How to efficiently tune the biradicaloid nature of acenes by chemical doping with boron and nitrogen

Abstract

Acenes are fascinating polyaromatic compounds that combine impressive semiconductor properties with an open-shell character by varying their molecular sizes. However, the increasing chemical instabilities related to their biradicaloid structures pose a great challenge for synthetic chemistry. Modifying the p-bond topology through chemical doping allows modulation of the electronic properties of graphene-related materials. In spite of the practical importance of these techniques, remarkably little is known about the basic question -the extent of the radical character created or quenched thereby. In this work, we report a high-level computational study on two acene oligomers doubly-doped with boron and nitrogen, respectively. These calculations demonstrate precisely which the chemical route is in order to either quench or enhance the radical character. Moving the dopants from the terminal rings to the central ones leads to a remarkable variation in the biradicaloid character (and thereby also in the chemical stability). This effect is related to a p-charge transfer involving the dopants and the radical carbon centers at the zigzag edges. This study also provides specific guidelines for a rational design of large polyaromatic compounds with enhanced chemical stability.

Elucidating the Relationship between Multiradical Character and Predicted Singlet Fission Activity

“…It can also be noted that the effect of heteroatom substitution in Ref. [13] is similarly observed for a small number of our entries i.e. the diradical character of pentacene derivatives increases markedly with nitrogen substitution into the outer rings and less so when doped into the inner rings; asymmetric nitrogen substitution appears to further increase the ground state diradical character in these examples.…”

“…[57] A reiteration of the SF/biradicaloid trend is given in Figure 3, similarly those points to the left (but closest) to the identity line ought to be the better candidates and have y 0 values in the weak biradicaloid range, whereas the closed shell structures lie further to the right of the line. An interesting point is to consider this typical result is now being shown for a large set of molecules and across a wide range of molecular classes that were outlined in our previous work, with SF known [13] biradicaloid 0.12 0.17 7 [13,50] biradicaloid 0.12 0.15 8 [51] biradicaloid 0.13 0.13 9 [50,52] biradicaloid 0.15 0.19 10 biradicaloid 0.16 0.16 11 [53] biradical 0.77 0.75 12 biradical 0.82 0.80 Figure 2. Scatter plot of the SF primary energy criterion versus diradical index.…”

“…[8] The typical oligoacenes such as tetracene and pentacene, which are strong candidates for SF, can be considered as mostly closed shell but do possess a weak diradical character in their ground states which can itself be adjusted by altering the number of rings, [12] or by heteroatom substitution. [13] It has been suggested that this necessarily weak, but non-negligible, to intermediate diradical character is a defining characteristic of efficient SF molecules. [14] The idea behind this comes from a quantifiable correlation proposed by Nakano et al using the valence configuration interaction two-site model.…”

Tuning the Biradicaloid Nature of Polycyclic Aromatic Hydrocarbons: The Effect of Graphitic Nitrogen Doping in Zethrenes

“…It has been demonstrated in theoretical studies of graphitic N‐doped polyacenes that the location of the heteroatoms in the acene chain affects the extent of biradicaloid character strongly. Insertion of a pair of N atoms symmetrically into different C positions in octaacene could either increase or decrease the biradicaloid character substantially as compared to the pristine octaacene.…”

“…Secondly, extended multireference calculations based on the afore‐mentioned MR‐AQCC method will be performed in order to discuss the biradicaloid character quantitatively. It has been shown in previous investigations on the pristine HZ, on amino and cyano substituted HZ and on the N‐doping of acenes (tetraacene and octaacene) that this method provides a well‐balanced description of quasi‐degenerate orbital effects as they occur in biradical systems. The analysis of the MR‐AQCC calculations in terms of natural orbitals (NOs) and densities of unpaired electrons provides to the chemist an instructive and pictorial insight into the electronic structure of these compounds and a straightforward assessment of the extent of the biradical character and thus the chemical reactivity.…”