Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe structural dynamics under thermal equilibrium conditions on time scales ranging from femtoseconds to approximately 100 ps and longer. One of the important uses of 2D-IR spectroscopy is to monitor the dynamical evolution of a molecular system by reporting the time dependent frequency fluctuations of an ensemble of vibrational probes. The vibrational frequency-frequency correlation function (FFCF) is the connection between the experimental observables and the microscopic molecular dynamics and is thus the central object of interest in studying dynamics with 2D-IR vibrational echo spectroscopy. A new observable is presented that greatly simplifies the extraction of the FFCF from experimental data. The observable is the inverse of the center line slope (CLS) of the 2D spectrum. The CLS is the inverse of the slope of the line that connects the maxima of the peaks of a series of cuts through the 2D spectrum that are parallel to the frequency axis associated with the first electric field-matter interaction. The CLS varies from a maximum of 1 to 0 as spectral diffusion proceeds. It is shown analytically to second order in time that the CLS is the T(w) (time between pulses 2 and 3) dependent part of the FFCF. The procedure to extract the FFCF from the CLS is described, and it is shown that the T(w) independent homogeneous contribution to the FFCF can also be recovered to yield the full FFCF. The method is demonstrated by extracting FFCFs from families of calculated 2D-IR spectra and the linear absorption spectra produced from known FFCFs. Sources and magnitudes of errors in the procedure are quantified, and it is shown that in most circumstances, they are negligible. It is also demonstrated that the CLS is essentially unaffected by Fourier filtering methods (apodization), which can significantly increase the efficiency of data acquisition and spectral resolution, when the apodization is applied along the axis used for obtaining the CLS and is symmetrical about tau=0. The CLS is also unchanged by finite pulse durations that broaden 2D spectra.

Hydrogen bond dynamics of water in NaBr solutions are studied by using ultrafast 2D IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments. The hydrogen bond structural dynamics are observed by measuring spectral diffusion of the OD stretching mode of dilute HOD in H2O in a series of high concentration aqueous NaBr solutions with 2D IR vibrational echo spectroscopy. The time evolution of the 2D IR spectra yields frequency-frequency correlation functions, which permit quantitative comparisons of the influence of NaBr concentration on the hydrogen bond dynamics. The results show that the global rearrangement of the hydrogen bond structure, which is represented by the slowest component of the spectral diffusion, slows, and its time constant increases from 1.7 to 4.8 ps as the NaBr concentration increases from pure water to Ϸ6 M NaBr. Orientational relaxation is analyzed with a wobbling-in-a-cone model describing restricted orientational diffusion that is followed by complete orientational randomization described as jump reorientation. The slowest component of the orientational relaxation increases from 2.6 ps (pure water) to 6.7 ps (Ϸ6 M NaBr). Vibrational population relaxation of the OD stretch also slows significantly as the NaBr concentration increases.ultrafast 2D IR spectroscopy ͉ water dynamics in ionic solutions W ater plays an important role in chemical and biological processes. In aqueous solutions, water molecules dissolve ionic compounds, charged chemical species, and biomolecules by forming hydration shells (layers) around them. Pure water undergoes rapid structural evolution of the hydrogen bond network that is responsible for water's unique properties (1). A question of fundamental importance is how the dynamics of water in the immediate vicinity of an ion or ionic group differ from those of pure water. For monatomic ions, molecular ions, charged groups of large molecules, or charged amino acids on the surfaces of proteins, the basic structure of hydration shells is determined by ion-dipole interactions between water molecules and the charged group (2, 3). Such ion-dipole interactions will influence both the structure and dynamics of water in the proximity of ions. Water dynamics in ion hydration shells play a significant role in the nature of systems such as proteins and micelles (3) and in processes such as ion transport through transmembrane proteins (4).Over the last several years, the application of ultrafast IR vibrational echo spectroscopy (5, 6), particularly 2D IR vibrational echo experiments, (7, 8) combined with molecular dynamics (MD) simulations (9-12) have greatly enhanced understanding of the hydrogen bond dynamics in pure water. These experiments directly examine the dynamics of water rather than study the indirect influence of water dynamics on a probe molecule (13). The ultrafast 2D IR vibrational echo experiments on water (7, 8) and other hydrogen-bonded systems (14) build upon earlier IR pump-probe experiments that have been extensively used to study ...

