Multichromophoric Dendrimers Research Articles

Light-Harvesting Nanoparticle Core-Shell Clusters with Controllable Optical Output.

We used DNA self-assembly methods to fabricate a series of core-shell gold nanoparticle-DNA-colloidal quantum dot (AuNP-DNA-Qdot) nanoclusters with satellite-like architecture to modulate optical (photoluminescence) response. By varying the intercomponent distance through the DNA linker length designs, we demonstrate precise tuning of the plasmon-exciton interaction and the optical behavior of the nanoclusters from regimes characterized by photoluminescence quenching to photoluminescence enhancement. The combination of detailed X-ray scattering probing with photoluminescence intensity and lifetime studies revealed the relation between the cluster structure and its optical output. Compared to conventional light-harvesting systems like conjugated polymers and multichromophoric dendrimers, the proposed nanoclusters bring enhanced flexibility in controlling the optical behavior toward a desired application, and they can be regarded as controllable optical switches via the optically pumped color.

The European Young Chemist Award 2012

The European Young Chemist Award 2012

Light harvesting in dendrimer materials: Designer photophysics and electrodynamics

Abstract

Design and synthesis of bichromophores for nonlinear optical applications in polymer films

Three bichromophores were designed and synthesized by linking a vinylthiophene-conjugated chromophore with different tether groups via esterification. Second harmonic generation measurements of nonlinear polymer films revealed that the second harmonic generation coefficients of the films containing the bichromophore and using 3-phenoxypropane-1,2-diyl succinic diester as the tether group were improved in comparison with the mono-chromophore-doped polymer films. However, the second harmonic generation coefficients of the polymer films containing bichromophores using isophthalic ester as the tether group were decreased. Overall, the results indicate that the 3-phenoxypropane-1,2-diyl succinic diester tether group has potential applications in the design and synthesis of multi-chromophoric dendrimers with large macroscopic optical nonlinearity.

A multichromophoric dendrimer: from synthesis to energy up-conversion in a rigid matrix

A dendrimer with a [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) complex as a core and four diphenylanthracene units at the periphery was prepared from a scaffold based on a bipyridyl ligand bearing four terminal alkyne groups. Upon green light excitation, the dendrimer shows blue luminescence even in a rigid matrix at 77 K thanks to the dendritic multichromophoric structure.

A 3-phenoxypropane-1, 2-diol based bichromophore for enhanced nonlinear optical properties

The structure of a novel bichromophore that comprised a readily hyperpolarisable chromophore with 3-phenoxypropane-1, 2-diol via esterification, was confirmed by elemental analysis, UV–visible absorption spectra and 1H NMR. Nonlinear optical polymer films were fabricated by doping the bichromophore and the corresponding monochromophore into poly(4-vinylphenol). In situ second harmonic generation measurements revealed that the bichromophore imparted a 2.2-fold enhancement in the second harmonic generation coefficients of the films, indicating that the novel 3-phenoxypropane-1, 2-diol derivative could be used in the design and synthesis of multi-chromophoric dendrimers with large macroscopic optical nonlinearity.

Electronic Excitation Energy Transfer in Multichromophoric Assemblies: A Single Molecule Insight

AbstractIn recent years, single molecule spectroscopy has provided novel insights into the fundamentals of electronic excitation energy transfer in molecular aggregates. In order of increasing structural complexity, we have studied simple molecular dimers and multichromophoric dendrimers. It will be shown that the combination of frequency‐selective single molecule spectroscopy and confocal fluorescence microscopy at 1.4 K is a unique tool to study energy transfer processes in these systems. In particular, from the line widths of single molecule excitation spectra, rate constants of energy transfer can be deduced directly. A detailed analysis shows that, for several cases, the mechanism of energy transfer cannot be described by the simple Förster model (dipole–dipole coupling). Furthermore, novel experiments with dual color pulsed excitation are discussed which allow for controlling the excitation energy flow in individual molecular dimers.

Energy Transfer Rates and Pathways of Single Donor Chromophores in a Multichromophoric Dendrimer Built around a Central Acceptor Core

An artificial light-harvesting dendrimer showing highly efficient electronic excitation energy transfer from four peripheral donors to one central acceptor has been investigated by single-molecule spectroscopy at low temperatures. Confocal imaging in combination with frequency selective excitation spectroscopy gives direct access to energy transfer rates of individual donors and allows the determination of energy transfer pathways within a single multichromophoric aggregate.

Fluorescence Lifetimes and Emission Patterns Probe the 3D Orientation of the Emitting Chromophore in a Multichromophoric System

In this Communication, we report on the fluorescence behavior of an individual first generation multichromophoric dendrimer. The fact that each of the chromophores in time acts as a fluorescent trap is demonstrated by directly probing the dipole orientation of the emitting chromophore by means of defocused wide-field imaging and comparing experimental and calculated emission patterns. It is shown that in such cases the electromagnetic boundary condition effect results in discrete changes in the fluorescence lifetime as a function of time for individual dendrimers.

