An unprecedented amplification of near-infrared emission in a Bodipy derived π-system by stress or gelation

A meso-substituted Bodipy derived π-gelator exhibits amplified near-infrared (NIR) emission upon shearing of its film from n-decane or drying of its gel from DMSO.


Synthesis -General Procedures
Unless otherwise stated, all starting materials and reagents were purchased from commercial suppliers and used without further purification. The solvents used for the reactions were purified and dried by employing the standard laboratory methods. The reactions were monitored using thin layer chromatography on silica gel 60 F254 (0.2 mm; Merck) or Al2O3 (0.2 mm; Merck) and visualization was accomplished using a hand held UVlamp (365 nm). Column chromatography was employed to purify the reaction products using glass column packed with silica gel of size range 100-200 microns.

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) (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 IR Prestige-21 Fourier Transform Infrared Spectrophotometer (FT-IR) using KBr pellet method.
Electrospray ionization (ESI) high-resolution mass spectra were recorded on using Thermo Scientific Exactive mass spectrometer. Matrix-assisted laser desorption ionization time-offlight (MALDI-TOF) mass spectra were obtained on a Shimadzu AXIMA-CFR PLUS spectrometer using -cyano-4-hydroxycinnamic acid as the matrix.

Measurements
The electronic absorption spectra were recorded on a Shimadzu spectrophotometer UV-2100. The fluorescence spectra were recorded on a SPEX-Fluorolog-3 FL3-221 spectrofluorimeter. Solid-state emission spectra were recorded using a front face sample holder. Optical studies in solution-state were carried out in a 1 cm quartz cuvette. Relative quantum yield measurements were carried out at two different excitation wavelengths (315 and 475 nm) for the monomeric state using fluorescein in 0.1 N NaOH as the standard which has a reported quantum yield of f = 0.91. Absolute fluorescence quantum yields in the film state were measured by using a calibrated integrating sphere attached to SPEX Fluorolog spectrofluorimeter and the quantum yield value was estimated based on the de Mello method. S1 Lifetime measurements were carried out using IBH (model 5000 DPS) timecorrelated single photon counting (TCSPC) system. Experiments were carried out with 375 nm LED excitation source (pulse width < 100 ps, repetition rate 1 MHz). Film state FT-IR spectra recorded on a Shimadzu IR Prestige-21 FT-IR spectrophotometer using attenuated total reflection (ATR)-8200HA accessory. SEM images were taken on a Zeiss EVO 18 cryo S4 SEM Special Edn with variable pressure detector working at 20-30 kV after sputtering with gold. Samples were prepared by drop cast the aggregates of Bodipy-1 in n-decane and DMSO gel over a smooth aluminium foil. The drop cast samples were allowed to dry under normal condition for one day and subsequently subjected to drying under vacuum for 12 h.
Fluorescence microscopy images were taken by using Leica DFC 290 microscope. The WAXS analysis was carried out using XEUSS SAXS/WAXS system by Xenocs, operated at 50 kV and 0.6 mA. The data were collected in the transmission mode geometry using Cu-K radiation of wavelength 1.54 Å. The single crystal analysis of the model derivative, Bodipy-2 was carried out on a Bruker SMART APEX CCD diffractometer with graphitemonochromatized Mo-K,  = 0.71073 Å radiation.

Preparation of 3,4,5-tris(dodecyloxy)-N-(4-ethynylphenyl)benzamide (9) s3
To a solution of 8 (2.25 mmol) in 10 ml dichloromethane, KF (44.9 mmol) in 50 ml methanol was added to it and allowed to stir at room temperature for 12 h. After completion of the reaction, the reaction mixture was extracted using chloroform, washed with water, brine and then dried over anhydrous sodium sulphate. Solvent was evaporated under reduced pressure. The residue thus obtained purified by silica gel column chromatography using 5% ethylacetate/n-hexane as an

Preparation of Bodipy-1 and Bodipy-2
Compound 3 (0.47 mmol), bis(triphenylphosphine)palladium (II) dichloride (10 mol%), and copper (I) iodide (10 mol%) were added to an oven-dried two-neck round bottom flask equipped with a magnetic stirring bar. The round bottom flask was then sealed with a rubber septum, evacuated and backfilled with argon three times. Degassed triethylamine (30 ml) was added followed by degassed THF (50 ml). After stirring for 5 minutes at room temperature, compound 9 (0.57 mmol) dissolved in 10 ml (1:1) mixture of degassed triethylamine and THF was added and the reaction mixture was stirred at 60 °C for 24 h. The reaction mixture was extracted using chloroform and washed with dilute hydrochloric acid. The organic layer was washed with brine and dried over anhydrous sodium sulphate and then evaporated under reduced pressure. The crude product was then purified by column chromatography using silica gel as adsorbent. The pure compound (Bodipy-1) was eluted with 40% dichloromethane-n-hexane solvent mixture.