Hydrogen bond dynamics of water in highly concentrated NaBr salt solutions and reverse micelles are studied using ultrafast 2D-IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments performed on the OD hydroxyl stretch of dilute HOD in H(2)O. The vibrational echo experiments measure spectral diffusion, and the pump-probe experiments measure orientational relaxation. Both experimental observables are directly related to the structural dynamics of water's hydrogen bond network. The measurements performed on NaBr solutions as a function of concentration show that the hydrogen bond dynamics slow as the NaBr concentration increases. The most pronounced change is in the longest time scale dynamics which are related to the global rearrangement of the hydrogen bond structure. Complete hydrogen bond network randomization slows by a factor of approximately 3 in approximately 6 M NaBr solution compared to that in bulk water. The hydrogen bond dynamics of water in nanoscopically confined environments are studied by encapsulating water molecules in ionic head group (AOT) and nonionic head group (Igepal CO 520) reverse micelles. Water dynamics in the nanopools of AOT reverse micelles are studied as a function of size by observing orientational relaxation. Orientational relaxation dynamics deviate significantly from bulk water when the size of the reverse micelles is smaller than several nm and become nonexponential and slower as the size of the reverse micelles decreases. In the smallest reverse micelles, orientational relaxation (hydrogen bond structural randomization) is almost 20 times slower than that in bulk water. To determine if the changes in dynamics from bulk water are caused by the influence of the ionic head groups of AOT or the nanoconfinement, the water dynamics in 4 nm nanopools in AOT reverse micelles (ionic) and Igepal reverse micelles (nonionic) are compared. It is found that the water orientational relaxation in the 4 nm diameter nanopools of the two types of reverse micelles is almost identical, which indicates that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

A novel strategy for designing highly efficient and activatable photosensitizers that can effectively generate reactive oxygen species (ROS) under both normoxia and hypoxia is proposed. Replacing both oxygen atoms in conventional naphthalimides (RNI-O) with sulfur atoms led to dramatic changes in the photophysical properties. The remarkable fluorescence quenching (Φ PL ≈ 0) of the resulting thionaphthalimides (RNI-S) suggested that the intersystem crossing from the singlet excited state to the reactive triplet state was enhanced by the sulfur substitution. Surprisingly, the singlet oxygen quantum yield of RNI-S gradually increased with increasing electron-donating ability of the 4-R substituents (MANI-S, Φ Δ ≈ 1.00, in air-saturated acetonitrile). Theoretical studies revealed that small singlet−triplet energy gaps and large spin−orbit coupling could be responsible for the efficient population of the triplet state of RNI-S. In particular, the ROS generation ability of MANI-S was suppressed under physiological conditions due to their self-assembly and was significantly recovered in cancer cells. More importantly, cellular experiments showed that MANI-S still produced a considerable amount of ROS even under severely hypoxic conditions (1% O 2 ) through a type-I mechanism.

We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

Electron dynamics in Au nanorods are studied with femtosecond nonlinear spectroscopic techniques, by directly exciting and probing the longitudinal surface plasmon resonance. The dispersive and absorptive parts of the third-order signal are measured using optical heterodyne detected four-wave-mixing spectroscopy. These signals are used to describe dynamics in Au nanorods in terms of frequency shift and broadening of the plasmon resonance. Pump−probe experiments are performed with a series of pump intensities. The results are treated in two ways:  (1) by calculating the temperature changes of electrons and phonons in the nanorods and the effects of these temperatures on the dielectric constant of Au; and (2) by a nonlinear least-squares fitting using a phenomenological response function. The first model agrees with the pump−probe experimental results for pump energies up to 2.0 nJ (2.5 GW/cm2) and for delays in the range of 150 fs to 150 ps, but does not reproduce three additional features present in the data and the phenomenological model:  (1) an “instantaneous” response, attributed to coherent plasmon oscillation; (2) a decaying component with an intensity-independent time constant of 170 fs, attributed to a nonthermal electron distribution or to two-photon-excited interband transitions; and (3) oscillations with a period of 71 ps, attributed to coherent vibration of the rods. Higher pump intensities yield substantial deviation at short delays from the lower-intensity response. Additional plasmon damping and higher-order nonlinear mechanisms are suggested to account for these deviations.

