All-Carbon-Linked Continuous Three-Dimensional Porous Aromatic Framework Films with Nanometer-Precise Controllable Thickness

Inherently porous materials that are chemically and structurally robust are challenging to construct. Conventionally, dynamic chemistry is thought to be needed for the formation of uniform porous organic frameworks, but dynamic bonds can limit the stability of these materials. For this reason, all-carbon-linked frameworks are expected to exhibit higher stability performance than more traditional porous frameworks. However, the limited reversibility of carbon–carbon bond-forming reactions has restricted the exploration of these materials. In particular, the challenges associated with producing uniform thin films of all-carbon-linked frameworks has inhibited the study of these materials in applications where well-defined films are required. Here, we synthesize continuous and homogeneous films of two different all-carbon-linked three-dimensional porous aromatic frameworks with nanometer-precision thickness (PAF-1 and BCMP-2). This was accomplished by kinetically promoting surface reactivity while suppressing homogeneous nucleation. Through connection of the PAF film to a gold substrate via a self-assembled monolayer and use of flow conditions to continually introduce monomers, smooth and continuous PAF films can be grown with controlled thickness. This strategy allows traditional transition metal mediated carbon–carbon cross-coupling reactions to form porous, organic thin films. We expect that the chemical principles uncovered in this study will enable the synthesis of a variety of chemically and structurally diverse carbon–carbon-linked frameworks as high-quality films, which are inaccessible by conventional methods.

THF:DMF 1:1. The third pump contained only the solvent mixture (THF:DMF 1:1, 24 ml). The syringe pumps pushed the fluids from syringe one and two (flow speed: 1 ml hr -1 each) through the system for 12-22 hr while the resonance frequency shift was constantly measured and the 11 th overtone was used for further analysis (see Figure S6 for a comparison between overtones). In the end, the two syringe pumps switched off and the tubing and the chamber was washed for 1-3 hours using the third syringe pump (1ml hr -1 ). After pushing air through the system, the sensor was removed and dried under a stream of nitrogen.

BCMP-2 film synthesis: A QCM Q-Sense E4 Quartz Crystal Microbalance (QCM-D / Q-Sense
Analyzer) from Biolin Scientific (with Kalrez® O-rings and gaskets) was used. QCM gold sensors with Cr adhesive layers were purchased from Biolin Scientific. The gold sensors were cleaned with a UV/ozone treatment for two hours and afterwards immersed in an 80°C hot 5:1:1 solution of mQ-water, ammonia (32%), and hydrogen peroxide (30%) for 45 min. Next, the cleaned gold chips were washed with mQ-water and EtOH, dried under a N2 stream, and stored for 1-4 days in a 24-well cell culture plate in a solution of 4-bromothiophenol (1 mM in EtOH). Before film synthesis, the coated sensor was taken out, rinsed with EtOH, blow dried under a stream of N2, and placed into the QCM chamber. When the temperature was stable at 25 o C, the whole set-up was flushed with the solvent mixture (DMF:DIPA, 1:1) until all air bubbles were pushed out. Three programmable Legato110 OEM syringe pumps from KD Scientific were connected to a computer and controlled using Adagio 1.0 software. The first pump was charged with a solution of the building blocks (TBPM, 1.6 mg, 2.5 µmol; 1,4-diethynylbenzene 1.0 mg, 7.9 µmol; in 24 mL DMF:DIPA 1:1) and the second pump was charged with a solution of catalysts tetrakis(triphenylphosphine)palladium(0) (2 mg, 1.7 µmol, 0.7 equiv.) and CuI (1 mg, 5.3 µmol, 2.1 equiv.) in 24mL DMF:DIPA 1:1. The third pump contained only the solvent mixture (DMF:DIPA 1:1, 24 ml). The syringe pumps pushed the fluids from syringe one and two (flow speed: 1 ml hr -1 each) through the system for 12-22 hr while the resonance frequency shift was constantly measured and the 11 th overtone was used for further analysis. In the end, the two syringe pumps switched off and the tubing and the chamber was washed for 1-3 hours using the third syringe pump (1 ml hr -1 ). After pushing air through the system, the sensor was removed and dried under a stream of nitrogen.

PAF films analysis:
Before observation, the chips were cleaned from potential dust using a stream of nitrogen. A Zeiss Axio Scope A1 and a Zeiss Axioscope 5 were used to analyze the macroscopic distribution of material on the QCM chips. Scanning electron microscopy (SEM) and Energy dispersive x-ray spectroscopy-scanning electron microscopy (EDX-SEM) were performed using a Quanta200 ESEM or a JEOL JSM-7800F Prime and a LEO Ultra 55 electron microscope, respectively. The microscopes (spatial resolution: <2 nm) were used with an acceleration voltage of 3-6 keV to avoid charging effects on the surface. To assess the cross section of films, the QCM chips were broken, cleaned from dust and potential small pieces of the cross section with a nitrogen stream, and placed endways in the sample holder. To break the chips, they were held in between two tweezers, and by increasing the tension on both sides, breakage occurred. To gain evidence that the SAM formation occurred, the water contact angle was measured 10 times on the templated gold surface, resulting in angles consistently between 90 and 99 degrees (average 95 degrees). The values are thus much larger than those for clean gold (60-65 degrees according to the literature) 2 , confirming the formation of the SAM. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Scientific ESCALAB XI + spectrometer with an Al K-Alpha source. Etching of the surface was performed with a Monatomic and Gas Cluster Ion Source (MAGCIS) accessory at 200 eV. AFM was performed using an NTEGRA AFM from NT-MDT Spectrum instruments. Water contact angle measurements were performed with a Theta OneAttension from Biolin Scientific. X-ray reflectivity measurements were collected on a Rigaku Smartlab using a Bragg-Brentano geometry with an X-ray wavelength of 1.54 Å.
Electron density profiles were fit using the REFLEX toolbox. 3 Fitting parameters were obtained by estimating roughness and thickness from SEM and AFM measurements and an iterative assignment of the electron density of the PAF Film. We attribute poor fitting in the high-q regime to substantial roughness at buried interfaces which precluded more precise intensity fits.

Building block synthesis
Scheme S1. Synthetic procedure of the building blocks. At this temperature, conc. sulfuric acid (96%, 5.5 mL) and isoamyl nitrite (4 mL, 3.46 g, 29.9 mmol, 1.67 equiv.) were slowly added and the reaction mixture were left to stir for 1 hr. Afterwards hypophosphorous acid (30 %, 15 mL) was added dropwise. When the addition was completed, the reaction mixture was heated to 50 C and stirred until the evolution of gas ceased. Then, water (15 ml) was added to precipitate the product and the suspension was filtered under suction and washed with water (15 ml). After drying in vacuo (70 °C, overnight), tetraphenylmethane (2) was obtained as a bright brown powder (5.45 g, 17.0 mmol, 95%). The proton NMR data matched with literature values.   Figure S1c).