Chromium(III) Luminophores: photochemical property enhancement and tuning
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Abstract
For the targeted optimization of emissive transition metal complexes, it is essential to systematically and reliably predict photophysical properties. Quantum chemical characterization of previously analyzed compounds aids in understanding structure-property relationships, while comparison with experimental data validates the predictive power of theoretical models for analogous systems. This acquired knowledge can then be applied to design new photoactive complexes with enhanced properties before experimental methods are accessible.
This work applies this fundamental approach to highly luminescent chromium complexes—referred to as molecular rubies—focusing on the synthesis, analysis, and optimization of photophysical properties. Since the discovery of the [Cr(ddpd)2]3+ complex in 2015, molecular rubies have become a pioneering class in 3d-metal photochemistry, recognized for their outstanding near-infrared luminescence efficiency, high photostability, and chemical versatility. Building on the design principles of [Cr(ddpd)2]3+, numerous derivatives demonstrate significant potential for applications in optical sensing, photocatalysis, and circularly polarized emission.
However, a systematic approach to the targeted modification of optical properties, such as shifting emission into the visible range, has been lacking for spin-flip systems like molecular rubies. Through quantum chemically supported structural design, this work achieves an increase in emission energy, leading to the synthesis of the first molecular chromium(III) complex with visible-light luminescence, high quantum yield of 20%, and millisecond-scale excited-state lifetime. The red phosphorescence of [Cr(bpmp)2]3+ can be reversibly switched on and off by pH regulation, enabling sensory applications like ratiometric optical pH measurements in combination with a pH-insensitive dye. The elevated energy of the photoactive state expands the applicability of molecular rubies in photocatalysis, as it allows the activation of a broader substrate scope. With the efficient sensitization of anthracenes for triplet-triplet annihilation photon upconversion and cycloadditions via a previously underexplored doublet-triplet energy transfer, this work demonstrates that molecular rubies are a viable alternative to precious metal complexes in selected photochemical systems.
The improved redox behavior of [Cr(bpmp)2]3+, with a favorable ligand-centered reduction, results in a high oxidation potential in the excited state, rendering it particularly attractive for photoredox catalysis. The metal-centered nature of spin-flip states generally enables largely independent optimization of electrochemical and optical properties, as proven in this work through the controlled adjustment of the ground-state redox potential via peripheral substituents in the analogous complexes [Cr(ddpdX)2]3+ (X = CF3, OMe, NMe2).
In contrast to charge-transfer states, which follow well-established design rules such as substitution effects, metal-centered spin-flip states have, to date, been tunable only through qualitative criteria. Predicting spin-flip energies depends critically on interelectronic interactions and metal-ligand bond covalency, as described by the nephelauxetic effect. Based on the isostructural complex series [Cr(ddpd)2]3+, [Cr(bpmp)2]3+, [Cr(bpop)2]3+ and [Cr(bptp)2]3+ with variations in the ligand backbone, this work provides unprecedented insights into the energies and dynamics of spin-flip states and reveals the influence of structural and electronic factors on their emission. It highlights how spin-orbit coupling of heavier atoms, Jahn-Teller distortions of excited states, and thermally activated multiphonon relaxation can promote non-radiative decay in molecular rubies and thus give valuable design criteria for future systems.
In addition to enhancing photophysical properties, this work focuses on developing new synthetic methods, particularly for heteroleptic molecular rubies. The complexes [CrLXLY]3+ (LX, LY = ddpd, bpmp, bptp), combined with their computational analysis, provide insights into spin-flip energy correlations based on an additive nephelauxetic effect of ligands, which allows for a precise prediction and fine-tuning of emission in analogous systems through strategic ligand pairing.
Finally, the study investigates the potential of additives such as Lewis acids for flexible modification of the optical properties in the complexes [Cr(ddad)2]3+ and [Cr(bptp)(ddad)]3+, which feature free donor sites, and introduces a design strategy for red-shifting spin-allowed absorption in molecular rubies.
In summary, this work builds a deeper understanding of spin-flip system photophysics through spectroscopic, electrochemical, and theoretical investigations of new and established chromium(III) complexes. By optimizing optical and electrochemical properties and assessing potential applications, it substantiates the viability of this class of compounds as a competitive and complementary alternative to noble metal complexes in photochemistry.