Mesomorphic and photoconductive properties of a mesogenic long-chain tetraphenylporphyrin nickel(II) complex

A tetraphenylporphinatonickel-based mesogenic compound 5,10,15,20-tetrakis( 4-n-pentadecylphenyl)porphinatonickel(II) (C15TPPNi) was synthesised and it s mesomorphic and photoconductive properties were investigated. The photoco nductive properties were compared with those of the metal-free compound. Al l measurements were carried out for symmetrical ITO/C15TPPNi/ITO type cells and steady-state currents in the constant temperature conditions were dete cted. A photocurrent rectification was seen in either the crystal phase or lamellar mesophase of C15TPPNi. The metal-free compound with pentadecyl cha ins showed a strong dependence of photocurrent rectification on mesomorphic phases: the rectification is shown in the cell of crystal and low-temperat ure lamellar phases and it disappears in the high-temperature lamellar meso phase. This phenomenon was interpreted in terms of the various mechanisms o f charged carrier generation with the phase transition, based on the result s of the action spectra and the illumination light intensity dependence of the photocurrent. The improvement of the contact for electric conduction at the interface upon heating was found, which is a special feature of this m esomorphic phase in the ITO/C15TPPNi/ITO cell in comparison with the metal- free compound.