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

ОГРАНИЧЕНИЯ ГАЗОХРОМАТОГРАФИЧЕСКОГО РАЗДЕЛЕНИЯ СОЕДИНЕНИЙ С АКТИВНЫМИ АТОМАМИ ВОДОРОДА. ДИАЛКИЛФОСФАТЫ

Dialkyl esters of inorganic phosphoric acid (RO)2PO-OH contain only one active hydrogen atom. Hence, in accordance with the contemporary viewings, there are no theoretical limitations on the possibilities of their gas chromatographic separation. Comparison of the physicochemical properties of dialkyl- and trialkyl (characterized in more detailed extent) phosphates confirms this proposition. The evaluation of di- and trialkyl phosphates polarity by comparing the so called indices of the molecular mass, normal boiling point, and molar refraction indicates that both esters are rather low polar compounds. Nevertheless, the analyses of three reaction mixtures of phosphoric anhydride (P2O5) with excesses of 1-propanol, 2-propanol, and 1-butanol indicate the absence of dialkyl phosphates among the constituents being detected. At the same time, the impu­­ri­ti­es of the corresponding trialkyl phosphates are revealed in all cases. This can be ex­p­lai­ned by the impossibility of the gas chromatographic separation of dialkyl esters due to the following reasons:– dialkyl phosphates are rather strong acids (рКа 1.3 – 1.7);– formation of inner molecular hydrogen bonds improves the chromatogra­­phic “properties” of analytes. However, the probability of its formation in dial­kyl phos­pha­tes seems to be less than those in monoalkyl esters of organic dicarbo­xylic acids;– esters of the phosphoric acid (including trialkyl esters) are known as the active alky­lation reagents even at the elevated temperatures. This can decrease their stability du­ring the chro­ma­tographic separation due to the possible interaction with the stationary pha­se of the chro­ma­tographic column.Every single factor from the ones mentioned above cannot be the principal reason of the inapplicability of the gas chromatography for the separation of dialkyl phosphates by itself. The joint acti­on of all of them seems to be more reasonable.Keywords:  Dialkyl phosphates, polarity criteria, impossibility of gas chroma­to­graphic separationDOI: http://dx.doi.org/10.15826/analitika.2016.20.4.007 Igor G. Zenkevich, Vlada E. NosovaSt. Petersburg State University, Universitetskii prosp., 26, St. Petersburg 198504, Russian Federation Диалкиловые эфиры неорганической трехосновной фосфорной кислоты (RO)2PO-OH содержат только один активный атом водорода. Следовательно, по современным представлениям теоретически не существует ограничений на возможности их газохроматографического разделения. Сравнение физико-хи­мических свойств диалкил- и более подробно охарактеризованных триалкил­­фос­­фатов соответствует такому предположению. Оценка полярности ди- и триалкиловых эфиров фосфорной кислоты путем сравнения индексов моле­ку­ляр­ных масс, нормальных температур кипения и молярных рефракций по­ка­зывает, что и те и другие эфиры относятся к достаточно слабополярным со­единениям. Тем не менее, в результате анализа трех реакционных смесей, получен­ных взаимодействием фосфорного ангидрида (Р2О5) с избытками 1-пропано­ла, 2-пропанола и 1-бутанола установлено, что диалкилфосфаты отсутствуют среди детектируемых компонентов, хотя во всех случаях обнаружены следы соответствующих триалкилфосфатов. Это обусловлено невозможностью га­зо­хроматографического разделения диалкиловых эфиров, что может быть свя­зано со следующими причинами:– диалкилфосфаты являются достаточно сильными кислотами (рКа = 1.3-1.7);– вероятность образования внутримолекулярных водородных связей, «улучшающих» хроматографические свойства аналитов, в ди­алкилфосфатах, в отли­чие от моноалкиловых эфиров органических дикарбоновых кислот, пре­дстав­ляется маловероятной;– эфиры фосфор­ной кислоты (в том числе триалки­ло­вые) даже при низких темпера­ту­рах являются активными алкилирующими агентами, что сни­жа­ет их стабильность в процессе раз­деле­ния за счет возможного взаимодей­ст­вия с неподвижной фазой колонки.Каждый из указанных факторов по отдельности не может служить причи­ной невозможности использования газовой хроматографии для разделения диалкилфосфа­тов; наиболее вероятным представляется их совместное дейст­вие.Ключевые слова:  Диалкиловые эфиры фосфор­ной кислоты, критерии полярности, невозможность газохроматографического разделения DOI: http://dx.doi.org/10.15826/analitika.2016.20.4.007

