enantio-Enriched CPL-active helicene–bipyridine–rhenium complexes

Abstract

The incorporation of a rhenium atom within an extended helical π-conjugated bi-pyridine system impacts the chiroptical and photophysical properties of the resulting neutral or cationic complexes, leading to the first examples of rhenium-based phosphors that exhibit circularly polarized luminescence.

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2,2′-Bipyridine (bipy) derivatives are widely used N,N′-bidentate ligands in coordination chemistry, giving access to a great variety of complexes.¹ The luminescence properties of d⁶ transition metal polypyridyl complexes have been increasingly studied for the development of new metal-based luminescent materials and sensing probes.² Among them, [Re(N,N′)(CO)₃X]^0/+ complexes (X = halide, pyridyl (py) or isocyanide (CNR)) exhibit room-temperature (RT) phosphorescence from triplet metal-to-ligand (ML) and/or intraligand charge-transfer (ILCT) states.^3,4 Such d⁶-complexes find applications as electroswitchable emissive systems,^5a cellular imaging agents,^5b,c chromophores for photoredox chemistry,^5detc. It would therefore be of great interest to develop chiral analogues⁶ in order to benefit from the chiral version of emission, namely circularly polarized luminescence (CPL) which may potentially be used in cryptography or for 3D-displays.^7,8

In this communication, we describe the synthesis of tricarbonyl Re^I complexes of general formula [Re(N^∧N′)(CO)₃X]^0/+ (X = halide, pyridyl or isocyanide) with N^∧N′ being either achiral 3-(2-pyridyl)-4-aza[4]-helicene (1a) or chiral 3-(2-pyridyl)-4-aza[6]-helicene (M- and P-1b) (Scheme 1). The stereochemical features of these novel d⁶-complexes are presented in detail. The chiroptical properties of enantio-enriched samples and the non-polarized and circularly polarized phosphorescence were measured experimentally and analyzed using quantum-chemical calculations.

Scheme 1 Synthesis of rhenium complexes 2a–4a and enantio-enriched (M,A_Re)-2b¹, (M,C_Re)-2b² and (M,A_Re*)-3b^1,2 from respectively [4]helicene-bipy, 1a and M-[6]helicene-bipy, M-1b. (i) Re(CO)₅Cl, toluene, reflux; (ii) AgOTf, EtOH/THF, then 2,6-dimethylphenyl isocyanide, THF, NH₄PF₆, 75–80%; (iii) AgBF₄, CH₃CN, reflux then pyridine, THF, 80%. X-ray structures of racemic 2b², 3a and 4a (only (M,C^Re) stereoisomers are shown).¹⁰

Re^I-complex 2a was obtained in 85% yield as a yellow-orange precipitate upon refluxing a solution of 1a^8g and Re(CO)₅Cl in toluene for 5 hours (Scheme 1). It was fully characterized by multinuclear NMR spectroscopy (one set of peaks), by elemental analysis, UV-vis and emission spectroscopies. As compared to ligand 1a, the ¹H NMR spectrum of 2a shows strongly deshielded signals (except for H², H¹² and H⁶′) with Δδ up to +0.8 ppm for H⁵ (see ESI†). The UV-vis spectrum of ligand 1a in CH₂Cl₂ displays a strong band at 295 nm (ε > 50 × 10³ M⁻¹ cm⁻¹), accompanied by several structured bands of lower intensity between 300 and 400 nm. Meanwhile, complex 2a shows several absorption bands between 230 and 370 nm (ε ∼ 30–43 × 10³ M⁻¹ cm⁻¹) that can be assigned to intraligand π–π* transitions and a broad, low-energy absorption band between 370 and 480 nm (λ_max = 398 nm, ε = 12 700 M⁻¹ cm⁻¹) related to the incorporation of the Re^I metal and predominantly assigned as ILCT with contributions of MLCT character (vide infra). The absorption maximum at 398 nm appears red-shifted compared to the corresponding band in Re(2,2′-bpy)(CO)₃Cl^3f (350 nm) indicating extended π-conjugation.

