Non-adiabatic dissociation dynamics of Ar⋯I2 (E, v) intermolecular vibrational levels probed using velocity-map imaging.

Ion time-of-flight velocity-map imaging was used to measure the kinetic-energy distributions of the I2 ion-pair fragments formed after photoexcitation of Ar⋯I2 complexes to intermolecular vibrational levels bound within the Ar + I2 (E, vE = 0-2) potential energy surfaces. The kinetic-energy distributions of the I2 products indicate that complexes in the Ar⋯I2 (E, vE) levels preferentially dissociate into I2 in the D and ß ion-pair states with no change in I2 vibrational excitation. The energetics of the levels prepared suggest that there is a non-adiabatic coupling of the initially prepared levels with the continuum of states lying above the Ar + I2 (D, vD = vE) and Ar + I2 (ß, vß = vE) dissociation limits. The angular anisotropies of the I2 product signals collected for many of the Ar⋯I2 (E, vE) levels have maxima parallel to the laser polarization axis. This contradicts expectations for the prompt dissociation of complexes with T-shaped geometries, which would result in images with maxima perpendicular to the polarization axis. These anisotropies suggest that there is a perturbation of the transition moment in these clusters or there are additional intermolecular interactions, likely those sampled while traversing above the attractive wells of the lower-energy potentials during dissociation. I2 (D', vD') products are also identified when preparing several of the low-lying levels localized in the T-shaped well of the Ar + I2 (E, vE = 0-2) potentials, and they are formed in multiple νD' vibrational levels spanning energy ranges up to 500 cm-1.

Bound-Ion Pair X-Type Ligation of Cadmium and Zinc Dithiocarbamates on Cadmium Selenide Quantum Belts.

Wurtzite CdSe quantum belts with L-type n-octylamine, L-type ammonia, or Z-type Cd(oleate)2 ligands are exchanged for several metal-dithiocarbamate ligands [M(S2CNR1R2)2]: Cd(S2CNPhMe)2, Cd(S2CNEt2)2, Zn(S2CNPhMe)2, and Zn(S2CNEt2)2. Successful ligand exchange with all M(S2CNR1R2)2 compounds occurs from {CdSe[Cd(oleate)2]0.19} quantum belts (QBs), which induce similar spectral shifts in the absorption spectra of the ligand-exchanged QBs. Spectroscopic data, experimentally determined lattice strains, and ligand exchanges with [Na][Et2NCS2] and [NH4][MePhNCS2] establish that the [M(S2CNR1R2)2] ligands bind as bound-ion-paired X-type ligands with (S2CNR1R2)- groups ligated directly to the QB surfaces and [M(S2CNR1R2)]+ groups serving as the charge-balancing ion-paired countercations. The X-type dithiocarbamate ligands do not impart any special electronic effects to the CdSe QBs.

Probing the Dependence of Long-Range, Four-Atom Interactions on Intermolecular Orientation. 4. The Dissociation Dynamics of H2/D2···ICl(B,v'=3) and the Observation of Efficient Vibrational-Rotational Energy Transfer.

The vibrational predissociation dynamics of H2/D2···I35Cl(B,v'=3) complexes containing both para- and ortho-hydrogen prepared in different intermolecular vibrational levels were investigated. The Δv = -1 I35Cl(B,v = 2,j) rotational product-state distributions measured for excitation to the lowest-energy T-shaped levels of these complexes are mostly bimodal. The rotational distributions measured for excitation of the H2···I35Cl(B,v'=3) complexes are colder than those of the D2···I35Cl(B,v'=3) complexes, and there are only slight differences between those measured for the para- and ortho-hydrogen containing complexes. Excitation of the delocalized bending levels results in slightly colder rotational product-state distributions. The distributions suggest the dynamics result from more than impulsive dissociation off of the inner repulsive wall of the lower-energy H2/D2 + I35Cl(B,v = 2) potential surfaces of the products. The depths of these potentials and the energies available to these products also contribute to the dynamics. The formation of the Δv = -2, I35Cl(B,v = 1) product channel was only identified for excitation of levels within the ortho(j = 0)-D2 + I35Cl(B,v'=3) potential. The formation of this channel occurs via I35Cl(B,v'=3) vibrational to D2 rotational energy transfer forming the ortho(j = 2)-D2 + I35Cl(B,v = 1,j) products.

