Cavan N Fleming

University of North Carolina at Chapel Hill, Chapel Hill, NC, United States

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Publications (13)69.88 Total impact

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    ABSTRACT: Energy transfer between the Metal-to-Ligand Charge Transfer (MLCT) excited states of [Pra [M<sup>II</sup>(bpy)<sub>2</sub>(4-Me-4'(_N(H)CO)bpy)](PF<sub>6</sub>)<sub>2</sub> units ([Pra(M<sup>II</sup>bpy<sub>2</sub>(mbpy)]<sup>2+</sup>: M<sup>II</sup> = Ru<sup>II</sup> or Os<sup>II</sup>, bpy = 2,2'-bipyridine, mbpy = 4'-methyl-2,2'-bipyridine-4-carboxamido, Pra = 4-M<sup>II</sup>-L-proline) linked covalently to oligoproline assemblies in room temperature acetonitrile occurs on the picosecond-nanosecond time scale and has been time-resolved by transient emission measurements. Three derivatized oligoprolines _ [CH<sub>3</sub>-CO-Pro<sub>6</sub>-Pra[Os<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>2</sub>-Pra[Ru<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>2</sub>-Pra[Ru<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>6</sub>-Glu-NH<sub>2</sub>]<sup>6+</sup> ( ORR-2 : Pro = L-proline and Glu = glutamic acid) _ [CH<sub>3</sub>-CO-Pro<sub>6</sub>-Pra[Os<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>3</sub>-Pra[Ru<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>3</sub>-Pra[Ru<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>6</sub>-Glu-NH<sub>2</sub>]<sup>6+</sup>, ( ORR-3 ), and CH<sub>3</sub>-CO-Pro<sub>6</sub>-Pra[Os<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>5</sub>-Pra[Ru<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-Pro<sub>5</sub>-Pra[Ru<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>Pro<sub>6</sub>-Glu<sub>2</sub>-NH<sub>2</sub>]<sup>6+</sup> ( ORR-5 ) were prepared by using solid-phase peptide synthesis. Given the helical nature of the resulting assemblies and the nature of the synthesis, composition, length and loading pattern are precisely controlled in the assemblies. In acetonitrile, they adopt a Proline I helical secondary structure, confirmed by circular dichroism, in which the appended chromophores are ordered in well-defined orientations and internuclear separation distances although helix formation for ORR-2 is incomplete. Quantitative comparison of oligoproline ground state absorption and steady-state emission spectra to those for the constituents, [Boc-Pra[M<sup>II</sup>(bpy)<sub>2</sub>(mbpy)]<sup>2+</sup>-OH](PF<sub>6</sub>)<sub>2</sub> (Boc = N<sup>α</sup>-(1,1-Dimethylethoxycarbonyl), shows that following Ru<sup>II</sup> light absorption, Ru<sup>II</sup>* undergoes facile energy transfer resulting in sensitization of Os<sup>II</sup>. Sensitization efficiencies are 93% for ORR-2 , 77% for ORR-3 , and 73% for ORR-5 . Picosecond-resolved emission measurements reveal complex, coupled dynamics that arise from excited state decay and kinetically competitive _Ru<sup>II</sup>*-Ru<sup>II</sup> _ → _Ru<sup>II</sup>-Ru<sup>II</sup>*_ energy transfer migration/exchange and downhill _Ru<sup>II</sup>*-Os<sup>II</sup> → _Ru<sup>II</sup>-Os<sup>II</sup>*, energy transfer. These processes were modeled simultaneously to extract rate constants for Ru<sup>II</sup>* → Ru<sup>II</sup> energy transfer migration, k<sub>Ru*-Ru</sub>, and Ru<sup>II</sup>* → Os<sup>II</sup> energy transfer, k<sub>Ru*-Os</sub>. For ORR-2 , k<sub>Ru*-Ru</sub> = 2.9 x 10<sup>7</sup> s<sup>-1</sup> and k<sub>Ru*-Os</sub> = 3.4 x 10<sup>8</sup> s<sup>-1</sub>. For ORR-3 , k<sub>Ru*-Ru</sub> = 1.2 x 10<sup>7</sup> s<sup>-1</sup> and k<sub>Ru*-Os</sub> = 1.3 x 10<sup>8</sup> s<sup>-1</sup>. For ORR-5 , k<sub>Ru*-Ru</sub> = 3.6 x 10<sup>6</sup> s<sup>-1</sup> and k<sub>Ru*-Os</sub> = 5.8 x 10<sup>7</sup> s<sup>-1</sup>, respectively all in acetonitrile at 22 °C. The data were analyzed by assuming Dexter energy transfer with the Franck-Condon factors arising from intramolecular structural and medium changes evaluated by use of an emission spectral fitting procedure. Fits of the data to the Dexter mechanism were consistent with the predicted distance dependence of energy transfer.
