KEYWORDS: Perovskite, Nanocrystals, Excitons, Optical properties, Nanophotonics, Polymethylmethacrylate, Metals, Bromine, Temperature metrology, Near field optics
Metal halide perovskites in the form of nanocrystals are highly efficient light emitters at visible-NIR wavelengths. In this work, the optical properties of single nanocrystals and ensembles will be discussed, as also several applications in nanophotonics. At low temperatures, single nanocrystals can be also single photon emitters if blinking and spectral diffusion is conveniently reduced. In the case of nanocrystal assemblies, stimulated emission can be observed with thresholds lower than 10 μJ/cm2 under nanosecond laser excitation at low temperatures, whose physical origin is attributed to single exciton recombination. Finally, the coupling of perovskite nanocrystals to the optical modes of hyperbolic metaldielectric metamaterials has been studied and demonstrated an important Purcell enhancement of the exciton radiative emission by more than a factor three for CsPbI3 and around factor two for FAPbI3 when the distance between the emitters and HMM is 10 nm.
Femtosecond laser fragmentation from preliminarily prepared water-dispersed Si microcolloids was used to synthesize bare (ligand-free) spherical silicon nanoparticles (Si-NPs) with low size dispersion and controllable mean size from a few nm to several tens of nm. In order to control the oxidation state of Si-NPs, the fragmentation was performed in normal oxygen-saturated water (oxygen-rich conditions) or in water disoxygenated by pumping with noble gases (Ag, He) before and during the experiment (oxygen-free conditions). XPS and TEM studies revealed that Si-NPs were composed of Si nanocrystals with inclusions of silicon oxide species, covered by SiOx (1 < x < 2) shell, while the total oxide content depended whether Si-NPs were prepared in oxygen-rich or oxygen-free conditions. When placed into a dialysis box, waterdispersed Si-NPs rapidly dissolved, which was evidenced by TEM data. In this case, NPs prepared under oxygen-rich conditions demonstrated much faster dissolution kinetics and their complete disappearance after 7-10 days, while the dissolution process of less oxidized counterparts could last much longer (25-30 days). Much fast dissolution kinetics of more oxidized Si-NPs was attributed to more friable structure of nanoparticle core due to the presence of numerous oxidation-induced defects. Laser-synthesized Si-NPs are of paramount importance for biomedical applications.
Nanometer-size colloidal semiconductor nanocrystals, or Quantum Dots (NQD), are very prospective active centers
because their light emission is highly efficient and temperature-independent. Nanocomposites based on the incorporation
of QDs inside a polymer matrix are very promising materials for application in future photonic devices because they
combine the properties of QDs with the technological feasibility of polymers. In the present work some basic
applications of these new materials have been studied. Firstly, the fabrication of planar and linear waveguides based on
the incorporation of CdS, CdSe and CdTe in PMMA and SU-8 are demonstrated. As a result, photoluminescence (PL) of
the QDs are coupled to a waveguide mode, being it able to obtain multicolor waveguiding. Secondly, nanocomposite
films have been evaluated as photon energy down-shifting converters to improve the efficiency of solar cells.
Excited-state dynamics of water-soluble Cu-5,10,15,20-tetrakis[4-(N- methylpyridyl)]porphyrin(Cu(TMpy-P4)) groove-bounded to poly(dA-dT) or dissolved in neat water phosphate buffer were investigated. In the first case, double-exponential decay kinetics were found with time constants of 35 +/- 7 ps and 3.2 +/- 0.5 ns. Spectral features of the transient absorption, a comparison with photophysical properties of CuP in oxygen-containing solvents as well as earlier RR investigations of an exciplex formation between Cu(TMpy-P4) and some polynucleotides enable us to attribute the two observed transient species to the triplet 2,4T1 state of four-coordinate Cu(TMpy-P4) and to the (d,d) state of five-coordinate Cu(TMpy-P4) axial liganded by CO group of thymine residue, respectively. In the case of Cu(TMpy-P4) in neat phosphate buffer, double-exponential kinetics of a transient species depopulation was observed with time constants of 21 +/- 3 ps and > 2 ns. Spectral feature of the transient absorption enables us to attribute the first transient species to the triplet 2,4T1 state. We believe, the reason of such short 2,4T1 state lifetime is liganding of the excited Cu(TMpy-P4) by molecules of H2O resulting in deactivation via downshifted CT state and/or (d,d) state. The second transient species corresponds, most likely, to the (d,d) state.
