| 1. |
- Coates, C. G., et al.
(författare)
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Picosecond time-resolved resonance Raman probing of the light-switch states of Ru(Phen)(2)dppz (2+)
- 2001
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 105:50, s. 12653-12664
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Tidskriftsartikel (refereegranskat)abstract
- Picosecond time-resolved resonance Raman (picosecond-TR3) spectroscopy has been used to conduct an extensive photophysical characterization of the light- switch complex [Ru(phen)(2)dppz](2+) as a function of environment, in which studies have been carried out in aqueous and nonaqueous media and in DNA. The results are considered in rotation to a previous report describing environment-sensitive lowest triplet MLCT states. Vibrational marker features and enhancement patterns were used to determine the rapid progression (< 20 ps) between two triplet MLCT states in aqueous environment, followed by subnanosecond, nonradiative deactivation to the ground state. In nonaqueous environment, the long-lived, emissive triplet MLCT state is spectrally identified as the short-lived first triplet MLCT state observed in water, in agreement with the earlier proposed mechanism. The present data are shown to correlate well with previous nanosecond RR findings for the complex in each environment. Interestingly, a precursor state has been identified upon excitation in both nonaqueous solvent and in DNA, which precedes the triplet MLCT state, and the lifetime of which appears to be environment dependent. Observation of this state is discussed in relation to other recent femtosecond spectroscopic studies on this complex.
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| 2. |
- Olofsson, J., et al.
(författare)
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Picosecond Kerr-gated time-resolved resonance Raman spectroscopy of the Ru(phen)(2)dppz (2+) interaction with DNA
- 2002
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Ingår i: Journal of Inorganic Biochemistry. - 0162-0134 .- 1873-3344. ; 91:1, s. 286-297
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Tidskriftsartikel (refereegranskat)abstract
- To investigate the basis of the 'light-switch' effect, the solvent dependence of the Kerr-gated picosecond-time resolved resonance Raman (TR3) spectra of [Ru(bpy),dppz](2+), [Ru(phen)(2)dppz](2+), and the modified complex [Ru(phen)(2)cpdppzOMe](2+) and a dimer [mu-C4(cpdppz)(2)-(phen)(4)Ru-2](4+) were studied. The investigation focussed on comparing the behaviour of [Ru(phen)(2)dppz](2+) in acetonitrile, ethanol, H2O, D2O, and DNA. The data are consistent with a model wherein excitation induces metal-to-ligand charge transfer (MLCT) to any of the ligands (termed the 'precursor' state) which, by interligand electron transfer (ILET), produces an excited state localised on the dppz ligand, MLCT1. In water this state relaxes with a characteristic time of similar to6 ps to a non-emissive state (MLCT2). The TR3 spectra in water, acetonitrile and DNA are all distinctly different. However. the early (4 ps) water spectrum resembles the spectrum in DNA. This interesting observation suggests that the DNA-bound excited state of the complex can be thought of as a model for the initial, poorly solvated state in water.
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| 3. |
- Olofsson, J., et al.
(författare)
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Three-state light switch of Ru(phen)(2)dppz (2+) : Distinct excited-state species with two, one, or no hydrogen bonds from solvent
- 2004
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 108:20, s. 4391-4398
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Tidskriftsartikel (refereegranskat)abstract
- The ruthenium complexes of dppz (dipyrido[3,2-a:2',3'-c]phenazine) have found wide interest due to their environment-sensitive luminescence and are used, for example, as spectroscopic probes for DNA. The deactivation process for the excited state of the light-switch complex [Ru(phen)(2)dppz](2+) (phen = 1,10-phenanthroline) has been studied in water, glycerol, ethylene glycol, and 1,2- and 1,3-propandiol by using fluorescence spectroscopy and single photon counting. In all solvents anomalous temperature dependence is found (increasing quantum yield and excited-state lifetime with increasing temperature). Model-independent analysis shows that only two emissive species, with solvent- and temperature-invariant emission spectral profiles, are sufficient to account for all the data in the polyol solvents. Van't Hoff plots of the ratio of the two species are linear at higher temperatures in all solvents, indicating rapid thermal equilibration of the two species, except for lower temperatures in the most viscous solvent glycerol. Kinetic modeling of the system with microscopic rate constants with positive Arrhenius activation energies requires a third nonemissive species, which is assigned to an excited state with two hydrogen bonds from the solvent, whereas the first two species are assigned to the mono-hydrogen-bonded and non-hydrogen-bonded excited-state species. This assignment is supported by the observation of a growing luminescence intensity as temperature is increased, but no wavelength shift, of high-purity [Ru(phen)(2)dppz](2+) in water solution.
