a These authors contributed equally to the work. AbstractThe understanding of excimer formation in organic materials is of fundamental importance, since it profoundly influences their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Frank-Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for at least 5 ps. The second excimer forming pathway involves large amplitude oscillations along the parallel shift 1 coordinate with a period of ≈ 900 fs that after intramolecular vibrational energy redistribution on a 2 ps time scale leads to the formation of a perfectly stacked dimer.The electronic relaxation within the excitonic manifold is mediated by the presence of conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6-8 ps, in excellent agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by collisions with the surrounding molecules leading to the slow excimer stabilization with a time constant of several picoseconds.2

Diborynes, molecules containing homoatomic boron-boron triple bonds, have been investigated by Raman spectroscopy in order to determine the stretching frequencies of their central B≡B units as an experimental measure of homoatomic bond strengths. The observed frequencies between 1600 and 1750 cm(-1) were assigned on the basis of DFT modeling and the characteristic pattern produced by the isotopic distribution of boron. This frequency completes the series of known stretches of homoatomic triple bonds, fitting into the trend established by the long-known stretching frequencies of C≡C and N≡N triple bonds in alkynes and dinitrogen, respectively. A quantitative analysis was carried out using the concept of relaxed force constants. The results support the classification of the diboryne as a true triple bond and speak to the similarities of molecules constructed from first-row elements of the p block. Also reported are the relaxed force constants of a recently reported diborabutatriene, which again fit into the trend established by the vibrational spectroscopy of organic cumulenes. As part of these studies, a new diboryne with decreased steric bulk was synthesized, and a computational study of the rotation of the stabilizing ligands indicated alkyne-like electronic isolation of the central B2 unit.

The self-reaction of the phenyl radical is one of the key reactions in combustion chemistry. Here we study this reaction in a high-temperature flow reactor by IR/UV ion dip spectroscopy, using free electron laser radiation as mid-infrared source. We identified several major reaction products based on their infrared spectra, among them indene, 1,2-dihydronaphthalene, naphthalene, biphenyl and para-terphenyl. Due to the structural sensitivity of the method, the reaction products were identified isomer-selectively.The work shows that the formation of indene and naphthalene, which was previously considered to be evidence for the HACA (hydrogen abstraction C 2 H 2 addition) mechanism in the formation of polycyclic aromatic hydrocarbons and soot can also be understood in a phenyl addition model.

In this work we investigate the excited-state structure and dynamics of the two molecules 1,8-naphthalimide (NI) and N-methyl-1,8-naphthalimide (Me-NI) in the gas phase by picosecond time- and frequency-resolved multiphoton ionization spectroscopy. The energies of several electronically excited singlet and triplet states and the S1 vibrational wavenumbers were calculated. Nonadiabatic dynamics simulations support the analysis of the radiationless deactivation processes. The origin of the S1 ← S0 (ππ*) transition was found at 30, 082 cm(-1) for NI and at 29, 920 cm(-1) for Me-NI. Furthermore, a couple of low-lying vibrational bands were resolved in the spectra of both molecules. In the time-resolved scans a biexponential decay was apparent for both Me-NI and NI. The fast time constant is in the range of 10-20 ps, whereas the second one is in the nanosecond range. In accordance with the dynamics simulations, intersystem crossing to the fourth triplet state S1 (ππ*) → T4 (nπ*) is the main deactivation process for Me-NI due to a large spin-orbit coupling between these states. Only for significant vibrational excitation internal conversion via a conical intersection becomes a relevant deactivation pathway.

We report a synthetic, spectroscopic and computational study of the polycyclic aromatic molecule pyracene, which contains aliphatic five-membered rings annealed to a naphthalene chromophore. An improved route to synthesize the compound is described. Gas-phase IR and solid-state Raman spectra agree with a ground-state D2h structure. The electronically excited S1 A(1)B3u state has been studied by resonance-enhanced multiphoton ionisation. An adiabatic excitation energy T0 = 30,798 cm(-1) (3.818 eV) was determined. SCS-ADC(2) calculations found a D2h minimum energy structure of the S1 state and yielded an excitation energy of +3.98 eV, including correction for zero point vibrational energy. The spectrum shows a rich low-frequency vibrational structure that can be assigned to the overtones of out-of-plane deformation modes of the five-membered rings by comparison with computations. The appearance of these modes as well as the frequency reduction in the excited state indicate that the potential in the S1 state is very flat. At higher excess energies most bands can be assigned to fundamentals, overtones and combination bands of either totally symmetric ag modes or of b2g modes that appear due to vibronic coupling. Lifetimes between 43 ns and 76 ns were measured for a number of vibronic bands. For the S2 state an equilibrium geometry with a non-planar carbon framework was computed. In addition a signal from the pyracene dimer was present. The spectrum shows several broad and structureless transitions. The origin band has a maximum at around 329 nm (30,400 cm(-1)). Again lifetimes between 60 ns and 70 ns were found. The dimer ion signal rises within less than 10 ps. Computations show that a crossed geometry with the long axis of one unit aligned with the short axis of the second constitutes the most stable structure. The broadening of the bands is most likely caused by excimer formation.