Vibrational fingerprint of localized excitons in a two-dimensional metal-organic crystal

Abstract

Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. Here, we describe a self-assembled two-dimensional metal-organic crystal, composed of graphene-supported macrocycles, each hosting a single FeN4 center, where a single carbon monoxide molecule can adsorb. In this heme-like biomimetic model system, excitons are generated by visible laser light upon a spin transition associated with the layer 2D crystallinity, and are simultaneously detected via the carbon monoxide ligand stretching mode at room temperature and near-ambient pressure. The proposed mechanism is supported by the results of infrared and time-resolved pump-probe spectroscopies, and by ab initio theoretical methods, opening a path towards the handling of exciton dynamics on 2D biomimetic crystals.

Tetrapyrroles at near-ambient pressure: porphyrins and phthalocyanines beyond the pressure gap

“…shifts, lifetimes of electronic excited states, and intersystem crossing and internal conversion mechanisms [3,37]. In the biology framework, the concept of 'conformational control' has been developed in order to exploit the macrocycle conformation to fine-tune its physicochemical properties (figures 5(d)-(f)) [38].…”

“…The aromatic nature of tetrapyrroles provides the basis for their unique light absorption properties in the UV/ Vis region (figure 8) [43], exploited by Nature for light harvesting, and of extreme interest in the view of singleand multi-exciton (ME) generation and charge separation in organic photovoltaics [3,37,44]. Absorption [36].…”

“…These elements are of paramount importance, since they can be exploited for tailoring purposes in biomimetic systems (figure 9) [10,45]. At surfaces, the tuning role of axial ligands is played by the surface trans-effect [10], where hybridization of the LUMO with the substrate states takes place [46,47] while the surrounding metalorganic coordination environment (lateral interactions among tectons) governs the selfassembly [9,26,30,48,49], the charge delocalization [37], and the chemical/thermal stability [50]. In order to draw a characterizing picture of natural systems in the view of steering the surface synthetic counterpart, we can make further representing examples [4], referring to heme-based enzymes all containing a single Fe atom.…”

“…Besides electronic and chemical properties, tetrapyrroles show also interesting magnetic character when the caged transition metal atom possesses unpaired electrons and, thus, non-zero magnetic moment [24]. Coordination [15], as well as adsorption on a surface [52,53], can influence the magnetic moment of the complex, with surprising magnetic behavior like for example spin-crossover [54][55][56] and singlet-fission [37,57], associated with the possibility of exploiting metalated tetrapyrroles in spintronic devices (figure 11) [54,58,59]. Spin-crossover (figure 11(a)) is a thermal transition between electronic states with different numbers of unpaired electrons, accompanied by changes of a material's properties like magnetic moment, color, dielectric constant, and electrical resistance [56].…”

“…In the specific, the role of the supporting, templating surface is extremely relevant since it also works as a charge reservoir within the picture of a surface-trans effect. This provides effective electronic, chemical, and (electro)-catalytic tuning capabilities [8,10,14], together with selected electron-dynamics properties in the case of donor/acceptor interfaces for light harvesting and charge separation purposes [37,66]. As an example, a strategic reaction from both environmental and technological points of view is the reduction and conversion of carbon dioxide [67].…”