image: a The simplified Jablonski diagram and design schematic of how to manage triplet exciton transitions without changing the luminophore structures based on Dexter triplet-triplet energy transfer (TTET) and Förster triplet-singlet energy transfer (TSET). PF: prompt fluorescence; TADF: thermally activated delayed fluorescence; RTP: room-temperature phosphorescence; ISC: intersystem crossing; RISC: reverse intersystem crossing. b Chemical structures of the host molecules in host engineering. c The schematic diagram of management of triplet exciton transitions to express Tn→S1 and Tn→T1 mainly and the chemical structure of guest. IC: internal conversion.
Credit: by Jiaqiang Wang, Yujie Yang, Xinnan Sun, Xiaoning Li, Liyao Zhang, and Zhen Li
Organic luminescent materials have achieved a significant increase in interest for their lighting, display, anti-counterfeiting and bioimaging applications, and the luminescent process is heavily related to excitons. Difficulties in observation of emission involving triplet excitons due to the spin-forbidden nature of them led to only partially understanding of management of triplet excitons. So far, the emission involving triplet states mainly includes thermally activated delayed fluorescence (TADF), triplet-triplet annihilation (TTA) and phosphorescence (Figure 1a), with great attention attracted for their wide applications. Considering that both TADF and RTP are related to triplet excitons, thus, perhaps, the emission intensity at different wavelength might help us to explore the management of triplet excitons. But, for an isolated system, management of triplet excitons with unchanged energy levels of emissive S1 and T1 is hardly realized, no matter it helps to study the inherent mechanism of excitons management and make the evaluation of transition more accurate. Fortunately, the different energy transfer mechanisms of Dexter exchange energy transfer and Förster resonance energy transfer (FRET) could help to control the transitions of triplet excitons to singlet or triplet states, according to different energy levels.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Zhen Li, and co-workers have successfully managed triplet excitons and studied exciton transition process through host-guest systems with dual emission of TADF and RTP. Distinct from the traditional RTP host-guest systems to change the guest molecules for different afterglow colors, in this work, host engineering was utilized to do the management of triplet excitons. Accordingly, a dibenzofuran substituted 1,8-naphthalimide derivative (NDOH) was synthesized to be the guest molecule and seven phthalimide derivatives were prepared to act as host (Figure 1b). The management results could be evaluated based on the intensity ratios between TADF and RTP. By finely tuning the conjugated structures of phthalimide derivatives, energy differences (ΔE) between T1 of host and guest were tuned from 0.03 to 0.17 eV, and the resultant host-guest system exhibited different intensity ratios between TADF and RTP (ITADF/IRTP) from 20 to 0.1, which decreased by 200 times, while the afterglow color also changed from cyan to orange-red (Figure 1c).
Moreover, it was found that larger ΔE would result in more efficient transition from T1 of host to that of guest. When ΔE was larger than 0.07 eV, ITADF/IRTP would be smaller than 1 and transitions to triplet state would dominate rather than to singlet state. Furthermore, the terminal group on the N-substituted alkyl chain of PBOH was further tuned from hydroxyl group to other groups. Interestingly, PBNC exhibited the loosest crystal packing and strongest spin-orbital coupling (SOC) originating from the bromide ions, resulting in no triplet-singlet transition accompanying with no TADF emission but only strongest RTP and further proving the mechanism of management of triplet excitons. Accordingly, scientists can not only predict the triplet-singlet and triplet-triplet transitions based on ΔE, but also develop efficient TADF or RTP materials by regulating ΔE to promote FRET or Dexter energy transfer. These scientists summarize the management principle of triplet excitons:
“With the extent of electron delocalization increasing, the absorption edge of the phthalimide derivative was red shifted and the band gap was decreased. In other words, energy gaps between highest occupied crystal orbital and lowest unoccupied crystal orbital were decreased with the extent of electron delocalization increasing. Large energy gap indicates the large optical gap between T1 and ground state. Thus, energy differences (ΔE) between T1 of host and guest were also increased with the extent of electron delocalization increasing, facilitating hole and electron transfer between the T1 states of host and guest materials, and giving rise to efficient TTET and inefficient TSET. In this way, management of triplet excitons was achieved. When a weak π-electron donating group, such as methoxy group, was connected to phthalimide, and the resultant ΔE of host-guest system was larger than 0.07 eV, more efficient transition to triplet state rather than singlet state would occur with stronger RTP emission intensity. As the result, various relative intensities and lifetimes of TADF and RTP for the host-guest systems were observed, resulting in different afterglow color.”
“By changing the stacking modes of host materials, it also becomes possible to control the triplet states of these materials and subsequently manipulate triplet excitons. For example, PBNC crystal with loosest stacking mode and largest ΔE (0.22 eV) exhibited most efficient TTET process when serving as host and no transition to singlet state was observed. Emission intensity around 448 and 599 nm of NDNC@PBNC powder both decreased with temperature increasing, further confirming that no TADF emission could be found on it. When other crystals served as host, the emission in short wavelength region was thermally activated while the emission in long wavelength region was thermally quenched, which corresponded to TADF and RTP nature of the sample, respectively. And the management of triplet excitons could also be achieved by adjusting temperature.” they added.
“A line chart of the relationship between the T1 emission wavelength of host materials and ln(ITADF/IRTP) can be provided, where ITADF and IRTP are the maximum emission intensities of TADF and RTP in the delayed spectra of host-guest systems, respectively. It will be conducive to predict the triplet-triplet and triplet-singlet transitions on the basis of the phosphorescence emission of host materials. Benefiting from it, we can just finely regulate the movement of triplet excitons and afterglow color by changing the host structure, as well as develop efficient TADF or RTP materials after the regulation of ΔE for promotion of FRET or Dexter energy transfer. Based on the multiple stimulus-responsive exciton transitions, the host-guest systems can also be utilized in multiple anti-counterfeiting and temperature or humidity monitoring.” the scientists forecast.