Publications by authors named "Jeffrey J Rack"

48 Publications

A Future Perspective on Phototriggered Isomerizations of Transition Metal Sulfoxides and Related Complexes.

J Am Chem Soc 2021 Jan 5;143(2):526-537. Epub 2021 Jan 5.

Department of Chemistry and Chemical Biology, 300 Terrace Street NE, University of New Mexico, Albuquerque, New Mexico 87131-001 United States.

Photochromic molecules are examples of light-activated bistable molecules. We highlight the design criteria for a class of ruthenium and osmium sulfoxide complexes that undergo phototriggered isomerization of the bound sulfoxide. The mode of action in these complexes is an excited-state isomerization of the sulfoxide from S-bonded to O-bonded. We discuss the basic mechanism for this transformation and highlight specific examples that demonstrate the effectiveness and efficiency of the isomerization. We subsequently discuss future research directions within the field of phototriggered sulfoxide isomerizations on transition metal polypyridine complexes. These efforts involve new synthetic directions, including the choice of metal as well as new ambidentate ligands for isomerization.
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http://dx.doi.org/10.1021/jacs.0c08820DOI Listing
January 2021

Finely Designed P3HT-Based Fully Conjugated Graft Polymer: Optical Measurements, Morphology, and the Faraday Effect.

ACS Appl Mater Interfaces 2020 Jul 25;12(27):30856-30861. Epub 2020 Jun 25.

Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States.

In this work, our group synthesized and characterized a fully conjugated graft polymer comprising of a donor-acceptor molecular backbone and regioregular poly(3-hexylthiophene) (RRP3HT) side chains. Here, our macromonomer () was synthesized via Kumada catalyst transfer polycondensation reaction based on ditin-benzodithiophene (BDT) initiator. The tin content of was then investigated by inductively coupled plasma-mass spectrometry (ICP-MS), which allowed for accurate control of donor/acceptor monomer ratio of 1:1 for the following Stille coupling polymerization toward our graft polymer (). The structures of the polymers were then characterized by gel permeation chromatography (GPC), NMR, and elemental analysis. This was followed by the characterization of optical, electrochemical, and physical properties. The magneto-optical activity of graft polymer was then measured. It was found that, despite the presence of the acceptor backbone, the characteristic large Faraday rotation of RRP3HT was maintained in polymer , which exhibited a Verdet constant of 2.39 ± 0.57 (10) °/T·m.
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http://dx.doi.org/10.1021/acsami.0c08170DOI Listing
July 2020

Luminescent lanthanide complexes with a pyridine-bis(carboxamide)-bithiophene sensitizer showing wavelength-dependent singlet oxygen generation.

Dalton Trans 2020 May 6;49(20):6661-6667. Epub 2020 May 6.

Department of Chemistry, University of Nevada, Reno, Nevada 89557, USA.

A new pyridine-bis(carboxamide)-based ligand with a bithiophene pendant, 2Tcbx, was synthesized. Its lanthanide ion (Ln) complexes, [Ln(2Tcbx)], were isolated and their photophysical properties were explored. Upon excitation at 360 nm, these complexes display emission in the near-infrared (NIR) with efficiencies of 0.69% for Ln = Yb, 0.20% for Ln = Nd, and 0.01% for Ln = Er, respectively. Concurrent O formation was seen for all complexes, with efficiencies of 19% for the Yb complex, 25% for the Nd complex, and 9% for the Er complex. When exciting at a longer wavelength, 435 nm, only Ln emission was observed and larger efficiencies of Ln-centered emission were obtained. The lack of O generation indicates that energy pathways involving different ligand conformations, which were investigated by transient absorption spectroscopy, are involved in the sensitization process, and enable the wavelength-dependent generation of O.
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http://dx.doi.org/10.1039/d0dt01034kDOI Listing
May 2020

Photoinduced ligand dissociation follows reverse energy gap law: nitrile photodissociation from low energy MLCT excited states.

Chem Commun (Camb) 2020 Apr 11;56(29):4070-4073. Epub 2020 Mar 11.

Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.

A series of Ru(ii)-terpyridine complexes containing electron-donating bidentate ligands are able to effectively photodissociate nitrile ligands using red light. A spectroscopic investigation of these complexes reveal that they follow anti-energy gap law behavior, providing further evidence that population of LF excited states is not necessary for photoinduced nitrile dissociation.
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http://dx.doi.org/10.1039/c9cc10095dDOI Listing
April 2020

Wavelength-Dependent Singlet Oxygen Generation in Luminescent Lanthanide Complexes with a Pyridine-Bis(Carboxamide)-Terthiophene Sensitizer.

