Efficient Vibrational Energy Transfer Research Articles

Proton-Coupled Electron Transfer in a Ruthenium(II) Bipyrimidine Complex in Its Ground and Excited Electronic States.

Proton-coupled electron transfer (PCET) was studied for the ground and excited electronic states of a [Ru(terpy)(bpm)(OH2)(PF6)2] complex, Ru-bpm. Cyclic voltammetry measurements show that the Ru(II)-aqua moiety undergoes PCET to form a Ru(IV)-oxo moiety in the anodic region, while the bpm ligand undergoes PCET to form bpmH2 in the cathodic region. The photophysical behavior of Ru-bpm was studied using steady-state and femtosecond transient UV-vis absorption spectroscopy, coupled with density functional theory (DFT) calculations. The lowest-lying excited state of Ru-bpm is described as a (Ru → bpm) metal-to-ligand charge-transfer (MLCT) state, while the metal-centered (MC) excited state was found computationally to be close in energy to the lowest-energy bright MLCT state (MC state was 0.16 eV above the MLCT state). The excited-state kinetics of Ru-bpm were found via transient absorption spectroscopy to be short-lived and were fit well to a biexponential function with lifetimes τ1 = 4 ps and τ2 = 65 ps in aqueous solution. Kinetic isotope effects of 1.75 (τ1) and 1.61 (τ2) were observed for both decay components, indicating that the solvent plays an important role in the excited-state dynamics of Ru-bpm. Based on the pH-dependent studies and the results from prior studies of similar Ru-complexes, we hypothesize that the 3MLCT state forms an excited-state hydrogen-bond adduct with the solvent molecules and that this process occurs with a 4 ps lifetime. The formation of such a hydrogen-bond complex is consistent with the electronic density accumulation at the peripheral N atoms of the bpm moiety in the 3MLCT state. The hydrogen-bonded state 3MLCT decays to the ground state with a 65 ps lifetime. Such a short lifetime is likely associated with the efficient vibrational energy transfer from the 3MLCT state to the solvent.

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.

Efficient non-resonant intermolecular vibrational energy transfer

Molecular excited vibrational states are metastable states and we incorporate their finite lifetimes into the theory of vibrational energy transfer between weakly interacting molecules, i.e., at internuclear distances at which they do not have a chemical bond. Expressions for the effective lifetime of an initially vibrationally excited molecule in the presence of a neighbouring molecule are derived in closed form. These expressions allow one to analyse the physics behind the energy transfer. It is shown that due to different finite lifetimes of the isolated excited molecules, a very efficient vibrational energy transfer can take place between them even if their energies are rather off-resonance. Examples are discussed.

Efficient Vibrational Energy Transfer through Covalent Bond in Indigo Carmine Revealed by Nonlinear IR Spectroscopy.

Ultrafast vibrational relaxation and structural dynamics of indigo carmine in dimethyl sulfoxide were examined using femtosecond pump-probe infrared and two-dimensional infrared (2D IR) spectroscopies. Using the intramolecularly hydrogen-bonded C═O and delocalized C═C stretching modes as infrared probes, local structural and dynamical variations of this blue dye molecule were observed. Energy relaxation of the vibrationally excited C═O stretching mode was found to occur through covalent bond to the delocalized aromatic vibrational modes on the time scale of a few picoseconds or less. Vibrational quantum beating was observed in magic-angle pump-probe, anisotropy, and 2D IR cross-peak dynamics, showing an oscillation period of ca. 1010 fs, which corresponds to the energy difference between the C═O and C═C transition frequency (33 cm-1). This confirms a resonant vibrational energy transfer happened between the two vibrators. However, a more efficient energy-accepting mode of the excited C═O stretching was believed to be a nearby combination and/or overtone mode that is more tightly connected to the C═O species. On the structural aspect, dynamical-time-dependent 2D IR spectra reveal an insignificant inhomogeneous contribution to time-correlation relaxation for both the C═O and C═C stretching modes, which is in agreement with the generally believed structural rigidity of such conjugated molecules.

Vibrational Dynamics, Mode Coupling, and Structural Constraints for Acetylproline-NH2

Vibrational relaxation, mode coupling, and peptide structural constraints were obtained by femtosecond two-dimensional vibrational spectroscopy of an acetylproline-NH2 model dipeptide. Three high-frequency vibrational modes were examined, two amide-I modes (1693 and 1633 cm-1) and the amide-II amino end mode (1580 cm-1). Efficient vibrational energy transfer between amide-I and amide-II bands located on the same amide has been clearly observed (τ ∼ 3.1 ps), which accounts for ∼36% of all deactivation pathways of the excited amide-I mode. The evolution of the transient spectral shape (cross-peak) as a result of vibrational energy transfer has been observed for the first time. The ET from the amide-I (acetyl end) band to the amide-II (amino end) band was detected (∼10 ps). However, no clear energy transfer between two amide-I bands was seen, yielding an upper limit for the rate constant of (20 ps)-1. A switch of the transient signal from a transition of the pumped state to an overtone (diagonal peak) to one involving a combination band (off-diagonal peak) was clearly demonstrated. This process provides an explanation for the nonexponential decay of the transient spectrum of each of the oscillators. A long-lived signal (cross-peak) was detected in each of the amide-I bands when the other amide-I band was excited, indicating the existence of at least one vibration in the molecule which remains excited for much longer than the average lifetime of the three vibrations being studied. The decay time (∼10 ps) of this signal represents one signature of the vibrational cooling of the dipeptide by energy transfer to the solvent. The angles between the transition dipoles of these modes given by the polarization measurements were close to those of a C7 conformation. The magnitudes of the coupling between the three vibrational modes were determined from a transition charge model of the off diagonal anharmonicity. The values obtained for the amide-I/amide-I and the amide-I/amide-II interactions are about twice those calculated from a dipole−dipole interaction potential based on a C7 conformation.

Structural and vibrational analysis of azodendrimers by molecular orbital methods

The structural optimization and normal-mode analysis are performed for the aryl ether azodendrimers on the basis of the semi-empirical molecular orbital methods. Through the geometrical characterization for the stable structures, the fractal dimension and the degree of azo core wrapping are found to provide key parameters related to the unique photoinduced isomerization. We also find that the normal-mode frequency distribution is virtually invariable irrespective of the molecular structure and generation of azodendrimers. The importance of normal-mode distribution gap in the range of 700–900 cm −1 is suggested regarding the efficient vibrational energy transfer to the azo core region.

Matrix studies of the products of laser vaporization of graphite

Laser vaporization is shown to be an efficient source of ionic species for matrix studies. C 2 − and other ions are produced by vaporization of graphite. A new spectrum characterized by T e = 19732 cm −, ω e′ = 1507±2 cm −1 and ω e″ = 1360±5 cm −1 is detected and assigned to C 2 + or, less probably, to C 3 −. An efficient vibrational energy transfer between C 2 and C 2 − is observed.

Microwave spectroscopy of OCS in highly excited vibrational states through energy transfer from N 2

The efficient vibrational energy transfer between the first excited vibrational state of N 2 and the asymmetric stretching vibrational state of OCS has allowed the observation of many pure rotational lines in different vibrational states of OCS up to 4101 cm −1: (00 01), (01 11), (02 l 1), (10 01), (00 02), (21 10), (03 l 0), (04 l 0), and (05 l 0). Accurate values of some rotational, centrifugal distortion and l-doubling constants are determined.