Eilers-König , Nina: Ultrafast relaxation after photoexcitation of the dyes DCM and LDS-750 in solution
Ultrafast relaxation after photoexcitation of the dyes DCM and LDS-750 in solution
Dissertation

zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat)
im Fach Chemie

eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
der Humboldt-Universität zu Berlin

von Nina Eilers-König ,
geb. 13.12.1968 in Bielefeld

Präsident der Humboldt-Universität zu Berlin : Prof. Dr. Dr. h.c. H. Meyer

Dekan der Mathematisch-Naturwissenschaftlichen Fakultät : Prof. Dr. J.P. Rabe

Gutachter:
Prof. Dr. Nikolaus P. Ernsting
Prof. Dr. Thomas Elsässer (Max-Born-Institut, Berlin-Adlershof)
Prof. Dr. Wolfgang Rettig

Tag der mündlichen Prüfung: 30.11.1999

Abstract

Relaxation of the stryryl dyes DCM and LDS-750 after photoexcitation in the liquid phase has been investigated by means of time-resolved optical spectroscopy. For this purpose, the broadband pump-probe technique was used. To characterize the relaxation of DCM in the electronic ground state, additionally the broadband dump-probe technique (stimulated emission pumping) was applied.

An approximately solvent-independent time constant was found typical for the observed fast relaxation of DCM, with values of 0.23 (± 0.04) ps in the excited and 0.28 (± 0.07 ps) in the electronic ground state. The relaxation was characterized as conformational change with only a small amount of charge transferred. The further spectral evolution in polar solvents was dominated by solvation dynamics.

For the ionic polymethine species LDS-750 solvent-dependent kinetics were found after photoexcitation. They could be accounted for by assuming the existence of three different conformers within the S1 state.

Keywords:
stilbene, vibrational relaxation, 2-photon absorption, isomerization

Abstrakt

Die Relaxation der Styrylfarbstoffe DCM und LDS-750 nach Photoanregung in flüssiger Phase wurde mittels zeitaufgelöster optischer Spektroskopie untersucht. Dabei wurde die Breitband-Pump-Probe-Technik angewandt. Zur Charakterisierung der Relaxation von DCM im elektronischen Grundzustand wurde außerdem die Breitband-Dump-Probe-Technik (stimuliertes Emissionspumpen) eingesetzt.

Für die beobachtete schnelle Relaxation von DCM wurde eine annähernd lösungsmittelunabhängige Zeitkonstante von 0.23 (± 0.04) ps im elektronisch angeregten und von 0.28 (± 0.07 ps) im elektronischen Grundzustand gefunden. Sie wurde als Konformationsänderung mit nur geringer Ladungsverschiebung charakterisiert. Die weitere spektrale Entwicklung wird in polarer Lösungsmittelumgebung vorwiegend von der Solvatation bestimmt.

Für das ionische Polymethin LDS-750 wurden nach der Anregung solvensabhängige Kinetiken beobachtet, die sich durch die Annahme dreier möglicher Konformationen im S1 erklären lassen.

Zusammenfassung

Die Relaxation der Styryl-Farbstoffe DCM und LDS-750 nach Photoanregung in flüssiger Phase wurde mittels optischer Spektroskopie, unter Anwendung der Breitband-Pump-Probe-Technik, untersucht. Zur Charakterisierung der Relaxation von DCM im elektronischen Grundzustand außerhalb des thermischen Gleichgewichts wurde außerdem die Breitband-Dump-Probe-Technik (stimuliertes Emissionspumpen) eingesetzt.

Das Donor-Akzeptor substituierte Stilbenderivat DCM zeigt im elektronisch angeregten Zustand eine im roten Spektralbereich verzögert anwachsende Emission. Sie ist zur direkt nach der Anregung beobachteten Emissionsbande bathochrom verschoben. Für ihren verzögerten Anstieg wurde eine annähernd von der Anregungsenergie und von der Lösungsmittelumgebung unabhängige Zeitkonstante von 0.23 ( 0.04) ps gefunden. Die beobachtete Abhängigkeit des Anteils der verzögert anwachsenden Emission von der Anregungsintensität wurde auf 2-Photonen-Anregung in höhere elektronische Zustände und Schwingungsrelaxation im S1 nach schneller innerer Konversion zurückgeführt.

