Vibrational Energy Redistribution Process Research Articles

Ultrafast intramolecular vibrational energy transfer in carbon nitride hydrocolloid examined by femtosecond two-dimensional infrared spectroscopy.

In this work, ultrafast vibrational and structural processes in a graphitic carbon nitride hydrocolloid system were studied using a combination of linear infrared and nonlinear two-dimensional infrared (2D IR) spectroscopies. The experimentally observed three IR line shapes in the C=N stretching vibration frequency region were analyzed and attributed to the rigid and conjugated molecular frame of the prepared g-CN molecular species, which is believed to be a dimeric tris-s-triazine, as well as attributed to insignificant solvent influence on the delocalized C=N vibrations. Vibrational transition density cubes were also computed for the proposed g-CN dimer, confirming the heterocyclic C=N stretching nature of the three IR absorption peaks. Intramolecular vibrational energy transfer dynamics and spectral diffusion of the g-CN system were characterized by examining a series of time-dependent 2D IR spectra. A picosecond intramolecular vibrational energy redistribution process was found to occur among these delocalized C=N stretching modes, acting as an efficient vibrational energy transfer channel. This work reasonably connects the experimentally observed IR signature to a specific g-CN structure and also provides the first report on the ultrafast intramolecular processes of such carbon nitride systems. The obtained results are fundamentally important for understanding the molecular mechanisms of such carbon-nitride based functional materials.

Tracking intra-molecular electron and vibrational energy redistribution by time and frequency resolved transient grating spectroscopy

Theoretical calculation results show that electron flow occurs from xanthene to phenyl ring within the Rh101+ after its electronic transition from ground state (S0) to first excited state (S1). Time and frequency resolved transient grating (TG) spectroscopy is then conducted to seek the experimental evidence. Through Fourier analysis at different wavelengths of TG signal and their specific value spectra, vibrational enhancement corresponding to electron acceptor chromophore (phenyl ring) is observed. So the ultrafast vibrational energy redistribution process is detected experimentally and the direction of electron flow is confirmed.

Understanding the connection between conformational changes of peptides and equilibrium thermal fluctuations.

Despite the increasing evidence that conformational transitions in peptides and proteins are driven by specific vibrational energy pathways along the molecule, the current experimental techniques of analysis do as yet not allow to study these biophysical processes in terms of anisotropic energy flows. Computational methods offer a complementary approach to obtain a more detailed understanding of the vibrational and conformational dynamics of these systems. Accordingly, in this work we investigate jointly the vibrational energy distribution and the conformational dynamics of trialanine peptide in water solution at room temperature by applying the Instantaneous Normal Mode analysis to the results derived from equilibrium molecular dynamics simulations. It is shown that conformational changes in trialanine are triggered by the vibrational energy accumulated in the low-frequency modes of the molecule, and that excitation is caused exclusively by thermal fluctuations of the solute-solvent system, thus excluding the possibility of an intramolecular vibrational energy redistribution process.

Unveiling Singlet Fission Mediating States in TIPS-pentacene and its Aza Derivatives.

Femtosecond pump-depletion-probe experiments were carried out in order to shed light on the ultrafast excited-state dynamics of triisopropylsilylethynyl (TIPS)-pentacene and two nitrogen-containing derivatives, namely, diaza-TIPS-pentacene and tetraaza-TIPS-pentacene. Measurements performed in the visible and near-infrared spectral range in combination with rate model simulations reveal that singlet fission proceeds via the extremely short-lived intermediate (1)TT state, which absorbs in the near-infrared spectral region only. The T1 → T3 transition probed in the visible region shows a rise time that comprises two components according to a consecutive reaction (S1 → (1)TT → T1). The incorporation of nitrogen atoms into the acene structure leads to shorter dynamics, but the overall triplet formation follows the same kinetic model. This is of particular importance, since experiments on tetraaza-TIPS-pentacene allow for investigation of the triplet state in the visible range without an overlapping singlet contribution. In addition, the pump-depletion-probe experiments show that the triplet absorption in the visible (T1 → T3) and near-infrared (T1 → T2) regions occurs from the same initial state, which was questioned in previous studies. Furthermore, an additional ultrafast transfer between the excited triplet states (T3 → T2) is identified, which is also in agreement with the rate model simulation. By applying depletion pulses, which are resonant with higher vibrational levels, we gain insight into internal vibrational energy redistribution processes within the triplet manifold. This additional information is of great relevance regarding the study of loss channels within these materials.

