L-Valine and Salicylaldehyde Derivative Schiff Base Zn(II) Complexes as UVA Sunscreen
Corresponding author: Dr. Takashiro Akitsu, 1-3 Kaguarzaka, Shinjuku-ku, Tokyo 162-8601, Japan;
Tel: +81-3-5228-8271, Email: email@example.com
Generally UV of sunlight is classified into UVC, UVB and UVA according to wavelength. Especially, UVB (280 nm-315 nm) and UVA (315 nm-400 nm) are attracting much attention because of influence on human health and environmental issues. Sunscreen cosmetics are required useful functions to protect our skin against damage such as stains and freckles by UVB light or wrinkles by UVA light.
Commonly sunscreen cosmetics are composed of inorganic UV-reflecting compounds and organic UV-absorbing ones. Conventionally, commercially available UV intercepting ones out of the skin are known as UV-reflecting ones, for instance, TiO2, ZnO and so on. While as for organic UV-absorbing ones, ethylhexylmethoxycinnamate, octylmrthoxycinnamate, tert-butylmethoxydibenzoylmethane, and so on are known. UVA denatures protein and lets skin age by arriving at the dermis layer of skin. In spite of serious hazard of UVA light, only a few food ones are known as organic UVA-absorbing ones,
though many good ones are known as UVB-absorbing (or reflecting) ones. Thus UVA-absorbing ones that can cover all range of UVA wavelength region are needed so far .
In this way, we focused on metal complexes, having both aspects of organic and inorganic compounds, as UVA-absorbing ones. Indeed, recently we have found that composite materials of L-amino acid derivative Schiff base Cu(II) complexes and TiO2 occur characteristic photo-induced electron transfer reactions by absorbing UV(A) light . Unfortunately, these Cu(II) complexes were greenish blue, which is not suitable for whitening cosmetics. Colourless Zn(II) ions or complexes may
be suitable for this purpose. Herein, we report on the analogous L-valine amino acid derivative Schiff base Zn(II) complexes (X- = H-, Cl-, and CH3O-; L- = methanol, imidazole) which are expected both biocompatibility (from amino acid moiety) and whitening effect (Scheme 1). We have discussed the substituent (X-, L-) effects on absorption and emission wavelength and intensity, and investigated photo-induced reaction as composites and anatase TiO2.
Scheme 1. Molecular structures of Zn(II) complexes (X= H-, Cl-, orCH3O-; L= methanol or imidazole).
Materials and Methods
Preparation of three Zn(II) complexes (L=methanol) was carried out by following conventional procedures . To a methanol solutions (50 mL) of L-valine (3 mmol) and salycylaldehyde (X=H) (or 5-chloro-salycylaldehyde (X=Cl), 5-methoxy-salycylaldehyde (X=CH3O) (3 mmol) stirred for 3 hr at 333 K, zinc(II) acetate monohydrate (3 mmol) was added and stirred for 3 hr to give rise to yellow crude products. After evaporation for concentration and recrystallization from methanol, yellow products were obtained. On the other hand, the corresponding three Zn(II) complexes (L=imidazole) were prepared in a similar procedure except for addition of imidazole (3 mmol) at 1 hr after addition of zinc(II) acetate monohydrate for easy isolation. Formation of complexes was confirmed by C=N bands of IR spectra around 1630 cm-1. Due to quite easy dissociation and/or di- or tri-merization of the monodentate “L” ligands in solution, 1H-NMR (not shown) could not observed properly like normal and stable compounds. Hence we managed to measure IR spectra in the solid state as follows (cm-1): X=H; L=imidazole: 1630 (C=N), 3414 (O-H). X=H; L=methanol: 1633 (C=N), 3306 (O-H). X=Cl; L=imidazole: 1629 (C=N), 3422 (OH). X=Cl; L=methanol: 1635 (C=N), 3441 (O-H). X=CH3O; L=imidazole: 1632 (C=N), 3422 (O-H). X=CH3O; L=methanol: 1634 (C=N), 3434 (O-H).
Infrared spectra were recorded on a JASCO FT-IR 2400 spectrophotometer in the range of 4000-400cm-1 at 298K as KBr pellets. Absorption electronic spectra were measured on a JASCO V-570 spectrophotometer in the range of 900-250 nm at 298K. Fluorescent spectra were measured on a JASCO FP- 6200 spectrophotometer in the range of 350-600 nm at 298K. 1H-NMR was measured on a JEOL JMN-300 spectrometer (300 MHz) in DMSO-d6. UV light source used was Hayashi LA-310UV with a visible light (> 350 nm) cut filter.
Prismatic pale yellow single crystal of X=H; “L=H2O” (as isolated crystals which were grown form X=H; L=methanol in methanol (containing water) solution) were glued on top of a glass fiber and coated with a thin layer of epoxy resin to measure the diffraction data. Intensity data were collected on a Bruker APEX2 CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Data analysis was carried out with a SAINT program package. The structures were solved by direct methods with a SHELXS-97  and expanded by Fourier techniques and refined by full-matrix least-squares methods based on F2 using the program SHELXL-97 . An empirical absorption correction was applied by a program SADABS . All non-hydrogen atoms were readily located and refined by anisotropic thermal parameters. All hydrogen atoms were located at geometrically calculated positions and refined using riding models.
