PROTONATION-DRIVEN FORMATION OF A DOUBLE-STRANDED STRUCTURE - A PHOTOPHYSICAL AND H-1-NMR STUDY

The electronic absorption spectrum and the fluorescence properties of a bischelating ligand made of two 2-p-anisyl-1,10-phenanthroline units linked in the 9 position by a 1,3-phenylene spacer (A-phen-P-phen-A) and of the model compound 2-p-anisyl-9-p-phenyl-1,10-phenanthroline (A -phen-P) have been investigated. In CH2Cl2 solution A-phen-P-phen-A sh ows a very strong (Phi = 0.37), short-lived (tau = 2.3 ns) fluorescenc e band with lambda(max) = 400 nm, At 77 K, a phosphorescence band with lambda(max) = 560 nm and tau = 1.2 s is also present. Addition of tri fluoroacetic acid to a CH2Cl2, solution of A-phen-P-phen-A and of the model compound A-phen-P causes strong changes in the absorption and fl uorescence 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 undergo es two distinct, reversible protonation steps. The absorption and fluo rescence results suggest, and H-1-NMR data fully confirm, that the fir st protonation step of A-phen-P-phen-A does not involve simple mono-pr otonation of one of the phen units, but implies the cooperative action of a pair of A-phen-P-phen-A molecules that organize themselves aroun d protons to form a dimeric structure. The highly organized, entwined structure of the A-phen-P-phen-A dimer is destroyed on addition of bas e, giving back the non-protonated A-phen-P-phen-A molecules, or on add ition of acid, leading to the di-protonated A-phen.H+-P-phen.H+-A spec ies.