Photoinduced proton transfer promoted by peripheral subunits for some Hantzsch esters.

It is noted that, for a small series of 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives and the corresponding Hantzsch esters, the presence of methyl groups at the 2,6-positions serves to extinguish fluorescence in solution but not in the solid state. Emission is weakly activated and affected by changes in solvent polarity. The latter situation arises because the optical transition involves intramolecular charge transfer. Calculations, both semiempirical and DFT, indicate that, in all cases, rotation of the carbonyl function is facile and that the dihydropyridine ring is planar. These calculations also indicate that the 2,6-methyl groups do not affect the generic structure of the molecule. It is proposed that illumination increases the molecular dipole moment and pushes electron density toward the carbonyl oxygen atom. Proton transfer can now occur from one of the methyl groups, leading to formation of a relatively low-energy, neutral intermediate, followed by a second proton transfer step that forms the enol. Reaction profiles computed for the ground-state species indicate that this route is highly favored relative to hydrogen transfer from the 4-position. The barriers for light-induced proton transfer are greatly reduced relative to the ground-state process but such large-scale structural transformations are hindered in the solid state. A rigid analogue that cannot form an enol is highly emissive in solution, supporting the conclusion that proton transfer is in competition to fluorescence in solution.

A hybrid bis(amino-styryl) substituted Bodipy dye and its conjugate diacid: synthesis, structure, spectroscopy and quantum chemical calculations.

A new type of fluorescent pH indicator has been developed whereby two dissimilar amino-styryl units are attached to a boron dipyrromethene (Bodipy) dye. The photophysical properties of this hybrid dye, and its simpler counterparts bearing only a single amino-styryl residue, depend on the polarity of the surrounding medium. Of the two terminal amines, DFT (B3LYP/6-31G**) calculations and spectroscopic measurements support the notion that julolidine is oxidised and protonated under milder conditions than is N,N-dimethylaniline. For the hybrid dye, similar DFT calculations carried out for the mono-protonated analogues indicate that the julolidine residue is the stronger base while the resultant conjugate acid is the weaker one. Absorption and fluorescence spectroscopic titrations show that protonation of the hybrid dye occurs in two well-resolved steps, whereby addition of the first proton introduces a thermodynamic barrier for entry of the second. In the hybrid dye, the pKA values for the respective conjugate acids differ markedly from those derived for the mono-amino-styryl dyes and display negative co-operativity. Effectively, this means that electronic interactions running along the molecular backbone make it more difficult, relative to the individual dyes, to protonate both amino sites. As such, this dye operates as a probe over an unusually wide pH range.