Enhanced thermal and photo-stability of a para-substituted dicumyl ketone intercalated in a layered double hydroxide


A ketodiacid, 4,4 '-dicarboxylate-dicumyl ketone (3), has been intercalated into a Zn, Al layered double hydroxide (LDH) by a coprecipitation synthesis strategy. The structure and chemical composition of the resultant hybrid material (LDH-KDA3) were characterized by powder X-ray diffraction (PXRD), FT-IR, FT-Raman and solid-state C-13{H-1} NMR spectroscopies, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and elemental analysis (CHN). PXRD showed that the dicarboxylate guest molecules assembled into a monolayer to give a basal spacing of 18.0 & Aring;. TGA revealed that the organic guest starts to decompose at a significantly higher temperature (ca. 330 degrees C) than that determined for the free ketodiacid (ca. 230 degrees C). Photochemical experiments were performed to probe the photoreactivity of the ketoacid in the crystalline state, in solution, and as a guest embedded within the photochemically-inert LDH host. Irradiation of the bulk crystalline ketoacid results in photodecarbonylation and the exclusive formation of the radical-radical combination product. Solution studies employing the standard myoglobin (Mb) assay for quantification of released CO showed that the ketoacid behaved as a photoactivatable CO-releasing molecule for transfer of CO to heme proteins, although the photoreactivity was low. No photoinduced release of CO was found for the LDH system, indicating that molecular confinement enhanced the photo-stability of the hexasubstituted ketone. To better understand the behavior of 3 under irradiation, a more comprehensive study, involving excitation of this compound in DMSO-d(6 )followed by H-1 NMR, UV-Vis and fluorescence spectroscopy, was undertaken and further rationalized with the help of time-dependent density functional theory (TDDFT) electronic quantum calculations. The photophysical study showed the formation of a less emissive compound (or compounds). New signals in the H-1 NMR spectra were attributed to photoproducts obtained via Norrish type I alpha-cleavage decarbonylation and Norrish type II (followed by CH3 migration) pathways. TDDFT calculations predicted that the formation of a keto-enol system (via a CH3 migration step in the type II pathway) was highly favorable and consistent with the observed spectral data.






Costa, AL; Monteiro, RP; Barradas, PDN; Ferreira, SCR; Cunha, C; Gomes, AC; Goncalves, IS; de Melo, JSS; Pillinger, M

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This research was supported by the Associated Laboratory CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020), CQC-Coimbra Chemistry Center (UIDB/00313/2020 and UIDP/00313/2020), the CENTRO 2020 Regional Operational Programme (project references SASCOT-CENTRO-01-0145-FEDER-028031 and PTDC/QUI-QOR/28031/2017), and the COMPETE 2020 Operational Thematic Program for Competitiveness and Internationalization (project reference HYLIGHT-POCI-010145-FEDER-031625), financed by national funds through the FCT (Fundacao para a Ciencia e a Tecnologia)/MEC (Ministerio da Educacao e Ciencia) (PIDDAC) and when appropriate cofinanced by the European Union through the European Regional Development Fund under the Portugal 2020 Partnership Agreement. The PhD grant held by RPM (ref. 2020.04758.BD) was funded by the FCT and the European Social Fund (ESF). NMR data collected at the UC-NMR facility are supported in part by the EDRF through the COMPETE Program and by national funds from the FCT through grants RECI/QEQ-QFI/0168/2012 and CENTRO-07-CT62-FEDER-002012, and also through support to Rede Nacional de Ressonancia Magnetica Nuclear (RNRMN) and to Coimbra Chemistry Centre through grant UID/QUI/00313/2019. AG thanks the FCT/MCTES for funding through the Individual Call to Scientific Employment Stimulus (CEECIND/02128/2017). CC thanks the FCT for a PhD Grant (ref. 2020.09661.BD).

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