Why Bistetracenes Are Much Less Reactive Than Pentacenes in Diels–Alder Reactions with Fullerenes

Abstract

The Diels-Alder (DA) reactions of pentacene (PT), 6,13-bis(2-trimethylsilylethynyl)pentacene (TMS-PT), bistetracene (BT), and 8,17-bis(2-trimethylsilylethynyl)bistetracene (TMS-BT) with the [6,6] double bond of [60]fullerene have been investigated by density functional theory calculations. Reaction barriers and free energies have been obtained to assess the effects of frameworks and substituent groups on the DA reactivity and product stability. Calculations indicate that TMS-BT is about 5 orders of magnitude less reactive than TMS-PT in the reactions with [60]fullerene. This accounts for the observed much higher stability of TIPS-BT than TIPS-PT when mixed with PCBM. Surprisingly, calculations predict that the bulky silylethynyl substituents of TMS-PT and TMS-BT have only a small influence on reaction barriers. However, the silylethynyl substituents significantly destabilize the corresponding products due to steric repulsions in the adducts. This is confirmed by experimental results here. Architectures of the polycyclic aromatic hydrocarbons (PAHs) play a crucial role in determining both the DA barrier and the adduct stability. The reactivities of different sites in various PAHs are related to the loss of aromaticity, which can be predicted using the simple Hückel molecular orbital localization energy calculations.

Unconventional, Chemically Stable, and Soluble Two-Dimensional Angular Polycyclic Aromatic Hydrocarbons: From Molecular Design to Device Applications

“…These computational predictions ( Figure 3) were later validated by monitoring the reaction of TIPS-PT and C 60 via 1 H NMR. 37 The monoadduct on site 3 was formed in the first 10 min at room temperature but disappeared after a substantial reaction time (24 h in refluxing CS 2 ), and then the monoadduct across diene site 2 became the dominant product.…”

“…36 We have reported DFT calculations of the Diels−Alder (DA) reactions of pentacene (PT), 6,13-bis(2-trimethylsilylethynyl)-pentacene (TMS-PT), bistetracene (BT), and 8,17-bis(2-trimethylsilylethynyl)bistetracene (TMS-BT) with the [6,6] double bond of C 60 (Table 1). 37 The introduction of the bulky silylethynyl substituents does not have a significant effect on reactivity, but influences the corresponding product stability substantially. The DA reaction on site 3 of PT is favored both kinetically, with the lowest barrier of 17.6 kcal/mol, and thermodynamically, with the most exergonic product of −18.6 kcal/mol (Table 1).…”

“…This accounts for the experimental observations. 37 The reaction of TIPS-PT with [6,6]-phenyl- C 61 -butyric acid methyl ester (PCBM) proceeds within a few minutes after mixture. In contrast, the reaction between TIPS-BT and PCBM is much slower, with almost no observable change in the first 36 h.…”

“…By comparing these two sets of data, we found that the differences for 6 reactive sites are in the range of 3.8 to −2.4 kcal/mol. 37 Therefore, the simple HMO method provides reliable estimations of the reactivities of different diene sites in various PAHs for DA reactions with fullerenes.…”

London’sche Dispersionswechselwirkungen in der Molekülchemie – eine Neubetrachtung sterischer Effekte

“…In ähnlicher Weise wurde gezeigt, dass bei der Diels-Alder-Reaktion von Pentacen mit C 60 die Hçhe der Barriere nur wenig beeinflusst wird, wenn in 6-und 13-Position zwei sperrige 2-Tr imethylsilylethinylsubstituenten eingeführt werden. [166] DFT-Rechnungen lassen sogar darauf schließen, dass die Übergangszustandsenergie um 1.2 kcal mol À1 durch die attraktive Wechselwirkung der Silylgruppen mit dem Buckyball gesenkt wird. Interessanterweise konstatieren die Autoren, dass diese Wechselwirkung attraktiv im Übergangszustand ist, aber zu einer ungünstigen Bindungsdehnung im Produkt und damit zu einer insgesamt destabilisierenenden sterischen Hinderung führt.…”