Excited state proton transfer in β-carboline

Sakurovs, Richard; Ghiggino, K. P.

doi:10.1016/0047-2670(82)80002-6

Cited by 74 publications

(59 citation statements)

References 9 publications

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“…This is expected with the bimolecular quenching of S 1 by O 2 . Indeed, these compounds have an S 1 lifetime (~20-25 ns) [6][7][8] that is long enough for O 2 -induced deactivation to play a prominent role.If O 2 indeed induces S 1 !T 1 ISC, one would then expect to see a corresponding increase in the triplet state yield. This was not the case.…”

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confidence: 99%

Fluorescence Quenching by Oxygen: “Debunking” a Classic Rule

et al. 2010

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It has long been accepted that oxygen-induced deactivation of an organic molecule's fluorescent state (i.e. the lowest excited singlet state, S 1 ) proceeds exclusively via intersystem crossing (ISC) to produce the triplet excited state, T 1 . [1][2][3][4] Oxygeninduced S 1 !S 0 internal conversion (IC) to produce the singlet ground state, S 0 , is thought not to occur. In a light-filled world where S 1 generation is commonplace and where molecular oxygen can be found at appreciable concentrations, this phenomenon can have significant implications. Among other things, it means that much O 2 -dependent photochemistry derives from the T 1 state and, as such, can yield unique and characteristic products.To our knowledge, only 9,10-diphenylanthracene has been presented as an exception to the rule that O 2 always induces S 1 !T 1 ISC.[5] Even in this case, however, the data are not as convincing as one might otherwise desire. We recently suggested that b-carbolines, bCs, may provide substantive evidence that an O 2 -induced S 1 !S 0 transition can actually occur. [6] We now demonstrate that the O 2 -dependent photophysics of selected bCs indeed provide an "exception to the rule" and that O 2 -induced S 1 !S 0 IC can efficiently compete with O 2 -induced S 1 !T 1 ISC. The present data, point to the possibility that other molecules will respond to O 2 like the bCs, limiting T 1 -derived chemistry or behavior which, in turn, opens new options for the design and application of unique photochemical, photophysical, and photobiological systems.The bCs used, norharmane (nHo, R 1 = R 2 = H), harmane (Ho, R 1 = H, R 2 = CH 3 ) and N-methylharmane (NMeHo, R 1 = R 2 = CH 3 ), Scheme 1, are water soluble compounds of significant biological importance whose behavior depends on pH.[6] Although the S 1 !S 0 phenomenon presented herein is observed in both alkaline and acidic media, it is more pronounced in the latter and, as such, we limit our discussion to data recorded at pH 5. We also limit our discussion to Ho; data for nHo and NMeHo can be found in the Supporting Information.For Ho, the fluorescence quantum yield, F F , decreases as the concentration of dissolved oxygen increases (Figure 1 a). This is expected with the bimolecular quenching of S 1 by O 2 . Indeed, these compounds have an S 1 lifetime (~20-25 ns) [6][7][8] that is long enough for O 2 -induced deactivation to play a prominent role.If O 2 indeed induces S 1 !T 1 ISC, one would then expect to see a corresponding increase in the triplet state yield. This was not the case. Rather, we observed a distinct and pronounced decrease in the T 1 yield as the O 2 concentration was increased (Figure 1 b). The latter data were recorded in time-resolved ab- [a] Dr.

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Fluorescence Quenching by Oxygen: “Debunking” a Classic Rule

et al. 2010

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“…[7][8][9][10] Also, time-correlated techniques have been employed to examine the fluorescence decays of norharmane in water as a function of pH. [11][12][13] An analysis of the excited state interconvertion kinetics between tautomers and an evaluation of the rate constants were presented.…”

Section: Introductionmentioning

confidence: 99%

β-Carbolines. 2. Rate Constants of Proton Transfer from Multiexponential Decays in the Lowest Singlet Excited State of Harmine in Water As a Function of pH

et al. 1996

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The -carbolines present a complex problem involving multiple equilibria in the excited state in hydrogenbonding solvents including water. Three excited state species exist: neutral, cation, and zwitterion. Here we examine the multiple equilibria and excited state kinetics of harmine, using time-resolved and steady state fluorescence techniques. From an analysis of the multiexponential decays, measured at the emission wavelengths of the three species as a function of the pH, seven unknowns (four rate constants and three reciprocal lifetimes) were determined. Data analysis was made both by a previously reported numerical method and by analytical solution of the differential equation set. The results obtained accurately describe the independently obtained steady-state fluorescence results. The dramatic modifications of the equilibria and rate constants between the ground and excited states can be understood on the basis of the significative changes in charge densities on the two nitrogen atoms of harmine upon excitation. Mechanisms are proposed for the formation of excited state cation and zwitterion beginning with the excited state neutral molecule.

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“…In recent years the influence of the pH on the electronic absorption and fluorescence emission spectra of the beta-carboline alkaloids has increasingly been studied, mostly because of the biological interest of many of them [1][2][3][4][5][6], and a wealth of information on this subject is now available [7][8][9][10][11][12][13][14][15][16]. Recently, we have reported a study on the fluorescence characteristics of some beta-carboline derivatives in moderately and highly concentrated hydroxide solutions [17].…”

Section: Introductionmentioning

confidence: 99%

Fluorescence of harmol, harmalol and 2-hydroxycarbazole in concentrated hydroxide solutions

Hidalgo

Carmona

Balón

et al. 1990

Pharmaceutisch Weekblad Scientific Edition

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Room temperature electronic absorption and fluorescence spectra of harmol, harmalol and 2-hydroxycarbazole have been obtained in concentrated aqueous potassium hydroxide solutions. The appearance of a new fluorescence band for all these compounds in media of H- greater than 16, has been ascribed to the emission of excited dianions formed by deprotonation. Acidity constants have been estimated from the Föster-Weller method.

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Excited state proton transfer in β-carboline

Cited by 74 publications

References 9 publications

Fluorescence Quenching by Oxygen: “Debunking” a Classic Rule

Fluorescence Quenching by Oxygen: “Debunking” a Classic Rule

β-Carbolines. 2. Rate Constants of Proton Transfer from Multiexponential Decays in the Lowest Singlet Excited State of Harmine in Water As a Function of pH

Fluorescence of harmol, harmalol and 2-hydroxycarbazole in concentrated hydroxide solutions

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