Chemistry 2020 Jun 19;26(32):7274-7280. Epub 2020 May 19.

Department of Chemistry, University of Nevada, Reno, Reno, NV, 89557, USA.

Lanthanide ion (Ln ) complexes, [Ln(3Tcbx) ] (Ln =Yb , Nd , Er ) are isolated with a new pyridine-bis(carboxamide)-based ligand with a 2,2':5',2''-terthiophene pendant (3TCbx), and their resulting photophysical properties are explored. Upon excitation of the complexes at 490 nm, only Ln emission is observed with efficiencies of 0.29 % at 976 nm for Ln =Yb and 0.16 % at 1053 nm for Ln =Nd . Er emission is observed but weak. Upon excitation at 400 nm, concurrent O formation is seen, with efficiencies of 11 % for the Yb and Nd complexes and 13 % for the Er complex. Owing to the concurrent generation of O , as expected, the efficiency of metal-centered emission decreases to 0.02 % for Yb and 0.05 % for Nd . The ability to control O generation through the excitation wavelength indicates that the incorporation of 2,2':5',2''-terthiophene results in access to multiple sensitization pathways. These energy pathways are unraveled through transient absorption spectroscopy.
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http://dx.doi.org/10.1002/chem.202000587DOI Listing
June 2020

Trifluoromethylated Phenanthroline Ligands Reduce Excited-State Distortion in Homoleptic Copper(I) Complexes.

Inorg Chem 2020 Mar 12;59(5):2781-2790. Epub 2020 Feb 12.

Department of Chemistry and Chemical Biology, The University of New Mexico, Albuquerque, New Mexico 87131, United States.

We report the synthesis and excited-state dynamics for a series of homoleptic copper(I) trifluoromethylated phenanthroline complexes with two, three, and four trifluoromethyl functional groups. Our analysis of the steady-state absorbance and emission, transient-absorption spectroscopy, and electronic-structure-theory calculations results enable in-depth analysis of the pseudo-Jahn-Teller distortion inhibition from increased steric hindrance of the trifluoromethyl functional group relative to the prototypical dimethyl phenanthroline complex. Surprisingly, our results demonstrate that the greatest degree of pseudo-Jahn-Teller distortion inhibition is achieved with trifluoromethylation of only the 2 and 9 positions by an unusual combination of steric hindrance and stabilization of a nondistorted MLCT manifold observed by transient kinetic lifetimes and optimized excited-state structures. The intersystem-crossing (ISC) lifetime for the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex is 69 ps, while the triplet excited-state lifetime and emission quantum yield are 106 ns and 4 × 10, respectively. Further trifluoromethylation of the phenanthroline yields a greater σ bond inductive withdrawing force on the phenanthroline nitrogens, ultimately resulting in weaker coordination to the copper. Last, the surprising success of the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex by adjusting both ligand sterics and electronic properties outlines a new strategy for developing long-lived Cu(I) charge-transfer complexes.
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http://dx.doi.org/10.1021/acs.inorgchem.9b03146DOI Listing
March 2020

Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an Fe to Ti Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States.

Inorg Chem 2019 Nov 5;58(22):15320-15329. Epub 2019 Nov 5.

Department of Chemistry and Chemical Biology , University of New Mexico , Albuquerque , New Mexico 87131 , United States.

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising Fe donors and Ti acceptors with the general formula CpTi(CFc) (where Cp = Cp*, Cp, and Cp) and CpTi(CFc)(CR) (where R = Ph or CF) are reported. The transient absorption spectra are consistent with an Fe/Ti metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (MMCT) excited state for the Fe/Ti complexes is not purely MMCT in nature but is contaminated with the higher-energy Fc (d-d) state. For the diferrocenyl complexes, CpTi(CFc), the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the Cp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased Fc character. The effect of Cu coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in CpTi(CFc)·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti-Fe-Cu.
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http://dx.doi.org/10.1021/acs.inorgchem.9b02316DOI Listing
November 2019

Large Excited-State Conformational Displacements Expedite Triplet Formation in a Small Conjugated Oligomer.

J Phys Chem Lett 2019 Mar 4;10(6):1259-1263. Epub 2019 Mar 4.

Department of Chemistry and Chemical Biology , University of New Mexico , Albuquerque , New Mexico 87131 , United States.

