Studying the local solvent surrounding nanoparticles is important to understanding the energy exchange dynamics between the particles and their environment, and there is a need for spectroscopic methods that can dynamically probe the solvent region that is in nearby contact with the nanoparticles. In this work, we demonstrate the use of time-resolved infrared spectroscopy to track changes in a vibrational mode of local water on the time scale of hundreds of picoseconds, revealing the dynamics of heat transfer from gold nanorods to the local water environment. We applied this probe to a prototypical plasmonic photothermal system consisting of organic CTAB bilayer capped gold nanorods, as well as gold nanorods coated with varying thicknesses of inorganic mesoporous-silica. The heat transfer time constant of CTAB capped gold nanorods is about 350 ps and becomes faster with higher laser excitation power, eventually generating bubbles due to superheating in the local solvent. Silica coating of the nanorods slows down the heat transfer and suppresses the formation of superheated bubbles.Keywords: electron ejection; heat transfer; plasmonic nanoparticle; silica coating; time-resolved infrared spectroscopy;

Highlights•Two-photon photoemission is used to probe the metal/organic interface.•The first four image-potential states (IPS's) are resolved in the energy domain.•Quantum beats evidence the presence of the n = 5 and n = 6 IPS's.•The quantum beats are observed for times greater than 1 ps.Time resolved two-photon photoemission (TPPE) is used to probe the unoccupied electronic structure of monolayer films of dicarbonitrile-quaterphenyl (NC-Ph4-CN) on Ag(1 1 1) and cobalt phthalocyanine (CoPc) on Ag(1 0 0). For both samples, photoelectron spectra show a well-formed series of electronic states near the vacuum level. These are assigned as the 1 ≤ n ≤ 4 image-potential states (IPS's) based on the scaling of their binding energies and lifetimes. Time domain measurements at energies near the vacuum level exhibit intensity oscillations (quantum beats) which are due to excitation of an electronic wave packet of the n ≥ 4 IPS's. The wave packets remain coherent until population decay renders them unobservable. These measurements clearly demonstrate that the classical image-potential state structure is retained to high order (n ∼ 6) in the presence of aromatic organic adlayers. This represents the first definitive observation via TPPE of quantum beats of electronic origin at the metal/organic interface.

•Picosecond time-resolved IR studies elucidate photochemistry of M–M bonds.•Photoexcitation of clusters leads to transient M–M bond cleavage.•CO-loss pathways compete with M–M bond cleavage.Metal carbonyl dimers and clusters constitute a diverse class of organometallic reagents and catalysts. The photochemistry of these complexes is a topic of significant and long-standing interest, as preparative-scale photolyses constitute many of the most synthetically powerful reactions in organometallic chemistry. The metal–metal bonding present in dimers and clusters is varied and significantly influences their overall reactivity. In this review we discuss the primary photochemical dynamics of transition metal carbonyl metal dimers and clusters, with a focus on the changes in metal–metal bonding that occur upon visible and/or ultraviolet photochemical excitation. The bulk of the results discussed here were obtained using ultrafast time-resolved infrared spectroscopy, a technique with high structural sensitivity afforded by the carbonyl reporter ligands of the complexes studied. Picosecond time resolution allows detailed monitoring of the photochemical reaction dynamics, including observation of initially excited complexes and short-lived transient metal–metal bond cleavage intermediates, as well as formation of the longer-lived, yet reactive, intermediates responsible for reactivity occurring on diffusion-limited time scales and beyond.The primary photochemistry of transition metal dimers and clusters is reviewed with an emphasis on the changes in metal-metal bonding that take place immediately following photoexcitation.

Dynamic IR peak coalescence and simulations based on the optical Bloch equations have been used previously to predict the rates of intramolecular electron transfer in a group of bridged mixed valence dimers of the type [Ru3(O)(OAc)6(CO)L]-BL-[Ru3(O) (OAc)6(CO)L]. However, limitations of the Bloch equations for the analysis of dynamical coalescence in vibrational spectra have been described. We have used ultrafast 2D-IR spectroscopy to investigate the vibrational dynamics of the CO spectator ligands of several dimers in the group. These experiments reveal that no electron site exchange occurs on the time scale required to explain the observed peak coalescence. The high variability in FTIR peak shapes for these mixed valence systems is suggested to be the result of fluctuations in the charge distributions at each metal cluster within a single-well potential energy surface, rather than the previous model of two-site exchange.

