Synthesis and chiroptical properties of hexa-, octa- and deca-azaborahelicenes: influence of the helicene’s size and of the number of boron atoms

Abstract

Four members of a new class of cycloborylated hexa- to deca-helicenes (1a–d) have been prepared in enantiopure forms, along with cycloplatinated deca-helicenes (1d′, 1d¹) further extending a family of cycloplatinated hexa- to octa-helicenes reported previously. The azabora[n]helicenes display intense electronic circular dichroism and strong optical rotations, whose dependence on the size of the helix (n = 6, 8, or 10) and number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (unpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives (1a¹–d¹).

Graphical Abstract

Cycloborylated hexa- to deca-helicenes have been prepared in enantiopure forms, along with cycloplatinated deca-helicenes. The chiroptical (electronic circular dichroism and optical rotation) and photophysical properties (unpolarized and circularly polarized luminescence) of these new chiral emissive helicenes have been studied as a function of the helicene’s size and boron atoms number.

Introduction

Heteroatomic polycyclic aromatic hydrocarbons (heteroatomic PAHs) are attractive due to their potential use as materials for organic electronics such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), or organic photovoltaics (OPVs).^[1] Representative achiral heteroarenes are, for instance, graphene analogues doped with heteroatoms (Si, S, B, N, P).^[2] Heterohelicenes, on the other hand, may be considered as chiral heteroatomic PAHs.^[3] Within this class of molecules, borahelicenes and azaborahelicenes have revealed strong emission (non-polarized and circularly polarized) properties^[4a,b] and interesting carrier-transport abilities.^[4c,d] In these molecular systems, the C=C units are replaced by isoelectronic B=C or B=N moieties, which represent two attractive means of modifying the electronic properties of achiral and chiral PAHs without changing their molecular size.^[5] Aside from the 3-coordinate boron derivatives, extended π-conjugated molecules incorporating 4-coordinate boron groups are also under current development and have demonstrated potential applications in OLEDs, OFETs, photoresponsive materials, sensors and imaging materials.^[6] For instance, Wang and co-workers synthesized a series of boron complexes bearing phenyl-pyridine-type C^N chelates which display a reversible photo-thermal color switching.^[7a–c] A similar type of compound has been recently used for two-photon absorption and fluorescence microscopy.^[7d] However, to our knowledge similar chiral 4-coordinate boron derivatives based on helicenes are still not known.

In this paper, we report the synthesis of azabora[n]helicenes of different size (from n = 6 up to 10 ortho-fused cycles) bearing one or two 4-coordinate boron atoms (1a–d in Figure 1). Strong chiroptical properties, blue fluorescence and circularly polarized luminescence (CPL) of these readily prepared novel heterohelicenes are examined and analyzed with the help of quantum-chemical calculations in terms of the size of the helicene and number of heteroatoms, and compared with previously described platina[n]helicenes (1a¹–c¹, Figure 1),^[8a,b,e,f] as well as the novel bis-platina[10]helicene 1d¹.

Figure 1.

Chemical structures of the azaborahelicenes studied and of their corresponding platina[n]helicenes analogues.

Results and Discussion

Synthesis and characterization of mono- and bis-azabora[n]helicenes (n = 6, 8, 10) and bis-platina[10]helicene

In 2010, our group reported the first helicenes incorporating a metal ion into the helical backbones, named metallahelicenes.^[8] For instance, we developed a straightforward strategy for the synthesis of platina[6]helicenes, from a benzophenanthrene substituted with a 2-pyridyl. This ortho-fused π-conjugated phenyl-pyridine type ligand underwent a cycloplatination with a platinum source such as K₂PtCl₄ or Pt(dmso)₂Cl₂ followed by a reaction with sodium acetylacetonate (Scheme 1a).^[8a–c] This synthetic strategy was generalized to the larger cycloplatinated hepta- and octa-helicenes,^[8e] and extended to other metal ions, such as iridium^[8a] or osmium.^[8g] Herein, we describe the extension of the approach to include heteroatoms such as boron in the helicene structures.

Scheme 1.

a) Synthesis of platina[6]helicene 1a¹. b) General synthetic route to the azaborole derivative studied here.

Murakami et al. have recently reported a simple synthesis^[9a] for azaborole derivatives in a two-step process starting from 2-phenylpyridine. First, a cycloborylation reaction of 2-phenylpyridine with BBr₃ in the presence of N(ⁱPr)₂Et affords a B,B-dibromo-azaborole system, which then can react with AlR₃, where the bromine atoms are replaced by alkyl groups to form a B,B-dialkyl-azaborole derivative (Scheme 1b). The method takes advantage of the nitrogen atom which directs the electrophilic aromatic borylation to the ortho-position. Interestingly, such dialkyl-azaborole derivatives display blue fluorescence both in solid and in solution states, with high quantum yields.^[9b–d]

Following a similar synthetic route, we obtained a series of azaborahelicenes containing different numbers of fused cycles in a helicene backbone and different numbers of boron atoms, namely azabora[6]helicene 1a, azabora[8]helicene 1b, bis-azabora[6]helicene 1c, and bis-azabora[10]helicene 1d (Figure 1 and Scheme 2).

Scheme 2.

