An S-Shaped Double Helicene Showing both Multi-Resonance Thermally Activated Delayed Fluorescence and Circularly Polarized Luminescence

We present the first example of a multi-resonant thermally activated delayed fluorescent (MR-TADF) helicene, Hel-DiDiKTa. This S-shaped double helicene exhibits sky-blue emission, a singlet-triplet energy gap, EST, of 0.15 eV and narrow emission at a peak maximum of 473 nm with a full-width at half-maximum of 44 nm in toluene. The MR-TADF character is confirmed by the small degree of positive solvatochromism and temperature-dependent increase in intensity of the delayed emission. The chiroptical properties of the separated enantiomers are similar to other large helicenes with comparable dissymmetry values, but with the added benefit of MR-TADF. Thus, this study further strengthens the burgeoning area of chiral TADF emitters for use in cutting-edge optoelectronic and photocatalytic molecules and materials.


Experimental Section
General Synthetic Procedures. All reagents and solvents were obtained from commercial sources and used as received. Air-sensitive reactions were performed under a nitrogen atmosphere using Schlenk techniques, no special precautions were taken to exclude air or moisture during work-up. DCM was obtained from a MBraun SPS5 solvent purification system. Flash column chromatography was carried out using silica gel (Silia-P from Silicycle, S-3 60 Å, 40-63 µm). Analytical thin-layer-chromatography (TLC) was performed with silica plates with aluminum backings (250 µm with F-254 indicator). TLC visualization was accomplished by 254/365 nm UV lamp. HPLC analysis was conducted on a Shimadzu LC-40 HPLC equipped with a Shim-pack GIST 3μm C18 reverse phase analytical column. GCMS analysis was conducted using a Shimadzu QP2010SE GC-MS equipped with a Shimadzu SH-Rtx-1 column (30 m × 0.25 mm). 1 H and 13 C NMR spectra were recorded on a Bruker Advance spectrometer (400 MHz for 1 H and either 101 or 126 MHz for 13 C). The following abbreviations have been used for multiplicity assignments: "s" for singlet, "d" for doublet, "t" for triplet, "m" for multiplet, and "dd" for doublet of doublets. 1 H and 13 C NMR spectra were referenced residual solvent peaks with respect to TMS (δ = 0 ppm). Melting points were measured using open-ended capillaries on an Electrothermal 1101D Mel-Temp apparatus and are uncorrected.
High-resolution mass spectrometry (HRMS) was performed in the School of Chemistry of the University of Leeds.

X-Ray Crystallography
X-ray diffraction data for (rac)-Hel-DiDiKTa were collected at 173 K using a Rigaku MM-007HF High Brilliance RA generator/confocal optics with XtaLAB P100 diffractometer [Cu Kα radiation (λ = 1.54187 Å)]. Intensity data were collected using both ω and φ steps accumulating area detector images spanning at least a hemisphere of reciprocal space. Data for all compounds analysed were collected using CrystalClear [12] and processed (including correction for Lorentz, polarization and absorption) using CrysAlisPro. [13] The structure was solved by dual-space methods (SHELXT [14] ) and refined by full-matrix least-squares against F 2 (SHELXL-2018/3 [15] ). Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. All calculations were performed using the Olex2 [16] interface. Deposition number 2105660 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data    S-12 Table S2. PLQY (FPL) study of (rac)-Hel-DiDiKTa at different doping concentration in mCP, lexc = 350 nm.     a) S1, T1 and DEST values of (rac)-Hel-DiDiKTa calculated from the onsets of the prompt fluorescence and phosphorescence spectra in toluene glass at 77 K, b) in 1 wt% (rac)-Hel-DiDiKTa doped in mCP, c) and in 1 wt% (rac)-Hel-DiDiKTa doped in PMMA.

Determination of photophysical parameters
The efficiencies (Φp and Φd) and constants (kp and kd) for prompt and delayed emission can be estimated according to their contributions to total ΦPL and their lifetimes (τp and τd) according to the triple exponential fitting of time-resolved decay curves at 300 K.
To determine the rates, we used the following equations according to previous studies [17] of TADF photophysical processes: where ΦPL is the absolute FPL; Φp and Φd are the prompt and delayed fluorescence efficiencies, respectively; kp and kd are the prompt and delayed fluorescence decay rate, respectively; 0 and 1 are the lifetimes of prompt and delayed fluorescence; nr, ISC and RISC are nonradiative conversion, intersystem crossing and reverse intersystem crossing processes.   Table S6. Electrochemical data of (rac)-Hel-DiDiKTa compared to the parent DiKTa. [2] Compound Eox a -5.93 [2] -3.11 [2] 2.82 [2] a) Potential values obtained for (rac)-Hel-DiDiKTa from the DPV peak values, measured in degassed dichloromethane with 0.1 M [ n Bu4N]PF6 as the supporting electrolyte and referenced with respect to SCE (Fc/Fc + = 0.46 eV). [18] b) HOMO and LUMO energy levels determined using the relation EHOMO/LUMO = −(Eox / Ered + 4.8) eV (using Fc/Fc + as the internal reference); [19] c) DE = │HOMO − LUMO│eV. [2] Value obtained from the literature in MeCN.

Resolution of (rac)-Hel-DiDiKTa.
Enantiomers were isolated using a Chiralpak IE column (10 mm I.D. x 250 mm) with Toluene:EtOAc 60:40 mobile phase and 5.0 mL min -1 flow rate on a recycling HPLC system (Japan Analytical Industry LC-908 HPLC) for 3 cycles and then collected ( Figure S14). The first enantiomer initially exhibited a retention time of 9.9 min while the second enantiomer exhibited a retention time of 11.5 min. Following separation, the purity of both samples was further checked by cHPLC and UV-Vis absorption, showing high purity for each enantiomer (Figures S15 and S16). Enantiomers exhibited enantiomeric ratios of (e.r.) > 99 % and > 97 % for (P) and (M) enantiomers, respectively and identical UV-Vis spectra at a concentration ~ 5×10 -5 M in toluene.

S-16
Figure S14. Chiral-HPLC trace of (rac)-Hel-DiDiKTa. In blue, recycled signals submitted to the next cHPLC cycle. In orange, collected peaks. Both enantiomeric peaks were collected on the third cHPLC cycle. where AL and AR represent the magnitude of absorbed left-and right-handed light, respectively.
Molecular orbitals were visualized using GaussView 5.0 software. [29] Spin-component scaling coupled-cluster singles-and-doubles model (SCS-CC2) was also used in conjunction with cc- pVDZ basis set. [30] We first optimized the ground state using SCS-CC2 method, then vertical excited states were performed on the ground state optimized structure using the SCS-CC2 method. Difference density plots were used to visualize change in electronic density between the ground and excited state and were visualized using the VESTA package.    Table S8. Computational data calculated in the gas phase for Hel-DiDiKTa, DiKTa and previously mentioned CPL-active triangulene, Hel-DiKTa-2. [31] Compound HOMO / eV LUMO / eV ΔE / eV S1 / eV (f) T1 / eV T2 / eV ΔEST / eV