Protonation-driven formation of a double-stranded structure: A photophysical and 1H-NMR study

The electronic absorption spectrum and the fluorescence properties of a bischelating ligand made of two 2-p-anisyl-1,10-phenanthro!ine units linked in the 9 position by a 1,3-phenylene spacer (A-phen-P-phen-A) and of the model compound 2-/>anisyl-9-p-phenyl1,10-phenanthroline (A-phen-P) have been investigated. In CH2C!2 solution A-phen-P-phen-A shows a very strong ($ = 0.37), short-lived (r = 2.3ns) fluorescence band with Amax = 400nm. At 77 K, a phosphorescence band with Amax = 560 nm and T = 1.2 s is also present. Addition of trifluoroacetic acid to a CHaCk solution of A-phen-P-phen-A and of the model compound A-phenP causes strong changes in the absorption and fluorescence spectra, lifetimes, and quantum yields, whereas it does not affect the phosphorescence properties. In the case of A-phen-P, only one protonation step can be evidenced, whereas A-phen-P-phen-A undergoes two distinct, reversible protonation steps. The absorption and fluorescence results suggest, and 1H-NMR data fully confirm, that the first protonation step of A-phen-P-phen-A does not involve simple mono-protonation of one of the phen units, but implies the cooperative action of a pair of A-phenP-phen-A molecules that organize themselves around protons to form a dimeric structure. The highly organized, entwined structure of the A-phen-P-phen-A dimer is destroyed on addition of base, giving back the non-protonated A-phen-P-phen-A molecules, or on addition of acid, leading to the di-protonated A-phen.H-P-phen.H-A species. © CNRS-Gaulhier-Villars.