Synthesis and Property of Dichroic Photochromism of Bipyridinium Bis(phosphonate)s

紫精，4，4 '-联吡啶的双季铵盐，通过光化学反应，由单电子转变为强烈的蓝色自由基阳离子。为了评价紫精的可逆光还原（光致变色）性能，研究了具有较好亲核性和各向异性的烷基膦阴离子紫精的合成及其二色性。本文报道了N，N ′-二烷基-4，4 ′-联吡啶鎓二卤化物的合成，并将其与烷基膦酸酯进行阴离子交换。1H-NMR和元素分析表明紫精由两个膦酸根阴离子组成，吡啶环上的质子向高场移动，推测是由于与庞大的二价膦酸根阴离子的静电相互作用较低。这使得膦酸盐具有热不稳定性，在相对较低的温度下分解而不熔化，在高电离势的有机溶剂中，近紫外光照射下自由基的形成量很小。在具有给电子性质的高稀释溶剂（低IP）中的光还原主要通过周围溶剂分子的ET-反应进行。自由基阳离子在玻璃夹层PVC膜观察到稳态紫外线照射，自由基阳离子的颜色持续了几天。红外光谱表明，光诱导的固体膦酸盐的还原收益从紫精分子内的抗衡阴离子。当膜通过逐渐冷却的熔融viologen-bisphosphonate与烷烃膦酸，在530 nm处的吸收对应于二聚体自由基阳离子相比，快速冷却的增加。虽然吸收差异很小，紫精双膦酸盐的膜显示二向色性的各向异性组织的烷烃膦酸中的紫精分子的取向。

The viologens, bis-quaternary salts of 4,4'-bipyridine turn to intensive blue radical cations by one-electron by photochemical reaction. With the intention to evaluate the reversible photo-reduction(photochromism), the synthesis and dichroism of viologens with alkanephophonate anions which possess relatively good nucleophilicity and anisotrophic character was investigated. N,N'-dialkyl-4,4'-bipyridinium dihalides were prepared by two steps of Menshutkin reactions with different alkyl halides and their counter ions were exchanged with alkanephosphonates using an anion-exchange resin. ^1H-NMR and elemental analyses indicated that the viologens consist of two phosphonate anions, and protons at pyridinium rings are shifted to higher field, presumably due to low electrostatic interaction with bulky bivalent phophonate anions. This causes that the phosphonate salts are thermally unstable and decompose at relative low temperature without melting.The radical formation by near-UV irradiation is small in organic solvents possessing high ionization potential. The photo-reduction in high-diluted solvents possessing electron-donating property (low IP) mainly proceeds by the ET-reaction from surrounding solvent molecules. The radical cations in glass sandwiched PVC-films were observed by steady state UV-irradiation, and the color of the radical cations persisted for several days. IR-spectra indicate that the photo-induced reduction of solid phosphonate salts proceeds from counter anion within a viologen molecule. When films were prepared by gradually cooling the melted viologen-bisphosphonate with alkanephosphonic acids, the absorption at 530 nm corresponds to the dimeric radical cation increased compared with rapidly cooling ones. While the absorption difference was small, the films of viologen-bisphosphonate salt showed dichroism by the orientation of viologen molecules in anisotropically organized alkanephosphonic acids.