Intramolecular Relaxation Dynamics Research Articles

Probing C-I bond fission in the UV photochemistry of 2-iodothiophene with core-to-valence transient absorption spectroscopy.

The UV photochemistry of small heteroaromatic molecules serves as a testbed for understanding fundamental photo-induced chemical transformations in moderately complex compounds, including isomerization, ring-opening, and molecular dissociation. Here, a combined experimental-theoretical study of 268nm UV light-induced dynamics in 2-iodothiophene (C4H3IS) is performed. The dynamics are experimentally monitored with a femtosecond extreme ultraviolet (XUV) probe that measures iodine N-edge 4d core-to-valence transitions. Experiments are complemented by density functional theory calculations of both the pump-pulse induced valence excitations and the XUV probe-induced core-to-valence transitions. Possible intramolecular relaxation dynamics are investigated by abinitio molecular dynamics simulations. Gradual absorption changes up to ∼0.5 to 1ps after excitation are observed for both the parent molecular species and emerging iodine fragments, with the latter appearing with a characteristic rise time of 160 ± 30fs. Comparison of spectral intensities and energies with the calculations identifies an iodine dissociation pathway initiated by a predominant π → π* excitation. In contrast, initial excitation to a nearby n⟂ → σ* state appears unlikely based on a significantly smaller oscillator strength and the absence of any corresponding XUV absorption signatures. Excitation to the π → π* state is followed by contraction of the C-I bond, enabling a nonadiabatic transition to a dissociative π→σC-I* state. For the subsequent fragmentation, a relatively narrow bond-length region along the C-I stretch coordinate between 230 and 280pm is identified, where the transition between the parent molecule and the thienyl radical + iodine atom products becomes prominent in the XUV spectrum due to rapid localization of two singly occupied molecular orbitals on the two fragments.

Intramolecular Relaxation Dynamics Mediated by Solvent-Solute Interactions of Substituted Fluorene Derivatives. Solute Structural Dependence.

Several fluorene derivatives exhibit excited-state reactivity and relaxation dynamics that remain to be understood fully. We report here the spectral relaxation dynamics of two fluorene derivatives to evaluate the role of structural modification in the intramolecular relaxation dynamics and intermolecular interactions that characterize this family of chromophores. We have examined the time-resolved spectral relaxation dynamics of two compounds, NCy-FR0 and MK-FR0, in protic and aprotic solvents using steady-state and time-resolved emission spectroscopy and quantum chemical computations. Both compounds exhibit spectral relaxation characteristics similar to those seen in FR0, indicating that hydrogen bonding interactions between the chromophore and solvent protons play a significant role in determining the relaxation pathways available to three excited electronic states.

Intramolecular relaxation of ring polymers in dilute solutions

The intramolecular relaxation dynamics of unconcatenated ring polymers in dilute solutions is theoretically investigated within the framework of the Rouse–Zimm theory. The excluded volume interactions (EVIs) between the nonbonded monomers are modeled by a harmonic potential, where the interaction parameter is evaluated from Flory’s mean-field approach. The hydrodynamic interactions (HIs) between the pairs of monomers are approximated by a preaveraged Oseen tensor. The mechanical moduli are dominated by the smaller relaxation rates corresponding to the collective relaxation modes in the low frequency regime, while they are governed by the higher relaxation rates corresponding to the local relaxation modes in the high frequency regime. EVI decreases the relaxation rates of the normal modes where the decrease for the collective modes is larger than that for the local modes, which consequently expands the width of the relaxation spectrum. The characteristic overall relaxation time is evaluated from the inverse of the crossover frequency, which is the same for rings of various sizes with HI and with both HI and EVI, while it shifts to lower frequencies with increasing ring size for the Rouse rings.

Fluorescence excitation and excited state intramolecular relaxation dynamics of jet-cooled methyl-2-hydroxy-3-naphthoate

Two distinct S0→S1 fluorescence excitation spectra of methyl-2-hydroxy-3-napthoate (MHN23) have been obtained by monitoring fluorescence separately in the short (∼410nm) and long (∼650nm) wavelength emission bands. The short wavelength fluorescence is assigned to two MHN23 conformers which do not undergo excited state intramolecular proton transfer (ESIPT). Analysis of the ‘long wavelength’ fluorescence excitation spectrum, which arises from the proton transfer tautomer of MHN23 indicates an average lifetime of τ⩾18±2fs for the initially excited states. Invoking the results of Catalan et al. [J. Phys. Chem. A, 1999, 103, 10921], who determined the N tautomer to decay predominantly via a fast non-radiative process, the limit of the rate of intramolecular excited proton transfer in MHN23 is calculated as, kpt⩽1×1012s−1.

