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1. Highly Efficient, Long-Lived Charge Separated States in Star-Shaped Ferrocene-Diketopyrrolopyrrole-Triphenylamine Donor-Acceptor-Donor Conjugates

   https://doi.org/doi.org/10.1002/chem.202000346  

   M. Poddar, Y. Jang (November 2020, Chemistry)

   null (Ed.)

   The significance of multiple number of donor-acceptor entities on a central electron donor in a star-shaped molecular system in improving light energy harvesting ability is reported. For this, donor-acceptor-donor type conjugates comprised up to three entities ferrocenyl (Fc)-diketopyrrolopyrrole (DPP) onto a central triphenylamine (TPA), (4-6) by the Pd-catalyzed Sonogashira cross–coupling reactions have been newly synthesized and characterized. Donor-acceptor conjugates possessing diketopyrrolopyrrole (1 to 3 entities) onto the central triphenylamine, (1-3) served as reference dyads while monomeric DPP and Fc-DPP served as control compounds. Both DPP and Fc-DPP carrying conjugates exhibited red-shifted absorption compared to their respective control compounds revealing existence of ground state interactions. Furthermore, DPP fluorescence in 4-6 was found to be quantitatively quenched while for 1-3, this property varied between 73-65% suggesting occurrence moderate amounts of excited state events. The electrochemical investigations exhibited an additional low potential oxidation in the case of Fc-DPP-TPA based derivatives (4-6) owing to the presence of ferrocene unit(s). This was in addition to DPP and TPA redox peaks. Using spectral, electrochemical and computational studies, Gibbs free-energy calculations were performed to visualize excited state charge separation (GCS) in these donor-acceptor conjugates as a function of different number of Fc-DPP entities. Formation of Fc+-DPP•--TPA charge separated states (CSS) in the case of 4-6 was evident. Using spectroelectrochemical studies, spectrum of CSS was deduced. Finally, femtosecond transient absorption spectral studies were performed to gather information on excited state charge separation. Increasing the number of Fc-DPP entities in 4-6 improved charge separation rates. Surprisingly, lifetime of the charge separated state, Fc+-DPP•--TPA was found to persist longer with an increase in the number of Fc-DPP entities in 4-6 as compared to Fc-DPP-control and simple DPP derived donor-acceptor conjugates in literature. This unprecedented result has been attributed to subtle changes in GCS and GCR and the associated electron coupling between different entities. 

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2. Radical Complexes of Nickel(II)/Copper(II) and Redox Non‐innocent MB‐DIPY Ligands: Unusual Stability and Strong Near‐Infrared Absorption at λ _max ∼1300 nm

   https://doi.org/10.1002/chem.202201181  

   Zatsikha, Yuriy_V; Shamova, Liliya_I; Shepit, Michael; Berry, Steven_M; Thomas, Fabrice; Herbert, David_E; van_Lierop, Johan; Nemykin, Victor_N (June 2022, Chemistry – A European Journal)

   Abstract The preparation of radicals with intense and redox‐switchable absorption beyond 1000 nm is a long‐standing challenge in the chemistry of functional dyes. Here we report the preparation of a series of unprecedented stable neutral nickel(II) and copper(II) complexes of “Manitoba dipyrromethenes” (MB‐DIPYs) in which the organic chromophore is present in the radical‐anion state. The new stable radicals have an intense absorption atλ_max∼1300 nm and can be either oxidized to regular [M^II(MB‐DIPY)]⁺(M=Cu or Ni) or reduced to [M^II(MB‐DIPY)]⁻compounds. The radical nature of the stable [M^II(MB‐DIPY)] complexes was confirmed by EPR spectroscopy with additional insight into their electronic structure obtained by UV‐Vis spectroscopy, electro‐ and spectroelectrochemistry, magnetic measurements, and X‐ray crystallography. The electronic structures and spectroscopic properties of the radical‐based chromophores were also probed by density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations. These nickel(II) and copper(II) complexes represent the first stable radical compounds with a MB‐DIPY ligand. 

