Donal Bradley

Abstract:

The nature of chain aggregation in solution always results in variable spin-coated film mesoscale morphology and uncontrollable device performance. The abundant variety and increasing chemical complexity of conjugated polymers induced additional diverse electrostatic and dispersion interactions (non-covalent interactions), although it is not fully understood how the interplay of these forces results in the observed conformational order, chain aggregates and film morphologies. Herein, we present a precise study on the role of non-covalent interaction in the self-organization behavior, conformational order and optoelectrical properties of polyfluorene (PPFOH) toward tuning its electroluminescence (EL). The supramolecular PPFOH system consisted of an intrinsically doped hydrogen-bond-assisted microstructure as a "guest" and a blue light-emitting backbone chain as a "host", which show a special binary emissive property of solution-induced self-dopant formation in the amorphous films. As a result of a strong non-covalent interaction between polymer chains and solvent molecules (type II solvent), a likely distorted or fold chain in rod-coil or branch cluster shows a narrow and strong aggregation emission at 525-540 nm. Low-polar solvents (called type I) can also induce a shoulder low-energy emission at 550-580 nm in the films, attributed to the extended and stretched chain complex for the tendency of interchain hydrogen-bonding interaction. Further evidence from nanoscale infrared (AFM-IR) analysis confirmed the stronger hydrogen-bonding interaction in the type II films than those in the type I films. Finally, supramolecular PPFOH electroluminescence colours can be tuned from blue to sky blue, green, white, yellow and orange.

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.