• Journal of Photoscience
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• v.12 no.2
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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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• v.12 no.3
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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.

• Journal of the Korean Chemical Society
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• v.63 no.2
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• pp.73-77
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• 2019

We compared methodologies based on the density functional theory (DFT), e.g., time-dependent DFT (TD-DFT), TD-DFT within Tamm-Dancoff approximation (TDA-DFT), and spin-unrestricted DFT (UDFT), that are usually employed to optimize the geometries of ${\pi}$-conjugated molecules in their lowest lying triplet excited ($T_1$) state. As a model system for ${\pi}$-conjugated molecules, we employed 1,2,3,4,5-pentacyano-6-phenyl-benzene. In conjunction with 6-31G(d) basis sets, we made use of gap-tuned range-separated ${\omega}B97X$ functional which is often employed recently in the calculations of molecular excited states. Near the equilibrium geometries, we found that the important difference between the geometries derived at UDFT level and those at TD-DFT or TDA-DFT methods: more stable ground-state energies but higher triplet excitation energies for UDFT derived geometries. In the studies, we discuss such differences in more detail.

• Journal of Electrical Engineering and Technology
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• v.11 no.4
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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.

• Journal of the Korean Vacuum Society
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• v.16 no.5
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• pp.353-358
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• 2007

J-shaped superluminescent diodes (SLD) utilizing trench structure have been fabricated on the multiple quantum dots epi-structure with its ground state energy wavelength of $1.3\;{\mu}m$. It was observed that optical power was drastically increased up to 20 times in comparison with that of SLD without trench structure, The electroluminescence characteristics showed that the peak intensity of excited state was several ten times higher in the SLD with trench than without trench structure. It is explained that the optical power enhancement of J-shaped SLD with trench structure resulted from the drastic increase of peak intensity of excited state.

• Journal of the Korean Chemical Society
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• v.67 no.6
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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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• v.23 no.9
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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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• v.7 no.4
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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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• v.6 no.4
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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.

• Journal of the Korean Chemical Society
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• v.29 no.4
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• pp.441-447
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• 1985

Several benzanilides were prepared by acylation of anilines with substituted benzoyl chlorides. While 2-chlorobenzanilides were photocyclized, 2-methylbenzanilide and 2'-methylbenzanilide were cleaved to give photo-Fries type products. 2-Nitrobenzanilide and 2'-nitrobenzanilide were inert in the above conditions due to lowering energy of the excited state by the nitro group. N, N-dibenzoylaniline and N, N-di-(2-chlorobenzoyl) aniline gave photo-Fries type reaction products effectively. In the benzanilide photo-Fries type reaction the excited singlet state was believed to be involved, since no oxygen effect was observed on the reaction rate. Quantum yield for 2-methylbenzanilide is higher in nonpolar and less viscous solvents than in polar and viscous solvents. The solvent cage radical pair is suggested in the photo-Fries type reaction of benzanilides.