
Paul Wagenknecht
Professor, Chemistry
- Email: paul.wagenknecht@furman.edu
- Phone: 864.294.2905
- Office: Plyler 244E, Townes Science Center
Paul Wagenknecht grew up in Clearwater, Florida and obtained his bachelor of science in chemistry from Furman University in 1986. He was awarded a graduate fellowship from the National Science Foundation to attend Stanford University, and received his Ph.D. in Inorganic Chemistry in 1991.
Following postdoctoral studies at Colorado State University, he accepted a one-year adjunct teaching position at Occidental College in Los Angeles, California before beginning a tenure track position at San Jose State University in 1996. In 2004, he moved back to his alma mater, Furman University. Over his career, he has secured more than $2 million in external funding for support of undergraduate research in his group and department from agencies including the National Science Foundation, National Institutes of Health, William F. Keck Foundation, Arnold and Mabel Beckman Foundation, Research Corporation, Camille and Henry Dreyfus Foundation, and American Chemical Society.
Since beginning his independent career with undergraduate researchers, he has published more than 25 peer-reviewed research articles (mostly with student coauthors) and two patents. When not in the classroom or laboratory, he enjoys competing on the tennis courts and BBQ circuit (as AlQuemy BBQ).
Education
- Ph.D., Stanford University
- B.S., Furman University
Research
Adventures in Transition Metal Photophysics
State-of-the-art phosphorescent materials are integral to devices such as flat-screen displays and modern energy efficient lighting. Such materials efficiently convert electricity to light. The reverse process, the conversion of light into electricity, is perhaps even more technologically desirable. Our group studies complexes of metals such as iron, titanium, ruthenium, chromium, iridium, and rhodium as possible materials to improve these technologies.
MMCT in donor-π-bridge-acceptor complexes
Charge-transfer (CT) excited states where charge is separated over a long distance are interesting from a fundamental standpoint and for applications involving solar energy conversion and photocatalysis. Metal-to-metal CT (MMCT) excited states fall into this class and have been exploited for such applications. In particular, photocatalysis involving MMCT excited states in solid state Ti-O-M architectures involving TiIV acceptors oxo-bridged to donors (M) such as FeII, CrIII, and MnII has recently been investigated. Of additional relevance is that ferrocyanide photosensitizes TiO2, with the photosensitization being ascribed to an FeII to TiIV charge transfer. Furthermore, FeII to TiIVcharge transfer has been implicated as being responsible for the color observed in minerals such as blue sapphire (a corundum based gemstone) and blue kyanite (an aluminosilicate based gemstone). Our group is interested in molecular examples of FeII to TiIV charge transfer (Figure 1) and is involved in a systematic investigation of such complexes. These investigations involve collaborations with research groups outside of Furman who are experts in computational chemistry and ultrafast spectroscopy.
Figure 1. Adjusting the electron density at TiIV tunes the low energy absorption in a manner consistent with the assignment of this band as an FeII to TiIV MMCT. The MMCT is solvatochromic, shifting to lower energy with increased solvent polarizability. The reduction potential suggests the transient TiIII species is appropriate for electron injection into TiO2.
Photophysics of complexes with electron deficient alkyne ligands
Many transition metal complexes with alkynyl ligands are investigated for their luminescent properties. Of particular interest is adjusting the electronic properties of the metal/ligands, as this provides for the variation of colors observed in many complexes used in organic light emitting diodes (OLEDs). One common alkynyl ligand used to diminish the electron density at the metal is the pentafluorophenylethynyl ligand (C6F5C≡C–). Our group is interested in even more electron deficient alkyne ligands and has turned to the trifluoropropynyl ligand (CF3C≡C–). Electronically, this ligand behaves much like the cyano ligand. However, emission from a Rh(III) complex with this ligand, trans-[Rh(cyclam)(C2CF3)2]+, shows intense metal centered luminescence (Figure 2). We are presently investigating the relative electronic properties of C6F5C≡C– and CF3C≡C– through spectroscopic, voltammetric, and computational methods.
Figure 2. Emission of a RhIII complex with the trifluoropropynyl ligand. The emission is metal centered 3A2g to 1A1g (D4h) phosphorescence with a quantum yield of 0.11. Upon N-H deuteration, the quantum yield increases to 0.24 (24%).