Intersystem crossing in conjugated organic molecules is most conveniently viewed from pure electronic perspectives; yet, vibrational displacements may often drive these transitions. We investigate an alkyl-substituted thienylene-vinylene dimer (dTV) displaying efficient triplet formation. Steady-state electronic and Raman spectra display large Stokes shifts (∼4000 cm) involving high-frequency skeletal symmetric stretching modes (∼900-1600 cm) in addition to large displacements of low-frequency torsional motions (∼300-340 cm). Transient absorption spectroscopy reveals the emergence of distorted singlet (S) and triplet signatures following initial vibrational relaxation dynamics that dominate spectral dynamics on time scales > 100 ps, with the latter persisting on time scales up to ca. 7 μs. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.
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http://dx.doi.org/10.1021/acs.jpclett.9b00495DOI Listing
March 2019

Generating Photonastic Work from Irradiated Dyes in Electrospun Nanofibrous Polymer Mats.

ACS Appl Mater Interfaces 2018 Oct 17;10(43):37470-37477. Epub 2018 Oct 17.

Department of Chemistry and Chemical Biology , The University of New Mexico , Albuquerque , New Mexico 87131 , United States.

For solar-driven macroscopic motions, we assert that there is a local heating that facilitates large-scale deformations in anisotropic morphologic materials caused by thermal gradients. This report specifically identifies the fate of heat generation in photonastic materials and demonstrates how heat can perform work following excitation of a nonisomerizing dye. Utilizing the electrospinning technique, we have created a series of anisotropic nanofibrous polymer mats that comprise nonisomerizing dyes. Polymers are chosen because of their relative glass transition temperatures, elastic moduli, and melting temperatures. Light irradiation of these polymer mats with an excitation wavelength matching the absorption characteristics of the dye leads to macroscopic deformation of the mat. Analysis of still images extracted from digital videos provides plots of angular displacement vs power. The data were analyzed in terms of a photothermal model. Analyses of scanning electron microscopy micrographs for all samples are consistent to local melting in low T polymers and softening in high T polymers. Dynamic mechanical analysis allowed for quantification of the modulus change under a given light fluence. We employ these data to calculate a energy conversion efficiency. These efficiencies for the polymer mats are compared to other nonmuscular systems, including a few natural, biological samples.
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http://dx.doi.org/10.1021/acsami.8b11294DOI Listing
October 2018

Unravelling the enigma of ultrafast excited state relaxation in non-emissive aggregating conjugated polymers.

Phys Chem Chem Phys 2018 Aug;20(34):22159-22167

Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA.

We investigate a class of non-emissive conjugated polymers with very short excited state lifetimes believed to undergo singlet fission and relaxation to mid-gap forbidden excited states. Poly(3-decylthieneylenvinylene) (P3DTV) and its heavy atom analog, poly(3-decylseleneylenvinylene) (P3DSV), are strongly aggregating conjugated polymers that experience large excited state displacements along multiple vibrational modes. We demonstrate this Franck-Condon vibrational activity effectively disperses excitation energy into multiple non-radiative channels that can be explained using a simple, two-state potential energy surface model. Resonance Raman spectroscopy is sensitive to early Franck-Condon vibrational activity and we observe rich harmonic progressions involving multiple high frequency CC backbone symmetric stretching motions (∼1000-1600 cm-1) in both systems reflecting mode-specific excited state geometrical displacements. Transient absorption spectra confirm that efficient non-radiative processes dominate excited state relaxation dynamics which are confined to π-stacked aggregated chains. Surprisingly, we found little influence of the heteroatom consistent with efficient vibrational energy dissipation. Our results highlight the importance of aggregation and multi-dimensional Franck-Condon vibrational dynamics on the ability to harvest excitons, which are not usually considered in materials design and optimization schemes.
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http://dx.doi.org/10.1039/c8cp04061cDOI Listing
August 2018

Controlling Photoisomerization Reactivity Through Single Functional Group Substitutions in Ruthenium Phosphine Sulfoxide Complexes.

J Am Chem Soc 2018 08 30;140(31):9819-9822. Epub 2018 Jul 30.

Department of Chemistry and Chemical Biology , 1 University of New Mexico , Albuquerque , New Mexico 87131 , United States.

We report the crystallography, emission spectra, femtosecond pump-probe spectroscopy, and density functional theory computations for a series of ruthenium complexes that comprise a new class of chelating triphenylphosphine based ligands with an appended sulfoxide moiety. These ligands differ only in the presence of the para-substitutent (e.g., H, OCH, CF). The results show a dramatic range in photoisomerization reactivity that is ascribed to differences in the electron density of the phosphine ligand donated to the ruthenium and the nature of the excited state.
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http://dx.doi.org/10.1021/jacs.8b05957DOI Listing
August 2018

Transient metal-centered states mediate isomerization of a photochromic ruthenium-sulfoxide complex.

