Photophysics of Phenylene Vinylene Polymers and Oligomers: The Influence of Nanocrystalline Domains

Abstract

This study looks at the nature of excited states and their photodynamics in phenylene vinylene molecules. Poly-p-(phenylene vinylene) (PPV), poly[(2-methoxy-5-hexyloxy-p-phenylene)vinylene] (MH-PPV), and 1,4-distyrylbenzene (DSB), a small molecule analog of PPV, are studied using various spectroscopic techniques including absorption and emission spectroscopy, transient emission spectroscopy, streak camera, fluorescence bleach-recovery measurements and confocal fluorescence microscopy. Strong temperature dependence of the fluorescence spectral shape is attributed to the existence of nanocrystalline domains in PPV and an exciton vibrational coupling theory shows that the vibronic peak distribution can be used as a measure of exciton delocalization. Transient emission spectroscopy shows that an initially localized exciton undergoes delocalization inside a single nanocrystalline domain on 10 ps time scale and the inter-domain energy transfer occurs over 100 ps. Isolated aggregates of DSB are prepared and studied using both confocal fluorescence microscopy and atomic force microscopy. A simple solvent annealing procedure is developed to change the morphology and optical properties of the aggregates. The fluorescence streak camera experiments of individual aggregate show that the emission has two different emissive states; one is a delocalized exciton and the other is a trap state. A global biexponential fitting algorithm separates the two species and the true exciton spectrum is obtained.