Heavy-Atom-Free Room-Temperature Phosphorescent Organic Light-Emitting Diodes Enabled by Excited States Engineering

Abstract

Room temperature phosphorescence materials offer great opportunities for applications in optoelectronics, due to their unique photophysical characteristics. However, purely organic emitters that can realize distinct electrophosphorescence are rarely exploited. Herein a new approach for designing heavy-atom-free organic room temperature phosphorescence emitters for organic light-emitting diodes is presented. The subtle tuning of the energy diagrams of singlet and triplet excited states by appropriate choice of host matrix allows tailored emission properties and switching of emission channels between thermally activated delayed fluorescence and room temperature phosphorescence. Moreover, an efficient and heavy-atom-free room temperature phosphorescence organic light-emitting diodes using the developed emitter is realized.

Revealing Internal Heavy Chalcogen Atom Effect on the Photophysics of Dibenzo[a,j]phenazine-Cored Donor–Acceptor–Donor Triad

“…As for the long-delayed emission (phosphorescence), the emission slightly shifted to the higher energy regime with the rise of temperature (red and green lines in Figure 5b). Interestingly, while the energy level and the spectrum shape of the phosphorescence from T1 of PSeZ-DBPHZ-PSeZ are similar to those of previously studied DBPHZ-cored D-A-D compounds, [24][25][26][27][28][29] the phosphorescence of PTeZ-DBPHZ-PTeZ appears to be much more similar to the phosphorescence of donor (PTeZ) (Figure S14b). But, the calculation suggests that similarity is coincidental.…”

“…This implies a quite different conformation of the Se compound from its sulfur analogue, where eq-eq conformer dominates and efficiently electron-decoupled absorption spectra displays. [26a] In a similar manner with our previously developed DBPHZ-cored D-A-D compounds, [25][26][27][28][29] an intramolecular charge-transfer (ICT) transition at around l 460 nm was observed (Figure 3a). However, when compared to the absorption spectra of the sulfur analogue, the molar coefficient (ca.…”

“…Such complex behavior is not usually observed, but as mentioned before, we probably observe the emission from T1 and a higher triplet state (T2). [29] The change of the host more affects the CT excited state energies to the lower level than LE states, thereby in CBP host both D-A-D compounds display almost the same energy level. OLED fabrication and characterization.…”

“…Design and Synthesis. Previously, we have developed multi-photofunctional organic materials displaying thermally activated delayed fluorescence (TADF), mechanochromic luminescence (MCL), [23] and room-temperature phosphorescence (RTP) properties, based on a donor-acceptor- donor (D-A-D) [24][25][26][27][28][29] or a D-A-D-A scaffold, [30] utilizing dibenzo[a,j]phenazine [31] as the key acceptor unit. In 2017, we reported the first example of TADF-active multi-color-changing MCL material (PTZ-DBPHZ-PTZ) based on the design concept of conformation-dictated regulation of luminescence properties, where the ax-and eq-conformer arising from the boat-chair geometry of phenothiazine plays the key role in giving divergent ground and excited state energies.…”

“…Given these energy alignments and the SOC between the S0-T2 states (1.56 cm -1 ), the temperature activated process (Figure 5a and e) could be related with the processes comprising of ISC (from S1 to T1) followed by thermal reverse internal conversion (from T1 to T2) and the photo-irradiation from the T1 and/or T2 states. [29] Given the PSeZ-DBPHZ-PSeZ is admixrue of ax-ax and eq-ax conforms (Figure 3b), the energy transfer from the triplet excited state of ax-ax conformer to eq-ax conformer follwed by TADF emission from the 1 CT state would also likely to occur.…”