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1. Excited state processes of dinuclear Pt( ii ) complexes bridged by 8-hydroxyquinoline

   https://doi.org/10.1039/D3DT00348E  

   Kromer, Sarah ; Roy, Subhangi ; Yarnell, James E. ; Taliaferro, Chelsea M. ; Castellano, Felix N. ( March 2023 , Dalton Transactions)

   Dinuclear d 8 Pt( ii ) complexes, where two mononuclear square planar Pt( ii ) units are bridged in an “A-frame” geometry, possess photophysical properties characterised by either metal-to-ligand-(MLCT) or metal–metal–ligand-to-ligand charge transfer (MMLCT) transitions determined by the distance between the two Pt( ii ) centres. When using 8-hydroxyquinoline (8HQH) as the bridging ligand to construct novel dinuclear complexes with general formula [C^NPt(μ-8HQ)] 2 , where C^N is either 2-phenylpyridine (1) or 7,8-benzoquinoline (2), triplet ligand-centered ( 3 LC) photophysics results echoing that in a mononuclear model chromophore, [Pt(8HQ) 2 ] (3). The lengthened Pt–Pt distances of 3.255 Å (1) and 3.243 Å (2) results in a lowest energy absorption centred around 480 nm assigned as having mixed LC/MLCT character by TD-DFT, mirroring the visible absorption spectrum of 3. Additionally, 1 and 2 exhibit 3 LC photoluminescence with limited quantum yields (0.008) from broad transitions centred near 680 nm. Photoexcitation of 1–3 leads to an initially prepared excited state that relaxes within 15 ps to a 3 LC excited state centred on the 8HQ bridge, which then persists for several microseconds. All the experimental results correspond well with DFT electronic structure calculations. 

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2. Balancing the interplay between ligand ejection and therapeutic window light absorption in ruthenium polypyridyl complexes

   https://doi.org/10.1039/D2DT01237E  

   McCullough, Annie B. ; Chen, Jiaqi ; Valentine, Nathaniel P. ; Franklin, Toney M. ; Cantrell, Andrew P. ; Darnell, Vayda M. ; Qureshi, Qasim ; Hanson, Kenneth ; Shell, Steven M. ; Ashford, Dennis L. ( July 2022 , Dalton Transactions)

   Ruthenium polypyridyl complexes have gained significant interest as photochemotherapies (PCTs) where their excited-state properties play a critical role in the photo-cytotoxicity mechanism and efficacy. Herein we report a systematic electrochemical, spectrochemical, and photophysical analysis of a series of ruthenium( ii ) polypyridyl complexes of the type [Ru(bpy) 2 (N–N)] 2+ (where bpy = 2,2′-bipyridine; N–N is a bidentate polypyridyl ligand) designed to mimic PCTs. In this series, the N–N ligand was modified through increased conjugation and/or incorporation of electronegative heteroatoms to shift the metal-to-ligand charge-transfer (MLCT) absorptions near the therapeutic window for PCTs (600–1100 nm) while incorporating steric bulk to trigger photoinduced ligand dissociation. The lowest energy MLCT absorptions were red-shifted from λ max = 454 nm to 564 nm, with emission energies decreasing from λ max = 620 nm to 850 nm. Photoinduced ligand ejection and temperature-dependent emission studies revealed an important interplay between red-shifting MLCT absorptions and accessing the dissociative 3 dd* states, with energy barriers between the 3 MLCT* and 3 dd* states ranging from 850 cm −1 to 2580 cm −1 for the complexes measured. This work demonstrates the importance of understanding both the MLCT manifold and 3 dd* state energy levels in the future design of ligands and complexes for PCT. 

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3. 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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4. Delocalization tunable by ligand substitution in [L 2 Al] n− complexes highlights a mechanism for strong electronic coupling

   https://doi.org/10.1039/D0SC02812F  

   Arnold, Amela ; Sherbow, Tobias J. ; Bohanon, Amanda M. ; Sayler, Richard I. ; Britt, R. David ; Smith, Allison M. ; Fettinger, James C. ; Berben, Louise A. ( January 2021 , Chemical Science)

   null (Ed.)

   Ligand-based mixed valent (MV) complexes of Al( iii ) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I 2 P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I 2 P − )(I 2 P 2− )Al and [(I 2 P 2− )(I 2 P 3− )Al] 2− charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I 2 P − )(I 2 P 2− )Al and [(I 2 P 2− )(I 2 P 3− )Al] 2− show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm −1 , respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I 2 P 2− )(I 2 P 3− )Al] 2− complexes containing –PhNMe 2 or –PhF 5 substituents. Minor localization of charge in [(I 2 P 2− )(I 2 P 3− )Al] 2− was observed when –PhOMe substituents are included. 

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5. 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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