Im elektronischen Grundzustand tritt nach stimuliertem Emissionspumpen Absorption rotverschoben zum stationären Absorptionsspektrum auf. Zu der zunächst beobachteten Absorptionsbande bildet sich hyposochrom verschoben eine weitere Absorptionsbande aus. Eine Zeitkonstante von 0.28 ( 0.07) ps wurde für ihren langsamen Anstieg und den Abfall der ersten Absorptionsbande ermittelt.

Diese beiden Relaxationen gehen jeweils mit einer abgeschätzten Änderung des Dipolmomentes von maximal 2_D in S1 und maximal 4.4 D in S0 einher, so daß sie als Konformationsänderung mit nur geringer Ladungsverschiebung charakterisiert wurden. Die weitere spektrale Entwicklung in polarer Lösungsmittelumgebung wird sowohl im elektronischen Grundzustand wie auch im angeregten Zustand vor allem von der Solvatation bestimmt.

Für das ionische Polymethin LDS-750 wurden nach optischer Anregung solvensabhängige Kinetiken beobachtet, die sich durch Annahme dreier möglicher Konformationen im S1 erklären lassen. In diesem Bild erfolgt die Gleichgewichtseinstellung zwischen dem durch Photoanregung erzeugten Konformer A und dem nicht fluoreszierenden Konformer B sehr schnell auf der Zeitskala der Inertialbewegung der Lösungsmittelmoleküle. Auf einer Pikosekunden-Zeitskala wird die Konformation C mit dem höchsten Dipolmoment durch die diffusive Reorientierung der Lösungsmittelmoleküle stabilisiert, so daß sie eine energetisch günstigere Lage gegenüber den anderen beiden Konformationen einnimmt. Die Pikosekunden-Relaxation von LDS-750 nach Photoanregung wurde demnach als Ladungstransfer-Reaktion gedeutet, während die schnelle Reaktion wahrscheinlich zu einem Intermediat der Isomerisierung führt.


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Inhaltsverzeichnis

TitelseiteUltrafast relaxation after photoexcitation of the dyes DCM and LDS-750 in solution
Selbständigkeitserklärung
1 Introduction
2 Background
2.1. Donor-acceptor substituted stilbenes
2.1.1. Relaxation pathways for stilbenes after photoexcitation
2.1.2. Photophysical properties of some donor-acceptor substituted stilbenes
2.1.3. The TICT concept
2.2. Electron transfer
2.3. Solvation dynamics
2.4. Vibrational relaxation
3 Experimental
3.1. Short Pulses
3.1.1. Generation
3.1.2. Amplification
3.1.3. Diagnostics
3.1.4. Handling
3.1.5. Tunability
3.2. Broadband Pump-Probe Measurement Set-up
3.3. Test, calibration and resolution
3.4. Synchronization and electronics
3.5. Data correction
3.6. Chemicals
3.7. Photostationary spectra
3.8. Molecular beam spectroscopy
3.9. Raman spectroscopy
4 Results
4.1. Time-resolved spectroscopy
4.1.1. Spectral decomposition
4.1.2. Spectral dynamics
4.1.2.1.Dump-probe experiments
4.1.2.2.Pump-probe experiments
4.1.3. Dynamic Stokes shift
4.1.4. Total intensity
4.1.5. Precursor-successor modelling
4.2. Stationary spectroscopy
4.2.1. Absorption and fluorescence characteristics of DCM
4.2.2. Jet spectrum of DCM
4.2.3. Raman spectra
4.2.4. Solvatochromic analysis
4.2.5. Simulation of stationary UV/VIS absorption and fluorescence spectra
4.3. Semiempirical calculations
4.4. Simulation of solvation and vibrational relaxation dynamics
4.5. LDS 750
5 Discussion
5.1. DCM
5.1.1. Excited-state dynamics
5.1.2. Ground state dynamics
5.1.3. Charge transfer reaction
5.1.4. Vibrational relaxation and solvent dynamics
5.1.5. Overall reaction scheme
5.2. LDS-750
5.3. Outlook
Bibliographie References
Anhang A Annex
Danksagung
Lebenslauf

