Zinc porphene is a two-dimensional material made of fully fused zinc porphyrins in a tetragonal lattice. It has a fully conju-gated π-system, making it similar to graphene. Zinc porphene has recently been synthesized and shown to be a semiconduc-tor (Nat. Comm., 2023, 14, 6308.). This is in contrast with all previous predictions of its electronic structure, which indi-cated metallic conductivity. We show that the gap-opening in zinc porphene is caused by a Peierls distortion of its unit cell from square to rectangular, thus giving the first account of its electronic structure in agreement with experiment. Accounting for this distortion requires a proper treatment of electron delocalization, which can be done using hybrid functionals with a substantial amount of exact exchange. Such a functional, PBE38, is then applied to predict the properties of many first tran-sition row metalloporphenes, some of which have already been prepared. We find that changing the metal strongly affects the electronic structure of metalloporphenes, resulting in a rich variety of both metallic conductors and semiconductors, which may be of great of interest to molecular electronics and spintronics. Properties of these materials are mostly governed by the extent of the Peierls distortion and the number of electrons in their π system, analogous to changes in aromaticity observed in cyclic conjugated molecules upon oxidation or reduction. These results give an account of how the concept of antiaromaticity can be extended to periodic systems.

Diradicals are of high current interest as emerging materials for next generation optoelectronic applications. To tune their excited-state properties it would be greatly beneficial to have a detailed understanding of the wave functions of the different states involved but this endeavour is hampered by formal and practical barriers. To tackle these challenges, we present a formal analysis as well as concrete results on diradical excited states. We start with a detailed investigation of the available states of a two-orbital two-electron model viewed from both the valence-bond and molecular orbital perspectives. We highlight the presence of diradical and zwitterionic states and illustrate their connections to the states found in closed-shell molecules. Subsequently, we introduce practical protocols for analysing states from realistic multireference computations applying these to the para-quinodimethane (pQDM) molecule. The analysis reveals four different categories of states - diradical, zwitterionic, HOMO-SOMO as well as biexciton - while also providing insight into their energetics and optical properties. Twisting the CH2 groups allows us to interconvert between the closed- and open-shell forms of pQDM illustrating the connection between the states in both forms. More generally, we hope that this work will lay the foundations for a more powerful rational design approach to diradicals for photophysical applications. A detailed classification scheme for the excited states of diradicals is presented highlighting the connections between the states of closed-shell and open-shell molecules.

Zinc porphene, a two-dimensional material made of fully fused zinc porphyrins and transferable to various substrates, has recently been synthesized on water surface and shown to be a semiconductor (10.26434/chemrxiv-2022-t84kd). This is in contrast with all previous calculations of its electronic structure, which predicted metallic conductivity. In this paper, we show how the Peierls distortion causes a gap-opening in zinc porphene and polymers with a square unit cell in general, thus giving an explanation of its electronic structure in agreement with experiment. Then, we explore the properties of first-row transition metalloporphenes, including some that carry a fifth or even a sixth ligand on the metal. Such highly tunable materials can be obtained by reversibly inserting different metal ions into the porphene macrocycles without removing any π centers from conjugation. We find that varying the metal produces both metallic conductors and semiconductors, with their electronic structure governed by the number of delocalized electrons and the extent of the Peierls distortion. The results suggest that it may be possible to advance flexible organic electronics by using lithography to pattern electronic circuitry in a monolayer or multilayer of metalloporphene, using it as a canvas for painting with a variety of metals and their ligands.

The possibilities for tuning of electronic, transport, and optical properties of the linear dinitrosobenzene polymer (1) are explored. The band gap (Eg) and optical spectrum of 1 are calculated using both GW-BSE corrected for zero-point vibrations and hybrid TD-DFT, with the former method predicting a value (2.41 eV) in excellent agreement with diffuse reflectance spectroscopy measurements (2.39 eV). GW-BSE is also used to evaluate the effects of solid-state packing, while comprehensive TD-DFT calculations are employed to study the effects of intra-polymer torsion, gold surface adsorption, substitution, and changes in the aromatic core of 1. Torsion is found to be an important factor in determining Eg and transport properties, and a strong effect of the environment on the exciton binding energies is identified. Extending the conjugation in the aromatic core is found to enhance transport properties and narrow Eg, identifying future synthetic targets. Atomic force microscopy and spectroscopic ellipsometry are used to study 1 adsorbed to a (111) gold surface (1@Au), with the latter method showing a significant narrowing of the band gap to 0.68 eV, in good agreement with TD-DFT predictions.