    The Journal of Physical Chemistry B 05/2013; · 3.61 Impact Factor
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    ABSTRACT: Results of CW and lifetime emission studies have been used to demonstrate facile intra-strand energy transfer in the derivatized polystyrene polymer [PS-4-CH(2)CH(2)NHC(O)-(Ru(II)(4,4'-(CONEt(2))(2)bpy)(2))(17)(Os(II)(bpy)(2)))(3)](PF(6))(40) in four rigid media: frozen 5:4 (v:v) propionitrile:butyronitrile solutions at 77 K, polymethyl-methacrylate (PMMA) and polyethylene glycol-dimethacrylate (PEG-DMA) films, and silica xerogel monoliths at room temperature. Continued rapid energy transfer in rigid media is in contrast to electron transfer which is inhibited. This can be explained by energy transfer theory and is due to a decrease in the energy transfer barrier because of the frozen nature of the medium. The abbreviation used for the polymer defines the chemical link to the polystyrene backbone and gives the extent of loading out of 20 available sites.
    Dalton Transactions 06/2009; · 4.10 Impact Factor
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    ABSTRACT: The ligand-bridged complex cis,cis-[(bpy)2ClRu(pz)RuCl(bpy)2]2+ as the PF6- salt, (1)(PF6)2, is stabilized toward photochemical ligand loss in poly(methyl methacrylate) (PMMA). Stabilization allows measurement of metal-to-ligand charge transfer (MLCT) photophysical properties--emission and transient absorption. This includes appearance of an intervalence transfer absorption band in the near IR spectrum of the photochemically prepared, mixed valence form, [(bpy)2ClRuIII(pz(-*))RuIICl(bpy)2](PF6)2* (1*(PF6)2). Comparison of its IT band properties with those of ground state cis,cis-(bpy)2ClRuIII(pz)RuIICl(bpy)2]3+ in CD3CN allows a comparison to be made between pz and pz(-*) as bridging ligands. A model based on differences between rigid and fluid media provides an explanation for decreased IT band energies and widths in PMMA and provides important insight into electron transfer in rigid media.
    Journal of the American Chemical Society 09/2007; 129(31):9622-30. · 10.68 Impact Factor
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    ABSTRACT: The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.
    The Journal of Physical Chemistry B 07/2007; 111(24):6930-41. · 3.61 Impact Factor
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    ABSTRACT: A general, nanosecond equilibrium method is described for determining thermodynamically meaningful oxidation potentials in organic media for compounds that form highly reactive cation radicals upon one-electron oxidation. The method provides oxidation potentials with unusually high precision and accuracy. Redox ladders have been constructed of appropriate reference compounds in dichloromethane and in acetonitrile that can be used to set up electron-transfer equilibria with compounds with unknown oxidation potentials. The method has been successfully applied to determining equilibrium oxidation potentials for a series of aryl-alkylcyclopropanes, whose oxidation potentials were imprecisely known previously. Structure-property trends for oxidation potentials of the cyclopropanes are discussed.