The importance of spin state changes in organometallic reactions is a topic of significant interest, as an increasing number of reaction mechanisms involving changes of spin state are consistently being uncovered. The potential influence of spin state changes on reaction rates can be difficult to predict, and thus this class of reactions remains among the least well understood in organometallic chemistry. Ultrafast time-resolved infrared (TRIR) spectroscopy provides a powerful tool for probing the dynamics of spin state changes in organometallic catalysis, as such processes often occur on the picosecond to nanosecond time scale and can readily be monitored in the infrared via the absorptions of carbonyl reporter ligands. In this Account, we summarize recent work from our group directed toward identifying trends in reactivity that can be used to offer predictive insight into the dynamics of coordinatively unsaturated organometallic reaction intermediates.In general, coordinatively unsaturated 16-electron (16e) singlets are able to coordinate to solvent molecules as token ligands to partially stabilize the coordinatively unsaturated metal center, whereas 16e triplets and 17-electron (17e) doublets are not, allowing them to diffuse more rapidly through solution than their singlet counterparts. Triplet complexes typically (but not always) undergo spin crossover prior to solvent coordination, whereas 17e doublets do not coordinate solvent molecules as token ligands and cannot relax to a lower spin state to do so. 16e triplets are typically able to undergo facile spin crossover to yield a 16e singlet where an associative, exothermic reaction pathway exists. The combination of facile spin crossover with faster diffusion through solution for triplets can actually lead to faster catalytic reactivity than for singlets, despite the forbidden nature of these reactions.We summarize studies on odd-electron complexes in which 17e doublets were found to display varying behavior with regard to their tendency to react with 2-electron donor ligands to form 19-electron (19e) adducts. The ability of 19e adducts to serve as reducing agents in disproportionation reactions depends on whether the excess electron density localized at the metal center or at a ligand site. The reactivity of both 16e and 17e complexes toward a widely used organic nitroxyl radical (TEMPO) are reviewed, and both classes of complexes generally react similarly via an associative mechanism with a low barrier to these reactions.We also describe recent work targeted at unraveling the photoisomerization mechanism of a thermal–solar energy storage complex in which spin state changes were found to play a crucial role. Although a key triplet intermediate was found to be required for this photoisomerization mechanism to proceed, the details of why this triplet is formed in some complexes (those based on ruthenium) and not others (those based on iron, molybdenum, or tungsten) remains uncertain, and further exploration in this area may lead to a better understanding of the factors that influence intramolecular and excited state spin state changes.

Conventional ultrafast spectroscopic studies on the dynamics of chemical reactions in solution directly probe the solute undergoing the reaction. We provide an alternative method for probing reaction dynamics via monitoring of the surrounding solvent. When the reaction exchanges the energy (in form of heat) with the solvent, the absorption cross sections of the solvent’s infrared bands are sensitive to the heat transfer, allowing spectral tracking of the reaction dynamics. This spectroscopic technique was demonstrated to be able to distinguish the differing photoisomerization dynamics of the trans and cis isomers of stilbene in acetonitrile solution. We highlight the potential of this spectroscopic approach for studying the dynamics of chemical reactions or other heat transfer processes when probing the solvent is more experimentally feasible than probing the solute directly.Keywords: heat transfer; photochemical reaction; probing solvent; reaction dynamics; stilbene; ultrafast infrared spectroscopy;

We report the excited-state electron dynamics for ultrathin films of NaCl on Ag(100). The first three image potential states (IPSs) were initially observed following excitation. The electrons in the spatially delocalized n = 1 IPS decayed on the ultrafast time scale into multiple spatially localized states lower in energy. The localized electronic states are proposed to correspond to electrons trapped at defects in the NaCl islands. Coverage and temperature dependence of the localized states support the assignment to surface trap states existing at the NaCl/vacuum interface. These results highlight the importance of electron trapping in ultrathin insulating layers.Keywords: electron dynamics; electron localization; trap state; two-photon photoemission (TPPE); ultrathin insulating films;