Conformation and intramolecular relaxation dynamics of semiflexible randomly hyperbranched polymers

The conformational and dynamic properties of semiflexible randomly hyperbranched polymers are investigated in dilute solutions within the framework of optimized Rouse-Zimm formalism. Semiflexibility is incorporated by restricting the directions and orientations of the respective bond vectors, while hydrodynamic interactions are modeled through the preaveraged Oseen tensor. The effect of semiflexibility is typically reflected in the intermediate frequency regime of the viscoelastic relaxation moduli where the bond orientation angle restores the characteristic power-law scaling in fractal structures, as in randomly hyperbranched polymers. Despite the absence of this power-law scaling regime in flexible randomly hyperbranched polymers and in earlier models of semiflexible randomly branched polymers due to weak disorder [C. von Ferber and A. Blumen, J. Chem. Phys. 116, 8616 (2002)], this power-law behavior may be reinstated by explicitly modeling hyperbranched polymers as a Vicsek fractals. The length of this power-law zone in the intermediate frequency region is a combined function of the number of monomers and the degree of semiflexibility. A clear conformational transition from compact to open structures is facilitated by changing the bond orientation angle, where the compressed conformations are compact, while the expanded ones are relatively non-compact. The extent of compactness in the compressed conformations are much less compared to the semiflexible dendrimers, which resemble hard spheres. The fractal dimensions of the compressed and expanded conformations calculated from the Porod's scaling law vary as a function of the bond orientation angle, spanning the entire range of three distinct scaling regimes of linear polymers in three-dimensions. The results confirm that semiflexibility exactly accounts for the excluded volume interactions which are expected to be significant for such polymers with complex topologies.

Intramolecular relaxation dynamics in semiflexible dendrimers

The intramolecular relaxation dynamics of semiflexible dendrimers in dilute solutions are theoretically investigated in the framework of optimized Rouse-Zimm formalism. Semiflexibility is implemented by modeling topological restrictions on the bond directions and orientations of the respective bond-vectors. Based on our recently developed approach for semiflexible dendrimers [A. Kumar and P. Biswas, Macromolecules 43, 7378 (2010)], the mechanical and dielectric relaxation moduli are studied as functions of local flexibility parameters and branching topology. It is rather interesting to observe that semiflexibility affects the local modes of G''(ω) and Δε''(ω), which have lower relaxation rate with increasing bond restrictions, while the collective modes with small relaxation rate remain almost constant. The relaxation dynamics of the flexible dendrimer is similar to that of the semiflexible dendrimer with unrestricted bond orientations (Φ = 0) and is flanked by the compressed (Φ = 30°) and expanded (Φ = 150°) conformations, respectively. The effect of semiflexibility is typically reflected in the intermediate frequency regime. The expanded conformations of semiflexible dendrimers display a power-law behavior in the intermediate frequency regime for both loss and storage modulus resembling fractal structures, while the compressed and unrestricted bond orientation conformations exhibit an approximately logarithmic dependence. The power-law exponent is found to be similar to the flexible dendrimers with excluded volume interactions. Thus, by tuning Φ, a spectrum of dynamic relaxation pattern is obtained spanning a broad range of conformations from a power-law fractal network to a non-fractal one. In certain limits, this highly generalized model captures the characteristics of flexible dendrimers and also resembles La Ferla's model semiflexible dendrimers. The influence of hydrodynamic interactions reduces the dynamical range and the width of the intermediate domain by decreasing the smaller relaxation rates and increasing the higher relaxation rates correspondingly.

Intramolecular relaxation dynamics and infrared spectra of diaza macrocyclic complexes with sodium and silver(I) in propylene carbonate

Ultrasonic relaxation spectra and infrared spectra (N-H stretch) are reported for the macrocycles 1,10-diaza-18-crown-6-(Kryptofix 22), monoaza-18-crown-6, and 18-crown-6 (18C6) and their complexes with Na + and Ag + in the solvent propylene carbonate (PC) at 25 o C. The relaxation spectra are attributed to intramolecular conversions on the microsecond to nanosecond time scale between various conformers of the macrocyclic ligands

Molecular quantum beat spectroscopy

The present account of high-resolution molecular quantum beat spectroscopy emphasizes the versatility of this powerful Doppler-free method. Illustrations are drawn from work on the six-atomic molecule propynal carried out in the authors' laboratory. Examples of anisotropic Zeeman, Stark and hyperfine quantum beats, of isotropic molecular quantum beats and of multi-level quantum beats are discussed with regard to intramolecular relaxation dynamics and the determination of structural parameters such as nuclear hyperfine constants, electric dipole moments, spin-orbit matrix elements and asymmetry-splittings.