Crystallographic data for X=H; L=H2O. C12H15NO4Zn. crystal size 0.30 mm × 0.17 mm × 0.11 mm, Mw = 834.75, trigonal, space group P31(#144), a = 12.655(3) Å, c = 6.7673(15) Å, V = 938.6(5) Å3, Z = 3, Dcalc = 1.606 mg/m3, F(000) = 468, R1 = 0.0259, wR2 = 0.0707 (2628 reflections), S = 1.490, Flack parameter = 0.026(7). (where R1 =Σ||Fo|-|Fc||/Σ|Fo|. Rw = (Σw(|Fo|-|Fc|)2 /Σw|Fo|2)1/2, w = 1/(σ2(Fo) + (0.1P)2+0.4679P), P = (Fo2 + 2Fc 2)/3).
Results and Discussion: As shown structure of a bridged five-coordinated complex in Figure 1, Schiff base Zn(II) complexes (X- = H-, Cl-, and CH3O-; L- = imidazole) were pale yellow showing red-shift of electronic spectra by electron donating group (Figure 2), and the corresponding CD spectra appeared negative Cotton effect around 280 and 380 nm. They also appeared blue emission around 380 nm and 470 nm by UV light excitation, of which intensity was strong for X= CH3O. It should be noted that absorption electronic spectra appeared broad (charge transfer) bands around 300-400 nm, which covers UVA wavelength region (315-400 nm) except for intense (Π-Π* or n-Π*) bands of UVB or UVC region. Contrary to known UVA-absorbing Zn(II) complexes reported in patents showing more intense Π-Π* or n-Π* bands but limited up to shorter wavelength (less than 400 nm), the present Zn(II) complexes are possible to cover fully wide range of UVA wavelength region. To our knowledge, this is the first example of such UVA-absorbing Zn(II) complexes.
Figure 1. Crystal structure of “X=H; L=H2O” one showing selected atom labeling scheme. Hydrogen atoms are omitted for clarity. Selected bond distance (Å) and bond angles (°): Zn1-O1 = 1.998(3), Zn1-O4 = 2.013(3), Zn1-N1 = 2.021(3), Zn1-O3i= 2.035(2), Zn1-O3= 2.151(3), O1-Zn1-O4 = 103.68(11), O1-Zn1-N1 = 91.82(12), O4-Zn1-N1 = 108.38(12), O1-Zn1-O3i = 91.80(11), O4-Zn1-O3i = 104.88(12), N1-Zn1-O3i = 144.61(12), O1-Zn1-O3 = 168.32(10), O4-Zn1-O3 = 84.75(10), N1-Zn1-O3 = 77.70(11), O3-Zn1-O3i = 93.83(11) . Symmetry operation: (i) 1-y, x-y+1, z+1/3.
Figure 2. Absorption electronic spectra (0.2 M methanol solutions)for Zn(II) complexes (X= H-, Cl-, or CH3O- ; L= methanol).
Consequently, composite materials of anatase TiO2 powder and these Zn(II) complexes can be also expected for potential application of sunscreen for wide range UVA-UVB-UBC. However, the spectra of composite materials (0.2 mM Zn(II) complexes and 0.05 mM TiO2in methanol suspension) were changes with decreasing Π-Π* and charge transfer bands and increasing intensity between them around 300 nm after UV light irradiation up to 60 nm (Figure 3).
Figure 3. Spectra changes of methanol suspension of TiO2 and Zn(II) complex (X= H-; L= imidazole) after UV light irradiation for 0, 20, 40, and 60 min.
Although spectral changes occurred, and serious change was observed for X= CH3O especially, they are attributed not to photo-induced electron transfer reactions (accompanying with reduction from Cu(II) to Cu(I)) like the analogous Cu(II) complexes  but to decomposition of monodentate ligand (L) of Zn(II) complex by exchanging imidazole ligand and methanol solvent only. It may be caused by unstability of imidazole ligands.
Therefore, we employed Zn(II) complexes (L=methanol) from the beginning. Figure 4 depicts absorption electronic spectra in methanol solutions. Similar to the complexes (L=imidazole), these also appeared characteristic band around UVA wavelength regions and substitution group effects on both absorption and emission spectra, though relatively smaller spectral changes could be observed after UV light irradiation up to 60 min as suspension with TiO2.
Figure 4. Absorption electronic spectra (0.2 M methanol solutions) for Zn(II) complexes (L= methanol).
Consequently, all of the present Zn(II) complexes showed relatively strong absorption fully over the wavelength range of UVA region (315nm-400nm). In addition, monodentate ligands (X=methanol) has advantage of hard to photo-decomposition (or changes) without losing function of UVA absorption. Since these complexes did not exhibit photo-induced electron transfer reactions under coexisting TiO2, they were suggested to utility to new sunscreen cosmetics especially against UVA light .
This work was partly supported by Network Joint Research Center for Materials and Devices (Tokyo Institute of Technology).
Conflict of Interest
CCDC 1510939 contains the supplementary crystallographic data. The data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: firstname.lastname@example.org.
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1. Takeshita Y, Akitsu T. Fragrance J. 2015, 43: 63-66.
4. Akitsu T, Yoshida N, Shimada T, patent, submitted.