Nat Commun 2018 05 18;9(1):1989. Epub 2018 May 18.

Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan, 46241, Republic of Korea.

Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.
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http://dx.doi.org/10.1038/s41467-018-04351-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5959936PMC
May 2018

Linkage Photoisomerization of an Isolated Ruthenium Sulfoxide Complex: Sequential versus Concerted Rearrangement.

Inorg Chem 2018 May 17;57(9):5701-5706. Epub 2018 Apr 17.

School of Chemistry , University of Melbourne , Parkville , Victoria 3010 , Australia.

Ruthenium sulfoxide complexes undergo thermally reversible linkage isomerization of sulfoxide ligands from S- to O-bound in response to light. Here, we report photoisomerization action spectra for a ruthenium bis-sulfoxide molecular photoswitch, [Ru(bpy)(bpSO)], providing the first direct evidence for photoisomerization of a transition metal complex in the gas phase. The linkage isomers are separated and isolated in a tandem drift tube ion mobility spectrometer and exposed to tunable laser radiation provoking photoisomerization. Direct switching of the S,S-isomer to the O,O-isomer following absorption of a single photon is the predominant isomerization pathway in the gas phase, unlike in solution, where stepwise isomerization is observed with each sulfoxide ligand switching in turn. The change in isomerization dynamics is attributed to rapid vibrational quenching that suppresses isomerization in solution. Supporting electronic structure calculations predict the wavelengths and intensities of the peaks in the photoisomerization action spectra of the S,S- and S,O-isomers, indicating that they correspond to metal-to-ligand charge transfer (MLCT) and ligand-centered ππ* transitions.
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http://dx.doi.org/10.1021/acs.inorgchem.8b00871DOI Listing
May 2018

Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA); Synthesis and Structure of MTTA (M = Zr, Hf, Ce, Th, U, Np, Pu) and M(TTA) (M = Ce, Nd, Sm, Yb).

Inorg Chem 2018 Apr 21;57(7):3782-3797. Epub 2018 Mar 21.

University of New Mexico , Albuquerque , New Mexico 87131 , United States.

Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating microscopic amounts of transuranic actinides (i.e., Np and Pu) from macroscopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extraction-or find alternatives-because little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (M = Zr, Hf, Ce, Th, U, Np, and Pu vs M = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula M(TTA). Meanwhile, +3 metals formed anionic M(TTA) species. Characterization of these M(TTA) ( x = 0, 1) compounds by UV-vis-NIR, IR, H and F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that Np(TTA) and Pu(TTA) were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between M(TTA) and M(TTA) are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np to Pu, Hf, and Zr.
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http://dx.doi.org/10.1021/acs.inorgchem.7b03089DOI Listing
April 2018

"Roller-Wheel"-Type Pt-Containing Small Molecules and the Impact of "Rollers" on Material Crystallinity, Electronic Properties, and Solar Cell Performance.

J Am Chem Soc 2017 10 21;139(40):14109-14119. Epub 2017 Aug 21.

Department of Chemistry & Chemical Biology, University of New Mexico , Albuquerque, New Mexico 87131, United States.

We report the synthesis, characterization, and detailed comparison of a series of novel Pt-bisacetylide containing conjugated small molecules possessing an unconventional "roller-wheel" shaped structure that is distinctly different from the "dumbbell" designs in traditional Pt-bisacetylide containing conjugated polymers and small molecules. The relationships between the chemical nature and length of the "rollers" and the electronic and physical properties of the materials are carefully studied by steady-state spectroscopy, cyclic voltammetry, differential scanning calorimetry, single-crystal X-ray diffraction, transient absorption spectroscopy, theoretical calculation, and device application. It was revealed that if the roller are long enough, these molecules can "slip-stack" in the solid state, leading to high crystallinity and charge mobility. Organic solar cells were fabricated and showed power conversion efficiencies up to 5.9%, out-performing all existing Pt-containing materials. The device performance was also found to be sensitive to optimization conditions and blend morphologies, which are a result of the intricate interplay among materials crystallinity, phase separation, and the relative positions of the lowest singlet and triplet excited states.
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http://dx.doi.org/10.1021/jacs.7b05801DOI Listing
October 2017

A "roller-wheel" Pt-containing small molecule that outperforms its polymer analogs in organic solar cells.

Chem Sci 2016 Sep 23;7(9):5798-5804. Epub 2016 May 23.

