L. Latterini

Photophysical study of a multi-chromophoric dendrimer by time-resolved fluorescence and femtosecond transient absorption spectroscopy

Abstract The photophysical properties of a dendrimer ( 1 ) bearing eight peryleneimide chromophores on a polyphenylene core were investigated by picosecond fluorescence and femtosecond transient absorption techniques. A peryleneimide attached to a hexaphenylbenzene unit ( 2 ) served as model compound. Multi-exponential fluorescence and fs-transient absorption decays were observed only for 1 , which could therefore be attributed to excimer-like interactions among neighbouring peryleneimides. The comparative time-resolved polarisation results on 1 and 2 indicate the occurrence of different depolarisation processes in the dendrimer assignable to the rotation of the molecule, a fractional motion maybe of a dendrimer branch and to an intramolecular energy transfer process.

STM imaging of a heptanuclear ruthenium(II) dendrimer, mono-add layer on graphite

Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.

Femtosecond polarized transient absorption spectroscopy of a C3-symmetric amino-substituted phenylbenzene derivative

Abstract The excited-state relaxation processes of an amino-substituted triphenylbenzene derivative (p-EFTP) and its biphenyl-model compound (p-EFBP) were investigated by femtosecond polarized transient absorption spectroscopy. The S 1 –S n absorption was detected upon pumping at 360 nm and probing at 470 nm. Both compounds presented a polarization-dependent component in the transient decay. For p-EFBP a decay time of 97±20 ps was obtained which agrees with the rotational diffusion time determined by time-resolved fluorescence anisotropy. For the symmetric p-EFTP a decay time of 8±2 ps was detected. This value correlates with the intramolecular dipole reorientation time deduced from time-resolved microwave conductivity measurements.

Merging of Hard Spheres by Phototriggered Micromanipulation

By careful design of the dendritic mol. the phys. properties of the self-assembled structure can be tuned in such a way that phototriggered micromanipulation and merging of hard spheres is possible. This possibility leads to the bottom-up synthesis of micrometer-sized objects through a combination of covalent synthesis and supramol. organization followed by micromanipulation.

Dynamic process in a hexameric benzo-porphyrin studied by femtosecond transient absorption

The fast kinetics of a hexameric benzo-porphyrin compound was investigated by femtosecond transient absorption spectroscopy. Using polychromatic detection, a sequence of spectra acquired at different delays revealed complex kinetic processes on the timescale of approximately 20 ps. The samples were then investigated at selected wavelengths by using a dual-colour femtosecond pump-probe setup. The same sequence of measurements was carried out under identical conditions on a benzo-porphyrin model sample. The comparison of the transient absorption decays recorded for both compounds revealed that only in the hexamer a 20 ps decay component is present which is attributed to an intramolecular interchromophoric excited-state process. q 1999 Elsevier Science B.V. All rights reserved.

Femtosecond transient dynamics of a heptametallic HAT–ruthenium(II) complex. A photophysical study

Abstract A photophysical study of a heptametallic HAT–ruthenium(II) complex, {Ru(HAT) 3 [Ru(phen) 2 ] 6 } 14+ (where phen=1,10-phenanthroline and HAT=1,4,5,8,9,12-hexa-azatriphenylene), has been carried out using steady state and time resolved techniques. The absence of luminescence suggests that polycomplexation changes the properties of the Ru-bridging HAT luminophore inducing a decrease of the radiative rate constant and an enhancement of the non-radiative decay channel. By femtosecond transient absorption measurements a decay time of 1.9 ns has been measured for the 3 MLCT state formed upon excitation at 590 nm. Furthermore the data give information on the relaxation processes leading to the formation of the MLCT triplet state. Upon excitaton, a single decay time of 1.4 ps has been determined. The lack of faster decays, within the instrumental time resolution, may indicate that this is a relaxation time from the Frank–Condon state produced upon excitation to the triplet MLCT state.

Microscopy and optical manipulation of dendrimer-built vesicles

A fifth-generation poly(propylene imine) dendrimer decorated with palmitoyl- and azobenzene-containing alkyl groups forms giant vesicles in aqueous solutions with diameters from 50 nm up to 20 mm and a multilaminar onion-like structure. Dense and ordered arrangement of the azobenzene chromophores in the bilayer structure leads to fluorescence with lmax= 600 nm. The fluorescence intensity can be increased by irradiation with blue light, and at low pH a distinctive blue shift of the spectrum is observed. With the aid of a single-beam optical tweezers it is possible to trap vesicles and direct them in a billiard-like fashion against each other using forces in the range of several pN. In collision experiments, the vesicles behave like hard spheres, and merging is not observed.

Excited State Probing of Supramolecular Systems on a Submicron Scale

Imaging techniques, such as Confocal Fluorescence Microscopy (CFM) or Near-field Scanning Optical Microscopy (NSOM) [1, 2] are essential techniques to study complex heterogeneous organic thin films by mapping their optical properties. They are complementary techniques having different advantages and disadvantages [3]. CFM is relatively easy to apply and combines a lateral resolution approaching λ/2 with the possibility of layered imaging in the z-direction. In contrast, NSOM has significantly better spatial resolution and offers simultaneous optical and topographic images. Although confocal microscopy has been mostly used for biological applications, the technique has proven to be useful, for example, in the study of colloids [4,5], polymer blends [6] and liquid crystals [7,8].