shrinkage of the coordination sphere around Fe^II (average Fe-N bond distance of 1.918 Å have been reported for [Fe(bbzpy)2]²⁺).^107,108 Short paramagnetic ¹H nuclear longitudinal relaxation time T1, H^para = 33 ms measured for the methyl signal in CrLFeLCr (Table S3) combined with specific ESI-MS peaks (Figures S34–S36) and diffusion coefficient DCrLFeLCr = 5.6(3) × 10^-10 m²s^-1 (Figure S37) corroborate similar cylindrical shapes and sizes for CrLFeLCr and CrLZnLCr complexes. Upon addition of more than 0.5 eq of Fe²⁺, the formation of the CrLFe complex could be evidenced by ESI-MS spectra (Figures S38–S39) and by the appearance of three strongly paramagnetically shifted ¹H NMR signals located at 55 ppm, 29 ppm and 10.5 ppm (Figure S28). These observations are diagnostic for the complexation of a high-spin (HS) Fe²⁺ in CrLFe, where the coordination sphere is completed by weak donor atoms in solution (solvent molecules and/or BF4- counter ions). A thorough analysis of the ¹H NMR spectra collected during the stepwise transformation of CrLFeLCr + Fe²⁺  2 CrLFe allowed to assign the two low-field signals (55 and 29 ppm) to aromatic protons connected to the benzimidazole unit while the signal at 10.5 ppm corresponds to the methyl group (Appendix S3). Speciation curves constructed from ¹H NMR titrations (Figure S40) provide estimations for the cumulative binding constants

Cr Fe Cr

log(1,2^{L L} ) 13(1) and log(_1,1^{Cr Fe}^L ) 5.8(5) , which highlight the lower affinity of high-spin Fe^II(HS) for the tridentate binding site (log(^{Cr Fe}_1,1^L ) 5.8(5) ) compared to either low-spin Fe^II(

Cr Fe Cr Cr Fe

1,2 1,1

log( ^{L L} ) log(  ^L ) log(2) 7.5  ) or Zn^II (log(_1,1^{Cr Zn}^L ) 7.3(2) ).

Photophysical properties.

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The molecular CrÎIIN6 chromophores are known to display unique long-lived emissions (from μs to ms) in the 700–800 nm range, which are assigned to the metal centered CrÎII(²E→⁴A2) and CrÎII(²T1→⁴A2) spin-flip transitions.35,36,51,109-113 Moreover, these chromophores offer versatile sensitization pathways since NIR emission can be induced upon ligand-centered ¹π*←¹π (UV domain) or ³π*←¹π (400–460 nm domain), ligand-to-metal charge transfer (400–500 nm domain) or metal-centered CrÎII(⁴T2←⁴A2) (400–550 nm domain) excitations (Figure 5).⁵¹ In addition, the direct feeding of low energy CrÎII(²T1) and CrÎII(²E) levels was proved to be accessible via irradiation of the ground CrÎII(⁴A2) state in molecular complexes.^65,42,43

Figure 5. Perrin-Jablonski diagram of Cr^IIIN6 complexes with strong ligand field, showing various excitation pathways (upward coloured full arrows) and ultimate metal-centered NIR emission (downward red dashed arrows). The black dashed arrows illustrate internal relaxation (vertical) or energy transfers (horizontal).

The absorption spectrum of CrL (Figure S41; Table S4) displays the expected ligand-centered high energy ¹π*←π bands (> 27000 cm^-1; ε ≈ 10000–70000 Lmol^-1cm^-1), some intense ligand- to-metal charge transfer absorptions (LMCT; 25000–27000 cm^-1; ε ≈ 5000–20000 Lmol^-1cm^-1) and moderately intense ligand-centered spin-forbidden transitions ³π*←π and metal-centered spin-allowed transition Cr^III(⁴T2←⁴A2) (20000–25000 cm^-1; ε ≈ 100–5000 Lmol^-1cm^-1).^39,114,115

1 4

[ ( )] A2 2 1[ ( )] E

1 2

[ ( )] T2

1 2

( T1

[ )]