    Journal of the American Chemical Society 12/2004; 126(43):14086-94. · 10.68 Impact Factor
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    ABSTRACT: Four new helical oligoproline assemblies containing 16, 17, 18, and 19 proline residues and ordered arrays of a Ru(II)-bipyridyl chromophore and a phenothiazine electron-transfer donor have been synthesized in a modular fashion by solid-phase peptide synthesis. These arrays are illustrated and abbreviated as CH(3)CO-Pro(6)-Pra(PTZ)-Pro(n)()-Pra(Ru(II)b(2)m)(2+)-Pro(6)-NH(2), where PTZ is 3-(10H-phenothiazine-10)propanoyl and (Ru(II)b'(2)m)(2+) is bis(4,4'-diethylamide-2,2'-bipyridine)(4-methyl,4'-carboxylate,2,2'-bipyridine)ruthenium(II) dication with n = 2 (2), 3 (3), 4 (4), and 5 (5). They contain PTZ as an electron-transfer donor and (Ru(II)b'(2)m)(2+) as a metal-to-ligand charge transfer (MLCT) light absorber and are separated by proline-to-proline through-space distances ranging from 0 (n = 2) to 12.9 A (n = 5) relative to the n = 2 case. They exist in the proline-II helix form in water, as shown by circular dichroism measurements. Following laser flash Ru(II) --> b'(2)m MLCT excitation at 460 nm in water, excited-state PTZ --> Ru(2+) quenching (k(2)) occurs by reductive electron transfer, followed by Ru(+) --> PTZ(+) back electron transfer (k(3)), as shown by transient absorption and emission measurements in water at 25 degrees C. Quenching with DeltaG degrees = -0.1 eV is an activated process, while back electron transfer occurs in the inverted region, DeltaG degrees = -1.8 eV, and is activationless, as shown by temperature dependence measurements. Coincidentally, both reactions have comparable distance dependences, with k(2)( )()varying from = 1.9 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4) and k(3) from approximately 2.0 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4). For both series there is a rate constant enhancement of approximately 10 for n = 5 compared to n = 4 and a linear decrease in ln k with the through-space separation distance, pointing to a significant and probably dominant through-space component to intrahelical electron transfer.
    Journal of the American Chemical Society 11/2004; 126(44):14506-14. · 10.68 Impact Factor
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    ABSTRACT: The energy migration dynamics have been studied in multi-centered assemblies embedded in poly(methyl methacrylate) (PMMA) films using steady-state and time-resolved emission techniques. The assemblies consist of twenty Ru(II) and Os(II) polypyridyl coordination complexes linked through a polystyrene backbone. Energy migration is initiated by photoexcitation of one of the Ru(II) sites and terminated upon sensitization of a low-energy Os trap. The inhomogeneous environment of the assembly results in a distribution of excited-state energies, which is frozen in time due to the rigidity of the PMMA film. Energy migration proceeds toward the lower energy sites, resulting in a time-dependent red-shift in the polymer emission band.
    Journal of Physical Chemistry B - J PHYS CHEM B. 01/2004; 108(7).
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    ABSTRACT: We report temperature-dependent excited-state lifetime measurements on [Ru(bpy)(2)dppz](2+) in both protic and aprotic solvents. These experiments yield a unifying picture of the excited-state photophysics that accounts for observations in both types of solvent. Our measurements support the notion of bpy-like and phz-like states associated with the dppz ligand and show that the ligand orbital associated with the bright state is similar in size to the corresponding orbital in the (3)MLCT state of [Ru(bpy)(3)](2+). In contrast to the current thinking, the experiments presented here indicate that the light-switch effect is not driven by a state reversal. Rather, they suggest that the dark state is always lowest in energy, even in aprotic solvents, and that the light-switch behavior is the result of a competition between energetic factors that favor the dark state and entropic factors that favor the bright (bpy) state.
    Journal of the American Chemical Society 01/2003; 124(50):15094-8. · 10.68 Impact Factor
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    ABSTRACT: Energy transfer between RuII and OsII polypyridyl complexes covalently attached to polystyrene has been in studied in CH3CN. The polymer is a 1:1 styrene-p-aminomethylstyrene copolymer derivatized by amide coupling with the acid-functionalized metal complexes [MII(bpy)2(bpy-COOH)](PF6)2 (MII = RuII, OsII; bpy is 2,2‘-bipyridine and bpy-COOH is 4‘-methyl-2,2‘-bipyridine-4-carboxylic acid). In the resulting polymer [co-PS−CH2NHCO−(RuII11OsII5)](PF6)32, 11 of, on the average, 16 polymer sites are derivatized by RuII and five by OsII. Photophysical properties compared to the homopolymers [co-PS−CH2NHCO−(RuII16)](PF6)32 and [co-PS−CH2NHCO−(OsII16)](PF6)32 reveal that excitation at RuII is followed by efficient energy transfer to the lower energy OsII sites with near unit efficiency (95%). Time-correlated single photon counting measurements with picosecond time resolution reveal that quenching of RuII* produced adjacent to an OsII trap site is quenched with an average rate constant ken = 4.2 × 108 s-1. RuII* decay and OsII* sensitization kinetics are complex because the polymer sample consists of a distribution of individual strands varying in chain length, loading pattern, and number of styryl spacers. The kinetics are further complicated by a contribution from random walk energy migration. An average energy transfer matrix element of Ven 2 cm-1 for RuII* → OsII energy transfer has been estimated by using emission spectral fitting parameters to calculate the energy transfer barrier.
