Structural Dynamics and Relaxation Processes with Participation of Excited Singlet and Triplet States in Sterically Hindered Porphyrins and Their Chemical Dimers

Paper presents an overview of mutual Belarussian-Russian collaboration in the field of sterically hindered porphyrins compared with the relevant literature data. Typically, the conformational dynamics of a non-planar tetrapyrrole macrocycle at 295 K manifests itself in bathochromic absorption and fluorescence shifts (∆ν ~ 1100 cm⁻¹), the increase of the Stokes shift (∆νₛ ~ 900 cm⁻¹), broadened emission and the fluorescence strong quenching. Nevertheless, the T₁ states dynamics upon steric interactions and porphyrin non-planarity remained to be not studied yet. We found for the first time that mono- and di-meso-phenyl substitution in octaethylporphyrins led to a drastic shortening of T₁ decays (~1.5 ms → 2–5 μs) in deaerated solutions at 295 K without considerable influence on spectral-kinetic parameters of singlet states. These effects have been systematically studied for porphyrins and their chemical dimers with a controllable structure of meso-phenyl substituents as well as for meso-phenyl-substituted octaarylporphyrins with increasing number (n=1÷4) of meso-phenyl rings. In some cases, quantum-chemical calculations have been used to explain experimental findings. Paper cont ains also some recent results showing what circumstances should be taken into account when using these compounds in various areas (excited state deactivation in multiporphyrin complexes, photoinduced electron transfer, singlet oxygen generation)

Deactivation of excited states in nanostructures containing cu-porphyrin subunit

Here, we present a semi-review of mutual Belarussian-German collaboration in the field of supramolecular chemistry and photophysics of tetrapyrrole compounds of various types: porphyrin chemical dimers, self-organized multiporphyrin complexes, ordered aggregates of photosynthetic pigments and nanoassemblies based on semiconductor CdSe/ZnS quantum dots and porphyrins. A special attention is paid to various nanostructures containing Cu-porphyrin subunits. Based on steady-state and time-resolved measurements, spectral properties as well as pathways and dynamics of non-radiative relaxation processes with participation of singlet and triplet excited states (energy transfer, photoinduced electron transfer, exchange d- p effects) are the subject of the analysis upon variation of the temperature (77-295 K) and polarity of the solvent. Finally, we consider recent results on “Quantum Dot-Porphyrin” nanoassemblies showing that self-assembly of only one Cu-porphyrin molecule with one CdSe/ZnS quantum dot modifies not only the photoluminescence intensity of quantum dot but creates new energetically clearly distinguishable electronic states opening additional effective relaxation pathways

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

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

Radiationless intermolecular energy transfer with participation of acceptor excited triplet states

Radiationless energy transfer between like and unlike molecules has been experimentally studied under conditions where acceptor molecules have been excited to the triplet state. Homogeneous singlet—triplet—triplet migration has been discovered in higbly concentrated chlorophyll “a” and pheophytin “a” solutions in castor oil at 183 K by measuring the variation of pigment relative quantum yields of fluorescence and triplet state formation as a function of exciting pulse intensity. Heterogeneous singlet—triplet—triplet energy transfer has been observed in solid solutions of different complex organic molecules (perylene +phenanthrene, Na-fluorescein +chlorophyll “a”, pyrene+ Mg-phthalocyanine) as the fluorescent donor state quenching in the presence of acceptor triplet-excited molecules. Primary emphasis is placed on a direct observation of the effect of energy transfer on the excited-state lifetime of the donor. The benzophenone phosphorescence quenching (shortening of phosphorescence lifetime) in the presence of Mg-mesoporphyrin triplet molecules has been found to be caused by the heterogeneous triplet—triplet—triplet energy transfer. Good agreement of the theoretical and experimental results permits us to conclude that all types of observed transfer processes are described by the Förster—Galanin theory for dipole—dipole radiationless energy transfer with no additional assumptions

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

ВЭЖХ-МС РАЗДЕЛЕНИЕ И ДЕТЕКТИРОВАНИЕ ДИАЛКИЛФОСФОНАТОВ И ТРИАЛКИЛФОСФИТОВ В РЕАКЦИОННЫХ СМЕСЯХ 1-АЛКАНОЛОВ С ТРИХЛОРИДОМ ФОСФОРА

