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OQX(Z)-a-cyano-~-(N-hexadecylquinolin-4-ylium)-4-styryldicyanomethanide, C16H3,-Q3CNQ, has two Langmuir-Blodgett (LB) phases that differ in thickness, tilt angle and second-order susceptibility. For phase I, /=2.2 nm layer-', 4 = 8 " and ~( ~) = 1 8 0 pm V-' at 1.064 pm, whereas for phase II, /=1.6 nm layer-', #=24" and ~( ~) = 1 0 0 pm V-'. The susceptibility of phase I is the highest so far reported for an LB multilayer; the structure is non-centrosymmetric (Ztype) and, as predicted by theory, the second-harmonic intensity varies as /2w(N)=/2w(,)N2 as N increases from 1 to 200 layers. In contrast, C10H2,-Q3CNQ has a low susceptibility (x,z(~)= 6 pm V-') and neutron reflectivity studies have indicated an antiparallel alignment of chromophores at the air/water interface.

Poly(1-methylpyrrol-2-ylsquaraine) precipitates from reaction solution as uniformly spherical particles with a diameter of 1.3 microm. Upon heating, the particles reduce in diameter until extinction at approximately 630 degrees C. Treatment of the particles with 9:1 tetraethoxysilane:ethanol solution, and subsequent hydrolysis in dilute acid, results in a polymer core-silica shell structure. Removal of the core, upon heating to 660 degrees C, results in an amorphous silica shell with a diameter half that of the initial template sphere. It has been found that the silica shells produced by this method are able to encapsulate organic dyes upon soaking of the shells in chloroform solutions of the dyes, and further washings with fresh chloroform did not remove the dyes. The production of crystalline titanium dioxide shells was also achieved through the use of the polysquaraine particles as a spherical template.

The crystal structure of 3,5-dinitrosalicylic acid monohydrate (1) and its adducts with 2- aminobenzoic acid (2-aba) [( dnsa )(2-aba)] (2), 3-aminobenzoic acid (3-aba) [( dnsa )(3-aba)] (3), and 4-aminobenzoic acid (4-aba) [( dnsa )(4-aba)2] (4), have been determined and the hydrogen bonding associations in each analysed . The acid (1), which is essentially planar, forms strong hydrogen-bonding network associations involving the carboxylic, nitro and phenolic oxygens as well as the lattice water. In all adducts, protonation of the amino group of the second acid occurs, with subsequent hydrogen bonding via all three alkyl ammonium hydrogens to the carboxylic, nitro and phenolic oxygens of dnsa. With the 1:2 adduct (4), only one of the two 4-aba molecules is protonated, these forming a secondary hydrogen-bonded cyclic dimer. A strong intramolecular hydrogen bond between the phenolic proton and the carboxylate group is found in all adducts.

In the 50+ years since the first squaraine dye, made by using 3,4‐dihydroxycyclobut‐3‐ene‐1,2‐dione (squaric acid), was reported, numerous analogues have not only been made and published, but similarly, a number of good review articles on squaraine dyes has also been published. In contrast, for the majority of this time, reports of croconaine dyes, made by using 4,5‐dihydroxy‐4‐cyclopentene‐1,2,3‐trione (croconic acid), have also been published, but they have not received the same attention as squaraine dyes. Furthermore, the only known review of croconaine dyes, which was recently published, is written in Chinese. This current review aims to document the history of croconaine dyes since their first report in a German patent in 1970. This review sources both the scientific literature and published patents, and attention is given to both synthesis and spectroscopic details.

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Crystals of the title compound exist as a green monoclinic phase [space group P21/c with u=9.046( l), b= 19.615(2), c =9.055( 1) A, p = 116.107( 5)", 2 = 21 and a purple triclinic phase. The chromophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation. The Langmuir-Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity from films of hemicyanine dyes. The anomalous non-linear optical properties are attributed to a serendipitous non-centrosymmetric packing arrangement within the films and to an intermolecular charge transfer contribution to the bulk second-order susceptibility.Current interest in organic materials for non-linear optics'-' stems from the high molecular hyperpolarizabilities of donor-(z-bridge)-acceptor molecules and the potential applications of such materials in the electronics and communications industries. The non-linearities arise from the dependence of the polarization on the electric field of the incident radiation: