Intramolecular Excitation Research Articles

Cherenkov Radiation-Mediated In Situ Excitation of Discrete Luminescent Lanthanide Complexes.

Lanthanide luminescence, while ideal for in vivo applications owing to sharp emission bands within the optical window, requires high-intensity, short-wavelength excitation of small organic "antenna" chromophores in the vicinity of the lanthanide complex to access excited f-orbital states through intersystem crossing. Herein, we explored Cherenkov radiation of the radioisotopes 18 F and 89 Zr as an in situ source of antenna excitation. The effective inter- and intramolecular excitation of the terbium(III) complexes of a macrocylic polyaminocarboxylate ligand (hydration number (q)=0, quantum yield (φ)=47 %) as well as its analogue functionalized to append an intramolecular Cherenkov excitation source (q=0.07, φ=63 %) was achieved. Using conventional small-animal fluorescence imaging equipment, we have determined a detection limit of 2.5 nmol of Tb(III) complex in presence of 10 μCi of 18 F or 89 Zr. Our system is the first demonstration of the optical imaging of discrete luminescent lanthanide complexes without external short-wave excitation.

The effect of electronic excitation on London dispersion

Atomic and molecular dispersion coefficients can now be calculated routinely using density-functional theory. In this work, we present the first determination of how electronic excitation affects molecular C6 London dispersion coefficients from the exchange-hole dipole moment (XDM) dispersion model. Excited states are typically found to have larger dispersion coefficients than the corresponding ground states, due to their more diffuse electron densities. A particular focus is both intramolecular and intermolecular charge-transfer excitations, which have high absorbance intensities and are important in organic dyes, light-emitting diodes, and photovoltaics. In these classes of molecules, the increase in C6 for the electron-accepting moiety is largely offset by a decrease in C6 for the electron-donating moiety. As a result, the change in dispersion energy for a chromophore interacting with neighbouring molecules in the condensed phase is minimal.

Ultrafast Coherent Control of Condensed Matter with Attosecond Precision.

Coherent control is a technique to manipulate wave functions of matter with light. Coherent control of isolated atoms and molecules in the gas phase is well-understood and developed since the 1990s, whereas its application to condensed matter is more difficult because its coherence lifetime is shorter. We have recently applied this technique to condensed matter samples, one of which is solid para-hydrogen ( p-H2). Intramolecular vibrational excitation of solid p-H2 gives an excited vibrational wave function called a "vibron", which is delocalized over many hydrogen molecules in a manner similar to a Frenkel exciton. It has a long coherence lifetime, so we have chosen solid p-H2 as our first target in the condensed phase. We shine a time-delayed pair of femtosecond laser pulses on p-H2 to generate vibrons. Their interference results in modulation of the amplitude of their superposition. Scanning the interpulse delay on the attosecond time scale gives a high interferometric contrast, which demonstrates the possibility of using solid p-H2 as a carrier of information encoded in the vibrons. In the second example, we have controlled the terahertz collective phonon motion, called a "coherent phonon", of a single crystal of bismuth. We employ an intensity-modulated laser pulse, whose temporal envelope is modulated with terahertz frequency by overlap of two positively chirped laser pulses with their adjustable time delay. This modulated laser pulse is shined on the bismuth crystal to excite its two orthogonal phonon modes. Their relative amplitudes are controlled by tuning the delay between the two chirped pulses on the attosecond time scale. Two-dimensional atomic motion in the crystal is thus controlled arbitrarily. The method is based on the simple, robust, and universal concept that in any physical system, two-dimensional particle motion is decomposed into two orthogonal one-dimensional motions, and thus, it is applicable to a variety of condensed matter systems. In the third example, the double-pulse interferometry used for solid p-H2 has been applied to many-body electronic wave functions of an ensemble of ultracold rubidium Rydberg atoms, hereafter called a "strongly correlated ultracold Rydberg gas". This has allowed the observation and control of many-body electron dynamics of more than 40 Rydberg atoms interacting with each other. This new combination of ultrafast coherent control and ultracold atoms offers a versatile platform to precisely observe and manipulate nonequilibrium dynamics of quantum many-body systems on the ultrashort time scale. These three examples are digested in this Account.