1 4

[ ( )] T2

Energy

Cr^III

1 4

[ ( )] A2

1 4

[ *( ) ( )] A   2

3 4

[ *( ) ( )] A   2

Ligand

3LMCT

1LMCT

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Measurements performed in highly concentrated solutions (green dashed trace in Figure 6;

Figure S41) gave access to two weak absorption bands at 729 nm (13725 cm^-1; ε = 0.16 Lmol^-

1cm^-1) and 691 nm (14472 cm^-1; ε = 0.17 Lmol^-1cm^-1) which are respectively assigned to the Cr^III(²E←⁴A2) and Cr^III(²T1←⁴A2) spin-flip transitions. The high-energy band shows a shoulder at 700 nm (14290 cm^-1; ε = 0.087 Lmol^-1cm^-1) probably arising from the splitting of the ²T1

energy level in local D3 symmetry.^38,51,116

Figure 6. Low energy part of the absorption spectrum recorded for the complex-as-ligand CrL at 293 K in methanol (green dashed trace, right vertical axis), and associated emission spectra (λexc

= 355 nm. left vertical axis) recorded at 10K in frozen acetonitrile/nitromethane (6/4) solution (blue full trace) and at 293 K in acetonitrile (red full trace).

The emission spectrum of CrL recorded at 10 K upon ligand excitation (blue full trace in Figure 6; Table S5) shows a single narrow band at 730 nm, which corresponds to the emission arising from the lowest doublet excited state Cr(²E). Time-resolved emission showed a bi- exponential decay due to the formation, upon freezing, of aggregates in which intermolecular energy migration processes provide self-quenching.47,51,117,118 The short and long lifetimes were

0 0.2 0.4 0.6 0.8 1 1.2

12000 13000 14000 15000

Normalized intensity / a.u.

Wavenumber / cm^‐1

Cr^III(²E → ⁴A2)

Cr^III(²T₁← ⁴A₂) Cr^III(²E ← ⁴A₂)

Cr^III(²E → ⁴A2)

Cr^III(²T1→⁴A2)

0.10

0.05

0 0.15

ε/ Lmol-1∙cm-1

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mono-exponential decay is recovered upon melting at room temperature. The CrL complex in frozen solution at 10 K displays an extremely long lifetime of 2.8(3) ms, which represents an outstanding advantage when using Cr^III as sensitizer at low temperature. Even if the construction of devices operating at low temperature offers few perspectives for technological applications, sensitizers operating at 10–77 K are of interest for the development and for the understanding of physical properties. For instance, low temperature sensitization was harnessed in photomagnetism,¹¹⁹ and allowed the first observation of linear upconversion phenomena implemented in molecular complexes.^42,43

Upon heating to room temperature, the CrÎII(²E→⁴A2) emission slightly broadens and is completed on its high-energy side by the appearance of the CrÎII(²T1→⁴A2) emission band (Figure 6, Table S5). This dual emission, which can be exploited for thermometric applications,¹²⁰ is due to thermal population of the excited Cr(²T1) state; an explanation supported by the identical excited state lifetimes of 112(8) and 120(8) μs, recorded for the respective CrÎII(²E→⁴A2) and CrÎII(²T1→⁴A2) emission bands in deaerated acetonitrile solutions.

The considerable drop of Cr(²E) lifetimes (one order of magnitude) upon increasing the temperature results from the operation of efficient phonon-assisted non-radiative relaxation pathways, a very common phenomenon in CrN6 complexes. Those room temperature emission lifetimes remain in the same order of magnitude as those reported for [Cr(phen)3]³⁺ derivatives^49-

51,113,121 despite the additional vibration modes brought by the bbzpy moiety. Lifetime enhancement is nonetheless conceivable upon (per)methylation of the phen unit, which reduces vibrational quenching and provides Cr(²E) lifetimes of several hundreds of microseconds.⁴⁸ Nevertheless most of the tri-didendate50,109,112,113 or di-tridentate^39,47 pseudo-octahedral chromium complexes described in the literature display shorter Cr(²E) lifetimes than CrL

