Red edge effect and chromoselective photocatalysis with amorphous covalent triazine-based frameworks

Chromoselective photocatalysis offers an intriguing opportunity to enable a specific reaction pathway out of a potentially possible multiplicity for a given substrate by using a sensitizer that converts the energy of incident photon into the redox potential of the corresponding magnitude. Several sensitizers possessing different discrete redox potentials (high/low) upon excitation with photons of specific wavelength (short/long) have been reported. Herein, we report design of molecular structures of two-dimensional amorphous covalent triazine-based frameworks (CTFs) possessing intraband states close to the valence band with strong red edge effect (REE). REE enables generation of a continuum of excited sites characterized by their own redox potentials, with the magnitude proportional to the wavelength of incident photons. Separation of charge carriers in such materials depends strongly on the wavelength of incident light and is the primary parameter that defines efficacy of the materials in photocatalytic bromination of electron rich aromatic compounds. In dual Ni-photocatalysis, excitation of electrons from the intraband states to the conduction band of the CTF with 625 nm photons enables selective formation of C‒N cross-coupling products from arylhalides and pyrrolidine, while an undesirable dehalogenation process is completely suppressed.


Light source
In this work the following light sources were used: blue LED module 1 (home-made steel cylinder photoreactor attached with self-adhesive LED strips which were purchased from JKL components, emission maximum λ = 468 nm, measured optical power 14 mW cm -2 at the central position); blue LED module 2 (emission maximum λ = 461 nm, measured optical power 101 mW cm -2 ); blue LED module 3 (M455F3, purchased from ThorLabs) (emission maximum λ = 455 nm) coupled with an optical fiber and controlled by the driver (DC2200, purchased from ThorLabs); green LED module (M530F2, purchased from ThorLabs) (emission maximum λ = 530 nm, measured optical power 41 mW cm -2 ) coupled with an optical fiber and controlled by the driver (DC2200, purchased from ThorLabs); red LED module (emission maximum λ = 620-625 nm, measured optical power 302 mW cm -2 ); white LED module (emission maximum λ = 400-760 nm, measured optical power 203 mW cm -2 ).
Irradiance of LED modules was measured using PM400 Optical Power and Energy Meter equipped with the integrating sphere S142C and purchased from Thorlabs.

Powder X-ray diffraction (PXRD)
PXRD patterns were recorded at room temperature on a Bruker D8 X-Ray Diffractometer using Cu Kα1 radiation.

Fourier-transformed infrared (FT-IR)
FT-IR spectra were recorded on Thermo Scientific Nicolet iD5 spectrometer equipped with an attenuated total reflection unit applying a resolution of 2 cm -1 .

XPS
XPS analysis was carried out on a Thermo Fisher Scientific ESCALAB spectrometer with Al Kα radiation.

Elemental analysis
Elemental analysis was accomplished by combustion analysis using a Vario Micro device.
Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) SEM images were obtained on JSM-7500F (JEOL) at an accelerating voltage of 3 kV. EDX) investigations were conducted using a Link ISIS-300 system (Oxford Microanalysis Group) equipped with a Si (Li) detector and an energy resolution of 133 eV.

Transmission electron microscopy (TEM)
The TEM study was performed using a double Cs corrected JEOL JEM-ARM200F (S)TEM operated at 80 kV equipped with a cold field emission gun.

Nitrogen adsorption/desorption
Nitrogen adsorption/desorption measurements were performed after degassing the samples at 150 °C for 20 hours using a Quantachrome Quadrasorb SI-MP porosimeter at 77 K. The specific surface areas were calculated by applying the Brunauer-Emmett-Teller (BET) model to adsorption isotherms for 0.05<p/p0<0.3 using the QuadraWin 5.05 software package. The pore size distribution was obtained by applying the quenched solid density functional theory (QSDFT) model for N2 adsorbed on carbon with cylindrical pore shape at 77 K.
Thermal gravimetric analysis (TGA) TGA measurement was performed using a thermo microbalance TG 209 F1 Libra coupled with a Thermostar Mass spectrometer (Pfeiffer Vacuum) with an ionization energy of 75 eV.
Analysis was conducted under N2 atmosphere.

