@article{garakyaraghi_crapps_mccusker_castellano_2016, title={Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics}, volume={55}, ISSN={["1520-510X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84992163171&partnerID=MN8TOARS}, DOI={10.1021/acs.inorgchem.6b01880}, abstractNote={In the interest of expanding the inventory of available long lifetime, photochemically robust, and strongly reducing Cu(I) MLCT sensitizers, we present detailed structural, photophysical, and electrochemical characterization of [Cu(dipp)2]+, dipp = 2,9-diisopropyl-1,10-phenanthroline, and its sterically encumbered tetramethyl analogue [Cu(diptmp)2]+, diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline. The achiral isopropyl substituents enable similar steric bulk effects to the previously investigated sec-butyl substituents while eliminating the complex NMR structural analyses associated with the presence of two chiral centers in the latter. The photophysical properties of [Cu(diptmp)2]+ are impressive, possessing a 2.3 μs lifetime in deaerated CH2Cl2 and a photoluminescence quantum yield of 4.7%, which were slightly attenuated in coordinating tetrahydrofuran (THF) solutions. Nanosecond transient absorption spectroscopy results matched the transient photoluminescence kinetics enabling complete characterization of MLCT excited-state decay in these molecules. The calculated excited-state potential for the Cu2+/Cu+* couple (E = -1.74 V vs Fc+/0) indicated that [Cu(diptmp)2]+* is a strong photoreductant potentially useful for myriad applications. Ultrafast transient absorption measurements performed in THF solutions are also reported, yielding the relative time scales for both the pseudo-Jahn-Teller distortion (0.4-0.8 ps in [Cu(dipp)2]+ and 0.12-0.5 ps in [Cu(diptmp)2]+) and singlet-triplet intersystem crossing (6.4-10.1 ps for [Cu(dipp)2]+ and 3.5-5.4 ps for [Cu(diptmp)2]+) within these molecules. The disparity in the time scales of pseudo-Jahn-Teller distortion and intersystem crossing between two complexes with different anticipated excited-state geometries suggests that strongly impeded structural distortion in the MLCT excited state (i.e., [Cu(diptmp)2]+) enables more rapid surface crossings in the initial deactivation dynamics.}, number={20}, journal={INORGANIC CHEMISTRY}, author={Garakyaraghi, Sofia and Crapps, Peter D. and McCusker, Catherine E. and Castellano, Felix N.}, year={2016}, month={Oct}, pages={10628–10636} }
 @article{yarnell_mccusker_leeds_breaux_castellano_2016, title={Exposing the Excited-State Equilibrium in an Ir-III Bichromophore: A Combined Time Resolved Spectroscopy and Computational Study}, volume={2016}, ISSN={["1099-0682"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84962833574&partnerID=MN8TOARS}, DOI={10.1002/ejic.201600194}, abstractNote={Abstract}, number={12}, journal={EUROPEAN JOURNAL OF INORGANIC CHEMISTRY}, author={Yarnell, James E. and McCusker, Catherine E. and Leeds, Alexander J. and Breaux, Josue M. and Castellano, Felix N.}, year={2016}, month={Apr}, pages={1808–1818} }
 @misc{mccusker_castellano_2016, title={Materials Integrating Photochemical Upconversion}, volume={374}, ISSN={["2364-8961"]}, url={https://doi.org/10.1007/s41061-016-0021-7}, DOI={10.1007/s41061-016-0021-7}, abstractNote={This review features recent experimental work focused on the preparation and characterization of materials that integrate photochemical upconversion derived from sensitized triplet-triplet annihilation, resulting in the conversion of low energy photons to higher energy light, thereby enabling numerous wavelength-shifting applications. Recent topical developments in upconversion include encapsulating or rigidifying fluid solutions to give them mechanical strength, adapting inert host materials to enable upconversion, and using photoactive materials that incorporate the sensitizer and/or the acceptor. The driving force behind translating photochemical upconversion from solution into hard and soft materials is the incorporation of upconversion into devices and other applications. At present, some of the most promising applications of upconversion materials include imaging and fluorescence microscopy, photoelectrochemical devices, water disinfection, and solar cell enhancement.}, number={2}, journal={TOPICS IN CURRENT CHEMISTRY}, publisher={Springer Science and Business Media LLC}, author={McCusker, Catherine E. and Castellano, Felix N.}, year={2016}, month={Apr} }
 @article{mccusker_castellano_2015, title={Efficient Visible to Near-UV Photochemical Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex}, volume={54}, ISSN={["1520-510X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84935843196&partnerID=MN8TOARS}, DOI={10.1021/acs.inorgchem.5b00907}, abstractNote={The current investigation compares the photochemical upconversion sensitization properties of two long lifetime Cu(I) metal-to-ligand charge transfer (MLCT) chromophores to 3 distinct anthryl-based triplet acceptors. The sensitizers [Cu(dsbtmp)2](PF6) (1, dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) and [Cu(dsbp)2](PF6) (2, dsbp = 2,9-di(sec-butyl-1,10-phenanthroline) were selectively excited in the presence of anthracene, 9,10-diphenylanthracene (DPA), and 9,10-dimethylanthracene (DMA) in degassed dichloromethane solutions. In all instances, triplet energy transfer was observed from selective excitation of the Cu(I) MLCT chromophore to each respective anthryl species. The bimolecular triplet-triplet energy transfer quenching rate constants were extracted from dynamic Stern-Volmer analyses in each case, yielding values below the diffusion limit in dichloromethane. However, the Stern-Volmer quenching constants (KSV's) were sizable enough (up to ∼2300 M(-1) with 1 as a sensitizer) to support efficient photochemical upconversion. As such, visible to near-UV photochemical upconversion was observed in every instance, along with the anticipated quadratic-to-linear incident light power dependence when pumping at 488 nm. The latter verified that it is indeed sensitized triplet-triplet annihilation responsible for the generation of the anthryl-based singlet fluorescence. Photochemical upconversion quantum efficiencies were evaluated using a relative actinometric method as both a function of incident light power density as well as anthryl acceptor/annihilator concentration. When 1 was used as the sensitizer, upconversion quantum yields as large as 9.2% and 17.8% were observed for DMA and DPA, respectively. Finally, the combination of 1 with DMA was shown to be quite robust, showing no obvious signs of decomposition during 12 h of continuous 488 nm photolysis.