Experimental Determination of Ionization Potentials of Organic Amines, β-Carotene and Chlorophyll a

Ionization potentials (Ip’s) of several organic compounds have been determined by the method of photocurrent measurements in gas and organic solvents. It has been found that the vapor-phase Ip’s, calculated from those determined from the onsets of the photo-currents in solutions by using Born equation for the solvation energies of the cations and a common term for the energy of the electron in solution, agree well with those measured in the vapor phase. Based on this result, the Ip’s of molecules in the gas phase can be derived from those measured in the solution. This method is useful for the determination of Ip’s of large molecules which cannot be obtained in the gas phase. Some of the Ip’s obtained are: 5.6 for 1,1,4,4-tetrakis(dimethylamino)butadiene, 5.7 for 3,3′-dimethyl-2,2′-bibenzothiazolinylidene, 6.4 for β-carotene, 6.1 for chlorophyll a, 6.3 for dihydridobis(π-cyclopentadienyl)tungsten (eV unit). The conditions of lowering the Ip’s of organic molecules are discussed.