• Bulletin of the Korean Chemical Society
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• v.22 no.9
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• pp.1005-1008
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• 2001

The lanthanide complexes have been anticipated to exhibit high efficiency along with a narrow emission spectrum. Photoluminescence for the lanthanide complex is characterized by a high efficiency since both singlet and triplet excitons are involve d in the luminescence process. However, the maximum external electroluminescence quantum efficiencies have exhibited values around 1% due to triplet-triplet annihilation at high current. Here, we proposed a new energy transfer mechanism to overcome triplet-triplet annihilation by the Eu complex doped into phosphorescent materials with triplet levels that were higher than singlet levels of the Eu complex. In order to show the feasibility of the proposed energy transfer mechanism and to obtain the optimal ligands and host material, we have calculated the effect depending on ligands as a factor that controls emission intensity in lanthanide complexes. The calculation shows that triplet state as well as singlet state of anion ligand affects on absorption efficiency indirectly.

• Bulletin of the Korean Chemical Society
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• v.23 no.2
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• pp.271-276
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• 2002

The energy relaxation dynamics of the lowest excited singlet and triplet states of the Zn(II)porphyrin monomer and its directly linked arrays were comparatively investigated with increasing the number of porphyrin moieties. While the fluorescence decay rates and quantum yields of the porphyrin arrays increased with the increase of porphyrin units, their triplet-triplet (T-T) absorption spectra and decay times remained almost the same. The difference in the trends of energy relaxation dynamics between the excited singlet and triplet states has been discussed in view of the electronic orbital configurations.

• Bulletin of the Korean Chemical Society
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• v.27 no.12
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• pp.2051-2054
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• 2006

The molecular structures of the ground and lowest triplet states of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH), 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) and their fused ring derivatives are investigated with an ab initio method and the density functional theory. Unlike the singlet DBH and DBO, the azo skeletal structures of the triplet counterparts are turned out to be quite sensitive to the change of the electronic structure of the fused ring. The B3LYP C-N=N-C dihedral angles of the triplet DBH and DBO are estimated to be about 28.0 and $40.4{^{\circ}}$, respectively. The B3LYP singlet-triplet energy gaps for DBH and DBO are predicted to be 58.4 and 48.4 kcal/mol, respectively. The triplet state energy can be lowered drastically by the presence of the remote $\Pi-\Pi$ interaction as in the case of 1bb'.

• Korean Chemical Engineering Research
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• v.55 no.6
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• pp.731-744
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• 2017

Triplet-triplet annihilation upconversion (TTA-UC) is a special photochemical process that converts low energy photons to higher energy photon via combination of organic chemicals which fulfill specific energetic criteria. TTA-UC has been known as attractive technology that is able to enhance energy conversion efficiency of the photonic devices based on sunlight, which is achieved by conversion of wasted low energy photons in solar spectrum into higher energy photon. In the present paper, we introduced the photochemical mechanism and characteristics of TTA-UC phenomenon, which is yet unfamiliar to the domestic academia, and investigated recent research status, application, and future research directions of TTA-UC technology.

• Applied Chemistry for Engineering
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• v.30 no.1
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• pp.88-94
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• 2019

Triplet-triplet annihilation upconversion (TTA-UC) is a photochemical process wherein two or more low-energy photons are converted to a high-energy photon through a special energy transfer mechanism. Herein, we report a strategy to enhance the efficiency of TTA-UC through the light-scattering effect induced by silica microparticles (SM) embedded in polymeric thin films. By incorporating monodisperse uniform silica microparticles with a uniform size of 950 nm synthesized by $St{\ddot{o}}ber$-based seeded growth method into UC polymeric thin films, the UC intensity in the 430-570 nm range was enhanced by as much as 64% when irradiated by 635 nm laser. Analyzing the lifetime of PdTPBP phosphorescence revealed that the presence of SM in the UC layer does not affect triplet-triplet energy transfer (TTET) between sensitizers and acceptors, supporting the enhancement of TTA-UC originated from the light-scattering effect. On the other hand, the incorporation of SM in UC layer is shown to enhance the triplet-triplet annihilation (TTA) efficiency, which results in a 1.5-fold increase of the ${\Phi}_{UC}$, by scattering light source and thus increasing the number of excited photons to be utilized in TTA-UC process.

