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

Abstract

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 ¹³C{¹H} 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 Å. TGA revealed that the organic guest starts to decompose at a significantly higher temperature (ca. 330°C) than that determined for the free ketodiacid (ca. 230°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-d₆ followed by ¹H 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 ¹H NMR spectra were attributed to photoproducts obtained via Norrish type I α-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.

Keywords: CO-releasing molecules; TDDFT calculations; hexasubstituted ketones; intercalation; ketodiacid; layered double hydroxides; myoglobin assay; photodecarbonylation.

SCHEME 1

Preparation of ketodiacid 3 via modified literature procedures (Resendiz and Garcia-Garibay, 2005; Family and Garcia-Garibay, 2009).

FIGURE 1

PXRD patterns of (A) ketodiacid 3, (B) LDH-NO₃, and (C) LDH-KDA3.

FIGURE 2

(A) Structural model (ball-and-stick diagram superimposed on a spacefilling, van der Waals-based representation) for the monolayer arrangement of 4,4′-(2,4-dimethyl-3-oxopentane-2,4-diyl)dibenzoate anions in the material LDH-KDA3. (B) View of the crystal packing of 2 down the crystallographic b axis.

FIGURE 3

FT-IR spectra in the range 280–1800 cm⁻¹ of (A) LDH-NO₃, (B) LDH-KDA3, and (C) ketodiacid 3. The frequencies of selected bands are indicated.

FIGURE 4

FT-Raman spectra in the ranges 200–1,700 and 2,700–3,600 cm⁻¹ of (A) ketodiacid 3 and (B) LDH-KDA3.

FIGURE 5

(A) Solution ¹³C{¹H} NMR spectrum of the ketodiacid 3 in DMSO-d₆. The resonance labelled with an asterisk is due to residual acetone. (B) ¹³C{¹H} CP MAS NMR spectrum of LDH-KDA3.

FIGURE 6

TGA (solid lines) and corresponding DTG (dashed lines) curves for the ketodiacid 3 (magenta) and LDH-KDA3 (blue).

FIGURE 7

Representative SEM images under different magnifications and EDS spectrum of LDH-KDA3.

FIGURE 8

Aromatic and mid-field regions of the ¹H NMR spectra obtained after irradiating a solution of the ketodiacid 3 in DMSO-d₆ for (A) 0 h, (B) 3 h, (C) 5 h, (D) 8 h, and (E) 12 h. For comparison, (F) shows the same regions of the ¹H NMR spectrum obtained in a DMSO-d₆ solution of the irradiated solid i3 (see main text for details).

SCHEME 2

Possible photoproducts of 3 in solution.

FIGURE 9

Normalized absorption (left) and fluorescence emission (right) spectra of ketodiacid 3 in DMSO-d₆ solution with different irradiation times. In the absorption spectra the shape of the spectra for wavelength values below 260 nm mirrors the solvent’s cut-off, and the vertical arrows indicate the increase (∼304 nm) and decrease (∼314 nm and ∼327 nm) of the absorption bands.

FIGURE 10

Mb assays for ketodiacid 3 (2.6 mM) using a deoxy-Mb (DxMb) concentration of approximately (A) 30 μM and (B) 60 μM.

FIGURE 11

Mb assays for LDH-KDA3 (0.8 g/L) using a deoxy-Mb (DxMb) concentration of approximately (A) 30 μM and (B) 60 μM.