Re^I complex 2a is red-phosphorescent in CH₂Cl₂ at RT (λ^phos_max = 678 nm, ϕ = 0.11%, τ = 25 ns, see ESI†). The phosphorescence originates from the triplet charge-transfer state. It is facilitated by spin–orbit coupling at the rhenium heavy atom and bathochromically shifted compared to that of Re(2,2′-bpy)(CO)₃Cl (λ^phos_max = 610 nm).^3f At 77 K, the phosphorescence of 2a is significantly shifted to shorter wavelengths (λ^phos_max = 550 nm, τ = 7.9 μs). Such a hypsochromic shift is usually explained by inversion of the energies of ³π–π* and ³MLCT triplet states and/or by rigidification of the system.³ Note that as usual in this class of complexes, the quantum yield at RT is rather low.³ In comparison, charged complexes of formula [Re(N^∧N′)(CO)₃py]⁺ or [Re(N^∧N′)(CO)₃CNR]⁺ typically display superior luminescence efficiency due to a stronger ligand field. For this reason, complexes 3a and 4a were prepared in good yields from 1a, according to Scheme 1. They were fully characterized by multinuclear NMR spectroscopy, elemental analysis, UV-vis spectroscopy, emission and X-ray crystallography. The 3a and 4a compounds crystallize in Fdd2 and P2₁/n centrosymmetric space groups respectively (Scheme 1). At this stage, it is worth noting that complexes 2a–4a are chiral at the rhenium centre,‡⁹ since the Re atoms adopt a slightly distorted octahedral geometry, with three carbonyl groups being fac-oriented around the Re^I, as classically seen in such rhenium(I) tricarbonyl diimine complexes.³ The equatorial planes are defined by the chelate bipyridine ligand and two trans carbonyls. A third carbonyl and either the chlorine, the isocyanide or the pyridine are placed in the apical positions. Note that in structures 3a,4a the [4]helicene-bpy ligand exhibits a helicity angle (defined as angle between the terminal rings of the helicene moiety) of ∼35° and the cyanide and pyridine ligand are directed towards it, thus defining the (P,A^Re) and (M,C^Re) stereochemistry.¹⁰ However, in solution, the helicene is not configurationally stable, and the Re center readily epimerizes (vide infra). As expected, the charged complexes displayed improved photophysical properties with similar UV-visible and emission spectra as for 2a (see ESI†), but with higher quantum yields (3a: 16%; 4a: 8.3%). These results prompted us to prepare tricarbonylrhenium(I) complexes bearing a configurationally stable enantiopure [6]helicene-bipy ligand.

Racemic 1b was reacted with Re(CO)₅Cl in refluxing toluene for 5 hours, yielding after purification by column chromatography two distinct diastereomeric Re(I) complexes (2b¹ and 2b², with 28% and 52% yields, respectively) as evidenced by ¹H and ¹³C NMR spectroscopy (for example H¹⁵: 6.7 ppm for 2b¹ and 6.9 ppm for 2b², see ESI†). Complex 2b² crystallizes in a centrosymmetric space group (P2₁/C) in which two enantiomeric structures, namely (M,C^Re)- and (P,A^Re)-2b² are present (Scheme 1).¹⁰ Note that a substantial distortion results from the bite angles between the chelating N atoms of the helicenic ligand, the rhenium centre and the chloride ligand ranging between 82.6 and 84.3°. In complex 2b² the chlorine atom is directed towards the helicene moiety, whereas it directs outwards from the helicene core in the enantiomeric complexes (M,A^Re) and (P,C^Re)-2b¹. The helicity of the aza[6]helicene moiety ranges between 47.0–66.2°, which is typical for aza[6]helicene derivatives (58° for carbo[6]helicene).⁸ Finally, complexation with Re affords an extended π-conjugation over the whole molecule, as evidenced by the small NCCN dihedral angles between the two chelating pyridine moieties (−3.1–6.0°). The extended π-conjugation and the metal-ligand interaction are evidenced by UV-vis spectroscopy since 2b^1,2 display similar UV-vis spectra with a set of several bands between 330 and 450 nm (ε ∼ 7–25 × 10³ M⁻¹ cm⁻¹) that are bathochromically shifted and more intense compared to ligand 1b, together with a very weak band observed between 450 and 500 nm (see Fig. S21, ESI†). Calculations at the BHLYP/SV(P) level with a continuum solvent model for CH₂Cl₂ reproduce well these data and show that the low-energy band of the spectrum is dominated by an ILCT transition, π(helicene) → π*(N^∧N′), while the medium-energy bands are mostly π-to-π* ‘CT-like’ transitions localized within the helicene moiety (vide infra, ESI†) in agreement with assignments of absorption spectra of related rhenium(I) systems, in particular for complexes with large π-conjugated ligands.^4c–e The overall contribution of the Re orbitals is low, meaning that the primary effect of the metal is to rigidify the system and induce strong charge transfer from the helical π-system to the bipy N^∧N′ part of the ligand. The simulated spectral shapes and band positions agree well with experiment. It is possible, though, that the overall involvement of Re orbitals in the absorption transitions is somewhat underestimated by the BHLYP functional (vide infra). Re^I complexes 2b^1,2 are red-phosphorescent emitters in CH₂Cl₂ at RT (2b¹: λ^phos_max = 680 nm, ϕ = 0.13%, τ = 27 ns; 2b²: λ^phos_max = 673 nm, ϕ = 0.16%, τ = 33 ns; for details see ESI†). At 77 K, these complexes display phosphorescence at shorter wavelengths (2b¹: λ^phos_max = 560 nm, τ = 46 μs; 2b²: λ^phos_max = 554 nm, τ = 43 μs) (vide supra). Note that the emission properties of diastereomers 2b^1,2 are only slightly different and (for τ and ϕ) within the uncertainty in the measurements (see ESI†).

