From the aristocracy to the common people – new captain afterglow luminous materials

Long Persistent Luminescence (LPL) material is also known as luminescent Luminescence material, commonly known as luminous powder or Long afterglow powder. Its luminescence principle is photoluminescence, that is, when excited by the light source, the excitation energy is stored in the excited state. When the excitation stops, the energy is slowly released in the form of light. In 1996, Matsuzawa et al. published a strontium aluminate (SrAl2O4) system doped with europium (Eu) and dysprosium (Dy), with a decay time of up to 10 hours and high durability. Subsequently, this rare earth activated aluminate long-afterglow material became the basis of luminescent coating and won the favor of the commercial industry. It has been widely used in instrument display, optoelectronic devices, night emergency instructions, national defense and military and other fields. However the LPL materials based on inorganic system needs not only expensive rare element, and make temperature as high as 1000 ℃ above, high energy consumption. In addition, many steps are required to move from an undissolved aluminate to a finished paint, such as grinding the compound into a micron-sized powder before it can be soaked into the solvent or substrate. In addition, the light scattering of powder also limits the transparency of LPL coating. In order to solve these problems, a lot of attention is focused on LPL materials based on organic system.

There have been very few reports of captain yu hui glowing (OLPL) materials. As we all know, the long-life luminescence of organic molecules is usually phosphorescence, most of which lasts less than 1 second, and the longest is only a few minutes, which cannot be compared with the long-life luminescence of inorganic LPL luminescence. The radiation transition of organic molecules from the photoexcited state usually only produces fluorescence, phosphorescence and delayed fluorescence, which is not suitable for the generation of OLPL. However, when ionized state (photoinduced ionization state) and charge separation state (photoinduced charge separation state) can be generated in organic molecules under light, they can also obtain a long luminous life.

So let’s talk about ionization. Photoionization of organic molecules was first reported in 1942 as a result of continuous two-photon absorption. The absorption of the first photon produces an excited state (usually a trilinear or singlet state), and the absorption of the second photon produces an ionized state whose energy exceeds the ionization potential. Then another research group studied the two-photon ionization of organic guest molecules dispersed in the polymer matrix which provided a rigid environment to stabilize the free radical cations of the guest. It was found that the system could generate LPL at a temperature of 20 K for more than 10 hours. In this luminescence process, the electrons from the photoionized guest molecules continuously accumulate in the polymer matrix, and then slowly recombine with the free radical cation of the object to form the excited state of the guest molecules, in which the singlet state accounts for 25% and the triplet state accounts for 75%. After recombination, the fluorescence and phosphorescence of the guest molecules can be observed. However, this particular two-photon ionization process requires a very strong excitation source and extremely low temperature.

Let’s talk about charge separation. The molecules commonly used in organic photocells, which have both a donor and a donor, offer another way of forming charge-separated states even under weak light radiation. When an electron is excited by light and jumps from the highest occupied molecular orbital (HOMO) of the donor to the lowest unoccupied molecular orbital (LUMO), it moves from the LUMO of the donor to the LUMO of the receptor, creating a charge transfer state that eventually splits into a charge separation state. It has been reported that a single molecule with a twisted donor and receptor produces a long-lived charge separation state, but OLPL under low light radiation has not yet been achieved.

Recently, researchers such as Chihaya Adachi, a professor at the center for advanced organic optoelectronics research (OPERA) at kyushu university in Japan, and Ryota Kabe, an assistant professor at the center for advanced organic optoelectronics research (OPERA), developed the world’s first long-tail luminescent material that can glow at room temperature for more than an hour under low radiation. This OLPL material can be standard white LED light source stimulated, even under the condition of higher than 100 ℃ can shine for a long time. Because it contains no expensive rare elements, but only a mixture of two simple organic molecules, TMB and PPT, and is easy to make, the cost can be reduced by about 90%, according to Japanese media. In addition, the researchers demonstrated that the luminescence mechanism of this OLPL material is caused by the charge combination of long-lived charge separation states (intermediates). The results were published in Nature.

First, in order to obtain a charge separation state (intermediate) necessary for the long-term luminescence of organic molecules, the researchers selected the strongly electron donor molecule TMB (N,N,N’,N’ -tetramethylbenzidine) and the strongly electron acceptor molecule PPT (2, 8-bis (diphenyl phosphoryl) dibenzo [b,d] thiophene). The former can form a very stable free radical cation; The latter has a high tri-linear energy and provides a rigid amorphous environment (substrate) to help inhibit radiation-free inactivation. Then the researchers by changing the TMB in PPT content made a lot of amorphous thin film, with the traditional casting method, under the environment of nitrogen heat the mixture of the two materials to more than PPT after melting point (250 ℃), rapid cooling to room temperature. In the process of photoexcitation, charge transfer states are formed between TMB and PPT. The PPT free radical anion generated then diffuses to all PPT molecules through charge jump, so that the TMB free radical cation and the corresponding PPT free radical anion are separated to form a stable charge separation state. After that, PPT free radical anion and TMB free radical cation slowly recombined to produce Exciplex luminescence, and the photoexcitation could remain long after termination.

