Victor V. Prezhdo

Dynamics of the Photoexcited Electron at the Chromophore–Semiconductor Interface

Electron dynamics at molecular-bulk interfaces play a central role in a number of different fields, including molecular electronics and sensitized semiconductor solar cells. Describing electron behavior in these systems is difficult because it requires a union between disparate interface components, molecules and solid-state materials, that are studied by two different communities, chemists and physicists, respectively. This Account describes recent theoretical efforts to bridge that gap by analyzing systems that serve as good general models of the interfacial electron dynamics. The particular systems that we examine, dyes attached to TiO2, are especially important since they represent the key component of dye-sensitized semiconductor solar cells, or Gratzel cells. Gratzel cells offer a cheap, efficient alternative to traditional Si-based solar cells. The chromophore-TiO2 interface is a remarkably good target for theorists because it has already been the subject of many excellent experimental investigations. The electron dynamics in the chromophore-semiconductor systems are surprisingly rich and involve a great variety of processes as illustrated in the scheme above. The exact rates and branching ratios depend on the system details, including the semiconductor type, its bulk phase, and its exposed surface, the chromophore type, the presence or absence of a chromophore-semiconductor bridge, the alignment of the chromophore and semiconductor energy levels, the surface termination, the active vibrational modes, the solvent, the type of electrolyte, the presence of surface defects, etc. Still, the general principles governing the electron dynamics at the bulk-semiconductor interface can be understood and formulated by considering a few specific examples. The ultrafast time scale of the electronic and vibrational processes at the molecule-bulk interface make it difficult to invoke traditional theories. Instead, we perform explicit time-domain simulations with an atomistic representation of the interface. This approach most directly mimics the time-resolved experimental data and provides a detailed description of the processes as they occur in real time. The simulations described in this Account take into consideration the chemical structure of the system, determine the role of the vibrational motion and non-adiabatic coupling, uncover a vast variety of electron dynamics scenarios, and ultimately, allow us to establish the basic criteria that provide an understanding of this complicated physical process. The insights attained in the theoretical studies let us formulate a number of practical suggestions for improving the properties of the dye-sensitized semiconductor solar cell and for controlling the electron transfer across molecular-bulk interfaces.

Covalent Linking Greatly Enhances Photoinduced Electron Transfer in Fullerene-Quantum Dot Nanocomposites: Time-Domain Ab Initio Study

Nonadiabatic molecular dynamics combined with time-domain density functional theory are used to study electron transfer (ET) from a CdSe quantum dot (QD) to the C60 fullerene, occurring in several types of hybrid organic/inorganic nanocomposites. By unveiling the time dependence of the ET process, we show that covalent bonding between the QD and C60 is particularly important to ensure ultrafast transmission of the excited electron from the QD photon-harvester to the C60 electron acceptor. Despite the close proximity of the donor and acceptor species provided by direct van der Waals contact, it leads to a notably weaker QD-C60 interaction than a lengthy molecular bridge. We show that the ET rate in a nonbonded mixture of QDs and C60 can be enhanced by doping. The photoinduced ET is promoted primarily by mid- and low-frequency vibrations. The study establishes the basic design principles for enhancing photoinduced charge separation in nanoscale light harvesting materials.

Confinement by Carbon Nanotubes Drastically Alters the Boiling and Critical Behavior of Water Droplets

Vapor pressure grows rapidly above the boiling temperature, and past the critical point liquid droplets disintegrate. Our atomistic simulations show that this sequence of events is reversed inside carbon nanotubes (CNT). Droplets disintegrate first and at low temperature, while pressure remains low. The droplet disintegration temperature is independent of the CNT diameter. In contrast, depending on CNT diameter, a temperature that is much higher than the bulk boiling temperature is required to raise the internal pressure. The control over pressure by CNT size can be useful for therapeutic drug delivery.

Spin–orbit coupling and luminescence characteristics of conjugated organic molecules. I. Polyacenes

Abstract Spin–orbit coupling (SOC) of the ground and ππ ∗ , σπ ∗ , πσ ∗ , σσ ∗ singlet and triplet excited states of naphthalene, anthracene, tetracene and pentacene are investigated in the all-valence-electron CNDO/S approximations. The SOC matrix elements are calculated including one- and two-electron contributions to the single- and multi-center integrals. It is shown that the two-electron contributions cannot be neglected in the ππ ∗ excitations. The rate constants of radiative decay of fluorescent and phosphorescent states are computed using several excited state models. The lifetime and polarization of the excited state phosphorescence are found to be strongly dependent on the number of considered intermediate singlet and triplet levels. Quantum yields of fluorescence and radiative lifetimes of phosphorescent states are estimated. It is shown that for all molecules under study phosphorescence is polarized primarily perpendicular to the molecular plane.

