Aggregates H-type

Irradiation of lower molecular weight samples in the fluid N phase at 313 or 366 nm led to an unusual result [21]. In the first few seconds of irradiation the perturbed spectrum of the N phase exhibited hyperchromism (an increase in absorbance) and its shape became similar to that of the spectrum of the isotropic melt. This effect is also observed upon triplet sensitization which, like 366-nm irradiation, suppresses photo-Fries rearrangement [28]. It has not yet been proved that this effect is accompanied by a phase change from N to I induced by photoproducts essentially acting as impurities in the mesophase. The effect could be at the microscopic level where formation of a cyclobutane dimer or other photoproduct could interrupt H-type aggregated chromophore stacks, or confor-... 

The influence of the counter anion on the excited state relaxation time of cationic polymethine dyes has also been reported . The fluorescence lifetime is dependent on the anion in weakly polar media but independent in polar media. The fluorescence behaviour of highly concentrated rhodamine GG solutions in methanol and water can be separated into monomer and dimer contributions2. Absorption emission and excitation spectral data support the view that the dye rose bengal forms H-type aggregates in water and polar protic solvents 1. The spectroscopic behaviour of rhodamine 6G in polar and nonpolar solvents as well as in thin glass and PMMA films shows dimer formation occurs and their stabilities have been compared under different conditions. The equilibrium between the neutral... 

The aggregation behavior of the squaraine dyes 26 and 27 (Structure 8) in water in the presence and absence of polyvinylpyrrolidone has been studied [66]. In aqueous solutions, these dyes show two absorption bands with maxima around 595 and 645 nm, which have been attributed to those of the aggregated and monomeric forms, respectively. The aggregated forms were identified as dimers from concentration dependent changes in the absorption spectra. The blue shift in absorption of the aggregated forms indicate the formation of H-type aggregates [82,83], imlike in the case of the bis(hydroxyphenyl)squaraine derivatives, where J-type aggregates were obtained [76]. 

Other H-type aggregates of various chromophores [40]. The arrangement of two porphyrins with a finite dihedral angle causes, in principle, a splitting in the energy bands due to the existence of both face-to-face and head-to-tail orientations of the two in-plane transition moments of the porphyrin. This is also the case for slipped cofacial arrangements, where the existence of two similar transition moments produces split Soret bands. 

The multiple hydrogen-bonded eomplex of the dithienylethene (107), bearing two melamine moieties, with merocyanine (108) showed J-type exciton eoupling, whieh eould be switched reversibly upon UV/vis irradiation. Upon UV irradiation the J-type exciton in the presence of the bismelamine (109) as a receptor led to the H-type aggregators between (108) and (109). ... 

The spectroscopic behavior observed for confined Si(OEt)3-ImPV-F (blue-shifted absorption, decreased intensity, and red-shifted emission) could be attributed to an H-type aggregation such an aggregation may reasonably be expected from a card pack orientation of the distyrylbenzene imide chromophores in the silicate network. The formation of the silyl network supports a face-to-face organization of chromophores. 

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