Biomedical subjects
Publications and source records attributed to Pavel Matousek.
In this paper we demonstrate the use of picosecond time-resolved infrared spectroscopy (ps-TRIR) to monitor the early structural dynamics of DNA bases and polydeoxynucleotides following UV excitation in solution.
We describe an apparatus that provides, for the first time, a seamless bridge between femtosecond and microsecond time-resolved Raman and infrared vibrational spectroscopy. The laser system comprises an actively Q-switched sub-nanosecond pulsed kilohertz laser electronically synchronized to an ultrafast titanium sapphire regenerative amplifier to within 0.2 ns. The ultrafast amplifier provides the stable probe light source enabling high-sensitivity infrared vibrational spectroscopy of transients. Time-resolved infrared spectra of the excited-state relaxation dynamics of metal carbonyl compounds are presented to illustrate the capability of the apparatus, and transient data is resolved from 1 picosecond to over 100 microseconds. The results are compared to conventional nanosecond Fourier transform infrared (FT-IR) and laser based flash photolysis time-resolved infrared technology.
Ultrafast photochemistry of the complexes trans(X,X)-[Ru(X)(2)(CO)(2)(bpy)] (X = Cl, Br, I) was studied in order to understand excited-state reactivity of equatorial CO ligands, coordinated trans to the 2,2'-bipyridine ligand (bpy). TD-DFT calculations have identified the lowest electronic transitions and singlet excited states as mixed X -->bpy/Ru --> bpy ligand to ligand/metal to ligand charge transfer (LLCT/MLCT). Picosecond time-resolved IR spectroscopy in the region of nu(CO) vibrations has revealed that, for X = Cl and Br, subpicosecond CO dissociation is accompanied by bending of the X-Ru-X moiety, producing a pentacoordinated intermediate trans(X,X)-[Ru(X)(2)(CO)(bpy)]. Final movement of an axial halide ligand to the vacant equatorial position and solvent (CH(3)CN) coordination follows with a time constant of 13-15 ps, forming the photoproduct cis(X,X)-[Ru(X)(2)(CO)(CH(3)CN)(bpy)]. For X = I, the optically populated (1)LLCT/MLCT excited state undergoes a simultaneous subpicosecond CO dissociation and relaxation to a triplet IRuI-localized excited state which involves population of an orbital that is sigma-antibonding with respect to the axial I-Ru-I bonds. Vibrationally relaxed photoproduct cis(I,I)-[Ru(I)(2)(CO)(CH(3)CN)(bpy)] is formed with a time constant of ca. 55 ps. The triplet excited state is unreactive, decaying to the ground state with a 155 ps lifetime. The experimentally observed photochemical intermediates and excited states were assigned by comparing calculated (DFT) and experimental IR spectra. The different behavior of the chloro and bromo complexes from that of the iodo complex is caused by different characters of the lowest triplet excited states.
The photochemistry of Fe(CO)5 (5) has been studied in heptane, supercritical (sc) Ar, scXe, and scCH4 using time-resolved infrared spectroscopy (TRIR). 3Fe(CO)4 ((3)4) and Fe(CO)3(solvent) (3) are formed as primary photoproducts within the first few picoseconds. Complex 3 is formed via a single-photon process. In heptane, scCH4, and scXe, (3)4 decays to form (1)4 x L (L = heptane, CH4, or Xe) as well as reacting with 5 to form Fe2(CO)9. In heptane, 3 reacts with CO to form (1)4 x L. The conversion of (3)4 to (1)4 x L has been monitored directly for the first time (L = heptane, kobs = 7.8(+/- 0.3) x 10(7) s(-1); scCH4, 5(+/- 1) x 10(6) s(-1); scXe, 2.1(+/- 0.1) x 10(7) s(-1)). In scAr, (3)4 and 3 react with CO to form 5 and (3)4, respectively. We have determined the rate constant (kCO = 1.2 x 10(7) dm3 mol(-1) s(-1)) for the reaction of (3)4 with CO in scAr, and this is very similar to the value obtained previously in the gas phase. Doping the scAr with either Xe or CH4 resulted in (3)4 reacting with Xe or CH4 to form (1)4 x Xe or (1)4 x CH4. The relative yield, [(3)4]:[3] decreases in the order heptane > scXe > scCH4 >> scAr, and pressure-dependent measurements in scAr and scCH4 indicate an influence of the solvent density on this ratio.
