Organic Composite Crystal with Persistent Room-Temperature Luminescence Above 650 nm by Combining Triplet–Triplet Energy Transfer with Thermally Activated Delayed Fluorescence

Abstract

Structural modifications are a successful and commonly used approach to tune the emission properties of diverse fluorophores, but extending this approach to heavy-atom-free persistent luminophores has so far been unsuccessful. Here we employed a novel strategy to demonstrate triplet-triplet energy transfer from an organic room-temperature phosphor (RTP) with persistent luminescence to an organic molecule with thermally activated delayed fluorescence (TADF). We illustrated this approach by preparing heavy-atom-free composite crystals of an RTP with a long-lifetime emission and a red emissive organic fluorophore with TADF to yield materials with emission above 650 nm. The emission arose from the triplet excited state of an acceptor undergoing thermally activated reverse intersystem crossing (RISC) to the emissive S 1 state. Such composite crystal is the first organic material with persistent TADF, achieved by triplet-triplet energy transfer.

Manipulation of Triplet Excited States for Long‐Lived and Efficient Organic Afterglow

“…It has been reported that triplet-to-triplet excited state energy transfer in several two-component systems can give rise to room-temperature afterglow properties. [93,94] In these reported studies, efficient afterglow properties can only be observed when the donor components are sufficiently excited. In the present study, when the spiroBF 2 -MeOBP samples are excited by 365 nm UV lamp, triplet-to-triplet excited state energy transfer from MeOBP to spiroBF 2 would occur, because triplet excited states of benzophenone derivatives can be easily populated upon UV excitation and the T 1 level of MeOBP is higher than the T 1 level of spiroBF 2 .…”

Achieving High Afterglow Brightness in Organic Dopant‐Matrix Systems

“…It is known that the population of triplet excited states in BP systems is very efficient. To test whether excited‐state energy transfer [ 65 ] from BP triplets to spiroBF 2 triplets is responsible for the afterglow in the present system, the decay profiles of spiroBF 2 ‐BP‐1% materials excited at 420 nm have been recorded to show persistent luminescence as well (Figure S20, Supporting Information). These can rule out the possibility that the afterglow properties are originated from excited‐state energy transfer from BP to spiroBF 2 since BP matrices have negligible absorption at 420 nm (Figure S21, Supporting Information).…”

Red-light excited efficient metal-free near-infrared room-temperature phosphorescent films

“…According to El-Sayed rules, the n→π * transition of the orange area could further allow the ISC process to induce RTP emission. Existing studies show that organic molecules with small HOMO and LUMO orbital overlaps have a small singlet-triplet energy gap ( ▵ E ST ) because of small exchange energy between them, thus making it easy to induce TADF [ 38 , 39 ]. Based on the DFT analysis, it was also found that minimal overlap between HOMO and LUMO orbitals for PSP molecules was conducive to generating TADF.…”