Synthesis and properties of blue-light-emitting Oligo(fluorene-co-dibenzothiophene-S,S-dioxide)s

Abstract

An effective synthetic route for preparation of oligo(fluorene-co- dibenzothiophene-S,S-dioxide)s (trimer SOF2, pentamer SOF4, heptamer SOF6 and nonamer SOF8) based on meta- or para-linked dibenzothiophene-S,S-dioxide (SO) unit as the core was developed by convergence method, for which the intermediates unilateral boric acid esters were prepared by one-step palladium-catalysed Suzuki cross-coupling reaction. The resulting oligomers exhibited excellent thermal stability with thermal decomposition temperature exceeding 415 °C, and high photoluminescent quantum yield in the solid state (52–81%). The UV–vis absorption and photoluminescence emission maxima of the oligomers exhibited gradual bathochromic shifts from the trimer to the nonamer, approaching those of the previously reported polymer PFSO10 containing 10 mmol% of the SO unit in the polyfluorene backbone, demonstrating the similar photophysical properties of the nonamer SOF8 and polymer PFSO10. The highest occupied molecular orbital energy level gradually decreased and the lowest unoccupied molecular orbital energy level steadily increased from the trimer SOF2 to the nonamer SOF8. The obtained oligomers displayed pure blue-light emission with the maximal emission peaked at about 440 nm and superior EL spectral stability compared with PFSO10 upon varying the current density from 100 mA cm−2 to 500 mA cm−2, in addition to more balanced hole and electron fluxes. With the device configuration of ITO/PEDOT:PSS/oligomer/CsF/Al, a maximum luminous efficiency of 2.75 cd A−1 with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.15) was obtained for the device based on 37SOF8. The efficiency increased by 88% compared with the device based on 37PFSO10 (1.46 cd A−1). A similar enhancement in efficiency was also achieved for the device based on 28SOF8. These results indicate that the obtained oligomers are very promising candidates for structurally well-defined blue-light- emitting materials with high efficiency and reproducibility.