To evaluate the rationality and separation conditions of alkyl esters of phosphorous P(III) and P(V) acids using reversed phase HPLC with mass spectrometric detection, the analyses of reaction mixtures of two 1-alkanols (1-butanol and 1-heptanol) with PCl3 in the presence of N,N-dimethyl aniline (for pH correction) have been carried out. In accordance with the previous results, these mixtures preferably contain trialkyl phosphites (RO)3P and dialkyl phosphonates (RO)2PH=O. However, the results of HPLC-MS analyses indicate the prevalence of oxidation products P(III) ® P(V) in the composition of these mixtures, namely trialkyl phosphates (RO)3OP. In accordance with this scheme, the appearance of diheptyl phosphate (C7H15O)2P(=O)-OH is caused by the oxidation of diheptyl phosphonate. Possible reasons of such discrepancies in the composition of reaction mixtures are discussed. One of them may be oxidation of P(III) derivatives by air oxygen during storage, or, tentatively, due to electrochemical reactions during electrospray ionization. The reaction mixture of 1-heptanol with PCl3 contains two components with the same molecular weights (M = 294) and identical mass spectra, but strongly different retention parameters (tR 5.4 and 16.0 min). The second of them is diheptyl phosphonate. To explain the appearance the first of them, existence of more hydrophilic prototropic tautomer (C7H15O)P-OH (contains hydroxyl group in the molecule) was proposed. Comparing the analytical results obtained in positive and negative modes of detection indicates that the first of them seems to be more effective for detection of alkyl esters of P(III) and P(V) phosphorous acids.Keywords: Trialkyl phosphites, dialkyl phosphonates, reversed phase HPLC, MS-detection, determination in reaction mixtures, oxidation productsDOI: http://dx.doi.org/10.15826/analitika.2018.22.3.014(Russian)Igor G. Zenkevich,   Tatiana I. Pushkareva,   Vlada E. NosovaSt. Petersburg State University,Universitetskii prosp., 26, St. Petersburg,   198504, Russian Federation  Для оценки целесообразности и характеристики особенностей разделения алкиловых эфиров фосфорных кислот P(III) и P(V) с использованием обращенно-фазовой высокоэффективной жидкостной хроматографии (ОФ ВЭЖХ) и их масс-спектрометри­чес­кого детектирования проведен анализ реакционных смесей двух 1-алканолов (ROH) (1-бута­нола и 1-гептанола) с трихлоридом фосфора (PCl3) в присутствии N,N-диметил­ани­лина для регулирования рН. В соответствии с предварительными данными, такие смеси содержат трилкилфосфиты (RO)3P и диалкилфосфонаты (RO)2PH=O. Однако по данным ВЭЖХ-МС-МС анализа в их составе преобладают продукты окисления производных Р(III) до производных P(V), а именно соответствующие триалкилфосфаты (RO)3P=O. В реакционной смеси 1-гептанола с PCl3 обнаружен дигептилфосфат (C7H15O)2P(O)-OH, вероятнее всего образующийся при окислении дигептилфосфоната. Обсуждаются возможные причины выявленных несоответствий в результатах определения состава реакционных смесей разными методами, в том числе окисление производных P(III) кислородом воздуха при их хранении или, предположительно, в результате электрохимических трансформаций аналитов в процессе электрораспылительной ионизации. В реакционной смеси 1-гептанола с PCl3 обнаружены два компонента с одинако­выми молекулярными массами (М = 294) и идентичными масс-спектрами, но значительно отличающимися по параметрам удерживания (tR = 5.4 и 16.0 мин). Второй из них однозначно соответствует дигептилфосфонату (C7H15O)2PH=O. Высказано предположе­ние, что первый пик может принадлежать прототропному таутомеру (C7H15O)P-OH, более гидрофильному за счет наличия гидроксильной группы в молекуле. Показано, что детектирование положительно заряженных ионов в условиях ВЭЖХ-МС значительно более эффективно при определении алкиловых эфиров кислот P(III) и P(V), чем отрицательных.Ключевые слова: Триалкилфосфиты, диалкилфосфонаты, обращенно-фазовая высоко­эффективная жидкост­ная хроматография, масс-спектрометрия, определение в реакционных смесях, продукты окисления  DOI: http://dx.doi.org/10.15826/analitika.2018.22.3.01

Fluorescence line narrowing study of cyclopentaneporphyrin chemical dimers at 4.2 K

The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycle=CH2) and their chemical dimers covalently linked via isocycles (Zn-cyclodimers) have been studied by the method of fluorescence line narrowing (FLN) in tetrahydrofurane-toluene (3:1) glassy matrixes at 4.2 K. Well-resolved fluorescence spectra upon laser excitation into the S₀→S₁ absorption band have been observed. In contrast to this, it has been shown that the excitation into the region of S₀→S₂ absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S₀→S₂ electronic transitions in the energy scale have been determined. The normal coordinate treatment of FLN spectra of ZnOEP-cycle=CH2 and those for the Zn-cyclodimers permitted us to determine the normal modes which are connected with the formation of the dimeric species. The weak interaction of Q-transitions of donor (D, ZnOEP-cycle) and acceptor (A, ZnOEP-cycle=CH2) subunits in Zn-cyclodimers manifests itself in the strong fluorescence quenching of D. Under excitation into the S₀→S₁ transition of D the FLN spectra of the dimers (belonging to their A subunits) were not observed. These facts are connected with the effective non-radiative singlet-singlet energy transfer in conditions of essential spectra inhomogeneity

Directed energy transfer due to orientational broadenning of energy levels in photosynthetic pigment solutions

The directed non-radiative energy transfer through monomeric molecules of chlorophyll “a” and pheophytin “a” at high concentrations (c ~ 10⁻²) in a rigid matrix of polyvinylbutyral has been found by using the nanosecond laser spectrofluorimeter. The phenomenon is caused by orientational broadening of pigment molecular spectra owing to its interaction with a solvent. The observed temporal shift of the luminescence spectrum to the red region in a nanosecond time scale as well as the red shift of the time integrated spectrum at a high concentration of pigment molecules and the monotonic growth of the luminescence lifetime with a shift to the red region of the spectrum served as indications of the directed energy transfer in the sample. The non-radiative energy transfer from monomeric molecules towards aggregates is also directly demonstrated by the deformation of instantaneous luminescence spectra in the long-wavelength range (λ > 700 nm). The role and the possibility of the directed energy transfer between molecules with orientationally broadened spectra in the biological systems are discussed