Photoluminescence of Solvent-Selected Fluorescent Moieties in MEH-PPV Solutions and Films

Antonio Ribeiro,Rogério Custódio,Douglas Pereira,Henrique Camargo,Tatiana Martins

doi:10.21577/0103-5053.20170167

Abstract

In this work, we examined how photoluminescence in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is affected by morphology, dictated by the mode of preparation. For that, MEH-PPV dilute solutions in good and non-solvents, thick cast films with poly[methylmethacrylate] (PMMA) as inert matrix and spin-coated thin films were prepared. These systems were studied by steady-state and time-dependent fluorescence spectroscopy and electronic microscopy. Unexpected photoluminescent behavior was detected in solutions and films and it was related to different excitons modulated by MEH-PPV conformation. By semi empirical calculations we obtained the singlet and triplet state energies of an MEH-PPV model system and, from time-resolved fluorescence spectroscopy, the existence of short-lived excited states was revealed, enabling us to infer on possible singlet fission in MEH-PPV-based systems.

Highlights

Since poly(phenylene-vinylene) (PPV) derivatives were found to be electroluminescent,[1] the search for a systematic method to determine their applications has been of great interest
Hao et al.[48] used time-resolved emission microscopy to evidence regions in MEH-PPV film with several types of aggregation. They observed a long-lived blue-shifted emission from a monomer-like regions and a short-lived, red-shifted emission, due to aggregates. Their observations agree with Peteanu et al.[49] work who used fluorescence lifetime imaging microscopy (FLIM) to access aggregates in films of oligomers produced by reprecipitation which are models for MEH-PPV films
This reinforces the assumption that there is a singlet excited state presenting characteristics of allowed intercrossing system, associated to a triplet excited state with half the energy. These energies were reported by Österbacka et al.[78]. These results suggest that exciton fission is possible to occur in MEH‐PPV, and that this property is not related to preparation procedures, such as solvent choice, but it is intrinsic to this molecule

Summary

Introduction

Since poly(phenylene-vinylene) (PPV) derivatives were found to be electroluminescent,[1] the search for a systematic method to determine their applications has been of great interest. Their mechanical, morphological and optical properties have been widely discussed and findings related these properties to preparation methods. There are many aspects regarding the effects of preparation methods on optical response, authors seem to agree that preparation method influences molecular conformation, enabling or inhibiting aggregation and, affecting inter and intrachain interactions. Processing parameters are believed to modify the conformation and assembling mechanisms of polymeric materials, resulting in a memory effect of the structural information that is replicated from solutions to solid films. As pointed out in an earlier work,[5] this red-shift indicates aggregation driven by π-π interactions as the conjugation length increases

Results

Discussion

Conclusion