Achievement and photodynamic of the high-level intersystem crossing in thermally activated delayed fluorescence materials

Abstract

Thermally activated delayed fluorescence (TADF) is widely applied in the luminescence materials and exhibits promising potential in organic light-emitting diode (OLED), time-resolved bioimaging, fluorescent sensors, and other areas. Among the TADF, the one resulting from the high-level intersystem crossing (hRISC) is appealing to the increasing interest recently. Traditionally, the emissive singlet state is generated from the lowest triplet state by reversed intersystem crossing (RISC), while the singlet exciton of hot exciton emission arises from the higher level of triplet state. The luminescence material facilitated with hot exciton emission feature will further enhance the utilization of the triplet exciton and can reach the maximum internal quantum efficiency (IQE) in theory. Although numerous cases of hot exciton emission material have been reported, the excited-state behaviors and photodynamic of them is still insufficient. Here, in this thesis, we investigated the photophysical properties of several luminescence molecules, and directly observed their hRISC process after excitation by femtosecond transient absorption (fs-TA) spectroscopy In Chapter 2, the difference in luminescence properties for three compounds comprised of benzophenone and triphenylamine was investigated, and the structural-property correlation was clarified. Among three molecules, o-TPA-FBP, m-TPA-FBP, p-TPA-FBP, only the para-substituted one presents the TADF feature. The results suggested that the p-TPA-FBP has a satisfactory ΔEST value and spin-orbital coupling (SOC) effect to realize hRISC, and a clear simultaneous decay of the signal for singlet state and high-level triplet state can be observed. The fs-TA spectra of o-TPA-FBP only presents singlet state signal, as it shows a too large ΔEST and the ISC is forbidden. The SOC effect between T2 and S1 state of m-TPA-FBP is too weak so it will quickly decay to T1 state by radiationless transition, and in the fs-TA spectra, the feature the lowest triplet state will immediately formed in few picoseconds. These results well unveil the mechanism of substituent-position-dependent hot exciton emission. In Chapter 3, the mechanism of aggregated-induced hot exciton emission was revealed. In this part, the solid-state luminescence properties and photodynamic of DPA-FBP and TPA-FBP were studied in the PMMA doped film with different weight fraction. For these two molecules, only the 50wt% doped film (aggregated state) presents TADF, whereas the 1wt% doped film (single-molecular state) only displays prompt fluorescence. Fs-TA spectroscopy reveals that the 50wt% doped film tends to generate charge transfer (CT) state, whereas as 1wt% doped film shows a relatively long-lived local excited (LE) state. The film of aggregated state shows a much narrower ΔEST than single-molecular state, illustrated from the comparison of fluorescence and phosphorescence peak in photoluminescence spectra, so the efficiency of ISC and RISC will enormously improve. Hence, the key factor for the aggregated-induced hot exciton emission is uncovered to be the CT process in aggregated state. In Chapter 4, the F-AQ comprised of fluorene and anthraquinone is reported, and it exhibits a polymorphism with muti-color emission and TADF from (hRISC). In the fs-TA result, an unambiguous signal of through space charge transfer (TSCT) was observed for the crystal with the π-π interaction between the fluorene and anthraquinone groups, whereas the other amorphous solids and crystal only show a conventional deactivation pathway of hot exciton emission. This is the first study that directly observed the morphism-dependent TSCT in a crystal to our knowledge, These studies provide a new insight into the deactivation mechanism and photodynamic of the TADF from hRISC and supplement the deficiency in the ultrafast spectroscopy investigation of solid-state materials.