In the unusual photooxidation of alkanes within the cavity of a self-assembled M6L4 cage complex (1), the proposed reaction mechanism involves the generation of the host anion radical and guest cation radical via guest-to-host photoinduced electron transfer (PET). In the present study, the postulated host anion radical was elucidated by in situ IR spectroscopy. The difference IR spectrum before and after the irradiation of the clathrate complex 1a superset of(2)(4) (2 = adamantane) features a dominant negative peak at similar to 1500 cm(-1), which is attributed to the decrease in the C=N stretching vibration and thus indicative of the structural change in the triazine core of the cage. Electrochemical measurements and theoretical calculations show the remarkably great electron-accepting ability of the triazine core. These results strongly support the proposal of guest-to-host PET.