Circularly Polarized Luminescence in a Möbius Helicene Carbon Nanohoop

Abstract We present the first helicene carbon nanoohop that integrates a [6]helicene into [7]cycloparaphenylene. The [6]helicene endows the helicene carbon nanohoop with chiroptical properties and configurational stability typical for higher helicenes, while the radially conjugated seven para‐phenylenes largely determine the optoelectronic properties. The structure of the helicene carbon nanoohop was unambiguously characterized by NMR, MS and X‐ray analysis that revealed that it possesses a topology of a Möbius strip in the solid state and in solution. The chirality transfers from the [6]helicene to the para‐phenylenes and leads to a pronounced circular dichroism and bright circularly polarized luminescence, which is affected by the structural topology of the nanohoop.


General Remarks
Materials: All commercially available chemicals were purchased from Acros, Alfa Aesar, Apollo Scientific, Fluorochem and Sigma-Aldrich and used without further purification. Anhydrous solvents were purchased from Acros and stored over molecular sieves (4 Å). THF was dried by distillation over sodium and benzophenone under Ar and used fresh. Concentration of organolithium reagents was determined via titration prior to use. Molecular sieves were dried at 300 °C and 1 mbar over 24 h in Büchi Glass Oven B-585. Column chromatography was performed on silica gel P60 (40-63 μm) from SilicycleTM. Celite® 545 (0.02-0.1 mm) from Supelco was used. Automated flash chromatography was performed using Biotage Isolera One. TLC was performed with silica gel 60 F254 aluminium plates purchased from Merck. High resolution mass spectra (HRMS) were measured on a maX-isTM4G instrument from Bruker for HR-ESI-ToF MS. UV-Vis absorption were measured using Jasco V-770 with Jasco ETCR-762 Peltier thermostatted cell holder at 25 °C in 1 cm cuvette. Excitation and emission spectra were recorded using Jasco FP-8600 with Jasco ETC-815 Peltier thermostatted cell holder at 25 °C in 1 cm cuvette. Circular dichroism was measured using Jasco J-1500 with Jasco PTC-517 Peltier thermostatted cell holder at 25 °C in 1 cm cuvette.
Time-correlated single photon counting (TCSPC)-based emission lifetime measurements were performed on a LifeSpec II spectrometer (Edinburgh Instruments) employing a picosecond pulsed diode laser (ca. 60 ps pulse width). Hamamatsu absolute photoluminescence quantum yield spectrometer C11347 Quantaurus-QY was used for quantum yield measurement. Circularly polarized luminescence (CPL) was recorded in an Olis DSM172 spectrophotometer equipped with a xenon lamp of 150 W.
Nuclear magnetic resonance: NMR experiments were performed on Bruker Avance III NMR spectrometers operating at 400, 500 or 600 MHz proton frequencies. The instruments were equipped with a direct-observe 5 mm BBFO smart probe (400 and 500 MHz), an indirect-detection 5 mm BBI probe (600 MHz). All probes were equipped with actively shielded z-gradients (10 A). The chemical shifts are reported in ppm relative to tetramethylsilane or referenced to residual solvent peak and the J values are given in Hz (±0.1 Hz). Standard Bruker pulse sequences were used, and the data was processed on Topspin 3.6 (Bruker) using twofold zero-filling in the indirect dimension. Variable temperature experiments were performed in 1,1,2,2-tetrachloro-ethane-d2 (δ 1 H = 6.00 ppm) between 368 K and 228 K or in tetrahydrofuran-d8 (δ 1 H = 3.58 ppm) between 298 K and 168 K. As far as possible, the temperature was calibrated by either a glycerol (high T) or a methanol (low T) sample and showed accuracy within +/-0.2 K. Compound 1 was characterized at slow exchange conditions (168 K) by 1   Spectral data matches the literature-reported values. [4] 2,7-bis(4-Bromostyryl)naphthalene (8) NaH (60% in oil; 587 mg; 14.7 mmol; 3.00 eq.) was suspended in dry THF (100 mL) and cooled to 0 °C. 7 (5.26 g; 10.3 mmol; 2.11 eq.) was added and the reaction was stirred for 10 min at 0 °C and for 2 h at room temperature. Color of the reaction mixture changed from white to orange. 