Probing the pi-stacking induced molecular aggregation in pi-conjugated polymers, oligomers, and their blends of p-phenylenevinylenes.

The role of pi-stack induced molecular aggregation on solution and solid-state luminescent properties was investigated for the tricyclodecane substituted bulky (p-phenylenevinylene)s (BTCD-60, with 60% bulky group), oligophenylenevinylenes (MEH-OPV and BTCD-OPV)s, and their polymer-oligomer binary blends. The natures of the solvent, concentration, solvent combinations (good or bad), and temperature were employed as stimuli to probe the origin of the molecular aggregates in bulky conducting polymers. Absorption, photoluminescence (PL), and time-resolved fluorescence spectroscopic techniques were employed as tools to trace aggregation in solvents such as toluene, tetrahydrofuran (THF), THF and methanol, or THF and water as well as in the solid state. The absorbance spectra of poly(2-methoxy-5-(2-ethylhexyloxy))-1,4-phenylenevinylene (MEH-PPV) and BTCD-60 indicated that the films obtained from polymers that were dissolved in aromatic solvents such as toluene were found to possess more pi-stacking as compared to that of films obtained from a good solvent such as THF. The solid-state emission spectrum of BTCD-60 was found to show almost a 5-6 times enhancement in PL intensity as compared to that of MEH-PPV. Concentration dependent excitation spectra of the polymers confirmed the presence of aggregated polymer chains in MEH-PPV, which is the main reason for the quenching of luminescence intensity in the polymer. Solvent induced aggregation studies of polymers in THF and methanol mixture further supports the existence of strong aggregation in MEH-PPV as compared to that of bulky BTCD-60. Variable temperature absorption studies confirmed the reversibility of molecular aggregation on heating/cooling cycles, and the extent of aggregation was found more in MEH-PPV chains as compared to that of BTCD-60. MEH-PPV/OPV binary blends were prepared in the entire composition range from 0 to 100% via solution blending techniques. Through selective PL excitation techniques, the effect of oligomer-to-polymer energy transfer and also luminescent enhancement in MEH-PPV via interchain separation were investigated. Both the energy transfer and the interchain separation were found to be more effective on the enhancement of luminescence properties in the BTCD blends as compared to that of MEH blends. Time-resolved fluorescence studies confirmed the existence of two types of species corresponding to the free and aggregated chains in the polymer matrix with lifetimes in the range of 0.5-2.0 ns. In the present investigation, we have successfully shown that the molecular aggregation of the pi-conjugated polymers, oligomers, and their binary blends can be controlled via suitable bulky substitution to tune their emission properties in solution as well as in the solid state.