Hans Peter Trommsdorff

Coupling of Chromophores: Carotenoids and Photoactive Diarylethenes – Photoreactivity versus Radiationless Deactivation

New photochromic carotenoid-like chromophores containing the dithienylperfluorocyclopentene fragment have been synthesized and characterized by UV/Vis spectroscopy. The quantum yields of the photochromic forward and back reactions of these compounds have been determined and are found to decrease sharply with increasing chain length of the substituent. This decrease in quantum yield can be rationalized in terms of a decrease of the excitation density at the central photoreactive unit and of the excited state lifetime, which is known to shorten in carotenoids with increasing chain length.

Tunneling splittings in vibrational spectra of non-rigid molecules: IX. Malonaldehyde and its isotopomers as a test case for fully coupled multidimensional tunneling dynamics

Abstract Twenty one dimensional potential energy surfaces (PES) and the tunneling coordinate dependent kinematic matrices of malonaldehyde and of several of its isotopomers (D, 13 C) are constructed in the low-energy region ( −1 ) using quantum-chemical data. Even though the barrier heights calculated with different methods differ strongly (from 2.8 to 10.3 kcalxa0mol −1 ), all PES become near identical after scaling, fitting only one semiclassical parameter γ , which defines the scales of potential, energy and action. The results of dynamical calculations carried out in dimensionless variables are therefore quite insensitive to the choice of the quantum-chemical method used for the construction of the Hamiltonian. Choosing the value of γ such that the calculated tunneling splitting in the ground state coincides with the experimental value, the corresponding barrier height is determined as 4.30 kcalxa0mol −1 and the vibrational spectrum of the transition state is obtained. The perturbative instanton approach developed in the previous papers of this series is used to solve the dynamical problem without reducing the number of degrees of freedom. The role of all 20 transverse vibrations in proton tunneling is characterized. The tunneling path and globally uniform semiclassical wave functions are evaluated from the fourth-order Hamilton–Jacobi equation and the second-order transport equation. Tunneling splittings in the ground and low-lying excited states are calculated and isotope effects of H/D and 13C/12C substitutions are predicted.

Tunneling splittings in vibrational spectra of non-rigid molecules: V. Reconstruction of the Hamiltonian from quantum chemical data

Abstract The complete set of potential and kinematic coupling constants, characterizing the Hamiltonian of a non-rigid molecule with one-wide-amplitude-coordinate is derived from standard quantum chemical data for the equilibrium geometries, normal frequencies and eigenvectors in the ground and transition states. This Hamiltonian is suitable for the perturbation instanton approach (PIA), developed in previous work [V.A. Benderskii et al., Chem. Phys., 219 (1997) 119, 143; 234 (1998) 153, 173]. The reconstruction of the Hamiltonian is made via the following steps: classification of the generalized coordinates according to the irreducible representations of the group of the non-rigid molecular model; linear transformation of the ground state normal coordinates into the frame of the transition state taking into account the Eckart conditions; perturbative solution of the inverse vibrational problem. As an example, the Hamiltonian for the internal C 2 -rotation in HNO 3 and its isotopomers has been reconstructed. The specific feature of this molecule is a Fermi resonance between the second torsional level (2 ν 9 ) and first level of the ONO bending mode ν 5 . Tunneling splittings in the ground and first excited states of all vibrations are calculated within the PIA and are found to be in good agreement with the available experimental data. Anomalous large isotope effects on the tunneling splittings of excited levels of active transverse vibrations under O 18 /O 16 and N 15 /N 14 substitution are predicted.

Tunneling splittings in vibrational spectra of non-rigid molecules: VI. Asymmetric double-well potentials

Abstract The perturbative instanton approach (PIA) developed in the previous papers of this series [V.A. Benderskii, E.V. Vetoshkin, S.Yu. Grebenshchikov, L. von Laue, H.P. Trommsdorff, Chem. Phys. 219 (1997) 119; V.A. Benderskii, E.V. Vetoshkin, L. von Laue, H.P. Trommsdorff, Chem. Phys. 219 (1997) 143; V.A. Benderskii, E.V. Vetoshkin, H.P. Trommsdorff, Chem. Phys. 234 (1998) 153; V.A. Benderskii, E.V. Vetoshkin, Chem. Phys. 234 (1998) 173] has been extended to asymmetric double-well potentials, characterized by an energy difference, D , between the two potential wells. For multidimensional double-well potentials with an asymmetry comparable to or smaller than the fundamental frequency, Ω , of the tunneling mode in the wells, the semi-classical wave functions and tunneling splittings in the ground and low-lying excited vibrational states are calculated. The semi-classical equations are written in a form, in which the energy asymmetry, D , and eigenvalues, as terms of first-order of ℏ , are replaced from the Hamilton–Jacobi equation (HJE) into the transport equation. This replacement, symmetrizing the HJE, makes it possible to avoid the construction of a tunneling trajectory between inequivalent wells and to use the extreme classical trajectory in the upside-down symmetric potential. Comparison of the results of the quantum and PIA calculations shows that the latter provide an evaluation of the tunneling matrix element with an accuracy of better than 10%, provided that D / ℏΩ ≤3 and the semi-classical parameter γ >5. A resonant increase of the tunneling splitting is predicted, if one of the transverse frequencies or the difference between longitudinal and transverse frequencies become equal to D . The relation between these resonances and the Flynn–Stoneham and Skinner–Trommsdorff mechanisms of vibrationally assisted incoherent tunneling is discussed.

