Solvent dependence of the photophysical properties of 2-chlorothioxanthone, the principal photoproduct of chlorprothixene

Abstract

2-chlorothioxanthone (CTX) is used as photoinitiator for the reticulation of synthetic resins and for the preparation of pharmaceuticals. It was previously determined that CTX is the primary photoproduct of z-chlorprothixene (CPTX) when irradiated at 313 nm and is formed in an autocatalyzed reaction through an energy-transfer mechanism (Piñero et al. [2009] Photochem. Photobiol., 85, 895-900). In this work, the photophysical properties of CTX were measured in acetonitrile/water solutions to determine if their magnitude can affect the side effects of CPTX. The results show that CTX has higher absorption coefficients in the visible region (400-420 nm) and higher triplet quantum yields than its parent compound. Similar to TX, both properties strongly depend on the solvent polarity/hydroxylicity. The quantum yield of the triplet intermediate is very close to the value of the phenothiazine triplets. The phenothiazines are the most phototoxic antidepressants. Therefore, given the appropriate microenvironment, the photosensitization side effects of CPTX can be intensified on the production of CTX.

Figure 1

Structures of the derivatives used in this work.

Figure 2

CTX ground-state molar absorption spectrum in: () MeCN, (▼) MeCN/Water (4:1), () MeCN/Water (2:1), () MeCN/Water (1:1). Inset: Molecular orbitals involved in the 386 nm transition.

Figure 3

Normalized emission and excitation spectra of CTX in different acetonitrile/water mixtures: () Excitation spectrum in MeCN; Emission spectra in () MeCN, (●) MeCN:H2O (4:1), () MeCN:H2O (2:1), () MeCN:H2O (1:1), and () MeCN:H2O (1:4).

Figure 4

Fluorescence decay curve for CTX in a MeCN/Water (2:1) solution (blue), the mono-exponential fitting (red, χ²=1.109), the IRF (black) and the corresponding residuals (bottom).

Figure 5

Dependence on the water content in MeCN/Water solutions of the fluorescence quantum yield (■), the triplet quantum yield (▲), and the triplet lifetime (●, right scale). The values for 100% water were extrapolated.

Figure 6

Transient absorption spectra of a 0.050 mM CTX solution in N₂-saturated MeCN after a 355 nm laser pulse (Energy < 3 mJ): (●) 0.83 μs, (○) 1.22 μs, (▼) 2.02 μs, and (△) 4.77 μs. Inset: Decay curves for the transients at 320 nm (●) and 630 nm (○).

Figure 7

Dependence of the absorption on the laser energy for CTX in MeCN (○, r² = 0.9885), TX in MeCN (●, r² = 1.0000, CTX in MeCH/Water (4:1) (▼, r² = 0.9749), and CTX in MeCN/Water (1:1) (△, r² =0.9802).