A.G. Vitukhnovsky

Formation of ZnSe and CdS quantum dots via laser ablation in liquids

Abstract Formation of nanoparticles of both ZnSe and CdS under ablation of corresponding bulk semiconductors in liquid environment (diethyleneglycol, ethanol, etc.) using radiation of a Cu vapor laser is reported. X-ray diffraction (XRD), resonance Raman scattering (RRS), and transmission electron microscopy (TEM) confirm that the nanoparticles are crystalline and have average size of ca 10–20 nm. RRS of nanoparticles are characterized by several peaks multiple to frequency of corresponding phonon (LO replicas).

Dimensionality and temperature dependence of the radiative lifetime of J-aggregates with Davydov splitting of the exciton band

Abstract The temperature dependence of the radiative exciton lifetime (τrad) of J-aggregates of 3,3′-bis(sulfopropyl)-5,5′-dichloro-9-ethylthiacarbocyanine (THIATS) characterized by a Davydov splitting of the exciton band has been determined over the temperature range from 4.2 to 130 K. The Davydov splitting of the exciton band was taken into account during the calculation of the coherent length (Nc) from the values of τrad. The temperature dependence of τrad(T) in units of (Nc)−1 was compared to that for PIC aggregates. In contrast to the observations for the PIC J-aggregates [Potma and Wiersma, J. Chem. Phys., 108 (1998) 4894] the τrad(T) of J-aggregates of THIATS can be rationalized within the framework of a 1-D exciton model.

NON-COHERENT EXCITON MIGRATION IN J-AGGREGATES OF THE DYE THIATS : EXCITON-EXCITON ANNIHILATION AND FLUORESCENCE DEPOLARIZATION

Abstract Exciton–exciton annihilation (EEA) in J-aggregates of 3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine in solution at room temperature and at 77 K was related to incoherent exciton migration within large aggregate domains. Inside a domain, the rate constant for EEA was estimated to be at least 1.5 times smaller than that of exciton decay. The experimental results were incompatible with one-dimensional incoherent exciton transport. The intensity dependence of the fluorescence anisotropy decay was assigned to EEA. At 77 K and in the absence of EEA, the average angle of the transition dipole changed less than 25° during exciton migration over a domain.

Electroluminescence and optical properties of poly(phenylenevinylene)/J-aggregate composites

A composite material of poly(phenylenevinylene) (PPV) doped by dye aggregates was prepared. A very efficient and temperature activated energy transfer (ET) from the PPV to the dye aggregates was attributed to F . orster ET accompanied by exciton diffusion. A clear complementary relation between the photoluminescence (PL) and electroluminscence (EL) images was observed for thin (15 nm) PPV-based OLEDs. So-called ‘‘black spots’’ in EL become bright ones when the photoluminescence of the same area was excited. This effect was attributed to the presence of an insulating layer between the polymer and aluminium. r 2001 Elsevier Science B.V. All rights reserved.

Reorientation of transition dipoles during exciton relaxation in J-aggregates probed by flourescence anisotropy

Abstract A negative fluorescence anisotropy is observed in J-aggregates of two thiacarbocyanine dyes in water-ethylene glycol low-temperature glass upon excitation outside the J-band. The fluoresence anisotropy gradually increases from −0.16 upon non-resonant excitation at 532 nm to +0.37 upon excitation at 610 nm (near the maximum of the J-band absorption). The fluorescence anisotropy is found to vary non-monotonically as a function of the emission wavelength within the J-band of the fluorescence. The dependence of the anisotropy on excitation frequency can be explained by a chain model with two molecules per unit cell. The transition dipole moment of each molecule is estimated to make an angle of 65–70° with the chain direction.

Relaxation and trapping of excitons in J-aggregates of a thiacarbocynine dye

Abstract Exciton relaxation processes in J-aggregates of 3,3′,9-triethyl-5,5′-dichlorothiacarbocyanine iodide (TDC) dye have been studied by means of site-selective, steady-state and time-resolved spectroscopy. We found that TDC forms three different types of J-aggregates (J1,J2, and J3) in frozen solutions. Fluorescence polarization measurements showed that exciton diffusion between randomly oriented segments of aggregates plays a minor role in exciton relaxation. Optical properties of J3-aggregate fluorescent states are well described by the theoretical model of barrierless self-trapping of an excitation in a strictly one-dimensional discrete lattice. Upon optical excitation J3-aggregates also show thermally activated transformation to J2-aggregates. The height of the related potential barrier is approximately equal to 30 cm−1. Hence, two different exciton relaxation processes are proposed to take place in the system under study: barrierless self-trapping in J3-aggregates and thermally activated J3 → J2 photorearrangement.

Relaxation dynamics of excitons in J-aggregates revealing a two-component Davydov splitting

Abstract The J-aggregates of the thiacarbocyanine dye THIATS revealing a clear-cut two-component Davydov splitting have been investigated by different optical methods. The presence of two molecules per unit cell was proved both by polarised fluorescence and linear dichroism in the absorption of aligned aggregates in a rotating cell. The fluorescence excitation spectrum of the aggregates and their quantum yield (QY) were measured across the entire exciton band. The QY was found to be 0.1 and 0.4 for excitation via the upper and lower Davydov components respectively. The radiative lifetime of emission of the J-band was estimated to be about 15 ns combining fluorescence decay data with the fluorescence quantum yield. The excitation intensity dependence of the fluorescence decays suggests the occurrence of exciton–exciton annihilation (EEA). EEA occurs even at an excitation photon fluxes below 1010 photons/(pulse cm2) corresponding to less than one absorbed photon per 106 monomer units.

Exciton dynamics and trapping in J-aggregates of carbocyanine dyes

Exciton relaxation processes in J-aggregates of two thiacarbocyanine dyes (TDC and THIATS) in water/ethylene glycol low temperature glass have been studied by means of steady-state, site-selective and time-resolved spectroscopy. The results of different experiments can be rationalized within the framework of exciton self-trapping (ST) in a 1D molecular chain. The measured Stokes shift is found to be proportional to the exciton - phonon coupling strength squared that is in accordance with the model of large radius ST excitons. A negative fluorescence anisotropy was observed for J-aggregates of both investigated dyes upon excitation outside the J-band. The observed monotonic dependence of the fluorescence anisotropy on the excitation wavelength is interpreted quantitatively within the framework of intraband absorption and Davydov splitting.

Absorption lineshape of J-aggregates of thiacarbocyanine dyes: Experiment and numerical study

Abstract The experimental absorption spectra of J-aggregates of the carbocyanine dye THIATS and TDC were fitted to a segment and a continuous energy disorder (CED) model. From 6 K to room temperature the best fits were obtained using an exponential distribution of segment lengths. Both models gave equally good fits when the average length in the segment model corresponded to the average localization length in the CED model. At room temperature homogeneous broadening or/and an enhanced contribution of the off-diagonal disorder must be taken into account.

Excitation dynamics of pyrenyl labeled polyallylcarbosilane dendrimers

Pyrenyl labeled anesiloxane dendrimers and carbosilane hyperbranched polymers have been investigated with the use of various types of fluorescence experiments. Both steady-state and time-resolved fluorescence quenching of a probe molecule, pyrenyl excimer formation as well as polarization measurements were employed. It was shown that different aspects of dendritic polymers structure and their dynamics may be evaluated and visualized.