Circularly polarised luminescence in an RNA-based homochiral, self-repairing, coordination polymer hydrogel

The aqueous equimolar reaction of Ag(i) ions with the thionucleoside enantiomer (−)6-thioguanosine, ((−)6tGH), yields a one-dimensional coordination polymer {Ag(−)tG}n, the self-assembly of which generates left-handed helical chains. The resulting helicity induces an enhanced chiro-optical response compared to the parent ligand. DFT calculations indicate that this enhancement is due to delocalisation of the excited state along the helical chains, with 7 units being required to converge the calculated CD spectra. At concentrations ≥15 mmol l−1 reactions form a sample-spanning hydrogel which shows self-repair capabilities with instantaneous recovery in which the dynamic reversibility of the coordination chains appears to play a role. The resulting gel exhibits circularly polarised luminescence (CPL) with a large dissymmetry factor of −0.07 ± 0.01 at 735 nm, a phenomenon not previously observed for this class of coordination polymer.


Methods
Materials. All reagents were obtained from Sigma-Aldrich. Deionised water (18 MΩ cm resistivity) was obtained from a Direct-Q® 3 UV Water Purification System (Merck).
Preparation of 1. Ag-thioguanosine, 1, prepared as follows. An aqueous suspension of 6-thioguanosine (6TG-H) was sonicated for 1 hour to achieve a fine dispersion to which was added a molar equivalent of AgNO3 (solid). The reaction mixture was stirred for hour. As an example, a gel sample at concentration of 30 mmol -1 was prepared by sonication of 18.0 mg of 6-thioguanosine in 2 ml of water for 1 hour. Then, the suspension was mixed with 10.2 mg of finely powdered AgNO3. After the addition the mixture was stirred for an hour. Over the course of this period a yellow pale gel formed, which was indicated by the inverted-vial test (Fig. 1). There is no effect of stir direction has been notice on the gel (SI Fig. 6c).
Atomic Force Microscopy (AFM). AFM data was acquired using a Multimode 8 atomic force microscope with a NanoscopeV controller (Bruker), and a ''E'' scanner. Nanoscope software version 9.1 was used to control the microscope. The system was operated in ScanAsyst in Air mode as a peak force tapping mode at ultra-low forces minimise damage to the samples. For reducing vibrational noise, an isolation table/acoustic enclosure was used (Veeco Inc., Metrology Group). Silicon tips on silicon nitride cantilevers (ScanAsyst, Bruker) were used for imaging. The nominal tip radius was approximately 2 nm, resonant frequency 150 kHz and spring constant k ~0.7 Nm -1 . The AFM data were analysed with NanoScope Analysis 1.5 software (Bruker). The sample was prepared by adding 2µl of freshly prepared dilute aqueous solution of 1 onto a clean silicon wafer and drying in air.

