Wachsmann-Hogiu, Sebastian: Vibronic coupling and ultrafast electron transfer studied by picosecond time-resolved resonance Raman and CARS spectroscopy
Vibronic coupling and ultrafast electron transfer studied by picosecond time-resolved resonance Raman and CARS spectroscopy
D i s s e r t a t i o n

zur Erlangung des akademischen Grades
d o c t o r r e r u m n a t u r a l i u m
( Dr. rer. nat.)
im Fach Physik

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

von Diplom Physiker Sebastian Wachsmann-Hogiu,
geboren am 1. Februar 1968 in Rumänien

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

Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I
Prof. Dr. Bernhard Ronacher

Gutachter:
Prof. Dr. Thomas Elsässer, Humboldt Universität Berlin
Prof. Dr. Beate Röder, Humboldt Universität Berlin
Prof. Dr. Wolfgang Kiefer, Universität Würzburg

Tag der mündlichen Prüfung: 18.10.00

This thesis deals with vibronic coupling effects between two excited electronic singlet states in Diphenylhexatriene (DPH), and with the role of vibrational modes in photoinduced ultrafast electron transfer in Betaine-30.

By using the picosecond time-resolved Coherent Antistokes Raman Spectroscopy method, it was possible to observe for the first time two very broad and unusual up-shifted vibrational frequencies in the excited singlet state of DPH, which have frequencies higher than frequency region of the C=C stretching mode. These two frequencies shift towards lower frequencies with increasing solvent polarizability. Two explanations have been discussed: (i) the simultaneous existence of two rotamers, where the two frequencies originate from "different molecules" and (ii) a model of vibronic coupling by an asymmetric low frequency bu-mode (pseudo-Jahn-Teller effect).

By using the picosecond time-resolved anti-Stokes Raman spectroscopy method, we observed for the first time mode-specific excitation of vibrational modes after back-electron transfer in Betaine-30. In the primary event, high frequency Raman active modes are most effective in accepting energy, which leads to a non-thermal distribution of the relative populations of Raman active modes. This is qualitatively in accordance with predictions derived from Fermi's Golden Rule. Although energy transfer between the Raman active modes has been finished after about 10 to 15 ps, thermalization is not yet complete in the whole molecule.

Diese Arbeit befasst sich mit der vibronischen Kopplung zweier angeregter Elektronenniveaus in Diphenylhexatrien (DPH) und mit der Rolle von Schwingungsmoden beim ultraschnellen photoinduzierten intramolekularen Elektronentransfer in Betain-30.

Mit Hilfe von Pikosekunden-zeitaufgelöster Kohärenter Antistokes Ramanspektroskopie im angeregten Zustand des DPH haben wir zum ersten Mal das Auftreten zweier extrem frequenzverbreiterter Ramanlinien beobachtet, die gegenüber dem C=C Streckschwingungsbereich zu höheren Wellenzahlen verschoben sind. Beide Ramanlinien lassen sich mit Erhöhung der Lösungsmittelpolarisierbarkeit um mehr als 50 cm-1 in Richtung niedrigerer Frequenzen verschieben.

Zur Erklärung des Sachverhalts werden zwei Modelle diskutiert: (i) die Existenz zweier Isomere im ersten angeregten Elektronenniveau des DPH und (ii) vibronische Kopplung der beiden Elektronenniveaus durch eine niederfrequente asymmetrische bu Schwingungsbewegung (pseudo-Jahn-Teller Effekt).

Mit Hilfe von stationärer Ramanspektroskopie und insbesondere Messungen der Stokes- und anti-Stokes-Ramanspektren mit Pikosekunden-Zeitauflösung, die Beteiligung von Molekülschwingungen beim Elektronentransfer in Betain-30 wurde untersucht.

Zum ersten Mal wurde eine modenspezifische Kinetik der Ramanaktiven Schwingungen nach Elektronen Rücktransfer in Betain-30 beobachtet. Die hochfrequenten Ramanaktiven Moden werden beim Elektronen Rücktransfer bevorzugt, was zu einer nicht-thermischen Besetzung der Schwingungen führt. Das ist zumindest qualitativ in Übereinstimmung mit Rechnungen die auf Fermi's Goldener Regel basieren. Eine Thermalisierung zwischen den beobachteten Ramanaktiven Moden stellt sich frühestens 10 ps nach Anregung ein. Die Thermalisierung in dem gesamten Molekül ist aber noch nicht beendet.

Keywords:
vibronic coupling, polyene, electron transfer, vibrational modes, mode-specificity

Schlagwörter:
vibronische Kopplung, Polyen, Elektronen Transfer, Schwingungsmoden


Pages: [1] [2] [3] [4] [5] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [101] [102] [103] [104] [105] [107] [108] [109] [110] [111]

Table of Contents

Front pageVibronic coupling and ultrafast electron transfer studied by picosecond time-resolved resonance Raman and CARS spectroscopy
Acknowledgements
1 Introduction
1.1Vibronic coupling in polyene-like molecules
1.2Photoinduced electron transfer in condensed phase
1.3Outline of the thesis
1.4References
2 Theoretical background
2.1Vibronic coupling in organic molecules
2.1.1The Born-Oppenheimer adiabatic representation
2.1.2Vibronic interactions and vibronic constants
2.1.3Symmetry selection rules
2.1.4The Jahn-Teller theorem
2.2Time-dependent formalism for absorption and Raman scattering
2.2.1Milestones in understanding of ET mechanism in solution
2.2.2The classical Marcus theory of ET
2.2.3Inner-sphere versus outer-sphere ET
2.2.4Adiabatic versus nonadiabatic ET
2.2.5Microscopic ET rates using the Fermi‘s Golden Rule
2.3References
3 Experiment
3.1Theoretical considerations of the experiments
3.1.1Introduction
3.1.2Raman scattering
3.1.2.1Molecular polarizability
3.1.2.2Raman intensity and selection rules
3.1.2.3The Resonance Raman effect
3.1.3Coherent Anti-Stokes Raman Scattering (CARS)
3.1.3.1General CARS
3.1.3.2Polarization CARS
3.1.4Second Harmonic Generation (SHG)
3.1.5Stimulated Raman Scattering (SRS)
3.1.6The pump-probe technique
3.2Experimental set-up
3.2.1Generation of the picosecond laser pulses
3.2.1.1The synchronously pumped dye laser
3.2.1.2Characterization of the synchronously pumped dye laser pulses
3.2.1.3Amplification of mode-locked picosecond light pulses
3.2.2CARS set-up
3.2.3Raman set-up
3.2.4Measuring of the time resolution in the experiment
3.3References
4 Vibronic coupling in the first excited singlet state of DPH
4.1Motivation
4.2Photophysics of DPH
4.3Results
4.3.1Raman spectra in ground state and time-resolved CARS spectra in excited states
4.3.2Depolarization ratios of the Raman vibrations
4.3.3
4.3.3Background free CARS measurements
4.3.3.1Background free CARS measurements of DPH in octane
4.3.3.2CARS measurements of DPH in different solvents
4.3.4Kinetics of the CARS spectra
4.4Discussion
4.4.1Molecular geometry and assignment of the Raman frequencies in the ground and excited states
4.4.2Bond order equalization in the first and second excited state
4.4.3Origin of the Raman resonances
4.4.4Mechanisms of vibronic coupling
4.4.4.1(i) Coexistence of two species in the 21Ag state
4.4.4.2(ii) Model of vibronic coupling with an asymmetric low-frequency mode
4.4.4.2.1Determination of the modes responsible for vibronic coupling
4.4.4.2.2Pseudo-Jahn-Teller effect in the excited states of DPH
4.4.4.2.3Frequency broadening by vibrational coupling under the condition of the pseudo-Jahn-Teller effect
4.5Conclusions
4.6References
5 Mode specific vibrational kinetics after intramolecular electron transfer in Betaine-30
5.1Motivation
5.2Absorption spectra and photophysics of the molecule
5.3Stationary vibrational spectra and ab initio calculations of geometry and vibrational spectra.
5.3.1Stationary Stokes-Raman and infrared spectra
5.3.2Molecule geometry in ground and excited electronic state
5.3.3Assignment of the Raman vibrations
5.3.4Discussion of the dispersion effects
5.3.5Overtones and combination tones
5.4Transient spectra and a view into the mechanism of back-ET
5.4.1Kinetics of the anti-Stokes Raman modes in slowly and fast relaxing solvents
5.4.2Selective excitation of the vibrations and IVR after back-ET
5.4.3Nonequilibrium vibrational populations of B-30 after back-ET
5.4.4Transient Stokes-Raman spectra in the first excited electronic singlet state
5.5Conclusions
5.6References
6 Summary
6.1Vibronic coupling in DPH
6.2Back-ET in B-30
6.3References
7 Appendix 1
7.1Vibrational population measured with anti-Stokes Raman intensity
8 Zusammenfassung
Vita
Declaration