6 (901 mg; 4.89 mmol; 1.00 eq.) was added and the color of reaction mixture changed from orange back to white. The reaction was stirred for 16 h at room temperature. At this point, the color of reaction mixture was back to orange. The reaction was quenched with H2O (50 mL), concentrated on rotavap to 1/3 of the volume and extracted with DCM (5 times 50 mL). Combined organic phases were washed with brine (50 mL) and dried with Na2SO4. Volatiles were evaporated. Crude product was purified by automated flash silica gel chromatography (10% DCM in cyclohexane). A mixture of Z,Z-and Z,E-isomer was isolated as a yellow solid (1.50 g; 63%).  Spectral data matches the literature-reported values. [1] 11,14-Dibromo [6]helicene (2) 8 (300 mg; 612 μmol; 1.00 eq.) was dissolved in toluene (1.4 L) and iodine (373 mg; 1.47 mmol; 2.40 eq.) was added and the solution was degassed by bubbling with Ar for 45 min. Propylene oxide (21.4 mL; 306 mmol; 500 eq.) was added and degassing was further continued for 15 min. The reaction mixture was irradiated with 150 W medium-pressure mercury lamp for 4.5 h. Volatiles were evaporated. Crude product was purified by column chromatography (DCM). The product was isolated as yellow solid (260 mg; 87%). Spectral data matches the literature-reported values. [1] Compound 4 3 (1.61 g; 1.26 mmol; 1.00 eq.) was dissolved in dry THF (30 mL) and the solution was cooled to -78 °C. n-BuLi (2.6 M; 1.1 mL; 2.77 mmol; 2.20 eq.) was added dropwise and the reaction was stirred for 1 h at -78 °C, i-PrOBpin (0.65 mL; 3.15 mmol; 2.5 eq.) was added and the reaction was stirred and left to warm to room temperature over 16 h. H2O (50 mL) and brine (50 mL) were added and the resulting mixture was extracted with EtOAc (3 times 50 mL). The combined organic phases were washed with brine (50 mL) and dried with Na2SO4. Volatiles were evaporated. The crude product isolated as a white amorphous solid (1.71 g) and used without further purification.
The reaction mixture was filtered through celite, the flask and celite were rinsed with DCM and the volatiles were evaporated and crude product dried in high vacuum. The residue was dissolved in dry THF (2.5 mL) and TBAF (1 M in THF; 0.62 mL; 618 μmol; 6.00 eq.) was added and the reaction was stirred for 1 h at room temperature and quenched with H2O (5 mL). The resulting mixture was concentrated on rotavap (40 °C; 160 mbar). Formed precipitate was filtered off, washed with DCM (2 times 5 mL) and dried in high vacuum.
The residue was dissolved in THF (10 mL) and H2SnCl4 (127 mM in THF; 2.8 mL; 356 μmol; 3.45 eq.) was added. The reaction was stirred for 1 h at room temperature and quenched with 10% aqueous NaOH (2.5 mL). The resulting mixture was diluted with H2O (25 mL) and extracted with DCM (6 times 25 mL). The combined organic phases were washed with brine (25 mL) and dried with Na2SO4. Volatiles were evaporated. The crude product was purified by column chromatography (50% toluene in n-heptane). The product was isolated as yellow solid (18.4 mg; 21%).

X-ray analysis
Single crystals were grown from chlorobenzene solution of racemic 1 (~1.5 mmol/L) by layering with methanol in an NMR tube under argon at room temperature. The grown crystals had an appearance of yellow needles. A suitable crystal with dimensions 0.37 × 0.23 × 0.13 mm 3 was selected and mounted on a mylar loop in perfluoroether oil on a STOE STADIVARI Cu diffractometer. The crystal was kept at a steady T = 150 K during data collection. The structure was solved with the ShelXT 2018/2 solution program [5] using dual methods and by using Olex2 1.3 as the graphical interface [6] . The model was refined with ShelXL 2018/3 [7] using full matrix least squares minimisation on F 2 .

