.beta.-Carboline photosensitizers. 1. Photophysics, kinetics and excited-state equilibria in organic solvents, and theoretical calculations

Abstract

Four &carbolines have been examined regarding absorption, emission, and excited-state equilibria in several classes of organic solvents. In hydrocarbon and nonprotic solvents, excitation results only in an excited-state neutral tautomer and fluorescence. In a neutral protic solvent (methanol), excitation of the neutral tautomer (except harmaline) results in multiple excited-state tautomers-neutral, cation, and zwitterion-and emission. The excited cation and zwitterion are formed from the neutral in and during the lifetime of the lowest ( r , r * ) singlet state. A kinetic model has been developed for the complex decays, and an analysis is presented for the neutral protic solvent case. Absorption, emission, and kinetic decay data for the cations are also presented. Only a single excited-state tautomer exists and emits independent of the solvent. Theoretical calculations are carried out regarding (1) state orders and (2) ground-and excited-state charge densities, the latter of which helps rationalize the existence of multiple excited-state tautomers. The impact of the results on photosensitizer aspects of the compounds is given.

β-Carbolines That Accumulate in Human Tissues May Serve a Protective Role against Oxidative Stress

“…The difficulty encountered in reconciling these two properties might lie in the fact that in a protic environment, particularly water, the fully aromatized compounds can exist in multiple states in equilibrium, namely neutral, cationic, and tautomeric both in the ground and excited states (36); the principal absorbing tautomer may be quite different from the principal emitting or photoactive state. Also, in molecules with reduced conjugation such as IIa, energy level calculations and fluorescence measurements (value of the parameter o ϭ / F ) indicate that the order of electronic energy levels changes, making the 1 (n*) state lower than the 1 (*) state, although the former transition is not observed since it is forbidden.…”

“…The emission quantum yields of these were measured to be 0. 27 (36), and suggest that in these cases too, the lowest state may be 1 (n*). Semiempirical molecular orbital calculations, using the Dewar approach (34), support this possibility though, as expected, the n* transitions are not seen.…”

Solid-phase fluorescence and ionization efficiency in negative-ion matrix-assisted laser desorption/ionization of neutral oligosaccharides: Interaction between β-carboline matrix and ammonium salt

“…The liquid-phase UVabsorption spectra of harmine were measured in methanol and in HCl-acidic methanol solution (Figure 6). Since the UV-spectra of harmine and harmine hydrochloride just dissolved in methanol are the same (data not shown), we measure the harmine in the HCl acidic methanol solution to obtain the spectrum of the protonated harmine form [29]. The absorption at 300 nm in harmine was shifted to close to 337 nm in the spectrum of harminium, a value which is the wavelength of the N 2 laser used in the MALDI-MS. From the liquid-phase data, the matrix of Harmaline ϩ 100 mM NH 4 harmine with ammonium chloride and its hydrochloride salt are excited by the MALDI laser more effectively than the harmine only.…”

“…Thus, we cannot relate these results to the ionization efficiency directly. In contrast, harminium in methanol absorbs 337 nm light more than harmine [29]. The liquid-phase UVabsorption spectra of harmine were measured in methanol and in HCl-acidic methanol solution (Figure 6).…”

β-Carboline Alkaloid Constituents from aThermoactinomyces SP.Strain Isolated from Livingston Island, Antarctica

“…The pyrrolic nitrogen is less basic than the pyridinic one. The latter becomes even more basic upon excitation (4,5); its basicity also depends on the presence of substituents (10). Some of the more important β-carbolines are: β-carboline, tryptoline, pinoline, harmane, harmine, harmaline and tetrahydroharmine.…”