@unpublished{AuerhammerSchulzSchmiedeletal.2019, author = {Auerhammer, Nina and Schulz, Alexander and Schmiedel, Alexander and Holzapfel, Marco and Hoche, Joscha and R{\"o}hr, Merle I. S. and Mitric, Roland and Lambert, Christoph}, title = {Dynamic exciton localisation in a pyrene-BODIPY-pyrene dye conjugate}, series = {Physical Chemistry Chemical Physics}, journal = {Physical Chemistry Chemical Physics}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-198718}, year = {2019}, abstract = {The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.}, language = {en} } @article{HocheFlockMiaoetal.2021, author = {Hoche, Joscha and Flock, Marco and Miao, Xincheng and Philipp, Luca Nils and Wenzel, Michael and Fischer, Ingo and Mitric, Roland}, title = {Excimer formation dynamics in the isolated tetracene dimer}, series = {Chemical Science}, volume = {12}, journal = {Chemical Science}, number = {36}, doi = {10.1039/D1SC03214C}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-251559}, pages = {11965 -- 11975}, year = {2021}, abstract = {The understanding of excimer formation and its interplay with the singlet-correlated triplet pair state \(^{1}\)(TT) is of high significance for the development of efficient organic electronics. Here, we study the photoinduced dynamics of the tetracene dimer in the gas phase by time-resolved photoionisation and photoion imaging experiments as well as nonadiabatic dynamics simulations in order to obtain mechanistic insight into the excimer formation dynamics. The experiments are performed using a picosecond laser system for excitation into the S\(_{2}\) state and reveal a biexponential time dependence. The time constants, obtained as a function of excess energy, lie in the range between ≈10 ps and 100 ps and are assigned to the relaxation of the excimer on the S\(_{1}\) surface and to its deactivation to the ground state. Simulations of the quantum-classical photodynamics are carried out in the frame of the semi-empirical CISD and TD-lc-DFTB methods. Both theoretical approaches reveal a dominating relaxation pathway that is characterised by the formation of a perfectly stacked excimer. TD-lc-DFTB simulations have also uncovered a second relaxation channel into a less stable dimer conformation in the S\(_{1}\) state. Both methods have consistently shown that the electronic and geometric relaxation to the excimer state is completed in less than 10 ps. The inclusion of doubly excited states in the CISD dynamics and their diabatisation further allowed to observe a transient population of the \(^{1}\)(TT) state, which, however, gets depopulated on a timescale of 8 ps, leading finally to the trapping in the excimer minimum.}, language = {en} } @unpublished{HumeniukBužančićHocheetal.2020, author = {Humeniuk, Alexander and Bužančić, Margarita and Hoche, Joscha and Cerezo, Javier and Mitric, Roland and Santoro, Fabrizio and Bonačić-Koutecky, Vlasta}, title = {Predicting fluorescence quantum yields for molecules in solution: A critical assessment of the harmonic approximation and the choice of the lineshape function}, series = {The Journal of Chemical Physics}, journal = {The Journal of Chemical Physics}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-199305}, year = {2020}, abstract = {For the rational design of new fluorophores, reliable predictions of fluorescence quantum yields from first principles would be of great help. However, efficient computational approaches for predicting transition rates usually assume that the vibrational structure is harmonic. While the harmonic approximation has been used successfully to predict vibrationally resolved spectra and radiative rates, its reliability for non-radiative rates is much more questionable. Since non-adiabatic transitions convert large amounts of electronic energy into vibrational energy, the highly excited final vibrational states deviate greatly from harmonic oscillator eigenfunctions. We employ a time-dependent formalism to compute radiative and non-radiative rates for transitions and study the dependence on model parameters. For several coumarin dyes we compare different adiabatic and vertical harmonic models (AS, ASF, AH, VG, VGF, VH), in order to dissect the importance of displacements, frequency changes and Duschinsky rotations. In addition we analyze the effect of different broadening functions (Gaussian, Lorentzian or Voigt). Moreover, to assess the qualitative influence of anharmonicity on the internal conversion rate, we develop a simplified anharmonic model. We adress the reliability of these models considering the potential errors introduced by the harmonic approximation and the phenomenological width of the broadening function.}, language = {en} } @article{HocheSchulzDietrichetal.2019, author = {Hoche, Joscha and Schulz, Alexander and Dietrich, Lysanne Monika and Humeniuk, Alexander and Stolte, Matthias and Schmidt, David and Brixner, Tobias and W{\"u}rthner, Frank and Mitric, Roland}, title = {The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes}, series = {Chemical Science}, volume = {10}, journal = {Chemical Science}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-198707}, pages = {11013}, year = {2019}, abstract = {Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM),i.e., merocyanine 4-(dicyanomethylene)-2-tert-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4H-pyran (DCBT). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law.This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.}, language = {en} }