Browsing by Author "Wang, TT"

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Bis(2,2-diphenylvinyl)spirobifluorene: An efficient and stable blue emitter for electroluminescence applications
(2005-08-15)
Wu, FI
;
Shu, CF
;
Wang, TT
;
Diau, EWG
;
Chien, CH
We have synthesized a spirobifluorene-based DPVBi [4,4'-bis(2,2-diphenylvinyl)-1,1'-biphenyl] analogue, DPVSBF [2,7-bis(2,2diphenylvinyl)-9,9'-spirobifluorene], in which the bis(2,2-diphenylvinyl) groups are connected through the 2 and 7 positions of the spirobifluorene framework, and have characterized its thermal properties, electronic properties (viz. absorption and photoluminescence), and electrochemical behavior. The presence of the rigid spirobifluorene linkage imparts significant improvement in the material's glass transition temperature and morphological stability, while preserving most of the photophysical and electronic properties of its non-spiro, biphenyl analogue, DPVBi. Organic electroluminescent devices having the structure ITO/NPB/DPVSBF/AlQ/LiF/Al display bright emissions with a lambda(max) at 474 nm (CIE coordinates: 0.16, 0.24) and exhibit maximum luminescence exceeding 40,000 cd/m(2). At a driving current density of 100 mAcm(2) (6.4 V), a luminance of 4110 cd/m(2) was obtained with external quantum efficiency of 2.54%, luminance efficiency of 4.1 cd/A, and power efficiency of 2.0 lm/W. Moreover, the DPVSBF-based device exhibits a 16-fold enhancement in the operation lifetime relative to that of a similar device based on DPVBi. (C) 2005 Elsevier B.V. All rights reserved.
Droplets oscillation and continuous pumping by asymmetric electrowetting
(2006)
Wang, TT
;
Huang, PW
;
Fan, SK
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材料科學與工程學系奈米科技碩博班
;
Graduate Program of Nanotechnology , Department of Materials Science and Engineering
This paper reports a new approach to digital microfluidic systems based on asymmetric electrowetting-on-dielectric (AEWOD) mechanism. On the AEWOD device, droplets can be oscillated or continuously pumped on an open surface by none or limited control electric circuits. This paper presents: (1) the AEWOD phenomenon, (2) droplet oscillation controlled by AEWOD force, and (3) the demonstration of unidirectional and bidirectional droplet pumping on AEWOD device.
Photoisomerization dynamics of azobenzene in solution with S-1 excitation: A femtosecond fluorescence anisotropy study
(2004-08-18)
Chang, CW
;
Lu, YC
;
Wang, TT
;
Diau, EWG
;
應用化學系
Measurements of anisotropy of femtosecond fluorescence after direct excitation of the S-1(npi(*)) state of azobenzene in hexane and ethylene glycol solutions have been carried out to address the controversy about inversion and rotation in the mechanism of photoisomerization. The observed anisotropies in hexane decay to a nonzero asymptotic level with a relaxation period the same as that for slow decay of the corresponding biexponential transient; this effect is attributed to involvement of the out-of-plane CNNC-torsional motion on approach to a twisted conical intersection along the "rotation channel" that depolarizes the original in-plane transition moment. In contrast, when the rotational channel becomes substantially hindered in ethylene glycol, the anisotropies show no discernible decay feature, but the corresponding transients show prominent decays attributed to involvement of in-plane symmetric motions; the latter approach a planar-sloped conical intersection along a "concerted inversion channel" for efficient internal conversion through vibronic coupling. The proposed mechanism is consistent with theoretical calculations and rationalizes both results on quantum yields and ultrafast observations.
Relaxation dynamics of 2,7-and 3,6-distyrylcarbazoles in solutions and in solid films: Mechanism for efficient nonradiative deactivation in the 3,6-linked carbazole
(2005-12-22)
Wang, TT
;
Chung, SM
;
Wu, FI
;
Shu, CF
;
Diau, EWG
We performed time-resolved spectral investigations of two distyrylcarbazole derivatives, 2,7- and 3,6-distyrylcarbazole (2,7-DPVTCz and 3,6-DPVTCz, respectively), in dilute toluene solution and in solid films mixed with poly(methyl methacrylate) (PMMA). The lifetime of 2,7-DPVTCz in its excited state in solution is similar to 100 times as great as that of 3,6-DPVTCz, consistent with their photophysical nature. The former shows intense emission, but the latter is nearly nonfluorescent in a free environment. Moreover, the lifetime of 3,6-DPVTCz in its excited state increased also similar to 100 times when the molecule was encapsulated in a 3,6-DPVTCz/PMMA solid film, indicating that intramolecular motion of the molecule significantly affects the observed relaxation dynamics in a confined environment. Calculations on the excited states indicate that ail efficient intersystern crossing is activated upon twisting of the bridged C-C single bond in a free 3,6-linked carbazole; such efficient deactivation is impractical in 2,7-linked carbazole or for 3,6-linked carbazole in a PMMA matrix. Information obtained from experiments on femtosecond fluorescence enables LIS to distinguish crucial relaxation processes in the excited state for a profound understanding of the details of vibrational and electronic relaxations of 3,6-DPVTCz in Solution.
Temperature dependence of rate coefficients of reactions of NO2 with CH3S and C2H5S
(2000-06-15)
Chang, PF
;
Wang, TT
;
Wang, NS
;
Hwang, YL
;
Lee, YP
Rate coefficients of reactions (1) CH3S + NO2 and (2) C2H5S + NO2 were determined using laser photolysis and laser-induced fluorescence, CH3S and C2H5S radicals were generated on photolysis of CH3SSCH3 and C2H5SSC2H5, respectively, with a KrF excimer laser at 248 nm. Their concentrations were monitored via fluorescence excited by emission from a dye laser at 371.4 nm (for CH3S) or 410.3 nm (for C2H5S) pumped with a pulsed XeCl excimer laser at 308 nm. Our measurements show that k(1) (297 K) = (1.01 +/- 0.15) x 10(-10) cm(3) molccule(-1) s(-1) is similar to a value reported by Balla ct al. and approximately 50% greater than other previously reported values; listed errors represent 95% confidence limits. Reaction 1 has a negative activation energy: k(1) = (4.3 +/- 1.3) x 10(-11) exp[(240 +/- 100)/T] cm(3) molecule(-1) s(-1) for T = 222-420 K. Reaction 2 has a rate coefficient similar to reaction 1 at room temperature: k(2)(296 K) = (1.05 +/- 0.16) x 10(-10) cm(3) molecule(-1) s(-1) but has a small positive activation energy with k(2) = (2.4 +/- 0.7) x 10(-10) exp[(-210 +/- 80)/T] cm(3) molecule(-1) s(-1) for T = 223-402 K. The temperature dependence of k(2) is determined for the first time. Reactions of NO2 with HS, CH3S, and C2H5S are compared.