J Luis Pérez Lustres

University of Santiago de Compostela, Santiago, Galicia, Spain

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Publications (23)100.07 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We studied the excited-state behavior of a family of mono- and diprotonated derivatives of 2-phenylbenzimidazole in different solvents, using steady-state and time-resolved fluorescence spectroscopy. The species investigated were 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), the diethylamino analogue 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2) and its N-methylated derivative 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3). The O-methoxy derivatives of 2 and 3 (2 OMe and 3 OMe), and the simpler models 2-phenylbenzimidazole (4) and its 4'-amino (5) and 4'-dimethylamino (6) derivatives were also studied. We found that the dications of 1, 2, and 3 (protonated at the benzimidazole N3 and at the amino group) were strong photoacids, which were deprotonated at the hydroxyl group upon excitation in aqueous solution (totally for 2 and 3) to give a tautomer of the ground-state monocation. In contrast, no photodissociation was observed for the monocations of these species. Instead, some of the monocations studied behaved as molecular rotors, for which electronic excitation led to a twisted intramolecular charge transfer (TICT) state. The monocations of 2, 3, 2 OMe, 3 OMe, and 6, protonated at the benzimidazole N3, experienced a polarity- and viscosity-dependent radiationless deactivation associated to a large-amplitude rotational motion. We propose that this process is connected to an intramolecular charge transfer from the dimethylaminophenyl or diethylaminophenyl moiety (donor) to the protonated benzimidazole group (acceptor) of the excited monocation, which yields a twisted charge-transfer species. No fluorescence from this species was detected except for 3 and 3 OMe in low-viscosity solvents.
    The journal of physical chemistry. B. 09/2014;
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    ABSTRACT: Proton transfer from strong photoacids to hydroxylic solvents is much under debate. Experimentally, the main issue stems from relaxation and diffusion processes that are concomitant with ultrafast proton transfer and blur population dynamics. To overcome this, we propose a fast photodissociation reaction that, however, proceeds slower than solvent relaxation. Fluorescence spectroscopy of the cationic photoacid 2-(1′-hydroxy-2′-naphtyl)benzimidazolium reveals a two-stage mechanism: (a) reversible elementary proton transfer inside the solvent shell and (b) irreversible contact-pair splitting. The time evolution of the fluorescence signal is complex, yet this is explained quantitatively by simultaneous, spectrally overlapping emission of the acid, the conjugate base, and the contact proton-transfer pair. The latter attains high transient concentration in linear alcohols. Microscopic rate constants of dissociation are determined.
    Journal of Physical Chemistry Letters 03/2014; 5(6):989–994. · 6.59 Impact Factor
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    ABSTRACT: Strong-acid dissociation was studied in alcohols. Optical excitation of the cationic photoacid N-methyl-6-hydroxyquinolinium triggers proton transfer to solvent, which was probed by spectral reconstruction of picosecond fluorescence traces. The process fulfills the classical Eigen--Weller mechanism in two stages: a) solvent-controlled reversible dissociation inside the solvent shell and b) barrierless splitting of the encounter complex. This can only be appreciated when fluorescence band integrals are used to monitor the time-evolution of reactant and product concentrations. Band integrals are insensitive to solvent dynamics and report relative concentrations directly. This was demonstrated by first measuring the fluorescence decay of the conjugate base across the full emission band and independently of the proton transfer reaction. Multiexponential decay curves at single wavelengths result from a dynamic red-shift of fluorescence in the course of solvent relaxation, whereas clean single exponential decays are obtained if the band integral is monitored instead. The extent of the shift is consistent with previously reported femtosecond transient absorption measurements, continuum theory of solvatochromism and molecular properties derived from quantum chemical calculations. In turn, band integrals show clean biexponential decay of the photoacid and triexponential evolution of the conjugate base in the course of the proton transfer to solvent reaction. The dissociation step follows the slowest stage of solvation, measured here independently by ps fluorescence spectroscopy in five aliphatic alcohols. Also, the rate constant of the encounter-complex splitting stage is compatible with proton diffusion. Thus, both stages reach for this photoacid the highest possible rates: solvation and diffusion control. In these conditions, the concentration of the encounter complex is substantial during the earliest nanosecond.
