Tuning electron transfer (ET) rates from catalysts to substrates is important for modulating photocatalytic organic reactions. In this work, we have taken pyrene-based photocatalysts (Py) for photocatalytic hydrodefluorination of polyfluoroarenes (FA) as model systems, and conducted a first-principle study on modulating ET rates from Py to FA via chemical modification of Py with different electron donating/withdrawing groups (EDGs/EWGs). The computed spatial distributions of frontier Kohn-Sham orbitals suggest that ET is energetically more favorable for Py-EDGs than for Py-EWGs. The estimated ET rates by a simplified Marcus model show that they are appreciably enhanced by EDGs substitution and weakened by EWGs substitution. Noticeably, the associated Gibbs free energy change plays a dominant role. Our findings of tuning ET rates for Py-FA complexes via chemical group modifications cast new insight into the rational design of metal-free photocatalysts for organic transformations. TOC Graphic