Intramolecular electron transfer in diastereomeric naphthalene—amine dyads: a fluorescence and laser flash photolysis study

Abstract

Two dyads containing a naphthalene-like chromophore linked to a pyrrolidine-derived moiety, namely (S,S)- and (R,S)-NPX-PYR, have been synthesised by esterification of (S)- or (R)-naproxen (NPX) with (S)-N-methyl-2-pyrrolidinemethanol (PYR) and submitted to photophysical studies (steady-state and time-resolved fluorescence, as well as laser flash photolysis). The emission spectra of the dyads in acetonitrile were characterised by a typical band centred at 350 nm, identical to that of the reference compound (S)-NPX. However the intensities were clearly different, revealing a significant intramolecular quenching in the dyads, as well as a remarkable stereodifferentiation (factor of 1.6). Accordingly, the fluorescence lifetimes of the two dyads were different from each other and markedly shorter than that of (S)-NPX. The quenching mechanism is intramolecular electron transfer, that is thermodynamically favoured. Exciplex formation, that is nearly thermoneutral, does not compete efficiently. The electron transfer rate constants for (S,S)- and (R,S)-(NPX-PYR) were 1.8 x 10(8) and 2.8 x 10(8) s(-1), respectively. By contrast, no significant intramolecular quenching was observed for the excited triplet states (lambda(max)= 440 nm), generated by laser flash photolysis; this is in agreement with the fact that intramolecular electron transfer is thermodynamically disfavoured, due to the lower energy of excited triplets.

Spin effects in intramolecular electron transfer in naproxen-N-methylpyrrolidine dyad

“…The foregoing is verified by measuring the MFE on the yield of the dyad's triplet state (λ obs = 440 nm [18,19]) with the help of laser flash photolysis technique. Figure 1 shows the kinetic curve for the NPX-Pyr triplet.…”

“…Note that the typical photoinduced processes, expected to occur between the excited naphthalene chromophores and the electron donors like amines, include both PET and exciplex formation [18,19,25,26,27]. Thus, these two pathways should be considered as the main quenching mechanisms.…”

“…The R,S and S,S naproxen -pyrrolidine ester dyads ((S)-N-methyl-2-pyrrolidinemethyl 2 (R) or (S)-(6-methoxy-2-naphthylpropanoate) were synthesized as described elsewhere [19]. 2-Methyl-naphthalene (Aldrich) was sublimated prior to using.…”

“…Later attempts to verify the magnetic field effect (MFE) on this reaction were unsuccessful [14,15,16,17]. The difference in the configuration-dependent fluorescence quenching in the R,S and S,S naproxen -pyrrolidine dyads ((S)-N-methyl-2-pyrrolidinemethyl 2 (R) or (S)-(6-methoxy-2-naphthylpropanoate) ( Chart 1) have been recorded quite recently [18,19]. The intramolecular electron transfer between the naproxen (NPX) and the Nmethylpyrrolidine (PYR) fragments was assumed to be the major mechanism of NPX excited state deactivation.…”

Impact of chirality on the photoinduced charge transfer in linked systems containing naproxen enantiomers

“…[19][20][21][22][23][24] Since the days of Jabotinsky's works, the photochemical generation of a pair of paramagnetic particles has been used for the modeling of biological processes such as the elementary steps of enzymatic oxygenation and drug-transport protein binding. [25][26][27] This approach is promising in several aspects.…”