U. Rempel

Formation and optical properties of self-organized pentameric porphyrin arrays

Abstract Principles of formation, electronic absorption and fluorescence spectra are reported for self-organized pentameric arrays of tetrapyrrolic macrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(II 1,4-bis[5-(10,15,20-tri-p-hexylphenylporphyrinyl)]-benzene ((ZnHTPP)2) are bound via one molecule of a tetrapyridyl-substituted free base of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions in the dimer with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature. The formation of the complexes can be followed by changes in the absorption bands of (ZnHTPP)2 characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spectral behavior of the free bases in the pentads is determined by a non-planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substituted free bases. This quenching effect is assigned to a singlet-singlet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad.

Competition between charge transfer via superexchange and thermally activated energy transfer in porphyrinheterodimerquinone systems

Abstract Electron transfer is observed in semi-rigid heterodimers of free base and zinc porphyrin covalently linked to the electron acceptor p -benzoquinone (Q). The rate of transfer critically depends on the relative spatial arrangement of the zinc and free-base porphyrin unit with respect to Q and varies up to two orders of magnitude in time. Complexation of the zinc porphyrin subunit with pyridine influences the energetics and transfer dynamics of the system. Excitation energy transport among the porphyrin subunits controls the efficiency of electron transfer to a large extent. Temperature-dependent measurements reveal the superposition of two processes, one of which is interpreted as superexchange mediated electron transfer. Electronic coupling elements for such a process are estimated from the analysis of the temperature dependence of electron transfer rates.

Self-assembled nanoscale photomimetic models: structure and related dynamics

Abstract Using static and time-resolved measurements, dynamics of non-radiative relaxation processes have been studied in self-assembled porphyrin triads of various geometry, containing the main biomimetic components, Zn–porphyrin dimers, free-base extra-ligands (porphyrin, chlorin or tetrahydroporphyrin), and electron acceptors A (quinone or pyromellitimide). The strong quenching of the dimer fluorescence is due to energy and sequential electron transfer (ET) processes to the extra-ligand (∼0.9–1.7 ps), which are faster than a slower ET (34–135 ps) from the dimer to covalently linked A in toluene at 293 K. The extra-ligand S 1 -state decay ( τ S =940–2670 ps) is governed by competing processes: a bridge (dimer) mediated long-range ( r DA =18–24 A) superexchange ET to an acceptor, and photoinduced hole transfer from the excited extra-ligand to the dimer followed by possible superexchange ET steps to low-lying charge transfer states of the triads. The subsequent ET steps dimer→monomer→A taking place in the triads, mimic the sequence of primary ET reactions in photosynthetic reaction centers in vivo.

Energy transfer and distance independent charge separation in self-organised porphyrin-quinone aggregates

Abstract Pyridyl-substituted free-base porphyrins form self-organised complexes with zinc-porphyrin dimers. Self-organisation is mediated by ligand formation at the Zn central ions of the porphyrin dimers. Fast energy transfer from the free-base-porphyrin to the zinc-porphyrin-dimer is observed. In case of additional substitution of the zinc-porphyrin-dimer with the electron acceptor quinone long-range electron transfer takes place from the free-base-porphyrin to the quinone. Variation of electron donor–acceptor distances by a factor of two does only have a marginal influence on charge separation rates. This finding may be explained by a superexchange mechanism.

Energy and charge transfer dynamics in self-organized multimolecular arrays

Abstract The main kinetic parameters of the nonradiative singlet-singlet energy transfer and the charge separation have been studied for well-defined supramolecular arrays with controllable number and photophysical properties of subunits in methylcyclohexane in the temperature range of 77–300 K (cw, picosecond time-resolved fluorescence spectroscopy and femtosecond pump-probe kinetic data). The arrays formation is based on covalently linked Zn-porphyrin dimers with or without electron acceptors (quinone, anthraquinone, pyromellitimide) and the noncovalent self-assembling with pyridyl substituted tetrapyrrolic extra-ligands (porphyrins, chlorins and tetrahydroporphyrins, fluorinated tetraphenylporphyrins). It has been found that the competition between the energy transfer (within ⩽ 10 ps) and charge transfer (within 300 fs–700 ps) processes in the systems under consideration depends on the structure, spectral and redox properties of interacting subunits and may be driven by the distance, temperature and solvent polarity. The possible mechanisms of the excited S-states quenching in the complexes (including ‘superexchange’ model) are discussed.

Fluorescence of plasma polymer films with embedded dye molecules

Abstract Plasma polymer thin films with embedded dye molecules (Rhodamine 6G) were deposited by alternating or simultaneous plasma polymerization and dye molecule sublimation. From films with different amounts of embedded dye molecules, fluorescence spectra as well as excitation spectra were recorded. At samples with very low amounts of embedded dye molecules, the fluorescence band was found at λ fl = 550 … 560 nm. At higher dye amounts, the fluorescence band shifts to λ fl = 590 … 600 nm. This was interpreted as the change from embedded isolated dye molecules at low concentrations to dye molecule clusters or nanocrystallites at higher concentrations. The appearance of a second fluorescence band at λ fl = 680 … 700 nm was interpreted as a result of a chemical modification of the Rhodamine 6G molecules during plasma polymer deposition.

