In situ observation of a stepwise [2 + 2] photocycloaddition process using fluorescence spectroscopy

Using highly sensitive and selective in situ techniques to investigate the dynamics of intermediates formation is key to better understand reaction mechanisms. However, investigating the early stages of solid-state reactions/transformations is still challenging. Here we introduce in situ fluorescence spectroscopy to observe the evolution of intermediates during a two-step [2 + 2] photocycloaddition process in a coordination polymer platform. The structural changes and kinetics of each step under ultraviolet light irradiation versus time are accompanied by the gradual increase-decrease of intensity and blue-shift of the fluorescence spectra from the crystals. Monitoring the fluorescence behavior using a laser scanning confocal microscope can directly visualize the inhomogeneity of the photocycloaddition reaction in a single crystal. Theoretical calculations allow us to rationalize the fluorescence behavior of these compounds. We provide a convenient strategy for visualizing the solid-state photocycloaddition dynamics using fluorescence spectroscopy and open an avenue for kinetic studies of a variety of fast reactions.


Characterization
Powder X-ray diffraction (PXRD) patterns were acquired on a PANalytical X'Pert PRO MPD system (PW3040/60) using Cu Kα radiation (λ = 1.5406Å) from 5 o to 50 o with a scanning step size of 0.02 o .Elemental analyses (C, H, N) were performed using a PE 2400 II elemental analyzer.The NMR spectra were recorded at ambient temperature on a Bruker AVANCEШ HD-400M spectrometer. 1 H NMR and 19 F NMR chemical shifts were referenced to the solvent signal in CDCl3 or DMSO-d6.Chemical shifts are reported in parts per million (ppm) and referenced with TMS for 1 H NMR and CFCl3 for 19 F NMR. Solid-state UV-vis absorption spectra were recorded on a Varian Cary-50 UV-vis spectrophotometer with an integrating sphere at room temperature in the range of 200 -800 nm.The electron spin (paramagnetic) resonance (ESR/EPR) spectra were obtained with an JES-X320 electron spin resonance spectrometer operating at the X-band (frequency 9.148 GHz) for samples sealed inside a 4 mm thick quartz capillary, with irradiation by a Xe light (500 W, equipped with a filter ≤360 nm).
Photoluminescence spectra and quantum yields were obtained on a HORIBA PTI QuantaMaster40 Spectrofluorometer and lifetimes were measured on a FLS980.Luminescence imaging was performed with a Leica TCS SP5 II confocal laser scanning microscope.The photo-irradiation experiments were conducted with a LED lamp NBT-LED4 (λ = 365 nm, 2 W).

Explanations of any A-or B-level alerts
The crystal used has cracks in this reaction, and thus the poor single-crystal quality of CP1-2β inevitably led to these B-level mistake in the Checkcif Report (PLAT342_ALERT_3_B Low Bond Precision on C-C Bonds ............... 0.02785 Ang; PLAT601_ALERT_2_B Unit Cell Contains Solvent Accessible VOIDS of .103 Ang**3).
After cooling to ambient temperature, the solid dark mass was dissolved in CH2C12 (100 mL), extracted thoroughly with water (3 × 50 mL), and dried over anhydrous Na2SO4.The organic phase was concentrated under vacuum to give F-1,3-bpeb ligand as a light yellow powder.Yield: 2.55 g (84.4%). 1

CP1-1:
The as-synthesized crystals of CP1 deposited on the quartz plate were placed in a long glass tube which was immersed in a low-temperature thermostatic reaction bath at -50 °C and irradiated with a LED lamp (365 nm, 2 W) for 10 min to form faint yellow crystals of CP1-1 (100% yield based on CP1). 1

CP1-2β:
The as-synthesized crystals of CP1 on the quartz plate were irradiated with a LED lamp (365 nm, 2 W) for 1 h at 25 °C or the as-synthesized crystals of CP1-1 on the quartz pieces were irradiated with a LED lamp (365 nm, 2 W) for 35 min at 25 °C to form yellow crystals of CP1-2β (100% yield based on CP1 or CP1-1).

Kinetic analysis of each step
In order to determine the kinetics of this reaction, we monitored the corresponding structural transformation upon UV irradiation by in situ time-dependent fluorescence spectra and 19 F NMR.The fitting of the conversion data calculated from fluorescence intensity versus irradiation time showed different kinetics for CP1 to CP1-1 and CP1-1 to CP1-2β, respectively.
4][5][6] The JMAK kinetics is described by equation ( 1): where y is the conversion (mole fraction) of the photoproduct formed in time t, k is the rate constant, and n is the dimensionality of growth (Avrami exponent).
The kinetics of the transformation from CP1 to CP1-1 was calculated by general equation 7,8 for zero-order reaction rate, which has been successfully applied previously to [2+2] photocycloaddition reactions.The kinetics is described by equation ( 2): where c0 and ct represent the conversion (mole fraction) of CP1 before and at any irradiation time (t) at 365 nm and -50 °C, respectively.k is the rate constant.
photoproduct calculated from 19 F NMR data sets of CP1-1 irradiated at 365 nm and 25 °C.k is the rate constant, and n is the dimensionality of growth (Avrami exponent).

CP1
irradiated under UV at -50 °C CP1-1 irradiated under UV at 25 °C Supplementary Figure 12│Conversion obtained from fluorescence intensity analysis.The conversion (mole %) calculated from fluorescence intensity data sets of CP1 under UV light (λ = 365 nm) irradiation at -50 °C and CP1-1 under UV light (λ = 365 nm) irradiation at 25 °C as a function of time.