The 1 L a and 1 L b classification of electronically excited states of cata-condensed hydrocarbons proposed by Platt in 1949 (PlattJ. R. Platt, J. R. J. Chem. Phys.194917484) is challenged by investigating a series of N-heteronaphthalenes and comparison of their low-lying ππ* excited states to those of naphthalene. The breakdown of Platt’s classification scheme for N-heterocycles is highlighted, and a reliable and versatile alternative using exciton analyses is presented. The strength of electron–hole correlation turns out to be the most reliable distinguishing feature, and thus, an alternative nomenclature of 1 L w (weakly correlated) and 1 L s (strongly correlated) is proposed. Furthermore, fundamental guidelines for their property modulation through N-atom substitution patterns are discussed.

The electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based field effect transistors and solar cells. Here, we investigated the adsorption and electronic properties of the N-heteropolycyclic aromatic compound 6,13-diazapentacene (DAP), a potential electron-transporting semiconductor on Au(111), using temperature-programmed desorption, vibrational and electronic high-resolution electron energy loss spectroscopy, two-photon photoemission spectroscopy, and state-of-the-art quantum chemical methods. In the mono- and multilayer regime DAP adsorbs in a planar fashion with the molecular backbone oriented parallel to the gold substrate. The energetic position of transport levels (electron affinities and ionization potentials) and singlet (S) as well as triplet (T) transition energies are quantitatively determined. The lowest affinity level is located at 3.48 eV, whereas the energetic position of the first excitonic state is at 4.00 eV, resulting in an exciton binding energy of 0.52 eV. Compared to pentacene, the optical gap is reduced by 0.1 eV and the α-band gains substantially in intensity, which is explained by a detailed analysis of the electronic structure. The optical gap, i.e., the S1 excitation energy, is determined to be 2.0 eV, and the T1 transition energy is 0.9 eV, making an exothermic singlet fission process relevant in organic photovoltaics feasible.

N-Heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field effect transistors. Here, we studied the structural and the electronic properties of an arrow-shaped N-heteropentacene derivative (triisopropylsilyl-dibenzodiazapentacene, TIPS-BAP) adsorbed on Au(111) in the monolayer and thin films using temperature-programmed desorption, vibrational and electronic high-resolution electron energy loss spectroscopy, and two-photon photoemission spectroscopy. In addition, we performed state-of-the-art quantum chemical calculations to further elucidate electronic properties. TIPS-BAP adopted an adsorption geometry in which the molecular backbone is oriented parallel to the Au(111) surface. We quantitatively determined the energetic position of several unoccupied as well as occupied molecular electronic states (transport states) with respect to the Fermi level of the gold substrate and resolved the optical gap (S0 → S1 transition) to be 1.9 eV. Compared to the corresponding polycyclic aromatic hydrocarbon TIPS-dibenzodipentacene (TIPS-BP), the optical gap is reduced by 0.2 eV due to nitrogen substitution. Based on our quantum chemical calculations, we attributed this effect to a stabilization of the first excited singlet state (S1) in the polar environment. Furthermore, an intense α-band (S0 → S2) in TIPS-BAP compared to TIPS-BP is observed due to an enhanced oscillator strength in the N-heteropolycyclic aromatic compound.

The oxidative cyclodehydrogenation (often named the Scholl reaction) is still a powerful synthetic tool to construct even larger polycyclic aromatic hydrocarbons (PAHs) by multiple biaryl bond formations without the necessity of prior installation of reacting functional groups. Scholl‐type reactions are usually very selective although the resulting products bear sometimes some surprises, such as the formation of five‐membered instead of six‐membered rings or the unexpected migration of aryl moieties. There are a few examples, where chlorinated byproducts were found when FeCl3 was used as reagent. To our knowledge, the direct functionalization of PAHs during Scholl‐type cyclization by triflyloxylation has not been observed. Herein we describe the synthesis of functionalized PAHs by the formation of five‐membered rings and a regioselective triflyloxylation in one step. The triflyloxylated PAHs can be used as reactants for further transformation to even larger contorted PAHs.

Homoconjugation and intramolecular “through-space” charge transfers are molecular phenomena that have been studied since the 1960s. A detailed understanding and control of these effects would provide a tool to tune the optoelectronic properties of organic molecules in respect of the necessities for applications such as for organic electronics. Triptycene is a perfect candidate to investigate homoconjugation effects due to its three-dimensional alignment of three aromatic phenylene units, separated by two methine bridges. Here, a series of 16 π-extended triptycenes with up to three different permuted electron-accepting units and an electron-rich veratrole unit are studied in detail by UV/vis spectroscopy and cyclovoltammetry in combination with DFT calculations to get a deeper understanding of homoconjugation and charge-transfer processes of triptycenes. Furthermore, the gained knowledge can be exploited to construct triptycene-based electron acceptors with fine-tuned adjustment of electronic properties, such as electron affinities, by thorough choice of the aromatic blades that interact through homoconjugation.