The nature and dynamics of the lowest excited states of fac-[Re(I)(L)(CO)(3)(phen)](+) and fac-[Re(I)(L)(CO)(3)(5-NO(2)-phen)](+) [L = Cl(-), 4-ethyl-pyridine (4-Etpy), imidazole (imH); phen = 1,10-phenanthroline] have been investigated by picosecond visible and IR transient absorption spectroscopy in aqueous (L = imH), acetonitrile (L = 4-Etpy, imH), and MeOH (L = imH) solutions. The phen complexes have long-lived Re(I) --> phen (3)MLCT excited states, characterized by CO stretching frequencies that are upshifted relative to their ground-state values and by widely split IR bands due to the out-of-phase A'(2) and A"nu(CO) vibrations. The lowest excited states of the 5-NO(2)-phen complexes also have (3)MLCT character; the larger upward nu(CO) shifts accord with much more extensive charge transfer from the Re(I)(CO)(3) unit to 5-NO(2)-phen in these states. Transient visible absorption spectra indicate that the excited electron is delocalized over the 5-NO(2)-phen ligand, which acquires radical anionic character. Similarly, involvement of the -NO(2) group in the Franck-Condon MLCT transition is manifested by the presence of an enhanced nu(NO(2)) band in the preresonance Raman spectrum of [Re(I)(4-Etpy)(CO)(3)(5-NO(2)-phen)](+). The Re(I) --> 5-NO(2)-phen (3)MLCT excited states are very short-lived: 7.6, 170, and 43 ps for L = Cl(-), 4-Etpy, and imH, respectively, in CH(3)CN solutions. The (3)MLCT excited state of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) is even shorter-lived in MeOH (15 ps) and H(2)O (1.3 ps). In addition to (3)MLCT, excitation of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) populates a (3)LLCT (imH --> 5-NO(2)-phen) excited state. Most of the (3)LLCT population decays to the ground state (time constants of 19 (H(2)O), 50 (MeOH), and 72 ps (CH(3)CN)); in a small fraction, however, deprotonation of the imH.+ ligand occurs, producing a long-lived species, [Re(I)(im.)(CO)(3)(5-NO(2)-phen).-]+.
Combined picosecond transient absorption and time-resolved infrared studies were performed, aimed at characterising low-lying excited states of the cluster [Os(3)(CO)(10)(s-cis-L)] (L=cyclohexa-1,3-diene, 1) and monitoring the formation of its photoproducts. Theoretical (DFT and TD-DFT) calculations on the closely related cluster with L=buta-1,3-diene (2') have revealed that the low-lying electronic transitions of these [Os(3)(CO)(10)(s-cis-1,3-diene)] clusters have a predominant sigma(core)pi*(CO) character. From the lowest sigmapi* excited state, cluster 1 undergoes fast Os-Os(1,3-diene) bond cleavage (tau=3.3 ps) resulting in the formation of a coordinatively unsaturated primary photoproduct (1 a) with a single CO bridge. A new insight into the structure of the transient has been obtained by DFT calculations. The cleaved Os-Os(1,3-diene) bond is bridged by the donor 1,3-diene ligand, compensating for the electron deficiency at the neighbouring Os centre. Because of the unequal distribution of the electron density in transient 1 a, a second CO bridge is formed in 20 ps in the photoproduct [Os(3)(CO)(8)(micro-CO)(2)(cyclohexa-1,3-diene)] (1 b). The latter compound, absorbing strongly around 630 nm, mainly regenerates the parent cluster with a lifetime of about 100 ns in hexane. Its structure, as suggested by the DFT calculations, again contains the 1,3-diene ligand coordinated in a bridging fashion. Photoproduct 1 b can therefore be assigned as a high-energy coordination isomer of the parent cluster with all Os-Os bonds bridged.
The lowest allowed electronic transition of fac-[Re(Cl)(CO)(3)(bopy)(2)] (bopy = 4-benzoylpyridine) has a Re --> bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re --> bopy (3)MLCT. Electronic depopulation of the Re(CO)(3) unit and population of a bopy pi orbital upon excitation are evident by the upward shift of nu(CO) vibrations and a downward shift of the ketone nu(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the (3)MLCT excited state is indicated by time-resolved visible and resonance Raman (TR(3)) spectra that show features typical of bopy(*)(-). In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)(3)(bpy)](+) (bpy = 2,2'-bipyridine) have been identified as Re --> bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. (3)MLCT excited states of both fac-[Re(Cl)(CO)(3)(bopy)(2)] and fac-[Re(bopy)(CO)(3)(bpy)](+) are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of nu(C(triple bond)O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)(3)(bpy)]PF(6).CH(3)CN has been determined.
