Intermolecular Energy Transfer Processes Research Articles

Comparison of Computational Strategies for the Calculation of the Electronic Coupling in Intermolecular Energy and Electron Transport Processes.

Electronic couplings in intermolecular electron and energy transfer processes calculated by six different existing computational techniques are compared to nonorthogonal configuration interaction for fragments (NOCI-F) results. The paper addresses the calculation of the electronic coupling in diketopyrrolopyrol, tetracene, 5,5'-difluoroindigo, and benzene-Cl for hole and electron transport, as well as the local exciton and singlet fission coupling. NOCI-F provides a rigorous computational scheme to calculate these couplings, but its computational cost is rather elevated. The here-considered ab initio Frenkel-Davydov (AIFD), Dimer projection (DIPRO), transition dipole moment coupling, Michl-Smith, effective Hamiltonian, and Mulliken-Hush approaches are computationally less demanding, and the comparison with the NOCI-F results shows that the NOCI-F results in the couplings for hole and electron transport are rather accurately predicted by the more approximate schemes but that the NOCI-F exciton transfer and singlet fission couplings are more difficult to reproduce.

Theoretically Revealing the Response of Intermolecular Vibration Energy Transfer and Decomposition Process of the DNTF System to Electric Fields Using Two-Dimensional Infrared Spectra.

The external electric field (E-field), which is an important stimulus, can change the decomposition mechanism and sensitivity of energetic materials. As a result, understanding the response of energetic materials to external E-fields is critical for their safe use. Motivated by recent experiments and theories, the two-dimensional infrared (2D IR) spectra of 3,4-bis (3-nitrofurazan-4-yl) furoxan (DNTF), which has a high energy, a low melting point, and comprehensive properties, were theoretically investigated. Cross-peaks were observed in 2D IR spectra under different E-fields, which demonstrated an intermolecular vibration energy transfer; the furazan ring vibration was found to play an important role in the analysis of vibration energy distribution and was extended over several DNTF molecules. Measurements of the non-covalent interactions, with the support of the 2D IR spectra, indicated that there were obvious non-covalent interactions among different DNTF molecules, which resulted from the conjugation of the furoxan ring and the furazan ring; the direction of the E-field also had a significant influence on the strength of the weak interactions. Furthermore, the calculation of the Laplacian bond order, which characterized the C-NO2 bonds as trigger bonds, predicted that the E-fields could change the thermal decomposition process of DNTF while the positive E-field facilitates the breakdown of the C-NO2 in DNTFⅣ molecules. Our work provides new insights into the relationship between the E-field and the intermolecular vibration energy transfer and decomposition mechanism of the DNTF system.

Polarization effects on intermolecular vibrational energy transfer analyzed by 2DIR spectroscopy

AbstractIn the present work, we analyze the influence of the polarization effects taking place during the course of a 2DIR spectroscopy experiment performed on a molecular system undergoing an intermolecular vibrational energy transfer process. When both donor and acceptor molecules participating in the vibrational energy transfer are embedded in a host solvent, they face rotational diffusion that strongly distorts the resulting 2DIR spectra. It could be expected that the difference between rotational diffusion constants will be of particular interest. For this purpose, the polarization effects are discussed according to the different orderings of the laser‐molecule interactions. Next, we study the distortions of the spectra as a function of the rotational diffusion constants of the individual molecules. The knowledge of these polarization effects is relevant to the interpretation of the spectra. Finally, the conclusions reached in this work for a vibrational energy transfer are valid for any other type of third‐order optical process performed on the same molecular system.

