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 have been designed, synthesized and explored as gain media for organic lasers. The resulting materials demonstrated excellent thermal stability with a high degradation temperature (Td) over 400 °C. The extension of 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 (Tg) and improved morphological stability in neat films. Amplified spontaneous emission (ASE) threshold (Eth) decreases with an extension in the conjugation length of the oligo(p-phenylene)s. Especially for 4L-Ph-F with the longest conjugation length, the ASE threshold is determined as low as 1.97 μJ cm−2 with 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 threshold 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 is noted that the Eth of all the ladder-type samples (n)L-Ph-F(n=2-4) remains almost the same with increasing the annealing temperature even up to 220℃. 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.