Inducing Open-Shell Character in Porphyrins through Surface-Assisted Phenalenyl π-Extension

Sun, Qiang; Mateo, Luis M.; Robles, Roberto; Ruffieux, Pascal; Lorente, Nicolas; Bottari, Giovanni; Torres, Tomás; Fasel, Roman (2020). Inducing Open-Shell Character in Porphyrins through Surface-Assisted Phenalenyl π-Extension. Journal of the American Chemical Society, 142(42), pp. 18109-18117. American Chemical Society 10.1021/jacs.0c07781

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Organic open-shell compounds are extraordinarily attractive materials for their use in molecular spintronics thanks to their long spin-relaxation times and structural flexibility. Porphyrins (Pors) have widely been used as molecular platforms to craft persistent open-shell structures through solution-based redox chemistry. However, very few examples of inherently open-shell Pors have been reported, which are typically obtained through the fusion of non-Kekulé polyaromatic hydrocarbon moieties to the Por core. The inherent instability and low solubility of these radical species, however, requires the use of bulky substituents and multistep synthetic approaches. On-surface synthesis has emerged as a powerful tool to overcome such limitations, giving access to structures that cannot be obtained through classical methods. Herein, we present a simple and straightforward method for the on-surface synthesis of phenalenyl-fused Pors using readily available molecular precursors. In a systematic study, we examine the structural and electronic properties of three surface-supported Pors, bearing zero, two (PorA2), and four (PorA4) meso-fused phenalenyl moieties. Through atomically resolved real-space imaging by scanning probe microscopy and high-resolution scanning tunneling spectroscopy combined with density functional theory calculations, we unambiguously demonstrate a triplet ground state for PorA2 and a charge-transfer-induced open-shell character for the intrinsically closed-shell PorA4.

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