Abstract
Excited state intramolecular proton transfer (ESIPT) in 3-hydroxyflavone (3HF) has been known for its dependence on excitation wavelength. Such a behavior violates Kasha’s rule, which states that the emission and photochemistry of a compound would only take place from its lowest excited state. The photochemistry of 3HF was studied using femtosecond transient absorption spectroscopy at a shorter wavelength excitation (266 nm), and these new experimental findings were interpreted with the aid of computational studies. These new results were compared with those from previous studies that were obtained with a longer wavelength excitation and show that there exists a pathway of proton transfer that bypasses the normal first excited state from the higher excited state to the tautomer from first excited state. The experimental data correlate with the electron density difference calculations such that the proton transfer process is faster on the longer excitation wavelength than compared to the shorter excitation wavelength.
Highlights
Kasha’s rule states that, in any condensed phase, photon emission occurs in appreciable yields only from the lowest state of a given multiplicity, irrespective of the initial photoexcited state [1,2]
The femtosecond transient absorption spectra show an initial appearance of a 362 nm band, and it reaches its maximum intensity at ~2 ps. (Figure 1b)
Since the femtosecond UV absorption spectra for the 3HF derivatives performed provided similar results, the following discussion will be focused on the results for 1b
Summary
Kasha’s rule states that, in any condensed phase, photon emission (fluorescence or phosphorescence) occurs in appreciable yields only from the lowest state of a given (singlet or triplet) multiplicity, irrespective of the initial photoexcited state [1,2]. Excited-state intramolecular proton transfer (ESIPT) is one of the fundamental reactions in chemistry and biology [4,5]. Even though the ESIPT process may have an extremely fast nature, two fluorescence emission bands from the reactant normal (N) and product tautomer (T) states can be observed in some cases. This results in interest from researchers who are seeking fluorescent probes, and, in this regard, the 3-hydroxyflavone (3HF) molecular family is one that has attracted considerable interest. This is because such dual emissions of 3HF are sensitive to the properties of the surrounding environment such as polarity and H-bond donor and acceptor ability of the condensed phase environment [6,7], and such unique properties have already found a variety of applications for studying polymers [8], host-guest complexes [9,10], reverse micelles [11,12], model lipid membranes [13], biomembranes [14] and proteins [15]
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