Vibrational predissociation versus intramolecular vibrational energy redistribution (IVR) in rare gas⋯dihalogen complexes: IVR identified in Ar⋯I2(B, ν') using velocity-map imaging.

The competition between multiple pathways sampled during the energetic relaxation of excited molecules can be difficult to experimentally decipher. The rare gas···dihalogen van der Waals complexes have remained key systems for exploring the competition between relaxation pathways, such as intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP). As these mechanisms can yield the same products, the relaxation pathways traversed are often deduced from the excitation spectra or product-state distributions. In addition to a brief perspective on IVR and VP in rare gas⋯dihalogen complexes, we present new results obtained using time-of-flight velocity-map imaging (VMI) on T-shaped Ar⋯I2(B, ν', n' = 0) complexes that illustrate how contributions from these two pathways can be separated. The angular anisotropies of the ion images collected for the I2(B, ν < ν') fragments indicate the products for certain Ar⋯I2(B, ν', n' = 0) levels are weighted along the direction perpendicular to the laser-polarization axis. These distributions are consistent with prompt dissociation of the T-shaped excited-state complexes, likely via direct VP. The distributions measured for other Ar⋯I2(B, ν', n' = 0) levels are preferentially along the laser-polarization axis. These initially prepared levels must undergo IVR with nearly resonant Ar⋯I2(B, ν < ν', n > 0) intermolecular vibrational levels that sample linear Ar-I-I orientations during dissociation.

Electronic predissociation in rare gas-dihalogen complexes.

The role of electronic predissociation (EP) in the dissociation dynamics of rare gas⋯dihalogen complexes (Rg⋯X2) prepared in the B electronic state was probed using ion time-of-flight velocity-map imaging. Specifically, EP of complexes prepared in the T-shaped Ar⋯I2, Ne⋯I2, He⋯I2, Ar⋯Br2, Ne⋯Br2, and He⋯Br2 levels with varying amounts of X2 vibrational excitation, ν', was investigated. The atomic I(2P3/2) or Br(2P3/2) EP fragments were probed using ion time-of-flight velocity-map imaging. Definitive evidence for EP was observed only for the Ar⋯I2 complex, and it occurs for all of the T-shaped intermolecular levels investigated, those with ν' = 12-22, 24, and 25. The relative yields for EP in these levels measured as a function of ν' are consistent with previously reported yields for the competing mechanism of vibrational predissociation. The anisotropies of the I+ images collected for Ar⋯I2 indicate that EP is occurring on timescales shorter than the rotational periods of the complex. The kinetic energy distributions of the departing I-atom fragments suggest that EP occurs from an asymmetric geometry rather than the rigid T-shaped geometry for many of the Ar⋯I2 levels prepared. These findings indicate that intramolecular vibrational redistribution of these initially prepared T-shaped levels to excited levels bound within a lower-energy intermolecular potential occurs prior to EP.

Cadmium Bis(phenyldithiocarbamate) as a Nanocrystal Shell-Growth Precursor.

Cadmium bis(phenyldithiocarbamate) [Cd(PTC)2] is prepared and structurally characterized. The compound crystallizes in the monoclinic space group P21/n. A one-dimensional polymeric structure is adopted in the solid state, having bridging PTC ligands and 6-coordinate pseudo-octahedral Cd atoms. The compound is soluble in DMSO, THF, and DMF and insoluble in EtOH, MeOH, CHCl3, CH2Cl2, and toluene. {CdSe[n-octylamine]0.53} quantum belts and Cd(PTC)2 react to deposit epitaxial CdS shells on the nanocrystals. With an excess of Cd(PTC)2, the resulting thick shells contain spiny CdS nodules grown in the Stranski-Krastanov mode. Stoichiometric control affords smooth, monolayer CdS shells. A base-catalyzed reaction pathway is elucidated for the conversion of Cd(PTC)2 to CdS, which includes phenylisothiocyanate and aniline as intermediates, and 1,3-diphenylthiourea as a final product.

Two-dimensional semiconductor nanocrystals: properties, templated formation, and magic-size nanocluster intermediates.