Synthetic routes to azabora[n]helicenes 1a-d. i) BBr₃, NⁱPr(Et)₂, CH₂Cl₂, 25°C, 24 hrs; ii) AlMe₃, CH₂Cl₂, 30 min.

The racemic azabora[6]helicene 1a was prepared in a two-step reaction from previously described 1-fluoro-4-(2-pyridyl)-benzo[g]phenanthrene 2a.^[8a] The fluoro substituent has a protective role, enforcing regioselective photocyclization during the preparation of 2a (see ref. [8a]). Furthermore, contrary to the methoxy group, commonly used in the platina[n]helicene series, the fluorine is inert to the borylation conditions. Using the procedure depicted in Scheme 2, racemic compound (±)-1a was thus obtained in 89% overall yield and then fully characterized (see Supplementary Information, SI). For example, the ¹H NMR shows a doublet at 7.64 ppm corresponding to the proton H³ (³J_HF = 8.9 Hz), and two signals at 0.09 and 0.37 ppm for the methyl groups that are diastereotopic due to the presence of the helicene. The ¹¹B NMR displays one broad signal at 0.3 ppm and the ¹⁹F NMR shows a doublet at −120.4 ppm, (³J_HF = 8.9 Hz, coupling with H³).

Similarly, racemic azabora[8]helicene 1b was synthesized from the racemic ligand 2b which corresponds to the [6]helicene substituted at position 1 by a 2-pyridyl group (Scheme 2). 2b was prepared as described in Scheme 3. A Wittig reaction starting from the benzo[c]phenanthrylmethyl-phosphonium bromide 3b^[10] and 2-fluoro-5-(pyridine-2-yl)benzaldehyde 4b^[8a] yielded stilbene derivative 5b in 92% yield. Then a photocyclization reaction in a highly diluted toluene solution under irradiation for 16 hours using a mercury lamp with in situ oxidation (using catalytic amounts of iodine under air) generated the target racemic ligand 2b in 56% yield. Note that 2b corresponds to a functionalized carbo[6]helicene; it is therefore configurationally stable and exists as a racemic mixture of both P and M enantiomers. Finally, the subsequent ortho-borylation and methylation of 2b yielded azabora[8]helicene 1b with 64% overall yield. The last two steps are convenient for increasing the helicene’s size from a hexa- to an octa-helicene. Both 2b and 1b were fully characterized (see SI). The ¹H NMR spectrum of the ligand 2b shows for example the proton H³ resonating at 6.79 ppm as a doublet of doublets due to coupling with the F atom (³J_HF = 9.3 Hz) and with H² (J = 8.3 Hz). For 1b, the ¹H NMR demonstrates two upfield shifted signals for the diastereotopic methyl groups at −0.40 and −0.43 ppm and a doublet at 7.21 ppm corresponding to the proton H³ (³J_HF = 8.9 Hz). The ¹¹B NMR displays one broad signal at −0.7 ppm and the ¹⁹F NMR shows a doublet at −121.0 ppm, with the same ³J_HF coupling constant of 9.3 Hz (coupling with H³). Note that the NMR signals for the [8]helicene derivative appear at higher field than for the [6]helicene one, presumably due to the anisotropy cone effect of the longer helicene core.

Scheme 3.

Synthesis of racemic ligands 2b, 2d and 2d¹. i) 4b or 4b¹, n-BuLi, THF, Ar, r.t., 2 hrs; ii) hv, cat. I₂, air, toluene, 16 hrs.^$

Racemic bis-azabora[6]helicene 1c was prepared in two steps from the already known ligand 2c^[8e] in 55% overall yield according to the procedure presented in Scheme 2. This double cycloborylation enables a [6]helicene incorporating two boron atoms to be obtained in a straightforward manner from a 1,8-bi-pyridyl-naphthalene scaffold. The ¹H NMR of 1c is in agreement with a C₂-symmetric molecule and shows two upfield shifted signals for the diastereotopic methyl groups at 0.31 and 0.10 ppm. The ¹¹B NMR displays one broad signal at 0.5 ppm. Single crystals of (±)-1c were grown by slow diffusion of pentane vapors into a CH₂Cl₂ solution and its structure was further ascertained by X-ray crystallography. Compound (±)-1c crystallized in the centro-symmetric I2/a space group (i.e. with the presence of both P and M enantiomers, Figure 2). The molecular structure shows a C₂ symmetry with a helicity (dihedral angle between terminal rings) of 53.1°, a typical value for a [6]helicene.^[3] The C11−B1 and the B1−N1 bond-lengths are 1.598 and 1.617 Å respectively, which is in the range of similar 4-coordinate boron derivatives.^[7] Heterochiral supramolecular columns formed of alternating M and P helices due to intermolecular π–π stacking are observed (Figure 2b). Furthermore, similarly to what was observed when comparing mono- and bis-platina[6]helicenes,^[8e] the [6]helicenic 1c has a more open helical structure than the mono-azabora[6]helicene 1a because of the presence of two azaboracycles. This structural feature, along with a possible lability of the B-N bond, lead to a low configurational stability of 1c in solution at room temperature and to its easy racemization (vide infra).

Figure 2.

a) X-ray structure of (±)-1c (only the P enantiomer is shown) along with b) its heterochiral supramolecular arrangement along the x axis.