ConspectusWater is ubiquitous in nature, but it exists as pure water infrequently. From the ocean to biology, water molecules interact with a wide variety of dissolved species. Many of these species are charged. In the ocean, water interacts with dissolved salts. In biological systems, water interacts with dissolved salts as well as with charged amino acids, the zwitterionic head groups of membranes, and other biological groups that carry charges. Water plays a central role in vast number of chemical processes because of its dynamic hydrogen bond network. A water molecule can form up to four hydrogen bonds in an approximately tetrahedral arrangement. These hydrogen bonds are continually being broken and new bonds are being formed on a picosecond time scale. The ability of water's hydrogen bond network to rapidly reconfigure enables water to accommodate and facilitate chemical processes. Therefore, the influence of charged species on water hydrogen bond dynamics is important.Recent advances in ultrafast coherent infrared spectroscopy have greatly expanded our understanding of water dynamics. Two dimensional infrared (2D IR) vibrational echo spectroscopy is providing new observables that yield direct information on the fast dynamics of molecules in their ground electronic state under thermal equilibrium conditions. 2D IR vibrational echoes are akin to 2D NMR but operate on time scales that are many orders of magnitude shorter. In a 2D IR vibrational echo experiment (see Conspectus figure), three IR pulses are tuned to the vibrational frequency of interest, which in this case is the frequency of the hydroxyl stretching mode of water. The first two pulses "label" the initial molecular structures by their vibrational frequencies. The system evolves between pulses two and three, and the third pulse stimulates the emission of the vibrational echo pulse, which is the signal. The vibrational echo pulse is heterodyne detected by combining it with another pulse, the local oscillator. Heterodyne detection provides phase and amplitude information, which are both necessary to perform the two Fourier transforms that take the data from the time domain to a two dimensional frequency domain spectrum. The time dependence of a series of 2D IR vibrational echo spectra provides direct information on system dynamics.Here we use two types of 2D IR vibrational echo experiments to examine the influence that charged species have on water hydrogen bond dynamics. Solutions of NaBr and NaBF 4 are studied. The NaBr solutions are studied as a function of concentration using vibrational echo measurements of spectral diffusion and polarization selective IR pump-probe measurements of orientational relaxation. Both types of measurements show the slowing of hydrogen bond network structural evolution with increasing salt concentration. NaBF 4 is studied using vibrational echo chemical exchange spectroscopy. In these experiments it is possible to directly observe the chemical exchange of water molecules switching their hydrogen bond partners between ...

Novel BODIPY photosensitizers were developed for imaging-guided photodynamic therapy. The introduction of a strong electron donor to the BODIPY core through a phenyl linker combined with the twisted arrangement between the donor and the BODIPY acceptor is essential for reducing the energy gap between the lowest singlet excited state and the lowest triplet state (DE ST ), leading to a significant enhancement in the intersystem crossing (ISC) of the BODIPYs. Remarkably, the BDP-5 with the smallest DE ST (ca. 0.44 eV) exhibited excellent singlet oxygen generation capabilities in both organic and aqueous solutions. BDP-5 also displayed bright emission in the far-red/near-infrared region in the condensed states. More importantly, both in vitro and in vivo studies demonstrated that BDP-5 NPs displayed a high potential for photodynamic cancer therapy and bioimaging.