Thomas Scheby Kuhlman

Interpretation of the Ultrafast Photoinduced Processes in Pentacene Thin Films

Ambiguity remains in the models explaining the photoinduced dynamics in pentacene thin films as observed in pump-probe experiments. One model advocates exciton fission as governing the evolution of the initially excited species, whereas the other advocates the formation of an excimeric species subsequent to excitation. On the basis of calculations by a combined quantum mechanics and molecular mechanics (QM/MM) method and general considerations regarding the excited states of pentacene we propose an alternative, where the initially excited species instead undergoes internal conversion to a doubly excited exciton. The conjecture is supported by the observed photophysical properties of pentacene from both static as well as time-resolved experiments.

Coherent Motion Reveals Non-Ergodic Nature of Internal Conversion between Excited States

We found that specific nuclear motion along low-frequency modes is effective in coupling electronic states and that this motion prevail in some small molecules. Thus, in direct contradiction to what is expected based on the standard models, the internal conversion process can proceed faster for smaller molecules. Specifically, we focus on the S(2) →S(1) internal conversion in cyclobutanone, cyclopentanone, and cyclohexanone. By means of time-resolved mass spectrometry and photoelectron spectroscopy the relative rate of this transition is determined to be 13:2:1. Remarkably, we observe coherent nuclear motion on the S(2) surface in a ring-puckering mode and motion along this mode in combination with symmetry considerations allow for a consistent explanation of the observed relative time-scales not afforded by only considering the density of vibrational states or other aspects of the standard models.

The Non-Ergodic Nature of Internal Conversion

The absorption of light by molecules can induce ultrafast dynamics and coupling of electronic and nuclear vibrational motion. The ultrafast nature in many cases rests on the importance of several potential energy surfaces in guiding the nuclear motion-a concept of central importance in many aspects of chemical reaction dynamics. This Minireview focuses on the non-ergodic nature of internal conversion, that is, on the concept that the nuclear dynamics only sample a reduced phase space, potentially resulting in localization of the dynamics in real space. A series of results that highlight the nonstatistical nature of the excited-state deactivation process is presented. The examples are categorized into four groups. 1) Localization of the energy in one degree of freedom in S2 →S1 transitions, in which the transition is either determined by the time spent in the S2 →S1 coupling region or by the time it takes to reach it. 2) Localization of energy into a single reactive mode, which is dictated by the internal conversion process. 3) Initiation of the internal conversion by activation of a single complex motion, which then specifically couples to a reactive mode. 4) Nonstatistical internal conversion as a tool to accomplish biomolecular stability. Herein, the discussion on nonstatistical internal conversion in DNA as a mechanism to eliminate electronic excitation energy is extended to include molecules with an S-S bond as a model of the disulfide bridge in peptides. All of these examples are summed up in Kashas rule. For systems with multiple degrees of freedom it will be possible to locate an appropriate motion somewhere in phase space that will take the wavepacket to the coupling region and facilitate an ultrafast transition to S1. Once at S1, the momentum of the wavepacket is lost and the only options left are the statistical processes of reaction or light emission.

Pulling the levers of photophysics: how structure controls the rate of energy dissipation.

A general picture of the structural parameters that control the rate of an internal conversion process leading to energy dissipation is not easily deducible. Herein, we demonstrate how the rate of such an internal conversion process can change by more than an order of magnitude for related molecules as a consequence of minor structural variations. We disentangle the complex process and identify one specific vibrational mode involved through the use of the techniques time-resolved mass-spectrometry (TRMS) and time-resolved photoelectron spectroscopy (TRPES) on four molecules: 2methylcyclobutanone (2-MeCB), 2-methylcyclopentanone (2-MeCP), 3-methylcyclopentanone (3-MeCP), and 3-ethylcyclopentanone (3-EtCP). These four molecules are structurally similar to cyclobutanone (CB), cyclopentanone (CP), and cyclohexanone (CH) investigated in our previous works (Figure 1). [6, 7] Following excitation to S2 ,( n,3s) state, the temporal evolution of the ion currents presented in Figure 2 closely resemble that of the (n,3s) photoelectron peak also given in the figure for 2-MeCB and 2-MeCP. Consequently, the decay

