Full Electron Delocalization across the Cluster in 1,12-bisBMes2-p-carborane Radical Anion.

Conjugation between three-dimensional (3D) carboranes and the attached substituents is commonly believed to be very weak. In this paper, we report that reducing 1,12-bis(BMes2)-p-carborane (B2pCab) with one electron gives a radical anion with a centrosymmetric semiquinoidal structure. This radical anion shows extensive electron delocalization between the two boron centers over the p-carborane bridge due to the overlap of carborane lowest unoccupied molecular orbital (LUMO) and the BMes2 LUMO. Unlike dianions of other C2B10H12 carboranes, which rearrange to a nido-form, two-electron reduction of B2pCab leads to a rearrangement into a basket-shaped intermediate.

Onset of spin entanglement in doped carbon nanotubes studied by EPR.

Nanoscale semiconductors with isolated spin impurities have been touted as promising materials for their potential use at the intersection of quantum, spin, and information technologies. Electron paramagnetic resonance (EPR) studies of spins in semiconducting carbon nanotubes have overwhelmingly focused on spins more strongly localized by sp3-type lattice defects. However, the creation of such impurities is irreversible and requires specific reactions to generate them. Shallow charge impurities, on the other hand, are more readily and widely produced by simple redox chemistry, but have not yet been investigated for their spin properties. Here, we use EPR to study p-doped (6,5) semiconducting single-wall carbon nanotubes (s-SWNTs) and elucidate the role of impurity-impurity interactions in conjunction with exchange and correlation effects for the spin behavior of this material. A quantitative comparison of the EPR signals with phenomenological modeling combined with configuration interaction electronic structure calculations of impurity pairs shows that orbital overlap, combined with exchange and correlation effects, causes the EPR signal to disappear due to spin entanglement for doping levels corresponding to impurity spacings of 14 nm (at 30 K). This transition is predicted to shift to higher doping levels with increasing temperature and to lower levels with increasing screening, providing an opportunity for improved spin control in doped s-SWNTs.

A CASSCF/MRCI trajectory surface hopping simulation of the photochemical dynamics and the gas phase ultrafast electron diffraction patterns of cyclobutanone.

We present the simulation of the photochemical dynamics of cyclobutanone induced by the excitation of the 3 s Rydberg state. For this purpose, we apply the complete active space self-consistent field method together with the spin-orbit multireference configuration interaction singles treatment, combined with the trajectory surface hopping for the inclusion of nonadiabatic effects. The simulations were performed in the spin-adiabatic representation, including nine electronic states derived from three singlet and two triplet spin-diabatic states. Our simulations reproduce the two previously observed primary dissociation channels: the C2 pathway yielding C2H4 + CH2CO and the C3 pathway producing c-C3H6 + CO. In addition, two secondary products, CH2 + CO from the C2 pathway and C3H6 from the C3 pathway, both of them previously reported, are also observed in our simulation. We determine the ratio of the C3:C2 products to be about 2.8. Our findings show that most of the trajectories reach their electronic ground state within 200 fs, with dissociation events finished after 300 fs. We also identify the minimum energy conical intersections that are responsible for the relaxation and provide an analysis of the photochemical reaction mechanism based on multidimensional scaling. Furthermore, we demonstrate a minimal impact of triplet states on the photodissociation mechanism within the observed timescale. In order to provide a direct link to experiments, we simulate the gas phase ultrafast electron diffraction patterns and connect their features to the underlying structural dynamics.

Conformational preferences of modified nucleobases in RNA aptamers and their effect on Förster resonant energy transfer.

Förster resonant energy transfer (FRET) can be utilized in the study of tertiary structures of RNA aptamers, which bind specific fluorophoric ligands to form a fluorogenic aptamer complex. By introducing the emissive nucleobase analog 4-cyanoindole into the fluorogenic Chili RNA aptamer a FRET pair was established. The interpretation of studies aiming to investigate those tertiary structures using FRET, however, relies on prior knowledge about conformational properties of the nucleobase, which govern exciton transfer capabilities. Herein we employed classical molecular dynamics combined with Förster exciton theory to elucidate the preferred orientation relative to proximate bases and the influence on exciton transfer efficiency in multiple substitution sites. We did this by comparing the chromophoric distances emergent from MD simulations with experimental FRET data based on structural data of the native aptamer. We present the outlined methodology as a means to reliably evaluate future nucleobase analogue candidates in terms of their structural behavior and emergent exciton transfer properties as exemplified in the study of the preferred orientation of 4-cyanoindole in the Chili RNA aptamer.

