Harnessing Fluorescence versus Phosphorescence Branching Ratio in (Phenyl)(n)-Bridged (n=0-5) Bimetallic Au(I) Complexes

We have designed and synthesized a series of Au(I) complexes bearing either an alkynyl-(phenylene)(n)-diphosphine (A-0-A-3) or a (phenylene)(n)-diphosphine (B-1-B-5) bridge, among which the effective distance between Au(I) and the center of the emitting pi pi* chromophore can be fine-tuned via the insertion of various numbers of phenylene spacers. We then demonstrated for the first time in a systematic manner the decrease of rate constant for S-1 -> T-1 intersystem crossing (ISC) k(isc) as the increase of the effective distance. The results also unambiguously showed that the phosphorescence could be harvested via higher S-0 -> S-n (n > 1) electronic excitation, followed by fast S-n -> T-m ISC and then the population at T-1 state, bypassing the relatively slow S-1 -> T-1 ISC. The results unify a recent report on higher excited-state relaxation dynamics for the late transition metal complexes (J. Am. Chem. Soc. 2012, 134, 7715-7724). The dual, far separated fluorescence and phosphorescence of the titled complexes make feasible the white light generation in a single molecule unit, as successfully demonstrated using complex B-3 as a dopant to fabricate organic light emitting diodes.