Tuning the triplet population of arylselanyl-BODIPY photosensitizers through substituents engineering for triplet-triplet annihilation photon upconversion with perylene

Organic triplet-triplet annihilation photon upconversion (TTA-UC) systems have attracted considerable attention owing to their promising applicability in solar energy harvesting, optoelectronic devices, photocatalysts, and bio-imaging. In this study, a series of BODIPYs prepared by incorporating substituted/ unsubstituted arylselenium groups, 1 (R = H), 2 (R = OMe), and 3 (R = F), were evaluated as triplet photosensitizers for TTA-UC. Direct Se-C bond formation on the BODIPY core provided a facile intersystem crossing (ISC) channel from the excited singlet state to the metastable triplet state, being the most effective in 2, as inferred from the singlet oxygen generation experiments, possibly because of the n-electron arising from the OMe group. The UC behavior of systems comprising the sensitizers and perylene as the acceptor in deaerated toluene was investigated using a 524 nm-wavelength laser to detect upconverted emission at 449 nm; thus, the UC yield decreased in the order of 21% for 3 > 16% for 2 > 12% for 1. This trend is consistent with the Stern-Volmer constants calculated from the quenched triplet state lifetimes of the sensitizers as a function of the concentration of the perylene quencher. This suggests that the UC efficiency was mainly governed by the intermolecular triplet-triplet energy transfer (TTET) process between arylselanyl-BODIPY photosensitizers and the perylene acceptor. This result was rationalized by the efficient population of the long-lived triplet excited state of the sensitizer, which is advantageous for diffusion-controlled TTA-UC behavior.

Influence of oxygen on the ro-vibrational kinetics of a non-equilibrium discharge in CO2–O2 mixtures

Storing excess renewable energy in hydrocarbons produced from CO2 potentially solves the intermittency issue of renewable energy sources in a green manner. The required reduction of CO2 to CO can be efficiently accomplished with non-equilibrium plasma conversion. On an industrial scale, effects of impurities on the reduction must be taken into account. During this study, the effects of oxygen are considered, as the impurity O2 is both a product of the reduction reaction and abundant in air. In this paper, the influence of O2 addition on the ro-vibrational kinetics of a pulsed DC CO2 glow discharge at 2.5–6.0 Torr—serving as a model non-equilibrium system—is studied in situ with quantum cascade laser infrared absorption spectroscopy. The temporal evolution of the ro-vibrational temperatures is measured, as well as the conversion of CO2 to CO. Trends in the temperature evolutions when increasing the flow rate from 7.4 to 30.0 sccm, varying the pressure, and increasing the O2 admixture up to 90% in increments of 10% are utilised to determine the underlying kinetic processes. Our results show that any decrease in the conversion of CO2 to CO caused by increasing O2 addition cannot be attributed to an induced change in the vibrational kinetics, since the asymmetric stretch mode of CO2—which is associated with dissociation via vibrational excitation—is not quenched. Measured changes in the temporal temperature trends are explained by species-dependent intra- and intermolecular collisional energy transfer processes.

Molecular Photothermal Effects on Time-Resolved IR Spectroscopy: Solute-Solvent Intermolecular Energy Transfer.

Time-resolved IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy are valuable tools for studying ultrafast chemical and biological processes in solutions. However, the corresponding signals at long times are obscured by the molecular photothermal effects resulting from the heat dissipation of vibrationally photoexcited molecules to the surroundings. Recently, a phenomenology model was used to describe molecular photothermal effects on IR-PP signals and the diagonal and cross-peaks of 2D-IR spectra at long pump-probe delay times. Here, we consider the thermal diffusion equation with a time-dependent heat source term to describe the solute-solvent energy transfer process. An approximate solution to the nonhomogeneous differential equation shows that the molecular photothermal effect is determined by the mean intermolecular distance between IR-absorbing molecules. We show that the time profile of heat dissipation from a vibrationally excited molecule to the surroundings, which provides information about the mechanisms involved in the solute-solvent intermolecular energy transfer process in solutions, can be directly measured by analyzing the molecular photothermal IR-PP and 2D-IR signals. We anticipate that the present work can be used to interpret local heating-induced time-resolved IR spectroscopic signals and understand the rate of and the mechanisms involved in the conversion from high-frequency molecular vibrational energy to solvent kinetic energy in condensed phases.