Department of Chemistry & Chemical Biology , University of New Mexico , MSC03 2060, 1 UNM , Albuquerque , NM 87131 , USA . Email:

A novel Pt-bisacetylide small molecule () featuring "roller-wheel" geometry was synthesized and characterized. When compared with conventional Pt-containing polymers and small molecules having "dumbbell" shaped structures, displays enhanced crystallinity and intermolecular π-π interactions, as well as favorable panchromatic absorption behaviors. Organic solar cells (OSCs) employing achieve power conversion efficiencies (PCEs) up to 5.9%, the highest reported so far for Pt-containing polymers and small molecules.
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http://dx.doi.org/10.1039/c6sc00513fDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6021780PMC
September 2016

Complexes with Tunable Intramolecular Ferrocene to Ti(IV) Electronic Transitions: Models for Solid State Fe(II) to Ti(IV) Charge Transfer.

Inorg Chem 2016 Mar 16;55(5):2200-11. Epub 2016 Feb 16.

Department of Chemistry, Furman University , Greenville, South Carolina 29613, United States.

Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti(IV) has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, (TMS)Cp, and comprise the complexes (TMS)Cp2Ti(C2Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)2, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti(IV/III) reduction wave and an Fe(II/III) oxidation that is irreversible for all complexes except (TMS)Cp2Ti(C2Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M(-1)cm(-1) < ε < 8000 M(-1)cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti(IV/III) and Fe(III/II) reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm(-1) < λ < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti(IV) and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)2 rapidly decomposes upon photolysis.
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http://dx.doi.org/10.1021/acs.inorgchem.5b02587DOI Listing
March 2016

Excited state dynamics and isomerization in ruthenium sulfoxide complexes.

Acc Chem Res 2015 Apr 11;48(4):1115-22. Epub 2015 Mar 11.

Nanoscale and Quantum Phenomena Institute, Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States.

Molecular photochromic compounds are those that interconvert between two isomeric forms with light. The two isomeric forms display distinct electronic and molecular structures and must not be in equilibrium with one another. These light-activated molecular switch compounds have found wide application in areas of study ranging from chemical biology to materials science, where conversion from one isomeric form to another by light prompts a response in the environment (e.g., protein or polymeric material). Certain ruthenium and osmium polypyridine sulfoxide complexes are photochromic. The mode of action is a phototriggered isomerization of the sulfoxide from S- to O-bonded. The change in ligation drastically alters both the spectroscopic and electrochemical properties of the metal complex. Our laboratory has pioneered the preparation and study of these complexes. In particular, we have applied femtosecond pump-probe spectroscopy to reveal excited state details of the isomerization mechanism. The data from numerous complexes allowed us to predict that the isomerization was nonadiabatic in nature, defined as occurring from a S-bonded triplet excited state (primarily metal-to-ligand charge transfer in character) to an O-bonded singlet ground state potential energy surface. This prediction was corroborated by high-level density functional theory calculations. An intriguing aspect of this reactivity is the coupling of nuclear motion to the electronic wave function and how this coupling affects motions productive for isomerization. In an effort to learn more about this coupling, we designed a project to examine phototriggered isomerization in bis-sulfoxide complexes. The goal of these studies was to determine whether certain complexes could be designed in which a single photon excitation event would prompt two sulfoxide isomerizations. We employed chelating sulfoxides in this study and found that both the nature of the chelate ring and the R group on the sulfoxide affect the photochemical reactivity. For example, this reactivity may be tuned such that two sulfoxide ligands isomerize sequentially following two successive excitations or that two sulfoxide ligands isomerize following a single excitation. This Account explains our understanding to date of this photochemistry.
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http://dx.doi.org/10.1021/ar500396aDOI Listing
April 2015

Ultrafast spectroscopy and structural characterization of a photochromic isomerizing ruthenium bis-sulfoxide complex.

Dalton Trans 2014 Dec;43(47):17847-55

Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University, Clippinger Laboratories, Athens, OH 45701, USA.

Irradiation of [Ru(bpy)2(bpSOp)](PF6)2 (where bpy is 2,2'-bipyridine and bpSOp is 1,3-bis(phenylsulfinyl)propane) results in the formation of two new isomers, namely the S,O- and O,O-bonded species. The crystal structure of the bis-thioether and bis-sulfoxide complexes are reported. NMR spectroscopy of the bis-thioether complex in solution is consistent with the molecular structure determined by diffraction methods. Further, NMR spectroscopy of the bis-sulfoxide complex reveals two conformers in solution, one that is consistent with the solid state structure and a second conformer showing distortion in the aliphatic portion of the chelate ring. Time-resolved visible absorption spectroscopy reveals isomerization time constants of 91 ps in dichloroethane (DCE) and 229 ps in propylene carbonate (PC). Aggregate isomerization quantum yields of 0.57 and 0.42 have been determined in DCE and in PC, respectively. The kinetics of the thermal reversion from the O,O- to S,O-bonded isomer are strongly solvent dependent, occurring with rates of 2.41 × 10(-3) and 4.39 × 10(-5) s(-1) in DCE, and 4.68 × 10(-4) and 9.79 × 10(-6) s(-1) in PC. The two kinetic components are assigned to the two isomers identified in solution.
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http://dx.doi.org/10.1039/c4dt01402bDOI Listing
December 2014

Investigating the effects of solvent on the ultrafast dynamics of a photoreversible ruthenium sulfoxide complex.