Vibrational energy relaxation of the amide I mode of N-methylacetamide in D₂O studied through Born-Oppenheimer molecular dynamics.

The vibrational relaxation of the amide I mode of deuterated N-methylacetamide in D2O solution is studied through nonequilibrium simulations using the semiempirical Born-Oppenheimer molecular dynamics (SEBOMD) approach to describe the whole solute-solvent system. Relaxation pathways and lifetimes are determined using the instantaneous normal mode (INM) analysis. The relaxation of the amide I mode is characterized by three different time scales; most of the excess energy (80%) is redistributed through intramolecular vibrational energy redistribution processes, with a smaller contribution (20%) of intermolecular energy flowing into the solvent. The amide II mode is found to contribute modestly (7%) to the relaxation mechanism. The amide I mode and the total vibrational energy decay curves obtained using SEBOMD and INM are in satisfactory agreement with the experimental measurements.

Ionization-induced π → H site-switching in phenol–CH4complexes studied using IR dip spectroscopy

IR spectra of phenol-CH4 complexes generated in a supersonic expansion were measured before and after photoionization. The IR spectrum before ionization shows the free OH stretching vibration (ν(OH)) and the structure of neutral phenol-CH4 in the electronic ground state (S0) is assigned to a π-bound geometry, in which the CH4 ligand is located above the phenol ring. The IR spectrum after ionization to the cationic ground state (D0) exhibits a red shifted ν(OH) band assigned to a hydrogen-bonded cationic structure, in which the CH4 ligand binds to the phenolic OH group. In contrast to phenol-Ar/Kr, the observed ionization-induced π → H migration has unity yield for CH4. This difference is attributed to intracluster vibrational energy redistribution processes.

Picosecond Rotational Interconversion Adjacent to a C═O Bond Studied by Two-Dimensional Infrared Spectroscopy

Molecular conformations around the C═O group of carbonyl compounds like ketones and aldehydes play an important role in determining their reaction properties in solutions, including reaction rate, mechanism, steric structure, and chirality of products. Investigating different rotational conformers and their rapid exchange at room temperature will provide information on the rotational barrier and insights into how different rotamers may contribute to fundamental reactions in chemistry. We applied two-dimensional infrared (2D IR) spectroscopy and polarization-dependent IR transient grating technique to the study of 4,4-dimethyl-2-pentanone in CCl(4). Spectroscopic evidence showed that the internal rotation around the single carbon-carbon bond adjacent to the C═O group takes place on a picosecond time scale. DFT calculations suggested the presence of three different rotational conformations, one eclipsed and two staggered forms. Spectral simulation utilized the stochastic Liouville equation with a three-state jump model and incorporated the polarization factors that take into account the different direction of transition dipole moment in the three rotamers. The effects of the intramolecular vibrational energy redistribution process on the waiting time dependence of the 2D absorptive spectra were also included. Through comprehensive simulation of the observed spectral features, the exchange time constants between the three rotamers were determined: 5.4 ps from the eclipsed to staggered forms and 1.7 ps for the reverse direction.

