Determination of the Geometry Change of the Phenol Dimer upon Electronic Excitation

Abstract

The change of the phenol dimer (PH2) structure upon electronic excitation is determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of seven different vibronic bands. A total of 547 emission band intensities are fitted, together with the changes of rotational constants upon electronic excitation of fi ve isotopomers. These rotational constants are taken from previously published [Schmitt et al. ChemPhysChem 2006, 7, 1241-1249] high-resolution LIF measurements. The geometry change upon electronic excitation of the pipi* state of the donor moiety can be described by a strong shortening of the hydrogen bond, a shortening of the CO bond in the donor moiety, an overall symmetric expansion of the donor phenol ring, and a nearly unchanged acceptor moiety. The resulting geometry changes are interpreted on the basis of ab initio calculations.

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“…For the p CR dimer α changes from 54° to 32° while β changes from 17° to 53°. The same observations were made earlier for the PH dimer27, 33 as well as the hydroquinone dimer 34. In the aromatic dimers, the smaller value of α in the electronic ground state compared to that of the p CR⋅H 2 O and p ‐CR⋅MeOH complexes is because of the C 2D H 2D ⋅⋅⋅O A hydrogen bonding, which is absent in case of the H 2 O and MeOH complexes.…”

“…The dispersion interaction plays a vital role in determining the structure and stabilization energy of the p CR dimer and the PH dimer, to which the π⋅⋅⋅π and CH⋅⋅⋅π interactions contribute significantly. Moreover recently has been demonstrated by Kleinermanns et al27 that the PH dimer structures obtained at these two levels of theory match well with the experimentally determined structure. In addition it was observed that the structural parameters of the complexes obtained using RICC2‐CP/TZVP level match well with the experimental values (vide infra).…”

“…The given values correspond to the p CR dimer. Similar changes were also noticed for the PH dimer27 as well as the hydroquinone dimer 9. The significant changes in these two dihedral angles affect the 2c‐R2PI spectrum of the p CR dimer (vide infra).…”

“…For the calculation of the binding energy in the S 1 state, it is assumed that the H‐bond donor is electronically excited, whereas the H‐bond acceptor is in the S 0 state. In fact, for the hydrogen‐bonded complexes of PH and substituted phenols it is reported that the electronic excitation is solely localized on the donor moiety 27. 29 Thus, the binding energy of the dimer in the S 1 state was calculated by taking the energy of the S 1 state of the donor and the energy of the S 0 state of the acceptor in the S 1 ‐state geometry of the hydrogen‐bonded complex.…”

Hydrogen bonds vs. π-stacking interactions in the p-aminophenol⋯p-cresol dimer: an experimental and theoretical study

“…In other cases, T, V and HB structures of (phenol) 2 35 and (p-cresol) 2 36 homodimers, and of 7-azaindole…fluoropyridines, 37 indole…pyridine, 38 indole…imidazole, 39 anisole…phenol, 40 and 7-azaindole…phenol 41 heterodimers have been experimentally observed. These structures are believed to be observed because of their remarkable stability in the ground state, as compared to other possible isomers.…”