The preparation of [n]helicenes with n higher than 8 is still very challenging.^[11] Taking advantage of the 4-cycles increase upon the double cycloborylation, we examined the preparation of bis-azabora[10]helicene 1d from the [6]helicene ligand 2d substituted with two 2-pyridyl groups in the inner 1,16 positions. 2d was prepared as demonstrated in Scheme 3. A double Wittig reaction performed on aldehyde 4b and naphtha-2,7-diyl-bis-triphenylphosphonium-bis-bromide salt 3d^[10b,c] yielded the bis-stilbenic compound 5d. Then, a double photocyclization under irradiation by a mercury lamp and in the presence of a catalytic amount of iodine in a highly diluted toluene solution produced racemic 2d in 11% yield. The low yield of the last photocyclization step can be explained by high steric hindrance since both of the inner positions, i.e. 1 and 16, are substituted by a 2-pyridyl group. As 2b, carbo[6]helicene derivative 2d is configurationally stable and exists as a racemic mixture of both P and M enantiomers. The ¹H NMR spectrum of 2d is typical of a C₂-symmetric [6]helicene system (see SI). Single crystals of (±)-2d were grown by slow diffusion of pentane vapors into a CH₂Cl₂ solution, and the structure was resolved by X-ray diffraction. The compound crystallizes in the centrosymmetric P2₁/a space group (Figure 3a,b) with the presence of both P and M enantiomers in the unit cell. The helicity of the 2d [6]helicene moiety is 31.9°, which is small and may be explained by π-π stacking interactions between each terminal pyridyl ring with the helicene backbone (centroid-centroid distances of 3.38–3.59 Å between the pyridyls and the second fused phenyls, see Figure 3a). Note also that the two nitrogen atoms are placed in the outer side and that the pyridyl rings and the helicene’s terminal rings are not coplanar (dihedral angles of 32.3–44.9°). Furthermore, heterochiral supramolecular columns formed of alternating M and P helices are observed (Figure 3b). The racemic bis-azabora[10]helicene (±)-1d was finally prepared from (±)-2d by a double cycloborylation (66% overall yield, Scheme 2). The ¹H NMR of 1d is in agreement with a C₂-symmetric structure and shows two shielded signals at 0.06 and −0.52 for the BMe₂ group. The ¹¹B NMR demonstrates one shielded signal at −0.6 ppm, and the ¹⁹F NMR displays a doublet due to the coupling with the H³ proton (−119.5 ppm, ³J_HF = 8.8 Hz). Complex (±)-1d crystallizes in the centrosymmetric P1̄ space group (Figure 4). The helicity of this molecule is very small, i.e. 2.6° which may be due to the strong π-π stacking between the 4 terminal rings of each side (smallest centroid-centroid distance: 3.67 Å). The distance between two boron atoms is 6.19 Å. Note that the deca-helicene 1d forms a helix bigger than 1½ turn. The supramolecular arrangement of 1d is similar to that of 2d and 1c, with heterochiral supramolecular columns formed of alternating M and P helices along the y axis. Furthermore, close CH₃-π contacts are present between the BMe₂ groups and one phenyl of the neighbouring helicene (Figure 4b).

Figure 3.

X-ray structures of a) (±)-2d and c) (±)-2d¹ (only the P enantiomers are shown) along with their corresponding b) heterochiral and d) homochiral supramolecular arrangement along the z and x axis, respectively.

Figure 4.

a) X-ray structure of (±)-1d (only the M enantiomer is shown) along with b) its corresponding heterochiral supramolecular arrangement along the y axis.

Similarly to (±)-2d, ligand (±)-2d¹ bearing two methoxy groups was synthesized in two steps from 3d and using 4d¹ as the aldehyde precursor (Scheme 3). As in the case of 2d, the yield of the photocyclization step was low (19%) due to high steric hindrance. Ligand 2d¹ displays the same characteristics as 2d (see SI). For example, the 2d¹ crystallizes in the centrosymmetric P1̄ space group with a helicity value of 36.9°, which is again relatively small due to the π-π stacking of each terminal pyridyl ring with the helicene backbone (centroid-centroid distances of 3.52 Å between the pyridyls and the second fused phenyls, see Figure 3c). Furthermore, the two nitrogen atoms are also placed in the outer side. In the case of 2d¹, homochiral supramolecular columns formed from either M or P enantiomers are observed (Figure 3d).

