Long persistent phosphors are widely commercialized as glow-in-the-dark paints for watch indicators and emergency signs. Although present glow-in-the-dark materials are made using inorganic materials, some organic molecules are also known to show long-lived emission called phosphorescence. Organic long-lived phosphorescent materials have advantages in areas such as solubility, color tunability, processability, and biocompatibility.
Phosphorescence, which is a spin-forbidden transition from the triplet excited state to the single ground state, is often quenched by the competing nonradiative deactivation process at room temperature. To observe long-lived phosphorescence from organic molecules at room temperature, the nonradiative deactivation needs to be minimized. The origin of this nonradiative deactivation can be divided into components from the guest emitters and host matrices. To suppress the nonradiative deactivation caused by the host matrices, host-guest systems using hosts that are polymers, hydroxyl compounds, and crystalline molecules have been reported. However, all of these systems are electrical insulators because they have nonaromatic backbones, so they are not suitable for use in semiconducting devices.
Here, Prof. Kabe, Prof. Adachi, and co-workers from Kyushu University report long-lived phosphorescence at room temperature through suppression of the nonradiative deactivation in conventional organic semiconducting host-guest systems, which are often used as the emitting layer of organic light-emitting diodes (OLEDs).
The researchers found that long-lived triplet excitons of the guest emitters can be transferred to host molecules even when the triplet energy level of the host is higher than that of the guest emitters. Because this reverse energy transfer is a thermally assisted process, nonradiative deactivation by the host matrix is significant at room temperature in the conventional host-guest system. To confine the long-lived triplet excitons to the emitting molecules, a very large triplet energy gap between the guest emitters and the host molecules of over 0.5 eV is required. By confining the long-lived triplet excitons in the semiconducting host-guest system, they demonstrated long-lived room temperature phosphorescence from OLEDs.
Because the triplet exciton dynamics is important for many optoelectronic devices such as OLEDs, organic solar cells, photon-upconversion systems, and non-linear optics, obtaining long-lived triplet excitons in semiconducting host-guest systems will help to advance the understanding of these devices.
This study, Confinement of long-lived triplet excitons in organic semiconducting host-guest systems was recently published in the journal Advanced Functional Materials and selected as a back cover.