Molecular triplet emitters - From design to assembly and functions

Abstract

Functional materials research is one of the top priority strategic areas of development in science and technology in the 21st century. The last decade has witnessed the rapid development of organic and metal-organic materials for various photonic, electronic and optoelectronic applications. Despite optoelectronic products using layered semiconductor superlattices being commonplace now, the constant search for materials with new and improved properties has led to a variety of investigations into molecular materials and a growing interest in the exploration of molecular-based materials research. Molecular functional materials are made up of molecules that could perform a specific function or task at the molecular level. This increasing interest in the search for new optoelectronics, electronics, photonics, sensors, devices and machineries fabricated based on pure organic or metal-organic molecular materials mainly stems from the versatility of materials development through rational design studies and elucidation of the structure-property relationship at the molecular level. Organic and metal-organic molecules can serve as versatile building blocks for molecular-based functional materials; they can be rationally engineered and prepared, their physical, mechanical and electronic properties are tunable with a proper understanding of structure-property relationships. Development of molecular-based materials is deemed to provide impetus and offers enormous potential for materials science research and development in the forthcoming decades. In recent years, there has been considerable growth of interest in new advanced molecular materials with unique properties for technological developments. This constant search for materials with new and improved properties has led to a variety of investigations into molecular materials. One such property that has attracted immense attention recently is luminescence. The interest has led to the search for novel luminescent molecular materials that exhibit intense photoluminescence (PL) and tunable emission colors, in particular metal-organic compounds with long-lived triplet phosphorescent states. Thus the molecular design and spectroscopic studies of these compounds represent an important and challenging area of research. There have also been significant improvements in OLED efficiencies by using phosphorescent materials to generate light emission from both singlet and triplet excitons. One approach, particularly for small molecule OLEDs, is to efficiently harvest triplet excitons through incorporation of heavy metal centres, which would increase spin-orbit coupling and hence intersystem crossing into the triplet state. Apart from the emission efficiency issue, an exploration of possible methods to bring about a variation in the emission colour would be important. Most of the common approaches involve the use of different light-emitting materials or multi-component blended mixtures of light-emitting materials with different emission characteristics for colour tuning. Recent reports have shown that the emission colours of OLEDs could be generated by using phosphorescent materials as dopants or non-doping emitters through a change in the dopant concentration or a variation of bias voltage. Despite recent interest in the exploration of electrophosphorescent materials, most of the works have been focused on those of iridium(III), ruthenium(II) and platinum(II) with other metal centres relatively less extensively explored. Recent works in our laboratory have shown that novel luminescent metal-based molecular materials of selected metal centres could be assembled through the use of various metal-ligand building blocks [1-8]. In this presentation, various design and synthetic strategies together with the successful isolation of a series of complexes of selected metals will be described. A number of these complexes have been structurally characterized and shown to display rich luminescence behaviour. The luminescence properties have been studied and their emissive origins elucidated. Correlations of the luminescence behaviour with the electronic and structural effects of the ligands and metal complexes have been made. Different approaches and assembly motifs have been employed to tune the absorption and emission characteristics of these materials based on an understanding of their spectroscopic origins [1,2]. Through rational design and assembly strategies, these molecular materials may find potential applications and functions as triplet light-emitters and functional materials. © 2005 IEEE.