On the vibron-polaron damping in quasi 1D macromolecular chains

The properties of the intramolecular vibrational excitation (vibron) in a quasi 1D macromolecular structure are studied. It is supposed that due to the vibron interaction with optical phonon modes, a vibron might form partially dressed small polaron states. The properties of these states are investigated in dependence on the basic system parameters and temperature of a thermal bath. We also investigate the process of damping of the polaron amplitude as a function of temperature and vibron-phonon coupling strength. Two different regimes of the polaron damping are found and discussed.

Excited state intramolecular proton transfer mechanism of o-hydroxynaphthyl phenanthroimidazole**Project supported by the Shandong Provincial Higher Educational Science and Technology Program, China (Grant No. J17KA186), the Taishan Scholar Project of Shandong Province, China, the Natural Science Foundation of Liaoning Province, China (Grant No. 20170540408), and the Science and Technology Plan Project of Shenyang City, China (Grant No. 17-231-1-06).

By utilizing the density functional theory (DFT) and the time-dependent density functional theory (TDDFT), the excited state intramolecular proton transfer (ESIPT) mechanism of o-hydroxynaphthyl phenanthroimidazole (HNPI) is studied in detail. Upon photo is excited, the intramolecular hydrogen bond is obviously enhanced in the S1 state, which thus promotes the ESIPT process. Hydrogen bond is shown to be strengthened via comparing the molecular structures and the infrared vibration spectra of the S0 and S1 states. Through analyzing the frontier molecular orbitals, we can conclude that the excitation is a type of the intramolecular charge transfer excitation, which also indicates the trend of proton transfer in S1 state. The vertical excitation based on TDDFT calculation can effectively repeat the absorption and fluorescence spectra of the experiment. However, the fluorescence spectrum of normal structure, which is similar to the spectrum of isomer structure is not detected in the experiment. It can be concluded that the fluorescence measured in the experiment is attributed to both structures. In addition, by analyzing the potential energy curves (PECs) calculated by the B3LYP functional method, it can be derived that since the molecule to cross the potential barrier in the S1 state is smaller than in the S0 state and the reverse proton transfer process in the S1 state is more difficult than in the S0 state, the ESIPT occurs in the S1 state.

Prediction of Intramolecular Charge-Transfer Excitation for Thermally Activated Delayed Fluorescence Molecules from a Descriptor-Tuned Density Functional

The reliability of time-dependent density functional theory (TDDFT) for the calculation of the charge-transfer (CT) excitation energy depends on the proportion of the Hartree–Fock (HF) exchange (α) in a hybrid functional. Here, we develop a new descriptor-tuning methodology to determine the optimal α (OHF) in a hybrid PBE functional (PBEα). The conventional transition-density-based descriptors are shown to be inappropriate to measure the degree of CT for our approach because of the functional-dependence. A new functional-independent descriptor K, defined as the negative growth rate of the exciton binding energy upon α, is then developed and found to be highly correlated with the OHF in the PBEα. Applying the so-called K-OHF method, the lowest singlet excitation energy and singlet–triplet energy splitting of various reported thermally activated delayed fluorescence (TADF) materials are successfully reproduced with mean absolute deviations of 0.07 and 0.09 eV, respectively, compared to the carefully measure...

Manifestation of cluster excitations in dielectric properties of water vapor and liquid water as well as their heat capacity

The main attention in this work is focused on the physical nature of the effective polarizability of water molecules in water vapor and liquid water. We will show that the main contribution to the effective polarizability of water vapor is formed by thermal vibration excitations of water dimers. At approaching the critical point this contribution becomes considerably greater than that caused by deformations of electron shells. In parallel with this we also show that the specific increase of the heat capacity of water vapor is mainly caused by the intramolecular vibrations and vibration excitations of water dimers. We propose a new method for the determination of dimer concentration in water vapor. This approach is generalized for the description of effective polarizability for liquid water. It is assumed that water can be considered as an ensemble of tetramers and dimers in wide temperature interval of liquid states. We analyze peculiarities of their thermal excitations in dense surroundings and their contributions to the effective polarizability of molecules. There is concentration of dimers, arising because of disintegration of tetramers, calculated.

The role of the CI expansion length in time-dependent studies.