J Phys Chem A 2014 Nov 19;118(45):10425-32. Epub 2014 Aug 19.

Nanoscale and Quantum Phenomena Institute, Department of Chemistry and Biochemistry, Ohio University , Athens, Ohio 45701, United States.

The photochromic complex [Ru(bpy)2(pySO)](2+) [pySO is 2-(isopropylsulfinylmethyl)pyridine] undergoes wavelength specific, photoreversible S → O and O → S linkage isomerizations. Irradiation of the ground state S-bonded complex with blue light produces the O-bonded isomer, while irradiation of the O-bonded isomer with green light produces the S-bonded isomer. Furthermore, isomerization time constants are solvent-dependent. Ultrafast transient absorption spectroscopy has been employed to investigate the relaxation processes that lead to S → O isomerization in 1,2-dichloroethane, propylene carbonate, and ethylene glycol. The isomerization is most rapid in 1,2-dichloroethane and slowest in ethylene glycol. Photochemical reversion of the O-bonded isomer in propylene carbonate has further been investigated and indicates similar relaxation or isomerization kinetics, though the excited states that lead to isomerization are distinct between the S- and O-bonded isomers.
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http://dx.doi.org/10.1021/jp504078gDOI Listing
November 2014

Sn(IV) Schiff base complexes: triplet photosensitizers for photoredox reactions.

Dalton Trans 2014 Dec;43(47):17754-65

Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.

We present the synthesis and characterization of a series of four fluorescent Sn(iv) Schiff base complexes, which also possess long-lived triplet excited states. The complexes absorb visible light (λmax = 420 to 462 nm) and the optical properties are easily tunable without laborious synthetic elaboration. The triplet excited states are not luminescent, but can be observed and followed using nanosecond transient absorption spectroscopy. The lifetimes of the triplet excited states are on the order of 500 μs-10 ms in PMMA matrices. The triplet state energies were estimated via energy transfer reactions with a series of organic triplet acceptors. In addition, the photoexcited complexes react with electron donors and acceptors in solution. These results demonstrate the potential for the development of photosensitizers based on main group elements with high spin orbit coupling constants.
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http://dx.doi.org/10.1039/c4dt01427hDOI Listing
December 2014

One photon yields two isomerizations: large atomic displacements during electronic excited-state dynamics in ruthenium sulfoxide complexes.

J Am Chem Soc 2014 Feb 27;136(5):1856-63. Epub 2014 Jan 27.

Nanoscale and Quantum Phenomena Institute, Department of Chemistry and Biochemistry, Ohio University , Athens, Ohio 45701, United States.

Photochromic compounds efficiently transduce photonic energy to potential energy for excited-state bond-breaking and bond-forming reactions. A critical feature of this reaction is the nature of the electronic excited-state potential energy surface and how this surface facilitates large nuclear displacements and rearrangements. We have prepared two photochromic ruthenium sulfoxide complexes that feature two isomerization reactions following absorption of a single photon. We show by femtosecond transient absorption spectroscopy that this reaction is complete within a few hundred picoseconds and suggest that isomerization occurs along a conical intersection seam formed by the ground-state and excited-state potential energy surfaces.
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http://dx.doi.org/10.1021/ja409262rDOI Listing
February 2014

Tuning excited state isomerization dynamics through ground state structural changes in analogous ruthenium and osmium sulfoxide complexes.

Chemistry 2013 Aug 16;19(35):11686-95. Epub 2013 Jul 16.

Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Clippinger Laboratories, Ohio University, Athens, OH 45701, USA.

