Synthesis and physico-chemical properties of a H-cardanol triazole zinc porphyrin conjugate

Abstract

Although a large number of natural and non-natural metalloporphyrins are known, examples with fluorescence and fat-soluble properties are rare. We have achieved the synthesis of a fluorescent and fat-soluble zinc porphyrin incorporating four units of hydrogenated cardanol (H-cardanol). The synthesis is sustainable since the product is derived from cashew-nut shell liquid (CNSL), which is a renewable and bio-waste material. The H-cardanol triazole zinc porphyrin conjugate (HTZPC) was synthesized through applying a copper(i) catalyzed azide-alkyne cycloaddition (CuAAC) reaction between a H-cardanol derived azide and a tetraarylporphyrin derived alkyne. The absorption and emission properties of the hydrocarbon solvent soluble HTZPC were evaluated using UV-vis and fluorescence emission spectra obtained in various solvents. The results were compared with related molecules like a triazole-zinc porphyrin conjugate (TZPC), zinc tetra-C(4)-methoxyphenyl porphyrin (ZP), and a H-cardanol-triazole conjugate (HTC). The results showed that HTZPC undergoes J-type aggregation in both non-polar and highly polar solvents, which is dictated by van der Waals attractive forces between H-cardanol units in polar solvents (e.g. methanol and dimethylformamide) and π-π stacking interactions between porphyrin units in non-polar solvents (hexane). Moreover, the spectra indicated that the triazole units could stabilize the zinc porphyrin via intermolecular coordinate-complex formation. We anticipate that fat-soluble HTZPC could find applications in medical fields (e.g. in the photodynamic therapy of fat tissue).

Fig. 1. Structures of the porphyrin free base 1 (selected numbering and three types of carbons in the porphyrin ring are highlighted), the metallated porphyrin 2, the metallated tetraphenylporphyrin (TPP) 3 and hydrogenated cardanol (H-cardanol) 4.

Fig. 2. The structure of the H-cardanol-triazole-porphyrin conjugate (HTZPC) 5.

Scheme 1. The synthesis of the H-cardanol triazole porphyrin conjugate (HTZPC) 5.

Fig. 3. The structures of the triazole-zinc porphyrin conjugate (TZPC) 6, zinc tetra(4-methoxyphenyl)porphyrin (ZP) 7, and the H-cardanol-triazole conjugate (HTC) 8.

Fig. 4. UV-vis spectra of HTZPC 5 (black), TZPC 6 (red), ZP 7 (blue), and ZP 7 + HTC 8 (olive) in hexane at room temperature.

Fig. 5. An ensemble of multiple porphyrin units in HTZPC 5.

Fig. 6. Absorption spectra of (A) HTZPC 5 (4.19 × 10⁻⁴ M), and (B) ZP 7 (1.25 × 10⁻³ M) in hexane (black), DCM (red), THF (green), EtOAc (blue), MeOH (purple) and DMF (magenta).

Fig. 7. Emission spectra (excited at 422 nm) of HTZPC 5 (left, 4.19 × 10⁻⁴ M) and ZP 7 (right, 1.25 × 10⁻³ M) soon after the preparation of the solutions (top row) and after two days (bottom row) in hexane (black), DCM (red), THF (green), EtOAc (blue), MeOH (cyan) and DMF (pink).

Fig. 8. Emission spectra (excited at 422 nm) of HTZPC 5 (4.19 × 10⁻⁴ M) in the non-polar solvents hexane (black), CHCl₃ (red), petrol (blue), 1-octanol (magenta), toluene (olive blue) and benzene (navy green).

Fig. 9. Photographs of 5 mg each of HTZPC 5, TZPC 6, ZP 7 and HTC 8 in hexane. While HTZPC 5 and HTC 8 were freely soluble, TZPC 6 and ZP 7 were practically insoluble.

Fig. 10. Photographs of HTZPC 5 dissolved in six different solvents, namely hexane (Hex), dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate (EtOAc), methanol (MeOH) and dimethylformamide (DMF).