A pass too far: dissociation of internal energy selected paracyclophane cations, theory and experiment.

The vacuum ultraviolet (VUV) photoionization and dissociative photoionization of three hydroxy-substituted [2.2]paracyclophane derivatives were studied yielding adiabatic ionization energies and dissociative photoionization energies (appearance energies). The simplest dissociation pathway is the breaking of both CH(2)-CH(2) bridge units and fragmenting the molecular ion in half to yield xylylene neutral and cationic fragments. The experimental data show that this process is outcompeted by a faster, higher energy channel, possibly yielding cyclooctatetraene derivatives. The role of the reaction coordinate, the effect of large amplitude motions on the density of states function at low and high energies and the temperature dependent 'population gap' in the internal energy distribution in large molecules are discussed in the context of applying statistical models to the dissociation. Computational approaches to the binding energy of paracyclophanes are marred with pitfalls. Noncovalent interactions play a major role in keeping paracyclophanes bound by some 200 kJ mol(-1) with respect to the two xylylene motifs, and the covalent CH(2)-CH(2) bonds are mostly counteracted by the geometric strain. The stabilizing effects are twofold: first, paracyclophanes are aromatic compounds, whereas xylylenes are not. Thus, the aromaticity of the molecule is induced by dimerization. Second, dispersive π-π interactions also stabilize the molecule. We evaluated 23 different computational chemistry approaches, and found that very few of the favorably scaling ones give an adequate description of this system. Among the DFT functionals tested, only PBE-D3 and perhaps M06-2X yielded consistently accurate results, comparable with MP3 and CCSD, or the G4 and CBS-QB3 composite methods. MP2 results in general suffer from significant overbonding.

Paracyclophanes as model compounds for strongly interacting π-systems, part 3: influence of the substitution pattern on photoabsorption properties.

The structures and energetics of the ground and first excited states of [2.2]paracyclophane (PC) and its pseudo-para- (p-DHPC) and pseudo-ortho-dihydroxy (o-DHPC) as well as monohydroxy derivates (MHPC) are investigated by quantum chemical calculations, X-ray crystallography, and resonance-enhanced multiphoton ionization spectroscopy (REMPI) in a free jet. We show that substitution of the aromatic hydrogens in PC causes significant changes of the structure and in particular its change between the ground and the excited state. The structural changes include a breathing mode as well as shift and rotation of the benzene moieties and are rationalized by the electronic structure changes upon excitation. Spin-component-scaled second-order Møller-Plesset perturbation method (SCS-MP2) reproduces the experimental X-ray structure correctly and performs significantly better than ordinary MP2 and the B3LYP methods. The parent propagation method, SCS-approximate coupled cluster second order (SCS-CC2), yields adiabatic excitation energies within 0.1 eV of the experimental values for PC and the investigated hydroxyl derivates as well as the related aromatic molecules benzene and phenol. It is shown that zero-point vibration energy corrections at the time dependent density functional (B3LYP) level are no more accurate enough for that level of theory and have to be substituted by SCS-CC2 values. While the structures of PC and o-DHPC are only slightly modified upon excitation, p-DHPC changes its structural parameters substantially. This is in line with [1 + 1] REMPI-spectra of these substances, which are interpreted with the help of Franck-Condon simulations.

Paracyclophanes as model compounds for strongly interacting pi-systems. Part 1. Pseudo-ortho-dihydroxy[2.2]paracyclophane.

In this work we describe a study of the ground and first excited state structures and energetics of a dihydroxy-derivative of [2.2]paracyclophane (PC), the pseudo-ortho-dihydroxy[2.2]paracyclophane (o-DHPC), also termed 4,12-dihydroxy[2.2]paracyclophane. In order to understand the electronic interactions between the two pi-systems, the molecule is investigated by REMPI spectroscopy in a free jet and by quantum chemical calculations. REMPI-spectra of the cluster with one water molecule were also obtained and aid in the interpretation. The origin of the S(1) <-- S(0) transition lies at 31,483 cm(-1) (3.903 eV) for o-DHPC and 31,263 cm(-1) (3.876 eV) for the o-DHPC x H(2)O cluster. An adiabatic excitation energy of 3.87 eV was computed for the S(1) <-- S(0) transition in o-DHPC. The SCS-CC2 calculations deviate by less than 0.1 eV for the adiabatic excitation energies of PC, o-DHPC and the related aromatic molecules benzene and phenol. Considerable activity in a breathing vibration of 190 cm(-1) is found in the S(1) state of o-DHPC and o-DHPC x H(2)O, in agreement with the computed SCS-CC2 value of 185 cm(-1). Further vibrations appear at +11 cm(-1) and +54 cm(-1) in o-DHPC. The computations and the available experimental data of the parent PC show that both PC and o-DHPC are rather flexible with respect to motions of the benzene moieties.While PC has a double minimum potential energy with respect to the torsional motion, a single-minimum structure is found for the ground state of o-DHPC. The geometry change upon excitation is less pronounced in o-DHPC as compared to PC. Two of the three possible rotational conformers of the OH groups were found to have similar energies, but spectral hole burning shows that the spectra are dominated by a single rotamer.

Electrochemistry and photophysics of donor-substituted triarylboranes: symmetry breaking in ground and excited state.

We synthesized a series of amino substituted triarylboranes (TABs) 1-3 by copper(I)-catalyzed cross-coupling reactions. The title compounds were investigated by means of cyclic voltammetry (CV) and UV-visible absorption and fluorescence spectroscopy. Electrochemical oxidation of tris(4-carbazolyl-2,6-dimethylphenyl)borane (3) leads to the formation of an electroactive polymer film on the electrode surface. The charge-transfer (CT) absorption band of all three TABs shows a pronounced negative solvatochromism, while the emission is positively solvatochromic. By combining Jortner's theory, AM1 computations, and electrooptical absorption measurements (EOAM), this unexpected behavior was shown to be due to a dipole inversion upon S0-->S1 excitation. Furthermore, polarized steady-state fluorescence spectroscopy and EOAM prove that the ground-state geometry of 3 is of lower symmetry than D3 and that the excitation energy can be transferred from one subchromophore to another within the lifetime of the excited state. Exciton-coupling theory was used to quantitatively analyze this excitation transfer.