The complexes [Ru(bpy)2(pyESO)](PF6)2 and [Os(bpy)2(pyESO)](PF6)2, in which bpy is 2,2'-bipyridine and pyESO is 2-((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by (1)H NMR, UV-visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X-ray structural analysis were grown for [Ru(bpy)2(pyESO)](PF6)2. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical-chemical electrochemical-chemical) mechanism in which isomerization follows Ru(2+) oxidation and Ru(3+) reduction. The S- and O-bonded Ru(3+/2+) couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)2(pyESO)](PF6)2, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge-transfer excitation of [Ru(bpy)2(pyESO)](PF6)2 results in a significant change in the absorption spectrum. The S-bonded isomer of [Ru(bpy)2(pyESO)](2+) features a lowest energy absorption maximum at 390 nm and the O-bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)2(pyESO)](2+) was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)2(pyESO)](2+). These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.
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http://dx.doi.org/10.1002/chem.201301553DOI Listing
August 2013

Sequential picosecond isomerizations in a photochromic ruthenium sulfoxide complex triggered by pump-repump-probe spectroscopy.

Inorg Chem 2013 Feb 6;52(4):2086-93. Epub 2013 Feb 6.

Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University, Clippinger Laboratories, Athens, Ohio 45701, United States.

The complex [Ru(bpy)(2)(bpSO)](PF(6))(2), where bpy is 2,2'-bipydine and bpSO is 1,2-bis(phenylsulfinyl)ethane, exhibits three distinct isomers which are accessible upon metal-to-ligand charge-transfer (MLCT) irradiation. This complex and its parent, [Ru(bpy)(2)(bpte)](PF(6))(2), where bpte is 1,2-bis(phenylthio)ethane, have been synthesized and characterized by UV-visible spectroscopy, NMR, X-ray crystallography, and femtosecond transient absorption spectroscopy. A novel method of 2-color Pump-Repump-Probe spectroscopy has been employed to investigate all three isomers of the bis-sulfoxide complex. This method allows for observation of the isomerization dynamics of sequential isomerizations of each sulfoxide from MLCT irradiation of the S,S-bonded complex to ultimately form the O,O-bonded metastable complex. One-dimensional (1-D) and two-dimensional (2-D) (COSY, NOESY, and TOCSY) (1)H NMR data show the thioether and ground state S,S-bonded sulfoxide complexes to be rigorously C(2) symmetric and are consistent with the crystal structures. Transient absorption spectroscopy reveals that the S,S to S,O isomerization occurs with an observed time constant of 56.8 (±7.4) ps. The S,O to O,O isomerization time constant was found to be 59 (±4) ps by pump-repump-probe spectroscopy. The composite S,S- to O,O-isomer quantum yield is 0.42.
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http://dx.doi.org/10.1021/ic302489aDOI Listing
February 2013

pH control of intramolecular energy transfer and oxygen quenching in Ru(II) complexes having coupled electronic excited states.

J Am Chem Soc 2012 May 24;134(17):7497-506. Epub 2012 Apr 24.

Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.

This work illustrates the control of excited state energy transfer processes via variation of pH in transition metal complexes. In these systems a Ru(II) complex having two carboxylated bipyridyl ligands is covalently linked to pyrene via one of two different pyrene derivitized bipyridyl ligands. The energy of the Ru to carboxy-bipyridine (3)MLCT state is pH dependent while the pyrene triplet energy remains unchanged with solution acidity. At pH 0 the (3)MLCT state is the lowest energy state, and as the pH is raised and the carboxy-bipyridyl ligands are successively deprotonated, the energy of the (3)MLCT state rises above that of the pyrene triplet, resulting in a significant increase in the lifetime of the observed emission. Detailed analysis of ultrafast and microsecond time-resolved excited state decays result in a description of excited state decay that involves initial equilibration of the (3)MLCT and pyrene triplet states followed by relaxation to the ground state. The lifetime of excited state decay is defined by the position of the equilibrium, going from 2 μs at pH 0 to >10 μs at higher pH as the equilibrium favors the pyrene triplet. In addition, quenching of the excited state by dissolved oxygen exhibits a pH dependence that parallels that of the excited state lifetime. The results illustrate the utility of exploiting excited state equilibria of this type in the development of highly effective luminescent oxygen sensors.
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http://dx.doi.org/10.1021/ja300866sDOI Listing
May 2012

Bending materials with light: photoreversible macroscopic deformations in a disordered polymer.

Adv Mater 2011 Oct;23(37):4312-7

Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Clippinger Laboratories, Ohio University, Athens, OH 45701, USA.

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http://dx.doi.org/10.1002/adma.201102157DOI Listing
October 2011

Ultrafast spectroscopy of a photochromic ruthenium sulfoxide complex.

Inorg Chem 2011 Aug 19;50(16):7586-90. Epub 2011 Jul 19.

Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701, USA.

