@article{rosko_espinoza_arteta_kromer_wheeler_castellano_2023, title={Employing Long-Range Inductive Effects to Modulate Metal-to- Ligand Charge Transfer Photoluminescence in Homoleptic Cu(I) Complexes}, volume={62}, ISSN={["1520-510X"]}, url={https://doi.org/10.1021/acs.inorgchem.2c04315}, DOI={10.1021/acs.inorgchem.2c04315}, abstractNote={Four Cu(I) bis(phenanthroline) photosensitizers formulated from a new ligand structural motif (Cu1-Cu4) coded according to their 2,9-substituents were synthesized, structurally characterized, and fully evaluated using steady-state and time-resolved absorption and photoluminescence (PL) measurements as well as electrochemistry. The 2,9-disubstituted-3,4,7,8-tetramethyl-1,10-phenanthroline ligands feature the following six-membered ring systems prepared through photochemical synthesis: 4,4-dimethylcyclohexyl (1), tetrahydro-2H-pyran-4-yl (2), tetrahydro-2H-thiopyran-4-yl (3), and 4,4-difluorocyclohexyl (4). Universally, these Cu(I) metal-to-ligand charge transfer (MLCT) chromophores display excited-state lifetimes on the microsecond time scale at room temperature, including the three longest-lived homoleptic cuprous phenanthroline excited states measured to date in de-aerated CH2Cl2, τ = 2.5-4.3 μs. This series of molecules also feature high PL quantum efficiencies (ΦPL = 5.3-12% in CH2Cl2). Temperature-dependent PL lifetime experiments confirmed that all these molecules exhibit reverse intersystem crossing and display thermally activated delayed PL from a 1MLCT excited state lying slightly above the 3MLCT state, 1050-1490 cm-1. Ultrafast and conventional transient absorption measurements confirmed that the PL originates from the MLCT excited state, which remains sterically arrested, preventing an excessive flattening distortion even when dissolved in Lewis basic CH3CN. Combined PL and electrochemical data provided evidence that Cu1-Cu4 are highly potent photoreductants (Eox* = -1.73 to -1.62 V vs Fc+/0 in CH3CN), whose potentials are altered solely based on which heteroatoms or substituents are resident on the 2,9-appended ring derivatives. It is proposed that long-range electronic inductive effects are responsible for the systematic modulation observed in the PL spectra, excited-state lifetimes, and the ground state absorption spectra and redox potentials. Cu1-Cu4 quantitatively follow the energy gap law, correlating well with structurally related cuprous phenanthrolines and are also shown to triplet photosensitize the excited states of 9,10-diphenylanthracene with bimolecular rate constants ranging from 1.61 to 2.82 × 108 M-1 s-1. The ability to tailor both photophysical and electrochemical properties using long-range inductive effects imposed by the 2,9-ring platforms advocates new directions for future MLCT chromophore discovery.}, number={7}, journal={INORGANIC CHEMISTRY}, author={Rosko, Michael C. and Espinoza, Eli M. and Arteta, Sarah and Kromer, Sarah and Wheeler, Jonathan P. and Castellano, Felix N.}, year={2023}, month={Feb}, pages={3248–3259} }
 @article{kromer_roy_yarnell_taliaferro_castellano_2023, title={Excited state processes of dinuclear Pt(II) complexes bridged by 8-hydroxyquinoline}, volume={3}, ISSN={["1477-9234"]}, url={https://doi.org/10.1039/D3DT00348E}, DOI={10.1039/d3dt00348e}, abstractNote={Two novel dinuclear Pt(ii) complexes featuring 8-hydroxyquinoline bridges were synthesized and found to have photophysical properties dominated by ligand-centered transitions mirroring that of a mononuclear model system, [Pt(8HQ)2].}, journal={DALTON TRANSACTIONS}, author={Kromer, Sarah and Roy, Subhangi and Yarnell, James E. and Taliaferro, Chelsea M. and Castellano, Felix N.}, year={2023}, month={Mar} }
 @article{rafiq_weingartz_kromer_castellano_chen_2023, title={Spin-vibronic coherence drives singlet-triplet conversion}, volume={7}, ISSN={["1476-4687"]}, url={https://doi.org/10.1038/s41586-023-06233-y}, DOI={10.1038/s41586-023-06233-y}, abstractNote={Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry 1-3 . Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings 4,5 . However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes 6-9  to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.}, journal={NATURE}, author={Rafiq, Shahnawaz and Weingartz, Nicholas P. and Kromer, Sarah and Castellano, Felix N. and Chen, Lin X.}, year={2023}, month={Jul} }
 @article{kim_kim_mills_chakraborty_kromer_valentine_castellano_li_chen_2022, title={Ligand-Structure-Dependent Coherent Vibrational Wavepacket Dynamics in Pyrazolate-Bridged Pt(II) Dimers}, volume={7}, ISSN={["1932-7455"]}, url={https://doi.org/10.1021/acs.jpcc.2c02256}, DOI={10.1021/acs.jpcc.2c02256}, abstractNote={Bimetallic transition metal complexes have gained increasing attention because of their versatile functions in solar energy conversion and photonics applications arising from intermetal electronic coupling. In bimetallic platinum (Pt) complexes, electronic communication between the Pt-centered and ligand-centered moieties has been shown to be critical for defining their excited-state dynamic trajectories undergoing either localized ligand-centered (LC)/metal-to-ligand charge-transfer (MLCT) transitions or delocalized metal–metal-to-ligand charge-transfer (MMLCT) transitions. The branching of the excited-state intersystem crossing (ISC) trajectories is modulated through structural factors that alter the relative energies of the different states. In this study, we investigated the correlation of the structural factors influencing the excited-state trajectories. With the use of femtosecond broad-band transient absorption (fs-BBTA) spectroscopy, ultrafast dynamics in the excited state of two select Pt(II) dimers have been mapped out using their coherent vibrational wavepacket signatures in the corresponding transient absorption spectra. To examine how the ligand moieties of the Pt(II) dimers influence excited-state dynamics and the coherent vibrational wavepacket behavior, we carried out comparative studies on two pyrazolate-bridged Pt(II) dimers of the general formula [Pt(tBu2Pz)(N^C)]2 [tBu2Pz is 3,5-di-tert-butylpyrazole; N^C is 7,8-benzoquinoline (bzq, 1) or 1-phenylisoquinoline (piq, 2)]. We found that photoexcitation into the low-energy absorption bands of 1 and 2, respectively, induces the formation of 1MMLCT states from which ultrafast ISC proceeds, resulting in stimulated emission quenching and decoherence of the vibrational wavepacket motions. The results obtained in this study suggest that both the energetics and the structural rigidity of the aromatic cyclometalating ligands in 1 and 2 can significantly influence the dynamics along the excited-state trajectory characterized by dephasing of the coherent oscillations. The collective results provide direct evidence of how ligand structure alters electronic dynamics along excited-state trajectories associated with ISC processes, providing insight into using ligand design to steer photochemical processes.}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Kim, Tae Wu and Kim, Pyosang and Mills, Alexis W. and Chakraborty, Arnab and Kromer, Sarah and Valentine, Andrew J. S. and Castellano, Felix N. and Li, Xiaosong and Chen, Lin X.}, year={2022}, month={Jul} }