Chiral Perylene Bisimide Dyes by Interlocked Arene Substituents in the Bay Area

Abstract A series of perylene bisimide (PBI) dyes bearing various aryl substituents in 1,6,7,12 bay positions has been synthesized by Suzuki cross‐coupling reaction. These molecules exhibit an exceptionally large and conformationally fixed twist angle of the PBI π‐core due to the high steric congestion imparted by the aryl substituents in bay positions. Single crystal X‐ray analyses of phenyl‐, naphthyl‐ and pyrenyl‐functionalized PBIs reveal interlocked π‐π‐stacking motifs, leading to conformational chirality and the possibility for the isolation of enantiopure atropoisomers by semipreparative HPLC. The interlocked arrangement endows these molecules with substantial racemization barriers of about 120 kJ mol−1 for the tetraphenyl‐ and tetra‐2‐naphthyl‐substituted derivatives, which is among the highest racemization barriers for axially chiral PBIs. Variable temperature NMR studies reveal the presence of a multitude of up to fourteen conformational isomers in solution that are interconverted via smaller activation barriers of about 65 kJ mol−1. The redox and optical properties of these core‐twisted PBIs have been characterized by cyclic voltammetry, UV/Vis/NIR and fluorescence spectroscopy and their respective atropo‐enantiomers were further characterized by circular dichroism (CD) and circular polarized luminescence (CPL) spectroscopy.


General Methods
Unless otherwise stated, all chemicals, reagents and solvents were purchased from commercial suppliers and used after appropriate purification. The 1,6,7,12-tetrachloroperylene bis(dicarboximides) 1a -e were synthesized according to literature. [S1] Toluene was of HPLC grade and dried prior to use by an Innovative Technology PureSolv solvent purification system.
Dichloromethane was distilled prior to use. Column chromatography was performed with commercial glass columns using silica gel 60 M (particle size 0.04 -0.063 mm; Merck KGaA) as stationary phase. Normal phase HPLC was performed on a Japan Analytical Industry (JAI) recycling preparative HPLC system LC-9105 equipped with a VP 250/21 NUCLEOSIL 100-7 column of Macherey-Nagel. For the chiral separation of the enantiomers, the same HPLC system was equipped with a Reprosil 100 Chiral-NR 8 µm column from Trentec. NMR spectra were recorded on a Bruker Avance III HD 400 MHz NMR spectrometer and are calibrated to the residual proton signal of the used deuterated solvent. The chemical shifts (δ) are reported in parts per million (ppm). Multiplicities for proton signals are abbreviated as s, bs, d, bd, t, sep, m and bm for singlet, broad singlet, doublet, broad doublet, triplet, septet, multiplet and broad multiplet respectively. MALDI-TOF mass spectra were recorded with a Bruker Daltonics GmbH ultrafleXtreme mass spectrometer using DCBT (2-[(2E)-3-(4-tertbutylphenyl)-2-methylprop-2-enylidene]malononitrile) as matrix. High resolution mass spectra were measured by electrospray ionization (ESI) with an ESI microTOF Focus mass spectrometer from Bruker Daltonics GmbH. The melting points were determined using a SMP50 from Stuart. UV/Vis/NIR absorption spectra were recorded on a Jasco V-770 or Jasco V-670 spectrometer. Fluorescence spectra were recorded on a FLS980 fluorescence spectrometer (Edinburgh Instruments) and were corrected against the photomultiplier sensitivity and the lamp intensity. CD spectra were recorded on a Jasco J-810 S3 spectropolarimeter. CPL spectra were recorded with a customised JASCO CPL-300/J-1500 hybrid spectrometer. Cyclic voltammetry measurements were conducted on an EC epsilon (BASi instruments, UK) potentiostat connected to a three-electrode single-compartment cell.
A Pt disc electrode was used as working electrode, a platinum wire as counter electrode and an Ag/AgCl reference electrode. The spectra were referenced using the ferrocenium/ferrocene redox couple as an internal standard. Single crystal X-ray diffraction data were collected at 100 K on a Bruker D8 Quest Kappa diffractometer with a Photon II CPAD detector and multilayered mirror monochromated CuKα radiation. The structures were solved using direct methods, expanded with Fourier techniques and refined with the Shelx software package. [S2] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the structure factor calculation on geometrically idealized positions. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. 2086018 (2c), 2086019 (4a) and 2086020 (6a). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.ac.uk/data.request/cif. Theoretical calculations were performed by the Gaussian software [S3] using B3LYP/6-31G(d) level of theory for structure optimization and B3LYP/def2-SVP level of theory for TD-DFT simulation of electron transitions.

Figure S4
Relative energies in kJ mol -1 of the transition states of the a) M/P-racemization and b) rotation of one phenyl ring relative to the ground state energies of the M-enantiomer of PBI derivative 2e calculated by the Gaussian 09 program using DFT B3LYP/6-31G (d).

Figure S5
Possible arrangements of the four 2-naphthalene substituents in the bay-region of the P-enantiomer of 4a.

Figure S11
Elugrams of the HPLC separation on a chiral stationary phase of a) the successful separation of the atropoenantiomers of 4a, and the non-successful separations of b) 6a, as well as c) and d) for 5a using mixtures of hexane and DCM as eluent. The collection times of the respective fractions are highlighted. Detection by UV/Vis analysis at 550 nm. According to CD spectral analysis, no separation of atropoenantiomers was achieved for 5a. Thus, 5a was for one run separated in c) two fractions and for another separated in d) five fractions. For each fraction a CD spectrum and a 1 H-NMR spectrum was measured, but no CD signals were obtained and the 1 H-NMR spectra all look identical to the spectrum shown in Figure S34. Similar results were obtained for 6a.

NMR spectra
Figure S18 1 H-NMR spectrum of 2a in CD2Cl2 recorded at 295 K.