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McCarthy, J.S.; McCormick, M.J.; Zimmerman, J.H.; Hambrick, H.R.; Thomas, W.M.; McMillen, C.D.; Wagenknecht, P.S. "Role of the Trifluoropropynyl Ligand in Blue-Shifting Charge-Transfer States in Emissive Pt Diimine Complexes and an Investigation into the PMMA-Imposed Rigidoluminescence and Rigidochromism." Inorg. Chem. 2022, 61, 29, 11366 – 11376.
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London, H.C.; Pritchett, D.Y.; Pienkos, J.A.; McMillen, C.D.; Whittemore, T.J.; Bready, C.J.; Myers, A.R.; Vieira, N.C.; Harold, S.; Shields, G.C.; Wagenknecht. P.S. "Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes." Inorg. Chem. 2022, 61, 28, 10986–10998.
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Carlton, E. S.; Sutton, J. J.; Gale, A. G.; Shields, G. C.; Gordon, K. C.; Wagenknecht, P. S. "Insights into the Charge-Transfer Character of Electronic Transitions in RCp2Ti(C2Fc)2 Complexes using Solvatochromism, Resonance Raman Spectroscopy, and TDDFT" Dalton Trans. 2021, 50, 2233-2242.
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London, H.C.; Whittemore, T.J.; Gale, A.G.; McMillen, C.D.; Pritchett, D.Y.; Myers, A.R.; Thomas, H.D.; Shields, G.C.; Wagenknecht, P.S. "Ligand-to-Metal Charge-Transfer Photophysics and Photochemistry of Emissive d0 Titanocenes: A Spectroscopic and Computational Investigation." Inorg. Chem., 2021, 60, 14399-14409.
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Nguyen, K. T.; Lane, E. E.; McMillen, C. D.; Pienkos, J. A.; Wagenknecht, P. S. "Is indenyl a stronger or weaker electron donor ligand than cyclopentadienyl? Opposing effects of indenyl electron density and ring slipping on electrochemical potentials" Organometallics 2020, 39, 670-678.
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McCarthy, J. S.; McMillen, C. D.; Pienkos, J. A.; Wagenknecht, P. S. "Synthesis and characterization of a tert-butyl ester substituted titanocene dichloride: t-BuOOCCp2TiCl2", Acta Cryst. E., 2020, 76, 1562-1565.
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Livshits, M. Y.; Turlington, M. D.; Trindle, C. O. ; Wang, L. ; Altun, Z.; Wagenknecht, P. S.; Rack, J. J. Picosecond to Nanosecond Manipulation of Excited State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge-Transfer: The Role of Ferrocene-Centered Excited States Inorg. Chem. 2019, 58, 15320 - 15329.
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Eddy, L. E.; Thakker, P. U.; McMillen, C. D.; Pienkos, J. A.; Cordoba, J. J.; Edmunds, C. E.; Wagenknecht, P. S. A comparison of the metal-ligand interactions of the pentafluorophenylethynyl and trifluoropropynyl ligands in transition metal cyclam complexes. Inorg. Chim. Acta, 2019, 486, 141-149.
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Pienkos, J. A.; Webster, A. B.; Piechota, E. J.; Agakidou, A. D.; McMillen, C. D.; Pritchett, D. Y.; Meyer, G. J.; Wagenknecht, P. S. Oxidatively stable ferrocenyl-π-bridge-titanocene D-π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states Dalton Trans., 2018, 47, 10953-10964.
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Vieira, N.C.; Pienkos, J. A.; McMillen, C. D.; Myers, A. R.; Clay, A. P.; Wagenknecht, P. S. A trans-bidentate bis-pyridinyl ligand with a transition metal hinge. Dalton Trans., 2017, 46, 15195-15199.
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Turlington, M. D.; Pienkos, J. A.; Carlton, E. S.; Wroblewski, K. N.; Myers, A. R.; Trindle, C. O.; Altun, Z.; Rack, J. J.; Wagenknecht, P. S. Complexes with Tunable Intramolecular Ferrocene to TiIV Electronic Transitions: Models for Solid-State FeII to TiIV Charge Transfer, Inorg. Chem., 2016, 55(5), 2200-2211.
