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Theoretical Study on the Photooxygenation and Photorearrangement Reactions of Cite This: RSC Adv.,2017,7,32185 3-Hydroxyflavone†

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Theoretical Study on the Photooxygenation and Photorearrangement Reactions of Cite This: RSC Adv.,2017,7,32185 3-Hydroxyflavone† RSC Advances View Article Online PAPER View Journal | View Issue Theoretical study on the photooxygenation and photorearrangement reactions of Cite this: RSC Adv.,2017,7,32185 3-hydroxyflavone† a b ac Zoltan´ Szakacs,´ Mihaly´ Kallay´ and Miklos´ Kubinyi * The photooxygenation of 3-hydroxyflavone (3HF) into O-benzoyl salicylic acid and the photorearrangement of 3HF into 3-hydroxy-3-phenyl-indane-1,2-dione have been studied using theoretical calculations. These are the main photodegradation reactions of this versatile fluorescent probe which exhibits excited state intramolecular proton transfer (ESIPT). The Gibbs free energies for the ground/excited state species were computed at the DFT/TD-DFT level of theory. The calculations on the 3 direct photooxygenation (T1 state phototautomeric form of 3HF + O2) as well as on the photosensitized 1 reaction (S0 state normal form of 3HF + O2) confirmed the feasibility of the reaction routes via an endoperoxide intermediate. In contrast, the alternative mechanism, via an exoperoxide intermediate Creative Commons Attribution 3.0 Unported Licence. proved kinetically inaccessible. In the calculation of the rearrangement reaction, the T1 state Received 24th April 2017 phototautomeric form was considered the reacting species (as indicated by photochemical experiments) Accepted 19th June 2017 and the free energy profile was calculated through T1 stationary points. The results suggested the DOI: 10.1039/c7ra04590e occurrence of an intermediate with a 1-indanone skeleton, formed through a transition state with rsc.li/rsc-advances a relatively high free energy barrier corresponding to the folding of the pyrone ring. Introduction usually referred to as the normal (N*) and the phototautomeric (PT*) species. Following the discovery of ESIPT in the 3HF 1 This article is licensed under a 3-Hydroxyavone (3HF) is a dually uorescent compound due to parent compound by Sengupta and Kasha, the phenomenon 2 the excited state proton transfer (ESIPT, see Fig. 1) from its was studied in numerous natural and synthetic derivatives. The hydroxyl to its carbonyl group. The two emissive forms are wavelength and intensity of the N* and PT* bands are sensitive to the local environment, which has led to the development of Open Access Article. Published on 23 June 2017. Downloaded 9/26/2021 11:39:38 PM. many 3HF based ratiometric uorescent probes aimed at studying membrane structures,3,4 micellar assemblies,5–7 biomolecular interactions,8–10 and cellular processes11,12 and for use as chemosensors for the detection of non-uorescent ana- lytes.13,14 Much research effort has also been devoted to the construction of novel type ESIPT compounds, recent examples for which are the benzoindolizine derivative showing proton transfer from a protonated amino group to a ring C-atom,15,16 and the amino-benzothiazols17,18 and amino-benzoquinolines19 with proton transfer along their (amino)NH/N(ring) hydrogen bonds. Fig. 1 The intramolecular proton transfer cycle of 3-hydroxyflavone. In case of 3HF and some derivatives, the main photo- degradation reactions have also been studied. The reaction routes proposed in the literature are summarized in Fig. 2. In aDepartment of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, 1521 Budapest, Hungary. E-mail: [email protected] the presence of dissolved oxygen, the direct photooxygenation bMTA-BME Lendulet¨ Quantum Chemistry Research Group, Department of Physical was found to produce O-benzoyl salicylic acid (SA) and car- 20 Chemistry and Materials Science, Budapest University of Technology and Economics, bonmonoxide as the main photoproducts. The oxygenation of 1521 Budapest, Hungary 3HF was also studied with the use of photosensitizers, and the cInstitute of Materials and Environmental Chemistry, Research Center for Natural same products (SA + CO) were obtained.21 The photo- Sciences, Hungarian Academy of Sciences, 1519 Budapest, P.O. Box 286, Hungary oxygenation of some metal complexes with (anionic) 3HF ligand † Electronic supplementary information (ESI) available: Excitation energies of the 22–24 normal and phototautomeric forms of 3-hydroxyavone. Descartes coordinates of also yields SA, releasing CO. The direct as well as the pho- the stationary points in the three reactions. See DOI: 10.1039/c7ra04590e tosensitized oxygenation of avonols, the natural 3HF This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7, 32185–32192 | 32185 View Article Online RSC Advances Paper Fig. 2 Routes of photochemical reactions of 3HF, proposed in the literature; (top): unsensitized photooxygenation, (middle): sensitized pho- Creative Commons Attribution 3.0 Unported Licence. tooxygenation, (bottom): photorearrangement. derivatives, lead to the respective SA-s.25,26 In deaerated solu- The photosensitized oxygenation of 3HF occurs only in