A phthalocyanine/Ag(111) interface state is observed for the first time using time- and angle-resolved two-photon photoemission. For monolayer films of metal-free (H2Pc) and iron phthalocyanine (FePc) on Ag(111), the state exists 0.23 ± 0.03 and 0.31 ± 0.03 eV above the Fermi level, respectively. Angle-resolved spectra show the state to be highly dispersive with an effective mass of 0.50 ± 0.15 me for H2Pc and 0.67 ± 0.14 me for FePc. Density functional theory calculations on the H2Pc/Ag(111) surface allow us to characterize this state as being a hybrid state resulting from the interaction between the unoccupied molecular states of the phthalocyanine ligand and the Shockley surface state present on the bare Ag(111) surface. This work, when taken together with the extensive literature on the 3,4,9,10-perylene tetracarboxylic dianhydride/Ag interface state, provides compelling evidence that the hybridization of metal surface states with molecular electronic states is a general phenomenon.Keywords: charge transfer; density functional theory; electron dynamics; organic thin films; phthalocyanine; two-photon photoemission (TPPE);

Picosecond time-resolved infrared spectroscopy (TRIR) was performed for the first time on a dithiolate bridged binuclear iron(I) hexacarbonyl complex ([Fe2(μ-bdt)(CO)6], bdt = benzene-1,2-dithiolate) which is a structural mimic of the active site of the [FeFe]-hydrogenase enzyme. As these model active sites are increasingly being studied for their potential in photocatalytic systems for hydrogen production, understanding their excited and ground state dynamics is critical. In n-heptane, absorption of 400 nm light causes carbonyl loss with low quantum yield (<10%), while the majority (ca. 90%) of the parent complex is regenerated with biexponential kinetics (τ1 = 21 ps and τ2 = 134 ps). In order to understand the mechanism of picosecond bleach recovery, a series of UV-pump TRIR experiments were performed in different solvents. The long time decay (τ2) of the transient spectra is seen to change substantially as a function of solvent, from 95 ps in THF to 262 ps in CCl4. Broadband IR-pump TRIR experiments were performed for comparison. The measured vibrational lifetimes (T1avg) of the carbonyl stretches were found to be in excellent correspondence to the observed τ2 decays in the UV-pump experiments, signifying that vibrationally excited carbonyl stretches are responsible for the observed longtime decays. The fast spectral evolution (τ1) was determined to be due to vibrational cooling of low frequency modes anharmonically coupled to the carbonyl stretches that were excited after electronic internal conversion. The results show that cooling of both low and high frequency vibrational modes on the electronic ground state give rise to the observed picosecond TRIR transient spectra of this compound, without the need to invoke electronically excited states.

Among the most popular and widely studied CO-releasing molecules (CO-RMs) for biological research is Ru2Cl4(CO)6 (CORM-2). When dissolved in DMSO (the solubilizing agent used in preclinical trials), the dimer cleaves into monomeric DMSO-ligated Ru subunits; the rate of this reaction and behavior of the products formed are thus highly relevant to the efficacy of CORM-2 in vivo. FT-IR spectroscopy was used to monitor this reaction in DMSO and also to monitor the reactivity of CORM-2 and of the monomeric DMSO-ligated derivatives in mouse serum. While the CORM-2 dimer readily liberates CO in mouse serum under ambient conditions, the monomeric Ru subunits formed in DMSO solution do not. This demonstrates that a substantial fraction of the CO-delivery agent being introduced via DMSO solutions would appear to be, in fact, inactive. Photochemical irradiation readily liberates CO from the inactive species, and thus a combined thermal–photochemical approach can greatly improve the yield of CO delivery. Ultrafast experiments indicate that CO loss from the DMSO-ligated monomers is a primary photochemical process. Viewing these results for the popular CORM-2 as a series of proof-of-principle observations, these results demonstrate that predissolution of a CO-RM in any solvent may generate products that are no longer physiologically active, and thus the time between dissolution and injection should always be carefully monitored; this has been seldom reported or considered in CO-RM studies to date. Although a combined thermal–photochemical CO-release approach has not been taken previously, we demonstrate that such an approach is practically quite useful for one of the most popular CO-RMs currently being studied, and thus it is clear that such an approach will be needed to achieve the full potential of many current and future CO-RMs.

