Ewelina Janiga

Surface-assisted reactions on nonmetallic substrates remain much less developed than their counterparts on metals, especially for heteroatom-doped precursors and intermolecular coupling processes. Here we show that TiO₂(110) can support not only cyclodehydrogenation of a nitrogen-rich nanographene precursor but also annulative intermolecular coupling on a semiconducting oxide surface. Under ultrahigh vacuum, hexapyrrolylbenzene (HPB) undergoes cyclodehydrogenation on rutile TiO₂(110) to give unsubstituted hexapyrrolohexaazacoronene (HPHAC, 1). Upon further annealing, 1 undergoes dehydrogenative dimerization to form a cyclooctatetraene-annulated HPHAC dimer (2), as supported by scanning tunneling microscopy and spectroscopy (STM/STS), density functional theory (DFT), and simulated STM images. The dimer, but not the monomer, undergoes single-electron transfer to the substrate to form [2]^•+, consistent with STM/STS observations, its lower ionization threshold, and computed frontier orbital energies. Gas-phase calculations are consistent with a plausible arenium-type pathway for annulative dimer formation and further indicate that oxidation increases global aromatic character across the fused framework.

Energy-transfer cassettes consisting of naphthaleneimide-fused metalloporphyrin acceptors (M=Zn and Pd) and BODIPY donors have been designed and synthesized. These systems have rigid pseudo-tetrahedral structures with a donor-acceptor separation of ca. 17.5 Å. Spectroscopic investigations, including femtosecond transient absorption measurements, showed efficient excitation energy transfer (EET) occurring according to the Förster mechanism. Strong fluorescence of the donor units and significant spectral overlap of the donor and acceptor subunits are prerequisites for the efficient EET in these systems.

The purpose of this study was to investigate 3,4-diarylpyrrole electropolymerization and simultaneous or subsequent intramolecular oxidative coupling leading to poly(phenanthropyrrole). Small amounts of π-conjugated products were obtained only under the conditions of increased concentration of the monomer and increased rate of potential sweep, which enabled both processes − monomer oxidation and deprotonation of the σ-dimer. The combined electrochemistry, UV-vis-NIR, ESR and TD-DFT results show, that the α,α'-coupling of 3,4-diarylpyrrole derivatives is inhibited by the stability of the σ-dimer dication, inhibition of the deprotonation and the reversal of the σ bond formation resulting in regeneration of the monomer. The addition of pyridine did not result in σ–dimer deprotonation in 2- and 2'-position.