Gerd Schweitzer

Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric molecules

We demonstrate measurements of the efficiency of competing Förster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homo-energy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data suggest that similar energy-transfer pathways have to be considered in the analysis of single-molecule trajectories of donor/acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.

New picosecond laser system for easy tunability over the whole ultraviolet/visible/near infrared wavelength range based on flexible harmonic generation and optical parametric oscillation

A new laser-based and time-correlated single photon counting (TCSPC) detection system which allows easy and fast tuning of excitation wavelengths over a broad range from 240 to 1300 nm, with small gaps from 335 to 360 nm and 660 to 720 nm, has been built. The unique combination of a mode-locked Ti:sapphire laser, an optical parametric oscillator, pulse selectors, and harmonic generators delivers ultrafast laser pulses (1–2 ps) with variable repetition rates and excitation wavelengths. Performance characteristics of the laser system at different excitation wavelengths are reported and the TCSPC setup, which is characterized by a total instrument response function of 25 ps full width at half maximum, is described. Typical TCSPC measurements demonstrate the capability of the system of deriving decay or species associated excitation spectra.

Photophysical study of bay substituted perylenediimides

A detailed investigation of the photophysical properties of a series of perylenediimide systems bearing three different types of bay substituents is presented. Single photon timing and femtosecond transient absorption experiments reveal that the dynamics of interconversion between two conformational arrangements of these substituents occurs with a time constant of 550 ps. In addition, charge transfer from the electron-rich units attached to the bay area of the electron-poor perylenediimide core is observed. This process leads to a fast non-radiative deactivation of the locally excited state of the perylenediimide in polar solvents. When the experimental results of the investigated systems are compared we observe a shift from a conformational dynamics towards competitive excited state processes involving charge transfer in the -meta and -para substituted perylenediimide chromophore.

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.

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.

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.

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.

Photoinduced electron-transfer in perylenediimide triphenylamine-based dendrimers: single photon timing and femtosecond transient absorption spectroscopy

The excited state dynamics of two generations perylenediimide chromophores substituted in the bay area with dendritic branches bearing triphenylamine units as well as those of the respective reference compounds are investigated. Using single photon timing and multi-pulse femtosecond transient absorption experiments a direct proof of a reversible charge transfer occurring from the peripheral triphenylamine to the electron acceptor perylenediimide core is revealed. Femtosecond pump-dump-probe experiments provide evidence for the ground state dynamics by populating excited vibronic levels. It is found by the means of both techniques that the rotational isomerization of the dendritic branches occurs on a time scale that ranges up to 1 ns. This time scale of the isomerization depends on the size of the dendritic arms and is similar both in the ground and excited state.

A double OPA femtosecond laser system for transient absorption spectroscopy

Abstract We demonstrate a novel approach for femtosecond time resolved transient absorption spectroscopy in which we use an amplified laser system to pump two identical optical parametric generators/amplifiers at the same time. This scheme brings about several advantages: first, both pump and probe laser are independently tunable over a wide spectral range from 300 nm to 3 μm, and second, a high time resolution (100 fs cross correlation width) is still achieved.

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.