Surface Second Harmonic Generation of Cationic Water-Soluble Porphyrins at the Polarized Water|1,2-Dichloroethane Interface

Abstract

The adsorption behavior of the free base and the zinc(II) complex of cationic meso-tetrakis(N-methyl-4-pyridyl)porphyrins, H2TMPyP4+ and ZnTMPyP4+, were studied at the water|1,2-dichloroethane (DCE) interface by surface second harmonic generation (SSHG) under electrochemical control. The examined porphyrins exhibited specific adsorption features at the water|DCE interface. The surface second harmonic (SH) spectrum and the polarization analysis of SH signals were performed to characterize the interfacial species. Although symmetrically substituted porphyrins show a rather small nonlinear optical response in general, the adsorbed species generated intense SH signals suggesting that the electronic structure of molecules adsorbed at the interface is modified from the bulk state. The surface SH spectrum for H2TMPyP4+ showed similar features to the absorption spectrum in the aqueous phase and the orientation parameter was independent of the bulk concentration. The adsorption equilibrium of H2TMPyP was measured at 295 K, and the Gibbs free energy of adsorption was determined as −29 ± 1 kJ mol-1. On the other hand, the surface SH spectrum of the zinc(II) complex system exhibited significant differences from the corresponding bulk absorption spectrum. The surface SH spectrum of ZnTMPyP showed the maximum intensity at 436 nm coinciding with the bulk aqueous species and, furthermore, the additional response centered at 452 nm. The dependence of the SH intensity on the applied potential suggested that the SH response at 452 nm is associated with the J-aggregation of ZnTMPyP monomers adsorbed at the interface.

Interfacial behavior of sulforhodamine 101 at the polarized water/1,2-dichloroethane interface studied by spectroelectrochemical techniques

“…In cases in which the monomer is preferentially adsorbed at the interface, the number density of monomers in the interfacial region is much higher than in the bulk solution, and the dimerization would occur more easily at the interface. The molecular association of ionic dyes at the liquid/liquid interface has been observed without formation of the associate in the bulk phase using surface-sensitive nonlinear spectroscopy [6,14,30]. Therefore, the spectroelectrochemical responses around −0.32 V possibly correspond to the formation and adsorption of the anionic dimer SR101 ð Þ 2À 2 even though the dimer formation was not observed in the bulk phase.…”

“…The analysis of electrochemical responses in the presence of surface-active species is complicated due to the primitive understanding of the potential distribution across the interface. Considerable efforts have been devoted to accessing the potential dependence of adsorption by means of spectroscopic techniques such as surface second-harmonic generation [3][4][5][6] and potential-modulated spectroscopy [7][8][9][10]. Potential-modulated fluorescence (PMF) spectroscopy has provided detailed information on the dynamics of charge transfer and adsorption as a function of the Galvani potential difference across the liquid/liquid interface.…”

Reactivity and Dynamics at Liquid Interfaces

“…at the water/DCE interface, and compared them to the UV-vis spectra of these two species in the bulk of the two liquids. 369 Although the molecular hyperpolarizability of the symmetrically substituted porphyrins is very small, 370 the adsorbed species give rise to intense SH signals, indicating that its electronic structure at 67 the interface is modified relative to the bulk structure.…”

Artificial Photosynthesis at Soft Interfaces

“…Photoreduction hv E le c t r o n t r a n s fe r c h a in cial molecules as a function of the applied potential; [7] -quasi-elastic light scattering to measure in situ the interfacial tension, thereby measuring the electrocapillary curves and determining the surface excess concentrations of adsorbed species, including nanoparticles. [8] In addition to spectroscopy techniques, we have developed methods based on Scanning Electrochemical Microscopy (SECM).…”