Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface

Abstract

On-surface synthesis provides a powerful method for the generation of long acene molecules, making possible the detailed investigation of the electronic properties of single higher acenes on a surface. By means of scanning tunneling microscopy and spectroscopy combined with theoretical considerations, we discuss the polyradical character of the ground state of higher acenes as a function of the number of linearly fused benzene rings. We present energy and spatial mapping of the tunneling resonances of hexacene, heptacene, and decacene, and discuss the role of molecular orbitals in the observed tunneling conductance maps. We show that the energy gap between the first electronic tunneling resonances below and above the Fermi energy stabilizes to a finite value, determined by a first diradical electronic perturbative contribution to the polyacene electronic ground state. Up to decacene, the main contributor to the ground state of acenes remains the lowest-energy closed-shell electronic configuration.

Dodecacene Generated on Surface: Reopening of the Energy Gap

“…2b. It is important to note that, as already reported for the on-surface formation of heptacene from the corresponding precursor 8 , the epoxy groups are visible in a STM image only at high bias voltage (V =-1 V in this case). The last step of the precursor deoxygenation leading to the formation of dodecacene on surface can be induced by inelastic tunneling electrons, as shown in Fig.…”

“…The maps visualize the spatial distribution of the molecular resonances and are very similar to those obtained for N = 10 (decacene) 8 , showing however the features of the two additional rings.…”

“…It is important to note that this ground state is neither the closed shell singlet |S0> nor a di-radical state. It is rather a quantum superposition between the neutral |S0> and the radical forms 8 . The conductance map recorded at R0 is the signature of the mono-electronic contribution of the highest occupied molecular orbital (HOMO) to this ground state, since both the |S0> and the di-radical states can be decomposed on Slaters determinants with a dominant weight for the acene HOMOs.…”

“…The on-surface investigation of single acenes is therefore developing as a promising complementary investigation method for exploring the high reactivity of higher acenes and unravelling their properties 5,6,7,8,9,10,11,12,13,14 .…”

A Practical General Method for the Preparation of Long Acenes

“…More recently, long acenes have been isolated by on‐surface synthesis at low‐temperature metallic surfaces under ultra‐high vacuum, through deoxygenation of epoxides, dehydrogenation of partially saturated precursors and bis‐decarbonylation of α‐diketones . These strategies allowed the investigation through scanning tunnelling spectroscopy of the electronic properties of polyacenes up to undecacene, and to follow the decrease in energy gap as the number of six‐membered rings increases.…”