Insulator-Metal Transition by Photochemical Doping Using Electron-Transfer Bond Fragmentation

四氰基喹啉二甲烷（TCNQ）具有高的电子亲和势，部分掺杂的π堆积TCNQ分子柱作为有机金属。通常，TCNQ与电子供体形成电荷转移（CT）复合物，如四硫富瓦烯，其显示出高电导率。通过光诱导电子转移（PET）反应，用牺牲给体掺杂适量的电子，可以使材料呈现出导电性。为了探索TCNQ的光化学电子掺杂，研究了TCNQ与一些给体在固相中的PET反应，四氰基喹啉二甲烷（TCNQ）和二聚体给体（D-D）的混合粉末在紫外光照射下形成TCNQ，表明TCNQ可以通过不可逆的光诱导电子转移和D-D^+的C-C断裂进行掺杂。供体（D-D）在C-C键裂解酶的作用下，经单电子氧化后迅速分解为阳离子（D^+）和自由基（D^-）。此外，氧化电位极低的自由基（D^-）可以作为TCNQ的另一种掺杂剂。然而，光谱结果表明，掺杂到TCNQ的电子可能不会在TCNQ分子柱上离域，而是定域在TCNQ^--D^+盐中;即，TCNQ晶体中的导电柱在掺杂牺牲电子供体时被破坏。用这种方法光生电导体可以通过使用自组装或非晶电子转移材料来实现。

Tetracyanoqunodimethane (TCNQ) has high electron affinity and partially doped π-stacking TCNQ molecular column acts as an organic metal. Typically, TCNQ forms charge-transfer (CT) complexes with electron donors likes as tetrathiafulvalene which show high electrical conductivity. Conductivity should be appeared if proper amount of electrons are doped by photoinduced electron transfer (PET) reactions with sacrificial donors. In order to explore the photochemical electron doping of TCNQ, the PET reaction between TCNQ and some donors in solid phase was investigated.The irradiation with UV light of mixed powder of tetracyanoqunodimethane (TCNQ) and dimeric donor (D-D) resulted in the formation of TCNQ indicating that TCNQ could be doped by irreversible photoinduced electron transfer and C-C cleavage of D-D^+. The donors (D-D) would be decomposed after one-electron oxidation into cations (D^+) and radicals (D^-) quickly by C-C bond cleavase. In addition, the radicals (D^-) with extremely low oxidation potential could act as another dopant to TCNQ.The spectroscopic results, however, indicated that electrons doped to TCNQ might not be delocalized over the TCNQ molecular column but localized in the TCNQ^--D^+ salt ; i.e., the conducting column in the TCNQ crystal was broken upon doping with sacrificial electron donors. Photo-generation of electrical conductor with this method might be achieved by use of self-assemble or amorphous electron transfer materials.