Fluorescence Characteristics of Methoxycoumarins as Novel Fluorophores

Abstract

The fluorescence characteristics of various methoxycoumarin fluorophores for the development of fluorescence reagents were examined in relation to their structures. The fluorescence emission mechanisms were also considered from the viewpoint of the intramolecular charge-transfer (CT) between methoxyl substituents and the coumarin ring. The arrangement of 6-methoxyl and 3-acetyl group pairs on the coumarin ring significantly contributed to the fluorescence enhancement through the intramolecular CT. We found that the structural features of methoxycoumarins required for intense fluorescence are to hold both diether bonds at the 6-and 7-positions and an electron-withdrawing group at the 3-position, as shown in 3-acetyl-6,7-dimethoxycoumarin with 0.52 in quantum yield.

Improvement of Fluorescence Characteristics of Coumarins: Syntheses and Fluorescence Properties of 6-Methoxycoumarin and Benzocoumarin Derivatives as Novel Fluorophores Emitting in the Longer Wavelength Region and Their Application to Analytical Reagents

“…Among them, benzocoumarin derivatives especially fluoresced in the longer wavelength around 540 nm with remarkably large Stokes shifts beyond 10000 cm donating groups at the 6-and 7-positions and an electronwithdrawing group at the 3-position develop intense fluorescence, [19][20][21] as shown in 3-acetyl-6,7-dimethoxycoumarin with a quantum yield of 0.52 and a large Stokes shift of 5200 cm Ϫ1 in methanol. 22) In previous studied, we examined in detail the fluorescence characteristics of coumarins from the viewpoint of the substituent effect in connection with two ICT in the coumarin skeleton and postulated that one ICT from an electron-donating group at the 6-position to an electron-withdrawing group at the 3-position contributed to the fluorescence wavelength and the other ICT from an electrondonating group at the 7-position to an electron-withdrawing lactone carbonyl group at the 2-position contributed to the fluorescence intensity. 22,23) Based upon the above hypotheses, only the former ICT may be allowed to shift the emission wavelength of coumarin to the longer wavelength region.…”

“…22) In previous studied, we examined in detail the fluorescence characteristics of coumarins from the viewpoint of the substituent effect in connection with two ICT in the coumarin skeleton and postulated that one ICT from an electron-donating group at the 6-position to an electron-withdrawing group at the 3-position contributed to the fluorescence wavelength and the other ICT from an electrondonating group at the 7-position to an electron-withdrawing lactone carbonyl group at the 2-position contributed to the fluorescence intensity. 22,23) Based upon the above hypotheses, only the former ICT may be allowed to shift the emission wavelength of coumarin to the longer wavelength region. Thus 6-methoxycoumarin derivatives with various substituents at the 3-position were synthesized and their fundamental fluorescence behaviors were investigated spectrophotometrically.…”

“…These large shifts in the fluorescence spectra in the fluorescence spectra may be due to the strong electronwithdrawing ability of a newly introduced functional group, which has two carbonyl groups at the 3-position of the coumarin skeleton due to the derivatization reaction. Al- though a fluorescence quantum yield for 2c in methanol has already been reported to be 0.25 (RFIϭ149 in Table 2), 22) those of 2b and 3b and c appear to be high as well. On the other hand, the RFIs of 3d-f obtained using reagent 3a were slightly low as compared with the above compounds, because thir electron-withdrawing ability at the 3-position were reduced by an amide group.…”

“…The results were in accordance with our hypothesis that an ICT from a methoxyl group at the 6-position to a substituent such as ethoxycarbonyl and acetyl groups at the 3-position greatly contributes to the fluorescence wavelength. Although the fluorescence quantum yields of 6-methoxycoumarins were lower than those of fluoresceins and BODIPYs, as shown in 1d with a quantum yield of 0.02 in acetonitrile, 22) their large D n values appear to be useful for fluorometric analysis. To confirm this hypothesis, we tried to synthesize novel fluorescence derivatizing reagents based on the 6-methoxycoumarin skeleton and investigated the fluorescence properties of the compounds derivatized by such reagents.…”

“…[19][20][21] In previous papers, we reported the fluorescence characteristics of methoxycoumarins and discussed the structural features of strongly fluorescing methoxycoumarins from the viewpoint of intramolecular charge transfer (ICT) between push-and pull-substituents in the ground and the excited states. 22,23) Based on the above findings, we succeeded in developing novel fluorescent reagents with both high sensitivity and functions such as self-catalytic reactivity. [24][25][26] In the course of those studies, we suspected that most coumarin derivatives showed large Stokes shifts in their fluorescence spectra as compared with other fluorophores.…”