R. Chang

Experimental and theoretical investigations of absorption and emission spectra of the light-emitting polymer MEH-PPV in solution

Abstract We report the absorption and photoluminescence spectra of the MEH-PPV polymer in a chloroform solvent. In the analysis, we assume that the polymer consists of short conjugated segments which are called oligomers with different lengths. The appearance of each chain length is controlled by a distribution function. Based on the conformational disorder caused by torsional motion, we found that the distribution function should take a Gaussian form with its center and width being adjustable parameters. While the energy level of the emissive S1 state of the oligomer and the corresponding oscillator strength are calculated from exciton theory. In addition, the associated vibrational motions are considered in determining the spectral shape in which both displacement and distortion of the potential surfaces are taken into account. With this information, the calculation result can fit the experiment reasonably well.

Aggregated states of luminescent conjugated polymers in solutions

Abstract In order to study the electronic properties of the interchain state in light-emitting polymers, we have performed spectroscopic experiments for 2,5-dioctyloxy PPV (DOO-PPV) in both good (chloroform) and poor (2-methyl-tetrahydrofuran) solvents. The concentration-dependent absorption, photoluminescence (PL) spectra and dynamics of time-resolved PL can be explained by the coexistence of an intrachain and one kind of aggregated states in poor solution. The estimated lifetime and transition moment of the aggregated state lead to significant quantum efficiency of PL which is about 0.75 times that of the intrachain state. The PL spectrum for the aggregated state resembles that of a thin film, implying that the emissive species in thin film is more likely to be an aggregated state.

Ultrafast dynamics of excitations in conjugated polymers: A spectroscopic study

We construct a microscopic model to describe the excited states of poly(2-methoxy, 5-(2′-ethylhexoxy)-p-(phenylenevinylene) in thin film. Within this model, we deduce that in the high energy region, the nature of excited states in the film is very similar to the species observed in solution phase. Moreover, we propose that the decay process of these excited states involves energy transfer, vibrational relaxation, and dissipation simultaneously, in contrast to the usual argument that assumes exciton migration occurs after vibrational motion reaches thermal equilibrium. As a result, the simulation of time-resolved photoluminescence spectra is in agreement with the experiment.

THEORETICAL INVESTIGATION OF NEAR-FIELD OPTICAL PROPERTIES OF TAPERED FIBER TIPS AND SINGLE MOLECULE FLUORESCENCE

We investigate herein the propagation and tunneling of monochromatic light through a tapered fiber tip, modeled as a tapered perfectly conducting waveguide. The opening aperture of the waveguide is much smaller than the wavelength of light. The transmission rate through the tip as well as the profiles of electric fields are obtained. A detailed formulation for calculating the fluorescent spectra of a single molecule under near-field excitation is also presented. In addition, the limitations of fluorescence measurement for obtaining near-field pattern is discussed on the basis of the present formulation.

Resonance fluorescence of a single molecule under near field excitation

We report here theoretical investigations on the resonance fluorescence of a two‐level molecule, probed by the near field optical microscopy technique in illumination mode. The proposed model considers the influence of a coated tapered fiber probe on the radiative properties of the molecule. It is observed that both the radiative decay rate as well as the fluorescence power depend sensitively on the position and direction of the transition dipole of the molecule.

Application of the density matrix method to spectroscopy and dynamics of photosynthetic reaction centers

The density matrix method is well known for being useful to theoretically treat complicated phenomena. In this work, this method will be applied to study the ultrafast energy and electron transfers in photosynthetic reaction centers. We present a microscopic model to describe the spectroscopy and dynamics for photosynthetic bacterial reaction centers (RCs). In this model, we propose eight vibrational modes and their Huang–Rhys factors for different electronic states, and the couplings between the electronic states. As applications, we have constructed steady-state and ultrafast time-resolved spectra for the wild-type RC of Rhodobacter (Rb.) sphaeroides.