Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals

Abstract Thermotropic ionic liquid crystals based on the flavylium scaffold have been synthesized and studied for their structure‐properties relationship for the first time. The mesogens were probed by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X‐ray diffraction (XRD). Low numbers of alkoxy side chains resulted in smectic (SmA) and lamello‐columnar (LamCol) phases, whereas higher substituted flavylium salts showed Colro as well as ordered and disordered columnar (Colho, Colhd) mesophases. Mesophase width ranged from 13 K to 220 K, giving access to room temperature liquid crystals. The optical properties of the synthesized compounds were probed towards absorption and emission properties. Strong absorption with maxima between 444 and 507 nm was observed, and some chromophores were highly emissive with quantum yields up to 99 %. Ultimately, mesogenic and dye properties were examined by temperature‐dependent emissive experiments in the solid state.

Measurements of the temperature-dependent X-ray diffraction were performed using a Bruker AXS Nanostar C with a ceramic tube generator (1500 W) having cross-coupled Goebel mirrors providing monochromatic Cu Kα radiation (1.5405 Å). Diffraction patterns were recorded with Bruker HI-STAR or VÅNTEC 500 detectors. Calibration was carried out using the diffraction pattern of silver behenate at room temperature. The compounds were examined in sealed glass capillaries from Hilgenberg GmbH (external diameter of 0.7 mm, wall thickness 0.01 mm).
Fibre samples were obtained by extrusion of the material at room temperature. Measured values were analysed with the software SAXS from Bruker. [3] The diffraction patterns were further processed using the software Datasqueeze [4] , Origin [6] and LCDiXray [7] .
For UV/Vis and fluorescence spectroscopy solvents were obtained in spectroscopic grade from the supplier and were not further purified. Solutions of flavylium salts were freshly prepared in order to avoid undesired reactions towards hemiketals and chalcones. For absorption spectra, a Perkin Elmer Lambda 35 spectrometer was employed. Emission spectra were recorded on a Perkin Elmer LS 55 spectrometer using slit lengths of 10 nm. Quantum yield were measured with a Hamamatsu C9920-03 system, equipped with a 150 W xenon lamp, monochromator and PMA-12 detector. Lifetime measurements were realized using a picosecond laser diode (HORIBA Jobin Yvon deltadiode, 375 nm) and a Hamamatsu C10910-25 streak camera mounted with a slow single sweep unit. Signals were integrated on a 30 nm bandwidth. Fits were obtained using origin software and the goodness of fit judged by the reduced χ 2 value and residual plot shape. Polarized Optical Microscopy and temperature dependent emission experiments were performed with a Nikon 80i polarized microscope equipped with a Linkam LTS420 hot stage, a Nikon Intensilight C-HGFI (UV 1 filter, 350 nm < λexc < 380 nm) irradiation source, a Nikon DS-FI2 digital camera and an ocean optics QE65000 photodetector connected by optical fibre.

Syntheses
General procedure for the syntheses of alkyloxy-substituted benzaldehydes (6a-e) by
After the suspension was stirred at 80 °C for 3 h water was added, and the aqueous phase was extracted with EtOAc (3 x 100 mL). The combined organic phases were dried over MgSO4.
The solvent was removed under reduced pressure and the crude product was coevaporated twice with toluene. The products 6a-e were used without further purification after drying under vacuum.

General procedure for the syntheses of phenols (7a-f) by acid-catalyzed Dakin oxidation (GP 2)
The arylaldehyde 6a-f (10.3 mmol for 7a) was dissolved in a mixture of CHCl3 (50 mL) and MeOH (20 mL). After the addition of H2O2 (35% in H2O, 25.8 mmol for 7a) and concentrated H2SO4 (4.13 for 7a) the reaction was stirred at room temperature. After 18 h water was added until the formation of two phases. The organic phase was separated, and the aqueous phase was extracted with CHCl3 (2 x 80 mL). The combined organic phases were dried over MgSO4 and the solvent removed under reduced pressure. The residue was purified by column chromatography (PE/EtOAc = 10 : 1). [8]

General procedure for the syntheses of ethynylalcohols (8a-f) by Grignard reaction (GP 3)
In a dried Schlenk flask the arylaldehyde 6a-f (6.89 mmol) was dissolved in abs. THF (30 mL) and cooled to 0 °C. To the solution was added ethynylmagnesium bromide solution (0. 5 M in THF,8.95 mmol) slowly and the reaction was stirred for additional 3 h at room temperature.
After the addition of saturated NH4Cl solution THF was removed under reduced pressure and the aqueous phase was extracted with EtOAc (3 x 50 mL). After drying over MgSO4 and the solvent was removed under reduced pressure and the product was used without further purification. [9] General procedure for the syntheses of ethynylketones (9a-f) by oxidation of
After cooling to room temperature, the mixture was filtrated over Celite ® and the filtration cake was washed with two additional portions of EtOAc. After evaporation of the solvent the crude product was purified by column chromatography (PE/EE = 20 : 1). [9] General procedure for the syntheses of flavylium salts (A-Fla-B) (GP 5) To a solution of phenol 7a-f (162 µmol for V-Fla-1) and ethynylketone 9a-f (162 µmol for V-Fla-1) in EtOAc (5 mL) an excess of triflic acid (600 µmol) was added and stirred for 18 h at room temperature. The product precipitated and was recrystallized directly from the reaction solution. For products with high solubility at room temperature the reaction mixture was stored at 4 °C or -18 °C until the product precipitated. Then 2 mL of absolute EtOH (p.a. grade) was added, the mixture was filtered and the obtained solid was washed with EtOH. The obtained product was dryed in a desiccator over phosphorous pentoxide for 3 days. [9] 1.1 Syntheses of arylaldehydes (6a-f)

3)
Synthesis according to

A quick quantum chemical survey
We have carried out a preliminary investigation of three selected Flavylium ions V-Fla-0, V-Fla-1 and V-Fla-2, replacing the long alkyl chains by methyl groups. This replacement reduces the conformational complexity, while it is expected to be of minor influence on the photophysical properties of the chromophore. All investigations were carried out with the Turbomole program package [19] and are based on density functional theory (DFT), while excited state energies and oscillator strength (velocity gauge) rely on the time-dependent DFT (TDDFT) formalism. The B3LYP functional, [20] the D3 dispersion correction with Becke-Johnson damping, [21] and the def2-TZVPP basis set [22] were used. Visualizations of the structures and orbitals were carried out with Tmolex. [23] Only the conformations as shown in Figure S25 along with the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) were considered here. For convenience, we started with planar structures (Cs point group symmetry). Interestingly, this assumption is confirmed by a normal mode analysis for V-Fla-1 and V-Fla-2, while for V-Fla-0 the ground state exhibits a small imaginary frequency for the torsion of the phenyl group. Reoptimization of V-Fla-0 in C1 indicates a very shallow torsional potential and an equilibrium torsion angle of around 7°, in agreement with previous computational studies. [24] Excitation energies were computed for the Cs constrained structures and geometry optimizations were also carried out for the excited state (again under Cs constraint). The estimates for the excitation energies are listed in Table S4. We We note, however, that our comparison is hampered by not considering solvent effects and conformational averaging, nonetheless the comparison reveals that the chosen approach (B3LYP/def2-TZVPP) gives reasonable energetics. Table S5 shows the computed structure changes (ground state equilibrium bond distances compared to excited state equilibrium bond distances), while keeping the Cs symmetry constraint. Most evident is the strong increase of the 2-1' interring bond distance by more than 6 pm for V-Fla-2, which is not observed for V-Fla-0 and V-Fla-1. This indicates a much reduced π-interaction between the rings in the excited state, possibly followed by a partial ring twist (not yet investigated). In any way, the computations show that there is a very subtle balance of substituent effects that enhance or diminish the conjugation between the two ring systems. For V-Fla-1, both ground and excited state seem to remain rather rigid, explaining the spectral shape and the enhanced fluorescence. All other flavylium salts are either non-rigid in the ground state (like V-Fla-0) or become non-rigid in the excited state, which likely leads to additional non-radiative pathways. (e, f) obtained by DFT calculations (B3LYP, def2-TZVPP, alkoxy groups were replaced by methoxy).

Table S4
Photophysical properties computed at the DFT/TDDFT level (functional B3LYP, basis def2-TZVPP). Vertical absorption means the vertical energy difference between the S0 and S1 surfaces at the ground state equilibrium geometry, vertical emission is the vertical energy difference at the excited state equilibrium geometry. The adiabatic excitation is the minimum to minimum energy difference. In all cases only one conformation is considered.

Table S5
Structure changes of the bond lengths r (in pm) of the ground state S0 upon excitation into the excited state S1, computed at the DFT/TDFT level (functional B3LYP, basis def2-TZVPP).