Finally, the bisplatina[10]helicene 1d¹ was synthesized in its racemic form by using a double cycloplatination (Scheme 4). First, intermediate (±)-1d′ bearing two chloro and two dimethyl sulfoxide (dmso) ancillary ligands was obtained with 84% yield by reacting the ligand 2d¹ with 3 eq. of Pt(dmso)₂Cl₂ in the presence of 6 eq. of Na₂CO₃ in refluxing toluene at 110 °C. The final racemic complex 1d¹ bearing two acetylacetonato ligands was then obtained by reacting (±)-1d′ with sodium acetylacetonate in refluxing toluene at 110 °C in ca. 57% yield. Both complexes 1d and 1d¹ were fully characterized (see SI). Single crystals of (±)-1d′ were obtained by slow diffusion of pentane vapors into a CH₂Cl₂ solution, and the structure was solved by X-ray diffraction. This intermediate system crystallizes in the centrosymmetric C2/c space group (both P and M enantiomers in the unit cell), and the structure shows ten ortho-fused rings with intramolecular π-π stacking between the last 4 terminal rings on each side (smallest centroid-centroid distance: 3.37 Å). For this reason, the helicity of 1d′ is only 18.7°. Furthermore, the distance between the two platinum atoms is small (4.83 Å). The ¹H NMR spectrum of 1d¹ is characteristic of a C₂-symmetric structure. For instance, 1d¹ displays the typical satellite peaks due to the coupling with ¹⁹⁵Pt (³J_Pt-H = 32 Hz) for the 6′-protons in the two pyridyl groups. Single crystals were obtained by slow diffusion of pentane vapors into CH₂Cl₂ solution of (±)-1d¹, and the structure was resolved by X-ray diffraction. The complex crystallized in the non-centrosymmetric P2₁ space group which means that there is a spontaneous resolution to pure M- and P-1d¹. The helicity of this [10]helicene is small (22.0°). Besides, π-π stacking can also be observed in this molecule between the 4 terminal rings in each side (smallest centroid-centroid distance: 3.46 Å). The distance between the two Pt atoms is 5.94 Å, almost 1 Å bigger than the value in 1d′, but similar to the B-B bond distance in 1d (6.19 Å). It is important to note that the deca-helicenes 1d′ and 1d¹ form a helix with more than 1½ turns and that, within the helix, the two platinacycles are facing each other and are therefore in an almost non-chiral local environment. This has a strong impact on the chiroptical properties (vide infra).

Scheme 4.

Synthesis and X-ray structures of bis-platina[10]helicenes 1d′ and 1d¹.^$ i) Pt(dmso)₂Cl₂, Na₂CO₃, toluene, reflux, 84%; ii) acacNa, toluene, reflux, 57%.

Photophysical properties

UV-vis spectroscopy

The UV-vis spectra of the azabora[n]helicenes 1a-d were measured in CH₂Cl₂ solution at concentrations ca. 10⁻⁴ mol·L⁻¹ and analyzed in detail with reference to corresponding platina[n]helicenes data (Table 1, Figures 5a,b and 6a,b, SI).^£ The 1a system displays a set of intense structured bands (ε > 26000) centered at 237 and 263 nm, and structured bands of moderate intensity (ε ~ 5000–10000) between 359–398 nm (Figure 5a, Table 1). The increase in the helicene’s size from a hexa- to an octa-helicene leads to an increase in the intensity of the absorption bands of 1b and a bathochromic shift (ca. 30 nm) of its lower-energy band as compared to 1a. Indeed, mono-azabora[8]helicene 1b demonstrates a sharp absorption band at 232 nm (ε > 53900) accompanied by a shoulder at 265 nm (ε ~ 47000), and a set of strong structured bands between 324–374 nm (ε ~ 10000–25000), and bands of weaker intensity at 404 and 429 nm (1500–2500). Overall, the bis-azabora[6]helicene 1c shows absorption bands similar to those of 1a but the intensities are weaker between 230–330 nm and stronger between 330–430 nm. Again, the increase in the helicene’s size from a hexa- to a deca-helicene results in much stronger absorption for 1d than for 1c, with intense bands at 269, 337 and 377 nm (ε ~ 60000, 31000 and 12400, respectively), and weaker bands at 415 and 440 nm (ε ~ 3000–4000) that are ca. 50 nm red-shifted compared to 1c. Note that overall, the UV-vis spectrum of bis-azabora[10]helicene 1d resembles that of the mono-azabora[8]helicene 1b, with increased intensity and red-shift consistent with an elongation of the π-conjugated system from an [8]helicene to a [10]helicene. Compared to 1a-d, the corresponding cycloplatinated derivatives 1a¹–d¹ show very similar (although generally of stronger intensity) UV-vis bands at higher-energy spectral regions, but with additional moderate or weak absorption bands in the low-energy region, i.e. above 430 nm (Figure 6a,b).^[8a,b,e] In particular, in newly synthesized bis-platina[10]helicene 1d¹ the lowest-energy band occurs at 480 nm with a tail down to 520 nm.

Table 1.

UV-vis and emission data for azaborahelicenes 1a-d and the related Pt₂ complex 1d¹ (in CH₂Cl₂ at 298 K or EPA at 77 K).

Compound	Absorption λmax/nm (ε/M−1cm−1)	Emission r.t. λmax/nm	Φ a	τ/ns	Emission 77 K
					λmax/nm	τ/ns
1a	237 (46000), 263 (26400), 295 (33100), 307 (30400), 344 (10300), 359 (9740), 377 (6910), 398 (5250)	404, 425, 450sh	0.21	4.1	F: 396, 417, 443, 471	7.2
					P: 543, 581, 626	1.1 × 109
1b	232 (53900), 265 (46700), 324 (25500), 352sh (15800), 374sh (10500), 404 (2190), 429 (1440)	435, 458	0.069	5.3	F: 427, 454, 484, 516	7.3
					P: 546, 593, 643	1.2 × 109
1c	256 (49300), 288 (18200), 342 (18100), 375 (15600), 391 (14300)	427	0.49	3.2	F: 397, 415, 443, 474sh	3.9
					P: 556, 604, 654	0.62 × 109
1d	269 (60200), 337 (31200), 377sh (12400), 415 (4020), 440 (3000)	443, 471, 502, 541	0.074	5.5	F: 437, 464, 496, 532	7.3
					P: 554, 599, 657	1.1 × 109
1d1	242 (72400), 297 (57000), 368sh (20800), 426 (9250), 446 (8380), 479 (6850)	639, 663	0.066	27 × 103	P: 615, 665, 738sh	61 × 103

^aQuantum yields of 1a-d/1d¹ were measured using quinine sulfate or [Ru(bpy)₃]²⁺ as standards: see S.I. ‘F’ – fluorescence. ‘P’ – phosphorescence.