The first use of Kerr gated fluorescence rejection Raman spectroscopy for the analysis of street samples of cocaine in a fluorescing matrix is reported.
The characters, dynamics, and relaxation pathways of low-lying excited states of the complexes [W(CO)(5)L] [L = 4-cyanopyridine (pyCN) and piperidine (pip)] were investigated using theoretical and spectroscopic methods. DFT calculations revealed the delocalized character of chemically and spectroscopicaly relevant molecular orbitals and the presence of a low-lying manifold of CO pi-based unoccupied molecular orbitals. Traditional ligand-field arguments are not applicable. The lowest excited states of [W(CO)(5)(pyCN)] are W --> pyCN MLCT in character. They are closely followed in energy by W --> CO MLCT states. Excitation at 400 or 500 nm populates the (3)MLCT(pyCN) excited state, which was characterized by picosecond time-resolved IR and resonance Raman spectroscopy. Excited-state vibrations were assigned using DFT calculations. The (3)MLCT(pyCN) excited state is initially formed highly excited in low-frequency vibrations which cool with time constants between 1 and 20 ps, depending on the excitation wavelength, solvent, and particular high-frequency nu(CO) or nu(CN) mode. The lowest excited states of [W(CO)(5)(pip)] are W --> CO MLCT, as revealed by TD-DFT interpretation of a nanosecond time-resolved IR spectrum that was measured earlier in a low-temperature glass (Johnson, F. P. A.; George, M. W.; Morrison, S. L.; Turner, J. J. J. Chem. Soc., Chem. Commun. 1995, 391-393). MLCT(CO) excitation involves transfer of electron density from the W atom and, to a lesser extent, the trans CO to the pi orbitals of the four cis CO ligands. Optical excitation into MLCT(CO) transition of either complex in fluid solution triggers femtosecond dissociation of a W-N bond, producing [W(CO)(5)(solvent)]. It is initially vibrationally excited both in nu(CO) and anharmonicaly coupled low-frequency modes. Vibrational cooling occurs with time constants of 16-22 ps while the intramolecular vibrational energy redistribution from the v = 1 nu(CO) modes is much slower, 160-220 ps. No LF excited states have been found for the complexes studied in a spectroscopically relevant range up to 6-7 eV. It follows that spectroscopy, photophysics, and photochemistry of [W(CO)(5)L] and related complexes are well described by an interplay of close-lying MLCT(L) and MLCT(CO) excited states. The high-lying LF states play only an indirect photochemical role by modifying potential energy curves of MLCT(CO) states, making them dissociative.
Monte Carlo simulation has been applied to study time-resolved Raman and Tyndall photon migration in opaque samples under isotropic and forward scattering conditions. For isotropic scattering, Raman and Tyndall intensities are predicted to decay according to t((1-n)) and t(-n), respectively, where the value of n depends on the ratio of the optical collection aperture to the mean scattering length. The simulation correctly reproduced the analytical results of n = 3/2 and n = 5/2 for a point source in infinite and semi-infinite media, respectively. In addition the model can be used to relate the time at which a Raman photon exits the sample to the mean depth at which it was generated. This could provide a useful tool for depth profiling the chemical composition of turbid systems, and hence be a useful addition to the established array of photon-migration techniques. The model was applied to analyze experimentally observed Raman and Tyndall decay profiles from powdered trans-stilbene. The transport mean free path (l(t)) was calculated to be approximately 400 microm, which was significantly larger than the particle sizes present in the sample (approximately 10-100 microm). This implies that the particles were highly forward scattering, as would be expected for this size range. When highly anisotropic scattering was introduced into the model a much more reasonable scattering length (l(s) approximately 40 microm) was obtained. Finally, a simple analytical model was developed that gives the correct relationship between the Raman and Tyndall decay exponents for isotropic scattering. To the best of our knowledge this work represents the first detailed study of Raman photon migration under time-resolved conditions.
