Comparing the structures and photophysical properties of two charge transfer co-crystals.

Organic co-crystals have emerged as a promising class of semiconductors for next-generation optoelectronic devices due to their unique photophysical properties. This paper presents a joint experimental-theoretical study comparing the crystal structure, spectroscopy, and electronic structure of two charge transfer co-crystals. Reported herein is a novel co-crystal Npe:TCNQ, formed from 4-(1-naphthylvinyl)pyridine (Npe) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) via molecular self-assembly. This work also presents a revised study of the co-crystal composed of Npe and 1,2,4,5-tetracyanobenzene (TCNB) molecules, Npe:TCNB, herein reported with a higher-symmetry (monoclinic) crystal structure than previously published. Npe:TCNB and Npe:TCNQ dimer clusters are used as theoretical model systems for the co-crystals; the geometries of the dimers are compared to geometries of the extended solids, which are computed with periodic boundary conditions density functional theory. UV-Vis absorption spectra of the dimers are computed with time-dependent density functional theory and compared to experimental UV-Vis diffuse reflectance spectra. Both Npe:TCNB and Npe:TCNQ are found to exhibit neutral character in the S0 state and ionic character in the S1 state. The high degree of charge transfer in the S1 state of both Npe:TCNB and Npe:TCNQ is rationalized by analyzing the changes in orbital localization associated with the S1 transitions.

Excited-State Dynamics during Primary C-I Homolysis in Acetyl Iodide Revealed by Ultrafast Core-Level Spectroscopy.

In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to nπC═O* states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed nπC═O* and nσC-I* character, leading to complex excited-state dynamics, ultimately resulting in dissociation. Using ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy and quantum chemical calculations, we present an investigation of the primary photodissociation dynamics of acetyl iodide via time-resolved spectroscopy of core-to-valence transitions of the I atom after 266 nm excitation. The probed I 4d-to-valence transitions show features that evolve on sub-100-fs time scales, reporting on excited-state wavepacket evolution during dissociation. These features subsequently evolve to yield spectral signatures corresponding to free iodine atoms in their spin-orbit ground and excited states with a branching ratio of 1.1:1 following dissociation of the C-I bond. Calculations of the valence excitation spectrum via equation-of-motion coupled cluster with single and double substitutions (EOM-CCSD) show that initial excited states are of spin-mixed character. From the initially pumped spin-mixed state, we use a combination of time-dependent density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N4,5 edge to reveal a sharp inflection point in the transient XUV signal that corresponds to rapid C-I homolysis. By examining the molecular orbitals involved in the core-level excitations at and around this inflection point, we are able to piece together a detailed picture of C-I bond photolysis in which d → σ* transitions give way to d → p excitations as the bond dissociates. We also report theoretical predictions of short-lived, weak 4d → 5d transitions in acetyl iodide, validated by weak bleaching in the experimental transient XUV spectra. This joint experimental-theoretical effort has thus unraveled the detailed electronic structure and dynamics of a strongly spin-orbit coupled system.

Ultrafast infrared transient absorption spectroscopy of gas-phase Ni(CO)4 photodissociation at 261 nm.

We employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of carbonyl ligands from gas-phase nickel tetracarbonyl following ultraviolet photoexcitation at 261 nm. Here, nickel tetracarbonyl undergoes prompt dissociation to produce nickel tricarbonyl in a singlet excited state; this electronically excited tricarbonyl loses another CO group over tens of picoseconds. Our results also suggest the presence of a parallel, concerted dissociation mechanism to produce nickel dicarbonyl in a triplet excited state, which likely dissociates to nickel monocarbonyl. Mechanisms for the formation of these photoproducts in multiple electronic excited states are theoretically predicted with one-dimensional cuts through the potential energy surfaces and computation of spin-orbit coupling constants using equation of motion coupled cluster methods (EOM-CC) and coupled cluster theory with single and double excitations (CCSD). Bond dissociation energies are calculated with CCSD, and anharmonic frequencies of ground and excited state species are computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT).

Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) Thin Films.

The coupling of inter- and intramolecular vibrations plays a critical role in initiating chemistry during the shock-to-detonation transition in energetic materials. Herein, we report on the subpicosecond to subnanosecond vibrational energy transfer (VET) dynamics of the solid energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) by using broadband, ultrafast infrared transient absorption spectroscopy. Experiments reveal VET occurring on three distinct time scales: subpicosecond, 5 ps, and 200 ps. The ultrafast appearance of signal at all probed modes in the mid-infrared suggests strong anharmonic coupling of all vibrations in the solid, whereas the long-lived evolution demonstrates that VET is incomplete, and thus thermal equilibrium is not attained, even on the 100 ps time scale. Density functional theory and classical molecular dynamics simulations provide valuable insights into the experimental observations, revealing compression-insensitive time scales for the initial VET dynamics of high-frequency vibrations and drastically extended relaxation times for low-frequency phonon modes under lattice compression. Mode selectivity of the longest dynamics suggests coupling of the N-N and axial NO2 stretching modes with the long-lived, excited phonon bath.

