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1. Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption

   https://doi.org/10.1073/pnas.2009996117  

   Jiang, Ting; Bai, Yusong; Zhang, Peng; Han, Qiwei; Mitzi, David B.; Therien, Michael J. (August 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Exploiting earth-abundant iron-based metal complexes as high-performance photosensitizers demands long-lived electronically excited metal-to-ligand charge-transfer (MLCT) states, but these species suffer typically from femtosecond timescale charge-transfer (CT)-state quenching by low-lying nonreactive metal-centered (MC) states. Here, we engineer supermolecular Fe(II) chromophores based on the bis(tridentate-ligand)metal(II)-ethyne-(porphinato)zinc(II) conjugated framework, previously shown to give rise to highly delocalized low-lying 3 MLCT states for other Group VIII metal (Ru, Os) complexes. Electronic spectral, potentiometric, and ultrafast pump–probe transient dynamical data demonstrate that a combination of a strong σ-donating tridentate ligand and a (porphinato)zinc(II) moiety with low-lying π*-energy levels, sufficiently destabilize MC states and stabilize supermolecular MLCT states to realize Fe(II) complexes that express 3 MLCT state photophysics reminiscent of their heavy-metal analogs. The resulting Fe(II) chromophore archetype, FeNHCPZn, features a highly polarized CT state having a profoundly extended 3 MLCT lifetime (160 ps), 3 MLCT phosphorescence, and ambient environment stability. Density functional and domain-based local pair natural orbital coupled cluster [DLPNO-CCSD(T)] theory reveal triplet-state wavefunction spatial distributions consistent with electronic spectroscopic and excited-state dynamical data, further underscoring the dramatic Fe metal-to-extended ligand CT character of electronically excited FeNHCPZn. This design further prompts intense panchromatic absorptivity via redistributing high-energy absorptive oscillator strength throughout the visible spectral domain, while maintaining a substantial excited-state oxidation potential for wide-ranging photochemistry––highlighted by the ability of FeNHCPZn to photoinject charges into a SnO 2 /FTO electrode in a dye-sensitized solar cell (DSSC) architecture. Concepts enumerated herein afford opportunities for replacing traditional rare-metal–based emitters for solar-energy conversion and photoluminescence applications. 

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2. Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems

   https://doi.org/10.1021/acs.inorgchem.3c01154  

   Rajh, Tijana; Masson, Eric; Latt, Kyaw Zin; Smith, Ashton; Brugh, Alexander M.; Dandu, Naveen; Trainer, Daniel; Curtiss, Larry A.; Ngo, Anh T.; Hla, Saw-Wai (August 2023, Inorganic Chemistry)

   Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g⊥ = 2.001 and g∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 (151Eu and 153Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors 151Eu/153Eu = 2.27. Illumination of water-soluble complex Eu(dipic)3 at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (gx = 2.006, gy = 1.995, gz = 1.988). The existence of LMCT affects the luminescence of Eu(dipic)3, and pre-reduction of the complex to Eu(II)(dipic)3 reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination. 

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3. Picosecond Metal-to-Ligand Charge-Transfer Deactivation in Co(ppy) ₃ via Jahn–Teller Distortion

   https://doi.org/10.1021/acs.inorgchem.4c01959  

   Malme, Justin T; Weaver, Jenelle N; Girolami, Gregory S; Vura-Weis, Josh (July 2024, Inorganic Chemistry)

   The excited-state dynamics of fac-Co(ppy)3, where ppy = 2-[2-(pyridyl)phenyl], are measured with femtosecond UV-Vis transient absorption spectroscopy. The initial state is confirmed with spectroelectrochemistry to have significant metal-to-ligand charge transfer (MLCT) character, unlike other Co complexes that generally have ligand-to-metal charge transfer or ligand-field transitions in this energy range. Ground-state recovery occurs in 8.65 ps in dichloromethane. Density functional theory (DFT) calculations show that the MLCT state undergoes Jahn-Teller distortion and converts to a 5-coordinate 3MC state in which one Co-N bond is broken. The results highlights a potential pitfall of heteroleptic-bidentate ligands when designing strong-field ligands for transition metal chromophores. 

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4. Impacts of extending the π-conjugation of the 2,2′-biquinoline ligand on the photophysics and reverse saturable absorption of heteroleptic cationic iridium( iii ) complexes

   https://doi.org/10.1039/D1TC03601G  

   Lu, Taotao; Lu, Cuifen; Cui, Peng; Kilina, Svetlana; Sun, Wenfang (November 2021, Journal of Materials Chemistry C)

   Two heteroleptic monocationic Ir( iii ) complexes bearing 6,6′-bis(7-benzothiazolylfluoren-2-yl)-2,2′-biquinoline as the diimine ligand with different degrees of π-conjugation were synthesized and their photophysics was investigated by spectroscopic techniques and first principles calculations. These complexes possessed two intense absorption bands at 300–380 nm and 380–520 nm in toluene that are predominantly ascribed to the diimine ligand-localized 1 π,π* transition and intraligand charge transfer ( 1 ILCT)/ 1 π,π* transitions, respectively, with the latter being mixed with minor 1 MLCT (metal-to-ligand charge transfer)/ 1 LLCT (ligand-to-ligand charge transfer) configurations. Both complexes also exhibited a spin-forbidden, very weak 3 MLCT/ 3 LLCT/ 3 π,π* absorption band at 520–650 nm. The emission of these complexes appeared in the red spectral region ( λ em : 640 nm for Ir-1 and 648 nm for Ir-2 in toluene) with a quantum yield of <10% and a lifetime of hundreds of ns, which emanated from the 3 ILCT/ 3 π,π* state. The 3 ILCT/ 3 π,π* state also gave rise to broad and moderately strong transient absorption (TA) at ca. 480–800 nm. Extending the π-conjugation of the diimine ligand via inserting CC triplet bonds between the 7-benzothiazolylfluoren-2-yl substituents and 2,2′-biquinoline slightly red-shifted the absorption bands, the emission bands, and the TA bands in Ir-2 compared to those in Ir-1 that lacks the connecting CC triplet bonds in the diimine ligand. The stronger excited-state absorption with respect to the ground-state absorption at 532 nm led to strong reverse saturable absorption (RSA) for ns laser pulses at this wavelength, with the RSA of Ir-2 being slightly stronger than that of Ir-1, which correlated well with their ratios of the excited-state to ground-state absorption cross sections ( σ ex / σ 0 ). These results suggest that extending the π-conjugation of the 2,2′-biquinoline ligand via incorporating the 7-benzothiazolylfluoren-2-yl substituents retained the broad but weak ground-state absorption at 500–650 nm, meanwhile increased the triplet excited-state lifetimes, which resulted in the much stronger triplet excited-state absorption in this spectral region and strong RSA at 532 nm. Thus, these complexes are promising candidates as broadband reverse saturable absorbers. 

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5. 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)

   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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