Light-Induced Self-Assembly and Decay of J Aggregates of Thiamonomethinecyanine Dyes

Excitonic Properties of Organic Dye Aggregates: Contribution of Ukrainian Science

Unexpected discovery that molecules of organic dyes when they form regular structures can change dramatically their light absorption and fluorescence properties were attracting the minds of researchers for more than eight decades. The progress in investigation of this unique phenomenon described in terms of H- and J-aggregation has led to many practical applications. Here the author expresses his personal view on the dramatic story of switching this research area from empirical knowledge to that standing on strong background of molecular exciton theory. The author was a witness of some of these events and acquainted with several great personalities involved. The major trends of future developments are highlighted.

Assembling Near-Infrared Dye on the Surface of Near-Infrared Silica-Coated Copper Sulphide Plasmonic Nanoparticles

Functionalization of colloidal nanoparticles with organic dyes, which absorb photons in complementary spectral ranges, brings a synergistic effect for harvesting additional light energy. Here, we show functionalization of near-infrared (NIR) plasmonic nanoparticles (NPs) of bare and amino-group functionalized mesoporous silica-coated copper sulphide (Cu_2-xS@MSS and Cu_2-xS@MSS-NH₂) with specific tricarbocyanine NIR dye possessing sulfonate end groups. The role of specific surface chemistry in dye assembling on the surface of NPs is demonstrated, depending on the organic polar liquids or water used as a dispersant solvent. It is shown that dye binding to the NP surfaces occurs with different efficiency, but mostly in the monomer form in polar organic solvents. Conversely, the aqueous medium leads to different scenarios according to the NP surface chemistry. Predominant formation of the disordered dye monomers occurs on the bare surface of mesoporous silica shell (MSS), whereas the amino-group functionalized MSS accepts dye predominantly in the form of dimers. It is found that the dye-NP interaction overcomes the dye-dye interaction, leading to disruption of dye J-aggregates in the presence of the NPs. The different organization of the dye molecules on the surface of silica-coated copper sulphide NPs provides tuning of their specific functional properties, such as hot-band absorption and photoluminescence.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Excimer Emission in J-Aggregates

An excimer in J-aggregates has been often considered as a self-trapped exciton originating from the free exciton excited on the same aggregate and relaxed through interaction with vibronic modes. Here we show that other types of excimers due to intermolecular off-diagonal interactions can be observed in J-aggregates of thiamonomethinecyanine dyes. These excimers arise owing to free excitons too, but they possess a longer formation time of more than 100 ps, indicating migration of free excitons to the excimer formation site, where they interact with a guest species in the ground state. Formation of the excimers occurs in solutions as a power law of concentration with an exponent of 1.5, showing that an excited aggregate should be twice longer than a ground-state guest species, consistent with the exciton coherence length of four molecules versus one dimer, respectively. Unlike the self-trapped exciton, lower temperatures lead to significant suppression of the observed excimer emission.