Reverse intersystem crossing (RISC) in purely organic molecules has become an attractive research topic since the demonstration of high efficiencies in organic light-emitting diodes using thermally activated delayed fluorescence (TADF). Although the intermolecular interactions have a significant impact on the exciton dynamics, it is generally difficult to identify the quantitative relationship associated with a specific factor. In this work, we used a clathrate crystal with TADF and H2O molecules to evaluate the effect of hydrogen bonding while maintaining molecular conformations and other intermolecular interactions. The hydrogen bonding shifted the charge transfer excited states to lower energies, resulting in superior TADF properties. Although the increase in the RISC rate is considered to enhance the stabilities of TADF molecules, photostability analysis revealed nearly the same degradation speed despite the 3 times faster RISC rate.