Intermolecular Vibration Energy Transfer Process in Two CL-20-Based Cocrystals Theoretically Revealed by Two-Dimensional Infrared Spectra.

Inspired by the recent cocrystallization and theory of energetic materials, we theoretically investigated the intermolecular vibrational energy transfer process and the non-covalent intermolecular interactions between explosive compounds. The intermolecular interactions between 2,4,6-trinitrotoluene (TNT) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and between 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and CL-20 were studied using calculated two-dimensional infrared (2D IR) spectra and the independent gradient model based on the Hirshfeld partition (IGMH) method, respectively. Based on the comparison of the theoretical infrared spectra and optimized geometries with experimental results, the theoretical models can effectively reproduce the experimental geometries. By analyzing cross-peaks in the 2D IR spectra of TNT/CL-20, the intermolecular vibrational energy transfer process between TNT and CL-20 was calculated, and the conclusion was made that the vibrational energy transfer process between CL-20 and TNTII (TNTIII) is relatively slower than between CL-20 and TNTI. As the vibration energy transfer is the bridge of the intermolecular interactions, the weak intermolecular interactions were visualized using the IGMH method, and the results demonstrate that the intermolecular non-covalent interactions of TNT/CL-20 include van der Waals (vdW) interactions and hydrogen bonds, while the intermolecular non-covalent interactions of HMX/CL-20 are mainly comprised of vdW interactions. Further, we determined that the intermolecular interaction can stabilize the trigger bond in TNT/CL-20 and HMX/CL-20 based on Mayer bond order density, and stronger intermolecular interactions generally indicate lower impact sensitivity of energetic materials. We believe that the results obtained in this work are important for a better understanding of the cocrystal mechanism and its application in the field of energetic materials.

Metal-Based Linear Light Upconversion Implemented in Molecular Complexes: Challenges and Perspectives.

The piling up of low-energy photons to produce light beams of higher energies while exploiting the nonlinear optical response of matter was conceived theoretically around 1930 and demonstrated 30 years later with the help of the first coherent ruby lasers. The vanishingly small efficacy of the associated light-upconversion process was rapidly overcome by the implementation of powerful successive absorptions of two photons using linear optics in materials that possess real intermediate excited states working as relays. In these systems, the key point requires a favorable competition between the rate constant of the excited-state absorption (ESA) and the relaxation rate of the intermediate excited state, the lifetime of which should be thus maximized. Chemists and physicists therefore selected long-lived intermediate excited states found (i) in trivalent lanthanide cations doped into ionic solids or into nanoparticles (2S+1LJ spectroscopic levels) or (ii) in polyaromatic molecules (triplet states) as the logical activators for designing light upconverters using linear optics. Their global efficiency has been stepwise optimized during the past five decades by using indirect intermolecular sensitization mechanisms (energy transfer upconversion = ETU) combined with large absorption cross sections.The induction of light-upconversion operating in a single discrete entity at the molecular level is limited to metal-based units and remained a challenge for a long time because coordination complexes possess high-frequency oscillators incompatible with the existence of (i) scales of accessible excited relays with long lifetimes and (ii) final high-energy emissive levels with noticeable intrinsic quantum yields. In contrast to intermolecular energy transfer processes operating in metal-based doped solids, which require statistical models, the combination of sensitizers and activators within the same molecule limits energy transfers to easily tunable intramolecular processes with first-order kinetic rate constants. Their successful programming in a trinuclear CrErCr complex in 2011 led to the first detectable near-infrared to green light upconversion induced in a molecular unit under reasonable excitation intensity. The subsequent progress in the modeling and understanding of the key factors controlling metal-based light upconversion operating in molecular complexes led to a burst of various designs exploiting different mechanisms, excited-state absorption (ESA), energy transfer upconversion (ETU), cooperative luminescence (CL), and cooperative upconversion (CU), which are discussed in this Account.