    The Journal of Physical Chemistry B 10/2013; · 3.61 Impact Factor
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    ABSTRACT: This paper deals with the interplay between solvent properties and isomerism of 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (1), and the proton and charge-transfer processes that the different isomers undergo in the first-excited singlet state. We demonstrate the strong influence of these processes on the fluorescence properties of 1. We studied the behavior of 1 in several neutral and acidified solvents, by UV-vis absorption spectroscopy and by steady-state and time-resolved fluorescence spectroscopy. The fluorescence of 1 showed a strong sensitivity to the environment. This behavior is the result of conformational and isomeric equilibria and the completely different excited-state behavior of the isomers. For both neutral and cationic 1, isomers with intramolecular hydrogen bond between the hydroxyl group and the benzimidazole N undergo an ultrafast excited-state intramolecular proton transfer (ESIPT), yielding tautomeric species with very large Stokes shift. For both neutral and cationic 1, isomers with the OH group hydrogen-bonded to the solvent behave as strong photoacids, dissociating in the excited state in solvents with basic character. The pyridine nitrogen exhibits photobase character, protonating in the excited state even in some neutral solvents. An efficient radiationless deactivation channel of several species was detected, which we attributed to a twisted intramolecular charge-transfer (TICT) process, facilitated by deprotonation of the hydroxyl group and protonation of the pyridine nitrogen.
    The Journal of Physical Chemistry B 12/2012; · 3.61 Impact Factor
  • Dominik Immeln, Alexander Weigel, Tilman Kottke, J Luis Pérez Lustres
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    ABSTRACT: Photoreceptors are chromoproteins that undergo fast conversion from dark to signaling states upon light absorption by the chromophore. The signaling state starts signal transduction in vivo and elicits a biological response. Therefore, photoreceptors are ideally suited for analysis of protein activation by time-resolved spectroscopy. We focus on plant cryptochromes which are blue light sensors regulating the development and daily rhythm of plants. The signaling state of these flavoproteins is the neutral radical of the flavin chromophore. It forms on the microsecond time scale after light absorption by the oxidized state. We apply here femtosecond broad-band transient absorption to early stages of signaling-state formation in a plant cryptochrome from the green alga Chlamydomonas reinhardtii. Transient spectra show (i) subpicosecond decay of flavin-stimulated emission and (ii) further decay of signal until 100 ps delay with nearly constant spectral shape. The first decay (i) monitors electron transfer from a nearby tryptophan to the flavin and occurs with a time constant of τ(ET) = 0.4 ps. The second decay (ii) is analyzed by spectral decomposition and occurs with a characteristic time constant τ(1) = 31 ps. We reason that hole transport through a tryptophan triad to the protein surface and partial deprotonation of tryptophan cation radical hide behind τ(1). These processes are probably governed by vibrational cooling. Spectral decomposition is used together with anisotropy to obtain the relative orientation of flavin and the final electron donor. This narrows the number of possible electron donors down to two tryptophans. Structural analysis suggests that a set of histidines surrounding the terminal tryptophan may act as proton acceptor and thereby stabilize the radical pair on a 100 ps time scale.
    Journal of the American Chemical Society 07/2012; 134(30):12536-46. · 10.68 Impact Factor
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    ABSTRACT: Photoisomerization around a central fulvene-type double bond is known to proceed through a conical intersection at the perpendicular geometry. The process is studied with an indenylidene-dihydropyridine model compound, allowing the use of visible excitation pulses. Transient absorption shows that 1) stimulated emission shifts to the red and loses oscillator strength on a 50 fs timescale, and 2) bleach recovery is highly nonexponential and not affected by solvent viscosity or methyl substitution at the dihydropyridine ring. Quantum-chemical calculations are used to explain point 1 as a result of initial elongation of the central C=C bond with mixing of S(2) and S(1) states. From point 2 it is concluded that internal conversion of S(1)→S(0) does not require torsional motion to the fully perpendicular state. The S(1) population appears to encounter a sink on the torsional coordinate before the conical intersection is reached. Rate equations cannot model the observed ground-state recovery adequately. Instead the dynamics are best described with a strongly damped oscillatory contribution, which could indicate coherent S(1)-S(0) population transfer.
