Ultraviolet UV and Diffuse Reflectance Spectra

Most distibines appear yellow, while dibismuthines are red, either in their liquid phases or in solution in organic solvents. A series of tetravinyldisti-bines (7) and dibismuthines (24) show absorption maxima in the range of 200-330 nm. 2,2, 5,5 -Tetramethylbistibole (30) shows a maximum at 346 nm (22), while the corresponding bibismole (31) absorbs at 320 nm (34). In all cases, the distibines show a low intensity, featureless absorption tail extending out to about 400 nm, while the tail extends to approximately 800 nm for the disbismuthines. Presumably, this feature corresponds to the observed yellow and red colors. No spectral assignments have been made. 

On crystallization, nonthermochromic distibines and dibismuthines show little visual change. The solid colors of two nonthermochromic distibines (8 and 9) and two nonthermochromic dibismuthines (45 and 46) have been characterized by diffuse reflectance. In each case, only very modest changes in the absorption maxima were observed and between solid and solution. On the other hand, the intense colors shown by the solid phases of the thermochromic distibines are red shifted by 200-250 nm from their solution phase maxima (see Table IV) (7,25,29,33,34,37, 38b,40). The dibismuthines are red shifted even further. There is complete correspondence between dibismuthines and the analogous distibines, with the dibismuthines being red shifted by around 100 nm (see Fig. 1). 

Diffuse Reflectance Absorption Maxima (nm) of Dibismuthines and Corresponding Distibines  

The five distibines (1, 22, 30, 36, and 40) that have been investigated by X-ray crystallography show very similar molecular structures (see Fig. 2 for the molecular structure of 2,2, 5,5 -tetramethylbistibole, 30). In each case the distibine adopts a staggered trans conformation so that there is an inversion center through the Sb—Sb bond. The Sb—Sb bond lengths vary between 2.84 and 2.88 A, values somewhat shorter than that of elemental antimony (2.90 A) (43). The bond angles about antimony are close to 90° and similar to those reported for other trivalent antimony compounds (44). In fact, none of the molecular parameters seems extraordinary. However, the four thermochromic distibines show unusual crystal packing (see Fig. 3, Table V) (22,25,36,40,41,45). In each case, the antimony atoms are 

Although the Sb chains are qualitatively similar, there is a variation in the linearity of the chain and in the intermolecular distances. Thus, the chains are nearly linear (4 Sb—Sb—Sb 179.2°) and the Sb—Sb distances are short (3.68 A) for the sterically undemanding tetramethyldistibine (22), while the chains are distinctly zig-zag (4 Sb—Sb---Sb 165.8°) and the distances (3.99 A) are longer for the more bulky tetrakis(trimethylsilyl)dis-tibine (36). While short intermolecular contacts seem necessary for the thermochromic effect, the UV absorption maxima of the solid distibines do not correlate with the intermolecular Sb---Sb distances. Thus, the red compounds 22,36, and 40 have nearly identical absorption maxima despite a variation of more than 0.3 A in the intermolecular Sb contacts. On the other hand, the intermolecular Sb—Sb separations of 22 and 30 are nearly the same although the absorption maxima differ by 75 nm. 

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