Table of Tables

Table 4.1: Depolarization ratios of the electronic ground state and of the first excited state of DPH. Spectra of the electronic ground state and depolarization ratios were obtained from DPH dissolved in acetone (1*10-2M) and measured at an excitation wavelength of 458.08nm. CARS frequencies of the first excited singlet state of DPH (5*10-4 M in cyclohexane) were recorded with 50 ps time delay after photochemical excitation at 355nm. For CARS generation we used an excitation wave length lambda1=710nm. Depolarization ratios rhoR of the excited electronic state were obtained in determining the planes of polarization: \|[PSgr ]\|=900-betaR for suppression of the CARS lines.
Table 4.2. Comparison of Raman spectra of DPH dissolved in cyclohexane electronic ground state with transient resonance CARS spectra originating from the first excited singlet state. An assignment related to Raman spectra of hexatriene (hx) and benzene (bz) is presented.
Table 5.1: Optimized geometry parameters for the complete B-30 molecule and the model structure. The dipole moments are given in D, bond lengths in pm, bond and dihedral angles in degrees.
a The model structure comprises the phenoxide and pyridinium rings (cf. Fig. 5.9).
b Experimental crystal structure of B-30 substituted with bromine at C13 and crystalized with ethanol hydrogen-bonded to O1 [28].
c Both rings are orthogonal to each other, the N-pyramidalization causes the deviation from 90°.
Table 5.2: Experimental Raman (in propylene carbonate, 1064 nm excitation) and infrared (in KBr) frequencies, calculated (HF/3-21G) vibrational frequencies (in cm-1), experimental and calculated depolarization ratios rho and assignments.
Abbreviations: Pyr = pyridinium ring; PhO = phenoxide ring; PhA,B,C = outer phenyl rings (cf. Figure 5.9); C-N = PhO-Pyr; ny = stretching; delta = in-plane deformation; gamma = out-of-plane deformation; tau = torsion.
According to calculation the ground state molecule possess C2-symmetry. Modes belonging to the B-species are designated (B), the other belong to A-species.
Table 5.3: Rise and decay times of anti-Stokes Raman kinetic curves measured for B-30 dissolved in propylene carbonate (PC), glycerol triacetin (GTA) and ethanol (ETH). They were approximated by an expression with two monoexponential terms (rise time t1, decay time t2). Back-ET times [3, 5] (tauback-ET) are given in parentheses.

