Photophysical Properties of Coumarin-500 (C500):  Unusual Behavior in Nonpolar Solvents

Abstract

Photophysical properties of 7-NHEt-4-CF 3 -1,2-benzopyrone dye (coumarin-500, C500), have been investigated in different solvents and solvent mixtures using optical absorption and fluorescence measurements. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 2-methylpentane, 3-methylpentane, and Decalin, the dye displays unusual properties in comparison to those in other solvents. Thus, in the former solvents, the Stokes' shifts ∆ν j and the fluorescence lifetimes (τ f ) for C500 are found to be unusually low. Though in these solvents the fluorescence quantum yields Φ f are seen to be only marginally higher, the radiative decay rate constants k f are found to be unusually higher than those in the other solvents. From comparing the results of C500 with those reported earlier for its lower analogue, C151 (coumarin-151, 7-NH 2 -4-CF 3 -1,2-benzopyrone; Nad, S.; Pal, H. J. Phys. Chem. A. 2001, 105, 1097, it is inferred that just like C151, in nonpolar solvents the dye C500 also exists in nonpolar structures, whereas in all other solvents the dye exists in a polar intramolecular charge transfer (ICT) structure. Unlike C151, the C500, however, does not show any activationcontrolled nonradiative deexcitation channel for the S 1 state in nonpolar solvents, which arises due to the flip-flop motion of the 7-amino group. Due to the heavier mass, the flip-flop motion of the 7-NHEt group in C500 is supposed to be much slower than that of the 7-NH 2 group in C151. Thus, such a motion cannot introduce any deexcitation channel, which can compete with the intrinsic fluorescence k f and the internal conversion k IC rates in the excited C500 dye. Thus, unlike C151, there is no activation-controlled nonradiative deexcitation channel for C500 in nonpolar solvents though both the dyes appear to exist in similar structural forms in these solvents, which are different than those existing in other solvents of lower to higher polarities.

Lifetime determination of materials that exhibit a stretched exponential luminescent decay

“…At 10 MHz a phase shift of À16.91 was obtained, corresponding to a lifetime of 4.8470.2 ns. The literature value for the lifetime of Coumarin 500/methanol is 4.83 ns [17]. Although this close agreement is fortuitous, the low noise in the averaged signals can be seen in the Lissajous plot of Fig.…”

Ultraviolet and yellow reflectance but not fluorescence is important for visual discrimination of conspecifics by Heliconius erato

“…We were therefore surprised that the quantum yield of 3-OHK in methanol indicated that the emission is unlikely to be visible under normal illumination (quantum yield=5.1×10 −4 ). By contrast, the quantum yield of our standard Coumarin 500 was 0.46 (Nad and Pal, 2003) or nearly a thousand times brighter than 3-OHK under similar conditions.…”

“…In order to measure the efficiency of this emission, and hence understand whether the fluorescence might contribute significantly to the signal, we determined the fluorescence quantum yield of 3-OHK. Quantum yield is characterized as the ratio of the number of photons emitted to the number of photons absorbed (Williams et al, 1983;Nad and Pal, 2003). Quantum yield of 3-OHK was obtained by comparing 3-OHK with a standard and well-characterized fluorescent molecule, Coumarin 500 (Dhami et al, 1995), which has similar absorbance and fluorescence peaks to 3-OHK (Fig.…”

Reply to ‘Comment on “Measuring the absolute quantum efficiency of luminescent materials”’

“…To bolster the 92% claim they cite papers by Kubin and Fletcher [4] and Jones et al, [5] but CM500 is not studied in these papers. Nad and Pal [6] is similarly cited, but they report QY ¼ 46%. Because A&W used CM500/MeOH as a QY standard during their tenure at SNL, the 92% value would have inflated their reported QY values.…”