Structure control of pi-conjugated polymers for enhanced solid-state luminescence: Synthesis and liquid crystalline and photophysical properties of new bulky poly(p-phenylenevinylene)s and oligo(phenylenevinylene)s bearing tricyclodecane pendants

Abstract

A series of novel highly luminescent bulky oligo(phenylenevinylene)s (MTCD-OPV, BTCD-OPV, and MEH-OPV), bulky pi-conjugated poly(p-phenylenevinylene)s (MTCD-PPV, BTCD-PPV), and symmetrically or unsymmetrically substituted bulky random PPV copolymers (BTCD-x and MTCD-x series, where x = 0-100 mol %) bearing tricylodecane (TCD) pendants were synthesized to trace the factors which control the molecular aggregation of pi-conjugated materials. The H-1 NMR signals of the OPVs were utilized to identify different types of repeating units in the copolymer backbone for determining their compositions. The thermal analysis revealed that all the polymers were highly amorphous, and the plots of T-g vs the composition of copolymers followed straight line Flory-Fox trend for a wider temperature ranging from 75 to 200 degrees C. The emission studies indicate that the copolymers with high rigidity (or high T-g) showed 4-5 times enhancement in photoluminescence (PL) intensity in the solid state and doubling of quantum yield in solution compared to MEH-PPV. The CPK space-filling model confirmed that bulky TCD substitution in the PPV induces strong steric hindrance and increases the intra- or interchain distances in the polymer backbone, which in turn control the pi-stack-induced molecular aggregation for high PL intensity. The luminescence of oligo(phenylenevinylene)s (OPVs) was unique to their pendant groups, and their absolute solid-state quantum efficiency determined by diffuse reflectance techniques revealed that TCD-substituted molecules MTCD-OPV and BTCD-OPV showed enhancement in the quantum efficiency (82 and 64%, respectively) compared to the MEH-OPV (27%). The thermal analysis and PLM observation of the OPV molecules indicate that while cooling from melt the TCD unit in MTCD-OPV hinders the packing of the molecules and forces the entire matrix to trap into amorphous glassy domain. The glassy nature of the molecule contributes to the 82% quantum yield of MTCD-OPV, which is the first OPV in the literature with more than 80% solid-state luminescent quantum yield. The symmetrical substitution enhances lateral packing of molecules in BTCD-OPV, and it was found as a thermotropic nematic liquid crystalline material with 64% quantum yield. In a nutshell, here for the first time, we have successfully demonstrated that the chemical structure of the bulky pi-conjugated materials plays a major role in the manipulation of solid-state luminescent intensity, quantum yield, liquid crystalline and amorphous glassy nature of PPVs, and its triad oligomers