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because of the considerable distortion of the coordination sphere brought by five membered chelate rings which reaches its maximum in di-tridentate complexes. On the contrary, tri- didendate CrN6 complexes with six membered chelate rings show close to perfect octahedral coordination spheres with Cr(²E) lifetimes in the millisecond range at room temperature.35,37,38,40

In summary, the maximization of room temperature Cr(²E) lifetimes in molecular complexes requires (i) important ligand field, (ii) weak distortion of the first coordination sphere compared to perfect Oh symmetry and (iii) the displacement of high-energy oscillators (C-H, N-H, O-H) from the Cr^III.^36,51

In order to have further insight into the emission process, the radiative rate constants (krad) of the two low emissive energy doublet excited states Cr(²E) and Cr(²T1) were deduced from the integration of the room-temperature absorption spectrum of CrL (dashed green trace in Figure 6) according to Einstein equation 2 (c is the light velocity in vacuum, n is the refractive index of the medium,  is the barycenter of the transition wavenumber, NA the Avogadro’s constant, gGS is the degeneracy of the ground state ( 4

Cr( A )2 4

g  ), g_ES is the degeneracy of the excited state (

21 Cr( T ) 6

g  and gCr( E)2 4) and ε is the molar absorption coefficient).^122-124

2 2 GS rad

A ES

2303 8 cn g ( )d

k N g

    

  ^



^ ^ ⁽²⁾

The resulting values of k_rad^{Cr( E)}² 33(3) s^-1 and k_rad^{Cr( T )}²¹ 43(4) s^-1indicate that the Cr(²T1) state is more emissive, which seem to mismatch the opposite relative intensities observed for the room temperature emission spectrum (red trace in Figure 6). In reality, the steady-state emitted intensity Ii reflects not only the radiative rate constant, but also the normalised excited state population Ni according to Ii = k_radⁱ Ni.⁴³ Thus harnessing Boltzmann partition for modelling the

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cm^-1 provides an intensity ratio I(²E)/I(²T1) = 18(3) (eq. 3), which is in good agreement with the experimental value of 16(3).

2 2

2 2 2

2 2

1 1

2 2 2

1 1 1

Cr( E) Cr( E)

Cr( E) rad Cr( E) rad Cr( E)

Cr( T ) Cr( T )

rad rad B

Cr( T ) Cr( T ) Cr( T )

exp Δ

I k N k g E

I k N k g k T

 

     

  (3)

In order to evaluate the importance of non-radiative processes, the intrinsic quantum yield of each emissive level (_int^Cr(i), Table 2) was computed using equation 4 and the experimental lifetimes τexp collected in Table 2. Non-radiative rate constants of k_non-rad^{Cr( E)}² = 8900(640) s^-1 and

21 Cr( T ) non-rad

k = 8300(560) s^-1 in deaerated solutions and of k_non-rad^{Cr( E)}² = 32000(2000) s^-1 and k_non-rad^{Cr( T )}²¹ = 34500(2400) s^-1 in aerated solutions can be then easily deduced (right part of eq. 4).

Cr(i)

Cr(i) Cr(i) Cr(i) rad

int rad exp Cr(i) Cr(i)

rad non-rad

k k

   

 (4)

The overall quantum yield Cr( E + T )² ² ¹

exp = 0.8(1) % measured in deaerated solution upon ligand- centered excitation (λexc = 435 nm, Table 2) compares well with _int^{Cr( E)}² _int^{Cr( T )}²¹ = 0.89(9) %, which implies that a close to quantitative Ligand-to-Cr^III sensitization process

 

2 2 2 2

1 1

Cr( E + T ) Cr( E ) Cr( T )

sens exp int int

    = 90(14)% operates in the complex-as-ligand CrL. Although the global quantum yield Cr( E + T )² ² ¹

exp is only around 1%, it remains among the largest reported for a tri-didentate Cr^IIIcomplex and overpasses most of di-tridentate based analogues except for the recent report of especially designed six-membered chelate rings.^36,40 According to literature, the [Cr(phen)3]³⁺ subunit displays a quantum yield of 0.15 % in air equilibrated water at 295 K,36,41,112,113 which does not make significant difference with the CrL quantum yield of 0.3 % measured at 295 K in air equilibrated acetonitrile solution, considering the difference of solvent and experimental errors.