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy.

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. We employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO)3 within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).

Sub-picosecond to Sub-nanosecond Vibrational Energy Transfer Dynamics in Pentaerythritol Tetranitrate.

The time scale associated with shock-induced detonation is a key property of energetic materials that remains poorly understood. Herein, we test aspects of one potential mechanism, the phonon up-pumping mechanism, where shock compression excites lattice phonon modes, transferring energy to intramolecular vibrations leading to chemical bond cleavage and reaction. Using ultrafast infrared pump-probe spectroscopy on pentaerythritol tetranitrate (PETN), we reveal sub-picosecond vibrational energy transfer (VET) from the photoexcited band at 1660 cm-1 into every other infrared-active mode in the probed frequency range 800-1800 cm-1. Energy transfer processes remain incomplete at 150 ps. Computational predictions from density functional theory are used in tandem to elucidate VET pathways in PETN.

Ultrafast photodissociation dynamics of pyrazole, imidazole and their deuterated derivatives using ab initio multiple cloning.

We present results obtained using the ab initio multiple cloning (AIMC) method to simulate fully quantum dynamics for imidazole and its structural isomer pyrazole along with their selectively deuterated species. We simulate the ultrafast dissociation of the N-H/D bond for these molecules along the repulsive 1πσ* state which agrees well with previous experimental results. Our results give evidence for a two-stage dissociation of the N-H/D bond on the sub-50 fs regime for imidazole, pyrazole and their selectively deuterated species, and give evidence for the importance of the repulsive 1πσ* state along the N-H/D bond coordinate for the relaxation of both imidazole and pyrazole. The ability of these calculations to reproduce experimental results lends confidence that larger complex systems could be explored with predictive capabilities with the AIMC method. These results also confirm the ability of the AIMC method to add detailed insights into which experiments are blind.

New insights into the dissociation dynamics of methylated anilines.

Aniline, an important model system for biological chromophores, undergoes ultrafast H-atom loss upon absorption of an ultraviolet photon. By varying the number and position of methyl substituents on both the aromatic ring and amine functional group, we explore the ultrafast production of photofragments as a function of molecular structure. Both N-methyl aniline and 3,5-dimethyl aniline show altered H-atom loss behaviour compared to aniline, while no evidence for CH3 loss was found from either N-methyl aniline or N,N-dimethyl aniline. With the addition of time-resolved photoelectron spectroscopy, the photofragment appearance times are matched to excited state relaxation pathways. Evidence for a sequential excited state relaxation mechanism, potentially involving a valence-to-Rydberg decay mechanism, will be presented. Such a global, bottom-up approach to molecular photochemistry is crucial to understanding the dissociative pathways and excited state decay mechanisms of biomolecule photoprotection in nature.

Ultrafast photodissociation dynamics of 2-ethylpyrrole: adding insight to experiment with ab initio multiple cloning.

The ultrafast photodissociation dynamics of 2-ethylpyrrole (2-EP) is simulated in a fully quantum manner on the S1 and S2 πσ* states by the ab initio multiple cloning (AIMC) method. AIMC treats electrons with accurate electronic structure methods "on the fly", and nuclear dynamics with wavefunction propagation via a basis set of Ehrenfest trajectory guided Gaussian wavepackets. Total kinetic energy release (TKER) spectra are produced, as well as velocity map images and N-H dissociation times. These are compared to results from time-resolved velocity map imaging studies, and the AIMC method is able to provide quantitative reproduction of experimental data, including dissociation times of 50-80 fs. Novel insight into the dissociation mechanism is then obtained, with the experimentally obtained time constant shown to be composed of two components. Firstly, there is a contribution in <50 fs from 2-EP molecules that have sufficient energy in the N-H stretch coordinate to dissociate almost immediately over the barrier, and this is followed by a second slower contribution from 2-EP molecules that must sample the potential energy surface before finding a way around the barrier to dissociate. This two component mechanism is not observed experimentally due to the temporal widths of the laser pulses obscuring the dynamics in the <50 fs window, and is shown for the first time via theory. Calculations are also performed on selectively deuterated 2-EP, demonstrating that AIMC is able to produce a kinetic isotope effect for the dissociation time constant, and correctly predict a shift to lower energy in the TKER spectrum. The S2 πσ* state is also shown to be unstable with respect to the S1 πσ* state, with the N-H dissociation proceeding along S1 when initially excited to S2. This work demonstrates that the combination of state of the art theory and experiments can provide unprecedented novel insight into the N-H dissociation mechanism, with the tantalising prospect of providing insight into more general heteroatom hydride bond dissociation.

