Harvesting highly electronically excited energy to triplet manifolds: State-dependent intersystem crossing rate in Os(II) and Ag(I) complexes

A series of newly synthesized Os(II) and Ag(I) complexes exhibit remarkable ratiometric changes of intensity for phosphorescence versus fluorescence that are excitation wavelength dependent. This phenomenon is in stark contrast to what is commonly observed in condensed phase photophysics. While the singlet to triplet intersystem crossing (ISC) for the titled complexes is anomalously slow, approaching several hundred picoseconds in the lowest electronic excited state (S₁ → T₁), higher electronic excitation leads to a much accelerated rate of ISC (10¹¹-10¹² s^-1), which is competitive with internal conversion and/or vibrational relaxation, as commonly observed in heavy transition metal complexes. The mechanism is rationalized by negligible metal d orbital contribution in the S₁ state for the titled complexes. Conversely, significant ligand-to-metal charge transfer character in higher-lying excited states greatly enhances spin-orbit coupling and hence the ISC rate. The net result is to harvest high electronically excited energy toward triplet states, enhancing the phosphorescence.