Browsing by Author "Niu, Yingli"

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Quantum Chemical Calculations of Intramolecular Vibrational Redistribution and Energy Transfer of Dipeptides (GlyTyr and LeuTyr) and Applications to the RRKM Theory
(2013-07-01)
Yang, Ling
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Niu, Yingli
;
Zhu, Chaoyuan
;
Fujimura, Yuichi
;
Shiu, Yingjen
Recently the quantum chemistry program can provide the anharmonic potentials for polyatomic molecules and clusters. It makes the quantum calculation of intramlecular vibrational redistribution (IVR) rates and intramolecular vibrational energy transfer possible. In the present paper, we apply the adiabatic approximation method to treat these two intramolecular energy redistribution processes. Using this method, we calculate the IVR rates of the dipeptides (glycyl tyrosine (GlyTyr) and leucyl tyrosine (LeuTyr)) and their cation radicals from the high frequency modes to the low frequency modes. Our theoretical results of the IVR lifetimes of the high frequency modes, like C-H, N-H, and O-H stretching modes, for both GlyTyr and LeuTyr molecules and their cation radicals are about 1 ps, 10 ps, and 100 ps, respectively. It provides theoretical support for the claim that IVR takes place prior to dissociation and the IVR rates are determined by the functional group rather than the degrees of freedom of the system. Thus the Rice-Ramsperger-Kassel-Marcus (RRKM) theory is suitable for investigating the photodissociation process of dipeptides. In addition, the calculated intramolecular energy transfer rate from O-H stretching mode of the C-termius of dipeptides to N-H mode of N-terminus is close to IVR rate.
Role of "dark" np* states in photoabsorption and relaxation processes of pyrazine
(2012-03-25)
Lin, Chih-Kai
;
Niu, Yingli
;
Lin, Sheng Hsien
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應用化學系
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Department of Applied Chemistry
The Role of the n pi* (1)A(u) State in the Photoabsorption and Relaxation of Pyrazine
(2011-11-04)
Lin, Chih-Kai
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Niu, Yingli
;
Zhu, Chaoyuan
;
Shuai, Zhigang
;
Lin, Sheng Hsien
The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD-DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet n pi* state, (1)A(u), which is conventionally regarded dark due to the dipole-forbidden (1)A(u) <- (1)A(g) transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring n pi* (1)B(3u) and pi pi* (1)B(2u) states through vibronic coupling. The scans on potential-energy surfaces further indicated that the (1)A(u) state intersects with the (1)B(2u) states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.
Theoretical design of polythienylenevinylene derivatives for improvements of light-emitting and photovoltaic performances
(2012)
Jiang, Yuqian
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Peng, Qian
;
Gao, Xing
;
Shuai, Zhigang
;
Niu, Yingli
Poly(thienylene vinylene) (PTV) is a low-bandgap polymer but shows poor performance in both light-emitting and photovoltaic applications. Recently a derivative of PTV with carboxylate substitution, poly(3-carboxylated thienylenevinylene) (P3CTV), has been synthesized and was shown to be fluorescent. The photovoltaic power conversion efficiency based on P3CTV has been found to be much larger than PTV, indicating an intrinsically concomitant relationship between light-emitting and photovoltaic properties. Employing quantum chemistry calculations coupled with our correlation function formalism for excited state decay and optical spectra, we have investigated a series of side-chain substituted PTVs targeting optimal optoelectronic performance. We predict that the carbonyl substituted PTV is a strongly fluorescent polymer with low bandgap, long exciton lifetime, and large spectral overlap between emission and absorption. It is expected that carbonyl PTV is a promising light-emitting and photovoltaic polymer. Methodology wise, we find that (i) our correlation function approach to calculate the optical spectrum has much lower computational scaling with respect to the system size than the conventional method; (ii) the harmonic oscillator approximation for the nonadiabatic decay works better for a large system than for a small system.
Theoretical Study on Structure and Sum-Frequency Generation (SFG) Spectroscopy of Styrene-Graphene Adsorption System
(2013-01-31)
Lin, Chih-Kai
;
Shih, Chun-Chi
;
Niu, Yingli
;
Tsai, Min-Yeh
;
Shiu, Ying-Jen
In this theoretical study, we aimed to simulate the sum-frequency generation (SFG) spectroscopy of a thin polystyrene layer physically adsorbed on the graphene sheet and to figure out the orientation distribution of the phenyl units. To simplify the problem, we started the investigation by constructing molecular models with styrene and ethylbenzene monomers and styrene oligomers up to four units adsorbed on a finite-sized graphene hexagon. Geometric optimization results showed that the phenyl rings of the adsorbate always orientate close to the surface normal with a small tilt angle. The adsorption is weak but not negligible. SFG spectra have been simulated based on these calculated structures, vibrational frequencies, and dipole and polarizability derivatives to compare with experimental reports of polystyrene adsorbed on other surfaces.