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1. Resolving electron injection from singlet fission-borne triplets into mesoporous transparent conducting oxides

   https://doi.org/10.1039/d1sc03253d  

   Gish, Melissa K.; Raulerson, Emily K.; Pekarek, Ryan T.; Greenaway, Ann L.; Thorley, Karl J.; Neale, Nathan R.; Anthony, John E.; Johnson, Justin C. (January 2021, Chemical Science)

   null (Ed.)

   Photoinduced electron transfer into mesoporous oxide substrates is well-known to occur efficiently for both singlet and triplet excited states in conventional metal-to-ligand charge transfer (MLCT) dyes. However, in all-organic dyes that have the potential for producing two triplet states from one absorbed photon, called singlet fission dyes, the dynamics of electron injection from singlet vs. triplet excited states has not been elucidated. Using applied bias transient absorption spectroscopy with an anthradithiophene-based chromophore ( ADT-COOH ) adsorbed to mesoporous indium tin oxide ( nanoITO ), we modulate the driving force and observe changes in electron injection dynamics. ADT-COOH is known to undergo fast triplet pair formation in solid-state films. We find that the electronic coupling at the interface is roughly one order of magnitude weaker for triplet vs. singlet electron injection, which is potentially related to the highly localized nature of triplets without significant charge-transfer character. Through the use of applied bias on nanoITO : ADT-COOH films, we map the electron injection rate constant dependence on driving force, finding negligible injection from triplets at zero bias due to competing recombination channels. However, at driving forces greater than −0.6 eV, electron injection from the triplet accelerates and clearly produces a trend with increased applied bias that matches predictions from Marcus theory with a metallic acceptor. 

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2. Structural and chemical properties of half-sandwich rhodium complexes supported by the bis(2-pyridyl)methane ligand

   https://doi.org/10.1039/C9DT01821B  

   Lionetti, Davide; Day, Victor W.; Blakemore, James D. (August 2019, Dalton Transactions)

   [Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally + iii and + ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)] n+ , L = Cl − , CH 3 CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me 2 dpma) ligand. Complex [Cp*Rh(dpma)(NCCH 3 )] 2+ displays a quasireversible rhodium( iii / ii ) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium( ii ) oxidation state. Further reduction to the formally rhodium( i ) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me 2 dpma enables reduction to rhodium( i ) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis. 

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3. Multi-stimuli responsive luminescent azepane-substituted β-diketones and difluoroboron complexes

   https://doi.org/10.1039/c7qm00137a  

   Wang, Fang; DeRosa, Christopher A.; Daly, Margaret L.; Song, Daniel; Sabat, Michal; Fraser, Cassandra L. (January 2017, Materials Chemistry Frontiers)

   Difluoroboron β-diketonate (BF 2 bdk) compounds show environment-sensitive optical properties in solution, aggregation-induced emission (AIE) and multi-stimuli responsive fluorescence switching in the solid state. Here, a series of 4-azepane-substituted β-diketone (bdk) ligands ( L-H , L-OMe , L-Br ) and their corresponding difluoroboron dyes ( D-H , D-OMe , D-Br ) were synthesized, and various responsive fluorescence properties of the compounds were studied, including solvatochromism, viscochromism, AIE, mechanochromic luminescence (ML) and halochromism. Compared to the β-diketones, the boron complexes exhibited higher extinction coefficients but lower quantum yields, and red-shifted absorption and emission in CH 2 Cl 2 . Computational studies showed that intramolecular charge transfer (ICT) dominated rather than π–π* transitions in all the compounds regardless of boron coordination. In solution, all the bdk ligands and boron dyes showed red-shifted emission in more polar solvents and increased fluorescence intensity in more viscous media. Upon aggregation, the emission of the β-diketones was quenched, however, the boronated dyes showed increased emission, indicative of AIE. Solid-state emission properties, ML and halochromism, were investigated on spin cast films. For ML, smearing caused a bathochromic emission shift for L-Br , and powder X-ray diffraction (XRD) patterns showed that the “as spun” and thermally annealed states were more crystalline and the smeared state was amorphous. No obvious ML emission shift was observed for L-H or L-OMe , and the boronated dyes were not mechano-active. Trifluoroacetic acid (TFA) and triethylamine (TEA) vapors were used to study halochromism. Large hypsochromic emission shifts were observed for all the compounds after exposure to TFA vapor, and reversible fluorescence switching was achieved using the acid/base pair. 