CONSPECTUS: Semiconductor nanocrystals having an extended length dimension and capable of efficiently transporting energy and charge would have useful applications in solar-energy conversion and other emerging technologies. Pseudocylindrical semiconductor nanowires and quantum wires are available that could potentially serve in this role. Sadly, however, their defective surfaces contain significant populations of surface trap sites that preclude efficient transport. The very large surface area of long wires is at least part of the problem. As electrons, holes, and excitons migrate along a nanowire or quantum wire, they are exposed to an extensive surface and to potentially large numbers of trap sites. A solution to this dilemma might be found by identifying "long" semiconductor nanocrystals of other morphologies that are better passivated. In this Account, we discuss a newly emerging family of flat semiconductor nanocrystals that have surprising characteristics. These thin, flat nanocrystals have up to micrometer-scale (orthogonal) lateral dimensions and thus very large surface areas. Even so, their typical photoluminescence efficiencies of 30% are astonishingly high and are 2 orders of magnitude higher than those typical of semiconductor quantum wires. The very sharp emission spectra of the pseudo-two-dimensional nanocrystals reflect a remarkable uniformity in their discrete thicknesses. Evidence that excitons are effectively delocalized and hence transported over the full dimensions of these nanocrystals has been obtained. The excellent optical properties of the flat semiconductor nanocrystals confirm that they are exceptionally well passivated. This Account summarizes the two synthetic methods that have been developed for the preparation of pseudo-two-dimensional semiconductor nanocrystals. A discussion of their structural features accounts for their discrete, uniform thicknesses and details the crystal-lattice expansions and contractions they exhibit. An analysis of their optical properties justifies the sharp photoluminescence spectra and high photoluminescence efficiencies. Finally, a bilayer mesophase template pathway is elucidated for the formation of the nanocrystals, explaining their flat morphologies. Magic-size nanocluster intermediates are found to be potent nanocrystal nucleants, allowing the synthesis temperatures to be decreased to as low as room temperature. The potential of these flat semiconductor nanocrystals in the form of nanoribbons or nanosheets for long-range energy and charge transport appears to be high.

Bright core-shell semiconductor quantum wires.

Colloidal CdTe quantum wires are reported having ensemble photoluminescence efficiencies as high as 25% under low excitation-power densities. High photoluminescence efficiencies are achieved by formation of a monolayer CdS shell on the CdTe quantum wires. Like other semiconductor nanowires, the CdTe quantum wires may contain frequent wurtzite-zinc-blende structural alternations along their lengths. The present results demonstrate that the optical properties, emission-peak shape and photoluminescence efficiencies, are independent of the presence or absence of such structural alternations.

Photoinduced Electron Transfer and Changes in Surface Free Energy in Polythiophene-Polyviologen Bilayered Thin Films.

Bipyridiniums, also known as viologens, are well-documented electron acceptors that are generally easy to synthesize on a large scale and reversibly cycle between three oxidation states (V2+, V•+, and V0). Accordingly, they have been explored in a number of applications that capitalize on their dynamic redox chemistry, such as redox-flow batteries and electrochromic devices. Viologens are also particularly useful in photoinduced electron transfer (PET) processes and therefore are of interest in photovoltaic applications that typically rely on electron-rich donors like polythiophene (PTh). However, the PET mechanism and relaxation dynamics between interfacing PTh and viologen-based thin films has not been well studied as a function of thickness of the acceptor layer. Here, a novel, bilayered thin film composite was fabricated by first spin-coating PTh onto glass slides, followed by spin-coating and curing polyviologen (PV)-based micron-sized films of variable thicknesses (0.5-11.3 µm) on top of the PTh layer. The electron-transfer mechanism and relaxation dynamics from the PTh sublayer into the upper PV film were investigated using femtosecond transient absorption (fTA) spectroscopy and electrochemistry to better understand how the charge-transfer/relaxation lifetimes could be extended using thicker PV acceptor films. The fTA experiments were performed under inert N2 conditions as well as in ambient O2. The latter shortened the lifetimes of the electrons in the PV layer, presumably due to O2 triplet-based trap sites. Contact angle measurements using H2O and MeI were also performed on top of the bilayered films to measure changes in surface free energy that would aid the assessment related to efficiency of the combined processes involving light penetration, photoexcitation, electron mobility, and relaxation from within the bilayered thin films. Insights gained from this work will support the development of future devices that employ viologen-based materials as an alternative electron-acceptor that is both easily processable and scalable.

Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 3. Hydrogen and iodine.

Two-laser, action spectroscopy experiments have been performed in the I(2)B-X, υ'-0 spectral region on H(2)···I(2) and D(2)···I(2) complexes to investigate the dependence of the H(2)/D(2) + I(2) intermolecular interactions on orientation. The spectra contain features associated with at least two different conformers of the ground-state H(2)/D(2)···I(2)(X,υ'' = 0) complexes; one conformer has a preferred T-shaped geometry with the H(2)/D(2) moiety localized in a potential minimum that is orthogonal to the I-I bond axis, and the second conformer has a linear geometry with the H(2)/D(2) moiety positioned in minima at either end of the I(2) molecule, along the bond axis. Those features associated with complexes containing para-H(2)(j = 0), ortho-H(2)(j = 1), ortho-D(2)(j = 0), and para-D(2)(j = 1) are also assigned. The linear conformers are found to be more strongly bound than the T-shaped conformers with binding energies of 118.9(1.9) cm(-1) versus 91.3-93.3 cm(-1) for the ortho-H(2)···I(2) complexes and 144.2(2.1) cm(-1) versus 107.9 cm(-1) for the para-D(2)···I(2) complexes, respectively. Electronic structure calculations of the complexes containing ICl and I(2) with H(2), He, Ne, and Ar were performed to reveal the nature of the interactions and to shed insight into the origins of the different binding energies. The most stable minima in the H(2)/D(2) + I(2)(B,υ') excited-state potentials have T-shaped geometries. Calculated energies and probability amplitudes of the excited-state levels provide insight into the different excited-state intermolecular vibrational levels accessed by transitions of the two ground-state conformers.

Origin of high photoluminescence efficiencies in CdSe quantum belts.

CdSe quantum belts (QBs) having lengths of 0.5-1.5 microm and thicknesses of 1.5-2.0 nm exhibit high photoluminescence (PL) efficiencies of approximately 30%. Epifluorescence studies establish the PL spectra to be uniform along single QBs, and nearly the same from QB to QB. Photogenerated excitons are shown to be effectively delocalized over the entire QBs by position-selective excitation. Decoration of the QBs with gold nanoparticles indicates a low density of surface-trap sites, located primarily on the thin belt edges. The high PL efficiencies and effective exciton delocalization are attributed to the minimization of defective {1100} edge surface area or edge-top/bottom (face) line junctions in QBs relative to quantum wires having roughly isotropic cross sections, for which very low PL quantum efficiencies have been reported. The results suggest that trap sites can be minimized in pseudo-one-dimensional nanocrystals, such that the facile transport of energy and charge along their long axes becomes possible.

Characterization of an FMO variant of Chlorobaculum tepidum carrying bacteriochlorophyll a esterified by geranylgeraniol.

The Fenna-Matthews-Olson light-harvesting antenna (FMO) protein has been a model system for understanding pigment-protein interactions in the energy transfer process in photosynthesis. All previous studies have utilized wild-type FMO proteins from several species. Here we report the purification and characterization of the first FMO protein variant generated via replacement of the esterifying alcohol at the C-17 propionate residue of bacteriochlorophyll (BChl) a, phytol, with geranylgeraniol, which possesses three more double bonds. The FMO protein still assembles with the modified pigment, but both the whole cell absorption and the biochemical purification indicate that the mutant cells contain a much less mature FMO protein. The gene expression was checked using qRT-PCR, and none of the genes encoding BChl a-binding proteins are strongly regulated at the transcriptional level. The smaller amount of the FMO protein in the mutant cell is probably due to the degradation of the apo-FMO protein at different stages after it does not bind the normal pigment. The absorption, fluorescence, and CD spectra of the purified FMO variant protein are similar to those of the wild-type FMO protein except the conformations of most pigments are more heterogeneous, which broadens the spectral bands. Interestingly, the lowest-energy pigment binding site seems to be unchanged and is the only peak that can be well resolved in 77 K absorption spectra. The excited-state lifetime of the variant FMO protein is unchanged from that of the wild type and shows a temperature-dependent modulation similar to that of the wild type. The variant FMO protein is less thermally stable than the wild type. The assembly of the FMO protein and also the implications of the decreased FMO/chlorosome stoichiometry are discussed in terms of the topology of these two antennas on the cytoplasmic membrane.