Table of Figures

Fig. 1.1: Polyene sequence (left) and diphenylpolyene (right) general structure. By increasing n, longer (diphenyl)polyene chains are obtained.
Fig. 1.2: Schematic diagram of vibrational excitation of high frequency modes of the product during ET. Electron-vibrational coupling described by the Hamiltonian Hif, and vibrational manifold of the reactant R (i) and product P (f) are also shown.
Fig. 2.1: Schematic illustration of two types of specific adiabatic potential behavior due to vibronic interactions: (a) Jahn-Teller effect in case of electronic degeneracy; (b) pseudo-Jahn-Teller effect in case of pseudo-degeneracy.
Fig. 2.2: Schematic view of physical processes that can take place after excitation with an external field.
Fig. 2.3: Three free energy regimes after Marcus (top) and the corresponding dependence of the transfer rates on DeltaG0 (bottom). By increasing the free energy of reaction -DeltaG0, the activation energy DeltaG* decreases leading to the increasing of the ET rate (normal regime). If -DeltaG0>lambda, increasing further -DeltaG0 leads to the increasing of DeltaG* and consequently to the decreasing of the ET rate (inverted regime). R and P represent the reactant and product, respectively. RC is the reaction coordinate.
Fig. 2.4: Adiabatic (left) and nonadiabatic (right) ET Vel is the same as defined in Eq. (2.25). The paths for ET are shown. P and R are the product and reactant diabatic states, respectively.
Fig. 2.5: Schematic view of the vibrational overlap between the reactant R and product P vibrational wavefunctions in the normal (left) and in the Marcus inverted regime (right). v represents the vibrational quantum number.
Fig. 3.1: Schema for Stokes (a) and anti-Stokes (b) Raman scattering. The vibrational levels are labeled with 0, 1, 2, ....
Fig. 3.2: Experimental schema of CARS and CSRS alignment (top) and the phase matching condition (bottom) for the wave vectors ki
Fig. 3.3: Schema of the CARS (left) and CSRS (right) generation
Fig. 3.4: Scheme of polarization CARS. E1(omega1) and E2(omega2) are the planes of polarization of the laser radiations at omega1 and omega2. PR(omega3) and PNR(omega3) are the planes of Raman resonant and Raman non-resonant contributions. lambdaR and lambdaNR are the projections of the polarizations of PR(omega3) and PNR(omega3) on the transmission plane \|[PSgr ]\| of the polarizer.
Fig. 3.5: Scheme of SHG generation in a nonlinear crystal
Fig.3.6: Schematic representation of the spontaneous (top) and stimulated (bottom) Raman scattering as a quantum process from the initial state i to the final state f.
Fig. 3.7: Picosecond time-resolved CARS spectrometer. DA is a three-stage dye amplifier, FR is a Fresnel romb, P is polarizer, F symbolized filter and CCD is a Charge Coupled Device.
Fig. 3.8: Picosecond time-resolved Raman spectrometer. DA is a three-stage dye amplifier, FR a Fresnel romb, P polarizer, F filter and CCD is a Charge Coupled Device. SRS is the cell for new frequency generation by Stimulated Raman Scattering .
Fig. 3.9: Cross-correlation between the pump and probe laser. Left: amplification in sulphorhodamine solution (top) and diff(-DeltaA) (bottom). Right: sum frequency mixing signal. Solid lines represent the fit with Gaussian expressions. Cross-correlation values are given in the inserts.
Fig. 4.1: Molecular structure and ordering of the three lowest electronic levels in trans-stilbene (TS), diphenylhexatriene (DPH) and diphenyloctatriene (DPO).
Fig. 4.2: Molecular structure of diphenylhexatriene (DPH). Photoexcitation to the 11Bu state by a pulse at lambdaUV=350 nm and CARS probing of a vibrational resonance of the excited electronic state are illustrated. The relaxation from the 11Bu to the 21Ag state occurs in about 500 fs. For simplicity, the excited state absorption is indicated as a 21Ag - n1Bu transition only.
Fig. 4.3: Absorption spectrum of DPH dissolved in cyclohexane (10-3 M). The excitation wavelength is marked with an arrow.
Fig. 4.4: Excited state resonance CARS spectrum (a) of DPH dissolved in cyclohexane (10-3 M) recorded in the frequency range 1050-2000 cm-1, and the Raman spectrum of the electronic ground state (b)-solid line, together with the CARS intensities determined from fit shown for comparison in (b)-column bars.
Fig. 4.5: Resonance Raman spectrum of DPH in the low-frequency range (lambdaexcit=334.5 nm), corrected for 0K with the Bose-Einstein formula. For comparison, the spectrum of the neat solvent is given. The solvent lines are marked by (*), the DPH Raman line which serves for normalizing to the solvent line is marked with a thin arrow and the DPH Raman line of interest is marked with a thick arrow.
Fig.4.6: Polarization CARS spectra of cyclohexane (spectra a, b, c, d) and excited state polarization CARS spectra of DPH dissolved in cyclohexane in a concentration of 5x10-4 M (spectra A, B, C, D). The spectra of DPH have been recorded at 50 ps time delay after excitation at 355 nm with circularly polarized light. The CARS spectra were measured with an angle =71.5o between the planes of polarizations of the laser beams of the frequencies (2 and (1 respectively. Spectra were recorded with different transmission planes \|[PSgr ]\| of the analyzer.
Fig. 4.7: Resonance CARS spectra of DPH in octane (5x10-4 M) measured under normal conditions (a) and with the background-free technique (b).
Fig. 4.8: Background-free CARS spectra of DPH after photochemical excitation obtained in the frequency range 1550-1900 cm-1. Dashed line: DPH (5x10-4 M/l) dissolved in methanol; solid line: DPH (5x10-4 M/l) dissolved in dimethylsulfoxide.
