Designing Cascades of Electron Transfer Processes in Multicomponent Graphene Conjugates

Abstract A novel family of nanocarbon‐based materials was designed, synthesized, and probed within the context of charge‐transfer cascades. We integrated electron‐donating ferrocenes with light‐harvesting/electron‐donating (metallo)porphyrins and electron‐accepting graphene nanoplates (GNP) into multicomponent conjugates. To control the rate of charge flow between the individual building blocks, we bridged them via oligo‐p‐phenyleneethynylenes of variable lengths by β‐linkages and the Prato–Maggini reaction. With steady‐state absorption, fluorescence, Raman, and XPS measurements we realized the basic physico‐chemical characterization of the photo‐ and redox‐active components and the multicomponent conjugates. Going beyond this, we performed transient absorption measurements and corroborated by single wavelength and target analyses that the selective (metallo)porphyrin photoexcitation triggers a cascade of charge transfer events, that is, charge separation, charge shift, and charge recombination, to enable the directed charge flow. The net result is a few nanosecond‐lived charge‐separated state featuring a GNP‐delocalized electron and a one‐electron oxidized ferrocenium.


Experimental Section
General method 1 H NMR spectra were measured on a Bruker AM-400 and Bruker 300 instruments. The chemical shifts were referenced to the residual peaks of the deuterated solvents: chloroform (7.26 ppm) and benzene (7.15 ppm). ESI-HRMS were performed with Orbitrap Q-Exactive (Thermo Scientific) in positive ion full scan mode by adopting the following parameters values: spray voltage between 5-6 kV depending on the compound, S-lens RF level = 100, sheath gas level = 50, auxiliary gas level = 10, capillary temperature = 320 °C, auxiliary gas heather = 300 °C, AGC target = 3x10e6, Max inj. Time = 120 ms, resolution power = 100000. Calculation of exact mass or each compound was performed by using Xcalibur software (Thermo Scientific) with an accuracy ≤ 5ppm. Porphyrin samples (≃ 0.5 mg) were dissolved in toluene, the obtained solution was further diluted 1/10 and 20 l of this final solution was then added to a 400 l of ACN:H2O=4:1 mixture (in case of compound 4c the final ACN/H2O ratio contained 0.1% g/vol of formic acid). Raman spectra were obtaining by using a confocal Raman microscope Invia Renishaw, endowed with a 633 nm laser, a RenCam CCD detector, 1024x256 pixels (200-1060 nm), an encoded xyz stage (replacement precision: 100nm), using an 1800 l/mm grating. About mapping two measurements for each point were accumulated using a sampling step of 100 nm. After obtaining the mapping, the spectra were analyzed using the Wire® 4.0 software to elaborate the curve fit. We choose to analyze the peak area data to decrease the error about the estimation of porphyrin localization. XPS measurements were obtained from a modified Omicron NanoTechnology MXPS system. The spectra were excited by achromatic Mg Kα and Al Kα photons (hν = 1253.6 and 1486.6 eV, respectively), generated operating the anode at 14-15 kV, 10-20 mA. All the samples were mounted on metal tips as thin layers of pressed powders. Experimental spectra were theoretically reconstructed by fitting the peaks to symmetric pseudo-Voigt functions (linear combination of gaussian and lorentzian peaks) and the background to a Shirley or a linear function. XPS atomic ratios (±10% associated error) between relevant core lines were estimated from experimentally determined area ratios corrected for the corresponding theoretical cross sections and for a square root dependence of the photoelectron's kinetic energies. All binding energies were referenced to the lowest lying C 1s peak component, taken at 285.0 eV, which was assumed to be related to the aromatic C atoms. SEM measurements were carried out by a scanning electron microscope TESCAN VegaII at 10 KV of voltage acceleration with a working distance of 9.788mm. It endowed with Bruker microanalysis and QUANTAX 400 software of analysis, as well as a detector STEM for the acquisition of images in brigthfield and in darkfield. Atomic force microscopy measurements were carried out with a JPK Nanowizard 4 Nanoscience microscope. The ACTA cantilever from Applied Nanostructures APPNANO with a resonance frequency of 300 kHz and a tip radius below 10 nm was used. The samples were drop casted onto silica wafers. Cyclic voltammetry (CV) experiments were performed with an EmStat potentiostat, using a platinum wire as counter electrode, Ag/AgCl as reference electrode and a glassy carbon (GCE) as working electrode. All the measurements were carried out at room temperature in a 0.1 M solution of tetra-n-butylammonium hexafluorophosphate (TBAPF6) in acetonitrile, with ferrocene as internal standard at 100 mV/s scan rate. Prior to each voltammetric measurement, the solution was degassed by bubbling with argon for about 20 mins. For each GNP-conjugate a dispersion was prepared by dissolving 1 mg in 1 mL of anhydrous N,N-dimethylformamide. The solutions were bath-sonicated for 30 min at 59 kHz. In the last step, 12 µL of the solutions were drop-casted onto the GCE working electrode and further drying. For the steady -satate and time resolved spectroscopic measurements the dispersions were prepared by sonicating the conjugates in THF for 30-40 mins at 30 kHz and 50 % power. The supernatant was obtained post centrifugation. A Perkin Elmer Lambda 2 spectrometer was used in order to collect the UV−Vis spectra at 298 K using a slit width of 2 nm and a scan rate of 480 nm /min. The data were recorded with the help of UV WinLab software. Steady-state fluorescence studies were conducted using Fluoromax 3 spectrometer (Horiba Scientific). Time-resolved absorption studies were performed by using the Clark MXR CPA 2101 and CPA2110 Ti: sapphire amplifier (775 nm, 1 kHz, 150 fs pulse width) as the laser source. Ultrafast Systems HELIOS femtosecond transient absorption spectrometer was used to acquire time resolved transient absorption spectra with 150 fs resolution and time delays from 0 to 5500 ps. The probe-visible white light (∼400−770 nm)was generated by focusing a fraction of the fundamental 775 nm output onto a 2 mm sapphire disk. And, for the (near) IR (780−1500 nm), a 1 cm sapphire was used. Non-collinear optical parameter (NOPA, Clark MXR) was used to generate the excitation wavelength at 568 nm; a bandpass filter ± 5 nm was used to exclude the fundamental 775 nm and 387 nm. All measurements were performed in 1 cm quartz cuvettes under argon atmosphere. The data points in the 450-470 nm, and 765-790 nm regimes were removed as they stem from the pump excitation and fundamental excitation wavelength, respectively. In order to deconvolute associated species, the spectral data were subjected to global-target analysis, which is based on an excited-state deactivation kinetic model. The plausible kinetic model which was used to fit the data is based on the information obtained on the energetics of the system from steady-state spectroscopy and electrochemistry. To carry out the analysis we used the free software program GloTarAn which is a graphical interface to the R-package TIMP. Origin 2018/2019/2020-pro version was used to plot the data obtained using different spectroscopic and analytical methods.
We performed the Tauc plot analysis in order to determine the optical band-gap of the GNP from the absorption characteristics using the following equation: where is the constant, is the band gap of the material and is the exponent which depends on the type of transition. For direct transition n = 1/2 and for indirect transition n = 2. Using the above relation and Tauc plots [1] , we extracted the band gap values by extrapolating the linear region of ℎ vs. ( ℎ ) 1 ⁄ to the energy axis.

