• Bulletin of the Korean Chemical Society
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• v.24 no.8
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• pp.1102-1106
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• 2003

In our previous work we developed the Finite Field method in order to calculate the fifth-order Raman response. The method was applied to calculate various polarization components of the two-dimensional response of liquid $CS_2$. So far, all calculations relied on the dipole-induced dipole. Accurate time-dependent density functional theory calculations have shown that this model has big discrepancies, when molecules are close together as in the liquid. We now report results of investigations on the importance of multipole and electron overlap effects on the polarizability and the fifth-order Raman response. It is shown that these collision effects, especially the induced multipoles, are crucial in the description of the fifth-order response. The impact is found to be especially pronounced for the χ_{mmzzzz}^{(5)}$response that is solely due to interaction induced effects. The calculated response will be compared with various experimental results.

• Bulletin of the Korean Chemical Society
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• v.28 no.12
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• pp.2382-2388
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• 2007

A series of dimethine cyanine dyes were synthesized in a fast, efficient and high yield by the condensation of quaternary salts with 1H-indole-3-carbaldehyde in the presence of piperidine under solvent-free microwave irradiation. The products were identified by 1H NMR, IR, UV-Vis spectra and elemental analysis. The absorption and fluorescence properties of these dyes were investigated both experimentally and theoretically. Calculations performed at a combination of time-dependent density functional theory (TD-DFT) and the polarizable continuum model (PCM) reproduced the π-π* type absorption bands of the dyes. Regression analysis was used for studying theoretical results of the absorption maxima in different solvents. Compared with experimental counterparts, estimated overall uncertainties in the absorption maxima were about ±2%.

• Bulletin of the Korean Chemical Society
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• v.31 no.4
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• pp.895-900
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• 2010

In order to find a relationship between firefly luciferases structure and bioluminescence spectra, we focus on excited substrate geometries which may be affected by rigid luciferases. Density functional theory (DFT) and time dependent DFT (TDDFT) were employed. Changes in only six bond lengths of the excited substrate are important in determining the emission spectra. Analysis of these bonds suggests the mechanism whereby luciferases restrict more or less the excited substrate geometries and to produce multicolor bioluminescence.

• Bulletin of the Korean Chemical Society
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• v.35 no.2
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• pp.587-596
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• 2014

A new ${\pi}$-stacked donor-acceptor (D-A) system, [Ru(1-([2,2'-bipyridine]-6-yl-methyl)-3-(2-cyclohexa-2',5'-diene-1,4-dionyl)-1H-imidazole)(2,2':6',2"-terpyridine)]$[PF_6]_2$ (ImQ_T), has been synthesized and characterized. Similar to its precedent, [Ru(6-(2-cyclohexa-2',5'-diene-1,4-dione)-2,2':6',2"-terpyridine)(2,2':6',2"-terpyridine)]$[PF_6]_2$ (TQ_T), this system has a cofacial alignment of terpyridine (tpy) ligand and quinonyl (Q) group, which facilitates an electron transfer through ${\pi}$-stacked manifold. Despite the presence of lowest-energy charge transfer transition from the Ru-based-HOMO-to-Q-based-LUMO (MQCT) predicted by theoretical calculations by using time-dependent density functional theory (TD-DFT), the experimental steady-state absorption spectrum does not exhibit such a band. The selective excitation to the Ru-based occupied orbitals-to-tpy-based virtual orbital MLCT state was thus possible, from which charge separation (CS) reaction occurred. The photo-induced CS and thermal charge recombination (CR) reactions were probed by using ultrafast visible-pump/mid-IR-probe (TrIR) spectroscopic method. Analysis of decay kinetics of Q and $Q^-$ state CO stretching modes as well as aromatic C=C stretching mode of tpy ligand gave time constants of <1 ps for CS, 1-3 ps for CR, and 10-20 ps for vibrational cooling processes. The electron transfer pathway was revealed to be Ru-tpy-Q rather than Ru-bpy-imidazol-Q.