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Pienkos, J. A.; Agakidou, D. A.; Trindle, C. O.; Herwald, D. W.; Altun, Z.; Wagenknecht, P. S. "Titanocene as a New Acceptor (A) for Arylamine Donors (D) in D-π-A Chromophores", Organometallics, 2016, 35, 2575-2578.
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Thakker, P. U.; Aru, R. G.; Sun, C.; Pennington, W. T.; Siegfried, A. M.; Marder, E. C.; Wagenknecht, P. S. Synthesis of trans bis-alkynyl complexes of Co(III) supported by a tetradentate macrocyclic amine: A spectroscopic, structural, and electrochemical analysis of π-interactions and electronic communication in the C≡C—M—C≡C structural unit. Inorg. Chim. Acta 2014, 411, 158-164.
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Thakker, P. U.; Sun, C.; Khulordava, L.; McMillen, C. D.; Wagenknecht, P. S. Synthetic control of the cis/trans geometry of M(cyclam)(CCR)2]OTf complexes and photophysics of cis-Cr(cyclam)(CCCF3)2]OTf and cis-Rh(cyclam)(CCCF3)2]OTf. J. Organomet. Chem. 2014, 772-773, 107-112.
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Burks, P. T.; Ostrowski, A. D.; Mikhailovsky, A. A.; Chan, E. M.; Wagenknecht, P. S.; Ford, P. C. Quantum dot photoluminescence quenching by Cr(III) complexes. Photosensitized reactions and evidence for a FRET mechanism. J. Am. Chem. Soc. 2012, 134, 13266-75.
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Forrest, W. P.; Cao, Z.; Hambrick, H. R.; Prentice, B. M.; Fanwick, P. E.; Wagenknecht, P. S.; Ren, T. Photoactive Chromium(III)-Cyclam Complexes with Axially Bound geminal-Diethynylethenes. Eur. J. Inorg. Chem. 2012, 2012, 5616-5620.
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Sun, C.; Thakker, P. U.; Khulordava, L.; Tobben, D. J.; Greenstein, S. M.; Grisenti, D. L.; Kantor, A. G.; Wagenknecht, P. S. Trifluoropropynyl as a surrogate for the cyano ligand and intense, room-temperature, metal-centered emission from its Rh(III) complex. Inorg. Chem. 2012, 51, 10477-9.
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Sun, C.; Turlington, C. R.; Thomas, W. W.; Wade, J. H.; Stout, W. M.; Grisenti, D. L.; Forrest, W. P.; VanDerveer, D. G.; Wagenknecht, P. S. Synthesis ofcis and trans bis-alkynyl complexes of Cr(III) and Rh(III) supported by a tetradentate macrocyclic amine: a spectroscopic investigation of the M(III)-alkynyl interaction. Inorg. Chem. 2011, 50, 9354-64.
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Wagenknecht, P. S.; Ford, P. C. Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices. Coord. Chem. Rev. 2011, 255, 591-616.
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Grisenti, D. L.; Smith, M. B.; Fang, L.; Bishop, N.; Wagenknecht, P. S. A convenient synthesis of isocyclam and [16]aneN4 and the photophysics of their dicyanochromium(III) complexes. Inorg. Chim. Acta 2010, 363, 157-162.
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Vagnini, M. T.; Rutledge, W. C.; Wagenknecht, P. S. Measurement of both the equilibrium constant and rate constant for electronic energy transfer by control of the limiting kinetic regimes. Inorg. Chem. 2010, 49, 833-8.
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Grisenti, D. L.; Thomas, W. W.; Turlington, C. R.; Newsom, M. D.; Priedemann, C. J.; VanDerveer, D. G.; Wagenknecht, P. S. Emissive chromium(III) complexes with substituted arylethynyl ligands. Inorg. Chem. 2008, 47, 11452-4.
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Carriker, J. L.; Wagenknecht, P. S.; Hosseini, M. A.; Fleming, P. E. Transition metal catalyzed D2/H2O exchange: Distinguishing between the single and double exchange pathways. J. Mol. Catal. A: Chem. 2007, 267, 218-223.
