Triplet States of Cyanine Dye Dimers
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TRIPLET STATES OF CYANINE DYE DIMERS M. V. Alfimov,1,2 A. K. Chibisov,1,2 G. V. Zakharova,1 T. V. Fedotova,2 and V. G. Plotnikov1
UDC 541.14+543.42
The results of investigations of the properties of cyanine dye dimers in the triplet state are presented. The formation of dimers is demonstrated as two absorption bands of differing intensities, which are due to splitting of the S*-level of monomers as a result of their resonance interaction. The cyanine dye dimers are characterized by a relatively high quantum yield of the intersystem crossing. In the triplet-triplet absorption spectra two bands appear, where the long-wavelength band is assigned to a charge transfer state. The cyanine dye dimers in the triplet state are involved in the energy and electron transfer and can act as the photosensitizes of redox reactions. Keywords: cyanine dyes, dimers, triplet-triplet absorption, electron transfer, energy transfer.
INTRODUCTION In this publication we present the results of the study of the triplet state of cyanine dye dimers. The principal result is the observation of the triplet dimers and identification of their properties from the triplet-triplet absorption spectra. Among the variety of dye classes, cyanine dyes occupy an outstanding place – they are widely used in manufacturing light-emitting diodes (LEDs) [1–3] and laser frequency conversion devices [4–6]. In contrast to dyes of other classes, cyanines manifest their properties in the ground state, forming aggregates primarily in aqueous solutions. Their aggregation results in the formation of dimers as the simplest associates. It should be noted that for many cyanine dimers they are the initial components (precursors) in the formation of more complex associates (J- and H-aggregates) [7]. From this perspective, it is quite appealing to study dimerization of cyanines. The properties of cyanine dye molecules in the excited states depend on the molecular structure, the state of their aggregation and the environment. The intersystem crossing of cyanine monomers is characterized by a low quantum yield and is observed primarily for cyanine dyes with bulky substituents in a meso-position in the polymethine chain or in the presence of heavy atoms (Br, I) [8, 9]. The population of the triplet level of cyanine monomers is often performed by the triplet-triplet energy transfer. In the triplet state, cyanine dye monomers are involved in redox reactions in the presence of electron donors/acceptors, acting as photosensitizers in the electron transfer reaction [10].
EXPERIMENTAL PROCEDURE The compounds used in this study were anionic and cationic cyanines, thia- and indocyanines (Table 1), and water was purified by means of a Direct-Q3 UV Millipore system. Their absorption spectra were measured in an Agilent 8453 spectrophotometer. The luminescence measurements were performed in a Varian Cary Eclipse
1
Photochemistry Centre of Federal Scientific Research Centre “Crystallography and Photonics” of the Russian Academy of Sciences, Moscow, Russia, e-mail: [email protected]; a
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