polar 1 tions, a photorearrangement takes place resulting in 3-hydroxy- solvents (methanol, pyridine), the dye is inert to O2 in apolar 3-phenyl-indan-1,2-dione (IN).27 These reactions are of impor- solvents.33 The reaction was interpreted by Matsuura21 in terms 1 2 tance in the characterization of the photostability of 3HF-based of the two-step mechanism seen in Fig. 2. O2 binds to the C - uorescent probes.28 In addition, there has been a particular atom of 3HF, yielding the exoperoxide intermediate, XP. This This article is licensed under a interest for the photosensitized oxygenation of avonols, as then transforms into the endoperoxide, NP, which decomposes a process contributing to their antioxidant function in living thermally with the rupture of the peroxide bond and the split- organisms.29,30 Much information on the mechanism of these ting of a carbonmonoxide (involving the C3 and O3(H) atoms in photoreactions has been collected via photolysis experiments. 3HF). In another version of this mechanism suggested by Chou Open Access Article. Published on 23 June 2017. Downloaded 9/26/2021 11:39:38 PM. The photooxygenation upon direct excitation of 3HF has et al.,33 the endoperoxide intermediate is formed directly from 20 1 2 4 been studied in air-saturated heptane and in low temperature 3HF, with an addition of O2 to the C and C atoms of 3HF and 31 O2 matrix in detail. It was established that the triplet state a concomitant thermally activated proton transfer from the 3- tautomer is the reacting species and it forms a peroxide inter- hydroxy to the 4-oxo group. 3 mediate with O2. In more detail, in apolar environments only The photorearrangement is thought to start from the triplet the uorescence of the PT* form of 3HF could be observed, state of the normal form of 3HF and goes through an epoxy indicating that the ESIPT is a fast and complete reaction. This intermediate, EP in Fig. 2. The triplet character of the reacting uorescence was not sensitive to dissolved oxygen, i.e. the 3HF was indicated by quenching experiments: the triplet 34 quenching of the S1 state tautomer was negligible. In contrast, quenchers piperilene and isoprene inhibit the reaction. The 3 the quenching of the T1 tautomeric form by O2 had been occurrence of EP as an intermediate was proposed by Matsuura detected in transient absorption experiments, and was et al.27 EP can be formed from the normal form of the avone in –3 20 explained in terms of physical quenching (T1 O2 energy a (2 + 2) cycloaddition. As was pointed out by Chou et al., the transfer) and chemical reaction. This result is also in accord vicinity of the proton on the 3-hydroxyl group to the epoxy with the spin correlation rules, as the reactions of the two triplet oxygen is favorable for the conversion of EP into the diketonic species are spin-allowed, the singlet–triplet reactions are spin- nal product. The cycloaddition of the phototautomer 3HF may forbidden. The reaction proceeds via a concerted addition of lead to a tautomeric epoxy intermediate, with the proton on the O2 and a concomitant proton transfer, yielding the endoper- 4-carbonyl group, remote from the epoxy oxygen atom, making oxide NP or the exoperoxide XP as primary product – the former the arrangement into the diketone unlikely. The reaction rate is was suggested by surface-enhanced Raman experiments,32 the increased in hydrogen bond donor solvents.35 31 latter by IR spectroscopic measurements in the O2 matrix. The Our target in the present work has been to calculate the peroxide NP or XP then decomposes into SA and potential energy surface (PES) theoretically, along the reaction carbonmonoxide. 32186 | RSC Adv.,2017,7, 32185–32192 This journal is © The Royal Society of Chemistry 2017 View Article Online Paper RSC Advances paths of these photoreactions and to analyze the above mech- states with minimal modications. The stability of the self- anisms proposed on the bases of experimental results. consistent eld (SCF) solutions was checked in case of the singlet optimized structures. The unstable solutions were recalculated on the singlet surface by the unrestricted Computational methods formalism, leading to spin contaminated states. The spin contamination was eliminated using Yamaguchi's method to The quantum chemical calculations were performed using the 39,40 calculate the corrected energies for the biradical structures. Gaussian09 package of programs.36 The initial geometries of Basis set superposition errors were not estimated for the 3HF in its N and PT forms and the reaction products were photooxygenation reaction because they are in the order of 1–2 computed with conformational analyses by MarvinSketch À kcal mol 1, which do not modify the hypothesized (version 16.4.25.0) using the MMFF94 force eld. For each mechanisms. species the lowest-energy conformer was selected for further The diradical character of the species, for which the SCF investigations. The M06-2X hybrid DFT functional37 with Pop- solutions were unstable, were described employing Complete le's 6-311++G** triple zeta basis set was employed for the 41 Active Space SCF (CAS-SCF) calculations.
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