• Proceeding of EDISON Challenge
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• 2017.03a
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• pp.132-144
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• 2017

현재, 빛을 이용한 화학 연구가 활발히 진행되고 있고 이러한 연구는 양자역학을 기반으로 화학에서 상당히 중요한 부분을 차지하고 있다. 또한 컴퓨터의 발전에 따라 여러 계산 모델들이 개발되고 있다. 본 논문에서는 회전파 근사(Rotating Wave Approximation, RWA)를 통해 라비 진동을 이론적으로 확인하고, 가장 간단한 연속파 레이저와 두 에너지 준위에서 시작하여 레이저 펄스와 두 에너지 준위, 레이저 펄스와 이원자분자인 $Na_2$ 분자의 두 전자에너지 퍼텐셜 준위, 그리고 실제 시간 밀도 범함수 이론(Real-Time Time Dependent Density Functional Theory, RT-TDDFT)이란 제일원리계산을 통해 연속파 레이저와 $H_2$ 분자와 $C_2H_4$ 분자에서까지 관찰하였다. 이 연구를 통해 공명 전이의 경우 펄스의 면적이 ${\pi}$의 홀수 배일 때 완전한 입자수 전이가 일어나는 펄스 면적 정리를 확인할 수 있었고, 이원자분자인 $Na_2$의 경우엔 펄스의 지속시간도 입자수 전이에 영향을 미친다는 것을 확인하였다. 더 나아가 $H_2$ 분자와 $C_2H_4$ 분자에서는 RT-TDDFT 계산을 통해 라비 진동을 확인할 수 있었고, 두 종류의 기저함수간의 대조를 통해 기저함수 선택의 중요성을 알아보았고, 가장 중요하게는 레이저를 잘 조작하면 입자를 원하는 상태로 들뜨게 할 수 있다는 것이란 결론을 얻게 되었다.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.246-246
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• 2016

Tungsten-nitrogen (W-N) co-doping has been known to enhance the photocatalytic activity of anatase titania nanoparticles by utilizing visible light. The doping effects are, however, largely dependent on calcination or annealing conditions, and thus, the massive production of quality-controlled photocatalysts still remains a challenge. Using density functional theory (DFT) thermodynamics and time-dependent DFT (TDDFT) computations, we investigate the atomic structures of N doping and W-N co-doping in anatase titania, as well as the effect of the thermal processing conditions. We find that W and N dopants predominantly constitute two complex structures: an N interstitial site near a Ti vacancy in the triple charge state and the simultaneous substitutions of Ti by W and the nearest O by N. The latter case induces highly localized shallow in-gap levels near the conduction band minimum (CBM) and the valence band maximum (VBM), whereas the defect complex yielded deep levels (1.9 eV above the VBM). Electronic structures suggest that substitutions of Ti by W and the nearest O by N improves the photocatalytic activity of anatase by band gap narrowing, while defective structure degrades the activity by an in-gap state-assisted electron-hole recombination, which explains the experimentally observed deep level-related photon absorption. Through the real-time propagation of TDDFT (rtp-TDDFT), we demonstrate that the presence of defective structure attracts excited electrons from the conduction band to a localized in-gap state within a much shorter time than the flat band lifetime of titania. Based on these results, we suggest that calcination under N-rich and O-poor conditions is desirable to eliminate the deep-level states to improve photocatalysis.

• Bulletin of the Korean Chemical Society
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• v.31 no.8
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• pp.2151-2157
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• 2010

Spectroscopic and quantum mechanical studies of the Et3PAuCl complex were performed to characterize the effect of aurophilicity on the optical properties. When excited with UV light at low temperature, the crystalline complex produced a deep luminescence in both the blue (high-energy) and red (low-energy) regions of the spectrum. The intensity of the low-energy luminescence was markedly reduced in the powdered state and quenched in the solution state. Time-dependent density functional theory (TD-DFT) calculations on electronic structures of both the ground and excited states of aggregates $[Et_3PAuCl]_n$ (n = 1 - 3) indicated that the low-energy luminescence was attributable to Au-centered transitions, which are significantly affected by aurophilic interactions. By contrast, the high-energy luminescence appeared to be independent of the state of the complex and was strongly associated with the charge transfer from Cl to Au.