Ultraviolet-Visible Spectroscopy (UV-Vis).
Absorption spectra (in the range of 190-850 nm) were recorded in a NanoDrop™ One C UV-Vis spectrophotometer. UV-Vis spectra of an aqueous solutions of 6-thioguanosine in at concentrations of 1 mmol l -1 and of Ag-thioguanosine (as an aqueous solution at concentrations of 1 mmol l -1 and as gel at concentration of 30 mmol l -1 ) were recorded in a quartz cuvette with path length 0.1 mm. The spectrometer was blanked using Nanopure water.
FTIR spectroscopy. FTIR Spectra (in the range of 600 to 4000 cm -1 wavenumbers) were recorded using the ATR accessory of an IRAffinity-1S Fourier transform infrared spectrophotometer (Shimadzu) at 4 cm -1 spectral resolution. For each spectrum, 64 scans were co-added and averaged. The bare ATR accessory was used as a background. A sample of Ag-6TG was dried in air for 1 day prior to analysis and then deposited on a clean p-Si(100) chip (1 cm 2 ).
Powder X-ray Diffraction (XRD). Very slow dehydration of the gel over 14 days yielded a fibrous green-yellow powder that exhibited a main Bragg diffraction ring at 3.03 Å.
Fluorescence spectroscopy. Emission spectroscopy (in the range 250-1000 nm) was recorded on a SPEX Fluoromax spectrofluorimeter. Emission spectra of solutions of 6-thioguanosine in 0.1 mol l -1 of NaOH (at concentrations of 1 mmol l -1 and 30 mmol l -1 ) and of Ag-thioguanosine (as an aqueous solution at concentrations of 1 mmol l -1 and as gel at concentration of 30 mmol l -1 ) were recorded in a quartz cuvette with pathlength 10 mm. The excitation wavelength was 350 nm for the solutions and 430 nm for the gel. Circular dichroism. Circular dichroism spectra were recorded on a Jasco J-810 Jasco J-810 Spectropolarimeter. CD spectra of 6-thioguanosine solution (10 mmol l -1 ) in 0.1 M of NaOH and Agthioguanosine as a gel (at concentration of 10 mmol l -1 ) were recorded in a quartz cell with pathlength 0.1 mm.
Computational details. All TDDFT calculations were performed with the ORCA quantum chemistry package 1, 2 within the Tamm-Dancoff approximation. 3 All atoms were modelled with an all-electron Gaussian basis set of triple-ζ valence quality (def2TZVP) 4 along with the hybrid-level PBE0 exchangecorrelation functional 5 and the semiempirical D3 Grimme dispersion correction. 6 The water solvent was simulated by employing the conductor-like polarizable continuum model. 7 Firstly, a geometry optimisation was performed on the crystal structure. From the optimised structure, we generated structures of increasing length, from 1 to 7 units -where one unit contains one silver atom and its associated organic neighbours. These unit structures were used to calculate all the excited states with a wavelength of 150 nm or more, along with the associated origin-independent velocity rotatory strengths, Ri. 8 We model the circular dichroism spectra, ∆ε, as the sum of Gaussian functions, each centred on the excitation energy Ei, whose amplitude is related to the rotatory strength and whose width, σ = 0:04E, is chosen empirically to best reproduce the experimental line shape, Rheology. Rheological measurements were performed with a HR-2 Discovery Hybrid Rheometer (TA Instruments) with a standard steel parallel-plate geometry of 20 mm diameter with a gap of 1 mm. The strain and the frequency were set to 1% and 1 Hz, respectively.

Scanning electron microscopy (SEM).
Samples of xerogel Xe1 were dried on silicon wafers or freezing dried. The SEM images were collected using a TESCAN VEGA LMU Scanning Electron Microscope, housed within EM Research Services, Newcastle University. Digital images collected with TESCAN supplied software.
Circularly polarized luminescence (CPL). CPL spectra were recording using a custom-build CPL spectrometer. Full details of this CPL spectrometer have been reported by Carr et al. 9 Non-standard protocols were followed to measure CPL from the gel samples. Total emission and CPL emission were sampled at 400 -800 nm in 5 nm increments, with 20 accumulated spectra per measurement. Roughly 500 micro-liters of the gel sample was contained in an open-topped quartz cuvette (101-10-40, Hellma). The gel adhered to a corner of the cuvette. Excitation was provided by a 410 nm laser source from directly above the sample. The laser and sample were positioned to maximise emission intensity from the sample. Data was processed using custom-written Matlab scripts (Matlab 2019b, Mathworks). Instrumental baselines for total emission intensity and CPL emission were subtracted to zero measurements as appropriate and were then smoothed using an intensity-preserving Savitzky-Golay filter. glum values for each repeated measurement were calculated from smoothed intensity and smoothed CPL emission. This was verified across 6 independent measurements. Quoted uncertainty is the standard deviation of glum of these six independent measurements. The uncertainty of the presented data is the standard deviation of 6 independent measurements. Supplementary Table 1. Wavelengths and rotatory strengths of the four excitations used to approximately reproduce the complete spectra. Also included is the angle between the electric and magnetic transition velocity dipoles.    Figure 11. (a) Fluorescence emission spectra of a solution of 6-TGH nucleoside in 0.1 mol l -1 of NaOH (black) and of an aqueous solution of Ag-thioguanosine (red), both at a concentration of 1 mmol l -1 . The excitation wavelength was 350 nm and the pathlength was 1 cm. (b) Fluorescence emission spectra of a solution of 6-TGH nucleoside in 0.1 mol l -1 of NaOH (black) and a gel of Agthioguanosine (red), both at a concentration of 30 mmol l -1 . The excitation wavelength was 350 nm for the 6-TGH solution and 430 nm for the Ag-thioguanosine gel and the pathlength was 10 mm.