Amorphous Metal-Free Organic Phosphors for Sensor Applications

Abstract

Phosphorescent organic light-emitting diodes are promising for many applications such as display and solid-state lighting because they can reach 100% theoretical internal quantum efficiency. In order to realize bright purely organic phosphors, efficiently promoting intersystem crossing from singlet to triplet and suppressing vibrational dissipation of triplets must be achieved. In this dissertation, I systematically investigated the two critical processes and devised some strategies to achieve bright room temperature purely organic phosphorescence in amorphous films and optical ozone sensors based on phosphorescence phenomena. Embedding organic phosphors into amorphous glassy polymer was investigated as the first strategy to efficiently suppress the triplet vibration by restricting molecular motion of the embedded organic phosphors. This system showed temperature dependent phosphorescence attributed to changing vibrational characteristics of the matrix polymer. An optical temperature sensor integrated in a microfluidic device was devised and demonstrated. Incorporating strong hydrogen bonding between a newly devised purely organic phosphor and hydrogen bonding capable matrix polymer was the second strategy, resulting in much brighter phosphorescence. Modulation of hydrogen bonding by water showed unique reversible phosphorescence-to-fluorescence switching behavior, which was utilized to develop a ratiometric water sensor. Based on the finding that the phosphorescence intensity of the purely organic phosphors is sensitive to environmental ozone concentration, I revealed that the origin of the ozone sensitivity is oxidation of the aldehyde moiety of the organic phosphors and devised highly sensitive and convenient optical ozone sensors by utilizing the observed inverse linear correlation between the phosphorescence emission intensity and the ozone concentration. Since manipulating conjugation length of organic phosphors is a powerful tool to tune the emission color, establishing an understanding on the conjugation length effects on the phosphorescent emission intensity is important. The effects of the conjugation length of the purely organic phosphors on their phosphorescence intensity were systematically studied using a combined experimental and computational approach. The obtained knowledge regarding the role of intermolecular interactions for vibration suppression was adapted to achieve high thermal conductivity in amorphous polymers by designing hydrogen bonding donating and accepting polymer pairs, providing uniformly distributed strong interpolymer linkage, and leading to high thermal conductivity of 1.5 Wm-1K-1.