Single Ensemble Non-exponential Photoluminescent Population Decays from a Broadband White-Light-Emitting Perovskite.

The mechanism of white-light emission from layered Pb-X (X = Cl or Br) perovskites following UV excitation has generated considerable interest. Prior time-dependent studies indicated that the broadband photoluminescence (PL) from (110) perovskites arises from a distribution of self-trapped excitonic sites emitting in different regions of the visible spectrum with different decay dynamics. Here, using time-correlated single photon counting to study single crystals, we show that the white-light emission decay from the (110) perovskite (EDBE)PbBr4 (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) behaves as a single ensemble. Following the rapid decay (0.6 ns) of a small spectral side band, the broad emission line shape is constant to 100 ns. We propose that rapid local structural fluctuations cause the self-trapped excitons (STEs) to experience a wide range of energies, resulting in the very broad PL. The STEs sample fluctuating local environments on time scales fast compared to the PL, which averages the PL decay at all emission wavelengths, yielding single ensemble PL dynamics. Although emission occurs from a very wide, inhomogeneously broadened spectral line with time-averaged single ensemble luminescence dynamics, the decay is tri-exponential. Two heuristic models for the tri-exponential decay involving defects are discussed. Spin-coated films show faster non-exponential decays with the slowest component of the crystal PL absent. Like the crystals, the film PL decays as a single ensemble. These results demonstrate that the broadband emission decay of (EDBE)PbBr4 arises from a time-averaged single ensemble and not from a set of excited states emitting with distinct luminescence decays at different wavelengths.

Proton Transfer Dynamics in the Aprotic Proton Accepting Solvent 1-Methylimidazole.

The dynamics of proton transfer to the aprotic solvent 1-methylimidazole (MeIm, proton acceptor) from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was investigated using fast fluorescence measurements. The closely related molecule, 8-methoxypyrene-1,3,6-trisulfonic acid trisodium salt (MPTS), which is not a photoacid, was also studied for comparison. Following optical excitation, the wavelength-dependent population dynamics of HPTS in MeIm resulting from the deprotonation process were collected over the entire fluorescence emission window. Analysis of the time-dependent fluorescence spectra revealed four distinct fluorescence bands that appear and decay on different time scales. We label these four states as protonated (P), associated I (AI), associated II (AII), and deprotonated (D). We find that the simple kinetic scheme of P → AI → AII → D is not consistent with the data. Instead, the kinetic scheme that describes the data has P decaying into AI, which mainly goes on to deprotonation (D), but AI can also feed into AII. AII can return to AI or decay to the ground state, but does not deprotonate within experimental error. Quantum chemistry and excited state QM/MM Born-Oppenheimer molecular dynamics simulations indicate that AI and AII are two H-bonding conformations of MeIm to the HPTS hydroxyl, axial, and equatorial, respectively.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy.

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Reorientation-induced Stokes shifts caused by directional interactions in electronic spectroscopy: Fast dynamics of poly(methyl methacrylate).

Dynamic Stokes shift measurements report on structural relaxation, driven by a dipole created in a chromophore by its excitation from the ground electronic state to the S1 state. Here, we demonstrate that it is also possible to have an additional contribution from orientational relaxation of the Stokes shift chromophore. This effect, called reorientation-induced Stokes shift (RISS), can be observed when the reorientation of the chromophore and the solvent structural relaxation occur on similar time scales. Through a vector interaction, the electronic transition of the chromophore couples to its environment. The orientational diffusive motions of the chromophores will have a slight bias toward reducing the transition energy (red shift) as do the solvent structural diffusive motions. RISS is manifested in the polarization-dependence of the fluorescence Stokes shift using coumarin 153 (C153) in poly(methyl methacrylate) (PMMA). A similar phenomenon, reorientation-induced spectral diffusion (RISD), has been observed and theoretically explicated in the context of two dimensional infrared (2D IR) experiments. Here, we generalize the existing RISD theory to include properties of electronic transitions that generally are not present in vibrational transitions. Expressions are derived that permit determination of the structural dynamics by accounting for the RISS contributions. Using these generalized equations, the structural dynamics of the medium can be measured for any system in which the directional interaction is well represented by a first order Stark effect and RISS or RISD is observed. The theoretical results are applied to the PMMA data, and the structural dynamics are obtained and discussed.

The influence of mesoscopic confinement on the dynamics of imidazolium-based room temperature ionic liquids in polyether sulfone membranes.

The structural dynamics of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (CnmimNTf2, n = 2, 4, 6, 10: ethyl-Emim; butyl-Bmim; hexyl-Hmim; decyl-Dmim) room temperature ionic liquids confined in the pores of polyether sulfone (PES 200) membranes with an average pore size of ∼350 nm and in the bulk liquids were studied. Time correlated single photon counting measurements of the fluorescence of the fluorophore coumarin 153 (C153) were used to observe the time-dependent Stokes shift (solvation dynamics). The solvation dynamics of C153 in the ionic liquids are multiexponential decays. The multiexponential functional form of the decays was confirmed as the slowest decay component of each bulk liquid matches the slowest component of the liquid dynamics measured by optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments, which is single exponential. The fact that the slowest component of the Stokes shift matches the OHD-OKE data in all four liquids identifies this component of the solvation dynamics as arising from the complete structural randomization of the liquids. Although the pores in the PES membranes are large, confinement on the mesoscopic length scale results in substantial slowing of the dynamics, a factor of ∼4, for EmimNTf2, with the effect decreasing as the chain length increases. By DmimNTf2, the dynamics are virtually indistinguishable from those in the bulk liquid. The rotation relaxation of C153 in the four bulk liquids was also measured and showed strong coupling between the C153 probe and its environment.

