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

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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.