• Proceedings of the Korean Society of Potoscience Conference
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• spring
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• pp.11-12
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• 1999

• Bulletin of the Korean Chemical Society
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• v.6 no.4
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• pp.234-238
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• 1985

The photoisomerization of 5-styryl-1,3-dimethyluracil is studied with nanosecond laser flash photolysis technique at room temperature. The laser flash photolysis of E-isomer produces the transient absorption spectrum regarded as the triplet-triplet absorption, but the transient absorption of Z-isomer does not show the typical decay curve, probably due to the facile photocyclization reaction during the laser flash photolysis. Using the energy transfer method on nanosecond laser spectroscopy, the energy of the lowest triplet state for E isomer is estimated to lie between 41.8 and 47 kcal/mol. The triplet lifetime for E-isomer obtained from the decay curve of the transient absorption is ca. 93ns. The $S_1 → T_1$ intersystem crossing of E-isomer on direct excitation is relatively inefficient at room temperature supporting the singlet mechanism for direct photoisomerization.

• Bulletin of the Korean Chemical Society
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• v.23 no.2
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• pp.281-285
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• 2002

The photophysical properties and the singlet oxygen generation efficiencies of meso-tetraphenyl-trithiasapphyrin $(S_3TPS)$ and meso-tetmkis(p-methoxy phenyl)-trithiasapphy rin ((p-MeO)-$S_3TPS$) have been investigated, utilizing steady-state and time-resolved spectroscopic methods to elucidate the possibility of their use as photosensitizers for photodynamic therapy (PDT). The observed photophysical properties were compared with those of other porphyrin-like photosensitizers in geometrical and electronic structural aspects, such as extended ${\pi}$ conjugation, structural distortion, and internal heavy atoms. The steady-state electronic absorption and fluorescence spectra were both red-shifted due to the extended ${\pi}$-conjugation. The fluorescence quantum yields were measured as very small. Even though intersystem crossing rates were expected to increase due to the increment of spin orbital coupling, the triplet quantum yields were measured as less than 0.15. Such characteristics can be ascribed to the more enhanced internal conversion rates compared with the intersystem crossing rates. Furthermore, the triplet state lifetimes were shortened to -1.0 ${\mu}s$ as expected. Therefore, the singlet oxygen quantum yields were estimated to be near zero due to the fast triplet state decay rates and the inefficient energy transfer to the oxygen molecule as well as the low triplet quantum yields. The low efficiencies of energy transfer to the oxygen molecule can be attributed to the lower oxidation potential and/or the energetically low lying triplet state. Such photophysical factors should be carefully evaluated as potential photosensitizers that have extended ${\pi}$-conjugation and heavy core atoms synthesized for red-shifted absorption and high triplet state quantum yields.

• Journal of the Korean Chemical Society
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• v.22 no.1
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• pp.45-51
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• 1978

The luminescence of N-methyllutidone is studied in ethanol matrix at $77^{\circ}C$K. No fluorescence is observed but a strong phosphorescence with the quantum yield of 0.1 and the lifetime of 0.2 sec is recorded. An unusually high triplet energy of 85.1 kcal/mole is determined for the compound from the O-O band of phosphorescence. The cis ${\leftrightarrow}$ trans photoisomerization of high triplet energy olefins such as 2-hexene and trans-1,4-dichlorobutene-2 is efficiently sensitized by N-methyllutidone substantiating the high triplet energy of the compound. The negative polarization of O-O band reveals the emitting triplet state to be $({\pi},{\pi}^*)^3$ state. Alkaline metal salts such as lithium chloride enhances the phosphorescence intensity through cation-N-methyllutidone coordination widening the gap between $({\pi},{\pi}^*)^3$and $(n,{\pi}^*)^3$ states.

• Bulletin of the Korean Chemical Society
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• v.35 no.9
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• pp.2738-2742
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• 2014

Geometry relaxation effects on the formation of benzene excimer were investigated by means of ab initio calculation at SOS-CIS($D_0$)/aug-cc-pVDZ level. In the case of T-shaped dimer configuration, intermolecular interactions in the excited states are found to be nearly the same as those in the ground state and structural deformations are limited within a single molecule; the geometry relaxation effects are then negligible and singlet-triplet energy gap remains constant. As for face-to-face eclipsed dimer, on the other hand, both molecules undergo structural change. As a result, intermolecular interactions in the excited states are significantly different than those in the ground state. Although the intermolecular distances obtained from potential energy curve calculation with frozen molecular structures are in qualitative agreement, the excited-state binding energies are notably overestimated with respect to those at optimized structures. In particular, the effects are calculated to be larger in $T_1$ state and hence singlet-triplet energy gap, which reduces markedly in this configuration, is underestimated without relaxation.