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Abstract:

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R1)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R2)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR1-R2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm2 V-1 s-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.

Abstract:

This paper reports state‐of‐the‐art fluorene‐based yellow‐green conjugated polymer blend gain media using Förster resonant‐energy‐transfer from novel blue‐emitting hosts to yield low threshold (≤7 kW cm−2) lasers operating between 540 and 590 nm. For poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) (15 wt%) blended with the newly synthesized 3,6‐bis(2,7‐di([1,1′‐biphenyl]‐4‐yl)‐9‐phenyl‐9H‐fluoren‐9‐yl)‐9‐octyl‐9H–carbazole (DBPhFCz) a highly desirable more than four times increase (relative to F8BT) in net optical gain to 90 cm−1 and 34 times reduction in amplified spontaneous emission threshold to 3 µJ cm−2 is achieved. Detailed transient absorption studies confirm effective exciton confinement with consequent diffusion‐limited polaron‐pair generation for DBPhFCz. This delays formation of host photoinduced absorption long enough to enable build‐up of the spectrally overlapped, guest optical gain, and resolves a longstanding issue for conjugated polymer photonics. The comprehensive study further establishes that limiting host conjugation length is a key factor therein, with 9,9‐dialkylfluorene trimers also suitable hosts for F8BT but not pentamers, heptamers, or polymers. It is additionally demonstrated that the host highest occupied and lowest unoccupied molecular orbitals can be tuned independently from the guest gain properties. This provides the tantalizing prospect of enhanced electron and hole injection and transport without endangering efficient optical gain; a scenario of great interest for electrically pumped amplifiers and lasers.

Abstract:

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.

Abstract:

Control of the hierarchical molecular organization of polydiarylfluorenes by synthetic strategies is significant for optimizing photophysical properties as well as the performance of light-emitting devices. Herein, for the suppression of molecular aggregation and enhancement of luminescence efficiency, a series of steric units were introduced into polydiarylfluorenes by copolymerization, with the aim of integrating the advantages of the steric-hindrance effect and of the β-phase. Optical and Raman spectroscopies revealed a β-phase conformation for a polymer copolymerized with spiro[fluorene-9,9'-xanthene] (SFX), with photoluminescence (PL) peaks at 454, 482, and 517 nm. Moreover, the morphological stability and electroluminescence (EL) stability were also improved without compromising the performance of the polymer light-emitting diodes (PLEDs). Furthermore, three steric-hindrance-functionalized copolymers showed significantly decreased thresholds for amplified spontaneous emission (EthASE) and enhanced stability following thermal annealing treatment. These results indicate that steric-hindrance functionalization is a superior approach to improve the overall stability and optoelectronic properties for blue-light-emitting π-conjugated polymers.