Fischer carbenes are commonly used as reagents in the synthesis of new carbon–carbon bonds, a reaction made possible by the unique chemistry of the formal metal–carbon double bond. Nevertheless, the photoinduced reactions of these complexes are relatively poorly understood. For instance, it has been postulated but not confirmed that visible irradiation leads to photocarbonylation, in which a CO ligand inserts into the metal–carbon bond to form a metal ketene intermediate. Here, we report the first direct observation of this intermediate following 400 nm photoexcitation of the model group 6 Fischer carbene Cr(CO)5[CCH3(OCH3)]. Using ultrafast time-resolved infrared spectroscopy (TRIR), we observe the formation of three distinct metal ketene structures, which we assign as a singlet and two isoenergetic triplet excited-state structures. The singlet relaxes to the ground state on a time scale of ∼35 ps, whereas the two triplets are long-lived (>2 ns). TRIR of the tungsten analogue yields no evidence for a metal ketene structure, consistent with the limited reactivity of this complex. The results directly elucidate the fundamental role of triplet metal ketenes in the photoreactivity of Fischer carbene complexes.

The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical (TEMPO) has been employed for an extensive range of chemical applications, ranging from organometallic catalysis to serving as a structural probe in biological systems. As a ligand in an organometallic complex, TEMPO can exhibit several distinct coordination modes. Here we use ultrafast time-resolved infrared spectroscopy to study the reactivity of TEMPO toward coordinatively unsaturated 16- and 17-electron organometallic reaction intermediates. TEMPO coordinates to the metal centers of the 16-electron species CpCo(CO) and Fe(CO)4, and to the 17-electron species CpFe(CO)2 and Mn(CO)5, via an associative mechanism with concomitant oxidation of the metal center. In these adducts, TEMPO thus behaves as an anionic ligand, characterized by a pyramidal geometry about the nitrogen center. Density functional theory calculations are used to facilitate interpretation of the spectra and to further explore the structures of the TEMPO adducts. To our knowledge, this study represents the first direct characterization of the mechanism of the reaction of TEMPO with coordinatively unsaturated organometallic complexes, providing valuable insight into its reactions with commonly encountered reaction intermediates. The similar reactivity of TEMPO toward each of the species studied suggests that these results can be considered representative of TEMPO’s reactivity toward all low-valent transition metal complexes.

Electron solvation is examined at the interface of a room temperature ionic liquid (RTIL) and an Ag(111) electrode. Femtosecond two-photon photoemission spectroscopy is used to inject an electron into an ultrathin film of RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr]+[NTf2]−). While much of current literature highlights slower nanosecond solvation mechanisms in bulk ionic liquids, we observe only a femtosecond response, supporting morphology dependent and interface specific electron solvation mechanisms. The injected excess electron is found to reside in an electron affinity level residing near the metal surface. Population of this state decays back to the metal with a time constant of 400 ± 150 fs. Electron solvation is measured as a dynamic decrease in the energy with a time constant of 350 ± 150 fs. We observe two distinct temperature regimes, with a critical temperature near 250 K. The low temperature regime is characterized by a higher work function of 4.41 eV, while the high temperature regime is characterized by a lower work function of 4.19 eV. The total reorganizational energy of solvation changes above and below the critical temperature. In the high temperature regime, the electron affinity level solvates by 540 meV at 350 K, and below the critical temperature, solvation decreases to 200 meV at 130 K. This study will provide valuable insight to interface specific solvation of room temperature ionic liquids.

A free program for the simulation of dynamic infrared (IR) spectra is presented. The program simulates the spectrum of two exchanging IR peaks based on simple input parameters. Larger systems can be simulated with minor modifications. The program is available as an executable program for PCs or can be run in MATLAB on any operating system. Source code is also provided to encourage computer coding projects based on the program. The program is useful for upper-division undergraduates and demonstrates several important concepts from physical chemistry. Example exercises based on the program are included.Keywords: Computer-Based Learning; IR Spectroscopy; Kinetics; Physical Chemistry; Upper-Division Undergraduate;

We demonstrate the capability of temperature-dependent 2D-IR to characterize sources of vibrational population transfer. In a model system of iron diene tricarbonyl “piano stool” complexes, this approach reveals symmetry breaking associated with equilibrium fluctuations and differentiates these from fluxional rearrangement. Tricarbonyl(1,3-butadiene)iron and tricarbonyl(1,5-cyclooctadiene)iron are shown to undergo intramolecular vibrational redistribution (IVR) coupled to the wagging motion of their carbonyl ligands. In the case of both molecules, these equilibrium fluctuations are distinguished from chemical exchange behaviors by their temperature dependence and arguments of molecular symmetry.