Simulation of exciton spectra in disordered supramolecular polymers: exciton localization in cisoid indolenine squaraine hexamers.

In order to understand the effects of disorder and defects in oligomers and polymers on the localization of excitons, we investigated the spectral properties of the squaraine B hexamer using long range corrected tight-binding TDDFT (lc-TDDFTB) and Frenkel-exciton model based calculations. Employing classical molecular dynamics, the cisoid indolenine squaraine hexamers helix was propagated in DCM and acetone to obtain ensembles of realistic structures, which naturally exhibit considerable disorder. The trajectories together with several model squaraine systems were studied to show the profound effects of disorder in the superstructure and disorder of the local monomer geometry on optical properties like absorption and exciton localization. We further compared lc-TDDFTB and exciton theory derived spectral data to related experimental data on absorption, exciton transfer and localization in squaraine polymers and oligomers.

Prediction of fluorescence quantum yields using the extended thawed Gaussian approximation.

Spontaneous emission and internal conversion rates are calculated within harmonic approximations and compared to the results obtained within the semi-classical extended thawed Gaussian approximation (ETGA). This is the first application of the ETGA in the calculation of internal conversion and emission rates for real molecular systems, namely, formaldehyde, fluorobenzene, azulene, and a dicyano-squaraine dye. The viability of the models as black-box tools for prediction of spontaneous emission and internal conversion rates is assessed. All calculations were done using a consistent protocol in order to investigate how different methods perform without previous experimental knowledge using density functional theory (DFT) and time-dependent DFT (TD-DFT) with B3LYP, PBE0, ωB97XD, and CAM-B3LYP functionals. Contrasting the results with experimental data shows that there are further improvements required before theoretical predictions of emission and internal conversion rates can be used as reliable indicators for the photo-luminescence properties of molecules. We find that the ETGA performs rather similar to the vertical harmonical model. Including anharmonicities in the calculation of internal conversion rates has a moderate effect on the quantitative results in the studied systems. The emission rates are fairly stable with respect to computational parameters, but the internal conversion rate reveals itself to be highly dependent on the choice of the spectral line shape function, particularly the width of the Lorentzian function, associated with homogeneous broadening.

HORTENSIA, a program package for the simulation of nonadiabatic autoionization dynamics in molecules.

We present a program package for the simulation of ultrafast vibration-induced autoionization dynamics in molecular anions in the manifold of the adiabatic anionic states and the discretized ionization continuum. This program, called HORTENSIA (Hopping Real-time Trajectories for Electron-ejection by Nonadiabatic Self-Ionization in Anions), is based on the nonadiabatic surface-hopping methodology, wherein nuclei are propagated as an ensemble along classical trajectories in the quantum-mechanical potential created by the electronic density of the molecular system. The electronic Schrödinger equation is numerically integrated along the trajectory, providing the time evolution of electronic state coefficients, from which switching probabilities into discrete electronic states are determined. In the case of a discretized continuum state, this hopping event is interpreted as the ejection on an electron. The derived diabatic and nonadiabatic couplings in the time-dependent electronic Schrödinger equation are calculated from anionic and neutral wavefunctions obtained from quantum-chemical calculations with commercially available program packages interfaced with our program. Based on this methodology, we demonstrate the simulation of autoionization electron kinetic energy spectra that are both time- and angle-resolved. In addition, the program yields data that can be interpreted easily with respect to geometric characteristics, such as bonding distances and angles, which facilitate the detection of molecular configurations important for the autoionization process. Furthermore, several useful extensions are included, namely, tools for the generation of initial conditions and input files as well as for the evaluation of output files, all of this both through console commands and a graphical user interface.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Time-resolved photoelectron spectroscopy of 4-(dimethylamino)benzethyne - an experimental and computational study.

