Pyrene-Cored Starburst Oligofluorenes with Diphenylamine End-Cappers: Design, Synthesis, Stabilized Optical Gain, and Lasing Properties

Abstract

A family of starburst conjugated molecules composed of a pyrene core and diphenylamine end-cappers with various oligofluorene bridge lengths, named as P1F, P2F, and P3F, were designed, synthesized, and characterized. The resulting materials exhibited good photoluminescence (PL) properties and excellent thermal stability with high degradation temperatures (T d, the temperature with 5% weight loss) over 400 °C. A combination of the thermal, photophysical, electrochemical, fluorescence transients, electroluminescence (EL), amplified spontaneous emission (ASE), and lasing measurements were carried out to reveal further the influence of the diphenylamine moieties as electron-donating end-cappers on their optoelectronic properties. It is interesting to find that the PL spectra are significantly blue-shifted for P2F (∼11–15 nm) and P3F (∼14–22 nm) relative to those of P1F in both dilute solutions and films with an extension in the conjugation length of the oligofluorenes, which is quite different from the common phenomenon of red-shifted PL spectra for the general starburst molecules with increasing the conjugation length. It is supposed that the introduction of strong electron-donating diphenylamine units helps to construct donor-π-acceptor structures that enable intramolecular excitation energy transfer transition from the diphenylamine donor to the pyrene acceptor. The diphenylamine units also play a key role in raising the highest occupied molecular orbital (HOMO) energy levels of the molecules, which is beneficial for improving the charge injection and transport properties. Consequently, efficient EL properties with rather low turn-on voltages of 2.7–3.0 V among nondoped blue OLEDs have been achieved. Moreover, stabilized EL and ASE with high net gain coefficients (around 73.6–81.9 cm–1) and low waveguide loss (about 2.10–4.90 cm–1) have been demonstrated. One dimensional distributed feedback (1D DFB) lasers demonstrated a low lasing threshold of 2.1 kW cm–2 (25 nJ pulse–1) at 475 nm for P2F. All the results confirm that the novel molecular design strategy on constructing diphenylamine-capped pyrene-centered starburst molecules can obviously improve the EL performance by fine modulating the HOMO energy levels, but did not largely sacrifice their optical gain properties. The high gain and low loss with excellent thermal and optical stability have rendered these donor-π-acceptor starburst conjugated molecules rather attractive as robust gain media for organic lasers.

Donor–Acceptor Type Pendant Conjugated Molecules Based on a Triazine Center with Depressed Intramolecular Charge Transfer Characteristics as Gain Media for Organic Semiconductor Lasers

“…The fluorescencet ransients of T-x and 2FEtz-x in solutions and thin films were investigated with the aim to unravel the excited state relaxation processes ( Figure 5, Table 1). In solution, both T-m and T-p exhibit double-exponential decaysw ith excited state lifetimes (t): [16] Generally,m igration-induced exciton quenching at nonradiative decay centers occurs at the initial excitonm igration stage; [31] localized excitons at low energy states avoid fast nonradiatived ecaysb ut show extended relaxation times. [32] As evident from the fluorescenceb ands of T-m and T-p,t he slow relaxation component with t 2 (3.18 ns for T-m and 3.30 ns for T-p)w as ascribed to aggregated states.…”

“…With this aim, it is an effective approacht oi ncorporate electron-donating units and/or electron-withdrawing groupsi ntot he conjugated molecules to finely adjust the highest occupied molecular orbital (HOMO) and/ort he lowest unoccupiedm olecular orbital( LUMO) levels, thus improving the carrier injectiona nd transport properties. [16,17] For example, several conjugated starburst molecules comprising truxene cores and oligofluorene bridges have been recently developed and demonstrated to be of improved hole injectionand transport ability andsuperior optical gain property with al ow threshold of 27.4 nJ pulse À1 ,b ecause of the incorporationo fd iphenylamine cappers. [18] This is an attractive strategy to enhance the hole and electron injection and trans-port at the same time with simultaneous incorporation of electron-donating and electron-withdrawing units into the molecular structures.…”

Design and Synthesis of Conjugated Starburst Molecules for Optoelectronic Applications

“…Electron‐rich diphenylamine units were also introduced as the end‐cappers into the starburst systems with a pyrene core with the aim to modulate the electrical properties, yielding the products 19 , named as P1F, P2F, and P3F (Figure a) . In this case, PL spectra are significantly blue‐shifted for P2F (∼11–15 nm) and P3F (∼14‐22 nm) compared with those of P1F in both solutions and films by increasing the conjugation length of the oligofluorenes, which behaves differently from the common phenomenon of red‐shifted PL spectra for the general starburst molecules with the increment of the conjugation length.…”

Low‐Threshold Non‐Doped Deep Blue Lasing from Monodisperse Truxene‐Cored Conjugated Starbursts with High Photostability

“…According to previousc ontributions, av ariety of conjugated starburst materials with various cores such as truxene, pyrene and triazatruxene, demonstrating efficient electroluminescence and prominent lasing behaviors have been designed and synthesized. [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] Because of the synergetic effects of differentm oieties, the molecules manifest good solubility andg reat film-forming behaviors as wella se xcellent stability and high electroluminescence, implying that such starburst molecular structures are promising to achieve excellent lasing behaviors with low thresholds. It is worthwhile to mention that the ability to adjust the architectures by modulation over the core and arms in the terms of am olecular level shows ad irect impact on the electronic and electroluminescent characteristics of the molecules.…”