'Remote' Adiabatic Photoinduced Deprotonation and Aggregate Formation of Amphiphilic N-Alkyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium Chloride Salts

摘要

The absorption and emission properties of a series of amphiphilic N-alkyl-N-methyl-3-(pyren-l-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10s M concentrations in tetrahydrofuran (THF), salts with at least one N-H bond exhibited broad, structureless emissions even though time-correlated single photon counting (TCSPC) experiments indicated negligible static or dynamic intermolecular interactions. Salts with a butylene spacer or lacking an N-H bond showed no discernible structureless emission; their emission spectra were dominated by the normal monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium, followed by the formation of an intramolecular exciplex consisting of amine and pyrenyl groups. The proposed mechanism involves excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic stabilization. At higher concentrations, with longer N-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static excimeric emissions. For example, whereas the emission spectrum of 10~(-3) M N-hexyl-N-methyl-3-(pyren-l-yl)propan-l-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission from 10~(-5) M N-dodecyl-N-methyl-3-(pyren-1-yl)propan-l-ammonium chloride in 1:9 (v:v) ethanol/water solutions was dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat solid of N-dodecyl-N-methyl-3-(pyren-l-yl)propan-l-ammonium chloride indicate that molecular packing in the microparticles and in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where conformational constraints hold ammonium groups over aromatic rings of peptide units.