Wavelength-dependent switching of the photocurrent direction at the surface of molecular semiconductor electrodes based on orbital-confined excitation and transfer of charge carriers from higher excited states

Abstract

Photoelectrochemical experiments are performed at evaporated thin films of unsubstituted phthalocyanines (Pc) and derivatives with electronegative substituent groups in the ligand. Light absorption in the B-band leads to occupation of a higher excited singlet state S 2 relative to the first excited singlet state S 1 that is populated by Q-band absorption. Charge carriers in S 2 can have sufficient lifetime to be transferred to adsorbed reactants at the electrode surface despite the competing relaxation into S 1 and to the ground state S 0. The assignment of the well-defined B- and Q-bands in the solid state to transitions between distinct molecular orbitals is thereby proven to be of practical relevance. If a material of a suitable position of energy levels is chosen, the direction of charge transfer can be switched by illumination with light of the two different wavelengths. According to the separate pathways of reaction starting from the S 2 and S 1 excited states different quantum efficiencies are obtained following absorption in either the B- or Q-band. Surface traps in the different reactions of charge transfer to the electrolyte are detected by charging and discharging photocurrent spikes. Implications of the present findings for the fundamental discussion of electrical and electrochemical properties involving illumination of molecular modified electrodes which are in part also of technical significance due to the related phenomena of photoconduction (photocopiers, laser printers), light emission (organic light emitting diodes) and charge transfer (solar energy conversion) are discussed.