Cycloaddition reactions1, 2 are cornerstones in carbon nanomaterial engineering. Early examples include Diels-Alder3, 4 [4 + 2] and Prato-type5 [3 + 2] cycloadditions in solution environments, and Huisgen-type6 [3 + 2] as well as related [2 + 2] Bergman7, 8 cyclization on surfaces. With the advent of nanographene synthesis9,10, 11, 12, a new chapter in organic chemistry has opened up, seeking a modular, highly selective, and high-yield synthesis6, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 of extended conjugated macromolecules at interfaces without byproducts.25 This endeavor embodying click-chemistry26 has notably driven the adaptation of a large variety of organic reactions at interfaces, including Ullmann coupling, Glaser coupling and polycondensations. Recently, we have shown that polycyclic aromatic azomethine ylides27, 28, 29 (PAMYs, Fig. 1a) can be employed to form diaza-hexabenzocoronenes and N-containing polycyclic aromatic chains20, 30 in the solid-state and on surfaces, opening an avenue to cycloaddition polymerizations for extended polycyclic aromatic hydrocarbons (PAHs) or related nanographenes. In solution, the PAMY precursor, namely 8H-isoquinolino[4,3,2-de]phenanthridin-9-ium tetrafluoroborate (DBAP salt, 1, see Methods and SI), undergoes selective 1,3-dipolar [3 + 2] cycloaddition to electron-deficient dipolarophiles yielding N-containing PAHs.31, 32 Towards the engineering of extended N-containing PAHs on-surface and in the solid-state, a mechanistic, broadly accessible understanding of the chemical reactions and pathways accessible to PAMY33, 34 and similar cycloadditions is desirable.35
In on-surface and solid-state thermochemistry, chemical reaction modeling beyond adiabatic energetic diagrams (e.g. nudge elastic band method for determining transition structure and energy barrier36) is rarely explored since the potential energy surfaces of ground states are often assumed to follow the noncrossing rule37 or void of electronic state hopping. Because of this limitation, it remains often unclear in the literature whether cyclization and dehydrogenation reactions are symmetry-allowed, that is, occur adiabatically or otherwise. Symmetry-forbidden reactions, that is, reactions in which molecular orbitals cross, are indicative of nonadiabatic dynamics that are known to play a fundamental role in PAH synthesis and cycloadditions38, 39. Several models are available to attempt to formally define forbidden nonadiabatic reaction pathways from on-surface reaction mechanisms. Woodward and Hoffmann (WH)40, 41, 42, 43, 44 attribute the reactivity of a chemical reaction to the atomic orbital symmetries under adiabatic conditions. The WH rules provide qualitative selection rules for pericyclic cycloaddition reactions45,46, 47, 48, 49 whereby molecular orbital symmetry and crossings are commonly treated by means of Fukui reaction theory50, Marcus theory of electron transfer and the surface hopping method51. More accurate mechanistic predictions relying on the Born-Oppenheimer52 approximation, are challenging when dealing with more than one reaction pathway in the presence of nonadiabatic quantum effects such as tunneling and surface hopping. These effects make it difficult to validate qualitative WH rules. Yet the WH approach remains a powerful well-known concept for reaction engineering. In this regard, quantitative and chemically-intuitive WH visualization tools treating ionic, radical and pericyclic reactions on the same footing would be desirable for the rapid prototyping of PAH reactions, and the study of nonadiabaticity. During the last decades, the classification of orbital crossings53, 54, 55 aided by differential geometry and algebraic topology has emerged as a promising method to study nonadiabaticity in condensed matter physics.56, 57, 58, 59, 60, 61, 62, 63 Topological physical properties64 in quantum matter and metamaterials can now be engineered with an extraordinary level of sophistication, aided by the interplay between effective (‘toy’) models, ab initio calculations and experiments. Such methodology has been rarely employed for the study of chemical phase transitions and corresponding reactions. Recently, the concept of topology classification for mirror-symmetric reaction pathway models to study reactions by means of topological invariants was introduced65, 66, whereby the reactions with distinct topologically classifiers are adiabatically forbidden (such as the [2 + 2] thermal reaction of two ethylene molecules)65, 66. Topological classifications could epitomize a turning point to accelerate cycloaddition reaction engineering, especially on-surface, where reactions are surface templated and highly symmetric. Particularly, such topological models expand and unify the WH-Fukui approach: They enforce the geometrical symmetry and concertedness of reaction pathways to summarize and formalize chemical notions, simplifying reaction interpreting and rational design. Additionally, they illustrate that reaction coordinates can be mathematically defined to study nonadiabaticity and (non-interacting) orbital intersections from a topological standpoint.67, 68
Here, we study the solid-state and on-surface cycloaddition of a PAMY precursor and pentacene to yield internally N-containing PAH with a tetracenoisoindole core (Fig. 1a) as characterized by ultra-high vacuum (UHV) scanning tunneling microscopy (STM) and matrix-assisted laser desorption-ionization mass spectrometry (MALDI-MS).69 By investigating the frontier orbital symmetries of gas-phase reaction pathways by means of intrinsic reaction coordinates which are assumed conclusive for the study of the on-surface and in the solid-state reactions, we describe the [3 + 2] reaction (Fig. 1b) between singlet diradicaloid PAMY (rPAMY) and pentacene and show that its de-aromatization pathway is adiabatic and therefore thermally allowed in the gas phase. We formally classify the WH rules via topological invariants, extending the recently proposed topological classification65 to a tight-binding Hückel reaction model combining first-principles calculations (Fig. 1c). Different from the frontier orbital model, our topological WH model differentiates the allowed, concerted adiabatic pathways from the nonadiabatic, crossings by a \(\:\mathbb{Z}\)-classified topological invariant \(\:C\left(t\right)={N}_{+}\left(t\right)-{N}_{-}\left(t\right)\), where \(\:{N}_{+}\left(t\right)\) (\(\:{N}_{-}\left(t\right)\)) is the number of mirror symmetric (antisymmetric) molecular orbitals (MOs) in all occupied Mos, and \(\:\varDelta\:C={C}_{react}-{C}_{prod}\) is the difference of the topological invariants between reactants and products (Fig. 1c). We find that singlet diradicaloid PAMY lateral addition to acenes is endothermic but topologically WH allowed, while central ring addition is exothermic but topological forbidden. Our work introduces a methodological and theoretical approach for the study of cycloaddition selectivity, particularly PAMY reactions which are relevant in development of N-containing PAHs as substrates for N-doped nanographenes, spin-chains70, 71, metal-free catalysis72, 73 and sensors74, 75.