Photophysics of N,N-dimethyl-3-(1-indolyl)propan-1-ammonium chloride and related derivatives.

The photophysical properties of two new indole derivatives have been examined by steady-state and dynamic spectroscopic methods. The ground-state structures and conformations of 3-(1-indolyl)-N,N-dimethylpropan-1-ammonium chloride (InCl) and 3-(1-indolyl)-N,N,N-trimethylpropan-1-ammonium chloride (MeInCl) have been examined through density functional theory calculations. These calculations reveal a preference for a 'closed' conformation which places the cationic ammonium group in proximity to the π-electron cloud in low polarity environments. This interaction is best described as an intramolecular hydrogen-π bond in the case of InCl and a cation-π interaction for MeInCl. The ground-state conformational equilibria are influenced by changes in the dielectric constant of the solvent, resulting in a variety of photophysical behaviors. The excitation/emission spectra, fluorescence quantum yields, and excited-state lifetimes, are reported for InCl, MeInCl, and a reference compound, 1-methylindole, in 1,4-dioxane (ε = 2), acetonitrile (ε = 37), and water (ε = 78) where solubility allows. Data from these solvents provide evidence for independent fluorescence quenching pathways for InCl and MeInCl. In addition, they lead to insights into the complexities of indole photophysics by demonstrating the sensitivity of the locally-excited states to changes in charge-density and solvent environment.

Proton-coupled charge-transfer reactions and photoacidity of N,N-dimethyl-3-arylpropan-1-ammonium chloride salts.

Photoinduced intermolecular proton-transfer processes from N,N-dimethyl-3-arylpropan-1-ammonium chloride salts (ArCl, with aryl as 1-pyrenyl, 9-anthryl, and 2-naphtyl) to a solvent molecule have been investigated by steady-state and dynamic spectroscopic methods. The intermolecular proton-transfers are coupled either to the formation of an exciplex or to a solvent-separated ion pair in what we have termed a 'proton-coupled charge-transfer reaction'. A range of solvents has been observed to mediate both the ground-state conformations of the ArCl and the extent of electron transfer. Unlike typical photoacids, in which through-bond interactions control photoacidity, through-space charge-transfer interactions are responsible in the excited singlet states of the ArCl. Transient absorption experiments reveal a range of electronic comportments in the excited-states of the ArCl. Excited-state pKa values of -3.4, 1.3, and -3.3 in THF were calculated using a Förster-like approach for the 1-pyrenyl, 9-anthryl, and 2-naphthyl salts, respectively. The observed rate of proton-transfer was found to be independent of the thermodynamic driving force and the short-term reversibility of these reactions has been approximated. The data suggest how other systems may be designed to facilitate this novel process.

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.

An intramolecular exciplex-mediated, proton-coupled, charge-transfer (PCCT) process has been investigated for a series of N,N-dimethyl-3-(1-pyrenyl)propan-1-ammonium cations with different anions (PyS) in solvents of low to intermediate polarity over a wide temperature range. Solvent mediates both the equilibrium between conformations of the cation that place the pyrenyl and ammonium groups in proximity (conformation C) or far from each other (conformation O) and the ability of the ammonium group to transfer a proton adiabatically in the PyS excited singlet state. Thus, exciplex emission, concurrent with the PCCT process, was observed only in hydrogen-bond accepting solvents of relatively low polarity (tetrahydrofuran, ethyl acetate, and 1,4-dioxane) and not in dichloromethane. From the exciplex emission and other spectroscopic and thermodynamic data, the acidity of the ammonium group in conformation C of the excited singlet state of PyS (pKa*) has been estimated to be ca. -3.4 in tetrahydrofuran. The ratios between the intensities of emission from the exciplex and the locally excited state (IEx/ILE) appear to be much more dependent on the nature of the anion than are the rates of exciplex formation and decay, although the excited state data do not provide a quantitative measure of the anion effect on the C-O equilibrium. The activation energies associated with exciplex formation in THF are calculated to be 0.08 to 0.15 eV lower than for the neutral amine, N,N-dimethyl-3-(1-pyrenyl)propan-1-amine. Decay of the exciplexes formed from the deprotonation of PyS is hypothesized to occur through charge-recombination processes. To our knowledge, this is the first example in which photoacidity and intramolecular exciplex formation (i.e., a PCCT reaction) are coupled.