Studies on the proton acceptor ability of phosphoryl compounds

Abstract Dipole moments and molar Kerr constants of complexes of phenols with phosphoryl compounds were studied to establish the structure of complexes and parameters characterizing the proton acceptor ability of these compounds. The structures of these complexes and parameters ( μ H , Δ ( mk ) S , log K and δ 0 ) were established. It has been suggested that a new parameter Δ ( mK ) S — structural additive difference of the molar Kerr constant — makes it possible to determine changes of polarity and polarizability of the systems during complex formation.

Solute−Solvent Interactions Determine the Effect of External Electric Field on the Intensity of Molecular Absorption Spectra

The study of the electronic absorption spectra of 4-aminoazobenzene subjected to an external electric field in nonpolar and polar solvents shows that the field-induced change in the absorption intensity is dominated by the solvent-solute interaction. Moreover, solvent can determine the sign of the change of the absorption intensity. These experimental observations are supported by ab initio electronic structure calculations and are rationalized by analytic theory. The results carry particular importance for the numerous fundamental and practical applications of electric fields to understanding and design of new materials and biological systems.

Methyl 3-(4-methoxyphenyl)prop-2-enoate

The title molecule, C(11)H(12)O(3), is almost planar, with an average deviation of the C and O atoms from the least-squares plane of 0.146(4)A. The geometry about the C=C bond is trans. The phenyl ring and -COOCH(3) group are twisted with respect to the double bond by 9.3(3) and 5.6(5) degree, respectively. The endocyclic angle at the junction of the propenoate group and the phenyl ring is decreased from 120 degree by 2.6(2) degree, whereas two neighbouring angles around the ring are increased by 2.3(2) and 0.9(2) degree. This is probably associated with the charge-transfer interaction of the phenyl ring and -COOCH(3) group through the C=C double bond. The molecules are joined together through C-H...O hydrogen bonds between the methoxy and ester groups to form characteristic zigzag chains extended along the c axis.

Vibrational Energy Transfer between Carbon Nanotubes and Nonaqueous Solvents: A Molecular Dynamics Study

We report molecular dynamics (MD) simulation of energy exchange between single-walled carbon nanotubes (CNTs) and two aprotic solvents, acetonitrile and cyclohexane. Following our earlier study of hydrated CNTs, we find that the time scales and molecular mechanisms of the energy transfer are largely independent of the nature of the surrounding medium, and therefore, should hold for other media including polymer matrices and DNA. The vibrational energy exchange between CNT and solvents exhibits two time-scales. Over half of the energy is transferred in less than one picosecond, indicating that the dominant exchange mechanism is inertial relaxation. It occurs by collisions of solvent molecules with CNT walls, facilitated by the short-range Lennard-Jones interaction. Additional several picoseconds are required for the remainder of the vibrational energy exchange, corresponding to the diffusive relaxation mechanism and involving collective molecular motions. The faster stage of the CNT-solvent energy exchange occurs on the same time-scale, and therefore, competes with the vibrational energy relaxation inside CNTs. The energy exchange time-scales are significantly influenced by the arrangement of solvent molecules inside CNTs. Generally, the effects of confinement on the dynamics can be rationalized by analysis of the solvent structure. For the same CNT diameter, the extent of the confinement effect strongly depends on the size of the solvent molecules. Icelike properties in water seen in small CNTs disappear in CNTs with intermediate diameters. In acetonitrile and cyclohexane, medium size CNTs still show strong confinement effects. Rotational motions of acetonitrile molecules are inhibited, and the cyclohexane density is dramatically decreased. The disbalance between the local temperatures of the inside and outside regions of the solvent equilibrates through a tube-mediated interaction, rather than by a direct coupling between the two solvent subsystems. In all cases, the CNT-solvent energy transfer is mediated by slow motions in the frequency range of CNT radial breathing modes.

The formation of mesophase composites under influence of the coal-tar pitch during coal carbonization

The influence of coal tar pitch on the formation of the mesophase composites from coal and pitch blends, the development of porous structure during carbonization, and the texture of matter in the pore walls has been studied using X-rays and scanning electronic microscopy (SEM). The influence of the coal-tar pitch on the structure of a coal plastic layer formed during the pyrolysis process of four Polish coals has been monitored. It was determined that, during pyrolysis of three mixtures of Z, S and K coals with coal-tar pitch, more porous composites are formed, and a denser composite is formed from P coal–pitch mixture. Based on a study of the texture of composites and carbonizates, it is suggested that during carbonization of coal Z with a coal-pitch in the plastic state there is a nematic mesophase formed, and when heating P coal–pitch mixtures, a cholesteric one is formed.

Studies on proton acceptor ability of SOx-containing compounds

Abstract IR spectra, dipole moments and molar Kerr constants of complexes of phenols with compounds containing SO x groups were studied to establish the structure of the complexes and the parameters characterizing the proton acceptor capability of these compounds. ( μ H , Δ ( mK ), log K and δ 0 ). It has been suggested that the new parameter Δ ( mK ) — the structural additive difference of the molar Kerr constant — makes it possible to determine changes of polarity and polarizability of the systems during complex formation.