• Journal of Photoscience
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• 제12권2호
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• pp.57-61
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• 2005

The ground and excited state dipole moments of Coumarin 450 (C 450) laser dye were measured at room temperature in several solvents of varying dipole moments. The ground state dipole moment (${\mu}_g$) is estimated by using the modified Onsagar model and the excited state dipole moments (${\mu}_e$) were estimated by the method of solvatochromism as well as by utilizing the microscopic solvent polarity parameter ($E^N_T$). Further, the deviation of some of the points from the linearity of the $E^N_T$ versus Stokes shift indicates the existence of specific type of solute-solvent interaction. The excited state dipole moment of C 450 were found to be higher than those of the ground state and is interpreted in terms of the resonance structure of the molecule. A reasonable agreement has been observed between the values obtained by the method of solvatochromism and modified Onsagar model. It is observed that, corresponding to cyclohexane solution, the fluorescence maxima shift towards the red region with increasing the polarity of the solvents, hence the transition involved are of ${\pi}-{\pi}^*$ type.

• Journal of Photoscience
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• 제12권3호
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• pp.119-122
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• 2005

The photophysical properties of coumarin 6 laser dye have been studied in two solvents; ethanol and acetonitrile using steady-state fluorescence depolarization technique. The experimentally measured reorientation time of coumarin 6 is more or less the same in given solvents at particular temperature. It is found that coumarin 6 rotates slower in acetonitrile than in ethanol especially at higher values of viscosity over temperature. We also measure the ground and excited state dipole moments of coumarin 6 by solvent perturbation method. The results found that excited state dipole moment is greater than ground state dipole moment, which indicates that excited state is more polar than the ground state.

• 대한화학회지
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• 제63권2호
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• pp.73-77
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• 2019

${\pi}$-공액계 분자에 대해, density functional theory (DFT) 방법에 기반한 일반적인 삼중항 구조 최적화 방법들, 즉, 시간 의존적 DFT (TD-DFT), Tamm-Dancoff 근사법에 기반한 TD-DFT (TDA-DFT), 그리고 스핀-비제한 DFT (UDFT)에 대한 점검을 수행하였다. 모델 분자로서 1,2,3,4,5-pentacyano-6-phenyl-benzene가 이용되었고, 6-31G(d) 기저 함수와 더불어 여기 상태 계산에 최근 자주 사용되는 에너지 차 조정 영역 분리 functional인 ${\omega}B97X$ functional이 사용되었다. 계산 결과 평형 구조 근처에서, UDFT 최적화된 구조는 TD-DFT 및 TDA-DFT 계산 구조와는 다른 차이점을 보인다. 즉, 보다 안정한 바닥 상태 에너지와 보다 높은 삼중항 여기 에너지가 UDFT 최적화 구조에서 보인다. 본 논문에서는 이러한 차이에 대해 보다 자세히 토의된다.

• Journal of Electrical Engineering and Technology
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• 제11권4호
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• pp.801-809
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• 2016

The electrically excited synchronous generator (EESG) is applied in wind turbine systems recently. In an EESG control system, electrical torque is affected by stator flux and rotor current. So the control system is more complicated than that of the permanent-magnet synchronous generator (PMSG). Thus, the higher demanding of the control system is required especially in case of wind turbine mechanical resonance. In this paper, the mechanism of rotor speed resonant phenomenon is introduced from the viewpoint of mechanics firstly, and the characteristics of an effective damping torque are illustrated through system eigenvalues analysis. Considering the variables are tightly coupled, the four-order small signal equation for torque is derived considering stator and rotor control systems with regulators, and the bode plot of the closed loop transfer function is analyzed. According to the four-order mathematical equation, the stator flux, stator current, and electrical torque responses are derived by torque reference step and ramp in MATLAB from a pure mathematical deduction, which is identical with the responses in PSCAD/EMTDC simulation results. At last, the simulation studies are carried out in PSCAD software package to verify the resonant damping control strategy used in the EESG wind turbine system.

• 한국진공학회지
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• 제16권5호
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• pp.353-358
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• 2007

기저준위의 중심 피크가 $1.3\;{\mu}m$인 다층 양자점 구조를 사용하여 트렌치 구조를 가진 J-형태의 고휘도 발광소자 (superluminescent diodes)를 제작하였다. 도파로와 트렌치 구조 사이의 간격이 좁아지면서 광출력이 최대 20배까지 증가하였음을 확인하였다. 전류의 증가에 의한 EL 피크 측정결과 트렌치 구조를 가진 경우에 여기준위의 피크가 기저준위의 피크보다 수 십배 증가하는 것을 확인하였고, 이로부터 트렌치 증가에 의한 광출력의 증가는 양자점의 여기준위에 의한 것으로 판단하였다.