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3. Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded‐tetracyanobutadiene Conjugates: Dark vs. Light‐Induced Electron Transfer

   https://doi.org/10.1002/chem.202302665  

   Guragain, Manan; Pinjari, Dilip; Misra, Rajneesh; D'Souza, Francis (October 2023, Chemistry – A European Journal)

   Abstract Using the popular metal‐ligand axial coordination self‐assembly approach, donor‐acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im‐TCBD) and cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded‐tetracyanobutadiene (Im‐DCNQ) as electron acceptors. The newly formed donor‐acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 10⁴–10⁵ M⁻¹in o‐dichlorobenzene. Free‐energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im‐DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im‐DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump‐probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 10¹¹ s⁻¹suggesting efficient processes. These findings show that the present self‐assembly approach could be utilized to build donor‐acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices. 

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4. Photophysical Characterization and Excited State Dynamics of Decamethylruthenocenium

   https://doi.org/10.1021/acs.jpca.4c06564  

   May, Ann Marie; Deegbey, Mawuli; Fosu, Emmanuel Adu; Danilov, Evgeny O; Castellano, Felix N; Jakubikova, Elena; Dempsey, Jillian L (February 2025, The Journal of Physical Chemistry A)

   Understanding the landscape of molecular photocatalysis is vital to enable efficient conversion of feedstock molecules to targeted products and inhibit off-cycle reactivity. In this study, the light-promoted reactivity of [RuCp*2]+ was explored via electronic structure, photophysical, and photostability studies and the reactivity of [RuCp*2]+ within a photocatalytic hydrogen evolution cycle was assessed. TD-DFT calculations support the assignment of a low-energy ligand-to-metal charge transfer transition (LMCT) centered at 500 nm, where an electron from a ligand-based orbital delocalized across both Cp* ligands is promoted to a dx2–y2-based β-LUMO orbital. Upon irradiating the LMCT absorption feature, ultrafast transient absorption spectroscopy measurements show that an initial excited state (τ1 = 1.3 ± 0.1 ps) is populated, which undergoes fast relaxation to a longer-lived state (τ2 = 12.0 ± 0.9 ps), either via internal conversion or vibrational relaxation. Despite the short-lived nature of these excited states, bulk photolysis of [RuCp*2]+ demonstrates that photochemical conversion to decomposition products is possible upon prolonged illumination. Collectively, these studies reveal that [RuCp*2]+ undergoes light-driven decomposition, highlighting the necessity to construct molecular photocatalytic systems resistant to off-cycle reactivity in both the ground and excited states. 

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5. Gold Nanoclusters: Bridging Gold Complexes and Plasmonic Nanoparticles in Photophysical Properties

   https://doi.org/10.3390/nano9070933  

   Zhou, Meng; Zeng, Chenjie; Li, Qi; Higaki, Tatsuya; Jin, Rongchao (July 2019, Nanomaterials)

   Recent advances in the determination of crystal structures and studies of optical properties of gold nanoclusters in the size range from tens to hundreds of gold atoms have started to reveal the grand evolution from gold complexes to nanoclusters and further to plasmonic nanoparticles. However, a detailed comparison of their photophysical properties is still lacking. Here, we compared the excited state behaviors of gold complexes, nanolcusters, and plasmonic nanoparticles, as well as small organic molecules by choosing four typical examples including the Au10 complex, Au25 nanocluster (1 nm metal core), 13 diameter Au nanoparticles, and Rhodamine B. To compare their photophysical behaviors, we performed steady-state absorption, photoluminescence, and femtosecond transient absorption spectroscopic measurements. It was found that gold nanoclusters behave somewhat like small molecules, showing both rapid internal conversion (<1 ps) and long-lived excited state lifetime (about 100 ns). Unlike the nanocluster form in which metal–metal transitions dominate, gold complexes showed significant charge transfer between metal atoms and surface ligands. Plasmonic gold nanoparticles, on the other hand, had electrons being heated and cooled (~100 ps time scale) after photo-excitation, and the relaxation was dominated by electron–electron scattering, electron–phonon coupling, and energy dissipation. In both nanoclusters and plasmonic nanoparticles, one can observe coherent oscillations of the metal core, but with different fundamental origins. Overall, this work provides some benchmarking features for organic dye molecules, organometallic complexes, metal nanoclusters, and plasmonic nanoparticles. 

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