4244
J . Phys. Ckem. 1985, 89, 4244-4249
Electron Mobility, Ion Yields, and Photoconductivity in Liquid Tetrakis(dimethylamino)ethylenet Richard A. Holroyd,* Stanton Ehrenson, and Jack M. Preses Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 (Received: May 7 , 1985)
Excess electrons in tetrakis(dimethy1amino)ethylene (TMAE) are observed to have a mobility of 2.2 cm2/(V s) at 20 'C, which is a factor of 100 higher than the mobility of electrons in other amines. This high value is attributed to the symmetry of the molecule and the presence of many methyl groups. The conduction band energy, V,, is determined from the ionization threshold at 326 nm and the polarization energy. The latter was determined experimentally to be -1.33 eV and theoretically by a multipole extension of the Born equation to be -1.5 eV. Based on the latter value Vois --0.1 eV for TMAE. The relationship of mobility and Vo,Le., that the mobility increases as Vodecreases, appears to apply to amines. Ion yields for exposure of TMAE to both X-rays and UV light are reported. The yield of fluorescencedecreases, and the ion yield increases, with decreasing wavelength and is not detectable below 240 nm. At longer wavelengths the lifetime of the excited state was determined to be 14.1 ns.
Introduction Previous studies have indicated that excess electrons in liquid amines are solvated, absorb in the infrared region, and exhibit low mobility.' For example, solvated electrons have been detected in n-propylamine; the absorption maximum2 is at 1900 nm, and the mobility is 1.3 X cm2/(V s).l Ethylamine is similar in these properties, but for tributylamine and n-butylamine even lower values of the electron mobility have been r e p ~ r t e d . ~These mobilities were derived indirectly; that is, drift time measurements were not made. Therefore, the suggestion4 that excess electrons may exhibit high mobility and that the conduction band energy (V,) may be significantly negative in tetrakis(dimethy1amino)ethylene (usually denoted as TMAE) contrasts with the known behavior of electrons in the amines investigated so far. On the other hand, it is conceivable that amines, like alkanes, may support a range of mobilities depending on their molecular structures. That is, solvated electrons are observed in n-hexanes and their mobility is low, yet in more symmetrical alkanes with many methyl groups electrons are not solvated and the mobility is high. By analogy, we might therefore expect symmetrical nonpolar amines with many methyl groups to exhibit high electron mobility. The present study was initiated to test this interesting possibility and to provide additional information about the relationship of molecular structure to the properties of excess electrons in liquids. Initially we were not certain that a drift mobility measurement would be possible, since no such measurement had been made for an amine. Therefore, the threshold of conductivity (Eth)of liquid TMAE was measured to ascertain the conduction band energy ( Vo),which would allow a prediction of the mobility from the known correlation of mobility with V0.6-7The drift time measurement was in fact done, which allowed a test of this correlation for amines. The energy of the conduction band can be measured in several ways. Determination of the work function of a metal immersed in the liquid8 is inappropriate here since TMAE absorbs UV light strongly. Furthermore, TMAE was reported to have a low conductivity threshold,' which would interfere with measurements of photoinjected currents. The method employed involved determination of the threshold (&) of conductivity of neat TMAE and calculation of Vo from eq 1, where I P is the gas-phase ionization potential (5.36 eV)Io and pTMAE is the abiabatic, electronic polarization energy of the medium (TMAE) resulting from ionization of the neutral to TMAE'. This latter energy term was deduced both by theory and by experiment. The theoretical +Denoted as octamethylethenetetramine in Chem. Abstr.
0022-365418512089-4244$01.50/0
calculation employed multipole corrections to the Born approximation for molecular ions (TMAE') where significant charge dispersal to substituent alkyl amine groups is expected.]' The experimental method involved measuring photoconductivity thresholds for TMAE in solvents with known V
