Herein, complexes [Ru(tpy)(qdppz)](PF6)2 (1) and [Ru(qdppz)2](PF6)2 (2) (qdppz = 8-quinyldipyridophenazine, tpy = terpyridine) have been characterized as potential models for photodynamic therapy through singlet oxygen, 1O2, sensitization. In particular, the contrast between the homoleptic and heteroleptic complexes provided insight into the effect of unfavorable bite angles tridentate ligands, such as terpyridine may have on the excited state lifetimes, τ, of ruthenium polypyridine complexes and thus any potential therapeutic applications. Upon visible light irradiation, complex 2 exhibited an ability to produce singlet oxygen with a quantum efficiency of 0.69 whereas 1 was relatively inactive, 0.02 consistent with the corresponding lifetimes of complex 2, τ = 570 ns compared to 1, τ = 10 ns in acetonitrile. The marked increase in 1O2 quantum yields and lifetimes in 1 versus 2 was initially thought to be a result of excited state deactivation through tpy-based excited states which are known to have a low activation energy Ea = 1700 cm-1 and high Arrhenius prefactor, A = 2×1013 s-1 for deactivation. In order to investigate the influence of tpy on the photophysics of 1 and 2 temperature dependent photoluminescence lifetimes of these complexes were recorded in acetone between 170 and 330 K. Kinetic analysis of the data indicated that the origin of the lifetime difference between 1 and 2, 10 ns to 570 ns respectively, was not due to the difference in activation energy because they were the same within experimental error, Ea ≈ 2500 cm-1. Instead, the data indicate that the difference in the lifetimes were a result of Arrhenius pre-factors that were faster by two orders of magnitude in complex 1, A = 2.3×1013 s-1, than in 2, A = 1.4×1011 s-1. The unexpectedly similar activation barriers and striking impediment of the pre-factors highlights the important role bite angles, π-expansive ligands, and molecular symmetry have on singlet oxygen formation and expected efficacy for therapeutic applications.

Improved photoreactivity through the expansion of unfavorable Ru-N bite angles on [Ru(dppz)(tpy)]2+ type complexes

ACS Spring Meeting 2021

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