Femtosecond Spectroscopic Study of Relaxation Processes of Three Amino-Substituted Coumarin Dyes in Methanol and Dimethyl Sulfoxide

Abstract

Time-resolved fluorescence spectra of three amino-substituted coumarin dyes have been recorded in methanol and dimethyl sulfoxide using the fluorescence upconversion technique with an apparatus response function of ≈200 fs fwhm. The three fluorinated coumarins are the 7-amino-4-trifluoromethylcoumarin (C151), the 7-diethylamino-4-trifluoromethylcoumarin (C35), and the rigidified aminocoumarin with a julolidine structure (C153). The dynamic Stokes shifts are found to be dominated by an ultrafast component with a characteristic time shorter than the present time resolution of ≈50 fs. The dynamic Stokes shifts are compared to estimations based on a “Kamlet and Taft” analysis of steady-state data in 20 solvents. It is found that the ultrafast component can be assigned mainly to intramolecular relaxation. The influences of photoinduced changes of solute−solvent hydrogen bonds on the observed spectral shifts are discussed. The breaking of hydrogen bonds at the amino group is very fast in both solvents and embedded in the ultrafast solvent inertial relaxation, while the reformation of hydrogen bonds at the carbonyl group is believed to occur on the 10−20 ps time scale in the hydrogen bond donating (HBD) solvent methanol. However, it is impossible to unambiguously correlate a particular experimental time constant with the breaking or the formation of a hydrogen bond.

Fluorescence of the DNA Double Helix (dA)20·(dT)20 Studied by Femtosecond SpectroscopyEffect of the Duplex Size on the Properties of the Excited States

“…9 The above mentioned studies dealing with DNA duplexes have been performed for model helices composed exclusively by one type of base-pair, adenine-thymine. In 2003, we reported the fluorescence decay of the oligomer (dA) 20 . (dT) 20 , recorded by the upconversion technique at the maximum of the emission spectrum.…”

“…In 2003, we reported the fluorescence decay of the oligomer (dA) 20 . (dT) 20 , recorded by the upconversion technique at the maximum of the emission spectrum. 10 It was found to be slower than those obtained for an equimolar mixture of the corresponding nucleotides (dAMP: 2'-deoxyadenosine monophosphate; TMP: thymidine monophosphate) and for each one of the single strands (dA) 20 and (dT) 20 .…”

“…(dT) 20 , recorded by the upconversion technique at the maximum of the emission spectrum. 10 It was found to be slower than those obtained for an equimolar mixture of the corresponding nucleotides (dAMP: 2'-deoxyadenosine monophosphate; TMP: thymidine monophosphate) and for each one of the single strands (dA) 20 and (dT) 20 . Despite this relative lengthening, the average fluorescence lifetime determined for (dA) 20 .…”

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AbstractThe fluorescence of the DNA double stranded oligomer (dA) 20 . (dT) 20 is studied at room temperature by fluorescence upconversion at times shorter than 10 ps.

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“…(dT) 20 is studied at room temperature by fluorescence upconversion at times shorter than 10 ps. The profile of the upconversion spectra is similar to that of the steady-state fluorescence spectrum, showing that the major part of the photons is emitted within the probed time-scale.…”