G. De Belder

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.

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.

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.

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.

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.

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.