Vibrational Energy Dynamics of Glycine,N-Methylacetamide, and Benzoate Anion in Aqueous (D2O) Solution

Ultrafast infrared-Raman spectroscopy is used to study vibrational energy dynamics of three molecules in aqueous solution (D(2)O) that serve as models for the building blocks of peptides. These are glycine-d(3) zwitterion (GLY), N-methylacetamide-d (NMA), and benzoate anion (BZ). GLY is the simplest amino acid, NMA a model compound with a peptide bond, and BZ a model for aromatic side chains. An ultrashort IR pulse pumps a parent CH-stretch on each solute. Anti-Stokes Raman monitors energy flow through the solutes' strongly Raman-active transitions. Stokes Raman of D(2)O stretching functions as a molecular thermometer to monitor energy dissipation from solute to solvent. A three-stage model is used to summarize the vibrational energy redistribution process and to provide a framework for discussing energy dynamics of different molecules. The initial CH-stretch excitation is found to be delocalized over some or all of the solute molecule in NMA and BZ but not in GLY. The overall time constants for energy dissipation are 7.2 ps for GLY, 4.9 ps for NMA, and 8.0 ps for BZ. CH-stretch energy in GLY is redistributed in a nearly statistical manner among observed GLY vibrations. In NMA the energy is distributed among about one-half of the observed vibrations, and in BZ much of the observed energy is channeled along a CH-stretch to the ring stretch pathway. The strongly Raman-active vibrations accurately represent the flow of vibrational energy through NMA but not through GLY or BZ.

Investigation of the Influence of Solute−Solvent Interactions on the Vibrational Energy Relaxation Dynamics of Large Molecules in Liquids

The influence of solute-solvent interactions on the vibrational energy relaxation dynamics of perylene and substituted perylenes in the first singlet excited-state upon excitation with moderate (<0.4 eV) excess energy has been investigated by monitoring the early narrowing of their fluorescence spectrum. This narrowing was found to occur on timescales ranging from a few hundreds of femtoseconds to a few picoseconds. Other processes, such as a partial decay of the fluorescence anisotropy and the damping of a low-frequency oscillation due to the propagation of a vibrational wavepacket, were found to take place on a very similar time scale. No significant relationship between the strength of nonspecific solute-solvent interactions and the vibrational energy relaxation dynamics of the solutes could be evidenced. On the other hand, in alcohols the spectral narrowing is faster with a solute having H-bonding sites, indicating that this specific interaction tends to favor vibrational energy relaxation. No relationship between the dynamics of spectral narrowing and macroscopic solvent properties, such as the thermal diffusivity, could be found. On the other hand, a correlation between this narrowing dynamics and the number of low-frequency modes of the solvent molecules was evidenced. All these observations cannot be discussed with a model where vibrational energy relaxation occurs via two consecutive and dynamically well-separated steps, namely ultrafast intramolecular vibrational redistribution followed by slower vibrational cooling. On the contrary, the results indicate that both intra- and intermolecular vibrational energy redistribution processes are closely entangled and occur, at least partially, on similar timescales.

Quantum Tunneling in the Midrange Vibrational Fundamentals of Tropolone

The Fourier transform infrared spectrum of tropolone(OH) vapor in the 1175-1700 cm(-1) region is reported at 0.0025 and 0.10 cm(-1) spectral resolutions. The 12 vibrational fundamentals in this region of rapidly rising vibrational state density are dominated by mixtures of the CC, CO, CCH, and COH internal coordinates. Estimates based on the measurement of sharp Q branch peaks are reported for 11 of the spectral doublet component separations DS(v) = |Delta(v) +/- Delta(0)|. Delta(0) = 0.974 cm(-1) is the known zero-point splitting, and three a(1) modes show tunneling splittings Delta(v) approximately Delta(0), four b(2) modes show splittings Delta(v) approximately 0.90Delta(0), and the remaining four modes show splittings Delta(v) falling 5-14% from Delta(0.) Significantly, the splitting for the nominal COH bending mode nu(8) (a(1)) is small, that is, 10% from Delta(0). Many of the vibrational excited states demonstrate strong anharmonic behavior, but there are only mild perturbations on the tautomerization mechanism driving Delta(0). The data suggest, especially for the higher frequency a(1) fundamentals, the onset of selective intramolecular vibrational energy redistribution processes that are fast on the time scale of the tautomerization process. These appear to delocalize and smooth out the topographical modifications of the zero-point potential energy surface that are anticipated to follow absorption of the nu(v) photon. Further, the spectra show the propensity for the Delta(v) splittings of b(2) and other complex vibrations to be damped relative to Delta(0).