The synthesis of new Pt(II) diimine complexes bearing perfluorinated thiolate ligands, Pt(II)(NN)(4-X-C(6)F(4)-S)(2), where NN = 2,2'-bipyridine or 1,10-phenanthroline and X = F or CN, is reported, together with an investigation of the nature and dynamics of their lowest excited states. A combined UV-vis, (spectro)electrochemical, resonance Raman, and time-resolved infrared (TRIR) study has suggested that the HOMO is mainly composed of thiolate(pi)/S(p)/Pt(d) orbitals and that the LUMO is largely localized on the pi*(diimine) orbital, thus revealing the [charge-transfer-to-diimine] nature of the lowest excited state. An enhancement of the thiolate ring vibrations, C-F vibrations, and the vibration of the CN-substituent on the thiolate moiety was observed in the resonance Raman spectra, whereas no such enhancement was seen for the nonfluorinated analogues. Thus, the introduction of fluorine substituents on the thiolate moiety probably leads to a more pronounced contribution of the intrathiolate modes to the HOMO compared to the analogous complexes with nonfluorinated thiolates. Furthermore, the introduction of the p-CN group into the thiolate moiety has allowed the dynamics of the lowest excited state of Pt(bpy)(4-CN-C(6)F(4)-S)(2) to be monitored by picosecond TRIR spectroscopy. The dynamics of the lowest [charge-transfer-to-diimine] excited state are governed by ca. 2-ps vibrational cooling and 35-ps back electron transfer.
Picosecond time-resolved resonance Raman spectroscopy, TR3, reveals an intense acetylenic band in the S1 state of the prototypical molecular wire 1,4-bis(phenylethynyl)benzene.
Explore the source record for details and available documents.
The excited-state properties of the transition metal complexes tris(2,2'-bipyridine) ruthenium(II) and tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) are examined using picosecond time-resolved luminescence spectroscopy. For both complexes, direct observation of a short-lived high-energy emission with a lifetime of less than 4 ps is reported. Upon deuteriation of the complexes the lifetime of the high-energy emission shows a marked increase with a biexponential decay (20 and approximately 300 ps components). Examination by time-resolved excited-state resonance Raman shows that for the perprotio complexes features attributable to the 3MLCT excited state are formed within 4 ps, while for the perdeuterio a rise time of approximately 20 ps is observed in the 3MLCT features. The results indicate that the emission in both cases may be 1MLCT in origin and are discussed with respect to heterogeneous electron transfer.
The photophysical properties of fac-[Re(CO)3(dppz)(py)]+ (1, where dppz = dipyrido[3,2-a: 2',3'-c]phenazine) in CH3CN have been investigated using a series of complementary techniques including visible and infrared transient absorption and resonance Raman spectroscopy on the picosecond and nanosecond timescales. The results confirm previous reports that the lowest-lying emissive state in 1 is a triplet intra-ligand (3IL) state localised on the dppz ligand and have provided detailed information on the dynamics of 1 upon photoexcitation, including the relative energies of the excited state species encountered and the electronic distribution within these. If the dppz ligand is viewed in terms of phenanthroline (phen) and phenazine (phz) moieties, the emissive state is probably more accurately described as a 3 pi-->pi *(phz) IL state. The picosecond studies have shown that this emissive state is formed, at least in part, within 30 ps of excitation from a precursor, which is possibly a 3 pi-->pi *(phen) IL state. On the nanosecond timescale, TRIR has been employed to elucidate further dynamics and reveal the presence of an energetically close-lying state in equilibrium with the emissive state. This has tentatively been assigned as being 3d pi(Re)-->pi *(phz) metal-to-ligand charge transfer (MLCT) in nature. A summary of the photophysics is proposed in the form of a Jablonski scheme. Time dependent density functional theory (TD-DFT) calculations support the relative ordering and suggested electronic character of the excited state species involved.