ХРОМАТОМАСС-СПЕКТРОМЕТРИЧЕСКАЯ ХАРАКТЕРИСТИКА ТРИАЛКИЛФОСФИТОВ

A set of analytical parameters for GC-MS identification of (RO)3P phosphorous acid tri­al­kyl esters has been determined. It included standard electron ionization mass spectra, statistically processed ion series spectrum of homologous series, gas chromatographic retention indices of semi-standard non-polar sta­tionary RTX-5 phase, and homologous incre­ments of retention indices. Tri­alkyl phos­phites have been discovered directly in the reaction mix­tu­res of aliphatic alcohols with phosphorous trichloride and without preparative isolation that required the iden­tification of all the constituents of these mixtures. In order to prevent the formation of corresponding (RO)2PHO dialkyl phosphonates in the results of Michaelis-Arbuzov rearrangement, the process was carried out by controlling the pH of the reaction mixture with N,N-dimethyl aniline additives. Using joint GC-MS parameters as homologous increments of retention indices (iRI) allowed distinguishing the trialkyl phosphates and the isobaric dialkyl phosphonates (both series belong to the same homologous group y = 12). Besides that, despite the absence of the signals of molecular ions in mass spectra of trialkyl phosphites, the estimation of their molecular masses became possible. The comparison of the average iRI values as a criterion of chromatographic polarity indicated that trialkyl phosphites are less polar than dialkyl phosphonates. iRI values for homologues of these series without branches in alkyl substituents were 29 ± 9 and -235 ± 52, respectively.Keywords: Trialkyl phosphites, revealing in reaction mixtures, EI mass spectra, gas chromatographic retention indices, homologous increments of retention indices.(Russian) DOI: http://dx.doi.org/10.15826/analitika.2018.22.2.006Igor G. Zenkevich, Vlada E. NosovaSt. Petersburg State University, Institute of ChemistryUniversitetskii prosp., 26, St. Petersburg 198504, Russian FederationОпределены аналитические параметры триал­ки­ло­вых эфи­ров фосфористой кислоты (RO)3P, необходимые для их хроматомасс-спектрометрической идентификации и до настоящего времени недостаточно подроб­но представленные в существующих базах справочных данных. Они включают стандартные масс-спектры ионизации электронами (ИЭ),  статистически об­работанный масс-спектр ионных серий гомологическо­го ряда, газохроматографические индексы удерживания на неполярной непод­виж­ной фазе RTX-5 и их гомологические инкременты. Триалкил­фос­фиты обнаружены не­по­с­ред­ственно в составе реакционных смесей алифатических спиртов с трихлоридом фосфора без предвари­те­ль­ного препара­тив­но­го вы­деления, что потребовало идентификации всех содержа­щих­ся в них компонен­тов. Для предотвращения образования соответствующих диалкилфосфонатов в результате перегруппировки Михаэлиса-Арбузова реакцию проводили в присутствии N,N-диметиланилина для поддержания рН на уровне ~5. Использование таких объединенных хро­мато­-масс-спектрометрических па­ра­мет­ров как гомологические инкременты инде­к­сов уде­р­живания (iRI) позволяет отли­чать три­алкилфосфиты (RO)3P от изобарных диалкилфос­фо­натов (RO)2PHO (соединения обоих рядов отно­сятся к одной и той же гомологической группе у = 12). Кроме того, не­смотря на отсутствие в масс-спектрах ИЭ триалкилфосфитов сигналов молекуляр­ных ионов, воз­можно определение их моле­ку­лярных мас­с. Сравнение средних значений iRI как критерия хроматографической по­лярности соединений разных классов показывает, что по сравнению с диалкил­фо­с­фонатами триалкил­фос­фиты значительно менее полярны. При отсутствии разветвлений в составе алкильных заместителей значения iRI для этих рядов составляют 29 ± 9 и -235 ± 52, соответственно.Ключевые слова: Триалкилфосфиты, обнаружение в составе реакционных сме­сей, масс-спектры ИЭ, газохроматографические индексы удерживания, гомологичес­кие инкременты индексов удерживанияDOI: http://dx.doi.org/10.15826/analitika.2018.22.2.00

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

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