Dr. Sergey Kovalenko

Profil

Zusammenfassung

Dr. Kovalenko entwickelt und wendet ultraschnelle spektroskopische Methoden an, um die Dynamik von Molekülen in Lösung auf Femtosekunden-Zeitskalen zu untersuchen. Seine Expertise umfasst die Charakterisierung von angeregten Zuständen, Ladungstransfer-Prozessen, Lösungsmittel-Wechselwirkungen und photochemischen Reaktionen wie Photoisomerisierung und Protonentransfer. Diese Kompetenzen ermöglichen es, molekulare Schalter und lichtresponsive Materialien rational zu optimieren.

Skills

Name

Dr. Sergey Kovalenko

Titel

Dr.

Fakultät

Mathematisch-Naturwissenschaftliche Fakultät

Institut

Institut für Chemie

Arbeitsgruppe

Physikalische Chemie - Spektroskopie

E-Mail

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HU-FIS-Profil

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Zuletzt gescrapt

28.6.2026, 01:08:22

• Angeregte Zustände und Spontane Polarisation Effekte in Biarylischen Farbstoffen

  Quelle ↗

  Förderer: DFG Sachbeihilfe Zeitraum: 09/2020 - 11/2021 Projektleitung: Dr. Sergey Kovalenko

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• Femtosecond spectroscopy of condensed phases with chirped supercontinuum probing

  1999

  Physical Review A · 498 Zitationen · DOI

  Pump--supercontinuum-probe (PSCP) spectroscopy with femtosecond time resolution is developed theoretically and experimentally. The connection to previous theoretical results on nonchirped probing is established. It is experimentally shown that the supercontinuum can be described as a single chirped pulse. A key problem of the technique---the precise time correction of transient spectra---is solved by monitoring the nonresonant electronic response from a pure solvent (liquids) or from a transparent substrate (solid films). This allows for an adequate characterization of the supercontinuum, in particular, for directly measuring the spectral dependence of the pump-probe cross correlation. For 50-fs pump pulses, a theoretical estimate gives an accuracy for the time correction of 10 fs, which is typically \ensuremath{\approx}1/30 of the supercontinuum pulse duration. Hence a time resolution of 10--20 fs can be experimentally realized. Contributions to the nonresonant transient signal from high-frequency Raman excitations and from low-frequency impulsive-stimulated Raman processes are discussed. The PSCP technique is illustrated by results from experiments with fused silica and several common solvents and with a chromophore in solution.

• <i>ortho</i>‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived <i>Z</i> Isomers

  2014

  Chemistry - A European Journal · 416 Zitationen · DOI

  Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ-electron-withdrawing F atoms ortho to the NN unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.

• Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy

  2001

  The Journal of Chemical Physics · 270 Zitationen · DOI

  The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in solution is studied experimentally and theoretically. Using the pump–supercontinuum probe (PSCP) technique we observed the complete spectral evolution of hot absorption induced by femtosecond optical pumping. In t-stilbene the hot S1 state results from Sn→S1 internal conversion with 50 fs characteristic time. The time constant of intramolecular thermalization or intramolecular vibrational redistribution (IVR) in S1 is estimated as τIVR≪100 fs. In PNA and DPNA the hot ground state is prepared by S1→S0 relaxation with characteristic time 0.3–1.0 ps. The initial molecular temperature is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency νm and width square Γ2 on molecular temperature T. Within this framework we derive analytic expressions for the differential absorption signal ΔOD(T(t),ν). After calibration with stationary absorption spectra in a low temperature range, the solute temperature T(t) may be evaluated from a transient absorption experiment. For highly polar PNA and DPNA, T(t) is well described by a biexponential decay which reflects local heating effects, while for nonpolar t-stilbene the local heating is negligible and the cooling proceeds monoexponentially. To rationalize this behavior, an analytic model is developed, which considers energy flow from the hot solute to a first solvent shell and then to the bulk solvent. Fastest cooling is found for PNA in water: a time constant of 0.64 ps (68%) corresponds to solute–solvent energy transfer while 2.0 ps (32%) characterizes the cooling of the first shell. In aprotic solvents cooling is slower than in alcohols and slows down further with decreasing solvent polarity. This contrasts with nonpolar t-stilbene which cools down with 8.5 ps both in acetonitrile and cyclohexane. Comparison of the cooling kinetics for PNA in water with those for DPNA in water-acetonitrile mixtures suggests that the solute–solvent energy transfer proceeds mainly through hydrogen bonds.

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