Enantiopure complexes (M,A^Re)-2b¹ and (M,C^Re)-2b² were then prepared from enantiopure M-1b (their mirror-images (P,C^Re)-2b¹ and (P,A^Re)-2b² from P-1b). Enantiopure complexes 2b^1,2 display similar molar rotation (MR) values to 1b in CH₂Cl₂ {(P,C^Re)-(+)-2b¹: [ϕ]²³_D = +9260 degree cm² dmol⁻¹ (±5%), calc. BHLYP +12721; (P,A^Re)-(+)-2b²: [ϕ]²³_D = +10 260 (±5%), calc. BHLYP +11 888; P-(+)-1b:^8g [ϕ]²³_D = +12 000 (±5%), calc. BHLYP +14176, see ESI†}. The ECD spectrum of P-2b¹ shows a strong negative band around 261 nm (Δε = −114 M⁻¹ cm⁻¹) and strong positive ones at 350 (+81 M⁻¹ cm⁻¹) and 368 nm (+76 M⁻¹ cm⁻¹) accompanied by weaker bands between 380 and 450 nm (20–40 M⁻¹ cm⁻¹) and an even weaker one around 480 nm but of opposite sign (Δε ∼ −0.6 M⁻¹ cm⁻¹). Diastereomeric complex (P,A^Re)-(+)-2b² exhibits the same ECD active bands as 2b¹ but they are more intense. A comparison with experimental ECD of 1b enantiomers is displayed in Fig. 1. The calculated spectra of 2b^1,2 (BHLYP/SV(P) with a continuum solvent model for CH₂Cl₂) qualitatively agree well with the experimental results (Fig. 3 and Fig. S5, ESI†). A detailed analysis of dominant excitations in the low- and medium-energy parts of the simulated spectra of 2b^1,2 indicates that the low-energy tail of the first positive ECD band is caused by excitation no. 1 calculated at E = 3.3 eV (375 nm). The excitation can be assigned as a π–π* ILCT transition involving the helicene-centered HOMO (H), H − 1, and the bipyridine N^∧N′-centered LUMO (L), for example for 2b¹: H–L 51% and H − 1–L 18% (see Fig. 3 and ESI†). The next dominant 2b^1,2 excitation is no. 5 calculated at E = 3.8 eV (330 nm) with the strongest rotatory strength. It involves two main contributions from π and π* orbital pairs localized mostly in the helicene moiety: H–L + 1 and H − 1–L + 1 (respectively 43% and 25% for 2b¹). The excitation reveals partial CT character.

