N-Doped polycyclic aromatic hydrocarbons have recently emerged as potential organic electronic materials. The function of such materials is determined not only by the intrinsic electronic properties of individual molecules but also by their supramolecular interactions in the solid state. Therefore, a proper characterization of the interactions between the individual units is of interest to materials science since they ultimately govern properties such as excitons and charge transfer. Here, we report a joint experimental and computational study of two azaphenanthrene dimers to determine the structure and the nature of supramolecular interactions in the aggregates. IR/UV double-resonance experiments were carried out using far- and mid-infrared free-electron laser radiation. The experimental spectra are compared with quantum chemical calculations for the lowest-energy π-stacked and hydrogen-bonded structures. The data reveal a preference of the π-stacked structure for the benzo­[f]­quinoline and the phenanthridine dimer.

Photo-initiated intramolecular charge transfer (ICT) processes play a pivotal role in the excited state reaction dynamics in donor-bridge-acceptor systems. The efficacy of such a process can be improved by modifying the extent of π-conjugation, relative orientation/twists of the donor/acceptor entities and polarity of the environment. Herein, 4-dimethylamino-4'-cyanodiphenylacetylene (DACN-DPA), a typical donor-π-bridge-acceptor system, was chosen to unravel the role of various internal coordinates that govern the extent of photo-initiated ICT dynamics. Transient absorption (TA) spectra of DACN-DPA in n-hexane exhibit a lifetime of > 2 ns indicating the formation of a triplet state while, in acetonitrile, a short time-constant of ~2 ps indicates the formation of charge transferred species. Ultrafast Raman loss spectroscopy (URLS) measurements show distinct temporal and spectral dynamics of Raman bands associated with C�C and C=C stretching vibrations. The appearance of a new band at ~1492 cm À 1 in acetonitrile clearly indicates structural modification during the ultrafast ICT process. Furthermore, these observations are supported by TD-DFT computations.

The compound 4-dimethylamino-4′-carbomethoxydiphenylacetylene (DACM-DPA) is a donor-π-acceptor system that exhibits significant charge transfer character in the excited state in polar solvents. Femtosecond transient absorption measurements in n-hexane, acetonitrile, and methanol exhibit excitedstate lifetimes of >2 ns, ∼500 ps, and ∼30 ps, respectively. Femtosecond Raman measurements of DACM-DPA elucidated the intricate structural dynamics during the excited-state evolution by revealing distinct vibrational signatures of DACM-DPA in these solvents. In the case of n-hexane solvent, vibrational bands at 2075 cm −1 (C�C) and 1600 cm −1 (C�C) stretch exist throughout the excited-state lifetime. However, in the case of polar solvents, the amplitudes of vibrational frequencies at 2098 cm −1 (C�C), 1618 cm −1 (C�C, acetonitrile), and 1603 cm −1 (C�C, methanol) diminish in a time period of 2.5 ps (acetonitrile) and 6.5 ps (methanol) along with the appearance of new bands at 2082 cm −1 (cumulene stretch) and 1582 cm −1 (quinoid stretch), clearly revealing the formation of an intramolecular charge-separated state. Furthermore, these experimental observations are corroborated by computations using DFT and TD-DFT.

Isolated 2‐phenylallyl radicals (2‐PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2‐PA is a resonance‐stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas‐phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self‐reaction as well as reactions with unimolecular decomposition products of 2‐PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2‐PA is compared with the one of the related 2‐methylallyl and phenylpropargyl radicals.

We investigated the excited-state dynamics of 4-(dimethylamino)benzethyne (DMABE) in a combined theoretical and experimental study using surface-hopping simulations and time-resolved ionisation experiments. The simulations predict a decay of the initially...