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Reports: ND351921-ND3: Photochemistry and Reactivity of Heterosubstituted Maltol Chelates

Patrick J. Farmer, Baylor University

This PRF funded work concerns a family of new chelating dyes derived from maltol, an FDA approved food additive that has the smell of cotton-candy. Metal complexes of these dyes display unusual reactivity and photochemistry, for example our initial publications showed that two dithiomaltol (ttma) complexes, [Ru(bpy)2(ttma)]+  and Zn(ttma)2, act as potent photo-reductants. To date have synthesized several families of metal complexes of hetero-substituted maltol derivatives including Pt(bpy)dye+, Ti(dye)xCly, and certain homoleptic complexes which show photochemical activity. Our goal is to illuminate the nature of aromaticity in these compounds when complexed with metal ions, as well as to correlate aromaticity with the observed photochemical and reactivity characteristics. 

Ligand synthesis. We are studying small heterocycles derived from maltol, Figure 1, which have chelating α-hydroxy ketone/thioketone functionalities analogous to that of anthocyanin, a red dye found in berries with unusually high efficiency for DSSCs. Other target ligands include the Se and Te analogues; selenomaltol has been reported, but in our hands the yeild is quite low and the product relatively unstable.

 

Figure 1. Heterocyclic ligands derived from maltol used in this work.                                                  

Ru and Zn complexes. In published work, we have shown that the bis-bipyridyl Ru(II) complex of dithiomaltol, [Ru(bpy)2(ttma)][PF6], undergoes C-H activation at a pendant alkyl position upon electrochemical or bulk oxidation, yielding [Ru(bpy)2(ttma-alcohol)]+ and [Ru(bpy)2(ttma-aldehyde)]+ products. The photo-initiation of similar oxidative reactivity for compound using flash quench methodology employing electron acceptors 1,1'-dimethyl-4,4'-bipyridinium, pentamminechlorocobalt(III) chloride, hexaammineruthenium(III) chloride, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Investigation of the photoreactivity identified fluorescence and phosphorecences that appeared ligand-based, Figure 2.  As a test the homoleptic Zn(ttma)2 complex was synthesized, which demonstrated similar photochemistry and also was shown to generate photo-initiated reductions of the electron acceptors.

Figure 2.  Comparison of characteristics of [Ru(bpy)2(ttma)][PF6] (left) and Zn(ttma)2 (right). Top) xtal structure; middle) normalized absorbance (solid line) and emission (dotted) spectra; bottom) emission trace of near IR phosphorescence.

Pt(II)( bipyridyl) complexes. A series of PtII(bpy)Ln complexes have been synthesized using thiomaltol (Htma), dithiomaltol (Httma) and hydroxypyridinthione (Hopto) ligands, Figure 3. The complexes [Pt(bpy)tma][PF6], [Pt(bpy)ttma][PF6], and [Pt(bpy)(hopto)][PF6 which represent the first Pt(bpy)(electron-donor) complexes with mixed O,S atom chelation and as will be shown, they exhibit some of the longest luminescence lifetimes for this structural motif.

 

Figure 3.  Structures and absorbance/emission spectra with emisison lifetimes for family of PtII(bpy)Ln complexes.

Ti and titania adducts. A possible use for these maltol-based compounds may be in as photoreductants absorbed onto TiO2 particles at the anode surface in DSSCs. In recent publications we have described the synthesis and characterizations of titania nanotwins using high temperature organic solvent methods, yielding two kinds of common high-quality rutile twinned nanocrystals, (101) and (301) twins, accompanied by minor rutile nanorods. In ongoing work we have synthesized Ti complexes of the maltol dyes to model their interaction on TiO2, Figure 4. We have isolated and crystallographically characterized a series of compounds of the composition Ti(dye)Cl3 and Ti(dye)2Cl2.  The absorbances of these monomeric complexes match well to the color of the dyes absorbed on TiO2. 

Figure 4. Top, family of Ti(L)Cl3 complexes studied; center, structure of Ti(ttma)Cl3; bottom, absorbance/emission spectra of Ti(ttma)Cl3 in frozen MeCN solution at 77K.

Novel octahedral phosphorus(III) dye complexes. We have recently synthesized novel six-coordinate phosphorous complexes of the formula P(dye)2X2 which exhibit a room temperature emissions, Figure 5. X-ray crystallography of P(C6H5O3H1/2)2F2 reveals the coordiation about the phosphorus of two bidentate maltolato ligands, and two fluorides in cis-configuration. There is a single hydroxyl proton with 50% occupancy on the two maltolato, shown in the structure. This complex exhibits a room temperature emission with lmax = 322 nm upon excitation at long wave UV radiation.

Figure 5. Top, family of P(dye)2X2 complexes studied; bottom left, structure of P(C6H5O3H1/2)2F2; bottom right, blue emission spectra of complex in solid state.