Porphyrin-based systems containing polyaromatic fragments: decoupling the synergistic effects in aromatic-porphyrin-fullerene systems

In this work, we report a two-step synthesis that allows the introduction of four pyrene or corannulene fragments at the para position of meso-tetraarylporphyrins using a microwave-assisted quadruple Suzuki–Miyaura reaction. Placing the PAHs at this position, further from the porphyrin core, avoids the participation of the porphyrin core in binding with fullerenes. The fullerene hosting ability of the four new molecular receptors was investigated by NMR titrations and DFT studies. Despite having two potential binding sites, the pyrene derivatives did not associate with C60 or C70. In contrast, the tetracorannulene derivatives bound C60 and C70, although with modest binding constants. In these novel para-substituted systems, the porphyrin core acts as a simple linker that does not participate in the binding process, which allows the system to be considered as two independent molecular tweezers; i.e., the first binding event is not transmitted to the second binding site. This behavior can be considered a direct consequence of the decoupling of the porphyrin core from the binding event.


Complexation measurements
In order to estimate the association constants (K a ) of the compounds Zn-PTetraCor and 2H-PTetraCor with fullerenes, the dilution method was applied. A 10 -4 M deuterated toluene solution of each compound was prepared, and a known volume was transferred to an NMR tube (500 μL). The titration was carried out by adding known portions of a stock solution of C 60 or C 70 (10 -3 M) in deuterated toluene to cover a wide range of equivalents. A 1 H NMR spectrum was recorded at room temperature after each addition. Once all data had been obtained, the changes in the chemical shifts (Δδ) of selected protons were plotted as a function of the molar fraction of the guest, and the resulting curve was fitted by a nonlinear method using the global analysis approach according to the following equations, depending on the type of equilibrium:      For additional information see: http://app.supramolecular.org/bindfit/view/f3cbc3e0-aacf-4516-852f-a773e3b87783

Computational methods
Optimized geometries of porphyrin 2H-PTetraCor and the supramolecular assemblies C 60 @2H-PTetraCor, C 70 @2H-PTetraCor, (C 60 ) 2 @2H-PTetraCor and (C 70 ) 2 @2H-PTetraCor were obtained by DFT methods with the B97D3 functional, which contains the Becke-Johnson damping empirical dispersion correction and was provided by Grimme and collaborators. 2 Pople's split valence set 6-31G(d,p) was chosen as the basis set. 3 Solvent corrections applied using the polarizable continuum model (PCM) using toluene (ε=2.3741). 4 The strategy to obtain the inclusion complexes C 60 @2H-PTetraCor and C 70 @2H-PTetraCor consisted of using the optimized structure of compound 2H-PTetraCor and manually placing the corresponding fullerene molecule halfway between the two corannulenes, rotating the single C-C bonds at the same time so that both PAH fragments matched with the fullerene surface. For the assembly C 70 @2H-PTetraCor, several attempts were carried out by imposing different orientations on the fullerene C 70 ; only the most stable one was considered and is reported here. Once both 1:1 adducts were optimized, their structures were used as starting geometries for a second round of optimizations to obtain the complexes (C 60 ) 2 @2H-PTetraCor and (C 70 ) 2 @2H-PTetraCor by placing a second fullerene molecule following the same protocol described above.
All minima were confirmed by vibrational analysis to show no imaginary frequencies. The electronic energies of the optimized geometries were further evaluated using a more extended 6-31+G(d,p) basis set that includes diffuse functions. 5 Deformation energies were estimated by subtracting the electronic energy of the optimized porphyrin 2H-PTetraCor (H) from the electronic energy of the porphyrin in the optimized structure of the adduct (HG, i.e., C 60 @2H-PTetraCor and C 70 @2H-PTetraCor) according to eq. 4. For the supramolecular assemblies (C 60 ) 2 @2H-PTetraCor and (C 70 ) 2 @2H-PTetraCor) (HG 2 ), the subtraction was carried out from the porphyrin structure shown in the parent 1:1 inclusion complexes (HG) according to eq. 5: Where the subscripts denote the geometry used and the letter between parentheses corresponds to the molecular entity studied (porphyrin in all cases).
Interaction energies were calculated taking into account basis set superposition error (BSSE) with the Boys-Bernardi functional counterpoise scheme 6 as follows (eq. 6): eq. 6 Where the subscripts denote the geometry used (inclusion complex in all cases) and the superscripts refer to the basis set (from the supramolecular assembly in all cases); H and G correspond to the host and guest molecular entities, respectively, and HG to the supramolecular adduct.

S41
All the above-described computational methods were performed using the Gaussian 16 package. 7 Non-covalent interactions were obtained from the location critical points at which the reduced density gradient decreases to low electronic density values according to the scheme of Yang et al. with the help of the NCIPlot package. 8 Calculations were performed with promolecular densities, and gradient isosurfaces were plotted with an isovalue of 0.3 a.u. and coloured on a blue-green-red scale according to the values of the sign of λ 2 (the second eigenvalue of the electron-density Hessian). Red indicates repulsion, green indicates weak attraction, and blue represents strong attraction. Graphics were visualized in Chimera 9 with the help of Tangram NCIPlot GUI built by Insilichem Group. 10  Figure S73: Plot of the reduced density gradient versus the electron density for the complex C 60 @2H-PTetraCor.