Comprehensive Picture of the Excited State Dynamics of Cu(I)- and Ru(II)-Based Photosensitizers with Long-Lived Triplet States

Abstract

Ru­(II)- and Cu­(I)-based photosensitizers featuring the recently developed biipo ligand (16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one) were comprehensively investigated by X-ray crystallography, electrochemistry, and especially several time-resolved spectroscopic methods covering all time scales from femto- to milliseconds. The analysis of the experimental results is supported by density functional theory (DFT) calculations. The biipo ligand consists of a coordinating 1,10-phenanthroline moiety fused with a 1,8-naphthalimide unit, which results in an extended π-system with an incorporated electron acceptor moiety. In a previous study, it was shown that this ligand enabled a Ru­(II) complex that is an efficient singlet oxygen producer and of potential use for other light-driven applications due to its long emission lifetime. The goal of our here presented research is to provide a full spectroscopic picture of the processes that follow optical excitation. Interestingly, the Ru­(II) and Cu­(I) complexes differ in their characteristics even though the lowest electronically excited states involve in both cases the biipo ligand. The combined spectroscopic results indicate that an emissive 3MLCT state and a rather dark 3LC state are populated, each to some extent. For the Cu­(I) complex, most of the excited population ends up in the 3LC state with an extraordinary lifetime of 439 μs in the solid state at 20 K, while a significant population of the 3MLCT state causes luminescence for the Ru­(II) complex. Hence, there is a balance between these two states, which can be tuned by altering the metal center or even by thermal energy, as suggested by the temperature-dependent experiments.

Unexpected Boost in Activity of a Cu(I) Photosensitizer by Stabilizing a Transient Excited State

“…Although the choice of the metal center is important, the proper selection and optimization of the surrounding ligands are crucial. ,, In this regard, we and others have found that particularly large diimine ligands with an extended π-system in their backbone can provide beneficial properties. − Consequently, several recent studies report on the successful application of 16 H -benzo-[4′,5′]-isoquinolino-[2′,1′:1,2]-imidazo-[4,5- f ]-[1,10]-phenanthrolin-16-one ( biipo ), a fully conjugated and planar phenanthroline-based ligand (see Figure ). − As a key feature of biipo , an imide unit is directly fused to the 1,10-phenanthroline core. − When coordinated to a photoactive metal center (see Figure ), this ligand forms a bichromophoric system consisting of a metal–phenanthroline unit and the fused naphthalimide unit.…”

“…The naphthalimide unit, as an organic chromophore, exhibits a long-lived and nonemissive excited triplet state ( 3 LC), which is reached only with a low quantum yield due to the absence of heavy atoms. , However, the functional interaction between the metal–phenanthroline part of the resulting complexes and the naphthalimide moiety can circumvent this bottleneck, combining the advantages of purely organic and transition-metal-based chromophores. ,,,, In these complexes, the absorption of a visible photon causes a transition to an electronically excited singlet state, which can then relax into a ligand-centered 3 LC state. ,,, This relaxation often involves a triplet metal-to-ligand charge-transfer state ( 3 MLCT). A fast population of the 3 LC state of biipo was demonstrated for a corresponding Ru­(II) and a Re­(I) complex (see Figure ).…”

“…A fast population of the 3 LC state of biipo was demonstrated for a corresponding Ru­(II) and a Re­(I) complex (see Figure ). , Interestingly, in the former case, even a thermal equilibrium between the 3 MLCT and the 3 LC state was found . As the first example of a complex based on biipo and a nonprecious metal, Cubiipo was recently studied, which exhibits also a fast relaxation into the 3 LC state .…”

“… , Interestingly, in the former case, even a thermal equilibrium between the 3 MLCT and the 3 LC state was found . As the first example of a complex based on biipo and a nonprecious metal, Cubiipo was recently studied, which exhibits also a fast relaxation into the 3 LC state . In this case, the 3 LC state is dark since it is completely localized on the ligand and the triplet state of an organic moiety has no transition dipole moment to the ground state.…”

“…Despite the rather long-lived 3 LC state of Cubiipo , the much lower singlet oxygen yield in MeCN compared to the corresponding Ru­(II) complex (tbbpy) 2 Ru­(biipo)­(PF 6 ) 2 ( Rubiipo , tbbpy = 4,4′-bis- tert -butyl-2,2′-bipyridine) indicates an unsatisfactory photocatalytic activity. Hence, the aim of the present study is to address this issue and to improve this very promising class of heteroleptic Cu­(I) photosensitizers further.…”