Influence of electron-withdrawing substituents on photoelectrochemical surface phenomena at phthalocyanine thin film electrodes

The influence of electron-withdrawing substituents on the photoelectrochemi cal properties of phthalocyanines is shown in a comparison between hexadeca fluorophthalocyaninatozinc(II) (FI6PcZn) and the unsubstituted phthalocyani natozinc(II) (PcZn). The role of surface states in the photoelectrochemistr y of both materials has been investigated by time-resolved photocurrent mea surements in the millisecond range. The charging and discharging of surface states could clearly be seen as spikes at the beginning and the end of ill umination. Surface states were filled with photogenerated electrons at PcZn and with photogenerated holes at F16PcZn. In the steady state under illumi nation only cathodic photocurrents were detected at PcZn, while at F16PcZn both cathodic and anodic photocurrents were observed. An adsorption step of electroactive species prior to the charge transfer was derived from the de pendence of the steady state photocurrents on the electrolyte concentration for both materials. The concentration dependence of the charging and disch arging currents, however, showed that charge transfer from surface states t o the electrolyte occurs at PcZn, while at F16PcZn the surface states only represent recombination centres. Copyright (C) 1999 John Wiley & Sons, Ltd.