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4. Ground- and Excited-State Properties of Charged Non-Stoichiometric Quantum Dots

   https://doi.org/10.1021/acs.chemmater.3c01414  

   Eniodunmo, Omolola; Gumber, Shriya; Prezhdo, Oleg; Ghosh, Dibyajyoti; Ivanov, Sergei A; Kilina, Svetlana; Tretiak, Sergei (January 2024, Chemistry of Materials)

   Charged excited states can accumulate on the surface of colloidal quantum dots (QDs), affecting their optoelectronic properties. In experimental samples, QDs often have non-stoichiometric structures, giving rise to cation-rich and anion-rich nanostructures. We explore the effect of charge on the ground- and excited-state properties of CdSe non-stoichiometric QDs (NS-QDs) of ∼1.5 nm in size using density functional theory calculations. We compare two cases: (i) NS- QDs with a charge introduced by direct hole or electron injection and (ii) neutral NS-QDs with one removed surface ligand (with a dangling bond). Our calculations reveal that a neutral dangling bond has an effect on the electronic structure similar to that of the electron injection for the Cd-rich NS-QDs or hole injection for the Se-rich NS-QDs. In Cd-rich structures, either the injection of an electron or the removal of a passivating ligand results in the surface-localized half-filled trap state inside the energy gap. For Se-rich structures, either the injection of a hole or the removal of a ligand introduces surface-localized unoccupied trap states inside the energy gap. As a result, the charge localization formed by these two approaches leads to an appearance of low-energy electronic transitions strongly red-shifted from the main excitonic band of NS-QDs. These transitions related to a negative charge or a dangling bond exhibit weak optical activity in Cd-rich NS-QDs. Transitions related to a positive charge or a dangling bond are optically forbidden in Se-rich NS-QDs. In contrast, electron injection in Se-rich NS-QDs strongly increases the optical activity of the lowest- red-shifted charge-originated states. 

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5. Insights into the charge-transfer character of electronic transitions in ^R Cp ₂ Ti(C ₂ Fc) ₂ complexes using solvatochromism, resonance Raman spectroscopy, and TDDFT

   https://doi.org/10.1039/d0dt04282j  

   Carlton, Elizabeth S.; Sutton, Joshua J.; Gale, Ariel G.; Shields, George C.; Gordon, Keith C.; Wagenknecht, Paul S. (February 2021, Dalton Transactions)

   null (Ed.)

   A series of complexes with low-energy Fe II to Ti IV metal-to-metal charge-transfer (MMCT) transitions, Cp 2 Ti(C 2 Fc) 2 , Cp* 2 Ti(C 2 Fc) 2 , and MeOOC Cp 2 Ti(C 2 Fc) 2 , was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters – solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) – was performed. The effect of SP indicates a greater degree of electron delocalization in the excited state (ES) than the ground state (GS). The small negative solvatochromism with respect to SdP indicates a smaller dipole moment in the ES than the GS. The effect of SB is consistent with charge-transfer to Ti. Upon excitation into the MMCT absorption band, the RRS data show enhancement of the alkyne stretching modes and of the out-of-plane bending modes of the cyclopentadienyl ring connected to Fe and the alkyne bridge. This is consistent with changes in the oxidation states of Ti and Fe, respectively. The higher-energy transitions (350–450 nm) show enhancement of vibrational modes consistent with ethnylcyclopentadienyl to Ti ligand-to-metal charge transfer (LMCT). The RRS data is consistent with the TDDFT predicted character of these transitions. TDDFT suggests that the lowest-energy transition in Cp 2 Ti(C 2 Fc) 2 CuI, where CuI is coordinated between the alkynes, retains its Fe II to Ti IV MMCT character, in agreement with the RRS data, but that the lowest-energy transitions have significant CuI to Ti character. For Cp 2 Ti(C 2 Fc) 2 CuI, excitation into the low-energy MMCT absorption band results in selective enhancement of the symmetric alkynyl stretching mode. 

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