Systematic approach of blue-light-emitting copolymers by introducing various naphthalene linkages into fluorene containing PPV derivatives

Abstract

We newly synthesized a series of naphthalene and fluorene containing alternating copolymers, poly[9,9-n-dihexyl-2,7-fluorenediylvinylene-alt-1,4-naphthalenevinylene] (1,4-PNFPV), poly[9,9-n-dihexyl-2,7-fluorenediylvinylene-alt-2,6-naphthalenevinylene] (2,6-PNFPV), and poly[9,9-n-dihexyl-2,7-fluorenediylvinylene-alt-2,3-naphthalene-vinylene] (2,3-PNFPV) through the well known Wittig polycondensation reaction. The conjugation lengths of the polymers were controlled by differently linked naphthalenes in the polymer main chain. The resulting polymers were completely soluble in common organic solvents and exhibited good thermal stability up to 400 °C. The synthesized polymers showed UV–visible absorbance and photoluminescence (PL) in the ranges 352–379 and 465–512 nm, respectively. The maximum emission peak of 1,4-PNFPV was found at 512 nm with green light. But, 2,6- and 2,3-PNFPV showed more blueshifted blue PL emission than that of 1,4-PNFPV at 477 and 465 nm, respectively. This result showed that 2,3- and 2,6-naphthalene linkage of the 2,3- and 2,6-PNFPV reduced π-conjugation length compared to 1,4-naphthalene linkage of the 1,4-PNFPV. The single-layer light-emitting device was fabricated which has a simple ITO (indium–tin oxide)/polymer/Al configuration. Electroluminescence (EL) maxima of 1,4-PNFPV and 2,6-PNFPV were shown at 523 and 484 nm, respectively. The measurement of current vs. electric field showed the threshold voltages of 1,4-PNFPV and 2,6-PNFPV to be about 1.7×108 and 2.1×108 V/m. The EL powers of 1,4-PNFPV and 2,6-PNFPV were reached 36 and 24 cd/m2 at 2.3×108 V/m (206 mA/cm2) and 2.8×108 V/m (190 mA/cm2), respectively.