    The Journal of Physical Chemistry A 02/2002; 106(10). · 2.77 Impact Factor
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    ABSTRACT: A detailed study of the excited state energy migration dynamics that take place within an assembly of Ru(II) and Os(II) polypyridyl complexes linked together through a polymer backbone is presented. The energy migration process is initiated by the photoexcitation of the metal-to-ligand charge transfer (MLCT) transition in one of the Ru(II) complexes and terminated by energy transfer to a lower energy Os(II) trap. Energy transfer sensitization of Os(II) can occur in a single step if the excited state is formed adjacent to a trap, or after a series of hops between isoenergetic rutheniums prior to reaching a trap. The dynamics of the energy transfer process are followed by monitoring the growth of Os(II) luminescence at 780 nm. The kinetics of the growth are complex and can be fit by a sum of two exponentials. This kinetic complexity arises both from the presence of a distribution of donor-acceptor distances and the variety of time scales by which Os(II) can be formed. We have augmented the time-resolved experiments with Monte Carlo simulations, which provide insight into the polymer array's structure and at the same time form the basis of a molecular-level description of the energy migration dynamics. The simulations indicate that the most probable Ru-->Os energy transfer time is approximately 400 ps while the time scale for Ru-->Ru hopping is approximately 1-4 ns. The time scale for Ru-->Ru hopping relative to its natural lifetime (1000 ns) suggests that this polymer system could be extended to considerably longer dimensions without an appreciable loss in its overall efficiency.
    Journal of the American Chemical Society 10/2001; 123(42):10336-47. · 10.68 Impact Factor
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    ABSTRACT: The decay properties of the metal-to-ligand charge transfer (MLCT) excited state(s) of [Ru(bpy)2(4,4‘-(PO3H2)2bpy)](Br)2 adsorbed in nanoporous, thin ZrO2 films are complex. Decay kinetics are comparable under Ar or Ar-deaerated CH3CN suggesting that the complexes are imbedded in the open porous structures of the films. Average lifetimes are dependent on the extent of fractional coverage (FRuII) and emission maxima are time dependent. A model is invoked involving complex surface relaxation dynamics arising from a heterogeneity in adsorption sites and cross-surface RuII*-to-RuII migration and quenching at low-energy trap sites. On mixed surfaces containing both adsorbed RuII and [Os(bpy)2(4,4‘-(CO2H)2bpy)](PF6)2 (OsII), RuII*-to-OsII energy transfer occurs with ΔG° = −0.40 eV. On the basis of CW emission and lifetime measurements, the extent of quenching varies with the fractional surface coverage of OsII, FOsII. The average rate constant for energy transfer ken is exponentially dependent on distance r according to the equation, ken(r) = ken(ro) exp(−βen(r − rO)), consistent with a dominant role for the Dexter (exchange) energy transfer mechanism. In this equation, the rate constant at close contact, ro, is ken(ro) = 2.7 × 107 s-1 and β = 0.35 Å-1. By using emission spectral fitting to evaluate the barrier parameters for energy transfer, the energy transfer matrix element at close contact is Vo = 0.4 cm-1. As shown by CW emission measurements, the extent of RuII* quenching is also dependent on the fractional coverage of RuII but in a complex way. A qualitative model is proposed to explain the data based on (1) RuII* → OsII energy transfer, (2) cross-surface energy migration by a random walk, and (3) RuII* → Os energy transfer following RuII* → RuII migration by percolation.
    Journal of Physical Chemistry B - J PHYS CHEM B. 08/2001; 105(37).
  • Journal of Photochemistry and Photobiology A Chemistry 01/2000; 137(2). · 2.42 Impact Factor
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    ABSTRACT: Spectroscopic evidence is presented for different structural isomers of trans-[Ru(bpy)2(L1)(L1)]2+ (L1 or L2=4-Etpy (1), py-PTZ) in which bpy is planar or nonplanar compared to cis-[Ru(bpy)2(py)2]2+ or [Ru(bpy)3]2+ in which bpy is planar. The nonplanar form is dominant in the ground states in low temperature glasses. The two forms appear to be in a temperature dependent equilibrium in 4:1 (v/v) ethanol/methanol at higher temperatures. The nonplanar form is converted into the planar form in the MLCT excited state(s) of 1.
    Journal of Photochemistry and Photobiology A-chemistry - J PHOTOCHEM PHOTOBIOL A-CHEM. 01/2000; 137(2):131-134.