Figure 5.

Experimental UV-vis (panel a) and ECD (panel c) spectra of azaborahelicenes P-1a-d and of bis-platina[10]helicene complex P-1d¹ (CH₂Cl₂, C 1.10⁻⁴ M) and their corresponding simulated spectra (panels b and d, respectively). TDDFT LC-PBE0* calculations with continuum solvent model for dichloromethane at BP-D3 optimized geometries. No spectral shift has been applied. Calculated excitation energies and rotatory strengths indicated as ‘stick’ spectra. Numbered excitations correspond to those analyzed in detail (see SI).

Figure 6.

Comparison of the experimental UV-vis (panels a and b) and ECD spectra (panels c and d) of P enantiomers of azaborahelicenes 1a-d and the corresponding platinahelicenes 1a¹–1d¹.^[8a,b,e]

UV-vis spectra of 1a-d and 1d¹ were calculated by time-dependent DFT (LC-PBE0* functional with a continuum solvent model for CH₂Cl₂ at DFT BP-D3 geometries)^[12] and agree very well with experiment (Figure 5b, see SI for computational details and a full set of computed data). A detailed excitation analysis in terms of molecular orbital (MO) pair contributions of azabora[n]helicenes 1a-d shows that the first electronic excitations are responsible for lowest-energy bands and that they are of π-π* type and involve the HOMO/LUMO pair (P-1a: 78%, P-1b: 56%, P-1c: 97%, P-1d: 42%) and the HOMO-1/LUMO+1 pair in the case of more π-extended azabora[8]helicene 1b (23%) and azabora[10]helicene 1d (32%). As shown in Figure 7a, for all azaborahelicenes studied here the HOMO and LUMO are π-conjugated throughout the molecule, including the boracycle and the pyridine moieties. As may be expected, orbitals of the electron-deficient boron atoms do not directly contribute to the π-electron system (vide infra). The additional contributions from the HOMO-1/LUMO+1 pairs in the excitations of 1b and 1d indicate a partial charge-transfer (CT) character. For the bis-azabora[6]helicene 1c there are other excitations that contribute to the longest-wavelength absorption band, e.g. no. 2 and 3. These excitations exhibit some contributions from HOMO-1. Interestingly, this orbital clearly shows a conjugation between the B-CH₃ σ-bonds and the π-system of the naphthalene scaffold (see Figure 7c). Other 1c MOs with similar features but involving the whole C^N π-skeleton were also identified, but they have much lower orbital energies and are not involved to any significant degree in the low-energy electronic excitations (Figure 7c, SI). Similar, albeit less pronounced, mixing of B-CH₃ σ-bonds and orbitals of the π-system of the C^N helical scaffold was found for all remaining azaborahelicenes considered here (SI).

Figure 7.

Isosurfaces (0.04 au) of HOMOs and LUMOs of the azaborahelicene systems 1a-d (panel a) and platinahelicene complex 1d¹ (panel b). Panel c: Isosurfaces (0.04 au) of selected MOs of 1c demonstrating σ-π mixing (for full set of MOs see SI).. LC-PBE0*//BP-D3 calculations with continuum solvent model for dichloromethane.

To gain further insight into the effect of the presence of the boron atom(s) on the helicene π-system, a quantum-chemical analysis of the electronic structures of 1a-d was performed with the natural bond orbital (NBO) method. The NBO analysis revealed a stabilizing electron delocalization between σ(B-C(CH₃)) and antibonding π*-(C^N) fragment orbitals. Such interactions are commonly associated with σ-π hyperconjugation (SI).

As already shown for platina[n]helicenes 1a¹–c¹, Pt centers may efficiently interact with extended azahelicene π-orbitals which is reflected in the presence of additional low-energy absorption bands observed for such systems.^[8a,b,e] The quantum-chemical analysis of 1d¹ revealed similar electronic features (see Figure 7b for frontier orbitals). The excitation at lower energy with sizable dipolar oscillator strength (no. 2) has a large contribution from the HOMO-1/LUMO (75%) pair and a secondary one from HOMO/LUMO+1 (13%) and may therefore be assigned as a π-π* transition with some participation of the platinum 5d orbitals. This finding is in full agreement with the previous assignments for 1a¹–c¹.^[8a,b,e]

To summarize: (i) All four novel heteroatomic helicenes reveal generally similar UV-vis spectra whose intensity and energy changes are typical for an enlargement of the [n]helicene π-electron system along the series. (ii) Unlike the platina[n]helicenes 1a¹–d¹ cases, where metal atoms do not only construct the helicoidal framework by double orthometallation but also markedly impact the electronic properties of the π-conjugated system, the effect of the cycloboration seems to be primarily to increase the size of the helicene skeleton by systematically adding two ortho-fused cycles and to involve these two cycles in the electronic π–π* type transitions. Overall, compounds 1a-d appear thus to behave as classical organic [n]helicene derivatives, but with additional σ-π hyperconjugation associated with the boron centers.

