UV-Vis and MCD Spectroscopy and TDDFT investigations into N-Confused porphyrins and properties of mono-functionalized tetraferrocenyl porphyrins in solution and on a gold surface

Abstract

N-confused porphyrin (NCP) and its externally methylated variant (MeNCP) were investigated using UV-visible and magnetic circular dichrosim (MCD) spectroscopies. In addition to evaluating the spectroscopy of the neutral compounds, the acid/base chemistry of these macrocycles was examined by the same methods. NCP exhibits two tautomeric states depending on the polarity of the solvent, and their protonation/deprotonation chemistries also differ depending on solvent polarity. DFT and TDDFT calculations were employed to evaluate the observed spectroscopic changes. Using both experimental and calculated results, we were able to determine the sites of protonation/deprotonation for both tautomeric forms of NCP. Inspection of the MCD Faraday B terms for all of the macrocycles presented in this report showed that the ΔHOMO > ΔLUMO condition is maintained in all cases and these observations were in good agreement with the DFT calculations. Various methylated N-confused tetraphenylporphyrins (MeNCTPP) were investigated using UV-Vis and magnetic circular dicrhoism (MCD) spectroscopy in polar and apolar solvents as well as the protonation and deprotonation of the compounds. The MeNCTPPs were also investigated using time-dependent density functional theory (TDDFT) methods in the gas phase. Experimentally, the MCD spectra showed that all the molecules investigated had transitions with the ΔHOMO > ΔLUMO. TDDFT results confirmed these energy gaps. TDDFT calculations also showed that nearly all low energy transitions were from the HOMO to the LUMO and LUMO+1, except in the case of 2c, 5a, and 5c. Otherwise, results show that the addition of a methyl group to the N-confused tautomer of tetraphenylporphyrin effectively regulates the proton tautomerization of the N-confused tautomer. MeNCTPPs are unaffected by the polarizability of the solvent. Two unsymmetric meso-tetraferrocenyl-containing porphyrins of general formula H2Fc3Fc(COR)P [Fc = ferrocenyl, R = -CH3 or -(CH2)5Br, P = porphyrin(2-)] have been prepared and characterized by variety of spectroscopic methods, while their redox properties were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches. Spectroscopic signature of the mixed-valence [H2Fc3Fc(COR)P ]n+ (n = 1, 3) were investigated using spectroelectrochemical as well as chemical oxidation methods and corroborated with DFT and TDDFT calculations. Inter-valence charge-transfer (IVCT) transitions in [H2Fc3Fc(COR)P ]+ were analyzed using band deconvolution analysis in the borders of Hush model. The resulting data from the mixed-valence [H2Fc3Fc(COR)P ]+ derivatives matched very closely to the previously reported MTFcP and metal-free poly(ferrocenyl)porphyrins complexes and were assigned as Robin and Day Class II mixed-valence compounds. Following previous results indicating the ability of gold supported TFcP monolayers in photo-catalytic reduction of dioxygen, self-assembled monolayers (SAMs) of a thioacetyl derivative (H2Fc3Fc(CO(CH2)5SCOCH3)P) were also prepared and characterized using UV-vis spectroscopy and CV methods. Photoelectrochemical properties of SAMs in different electrolyte systems were investigated by electrochemical techniques and photocurrent generation experiments, showing that the choice of electrolyte is critical for efficiency of redox-active SAMs.