Harvesting Fluorescence from Efficient T-k -> S-j (j, k > 1) Reverse Intersystem Crossing for pi pi* Emissive Transition-Metal Complexes

Using a bimetallic Au(I) complex bearing alkynyl-(phenylene)(3)-diphosphine ligand (A-3), we demonstrate that the fluorescence can be exquisitely harvested upon T-1 -> T-k (k > 1) excitation followed by T-k -> S-j Si (j, k > 1) intersystem crossing (ISC) back to the S-1 state. Upon S-0 -> S-1 355 nm excitation, the S-1 -> T-1 intersystem crossing rate has been determined to be 8.9 X 10(8) s(-1). Subsequently, in a two-step laser pump-probe experiment, following a 355 nm laser excitation, the 532 nm T-1 -> T-k probing gives the prominent blue 375 nm fluorescence, and this time-dependent pump-probe signal correlates well with the lifetime of the T-1 state. Careful examination reveals the efficiency of T-k -> S-j (j, k > 1) reverse intersystem crossing to be 5.2%. The result is rationalized by a mechanism incorporating substantial involvement of metal-to-ligand charge transfer (MLCT) in the T-k (S-j) states, enhancing the rate of T-k -> S-j ISC, which is competitive with the rate of T-k -> T-1 internal conversion. This mechanism is also proven to be operative in the A-3 solid film and should be universally applicable to the transition-metal complexes possessing a dominant pi pi* configuration in the lowest-lying states. From an energy point of view, the UV fluorescence (375 nm) generated by green (532 nm) excitation can be recognized as a signal up-conversion process.