John M. Papanikolas

Chemical approaches to artificial photosynthesis

In the early 1970s, the works by Fujishima and Honda (1) and Honda et al. (2) reported on the results of a now famous experiment. They showed that band gap excitation of anatase TiO2 in a photoelectrochemical cell with a Pt counter electrode and an applied bias resulted in water splitting into hydrogen and oxygen. The timing of the result was impeccable. In 1973, the Organization of the Petroleum Exporting Countries (OPEC) declared an embargo on oil imports to the West, resulting in gasoline shortages and long lines at gas pumps. Suddenly, there was a pressing need for energy independence and new ways of providing for the energy-hungry economies of Western Europe, Japan, and the United States. The international research community responded. There was a short lived explosion of interest in converting sunlight into high-energy molecules by what we now call artificial photosynthesis to make solar fuels. Target reactions were water splitting into hydrogen and oxygen (1) and light-driven reduction of CO2 by water to give CO, other oxygenates, or hydrocarbons. Methane is shown as the product in equation 2, but the ultimate target is liquid hydrocarbons to power our existing energy infrastructure (1 and 2):

Molecular Chromophore–Catalyst Assemblies for Solar Fuel Applications

Applications Dennis L. Ashford,† Melissa K. Gish,† Aaron K. Vannucci,‡ M. Kyle Brennaman,† Joseph L. Templeton,† John M. Papanikolas,† and Thomas J. Meyer*,† †Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599, United States ‡Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

I−2 photodissociation and recombination dynamics in size‐selected I−2(CO2)n cluster ions

Pump–probe techniques are used in conjunction with a tandem time‐of‐flight mass spectrometer to investigate the I...I− cage recombination dynamics following I−2 photodissociation in size‐selected I−2(CO2)n cluster ions. The absorption recovery, which reflects the recombination and vibrational relaxation of the photodissociated I−2, exhibits a strong cluster size dependence in the range of n=13–15. Over this limited cluster size range, the absorption recovery time decreases from ∼40 ps (n≤12) to ∼10 ps (n≥15). In addition, a recurrence is observed at ≊2 ps in the absorption recovery of the larger clusters (n=14–17). This feature results from coherent I...I− motion following photodissociation. Measurement of the absorption recovery with both parallel and perpendicular pump–probe polarizations demonstrates that the pump and probe transition dipoles lie in the same direction. Analysis of the I−2 transition dipole directions shows that the coherent motion takes place on the first two repulsive excited potentia...

I2− photofragmentation/recombination dynamics in size-selected I2−(CO2)n cluster ions: Observation of coherent I...I− vibrational motion

We have employed picosecond pump–probe techniques in conjunction with a tandem time‐of‐flight mass spectrometer to investigate the caging dynamics of photodissociated I2− solvated with a specific number of CO2 molecules. In this paper, we report the observation of a recurrence at ≊2 ps in the I2− absorption recovery, a feature which is attributed to coherent I...I− nuclear motion following I2− photoexcitation.

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck–Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated +H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.

Direct imaging of free carrier and trap carrier motion in silicon nanowires by spatially-separated femtosecond pump-probe microscopy.

We have developed a pump-probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of ∼10(4) cm/s. The free carrier dynamics are followed by trap carrier migration on slower time scales.

Time‐resolved measurements of the photodissociation and recombination dynamics of I−2 in mass selected cluster ions

We present picosecond time‐resolved pump‐probe measurements of the photodissociation and recombination dynamics of I−2 surrounded by a specific number of CO2 molecules in mass selected I−2 (CO2)n clusters. The transient bleaching data can be fit to an exponential absorption recovery of 30±10 ps in I−2 (CO2)9 clusters and 10±5 ps in I−2 (CO2)16 clusters. The data demonstrate the feasibility of measurements of real‐time reaction dynamics in microsolvent environments.

Recombination and relaxation of molecular ions in size‐selected clusters: Monte Carlo and molecular dynamics simulations of I−2 (CO2)n

The equilibrium structures and the recombination dynamics of I−2 molecular ions embedded in clusters of 3–17 CO2 molecules are studied by Monte Carlo and molecular dynamics simulations. The potential model incorporates, in a self‐consistent manner, a description of the I−2 electronic structure that depends on both the I−2 bond length and the solvent degrees of freedom. The influence of the solvent upon the I−2 electronic structure is treated by means of a single effective solvent coordinate, in a manner reminiscent of the theory of electron transfer reactions. This causes the electronic charge to localize on a single I atom when the I–I bond is long or when the solvent cage has become highly asymmetric. The primary focus is the I−2 vibrational relaxation that follows recombination. Simulations of I−2(CO2)16 and I−2(CO2)9 yield vibrational relaxation times of less than 3 ps, even faster than the experimentally observed absorption recovery time of 10–40 ps. It is suggested that the latter time scale is dete...

Phase and amplitude control in the formation and detection of rotational wave packets in the E 1Σg+ state of Li2

Femtosecond laser pulse amplitude/phase masking techniques are employed to control the formation and detection of rotational wave packets in the electronic E 1Σg+ state of lithium dimer. The wave packets are prepared by coherent excitation of rovibronic E 1Σg+(νE,JE) states of Li2 from a single intermediate state, A 1Σu+(νA=11, JA=28), and probed by time-resolved photoionization. In the detection step, the wave packet is projected onto the X 2Σg+ state of Li2+. New resonance structure in the X 2Σu+ ionic state continuum is obtained by measuring the wave packet signal modulation amplitude as a function of the frequencies removed from the spectrally dispersed probe pulse by insertion of a wire mask in a single-grating pulse shaper. A split glass phase mask inserted into the pulse shaper is used to produce step function changes in the spectral phase of the pulse. The phase relation among the wave packet states is varied by changing the relative phases of spectral components in the pump pulse and is monitored...