Multichromophoric Dendrimers as Single-Photon Sources: A Single-Molecule Study

Single-molecule fluorescence spectroscopy was performed on a number of multi-peryleneimide substituted polyphenylene dendrimers for the purpose of investigating the parameters which influence the efficiency of the dendrimers as single-photon sources at room temperature. Analysis of fluorescence intensity, lifetime, and interphoton arrival time distribution revealed that all measured first-generation dendrimers behave as single-photon emitters when more than one chromophore is excited by a single excitation pulse regardless of the number of constituent chromophores. This is a result of efficient singlet-singlet annihilation, which becomes less efficient in higher-generation dendrimers when the interchromophoric distance increases. The efficiency of the investigated dendrimers as single-photon sources depends on several other parameters, such as the nature of the surrounding polymer matrix, the number of chromophores, and the extent of interchromophoric interactions. These parameters mainly affect the frequency of singlet-triplet annihilation, which in turn dominates the quality of these multichromophoric dendrimers as single-photon sources. The results reported here are important not only for the design of single-photon sources based on single organic molecules but also for a fundamental understanding of natural and artificial multichromophoric systems.

Generation dependent singlet-singlet annihilation within multichromophoric dendrimers studied by polychromatic transient absorption.

Intramolecular kinetic processes in a series of shape-persistent meta- and para-substituted polyphenylene dendrimers bearing different peryleneimide chromophores at the rim have been investigated using time-resolved polychromatic transient absorption measurements. The influence of the generation number and different substitution patterns upon these processes was revealed by comparing different compounds. In particular, in multichromophoric systems a singlet-singlet annihilation process was detected. The corresponding time constant was dependent on the generation number.

Frequency selective excitation of single chromophores within shape-persistent multichromophoric dendrimers

Isolated multichromophoric dendrimers containing four perylenemonoimide chromophores were investigated in a confocal microscope which allowed imaging of the same set of dendrimers over a wide temperature range. Emission spectra at low temperature (2 K) reveal narrow zero-phonon lines although the electron-phonon coupling is not negligible. By using fluorescence excitation spectroscopy, the sharp purely electronic zero-phonon lines of single chromophores within a single multichromophoric unit could be isolated in the frequency domain. Selective excitation into a higher energy chromophoric site is followed by rapid directional energy transfer into the lowest energy site from which emission then occurs.

Generation-Dependent Energy Dissipation in Rigid Dendrimers Studied by Femtosecond to Nanosecond Time-Resolved Fluorescence Spectroscopy

Intramolecular kinetic processes in a series of second-generation polyphenyl dendrimers with multiple peryleneimide chromophores attached to the para position of the outer phenyl ring were investigated by steady-state and femtosecond to nanosecond time-resolved fluorescence spectroscopy. The results obtained were compared to the ones of the corresponding first-generation dendrimer series. The energy-hopping rate constant, khopp, observed from anisotropy decay times was found to be 5 times smaller than that of the first-generation series and scales well with the difference in average distance between the chromophores. In addition to the processes observed in first-generation dendrimers in the ultrafast time domain by fluorescence up-conversion, a second annihilation process is found in the second-generation multichromophoric dendrimer. The observation of two singlet-singlet annihilation processes in this compound can be explained by the presence of a mixture of constitutional isomers leading to a broader distribution of distances between neighboring chromophores compared to first-generation multichromophoric dendrimers.

Probing Photophysical Processes in Individual Multichromophoric Dendrimers by Single-Molecule Spectroscopy

Individual multichromophoric dendrimer molecules, bearing eight perylenecarboximide chromophores at the rim, immobilized in a thin polyvinylbutyral (PVB) film were studied by far-field fluorescence microscopy. Fluorescence intensity trajectories as a function of time (transients), spectra, and decay traces were recorded separately or simultaneously. For comparison, similar measurements have been performed on a model compound containing one perylenecarboximide chromophore. Collective on/off jumps of the fluorescence intensity were observed for single dendrimer molecules, resembling previously reported collective jumps for the emission of single light-harvesting antenna systems. Spectra and decays of both non-interacting and dimer-like interacting chromophoric sites could be distinguished within an individual dendrimer. Transitions between the different spectral forms and decay times, observed for a single molecule, underline the dynamic character of the interactions among the chromophores. Evidence for a stepwise bleaching process of the multichromophoric system was found. Furthermore, the single-molecule data incontestably prove the assumptions stated in the ensemble model.

Correlation between Ground State Conformation and Excited State Dynamics in a Multichromophoric Dendrimer Studied by Excitation Wavelength Dependent Fluorescence Upconversion

A time-resolved fluorescence upconversion study on a polyphenylene dendrimer with 16 peryleneimide chromophores at the rim, in which the excitation wavelength is systematically varied while the detection wavelength is kept constant, is reported. A new setup is described which allows excitation at wavelengths as short as 320 nm and throughout the visible range up to 900 nm with a system prompt response on the order of 300 fs. In addition to results reported previously13 which were obtained at shorter wavelengths indicating the initial formation of a locally excited state which then evolves further into a locally excited state on a 5 ps time scale, the newly obtained data at longer wavelength excitation show the existence of a ground state interaction leading under these conditions of excitation predominantly to a directly formed complex.

Intramolecular evolution from a locally excited state to an excimer-like state in a multichromophoric dendrimer evidenced by a femtosecond fluorescence upconversion study

Abstract A time-resolved fluorescence upconversion study on a polyphenylene dendrimer with eight peryleneimide chromophores on the surface (1) and on a monochromophoric model compound (2) is reported. The time-dependent fluorescence spectra of the dendrimer show that the initial excitation is into a locally excited chromophore. They further indicate the existence of a decay channel that leads to excited state interaction between chromophores in one dendrimer which takes place on a 5 ps timescale.

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

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.