Tabellenverzeichnis

Table 2.1.1: Maxima of the absorption and emission spectra of donor-acceptor substituted stilbenes. Structured spectra are denoted by asterisks.
Table 2.1.2: Dipole moments of donor-acceptor substituted stilbenes. Franck-Condon excited states are indicated by brackets. Values from electrooptical measurements are denoted by asterisks; values from semiempirical calculations are marked by double asterisks. All other dipole moments are derived from the Stokes shift between absorption and fluorescence/stimulated emission bands.
Table 2.1.3: Quantum yields of fluorescence and trans cis-isomerization and fluorescence decay times for donor-acceptor substituted stilbenes in different solvents.
Table 3.1.4 : Values for the refractive index and its wavelength derivatives for different media at 620 nm [Salin 87].
Table 4.1.1: Time coefficients (in ps) from precursor-successor fits to the transient spectra ("spectra") and fits to their integrated area ("bandintegral") in highly dipolar solvents. Relative amplitude fractions are shown in brackets.
Table 4.1.2: Time coefficients (in ps) from precursor-successor fits to the transient spectra ("spectra") and fits to their integrated area ("bandintegral") in moderately polar solvents. Relative amplitude fractions are shown in brackets.
Table 4.2.1 : Resonance Raman frequencies (in cm-1) for DCM from Stokes resonance Raman spectra in methanol and of DCM crystals. Also shown are relative displacements estimated from the intensities and frequencies of the peaks of the crystal spectrum.
Table 4.2.2 : Vibrational frequencies obtained from the structure of the early spectra in pump-probe and stimulated emission experiments. Values from the analysis of quantum beats (see 5.1.4) are denoted by asterisks.
Table 4.2.3 : Solvent dielectric properties [Horng 95], peak maxima and widths of DCM stationary absorption and fluorescence spectra.
Table 4.2.4 : Dimensionless displacement Delta, (0,0)-transition energy E(0,0), width sigma and frequencies omegaexc respective omegaground of an effective harmonic vibrational mode as obtained from fits to the stationary absorption and fluorescence spectra of DCM.
Table 4.5.1: Decay and rise times (in ps) from fits of sums of exponential functions to wavelength traces of LDS 750 in different solvents. Solvent relaxation times from [Horng 95] are also given.

