Ladder-type oligo(p-phenylene)s with D–π–A architectures: design, synthesis, optical gain properties, and stabilized amplified spontaneous emission

Abstract

A novel family of rigid ladder-type oligo(p-phenylene)s with donor–π–acceptor (D–π–A) architectures (n)L-F/(n)L-Ph-F (n = 2–4) end-capped with diphenylamino and fluorophenyl/fluorine units have been designed, synthesized and explored as gain media for organic lasers. The resulting materials demonstrated excellent thermal stability with a high degradation temperature (T_d) over 400 °C. The extension of the π-conjugated bridge length between the donor and acceptor units successfully depressed the crystallization tendency of oligo(p-phenylene)s, resulting in enhanced glassy temperature (T_g) and improved morphological stability in neat films. The amplified spontaneous emission (ASE) threshold (E_th) decreases with an extension in the conjugation length of the oligo(p-phenylene)s. In particular, for 4L-Ph-F with the longest conjugation length, the ASE threshold is determined to be as low as 1.97 μJ cm⁻² with a high net gain coefficient over 90 cm⁻¹ and a rather low loss coefficient of α = 2.0 cm⁻¹. One dimensional distributed feedback (1D DFB) lasers demonstrated lasing thresholds of 5.3 nJ pulse⁻¹ (0.44 kW cm⁻², 2.2 μJ cm⁻²) and 1.3 nJ pulse⁻¹ (0.1 kW cm⁻², 0.5 μJ cm⁻²) for 4L-F (at 460 nm) and 4L-Ph-F lasers (at 471 nm), respectively. It should be noted that the E_th value of all the ladder-type samples (n)L-Ph-F (n = 2–4) remains almost the same upon increasing the annealing temperature even up to 220 °C. The high gain and low loss with excellent thermal and optical stability have rendered these rigid D–π–A ladder-type materials advantageous as robust gain media for organic lasers.