Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

Abstract

Many CuI complexes have luminescent triplet charge-transfer excited states with diverse applications in photophysics and photochemistry, but for isoelectronic ZnII compounds, this behavior is much less common, and they typically only show ligand-based fluorescence from singlet π–π* states. We report two closely related tetrahedral ZnII compounds, in which intersystem crossing occurs with appreciable quantum yields and leads to the population of triplet excited states with intraligand charge-transfer (ILCT) character. In addition to showing fluorescence from their initially excited 1ILCT states, these new compounds therefore undergo triplet–triplet energy transfer (TTET) from their 3ILCT states and consequently can act as sensitizers for photo-isomerization reactions and triplet–triplet annihilation upconversion from the blue to the ultraviolet spectral range. The photoactive 3ILCT state furthermore facilitates photoinduced electron transfer. Collectively, our findings demonstrate that mononuclear ZnII compounds with photophysical and photochemical properties reminiscent of well-known CuI complexes are accessible with suitable ligands and that they are potentially amenable to many different applications. Our insights seem relevant in the greater context of obtaining photoactive compounds based on abundant transition metals, complementing well-known precious-metal-based luminophores and photosensitizers.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…Keeping an eye on possible future applications in solar energy conversion, lighting, sensing, or photocatalysis, it seems desirable that the full diversity of electronically excited states occurring in metal complexes is accessible, including LMCT, , LLCT, ILCT, and MC states in addition to MLCT excited states . For complexes with a partially filled 3d subshell, rapid nonradiative MLCT excited state deactivation is particularly prevalent, and to date luminescent MLCT excited states with nanosecond lifetimes have remained very rare among 3d 6 compounds. ,,,, Consequently, MLCT-based photocatalysis has remained extremely scarce in this compound class .…”

“…Zn II complexes are better explored, though they typically feature ligand-based fluorescent excited states rather than photoactive MLCT states . Recently, there has been increasing focus on harnessing triplet excited states of Zn II complexes and on introducing charge transfer character into the photoactive excited states, − including the development of emitters displaying thermally activated delayed fluorescence (TADF) . Lacking ligand field stabilization energy, Zn II coordination compounds are often substitution-labile and tend to form pentacoordinate or even polynuclear compounds, and multiple species can exist in dynamic equilibrium with each other .…”

Nanoengineering Triplet–Triplet Annihilation Upconversion: From Materials to Real-World Applications

“…Numerous tactics for increasing UC QYs have been pursued from a molecular design perspective . Molecular engineering of standalone sensitizers − and annihilators, ,, as well as covalently tethering sensitizers to annihilators ,− /annihilators to annihilators, − has been widely studied with varying degrees of success with regard to photophysical property manipulation and synthetic simplicity. Leveraging localized surface plasmon resonance (LSPR) , effects has recently been an area of intrigue.…”

An Air‐ and Moisture‐stable Zinc(II) Carbene Dithiolate Dimer Showing Fast Thermally Activated Delayed Fluorescence and Dexter Energy Transfer Catalysis**

“…The population of triplet excited states, of which the broad emission covers an energy range from 2.48-1.6 eV, with associated long lifetimes of hundreds of nanoseconds in solution suggests that Zn1 may be employed in photocatalytic transformations as triplet sensitizer, similar to two recently reported Zn(II) complexes with dark (non-emissive) triplet excited states. [41] Fulfilling another important prerequisite for photocatalysis, the continuous irradiation of Zn1 for one hour with high-power 450 nm LEDs revealed excellent photostability by maintaining 96% and 99% of the original absorbance in toluene and THF, respectively (Figure S23). The photo-isomerization (E)-stilbene®(Z)-stilbene requires the population of the organic T1 state at 2.2 eV, of which sensitization with our zinc(II) complex appears feasible (see above), and was thus tested in degassed toluene with 2 mol-% of Zn1, giving 93% conversion after four hours of irradiation.…”