We investigated the excited-state dynamics of 4-(dimethylamino)benzethyne (4-DMABE) in a combined theoretical and experimental study using surface-hopping simulations and time-resolved ionisation experiments. The simulations predict a decay of the initially excited S2 state into the S1 state in only a few femtoseconds, inducing a subsequent partial twist of the dimethylamino group within ∼100 fs. This leads to drastically reduced Franck-Condon factors for the ionisation transition to the cationic ground state, thus inhibiting the effective ionisation of the molecule, which leads to a vanishing photoelectron signal on a similar timescale as observed in our time-resolved photoelectron spectra. From the phototoelectron spectra, an adiabatic ionisation energy of 7.17 ± 0.02 eV was determined. The experimental decays match the theoretical predictions very well and the combination of both reveals the electronic characteristics of the molecule, namely the role of intramolecular charge transfer (ICT) states in the deactivation pathway of electronically excited 4-DMABE.

Long-range corrected fragment molecular orbital density functional tight-binding method for excited states in large molecular systems.

Herein, we present a new method to efficiently calculate electronically excited states in large molecular assemblies, consisting of hundreds of molecules. For this purpose, we combine the long-range corrected tight-binding density functional fragment molecular orbital method (FMO-LC-DFTB) with an excitonic Hamiltonian, which is constructed in the basis of locally excited and charge-transfer configuration state functions calculated for embedded monomers and dimers and accounts explicitly for the electronic coupling between all types of excitons. We first evaluate both the accuracy and efficiency of our fragmentation approach for molecular dimers and aggregates by comparing it with the full LC-TD-DFTB method. The comparison of the calculated spectra of an anthracene cluster shows a very good agreement between our method and the LC-TD-DFTB reference. The effective computational scaling of our method has been explored for anthracene clusters and for perylene bisimide aggregates. We demonstrate the applicability of our method by the calculation of the excited state properties of pentacene crystal models consisting of up to 319 molecules. Furthermore, the participation ratio of the monomer fragments to the excited states is analyzed by the calculation of natural transition orbital participation numbers, which are verified by the hole and particle density for a chosen pentacene cluster. The use of our FMO-LC-TDDFTB method will allow for future studies of excitonic dynamics and charge transport to be performed on complex molecular systems consisting of thousands of atoms.

Ultrafast Ring Closure Reaction of Gaseous cis-Stilbene from S1(ππ*).

cis-Stilbene (cis-St) is a well-known benchmark system for cis-trans photoisomerization. cis-St also produces 4a,4b-dihydrophenanthrene (DHP) in solution with a quantum yield of less than 0.19. The ring closure reaction, however, has never been identified for gaseous cis-St, and a recent computational simulation predicted the quantum yield of DHP to be only 0.04. In the present study, we identified an ultrafast ring closure reaction of gaseous cis-St for the first time using extreme ultraviolet time-resolved photoelectron spectroscopy. Surface hopping trajectory calculations at the SA3-XMS-CASPT2(2,2) level of theory reproduce the features of the observed time-resolved photoelectron spectra and predict the cis-St:DHP:trans-St branching ratio to be 0.55:0.41:0.04, in contrast with previous estimates. The results indicate that photoexcited cis-St favors ring closure over cis-trans isomerization under the isolated condition.

Quantum-classical dynamics of vibration-induced autoionization in molecules.

We present a novel method for the simulation of the vibration-induced autoionization dynamics in molecular anions in the framework of the quantum-classical surface hopping approach. Classical trajectories starting from quantum initial conditions are propagated on a quantum-mechanical potential energy surface while allowing for autoionization through transitions into discretized continuum states. These transitions are induced by the couplings between the electronic states of the bound anionic system and the electron-detached system composed of the neutral molecule and the free electron. A discretization scheme for the detached system is introduced, and a set of formulas is derived that enable the approximate calculation of couplings between the bound and free-electron states. We demonstrate our method on the example of the anion of vinylidene, a high-energy isomer of acetylene, for which detailed experimental data are available. Our results provide information on the time scale of the autoionization process and give insight into the energetic and angular distribution of the ejected electrons, as well as the associated changes in the molecular geometry. We identify the formation of structures with reduced C-C bond lengths and T-like conformations through bending of the CH2 group with respect to the C-C axis and point out the role of autoionization as a driving process for the isomerization to acetylene.

