Theis I. Sølling

Interpretation of the Ultrafast Photoinduced Processes in Pentacene Thin Films

Ambiguity remains in the models explaining the photoinduced dynamics in pentacene thin films as observed in pump-probe experiments. One model advocates exciton fission as governing the evolution of the initially excited species, whereas the other advocates the formation of an excimeric species subsequent to excitation. On the basis of calculations by a combined quantum mechanics and molecular mechanics (QM/MM) method and general considerations regarding the excited states of pentacene we propose an alternative, where the initially excited species instead undergoes internal conversion to a doubly excited exciton. The conjecture is supported by the observed photophysical properties of pentacene from both static as well as time-resolved experiments.

3-Deoxy-glucosone is an Intermediate in the Formation of Furfurals from D-Glucose.

There is a consensus that the current heavy dependence on fossil fuels is untenable because it both leads to an increase in atmospheric CO2 and climate change, and because fossil fuels are a limited resource. Therefore much attention has been given to research in biofuels, combustable organic compounds obtained from the biosphere, that is, plants. Because most of the organic material in the biosphere consists of carbohydrates, especially cellulose, the majority of this research is directed at solving the problems associated with the conversion of the carbohydrate biomass into fuels. Methods based on established fermentation technologies that convert carbohydrates into bioethanol are a more immediate solution. However, it is by no means certain that ethanol is a good or even efficient solution to the problem. Ethanol is corrosive and fermentation has a poor carbon economy (glucose = 2 EtOH + 2 CO2). [2] Therefore, much attention has been paid to the direct chemical conversion of carbohydrates, and particularly dehydrative reactions to furfurals are considered promising. The main furfural of interest is the product of the acidic dehydration of glucose: hydroxymethylfurfural (HMF). This compound has generated particular recent interest as an intermediate for new biofuels such as dimethylfuran and as a platform for biobased chemicals. Of course, realizing efficient ways to obtain HMF from biomass is the key to success, and this requires a deep understanding of the process. The acid-catalyzed dehydration of glucose to HMF is a multistep process that has been known for many years. From older literature it is clear that there are two possible mechanisms of the conversion, between which it was not possible to distinguish: one mechanism is the 3-deoxy-2-keto pathway shown in Scheme 1, where elimination of the 3-OH of glucose (1) leads to 3-deoxy-d-erythro-hex-2-ulose 3 (3-deoxyglucosone) that undergoes ring-closure and eliminations to HMF (6). The alternative mechanism is the fructose pathway shown in Scheme 2, where 1 isomerizes to fructose (8) that undergoes cyclization, reisomerization, and elimination to form 6. The latter mechanism is supported by observations that 6 is formed much faster and in higher yield from 8 than from 1, and that 8 has been observed in dehydrations of 1, so in most recent literature reports the fructose mechanism appears to prevail. Yet, the 3-deoxy mechanism is in fact more logical because it does not include any “back and forth” isomerization between C-1 and C-2. In this paper we ask the question: How good a source of HMF is 3? Older work shows that 3 does dehydrate to HMF, but otherwise little has been done in the area. Compound 3 has previously been prepared by El Khadem et al. by reaction of glucose with benzoyl hydrazine to form the oxazone S1 (Scheme S1, Supporting Information) followed by rehydrazoniation with benzaldehyde. We followed this procedure with minor changes, obtaining S1 in 75 % yield, while the second step gave us 80 % yield of 3. The product gave very poor NMR spectra consisting of at least eight compounds in aqueous solution, and previously 3 has only been characterized by conversion to the dinitrophenyl hydrazone. We performed a NaBH4 reduction of the material and obtained a mixture of d-riboand d-arabino-3-deoxy-hexitols (S2 and S3) in 74 % yield, which is consistent with the compound being essentially only 3. [a] Dr. H. Jadhav, Dr. C. M. Pedersen, Prof. T. Sølling, Prof. M. Bols Department of Chemistry, University of Copenhagen Universitetsparken 5, 2100 Kbh Ø (Denmark) Fax: (+ 45) 35 32 02 12 E-mail : bols@chem.ku.dk Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100249. Scheme 1. 3-Deoxy-2-ketohexose mechanism for formation of 6.

