Deciphering the Ligand's geometric effect on the photophysical properties of osmium complex and its application in triplet-triplet annihilation upconversion

Abstract

Osmium complex, which featured with direct S0→T1 absorption, are of particular interest in several photochemical applications including triplet-triplet annihilation (TTA) upconversion but less investigated as compared to Ru(II) analogues and the structural correlation to the photophysical properties, especially intersystem crossing (ISC) efficiency is still ambiguous. Herein, a series of Os(II) complexes (Os-1, Os-2, Os-3 and Os-4) bearing different ligands (bipyridine, terpyridine and 2,6-bis(8′-quinolinyl)-pyridine) have been prepared to explore the effect of structural variation/geometry on the photophysical properties. The photophysics of current Os(II) complexes were studied using steady-state as well as transient absorption spectroscopy and theoretical computations. Os-2 complex, bearing the distorted octahedral geometry, showed the higher singlet oxygen sensitizing ability (ФΔ = 68%) and longer triplet lifetime (τT = 281 ns) compared to Os-3 which exhibited the fastest triplet decay (τT = 57 ns), although it is with close-to-perfect octahedral coordination geometry. The large difference in excited-state features of Os(II) complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the energy gap between 3MLCT and 3MC states. Our results show that, in contrast to Ru(II) analogues, the perfect octahedral coordination geometry in Os(II) complexes leads to a decrease in the energy gap between 3MLCT and 3MC by destabilizing the 3MLCT energy, which resulted in drastically different photophysical properties. Finally, Os(II) complexes with different geometric features were implemented in triplet-triplet annihilation upconversion and a moderate upconversion quantum yield of 1.4% was observed. This study will be helpful for the future designing of osmium derived triplet photosensitizers.