Heat or light? Tools of choice for on-surface synthesis

In recent years, direct on-surface synthesis in UHV has been exploited as a promising strategy to obtain thermally and chemically stable structures by covalent bonding of suitable precursors. So far, covalent linking of organic molecules onto metal, semiconducting and bulk insulator surfaces has been mostly carried out thermally. Heat supplied to the system promotes the formation of covalent bonds between the monomeric building units either simultaneously with the surface diffusion phenomena it promotes (i.e. under dynamic bond-forming conditions) or as a trigger, after a pre-assembly step into a surface-supported supramolecular, non-covalent network. However, heat as a tool for on-surface synthesis has the ambivalent status of a pharmakon, since it has the capacity to be beneficial and detrimental to the production and to the structural quality of the covalently-linked network at the same time. The close interplay between molecular surface diffusion, chemical reactivity and temperature, in fact, often hampers the possibility to independently control reaction initiation and surface mobility, ultimately leading to highly defective covalent networks, poorly ordered on the long range. Photochemically activated reactions are a potentially powerful tool to stabilize self-organized structures without disrupting the long-range order. Yet, to date photo-initiated on-surface reactions are still rather uncommon, since the processes following light absorption are not completely understood and, in particular, the role of the substrate is still poorly characterized in quantitative terms: the high quenching rate of electronically excited species on metal surfaces, which inhibits the photophysical processes commonly observed in the gas phase and in solution; the presence of new, surface-related excitation and relaxation pathways; the occurrence of charge-transfer-mediated photochemistry are but a few important issues. Based on the recent experience of our group in this field [1–6], in this talk I will provide some hints on the necessary conditions to be fulfilled to make either heat or light – or a combination of both – the tool of choice for the successful on-surface synthesis of long range-ordered covalent networks. References [1] F. Sedona, M. Di Marino, M. Sambi, T. Carofiglio, E. Lubian, M. Casarin, and E. Tondello. ACS Nano 4, 5147 (2010) [2] A. Basagni, L. Colazzo, F. Sedona, M. Di Marino, T. Carofiglio, E. Lubian, D. Forrer, A. Vittadini, M. Casarin, A. Verdini, A. Cossaro, L. Floreano and M. Sambi. Chem. Eur. J. 20, 14296 (2014) [3] A. Basagni, F. Sedona, C. A. Pignedoli, M. Cattelan, L. Nicolas, M. Casarin, and M. Sambi. J. Am. Chem. Soc. 137, 1802 (2015) [4] A. Basagni, L. Ferrighi, M. Cattelan, L. Nicolas, K. Handrup, L. Vaghi, A. Papagni, F. Sedona, C. Di Valentin, S. Agnoli and M. Sambi. Chem. Commun. 51, 12593 (2015) [5] S. Tognolini, S. Ponzoni, F. Sedona, M. Sambi and S. Pagliara. Phys. Chem. Lett. 6, 3632 (2015) [6] A. Basagni, G. Vasseur, C. A. Pignedoli, M. Vilas-Varela, D. Peña, L. Nicolas, L. Vitali, J. Lobo-Checa, D. G. de Oteyza, F. Sedona, M. Casarin, J. E. Ortega, and M. Sambi. ACS Nano 10, 2644 (2016)