Photoinduced relaxation processes in self-assembled nanostructures: multiporphyrin complexes and composites "CdSе/ZnS quantum dot-porphyrin"

Here, we discuss self-assembled multicomponent organic/inorganic nanostructures. Self-assembled multiporphyrin triads were formed via non-covalent interactions of meso-phenyl bridged ZnOEP chemical dimer, (ZnOEP)₂Ph, with dipyridyl substituted tetrapyrrole extra-ligand. In tetrads, the dimer (ZnOEP)₂Ph is covalently linked via 5-mesoposition to additional electron acceptors (quinone Q, pyromellitimide Pim). Using steady-state, time-resolved fluorescent and pump-probe results, main relaxation pathways have been elucidated: competing energy migration and photoinduced electron transfer (PET) in normal triads within ≤1.4 ps; very fast (within ~ 700 fs) PET in porphyrin triads containing pentafluorinated porphyrin remaining still efficient at 77-120 K; a bridge-dimer mediated long-range (r_DA=18-24 Å) superexchange PET "extra-ligand→Q or Pim" in tetrads. Self-assembly of nanostructures from semiconductor CdSe/ZnS quantum dots (QD) and tetra-meso-pyridyl- substituted porphyrins is also based on extra-ligation interactions and results in a strong quenching of QD photoluminescence (PL). At the same molar ratios x =[H₂P(m-Pyr)₄]/[QD], the quenching is more effective for small QDs than for larger ones. From experimental Stern-Volmer PL quenching plots I₀/I(x) and the quantum mechanical calculations for the electron wave functions it follows that the specificity of the exciton non-radiative decay in "QD-porphyrin" nanocomposites is due to the manifestation of inductive and mesomeric effects leading to the charge tunnelling through ZnS barrier in quantum confinement conditions

Self-organization principles in the formation of multiporphyrin complexes and “semiconductor quantum dot-porphyrin” nanoassemblies

In this paper, we review several aspects of molecular recognition (based on non-covalent binding interactions) occurring between meso-pyridyl substituted tetrapyrrole extra-ligands and chemical dimers of tetrapyrrolic macrocycles containing central Zn ions and spacers of various nature and flexibility. Experimental results obtained by us earlier are analyzed using a novel approach (based on steady-state absorption/fluorescence measurements) for the evaluation of complexation constants KC for the formation of porphyrin triads. It was found that KC values [KC ~ (0.5 – 70) × 10⁶ M⁻¹] show noticeable dependence on the structural parameters of the interacting subunits as well as on the solvent nature. The same self-assembly approach has been used to attach meso-pyridyl substituted porphyrins to the surface of semiconductor CdSe/ZnS quantum dots (QD). It was comparatively found that in contrast to self-assembled porphyrin triads, the formation of “QD-porphyrin” nanoassemblies takes place in competition with surface stabilizing tri-n-octyl phosphine oxide (TOPO) ligand molecules and attached porphyrin molecules. It manifests in a temporal dynamics of QD photoluminescence caused by ligand exchange, TOPO layer reorganization, QD surface reconstruction, solvent properties. It was shown that the sensitivity of QD surface morphology to attached organic ligands (e.g. porphyrins) provides an opportunity to control the dynamics and pathways of the exciton relaxation in “QD-dye” nanoassemblies by changing the structure and electronic properties of these ligands

Dynamics of photoinduced electron transfer in multiporphyrin nanoassemblies

In self-assembled nanoscale porphyrin triads based on Zn-octaethylporphyrin chemical dimer (donor, D) and dipyridyl substituted porphyrin free base (acceptor, A), fluorescence quenching of D (down to 1.7-10 ps) and A (by ~1.3-1.6 times) subunits is strongly dependent on the solvent polarity (toluene-acetone mixtures) and temperature (77-350 K). The obtained experimental findings are analyzed using the reduced density matrix formalism in the frame of Haken-Strobl-Reineker approach taking into account the energy transfer, charge separation, and the dephasing of coherence between the excited electronic states of the triad