The photochemistry of [CpRu(CO)2]2 in P(OMe)3/CH2Cl2 solution has been studied using picosecond time-resolved infrared (TRIR) spectroscopy. Photolysis at 400 nm leads to the formation of 17-electron CpRu(CO)2• radicals, which react on the picosecond time scale to form 19-electron CpRu(CO)2P(OMe)3• adducts. The TRIR spectra of this adduct display an unusually low CO stretching frequency for the antisymmetric CO stretching mode, suggesting that one carbonyl ligand adopts a bent configuration to avoid a 19-electron count at the metal center. This spectral assignment is supported by analogous experiments on [CpFe(CO)2]2 in the same solvent, combined with DFT studies on the structures of the 19-electron adducts. The DFT results predict a bent CO ligand in CpRu(CO)2P(OMe)3•, whereas approximately linear Fe–C–O bond angles are predicted for CpFe(CO)2P(OMe)3•. The observation of a bent CO ligand in the 19-electron ruthenium adduct is a surprising result, and it provides new insight into the solution-phase behavior of 19-electron complexes. TRIR spectra were also collected for [CpRu(CO)2]2 in neat CH2Cl2, and it is interesting to note that no singly bridged [CpRu(CO)]2(μ-CO) photoproduct was observed to form following 400- or 267-nm excitation, despite previous observations of this species on longer time scales.

The reactivity of five transition metal dimers toward photochemical, in-solvent-cage disproportionation has been investigated using picosecond time-resolved infrared spectroscopy. Previous ultrafast studies on [CpW(CO)3]2 established the role of an in-cage disproportionation mechanism involving electron transfer between 17- and 19-electron radicals prior to diffusion out of the solvent cage. New results from time-resolved infrared studies reveal that the identity of the transition metal complex dictates whether the in-cage disproportionation mechanism can take place, as well as the more fundamental issue of whether 19-electron intermediates are able to form on the picosecond time scale. Significantly, the in-cage disproportionation mechanism observed previously for the tungsten dimer does not characterize the reactivity of four out of the five transition metal dimers in this study. The differences in the ability to form 19-electron intermediates are interpreted either in terms of differences in the 17/19-electron equilibrium or of differences in an energetic barrier to associative coordination of a Lewis base, whereas the case for the in-cage vs diffusive disproportionation mechanisms depends on whether the 19-electron reducing agent is genuinely characterized by 19-electron configuration at the metal center or if it is better described as an 18 + δ complex. These results help to better understand the factors that dictate mechanisms of radical disproportionation and carry implications for radical chain mechanisms.

The picosecond photochemical dynamics of two group 8 transition metal carbonyl clusters, Fe3(CO)12 and Os3(CO)12, have been studied using ultrafast time-resolved infrared spectroscopy. In both the iron and osmium clusters, no trimetallic photoproducts containing bridging carbonyls appear to be formed upon 267 or 400 nm photolysis of nonbridged parent molecules. This directly contrasts with the results of observations made previously for the Ru3(CO)12 congener, in which photolysis of the nonbridged parent complex led exclusively to the formation of trimetallic photoproducts containing bridging carbonyls. In the present study, the only complex for which photolysis led to bridging carbonyl photoproducts was the bridged, C2v isomer of Fe3(CO)12. For the iron cluster, excitation leads primarily to a mixture of transient metal–metal bond cleavage photoproducts with lifetimes on the picosecond time scale, along with Fe(CO)4 and Fe2(CO)8 photoproducts arising from fragmentation of the cluster. For the osmium cluster, similar metal–metal cleavage transients are observed to recover on the picosecond time scale, and a longer-lived carbonyl loss complex is also observed. Taken in conjunction with the existing literature on the photochemistry of the ruthenium congener, the results of this study highlight the nuanced nature of group 8 transition metal cluster photochemistry.

