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Young Jong Lee

Publications and source records attributed to Young Jong Lee.

6 recordsLinked to original sources
PubMed2006

Effect of sample preparation and excitation conditions on the single molecule spectroscopy of conjugated polymers.

Extensive new single molecule spectroscopy (SMS) data on the conjugated polymer MEH-PPV at low temperature were obtained. In particular, the combined effects of sample preparation and excitation condition were explored in detail. The data confirm previous observations from this laboratory that (i) the distribution of emission maxima of single MEH-PPV molecules has a bimodal distribution and (ii) the single molecule emission spectrum of MEH-PPV exhibits few time-dependent fluctuations of the emission intensity, band shape, or spectral maxima. These data also help explain the discrepancy among the various published SMS data on this compound and suggest that environmental impurities, long irradiation times, nearby interfaces, and incomplete data sampling may account for some of the discrepancies among the published data.

Journal Article↗
PubMed2005

Variable temperature single-molecule dynamics of MEH-PPV.

Herein, we continue our investigation of the single-molecule spectroscopy of the conjugated polymer poly[2-methoxy,5-(2'-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV) at cryogenic temperatures. First, the low temperature microsecond dynamics of single MEH-PPV conjugated polymer molecules are compared to the dynamics at room temperature revealing no detectible temperature dependence. The lack of temperature dependence is consistent with the previous assignment of the dynamics to a mechanism that involves intersystem crossing and triplet-triplet annihilation. Second, the fluorescence spectra of single MEH-PPV molecules at low temperature are studied as a function of excitation wavelength (i.e. 488, 543, and 568 nm). These results exhibit nearly identical fluorescence spectra for different excitation wavelengths. This strongly suggests that electronic energy transfer occurs efficiently to a small number of low-energy sites in the multichromophoric MEH-PPV chains.

Journal Article↗
PubMed2005

Single molecule modulation spectroscopy of conjugated polymers.

This paper describes a new single molecule spectroscopy approach for the investigation of triplet-triplet and singlet-triplet interactions in conjugated polymers. The technique involves the irradiation of isolated single, mulitchromophoric, conjugated polymer molecules by a repetitive sequence of variable-intensity microsecond time scale excitation pulses. The fluorescence intensity is synchronously time-averaged for thousands of cycles of the pulse sequence to yield a high signal-to-noise fluorescence transient on the microsecond time scale. The transient can be analyzed with kinetic models to obtain quantitative information about the kinetics of triplet-triplet exciton annihilation and the quenching of singlet excitons by triplet excitons in conjugated polymers.

Journal Article↗
PubMed2005

Single-molecule spectroscopy of conjugated polymers.

The molecular structure, photochemistry, and device physics of conjugated polymers have been investigated by single-molecule spectroscopy (SMS), using the unique ability of this technique to unravel complex spectra and dynamics. Surprisingly efficient and directional electronic energy funneling was observed for conjugated polymer molecules due to highly ordered conformations. Furthermore, recent studies on the SMS of conjugated polymers embedded in electronic devices demonstrate that SMS is a powerful tool for studying the photophysics and charge-transfer processes of conjugated polymers, giving new insights into the complex interactions among excited and charged species that exist in a device environment.

Journal Article↗
PubMed2003

Upregulation of cellular NF-kappa B activity by alkylating carcinogens in human epidermal keratinocytes.

Effect of alkylating carcinogens, i.e., N-nitroso-N-methylurea (NMU) and N-nitroso-N-ethylurea (NEU), as well as the simpler alkylating agents, methyl iodide and ethyl iodide, on the activation of NF-kappaB was evaluated in human epidermal squamous cell carcinoma (SCC-13) keratinocytes in order to investigate the possible correlation of cellular NF-kappaB activity with chemical carcinogenesis. The activities of NF-kappaB induced by chemical carcinogens were determined in human SCC-13 keratinocytes transfected with pNF-kappaB-SEAP-NPT plasmid, permitting expression of the secretory alkaline phosphatase (SEAP) reporter gene in response to the NF-kappaB activity and contains the neomycin phosphotransferase (NPT) gene conferring resistance to the geneticin. In this cell-based assay system, all alkylating carcinogens significantly upregulated the cellular NF-kappaB activations in a time- and dose-dependent manner until 72 h, at concentrations of 0.5-5 microM. These results suggest that carcinogenicity by alkylating chemicals may be associated with the modulation of cellular NF-kappaB activity in human skin cells.

Alkylating Agents↗