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Photo-Annealing of Merocyanine Aggregates
摘要： In this work we elucidate the fundamental difference between aggregate formation of donor-π-acceptor merocyanines in their electronic ground and excited states. While increasing the π-bridge size favors formation of π-stacked aggregates in the dark, irradiation with visible light causes reorientation of the dyes to form prototype H-aggregates with compensating dipole moments. This photo-annealing changes the supramolecular structure and its UV-vis spectroscopic properties dramatically, thus being of importance for the function of active layers composed of these dyes. Aggregates of the ground state dyes are bound cooperatively through ππ-London dispersion interactions and hydrogen bonds between the polar α-cyano-carboxylic acid groups. However, charge transfer upon photoexcitation leads to repulsion of the polar acid groups. Electronic excitation of the dyes approximately doubles the ground state dipole moment, thus driving molecular reorientation into prototype H-aggregate structures. We show that this photo-induced supramolecular rearrangement can disrupt the large polymeric aggregates formed in the dark. The photo-induced supramolecular structural changes reported in this work will influence the performance of optoelectronic devices composed of these structures and must be controlled to avoid morphological decomposition of active layers upon operation.
关键词： merocyanines,photo-annealing,H-aggregates,supramolecular structure,UV-vis spectroscopy,dipole moments,aggregate formation
更新于2025-09-23 15:21:21
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Low-Temperature Effect on the Electronic Structure and Spectral-Fluorescent Properties of Highly Dipolar Merocyanines
摘要： Absorption and fluorescence spectra of a vinylogous series of reversely solvatochromic merocyanines based on benzimidazole and malononitrile have been studied in frozen ethanol solutions at 77 K. It is found that they possess negative thermochromism – in contrast to both positively solvatochromic merocyanines and negatively solvatochromic symmetrical ionic polymethines – and even stronger negative thermofluorochromism. It has been deduced from the spectral data, that at low temperature their electronic structure becomes more dipolar, deviating substantially from the virtual ideal polymethine in both the ground and excited states. At that, owing probably to high polarity and ordering of frozen ethanol, the dipolarity of the studied merocyanines increases with the polymethine chain lengthening – the tendency not observed for them in common solvents. The conclusions, based on the spectra data analysis, have been verified by the (TD)DFT–PCM simulations of the dyes within the four-level scheme of electronic transitions.
关键词： Low-Temperature Effect,Spectral-Fluorescent Properties,Merocyanines,Quantum Chemistry,Electronic Structure,Molecular Structure,Spectroscopy
更新于2025-09-10 09:29:36