• Bulletin of the Korean Chemical Society
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• v.28 no.12
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• pp.2343-2353
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• 2007

We report the application of time-dependent density functional theory (TDDFT) to the calculation of potential energy profile relevant to the excited state intramolecular proton transfer (ESIPT) processes in title molecules. The TDDFT single point energy calculations along the reaction path have been performed using the CIS optimized structure in the excited state. In addition to the Stokes shifts, the transition energies including absorption, fluorescence, and 0-0 transition are estimated from the TDDFT potential energy profiles along the proton transfer coordinate. The excited state TDDFT potential energy profile of SA and 3ASA resulted in very flat function of the OH distance in the range ROH = 1.0-1.6 A, in contrast to the relatively deep single minimum function in the ground state. Furthermore, we obtained very shallow double minima in the excited state potential energy profile of SA and 3ASA in contrast to the single minimum observed in the previous work. The change of potential energy profile along the reaction path induced by the substitution of electron donating groups (-NH2 and -OCH3) at different sites has been investigated. Substitution at para position with respect to the phenolic OH group showed strong suppression of excited state proton dislocation compared with unsubstitued SA, while substitution at ortho position hardly affected the shape of the ESIPT curve. The TDDFT results are discussed in comparison with those of CASPT2 method.

• Bulletin of the Korean Chemical Society
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• v.33 no.11
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• pp.3618-3624
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• 2012

Photodissociation dynamics of propiolic acid ($HC{\equiv}C-COOH$) at 212 nm in the gas phase was investigated by measuring rotationally resolved laser-induced fluorescence spectra of OH ($^2{\Pi}$) radicals exclusively produced in the ground electronic state. From the spectra, internal energies of OH and total translational energy of products were determined. The electronic transition at 212 nm responsible for OH dissociation was assigned as the ${\pi}_{C{\equiv}C}{\rightarrow}{\pi}^*{_{C=O}}$ transition by time-dependent density functional theory calculations. Potential energy surfaces of both the ground and electronically excited states were obtained employing quantum chemical calculations. It was suggested that the dissociation of OH from propiolic acid excited at 212 nm should take place along the $S_1/T_1$ potential energy surfaces after internal conversion and/or intersystem crossing from the initially populated $S_2$ state based upon the potential energy calculations and model calculations for energy partitioning of the available energy among products.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.310-310
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• 2010

Electronic and photovoltaic characteristics of two sensitizers (TA-BTD-CA and TA-BTD-St-CA), composed of a different $\pi$-conjugation in the linker group, have been investigated by theoretical and experimental methods. The electronic structure, transition dipole moment and oscillator strengths of two sensitizers have been scrutinized by using density functional theory (DFT) and time-dependent DFT (TD-DFT) method. The LUMO level and the oscillator strength of TA-BTD-St-CA was higher than that of TA-BTD-CA, which may facilitate the electron injection process as well as increase the absorption coefficient. The relative efficiencies of the electron injection from the excited sensitizer to nanocrystalline TiO2 and SnO2 films have also been investigated by nanosecond transient absorption spectroscopy. The relative electron injection efficiency of TA-BTD-St-CA exhibited similar injection efficiency for two different semiconductors. However, in the case of TA-BTD-CA sensitizer, electron injection into SnO2 was approximately three times larger than that into TiO2. This enhancement of electron injection of TA-BTD-CA for the SnO2 is due to the increment of the driving force caused by positive shift of conduction band of semiconductor, which was also confirmed from the investigation for the photovoltaic characteristics according to the electrolyte additive, such as LiI additive.