Abbildungsverzeichnis

Figure 1-1 : Scheme of the pump-probe technique. Q may denote an intramolecular coordinate or the solvent configuration.
Figure 1-2: Scheme of the (pump)-dump-probe technique. Q may denote an intramolecular coordinate or the solvent configuration. The dump pulse is applied only after relaxation in S1.
Figure 1-3: Structure of DCM.
Figure 1-4 : Structure of LDS-750.
Figure 2.1-1: Trans-cis isomerization scheme of stilbene.
Figure 2.1-2: Structure of 4,4’-donor-acceptor substituted stilbenes.
Figure 2.1-3: Stationary spectra of DCM. The fluorescence spectra have been converted to emission cross section.
Figure 2.2-1 : Diabatic (a) and adiabatic (b) reactant and product free energy surfaces.
Figure 2.2-2 : Energy-gap dependence of the electron transfer rate coefficient and its relation to the relative position of the free energy surfaces of reactant and product, from [Yosh 95].
Figure 2.2-3 : Two-dimensional free energy surfaces in electron transfer reaction, from [Sum 86].
Figure 3.1-1 : CPM-Laser set-up.
Figure 3.1-2 : Laser pulse dye amplification set-up, after [Loch].
Figure 3.1-3: Autocorrelation trace of the amplified CPM pulses.
Figure 3.1-4: Prism pair used for pulse compression.
Figure 3.1-5: Pulse compression, second harmonic generation and measurement set-up.
Figure 3.2-1: Transmission of the beamsplitter for different angles of incidence. Solid and dotted curves alternate for successive angles.
Figure 3.3-1: Malachite green kinetics demonstrating the system temporal resolution. Rise and decay times as obtained from fits to (sums of) exponentials are given in ps.
Figure 3.3-2: Spectrum of a mercury lamp recorded with the home-built polychromators.
Figure 3.4-1: Synchronization scheme.
Figure 4.1-2: a) Pump-probe spectra of spectra of DCM in acetonitrile after excitation at 530 nm with 0.4 µJ excitation pulse energy. b) Spectral decomposition of the pump-probe spectra in a). The fluorescence spectrum has been converted to stimulated emission (gain). c) Isolated stimulated emission spectra obtained from the spectral decomposition.
Figure 4.1-3 : a) Dump-probe spectra of spectra of DCM in methanol after stimulated emission pumping at 630 nm. b) Spectral decomposition of the dump-probe spectra in a). The fluorescence spectrum has been converted to stimulated emission (gain).
Figure 4.1-4 : Comparison of pump-probe (dashed) and dump-probe spectra (solid) of DCM in methanol at a delay time of 20 ps.
Figure 4.1-5 : Isolated absorption spectra of DCM in methanol for different delays after stimulated emission pumping at 630 nm (dump wavelength indicated by large arrow in a).
Figure 4.1-6: Isolated absorption spectra of DCM in acetonitrile for different delays after stimulated emission pumping at 630 nm (indicated by large arrow in a).
Figure 4.1-7 : Isolated absorption spectra of DCM in propylene carbonate for different delays after stimulated emission pumping at 630 nm (indicated by large arrow in a).
Figure 4.1-8: Early isolated stimulated emission spectra of DCM in methanol after excitation at 530 nm with 0.4 µJ excitation pulse energy. The excitation wavelength is indicated by an arrow.
Figure 4.1-9: Isolated stimulated emission spectra of DCM in methanol for different delays after excitation at 530 nm a) with 0.4 µJ excitation pulse energy, b) with 0.8 µJ exc. pulse energy.
Figure 4.1-10: Isolated stimulated emission spectra of DCM in methanol for different delays on a picosecond timescale after excitation at 530 nm with 0.8 µJ excitation pulse energy.
Figure 4.1-11: Isolated stimulated emission spectra of DCM in acetonitrile for different delays after excitation at 470 nm with a) 0.2 µJ excitation pulse energy, b) 0.4 µJ exc. pulse energy, c) 0.9 µJ exc. pulse energy.
Figure 4.1-12: Isolated stimulated emission spectra of DCM in acetonitrile for different delays on a picosecond timescale after excitation at 470 nm with excitation energies of 0.4 and 0.9 µJ.
Figure 4.1-13: Pump-probe spectra of DCM in chloroform for different delays after excitation at 470 nm : a) with 0.4 µJ excitation pulse energy; the dotted curve indicates the spectrum after 20 ps, b) with 0.8 µJ exc. pulse energy, c) with 0.8 µJ exc. pulse energy on a picosecond timescale.
Figure 4.1-14 : Pump-probe spectra of DCM in chloroform for different delays after excitation at 530 nm with 0.7 µJ excitation pulse energy.
Figure 4.1-15: Pump-probe spectra of DCM in toluene for different delays after excitation at 450 nm with a) 0.7 µJ excitation pulse energy, b) 1.4 µJ exc. pulse energy.
Figure 4.1-16: Pump-probe spectra of DCM in toluene for different delays on a picosecond timescale after excitation at 450 nm with 1.4 µJ excitation pulse energy.
Figure 4.1-17: Pump-probe spectra of DCM in tetrachloromethane for different delays after excitation at 450 nm with a) 0.3 µJ excitation pulse energy, b) 0.6 µJ exc. pulse energy, c) 0.9 µJ exc. pulse energy.
Figure 4.1-18 : Pump-probe spectra of DCM in cyclohexane for different delays after excitation at 450 nm with 0.8 µJ excitation pulse energy.
Figure 4.1-19 : Spectral (or solvent) response function SÄE(t) of DCM in acetonitrile calculated from the peak frequency ny0 of a) isolated transient ground state absorption spectra (solid and hollow symbols refer to different dump-probe measurements) b) isolated stimulated emission spectra (solid and hollow symbols refer to different pump-probe measurements). Also shown (solid lines) is a fit to the spectral response function of coumarin 153 from fluorescence spectra in acetonitrile [Horng 95].