With the recent advances in experimental attosecond science, theoretical predictions of electron dynamics can now be validated against experiment. Time-dependent studies of the electron motion in molecules can be used to obtain information about electronic transitions and the interaction of the electrons with electromagnetic fields. Often, these approaches rely on single-excited wave functions. Presented here is a first attempt to evaluate the accuracy of the time-dependent configuration interaction method so that the optimal representation of the electronic wave function for time-dependent studies can be assessed. A quantifier is determined that can aid in finding this optimal representation. The approach is demonstrated on a variety of molecules that include both localized and intramolecular charge transfer electron excitations. Observables including excitation energies, dipole moments, strengths, and static polarizabilities are obtained from time-independent and time-dependent calculations and are compared to experimental data. In this way, a rigorous routine is developed by which the reliability and accuracy of the CI wave function can be assessed and which represents a first step to a more quantitative description of electron dynamics in molecules.

Charge transfer excitations in TDDFT: A ghost-hunter index.

This work presents a new index, MAC , enabling the on-the-fly detection of ghost charge transfer (CT) states, a major problem in time-dependent density-functional theory calculations. This computationally inexpensive index, derived as a modification of the Mulliken estimation of transition energy for CT excitations, relies on two basic ingredients: an effective CT distance, computed using our density-based index (DCT ), and an orbital weighted estimation of the Ionization Potential and Electron Affinity. Some model systems, representative of both intermolecular and intramolecular CT excitations, were chosen as test cases. The robustness of our approach was verified by analyzing the behavior of functionals belonging to different classes (GGA, global hybrids and range separated hybrids). The results obtained show that ghost states are correctly spotted, also in the delicate case of intramolecular excitations displaying substantial donor-bridge-acceptor delocalization, in a regime for which the standard Mulliken formulation attends its limits. © 2017 Wiley Periodicals, Inc.

New Infrared Bands of the C-Bonded Isomer of OC-N2O and Determination of Two Intermolecular Frequencies.

Three combination bands involving intermolecular modes of the most stable isomer of the OC-N2O van der Waals complex have been observed, two in the carbon monoxide CO stretch region (∼2150 cm-1) and one in the ν3 asymmetric stretch region of N2O (∼2223 cm-1). Vibrational assignment is achieved by comparison with data recently published ( Barclay , A. ; et al. Chem. Phys. Lett. 2016 , 62 , 651 ), concerning OC-CO2. Two of these bands involve the same intermolecular mode, one with the CO stretch and the other with the ν3 asymmetric stretch of N2O. The values of intermolecular frequency deduced from these two bands agree very well, showing no significant dependence on intramolecular vibrational excitation.

Dynamics of Intramolecular Energy Hopping in Multi-Bodipy Self-Assembled Metallocyclic Species: A Tool for Probing Subtle Structural Distortions in Solution

The intramolecular excitation energy transfer (EET) processes in a series of fluorescent-unquenched, self-assembled metallocycles consisting of spatially fixed-separated and parallel-aligned Bodipy chromophores, are investigated here by steady-state and femtosecond-fluorescence upconversion measurements in the solution phase. These multi-Bodipy macrocycles, namely, the rhomboid (A1), the tetragon (A2) and the hexagon (A3), are formed via temperature-regulated Pt(II)–pyridyl coordination and consist, respectively, of two, four, and six Bodipy subunits, which are locked at the corners and aligned with their long molecular axes perpendicular to the rigid polygonal frame formed by the alternating B···Pt(II) connectivities. Extensive simulations and fits to the experimental fluorescence anisotropy decays r(t) show that EET within the cyclic scaffolds is quite uniform and much faster than the intrinsic decay rate of the Bodipy’s. The equalization of the excitation survival probabilities over time of all chromop...

Triphenylamine corrole dyads: Synthesis, characterization and substitution effect on photophysical properties

We present our results on the effect of substitution on the photophysical properties of donor-acceptor (D-A) systems in which triphenylamine is the donor and substituted corroles i.e., 5,15-phenyl-10-triphenylaminecorrole TPACor 1, 5,15-di(3,5-ditertbutylphenyl)-10-triphenylaminecorrole TPACor 2, and 5,15-(4-nitrophenyl)-10-triphenylaminecorrole TPACor 3 is the acceptor. All three dyads have been characterized by elemental analysis, MALDI-MS, cyclic voltammetry, UV-Vis and fluorescence (steady state and time-resolved) spectroscopies. Both Soret and Q bands of TPACor 3 are red-shifted when compared to other two dyads due to the presence of electron withdrawing nitro group. Similarly, redox properties of TPACor 3 are altered, when correlated to TPACor 1 and TPACor 2 dyads. However, the fluorescence emission of triphenylamine in all three dyads was quenched significantly (>90%) compared to its monomeric unit. The presence of either electron releasing or electron withdrawing group on corrole moiety has not much effect on the photophysical properties. The quenched emission was attributed to intramolecular excitation energy transfer and the photoinduced electron transfer reactions contested in these dyads. Intramolecular photoinduced reactions were monitored in three 5,15-diaryl-10-triphenylaminecorroles, represented as TPACor 1 (phenyl), TPACor 2 (3,5-ditertiarybutylphenyl), and TPACor 3 (p-nitrophenyl). In these dyad systems, energy transfer from triphenylamine to corrole is the major pathway, which is explained with optical, electrochemical, and DFT studies.

