Structure and dynamics of molecules and molecular aggregates in gas phase studied by femtosecond spectroscopy Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften vorgelegt beim Fachbereich Biochemie, Chemie und Pharmazie (FB 14) der Goethe-Universität in Frankfurt am Main von Maksim Kunitski aus Gresk, Weißrussland Frankfurt 2009 (D30) vom Fachbereich Biochemie, Chemie und Pharmazie (FB 14) der Goethe-Universität als Dissertation angenommen. Dekan: Herr Prof. Dr. Dieter Steinhilber 1. Gutachter: Herr Prof. Dr. B. Brutschy 2. Gutachter: Herr Prof. Dr. J. Wachtveitl Datum der Disputation:………… 22 Juni 2010 …………………………….. to my mother and wife… Abstract Abstract Large amplitude intramolecular motions in non-rigid molecules are a fundamental issue in chemistry and biology. The conventional approaches for study these motions by far-infrared and microwave spectroscopy are not applicable when the molecule is non-polar. Therefore, in the current thesis an alternative approach for the investigation of large amplitude intramolecular motions was developed and tested. This new method is based on femtosecond rotational degenerate four-wave mixing spectroscopy (fs DFWM), which is a particular implementation of rotational coherence spectroscopy. The method was successfully applied for the investigation of pseudorotation in pyrrolidine and the ring-puckering vibration in cyclopentene. Another important subject is the photophysics of molecules and molecular clusters which have an ultrashort lifetime of their electronically excited state (photoreactivity). These ultrashort lifetimes often represent a protective mechanism causing photostability. The photoreactivity is usually the manifestation either of an “elementary” reaction, such as proton or electron transfer, which occurs in the excited state or of a fast non-radiative deactivation processes, such as internal conversion via conical intersection of the electronically excited and ground state. Due to a short-lived excited state, the conventional vibrational spectroscopic methods, such as IR depletion detected by resonance two-photon ionization spectroscopy (IR/R2PI), are not applicable for the structural investigation of these systems. Therefore, new approach, termed IR depletion detected by multiphoton ionization with femtosecond laser pulses (IR/fsMPI), was developed for studying the structure of photoreactive microsolvated molecules. The IR/fsMPI technique was applied for investigating the clusters of 1H-pyrrolo[3,2-h]quinoline with water/methanol as well as adenine- and 9-methyl-adenine-hydrates. In addition, the excited state dynamics of bifunctional azaaromatic molecule 7-(2-pyridyl)indole (7PyIn) was studied by femtosecond pump-probe resonance excitation multiphoton ionization technique (fs REMPI). Under electronic excitation of this molecule a fast proton transfer (phototautomerization) takes place, which is followed by radiationless excited state deactivation process. The fs REMPI spectra lead to the conclusion that the phototautomerization in 7PyIn is coupled with a twisting of the molecule, and that the twisting provides an efficient channel for ultrafast radiationless excited state deactivation. This pattern of excited-state tautomerization/deactivation might be quite general. Contents Contents 1 Introduction ....................................................................................................... 1 2 Methods and Instruments ................................................................................ 5 2.1 Generation of femtoseconds pulses ................................................................................... 6 2.1.1 Femtosecond laser setup ................................................................................................ 6 2.1.2 Generation of the second and the third harmonics ......................................................... 7 2.1.3 TOPAS ........................................................................................................................... 8 2.1.4 Inverted autocorrelator ................................................................................................... 9 2.2 Femtosecond Degenerate Four-Wave Mixing ............................................................... 12 2.2.1 Introduction .................................................................................................................. 12 2.2.2 Technique ..................................................................................................................... 13 2.2.3 Theory of the pump-probe non-resonant degenerate four wave mixing signal ........... 15 2.2.4 Simulation of fs DFWM spectra .................................................................................. 17 2.2.5 Preparation of the gas phase sample ............................................................................ 21 2.2.5.1 Gas cell with the heat-pipe principle ................................................................... 21 2.2.5.2 Molecular beams.................................................................................................. 24 2.3 Vibrational analysis of molecular clusters by IR/fsMPI .............................................. 25 2.3.1 Introduction .................................................................................................................. 25 2.3.2 Conventional ns IR/R2PI ............................................................................................. 26 2.3.3 IR/fsMPI principle ....................................................................................................... 27 2.3.4 Experimental IR/fsMPI setup ....................................................................................... 28 2.3.5 The source of infrared radiation ................................................................................... 30 2.3.6 Nozzle source ............................................................................................................... 32 2.3.7 Dual beam technique .................................................................................................... 35 2.4 Pump-probe Resonant Excitation Multiphoton Ionization .......................................... 37 Contents 3 Results ............................................................................................................... 38 3.1 Femtosecond Degenerate Four-Wave Mixing experiments ........................................ 38 3.1.1 Missing levels lead to additional lines: the influence of nuclear spin statistics on femtosecond degenerate four wave mixing spectroscopy of polyatomic systems ...... 39 3.1.2 Large amplitude intramolecular motions studied by fs DFWM: pyrrolidine and cyclopentene molecules............................................................................................... 45 3.2 Structure and Dynamics of short-lived excited state molecules and clusters ............ 54 3.2.1 Structural characterization of 1H-pyrrolo[3,2-h]quinoline microhydrates with ultrashort-lived electronically excited states ............................................................... 54 3.2.2 Proton transfer with a twist? Femtosecond dynamics of 7-(2´-pyridyl)indole in supersonic jets ............................................................................................................. 61 3.2.3 Structure of adenine monohydrates by femtosecond multiphoton ionization detected IR spectroscopy ........................................................................................................... 65 4 Overview and Future Perspectives ................................................................ 70 5 Bibliography ..................................................................................................... 72 6 Summary .......................................................................................................... 81 7 Zusammenfassung ........................................................................................... 85 8 Personal contribution to the papers .............................................................. 90 9 Publications ...................................................................................................... 93 9.1 Missing levels lead to additional lines: the influence of nuclear spin statistics on femtosecond degenerate four wave mixing spectroscopy of polyatomic systems ...... 93 9.2 Pseudorotation in pyrrolidine: rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion .......................................... 103 9.3 Ring-puckering motion in cyclopentene studied by time-resolved rotational coherence spectroscopy and ab initio calculations ..................................................... 115 9.4 Detection and structural characterization of clusters with ultrashort-lived electronically excited states: IR absorption detected by femtosecond multiphoton ionization ........................................................................................................................ 129 9.5 Separation of different hydrogen-bonded clusters by fs UV-ionization-detected infrared spectroscopy: 1H-pyrrolo[3,2-h]quipoline(H2O)(n=1,2) complexes ....... 133 Contents 9.6 Proton transfer with a twist? Femtosecond dynamics of 7-(2-pyridyl)-indole in condensed phase and in supersonic jets ....................................................................... 143 9.7 The structure of adenine monohydrates studied by femtosecond multiphoton ionization detected IR