Eduard I. Zenkevich

Photophysical and photochemical properties of potential porphyrin and chlorin photosensitizers for PDT

Structural and optical properties as well as photophysical and photochemical parameters (excited S1 and T1 state lifetimes at 77 K and in the presence of O2 in solution at 293 K; efficiencies of singlet oxygen, 1Δg, generation) are presented for porphyrins and chlorins with potential for the PDT of cancer: chlorin p6 and its trimethyl ester, chlorin e6 and its Na3 and K3 salts, purpurin-18 and its monomethylester, 5,10,15,20-tetrakis(3-methoxyphenyl)porphyrin (TPPM), 5,10,15,20-tetrakis(2,4-difluoro-3-methoxyphenyl)porphyrin (TPPMF) and GaTTP in different solvents (ethanol, toluene, pyridine and buffer pH 7.4) at 77–300 K. It has been shown that for monomeric chlorin e6, chlorin p6 and its derivatives the photophysical parameters are similar, as follows: fluorescence lifetimes τs in the presence of oxygen are 3.2–4.5 ns at 293 K; fluorescence quantum yields φ1 vary from 0.1 to 0.2 depending on the solvent; phosphorescence quantum yields φ1 are of t order 10−5; T1 state lifetimes τT = 1.5–2.0 ms at 77 K and 250–390 ns at 293 K in the presence of O2. By use of the direct kinetic measurement of singlet oxygen emission at 1.27 μm on laserexcitation the quantum yields of 1Δg generation by chlorins have been measured: φΔ = 0.35−0.68. In this case values of φ1 and φΔ depend strongly on the solve probably because of the formation of aggregates. For TPPM, TPPMF and Ga-TPP the φΔ values measured are higher (0.87–0.98) and are explained by the higher intersystem crossing S1 → T1 quantum yields.

Interaction of multiporphyrin systems with molecular oxygen in liquid solutions: extra-ligation and screening effects

Abstract Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn-cyclodimer→(ZnOEP) 2 Ph→(ZnOEP) 2 →(ZnHTPP) 2 →(ZnOEChl) 2 with relative lowering of excited S 1 - and T 1 -states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T 1 -states non-radiative decay (the most pronounced for dimers with higher lying T 1 -levels). For pyridinated dimers at 293 K T 1 -states quenching by molecular oxygen depends on the spacer flexibility and donor–acceptor interactions with PYR. In self-assembled triads and pentads energy and electron transfer (within a few ps) takes place from Zn-dimers to pyridyl substituted porphyrin extra-ligand, H 2 P, followed by the effective population of H 2 P T 1 -state. For these systems, bimolecular constants of H 2 P T 1 -states quenching by O 2 decrease by 1.4–1.8 times with respect to those found for individual monomeric porphyrins. This effect is explained by the screening action of a strongly quenched Zn–porphyrin dimer subunit limiting the access of oxygen molecule to the excited extra-ligand.

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E2(high) and A1(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 103) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.

Tuning electronic states of a CdSe/ZnS quantum dot by only one functional dye molecule.

Self-assembly of only one functionalized porphyrin dye molecule with one CdSe/ZnS quantum dot (QD) not only modifies the photoluminescence (PL) intensity but also creates a few energetically clearly distinguishable electronic states, opening additional effective relaxation pathways. The related energy modifications are in the range of 10-30 meV and show a pronounced sensitivity to the specific nature of the respective dye. We assign the emerging energies to surface states. Time-resolved PL spectroscopy in combination with spectral deconvolution reveals that surface properties of QDs are a complex interplay of the nature of the dye molecule and the topography of the ligand layer across a temperature range from 77 to 290 K. This includes a kind of phase transition of trioctylphosphine oxide ligands, switching the nature of surface states observed below and above the phase transition temperature. Most importantly, our findings can be closely related to recent calculations of ligand-induced modifications of surface states of QDs. The identification of the optical properties emerged from a combination of spectroscopy on single QDs and QDs in an ensemble.

Size-Dependent Non-FRET Photoluminescence Quenching in Nanocomposites Based on Semiconductor Quantum Dots CdSe/ZnS and Functionalized Porphyrin Ligands

We review recent experimental work to utilize the size dependence of the luminescence quenching of colloidal semiconductor quantum dots induced by functionalized porphyrin molecules attached to the surface to describe a photoluminescence (PL) quenching process which is different from usual models of charge transfer (CT) or Foerster resonant energy transfer (FRET). Steady-state and picosecond time-resolved measurements were carried out for nanocomposites based on colloidal CdSe/ZnS and CdSe quantum dots (QDs) of various sizes and surfacely attached tetra-mesopyridyl-substituted porphyrin molecules (“Quantum Dot-Porphyrin” nanocomposites), in toluene at 295 K. It was found that the major part of the observed strong quenching of QD PL in “QD-Porphyrin” nanocomposites can neither be assigned to FRET nor to photoinduced charge transfer between the QD and the chromophore. This PL quenching depends on QD size and shell and is stronger for smaller quantum dots: QD PL quenching rate constants 𝑘𝑞 scale inversely with the QD diameter. Based on the comparison of experimental data and quantum mechanical calculations, it has been concluded that QD PL quenching in “QD-Porphyrin” nanocomposites can be understood in terms of a tunneling of the electron (of the excited electron-hole pair) followed by a (self-) localization of the electron or formation of trap states. The major contribution to PL quenching is found to be proportional to the calculated quantum-confined exciton wave function at the QD surface. Our findings highlight that single functionalized molecules can be considered as one of the probes for the complex interface physics and dynamics of colloidal semiconductor QD.