The importance of dynamic effects in chemical reactions in condensed phase

Energy transfer and spatial diffusion are two significant sub-processes in chemical reaction. Traditional rate theory is based on two assumptions: (1) energy transfer is faster than chemical reaction so that specific energy transfer channel is not important. (2) The solvents can respond to the change of solute quickly resulting in equilibrium solvation. Then once system cross the barrier, it will enter the product basin. But when energy transfer is fast enough to distribute the energy into all degrees of freedom according to certain ratio, the specific energy transfer pathways should be considered. Our previous studies have found that the polar functional groups play the major role in the intermolecular energy transfer process. And the energy accumulation and dissipation are completed by the motion with specific frequency. Furthermore, the major energy transfer channel is different in different solutions. Moreover, the length of transition path is very short, leading to the existence of different transient solvation configurations with distinctly different strengths of interaction between solutes and solvents. Consequently, the motion of solvents modulates the reaction dynamics and results in heterogeneous reaction paths. Therefore, the dynamic effects are vital for understanding and controlling the chemical reaction.

Synthesis and fluorescence photoswitching of a diarylethene derivative having a dibenzoylmethanato boron difluoride complex

A fluorescent diarylethene derivative covalently linked to a dibenzoylmethanato boron difluoride complex (DAE-BF2DBM dyad 1) was synthesized and its fluorescence photoswitching properties were examined in solution, in nanoparticles and in powder solid. DAE-BF2DBM dyad 1 underwent reversible fluorescence photswitching associated with the photochromic reactions in THF solution as well as in the solid states such as nanoparticles and powders upon alternate irradiation with ultraviolet (UV) and visible light. In addition, the efficient fluorescence photoswitching property based on the intermolecular energy transfer process in the densely packed solid states was clearly observed.

Vibronic Quantum Beating between Electronic Excited States in a Heterodimer.

Energy transfer in multichromophoric molecules can be affected by coherences that are induced by the electronic and vibrational couplings between chromophore units. Coherent electron-vibrational dynamics can persist at the subpicosecond time scale even at room temperature. Furthermore, wave-like localized-delocalized motions of the electronic wave function can be modulated by vibrations that actively participate in the intermolecular energy transfer process. Herein, nonadiabatic excited state molecular dynamics simulations have been performed on a rigid synthetic heterodimer that has been proposed as a simplified model for investigating the role and mechanism of coherent energy transfer in multichromophoric systems. Both surface hopping (SH) and Ehrenfest approaches (EHR) have been considered. After photoexcitation of the system at room temperature, EHR simulations reveal an ultrafast beating of electronic populations between the two lowest electronic states. These oscillations are not observed at low temperature and have vibrational origins. Furthermore, they cannot be reproduced using SH approach. This periodic behavior of electronic populations induces oscillations in the spatial localization of the electronic transition density between monomers. Vibrations whose frequencies are near-resonant with energy difference between the two lowest electronic excited states are in the range of the electronic population beating, and they are the ones that contribute the most to the coherent dynamics of these electronic transitions.

Ultrafast Vibrational Energy Transfer through the Covalent Bond and Intra- and Intermolecular Hydrogen Bonds in a Supramolecular Dimer by Two-Dimensional Infrared Spectroscopy.

In this work, the structural fluctuations and vibrational energy transfer dynamics in a supramolecular homodimer model, which is composed of 2-(9-anthracene)ureido-6-(1-undecyl)-4[1H]-pyrimidinone (UPAn) with quadruple intermolecular and single intramolecular hydrogen bonds (HBs), have been examined using ultrafast two-dimensional infrared (2D IR) and steady-state IR spectroscopies. A less structurally fluctuating intermolecular HB is found between the pyrimidinone C═O and ureido N-H groups. However, a larger structurally fluctuating intramolecular HB is suggested by the equilibrium and dynamical line-shape measurements of the ureido C═O stretch. Further, dynamical time-dependent 2D IR diagonal and off-diagonal signals show that intra- and intermolecular vibrational energy transfer processes occur on the picosecond timescale, where the latter is more efficient due to intermolecular hydrogen bonding interaction and through-space interaction.

