The ultrafast energy transfer process in Naphtole - Nitrobenzofurazan bichromophoric molecular systems. A study by femtosecond UV-VIS pump-probe spectroscopy

This work presents an experimental and computational study of the intramolecular electronic energy transfer process occurring in two newly synthesized bichromophoric species: N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino-bis-ethyl-2-[(4-chloro-1-napht hyl)oxy]acetate (f-Bi) and N-(7-nitrobenzo[c][1,2,5]oxadiazole-4-yl)-(3S, 4S)-pyrrolidin-3,4-bis-yl-2-[(4-chloro-1-naphthyl)oxy]acetate (r-Bi). In both f-Bi and r-Bi the donor chromophore is the [(4-chloro-1-naphthyl)oxy]acetate moiety, whereas the acceptor units belong to the family of the 4-dialkylaminonitrobenzoxadiazoles, well-known fluorescent probes. The two bichromophores differ in the structural flexibility. In f-Bi, acceptor and donors are linked by a diethanolamine moiety, whereas in r-Bi through a (3S, 4S)3,4-dihydroxypyrrolidine ring. By means of steady-state and time-resolved UV-vis spectroscopies we carried out a detailed analysis of the photo-response of donor and acceptor chromophores as individual molecules and when covalently linked in f-Bi and r-Bi. The intramolecular energy transfer process occurs very efficiently in both the bichromophores. The rate constant and the quantum efficiency of the process are kET = (2.86 ± 0.16) × 1011 s-1 and Q = 0.998 in f-Bi, and kET = (1.25 ± 0.08) × 1011 s-1 and Q = 0.996 in r-Bi. Semiempirical calculations were utilized to identify the energy and the nature of the electronic states in the isolated chromophores. Molecular mechanics calculations have been performed to identify the most stable structures of the bichromophoric compounds. The predictions of Förster theory are consistent with the experimental results and provide a suitable way to evaluate the structural differences between the two compounds.