Optical Properties
We investigated the photophysical properties of 1 and 5.  l / nm Figure S9. Fluorescence spectra of 1 in 2-MeTHF at 77 K with excitation at 339 nm (black), 355 nm (red) and 405 nm (blue). Peak at 420 nm is a scattering artifact.

11,14-Di-p-tolyl[6]helicene (5)
The spectra were recorded at ca. 2.25×10 -5 M (for P and M) and were investigated in dichloromethane. For CPL measurements, a fixed wavelength of 355 nm (for M) and 345 nm (for P) provided by a Lamp source and 1.0 s of integration time were selected, the CPL calculated is an average spectra calculated after 100 scans.
[6]Helicene nanohoop (1) The spectra were recorded at ca. 2.25×10 -5 M (for M) and ca. 3.18x10 -5 M (for P) and were investigated in dichloromethane. For CPL measurements, a fixed wavelength of 370 nm (for M and P) provided by a Lamp source and 1.0 s of integration time were selected, the CPL calculated is an average spectra calculated after 100 scans.

DFT Calculations
Calculations were performed with Gaussian 09 [8] (release D.01) suite of electronic structure programs. The geometries of the potential energy minima or transition states were optimized at D3-B3LYP/6-31G(d) level of theory. The nature of the stationary points was confirmed by a subsequent frequency calculation.
UltraFine integration grid was used in all calculations. The nature of the transition states was verified by following the IRC path to connect the respective energy minima. Due to low force constants of the reaction coordinate, the IRC paths did not converge to the corresponding energy minima. Therefore, the last point of the IRC path was optimized calculating the full Hessian matrix in each optimization step to make sure that the optimization converged to the correct energy mimimum. The single point energies were then calculated with a number of DFT functionals using cc-pVTZ basis set. NMR chemical shifts (referenced to TMS) and NICS values were calculated using GIAO at B3LYP/6-31G(d)/gas phase level of theory.
Scheme S3. Homodesmotic reaction used for total strain energy calculation. Additionally, we calculated strain energy using program StrainViz, recently developed by Jasti group. [9] 19 and used as an input for StrainViz. We obtained a total strain of 55.6 kcal mol -1 , which is comparable to strain obtained via homodesmotic reaction. Figure S14. Overlay of the optimized structures of (P)-1 in BMöbius conformation (grey) and m [8]CPP (blue).    Figure S16. b Energies (0K, Eel + ZPVE ) with relative to conformer A obtained with the cc-pVTZ basis set on D3-B3LYP/6-31G(d) geometries. Zeropoint vibrational energy (ZPVE) correction is unscaled. c Transition state of full phenyl A (see Table 1 in the manuscript and Figure S16) rotation resulting in the proton exchange as observed in the VT 1 H NMR experiments (see Figure 2 in the manuscript and Figure S59).

CD Spectra
The absolute configurations of respective enantiomers of 1 and 5 were assigned by comparison of the calculated and measured CD spectra. BMöbius conformation of 1 was used. The spectra were obtained using TD-CAM-B3LYP method with 6-31G(d) basis set calculating first 75 singlet excitations. Solvent effects were simulated using PCM model. The ECD spectra were compared using SpecDis (release 1.71). [10] Empirical vibrational broadening of 0.2 eV was used and the calculated spectra were shifted by -0.

Dissymmetry Factors Calculation
Electric and magnetic transition dipole moment vectors were extracted from a TD-DFT calculation run on geometries of ground, 1st excited and 2nd excited states. Excited state geometries were optimized at CAM-B3LYP/6-31g* level of theory. Compound H6(H)2 was previously studied by Favereau et al. [11]

NICS Values
Although macrocycles with Möbius toplogy may display a global Möbius aromaticity, in our case, there is an odd number of sp 2 -hybridized carbon atoms in the path between the two positions in the [6]helicene unit, via which the p-phenylene half-loop is connected. This is in agreement with our calculations of the nucleusindependent chemical shifts (NICS, Figure S33) that show only small differences in NICS (1)