Intraband Relaxation Dynamics of Charge Carriers within CdTe Quantum Wires.

The state-to-state intraband relaxation dynamics of charge carriers photogenerated within CdTe quantum wires (QWs) are characterized via transient absorption spectroscopy. Overlapping signals from the energetic-shifting of the quantum-confinement features and the occupancy of carriers in the states associated with these features are separated using the quantum-state renormalization model. Holes generated with an excitation energy of 2.75 eV reach the band edge within the instrument response of the measurement, ∼200 fs. This extremely short relaxation time is consistent with the low photoluminescence quantum yield of the QWs, ∼0.2%, and the presence of alternative relaxation pathways for the holes. The electrons relax through the different energetically available quantum-confinement states, likely via phonon coupling, with an overall rate of ∼0.6 eV ps-1.

Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities.

The excitation energy dependence (EED) of the photoluminescence quantum yield (ΦPL) of semiconductor nanoparticles with varying dimensionalities is reported. Specifically, the EEDs of CdSe quantum dots, CdSe quantum platelets, CdSe quantum belts, and CdTe quantum wires were determined via measurements of individual ΦPL values and photoluminescence efficiency (PLEff(E)) spectra. There is a general trend of overall decreasing efficiency for radiative recombination with increasing excitation energy. In addition, there are often local minima in the PLEff(E) spectra that are most often at energies between quantum-confinement transitions. The average PL lifetimes of the samples do not depend on the excitation energy, suggesting that the EED of ΦPL arises from charge carrier trapping that competes efficiently with intraband carrier relaxation to the band edge. The local minima in the PLEff(E) spectra are attributed to excitation into optically coupled states that results in the loss of carriers in the semiconductor. The EED data suggest that the PLEff(E) spectra depend on the sample synthesis, preparation, surface passivation, and environment.

Photodissociation of the linear Ar-I2 van der Waals complex: velocity-map imaging of the I2 fragment.

Photodissociation of the Ar-I(2) 1:1 linear van der Waals complex is studied over the 490-520 nm region using the velocity-map imaging technique. Molecular iodine, and both the T-shaped and linear Ar-I(2)(X,v(")=0) ground-state complexes absorb strongly in this range, and these transitions access both the bound and dissociative regions of the I(2)(B) state. We measure the angle-speed distribution of vibrationally excited I(2)(B,v(')) state products by resonant 1+1 ionization via the E and f ion-pair states, forming I(2) (+), which is imaged under velocity-mapping conditions. The images show a strong angular anisotropy, which is the same for all excitation energies, spanning from the bound region to above the molecular dissociation limit. The observed angular anisotropy of the I(2) fragments is consistent with a direct dissociation of linear Ar-I(2) complexes promoted to the inner repulsive potential wall of the Ar+I(2)(B,v(')) potential energy surface.

Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 2. Molecular deuterium and iodine monochloride.

Laser-induced fluorescence and action spectroscopy experiments have identified multiple conformers of the D2...ICl van der Waals complex for both ortho-D2 (o-D2) and para-D2 (p-D2). As with the analogous H2...ICl van der Waals complexes [Darr, J. P.; Crowther, A. C.; Loomis, R. A.; Ray, S. E.; McCoy, A. B. J. Phys. Chem. A 2007, 111, 13387], the C2v conformer with the deuterium molecule localized at the iodine atom end of the dihalogen is significantly more stable than the asymmetric conformer that has the deuterium positioned orthogonally to the ICl bond axis, D0'' = 223.9(2.4) versus 97.3(8)-103.9(3) cm(-1) for p-D2...I(35)Cl(X, v''=0). For both conformers, complexes containing p-D2 are found to be more strongly bound than those with o-D2. The electronically excited D2...ICl(A, v') and D2...ICl(B, v') complexes are found to have equilibrium geometries that are nearly the same as those of the ground-state asymmetric structures. Calculated D2...ICl(B, v'=3) energies and probability amplitudes obtained using a simple scaled He + ICl(B, v'=3) potential provide clues to the nature of the different excited-state levels accessed.