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| 4. |
- Tuite, E., et al.
(författare)
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Probing DNA conductivity with photoinduced electron transfer and scanning tunneling microscopy
- 2000
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Ingår i: Journal of Biomolecular Structure and Dynamics. - : Informa UK Limited. - 0739-1102 .- 1538-0254. ; , s. 277-283
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Tidskriftsartikel (refereegranskat)abstract
- The possibility that the stacked DNA bases can mediate vectorial electron transfer has been examined using two different approaches. Experiments on photoinduced electron transfer with intercalated donors and accepters (either randomly bound or linked dyads of ruthenium complex and viologen) indicate that while DNA may be a better medium than acetonitrile for electron transfer over short distances (2-3-base pair, equivalent to 10-14 Angstrom centre-to-centre separation), it is a poor medium for transport over larger separations. Attempts to measure conductivity of individual DNA molecules using scanning tunneling microscopy to image mixed monolayers of mercaptohexanol (MCH) and 30-mer or 10-mer DNAs with alkanethiol linkers also indicate that DNA in its native state is a poor conductor. AFM images of the DNA/MCH mixed monolayers show that the DNA molecules extend vertically upward from the surface in such surface architectures.
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| 5. |
- Önfelt, Björn, et al.
(författare)
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Picosecond and steady-state emission of Ru(phen)(2)dppz (2+) in glycerol : Anomalous temperature dependence
- 2003
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:7, s. 1000-1009
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Tidskriftsartikel (refereegranskat)abstract
- The excited-state deactivation of the light-switch compound [Ru(phen)(2)dppz](2+), where phen = 1,10-phenanthroline and dppz = dipyrido[3,2-a:2',3'-c]phenazine, has been investigated in glycerol using single-photon counting at picosecond time resolution. Relaxation back to the ground state occurs in about 8 ns at 20 degreesC, which is much faster than previously reported in monohydric alcohols, though still slow compared to that in water. Multivariate kinetic analysis reveals three distinct excited species involved in the relaxation process in glycerol. Using a matrix exponential approach for the kinetic data analysis, including global fitting of the relaxation data collected at many wavelengths, individual emission spectra for all three excited species could be resolved. The resolved emission profile for the most short-lived species was found to resemble the steady-state emission spectrum of [Ru(phen)(3)](2+) in glycerol whereas the emission profile of the intermediate species resembled that of [Ru(phen)(2)dppz](2+) in ethanol. The spectrum of the third species is considerably red-shifted compared to those of the other two. The longest lifetime as well as the emission quantum yield show pronounced nonmonotonic variations with temperature in apparent conflict with the Arrhenius equation. This anomalous temperature dependence can be accounted for by a model based on the equilibrium between two excited species, corresponding to the two resolved emission spectra retrieved at 20 degreesC. Thermodynamic data indicates that transfer to the fast-relaxing, red-shifted species is accompanied by a substantial lowering in enthalpy. The thermodynamic data, as well as an abnormally high preexponential factor for the back reaction from the third to the second excited species, could be explained in terms of the formation of two hydrogen bonds, one to each of the aza nitrogens of the dppz moiety.
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