Friedel–Crafts-type reaction of pyrene with diethyl 1-(isothiocyanato)alkylphosphonates. Efficient synthesis of highly fluorescent diethyl 1-(pyrene-1-carboxamido)alkylphosphonates and 1-(pyrene-1-carboxamido)methylphosphonic acid

Summary Friedel–Crafts-type reaction of pyrene with diethyl 1-(isothiocyanato)alkylphosphonates promoted by trifluoromethanosulfonic acid afforded diethyl 1-(pyrene-1-carbothioamido)alkylphosphonates in 83–94% yield. These compounds were transformed, in 87–94% yield, into the corresponding diethyl 1-(pyrene-1-carboxamido)alkylphosphonates by treatment with Oxone®. 1-(Pyrene-1-carboxamido)methylphosphonic acid was obtained in a 87% yield by treating the corresponding diethyl phosphonate with Me3Si-Br in methanol. All of the synthesized amidophosphonates were emissive in solution and in the solid state. The presence of a phosphonato group brought about an approximately two-fold increase in solution fluorescence quantum yield in comparison with that of a model N-alkyl pyrene-1-carboxamide. This effect was tentatively explained by stiffening of the amidophosphonate lateral chain which was caused by the interaction (intramolecular hydrogen bond) of phosphonate and amide groups. The synthesized phosphonic acid was soluble in a biological aqueous buffer (PBS, 0.01 M, pH 7.35) and was strongly emissive under these conditions (λem = 383, 400 nm, τ = 18.7 ns, ΦF > 0.98). Solid-state emission of diethyl 1-(pyrene-1-carboxamido)methylphosphonate (λmax = 485 nm; ΦF = 0.25) was assigned to π–π aggregates, the presence of which was revealed by single-crystal X-ray diffraction analysis.

Oxone® (1.5 mmol in 10 mL of water) was added to the thioamides 2a-d (1 mmol) dissolved in a mixture of acetonitrile (30 mL) and water (20 mL). The resulting solution was stirred at room temperature overnight, poured into water (100 mL) and extracted several times with CH 2 Cl 2 . The organic phase was dried over anhydrous Na 2 SO 4 and concentrated. The products were purified by silica gel column chromatography (CH 2 Cl 2 /MeOH 100 : 0 /99 : 1).

X-ray diffraction analysis of diethyl 1-(pyrene-1-carboxamido)methylphosphonate 3a
Crystals suitable for X-ray analysis were obtained by slow diffusion of pentane into a solution of this compound in dichloromethane at room temperature. The crystals were clear colorless plates.
The data for were collected on Agilent Supernova 4 circle diffractometer system equipped with molybdenum microsource and Eos CCD detector. The data were collected using MoKα radiation with CrysAlis171 software and integrated with the CrysAlisPRO software. Data were corrected for absorption effects using the numerical methods (SCALE3 ABSPACK).
The structure was solved by direct methods using SXELXS and refined by the full-matrix least squares procedure with SHELXL within the OLEX2 graphical interface. Figures were produced with Ortep3v2 and Mercury_3.3 software.
All H atoms were visible in the residual density map, but were added geometrically and refined in riding approximation. Positions of the H atoms involved in hydrogen bonds were refined with restraint applied to the N-H distance as suggested by SHELXL for 100 K.
The quantitative descriptors of the data processing and structure refinement for all compounds are presented in Table S1.  Figure S1: Variable concentration 1 NMR spectra of 3a in CDCl 3

Photophysical study
Electronic absorption spectra were run on a Perkin Elmer Lambda 45 UV-vis spectrometer.
Corrected emission spectra were obtained on a Fluorolog FL3-221 spectrofluorometer from Horiba Jobin-Yvon, including an integration sphere accessory which allows recording excitation and emission spectra and determining absolute quantum yield values in the powder state.
Fluorescence decay curves were obtained by the time-correlated single-photon counting (TCSPC) method with femtosecond laser excitation using a Spectra-Physics set-up composed of a Titanium Sapphire laser (Tsunami, Spectra-Physics) pumped by a doubled YAG laser (Millennia, Spectra-Physics), itself pumped by two laser diode arrays. Light pulses at 720 nm were selected by optoacoustic crystals at a repetition rate of 4 MHz, and then doubled to 360 nm by non-linear crystals. Fluorescence photons were detected through a monochromator by s9 means of a Hamamatsu MCP R3809U photomultiplier connected to a constant-fraction discriminator. The time-to-amplitude converter was purchased from Tennelec. The instrumental response function was recorded before each decay measurement. The fluorescence data were analysed using the Globals software package developed at the Laboratory for Fluorescence Dynamics at the University of Illinois at Urbana-Champaign, which includes deconvolution analysis and the global non-linear least-squares minimization method.