We report the observation of a solvent-dependent spin state equilibrium in the 16-electron photoproduct CpCo(CO). Time-resolved infrared spectroscopy has been used to observe the concurrent formation of two distinct solvated monocarbonyl photoproducts, both of which arise from the same triplet CpCo(CO) precursor. Experiments in different solvent environments, combined with electronic structure theory calculations, allow us to assign the two solvated photoproducts to singlet and triplet CpCo(CO)(solvent) complexes. These results add to our previous picture of triplet reactivity for 16-electron organometallic photoproducts, in which triplets were not believed to interact strongly with solvent molecules. In the case of this photoproduct, it appears that spin crossover does not present a significant barrier to reactivity, and relative thermodynamic stabilities determine the spin state of the CpCo(CO) photoproduct in solution on the picosecond time scale. While the existence of transition metal complexes with two thermally accessible spin states is well-known, this is, to our knowledge, the first observation of a transient photoproduct that exhibits an equilibrium between two stable spin states, and also the first observed case in which a solvent has been able to coordinate as a token ligand to two spin states of the same photoproduct.

The picosecond dynamics of CpCo(CO), a catalytic intermediate in the cyclotrimerization of alkynes, have been studied using time-resolved infrared spectroscopy and density functional theory calculations. In neat 1-hexyne or 1-hexene solution, the first intermediate to form is a triplet η2-coordinated species, which then converts to a singlet η2 species in ca. 30–40 ps. The η2 triplet is the only solvent-coordinated species observed at early times, suggesting that this is the dominant mechanistic pathway for coordination of alkynes and alkenes to the CpCo(CO) catalyst. These new results complement the recent discovery that CpCo(CO) can coordinate to certain solvent molecules in both singlet and triplet spin states and further support previous studies implicating triplet intermediates in cobalt-catalyzed cyclotrimerization and cyclo-oligomerization reaction mechanisms.

The photochemical rearrangement dynamics of Co4(CO)12 were studied using picosecond time-resolved infrared spectroscopy. In cyclohexane and CH2Cl2 solvents, monitoring the kinetics of absorptions in the bridging carbonyl region reveals the formation of two transient rearrangement intermediates, both of which revert to the parent complex on the picosecond time scale. Density functional theory calculations are used to identify the structures of the rearrangement products, which arise from cleavage of an apical–basal Co–Co bond. While the lifetimes of both species exhibit a solvent dependence, the experimental kinetics and density functional calculations suggest that these species do not form solvent-coordinated complexes with cyclohexane or CH2Cl2, and instead, the solvent effect is believed to arise from differences in polarity, with the more polar CH2Cl2 solvent stabilizing the rearrangement intermediates, relative to when cyclohexane is the solvent. Carbonyl dissociation products are also observed and investigated by DFT calculations. No fragmentation products, such as Co(CO)4 or Co2(CO)8, are observed to form on the picosecond time scale, suggesting that subsequent chemistry of this cluster will occur via the single carbonyl-loss products. The experimental and computational results of this study provide insight into the role and nature of bridging carbonyl intermediates formed upon photoexcitation, as well as the formation of carbonyl-loss products and the role of solvation of transient species. To our knowledge, this study represents the first investigation into the dynamics of an M4L12 complex on the ultrafast time scale.

Time-resolved IR spectroscopy and density functional theory calculations indicate that the 14-electron, triplet species, 3Fe(CO)3, generated from photolysis of Fe(CO)5, appears to exist uncoordinated to alkyl groups in alkane solvents. In alcohols of varying lengths, triplet 3Fe(CO)3 forms a hydroxyl-coordinated complex on the time scale of tens of picoseconds, implying that its solvation kinetics are diffusion-limited. Surprisingly, the hydroxyl-coordinated complex remains in a triplet state, in contrast to the activity of triplet 3Fe(CO)4, which must convert to a singlet state to coordinate to a solvent molecule. To our knowledge, this study represents the first investigation into the detailed metal–solvent interactions and rearrangement kinetics of a 14-electron complex on the ultrafast time scale.

The activation of Sn–H bonds in tributylstannane by three triplet organometallic photoproducts (Fe(CO)4, CpCo(CO), and CpV(CO)3) has been studied using picosecond time-resolved infrared spectroscopy. Consistent with previous studies of triplet reactivity, the results suggest that triplet intermediates coordinate weakly at best with the alkyl groups in the solvent, allowing them to rearrange to form Sn–H bond activated products at, or near, diffusion-limited rates. For CpV(CO)3, an alkyl-coordinated singlet is initially formed along with the unsolvated triplet photoproduct, allowing for direct observation of the slower rate of bond activation by the alkyl-coordinated singlet species. Electronic structure theory calculations are used to investigate the potential energy surfaces, as well as to consider whether an external heavy atom effect may be important in mediating the extent of nonadiabatic behavior as the Sn–H bond approaches the metal center. Interestingly, we find no evidence for an external heavy-atom effect in the calculated spin–orbit coupling values, and we offer an explanation for the results of these calculations. To our knowledge, this study represents the first ultrafast investigation into Sn–H bond activation by organometallic catalysts.

