Synthesis and characterization of photoluminescent Tetrel(II) complexes
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Abstract
Over the past few decades, the utilization of light has gained significant attention as an alternative for reducing fossil fuel consumption. Sunlight can be used as an energy source by converting solar energy into electricity as in photovoltaic technology or by switching solar energy to chemical energy as in photochemistry and photocatalysis. Further, energy efficient devices which transform electricity into light like light-emitting diodes reduce the general energy demand.
In these applications, highly efficient photoactive transition metal complexes are mostly implemented. They provide high photoluminescence quantum yields which are crucial for light-emitting devices and/or long-lived excited states that are necessary to achieve bimolecular reactions in photocatalysis. The prerequisite for the implementation of transition metal complexes in applications was the research and understanding of the fundamental photophysical properties. Accordingly, considerable progress has been made in recent years for transition metal complexes based on both precious and earth-abundant 3d metals.
However, similar understanding of the fundamental photophysical properties with heavy main group complexes is missing. Photoactive complexes based on group 14 elements, the so-called tetrel complexes, are particularly rare. The most notable oxidation states for tetrel complexes in this context are +IV and +II. Especially photoluminescent tetrel(II) complexes, i.e. tetrylenes, are very scarce, but enable a higher variety of specific excited state characters than tetrel(IV) complexes.
The known examples mostly show weak phosphorescence with fast and efficient non-radiative deactivation. Further, the knowledge about the respective photophysical processes, including the nature of the excited state with its relevant deactivation pathways, has remained essentially unknown.
This work deals with phosphorescent heavy tetrel(II) complexes and the investigation of their excited state dynamics, in particular with complexes based on tin(II) and lead(II). Both elements should enhance efficient intersystem crossing due to the heavy-atom effect and subsequently facilitate phosphorescence.
The first project explores the excited state energy landscape of the tin(II) and lead(II) complexes E(bpep) as intramolecularly donor-stabilized tetrylenes with the dianionic tripodal ligand bpep2– (E = Sn, Pb; H2bpep = 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine). Detailed time-resolved and steady-state spectroscopic as well as (time-dependent) density functional theory studies provide new insights into the excited state dynamics of both complexes. The tin(II) complex displays green intraligand charge transfer phosphorescence in solution as well as in the solid state. Despite its larger heavy-atom effect, the lead(II) complex solely displays very weak red phosphorescence at low temperatures in the solid state, originating from strongly distorted ligand-to-metal charge transfer states. Quantum chemical calculations elucidate these observations and corroborate the deactivation pathways through strongly distorted excited states. These new findings basically help to establish rational ligand design concepts for tetrel(II) complexes with long-lived phosphorescence.
Based on the knowledge from the prior project, a conceptional topological approach combined with rational ligand design addresses specific excited state characters. The tripodal complex Sn(bpep) showed an intraligand charge transfer due to the additional
ii
intramolecular donor. In opposition, the Sn(t-BuPDPt-Bu) complex with the meridional coordinating pincer-type ligand t-BuPDPt-Bu2‒ (H2t-BuPDPt-Bu = 2,6-bis(3,5-di-tert-butylpyrrol-2-yl)pyridine) provides no additional donor stabilization and should show photoluminescence from a ligand-to-metal charge transfer. Indeed, the tin(II) complex displays red phosphorescence in the solid state from the assumed excited state, underlining the success of the conceptional approach. However, the phosphorescence is surprisingly weak, and its respective radiative rate presents unusual temperature dependence. This aspect can be explained by aggregation effects in the solid state, which are concluded from quantum chemical investigations.
The third project covers the well-known Lappert’s diamino stannylene Sn[N(SiMe3)2]2, which was reported in 1974 for the first time. Since then, it has been broadly studied with respect to the ground state properties and reactivity. However, its photophysical properties have been overlooked for over fifty years. The molecule provides a sufficient stability in solution, which enables detailed spectroscopic investigations such as temperature-dependent and time-resolved emission or absorption spectroscopy. The results are further verified by quantum chemical calculations. The yellow-coloured Lappert’s diamino stannylene Sn[N(SiMe3)2]2 exhibits thermally activated dual orange/green phosphorescence with microsecond lifetimes in fluid solution at room temperature, which can be attributed to conformational changes on the excited state hypersurface. In addition, the mechanism of the photochemical radical formation of Sn[N(SiMe3)2]3 has been elucidated. Two relevant excimers with second-order Jahn-Teller distorted structures are discussed in this aspect. Based on the results from this project, further rational ligand design concepts for tetrel(II) complexes with long-lived emissive excited states can be developed.
Alongside the stannylene, Lappert’s diamino plumbylene Pb[N(SiMe3)2]2 was also probed regarding the photophysical properties. In the first place, it provides strong thermal and photosensitivity with subsequent elemental lead formation. In the last project of the present work, this challenge was concerned with temperature-dependent emission and absorption spectroscopy. This plumbylene displays a weak red photoluminescence from possibly two triplet ligand-to-metal charge transfer states at low temperatures in frozen solution. Quantum chemical calculations indicate that a brunching into two separated excited states takes place after photoexcitation. Furthermore, both excited states might play a role in the photodegradation mechanisms of Pb[N(SiMe3)2]2 to elemental lead, which was also examined in this work.
Overall, the present work sheds more light on the fundamental photophysical and photochemical properties of heavy tetrel(II) complexes, including the investigation of the excited state characters and excited state dynamics or the examination of photochemical mechanisms. Complexes with several ligand geometries and coordination numbers were investigated with sophisticated spectroscopic experiments and quantum chemical calculations, to gather sufficient knowledge of the basic photophysics and their reactivities. The present studies revealed new results and challenges which might mark a step into the right direction for first applications.