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Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units. [ABSTRACT FROM AUTHOR]

In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo-simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV. [ABSTRACT FROM AUTHOR]

The excited-state dynamics of J aggregates of PIC have been studied by means of picosecond and subpicosecond absorption spectroscopy as well as integrated fluorescence yield measurements. The results of these measurements show that both the lifetime and the fluorescence yield are strongly dependent on excitation pulse intensity. At relatively high light intensity (1014 photons cm-2 pulse-1) the S1 lifetime is essentially pulse limited (<1 ps) and the fluorescence yield is very low. Upon decreasing the light intensity a gradual increase of the excited-state lifetime and fluorescence yield is observed. At very low excitation intensity (1010 photons cm-2 pulse-1) a single exponential lifetime of 400 ps is observed. At intermediate intensities the excited-state decay is strongly nonexponential. The observed intensity dependence of the excited-state dynamics is attributed to efficient exciton–exciton annihilation between the highly mobile singlet excitons. By applying an expression for bimolecular exciton annihilation, derived and used for photosynthetic antenna systems [Paillotin et al., Biophys. J. 25, 513 (1979)], and treating the energy migration as a hopping motion, information about aggregate size and exciton hopping rate was obtained. [ABSTRACT FROM AUTHOR]

In this work we have studied the isomerization dynamics of a cyanine dye molecule in solution. The viscosity and temperature dependencies of the isomerization rate have been measured in the series of n-alcohols for three different sizes of the isomerizing group. From these measurements we conclude that the shear viscosity of the solvent is not a good measure of the microscopic friction experienced by the isomerizing groups. The friction is varying in a nonhydrodynamic manner with viscosity, which shows that the relative volume of the isomerizing group and solvent molecules (Vp/Vs) is a critical parameter determining the microscopic friction. When the microscopic friction is calculated using a model for molecular rotational relaxation proposed by Dote, Kievelson, and Schwartz [J. Phys. Chem. 85, 2169 (1981)], good fits to Kramers’ equation is obtained. Similar models for microscopic rotational and translational friction combined with Kramers’ equation also yield an apparent improvement over the hydrodynamic Kramers description. The measurements also show that the non-Kramers behavior of the reaction rates have a more complex origin than the (Vp/Vs) dependence of the microscopic friction, that possibly can be traced back to a more general failure of the hydrodynamic description of friction (frequency dependent friction), or to a temperature and solvent dependence of the potential surface parameters. The results also suggest that the detailed nature of the reaction coordinate plays an important role in determining the detailed viscosity dependence of the isomerization. Thus a reactive motion mainly experiencing rotational friction is much more sensitive to the molecular size and free-volume effects, than is the isomerization controlled by translational friction. [ABSTRACT FROM AUTHOR]