Fig.4.9: Dependences of the two high-frequency vibrations above 1600 cm-1 of the excited electronic state of DPH on polarizability of neat solvents (°) and of mixture of glycerol and methanol (·). The solvents used are: (1) methanol, (2) acetone, (3) hexane, (4) octane, (5) cyclohexane, (6) dimethylsulfoxide, G/M 10=10 vol% glycerol in methanol, G/M 20=20 vol% glycerol in methanol, etc.
Fig.4.10: Dependences of the two high-frequency vibrations above 1600 cm-1 of the excited electronic state of DPH on solvent polarity (a) and viscosity (b). The solvents used are: (1) methanol, (2) acetone, (3) hexane, (4) octane, (5) cyclohexane, (6) dimethylsulfoxide.
Fig.4.11: Excited-state resonance CARS spectra of DPH dissolved in cyclohexane (10-3 M) recorded at 726 nm to different delays after UV (363 nm) excitation.
Fig. 4.12: Calculated bond length of DPH along the pi-chain. (a)-electronic ground state; (b) local minima of the 21Ag and 11Bu excited electronic states. The arrow between these two states indicate mixing.
Fig. 4.13: Decomposition of geometrical displacements in the two lowest excited singlet states according to normal coordinates of in-plane modes. Modes number 3 and 78 are those taken as involved in the vibronic coupling.
Fig. 4.14: (a) Effective potential for the totally symmetric C=C stretching coordinate around 1700 cm-1 in the two vibronically coupled states. In the 21Ag state the pseudo-Jahn-Teller effect leads to a double-well potential; (b) Enlarged segment of the range of avoided level crossing between the 11Bu and 21Ag singlet states. The dashed line represents the potential curve for a smaller gap. Horizontal lines indicate the energy positions for the lowest vibrational levels in the respective diabatic potentials (left 11Bu - section, right 21Ag - section). The assumed Raman transitions are indicated; (c) Contour diagram of the effective 2-d vibrational potential for the first excited singlet state of DPH resulting from strong vibronic coupling (the cut at Q2= Q20/2 represents the lower potential in (b).
Fig.4.15: Dependences of the two high-frequency vibrations above 1600 cm-1 of the excited electronic state of DPH on the polarizability of neat solvents (ç) and of mixture of glycerol and methanol (æ). The solvents used are: (1) methanol, (2) acetone, (3) hexane, (4) octane, (5) cyclohexane, (6) dimethylsulfoxide, G/M 10=10 vol% glycerol in methanol, G/M 20=20 vol% glycerol in methanol, etc. Straight lines represent calculated frequency dependences on the vibronic coupling strength of the bu-mode between the excited electronic states.
Fig.5.1: Molecular structure of B-30
Fig. 5.2: Absorption spectra of Betaine-30 in Glycerol triacetate (GTA), Propylene Carbonate (PC) and Ethanol (ETH). In the insert is marked the change in the position of the potential surfaces of the ground state (S0) and first excited singlet state (S1).
Fig. 5.3: Fit of the CT absorption band of B-30 dissolved in ETH, PC and GTA. Dot line: experimental curve; solid line: fit with Eq. 5.1. The parameters obtained from the fit are seen in the inserts.
Fig. 5.4: Photophysics of B-30: (1) direct excitation in the CT band, (2) reorganization in the excited state, (3) back-ET and (4) vibrational relaxation.
Fig. 5.5: Stationary Resonance Raman spectra of B-30 dissolved in GTA recorded by excitation in the charge transfer band (top) and in the locally excited band (bottom). Solvent lines are marked with *, and contributions from both solvent and solute lines with +.
Fig. 5.6: Stokes-Raman spectra of B-30 dissolved in methanol (solid line) and in PC (dashed line).
Fig. 5.7: Raman spectra of B-30 obtained after excitation in the CT band (600 nm) and off-resonance (1064 nm).
Fig. 5.8: Infrared spectrum of B-30 embedded in KBr (top) and Raman spectrum of B-30 dissolved in PC (bottom) for comparison.
Fig. 5.9: Molecular structure of betaine-30 (B-30): see the numbering of atoms and naming of outer phenyl rings.
Fig. 5.10: Geometry change of B-30 after excitation in the CT band
Fig. 5.11: Overtones and combination tones observed in B-30 dissolved in PC measured by 303 nm excitation.
Fig. 5.12: Schema for excitation and probing of anti-Stokes Raman mode in B-30.
Fig.5.13: Transient anti-Stokes Raman spectra of B-30 dissolved in PC (left) and GTA (right).
Fig. 5.14: Kinetics of the vibrational modes at 1603, 1360 and 1200/1245 cm-1 measured for B-30 dissolved in PC (left), GTA (middle) and in ETH (right), respectively.
Fig. 5.15: Relative vibrational populations of the excited vibrational levels different vibration determined at different delays. Left: B-30 dissolved in PC; right: B-30 dissolved in GTA. Vibrational temperatures for relative vibrational populations, which have been fitted by Eq. (5.8), are given in the insert.
Fig.5.16: Schema for excitation and probing of Stokes Raman scattering in the excited singlet state of B-30.
Fig. 5.17: Stokes-Raman spectrum of B-30 in pentanol in the electronic ground state with 364 nm excitation (a) and in the first excited electronic state by pump with 600 nm and probe with 364 nm (b). For comparison, a Raman spectrum of neat pentanol is shown, by excitation with 364 nm laser light (c).
Fig. 5.18: Kinetic of the Stokes-Raman spectra in the first excited electronic state of B-30 dissolved in pentanol (solid squares) and cross-correlation between pump and probe beams (open squares). The solid lines are the fits of the experimental points.

[Front page] [Acknowledgements] [1] [2] [3] [4] [5] [6] [7] [8] [Vita] [Declaration]

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