S5
Furthermore, we considered the Stern-Volmer analysis [2] for the reported conjugates, using the following equation.
In the equation above, is the Stern-Volmer constant, is the bimolecular rate constant, 0 and 0 stand for the fluorescence intensity and lifetime, respectively, of the fluorophore in absence of the quencher [Q], and and represents the same in presence of the quencher.
This was done, in order to comment on the impact of GNP and Fc as the two quenchers, on the intrinsic fluorescence and singlet excited state lifetime of the ZnP.

Chemicals
Silica gel 60 (70-230 mesh, Sigma Aldrich) was used for column chromatography. High-purity-grade nitrogen gas was purchased from Rivoira. When anhydrous conditions were required the solvents were freshly distilled under nitrogen atmosphere and at ambient pressure, following the literature procedures: toluene was distilled over sodium and THF over LiAlH4. All the reagents and solvents were from Fluka Chem. Co., Aldrich Chem. Co. or Carlo Erba and were used as received.

Compound 4c
50 mg (0.054 mmol) of 3c, 42 mg (0.11 mmol) of c and 15 mg (0.047 mmol) of AsPh3 were dissolved in 25 mL of anhydrous toluene and 5 mL of anhydrous triethylamine under nitrogen atmosphere. The solution was deaerated using argon bubbling for 30 min., then 6.5 mg (0.0063 mmol) of Pd2(dba)3 were added. The solution was deaerated for further 20 min., after that the argon inlet was place 1 cm above the solution. The flow rate was increased slightly, and the reaction was left under nitrogen at 40°C for 16 h. The solvent was evaporated, and the crude product was purified by column chromatography on silica gel eluting with toluene. The fraction containing the desired product was collected and the solvent was removed under vacuum to give compound 4c (35 mg, 0.03 mmol, 53%).

Compound 5c
32 mg (0.026 mmol) of 4c were dissolved in 34 mL of chloroform and 2 mL of saturated solution of Zn(AcO)2 in methanol was added. The mixture was refluxed and stirred for 3 h. The solvent was removed under vacuum and the crude product was purified by a plug of silica gel eluting with chloroform. The fraction containing the desired product was collected and the solvent was evaporated. The compound was crystallized from dichloromethane/methanol to give compound 5c (30 mg, 0.023 mmol, 88%).

5-GNP
In Figures S12a and S12b the N 1s and Zn 2p3/2,1/2 photoemission regions for 5-GNP are reported (Fe is not present in the compound).
The two N 1s distinct components shown in Figure S12a