Wavepacket insights into the photoprotection mechanism of the UV filter methyl anthranilate.

Meradimate is a broad-spectrum ultraviolet absorber used as a chemical filter in commercial sunscreens. Herein, we explore the ultrafast photodynamics occurring in methyl anthranilate (precursor to Meradimate) immediately after photoexcitation with ultraviolet radiation to understand the mechanisms underpinning Meradimate photoprotection. Using time-resolved photoelectron spectroscopy, signal from the first singlet excited state of methyl anthranilate shows an oscillatory behavior, i.e., quantum beats. Our studies reveal a dependence of the observed beating frequencies on photoexcitation wavelength and photoelectron kinetic energy, unveiling the different Franck-Condon overlaps between the vibrational levels of the ground electronic, first electronic excited, and ground cationic states of methyl anthranilate. By evaluating the behavior of these beats with increasing photon energy, we find evidence for intramolecular vibrational energy redistribution on the first electronic excited state. Such energy redistribution hinders efficient relaxation of the electronic excited state, making methyl anthranilate a poor choice for an efficient, efficacious sunscreen chemical filter.

Ultrafast Dissociation Dynamics of 2-Ethylpyrrole.

To explore the effects of ring substitution on dissociation dynamics, the primary photochemistry of 2-ethylpyrrole was explored using ultrafast ion imaging techniques. Photoexcitation to the S1 state, a πσ* state, in the range from 238 to 265 nm results in cleavage of the N-H bond with an H atom appearance lifetime of ca. 70 fs. The insensitivity of this lifetime to photon energy, combined with a small kinetic isotope effect, suggests that tunneling does not play a major role in N-H bond cleavage. Total kinetic energy release spectra reveal modest vibrational excitation in the radical counter-fragment, increasing with photon energy. At wavelengths less than or equal to 248 nm, an additional low kinetic energy H atom loss mechanism becomes available with an appearance lifetime of ∼1.5 ps, possibly due to the population of higher-lying 1ππ* states.

Bottom-up excited state dynamics of two cinnamate-based sunscreen filter molecules.

Methyl-E-4-methoxycinnamate (E-MMC) is a model chromophore of the commonly used commercial sunscreen agent, 2-ethylhexyl-E-4-methoxycinnamate (E-EHMC). In an effort to garner a molecular-level understanding of the photoprotection mechanisms in operation with E-EHMC, we have used time-resolved pump-probe spectroscopy to explore E-MMC's and E-EHMC's excited state dynamics upon UV-B photoexcitation to the S1 (11ππ*) state in both the gas- and solution-phase. In the gas-phase, our studies suggest that the excited state dynamics are driven by non-radiative decay from the 11ππ* to the S3 (11nπ*) state, followed by de-excitation from the 11nπ* to the ground electronic state (S0). Using both a non-polar-aprotic solvent, cyclohexane, and a polar-protic solvent, methanol, we investigated E-MMC and E-EHMC's photochemistry in a more realistic, 'closer-to-shelf' environment. A stark change to the excited state dynamics in the gas-phase is observed in the solution-phase suggesting that the dynamics are now driven by efficient E/Z isomerisation from the initially photoexcited 11ππ* state to S0.

Production and Photodissociation of the Methyl Perthiyl Radical.

The photodissociation dynamics of the methyl perthiyl (CH3SS) radical are investigated via molecular beam photofragment translational spectroscopy, using "soft" electron ionization to detect the radicals and their photofragments. With this new capability, we have shown that CH3SS can be generated from flash pyrolysis of dimethyl trisulfide. Utilizing this source of radicals and the advantages afforded by soft electron ionization, we have reinvestigated the photodissociation dynamics of CH3SS at 248 nm, finding CH3S + S to be the dominant dissociation channel with CH3 + SS as a minor process. These results differ from previous work reported in our laboratory in which we found CH3 + SS and CH2S + SH as the main dissociation channels. The difference in results is discussed in light of our new capabilities for characterization of radical production.

Benzyl Radical Photodissociation Dynamics at 248 nm.