• 대한화학회지
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• 제67권6호
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• pp.407-414
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• 2023

The present study uses quasi-classical trajectory procedures to examine the vibrational relaxation and dissociation of the methyl and ring C-H bonds in excited methylpyrazine (MP) during collision with either N₂ or O₂. The energy-loss (-ΔE) of the excited MP is calculated as the total vibrational energy (E_T) of MP is increased in the range of 5,000 to 40,000cm^-1. The results indicate that the collision-induced vibrational relaxation of MP is not large, increasing gradually with increasing E_T between 5,000 and 30,000 cm^-1, but then decreasing with the further increase in E_T. In both N₂ and O₂ collisions, the vibrational relaxation of MP occurs mainly via the vibration-to-translation (V→T) and vibration-to-vibration (V→V) energy transfer pathways, while the vibration-to-rotation (V→R) energy transfer pathway is negligible. In both collision systems, the V→T transfer shows a similar pattern and amount of energy loss in the ET range of 5,000 to 40,000cm^-1, whereas the pattern and amount of energy transfer via the V→V pathway differs significantly between two collision systems. The collision-induced dissociation of the C-H_methyl or C-H_ring bond occurs when highly excited MP (65,000-72,000 cm^-1) interacts with the ground-state N₂ or O₂. Here, the dissociation probability is low (10^-4-10^-1), but increases exponentially with increasing vibrational excitation. This can be interpreted as the intermolecular interaction below E_T = 71,000 cm^-1. By contrast, the bond dissociation above E_T = 71,000 cm^-1 is due to the intramolecular energy flow between the excited C-H bonds. The probability of C-H_methyl dissociation is higher than that of C-H_ring dissociation.

• Bulletin of the Korean Chemical Society
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• 제23권9호
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• pp.1309-1338
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• 2002

The fluorescence and photoisomerization quantum yields of trans-1-(9-anthryl)-2-(4-pyridyl)ethene (t-4-APyE), 1-(10-methyl-9-anthryl)-2-(4-pyridyl)ethene (t-4-MeAPyE), and 1-(10-chloro-9-anthryl)-2-(4- pyridyl)ethene (t-4-ClAPyE) were measured in cyclohexane, acetonitrile, and methanol at room temperature.Polar solvents result in the drastic reduction of fluorescence quantum yield and increase of photoisomerization quantum yield for all three compounds. These results are probably due to the stabilization of intramolecular charge transfer (ICT) excited state in polar solvent. The higher contribution of ICT in the presence of more electron-donating methyl substituent, manifested by largest positive fluorescence solvatochromism, indicates that the pyridine ring acts as an electron acceptor. Protonation or methylation makes pyridine ring stronger electron acceptor and causes long-wavelength ground state charge transfer absorption band and complete quenching of fluorescence. The fluorescence from t-4-APyE derivatives can be switched off responding external stimuli viz. medium polarity, protonation, or methylation.

• Bulletin of the Korean Chemical Society
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• 제7권4호
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• pp.289-293
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• 1986

The photocycloaddition reactivity of 5,7-dimethoxycoumarin (DMC) to thymidine is much higher than that of cis-syn 8-methoxypsoralen $<\array{4^{\prime},5\\5^{\prime},6}>$ thymidine monoadducts, due to the steric effect. The different rate is also due to the relative reactivity of the singlet rather than the triplet excited state of the compounds. The biadducts of 8-methoxypsoralen and thymidine are first splitted into 4',5'-monoadducts and thymidine, not into 3,4'-monoadducts and thymidine by 254 nm light.

• Bulletin of the Korean Chemical Society
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• 제6권4호
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• pp.186-191
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• 1985

The effect of pressure and temperature on the stabilities of the charge transfer complexes of hexamethyl benzene with iodine in n-hexane has been investigated by UV-spectrophotometric measurements. In this experiment the absorption spectra of mixed solutions of hexamethyl benzene and iodine in n-hexane were measured at 25, 40 and $60^{\circ}C$ under 1,200, 600, 1200 and 1600 bar. The equilibrium constant of the complex formation was increased with pressure while being decreased with temperature raising. Changes of volume, enthalpy, free energy and entropy for the formation of the complexes were obtained from the equilibrium constants. The red shift at higher pressure, the blue shift at higher temperature and the relation between pressure and oscillator strength were discussed by means of thermodynamic functions. In comparison with the results in the previous studies, it can be seen that the pressure dependence of oscillator strength has a extremum behavior in durene as the variation of ${\Delta}H$ or ${\Delta}S$ with the number of methyl groups of polymethyl benzene near atmospheric pressure in the previous study. The shift or deformation of the potential in the ground state and in the excited state of the complexes formed between polymethyl benzene and iodine was considered from the correlation between the differences of the electron transfer energies and the differences of free energies of the complex formation for the pressure variation.

• 대한화학회지
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• 제29권4호
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• pp.441-447
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• 1985

몇가지 벤즈아닐리드 유도체를 합성하여 광반응될 때 생성물의 구조를 밝혔으며, 메카니즘을 연구하였다. 치환된 아닐린에 benzoyl chloride 유도체를 넣어 아실화하여 benzanilide 유도체를 합성하였다. N, N-dibenzoylaniline 유도체는 뜨거운 benzoyl chloride 유도체에 아닐린을 소량씩 가하여 얻었다. 2-chlorobenzanilide와는 달리 2-methylbenzanilide나 2'-methylbenzanilide는 photo-Fries형 생성물을 주었다. 2-위치에 니트로기가 도입된 benzanilide는 광반응을 하지 않았다. 들뜬 상태가 광반응을 할만큰 충분한 에너지를 갖고 있지 않기 때문이라고 생각한다. N, N-dibenzoylaniline이나 N, N-di-(2-chlorobenzoyl) aniline의 광반응으로 부터 쉽게 photo-Fries형 반응을 할때 단일선 상태가 상관하는것 같다. 왜냐하면 산소가 반응에 아무런 영향이 없기 때문이다. 극성이 작은, 또 점성이 낮은 용매에서 양자수득률이 좋았다. 단일 들뜬상태에서 카르보닐탄소와 질소사이 결합이 끊어져 용매 바구니라디칼 쌍이 되었다가 반전되어 재결합하면 생성물이 된다고 생각된다.