Spectroscopic identification of higher-order rare gas-dihalogen complexes with different geometries: He(2,3)...Br(2) and He(2,3)...ICl.

Rovibronic transitions of multiple conformers of the He(2)...(79)Br(2)(X, v'' = 0), He(3)...(79)Br(2)(X, v'' = 0), He(2)...I(35)Cl(X, v'' = 0), and He(3)...I(35)Cl(X, v'' = 0) complexes stabilized in a pulsed, supersonic expansion are observed in action spectra recorded in the B-X region of the dihalogens. In addition to features associated with He(2)...(79)Br(2) and He(2)...I(35)Cl complexes with the rare gas atoms localized in the toroidal potential well lying in a plane perpendicular to the dihalogen bond, those associated with a ground-state conformer that has one He atom localized in the toroidal potential and the other He atom localized in the linear well at the end of the dihalogen moiety are also identified. Transitions of at least three conformers of the He(3)...Br(2) complex and two conformers of the He(3)...ICl complex are also observed. The relative populations of the different conformers are found to depend on where along the supersonic expansion the spectra are recorded, and thus on the local temperature regime sampled. The He(2)...(79)Br(2) and He(2)...I(35)Cl conformers with one He atom in each well are found to be the more stable conformers.

Experimental interrogation of the multidimensional He + ICl(E, v(dagger)) and He + ICl(beta, v(dagger)) intermolecular potential energy surfaces.

Resonant two-photon excitation of the T-shaped and linear He...I(35)Cl(X,v(") = 0) complexes is used to access the intermolecular vibrational levels bound within the He + ICl(beta,v(dagger) = 0-2) and He + ICl(E,v(dagger) = 11,12) intermolecular potentials. The excitation utilizes different metastable intermolecular vibrational levels within the He + ICl(A,v(') = 15) and He + ICl(B,v(') = 2,3) potentials to access levels with varying intermolecular vibrational excitation in the ion-pair states. In addition to providing data revealing properties of the He + ICl(E,v(dagger)) and He + ICl(beta,v(dagger)) potentials, the transition energies of the observed features permit the relative binding energies of the T-shaped and linear ground-state He...ICl(X,v(") = 0) conformers to be accurately measured. The binding energies of the T-shaped and linear He...I(35)Cl(X,v(") = 0) conformers are 16.6(3) and 22.0(2) cm(-1), respectively. These values and the observed transition energies are then used to set the binding energies of the T-shaped He...I(35)Cl complexes in the He + ICl(A,v(') = 15), He + ICl(B,v(') = 3), He + ICl(beta,v(dagger) = 1), and He + ICl(E,v(dagger) = 12) potentials as 13.4(3), 13.3(3), 41(1), and 39.2(4) cm(-1), respectively. Nonadiabatic coupling between specific intermolecular vibrational levels within the He + ICl(beta,v(dagger)) state and the ICl(D('),v(dagger)) molecular state is observed.

Wave Function Engineering in CdSe/PbS Core/Shell Quantum Dots.

The synthesis of epitaxial CdSe/PbS core/shell quantum dots (QDs) is reported. The PbS shell grows in a rock salt structure on the zinc blende CdSe core, thereby creating a crystal structure mismatch through additive growth. Absorption and photoluminescence (PL) band edge features shift to lower energies with increasing shell thickness, but remain above the CdSe bulk band gap. Nevertheless, the profiles of the absorption spectra vary with shell growth, indicating that the overlap of the electron and hole wave functions is changing significantly. This leads to over an order of magnitude reduction of absorption near the band gap and a large, tunable energy shift, of up to 550 meV, between the onset of strong absorption and the band edge PL. While the bulk valence and conduction bands adopt an inverse type-I alignment, the observed spectroscopic behavior is consistent with a transition between quasi-type-I and quasi-type-II behavior depending on shell thickness. Three effective mass approximation models support this hypothesis and suggest that the large difference in effective masses between the core and shell results in hole localization in the CdSe core and a delocalization of the electron across the entire QD. These results show the tuning of wave functions and transition energies in CdSe/PbS nanoheterostructures with prospects for use in optoelectronic devices for luminescent solar concentration or multiexciton generation.