Abstract
Acenes and azaacenes lie at the core of molecular materials’ applications due to their important optical and electronic features. A critical aspect is provided by their heteroatom multiplicity, which can strongly affect their properties. Here we report pyrazinacenes containing the dihydro-decaazapentacene and dihydro-octaazatetracene chromophores and compare their properties/functions as a model case at an oxidizing metal substrate. We find a distinguished, oxidation-state-dependent conformational adaptation and self-assembly behaviour and discuss the analogies and differences of planar benzo-substituted decaazapentacene and octaazatetracene forms. Our broad experimental and theoretical study reveals that decaazapentacene is stable against oxidation but unstable against reduction, which is in contrast to pentacene, its C–H only analogue. Decaazapentacenes studied here combine a planar molecular backbone with conformationally flexible substituents. They provide a rich model case to understand the properties of a redox-switchable π-electronic system in solution and at interfaces. Pyrazinacenes represent an unusual class of redox-active chromophores.
Highlights
Acenes and azaacenes lie at the core of molecular materials’ applications due to their important optical and electronic features
The non-planar molecular morphology together with the three-state redox system are key to the complexity observed and have been confirmed independently using high-resolution scanning tunneling microscopy (STM) and ab initio density functional theory (DFT) simulations (see Supplementary Information (SI) for experimental and computational details)
Octaazatetracene 2 is available by condensation of dichloro-3 with 2,3-diaminopyrazine 6
Summary
Acenes and azaacenes lie at the core of molecular materials’ applications due to their important optical and electronic features. Established structure–function relationships of molecular materials known from solution need to be re-evaluated to predict and understand the interface-specific chemical, electronic, optical and mechanical properties of any newly synthesized molecules Along these lines, we here discuss a special class of heteroacenes in-solution and on surfaces, in particular for their complex and interesting redox activity and supramolecular chemistry by taking a broad experimental approach in conjunction with ab initio property prediction techniques. The physicochemical properties of these molecules are discussed in the context of their modified redox state before and after on-surface reactions: we link their different observed oxidation states to their conformation, their configurations and their aggregation on surfaces The complexity of this system derives from its molecular architecture, which combines a planar molecular backbone with substituents exhibiting conformational freedom and a redox-variable π-electronic system. Delocalized electron systems comprising N and C π-electronic states[23,24] qualify the pyrazinacenes for the investigation of their light absorption as a chromophore and their redox behaviour, within specific supramolecular and on-surface architectures
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