Designed Suppression of Aggregation in Polypyrene: Toward High‐Performance Blue‐Light‐Emitting Diodes

Abstract

Pyrene is one of the most important and thoroughly investigated organic chromophores. Among the attractive features of pyrene is its exceptionally long fluorescence lifetime, [1] the sensitivity of its excitation spectra to microenvironment changes, [2] and its high propensity for forming excimers. [3] This excimer formation has been utilized over the last 50 years in the investigation of water-soluble polymers, making pyrene, by far, the most frequently applied dye in fluorescence-labeled polymers. [4] Despite its chemical stability and high charge carrier mobility, its strong tendency to form excimers leads to a red-shift of its emission as well as a decrease in its fluorescence efficiency in condensed media, which has prohibited its use as an emissive material in organic light-emitting devices (OLED)s.Since the report of the first double-layer thin-film OLED by the Kodak Company in 1987, [5] OLEDs have attracted enormous attention in the scientific community thanks to their high technological potential for the next generation of fullcolor-flat-panel displays and lighting applications. [6,7] Whether polymers [8] or small molecules, [5] to date only red-and greenemitters have shown sufficient efficiencies and lifetimes to be of commercial value. [7] Here, we present a novel non-aggregating polypyrene, which can be synthesized via a simple three-step chemical route from pyrene in good yields. Our poly-7-tert-butyl-1,3-pyrenylene (3) shows a high solid-state fluorescence quantum yield with blue emission, excellent solubility and stability, no aggregation in thin films, and good electro-optical performance in single-layer OLEDs.The use of pyrene, a large conjugated aromatic system, as emitting material in OLED applications has been limited, due to aggregation between planar pyrene molecules. The formation of p-aggregates/excimers causes the quenching of fluorescence, resulting in low solid-state fluorescence quantum yields. In recent years, there has been an increasing interest in the use of pyrene units in OLEDs, including oligothiophenes with pyrenyl side groups, [9] bipyrenylbenzene molecules, [10] as well as pyrene-carbazole [11] and pyrene-fluorene systems, [12,13] due to their emissive properties combined with high charge carrier mobility. However, the pyrene derivatives that have been reported so far as efficient blue-emitters for OLED applications still present some degree of aggregation in the solid state. [13] A successful effort in the prevention of aggregation in small molecules was achieved with tetrasubstituted, highly sterically hindered pyrenes, which can emit blue light in solution as well in the solid state and with high quantum yield.[14] The well-known 1,3,6,8-tetraphenylpyrene, for example, has been used in OLEDs, [14] organic field-effect transistors (OFET)s, [15] as well in organic light-emitting field-effect transistors (OLEFET)s. [16] Additional tetrasubstituted systems including different phenyl derivatives [14] or pyridyl units [17] at the 1-, 3-, 6-, and 8-positions have been reported ...

Deep blue polymer light emitting diodes based on easy to synthesize, non-aggregating polypyrene

“…2700 times) than those of pyrene molecules in thin films. As found for solutions with different polymer content [16] also the PL results of films show no evidence for excimer formation. A comparison of the TE lifetimes of isolated pyrene molecules and PPyr in thin films shows a 50 times shorter TE lifetime in the polymer.…”

“…Moreover the extraordinary long lifetime of the excited state leads to enhanced luminescence quenching thus resulting in a low solid state fluorescence quantum yield. Nevertheless, the π-π-stacking can be prevented in films by the introduction of spatially demanding substituents [14,15] or directly through avoiding the pyrene interaction via the implementation of a highly twisted structure in a pyrene polymeric chain [16]. In particular we have been able to show that in a pyrene-based polymer comprising 1,3-coupled pyrene-molecules bearing a tert-butyl group at the 7-position the π-π-stacking can be fully prevented.…”

“…The PL emission in a TOL solution is characterized by a broad structureless spectrum peaking at 475 nm for PPyr dissolved in TOL as well as in the film cast from these solutions. The distinct similarity of the fluorescence spectra in solution and in solid state provides good evidence for the absence of aggregation in the excited state of the compound which on the one hand stems from the strong steric hindrance caused by the tert-butyl-groups at the 7-position and on the other hand is based on the high dihedral angle between two adjacent monomer units [16]. As documented by many examples in literature, unsubstituted pyrene [18], and macromolecules containing pyrene moieties [19,20] typically display a strong solvatochromic effect as a consequence of the influence of solvent polarity on the molecular conformation.…”

“…Poly(pyrene-co-TPA) (PPyr/TPA) was synthesized by a Yamamoto coupling with Ni(0) from 1,3-dibromo-7-tert-butylpyrene and commercially available 4-bromo-N-(4-bromophenyl)-Nphenylaniline. The 1,3-dibromo-7-tert-butylpyrene was prepared in three steps as described for the homo-polymer in [16]. The dibromotriphenylamine was purified by three-time crystallization from n-hexane.…”

“…Poly-7-tert-butyl-1,3-pyrenylene (PPyr) was synthesized as reported elsewhere [16]. Poly(pyrene-co-TPA) (PPyr/TPA) was synthesized by a Yamamoto coupling with Ni(0) from 1,3-dibromo-7-tert-butylpyrene and commercially available 4-bromo-N-(4-bromophenyl)-Nphenylaniline.…”