Between ethylene and polyenes - the non-adiabatic dynamics of cis-dienes

Using Ab Initio Multiple Spawning (AIMS) with a Multi-State Multi-Reference Perturbation theory (MS-MR-CASPT2) treatment of the electronic structure, we have simulated the non-adiabatic excited state dynamics of cyclopentadiene (CPD) and 1,2,3,4-tetramethyl-cyclopentadiene (Me4-CPD) following excitation to S1. It is observed that torsion around the carbon-carbon double bonds is essential in reaching a conical intersection seam connecting S1 and S0. We identify two timescales; the induction time from excitation to the onset of population transfer back to S0 (CPD: -25 fs, Me4-CPD: -71 fs) and the half-life of the subsequent population transfer (CPD: -28 fs, Me4-CPD: -48 fs). The longer timescales for Me4-CPD are a kinematic consequence of the inertia of the substituents impeding the essential out-of-plane motion that leads to the conical intersection seam. A bifurcation is observed on S1 leading to population transfer being attributable, in a 5 : 2 ratio for CPD and 7 : 2 ratio for Me4-CPD, to two closely related conical intersections. Calculated time-resolved photoelectron spectra are in excellent agreement with experimental spectra validating the simulation results.

Symmetry, vibrational energy redistribution and vibronic coupling: The internal conversion processes of cycloketones

In this paper, we discern two basic mechanisms of internal conversion processes; one direct, where immediate activation of coupling modes leads to fast population transfer and one indirect, where internal vibrational energy redistribution leads to equidistribution of energy, i.e., ergodicity, and slower population transfer follows. Using model vibronic coupling Hamiltonians parameterized on the basis of coupled-cluster calculations, we investigate the nature of the Rydberg to valence excited-state internal conversion in two cycloketones, cyclobutanone and cyclopentanone. The two basic mechanisms can amply explain the significantly different time scales for this process in the two molecules, a difference which has also been reported in recent experimental findings [T. S. Kuhlman, T. I. Sølling, and K. B. Møller, ChemPhysChem. 13, 820 (2012)].

Surprising Intrinsic Photostability of the Disulfide Bridge Common in Proteins

For a molecule to survive evolution and to become a key building block in nature, photochemical stability is essential. The photolytically weak S-S bond does not immediately seem to possess that ability. We mapped the real-time motion of the two sulfur radicals that result from disulfide photolysis on the femtosecond time scale and found the reason for the existence of the S-S bridge as a natural building block in folded structures. The sulfur atoms will indeed move apart on the excited state but only to oscillate around the S-S center of mass. At long S-S distances, there is a strong coupling to the ground state, and the oscillatory motion enables the molecules to continuously revisit that particular region of the potential energy surface. When a structural feature such as a ring prevents the sulfur radicals from flying apart and thus assures a sufficient residence time in the active region of the potential energy surface, the electronic energy is converted into less harmful vibrational energy, thereby restoring the S-S bond in the ground state.

Comment on "Theoretical investigation of perylene dimers and excimers and their signatures in X-ray diffraction".

The structures of the ground and excimer states of perylene pairs are calculated [using density functional theory (DFT) and time-dependent DFT techniques] in a free as well as a crystal environment, and their spectroscopic properties are studied for the most stable configurations. The vertical transition energies for the absorption and emission bands are obtained, and they are in good agreement with experimental data. In these calculations, up to six excited states are considered. With the calculated structures of the ground and excimer states, the scattering factors are analyzed as a function of the concentration of excimers in a crystal. The intensity of the 110, 005, and 0 10 0 reflections are found to be fairly sensitive to the presence of excimers in the crystal. The finite (nanosecond) lifetime of the excimer may make it possible to observe this state using time-resolved X-ray diffraction techniques.

Tuning and Tracking of Coherent Shear Waves in Molecular Films

We have determined the time-dependent displacement fields in molecular sub-micrometer thin films as response to femtosecond and picosecond laser pulse heating by time-resolved X-ray diffraction. This method allows a direct absolute determination of the molecular displacements induced by electron–phonon interactions, which are crucial for, for example, charge transport in organic electronic devices. We demonstrate that two different modes of coherent shear motion can be photoexcited in a thin film of organic molecules by careful tuning of the laser penetration depth relative to the thickness of the film. The measured response of the organic film to impulse heating is explained by a thermoelastic model and reveals the spatially resolved displacement in the film. Thereby, information about the profile of the energy deposition in the film as well as about the mechanical interaction with the substrate material is obtained.

Time-Resolved Photoelectron Spectra

Through the time evolution of the wavefunction, dynamics simulations provide insight into the system under investigation. Often, it can be beneficial to consider partially integrated quantities such as electronic populations or coordinate expectation values to map out the dynamics. However, if we are interested in comparing simulations to experiment, it can be necessary to calculate the experimental observable from the dynamics simulations to allow for a direct comparison between theory and experiment.