Unusual absorption and photophysical properties are revealed for a novel fixed-distance symmetric ethylene-bridged porphyrin dimer, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbis equals OEP in short), investigated by the methods of absorption and fluorescence steady- state spectroscopies as well as picosecond fluorescence and femtosecond transient absorption time-resolved spectroscopies. It is found that in solutions of organic solvents, tbis equals OEP exists in two isomeric forms: an isomer P, and an isomer U (ca. 85% and 15%, respectively, in toluene). It was found that lifetimes of the lowest excited singlet states of both isomers are strongly dependent on solvent viscosity, the isomer U displaying the stronger dependence. Particularly, the fluorescence lifetime of the isomer U was found to increase to approximately 0.5 ns in polymer films at room temperature or in rigid glasses of organic solvents at 77 K. To the best of our knowledge, this is the first example of such viscosity-dependent photophysics for porphyrins. A qualitative explanation of these observations is proposed.
It is well known that methanol and ethanol at low concentrations associate with ferrihemoglobin and ferrimyoglobin to form complexes in which the alcohol groups are bound directly to the ferric ions. The nature of the complexes [heme iron-alcohol ligand] and their relations to the function of hemoprotein molecules have not been satisfactorily explained so far. Octaethylporphynatoiron complexes (Fe-OEP), taking into account their molecular structure, are often used as model systems for studies of heme properties. Ogura et. al., while investigating the photoreduction mechanism of Fe(III)-OEP complex with 2- methylimidazole in dichloromethane (CH2Cl2) solution by resonance Raman and absorption spectroscopies, have found that a trace amount of alcohol present in CH2Cl2 as a stabilizer can initiate the photoreduction process. An unexpected finding in this study was the observation that laser irradiation into the CT band at 585 nm didn't induce the photoreduction. Recently we observed the same photoreduction process, when the starting materials were alkoxocomplexes of Fe-OEP [Fe(OR)-OEP, were R equals CH3, C2H5]. This paper contains new data regarding the structure, spectroscopic, photochemical and photophysical properties of the photoactive products, generated in the reactions of Fe(III)-porphyrins with alcohols.
Excited electronic states of Zn-porphyrin (pi) -radical anions are studied by methods of picosecond absorption spectroscopy and quantum chemistry. An optical absorption spectrum of Zn-octaethylporphin (pi) -radical anions in its lowest excited doublet D1 state is obtained experimentally. Excited states energies are calculated by CNDO/S-CI method. Two allowed transitions from the D1 state are found in calculated spectrum. These theoretical data agree with the experimental spectrum which exhibits two absorption bands at ca. 500 and ca. 600 nm.
The steady interest in metalloporphyrins (Me-P) which have a central metal with an unfilled d-electronic shell
is due to the rich variety of photoinduced physico-chemical processes they participate in, and by the possibility of
modelling on the basis of them, the behavior of related native biological complexes.
Me-Ps with an unfilled d-shell of the central atom have typical two-band absorption spectra in the ground electronic
state: intense Soret or B band in the near UV and less intense Q band in the visible. According to Gouterman's fourorbital
model,' the Me-P absorption spectrum arises from the promotion of ir-electrons from the highest occupied
molecular orbitals a, , a2 to the lowest unoccupied orbitals e of the porphyrin conjugated macrocycle. The a,
and a2 orbitals are quasi-degenerate and the electron excited configurations e have the same symmetry. That is
why there is a strong mixing between the two orbital excitations a, -e and a, - e due to the configurational
interaction via the electron-electron repulsion, resulting in a two-band absorption spectrum.
The effect of the central atom is interpreted in this model as a perturbation of the ir-states of the porphyrin ring.
The perturbation is largely determined by the electrons of the unfilled shells of the metal ion (e.g. the md'-shell).
This effect is quite pronounced for the ground electronic state, nevertheless the interactions of excited ir-electronic
configurations with d-electron ones cause more dramatic changes in the spectroscopic properties of Me-P excited
states. New low-lying excited states are possible: the charge-transfer (CT) levels related to the electrodensity
transfering from the ring a, , a2 to the d-orbitals of central metal (ir-d states) or from the d-orbitals to the ring
e(d - ir states), and (d - d) levels bound up with the d-electron excitations. The present paper deals with the
investigations of porphyrin metallocomplexes with nickel (Ni-P) and copper (Cu-P) having electron configurations
3d8 and 3d9 , respectively.
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.
ir - anions of metalloporphyrins attract special attention due to the important role metalloporphyrins play in the
redox processes in biology. The porphyrin ir - anions are known to be short-lived intermediates of photoreduction
reactions. As the anions produced by alkalimetal reduction in aprotic solvent are stable products, their properties are
investigated by various spectroscopic methods. However there are no data available on the photophysical processes
of ir - anions. Information about the properties of excited states is undoubtedly nesessary for the description of a
complete picture of ir - anions photochernistry.
In this paper the photophysical processes in Zn-octaethylporphin and Zn-octaethylphlorin ir - monoanions (P
and PH- , structural formulas I and II, respectively, Fig. 1) are investigated by picosecond absorption spectroscopy.
The phiorin anions can be treated as a protonated species of porphyrin ir - dianions: P2 +H+ PH-.
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