Tether-entangled conjugated helices

Abstract

A new design concept, tether-entangled conjugated helices (TECHs), is introduced for helical polyaromatic molecules. TECHs consist of a linear polyaromatic ladder backbone and periodically entangling tethers with the same planar chirality. By limiting the length of tether, all tethers synchronously bend and twist the backbone with the same manner, and change it into a helical ribbon with a determinate helical chirality. The 3D helical features are customizable via modular synthesis by using two types of synthons, the planar chiral tethering unit (C ₂ symmetry) and the docking unit (C _2h symmetry), and no post chiral resolution is needed. Moreover, TECHs possess persistent chiral properties due to the covalent locking of helical configuration by tethers. Concave-type and convex-type oligomeric TECHs are prepared as a proof-of-concept. Unconventional double-helix π-dimers are observed in the single crystals of concave-type TECHs. Theoretical studies indicate the smaller binding energies in double-helix π-dimers than conventional planar π-dimers. A concentration-depend emission is found for concave-type TECHs, probably due to the formation of double-helix π-dimers in the excited state. All TECHs show strong circularly polarized luminescence (CPL) with dissymmetric factors (|g _lum|) generally over 10^-3. Among them, the (P)-T4-^tBu shows the highest |g _lum| of 1.0 × 10^-2 and a high CPL brightness of 316 M^-1 cm^-1.

Fig. 1. (a) Classic helical polyaromatics, helicene and twistacene, and tether-entangled conjugated helix in this work. (b) The design concept of TECHs. Note: the geometries were optimized at PM7 level; hydrogens are omitted for clarity; the tethers are highlighted in sky blue.

Fig. 2. The synthesis analysis of TECHs: (a) the chiral tethering unit and the docking unit; (b) modular synthesis of polymeric TECHs; (c) modular synthesis of oligomeric TECHs.

Scheme 1. Synthesis of the chiral tethering units (S_p)-3 and (R_p)-3. Reaction conditions: (i) (PhO)₂PON₃/Et₃N/(S)-1-phenylethanol, toluene, 80 °C; (ii) Cs₂CO₃/KI/1,8-dibromooctane, DMF, 80 °C; (iii) TFA, CH₂Cl₂, r.t.

Scheme 2. (a) Synthesis of P-chirality concave-type TECHs starting from (S_p)-3. (b) Synthesis of M-chirality concave-type TECHs starting from (R_p)-3. Reaction conditions: (i) Et₃N/ArCOCl, THF, 70 °C; (ii) Pd(OAc)₂/PCy₃·HBF₄/Cs₂CO₃, DMA, 130 °C; (iii) Et₃N/terephthaloyl chloride, CH₂Cl₂, r.t.; (iv) KOH, THF/EtOH/water, 70 °C; (v) 1. oxalyl chloride, CH₂Cl₂, r.t.; 2. Et₃N/(S_p)-3, CH₂Cl₂, r.t.

Scheme 3. Synthesis of P-chirality and M-chirality convex-type TECHs. Reaction conditions: (i) Pd(PPh₃)₄/K₂CO₃/2-(2,5-difluorophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF/water, 70 °C; (ii) KO^tBu, DMF, 80 °C; (iii) Pd(PPh₃)₄/K₂CO₃/2,5-difluorobenzene-1,4-diboronicacidbis(pinacol)ester, THF/water, 70 °C; (iv) KO^tBu, DMF, 100 °C.

Fig. 3. The chemical structure (left), single crystal structure profile (middle) and the backbone profile viewing along the helical axis (right) for (P)-T1, (P)-T2-^tBu, (S_p,P,S_p)-11, (R_p,M,R_p)-11 and (P)-T2-convex. Note: hydrogens and solvent molecules in the single crystal structures are omitted for clarity.

Fig. 4. Double-helix dimers in the crystals of (a) (P)-T2-^tBu, (b) (S_p,P,S_p)-11, and (c) (R_p,M,R_p)-11. (d) The DFT-optimized structures and IGMH analysis for (P)-T2 dimer and (P)-T3 dimer; the green isosurfaces representing the interfragment interactions were drawn at a δ^inter_g value of 0.004.

Fig. 5. The absorption spectra for (a) concave-type TECHs and (b) convex-type TECHs in solution. Note: all measurements except that for (P)-T5-^tBu were taken in toluene; the measurement for (P)-T5-^tBu was taken in toluene : CHCl₃ (5 : 1) due to the solubility issue.

Fig. 6. The photoluminescence spectra for TECHs in solution with different concentration and in film: (a) (P)-T1-^tBu; (b) (P)-T1-convex; (c) (P)-T2-^tBu; (d) (P)-T2-convex; (e) (P)-T3-^tBu; (f) (P)-T4-^tBu; (g) (P)-T5-^tBu. (h) The calculated fluorescence spectra for (P)-T2, (P)-T2 dimer, (P)-T3 and (P)-T3 dimer by TD-DFT theory at the PBE0(D3)/6-31G(d,p) level.

Fig. 7. The experimental CD (left), calculated CD (middle) and experimental CPL (right) spectra for TECHs in solution with a medium concentration: (a) (P)-T1-^tBu and (M)-T1-^tBu (5.6 × 10⁻⁵ M); (b) (P)-T1-convex and (M)-T1-convex (3.1 × 10⁻⁵ M); (c) (P)-T2-^tBu and (M)-T2-^tBu (9.2 × 10⁻⁵ M); (d) (P)-T2-convex and (M)-T2-convex (3.6 × 10⁻⁵ M); (e) (P)-T3-^tBu and (M)-T3-^tBu (6.0 × 10⁻⁵ M); (f) (P)-T4-^tBu and (M)-T4-^tBu (1.2 × 10⁻⁵ M); (g) (P)-T5-^tBu and (M)-T5-^tBu (6.3 × 10⁻⁶ M).

Fig. 8. (a) The variation of |g_lum| along with concentration for (P)-T1-^tBu, (P)-T2-^tBu, (P)-T3-^tBu, (P)-T4-^tBu, (P)-T5-^tBu, (P)-T1-convex, and (P)-T2-convex. (b) The optimized geometry of S₁ state and the S₁ → S₀ transition characteristics for (P)-T2, (P)-T3, (P)-T2 dimer, and (P)-T3 dimer. Note: the S₁ state geometries were optimized by TD-DFT theory at the PBE0(D3)/6-31G(d,p) level; the g-factor was calculated according to the equation g = 4cosθ|m|/|μ|, where μ is the electric transition dipole moment, m is the magnetic transition dipole moment, and θ is the angle between μ and m.