Observation of a simple vibrational wavepacket in a polyatomic molecule via time-resolved photoelectron velocity-map imaging: A prototype for time-resolved IVR studies

We have prepared a coherent superposition of the two components of a Fermi resonance in the S1 state of toluene at approximately 460 cm(-1) with a approximately 1 ps laser pulse and monitored time-resolved photoelectron velocity-map images. The photoelectron intensities oscillate with time in a manner that depends on their kinetic energy, even though full vibrational resolution in the cation is not achieved. Analysis of the time-dependent photoelectron spectra enables information on the composition of the S1 wavepacket to be deduced. Such an experiment, in which a whole set of partially dispersed cation vibrational states are detected simultaneously, suggests an efficient method of studying intramolecular vibrational energy redistribution processes in excited states.

Classical trajectory calculations of intramolecular vibrational energy redistribution. II. Phenol-water complex.

Ab initio classical trajectory calculations have been applied to the intramolecular vibrational energy redistribution process of an O-H stretching vibration for phenol cation, [phenol]+, and its hydrogen-bonded water complex, [phenol-water]+. In phenol cation, a single narrow peak in the power spectrum, obtained by Fourier transformation of the autocorrelation function of its total momentum, indicates that the initial energy given to the O-H stretching oscillator of the phenol moiety is conserved and no energy flow occurs. On the other hand, for phenol-water cation, the calculated broadened power spectrum implies that the initial energy is not conserved and the energy flow causes an energy redistribution among various vibrational modes.

Assignment of the low frequency vibrations in electron donor-acceptor complexes by ultrafast coherent spectroscopy

Abstract Coherent oscillations in a spontaneous fluorescence of electron donor-acceptor (EDA) complexes in solution were observed with three different acceptors: tetracyanoethylene (TCNE), chloranil, and fluoranil with various electron donors. Different oscillatory frequencies were observed for the complexes with different acceptors. The similar frequencies are observed for the complexes with different donors and a common acceptor: ∼155 cm−1 (period of ∼215 fs) for the complexes with TCNE, 178 cm−1 (187 fs) for the complexes with chloranil, and 212 cm−1 (157 fs) for the complex with fluoranil. It is concluded, that the out-of-plane vibrational mode of the acceptor, having b3u symmetry, is responsible for the observed oscillations in all three cases (three acceptors). The time-dependent fluorescence spectrum for the TCNE - HMB complex is reconstructed and the fluorescence peak-shift correlation function is obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mainly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory behavior of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The modulation of the mean transition moment is observed as well. Thus two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.

Vibrational Motion of Electron Donor-Acceptor Complexes Time-Resolved by Femtosecond Fluorescence Spectroscopy

The excited-state dynamics of several electron donor-acceptor complexes were studied by a femtosecond fluorescence up-conversion technique. The spontaneous fluorescence decay of the complexes contains an oscillatory component superimposed with an ultrafast decay component. The oscillatory component was assigned to the out-of-plane vibration of the acceptors (tetracyanoethylene (TCNE), chloranil, fluoranil) having b3u symmetry. The time-dependent fluorescence spectrum for the TCNE - hexamethylbenzene (HMB) complex was reconstructed from the fluorescence dynamics at different wavelengths and the fluorescence peak-shift correlation function was obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mainly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory behavior of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The time dependence of the mean transition moment was constructed. It oscillates with the same frequency (155 cm−1), which is an indication of the non-Condon transition. Thus, two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.

