Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials.

Min Sang Kwon,Stephen R Forrest,Caleb Coburn,Kyeongwoon Chung,Jaehun Jung,Jinsang Kim,Kevin Pipe,Youngchang Yu,Gunho Kim,Johannes Gierschner,Andrew W Phillips,Ji Ho Youk,Apoorv Shanker

doi:10.1038/ncomms9947

Abstract

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin–orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. Here we present a general design rule and a method to effectively reduce radiationless transitions and hence greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via Diels–Alder click chemistry is devised as a method. A sharp increase in phosphorescence quantum efficiency is observed in a variety of polymer matrices with this method, which is ca. two to five times higher than that of phosphor-doped polymer systems having no such covalent linkage.

Highlights

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin–orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes
intersystem crossing (ISC) from S1 to triplet manifold (Tn) can be greatly promoted in special organic moieties through efficient spin–orbit coupling (SOC) by the effective mixing of the singlet and triplet states of different molecular orbital (MO) configurations (El-Sayed rule; for example, aromatic carbonyls and nitrogen heterocycles11–13) and by heavy halogens[14], and/or through small singlet–triplet splitting energy (DEST)[15]
Deuterium substitution has been another route developed to enhance room-temperature phosphorescent (RTP) in metal-free phosphors, it requires the rather synthetically difficult deuterium substitution process[6,9,25]. These studies have indicated that oxygen permeability, triplet energy and rigidity of the matrix, and deuterium substitution of the phosphor are all important contributors to the radiationless transitions

Summary

Introduction

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin–orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. We present a general design rule and a method to effectively reduce radiationless transitions and greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. ISC from S1 to Tn can be greatly promoted in special organic moieties through efficient spin–orbit coupling (SOC) by the effective mixing of the singlet and triplet states of different molecular orbital (MO) configurations (El-Sayed rule; for example, aromatic carbonyls and nitrogen heterocycles11–13) and by heavy halogens[14], and/or through small singlet–triplet splitting energy (DEST)[15] In such compounds, the rate constant for phosphorescence is still small (kPB102 s À 1), whereas in organometallic complexes, on the other hand, rate constants are as high as kPB105 s À 1. We present a general design principle and a method to effectively reduce radiationless relaxation pathways and to achieve highly efficient RTP of metal-free organic materials in a a

Methods

Results

Conclusion