Electron transfer (ET) from donor to acceptor is often mediated by nuclear-electronic (vibronic) interactions in molecular bridges. Using an ultrafast electronic-vibrational-vibrational pulse-sequence, we demonstrate how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridge-acceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely switching a charge separation pathway off. These results offer a step toward quantum control of chemical reactivity by IR excitation.

The H+O2→OH+O reaction has been studied with a time-dependent wave packet method for total angular momentum J=0, 1, 2, and 5, using the Coriolis coupled method [E. M. Goldfield and S. K. Gray, Comp. Phys. Commun. 98, 1 (1996)] on parallel computers. We find that at higher energies the total reaction probability decreases by a factor of 2 in going from a J=0 calculation to a J=1 calculation. The effect for higher J with respect to J=1 is less dramatic. We investigated the decrease in reaction probability for J>0 by examining the different initial conditions with respect to Ω, the projection of J onto the body-fixed z axis for the J>0 calculations. We conclude that the reaction probability is a strong function of Ω. If Ω=0 for J>0, collision geometries are accessible that lead to an enhanced reaction probability.

Nuclear-electronic (vibronic) coupling is increasingly recognized as a mechanism of major importance in controlling the light-induced function of molecular systems. It was recently shown that infrared light excitation of intramolecular vibrations can radically change the efficiency of electron transfer, a fundamental chemical process. We now extend and generalize the understanding of this phenomenon by probing and perturbing vibronic coupling in several molecules in solution. In the experiments an ultrafast electronic-vibrational pulse sequence is applied to a range of donor-bridge-acceptor Pt(II) trans-acetylide assemblies, for which infrared excitation of selected bridge vibrations during ultraviolet-initiated charge separation alters the yields of light-induced product states. The experiments, augmented by quantum chemical calculations, reveal a complex combination of vibronic mechanisms responsible for the observed changes in electron transfer rates and pathways. The study raises new fundamental questions about the function of vibrational processes immediately following charge transfer photoexcitation, and highlights the molecular features necessary for external vibronic control of excited-state processes.

The associative desorption of H2 (ν,j) on a graphite(0001) surface via an Eley−Rideal mechanism has been studied theoretically. In our calculations we used a time-dependent wave packet method treating three degrees of freedom quantum mechanically. A newly developed potential energy surface based on plane-wave density functional calculations was employed. In our 3D calculations we find less vibrational excitation for the product H2 molecules than in calculations that used only two degrees of freedom. However, the product H2 molecules are formed rotationally excited. This could have important implications for the chemistry of H2 in the interstellar medium and the interpretation of astronomical data.

The host-guest chemistry of the octanuclear cubic coordination cage [Co(8)L(12)](16+) (where L is a bridging ligand containing two chelating pyrazolyl-pyridine units connected to a central naphthalene-1,5-diyl spacer via methylene "hinges") has been investigated in detail by (1)H NMR spectroscopy. The cage encloses a cavity of volume of ca. 400 Å(3), which is accessible through 4 Å diameter portals in the centers of the cube faces. The paramagnetism of the cage eliminates overlap of NMR signals by dispersing them over a range of ca. 200 ppm, making changes of specific signals easy to observe, and also results in large complexation-induced shifts of bound guests. The cage, in CD(3)CN solution, acts as a remarkably size- and shape-selective host for small organic guests such as coumarin (K = 78 M(-1)) and other bicyclic molecules of comparable size and shape such as isoquinoline-N-oxide (K = 2100 M(-1)). Binding arises from two independent recognition elements, which have been separately quantified. These are (i) a polar component arising from interaction of the H-bond accepting O atom of the guest with a convergent group of CH protons inside the cavity that lie close to a fac tris-chelate metal center and are therefore in a region of high electrostatic potential; and (ii) an additional component arising from the second aromatic ring (aromatic/van der Waals interactions with the interior surface of the cage and/or solvophobic interactions). The strength of the first component varies linearly with the H-bond-accepting ability of the guest; the second component is fixed at approximately 10 kJ mol(-1). We have also used (1)H-(1)H exchange spectroscopy (EXSY) experiments to analyze semiquantitatively two distinct dynamic processes, viz. movement of the guest into and out of the cavity and tumbling of the guest inside the host cavity. Depending on the size of the guest and the position of substituents, the rates of these processes can vary substantially, and the rates of processes that afford observable cross-peaks in EXSY spectra (e.g., between free and bound guest in some cases; between different conformers of a specific host·guest complex in others) can be narrowed down to a specific time window. Overall, the paramagnetism of the host cage has allowed an exceptionally detailed analysis of the kinetics and thermodynamics of its host-guest behavior.

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A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl 2 ( 1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a Pt (II)-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl 2 is charge-transfer-to-diimine of a largely Pt-->phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355-395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps ( (1)NDI*), 3 ps ( (3)MLCT), 19 ps (vibrational cooling of "hot" (3)NDI and of "hot" NDI ground state), and 520 mus ( (3)NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the (1)MLCT and the (1)NDI* excited states. A thermodynamically possible decay of the initially populated (1)MLCT to the charge-transfer-to-NDI excited state, [Pt (III)(phen-NDI (-*))Cl 2], is not observed. This finding could be explained by an ultrafast ISC of the (1)MLCT to the (3)MLCT state which lies about 0.4 eV lower in energy than [Pt (III)(phen-NDI (-*))Cl 2]. The predominant decay pathway of the (3)MLCT is a back electron transfer process with approximately 3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the (1)NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot (3)NDI. The latter relaxes to form (3)NDI state, that is, [Pt(phen- (3)NDI)Cl 2]*, which possesses a remarkably long lifetime for a Pt (II) complex in fluid solution of 520 mus. The IR signature of this excited state includes the nu(CO) bands at 1607 and 1647 cm (-1), which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH 2Cl 2. Pt(phen-NDI)Cl 2 acts as a modest photosensitizer of singlet oxygen.