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Table 2. Photophysical data of the Cr^III complexes in acetonitrile.

Complex

CrÎII(²E → ⁴A2)â CrÎII(²T1 → ⁴A2)^b

Cr exp, Ar

 (293 K)

i / %

Cr exp, Air

 (293 K)

j / %

Cr( E)2

krad c / s^-1

Cr( E)2

exp

(10 K)

d / ms

Cr( E)2

exp, Ar

 (293 K)

e / μs

Cr( E)2

exp, Air

 (293 K)

f / μs

Cr( E)2

int, Ar

 (293 K)

g / %

Cr( E)2

int, Air

 (293 K)

h / %

21 Cr( T )

krad c / s^-1

2 1

Cr( T ) exp, Ar

 (293 K)

e / μs

2 1

Cr( T ) exp, Air

 (293 K)

f / μs

2 1

Cr( T ) int, Ar

 (293 K)

g / %

2 1

Cr( T ) int, Air

 (293 K)

h / %

CrL ³³⁽³⁾ ^2.8(3) ¹¹²⁽⁸⁾ ³²⁽²⁾ ^0.37(6) ^0.10(2) ⁴³⁽⁴⁾ ¹²⁰⁽⁸⁾ ²⁹⁽²⁾ ^0.52(6) ^0.13(2) ^0.8(1) ^0.3(1)

CrLZnLCr - 3.2(3) 42(2) 36(2) 0.14(2)^k 0.12(2)^k - 41(2) 34(2) 0.18(2)^k 0.15(2)^k 0.4(1) 0.3(1)

CrLZn ^- ^3.8(3) ³⁸⁽²⁾ ³⁷⁽²⁾ ^0.12(2)^k^0.12(2)^k ^- ³⁸⁽²⁾ ³⁶⁽²⁾ ^0.16(2)^k^0.16(2)^k ^0.4(1) ^0.3(1)

a Measurements at 293 K using λem = 726 nm. ^b Measurements at 293 K done using λem = 690 nm.^c Radiative rate constants calculated using eq. 2. ^d Experimental lifetimes of solvated complexes measured at 10 K in frozen acetonitrile/nitromethane solution (5×10^-3 mol/L; λem = 730 nm, λex = 355 nm). ^e,f Experimental lifetimes measured at 293 K in acetonitrile solution (1.5×10^-4 mol/L; λex

= 355 nm) ê deaerated under argon and ^f air equilibrated. ^g,h Intrinsic quantum yields at 293 K for the low energy doublet states calculated with eq. 4 for ^g deaerated solution under argon and ^h air equilibrated solution. î,j Overall quantum yields measured at 293 K for the entire emission band (CrÎII(²E,²T1→⁴A2); λex = 435 nm) using comparative method with [Cr(ddpd)2]³⁺ as reference in acetonitrile solution (1.5×10^-4 mol/L) ⁱ deaerated under argon and ^jair equilibrated. ^kCalculated using the krad value determined for CrL.

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If one takes care of using reasonably diluted solutions (1.510^-4 to 510^-3 M) in order to avoid self-quenching processes while limiting complex dissociation in solution (Table S7), related photophysical data can be collected for the trinuclear CrLZnLCr and di-metallic CrLZn adducts.

Compared to the CrL building block, the absorption (Figure S42 and Table S4) and emission (Figure S43 and Table S5) spectra of CrLZnLCr and CrLZn are not significantly modified. The impressive long Cr(²E) excited state lifetime measured for CrL in frozen acetonitrile at 10 K (2.8(3) ms) is even slightly enhanced for CrLZnLCr (3.2(2) ms) and CrLZn (3.8(2) ms, Table 2).

