Substituent effects on fluorescence properties of thiazolo[4,5-b]pyrazine derivatives

Knoevenagel Condensation between 2-Methyl-thiazolo[4,5-b]pyrazines and Aldehydes

Knoevenagel condensation, an olefin-forming reaction from active methyl/methylene-containing compounds and aldehydes, is a fundamental and useful synthetic method. Benzothiazoles are, however, out of the scope of Knoevenagel condensation. Here, we report that Knoevenagel condensation between aldehydes and 2-methyl-thiazolo[4,5-b]pyrazines (MeTPy), a fused ring structure comprising pyrazine and thiazole, proceeded smoothly, despite minor structural differences from benzothiazoles. This finding will be useful for short synthesis of MeTPy-containing functional molecules, such as a tau probe analog 1.

LC-BLYP Calculations of the Structures and Photophysical Properties of [1,3]Thiazolo[4,5-b]pyrazine Derivatives in Cyclohexane and Methanol

Applications of the B3LYP and LC-BLYP functionals with the 6-311+G(d,p) basis set for predicting absorption and fluorescence spectra of benzothiazole and 11 [1,3]thiazolo[4,5-b]pyrazine (TPy) derivatives in implicit solvents (cyclohexane and methanol) were tested and compared with experimental results. The damping parameter of LC-BLYP was tuned for simulating absorption and fluorescence spectra of each compound. For the 11 TPy derivatives, four and seven compounds are small- and medium-sized TPy derivatives, respectively. From this calculation, TD-B3LYP can be used to simulate the absorption spectra of the small-sized compounds in cyclohexane and methanol, but it gives poor results for the fluorescence spectra in cyclohexane and methanol. Contrarily, the LC-BLYP functional with the optimal damping parameter can reproduce the absorption and fluorescence spectra of all compounds in cyclohexane and methanol. The values of the damping parameter of LC-BLYP depend on the types of substituents and solvents. Moreover, the conformations of the compounds do not affect the spectra, and the molecular size and molecular dipole moment do not affect the damping parameter. In addition, types and the number of substitutions attached to the phenyl rings of TPy as well as solvent polarity play the key roles in the long-wavelength fluorescence emission and fluorescence brightness of the compounds. The intramolecular charge transfer of the compounds containing the -NMe₂ and -CN groups is predominant in methanol, resulting in a large Stokes shift and oscillator strength.

Diiron Dithiolate Hydrides Complemented with Proton-Responsive Phosphine-Amine Ligands

The reaction of Fe₂(pdt)(CO)₆ with two equivalents of Ph₂PC₆H₄NH₂ (PNH₂) affords the amido hydride HFe₂(pdt)(CO)₂(PNH₂)(PNH) {[H1H]⁰, pdt^2- = CH₂(CH₂S^-)₂}. Isolated intermediates in this conversion include Fe₂(pdt)(CO)₅-(κ¹-PNH₂) and Fe₂(pdt)(CO)₄(κ²-PNH₂). X-ray crystallographic analysis of [H1H]⁰ shows that the chelating amino/amido-phosphine ligands occupy trans-dibasal positions. The ³¹P NMR spectrum indicates that [H1H]⁰ undergoes rapid proton exchange between the amido and amine centers. No exchange was observed for the hydride. Protonation of [H1H]⁰ gives [HFe₂(pdt)(CO)₂(PNH₂)₂]⁺ ([H₂1H]⁺), which contains two equivalent amino-phosphine ligands. Single-crystal X-ray crystallographic analysis of [H₂1H]⁺ also reveals hydrogen bonds between the exo amine protons with a THF molecule and BF₄. Deprotonation of [H1H]⁰ with potassium tert-butoxide gave [HFe₂(pdt)(CO)₂(PNH)₂]^- ([1H]^-), which was characterized spectroscopically. The complex has time-averaged C₂ symmetry with two amido-phosphine ligands. FTIR spectroscopic measurements show that υ_CO shifts by approximately 20 cm^-1 in the series [1H]^-, [H1H]⁰, and [H₂1H]⁺. These shifts are comparable to those seen for the S-protonation of the (NC)₂(CO)Fe-(μ-Scys)₂Ni(Scys)₂ site in the [NiFe]-hydrogenases.^[1].

Fluorescence Properties of Diphenylthiazolo[4,5-b]pyrazines Tuned by Donor-Acceptor Substituent Effects

Fluorescence properties of 2,6- and 2,5-diphenylthiazolo[4,5-b]pyrazine (TPy) derivatives having an electron-donating substituent (methoxy and dimethylamino) on the 6- and 5-phenyl groups were studied. It was found that 2,6-diphenyl derivatives fluoresce more efficiently than 2,5-diphenyl derivatives. Furthermore, a 2,6-diphenyl derivative having an additional cyano group on the 2-phenyl ring was an excellent fluorophore showing a wide solvatochromism with great fluorescence yields. Based on the obtained spectroscopic data and mechanistic explanations concerning the substituent effects on the fluorescence properties, useful information on designing new TPy fluorophores is provided.