Luminescence properties

The luminescence properties of azabora[n]helicenes 1a-d and bis-platina[10]helicene 1d¹ were measured in CH₂Cl₂ at 298 K (room temperature, r.t.) and in EPA at 77 K (EPA = diethyl ether/isopentane/ethanol, 2/2/1 v/v). The results are summarized in Table 1 and the r.t. emission spectra are displayed in Figure 8. Additional spectra including those at 77 K are provided in the SI. All four azaborahelicenes reveal a moderate to intense blue fluorescence around 430–460 nm at 298 K, with relaxation times of a few nanoseconds, and quantum yields between 7–49%. The [6]helicene derivatives with one (1a) and two azaboracycles (1c) demonstrate higher quantum yield, 21% and 49%, respectively; the latter being quite high for [6]helicenes^[3f,g] and consistent with the results for other azaborole derivatives.^[9b–d] The increase in the helicene’s size from hexa- to octa- and deca-helicenes leads to smaller quantum yields (~ 7%). At low temperature, all four borahelicenes show green phosphorescence with relaxation times around 1 s, a behavior previously observed in organic helicenes.^[3f,g] The Pt₂-[10]helicene 1d¹ displays a red phosphorescence (see Table 1) around 640 nm at 298 K with a relaxation time of 27 μs and a moderate quantum yield (~ 7%). This corresponds to the typical emission properties of platinahelicenes, such as the previously studied 1a¹–c¹.^[8e]

Figure 8.

The CPL spectra (upper) and the luminescence spectra (lower) for both enantiomers of azaborahelicene 1a (λ_ex = 395 nm), 1b (λ_ex = 405 to 409 nm), 1c (λ_ex = 405 nm), and 1d (λ_ex = 420 nm) measured in CH₂Cl₂ at r.t. (see also SI).

The calculated (LC-PBE0*, CH₂Cl₂ solvent model) fluorescence and phosphorescence energies for 1a-d correspond well with measured low-temperature data, supporting the experimental assignment (see SI). Similarly, in the case of bis-platina[10]helicene 1d¹ the calculated energy of S₁→S₀ fluorescence transition differs significantly from that of the experimental luminescence, while the phosphorescence energy T₁→S₀ agrees very well with experiment. For all the systems the calculated optimized excited states S₁ and T₁ and ground states S₀ are structurally very similar (SI).

Chiroptical properties (electronic circular dichroism, optical rotation)

The electronic circular dichroism (ECD) spectra and optical rotation (OR) parameters of azabora[n]helicene derivatives 1a-d and bis-platina[10]helicene complex 1d¹ were examined next (Figures 5c,d and 6c,d, Table 2, SI).^£ The M and P enantiomers of each helicene derivative were obtained with ee’s between 96.0–99.5% through HPLC separations over a chiral stationary phase, and, as expected, they displayed mirror-image ECD spectra (see SI). Note that bis-azabora[6]helicene 1c easily racemizes in solution and a 25% loss of the optical rotation was observed for this compound, upon leaving the enantiopure M-1c or P-1c in CH₂Cl₂ solution for one week at room temperature. An inversion barrier ΔG^≠ of 114.9 kJ mol⁻¹ (78°C, ethanol) was experimentally estimated by racemization kinetic studies (see SI). Consequently, a freshly prepared solution was used systematically for the optical measurements.

Table 2.

Experimental and calculated molar rotations, [ϕ],^£ of enantiopure azabora[n]helicene systems P-1a-d and of bis-platina[10]helicene complex P-1d¹.^a

Method	1a	1b	1c	1d	1d1
BHLYP//BP	6557	19359	5377	30601	35752
BHLYP//BP-D3	6091	16916	5146	25970	27491
LC-PBE0*//BP	6357	20011	5265	33194	38106
LC-PBE0*//BP-D3	5782	17134	4987	28008	29308
Expt. (23°C)	5062	13953	5209	32134	35500

^aTDDFT response calculation//DFT geometry optimization method. Continuum solvent model for dichloromethane (DCM, ε = 8.9). See SI for a full set of data.

The azabora[6]helicene P-1a shows typical ECD^£ of [6]helicene derivatives, i.e. a strong negative band at 237 nm (Δε = −95) along with two strong positive bands at 296 (+89) and 307 nm (+110), accompanied with an additional weak negative one at 397 nm (−8.7), see Figure 5c. The π-elongated azabora[8]helicene P-1b displays an ECD spectrum that is more intense and significantly red-shifted (ca. 50 nm) compared to 1a, with for example two strong negative ECD-active bands at 267 (Δε = −116) and 307 nm (−77), three strong positive bands at 340 (+122), 353 (+159), and 379 nm (+100), and, similarly to P-1a, an additional very weak negative band at 427 nm (−4.8). In spite of a general similarity between the 1a and 1c UV-vis spectra (vide supra), the bis-azabora[6]helicene P-1c exhibits an ECD spectrum that significantly differs from those for typical helicene derivatives. It displays a strong positive band at 277 (Δε = +71), a very weak negative intensity at 303 (−4.8) that is followed by three positive bands of medium intensity at 358 (+15), 374 (+20), and 392 nm (+16). Overall the ECD of 1c is relatively weak for a [6]helicene derivative. On the contrary, bis-azabora[10]helicene P-1d shows very strong and red-shifted (> 30 nm) ECD compared to P-1a-c, fully consistent with an elongation of the π-conjugated system. The compound reveals two strong negative bands at the 251 (Δε = −95) and 268 nm (−165), positive and negative ones of medium intensity at respectively 294 (+59) and 331 nm (−47), and strong positive band at 379 nm (+249). As for mono-azaborahelicenes P-1a and P-1b, an additional weak negative intensity occurs at lowest-energy part of the P-1d spectrum (439 nm, Δε = −7.9).