Разработка приспособления для сборки и сварки балки навески

Цель работы –разработка приспособления для сборки и сварки балки навески, которое позволит ускорить процесс подготовки деталей перед сваркой, будет иметь возможность поворота изделия и доступ к сварке со всех сторон. Проведен технико-экономический анализ процесса сборки и сварки балки навески. Дано обоснование эффективности предлагаемого приспособления и малый срок его окупаемости.The object of this study is to build technology and welding hinge beams. Purpose - improving the technology of assembling and welding of sample beams, so as to speed up the process of preparing the parts prior to welding, and have the ability to rotate the product and access to welding on all sides. Spend a technical and economic analysis of the process of assembly and welding hinge beams. The substantiation of the effectiveness of the proposed adaptations and short term payback

Multistep photoinduced electron transfer in self-organised nano-scale porphyrin triads

Well-defined structurally organised porphyrin triads of a controlled geometry andnanoscale size have been formed in liquid solutions using the combination of a covalent approach and non-covalent self-assembly. The triads contain zinc-octaethylporphyrin chemical dimer, (ZnOEP)₂Ph, with covalently linked electron acceptors (p-benzoquinone, Q or pyromellitimide, Pim), and additional dipyridyl-substituted tetrapyrrole extra-ligands. Steady-state, picosecond fluorescence (∆ₜ½ ≈ 75 ps) and femtosecond pump–probe (⊿½ ≈ 280 fs) data show that non-radiative deactivation of the dimer S₁-states (τₛ < 1 ps) is due to both the S–S energy transfer (ZnOEP)₂Ph→extra-ligand and the sequential photoinduced electron transfer (ZnOEP)₂Ph→Q (or Pim) at r_DA = 10.8 Å. The additional decay shortening of the extra-ligand S₁-states by 3–6 times (toluene, 293 K) is attributed to the increased “superexchange” mediated long distant (r_DA ≈ 18–21 Å) one-step electron transfer extra-ligand→Q (or Pim). © 2002 Elsevier Science B.V. All rights reserved

Time-resolved spectroscopy of single quantum dots.

We have performed a series laser time-resolved experiments for single CdSe/ZnS QDs spin coated onto a quartz substrate at 293 K.DFG Priority Unit FOR 877 and BSPSR “Convergence 3.2.08

Influence of Single Dye Molecules on Temperature and Time Dependent Optical Properties of CdSe/ZnS Quantum Dots: Ensemble and Single Nanoassembly Detection

Optical spectroscopy on ensembles and single CdSe/ZnS semiconductor quantum dots (QDs) demonstrates a competition of trap and near band edge photoluminescence (PL). This competition can be markedly influenced by a few surface attached pyridyl functionalized dye molecules (porphyrins or perylene diimides) forming nanoassemblies with well defined geometries. Temperature variation and related changes in absorption and emission reveal sharp changes of the ligand shell structure in a narrow temperature range for organic (TOPO and amine) surfactants (phase transition). The effects on QD PL at this transition become considerably pronounced upon attachment of only a few dye molecules to QD surface. Moreover, under ambient conditions amine capped QDs are photodegraded in the course of time. This process is enhanced by attached dye molecules both on the ensemble and single particle/dye level. This investigation elaborates the importance of (switchable) surface states for the characterization of the PL of QDs. © 2012 Elsevier B.V. All rights reserved

Time resolved photoluminescence anisotropy of CdSe/ZnS nanoparticles in toluene at 300 K

For CdSe nanoparticles it has been theoretically and experimentally shown that at low temperatures the photoluminescence is circularly polarized in accordance with a wurtzite structure and the corresponding allowed optical transitions. In the present Letter, we report on related investigations on CdSe/ZnS colloids in toluene solution. From time resolved photoluminescence anisotropy we conclude that also at room temperature the results are in good agreement with structure and related electronic states as determined from crystals at low temperature. © 2004 Elsevier B.V. All rights reserved

Formation and optical properties of self-organized pentameric porphyrin arrays

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)l,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 !n 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-silaglet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad

Identification and assignment of porphyrin-CdSe Hetero-nanoassemblies

Hetero-nanoassemblies in toluene solution are formed via anchoring pyridyl substituted free base porphyrin molecules on the colloidal core-shell semiconductor nanocrystals CdSe/ZnS. The formation can be identified via quenching of semiconductor photoluminescence and followed via spectral changes of porphyrin spectral properties such as fluorescence, fluorescence decay and absorption. Interpreting these changes we estimate that even at high molar ratios on average only one molecule is anchored on one nanocrystal. Experimentally determined complexation constants are comparable to those observed for multi-porphyrin complexes. © 2007 Elsevier B.V. All rights reserved