The dynamics and pathways of relaxation processes in meso-ortho-nitrophenyl substituted octaethylporphyrins OEP-OEPPh(o-NO2) and PdOEP-Ph(o-NO2) occurring with the participation of the S1 and T1 states was studied in polar
Ph((dimethylformamide) and nonpolar (toluene) solvents at 295 and 77 K using pico- and nanosecond laser kinetic
spectroscopy. At 295 K, steric interactions between bulky β-alkyl substituents and ortho-nitro groups of meso-phenyl in
these compounds create optimal conditions for overlapping of molecular orbitals of the porphyrin macrocycle (donor) and
NO2-group (acceptor), thus leading to an efficient photoinduced electron transfer (PET). For free-base OEP-Ph(o-NO2),
PET occurs only via the porphyrin S1 state within 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing
intersystem crossing S1 ~~>T1 is low probable. For metallocomplex PdOEP-Ph(o-NO2), PET involves both S1 and T1 states. In
the last case, the direct PET from the T1 state to CT state also occurs within picosecond range (20 and 46 ps for
dimethylformamide and toluene, respectively, at 295 K). Rate constants for PET with participation of T1 states are by 3-5
times smaller with respect to those found for PET occurring via the S1 state. For both compounds, the observed long-lived
component (250-700 ns) in decays of the transient T1-Tn absorption is due to the recombination processes of radical-ion
pairs whose lifetime decreases with an increase of the surrounding polarity and is hardly dependent on the presence of
molecular oxygen in the solution. For both compounds, PET is completely absent in rigid solutions at 77 K.
The directed surface passivation of semiconductor CdSe, 0r CdSe/ZnS quantum dots (QD) by meso-pyridyl substituted porphyrins (H2P) has been realized via a reversible non-covalent self-assembly interaction of H2P meso-pyridyl nitrogens with ions of the ZnS shell or Cd atoms of the CdSe core in various solvents at ambient temperature. The formation of "QD-porphyrin" nanoassemblies leads to a QD photoluminescence (PL) quenching (intensity decrease and PL decay shortening) accompanied by a H2P fluorescence enhancement. The analysis of experimental Foerster resonance energy transfer efficiencies EFRET (FRET) found via acceptor (H2P) sensibilization and donor (QD) PL quenching shows that EFRET values obtained from fluorescence enhancement are of the order of 6 - 8 % for most QD studied and are thus much smaller as compared to the PL quenching efficiency. With respect to QD PL quenching efficiencies, smaller values of EFRET might be due to different competing reasons: the presence of two independent quenching processes in the nanoassemblies, energy transfer QD -> H2P and photoinduced (electron/hole) charge transfer (CT) or time-dependent QD interface dynamics leading to a noticeable QD PL quenching. The analysis of spectroscopic and kinetic findings reveals that a limited number of "vacancies" accessible for porphyrin attachment is available on the QD surface. Simultaneous presence of porphyrin triads/pentads and QDs in a solution leads to the formation of higly organzed nanoassemblies.
Self-assembled nanoscale arrays of controllable geometry and composition (up to 8 tetrapyrroles) have been formed via non-covalent binding interactions of the meso-phenyl bridged Zn-octaethylporphyrin chemical dimers or trimers with di- /tetrapyridyl substituted porphyrin extra-ligands. In these complexes using steady-state and time-resolved (ps fluorescence and fs pump-probe) measurements pathways and efficiencies of the energy transfer photoinduced charge separation as well as exchange d-π effects have been studied in solutions of variable polarity at 77-293 K. The same principles of aggregation via the key-hole scheme "Zn-pyridyl" have been used also for the surface passivation of pyridylsubstituted tetrapyrroles on the coreshell semiconductor CdSe/ZnS quantum dots (QD) showing quantum confinement effects. Picosecond time-resolved and steady-state data reveal that CdSe/ZnS QD emission is multiexponential and the efficiency of its quenching by attached porphyrins (due to energy transfer and photoinduced charge separation) depends strongly on the number of anchoring groups their arrangement in the porphyrin molecule as well as on QD size and number of ZnS monolayers. The analysis of spectroscopic and kinetic findings reveals that on average only ~l/5 porphyrin molecules are assembled on the QD and a limited number of "vacancies" accessible for porphyrin attachment is available on the QD surface.
In this work we present the results showing how the extra-ligation, spacer properties and porphyrin macrocycle screening may influence on the conformational dynamics and photophysical properties of multiporphyrin arrays as well as on their interaction with molecular oxygen in solutions at 293 K. Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn- cyclodimer yields (ZnOEP)2Ph yields (ZnOEP)2 yields (ZnOEChl)2 with relative lowering of excited S1- and T1-states, the extra-ligation by pyridine does not influence essentially on fluorescence parameters but leads to an increase of T1-states non-radiative decay. At 293 K the T1-state quenching by O2 for Zn-dimer- pyridine complexes depends on the nature and flexibility of the spacer between macrocycles and donor-acceptor interactions with pyridine. In triads and pentads the dimeric subunit plays the role of screen weakening O2 interaction with the second subunit. As a result, the T1-state quenching by O2 in triads and pentads is decreased by 50 divided by 70 percent with respect to that for the corresponding individual monomers.
KEYWORDS: Molecules, Luminescence, Picosecond phenomena, Absorption, Atomic, molecular, and optical physics, Thermodynamics, Temperature metrology, Energy transfer, Performance modeling, Life sciences
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.
The methods of picosecond absorption spectroscopy require theuse of sufficiently high laser pulse intensities
because of low sensitivity of these methods. The excitation of aggregated systems containing a large number of
interacting chromophores, localized in a small volume, by intense laser pulses may lead to side effects which are
observed as changes in absorption spectra but are not connected with the processes of electronic excitation energy
relaxation.
In this paper we present the results of such investigations through the examples of polymer ordered aggregates
of photosynthetic pigments (chlorophyll, protochlorophyll, pheophytin) in binary mixtures of water with organic
solvent (T=293 K) upon excitation by 20 ps pulses. The observable reversible spectral changes in ps and ns time
scales are attributed to the effects of laser heating, rather than to the population of aggregate excited electronic
states.
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