Photoinduced Electron Transfer at Liquid|Liquid Interfaces:  Dynamics of the Heterogeneous Photoreduction of Quinones by Self-Assembled Porphyrin Ion Pairs

Abstract

The initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the water|1,2-dichloroethane (DCE) interface have been studied by ultrafast time-resolved spectroscopy and dynamic photoelectrochemical measurements. Photoexcitation of the water-soluble ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS(4)(-)) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP(4+)) leads to a charge-separated state of the form ZnTPPS(3)(-)-ZnTMPyP(3+) within 40 ps. This charge-separated state is involved in the heterogeneous electron injection to acceptors in the organic phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estimations of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodynamic driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This analysis also suggests that the effective redox potential of BQ at the liquid|liquid boundary is shifted by 0.6 V toward positive potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is associated with the formation of hydrogen bonds at the liquid|liquid boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed.

Molecular electrocatalysis at soft interfaces

“…For example, we have shown that anionic zinc meso-tetrakis(p-sulfonatophenyl)-porphyrin and cationic zinc tetrakis(N-methylpyridyl)porphyrin dimers can be used as photocatalysts. [42][43][44] In the case of oxygen reduction, it could be interesting to self assemble porphyrin dimers to drive the 4-electron reduction of oxygen thereby avoiding the lengthy synthesis of cofacial porphyrin dyads. Indeed, Anson et al had shown that CoP could drive such a 4-electron reduction on solid electrodes, 45 and it should be very interesting to see if similar reactions could be observed at ITIES.…”

Photoinduced Excited State Electron Transfer at Liquid/Liquid Interfaces

“…This includes scanning electrochemical microscopy (SECM), [15][16][17][18][19][20][21][22][23][24][25][26] thin-layer cyclic voltammetry, [27][28] and spectro-electrochemical methods (some utilizing recent advances in non-linear optics). [5][6][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] 3…”

“…16,19,25,[45][46] Spectroelectrochemical methods have been used in recent years to study fast photoinduced electron transfer (PET) at the liquid/liquid (L/L) interface. [32][33][34][35][36]47 Of particular importance is extending to the L/L interface the idea of using a solvent, such as N,N-dimethylaniline (DMA) as an electron donor. 37,[48][49][50][51] The advantage of this approach is that complications due to ion transfer across the interface and to diffusion are removed.…”

Reactivity and Dynamics at Liquid Interfaces

“…These methods include scanning electrochemical microscopy (SECM), [552][553][554][555][556][557][558][559][560] thin-layer cyclic voltammetry 561,562 and spectro-electrochemical methods (some taking advantage of recent advances in non-linear optics). [563][564][565][566][567][568][569][570][571][572][573][574][575][576][577][578] In the SECM technique, one has the ability to widen the potential window and that increases significantly the range of the driving force (ΔG rxn ) for the ET reactions studied at the L/L interface. Because of that it has enabled the observation of the Marcus-inverted region (see below) and provided a reliable determination of the reorganization free energy.…”