The dielectric constant, and the ferroelectric, pyroelectric, and piezoelectric properties of thiourea crystals have been measured in the temperature range 90°K to 300°K. At least three dielectric anomalies are found at 169°K, 177°K, and 202°K, the lowest of these corresponding to a pronounced discontinuity. The crystals are ferroelectric in two regions, below 169°K and between 176°K and 180°K. Substitution of deuterium for hydrogen causes the anomalies to move upwards in temperature by 16°, 16°, and 11°, respectively. The crystal structure has been determined in detail at 120°K in the lower ferroelectric region. The transition from the antiferroelectric room temperature structure to the ferroelectric state is accomplished by small rotations of the molecules such that two of the molecules in the crystal unit cell have tilts to the ferroelectric b axis appreciably different from the other two, and the resultant of the molecular dipoles along [010] is no longer zero. The ferroelectric reversal is thus easily accomplished by interchanging the tilts of the two pairs of molecules.

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The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H z Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0-5 and 0.6eV for H2Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0'3 and 0-2eV. An energy level scheme for single crystals of H:Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H2Pc (CuPc), one trapping level at 0-5 eV (0-6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0-3 eV (0-2eV) below the conduction band edge where all the optical transitions terminate. In H 2 Pc (CuPc), the forbidden gap is 1.44eV (1.34eV) wide; the five valence bands are at the band edge, and 0"09 (0-22), 0-42 (0"63), 0"69 (0"90), 1-32 (2-17) eV below the band edge.

The absorption spectrum of naphthacene vapor is similar to that in solution. There is an absorption band with pronounced vibrational structure running from about 5000 to 3500 Å and a second stronger band from 2700 Å to shorter wavelengths. We have examined the fluorescence spectrum of the vapor at low pressures, such that molecules have no collisions during the lifetime of the excited state. By selective excitation with either the 3650 Å line or the 2537 Å line of the mercury lamp it is possible to excite molecules into either the first excited singlet state or into the second excited singlet state. The fluorescence emission appears to be from the first excited singlet state in both cases even though the molecules suffer no intermolecular collisions in the excited state.

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