No abstract

Recent advances in fluorescence excitation spectroscopy with high efficiency have produced greatly improved optical spectra for the first electronic transition of individual single molecules of pentacene in p-terphenyl crystals at low temperatures (1.5 to 10 K). Two classes of single molecule behavior are observed: class I molecules have time-independent resonance frequencies, and class II molecules show a diffusive motion among several resonant frequencies with time which we term ‘‘spectral diffusion’’ by analogy with a similar effect which is common in amorphous materials. The temperature dependence of the linewidth and the power dependence of the fluorescence emission rate and of the linewidth are reported and analyzed. Various forms of the surprising class II behavior are described, including jumping among several discrete frequencies, creeping toward the center of the inhomogeneous line in many small steps, and a wandering among many possible resonance frequencies. The occurrence of class II behavior is restricted to the wings of the inhomogeneous line suggesting that the effect is correlated with some form of local disorder. The spectral diffusion rate increases with increasing temperature, suggesting that the effect may be due to phonon-assisted transitions of local degrees of freedom around the pentacene defect whose source remains to be identified conclusively.

No abstract

Perylenemonoimide chromophores, attached to a small dendron arm embedded in the polymer hosts Zeonex, poly(vinylbutyral), and poly(methyl methacrylate), are studied by means of laser-induced confocal fluorescence detection. Transient fluorescence intensity exhibits dark periods on two different time scales: on a 100 µs time scale and on a considerably longer time scale, ranging from 100 ms to as much as tens of seconds under high-vacuum conditions. The exponentially distributed short "off" times are attributed to triplet excursions of the molecule. The long-lived dark states follow a power-law distribution and are discussed in terms of the formation of radical anions/cations via electron tunneling processes.

By use of low-temperature (20 K) confocal microscopy, fluorescence spectra of isolated single CdS nanocrystals have been recorded. The narrowest measured line width of the main fluorescence band has been determined to be about 5 meV at the lowest excitation intensities. At higher intensities at least three additional observations are made:  the main fluorescence band broadens, a line to the blue of the main peak evolves, and a peak due to the coupling to a longitudinal optical phonon appears.