The photodissociation of jet cooled benzyl radicals, C7H7, at 248 nm has been studied using photofragment translational spectroscopy. Two dissociation channels were observed, H + C7H6 and CH3 + C6H4. The translational energy distribution determined for each channel suggests that both dissociation mechanisms occur via internal conversion to the ground state followed by intramolecular vibrational redistribution and dissociation. The branching ratio between these two channels has been measured to be (CH3 + C6H4)/(H + C7H6) = 0.011 ± 0.004. The dominance of the H + C7H6 channel is corroborated by the branching ratio calculated using Rice-Ramsperger-Kassel-Marcus theory.

Revisiting the photodissociation dynamics of the phenyl radical.

We have reinvestigated the photodissociation dynamics of the phenyl radical at 248 nm and 193 nm via photofragment translational spectroscopy under a variety of experimental conditions aimed at reducing the nascent internal energy of the phenyl radical and eliminating signal from contaminants. Under these optimized conditions, slower translational energy (P(E(T))) distributions for H-atom loss were seen at both wavelengths than in previously reported work. At 193 nm, the branching ratio for C2H2 loss vs. H-atom loss was found to be 0.2 ± 0.1, a significantly lower value than was obtained previously in our laboratory. The new branching ratio agrees with calculated Rice-Ramsperger-Kassel-Marcus rate constants, suggesting that the photodissociation of the phenyl radical at 193 nm can be treated using statistical models. The effects of experimental conditions on the P(E(T)) distributions and product branching ratios are discussed.

Photodissociation dynamics of the methyl perthiyl radical at 248 nm via photofragment translational spectroscopy.

Photofragment translational spectroscopy was used to study the photodissociation of the methyl perthiyl radical CH(3)SS at 248 nm. The radical was produced by flash pyrolysis of dimethyl disulfide (CH(3)SSCH(3)). Two channels were observed: CH(3) + S(2) and CH(2)S + SH. Photofragment translational energy distributions indicate that CH(3) + S(2) results from C-S bond fission on the ground state surface. The CH(2)S + SH channel can proceed through isomerization to CH(2)SSH on the ground state surface but also may involve production of electronically excited CH(2)S.

Formation and stability of atmospherically relevant isoprene-derived organosulfates and organonitrates.

Isoprene is the precursor for number of alcohol, organosulfate, and organonitrate species observed in ambient secondary organic aerosol (SOA). Recent laboratory and field work has suggested that isoprene-derived epoxides may be crucial intermediates that can explain the existence of these compounds in SOA. To confirm this hypothesis, the specific hydroxy epoxides observed in gas phase isoprene photooxidation experiments (as well as several other related species) were synthesized and the bulk phase aqueous reactions of these species in the presence of sulfate and nitrate were studied via nuclear magnetic resonance (NMR) techniques. The results indicate that both primary and tertiary organosulfates and organonitrates are efficiently formed from the potential SOA reactions of isoprene-derived epoxides. However, the tertiary organonitrates are shown to undergo rapid nucleophilic substitution reactions (in which nitrate is substituted for by water or sulfate) over the whole range of SOA pH, while the tertiary organosulfates are found to undergo a much slower acid-dependent hydrolysis reaction. The primary organonitrates and organosulfates under study were found to be stable against nucleophilic substitution reactions, even at low pH. This finding provides a potential explanation for the fact that organosulfates are more commonly detected in ambient SOA than are organonitrates.

Kinetics of the hydrolysis of atmospherically relevant isoprene-derived hydroxy epoxides.

Isoprene (the most abundant nonmethane hydrocarbon emitted into the atmosphere) is known to undergo oxidation to 2-methyl-1,2,3,4-butanetetraol, a hydrophilic compound present in secondary organic aerosol (SOA) in the atmosphere. Recent laboratory work has shown that gas phase hydroxy epoxides are produced in the low NOx photooxidation of isoprene and that these epoxides are likely to undergo efficient acid-catalyzed hydrolysis on SOA to 2-methyl-1,2,3,4-butanetetraol at typical SOA acidities. In order to confirm this hypothesis, the specific hydroxy epoxides observed in the isoprene photooxidation experiment (as well as several other related species) were synthesized, and the hydrolysis kinetics of all species were studied via nuclear magnetic resonance (NMR) techniques. It was determined that the isoprene-derived hydroxy epoxides should undergo efficient hydrolysis under atmospheric conditions, particular on lower pH SOA. An empirical structure-reactivity model was constructed that parametrized the hydrolysis rate constants according to the carbon substitution pattern on the epoxide ring and number of neighboring hydroxy functional groups. Compared to the previously studied similar nonfunctionalized epoxides, the presence of a hydroxy group at the alpha position to the epoxy group was found to reduce the hydrolysis rate constant by a factor of 20, and the presence of a hydroxy group at the beta position to the epoxy group was found to reduce the hydrolysis rate constant by a factor of 6.