@phdthesis{Kullmann2013,
  author    = {Kullmann, Martin Armin},
  title     = {Tracing Excited-State Photochemistry by Multidimensional Electronic Spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-81276},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {Light-induced excitation of matter proceeds within femtoseconds, resulting in excited states. Originating from these states chemical reaction mechanisms, like isomerization or bond formation, set in. Photophysical mechanisms like energy distribution and excitonic delocalization also occur. Thus, the reaction scheme has to be disentangled by assessing the importance of each process. Spectroscopic methods based on fs laser pulses have emerged as a versatile tool to study these reactions. Within this thesis time-resolved experiments with fs laser pulses on various molecular systems were performed. Novel photosystems, with possible applications ranging from ultrathin molecular wires to molecular switches, were extensively characterized. To resolve the complex kinetics of the investigated systems, time-resolved techniques had to be newly developed. By combining a visible excitation pulse pair with an additional pulse and a continuum probe electronic triggered-exchange two-dimensional spectroscopy (TE2D) was demonstrated for the first time. This goal was accomplished by combining a three-color transient-absorption setup with a pulse shaper. Hence, 2D spectroscopy with a continuum probe was also implemented. Using these methods two different molecular systems in solution were characterized in a comprehensive manner. (ZnTPP)2, a directly beta,beta'-linked Zn-metallated bisporphyrin, and a spiropyran-merocyanine photosystem, 6,8-dinitro BIPS, were characterized. (ZnTPP)2 is a homodimer, featuring strong excitonic effects. These manifest themselves in a twofold splitting of the Soret band (S2). 6,8-Dinitro BIPS exists in one of two possible conformations. The ring closed spiropyran absorbs only in the UV, while the ring open merocyanine also absorbs in the visible. For both molecular systems photodynamics upon illumination were monitored using transient-absorption. However, the obtained results were ambiguous, necessitating more complex methods. In the case of (ZnTPP)2 first the monomeric building block was characterized. There, population transfer from the S2 state into S1 within 2 ps was identified. Afterwards, intersystem crossing proceeds within 2 ns. For (ZnTPP)2 similar pathways were found, albeit the relaxation is faster. The intersystem crossing with 1.5 ns was not only indirectly deduced but directly measured by probing in the NIR spectral range. The excitonic influence of was investigated by coherent 2D spectroscopy in the Soret band. Population transfer within S2 was directly visualized on a time-scale of 100 fs. Calculation of the 2D spectra of a simple homodimer confirmed the results. After this analysis of the distinct excitonic character, this molecule may serve as a building block for larger porphyrin arrays with applications ranging from asymmetric catalysis over biomimicry of electron-transfer to organic optical devices. The second photosystem was the molecular switch 6,8-dinitro BIPS, existing in two conformations. Merocyanine is the more stable form in thermal equilibrium. Transient-absorption measurements uncovered that the sample consisted of a mixture of two merocyanine isomers, referred to as TTC and TTT. However, both isomers are capable of ring-closure forming spiropyran. The remaining excited molecules return to the ground state radiatively. Conducting 2D measurements utilizing a continuum probe the differing photochemistry of both isomers was examined in a single measurement. No isomerization between these conformations was detected. Therefore, 6,8-dinitro BIPS performs a concerted switching without long-living intermediates. This was confirmed by a pump-repump-probe scan. 6,8-DinitroBIPS can be closed by visible and opened by UV pulses using subsequent pulses and vice versa. These mechanisms via singlet pathways satisfy an important criterion for a unimolecular switching device. A second pump-repump-probe experiment showed that the sample is ionized, resulting in a merocyanine radical cation, when the first excited state is resonantly excited. Furthermore, by implementing TE2Dspectroscopy, it was elucidated that only TTC was ionized. Taking all this into account new techniques were developed and complex molecular systems were characterized within this thesis. Deeper insight into the photodynamics of (ZnTPP)2and 6,8-dinitro BIPS was gained by adapting transient absorption for the NIR spectral range, constructing a 2D setup in pump-probe geometry, and combining it with multipulse excitation to coherent TE2D. All techniques solved the questions for which they were constructed, but they are not limited to these cases. Especially TE2D opens new roads in photochemistry. By connecting reactant, product and the corresponding intermediates, a chemical reaction can be tracked through all stages, making unambiguous identification of the reactive states feasible. Thus, fundamental insight into the photochemistry of molecular compounds is gained.},
  subject      = {Femtosekundenspektroskopie},
  language  = {en}
}
@article{BrixnerKochKullmannetal.2013,
  author    = {Brixner, Tobias and Koch, Federico and Kullmann, Martin and Selig, Ulrike and Nuernberger, Patrick and G{\"o}tz, Daniel C. G. and Bringmann, Gerhard},
  title     = {Coherent two-dimensional electronic spectroscopy in the Soret band of a chiral porphyrin dimer},
  series = {New Journal of Physics},
  journal   = {New Journal of Physics},
  doi       = {10.1088/1367-2630/15/2/025006},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-96139},
  year      = {2013},
  abstract  = {Using coherent two-dimensional (2D) electronic spectroscopy in fully noncollinear geometry, we observe the excitonic coupling of β,β'-linked bis[tetraphenylporphyrinato-zinc(II)] on an ultrafast timescale in the excited state. The results for two states in the Soret band originating from an excitonic splitting are explained by population transfer with approximately 100 fs from the energetically higher to the lower excitonic state. This interpretation is consistent with exemplary calculations of 2D spectra for a model four-level system with coupling.},
  language  = {en}
}