Quantum Yield‐Engineered Biocompatible Probes Illuminate Lung Tumor Based on Viscosity Confinement‐Mediated Antiaggregation

AbstractLow quantum yield and aggregation‐mediated quenching are two concerns for fluorescence imaging. However, there are not yet general means available for addressing these issues. Herein, a viscosity confinement‐mediated antiaggregation strategy is established to enable the improved fluorescence properties of entrapped fluorophores in dye‐encapsulation nanotechnology including quantum yield, fluorescence lifetime, and photostability. To instantiate this strategy, solid DL‐menthol (DLM) is introduced to disperse entrapped indocyanine green (ICG) fluorophores when coencapsulating DLM and ICG molecules in organic poly(lactic‐co‐glycolic acid) carriers. Depending on the robust ability of highly viscous DLM to augment the migration barrier and diminish diffusion coefficient, ICG aggregation and aggregation‐mediated quenching are demonstrated to be theoretically and experimentally inhibited, resulting in prolonged fluorescence lifetime, increased quantum yield, and facilitated radiative process. Consequently, the fluorescence imaging ability and photostability are significantly improved, enabling the in vitro, cellular‐level, and in vivo fluorescence imaging. More significantly, this solid DLM‐mediated antiaggregation strategy can act as a general method to extend to the intermolecular fluorescence resonance energy transfer (FRET) process and improve FRET efficiency via inhibiting the aggregation‐mediated quenching.

Triplet Energy Transfer between a Sacrificial PMP and Blue TPF2 Iridium Dopants Leading to Enhancement of OLED Device Performance

In this study, we prepared phenylimidazole-based C^N-cyclometalated Ir(III) complexes (DMP, TPF2) and a C^C-cyclometalated Ir(III) complex (PMP), and investigated the energy transfer process by examining the intermolecular interactions between the two cyclometalated Ir(III) complexes. In films doped with 3% Ir(C^C)3 complex (PMP) and 15% Ir(C^N)3 complex (DMP or TPF2), the PMP effectively induced energy transfer to the DMP or TPF2. This intermolecular energy transfer process was investigated using a picosecond time-resolved emission spectroscopic method. In the case of mixing PMP with DMP, where two types of luminescence were observed at 470 and 580 nm, the emission at 470 nm was due to DMP, while the emission at 580 nm can be assigned as the intermolecular exciplex emission. By contrast, in the case of mixing PMP with TPF2, the emission at 465 nm corresponding to the PMP emission region decreased for 18.5 ns, while the emission at 530 nm corresponding to TPF2 increased. This emission can be attributed to...

Intermolecular electron energy transfer processes in the quenching of N2(C3Πu,v = 0–4) by collisions with N2 molecules

The removal rates of electronically excited molecular nitrogen N2(C3Πu,v = 0–4) in the collisions with the ground state N2(X1Σg+,v* = 0–4) molecules are calculated according to analytical expressions. The study includes a consideration of intermolecular electron energy transfers with the electronic excitation of target molecules. Reasonable agreement of the calculated rate coefficients with data of experimental measurements is obtained.

Damaging Intermolecular Energy and Proton Transfer Processes in Alpha‐Particle‐Irradiated Hydrogen‐Bonded Systems

AbstractAlthough the biological hazard of alpha‐particle radiation is well‐recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionized states produced by an alpha‐particle impact. Two such processes were identified in the present study by considering an acetylene dimer, a biologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well‐established intermolecular Coulombic decay of inner‐valence‐ionized states. The other is a novel relaxation mechanism of dicationic states involving intermolecular proton transfer. Both processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge‐separated states. These processes are general and predicted to occur also in alpha‐particle‐irradiated nucleobase pairs in DNA molecules.

Damaging Intermolecular Energy and Proton Transfer Processes in Alpha-Particle-Irradiated Hydrogen-Bonded Systems.