Ultrafast to Ultraslow Dynamics of a Langmuir Monolayer at the Air/Water Interface Observed with Reflection Enhanced 2D IR Spectroscopy.

Monolayers play important roles in naturally occurring phenomena and technological processes. Monolayers at the air/water interface have received considerable attention, yet it has proven difficult to measure monolayer and interfacial molecular dynamics. Here we employ a new technique, reflection enhanced two-dimensional infrared (2D IR) spectroscopy, on a carbonyl stretching mode of tricarbonylchloro-9-octadecylamino-4,5-diazafluorenerhenium(I) (TReF18) monolayers at two surface densities. Comparison to experiments on a water-soluble version of the metal carbonyl headgroup shows that water hydrogen bond rearrangement dynamics slow from 1.5 ps in bulk water to 3.1 ps for interfacial water. Longer time scale fluctuations were also observed and attributed to fluctuations of the number of hydrogen bonds formed between water and the three carbonyls of TReF18. At the higher surface density, two types of TReF18 minor structures are observed in addition to the main structure. The reflection method can take usable 2D IR spectra on the monolayer within 8 s, enabling us to track the fluctuating minor structures' appearance and disappearance on a tens of seconds time scale. 2D IR chemical exchange spectroscopy further shows these structures interconvert in 30 ps. Finally, 2D spectral line shape evolution reveals that it takes the monolayers hours to reach macroscopic structural equilibrium.

Proton Transfer in Perfluorosulfonic Acid Fuel Cell Membranes with Differing Pendant Chains and Equivalent Weights.

Proton transfer in the nanoscopic water channels of polyelectrolyte fuel cell membranes was studied using a photoacid, 8-hydroxypyrene-1,3,6-trisulfonic acid sodium salt (HPTS), in the channels. The local environment of the probe was determined using 8-methoxypyrene-1,3,6-trisulfonic acid sodium salt (MPTS), which is not a photoacid. Three fully hydrated membranes, Nafion (DuPont) and two 3M membranes, were studied to determine the impact of different pendant chains and equivalent weights on proton transfer. Fluorescence anisotropy and excited state population decay data that characterize the local environment of the fluorescent probes and proton transfer dynamics were measured. The MPTS lifetime and anisotropy results show that most of the fluorescent probes have a bulk-like water environment with a relatively small fraction interacting with the channel wall. Measurements of the HPTS protonated and deprotonated fluorescent bands' population decays provided information on the proton transport dynamics. The decay of the protonated band from ∼0.5 ns to tens of nanoseconds is in part determined by dissociation and recombination with the HPTS, providing information on the ability of protons to move in the channels. The dissociation and recombination is manifested as a power law component in the protonated band fluorescence decay. The results show that equivalent weight differences between two 3M membranes resulted in a small difference in proton transfer. However, differences in pendant chain structure did significantly influence the proton transfer ability, with the 3M membranes displaying more facile transfer than Nafion.

The Influence of Water on the Alkyl Region Structure in Variable Chain Length Imidazolium-Based Ionic Liquid/Water Mixtures.

Solutions of room temperature ionic liquids (RTILs) and water were studied by observing the reorientational dynamics of the fluorescent probe perylene. Perylene is solvated in the alkyl regions of the RTILs. Its D2h symmetry made it possible to extract dynamical information on both in-plane and out-of-plane reorientation from time-resolved fluorescence anisotropy measurements. Perylene reorientation reports on its interactions with the alkyl chains. The RTILs were a series of 1-alkyl-3-methylimidazolium tetrafluoroborates (CnmimBF4, where n is the number of carbons in the alkyl chain), and the effects on perylene's dynamics were observed when varying the alkyl chain length of the cation (n = 4, 6, 8, and 10; butyl, hexyl, octyl, decyl) and varying the water content from pure RTIL to roughly three water molecules per RTIL ion pair. Time correlated single photon counting was used to measure the fluorescence anisotropy decays to determine the orientational dynamics. The friction coefficients for both the in-plane and out-of-plane reorientation were determined to eliminate the influence of changes in viscosity caused by both the addition of water and the different alkyl chain lengths. The friction coefficients provided information on the interactions of the perylene with the alkyl environment and how these interactions changed with chain length and water content. As the chain length increased, the addition of water had less of an effect on the local alkyl environment surrounding the perylene. The friction coefficients generally increased with higher water contents; the in-plane orientational motion was hindered significantly more than the out-of-plane motion. The restructuring of the alkyl regions is likely a consequence of a rearrangement of the ionic imidazolium head groups to accommodate partial solvation by water, which results in a change in the arrangement of the alkyl chains. At very high water content, BmimBF4 broke this general trend, with both in-plane and out-of-plane rotational friction decreasing above a water content of one water per ion pair. This decrease indicates a major reorganization of the overall liquid structure in high water content mixtures. In contrast to BmimBF4, the longer chain length RTILs are not infinitely miscible with water, and do not show evidence of a major reorganization before reaching saturation and phase-separating. The results suggest that phase separation in longer chain length BF4 RTILs is a consequence of their inability to undergo the reorganization of the alkyl regions necessary to accommodate high water concentrations.