Vibrational Coherence in Electron Donor−Acceptor Complexes

The excited-state dynamics of several electron donor−acceptor complexes were studied by a femtosecond fluorescence up-conversion technique. The spontaneous fluorescence decay of the complexes contains an oscillatory component superimposed with a ultrafast decay component. The oscillations in fluorescence are observed for the complexes with three different electron acceptors, tetracyanoethylene (TCNE), chloranil, and fluoranil, with various electron donors, hexamethylbenzene, carbazole, 1-chloronaphthalene, N-methylaniline, and some others. The similar oscillatory frequencies are observed for the complexes with different donors and a common acceptor: ∼155 cm-1 (period of ∼215 fs) for complexes with TCNE, 178 cm-1 (187 fs) for the complexes with chloranil, and 212 cm-1 (157 fs) for the complex with fluoranil. The assignment of the oscillatory component to a particular vibration is proposed. It is concluded that the out-of-plane vibrational mode of the acceptor, having b3u symmetry, is responsible for the observed oscillations in all three cases (three acceptors). The time-dependent fluorescence spectrum for the TCNE−HMB complex is reconstructed from the fluorescence dynamics at different wavelengths, and the fluorescence peak shift correlation function is obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mostly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory component of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The modulation of the mean transition moment is observed as well. Thus, two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.

Two-dimensional vibrational spectroscopy. I. Theoretical calculation of the nonlinear Raman response function of CHCl3

The two-dimensional Raman response function of CHCl3 is theoretically considered with interpretations of each peak in terms of the associated vibrational transition pathways. In order to numerically calculate the 2D Raman spectrum, ab initio calculations of necessary quantities, such as the first- and second-order derivatives of the molecular polarizability with respect to vibrational coordinates and cubic potential anharmonic coefficients, were carried out by using the basis set 6-311++G(2df,2pd) at the Hartree–Fock level. Quantitative comparison between the two nonlinear response functions associated with the mechanical and electronic anharmonicities shows that the 2D Raman response from the high-frequency intramolecular vibrational modes of CHCl3 is mainly determined by the mechanical (potential) anharmonicity contributions. On the other hand, it is found that the two distinctive contributions originating from the mechanical and electronic anharmonicities interfere in the low-frequency region of the 2D spectrum. Overall, it is suggested that the high-frequency 2D Raman spectrum could provide a map of the mechanical anharmonic mode couplings. We briefly discuss how the 2D Raman spectrum can be used to elucidate the potential energy hypersurface and in turn to study the intramolecular vibrational energy redistribution process.

Relaxation and Vibrational Energy Redistribution Processes in Polyatomic Molecules

Relaxation and Vibrational Energy Redistribution Processes in Polyatomic Molecules

Vibrational predissociation in van der Waals complexes of glyoxal with Ar and Kr

Vibrational predissociation (VP) of glyoxal–Ar and glyoxal–Kr complexes formed in a supersonic expansion has been studied in several vibrational levels of the first excited singlet electronic state S1 (1Au) of glyoxal. Analysis of the fluorescence spectrum, together with lifetime measurements, has shown that VP is in competition with other intramolecular vibrational energy redistribution processes in these systems, as well as intersystem crossing (ISC). Glyoxal–Ar and glyoxal–Kr behave very similarly. In particular, two isomers exist with different dissociation schemes (but similar for the same isomer of Ar or Kr complexes), and Kr does not seem to enhance ISC. A comparison with collisional results is presented. Another interesting result of this work is that 710, a forbidden transition of free glyoxal, was found in the excitation spectrum of complexed glyoxal. The corresponding 7̄01 emission line of the complex was also observed, as well as combination bands. This suggests a strong interaction between mode 7 and one of the van der Waals modes. This interaction is the basis for a simple theoretical model which reproduces the experimental lifetime of the excited complex (7̄2).

Relaxation and Vibrational Energy Redistribution Processes in Polyatomic Molecules

Relaxation and Vibrational Energy Redistribution Processes in Polyatomic Molecules