Two of the primary hurdles facing organic electronics and photovoltaics are their low charge mobility and the inability to disentangle morphological and molecular effects on charge transport. Specific chemical groups such as alkyl side chains are often added to enable spin-casting and to improve overall power efficiency and morphologies, but their exact influence on mobility is poorly understood. Here, we use two-photon photoemission spectroscopy to study the charge transport properties of two organic semiconductors, one with and one without alkyl substituents (sexithiophene and dihexyl-sexithiophene). We show that the hydrocarbon side chains are responsible for charge localization within 230 fs. This implies that other chemical groups should be used instead of alkyl ligands to achieve the highest performance in organic photovoltaics and electronics.

Here we report nano- through microsecond time-resolved IR experiments of iron-catalyzed alkene isomerization in room-temperature solution. We have monitored the photochemistry of a model system, Fe(CO)4(η2-1-hexene), in neat 1-hexene solution. UV photolysis of the starting material leads to the dissociation of a single CO to form Fe(CO)3(η2-1-hexene), in a singlet spin state. This CO-loss complex shows a dramatic selectivity to form an allyl hydride, HFe(CO)3(η3-C6H11), via an internal C−H bond-cleavage reaction in 5−25 ns. We find no evidence for the coordination of an alkene molecule from the bath to the CO-loss complex, but do observe coordination to the allyl hydride, indicating that it is the key intermediate in the isomerization mechanism. Coordination of the alkene ligand to the allyl hydride leads to the formation of the bis-alkene isomers Fe(CO)3(η2-1-hexene)(η2-2-hexene) and Fe(CO)3(η2-1-hexene)2. Because of the thermodynamic stability of Fe(CO)3(η2-1-hexene)(η2-2-hexene) over Fe(CO)3(η2-1-hexene)2 (ca. 12 kcal/mol), nearly 100% of the alkene population will be 2-alkene. The results presented herein provide the first direct evidence for this mechanism in solution and suggest modifications to the currently accepted mechanism.

Ground-state structures with side-on nitrosyl (η2-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited-state structures with bent-NO ligands have been proposed for years but never directly observed. Here, we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO)3(NO), a model transition-metal−NO compound. Surprisingly, we have observed no evidence for ON and η2-NO structural isomers, but we have observed two bent-NO complexes. DFT modeling of the ground- and excited-state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO)3(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited-state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.

The photochemical disproportionation mechanism of [CpW(CO)3]2 in the presence of Lewis bases PR3 was investigated on the nano- and microsecond time-scales with step-scan FTIR time-resolved infrared spectroscopy. Laser excitation (532 nm) was used to homolytically cleave the W–W bond, forming the 17-electron radicals CpW(CO)3 and initiating the reaction. With the Lewis base PPh3, disproportionation to form the ionic products CpW(CO)3PPh3+ and CpW(CO)3- was directly monitored on the microsecond time-scale. Detailed examination of the kinetics and concentration dependence of this reaction indicates that disproportionation proceeds by electron transfer from the 19-electron species CpW(CO)3PPh3 to the 17-electron species CpW(CO)3. This result is contrary to the currently accepted disproportionation mechanism which predicts electron transfer from the 19-electron species to the dimer [CpW(CO)3]2. With the Lewis base P(OMe)3 on the other hand, ligand substitution to form the product [CpW(CO)2P(OMe)3]2 is the primary reaction on the microsecond time-scale. Density functional theory (DFT) calculations support the experimental results and suggest that the differences in the reactivity between P(OMe)3 and PPh3 are due to steric effects. The results indicate that radical-to-radical electron transfer is a previously unknown but important process for the formation of ionic products with the organometallic dimer [CpW(CO)3]2 and may also be applicable to the entire class of organometallic dimers containing a single metal–metal bond.