Although the biological hazard of alpha-particle radiation is well-recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionized states produced by an alpha-particle impact. Two such processes were identified in the present study by considering an acetylene dimer, a biologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well-established intermolecular Coulombic decay of inner-valence-ionized states. The other is a novel relaxation mechanism of dicationic states involving intermolecular proton transfer. Both processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge-separated states. These processes are general and predicted to occur also in alpha-particle-irradiated nucleobase pairs in DNA molecules.

White‐Light‐Emitting Materials Constructed from Supramolecular Approaches

AbstractWhite‐light emissive materials are gaining increasing attentions, ascribed to the potential applications in display and lighting devices as well as sensors. The realization of white‐light emission demands either the simultaneous emission of red, green, and blue colors or at least two complementary colors with similar distribution of emission intensities covering the entire visible spectrum. The introduction of supramolecular approaches not only relieves the negative effects of intermolecular interactions and energy transfer processes, but also endows the white‐light emissive materials with the capacity of color tunability and stimuli‐responsiveness, due to the dynamic and reversible nature of noncovalent interactions. Supramolecular materials with well‐ordered structures can regulate the energy transfer by the assembly–disassembly process, and meanwhile bring in extra fascinating attributes. This review mainly describes the recent reports on fabricating white‐light‐emitting materials from supramolecular approaches such as hydrogen bonding, host–guest interaction, π–π stacking, and metal coordination, which enriches the conceptions and pathways to afford such materials. Moreover, some perspectives together with pressing challenges are presented in the end, synchronously forecasting the potential trends of white‐light emissive materials.

Divide-to-Conquer: A Kinetic Model for Singlet Oxygen Photosensitization.

Photosensitized singlet oxygen generation occurring in a PS-O2 complex, where PS is a photosensitizer chromophore, is a weakly coupled intermolecular energy-transfer process, a still challenging problem for theoretical chemistry. To investigate the reaction rate directly from quantum-chemical calculations, we built a semiclassical kinetic model that minimizes the computational effort for the calculation of diabatic couplings, activation energies, and reorganization energies, which are the components of the rate. The model splits the system into sets of orthogonal coordinates, which are then explored to compute the reaction rate. This model offers an effective way to evaluate the reaction probability of singlet oxygen generation along different directions and intramolecular distances of the PS-O2 complex. The model can also be applied to other similar intermolecular energy-transfer problems, to connect the reaction kinetics and quantum-chemical calculations.

Linking ReI and PtII Chromophores with Aminopyridines: A Simple Route to Achieve a Complicated Photophysical Behavior.

The bifunctional aminopyridine ligands H2 N-(CH2 )n -4-C5 H4 N (n=0, L1; 1, L2; 2, L3) have been utilized for the preparation of the rhenium complexes [Re(phen)(CO)3 (L1-L3)]+ (1-3; phen=phenanthroline). Complexes 2 and 3 with NH2 -coordinated L2 and L3, respectively, were coupled with cycloplatinated motifs {Pt(ppy)Cl} and {Pt(dpyb)}+ (ppy=2-phenylpyridine, dpyb=dipyridylbenzene) to give the bimetallic species [Re(phen)(CO)3 (μ-L2/L3)Pt(ppy)Cl]+ (4, 6) and [Re(phen)(CO)3 (μ-L2/L3)Pt(dpyb)]2+ (5, 7). In solution, complexes 4 and 6 show 3 MLCT {Re}-based emission at 298 K, which changes to the 3 IL(ppy) state at 77 K. The photophysical properties of compounds 5 and 7 display a pronounced concentration dependence, presumably due to the formation of bimolecular aggregates. Analysis of the spectroscopic data, combined with TD-DFT simulations, suggest that unconventional heteroleptic {Re(phen)}⋅⋅⋅{Pt(dpyb)} π-π stacking operates as the driving force for ground-state association. The latter, together with intra- and intermolecular energy-transfer processes, determines the appearance of multiple emission bands and results in nonlinear relaxation kinetics of the excited states.