ELSEVIER Synthetic Metals 73 (1995) 239-245 Anomalous photoconductivity of ferrocene Aloke Kumar Chakraborty, Biswanath Mallik * Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032. India Received 19 December 1994; revised 21 March 1995; accepted 19 April 1995 Abstract Photoconductivity behaviour of ferrocene, a very useful metallo-organic sandwich compound, has been investigated at different constant temperatures using powdery material in a sandwich type of cell configuration and with the exposure of a polychromatic light source (mercury lamp of 125 W). Measurements with a constant d.c. bias voltage (27 V) across the sample cell and a fixed intensity of the exciting light source have shown a drastic change in the photocurrent versus time profile with the increase in temperature. Anomalous changes have been observed in the plot of the photocurrent versus reciprocal of temperature. Such changes are completely absent in the corresponding dark current behaviour. The photoinduced changes have been observed to be almost reversible in the entire temperature range. In a particular temperature range the reversibility of photocurrent is accompanied by fluctuations in equilibrium current obtained after switching off the light source. The observed anomalous changes in photocurrent have been explained by photoinduced phase transition in ferrocene. The possible origin and implications of this photoinduced phase transition are discussed. Keywords: Ferrocene; Conductivity; Photoinduced phase transition; Kinetics 1. Introduction Ferrocene (bis(cyclopentadieny1) iron; (C,H,),Fe; abbreviated as FcH), the first synthesized and perhaps the best known metallo-organic sandwich compound, is well known for its important electrochemical, photochemical and photophysical properties [ 141. This material has wide appli- cations in various fields, e.g., it has been used (i) efficiently as mediators in various electron transfer processes [ 21, (ii) in the development of biosensors [ 21, (iii) for the synthesis [ 51 of new materials of higher electrical conductivity from poly(vinylidine chloride) and (iv) for the preparation of charge transfer compounds with organic solids [ 4,6]. There is a renewed research interest in the semi- and photoconduc- tive properties of this material [ 7-91. Recent studies [ 91 on the dark and photoconductive properties of FcH have offered some interesting results. One striking observation is that the nature of the distribution of traps in FcH determined from the photoconductive studies using a polychromatic light source (mercury lamp of 125 W) agrees with results obtained from the dark conductivity measurements. In the course of the photoconductivity studies, we have recorded some interesting new observations regarding the photoinduced changes in pure FcH, which are presented in this paper. * Corresponding author. 0379-6779/9.5/.$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI0379-6779(95)03341-G 2. Experimental High purity microcrystalline FcH (in powder form) was obtained from the Aldrich Chemical Co. (Milwaukee, WI, USA) and was used after further purification by repeated crystallization. The experiment was performed by the usual sandwich-cell technique [ 10-I 31. The pure dry powdery material ( 10 mg) was placed between a conducting glass and a stainless-steel electrode, and kept inside a chamber made of brass and fashioned with Teflon having a quartz window at the top for photoconductivity studies. Teflon spacers, 2 mil (0.005 08 cm) thick, maintained the separation between the electrodes. To maintain the sandwich cell, two spring clips were attached at a moderate pressure (about 0.035 MPa) at the two sides of the electrodes. The, sample cells were pre- pared in air in safe light illumination. All the measurements were done in controlled dry nitrogen atmosphere. Before the conductivity measurements, desorption of water vapour, oxy- gen or any other pre-adsorbed (if adsorbed during sample preparation) vapours from the sample cell was ensured by repeated heating and cooling treatments of the cell, initially in vacuum and finally in dry nitrogen gas atmosphere. The bias potential 27 V (d.c.) across the electrodes was applied from the voltage source of a programmable electrometer (model 617, Keithley Inst. Inc., Cleveland, OH, USA) and the current was registered by the same electrometer. The data