Photoelectrochemistry vs. Time-Resolved Spectroscopy

The photoredox properties of porphyrin molecules at interfaces can be studied by time-resolved spectroscopic and photoelectrochemical techniques. A key difference between both approaches is that the latter only probes molecules located at distances from the interface not larger than the characteristic diffusion length of the excited state. According to the Einstein-Smoluchowski equation, the 

As in all potentiostatic techniques, the double layer charging is a parallel process to the faradaic reaction that can substantially attenuate the photocurrent signal at short-time scale (see Section 5.3) . This element introduces another important difference between fully spectroscopic and electrochemical techniques. Commercially available optical instrumentation can currently deliver time resolution of 50 fs or less for conventional techniques such as transient absorption. On the other hand, the resistance between the two reference electrodes commonly employed in electrochemical measurements at the liquid/liquid interfaces and the interfacial double layer capacitance provide time constants of the order of hundreds of microseconds. Consequently, direct information on the rate of heterogeneous electron injection from/to the excited state is not accessible from photocurrent measurements. These techniques do allow sensitive measurements of the ratio between electron injection and decay of the excited state under pho-tostationary conditions. Other approaches such as photopotential measurements, i.e. relative changes in the Fermi levels in both phases, can provide kinetic information in the nanosecond regime. 

These considerations emphasize that the information extracted from time resolved spectroscopy and photoelectrochemical measurements are not alternative but complementary. As discussed in Section 5, the spectroscopic information reviewed so far will allow rationalising the photocurrent signals obtained in a considerably longer time scale. Before analysing the dynamic photocurrent responses, we shall demonstrate that the photoreactions are strongly connected to the specific adsorption of the porphyrin at the liquid/liquid interface. 

Organisation of Water-Soluble Porphyrins at the Liquid/Liquid Interface 

Depending on the charge of the porphyrin species, polarisation of the liquid/liquid boundary to either positive or negative potentials will affect the concentration ratio of the ionic species in both phases. 

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