Materials exhibiting thermally activated delayed fluorescence (TADF) based on transition metal complexes are currently gathering significant attention due to their technological potential. Their application extends beyond optoelectronics, in particular organic light‐emitting diodes (OLEDs) and light‐emitting electrochemical cells (LECs), and include also photocatalysis, sensing, and X‐ray scintillators. From the perspective of sustainability, earth‐abundant metal centers are preferred to rarer second‐ and third‐transition series elements, thus determining a reduction in costs and toxicity but without compromising the overall performances. This review offers an overview of earth‐abundant transition metal complexes exhibiting TADF and their application as photoconversion materials. Particular attention is devoted to the types of ligands employed, helping in the design of novel systems with enhanced TADF properties.

The possibility of exploiting visible light to induce polymerizations is extremely appealing from a technological point of view as it improves the sustainability of the overall process. To achieve this objective, it is necessary to employ single‐ or multicomponent systems containing a photoinitiator, and in some cases, a photosensitizer in combination. Due to their long‐lived excited states and reversible redox properties, transition metal complexes are a valid choice to be applied in photoinitiating systems, often exhibiting enhanced conversions compared to purely organic compounds. This review presents an overview of the transition metal complexes exploited in photopolymerization reactions. Particular attention will be devoted to recent applications in 3D printing, highlighting the possible challenges that need to be faced to achieve highly efficient and more sustainable processes.

Excitation of the coordinated ligands in tetrahedral complexes having the general formula [MnX ₂ L ₂ ] (X = Cl, Br, I; L = N , N ′, N ′-tetramethyl- P -indol-1-ylphosphonic diamide) causes bright green luminescence from the Mn( ii ) centre.

Photoluminescent Mn(ii) tetrahedral complexes characterized by intense emission in the green region were isolated from the reaction of MnX₂ (X = Cl, Br, I) and the ligand 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide.

Manganese(II) halide complexes with N,N,N′,N′-tetramethyl-P-naphtalen-2-ylphosphonic diamide were synthesized from anhydrous MnX₂ salts (X = Cl, Br, I) and characterized. Single-crystal X-ray diffraction revealed in all the cases slightly distorted tetrahedral geometry of the coordination sphere. The photoluminescence spectra showed the superimposition of a green emission, related to the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn(II), with a band in the red range. Different possible attributions to the lowest-energy emission were taken into account. The emission spectra of the isolated products are dependent upon the nature of the halide and, in the case of X = Br, also upon the excitation wavelength.

Tetrahedral Mn(II) bromo- and iodo-complexes with phosphoramidate and phosphonate ligands were synthesized and characterized. Mononuclear complexes with the general formula [MnX₂L₂] were isolated by using the monodentate ligands OP(OPh)₂(NMe₂) and OP(OPh)₂Ph. The bidentate resorcinol derivative 1,3-OP(NMe₂)O–C₆H₄–O(NMe₂)₂PO behaved as a bridging ligand, and afforded, by reaction with MnX₂ salts, a dinuclear complex in the case of X = Br and a coordination polymer for X = I, as revealed by single crystal X-ray diffraction. All the compounds showed the characteristic green photoluminescence of Mn(II) in a tetrahedral environment. The resulting lifetime values were influenced by the symmetry of the compounds. The complexes with OP(OPh)₂(NMe₂) in the coordination sphere showed high photoluminescence quantum yields.