Measurement of very long (107 s) spin conversion times: dimethyl-s-tetrazine in durene

Abstract A newly developed optical technique gives direct access to the rate of nuclear spin conversion, i.e. the rate of relaxation between tunneling levels, for methyl groups in the solid state. In dimethyl-s-tetrazine (DMST) doped single crystals of durene, spin conversion times as long as 7.0×10 6 s (81 days) could be measured with an estimated error less than 25%. Study of the temperature dependence of the spin conversion rate shows that an Orbach type temperature law predominates down to 2 K, while existing theories predict Raman or direct processes to become predominant below 4 to 6 K. The assumptions of the theoretical predictions are questioned, based on the assignment of librational modes of the methyl group in the fluorescence spectra of protonated and perdeuterated DMST.

Reaction and excited state relaxation dynamics of photochromic dithienylethene derivatives

Abstract The spectroscopic properties as well as the reaction and relaxation dynamics of photochromic dithienylethene derivatives have been characterized. An in-depth understanding of these photochemical electrocyclic reactions, involving large conformational changes, is required for the optimization of the performance of these compounds as optical switches in photonic applications. Substituents, more or less strongly coupled to the electronic structure of dithienylethene unit were introduced. A threshold of the reactivity as a function of excitation energy has been established and the rate of the barrier-less ring closure reaction could be related to the spectral and steric properties of the substituent. The interplay of photochromism and fluorescence was characterized by attaching anthracene. The blue fluorescence of the open isomer is strongly suppressed in the closed isomer.

Proton tunnelling reactions in pentacene doped benzoic acid crystals

Abstract Optical excitation of pentacene, doped into a benzoic acid crystal matrix, induces a reversible proton transfer reaction between the host and the guest. This reaction occurs at low temperatures and leads to the creation of defects corresponding to the displacement, from its regular position, of an acid proton of the host matrix. The formation and evolution of these defects is monitored via the electronic S 0 → S 1 transition of the pentacene guest. Here we report and discuss a variety of measurements made with the aim of obtaining information about the first step of the reaction. It is shown that the rate for the first step of this reaction is reduced by a factor of about 10 4 upon deuteration of the host matrix, demonstrating that this step occurs by tunnelling. Other unsuccessful experiments (optical, ESR, magnetic field effects) made to identify the first intermediate of the reaction as well as molecular orbital calculations of potential intermediates are also reported briefly. It is shown that the formation of the pentacene cation, protonated in the centre position, is consistent with all observations. This species is proposed as the most likely first intermediate of the reaction.

Tunneling systems in molecular crystals: studies by optical spectroscopy and neutron scattering

Abstract Two-level tunneling systems have been identified in a number of pure and doped molecular crystals. The tunneling dynamics of translations of protons along symmetric hydrogen bonds and of hindered rotations of methyl groups have been characterized by a variety of experimental techniques: optical as well as vibrational spectroscopy, neutron scattering, and proton NMR T1 relaxation. Molecular mechanics/dynamics calculations have been made to quantify the potential energy surface of two-level-systems and to identify substitutional sites of guest molecules in host crystals and to evaluate the structural and energetic disorder of these sites.

Rotational and translational proton tunneling and relaxation in condensed molecular systems

Abstract Quantum motions of protons in organic crystalline solids are monitored via the optical transitions of suitable dye molecules coupled to the potential governing the proton motion. Tunneling splittings are determined using line-narrowing techniques and the relaxation dynamics is measured on timescales extending from picoseconds to hours. The relevance of these properties for material uses is discussed.

Laser spectroscopy of proton dynamics in hydrogen bonds

In this paper a summary of recent work on proton tunneling in intermolecular hydrogen bonds in the condensed phase is presented and its relevance to biological systems is discussed. Dye molecules are used to influence the proton structure in neighboring hydrogen bonds. The optical transitions of the dye serve as a probe of this structure and the dynamics. A variety of optical spectroscopic measurements at low temperatures leads to a determination of the tunneling splitting and the rate of proton transfer in benzoic acid dimers. A theoretical analysis of these data within a simplified model suggests that fluctuadons of the energy difference of the tautomers determine the relaxation behavior and that multiphonon processes dominate the relaxation of delocalized protons. 1.