Distribution of averages in a correlated Gaussian medium as a tool for estimation of the cluster size distribution.

Calculation of the distribution of the average value of a Gaussian random field in a finite domain is carried out for different cases. The results of the calculation demonstrate a strong dependence of the width of the distribution on the spatial correlations of the field. Comparison to the simulation results for the distribution of the size of the cluster indicates that the distribution of an average field could serve as a useful tool for the estimation of the asymptotic behavior of the distribution of the size of the clusters for "deep" clusters where value of the field on each site is much greater than the rms disorder.

Hopping approach towards exciton dissociation in conjugated polymers.

By employing random walk an analytic theory for the dissociation of singlet excitons in a random organic solid, for instance, a conjugated polymer, has been developed. At variance of conventional three-dimensional Onsager theory, it is assumed that an exciton with finite lifetime can first transfer endothermically an electron to an adjacent site, thereby generating a charge transfer state whose energy is above the energy of that of the initial exciton. In a second step the latter can fully dissociate in accordance with Onsager's concept Brownian motion. The results indicate that, depending of the energy required for the first jump, the first jump contributes significantly to the field dependence of the dissociation yield. Disorder weakens the temperature dependence of the yield dramatically and precludes extracting information on the exciton binding energy from it.

Fluorescence lifetime fluctuations of single molecules probe the local environment of oligomers around the glass transition temperature.

Single molecule fluorescence experiments have been performed on a BODIPY-based dye embedded in oligo(styrene) matrices to probe the density fluctuations and the relaxation dynamics of chain segments surrounding the dye molecules. The time-dependent fluorescence lifetime of the BODIPY probe was recorded as an observable for the local density fluctuations. At room temperature, the mean fraction of holes surrounding the probes is shown to be unaffected by the molecular weight in the glassy state. In contrast, the free volume increases significantly in the supercooled regime. These observations are discussed in the framework of the entropic theories of the glass transition.

Fluorescence lifetime of a single molecule as an observable of meta-basin dynamics in fluids near the glass transition.

Using single molecule spectroscopy, we show that the fluorescence lifetime trajectories of single probe molecules embedded in a glass-forming polymer melt exhibit strong fluctuations of a hopping character. Using molecular dynamics simulations targeted to explain these experimental observations, we show that the lifetime fluctuations correlate strongly with the average square displacement function of the matrix particles. The latter observable is a direct probe of the meta-basin transitions in the potential energy landscape of glass-forming liquids. We thus show here that single molecule experiments can provide detailed microscopic information on system properties that hitherto have been accessible via computer simulations only.

Energy dissipation in multichromophoric single dendrimers.

Single-molecule spectroscopy of well-chosen dendritic multichromophoric systems allows investigation of fundamental photophysical processes such as energy or electron transfer in much greater detail than the respective ensemble measurements. In dendrimers with multiple chromophores, energy hopping and transfer to the chromophore with the energetically lowest S(1) state was observed. If more than one chromophore is in an excited state in one molecule, annihilation, either singlet-triplet or singlet-singlet, can occur. In the latter case, a higher singlet state is populated opening new deactivation pathways. In the presence of an electron donor, reversible electron transfer could be observed, and the rate constants of forward and backward electron transfer were established. The value of these rate constants fluctuates time-correlated with the rotational motion of the dendrimer arms and the mobility of the embedding matrix.

Direct proof of electron transfer in a rigid first generation triphenyl amine core dendrimer substituted with a peryleneimide acceptor.

The combination of nanosecond transient absorption experiments and single photon timing experiments proved the occurrence of an electron transfer process in the triphenyl amine core dendrimer, N1P1, by demonstrating the presence of an ion-pair absorption for N1P1 in solvents of medium polarity. By means of femtosecond transient absorption measurements the rise time of this ion-pair absorption dominated by the radical anion absorption could be determined, resulting in a value of 180 ps in MeTHF and 138 ps in THF. Furthermore, in femtosecond fluorescence upconversion as well as in monochromatic femtosecond transient absorption, a few ps component was resolved which was assigned to a vibrational and solvent relaxation process of the locally excited singlet state of the peryleneimide.

Single-molecule spectroscopy of a dendrimer-based host-guest system.

We report on a single-molecule study of a host-guest system that consists of a second-generation polyphenylene dendrimer and the cyanine dye Pinacyanol. The use of single-molecule spectroscopy enables us to obtain more detailed information on the properties of the host-guest system and can be used to confirm solution data. At low dye to dendrimer ratios the system is present as a one-to-one complex, while for higher ratios an ion-pair system is formed. Changes in the spectral properties of the single molecules are explained by differences in local polarisability. The difference of the triplet lifetimes of single free dye molecules and of associated ones is interpreted as deriving from a larger free volume for the dye molecules in the dendritic host relative to the rigid polymer matrix.

Photophysical properties of 2-nitro-5,10,15,20-tetra-p-tolylporphyrins.

Tetraarylporphyrins substituted with nitro groups at beta-pyrrolic positions are potential candidates for electron-accepting pigments in model systems for photosynthesis. The photophysics of 2-nitro-5,10,15,20-tetra-p-tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet-triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature- and solvent-dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+.-NO2-. in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.