Temperature dependence of conformation, chemical state, and metal-directed assembly of tetrapyridylporphyrin on Cu (111) Two-dimensional localization of adsorbate/substrate charge-transfer excited states of molecules adsorbed on metal surfaces

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1-4 were investigated using steady-state UV-vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1-T1 energy gap (ΔES-T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV-vis-NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 1(2+) and 2(2+) was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene 3(2+) and triazacoronene 4(2+) with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized.

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A precise, one-pot synthesis of end-functionalized block copolymers consisting of poly(9,9-di-n-octylfluorene-2,7-vinylene)s (PFVs) and oligo(2,5-dialkoxy-1,4-phenylenevinylene) or terthiophene units as the middle segment have been prepared by olefin metathesis of the vinyl group in the PFV chain ends followed by subsequent Wittig-type coupling. Formation of the block copolymers and the quantitative efficiency in the end-functionalization can be confirmed by synthesis of amphiphilic block copolymers containing poly-(ethylene glycol) at the both polymer chain ends. Effect of PFV conjugation length, middle segment, and the end groups toward the emission properties have been studied: the polymers containing ferrocene moiety at the chain ends displayed unique emission/quenching properties.