Fig. 1 Experimental CD spectra of enantiopure M-(−)-(dotted red) and P-(+)-1b (plain red) and enantiopure Re^I complexes (M,A^Re)-(−)-2b¹ and (P,C^Re)-(+)-2b¹ (light blue) and (M,C^Re)-(−)-2b² and (P,A^Re)-(+)-2b² (dark blue). Inset: CD spectra of (−)- and (+)-2b¹ between 450–550 nm.

A novel aspect of these rhenium(I) helicene-based complexes is that they are CPL active (Fig. 2, top panels).^7,8 To the best of our knowledge, these are the first examples of CPL-active phosphorescent rhenium complexes. Indeed phosphorescent (P,A^Re) and (M,C^Re)-2b² enantiomers displayed mirror-imaged CPL spectra (Fig. 2) with opposite g_lum values ((P,A^Re)-2b²: +3.1 × 10⁻³ and (M,C^Re)-2b²: −2.8 × 10⁻³) around the emission maximum (∼670 nm). These values are of the same order as for the 1b ligand enantiomers (g_lum∼ ±10⁻³) but lower than those of previously published platina[6]helicenes (g_lum ∼ ±10⁻²),^8e because Re orbitals are less involved in the helical π-system of the molecule (vide supra).

Fig. 2 CPL (upper curves within each panel) and total luminescence (lower curves within each panel) spectra of M-(−)-1b (left red), P-(+)-1b (left black), M-(−)-2b² (middle red), P-(+)-2b² (middle black), M-(−)-3b^1,2 (right red), P-(+)-3b^1,2 (right black) in degassed CH₂Cl₂ solution (1 mM) at 295 K, upon excitation at 400, 456–461, and 458–468 nm, respectively.

In order to improve the efficiency of the chiroptical and photophysical properties, tricarbonyl-isocyanide-helicene-bipy-Re^I complex M-3b was prepared (see Scheme 1) in 75% yield from either (M,A^Re)-(−)-2b¹ or (M,C^Re)-(−)-2b². In this complex, the Re center appeared labile and 3b was obtained as a mixture of (M,A^Re)-3b¹ and (M,C^Re)-3b² as observed by ¹H and ¹³C NMR spectroscopy (diastereomeric ratio 50 : 50, see Fig. S27, ESI†) regardless of the diastereomeric purity of the starting compound used (either 2b¹ or 2b² or 2b^1,2). Nevertheless, as expected, this diastereomeric mixture exhibited an improved quantum yield (λ^phos_max = 598 nm, ϕ = 6%, τ = 79 μs; see ESI†) as compared to 2b^1,2. The UV-vis spectrum of 3b^1,2 displays the same shape as 2b¹ (see Fig. S21, ESI†). Compared to (P,C^Re)-2b¹ and (P,A^Re)-2b², cationic diastereomeric mixture of Re^I complexes P-3b^1,2 demonstrates an additional positive CD-active band around 450 nm (Δε = 17.5 M⁻¹ cm⁻¹). As for 2b^1,2, this latter band does not involve the Re center, but corresponds to the H–L transition (>74%) with strong charge transfer from the π-helicene to the bipy moiety, as evidenced by BHLYP calculations (see Fig. 3 and ESI†). The appearance of the 450 nm band is caused mainly by a bathochromic shift of the first singlet excitation. This charge-transfer excitation is likely responsible for the enhancement of molar rotations as compared to 2b^1,2 {(P,AC^Re)-(+)-3b: [ϕ]²³_D = 15 040 degree cm² dmol⁻¹ (±5%) (C = 8.8 × 10⁻⁵ M, CH₂Cl₂); (M,AC^Re)-(−)-3b: [ϕ]²³_D = −14 230 (±5%) (C = 9.7 × 10⁻⁵ M, CH₂Cl₂); calc. BHLYP Boltzmann average for 3b^1,2 conformers is +14 034 degree cm² dmol⁻¹ for the P-isomers, see ESI†}.