Secondly, the spectral properties of the new OLPL material and its components were studied and compared. Figure 3a shows the uv-visible absorption spectrum, fluorescence spectrum and phosphorescence spectrum of TMB and PPT in toluene solution of 10-5 M respectively. Due to their short conjugated chains, the absorption of TMB and PPT is below 350 nm, the maximum fluorescence is at 394 nm and 346 nm respectively, while the emission peak of TMB is at 486 nm and 520 nm in phosphor spectrum (77 K), and the emission peak of PPT is at 425 nm and 457 nm. In contrast, the luminescence spectrum of 1 mol% TMB/PPT mixture shows a wide emission peak, which is actually redshifted compared to the fluorescence and phosphorescence peaks of each component. This photoluminescence spectrum indicates the formation of an excitation complex between TMB and PPT. In addition, the absorption spectra of the 1 mol% TMB/PPT composite film were slightly extended to 450 nm, which was well consistent with the excitation spectra of the excitation complex. This extended absorption spectrum may originate in the charge transfer absorption band, or may be due to the presence of a small number of TMB free radical cations (which are absorbed in the long wave region).

 

After the excitation light irradiated on the TMB/PPT composite film was turned off, the researchers observed LPL, which also had the same spectrum as the photoluminescence, which further indicated that the luminescence of LPL came from the excitation complex formed between TMB and PPT. Figure 3b shows the luminescence attenuation curve of 1 mol% TMB/PPT thin film. Surprisingly, LPL can still maintain for more than an hour at 300 K. In contrast, the phosphor lifetime calculated from TMB and PPT instantaneous luminescence attenuation curve (in toluene solution at 77 K) is 2.20 s and 0.63 s, respectively. Although the activated delayed fluorescence (TADF) was often present in the activation complex, it entered a multiexponential decay state with a duration of less than 1 second. However, for a 1 mol% TMB/PPT film, the attenuation of LPL intensity was an exponential function of time t-1, with no exponential decay. In addition, the attenuation curve also conforms to debye-edwards law (t-m, m=1), which is similar to the reported two-photon ionization of TMB in polymer matrix. However, the photoluminescence and LPL intensity in this system have a linear relationship with the excitation intensity, showing the single-photon process.

The TMB two-photon ionization in the polymer matrix also produced fluorescence and phosphorescence, while the TMB/PPT hybrid film only showed the luminescence of the excitation complex. Because the energy gap between the lowest excited state and the lowest excited state is small, the excitation complex can capture excitons in delayed luminescence by reversible inter-gap hopping. Therefore, if the non-radiative attenuation is completely suppressed, the OLPL system may also capture all excitons generated by recombination. However, the researchers could not obtain direct evidence to prove the existence of delayed fluorescence (from reversible gap hopping), because the LPL luminescence attenuation masked any single exponential attenuation of delayed fluorescence.

Because photoinduced free radical anions and cations are unstable in the presence of water and oxygen, LPL luminescence is easy to quench in air, which requires sealing. The luminescent quantum yield in nitrogen (which simulates the seal) is 21% 3%, but in air it drops to 7% 2%. In the sealed state, the OLPL system can exist stably in the dark for more than a month.

At 300 K, the TMB/PPT film OLPL luminescence can be detected for more than 30 minutes, and the luminescence at the crack caused by rapid cooling of the molten casting film is the strongest. Since effective charge separation can also occur under weak light radiation, the OLPL system can also be excited by standard white LED lights. In order to prove that charge separation can also be formed under this level of light, the transient absorption spectra of TMB/PPT membrane were measured after being stimulated by light, and it was found that there was a wide absorption between 600 nm and 1400 nm, which was corresponding to the absorption of TMB free radical cations. Thus, it is clear that TMB free radical cations are generated under light radiation.

Finally, the researchers investigated the variation of the duration of OLPL with excitation intensity, excitation time, sample temperature and TMB doping concentration. The results show that the duration of OLPL increases with the intensity of excitation light source, but LPL can still be observed when the intensity is as low as 10 W (even at 300K). The duration of OLPL increases as the excitation time increases. Although the luminescence duration was only 200 s when excited for 1 s, the duration of OLPL exceeded 5000 s when excited for 180 s. This change with excitation time clearly confirms the generation and accumulation of charge carriers under weak illumination. The intensity and duration of OLPL reached saturation under stronger light sources and longer illumination, as the number of accumulated charge carriers reached its upper limit. The duration of LPL from 1 mol% TMB/PPT film also depends on the temperature of the sample because of the nonradiative inactivation caused by molecular vibration. Below 300K, the duration of OLPL is close to a constant, and decreases when it is greater than 300K. This shows that even at room temperature, non-radiation inactivation caused by molecular vibration is inhibited, and non-radiation inactivation begins to increase at higher temperature. This dependence of OLPL duration on temperature is also partly related to the increased charge motion at higher temperatures, which increases the probability of charge recombination. The doping concentration of TMB also has a great influence on the duration of OLPL. When the concentration of TMB increased from 1 mol% to 50 mol%, the luminescence spectrum shifted slightly to the red (from 526 nm to 550 nm), and the luminescence duration decreased significantly from 10-4 s to 10-2 s. This is because at a higher concentration, the distance between TMB and PPT decreases, leading to an increased probability of the combination of TMB free radical cations and PPT free radical anions.

In conclusion, the researchers from kyushu university have demonstrated that the luminescence mechanism of the new OLPL material is generated by the charge combination of long-lived charge separation states (similar to organic solar cells) (similar to organic electroluminescence). New OLPL materials require no activation of rare elements, are transparent, and are easy to manufacture and process, greatly reducing manufacturing costs. These characteristics will enable OLPL materials to create new applications in fiber, glass and large-area flexible coatings, and it is expected to play an important role in the fields of information processing, new energy, biological imaging technology and cutting-edge science and technology in the universe. In addition, the study of long-lived charge separation states in this system can also promote people to have a better understanding of various organic semiconductor devices and artificial light synthesis. The researchers are now working on how to further increase the intensity and luminescence duration of OLPL, as well as how to improve the stability of OLPL in the air through the development of sealing technology, so as to make it practical as soon as possible.

From http://www.x-mol.com