Spectroscopy of interchromophoric interactions in self-organized porphyrin and chlorin complexes

Abstract Spectral-luminescent properties of multimolecular complexes (triads and pentads, complexation constants range from 5·10 6 to 5·10 7 M −1 ) formed by two-fold coordination of dipyridyl and tetra-pyridyl substituted porphyrin or related molecules with Zn-porphyrin and Zn-chlorin dimers having various spacers between macrocycles (-CH 2 -CH 2 - or phenyl ring) have been studied in methylcyclohexane solutions in a temperature range from 140 to 360 K. The red shift of Zn-dimer electronic Q- and B-bands ( Δv ≤550 cm −1 ) upon complexation with pyridyl containing ligands is explained in terms of extra-ligation which influences the relative position for HOMOs a 1u and a 2u according to the four-orbital model. The splitting of Zn-dimer B-bands ( Δ E≥600 cm −1 ) as well as the significant transformation of these bands in trimeric and pentameric complexes (redistribution of the absorption band intensities in Soret region and appearance of complicated splitted spectral manifolds Δv ≤1900 cm −1 ) is connected with excitonic interactions of strong B-transitions of π-conjugated macrocycles included in the complex. Observed experimental splittings are compared with theoretical values calculated using the point dipole approximation and a computer-simulated geometry of the complexes under investigation. It has been observed that mutual influence of π-electronic macrocycles in the complexes leads to the red shift of porphyrin extra-ligand free base Q-bands ( Δv ≈120 cm −1 ) and is accompanied by quenching a fluorescence of certain components.

Competition between electron transfer and energy migration in self-assembled porphyrin triads

Abstract The photoinduced electron transfer (ET) and the energy migration (EM) processes have been studied in liquid solutions and polymeric (PMMA) films for the triads consisting of the Zn-octaethylporphyrin chemical dimer (the energy and electron donor, D ) and dipyridyl substituted tetrapyrrole extra-ligands (porphyrins, chlorin, tetrahydroporphyrin) as the acceptors, A . On the basis of the time correlated single photon counting technique and femtosecond pump-probe spectroscopy, it has been shown that D fluorescence quenching with time constant ranging from 1.7 to 10 ps is due to competing EM and ET processes from the dimer to A s. In addition, the fluorescence decay time shortening (by ∼1.3–1.6 times in toluene at 293 K) is observed for electron accepting extra-ligands in the triads. The acceptor fluorescence quenching is hard dependent on the mutual spatial arrangement of the triad subunits, but becomes stronger upon the solvent polarity increase (addition of acetone to toluene solutions) as well as the temperature lowering (from 278 to 221 K). The possible reasons and mechanisms of the non-radiative deactivation of locally excited S 1 -states in the triads are discussed taking into account a close lying charge-separated state. The obtained experimental data are analyzed using the reduced density matrix formalism in the frame of Haken–Strobl–Reineker approach. This model includes EM and ET processes as well as the dephasing of coherence between the excited electronic states of the triad.

Electron transfer in porphyrin multimolecular self-organized nanostructures

Abstract On the base of of covalent and non-covalent bonds nanoscale self-assembling multiporphyrin arrays with well-defined geometry, the controllable number of interacting components and their spectral and photophysical properties were formed. The deactivation of excited singlet and triplet states was studied using steady-state, time-resolved picosecond fluorescence (Δ½≈30 ps) and femtosecond pump-probe (Δ½≈280 fs) spectroscopy in solvents of various polarity at 77–300 K. It has been found that the competition between the non-radiative energy transfer (within ≤10 ps) and charge transfer (within 300 fs - 700 ps) processes in the systems depends on the structure, spectral and redox properties of interacting subunits and may be driven by the distance, temperature and solvent polarity. The possible pathways and mechanisms of the electron transfer in the systems of various types are discussed (Marcus theory for the “normal” region and the non-adiabatic case., the “superexchange” mechanism).

Dynamics of self-organized porphyrin aggregates

Supramolecular ensembles stable at room temperature (complexation constant and activation energy range from 5 (DOT) 106 M-1 to 5 (DOT) 107 M-1 and from 0.5 to 1.0 eV correspondingly) containing up to five macrocyclic fragments have been constructed using two-fold ligation of Zn-porphyrin and Zn-chlorin chemical dimers by pyridyl substituted porphyrin or related molecules. Spectral, photophysical and thermodynamic properties of triadic and pentadic arrays have been studied in a temperature range from 140 to 360 K. Kinetic behavior of the complexes was investigated using a fluorescent picosecond laser setup ((Delta) t approximately equals 30 ps) with 2-D (wavelength-lifetime) registration. Observed spectral properties are explained in terms of extra-ligation (red shift of all electronic bands <EQ 550 cm-1)) and excitonic splitting ((Delta) E < 1900 cm-1). Nonradiative for- and backward excitation energy transfer (K > 1010 c-1), electron transfer and d-(pi) interactions are discussed as the main paths of electronic excitation deactivation in the complexes.