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.

Adsorption energies of chemisorbed molecules on inorganic solids usually scale linearly with molecular size and are well described by additive scaling laws. However, much less is known about scaling laws for physisorbed molecules. Our temperature-programmed desorption experiments demonstrate that the adsorption energy of acenes (benzene to pentacene) on the Au(111) surface in the limit of low coverage is highly nonadditive with respect to the molecular size. For pentacene, the deviation from an additive scaling of the adsorption energy amounts to as much as 0.7 eV. Our first-principles calculations explain the observed nonadditive behavior in terms of anisotropy of molecular polarization stemming from many-body electronic correlations. The observed nonadditivity of the adsorption energy has implications for surface-mediated intermolecular interactions and the ensuing on-surface self-assembly. Thus, future coverage-dependent studies should aim to gain insights into the impact of these complex interactions on the selfassembly of π-conjugated organic molecules on metal surfaces.

Naphthothiadiazoles are promising electron acceptors for applications in organic semiconductor-based (opto)­electronic devices. Here, we studied the structural and electronic properties of naphthothiadiazole (NTD) derivatives adsorbed on Au(111) in the monolayer and thin films using temperature-programmed desorption as well as vibrational and electronic high-resolution electron energy loss spectroscopy. In addition, we performed state-of-the-art quantum chemical calculations to further illuminate electronic properties. In the monolayer and multilayer coverage regime, the NTD derivatives adsorbed in a planar fashion with the molecular backbone oriented parallel to the gold surface. Several singlet and the first triplet transition energies are determined. The optical gap (S0 → S1 transition) in the nonhalogenated parent NTD is found to be 2.6 eV, whereas it is reduced by 200 meV in the chlorinated and brominated NTD. All experimentally observed singlet and triplet transition energies are reduced due to halogenation, which is underlined by theory.

N-heteropolycyclic aromatic compounds are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the...