    ChemPhysChem 06/2011; 12(10):1860-71. · 3.35 Impact Factor
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    ABSTRACT: Femtosecond dynamics of riboflavin, the parent chromophore of biological blue-light receptors, was measured by broadband transient absorption and stationary optical spectroscopy in polar solution. Rich photochemistry is behind the small spectral changes observed: (i) loss of oscillator strength around time zero, (ii) sub-picosecond (ps) spectral relaxation of stimulated emission (SE), and (iii) coherent vibrational motion along a' (in-) and a'' (out-of-plane) modes. Loss of oscillator strength is deduced from the differences in the time-zero spectra obtained in water and DMSO, with stationary spectroscopy and fluorescence decay measurements providing additional support. The spectral difference develops faster than the time resolution (20 fs) and is explained by formation of a superposition state between the optically active (1pi pi*) S1 and closely lying dark (1n pi*) states via vibronic coupling. Subsequent spectral relaxation involves decay of weak SE in the blue, 490 nm, together with rise and red shift of SE at 550 nm. The process is controlled by solvation (characteristic times 0.6 and 0.8 ps in water and DMSO, respectively). Coherent oscillations for a' and a'' modes show up in different regions of the SE band. a'' modes emerge in the blue edge of the SE and dephase faster than solvation. In turn, a' oscillations are found in the SE maximum and dephase on the solvation timescale. The spectral distribution of coherent oscillations according to mode symmetry is used to assign the blue edge of the SE band to a 1n pi*-like state (A''), whereas the optically active 1pi pi* (A') state emits around the SE maximum. The following model comes out: optical excitation occurs to the Franck-Condon pi pi* state, a pi pi*-n pi* superposition state is formed on an ultrafast timescale, vibrational coherence is transferred from a' to a'' modes by pi pi*-n pi* vibronic coupling, and subsequent solvation dynamics alters the pi pi*/n pi* population ratio.
    The Journal of Physical Chemistry A 12/2008; 112(47):12054-65. · 2.77 Impact Factor
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    ABSTRACT: Femtosecond transient UV/vis absorption spectra of molecules in solution show vibronic structure initially while pump and probe pulses overlap, superimposed on bands which broaden and shift on several time scales. Third-order perturbation theory is used simulate and understand such spectra under experimental conditions, i.e., with chirped pump pulses and 40–80 fs pump/probe correlation time. The electronic system is coupled to a solvent bath whose fluctuations enter through a lineshape function g(t). This approach has two advantages. (i) In the general case when g(t) is modelled realistically it provides a distinction of coherent and sequential contributions to early transient spectra. The time-zero stimulated emission band can be extracted, giving access to the relaxation function for the emission frequency. (ii) For limiting forms of g(t) corresponding to the Bloch approximation and to inhomogeneous broadening the threefold time integrals underlying the theory reduce to a single integral. In such cases the lineshape formulation of transient absorption spectra can be used for data fitting.
    Chemical Physics 01/2008; · 1.96 Impact Factor
  • Angewandte Chemie International Edition 02/2007; 46(20):3758-61. · 11.34 Impact Factor
  • O Shoshana, J L Pérez Lustres, N P Ernsting, S Ruhman
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    ABSTRACT: Using multichannel femtosecond spectroscopy we have followed Na- charge transfer to solvent (CTTS) dynamics in THF solution. Absorption of the primary photoproducts in the visible, resolved here for the first time, consists of an asymmetric triplet centered at 595 nm, which we assign to a metastable incompletely solvated neutral atomic sodium species. Decay of this feature within approximately 1 ps to a broad and structureless solvated neutral is accompanied by broadening and loss of spectral detail. Kinetic analysis shows that both the spectral structure and the decay of this band are independent of the excitation photon frequency in the range 400-800 nm. With different pump-probe polarizations the anisotropy in transient transmission has been charted and its variation with excitation wavelength surveyed. The anisotropies are assigned to the reactant bleach, indicating that due to solvent-induced symmetry breaking, the CTTS absorption band of Na- is made up of discreet orthogonally polarized sub bands. None of the anisotropy in transient absorption could be associated with the photoproduct triplet band even at the earliest measurable time delays. Along with the documented differences in the spatial distribution of ejected electrons across the tested excitation wavelength range, these results lead us to conclude that photoejection is extremely rapid, and that loss of correlations between the departing electron and its neutral core is faster than our time resolution of approximately 60 fs.
    Physical Chemistry Chemical Physics 07/2006; 8(22):2599-609. · 4.20 Impact Factor
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    ABSTRACT: Ultrashort UV laser pulses were used to excite tryptophan residues of heparan sulfate proteoglycan (HS-PG) in blood substitute Krebs solution. Tryptophan fluorescence is sensitive to the environment, so its shift and decay indicate the conformation and solvation state of the protein. We monitored stimulated emission and excited-state absorption by probing with delayed white-light femtosecond pulses. Comparison with bare tryptophan revealed transient absorption features which are characteristic for HS-PG. Furthermore, the effect of adding calcium salt was investigated. Differences in the spectra from solutions with and without calcium developed during several minutes, which points to changes in protein conformation, but could only be measured in the sub-ps regime. These results provide a first step to a better understanding of the molecular formation of nanoplaques in blood vessels. The goal of this work is to open a way towards biosensing of the initial stages in atherogenesis allowing for a risk assessment in cardiovascular disease.
