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

Abstract

The absorption and emission properties of a series of amphiphilic N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10(-5) 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-1-yl)propan-1-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-1-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-1-yl)propan-1-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.

Intramolecular, Exciplex-Mediated, Proton-Coupled, Charge-Transfer Processes in N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Cations: Influence of Anion, Solvent Polarity, and Temperature

“…Importantly, Ex formation was observed only in solvents that can act as effective hydrogen-bond acceptors (N.B., THF, ethyl acetate, and 1,4-dioxane). Very little if any exciplex emission was observed in dichloromethane, or previously in solvents of much higher polarity . These results substantiate that solvent molecules, rather than the anion of PyS, are acting as the dominant proton acceptors in solution.…”

“…The synthesis and characterization of N , N -dimethyl-3-(1-pyrenyl)­propan-1-ammonium chloride (PyCl) have been described previously. , Salts with other anions were prepared by mixing dilute (∼10 –3 M) solutions of PyA in diethyl ether and 3 equiv of the corresponding concentrated acid under N 2 atmospheres. The precipitates were filtered and recrystallized from dichloromethane and diethyl ether.…”

“…Previously, we demonstrated a unique, adiabatic proton transfer for a series of N,N -dialkyl-3-(1-pyrenyl)­alkan-1-ammonium chlorides . Solutions of these salts in tetrahydrofuran (THF) exhibited strong intramolecular exciplex emission, despite the unavailability of a lone pair of electrons on the ammonium nitrogen atom.…”

“…14,15 Previously, we demonstrated a unique, adiabatic proton transfer for a series of N,N-dialkyl-3-(1-pyrenyl)alkan-1ammonium chlorides. 16 Solutions of these salts in tetrahydrofuran (THF) exhibited strong intramolecular exciplex emission, despite the unavailability of a lone pair of electrons on the ammonium nitrogen atom. The implied proton transfer was hypothesized to occur in the first few nanoseconds after excitation, giving rise to a stable, uncharged intermediate.…”

Axially chiral 1,4-dihydropyridine derivatives: aggregation-induced emission in exciplexes and application as viscosity probes

“…6 Although several examples of exciplexes are well known, [7][8][9] the examples of exciplexes with AIE properties are extremely rare. 10 Weiss et al 11 synthesized a series of N-alkyl-N-methyl-3-(pyren-1-yl) propan-1-ammonium chloride salts with spacers of propylene or butylene. Among these compounds, the compounds with propylene spacers could lead to intramolecular exciplex emissions.…”