Ladder-Type oligo(: P-phenylene)s with D-π-A architectures: Design, synthesis, optical gain properties, and stabilized amplified spontaneous emission

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.