The Influence of Push−Pull States on the Ultrafast Intersystem Crossing in Nitroaromatics

The photochemistry of nitro-substituted polyaromatic compounds is generally determined by the rapid decay of its S1 state and the rapid population of its triplet manifold. Previous studies have shown that such an efficient channel is due to a strong coupling of the fluorescent state with specific upper receiver states in the triplet manifold. Here we examine variations in this mechanism through the comparison of the photophysics of 2-nitrofluorene with that of 2-diethylamino-7-nitrofluorene. The only difference between these two molecules is the presence of a diethylamino group in a push-pull configuration for the latter compound. The femtosecond-resolved experiments presented herein indicate that 2-nitrofluorene shows ultrafast intersystem crossing which depopulates the S1 emissive state within less than a picosecond. On the other hand, the amino substituted nitrofluorene shows a marked shift in its S1 energy redounding in the loss of coupling with the receiver triplet state, and therefore a much longer lifetime of 100 ps in cyclohexane. In polar solvents, the diethylamino substituted compound actually shows double peaked fluorescence due to the formation of charge transfer states. Evaluation of the Stokes shifts in different solvents indicates that both bands correspond to intramolecular charge transfer states in equilibrium which are formed in an ultrafast time scale from the original locally excited (LE) state. The present study addresses the interplay between electron-donating and nitro substituents, showing that the addition of the electron-donating amino group is able to change the coupling with the triplet states due to a stabilization of the first excited singlet state and the rapid formation of charge transfer states in polar solvents. We include calculations at the TD-DFT level of theory with the PBE0 and B3LYP functionals which nicely predict the observed difference between the two compounds, showing how the specific S(π-π*)-T(n-π*) coupling normally prevalent in nitroaromatics is lost in the push-pull compound.

Coherent Motion Reveals Non-Ergodic Nature of Internal Conversion between Excited States

We found that specific nuclear motion along low-frequency modes is effective in coupling electronic states and that this motion prevail in some small molecules. Thus, in direct contradiction to what is expected based on the standard models, the internal conversion process can proceed faster for smaller molecules. Specifically, we focus on the S(2) →S(1) internal conversion in cyclobutanone, cyclopentanone, and cyclohexanone. By means of time-resolved mass spectrometry and photoelectron spectroscopy the relative rate of this transition is determined to be 13:2:1. Remarkably, we observe coherent nuclear motion on the S(2) surface in a ring-puckering mode and motion along this mode in combination with symmetry considerations allow for a consistent explanation of the observed relative time-scales not afforded by only considering the density of vibrational states or other aspects of the standard models.

On the condensed phase ring-closure of vinylheptafulvalene and ring-opening of gaseous dihydroazulene.

Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1-2 ps and return to the ground state is probably driven through a cis-trans isomerization about the exocyclic C1═C2 double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.