The fluorescence spectra of acidified aqueous solutions of tetracycline (tcH3+) exhibit three components with slightly different degrees of anisotropy: the 'blue' component (lambdad approximately equal to 475 nm) decays on the timescale of a few picoseconds; the second, most intense component (lambdad approximately equal to 530 nm) shows decay times of about 25 (H3O+) and 70 ps (D3O+); the third component (lambdad approximately equal to 650 nm) is longer lived (tau approximately equal to 200 ps). All three fluorescence components appear quasi-instantaneously, thus providing evidence that the relaxation processes which give rise to the unusually large Stokes shifts occur on a (sub-)picosecond timescale. The effect of H/D exchange suggests that these relaxation processes involve excited-state intramolecular proton transfer (ESIPT) of OH10 and/or OH12, but does not exclude a change in the hydrogen-bonding pattern to the solvent molecules. The low overall fluorescence yield of the fully protonated form must be correlated to the presence of a very fast decaying species. In alkaline aqueous solution, the fluorescence of the dianion (tc2-) essentially comprises two components; the decay time of the stronger, shorter-lived component is about 30 ps, that of the weaker, longer-lived one about 160 ps. The relative amplitude of the latter is larger at pH 11 than at pH 8.5, in accordance with the increase in the steady-state fluorescence intensity upon increasing the pH from 8.5 to 11. Complexation of the dianion with divalent metal ions like Mg2+ or Ca2+ leads to a strong enhancement of the steady-state fluorescence. In the time-resolved spectra, the decay time of the major fluorescence component exhibits approximately a five-fold increase in comparison to the major component of the dianion. It is about 150 ps in both types of complexes. The decay times of the minor component are increased to about 500 (Mg2+) and 320 ps (Ca2+). The absence of the ultra-fast component in the fluorescence of the dianion and its metal complexes provides evidence that a reaction of OH 12 must be responsible for the ultra-fast fluorescence component in tcH3+. The existence of a component with a lifetime of several tens of picoseconds in all samples suggests the involvement of hydrogen bonding at OH10 during the formation of the emitting species. DFT calculations for the isolated molecule provide evidence that ESIPT is indeed an energetically allowed relaxation process for those isomers that have only one intramolecular hydrogen bond to O11. The ESIPT process yields primary photoproducts that should emit at much longer wavelengths, thereby explaining the unusually large fluorescence Stokes shift.
We have constructed a broadband ultrafast time-resolved infrared (TRIR) spectrometer and incorporated it into our existing time-resolved spectroscopy apparatus, thus creating a single instrument capable of performing the complementary techniques of femto-/picosecond time-resolved resonance Raman (TR3), fluorescence, and UV/visible/infrared transient absorption spectroscopy. The TRIR spectrometer employs broadband (150 fs, approximately 150 cm(-1) FWHM) mid-infrared probe and reference pulses (generated by difference frequency mixing of near-infrared pulses in type I AgGaS2), which are dispersed over two 64-element linear infrared array detectors (HgCdTe). These are coupled via custom-built data acquisition electronics to a personal computer for data processing. This data acquisition system performs signal handling on a shot-by-shot basis at the 1 kHz repetition rate of the pulsed laser system. The combination of real-time signal processing and the ability to normalize each probe and reference pulse has enabled us to achieve a high sensitivity on the order of deltaOD approximately 10(-4) - 10(-5) with 1 min of acquisition time. We present preliminary picosecond TRIR studies using this spectrometer and also demonstrate how a combination of TRIR and TR3 spectroscopy can provide key information for the full elucidation of a photochemical process.
A series of substituted benzoyl radicals has been generated by laser flash photolysis of alpha-hydroxy ketones, alpha-amino ketones, and acyl and bis(acyl)phosphine oxides, all of which are used commercially as photoinitiators in free radical polymerizations. The benzoyl radicals have been studied by fast time-resolved infrared spectroscopy. The absolute rate constants for their reaction with n-butylacrylate, thiophenol, bromotrichloromethane and oxygen were measured in acetonitrile solution. The rate constants of benzoyl radical addition to n-butylacrylate range from 1.3 x 10(5) to 5.5 x 10(5) M(-1) s(-1) and are about 2 orders of magnitude lower than for the n-butylacrylate addition to the counterradicals that are produced by alpha-cleavage of the investigated ketones. Density functional theoretical calculations have been performed in order to rationalize the observed reactivities of the initiating radicals. Calculations of the phosphorus-centered radicals generated by photolysis of an acyl and bis(acyl)phosphine oxide suggest that P atom Mulliken spin populations are an indicator of the relative reactivities of the phosphorus-centered radicals. The alpha-cleavage of (2,4,6-trimethylbenzoyl)phosphine oxide was studied by picosecond pump-probe and nanosecond step-scan time-resolved infrared spectroscopy. The results support a mechanism in which the alpha-cleavage occurs from the triplet excited state that has a lifetime less than or equal to the singlet excited state.