}, number={12}, journal={INORGANIC CHEMISTRY}, author={McCusker, Catherine E. and Castellano, Felix N.}, year={2015}, month={Jun}, pages={6035–6042} }
 @article{castellano_mccusker_2015, title={MLCT sensitizers in photochemical upconversion: past, present, and potential future directions}, volume={44}, ISSN={["1477-9234"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84944184428&partnerID=MN8TOARS}, DOI={10.1039/c5dt03212a}, abstractNote={This frontier presentation highlights the historical development of MLCT sensitizers in photochemical upconversion while indentifying current state-of-the-art and exciting opportunities in this arena moving towards the future.}, number={41}, journal={DALTON TRANSACTIONS}, author={Castellano, Felix N. and McCusker, Catherine E.}, year={2015}, pages={17906–17910} }
 @article{garakyaraghi_danilov_mccusker_castellano_2015, title={Transient Absorption Dynamics of Sterically Congested Cu(I) MLCT Excited States}, volume={119}, ISSN={["1089-5639"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84926435446&partnerID=MN8TOARS}, DOI={10.1021/acs.jpca.5b00901}, abstractNote={Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.}, number={13}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Garakyaraghi, Sofia and Danilov, Evgeny O. and McCusker, Catherine E. and Castellano, Felix N.}, year={2015}, month={Apr}, pages={3181–3193} }
 @article{mccusker_chakraborty_castellano_2014, title={Excited State Equilibrium Induced Lifetime Extension in a Dinuclear Platinum(II) Complex}, volume={118}, ISSN={1089-5639 1520-5215}, url={http://dx.doi.org/10.1021/JP503827E}, DOI={10.1021/jp503827e}, abstractNote={Covalently linking two square planar platinum(II) centers using two pyrazolate bridging ligands allows the filled dz(2) orbitals on each Pt center to overlap, producing a Pt-Pt σ interaction and new low energy dσ* → π* metal-metal-to-ligand charge transfer (MMLCT) transitions terminating on an appropriate π-acceptor ligand such as 2-phenylpyridine (ppy). In an effort to extend the lifetime of the associated MMLCT excited state, we decided to append piperidinyl naphthalimide (PNI) chromophores to the 2-phenylpyridine charge transfer ligands. This structural modification introduces low-lying PNI-based triplet states serving as long-lived triplet population reservoirs, thermally capable of repopulating the charge transfer state at room temperature (RT), thereby extending its excited state lifetime. Specifically, [Pt(PNI-ppy)(μ-Ph2pz)]2 (1), where PNI-ppy is N-(2-phenylpyridine)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide and Ph2pz is 3,5-diphenylpyrazolate, was synthesized and structurally characterized. The static and dynamic photophysical behavior of 1 was directly compared to the MMLCT complex [Pt(ppy)(μ-Ph2pz)]2 (2), lacking the PNI substituents, as well as the naked PNI-ppy ligand 3, intended to independently model the MMLCT and NI excited state properties, respectively. Ultimately, experimental evidence for the presence of both the (3)PNI and (3)MMLCT excited states in 1 were revealed at RT in nanosecond transient absorbance and time-resolved photoluminescence spectroscopy, respectively. Temperature-dependent transient absorption spectroscopy permitted the extraction of an energy gap of 1740 cm(-1) between the MMLCT and PNI triplet states in 1 along with the time constants associated with the interconversions between the various excited states resident on this complex chromophore, ultimately decaying back to the ground state with a time constant of 65 μs at RT.}, number={45}, journal={The Journal of Physical Chemistry A}, publisher={American Chemical Society (ACS)}, author={McCusker, Catherine E. and Chakraborty, Arnab and Castellano, Felix N.}, year={2014}, month={Jun}, pages={10391–10399} }
 @article{mccusker_hablot_ziessel_castellano_2014, title={Triplet State Formation in Homo- and Heterometallic Diketopyrrolopyrrole Chromophores}, volume={53}, ISSN={["1520-510X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84914165949&partnerID=MN8TOARS}, DOI={10.1021/ic502169a}, abstractNote={The synthesis, structural characterization, and excited-state dynamics of series of diketopyrrolopyrrole (DPP) bridged homodinuclear Ir(III) and heterodinuclear Ir(III)/Pt(II) complexes is described. Steady-state and time-resolved photoluminescence along with transient absorption measurements were used to probe the nature of the emissive and long-lived excited states. Upon excitation into the (1)DPP ligand-localized excited state in the presence of coordinated Ir(III) or Pt(II) metal centers, the intersystem crossing is enhanced, leading to a quenching of the (1)DPP fluorescence and the formation of the long-lived (τ ≈ 30-40 μs) (3)DPP excited state in all instances.}, number={23}, journal={INORGANIC CHEMISTRY}, author={McCusker, Catherine E. and Hablot, Delphine and Ziessel, Raymond and Castellano, Felix N.}, year={2014}, month={Dec}, pages={12564–12571} }