203 research outputs found
Charge Photoinjection in Intercalated and Covalently Bound [Re(CO)_(3)(dppz)(py)]^(+)âDNA Constructs Monitored by Time-Resolved Visible and Infrared Spectroscopy
The complex [Re(CO)_(3)(dppz)(pyâ˛-OR)]+ (dppz = dipyrido[3,2-a:2â˛,3â˛-c]phenazine; pyâ˛-OR = 4-functionalized pyridine) offers IR sensitivity and can oxidize DNA directly from the excited state, making it a promising probe for the study of DNA-mediated charge transport (CT). The behavior of several covalent and noncovalent ReâDNA constructs was monitored by time-resolved IR (TRIR) and UV/visible spectroscopies, as well as biochemical methods, confirming the long-range oxidation of DNA by the excited complex. Optical excitation of the complex leads to population of MLCT and at least two distinct intraligand states. Experimental observations that are consistent with charge injection from these excited states include similarity between long-time TRIR spectra and the reduced state spectrum observed by spectroelectrochemistry, the appearance of a guanine radical signal in TRIR spectra, and the eventual formation of permanent guanine oxidation products. The majority of reactivity occurs on the ultrafast time scale, although processes dependent on slower conformational motions of DNA, such as the accumulation of oxidative damage at guanine, are also observed. The ability to measure events on such disparate time scales, its superior selectivity in comparison to other spectroscopic techniques, and the ability to simultaneously monitor carbonyl ligand and DNA IR absorption bands make TRIR a valuable tool for the study of CT in DNA
Tracking a Paterno-Buchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies
Relaxation Dynamics of Pseudomonas aeruginosa Re^I(C)O_3(Îą-diimine)(HisX)^+ (X=83, 107, 109, 124, 126)Cu-^(II) Azurins
Photoinduced relaxation processes of five structurally characterized Pseudomonas aeruginosa Re^I(CO)_3(Îą-diimine)(HisX) (X = 83, 107, 109, 124, 126)Cu^(II) azurins have been investigated by time-resolved (psâns) IR spectroscopy and emission spectroscopy. Crystal structures reveal the presence of Re-azurin dimers and trimers that in two cases (X = 107, 124) involve van der Waals interactions between interdigitated diimine aromatic rings. Time-dependent emission anisotropy measurements confirm that the proteins aggregate in mM solutions (D2O, KPi buffer, pD = 7.1). Excited-state DFT calculations show that extensive charge redistribution in the ReI(CO)_3 â diimine ^3MLCT state occurs: excitation of this ^3MLCT state triggers several relaxation processes in Re-azurins whose kinetics strongly depend on the location of the metallolabel on the protein surface. Relaxation is manifested by dynamic blue shifts of excited-state ν(CO) IR bands that occur with triexponential kinetics: intramolecular vibrational redistribution together with vibrational and solvent relaxation give rise to subps, 2, and 8â20 ps components, while the ~10^2 ps kinetics are attributed to displacement (reorientation) of the Re^I(CO)_3(phen)(im) unit relative to the peptide chain, which optimizes Coulombic interactions of the Re^I excited-state electron density with solvated peptide groups. Evidence also suggests that additional segmental movements of Re-bearing β-strands occur without perturbing the reaction field or interactions with the peptide. Our work demonstrates that time-resolved IR spectroscopy and emission anisotropy of Re^I carbonylâdiimine complexes are powerful probes of molecular dynamics at or around the surfaces of proteins and proteinâprotein interfacial regions
Ultrafast Excited-State Dynamics of Rhenium(I) Photosensitizers [Re(Cl)(CO)_(3)(N,N)] and [Re(imidazole)(CO)_(3)(N,N)]^+: Diimine Effects
Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)_(3)(N,N)]^n (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650â285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (~100 fs)^(â1) than in imidazole (~150 fs)^(â1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UVâvis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N^(â˘â) â Re^(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ĎĎ*(bpy^(â˘â)) transitions. For phen and dmp, the UV excited-state absorption occurs at 305 nm, originating from a series of mixed ĎĎ* and Re â CO;N,Nâ˘â MLCT transitions. UVâvis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1â5 ps range, while TRIR bands show small intensity changes (â¤5 ps) and shifts (~1 and 6â10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (âĽ15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed
Ultrafast Wiggling and Jiggling: Ir_2(1,8-diisocyanomenthane)_4^(2+)
Binuclear complexes of d^8 metals (Pt^(II), Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that MâM bonding is strengthened upon dĎ* â pĎ excitation. We report the bridging ligand dynamics of Ir2(1,8-diisocyanomenthane)_4^(2+)(Ir(dimen)), investigated by fsâns time-resolved IR spectroscopy (TRIR) in the region of CâĄN stretching vibrations, ν(CâĄN), 2000â2300 cm^(â1). The ν(CâĄN) IR band of the singlet and triplet dĎ*pĎ excited states is shifted by â22 and â16 cm^(â1) relative to the ground state due to delocalization of the pĎ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ^1dĎ*pĎ state depend on the initially excited FranckâCondon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale IrâIr contraction and 5 ps IrâIr contraction/intramolecular rotation. Exciting the short/twisted isomer induces a âź5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ^3dĎ*pĎ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν(CâĄN) IR bandwidth oscillates with the frequency of the ν(IrâIr) wave packet, ca. 80 cm^(â1), indicating that the dephasing time of the high-frequency (16 fs)^(â1) CâĄN stretch responds to much slower (âź400 fs)^(â1)IrâIr coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metalâmetal unit and that the coherent IrâIr motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation
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