Zeitaufgelöste Fluoreszenzspektroskopie unimolekularer Reaktionen im Überschalldüsenstrahl: <i>trans-cis</i>-Photoisomerisierung, Phenylringtorsion, intramolekularer Wasserstoffatomtransfer

Abstract

In this work three types of unimolecular reactions were studied spectroscopically and theoretically with model systems: (1) the trans-cis-photoisomerization of trans-stilbene and its isotopomer trans-stilbene-d10 (2) the phenylring torsion in 2-phenylindene (3) the intramolecular proton transfer in 1,8-dihydroxyanthraquinone, 1-aminoanthraquinone and 9-hydroxyphenalenone.The molecular beam technique was employed in order to obtain cooled isolated molecules. Energy- and time-resolved as well as polarization-analyzed fluorescence detection techniques (fluorescence excitation spectroscopy, time-correlated single photon counting, rotational coherence spectroscopy) were employed in order to obtain spectroscopic and kinetic information on microcanonical ensembles of the model systems.The fluorescence excitation spectrum of trans-stilbene was determined in the excess energy range -50-1950 cm-1. It was attempted to assign the vibronic transitions using ab initio calculated S1-frequencies (CIS/6-311+G(d)).The fluorescence lifetimes of the phenylring deuterated isotopomer trans-stilbene-d10 were determined in the excess energy range 0-2700 cm-1 using the time-correlated single photon counting technique. The specific rate constants k(E) of the photoisomerization were analyzed according to an optimized RRKM procedure (barrier height E0 and frequency scaling factor F as fit parameters).The quantitative analysis of the rotational coherence signals of 2-phenylindene obtained by polarization-analyzed fluorescence detection yielded the sum of the rotational constans B+C in the S1-state as well as the rotational temperature prevailing in the supersonic jet expansions of this work. The torsional potentials (MP2/6-31G and CIS/6-31G(d)) and the transition state structures and frequencies (STQN-B3LYP/6-31G(d) and STQN-CIS/6-31G(d)) in both the S0- and the S1-state were determined by ab initio methods. Moreover, using the assignments of the vibronic transitions of the fluorescence excitation spectrum and the fluorescence dispersion spectrum (taken from the literature), the torsional potentials of both electronic states were simulated.The fluorescence excitation spectrum of 1,8-dihydroxyanthraquinone was determined in the excess energy range -250-1400 cm-1. The rotational band contour of the alleged electronic transition (taken from the literature) was quantitatively analyzed. The attempt to determine the fluorescence lifetimes of several vibronic transitions in the excess energy range 0-1000 cm-1 was not successful. All fluorescence decay curves showed strong non-exponential behavior. Moreover, all estimated fluorescence lifetimes were near the time resolution of the time-correlated single photon counting apparatus (42-327 ps).The fluorescence excitation spectrum of 1-aminoanthraquinone was determined in the excess energy range -75-1450 cm-1. All fluorescence decay curves could be fitted satisfactorily partly by monoexponential functions, partly by biexponential functions. The state-specific scatter of the fluorescence lifetimes is superposed by a distinct energy dependence, which could satisfactorily be accounted for by a fit according to Fermi's Golden Rule.The fluorescence excitation spectrum of 9-hydroxyphenalenone was determined in the excess energy range -150-1850 cm-1. The measurement of some fluorescence decay curves revealed the same difficulties mentioned in the case of 1,8-dihydroxyanthraquinone. The symmetric double-minimum potentials of the intramolecular proton transfer in both the S0- und the S1-state were determined by ab initio methods (HF/6-31G(d,p), B3LYP/6-31G(d,p) and CIS/6-31G(d,p)). The tunneling splittings of the vibrationless levels of both electronic states were calculated.