@article{wells_palmer_yarnell_garakyaraghi_pemberton_favale_valchar_chakraborty_castellano_2021, title={Understanding the influence of geometric and electronic structure on the excited state dynamical and photoredox properties of perinone chromophores}, volume={23}, ISSN={["1463-9084"]}, url={https://doi.org/10.1039/D1CP03870B}, DOI={10.1039/d1cp03870b}, abstractNote={Modulating electronic and geometric structure of perinone chromophores is easily achieved via systematic alteration of aromatic diamine and anhydride building blocks, eliciting deterministic photoredox and excited state dynamical properties.}, number={42}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Wells, Kaylee A. and Palmer, Jonathan R. and Yarnell, James E. and Garakyaraghi, Sofia and Pemberton, Barry C. and Favale, Joseph M. and Valchar, Mary Katharine and Chakraborty, Arnab and Castellano, Felix N.}, year={2021}, month={Nov}, pages={24200–24210} }
 @article{zhang_lee_favale_leary_petersen_scholes_castellano_milsmann_2020, title={Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states}, volume={12}, ISSN={1755-4330 1755-4349}, url={http://dx.doi.org/10.1038/s41557-020-0430-7}, DOI={10.1038/s41557-020-0430-7}, abstractNote={Advances in chemical control of the photophysical properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on molecules containing precious metals such as ruthenium and iridium. Although the application of earth-abundant ‘early’ transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible light-absorbing Zr(iv) photosensitizer, Zr(MesPDPPh)2, where [MesPDPPh]2− is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This molecule has an exceptionally long-lived triplet LMCT excited state (τ = 350 μs), featuring highly efficient photoluminescence emission (Ф = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr(MesPDPPh)2 engages in numerous photoredox catalytic processes and triplet energy transfer. Our investigation provides a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions. Understanding the photophysical properties of transition-metal complexes is paramount to advances in photocatalysis, solar energy conversion and light-emitting diodes. Now, long-lived emission via thermally activated delayed fluorescence has been demonstrated from an air- and water-stable zirconium complex featuring excited states with significant ligand-to-metal charge transfer character.}, number={4}, journal={Nature Chemistry}, publisher={Springer Science and Business Media LLC}, author={Zhang, Yu and Lee, Tia S. and Favale, Joseph M. and Leary, Dylan C. and Petersen, Jeffrey L. and Scholes, Gregory D. and Castellano, Felix N. and Milsmann, Carsten}, year={2020}, month={Mar}, pages={345–352} }
 @article{favale_hauke_danilov_yarnell_castellano_2020, title={Ligand-triplet migration in iridium(iii) cyclometalates featuring π-conjugated isocyanide ligands}, volume={49}, ISSN={1477-9226 1477-9234}, url={http://dx.doi.org/10.1039/D0DT02100H}, DOI={10.1039/d0dt02100h}, abstractNote={The degree of ancillary ligand conjugation determines Ir(iii) polychromophoric photophysical properties via manipulation of the triplet excited state manifolds.}, number={29}, journal={Dalton Transactions}, publisher={Royal Society of Chemistry (RSC)}, author={Favale, Joseph M. and Hauke, Cory E. and Danilov, Evgeny O. and Yarnell, James E. and Castellano, Felix N.}, year={2020}, pages={9995–10002} }
 @article{palmer_wells_yarnell_favale_castellano_2020, title={Visible-Light-Driven Triplet Sensitization of Polycyclic Aromatic Hydrocarbons Using Thionated Perinones}, volume={11}, ISSN={1948-7185 1948-7185}, url={http://dx.doi.org/10.1021/acs.jpclett.0c01634}, DOI={10.1021/acs.jpclett.0c01634}, abstractNote={Metal-free chromophores that efficiently generate triplet excited states represent promising alternatives with respect to transition metal containing photosensitizers, such as those featuring metal-to-ligand charge transfer (MLCT) excited states. However, such molecular constructs have remained underexplored due to the unclear relationship(s) between molecular structure and efficient/rapid intersystem crossing. In this regard, we present a series of three thionated perinone chromophores serving as a newly conceived class of heavy-metal-free triplet photosensitizers. We demonstrate that thionation of the lone C=O substituent in each highly fluorescent perinone imparts red-shifted absorbance bands that maintain intense extinction coefficients across the visible spectrum, as well as unusually efficient triplet excited state formation as inferred from the measured singlet O2 quantum yields at 1270 nm (Φ∆ = 0.78 - 1.0). Electronic structure calculations revealed the emergence of a low energy S1 (n → π*) excited state in close proximity to a slightly higher energy S2 (π → π*) excited state. The distinct character in each of the two lowest lying singlet state manifolds resulted in the energetic inversion of the corresponding triplet excited states due to differences in electron exchange interactions. Rapid S1 → T1 intersystem crossing (ISC) was thereby facilitated in this manner through spin-orbit coupling as predicted by the El Sayed rules. The lifetimes of the resultant triplet excited states persisted into the microsecond time regime, as measured by transient absorbance spectroscopy, enabling effective bimolecular triplet sensitization of some common polycyclic aromatic hydrocarbons. The synthetically facile interchange of a single O atom to an S atom in the investigated perinones resulted in marked changes to their photophysical properties, namely, conversion of dominant singlet-state fluorescence in the former to long-lived triplet excited states in the latter. The combined results suggest a general strategy for accessing long-lived triplet excited states in organic chromophores featuring a lone C=O moiety resident within its structure, valuable for the design of metal-free triplet photosensitizers.}, number={13}, journal={The Journal of Physical Chemistry Letters}, publisher={American Chemical Society (ACS)}, author={Palmer, Jonathan R. and Wells, Kaylee A. and Yarnell, James E. and Favale, Joseph M. and Castellano, Felix N.}, year={2020}, month={Jun}, pages={5092–5099} }
 @article{favale_danilov_yarnell_castellano_2019, title={Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States}, volume={58}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/acs.inorgchem.9b01155}, DOI={10.1021/acs.inorgchem.9b01155}, abstractNote={We present a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal-organic chromophores of the generic molecular formula [Re(phen)(CO)3(CNAr)]+ [CNAr = 2,6-diisopropylphenyl isocyanide (1), 4-phenyl-2,6-diisopropylphenyl isocyanide (2), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide (3), and 4-biphenyl-2,6-diisopropylphenyl isocyanide (4)]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8-700 μs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states (λmax,em = 500-518 nm; Φ = 14-64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules (2-4), wherein the triplet metal-to-ligand charge-transfer (3MLCT) state along with two distinct triplet ligand-centered (3LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1, the phenanthroline 3LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit (2-4), the initially generated 3LC(phen)/3MLCT excited state ultimately migrates to the CNAr 3LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as evidenced by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.}, number={13}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Favale, Joseph M., Jr. and Danilov, Evgeny O. and Yarnell, James E. and Castellano, Felix N.}, year={2019}, month={Jun}, pages={8750–8762} }