9:15 AM - 9:30 AM
[5a-A504-1] [JSAP Young Scientist Award Speech] Direct visualization of triplet exciton diffusion in molecular crystals
Keywords:Photochemistry, Photon upconversion, Triplet exciton diffusion
Photon up-conversion (UC) materials which convert low-energy photons to high-energy ones are expected to improve the efficiency of solar energy-conversion devices. Photochemical UC materials using triplet-triplet annihilation process consist of donor molecules and acceptor molecules; such materials can potentially be used under low excitation intensity. In order to improve the efficiency of UC under low-light conditions, it is important to extend triplet exciton diffusion of acceptor molecules because triplet diffusion length is inversely related to excitation threshold. Therefore, a method to measure exactly the triplet diffusion length exactly is needed. So far, conventional methods used to measure the diffusion length are indirect in nature and are not suitable for solid-state UC materials due to the material inhomogeneity. Therefore, a method to measure the diffusion length directly is desirable.
In this study, 9,10-diphenyl anthracene or anthracene were used as an acceptor. Pt (II) Mesoporphyrin IX (D1) as a donor together with Mesoporphyrin IX- dihydrochloride (FD) as a fluorescence dye were adsorbed on alumina nanoparticles (30 nm) and these were used as hybrid donor particles (HDPs). The HDPs were doped in polycrystals of the acceptor molecules prepared on a glass substrate by spin-coating from tetrahydrofuran solution. A 532 nm irradiation light is absorbed by D1 as well as by FD in the HDP. The excited states in FD generate fluorescence which is used to determine the location of each HDP. The excited state in D1 undergoes efficient intersystem crossing and the excitation energy is transferred to the acceptor via triplet-triplet (T-T) energy transfer. Thus generated triplet excitons of the acceptor diffuse, encounter each other and can undergo triplet-triplet annihilation which results in the generation of the UC photons. Due to the diffusion of the triplet excitons of acceptor molecules, the UC photons can be emitted away from the HDP. As a result, the size of a microscopic image of UC emission can be larger than that of emission from the HDP, as determined from the size of fluorescence image of FDs attached to the HDP. Analysis of the images in terms of fitting by theoretical diffusion equations provides directly the values of the triplet diffusion length, which ranges between tens and hundreds of nm for the acceptor molecules used. This novel technique is expected to provide novel insight into the triplet diffusion mechanism in UC materials.
In this study, 9,10-diphenyl anthracene or anthracene were used as an acceptor. Pt (II) Mesoporphyrin IX (D1) as a donor together with Mesoporphyrin IX- dihydrochloride (FD) as a fluorescence dye were adsorbed on alumina nanoparticles (30 nm) and these were used as hybrid donor particles (HDPs). The HDPs were doped in polycrystals of the acceptor molecules prepared on a glass substrate by spin-coating from tetrahydrofuran solution. A 532 nm irradiation light is absorbed by D1 as well as by FD in the HDP. The excited states in FD generate fluorescence which is used to determine the location of each HDP. The excited state in D1 undergoes efficient intersystem crossing and the excitation energy is transferred to the acceptor via triplet-triplet (T-T) energy transfer. Thus generated triplet excitons of the acceptor diffuse, encounter each other and can undergo triplet-triplet annihilation which results in the generation of the UC photons. Due to the diffusion of the triplet excitons of acceptor molecules, the UC photons can be emitted away from the HDP. As a result, the size of a microscopic image of UC emission can be larger than that of emission from the HDP, as determined from the size of fluorescence image of FDs attached to the HDP. Analysis of the images in terms of fitting by theoretical diffusion equations provides directly the values of the triplet diffusion length, which ranges between tens and hundreds of nm for the acceptor molecules used. This novel technique is expected to provide novel insight into the triplet diffusion mechanism in UC materials.