LIGHT-ABSORPTION IN THE NEAR-INFRARED AND PHOTOCURRENT SYNERGISM OBSERVED WITH VACUUM-SUBLIMED FILMS OF ZINCTETRAPHENYLPORPHYRIN

Photocurrent peaks are found in the near-infrared (NIR) as well as in the visible for zinc(LI) tetraphenylporphyrin (ZnTPP) films contacting redox solutions, in contrast to no appreciable photocurrent in the NI R for a sandwich-type cell of Al/ZnTPP/Au. The spectral responses of p hotocurrents in the NIR for the wet cells resemble that of photocurren t synergism reported earlier for the sandwich-type cell, although the ZnTPP solid is almost transparent in this wavelength region. The obser ved electronic transition in the NIR is ascribed not to a singlet-to-t riplet excitation which is commonly observed with phthalocyanine solid s, but to the absorption of the NIR light due to a trace of a ZnTPP ph otoproduct, most probably an isoporphyrin. The photoproduct, which dim inishes a generation efficiency of photocurrents by acting as a recomb ination center, is responsible for the increase in photocurrents due t o Soret band excitation by simultaneous illumination with the NIR ligh t. The difference in the Ic?NIR photoresponse between the wet cells an d the dry cell is explained on the basis of an energy-level considerat ion. (C) 1997 Elsevier Science S.A.