Influence of the nature of the anchoring group on electron injection processes at dye-titania interfaces.

We report (time-dependent) density functional theory calculations characterizing the changes in the electronic and optical properties of oligothiophene dyes when grafted on a titania surface via a carboxylic acid or catechol moiety as anchoring group, in relation to their use in dye-sensitized solar cells. The broadening of the LUMO level of the compounds upon adsorption has been extracted from the computed electronic structures and used to estimate electron injection times into the conduction band of the oxide. The strongly coupled carboxylic-containing dyes lead to faster electron injection times compared to catechol-substituted dyes. This difference is ascribed to the electron-donating character of the catechol moiety that polarizes the dye LUMO away from the dye@titania interface. The absorption spectra simulated at the TD-DFT level indicate that the grafted carboxylic-thiophene dyes undergo an indirect injection mechanism (type I) in which an intramolecular excitation is created before the charge is transferred to titania. In contrast, catechol dyes with a short conjugation length for the thiophene backbone are type II sensitizers exhibiting a direct injection mechanism leading to a direct photoexcitation from the dye HOMO to the titania conduction band. A mixed character prevails for the injection in the case of catechol dyes containing a longer oligothiophene chain.

Graphene-modulated photo-absorption in adsorbed azobenzene monolayers.

The impact of graphene on the photo-absorption properties of trans- and cis-azobenzene monolayers is studied in the framework of density-functional theory and many-body perturbation theory. We find that, despite the weak hybridization between the electronic bands of graphene and those of the azobenzene monolayers, graphene remarkably modulates the absorption spectra of the adsorbates. The excitation energies are affected via two counteracting mechanisms: substrate polarization reduces the band-gap of azobenzene, and enhanced dielectric screening weakens the attractive interaction between electrons and holes. The competition between these two effects gives rise to an overall blueshift of peaks stemming from intramolecular excitations, and a redshift of peaks from intermolecular ones. Even more interesting is that excitations corresponding to intermolecular electron-hole pairs, which are dark in the isolated monolayers, are activated by the graphene substrate. Our results demonstrate that the photoisomerization process of weakly adsorbed azobenzene undergoes notable changes on a carbon-based substrate.

Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission.

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C60 fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp2-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C60 film: after hot excitons are nascently formed in C60 via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C60 film into the HOPG is evaluated to be 1.31 × 1012 s−1, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C60. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C60 layers is 3.3 ± 1.2 × 1013 s−1.

Light induced intramolecular electron and energy transfer events in rigidly linked borondipyrromethene: Corrole Dyad

We have designed and synthesized a photo-induced energy/electron donor–acceptor conjugate comprising of corrole linked to BODIPY at the 5-position via ester linkage. The dyad was characterized by elemental analysis, MALDI-MS, UV-Visible, 1H NMR fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical methods. A comparison of the UV–visible and 1H NMR spectra of the dyad with those of the corresponding individual model compounds (i.e., BODIPY-CO2H and BPFC-OH) reveal that there exist minimum π–π interactions between BODIPY and corrole π-planes. Quenched emission of BODIPY and corrole part of the dyad has been observed in five different solvents. Excitation spectral data provided evidence for an intramolecular excitation energy transfer (EET) from the singlet BODIPY to the corrole and an intramolecular photoinduced electron transfer (PET) from singlet state of corrole to ground state of BODIPY. Detailed analysis of the data suggests that Forster׳s dipole–dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET.

Effect of Intramolecular High-Frequency Vibrational Mode Excitation on Ultrafast Photoinduced Charge Transfer and Charge Recombination Kinetics.