Internal conversion rates from the extended thawed Gaussian approximation: Theory and validation.

The theoretical prediction of the rates of nonradiative processes in molecules is fundamental in assessing their emissive properties. In this context, global harmonic models have been widely used to simulate vibronic spectra as well as internal conversion rates and to predict photoluminescence quantum yields. However, these simplified models suffer from the limitations that are inherent to the harmonic approximation and can have a severe effect on the calculated internal conversion rates. Therefore, the development of more accurate semiclassical methods is highly desirable. Here, we introduce a procedure for the calculation of nonradiative rates in the framework of the time-dependent semi-classical Extended Thawed Gaussian Approximation (ETGA). We systematically investigate the performance of the ETGA method by comparing it to the adiabatic and vertical harmonic methods, which belong to the class of widely used global harmonic models. Its performance is tested in potentials that cannot be treated adequately by global harmonic models, beginning with Morse potentials of varying anharmonicity followed by a double well potential. The calculated radiative and nonradiative internal conversion rates are compared to reference values based on exact quantum dynamics. We find that the ETGA has the capability to predict internal conversion rates in anharmonic systems with an appreciable energy gap, whereas the global harmonic models prove to be insufficient.

Stacking Is Favored over Hydrogen Bonding in Azaphenanthrene Dimers.

N-Doped polycyclic aromatic hydrocarbons have recently emerged as potential organic electronic materials. The function of such materials is determined not only by the intrinsic electronic properties of individual molecules but also by their supramolecular interactions in the solid state. Therefore, a proper characterization of the interactions between the individual units is of interest to materials science since they ultimately govern properties such as excitons and charge transfer. Here, we report a joint experimental and computational study of two azaphenanthrene dimers to determine the structure and the nature of supramolecular interactions in the aggregates. IR/UV double-resonance experiments were carried out using far- and mid-infrared free-electron laser radiation. The experimental spectra are compared with quantum chemical calculations for the lowest-energy π-stacked and hydrogen-bonded structures. The data reveal a preference of the π-stacked structure for the benzo[f]quinoline and the phenanthridine dimer.

(De)localization dynamics of molecular excitons: comparison of mixed quantum-classical and fully quantum treatments.

Molecular excitons play a central role in processes of solar energy conversion, both natural and artificial. It is therefore no wonder that numerous experimental and theoretical investigations in the last decade, employing state-of-the-art spectroscopic techniques and computational methods, have been driven by the common aim to unravel exciton dynamics in multichromophoric systems. Theoretically, exciton (de)localization and transfer dynamics are most often modelled using either mixed quantum-classical approaches (e.g., trajectory surface hopping) or fully quantum mechanical treatments (either using model diabatic Hamiltonians or direct dynamics). Yet, the terms such as "exciton localization" or "exciton transfer" may bear different meanings in different works depending on the method in use (quantum-classical vs. fully quantum). Here, we relate different views on exciton (de)localization. For this purpose, we perform molecular surface hopping simulations on several tetracene dimers differing by a magnitude of exciton coupling and carry out quantum dynamical as well as surface hopping calculations on a relevant model system. The molecular surface hopping simulations are done using efficient long-range corrected time-dependent density functional tight binding electronic structure method, allowing us to gain insight into different regimes of exciton dynamics in the studied systems.

Transforming Dyes into Fluorophores: Exciton-Induced Emission with Chain-like Oligo-BODIPY Superstructures.

Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene-bridged oligo-BODIPYs. A set of non- and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain-like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non-conjugated, Coulomb-coupled oscillators. The non-radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non-)radiative decay rates studied along the oligomeric series. We demonstrate that a "masked" fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions.

Real-time observation of photoionization-induced water migration dynamics in 4-methylformanilide-water by picosecond time-resolved infrared spectroscopy and ab initio molecular dynamics simulations.

