Photophysical characterization of the plant growth regulator 2-(1-naphthyl) acetamide

• A full photophysical characterization has been carried out on the plant growth regulator 2-(1-naphthyl) acetamide.
• These show a marked solvent dependence.
• The compound acts as a relatively strong excited state oxidant.
• Sensitized singlet oxygen reactions compete with electron transfer in the excited state reactivity.
• This may be important in the photoreactivity under environmental conditions.

The photophysical properties of the widely used plant growth regulator 2-(1-naphthyl) acetamide (NAD) were studied in water and representative organic solvents (ethanol, ethylene glycol, acetonitrile, chloroform, 1,4-dioxane) employing steady-state and time-resolved spectroscopy. Quantum yields and lifetimes of fluorescence, phosphorescence and triplet formation and triplet–triplet absorption spectra were obtained. From these, all radiative and radiationless rate constants have been determined, together with singlet and triplet excited state energies (4.00 and 2.69 eV, respectively). The fluorescence quantum yield and lifetime increased on going from water (ΦF 0.066, τF 35.0 ns) to non-hydrogen bonding solvents (ΦF 0.357, τF 51.0 ns in 1,4-dioxane), probably due to decreased internal conversion. Fluorescence was quenched by several anions through an electron transfer process. A limit on the reduction potential of 1NAD* of E0 2.1 ± 0.2 V was estimated. The attribution of the transient absorption seen in nanosecond laser flash photolysis to 3NAD* was confirmed by energy transfer and oxygen quenching. Quenching of triplet states leads to singlet oxygen formation, with quantum yields varying from 0.097 in water to 0.396 in chloroform. However, these are lower than the triplet state quantum yields, particularly in water (ΦT 0.424), indicating competing quenching pathways, probably involving electron transfer. The relevance of these results to the photoreactivity of NAD under environmental conditions is discussed.

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