Organic afterglow materials, which feature long emission lifetimes, display promising applications in oxygen sensing and mapping, biological imaging and anti-counterfeiting. For those with emission lifetimes longer than 0.1 s, inexpensive optical equipment, mobile phone cameras and even human eyes can distinguish afterglow color, afterglow duration and their change. Using these afterglow materials and in conjugation with supramolecular recognition or chemodosimeter technique, we can envisage the emergence of a portable platform for optical sensing, analysis and imaging, which would function at hospital bedside, at home and even outdoors, rather than that only work in well-equipped laboratory.
Despite of the bright future, due to the spin-forbidden nature and weak spin-orbit coupling in organic systems, it is difficult for constructing highly-efficient and long-lived afterglow materials under ambient conditions. The past decade has witnessed the remarkable progress in this field, with diverse structures being synthesized, various strategies being developed, afterglow performance being enhanced, and intriguing functions being explored. We and other research groups have developed two-component strategy, where a second component is used to modulate the excited state property of luminescent component (the first component), for constructing high-performance organic afterglow materials1.
In these years, we have synthesized approximately 300 difluoroboron β-diketonate compounds via our cascade reactions for afterglow material fabrication2–7. Among them, three benzophenone-containing difluoroboron β-diketonate (BPBF2) compounds attract our attention. Upon doping into phenyl benzoate (PhB) matrices, the resultant BPBF2-PhB two-component materials exhibit phosphorescence bands with emission maxima (λP) in their delayed emission spectra (1 ms delay) shorter than the fluorescence maxima (λF) in their steady-state emission spectra. This is an unexpected and serendipitous finding; conventionally, λP should be longer than λF. Up-converted room-temperature phosphorescence (RTP) with λP < λF and emission lifetimes longer than 0.1 s has not been observed by conventional experimental setups in the reported studies8. Initially, we highly doubted the identity of this up-converted RTP. Many experimental and theoretical studies, as well as diverse control experiments, have been performed to reveal the nature of this up-converted RTP. Finally, we confirm that the up-converted RTP is the only possible mechanism to explain all the observations in the present study.
Figure 1. Serendipitous finding in BPBF2-PhB two-component materials. a) Chemical structures of three BPBF2 compounds and organic matrix; b) Steady-state and delayed emission spectra (1 ms delay) of 1-PhB-0.1% powder at room temperature; c) Proposed dual RTP emission mechanism.
The up-converted RTP is originated from BPBF2’s Tn (n ≥ 2) states which show typical 3n-π* characters from benzophenone moieties. Detailed studies exhibit that, upon intersystem crossing from BPBF2’s S1 states of charge transfer characters, the resultant T1 and Tn states build T1-to-Tn equilibrium under ambient conditions. Because of their 3n-π* characters, the Tn states possess large phosphorescence rates (kP(Tn)) that can strongly compete RTP(T1) to directly emit RTP(Tn) which violates Kasha’s rule.
The up-converted RTP features smaller Stokes shift than fluorescence, which is useful for devising visible-light-excitable deep-blue afterglow emitters. The ΔE(Tn-T1) of the BPBF2-PhB systems have been found to be around 0.3 eV. The direct observation of up-converted RTP demonstrates that it is still possible to form T1-to-Tn equilibrium under ambient conditions in organic systems with high ΔE(Tn-T1) value of around 0.3 eV. Theoretical studies reveal that the electron-vibrational coupling can increase the population of Tn states, and the large kP(Tn)/kP(T1) ratios can compensate the small population of Tn states, leading to anti-Kasha RTP(Tn) emission. Here the clearly resolved RTP(Tn) and RTP(T1) bands endow the BPBF2-PhB materials with stimuli-responsive functions via RTP(Tn)/RTP(T1) ratiometric change towards mechanical force and temperature variation.
The direct observation of up-converted RTP provides deep understanding on triplet excited state dynamics. The involvement of benzophenone functional groups on BPBF2 molecules is very important to achieve such up-converted RTP in the dopant-matrix systems since it not only facilitates intersystem crossing but also endows Tn (n ≥ 2) states with n-π* character and large phosphorescence rates. Given that the energy levels of the Tn states are mainly determined by the benzophenone groups, here the use of difluoroboron β-diketonate functional groups (with suitable LUMO level and electron-accepting strength) is also very important to result in a proper ΔE(Tn-T1) in BPBF2 system. Besides, the involvement of two-component strategy is also crucial for the occurrence of the up-converted RTP. Unlike the conventional observation of organic phosphorescence where nonradiative decay and oxygen quenching of organic triplets are suppressed at low temperature such as at 77 K, in two-component afterglow systems, the nonradiative decay and oxygen quenching can be “frozen” by rigid organic matrices at room temperature and even at higher temperature. At room temperature or higher, some photophysical processes can be thermally activated to a certain extent. Consequently, the interplay of multiple slow photophysical processes related to triplet excited states would give rise to the emergence of interesting photophysical properties in organic afterglow systems, as well as high-performance organic afterglow materials with intriguing properties. These are our understanding on the manipulation of triplet excited state properties in two-component systems. We are performing more studies to explore the potentials of two-component afterglow systems.
More information of this study entitled “A direct observation of up-converted room-temperature phosphorescence in an anti-Kasha dopant-matrix system” can be found in the Nature Communications journal (https://doi.org/10.1038/s41467-023-37662-y).
- Li, J. et al. Manipulation of Triplet Excited States in Two-Component Systems for High-Performance Organic Afterglow Materials. Chem. Eur. J. 28, e202200852 (2022).
- Zhou, B. et al. Highly efficient room-temperature organic afterglow achieved by collaboration of luminescent dimeric TADF dopants and rigid matrices. J. Mater. Chem. C 9, 3939–3947 (2021).
- Sun, Y., Wang, G., Li, X., Zhou, B. & Zhang, K. Achieving High Afterglow Brightness in Organic Dopant-Matrix Systems. Adv. Opt. Mater. 9, 2100353 (2021).
- Wang, X. et al. TADF-Type Organic Afterglow. Angew. Chem. Int. Ed. 60, 17138–17147 (2021).
- Pan, Y. et al. Highly Efficient TADF-Type Organic Afterglow of Long Emission Wavelengths. Adv. Funct. Mater. 32, 2110207 (2022).
- Deng, X., Huang, J., Li, J., Wang, G. & Zhang, K. Sonication-Responsive Organic Afterglow Emulsions. Adv. Funct. Mater. 2214960 (2023).
- Li, J. et al. Cascade Synthesis of Luminescent Difluoroboron Diketonate Compounds for Room-Temperature Organic Afterglow Materials. Chin. J. Chem. 40, 2507–2515 (2022).
- Kukhta, N. A. & Bryce, M. R. Dual emission in purely organic materials for optoelectronic applications. Mater. Horiz. 8, 33–55 (2021).