Acidity constants of β-carbolines in the ground and excited singlet states

Prototropic Transformation and Rotational–Relaxation Dynamics of a Biological Photosensitizer Norharmane inside Nonionic Micellar Aggregates

“…This leads to inherently complicated pH-dependent prototropic equilibria in these compounds in the ground as well as excited-states. ,− ,− Again it is well reported that acid–base equilibrium of a compound is also sensitive to solvent properties like the polarity, dielectric constant etc. ,− ,− The medium polarity-dependent prototropic activities of β-carboline and some of its derivatives have been reported in the literature − ,− ,− and also found in the present investigation with NHM as described in a forthcoming discussion (Section ). The representative absorption and emission spectral properties of NHM in aqueous solution as a function of medium pH as displayed in Figure S1 of the Supporting Information are found to be in good agreement with reported literatures. − ,− ,− In aqueous solution at acidic pH, NHM yields absorption band for the cationic species. The transformation from the cationic to the neutral species in alkaline pH is characterized by a blue-shift in the absorption profile, whereas at neutral pH (pH = 7.2), the absorption profile of NHM is designated by band features which are typical of both the neutral and cationic species as exemplified by Figure S1a. − ,− ,− The fluorescence emission spectra of NHM in medium of varying pH are displayed in Figure S1b.…”

“…The fluorophore NHM exhibits two distinct absorption bands centered at ∼340 nm and ∼370 nm in aqueous buffer. According to literature reports, − ,− ,− these two absorption bands are ascribed to the neutral and cationic species of NHM (Scheme ), respectively. The β-carboline systems (e.g., NHM) are well-known to undergo tautomeric equilibria in aqueous medium as a function of the medium pH. − ,− ,− The presence of different types of nitrogen atoms (namely, pyridinic and pyrrolic nitrogen atoms) within the same molecule is often invoked to lead to considerably different electronic charge density modulations on them following photoexcitation.…”

“…According to literature reports, − ,− ,− these two absorption bands are ascribed to the neutral and cationic species of NHM (Scheme ), respectively. The β-carboline systems (e.g., NHM) are well-known to undergo tautomeric equilibria in aqueous medium as a function of the medium pH. − ,− ,− The presence of different types of nitrogen atoms (namely, pyridinic and pyrrolic nitrogen atoms) within the same molecule is often invoked to lead to considerably different electronic charge density modulations on them following photoexcitation. This leads to inherently complicated pH-dependent prototropic equilibria in these compounds in the ground as well as excited-states. ,− ,− Again it is well reported that acid–base equilibrium of a compound is also sensitive to solvent properties like the polarity, dielectric constant etc. ,− ,− The medium polarity-dependent prototropic activities of β-carboline and some of its derivatives have been reported in the literature − ,− ,− and also found in the present investigation with NHM as described in a forthcoming discussion (Section ).…”

“…The β-carboline systems (e.g., NHM) are well-known to undergo tautomeric equilibria in aqueous medium as a function of the medium pH. − ,− ,− The presence of different types of nitrogen atoms (namely, pyridinic and pyrrolic nitrogen atoms) within the same molecule is often invoked to lead to considerably different electronic charge density modulations on them following photoexcitation. This leads to inherently complicated pH-dependent prototropic equilibria in these compounds in the ground as well as excited-states. ,− ,− Again it is well reported that acid–base equilibrium of a compound is also sensitive to solvent properties like the polarity, dielectric constant etc. ,− ,− The medium polarity-dependent prototropic activities of β-carboline and some of its derivatives have been reported in the literature − ,− ,− and also found in the present investigation with NHM as described in a forthcoming discussion (Section ). The representative absorption and emission spectral properties of NHM in aqueous solution as a function of medium pH as displayed in Figure S1 of the Supporting Information are found to be in good agreement with reported literatures. − ,− ,− In aqueous solution at acidic pH, NHM yields absorption band for the cationic species.…”

Fluorescence quenching of β‐carboline alkaloids in micellar media. A study to select the adequate surfactant to use in analytical techniques

“…ethanol) it is possible to observe the pale blue fluorescence corresponding to the neutral forms, but the addition of small amounts of HCl induces the appearance of the intense blue-violet fluorescence of the cation forms (24,25). In aqueous solution the emission corresponding to the cationic and zwitterionic species was explained, considering that the excitation of the neutral species produces the β-carboline cation due to the rapid proton transfer in excited state (26,27). The anionic species are originated in highly basic media due to the deprotonation of pyrrolic nitrogen (28).…”