Photochromic [Ru(bpy)(2)(OSO)](PF(6)), where bpy is 2,2'-bipyridine and OSO is 2-methylsulfinylbenzoate, was investigated by femtosecond transient absorption spectroscopy. The results show that for both S- and O-bonded isomers, a (3)MLCT state is formed on a femtosecond time scale. Also observed is the formation of multiple metal-to-ligand charge-transfer (MLCT) states, representing different conformers, prior to isomerization on the picosecond time scale. These results and others are compiled in an energy diagram depicting these results.
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http://dx.doi.org/10.1021/ic200532pDOI Listing
August 2011

Thermal stability, photochromic sensitivity and optical properties of [Ru(bpy)(2)(OSOR)]+ compounds with R = Bn, BnCl, BnMe.

Opt Express 2010 Oct;18(22):23495-503

Department of Physics, University of Osnabrück, Barbarastr 7, 49069 Osnabrück, Germany.

The influence of ligand substitution on the photochromic properties of [Ru(bpy)(2)(OSOR)]∙PF(6) compounds (bpy = 2,2'-bipyridine, OSO = 2-methylsulfinylbenzoate) dissolved in propylene carbonate is studied by UV/VIS laser-spectroscopy as a function of temperature and exposure. The group R is either Bn (CH(2)C(6)H(5)), BnCl or BnMe. The photochromic properties originate from a phototriggered linkage isomerization located at the SO ligand. It is shown that the thermal stability of the studied compounds can be varied by ligand substitution in the range of 1.6 × 10(3) s to 3.9 × 10(4) s. In contrast, absorption spectra of ground and metastable states as well as the characteristic exposure of the photochromic response remain unchanged. The results are explained in the frame of photoinduced linkage isomerization located at the SO ligand.
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http://dx.doi.org/10.1364/OE.18.023495DOI Listing
October 2010

Sulfoxide stretching mode as a structural reporter via dual-frequency two-dimensional infrared spectroscopy.

J Chem Phys 2010 Oct;133(14):144513

Department of Chemistry, Tulane University, New Orleans, Los Angeles 70118, USA.

The S=O stretching mode in sulfoxides, having a frequency in the 950-1150 cm(-1) range, is tested as a structural label via dual-frequency two-dimensional infrared (2DIR) spectroscopy. The properties of this structural reporter are studied in several compounds, including (4,4(')-dimethyl-2,2(')-bipyridyl)(o-methylsulfinylbenzoate) ruthenium II, [Ru(dmb)(2)(BzSO)](+), (RuBzSO), octylsulfinylpropionic acid (OSPA), and o- and p-methylsulfinyl-benzoic acid (oMSBA and pMSBA). The mode assignment in the fingerprint region for these compounds is made using a combination of density functional theory calculations and 2DIR and relaxation-assisted 2DIR (RA 2DIR) spectroscopies. The SO stretching mode frequency and IR intensity demonstrate substantial sensitivity to the molecular structure. Multiple cross peaks of the C=O and S=O stretching modes with modes in the fingerprint region (930-1450 cm(-1)) were recorded. The 2DIR and RA 2DIR spectra focusing at interactions of a high-frequency mode of a ligand with the modes in the fingerprint region provide a spectral fingerprint of a compound and help mode assignment in the often congested fingerprint region. The cross-peak amplitudes in oMSBA, pMSBA, and OSPA were compared with the theoretical predictions based on the computed values for the off-diagonal anharmonicities and a reasonable match is found. The SO stretching mode provides means for assigning other modes in the fingerprint region and constitutes a promising structural reporter for the 2DIR and RA 2DIR spectroscopy measurements.
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http://dx.doi.org/10.1063/1.3482708DOI Listing
October 2010

Solvent effects on isomerization in a ruthenium sulfoxide complex.

Inorg Chem 2010 May;49(10):4466-70

Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701, USA.

We report the structure, electrochemistry, and isomerization kinetics for [Ru(bpy)(biq)(OSO)](PF(6)), where bpy is 2,2'-bipyridine, biq is 2,2'-biquinoline, and OSO is 2-methylsulfinylbenzoate. UV-visible and infrared data are suggestive of intramolecular S-->O and O-->S isomerization of the sulfoxide. Cyclic voltammetry reveals evidence for isomerization triggered by oxidation and reduction. Of particular note is the variation of the S-->O isomerization rate constant in different solvents. The rates were found to be 3.2 (+/-0.4) s(-1) in propylene carbonate, 0.80 (+/-0.03) s(-1) in acetonitrile, and 0.26 (+/-0.01) s(-1) in dichloromethane.
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http://dx.doi.org/10.1021/ic902047fDOI Listing
May 2010