However, the phonon-assisted non-radiative relaxation pathways are slightly more efficient in the two ZnÎI adducts, which might be due (i) to additional vibration modes arising from the Zn(bbzpy)2 moiety and (ii) to the coordination/decoordination dynamics in solution, thus resulting in room temperature Cr(²E) excited state lifetimes and associated quantum yields twice smaller than those found for the parent complex-as-ligand CrL (Table 2). The introduction of FeÎI as an acceptor in CrLFeLCr complex induces the complete quenching of the chromium- centered emission in the whole range of temperature (from 10 to 293K). Interestingly, a quantitative intramolecular Cr(²E)FeÎI energy transfer occurs despite (i) the long intermetallic Cr∙∙∙Fe distance (13.9 Å in the analogous CrLZnLCr complex) and (ii) the weak spectral overlap between the emission spectrum of the CrÎII donor and the absorption spectrum of the FeÎI acceptor (Figure S45). The latter energy transfer might involve the intense FeÎI(MLCT) transition, however participation of the weaker FeÎI(³T1 ← ¹A1) and FeÎI(⁵T2 ← ¹A1) transitions cannot be ruled out, which may be of interest for spin crossover purposes. We conclude that the alkyne bridges are probably particularly efficient for promoting intermetallic electronic communication and Dexter-type energy transfers, a strong point for the use of CrL as a photophysically active sensitizer in polymetallic self-assemblies.

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CONCLUSION

The selective coordination of the ditopic Janus-type unsymmetrical ligand L to a Cr^III precursor provides the CrL building block, which (i) holds a photophysically active Cr^IIIN6

chromophore connected to a free tridentate binding unit and (ii) can be used in a “complex-as- ligand” strategy for the formation of polymetallic architectures. Thanks to the use of long alkyne wires, the positively charged Cr^IIIN6 chromophore is sufficiently set aside from the active binding site to allow (i) the formation of stable CrLM and CrLMLCr adducts with additional guest cations (M = Fe²⁺, Zn²⁺) and (ii) the rare NMR monitoring of successive chemical equilibria involving slow-relaxing paramagnetic Cr^III-containing partners. Hence the CrL

“complex-as-ligand” paves the way toward the rational introduction of long-lived CrÎIIN6 NIR chromophores into designed and predictable polymetallic architectures in solution. This affords a breakthrough for replacing the serendipitous crystallizations of CrÎII-containing systems. As shown by our thorough photophysical studies, this CrL complex-as-ligand is particularly suitable for being exploited as a NIR sensitizer for lanthanide activators in future d-f luminescent devices since (i) it allows efficient energy transfers through the alkyne connector, (ii) it displays an extremely long excited state lifetime at low temperature (several milliseconds), which remains of hundred microseconds at room temperature in solution and (iii) its sensitizing characteristics are only weakly affected by the presence of closed-shell ion coordinated to the binding unit. Finally the CrL building block opens wide perspectives for the reasoned design, monitoring, and construction of polymetallic architectures for energy conversion harnessing the outstanding emission lifetime of CrÎIIN6 NIR sensitizer.

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EXPERIMENTAL SECTION

General procedures, synthetic procedures, titration procedures and photophysical details are reported in the associated electronic supporting information

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge and contains synthetic procedures, NMR characterizations, NMR titration spectrum, ESI-HRMS

characterization, crystallographic data, absorption and emission spectrum, list of transition band and list of emission lifetimes. The following files are available free of charge.

Supporting Information (PDF)

AUTHOR INFORMATION Corresponding Author

*Benjamin Doistau, E-mail: benjamin.doistau@unige.ch

*Claude Piguet, E-mail: claude.piguet@unige.ch Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources

Financial support from the Swiss National Science Foundation is gratefully acknowledged.

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ACKNOWLEDGMENT

K. L. Buchwalder is acknowledged for performing elemental analysis. Financial support from the Swiss National Science Foundation is gratefully acknowledged.

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