Mechanism of Visible-Light Photooxidative Demethylation of Toluidine Blue O

Abstract

Experiments and theoretical calculations by density functional theory (DFT) have been carried out to examine a self-sensitized type I photooxidation of toluidine blue O (TBO+). This study attempts to build a connection between visible-light photolysis and demethylation processes of methylamine compounds, such as TBO+. We show that controlled photoinduced mono- and double-demethylation of TBO+ can be achieved. The kinetics for the appearance rate of the mono-demethylated TBO+ and the double-demethylated TBO+ were found to fit pseudo-first-order kinetics. DFT calculations have been used to examine the demethylation of TBO+ and included N,N-dimethylaniline as a model compound for TBO+. The results show an oxygen-dependent demethylation process. The mechanism for the sequential methyl loss is proposed to be due to H • or e–/H+ transfer to 3TBO+* followed by a reaction of TBO+• with O2, yielding a C-peroxyTBO+• intermediate. Instead of aminyl radical peroxyl formation, i.e., N-peroxyTBO+• , the C-centered peroxyTBO+• is favored, that upon dimerization (Russell mechanism) leads to dissociation of formaldehyde from the methylamine site.

Fatty Acid Conjugates of Toluidine Blue O as Amphiphilic Photosensitizers: Synthesis, Solubility, Photophysics and Photochemical Properties†

“…It is noteworthy that a sizable blueshift was observed for the derivatives compared to TBO. The absorption maxima for TBOC6 and TBOC14 in CH 3 CN could be reproduced quite well by computational calculus in acetonitrile (38), suggesting the amidation of the amino group of TBO produces hypsochromic shifts in the absorption to higher energies (see Figs. and ).…”

“…For both derivatives, the first fluorescence lifetime is equivalent to that of TBO and is attributed to an internal charge‐transfer state based on the calculations of the HOMO‐LUMO transition (see Figs. ) (38). In contrast, the second fluorescence lifetime could be due to an aggregated state in aqueous solution, showing that TBOC14 forms less aggregates than TBOC6.…”

“…Time-dependent DFT (TD-DFT) was used to study the electronic absorption spectra for each molecular system (50,51). The functional B97D was chosen based on its good performance in a previous study for TBO by TD-DFT calculations (38). The UV-Visible spectra were obtained by singlet-singlet vertical transitions of Franck-Condon type using 30 excited states.…”

“…Detection was performed in full scan mode in 100-1000 m/z range in positive mode. The MS 2 results were performed using He as collision gas for collision-induced dissociation (CID) with a normalized collision energy of 35 units and detection of fragments in full scan mode for all samples (38). Synthesis of myristic acid derivative (TBOC14).…”

“…We have previously reported that TBO-sensitized generation of singlet oxygen can be controlled by macrocyclic supramolecular encapsulation in an "on-off" switch manner (18). Also, we recently reported a previously neglected visible-light photooxidative demethylation process for TBO that involved type I photoprocesses (38). Wang et al showed that TBO can be derivatized with biotin to increase its selectivity for cancer cells (39).…”