The Interaction of Ground and Excited States of Lumichrome with Aliphatic and Aromatic Amines in Methanol

Abstract

Dedicated to Prof. Andre¬ M. Braun on occasion of his 60th birthdayThe reaction of the ground and excited states of lumichrome ( 7,8-dimethylalloxazine 7,8-dimethylbenzo[g]pteridine-2,4(1H,3H)-dione) with aliphatic and aromatic amines was investigated in MeOH. In the presence of aliphatic amines of high basicity, new bands are observed in the absorption and fluorescence spectra. These bands arise in a proton-transfer reaction from lumichrome, in the ground and in the singlet excited states, to the amine. On the other hand, amines with lower basicity such as triethanolamine ( 2,2',2''-nitrilotris- [ethanol]) and aromatic amines are not able to deprotonate lumichrome, and hence a quenching of the fluorescent emission takes place without changes in the spectral shape. In this case, bimolecular-quenching rate constants were determined for the excited singlet and triplet states. Based on laser-flash-photolysis experiments, an electron-transfer mechanism is proposed. Aliphatic amines yield lower rate constants than the aromatic ones for the same driving force. A notable difference arises in the limiting value reached by the singlet and triplet quenching rate constants by aromatic amines. For the singlet quenching, the limit is coincident with a diffusioncontrolled reaction, while those for triplet quenching reach a lower constant value, independent of the driving force. This is explained by an electron-transfer mechanism, with a lower frequency factor for the triplet-state process.

Electron transfer quenching of singlet and triplet excited states of flavins and lumichrome by aromatic and aliphatic electron donors

“…An electron transfer reaction was previously suggested for the quenching of flavins and LC by different electron donors. 4,5,12 Therefore, a correlation of the quenching rate constants with the Gibbs energy change for the electron transfer process, DG , is expected. In Figs.…”

“…Recently we have reported that the rate constant for triplet quenching of LC by triethanolamine is lower than the corresponding values for aromatic amines of similar DG . 12 It is of interest to extent this comparison to other aliphatic amines. The electron transfer quenching of LC cannot be investigated for aliphatic amines of high basicity, due to the proton transfer process leading to the anionic form of LC.…”

Photophysics, Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

“…Although extensive studies have been performed on the spectral and photophysical properties of a series of isoalloxazines (benzo[g]pteridine‐2,4(3 H ,10 H )‐diones) and alloxazines (benzo[ g ]pteridine‐2,4(1 H ,3 H )‐diones), including lumichrome (7,8‐dimethylbenzo[ g ]pteridine‐2,4(1 H ,3 H )‐dione) , there is very little data on such studies on 5‐deazaalloxazines.…”

Absorption and Emission Spectroscopic Characterization of Lumichrome in Aqueous Solutions†

“…Photophysical properties of lumichrome and lumichrome derivatives in organic solvents were investigated in (12,22–27). The molecule has several active sites for hydrogen bonding (N1, O2, N3, O4, N5, N10) with appropriate hydrogen bond forming solvents like water (12,25,27–29), pyridine (12,23,25,27), carboxylic acids (12,25,27), phosphoric acids (30), aliphatic and aromatic amines (22).…”

“…1) thereby changing the lumichrome‐like emission to an isoalloxazine‐like emission (7,12,23,25–35). The phototautomerization was observed for lumichrome in pyridine (27), glacial acetic acid (27), methanol‐acetic acid (34), ethanol‐acetic acid (28,29,31,32), dioxane‐pyridine (23,26,31,32), water‐pyridine (27), acetone‐water (27), 1,2‐dichloromethane‐hexafluoropropanol (25), trifluoroacetic acid (25), acetic acid (25), methanol‐aliphatic amine, and methanol‐aromatic amine mixtures (22). It was found for N(3)‐undecyllumichrome in dodecylammonium propionate reversed micelles (33).…”