NCAFM2023 Programme Booklet

THE ON-SURFACE SYNTHESIS OF HIGH-SPIN AZA-[5]TRIANGULENE

James Lawrence 1 , Yuanyuan He 1,2 , Haipeng Wei 1 , Jishan Wu 1 , Jiong Lu 1*

1 Department of Chemistry, National University of Singapore, 117543 Singapore 2 Department of Physics, Jiaxing University, 314001 China Email: jmlaw91@nus.edu.s

Nanographenes are the target of an intense research effort in the on-surface synthesis community due to promising potential applications that often relate to their unique optoelectronic properties 1 . Triangulenes are an exciting open-shell sub-class of these, with electronic properties/spin states that depend on their size and the presence of dopant atoms/functional groups 2,3 . Using on-surface synthesis under ultra-high vacuum conditions allows for the precise characterisation of such pristine high-spin species via scanning probe microscopy and spectroscopy methods. In particular, a combination of non-contact atomic force microscopy (nc-AFM) and scanning tu¬¬nnelling microscopy/spectroscopy (STM/S) allow the user to visualise the bond structure, adsorption conformation and electronic properties of adsorbed molecules. In this study, we demonstrate the synthesis of aza-[5]triangulene, a centrosymmetric nitrogen-doped triangulene molecule. By utilising a similar precursor to our previous synthesis 4 of [5]triangulene, we show that aza-[5]triangulene can be synthesised on both Cu(111) and Au(111) surfaces via annealing or tip-induced reactions. Using nc-AFM and STM/S in combination with density functional theory, we examine the structural and electronic properties of this molecule. In particular, we show that aza-[5]triangulene is positively charged on Au(111) in a similar manner to its previously studied smaller N-doped relative 5 , with a resulting quintet high-spin state. Accompanying theoretical calculations allow us to identify a pattern in the spin states of N-doped triangulenes of varying sizes, separating them into low- and high-spin classes. Finally, we also examine the properties of interesting common by-products of the on-surface reaction.

Fig 1. (a) Reaction scheme for the formation of aza-[5]triangulene on Cu(111) and Au(111). (b)-(d) STM and nc-AFM imaging of aza-[5]triangulene on Cu(111). The scale bars in (c) and (d) are both 500 pm.

References [1] Gu et al., J. Am. Chem. Soc., 2022, 144 , 11499–11524 [2] Su et al., Angew. Chem. Int. Ed., 2020, 59 , 7658-7668 [3] Chen et al., Angew. Chem. Int. Ed., 2022, 61 , e202212594 [4] Su et al., Sci. Adv., 2019, 5, eaav7717 [5] Wang et al., J. Am. Chem. Soc., 2022, 144 , 4522−4529

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