The corresponding TDDFT simulated ECD envelopes for P-1a-d (LC-PBE0*//BP-D3, CH₂Cl₂ solvent model) agree in general, very well with the experiments. As shown in Figure 5d, important experimentally observed trends are correctly reproduced by the calculations. Namely, there is an increasing red-shift of the ECD spectra with the increase in the helicene’s size from the hexa- (1a) to the octa- (1b) to the deca-helicene (1d), and the calculated 1c spectrum significantly differs from the others. No low-energy bands of weak negative intensity were observed in the broadened calculated vertical ECD spectra of P-1a,b,d. The excitations data shows that the lowest-energy excitation (no. 1, SI) in each of these systems has a negative rotatory strength. However, this weak negative rotatory strength is over-powered by excitations of strong positive intensity at higher energy, and the broadened spectra do not afford a low-energy ECD band. As already mentioned, the lowest-energy excitations are predominantly involving the HOMO/LUMO pair, with additional HOMO-1/LUMO+1 contributions in the case of 1b and 1d (vide supra, Figure 7 and SI). The intense positive ECD band of P-1a is dominated by excitations no. 3 and 4, calculated at 315 and 299 nm, respectively. Excitation no. 3 mainly involves the HOMO/LUMO+1 pair (70%) and is therefore assigned to a CT π-π* transition as it involves orbitals centered in different parts of the π-system. Excitation no. 4 has two contributions, from HOMO/LUMO+2 (53%) and from HOMO-1/LUMO (33%), and can be assigned as π-π* with some CT character. In the case of P-1b, low-energy positive ECD band originates from two nearly degenerate excitations, no. 2 (calculated at 357 nm) and no. 3 (353 nm). These excitations involves predominantly the π-π* HOMO/LUMO+1 pair (no. 2/3: 71%/12%) along with the CT- π-π* pairs HOMO/LUMO (no. 2/3: 17%/59%) and HOMO-2/LUMO (no. 3: 18%). Accordingly, the P-1b excitation with the strongest rotatory strength (no. 3) reveals a higher degree of CT character than in the case of 1a, which may be responsible for a corresponding red-shift of the ECD band. A similar conclusion can also be drawn for P-1d, for which the first ECD band is dominated by excitation no. 3 with a very large-magnitude rotatory strength calculated at 371 nm involving π-π* orbital pairs with some CT character (HOMO-1/LUMO+1 (52%) and HOMO/LUMO (32%)).

The substantial difference between the ECD of P-1c and the P-1a,b,d spectra can easily be related to the underlying excitations. The first broad positive ECD band of P-1c, of moderate intensity, is caused by three excitations calculated at 355 (no. 1), 336 (no. 2), and 313 nm (no. 3) with positive rotatory strength values that are lower than in the case of 1a,b,d (see SI). Their general assignment of the excitations is similar to those of 1a,b,d: Excitation no. 1 has almost pure π-π* HOMO/LUMO character, the excitations no. 2 and 3 can be assigned as π-π* with some CT character (HOMO/LUMO+1 (no. 2/3: 75%/17%) and HOMO/LUMO (no. 2/3: 18%/57%)). However, unlike for the other azaborahelicenes, the CT contributions in the latter excitations involve B-CH₃ σ-bonds due to involvement of HOMO-1 (vide supra).

As shown in Figure 6c,d, the previously studied^[8a,b,e] platina[n]helicene derivatives 1a¹–c¹ have ECD spectra of similar magnitude as in 1a-c, but strongly red-shifted (20–50 nm), and with an additional characteristic low-energy tail of the first ECD band (which is positive for the P-isomers). Similar features are also observed for the novel bis-platina[10]helicene 1d¹ complex, although in this case a decrease in the intensity compared to 1d can be noticed (Figures 5c, 6d). The assignment of first positive ECD bands in 1a¹ and 1b¹ is similar to those described above for 1a and 1b, although for the π-π* transitions with CT character localized in the C^N π-system of platinahelicene complexes there is a clear involvement of metal orbitals. In particular, π-π* transitions with some participation of the Pt 5d orbitals are responsible for low-energy tails of ECD spectra in 1a¹,b¹.^[8a,b,e] Similarly to P-1c, its platina-based analogue P-1c¹ reveals moderate positive ECD intensity around 350 nm that is also dominated by metal-enhanced π-π* transitions with partial CT character. Interestingly, the excitation involves an MO that demonstrates a noticeable contribution from non-bonding orbitals of both Pt centers which become a part of the naphthalene π-system,^[8e] thereby resembling features of the 1c HOMO-1 shown in Figure 7c. It therefore appears that the structural arrangement is conducive to an electronic (hyper)conjugation. The assignment of the ECD spectrum of newly synthesized 1d¹ remains in line with those of 1a¹–c¹, i.e. it is dominated by π-π* transitions with some CT character and involvement of Pt centers orbitals. The low-energy tail of the first 1d¹ ECD band is caused by the first electronic excitation (455 nm) which nearly purely involves the HOMO/LUMO pair (94%).