    Biochemical and Biophysical Research Communications 07/2006; 345(2):886-93. · 2.28 Impact Factor
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    ABSTRACT: The dynamics of the electric fields in the interior of DNA are measured by using oligonucleotides in which a native base pair is replaced by a dye molecule (coumarin 102) whose emission spectrum is sensitive to the local electric field. Time-resolved measurements of the emission spectrum have been extended to a six decade time range (40 fs to 40 ns) by combining results from time-correlated photon counting, fluorescence up-conversion, and transient absorption. Recent results showed that when the reporter is placed in the center of the oligonucleotide, the dynamics are very broadly distributed over this entire time range and do not show specific time constants associated with individual processes (Andreatta, D.; et al. J. Am. Chem. Soc. 2005, 127, 7270). This paper examines an oligonucleotide with the reporter near its end. The broadly distributed relaxation seen before remains with little attenuation. In addition, a new relaxation with a well-defined relaxation time of 5 ps appears. This process is assigned to the rapid component of "fraying" at the end of the helix.
    Journal of the American Chemical Society 06/2006; 128(21):6885-92. · 10.68 Impact Factor
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    ABSTRACT: Fluorescence of the cyanine dye Thiazole Orange (TO) is quenched by intramolecular twisting in the excited state. In polypeptide nucleic acids, a vibrational progression in a 1400 cm(-1) mode depends on base pairing, from which follows that the high-frequency displacement is coupled to the twist coordinate. The coupling is intrinsic to TO. This is shown by femtosecond fluorescence upconversion and transient absorption spectroscopy with the dye in methanol solution. Narrow emission from the Franck-Condon state shifts to the red and broadens within 100 fs. The radiative rate does not decrease during this process. Vibrational structure builds up on a 200 fs time scale; it is assigned to asymmetric stretching activity in the methine bridge. Further Stokes shift and decay are observed over 2 ps. Emission from the global S(1) minimum is discovered in an extremely wide band around 12 000 cm(-1). As the structure twists away from the Franck-Condon region, the mode becomes more displaced and overlap with increasingly higher vibrational wave functions of the electronic ground state is achieved. Twisting motion is thus leveraged into a fast-shrinking effective energy gap between the two electronic states, and internal conversion ensues.
    Journal of the American Chemical Society 04/2006; 128(9):2954-62. · 10.68 Impact Factor
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    ABSTRACT: Ground-state tautomerism and excited-state proton-transfer processes of 2-(6'-hydroxy-2'-pyridyl)benzimidazolium in H2O and D2O have been studied by means of UV-vis absorption and fluorescence spectroscopy in both steady-state and time-resolved modes. In the ground state, this compound shows a tautomeric equilibrium between the lactim cation, protonated at the benzimidazole N3, and its lactam tautomer, obtained by proton translocation from the hydroxyl group to the pyridine nitrogen. Direct excitation of the lactam tautomer leads to its own fluorescence emission, while as a result of the increase of acidity of the OH group and basicity at the pyridine N upon excitation, the lactim species undergoes a proton translocation from the hydroxyl group to the nitrogen, favoring the lactam structure in the excited state. No fluorescence emission from the initially excited lactim species was detected due to the ultrafast rate of the excited-state proton-transfer processes. The lactim-lactam phototaumerization process takes place via two competitive excited-state proton-transfer routes: a one-step water-assisted proton translocation (probably a double proton transfer) and a two-step pathway which involves first the dissociation of the lactim cation to form an emissive intermediate zwitterionic species and then the acid-catalyzed protonation at the pyridine nitrogen to give rise to the lactam tautomer.