Figure 4.1-20 : Spectral response function SIE(t) of DCM in propylene carbonate from the peak frequency ny0 of isolated transient ground state absorption spectra. Also shown (solid line) is a fit to the spectral response function of coumarin 153 from fluorescence spectra in propylene carbonate [Horng 95].
Figure 4.1-21: a) As in Figure 4.1-20 , but for a picosecond timescale. b) Intensity correlation function SI(t) of dump-probe spectra of DCM in propylene carbonate.
Figure 4.1-22: Total intensity of a) dump-probe b) pump-probe spectra of DCM in different solvents and for several measurements.
Figure 4.1-23 : Total intensity of pump-probe spectra of DCM in methanol (symbols) and fits (solid lines) for different pulse energies (excitation at 470 nm).
Figure 4.1-24: Amplitude fraction of exponential decay of the total intensity of pump-probe spectra of DCM in acetonitrile (solid squares) and its time coefficient tau in ps (hollow circles) on the same axis as a function of excitation pulse energy.
Figure 4.1-25 : Fit (solid lines) with precursor-successor modelling to isolated transient ground state absorption spectra (squares) of DCM in acetonitrile.
Figure 4.1-26 : Precursor and successor spectra from the analysis of isolated absorption spectra of DCM in acetonitrile of the previous figure.
Figure 4.1-27 : a) Fit (solid lines) to isolated stimulated emission spectra of DCM in methanol (squares). b) Pecursor and successor spectra as obtained from analysis in a).
Figure 4.2-1: Absorption spectra of irradiated DCM in chloroform. The number of laser shots between subsequent spectra are indicated by arrows.
Figure 4.2-2: Fluorescence spectrum of photostationary DCM solution compared to that of a solution maintained in the dark. Excitation was at 375 nm. The new fluorescence band is assigned to cis-DCM.
Figure 4.2-3 : Fluorescence excitation spectrum of isolated DCM, from [Mühl 99b].
Figure 4.2-4: Resonance Raman spectra of DCM a) in methanol b) of DCM crystals. For the latter, the fluorescence background has been subtracted.
Figure 4.2-5: Fits to the profile of the C=C stretch around 1550 cm-1 in the resonance Raman spectrum of DCM. a) In methanol, shown are the adaptation of a Gaussian (solid) and a Lorentzian (dotted) lineshape. b) In crystallic form, shown is the adaptation of a Lorentzian lineshape.
Figure 4.2-6: Raman spectrum of DCM crystals.
Figure 4.2-7 : Absorption (circles) and emission peak frequencies (squares) of DCM against the Debye reaction field factor F in different solvents. Also shown is modelling according to a reaction field treatment with µg = 9 D and µe = 23 D.
Figure 4.2-8 : Stationary emission (left) and absorption (right) spectra of DCM in various solvents of low polarity (dots) and modelling (lines) with a progression of an effective harmonic vibrational mode. Fluorescence quantum distributions have been converted to cross sections; all spectra are normalized.
Figure 4.3-1: Heat of formation of DCM as a function of dimethylamino group rotation.
Figure 4.3-2 : Oscillator strength of transitions from trans-DCM electronic ground state. For comparison, the stationary absorption spectrum in 2-methyl pentane is also shown.
Figure 4.3-3: Dipole moments of DCM (in Debye) for Franck-Condon and relaxed electronic ground and excited states.
Figure 4.4-1: Time-dependent fluorescence lineshapes of DCM in acetonitrile after eq. 4.11 for a fast vibrational relaxation rate.
Figure 4.4-2: a) Time-dependent fluorescence lineshapes of DCM in acetonitrile after eq. 4.11 for a vibrational relaxation rate of (200 fs)-1. Also shown are isolated stimulated emission spectra in the same solvent after 470 nm excitation for an excitation pulse energy of a) 0.4 µJ and b) 0.9 µJ.
Figure 4.4-3: a) and b) Time-dependent fluorescence lineshapes of DCM in chloroform after eq. 4.11 for a vibrational relaxation rate of (200 fs)-1 on different timescales. c) Isolated stimulated emission spectra in the same solvent after 450 nm excitation for an excitation pulse energy of 0.8 µJ.
Figure 4.4-4: a) Time-dependent fluorescence lineshapes of DCM in cyclohexane after eq. 4.11. b) Isolated stimulated emission spectra in the same solvent after 450 nm excitation.
Figure 4.5-1: Stationary emission (left) and absorption (right) spectra of LDS-750 in chloroform and in acetonitrile.
Figure 4.5-2: Pump-probe spectra of LDS-750 in acetonitrile after excitation at 630 nm.
Figure 4.5-3: Pump-probe spectra of LDS-750 in propylene carbonate for different delays after excitation at 630 nm.
Figure 4.5-4: Pump-probe spectra of LDS-750 in chloroform after excitation at 630 nm.
Figure 4.5-5: Pump-probe spectra of LDS-750 in methanol for different delays after excitation at 630 nm.
Figure 5.1-1: Three-state scheme for DCM relaxation in methanol as proposed by Ruthmann.
Figure 5.1-2: Quantum beats in the kinetic trace at 577 nm of DCM in methanol from pump-probe measurement with excitation at 470 nm and an excitation pulse energy of 0.4\µJ.
Figure A-1: Frequency divider and delay generator.

[Titelseite] [Selbständigkeitserklärung] [Einleitung] [1] [2] [3] [4] [5] [Bibliographie] [Anhang] [Danksagung] [Lebenslauf]

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