O.P. Varnavsky

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.

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.

A comparative study of temperature broadening of spectral holes burnt in absorption bands of monomers and J-aggregates of a thiacarbocyanine dye

Abstract J -aggregates and monomers of a thiacarbocyanine derivative are investigated in glassy matrices by spectral hole burning. The temperature dependence (at T = 4.6−20 K) of the width of the hole burnt in the absorption band of aggregates differs qualitatively from that of the hole in the absorption band of monomers. The hole width for J -aggregates does not depend on temperature below 9 K and is equal to 3.3 cm −1 . This residual hole width is suggested to be determined by exciton-phonon scattering with phonon emission.

DEGRADATION OF J-AGGREGATES IN SOLUTIONS EXPOSED TO LIGHT

The degradation ofJ-aggregates of thiacarbocyanine dye (THIATS) exposed to visible radiation is experimentally investigated. A model that allows a consistent description of photodegradation ofJ-aggregates in a solution is proposed. Luminescence quencher molecules are formed with the participation of monomer dye molecules in a solution under action of the light. These molecules, when diffusing to the aggregates for a time of nearly 10 s, are adsorbed on their surface and participate in quenching of exciton luminescence of theJ-aggregates.

Thiacarbocyanine Dye J-Aggregates as an Example of Restricted Molecular System

The results of a study of exciton dynamics of thiacarbocyanine derivative J-aggregates in glassy matrices at low temperatures are reported. Temperature dependences of fluorescence spectra and homogeneous absorption linewidth have been experimentally investigated. Spectral hole burning experiments show the homogeneous linewidth to be independent of temperature at T<9 K being as large as 1.7 cm-1 corresponding to the dephasing time 6.2 ps. The results are discussed in a framework of exciton-phonon scattering process. Temperature dependence of coherent length of exciton is considered on the base of fluorescence spectral data.