Metal-free photochemical silylations and transfer hydrogenations of benzenoid hydrocarbons and graphene

The first hydrogenation step of benzene, which is endergonic in the electronic ground state (S(0)), becomes exergonic in the first triplet state (T(1)). This is in line with Baird's rule, which tells that benzene is antiaromatic and destabilized in its T(1) state and also in its first singlet excited state (S(1)), opposite to S(0), where it is aromatic and remarkably unreactive. Here we utilized this feature to show that benzene and several polycyclic aromatic hydrocarbons (PAHs) to various extents undergo metal-free photochemical (hydro)silylations and transfer-hydrogenations at mild conditions, with the highest yield for naphthalene (photosilylation: 21%). Quantum chemical computations reveal that T(1)-state benzene is excellent at H-atom abstraction, while cyclooctatetraene, aromatic in the T(1) and S(1) states according to Baird's rule, is unreactive. Remarkably, also CVD-graphene on SiO(2) is efficiently transfer-photohydrogenated using formic acid/water mixtures together with white light or solar irradiation under metal-free conditions.