Study On Triplet Energy Transfer For Improving Molecular Self-assembly Triplet Annihilation Upconversion Performance

Triplet-triplet annihilation（TTA）photon upconversion is a technology that can convert light with lower energy（longer wavelength）into light with higher energy（shorter wavelength）.It has been applied in various fields from energy to biology.Compared with the other two upconversion techniques（two-photon absorption and rare earth upconversion）,TTA upconversion can utilize lower intensity incoherent excitation light,and this technique has a higher upconversion quantum yield and easily tunable excitation and emission wavelength,which makes it have unique advantages in photovoltaic,bioimaging,photocatalysis and other applications.Thus it has received extensive attention from many researchers.Early based TTA upconversion in organic solvents can generally achieve high upconversion efficiency,but the quenching of excited triplet states by oxygen greatly limits the application of TTA upconversion technology.In recent years,many researchers have used specific solid polymers or viscous liquids as matrices to block oxygen,however,this inevitably limits the free diffusion of excited triplet molecules in TTA upconversion,which not only results in the use of higher power excitation light,but also reduces the efficiency of TTA upconversion.Based on these problems,some scholars have recently introduced the concept of molecular self-assembly and proposed a new strategy"photon upconversion based on triplet energy transfer（TEM）".That is,the photosensitizer is combined with densely accumulated or ordered receptor molecules through molecular self-assembly.When the photosensitizer is excited,even without free molecular diffusion,the photosensitizer to acceptor triplet-triplet energy transfer（TTET）process,the triplet energy transfer（TEM）process in the acceptor molecule and the triplet-triplet annihilation（TTA）process between triplet acceptors can be efficiently performed,resulting in higher-efficiency upconversion emission at low excitation thresholds.Here,we designed and synthesized a series of upconversion luminescent materials by using the TEM strategy.The works are mainly divided into three parts:1.The acceptor molecule in TTA upconversion has a great influence on the upconversion efficiency.As a typical acceptor molecule,9,10-diphenylanthracene（DPA）can be combined with a transition metal complex photosensitizer to achieve efficient upconversion.However,after such up-conversion systems are immobilized,the required excitation power increases and the up-conversion efficiency decreases.To this end,based on the TEM strategy,we designed and synthesized two novel anthracene ionotropic acceptors DPA-1 and DPA-2 for TTA upconversion by introducing imidazolium cation groups with different chain lengths on DPA molecules.The study found that the fluorescence quantum yields of these two ionotropic acceptors were slightly lower than that of DPA,which made the upconversion quantum yields of the upconversion systems combined with the photosensitizer platinum octaethylporphyrin（Pt OEP）decreased.However,the excitation threshold of the DPA-1/Pt OEP upconversion system was as low as 6.02 m W/cm~2,which is close to the solar radiation intensity,and is nearly twice lower than the excitation threshold of the DPA/Pt OEP system（11.08 m W/cm~2）.This indicates that the system can realize the efficient triplet energy transfer and migration process.In order to further prove,we quantitatively studied the efficiency of triplet-triplet energy transfer process in the system through quenching experiments.The results showed that the acceptor DPA-1 has higher TTET and TTA efficiencies compared with DPA and DPA-1,and the larger alkyl chain of DPA-2 increases the distance between Pt OEP and the acceptor,which reduces the TTET efficiency,but its TTA efficiency is still higher than that of DPA.These studies demonstrate that the synthesized ionic acceptors are similar to chromophore ionic liquids and can utilize the ionic polar network formed in the solvent to facilitate triplet energy transfer and migration.2.According to the two synthesized receptors DPA-1 and DPA-2 with imidazolium cations introduced,we designed and synthesized three anionic amphiphilic block polymers PMAA₆₅-PBz MA_30/70/170 based on the concept of molecular self-assembly.The positively charged acceptor molecules can assemble in an orderly manner through electrostatic interaction,and self-assembled in a solvent to form upconversion nanoparticles under the interaction of hydrophilic and hydrophobic forces,realizing efficient TTA upconversion in solution in an air environment.It was found that these three polymers can form nanoparticles with different morphologies in solvent due to the different proportions of hydrophobic blocks.Their morphologies are spherical micelles,worm micelles and vesicles,respectively.We investigated and compared the upconversion performance in the three nanoparticles loaded with different upconversion dye pairs and found that the upconversion efficiency was highest in the upconversion vesicle system.Among them,theΦ_UC of PMAA₆₅-PBz MA₁₇₀/DPA-1/Pt OEP system was as high as 10.86%,which is higher than that of PMAA₆₅-PBz MA₁₇₀/DPA/Pt OEP system（8.21%）,and the excitation threshold（11.98m W/cm~2）was lower than that of general polymer-immobilized upconversion material.The experimental results shown that the upconversion dye molecules can achieve efficient triplet energy transfer and migration in the self-assembled nanostructures.Moreover,the system had strong stability,and the up-conversion emission intensity can also maintain 95%of the original intensity after the excitation light was continuously irradiated for 2 hours.We also analyzed the structure of different upconversion nanoparticles,and found that controlling the ratio of the hydrophobic block of the polymer can lead to different structures of the nanoparticles,resulting in different distribution of acceptor molecules in the hydrophobic layer of the polymer,and then the upconversion efficiency can be regulated.It has certain guiding significance for the study of self-assembled upconversion systems based on nanostructured polymers.3.In order to further realize the immobilization of TTA upconversion,based on molecular self-assembly,we used the hydrophilic and hydrophobic interaction of the triblock polymer Pluronic F127 to form a structurally stable upconversion nanomicellar aqueous solution with the upconversion dye Pt OEP/DPA.Then,an upconversion hydrogel was designed and synthesized using sodium alginate as an immobilization matrix.The study found that theΦ_UC of the system was as high as 5.22%,and the excitation threshold was16.62 m W/cm~2.It indicated that the triplet energy could be efficiently migrated and transferred in the micellar core in the gel.At the same time,we used the total reflection mechanism of the optical fiber to realize the light transmission of the up-conversion signal,and the distance can reach 35 mm.The discovery of the upconversion phototransduction phenomenon in this system and its good biocompatibility will help to develop new TTA upconversion technology applications in the biological field.