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Hembre, R. T.; Wagenknecht, P. S.; Penney, J. M. Process for Stabilizing Enzymes with Phosphine or Phosphite Compounds. U.S. Patent 7,195,889, March 27, 2007.
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Vagnini, M. T.; Rutledge, W. C.; Hu, C.; VanDerveer, D. G.; Wagenknecht, P. S. Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 4: Comparison of the trans-difluoro-complexes of tet a, 1,4-C2-cyclam, and 1,11-C3-cyclam. Inorg. Chim. Acta 2007, 360, 1482-1492.
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Vagnini, M. T.; Kane-Maguire, N.; Wagenknecht, P. S. Effects of steric constraint on Chromium(III) complexes of tetraazamacrocycles. 3. Insights into the temperature-dependent radiationless deactivation of the 2Eg (Oh) excited state of trans-Cr(N4)(CN)2]+ complexes. Inorg. Chem. 2006, 45, 3789-93.
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Watson, R. T.; Hu, C.; VanDerveer, D. G.; Musashe, D. T.; Wagenknecht, P. S. Synthesis and characterization of a highly topologically constrained tetraazamacrocyclic ligand. Inorg. Chem. Commun. 2006, 9, 180-182.
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Wagenknecht, P. S.; Hu, C.; Ferguson, D.; Nathan, L. C.; Hancock, R. D.; Whitehead, J. R.; Wright-Garcia, K.; Vagnini, M. T. Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2. Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam. Inorg. Chem. 2005, 44, 9518-26.
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Wagenknecht, P. S.; Sambriski, E. J. Transition Metal Catalyzed Reduction of Nicotinamide Coenzymes and Their Analogues. ChemInform 2005, 36.
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Watson, R. T.; Basinger, J.; Min, E. Y.; Wagenknecht, P. S. Synthesis of a Lewis acid bearing cyclopentadienyl ligand and its tricarbonylmanganese(I) complex. J. Organomet. Chem. 2005, 690, 2159-2162.
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Hembre, R. T.; Wagenknecht, P. S.; Penney, J. M. Enzymatic Reductions with Dihydrogen via Metal Catalyzed Cofactor Regeneration. U.S. Patent 6,599,723, July 29, 2003.
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Hu, C.; Chin, R. M.; Nguyen, T. D.; Nguyen, K. T.; Wagenknecht, P. S.; Nathan, L. C. Chemistry of constrained dioxocyclam ligands with Co(III): unusual examples of C-H and C-N bond cleavage. Inorg. Chem. 2003, 42, 7602-7.
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Hu, C.; Nguyen, T. D.; Wagenknecht, P. S.; Nathan, L. C. Solution-state and solid-state structural characterization of complexes of a new macrocyclic ligand containing the 1,5-diazacyclooctane subunit. Inorg. Chem. 2003, 42, 742-9.
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Wagenknecht, P. S.; Penney, J. M.; Hembre, R. T. Transition-Metal-Catalyzed Regeneration of Nicotinamide Coenzymes with Hydrogen1. Organometallics 2003, 22, 1180-1182.
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Wright-Garcia, K.; Basinger, J.; Williams, S.; Hu, C.; Wagenknecht, P. S.; Nathan, L. C. Effects of Steric Constraint on chromium(III) Complexes of Tetraazamacrocycles. Chemistry and Excited-State Behavior of 1,4-C2-Cyclam Complexes. Inorg. Chem. 2003, 42 (16), 4885-4890.
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Wagenknecht, P. S.; Kane-Maguire, N.; Speece, D. G.; Helwic, N. Electronic Energy Self-Exchange with Macrocyclic Chromium(III) Complexes. Inorg. Chem. 2002, 41, 1229-1235.
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Garizi, N.; Macias, A.; Furch, T.; Fan, R.; Wagenknecht, P. S.; Singmaster, K. A. Cigarette Smoke Analysis Using an Inexpensive Gas-Phase IR Cell. J. Chem. Educ. 2001, 78, 1665-1665.
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Wong, G.; Mark, B.; Chen, X.; Furch, T.; Singmaster, K. A.; Wagenknecht, P. S. Preparation and Use of a Room-Temperature Catalytic Converter. J. Chem. Educ. 2001, 78, 1667-1667.