A Compact Tetrathiafulvalene–Benzothiadiazole Dyad and Its Highly Symmetrical Charge‐Transfer Salt: Ordered Donor π‐Stacks Closely Bound to Their Acceptors

Abstract

A compact and planar donor-acceptor molecule 1 comprising tetrathiafulvalene (TTF) and benzothiadiazole (BTD) has been synthesised and experimentally characterised by structural, optical and electrochemical methods. Solution processed and thermally evaporated thin films of 1 have also been explored as active material in organic field-effect transistors (OFETs). For these devices, a hole field-effect mobility of FE = (1.3 ± 0.5) ×10-3 cm 2 /Vs and of FE = (2.7 ± 0.4) ×10 -3 cm 2 /Vs could be determined for the solution processed and thermally evaporated thin films, respectively. An intense intramolecular charge-transfer (ICT) transition around 495 nm dominates the optical absorption spectrum of the neutral dyad, which also shows a weak emission from its ICT state. The iodine induced oxidation of 1 leads to a partially oxidised crystalline charge-transfer (CT) salt {(1) 2 I 3 }, and eventually also to a fully oxidized compound {1I 3 } ½I 2 .Single crystals of the former CT compound, exhibiting a highly symmetrical crystal structure, reveal a fairly good room temperature electrical conductivity of the order of 2 S cm -1 . The one-dimensional spin system bears compactly bonded BTD acceptors (spatial localisation of LUMO) along its ridge.

Exploring the Electronic Structure of an Organic Semiconductor Based on a Compactly Fused Electron Donor–Acceptor Molecule

“…Notably, at approximately 120 K and 50 K, two clear‐cut irregularities occur in the susceptibility data. This kind of behavior is intriguing; the sudden drop in χ occurs at approximately 120 K, close to the observed discontinuity in the conductivity regime,9c thus suggesting that these two observations are directly linked.…”

“…Owing to an energetically low‐lying charge‐transfer state, they absorb strongly in the visible spectral region and are characterized by a HOMO–LUMO gap in the order of 2 eV. It was also found that the partially oxidized and crystalline compound, {(TTF–BTD) 2 I 3 }, is semiconducting, exhibiting a fairly large room‐temperature conductivity of 2 S cm −1 and activation energies in the range of 150–100 meV 9c. This observation is accounted for by the slightly dimerized π stacking of (TTF–BTD) +0.5 molecules within the crystalline solid.…”

“…It should be noted that the activated, yet large, conductivity of the actual compound, seems consistent with the partial dimerization of the structure and the presence of just one electron per dimer. Significantly, however, the conductivity versus temperature curve shows a break upon reaching temperatures below 150 K,9c suggesting that at least two different transport regimes occur in {(TTF–BTD) 2 I 3 }; thus, we felt it was necessary to consider the low‐temperature behavior for this phase in more detail. Consequently, we report herein the magnetic susceptibility measurement and the 90 K X‐ray structure of the compound.…”

“…To assess whether any structural change is responsible for the abrupt susceptibility change of {(TTF–BTD) 2 I 3 } at approximately 120 K, we performed a parallel study to a former X‐ray study conducted at 173 K,9c with the new data taken at 90 K. At both temperatures, the structure conforms to the space group C 2 m. The asymmetric unit comprises half of the TTF–BTD molecule (together with two iodine atoms of I 3 − ), which results in only one crystallographically unique TTF–BTD molecule that bears a formal positive charge of +0.5. The (TTF–BTD) +0.5 moiety is essentially planar and the root‐mean‐square deviation from a least‐squares plane through the atoms of the TTF core alone is 0.0465 Å (90 K) [0.0452 Å (173 K)].…”

Understanding the Influence of the Electronic Structure on the Crystal Structure of a TTF-PTM Radical Dyad

“…This conducting behavior is more often accomplished in materials displaying segregated stacks, in which the D-A pairs are better overlapped. [8][9] For the opposite reason, a polymorph with alternated stacks will mostoften be insulator. 10 In other words, the conducting properties of charge transfer salts based on two components are highly susceptible to the presence of polymorphism.…”