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Schmidhammer, Uli (2008): Ladungstransfer im Wechselspiel mit der Nanoumgebung: Organische Reaktivität auf der Femto- bis Millisekunden Skala. Dissertation, LMU München: Faculty of Physics
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Abstract

Femtosecond pump-probe spectroscopy in the UV to near infrared region has been implemented to monitor the energetic changes of valence electrons in molecules during chemical reaction, and to identify the intermediates and their stabilization by the surrounding. New laser flash-techniques with semiconductor probe sources have been developed to access reaction steps on the nano- to (milli-)second scale. As the complementary methods use the same spectrally tunable femtosecond pump, the reaction course can be followed for the first time over 14 magnitudes under identical starting conditions. With these techniques the influence of substituents and solvent on the reaction of medium sized molecules is studied to relate the structure to the reactivity and to investigate the physical basis of this relation. Based on a systematic variation of substituents the ground state reactivity of polar reactants can be classified in a general scale that allows the quantitative prediction of reaction speed: The reactivity of nucleophiles is parameterized via the kinetics of their combinations with a reference set of diarylcarbenium cations. Here the underlying empiric linear free energy relation is confirmed for fast bimolecular reactions from milliseconds to the diffusion limit. Three highly reactive nucleophiles are classified. For anions with two reaction centres, strong limitations of the HSAB principle and related theoretical concepts are revealed as the observed reactivity contradicts their predictions. In contrast to these bimolecular reactions, ultrafast structural reorganisation within one molecule initiated by a femtosecond light pulse are not superposed by diffusive and orientational motions or thermal activation. Such unimolecular reactions are studied in three molecular families that exhibit electron transfer (ET) between donor and acceptor units arranged via a central sp3-hybridized carbon. In spite of the common reaction centre the photo-processes span pure physical deactivation to complete chemical conversion. ET without bond cleavage is studied in low to high-polar solvents for a lactonic derivative of triarylmethane. The optically populated charge transfer state is converted into a highly polar charge transfer state. The observed kinetics of this conversion on the picosecond-scale is strongly dependent on the solvent but not directly correlated to the known solvation times. To explain these findings a quantitative model is established that takes into account the relaxation processes of the solute-solvent system after optical excitation. According to their unequal dipole moments, the donor and acceptor level of the ET are stabilized during the solvation to a different extent. In sufficient polar solvents the product state becomes that way energetically accessible and even favoured. This dynamic behaviour of the energetics renders the barrier of the ET and in turn its rates in forward and backward direction time-dependent. So the efficiency and kinetics of the ET are fully determined by the solvation. The dissociative ET of diarylmethane derivatives towards its ionic fragments is resolved for the first time also spectrally – here for the precursor compounds of the nitrite ion and diarylcarbenium cations of low and medium reactivity in the afore mentioned scale. The main component of the absorption of the sp2-hybridized cation emerges within a few 100 fs after excitation. The final band shape develops on the scale of 10 ps predominantly by vibrational cooling as revealed by the spectral signatures. While the sub-ps dynamics of the heterolytic dissociation is comparable with the reported one of diphenylmethyl chloride, the channel via the radical pair and successive intermolecular ET is reduced or even suppressed. The photochromic transformation from dihydroazulene (DHA) derivatives to their vinylhaptafulvene (VHF) isomers is found to proceed via the ring-opening on the fs- to ps-scale followed by internal conversion on the VHF geometry. Due to its dissociative ET character, the ring-opening is accelerated by the dynamic solvation of the VHF excited state in analogy to the solvent control in the lactonic triarylmethane derivative. Coherent wave packet oscillations reveal motions of the molecular frame around its central tetrahedral carbon. These vibrations bring the electron acceptor unit in plane with the excited conjugated system and promote the ET to the sigma-bond to be broken. In spite of the common general reaction path, DHA derivatives can differ strongly in their switching properties. A comparative study for compounds from three DHA classes correlates the substitution pattern with the reactivity. Electronically unsaturated substituents cause a delocalisation of the DHA wave function reducing the driving force and speed of the initial dissociative ET. For such compounds a non-reactive deactivation channel of the DHA excitation is observed in the UV bleaching region. The structural constraint of the rotation of the ring-opened isomer is shown to introduce a conical intersection on the deactivation pathway of the excited VHF. These insights explain the occurrence of solvent dependent quantum yields or the lack of the photoinduced reaction from VHF on and open the route towards an optical DHA switch. Host-guest-systems of 2-(2’-hydroxyphenyl)benzothiazole (HBT) in zeolites are studied in the form of colloidal suspension by optical transmission spectroscopy. Depending on the acid-base properties of the surrounding, two different species are found in the zeolite voids: enol-HBT is converted into the keto-form in a base catalyzed reaction with the anion of HBT as intermediate. Photoexcitation converts the ketotautomer back to the enol-tautomer. This transformation was observed with a for zeolite samples unprecedented resolution of sub-200 fs to proceed via ultrafast deprotonation mediated by the protic surrounding on the ps-scale. The photo-cycle is completed by nanosecond relaxation of the excited anion to the ground state. As this first application shows, the colloidal zeolite suspensions are well suited for developing host-guest-systems with medium sized molecules. Particularly, femtosecond transient transmission spectroscopy has proven to resolve the photodynamics in real-time, which opens new perspectives for the design of nano-structured supramolecular functional materials.