A novel time-resolved pump-probe spectroscopic approach that enables to keep high resolution in both the time and energy domain, nanosecond excitation-picosecond ionization-picosecond infrared probe (ns-ps-ps TRIR) spectroscopy, has been applied to the trans-4-methylformanilide-water (4MetFA-W) cluster. Water migration dynamics from the CO to the NH binding site in a peptide linkage triggered by photoionization of 4MetFA-W is directly monitored by the ps time evolution of IR spectra, and the presence of an intermediate state is revealed. The time evolution is analyzed by rate equations based on a four-state model of the migration dynamics. Time constants for the initial to the intermediate and hot product and to the final product are obtained. The acceleration of the dynamics by methyl substitution and the strong contribution of intracluster vibrational energy redistribution in the termination of the solvation dynamics is suggested. This picture is well confirmed by the ab initio on-the-fly molecular dynamics simulations. Vibrational assignments of 4MetFA and 4MetFA-W in the neutral (S0 and S1) and ionic (D0) electronic states measured by ns IR dip and electron-impact IR photodissociation spectroscopy are also discussed prior to the results of time-resolved spectroscopy.

Excimer formation dynamics in the isolated tetracene dimer.

The understanding of excimer formation and its interplay with the singlet-correlated triplet pair state 1(TT) is of high significance for the development of efficient organic electronics. Here, we study the photoinduced dynamics of the tetracene dimer in the gas phase by time-resolved photoionisation and photoion imaging experiments as well as nonadiabatic dynamics simulations in order to obtain mechanistic insight into the excimer formation dynamics. The experiments are performed using a picosecond laser system for excitation into the S2 state and reveal a biexponential time dependence. The time constants, obtained as a function of excess energy, lie in the range between ≈10 ps and 100 ps and are assigned to the relaxation of the excimer on the S1 surface and to its deactivation to the ground state. Simulations of the quantum-classical photodynamics are carried out in the frame of the semi-empirical CISD and TD-lc-DFTB methods. Both theoretical approaches reveal a dominating relaxation pathway that is characterised by the formation of a perfectly stacked excimer. TD-lc-DFTB simulations have also uncovered a second relaxation channel into a less stable dimer conformation in the S1 state. Both methods have consistently shown that the electronic and geometric relaxation to the excimer state is completed in less than 10 ps. The inclusion of doubly excited states in the CISD dynamics and their diabatisation further allowed to observe a transient population of the 1(TT) state, which, however, gets depopulated on a timescale of 8 ps, leading finally to the trapping in the excimer minimum.

On the quantum and classical control of laser-driven isomerization in the Wigner representation.

We investigate the validity of the classical approximation to the numerically exact quantum dynamics for infrared laser-driven control of isomerization processes. To this end, we simulate the fully quantum mechanical dynamics both by wavepacket propagation in position space and by propagating the Wigner function in phase space employing a quantum-mechanical correction term. A systematic comparison is made with purely classical propagation of the Wigner function. On the example of a one-dimensional double well potential, we identify two complementary classes of pulse sequences that invoke either a quantum mechanically or a classically dominated control mechanism. The quantum control relies on a sequence of excitations and de-excitations between the system's eigenstates on a time scale far exceeding the characteristic vibrational oscillation periods. In contrast, the classical control mechanism is based on a short and strong few-cycle field exerting classical-like forces driving the wavepacket to the target potential well where it is slowed down and finally trapped. While in the first case, only the quantum mechanical propagation correctly describes the field-induced population transfer, the short pulse case is also amenable to a purely classical description. These findings shed light on the applicability of classical approximations to simulate laser-controlled dynamics and may offer a guideline for novel control experiments in more complex systems that can be analyzed and interpreted utilizing efficient state-of-the-art classical trajectory simulations based on ab initio molecular dynamics.

Ultrafast Ring-Opening Reaction of 1,3-Cyclohexadiene: Identification of Nonadiabatic Pathway via Doubly Excited State.

The photoinduced ring-opening reaction of 1,3-cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays an essential role in the photobiological synthesis of vitamin D3 in the skin. This reaction follows the Woodward-Hoffmann rule, and C5-C6 bond rupture via an electronically excited state occurs with conrotatory motion of the end CH2 groups. However, it is noted that the photoexcited S1(π,π*) state of CHD is not electronically correlated with the ground state of HT, and the reaction must proceed via nonadiabatic transitions. In the present study, we have clearly observed the nonadiabatic reaction pathway via the doubly excited state of CHD using ultrafast extreme UV photoelectron spectroscopy. The results indicate that the reaction occurs in only 68 fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.