A model of photoinduced ultrafast charge separation and ensuing charge recombination into the ground state has been developed. The model includes explicit description of the formation and evolution of nonequilibrium state of both the intramolecular vibrations and the surrounding medium. An effect of the high-frequency intramolecular vibrational mode excitation by a pumping pulse on ultrafast charge separation and charge recombination kinetics has been investigated. Simulations, in accord with experiment, have shown that the effect may be both positive (the vibrational mode excitation increases the charge-transfer rate constant) and negative (opposite trend). The effect on charge separation kinetics is predicted to be bigger than that on the charge recombination rate but nevertheless the last is large enough to be observable. The amplitude of both effects falls with decreasing vibrational relaxation time constant, but the effects are expected to be observable up to the time constants as short as 200 fs. Physical interpretation of the effects has been presented. Comparisons with the experimental data have shown that the simulations, in whole, provide results close to that obtained in the experiment. The reasons of the deviations have been discussed.

Intermolecular electron transfer from intramolecular excitation and coherent acoustic phonon generation in a hydrogen-bonded charge-transfer solid.

Organic materials that produce coherent lattice phonon excitations in response to external stimuli may provide next generation solutions in a wide range of applications. However, for these materials to lead to functional devices in technology, a full understanding of the possible driving forces of coherent lattice phonon generation must be attained. To facilitate the achievement of this goal, we have undertaken an optical spectroscopic study of an organic charge-transfer material formed from the ubiquitous reduction-oxidation pair hydroquinone and p-benzoquinone. Upon pumping this material, known as quinhydrone, on its intermolecular charge transfer resonance as well as an intramolecular resonance of p-benzoquinone, we find sub-cm(-1) oscillations whose dispersion with probe energy resembles that of a coherent acoustic phonon that we argue is coherently excited following changes in the electron density of quinhydrone. Using the dynamical information from these ultrafast pump-probe measurements, we find that the fastest process we can resolve does not change whether we pump quinhydrone at either energy. Electron-phonon coupling from both ultrafast coherent vibrational and steady-state resonance Raman spectroscopies allows us to determine that intramolecular electronic excitation of p-benzoquinone also drives the electron transfer process in quinhydrone. These results demonstrate the wide range of electronic excitations of the parent of molecules found in many functional organic materials that can drive coherent lattice phonon excitations useful for applications in electronics, photonics, and information technology.

On the vibron nature in the system of two parallel macromolecular chains: The influence of interchain coupling

We studied the properties of the intramolecular vibrational excitation (vibron) at finite temperature in a system which consists of two parallel macromolecular chains. It was assumed that vibron interacts exclusively with dispersionless optical phonons and the whole system is considered to be in thermal equilibrium. Particular attention has been paid to the examination of the impact of the temperature and strength of the interchain coupling on the small polaron crossover. For that purpose we employed partial dressing method which enables the study of the degree of the phonon dressing of the vibron excitations in a wide area of system parameter space. We found that in the non-adiabatic regime the degree of dressing as a function of coupling constant continuously increases reflecting the smooth transition of the slightly dressed, practically free vibron, to a heavily dressed one: small polaron. As “adiabaticity” rises this transition becomes increasingly steeper, and finally, in the adiabatic limit, a discontinuous “jump” of the degree of dressing is observed. The interchain coupling manifests itself through the increase of the effective adiabatic parameter of the system.

Boron Arylations of Subporphyrins with Aryl Zinc Reagents.

Boron arylations of B-(methoxo)triphenylsubporphyrin have been developed with a combined use of ArZnI⋅LiCl and trimethylsilyl chloride. Aryl zinc reagents bearing bromo, cyano, amide, and ester groups can be employed for the B-arylation reaction to provide the corresponding B-arylated subporphyrins in moderate yields. Postmodifications of B-arylated subporphyrins have been demonstrated without loss of the B-C bond. These modifications include conversion of the cyano group into a benzoyl group with PhMgBr, hydrolysis of the ester group to give B-(4-carboxyphenyl)subporphyrin, and Pd-catalyzed Suzuki-Miyaura coupling of the 4-bromophenyl group to give a 1,4-phenylene-bridged subporphyrin-ZnII porphyrin hybrid that displays intramolecular excitation energy transfer from the subporphyrin to the porphyrin. The newly synthesized B-arylated subporphyrins have been fully characterized by NMR, UV/Vis absorption and fluorescence spectroscopies, mass spectrometry, electrochemical measurements, and X-ray diffraction analysis.