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

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.