A reaction mechanism for the electrocatalytic H2 evolution using BT-DCAF is proposed based on the experimental results and the DFT calculations (Schemes 1 and S3). The UV-Vis spectrum of BT-DCAF remains unchanged in the presence of a large excess of TFA (Figure ) suggesting that protonation does not take place for neutral BT-DCAF and therefore is not the first step of the reaction. The LUMO of BT-DCAF is located at the BT core of the molecule suggesting that injection of electrons takes place on the BT core first (Figure ). The HOMO of [BT-DCAF] •-suggests that the first protonation step probably takes place at the BT unit where the electron density is located (Figure ).