Fig. 3 Left: experimental CD spectra of enantiopure complexes (P,C^Re)-(+)-2b¹ (light blue), (P,A^Re)-(+)-2b² (dark blue) and of (M,CA^Re)-(−)-3b^1,2 (dotted green) and (P,CA^Re)-(+)-3b^1,2 (plain green). Right: calculated CD spectra of (P,C^Re)-2b¹, and (P,A^Re)-2b² and Boltzmann-averaged spectrum for P-3b^1,2 conformers. No spectral shifts were applied. View of HOMO and LUMO of 2b¹ and 3b¹ (0.04 au). First excitation energies indicated by dots on the abscissa.

Quantum-chemical calculations of luminescence properties have been performed for 2b^1,2 and 3b^1,2. The results support the experimental assignments: The energies of T₁ → S₀ phosphorescence transitions (∼2.1 eV) are similar for both 2b^1,2 and 3b^1,2 and agree fairly well with the experimental data (Table S5, ESI†). An overestimation of the calculated versus measured energies is consistent with a blue-shift of calculated 2b^1,2 and 3b^1,2 absorption and CD spectra. The emission energies from spin–orbit (SO) calculations agree with non-SO calculations but the former allow predictions of the phosphorescence lifetimes. Application of the BHLYP functional along with the Tamm-Dancoff approximation (see ESI†) resulted in much too high emission lifetimes (Table S6, ESI†). As the involvement of Re orbitals facilitates the formally spin-forbidden T₁ → S₀ phosphorescence transitions via spin–orbit coupling, decreasing the corresponding lifetimes, too high τ calculated with BHLYP may indicate that the metal orbital contributions to the frontier MOs are somewhat too small. The performance of a given functional for singlet vs. triplet transitions is not necessarily the same. When applying a computational protocol for emission lifetimes devised recently by Mori et al.¹¹ for organometallic complexes (full TDDFT with the B3LYP functional), a dramatic improvement of the lifetimes and some lowering of the emission energies (to ∼1.9 eV) was obtained (Table S7, ESI†), which correlates with increased participation of Re orbitals in the frontier MOs at the triplet geometries. Notably the experimental trend of an increase in emission lifetime by roughly an order of magnitude when going from 2b^1,2 to 3b^1,2 is correctly reproduced with B3LYP and qualitatively consistent with lesser metal orbital participation (lesser MLCT character) in the T₁ emission transitions for 3b^1,2 as compared to 2b^1,2 (see ESI†).§^4a,b

Finally, mirror-imaged CPL spectra were obtained in CH₂Cl₂ for (M,AC^Re)-3b^1,2 and (P,AC^Re)-3b^1,2 (Fig. 2) with respective g_lum values of −0.0015 and +0.0013. Overall, cationic Re^I complexes display similar CPL characteristics as neutral ones, but combined with a higher quantum yield, the polarized emitted light is stronger. Although the Re d orbitals are not strongly involved in the electronic π-systems of these novel metallo-helicenes, the metal helps to increase the π-conjugation pathway and promotes charge-transfer excitations within the π-helical ligand. In addition, the presence of the rhenium heavy atom makes these complexes chiral phosphors with unprecedented CPL activity.

We thank the Ministère de l′Education Nationale, de la Recherche et de la Technologie, the CNRS, the ANR (10-BLAN-724-1-NCPCHEM and 12-BS07-0004-METALHEL-01) and the LIA Rennes-Durham for financial support. J.A. acknowledges the UB Center for Computational Research and thanks the National Science Foundation (CHE 1265833) for financial support. M.S. thanks the Foundation for Polish Science Homing Plus program co-financed by the European Regional Development Fund and the Ministry of Science and Higher Education in Poland scholarship. G.M. thanks NIH MBRS (1 SC3 GM089589-05 and 3 S06 GM008192-27S1) and the Henry Dreyfus Teacher-Scholar Award for financial support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Synthetic, spectroscopic and computational details. CCDC 857156, 939180 and 942143. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc00453e

‡ Note that enantiopure (+) and (−) complexes (1a)Re(CO)₃Br were prepared by chiral HPLC and they displayed very weak CD activity (Δε_max = 20 M⁻¹ cm⁻¹, see Fig. S23, ESI†).

§ The direct comparison between experimental and calculated lifetimes must be treated with some caution, as the former are also affected by non-radiative decay pathways, which may be non-negligible even at 77 K. We assume that the non-radiative decay rates for the complexes are similar under these conditions.

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