The Non-Ergodic Nature of Internal Conversion

The absorption of light by molecules can induce ultrafast dynamics and coupling of electronic and nuclear vibrational motion. The ultrafast nature in many cases rests on the importance of several potential energy surfaces in guiding the nuclear motion-a concept of central importance in many aspects of chemical reaction dynamics. This Minireview focuses on the non-ergodic nature of internal conversion, that is, on the concept that the nuclear dynamics only sample a reduced phase space, potentially resulting in localization of the dynamics in real space. A series of results that highlight the nonstatistical nature of the excited-state deactivation process is presented. The examples are categorized into four groups. 1) Localization of the energy in one degree of freedom in S2 →S1 transitions, in which the transition is either determined by the time spent in the S2 →S1 coupling region or by the time it takes to reach it. 2) Localization of energy into a single reactive mode, which is dictated by the internal conversion process. 3) Initiation of the internal conversion by activation of a single complex motion, which then specifically couples to a reactive mode. 4) Nonstatistical internal conversion as a tool to accomplish biomolecular stability. Herein, the discussion on nonstatistical internal conversion in DNA as a mechanism to eliminate electronic excitation energy is extended to include molecules with an S-S bond as a model of the disulfide bridge in peptides. All of these examples are summed up in Kashas rule. For systems with multiple degrees of freedom it will be possible to locate an appropriate motion somewhere in phase space that will take the wavepacket to the coupling region and facilitate an ultrafast transition to S1. Once at S1, the momentum of the wavepacket is lost and the only options left are the statistical processes of reaction or light emission.

A high-level ab initio investigation of identity and nonidentity gas-phase SN2 reactions of halide ions with halophosphines

Abstract The high-level ab initio procedures G2(+) and G2(+)[ECP(S)]have been employed in an investigation of S N 2 reactions at neutral tri-coordinated phosphorus. The process has been modeled by identity and nonidentity substitution reactions involving a series of halophosphines PH 2 X and halide ions. We find that the reaction proceeds without an intervening barrier by way of a tetra-coordinated phosphorus anion intermediate (XPH 2 Y − ). This contrasts with the corresponding process for the carbon and nitrogen analogues, where the tetra-coordinated species is a transition structure. The threshold for inversion of the phosphorus intermediate is found to lie below the reaction energy in the cases where F − is the leaving group, but above it in all the other cases. The S N 2 reaction will therefore lead to racemization when F − is expelled. In the other cases, it is possible in principle that the reaction can be controlled to proceed with inversion, but because of the relatively low barriers for inversion, this is not a very likely outcome. We predict that the tetra-coordinated intermediate should be detectable, if not isolable, and that the S N 2 reaction at neutral tri-coordinated phosphorus is exothermic when the reactant halide ion is more electronegative than the product halide ion, and endothermic when the reverse applies.

ARE PI-LIGAND EXCHANGE REACTIONS OF THIIRENIUM AND THIIRANIUM IONS FEASIBLE? AN AB INITIO INVESTIGATION

Pi-ligand exchange is predicted to be experimentally feasible for thiirenium and thiiranium ions by a G2 ab initio investigation (see figure). The reaction is also predicted to be stereospecific with respect to the configuration at sulfur. Comparisons are made with the analogous phosphorus systems.

Towards Multireference Equivalents of the G2 and G3 Methods

The authors would also like to thank the National Science Foundation International Division for providing travel funds to ~M.S.G. and M.A.F.! and the National Science Foundation Chemistry Division for supporting the research.

Initial dynamics of the Norrish Type I reaction in acetone: probing wave packet motion.

The Norrish Type I reaction in the S(1) (nπ*) state of acetone is a prototype case of ketone photochemistry. On the basis of results from time-resolved mass spectrometry (TRMS) and photoelectron spectroscopy (TRPES) experiments, it was recently suggested that after excitation the wave packet travels toward the S(1) minimum in less than 30 fs and stays there for more than 100 picoseconds [Chem. Phys. Lett.2008, 461, 193]. In this work we present simulated TRMS and TRPES signals based on ab initio multiple spawning simulations of the dynamics during the first 200 fs after excitation, getting quite good agreement with the experimental signals. We can explain the ultrafast decay of the experimental signals in the following manner: the wave packet simply travels, mainly along the deplanarization coordinate, out of the detection window of the ionizing probe. This window is so narrow that subsequent revival of the signal due to the coherent deplanarization vibration is not observed, meaning that from the point of view of the experiment the wave packets travels directly to the S(1) minimum. This result stresses the importance of pursuing a closer link to the experimental signal when using molecular dynamics simulations in interpreting experimental results.