Benjamin M Lovaasen

University of Chicago, Chicago, IL, United States

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Publications (3)11.6 Total impact

  • Davis B. Moravec, Benjamin M. Lovaasen, Michael D. Hopkins
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    ABSTRACT: Picosecond transient-absorption spectra are reported for the S1 excited state of zinc tetraphenylporphyrin (ZnTPP) and related compounds in the near-infrared region (λ ≤ 1400 nm). The spectra exhibit a prominent absorption band (ɛ ≌ 4400 M−1 cm−1 for ZnTPP) between 1200 and 1300 nm whose position is sensitive to solvent (toluene and THF), zinc coordination number (ZnTPP(py-4-CCPh)), the metal center (GaTPP(Cl)), and the positions of the porphyrin substituents (zinc octaethylporphyrin, ZnOEP). The transient-absorption profile between 850 and 1200 nm is also sensitive to these variations. The near-infrared bands for the TPP compounds are assigned to transitions from the S1 state to gerade-symmetry dark states based on their correspondence to relative energies predicted by previously reported time-dependent density functional theory calculations and their constant energy gap relative to the S2 state. The characteristics of the prominent near-infrared transient-absorption band make it well suited for selectively probing photophysical and photoredox processes of the S1 state because it does not overlap with absorption bands of the T1 excited state or of [ZnTPP]+ or [ZnTPP]−, in contrast to the features in the typically probed visible-wavelength region.
    Journal of Photochemistry and Photobiology A Chemistry 02/2013; 254:20–24. · 2.42 Impact Factor
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    ABSTRACT: The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.
    Inorganic Chemistry 02/2012; 51(10):5660-70. · 4.59 Impact Factor
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    ABSTRACT: The new zinc porphyrin/tungsten alkylidyne dyad Zn(TPP)-C[triple bond]CC(6)H(4)C[triple bond]W(dppe)(2)Cl (1) possesses novel photophysical properties that arise from a tunable excited-state triplet-triplet equilibrium between the porphyrin and tungsten alkylidyne units. Dyad 1 exhibits (3)(d(xy) <-- pi*(WCR)) phosphorescence with a lifetime that is 20 times longer than that of the parent chromophore W(CC(6)H(4)CCPh)(dppe)(2)Cl (2). The triplet-triplet equilibrium can be tuned by the addition of ligands to the Zn center, resulting in phosphorescence lifetimes for 1(L) that are up to 1300 times longer than that of 2. The "lifetime reservoir" effect exhibited by 1(L) is approximately 1 order of magnitude larger than previously reported examples of the phenomenon.
    Inorganic Chemistry 07/2010; 49(13):5777-9. · 4.59 Impact Factor