Marc Lor

Singlet - Singlet Annihilation in Multichromophoric Peryleneimide Dendrimers, Determined by Fluorescence Upconversion

To gain better control over the distribution ofchromophores and their interaction in the excited state, it isdesirable to have them distributed on a spherical surface.Recently, this goal has been achieved with the synthesis of aseries of first-generation peryleneimide dendrimers with a rigidtetrahedral central core, as shown in Scheme 1.

Femtosecond fluorescence upconversion study of rigid dendrimers containing peryleneimide chromophores at the rim

The kinetics of a newly synthesized series of a first generation of polyphenylene dendrimers in which one phenyl in a dendritic arm was para-substituted by a peryleneimide chromophore are reported. One such peryleneimide chromophore is attached to 1, 3 or 4 arms. The results are compared to a series of polyphenylene dendritic compounds, which are identical except for the substitution at a meta-position of a phenyl ring. The para-substitution yields a better spatial definition of the peryleneimide units relative to one another and the influence of this aspect on the kinetics is studied. Four different kinetic components were resolved for both groups of dendrimers. An ultra-short component varying from 500 fs to 2 ps and attributed to intramolecular vibrational redistribution (IVR) is identical for both series. The decay time of a second component, which is comprized of the vibrational relaxation and a singlet–singlet annihilation process observed in both substituted dendrimer series, is shorter in the para-substituted dendrimers compared to the meta-substituted ones. It is also shown that the annihilation process, which is only present in the multi-chromophoric compounds of both the series and resolved with an excitation energy dependent study, has definitely a larger contribution in the partial amplitudes for the para-substituted compounds. This is related to the relative orientation of the transition dipoles of the chromophores in both the series.

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.

Energy transfer within perylene-terrylene dendrimers evidenced by polychromatic transient absorption measurementsThis paper is dedicated to Professor Jean Kossanyi on the occasion of his 70th birthday.

The time dependent spectral properties of a first and a second generation dendrimer with peryleneimide chromophores at the rim and a terrylenediimide chromophore in the core were investigated by time resolved polychromatic transient absorption measurements. The obtained results of the dendritic structures were compared with those of three model compounds. In the perylene-terrylene dendrimers a very fast energy transfer process was observed. Besides energy transfer singlet-singlet annihilation could be observed. It could be concluded that competition between the rates of energy transfer and of singlet-singlet annihilation is dependent on the dendrimer generation.

Generation dependent singlet?singlet annihilation within multichromophoric dendrimers studied by polychromatic transient absorptionThis paper is dedicated to Professor Fred Lewis on the event of his 60th birthday.

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.

Chapter 1 Photophysical processes in multichromophoric systems at the ensemble and single molecule level

Publisher Summary This chapter discusses photophysical processes in multichromophoric systems at the ensemble and single molecule level. Photophysical processes play a key role in the excited state dynamics of biological multichromophoric systems, such as light harvesting complexes and autofluorescent proteins. As a model system to study these photophysical pathways in multichromophoric entities, specially designed rigid dendrimers with a well defined number of chromophores are synthesized. Using the combination of ensemble and single molecule fluorescence techniques, a better picture of the excited state processes, such as energy hopping, singlet-singlet annihilation, singlet-triplet annihilation, and anion quenching can be established. Stationary absorption and fluorescence measurements in toluene are performed at room temperature. The fluorescence decay times for all the dendrimers are measured in toluene by the single photon counting method, detecting the emission under magic angle condition. The dendrimers are excited at 488 nm and detection is made at 600 nm. Besides time resolved measurements under magic angle condition, time resolved anisotropy measurements are also carried out.

Energy transfer process in dendrimers containing a perylene-terrylene donor-acceptor system investigated by femtosecond multicolor transient absorption spectroscopy

Dendrimers are highly branched macromolecular systems whose structure can be defined on a molecular level. Recent investigations of different generations of rigid dendrimers containing a terrylenediimide chromophore in the center and peryleneimide chromophores at the rim gave evidence for a directional energy transfer between a peryleneimide donor and the terrylenediimide acceptor chromophore within these dendrimers. By means of polychromatic transient absorption measurements and by comparing transient absorption spectra of these dendrimers to different model compounds, this process is further investigated in detail. Two different generations of dendrimers are investigated along with a set of three different model compounds, which only contain one of the donor-acceptor pair chromophores. For excitation, 250 fs laser pulses at 495 nm generated in an OPA pumped by a regeneratively amplified fs laser system are used. The multi-colour transient absorption changes are probed using a fs white light continuum and a CCD camera detection system. In order to account for possible multi-photonic processes, a series of excitation energy dependent measurements are performed.

Systematic fluorescence upconversion investigation of intramolecular processes in peryleneimide dendrimers containing one to four chromophores

First generation dendrimers bearing 1,2,3 or 4 peryleneimide chromophores at the rim have been investigated using fluorescence upconversion detection with 250 fs time resolution. For all samples, intramolecular vibrational reorganisation and vibrational relaxation processes could be attributed. Moreover, in multichromophoric samples, an additional lOps time constant was found and attributed to singlet singlet annihilation.