Optical absorbance
Optical absorbance spectra were measured on Shimadzu UV 2600 spectrophotometer equipped with an integrating sphere.

Time-resolved (TR) PL spectra
TR-PL spectra were recorded on fluorescence lifetime spectrometer (Fluo Time 250, PicoQuant) equipped with PDL 800-D picosecond pulsed diode laser driver. The decay curves were fitted using a nonlinear method with a multicomponent decay law given by The solid-state TR PL spectra were obtained with λexc = 375, 470 and 640 nm, respectively.
The following settings were used for the spectra acquisition: Laser Frequency 40 MHz, Emission Monochromator Bandwidth 2 nm, Delta (step between λem) 10 nm. Long pass filters of 495 and 665 nm were used for λexc = 470 and 640 nm, respectively.
For the quenching experiment, sample suspension containing 8 mg of PYT in 20 mL MeCN/H2O (volume ratio 5:1) solvent was prepared and sonicated for 1 h before use. The TR PL spectra were obtained with λexc = 470 nm and λem = 580 nm. The following settings were used for the spectra acquisition: Laser Frequency 40 MHz, Emission Monochromator Bandwidth 10 nm.

Nuclear magnetic resonance (NMR)
1 H NMR spectra were recorded on Agilent 400 MHz. Chemical shifts are reported in ppm versus solvent residual peak: chloroform-d 7.26 ppm.

Mott-Schottky measurement
The Mott-Schottky measurement was carried out with Arbin electrochemical testing station (Arbin Instrument) in a standard three-electrode quartz cell. The working electrode was prepared as follows: 2 mg of sample was suspended in 0.2 mL of deionized water containing 0.02 mL of 5 wt% Nafion D-520 dispersion, and the mixture was then dispersed by ultrasonication and spread onto an FTO glass. After being dried naturally, the FTO glass was where ERHE is the converted potential vs. RHE, E o Ag/AgCl = 0.1976 at 25 °C, and EAg/AgCl is the experimentally measured potential against Ag/AgCl reference. EPR spectra measurement of CTFs and PYT/anisole mixture Two capillaries were sealed from one side with the flame of gas burner. Each capillary was filled with PHT and PYT powder (3 mg). EPR spectra were acquired in dark. Dynamic EPR spectra under illumination were recorded using a '2D field delay' mode, with the first spectrum recorded when LED is OFF, the rest under illumination. The total illumination time was ~ 40 min.
Anisole (20 μL) was then added to the PYT capillary which was subsequently centrifuged at 3000 rpm for 5 min to make sure the catalyst was completely immersed in anisole. Acquisition of EPR in dark and dynamic EPR under light irradiation was repeated.

DMPO-O2 •− adduct detection
PYT (3 mg) and 40 mM DMPO in methanol (40 μL) were added to the capillary. The capillary was centrifuged at 3000 rpm for 5 min. EPR spectra in dark and dynamic EPR spectra under illumination were acquired (~ 40 min).

TEMPO detection
A mixture of PYT (5 mg) and TEMP (8.5 μL) in MeCN (3 mL) was added to a glass tube with an inlet for gas connection and a ground joint for 'cold finger' connection. The mixture was vigorously stirred at room temperature for 10 min. A capillary was charged with the reaction mixture (40 μL). EPR spectrum in dark was acquired. The reaction mixture was purged with O2 for 30 s. A cold finger was immersed into the suspension and cooling water circulation was enabled maintaining the reaction mixture temperature at 20-25 o C. A balloon with O2 was connected to the reactor headspace via gas inlet. The reaction mixture was vigorously stirred under blue light irradiation (blue LED module 2, 101 mW cm -2 ) for 3 h. A capillary was charged with the reaction mixture (40 μL) and EPR spectrum acquisition in dark was repeated.

Low-temperature EPR spectra measurement
Low-temperature EPR spectra were acquired with liquid nitrogen cooling which allowed stepwise decrease of temperature from 220 to 90 K. The following settings were used for the

Ultraviolet photoelectron spectroscopy
The ultraviolet photoelectron spectroscopy (UPS) spectra were acquired with a He I (21.2 eV) radiation source. The detector was a combined lens with an analyzer module thermoVG (TLAM). VB potential (U, V vs SHE, electrochemical scale) of the materials was calculated using the UPS data (physical scale) according to the equation: where E -energy of the VB determined from the UPS, eV; k -conversion factor, 1 V eV -1 .