The trends in the ECD-active bands observed for P-1a-d and P-1d¹ are clearly reflected in the corresponding molar optical rotation (MR) data (Table 2). The MR generally doubles with increase in the helicene size from a hexa- (1a) to an octa- (1b) to a deca-helicene (1d), which is in line with the red-shift and increasing intensity of first positive ECD bands along the series. Similarly, the bis-azabora[6]helicene 1b and bis-platina[10]helicene 1d¹ demonstrate optical rotations of slightly increased magnitude compared to their mono-azabora[6]helicene and bis-azabora[10]helicene analogues 1a and 1d. The calculated data listed in Table 2 (see also SI) show a strong dependence of the 1a-d and 1d¹ MRs on the functional and the optimized geometries used in the OR calculations, which is in agreement with previous observations for other helicene derivatives.^[13] With all computational model considered, the results remain however qualitatively the same and demonstrate (i) a strong increase in MR when extending the π-chromophore within the series 1a < 1b < 1d < 1d¹ in line with experiment, and (ii) a noticeable lower MR value of 1c compared to 1a.

To summarize: (i) Azabora[n]helicenes 1a, 1b, and 1d reveal an overall similar shape of the ECD spectra which resembles that of classical organic helicene derivatives. Furthermore, the increase in the helicene size when going from a [6] (1a) to an [8] (1b) to a [10]helicene (1d) leads to more intense and more red-shifted spectra, reflecting a well-known tendency for helicenes that is attributed to the enlargement of the π-electron system. This confirms that the primary role of the boron atom(s) is to assemble additional helicoidal cycles that can be involved in the electronic π-π* type transitions. (ii) Bis-azabora[6]helicene 1c displays a significantly different ECD spectrum from 1a,b,d. The TDDFT results indicate that this is due to excitations with CT character involving B-CH₃ σ-bonds and the π-conjugated system, mainly via HOMO-1 which shows a conjugation of B-CH₃ σ-orbitals and π-orbitals of the C^N backbone. (iii) The strong red-shift of the ECD spectra along with appearance of additional low-energy tail of first positive ECD band for platina[n]helicenes 1a¹–1d¹ compared to their azaborahelicene analogues 1a-1d can be rationalized by an extended π-conjugation involving metal orbitals from one or two Pt centers.

Circularly polarized luminescence (CPL)

Circularly polarized luminescence (CPL) activity, which combines emission and chirality, is another appealing chiroptical property of organic chiral chromophores such as helicenic derivatives,^[14] including borylated ones.^[14i,j] There is a growing interest in CPL,^[15] and CPL emitters have recently been incorporated into OLEDs to obtain circularly polarized (CP) OLEDs.^[8d,g] The CPL of enantiopure P- and M-azaborahelicene derivatives 1a-d and bis-platina[10]helicene 1d¹ was examined (Figure 8, Table 1, SI). For the azabora[n]helicenes studied, the measured g_lum are of the order of 10⁻³ to 10⁻⁴ (see Table S5), which is classical for organic helicenes. However, these heterohelicenes show overall more efficient emission properties than most organic helicene derivatives^[14] since their quantum yields are rather high, especially in the case of 1a and 1c (vide supra). On the other hand, the CPL activity of the azaborahelicenes 1a-c is different from their corresponding platinahelicenes 1a¹–c¹ which displayed CPL red-phosphorescence with g_lum values up to +10⁻² for P-1a¹ that recently proved very efficient for producing CP-OLEDs.^[8g] The signs of the CPL signals are interesting to note since for 1a, 1b and 1c, the CPL is positive for the M-enantiomers while it is negative for the P-enantiomers, which is the opposite situation for the bis-azabora[10]helicene 1d. Note that negative CPL signals are often obtained in substituted organic P-helicenes.^[16] Finally, the CPL was found to be zero in the P- and M-1d¹ which may be due to the square-planar geometry of the Pt atom that is mainly placed in a non-chiral environment and/or degradation upon the xenon lamp illumination. CPL simulations based on vertical transition energies and transition moments were also performed, but with inconsistent results (see SI). It is likely that the vibronic emission spectra need to be simulated^[17] in order to replicate the experimental dissymmetry factors, which is beyond the scope of the present work.

Conclusions

In this paper, we have reported the synthesis of the first helical azaboroles by a straightforward cycloborylation of phenyl-pyridyl-type helicene π-ligands. The role of the boron atoms has been emphasized in (i) increasing the size of the helicenic structure up to an azabora[10]helicene, (ii) increasing the fluorescence quantum yield efficiency with quantum yields up to 50%, (iii) obtaining strong ECD and MR responses, (iv) introducing σ–π conjugation and π–π* charge transfers, (v) displaying CPL activity. These enantiopure compounds therefore demonstrate a behavior that is complementary to their corresponding cycloplatinated helicenes and may find applications in chiral optoelectronic devices.

Experimental Section

Experimental Details are described in the Supplementary Information.