Room-Temperature Pentacene Fluids: Oligoethylene Glycol Substituent-Controlled Morphologies and Singlet Fission

Abstract

We report the first synthesis of solvent-free pentacene fluids at room temperature together with observation of singlet fission (SF). Three pentacenes with different number of ethylene glycol (EG) side chains (n) were employed (denoted as (EG) n -Pc-(EG) n : n = 2, 3, and 4). The morphologies of these pentacenes largely depend on the lengths of EG chains (n). (EG)3-Pc-(EG)3 and (EG)4-Pc-(EG)4 indicate fluid compounds at room temperature, whereas (EG)2-Pc-(EG)2 is a solid compound. Microscopic clustering with short-range interactions between pentacene chromophores was confirmed in X-ray diffraction profiles of solvent-free fluids. Such a structural trend is an important origin of SF and consistent with the steady-state spectroscopic results. To one’s surprise, femtosecond transient absorption spectroscopy demonstrated that SF occurred in thin films prepared from solvent-free fluids of (EG)3-Pc-(EG)3 and (EG)4-Pc-(EG)4 in spite of such excessive EG chains.

Molecular Design Strategy for High-Yield and Long-Lived Individual Doubled Triplet Excitons through Intramolecular Singlet Fission

“…18 Thus, judging from the current research progress of SF systems (vide infra), it is very reasonable to compare the photophysical properties using acene derivatives whose molecular structures and electronic properties can be systematically controlled by changing the number of benzene rings. Although high-yield or quantitative SF has been observed under various experimental conditions, for example in covalently linked dimers 12−14,19−26 and oligomers, 27−32 supramolecular assemblies, 33−37 solid-state films, 38−41 and fluid molecules, 42 many cases of much shorter lifetimes of triplet excited states relative to the conventional intersystem crossing (ISC) pathway have been observed. The plausible reason is the direct deactivation process of a correlated triplet pair (TT) but not individual triplet excitons (T 1 + T 1 ).…”

“…Although high-yield or quantitative SF has been observed under various experimental conditions, for example in covalently linked dimers − ,− and oligomers, − supramolecular assemblies, − solid-state films, − and fluid molecules, many cases of much shorter lifetimes of triplet excited states relative to the conventional intersystem crossing (ISC) pathway have been observed. The plausible reason is the direct deactivation process of a correlated triplet pair (TT) but not individual triplet excitons (T 1 + T 1 ).…”

Ultrafast Singlet Fission and Efficient Carrier Transport in a Lamellar Assembly of Bis[(trialkoxyphenyl)ethynyl]pentacene

“…55,57−59 We have reported that high-yield SF can be achieved by controlling the distances and orientations between neighboring pentacene chromophores using covalent linkages and supramolecular interactions. 60−69 We also showed the first example of room-temperature pentacene fluids capable of SF in the nonsolid phase, 70 where the molecular design relied on the incorporation of many oligoethylene glycol side chains to pentacene. As part of our continued research on the development of pentacene-based electronic materials, in the present study, we newly synthesized pentacene derivatives carrying 3,4,5-trialkoxyphenylethynyl groups (PcC n ) with different alkyl chain lengths (n = 6, 12, and 16) at the 6and 13-positions of pentacene to examine chain-length effects on the self-assembly, optical, and electronic properties (Figure 1).…”