Ultrafast pump-probe transient absorption spectroscopy (TAS)
Investigation was performed using a Clark MXR CPA 2101 Ti:sapphire as the laser source  Table 2). The reaction progress was checked with thin layer chromotography every 24 h.
After the substrate was completely consumed, the product was extracted with CHCl3, dried over Amplitude average lifetime where τ̅ is the average fluoresence lifetime, ai is the amplitude fraction and τi is the fluorescence lifetime.
Intensity average lifetime τ̅ = ∑ a i τ i 2 ∑ a i τ i (5) where τ̅ is the average fluoresence lifetime, ai is the amplitude fraction and τi is the fluorescence lifetime.

AQY
The AQY was calculated as:

Calculation of product yield and conversion of reagent in photocatalytic experiments
Yield of a product was calculated according to the equation: where nexp -amount of a product formed in a photocatalytic experiment and determined using either 1 H NMR or GC-MS, mol; ntheor -amount of a product expected to form in a photocatalytic experiment according to the reaction stoichiometry, mol.
Conversion of a reagent was calculated according to the equation: Where nrxn -amount of a reagent remaining in the reaction mixture after a photocatalytic experiment and determined using either 1 H NMR or GC-MS, mol; nload -amount of a reagent taken for a photocatalytic experiment, mol.  (Table 1), is that increase in crystallinity may be associated with an easier charge transfer to possible surface states. Figure 39), we find significantly weaker signals as well as strong scattering upon photoexcitation at 387 nm as compared to PYT and PYTnc.

Moving to dispersions of PHT (Supplementary
This likely correlates to the smaller size of the particles. Nonetheless, directly after photoexcitation with 387 nm, we find a negative transient from <460 nm to approximately 1000 nm together with the formation of an ESA in the nIR maximizing at around 1300 nm. The latter decays to zero within 1.5 ns. Furthermore, it must be noted that after photoexcitation, an ESA becomes apparent superimposed to the GSB in the region between <460 and 620 nm. After approximately 75 ps, these ESA increase in intensity giving rise to a positive differential absorption which converts into a broad GSB within 20 ns. Commencing with delay times >250 µs, all differential absorptions stemming from the GSB have returned to zero.

Supplementary Note 4
Visible-light absorption of the CTF generates excited electron-hole pairs, followed by their separation into free charges (Supplementary Figure 31). In the electrophilic substitution pathway, the photoexcited electrons reduce O2 to H2O2, which further reacts with HBr to produce the active electrophilic species, denoted collectively as "HOBr". Meanwhile, Br2 produced by oxidation of Br − reacts with H2O to give "HOBr" species. The "HOBr" The suggested mechanism can rationalize the fact that in acidic environment we obtained 4bromoanisole from anisole, despite its oxidation potential +1.81 V vs SCE 4 is more positive that the VB potentials of PHT (+1.55 V vs SCE) and PYT (+0.75 V vs SCE). Therefore, in acidic environment the VB potential of PHT is shifted by at least 0.26 V to more positive values.
As a result, oxidation of anisole by the photocatalyst becomes feasible. Taking into account the band diagram of PYT, such shift is even larger, ca. 1.06 V, but due to higher content of pyridinic-nitrogen atoms (basic sites where protons reside) and 4 times larger surface area, is apparently feasible.

Supplementary Note 6
To check stability of PHT, we recovered the CTF and characterized by a series of techniques.
FT-IR revealed that chemical structure of PHT was not altered (Supplementary Figure 46).  Table 12). Moreover, C/N ratio in fresh PHT (3.81) was close to that in PHT recovered after the photocatalytic experiment (3.86).

Supplementary Note 7
Stronger oxidation power of the PHT excited state, 1.55 V compared to 0.75 V vs SCE in PYT, is advantageous for one-electron oxidation of the Ni(II)-intermediate (Supplementary Figure   41).