Propriedades eletrônicas, eletroquímicas e espectroscópicas de complexos interfaciais: a coordenação de bifenilas sulfuradas sobre substratos metálicos

Abstract

In this work, theoretical, spectroscopic and electrochemical studies are shown aiming to understand the structural and electronic changes of the metallic surfaces induced by the formation of Self-Assembled Monolayers (SAMs) produced with 1’-biphenyl-4-thiol (HSBPH, μ ⃗= 1.13), 4′-mercapto-[1,1′- biphenyl]-4-carbonitrile (HSBPCN, μ ⃗=5,3 Debye) and 4,4′-dimercaptobiphenyl (HS2BP, μ ⃗= 0,46) on copper, silver and gold. Such surfaces, spontaneously modified upon immersion in the modifier solutions, showed maxima values of surface coverage fraction () of 0.96 (HSBPH), 0.99 (HSBPCN) and 0.98 (HS2BP) after 1h of modification at 20 oC. The values of of the SAMs as well as those of the heterogeneous charge transfer rate constants (kapp) of the redox probe molecule [Fe(CN)6]3/4, were determined by impedance and shown to be sensitive to the modification time. Considering the maximum of surface coverage fraction, the values of kapp were calculated as 6.8 x105, 1.3 x105 and 9.4 x106 cm s1 for the SAMs of HSBPH, HSBPCN and HS2BP on gold, respectively. The impedimetric responses in KF solution without probe molecules, indicated an increase of the surface coverage up to 5.3x1011 mol cm2 (HSBPH), 1.8x1011 mol cm2 (HSBPCN) and 1.4x1010 mol cm-2 (HS2BP). For the SAM of HSBPCN on gold, the voltammetric results within the double layer showed a meaningful decrease of the differential capacitance (8.9 F cm2) in comparison to the bare gold (35.3 F cm2). The SERS in situ spectra obtained for the SAM of HSBPCN on Cu, Ag and Au indicated the existence of a chemical mechanism of charge transfer between the frontier orbitals HOMO and LUMO of the interfacial complexes, with the transition going from the metal to the molecule for Cu and Ag, and in the opposite direction, from the molecule to the metal, for the Au substrate. According to the computational calculations, the LUMO orbitals of the interfacial complexes are composed by 44, ~3 and 60% of the ns atomic orbitals of Cu, Ag and Au, respectively. In comparison to Cu and Ag, the largest contribution of the ns atomic orbital of Au for the LUMO orbital of the complex is assigned to the relativistic effect that decrease the energy of the atomic orbital making it closer to the resulting molecular orbitals. The charge transfer transition HSBPCN  Au is, thus, justified since this process occurs between the frontier orbitals. With no exciting radiation, however, the polarity of the molecules within the SAM, established by the terminal group (electron withdrawing in the case of HSBPCN SAM), determines the interfacial properties. Indeed, based on the measurements of scanning electrochemical microscopy, the values of the rate constants of the heterogeneous charge transfer reactions (k0) indicate faster processes for the neutral (FcMeOH, k0 = 1.3 cm s1) and cationic ([Ru(NH3)6]3+, k0 > 3 cm s1) species, comparatively to the negatively charged mediators [Fe(CN)6]4- and [Fe(CN)6]3- (k0 ~1,2x104 cm s1). This result indicates that the interfacial polarity seems to be preponderant in the studied systems, taking apart from the interface the negatively charged complexes and approaching the cationic complex. This behavior suggests that the heterogeneous electron transfer reaction at the interface of HSBPCN SAM follows a nonadiabatic pathway. The applicability of the SAMs of HSBPCN, HSBPH e HS2BP on gold was evaluated in relation to the recognition/detection of D-mannose sugar after immobilization of the lectin ConBr. The voltammetric and impedimetric data, obtained in electrolytic medium containing [Fe(CN)6]3-/4-, showed an increase of the faradaic current assigned to the FeIII/II redox pair of the iron complex with the increase of the sugar concentration. Two hypotheses were considered to explain the voltammetric profiles: (1) existence of micropores and (2) interfacial dipole. The existence of micropores resulting from the possible leaching of the immobilized ConBr molecules on the SAMs implies lower resistance to the heterogeneous charge transfer reaction due to the easy access to the underlying gold surface. The role of the interfacial dipole generated by the polar species adsorbed on the substrate, in turn, justifies, more effectively, the current response associated with the heterogeneous charge transfer reaction due to the repulsive effects at the interface.