Supercoiled fibres of self-sorted donor–acceptor stacks: a turn-off/turn-on platform for sensing volatile aromatic compounds

Supercoiled fibres of self-sorted donor–acceptor assemblies that allow the sensing of nitroaromatics and aromatic amines using a “turn-off/turn-on” mechanism are reported.


Synthesis-General Procedures.
Unless otherwise stated, all starting materials and reagents were purchased from commercial suppliers and used without further purification. The solvents were purified and dried by standard methods prior to use. The reactions were monitored using thin layer chromatography (TLC) on silica gel 60 F 254 (0.2 mm; Merck).
Visualization was accomplished using UV lamp (365 nm). Column chromatography was performed on glass columns of different sizes hand packed with silica gel 60 (particle size 0.040-0.063 mm, Merck). Molecules C 3 OPV and C 3 PBI were synthesized according to Scheme S1 and S2 based on standard protocols.

Synthesis-Characterization
Techniques. NMR spectra were measured on a 300 or 500 MHz Bruker Avance DPX spectrometer. Chemical shifts are reported in parts per million (ppm) using tetramethylsilane (TMS) (δ H = 0 ppm) as an internal reference. The resonance multiplicity is described as s (singlet), d (doublet), t (triplet) and m (multiplet). FT-IR spectra were recorded on a Shimadzu IRPrestige-21 Fourier Transform Infrared Spectrophotometer using KBr pellet method. Mass spectra (MS) were recorded on a JEOLJSM 600 fast atom bombardment (FAB) high-resolution mass spectrometer. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were obtained on a Shimadzu AXIMA-CFR PLUS spectrometer using α-cyano-4-hydroxycinnamic acid as the matrix.

Measurements.
The electronic absorption spectra were recorded on a Shimadzu UV-3101 or 2401PC UV-Vis-NIR scanning spectrophotometer. The fluorescence spectra were recorded on a SPEX-Fluorolog F112X spectrofluorimeter. AFM images were recorded under ambient conditions using a NTEGRA (NT-MDT) operating with a tapping mode regime. Micro-fabricated TiN cantilever tips (NSG10) with a resonance frequency of 299 kHz and a spring constant of S8 20-80 Nm -1 was used. Samples for the imaging were prepared by drop casting the sample (1 × 10 -4 M) prepared in toluene on freshly cleaved mica surface after drying in vacuum. SEM images were taken on a Zeiss EVO 18 cryo SEM Special Edn with variable pressure detector working at 20-30 kV after sputtering with gold. Samples were prepared by drop casting the aggregates of C 3 OPV, C 3 PBI and 1:1 mixture in toluene on freshly cleaved mica substrate. It was kept for overnight to allow slow evaporation of the solvent and then further dried in a vacuum desiccator for 12 h. In order to carry out the sensing studies in vapor phase the selfassembled solution were drop cast over glass plates and dried under vacuum. The film was placed in a chamber containing saturated vapor of the analytes. Emission spectra were collected in a front face geometry using a film sample holder. The samples for photoelectron yield spectroscopy (PYS) were prepared on ITO coated glass plates by solution drop cast (chlorobenzene solution). Prior to the measurements, substrates were dried in a vacuum oven for 2 h at 50 °C. The PYS experiments were done on a RIKEN Keiki Co., Ltd., model AC-3 under high vacuum (1 × 10 -3 M pa). The instrument was calibrated using standard aluminium substrate.

Preparation of 4-(bromomethyl) benzonitrile (2):
To a solution of 4-methylbenzonitrile (1) (1 g, 8.32 mmol) in 30 mL of dry CCl 4 were added N-bromosuccinimide (1.77 g, 10 mmol) and AIBN. The reaction mixture was refluxed for 8 h. After the completion of the reaction, the hot reaction mixture was cooled and filtered. The solution was then concentrated under reduced pressure, kept overnight for recrystallisation and the product crystallised was filtered and dried.
After checking the completion of the reaction by TLC, the reaction mixture was concentrated under reduced pressure. The residue thus obtained was extracted with chloroform, washed with water and dried over anhydrous Na 2 SO 4 . The solvent was removed under reduced pressure.
The product was then precipitated from chloroform by adding excess amount of methanol. The crude product thus obtained was further purified by column chromatography over silica gel (40% CHCl 3 -n-hexane

Synthesis of C 3 OPV:
To a solution of OPV amine 11 (0.2g, 0.15mmol) and triethylamine (0.5 ml) in dry CH 2 Cl 2 (10 mL) a solution of 1,3,5-benzenetricarboxylic acid chloride (0.012g, 0.045 mmol) in dry CH 2 Cl 2 (20 mL) was added dropwise. The reaction mixture was stirred for 6 h at room temperature. After completion of the reaction, the solvent was evaporated in vacuo.

Synthesis of (16):
To a solution of 16 (0.5 g, 0.69 mmol) in CH 2 Cl 2 , trifluoroacetic acid (10 mL) was added and the mixture was stirred for 4 h at room temperature. After evaporation of the solvent in vacuo, the product was dissolved in CH 2 Cl 2 and subsequently washed with an aqueous 10% NaHCO 3 solution and brine. The organic layer was dried over anhydrous Na 2 SO 4 and finally evaporated in vacuo.

Isodesmic or Equal-K Self-Association Model
The temperature dependent absorption spectrum of C 3 OPV is fitted with isodesmic or equal-K model in which the binding constant for each addition of monomer to the growing assembly is the same. Standard isodesmic model is used for analyzing the data. S2 According to this model, the degree of polymerization or the molar fraction of aggregated species α agg (T) is given by the equation (1).
By using equation (1), T m, the melting temperature defined as the temperature for which α agg = 0.5 and ΔH, the molar enthalpy release related to the formation of noncovalent intermolecular interactions were determined. The number-averaged degree of polymerization DP N can be calculated from α agg (T): This expression can be related to the equilibrium constant K and the total concentration of molecules C T via: (3)

Nucleation-Elongation Model
In order to demonstrate the involvement of nucleation and growth process in the observed selfassembly of C 3 PBI, we have attempted to analyze the curve on the basis of the model proposed by van der Schoot, Schenning and Meijer. S3 According to this model, in the elongation regime, the fraction of aggregated species (α agg ) can be defined by the following equation: where, ΔH e is the enthalpy corresponding to the aggregation (elongation) process, T the absolute temperature, T e the elongation temperature, R the ideal gas constant. α SAT is a parameter introduced to ensure that α agg /α SAT does not exceed unity.
On the other hand in the nucleation regime the fraction of aggregated species (α agg ) can be defined by: where K a is the dimensionless equilibrium constant of the activation step at the elongation temperature.
The average length of the stack <N n > averaged over the nucleated species at the T e is given by: The substitution of K a in equation 6 enables the calculation of the number of aggregated molecules, i.e., the nucleus size, at the elongation temperature.