    The Journal of Physical Chemistry A 12/2005; 109(45):10189-98. · 2.77 Impact Factor
  • Angewandte Chemie International Edition 10/2005; 44(35):5635-9. · 11.34 Impact Factor
  • J L Pérez Lustres, Vadim M Farztdinov, Sergey A Kovalenko
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    ABSTRACT: Photoinduced excited-state relaxation of trans-3-phenylprop-2-enaldehyde (cinnamaldehyde) and three derivatives was studied in hexane and acetonitrile with the pump-supercontinuum-probe technique. Transient spectra were measured with 50 fs resolution in the range 260-660 nm after S3<--S0 excitation at 288 nm. The early spectra reveal an ultrafast, 0.1-0.5 ps, interchange of intensity between two excited-state absorption (ESA) bands followed by a approximately 3 ps decay of ESA. This behaviour is interpreted with the help of semiempirical calculations, which indicate that the sub-picosecond evolution is consistent with S3-->S1 excited-state relaxation while the picosecond decay should be due to a structural intramolecular rearrangement. The latter may consist of a twist of the phenyl ring and rotation around the C==C bond to a perpendicular conformation which corresponds to the global energy minimum in the S1 state and serves as source for trans and cis isomers in the ground state.
    ChemPhysChem 08/2005; 6(8):1590-9. · 3.35 Impact Factor
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    ABSTRACT: Time-resolved emission bands can be observed by simultaneous sum-frequency mixing with femtosecond near-infrared gate pulses over a wide frequency range. Using tilted gate pulses in a noncollinear geometry we achieve 80 fs time resolution and background-free measurement of upconverted spectra. All components were optimized resulting in a spectrometer for routine applications. As example we report the initial fluorescence from Coumarin 153 in acetonitrile after excitation at 400 nm. The fluorescence is modulated by nuclear coherence in the excited state which affects band width and asymmetry more than the energy gap itself.
    Physical Chemistry Chemical Physics 04/2005; 7(8):1716-25. · 4.20 Impact Factor
  • Angewandte Chemie 01/2005; 117(35):5779-5783.
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    ABSTRACT: The ground- and excited-state tautomerism of 2-(6‘-hydroxy-2‘-pyridyl)benzimidazole (1) and 1-methyl-2-(6‘-hydroxy-2‘-pyridyl)benzimidazole (2) in various solvents was investigated by means of UV−vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and quantum-mechanical ab initio calculations. A solvent-dependent tautomeric equilibrium was observed for both compounds in the ground state between the lactim or normal form and the lactam tautomer resulting from a proton translocation between the hydroxyl group and the pyridine nitrogen. Here, we report evidences for a solvent-dependent switching in the nature of the excited-state proton transfer (ESPT) reactions undergone by 1 and 2. In the aprotic solvent acetonitrile, no significant ESPT takes place. The protic solvents ethanol and water facilitate the proton transfer through bridges of solvent molecules, but each solvent catalyzes specifically a different ESPT process. In aqueous solution, the excited lactim species of 1 and 2 undergo a proton transfer from the hydroxyl group to the pyridine nitrogen, favoring the lactam tautomer in the first excited singlet state. In ethanol the ESPT takes place for 1 from the benzimidazole NH to the pyridine nitrogen, originating a new tautomer not observed in the ground state. Compound 2, without a NH group, does not tautomerize in the excited state in ethanol.
    Journal of Physical Chemistry A - J PHYS CHEM A. 01/2004; 108(29):6117-6126.
  • S. A. Kovalenko, J. L. Pérez Lustres, N. P. Ernsting, W. Rettig
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    ABSTRACT: Photoinduced intramolecular electron transfer (ET) in symmetric 9,9‘-bianthryl (BA) was studied by femtosecond pump-supercontinuum-probe spectroscopy in solvents covering the whole polarity scale. Results are compared with those obtained for the nonsymmetric 10-cyano derivative (CBA). The behavior exhibited by both compounds is qualitatively the same, which suggests that symmetry plays no essential role. Our measurements provide novel aspects:  (i) electron-transfer proceeds in CBA within a time on the order of 10 fs and therefore along a high-frequency coordinate, (ii) the observation of both educt and product bands allows an estimate of the ET equilibrium, which shifts through solvent relaxation. We find no evidence of dynamics along the intramolecular twist coordinate before ET.
    Journal of Physical Chemistry A - J PHYS CHEM A. 11/2003; 107(48).

Publication Stats

138 Citations
100.07 Total Impact Points

Institutions

  • 2011–2013
    • University of Santiago de Compostela
      • Department of Physical Chemistry
      Santiago, Galicia, Spain
    • Lomonosov Moscow State University
      • Department of Soil Chemistry
      Moscow, Moscow, Russia
  • 2012
    • Bielefeld University
      • Physical Chemistry
      Bielefeld, North Rhine-Westphalia, Germany
  • 2002–2008
    • Humboldt-Universität zu Berlin
      • Department of Chemistry
      Berlin, Land Berlin, Germany