Unveiling excited state energy transfer and charge transfer in a host/guest coordination cage.

Host-guest charge transfer (HGCT) plays a key role in applications from solar energy conversion to photocatalysis. Herein, a HGCT system, a pillared Pt(ii) metallacage with encapsulated coronene was synthesized and the ultrafast excited-state dynamics were investigated by combination of femtosecond transient absorption spectroscopy, nanosecond transient emission spectrocopy and quantum chemistry calculations. Two significant ultrafast dynamic processes were unveiled: (i) charge transfer from a singlet local excited (1LE) state associated with the coronene moiety to a 1HGCT state with τ = 9.5 ps; and (ii) triplet-triplet energy transfer from a high 3HGCT state to a 3LE state with τ = 139.5 ps. The resulting long-lived species, the lowest 3LE and 3HGCT states eventually decay to the ground state in microsecond time scales of 5.2 and 43.4 μs respectively. Moreover, a clear mechanism depicting the main excited-state decay pathways connecting the initial photoexcited transients with the resulting species was proposed.