Electron transfer reactions of rigid, cofacially compressed, π-stacked porphyrin–bridge–quinone systems

A 1,8-naphthyl pillaring motif can be utilized to enforce sub van der Waals interplanar separations between juxtaposed porphyryl, aromatic bridge, and quinonyl components of donor–spacer–acceptor (D–Sp–A) compounds. Such structures, synthesized via metal-mediated cross-coupling of appropriately functionalized (porphinato)zinc(II), arene, and quinone precursors, manifest unusual conformational rigidity in the condensed phase, and significant electronic communication between the cofacially aligned D, Sp, and A components. NMR experiments provide rigorous determination of the ambient temperature structures of these species in solution, while computational methods offer insight into the fragment molecular orbital interactions that give rise to the strong D–A coupling evident in these assemblies. The distance-, temperature-, and solvent-dependences of photoinduced charge separation (CS) and thermal charge recombination (CR) rate constants in these systems have been evaluated using femtosecond visible pump/vis–NIR probe and visible pump/mid-IR probe transient dynamical methods. These experiments: (i) demonstrate that simple aromatic building blocks like benzene, which are characterized by highly stabilized filled molecular orbitals and large HOMO–LUMO gaps, provide substantial D–A electronic coupling when organized within a π-stacked structural motif that features a modest degree of arene–arene interplanar compression; (ii) assess directly the degree of ground and excited state charge transfer (CT) in these donor–spacer–acceptor (D–Sp–A) structures, (iii) reveal unusual CS dynamics from vibrationally relaxed and unrelaxed S1 states, and (iv) show a photoinduced CS mechanistic transition from the nonadiabatic to the solvent-controlled adiabatic regime, to one where CS becomes decoupled from solvent dynamics and is determined by the extent to which the vibrationally unrelaxed S1 state is populated.