Improving the Stability of Organic Semiconductors: Distortion Energy versus Aromaticity in Substituted Bistetracene

Abstract

Polycyclic aromatic hydrocarbons (PAHs) have been widely explored as molecular semiconductors in organic electronic devices such as field-effect transistors or solar cells. However, their tendency to undergo photooxidation is a primary limitation to their practical applications. Bistetracene derivatives have recently been demonstrated to possess much larger photooxidation stability than the widely investigated pentacene and rubrene, while maintaining high charge-carrier mobilities. Here, using several levels of density functional theory, we identify the origin of the increased stability of bistetracene with respect to molecular oxygen by systematically investigating the [4 + 2] cycloaddition (Diels–Alder) photooxidation reaction mechanism. Importantly, our computational results indicate that endoperoxide formation in bis­(2-(trimethylsilyl)­ethynyl) bistetracene (BT) occurs not on the ring with least aromaticity, but rather on the ring with smallest distortion energy. This feature was subsequently confirmed by experimental NMR analyses. The oxidation activation barriers of bistetracene, pentacene, and rubrene are found to be 17.7, 13.6, and 14.4 kcal/mol, respectively, in agreement with the observed order of stability of these molecules with respect to oxidation reactions in solution. In the cases of BT and pentacene, the rates of electron transfer to create charged species (PAH+ and O2 –) are at least two orders of magnitude lower than that of the charge recombination process (back to PAH and O2); for rubrene, both of these processes are calculated to be of the same order of magnitude, in agreement with experimental electron paramagnetic resonance spectroscopy observations.

Novel Fluorophores based on Regioselective Intramolecular Friedel–Crafts Acylation of the Pyrene Ring Using Triflic Acid

“…[6] The great stability has been ascribed to the presence of two aromatic sextetsi nt he molecule leadingt oh igh distortion energy in interaction with oxygen. [7] Indeed, the change of aromaticityf rom BT to its oxidizedf orm is comparable to that observed from anthracene to its oxidized form. [8] The synthesiso fB Tskeletonsr elies on the preparation of the correspondingb istetracene-diones (BT-diones) that have been obtained from ad ouble Friedel-Crafts reaction of the naphthalene or pyrene rings functionalizedw ith aromatic acid (Figure 1).…”

“…In particular, materials so‐called bistetracenes ( BT‐I and BT ‐ II ) corresponding to two tetracenes fused at the zigzag edge in different manners have shown similar HOMO and LUMO energy levels than TIPS‐PEN associated with high charge mobility and stability . The great stability has been ascribed to the presence of two aromatic sextets in the molecule leading to high distortion energy in interaction with oxygen . Indeed, the change of aromaticity from BT to its oxidized form is comparable to that observed from anthracene to its oxidized form .…”

Synthesis, Aromaticity, and Application of peri‐Pentacenopentacene: Localized Representation of Benzenoid Aromatic Compounds

“…Recently, the lateral benzenoid extension of peri ‐fused systems like pyrene, perylene and anthanthrene has provided low band gap materials with excellent semiconducting characteristics in OFETs with enhanced stability [16–19] . The high stability was ascribed to the enhancement of the global aromaticity thanks to the peri ‐fusion that allows drawing the molecules with two aromatic sextets leading to high distortion energy in interactions with oxygen [20] . An extended Clar's representation for acenoacenes was also proposed [19] .…”

Synthesis, Aromaticity, and Application of peri‐Pentacenopentacene: Localized Representation of Benzenoid Aromatic Compounds

“…[16][17][18][19] The high stability was ascribed to the enhancement of the global aromaticity thanks to the peri-fusion that allows drawing the molecules with two aromatic sextets leading to high distortion energy in interactions with oxygen. [20] An extended Clars representation for acenoacenes was also proposed. [19] Nevertheless,i ft he Clarst heory gives uncontested pictures of polyfused systems that are drawn with at least two aromatic sextets (phenanthrene,p yrene,t riphenylene,e tc.…”