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1. Excited-state properties of Cu-TADF complexes: A density functional theory study

   https://doi.org/10.1063/5.0207506  

   Ashokan, Ajith; Coropceanu, Veaceslav; Salman, Seyhan (May 2024, AIP Advances)

   Johnson, AT Charlie (Ed.)

   Organometallic complexes, including copper atom, have attracted great interest as thermally activated delayed fluorescence (TADF) emitters for light emitting diode (LED) applications. This is ascribed to the potential low-cost, abundant availability of copper and most importantly to the ability of copper to enhance the spin–orbit couplings and, consequently, increase the reverse intersystem crossing rates. In this article, we use density functional theory (DFT) to investigate the excited state properties of six copper complexes based on N-heterocyclic carbene ligand, monoamido-amino carbene and diamido carbene, and carbazole ligand. The DFT calculations show that the lowest excited states consist of three groups, i.e., (i) local carbazole excitations, (ii) carbazole-to-carbene intramolecular charge transfer states, and (iii) metal-to-ligand charge transfer states. Only the latter states are characterized with large spin–orbit couplings. The DFT calculations show that the surrounding medium could have a major effect on electronic spectrum by reordering the states. Our results suggest that the TADF properties of the investigated complexes can be affected by the chemical structure of the ligands as well as by the dielectric properties of the LED device active layer. 

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2. Synthesis and Intramolecular Charge Transfer Studies on meso-Tetracyanobutadine-Functionalized Diphenylporphyrin Complexes Incorporating Copper and Nickel Metals

   https://doi.org/10.3390/molecules31060934  

   Yadav, Sumit Kumar; Sharma, Jatan K; Sankar, Muniappan; D’Souza, Francis (March 2026, Molecules)

   Liao, D (Ed.)

   This study presents the synthesis and electrochemical characterization of meso-tetracyanobutadiene (TCBD)-functionalized diphenylporphyrin (DPP) complexes incorporating copper (Cu) and nickel (Ni) metals. These push–pull metallo diphenylporphyrin–TCBD complexes were synthesized via a [2 + 2] cycloaddition–retroelectrocyclization reaction between 5-bromo-15-formyl-10,20-diphenylporphyrin metal(II) complexes (M = Cu, Ni) and tributyl(phenylethynyl)stannate, followed by tetracyanoethylene (TCNE) addition. The resulting TCBD-functionalized porphyrins were obtained in moderate yields (70–75%) and thoroughly characterized by 1H and 13C NMR, UV-Vis spectroscopy, MALDI-TOF-MS, and single-crystal XRD. Although the single-crystal X-ray structure of NiDPP was solved, DFT calculations were used to determine the structures of the donor–acceptor MDPP-TCBD systems and to visualize their electronic structures. HOMO on the porphyrin π system and LUMO on the TCBD entity were observed, and energy level diagrams clearly laid out the electron donor and acceptor parts of the molecular systems. As expected, these novel donor–acceptor porphyrinoid assemblies exhibited enhanced push–pull properties in both the ground and excited states. Femtosecond transient absorption studies revealed that both NiDPP-TCBD and CuDPP-TCBD populate the charge-transfer state upon photoexcitation, with lifetimes of 383.1 ps and 484.7 ps, respectively, in benzonitrile. The charge-transfer states populated the triplet or doublet states (in the case of CuDPP) before returning to the ground state. 

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3. Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear Cu ^I Cluster as Blue- and Green/Yellow-Light Emitters

   https://doi.org/10.1021/acs.inorgchem.4c01646  

   Arras, Janet; Calderón-Díaz, Alvaro; Lebedkin, Sergei; Gozem, Samer; McMillen, Colin D; Bhuvanesh, Nattamai; Stollenz, Michael (June 2024, Inorganic Chemistry)

   Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several Cu(I) complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected Cu(I)2···Cu(I)2 chain and a continuous helically bent Cu(I)4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized Cu(I)10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties. 

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4. Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

   https://doi.org/10.1021/jacs.3c02825  

   Muniz, Collin N.; Archer, Claire A.; Applebaum, Jack S.; Alagaratnam, Anushan; Schaab, Jonas; Djurovich, Peter I.; Thompson, Mark E. (June 2023, Journal of the American Chemical Society)

   Generating a sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes can be used as sensitizers to promote the light driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (vis > 103 M-1cm-1), maintain long excited state lifetimes (~ 0.2-1 s) and perform stable photo-induced charge transfer to a target substrate with high photoreducing potential (E+/* up to 2.33 V vs. Fc+/0 based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that these two-coordinate complexes presented can perform photo-driven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this “catalyst free” system the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. This work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuels photosensitizers that offer exceptional tunability and photoredox properties. 

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5. Symmetrically Functionalized Copper and Silver Corrole‐Bis‐Tetracyanobutadiene Push‐Pull Conjugates: Efficient Population of Triplet States via Charge Transfer

   https://doi.org/10.1002/chem.202301341  

   Yadav, Inderpal; Sharma, Jatan K; Sankar, Muniappan; D'Souza, Francis (July 2023, Chemistry – A European Journal)

   Abstract Copper and silver tritolylcorroles (TTC) are symmetrically functionalized to carry two tetracyanobutadiene (TCBD) entities via [2+2] cycloaddition‐retroeletrocyclization reaction involving ethynyl functionalized corroles with an electron acceptor, tetracyanoethylene (TCNE) in excellent yields, as the first examples of corrole‐TCBD push‐pull systems. The strong push‐pull effect resulted in charge polarization in the ground state resulting in a considerable hypsochromic shift of the spectrum extending it into the near‐IR region. Electrochemical studies coupled with computational studies revealed considerable interactions between the two TCBD entities via the corrole π‐system and the degree of such interactions was found to depend on the metal ion present in the corrole cavity. Energy considerations suggested charge transfer (CT) from the S₂or vibrationally hot S₁state but not the relaxed S₁state in the case of CuTTC(TCBD)₂while CT to occur from all these states in the case of AgTTC(TCBD)₂. Additionally, the high‐energy CT states populate the low‐lying triplet states. Systematic femtosecond pump‐probe studies provided the ultimate proof for the occurrence of excited CT as a function of excitation wavelength followed by the efficient population of the triplet states. The present study brings out the significance of charge transfer in efficiently populating the triplet states in rather unusual copper and silver corroles carrying two TCBD entities. 

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