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.

Structure and excited state properties of multiporphyrin arrays formed by supramolecular design

Structurally defined nanoscale self-assembled multiporphyrin arrays of variable geo-metry and composition (up to eight tetrapyrrole macrocycles) have been formed via two-fold extra-ligation in solutions at 77-293 K. The array formation is based on non-covalent binding interactions of the phenyl bridged Zn octaethylporphyrin chemical dimers or trimers, (ZnOEP)2Ph or (ZnOEP)3Ph2, with di- and tetrapyridyl substituted tetrapyrrole extra-ligands (porphyrin, pentafluorophenyl substituted porphyrin, Cu porphyrin, tetrahydroporphyrin). Using steady-state and time-resolved measurements, spectral properties as well as pathways and dynamics of non-radiative relaxation processes (energy migration, photoinduced electron transfer, exchange d-π effects, realized in nano-femtosecond time scale) have been studied in these complexes upon variation of the composition, mutual geometry, redox and photophysical properties of interacting subunits as well as on the tempera-ture and polarity of surrounding.

Spectroscopic and photophysical properties of covalent ether-bonded porphyrin-chlorin heterodimers

Abstract Comprehensive experimental and theoretical studies of structural, spectral and photophysical properties of the porphyrin-chlorin heterodimers 1 and 2 with a simple ether bond between macrocycles, P-O-Chl, characterised by different geometries, have been provided in toluene, tetrahydrofurane and a mixture of diethyl ester: petroleum ether: isopropanol 5:5:2 in a temperature range 77–293 K. In absorption porphyrin and chlorin subunits of the P-O-Chl heterodimers act as discrete entities rather than one large, delocalized π-electron system. The weak interaction of π-conjugated P and Chl macrocycles lead to small red shifts (by 2–5 nm) of electronic spectra of the dimers, but does not change the main photophysical parameters of Chl subunit in the P-O-Chl heterodimers 1 and 2 in a temperature range 77–293 K. Upon the optical excitation of covalently coupled π macrocycles of P-O-Chl heterodimers in solvents of various polarity, the P and Chl subunits do not interact via charge transfer mechanism, because the energies of charge-transfer (CT) states are essentially higher than the locally excited S1-states of interacting macrocycles. The strong porphyrin fluorescence quenching in P-O-Chl system is caused by the effective singlet-singlet nonradiative energy transfer to chlorin subunit with probabilities of fda = 1.35 × 1011 − 6.8 × 1010s−1 depending on temperature. This process takes place without quantum losses and occurs after the complete vibrational relaxation within the lifetime of the donor S1 excited state. The dynamics of singlet-singlet nonradiative energy transfer in a whole temperature range and the main experimental findings for the P-O-Chl heterodimer 2 are appropriately described by the Forster-Galanin inductive-resonant mechanism without any additional assumptions.

Formation Principles and Exciton Relaxation in Semiconductor Quantum Dot–Dye Nanoassemblies

In this chapter we discuss “bottom-up” non-covalent self-assembly principles which define a strategy for the formation of organic–inorganic nanoassemblies containing colloidal semiconductor quantum dots (QD) of different types (based on a CdSe core) and various heterocyclic molecules (dyes) with functionalized anchoring side substituents (meso-pyridyl substituted porphyrins and perylene diimides). Using a combination of ensemble and single molecule spectroscopy of “QD–Dye” nanoassemblies, we show that single functionalized molecules can be considered as extremely sensitive probes for studying the complex interface physics and chemistry (influence of the embedding environment and temperature) and related exciton relaxation processes in QDs. It will be quantitatively laid out that the major part of the observed QD photoluminescence (PL) quenching in nanoassemblies can be understood, on the one hand, in terms of exciton wave function tunneling under the condition of quantum confinement and, on the other hand, by the influence of ligand dynamics. In nanoassemblies, photoinduced Foerster-type energy transfer (FRET) QD → Dye is often only a small contribution to the PL quenching and is effectively suppressed already in slightly polar solvents which is often overlooked in literature. Finally we would like to point out that properties of “QD–Dye” nanoassemblies are not only interesting in themselves but also provide a valuable tool to study surface-related phenomena in QDs on an extremely low level of surface modification, thus providing the data for a further development of defined multi-component structures for exploitation as artificial light-harvesting complexes, electro- and photochemical devices or nanosensors.

Fluorescence line narrowing study of cyclopentaneporphyrin chemical dimers at 4.2 K

Abstract The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycleCH 2 ) 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 S 0 → S 1 absorption band have been observed. In contrast to this, it has been shown that the excitation into the region of S 0 → S 2 absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S 0 → S 2 electronic transitions in the energy scale have been determined. The normal coordinate treatment of FLN spectra of ZnOEP-cycleCH 2 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CH 2 ) subunits in Zn-cyclodimers manifests itself in the strong fluorescence quenching of D . Under excitation into the S 0 → S 1 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.