On the dual phosphorescence of xanthone and chromone in glassy hydrocarbon hosts.

Trace quantities of hydrogen-bonding impurities in otherwise highly purified and dried glassy hydrocarbon matrices at 77 K can modify the relative triplet state energy levels, and hence the photophysical properties of two aromatic ketones, xanthone and chromone, to the extent that the intrinsic spectroscopic properties are obscured. The intrinsic spectroscopic properties of each are revealed in multicrystalline n-alkane Shpol'skii matrices, and also can be observed in rigorously purified and dried hydrocarbon glasses at 77 K. The extreme sensitivity to stoichiometric, and even substoichiometric quantities of hydrogen-bonding impurities arises from the near-degeneracy of the two lowest-lying triplet states, and the sensitive nature of the n→π* blueshift phenomena to specific hydrogen-bonding interactions.

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: a model for the secondary evolution of the classic C2h dimer and comparison of four mechanisms.

A mechanism is proposed for the formation in gas phase, during a short time, of the delicately symmetrical coplanar C(2h) classic 7-azaindole (7AI) doubly hydrogen-bonded dimer. Of the five card-pack or otherwise random geometry structures most likely to be formed in the supersonic jet expansion molecular beam, none would be an obvious precursor to the C(2h) dimer. One unstable dimer with dipole-dipole, van der Waals, and plane-to-plane hydrogen bonding is shown to be capable of unhinging about the hydrogen-bond pair as an axis, from 0 degrees to 90 degrees to 180 degrees, yielding a deep minimum for the C(2h) structure with its delicate geometry and symmetry. This relaxation mechanism is feasible in the 3-micros interval between the nozzle escape and the first laser pulse interception of the molecular beam. In the second part of the paper four published mechanisms are compared for concerted vs. two-step biprotonic phototransfer for the 7AI dimers. The dependence of the latter two models on H-atom instead of proton-transfer as an intermediate step negates the mechanism in a singlet (pi,pi*) electronic state by the valency repulsion, in the 3-electron orbital that would be generated. The concerted mechanism for biprotonic phototransfer is reaffirmed by the analysis of the quantum mechanical conditions set on the biprotonic transfer in the photo-excited molecular 7AI pair.

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: structure, quantum-theoretical analysis, and simultaneity principles.

Six stable dimer models for 7-azaindole (including the classic C(2h) doubly hydrogen-bonded, coplanar, centrosymmetric dimer) are considered to be observable in adiabatic nozzle jet molecular beams. They are analyzed by hybrid density functional theory (DFT), the MP2 ab initio method for the ground electronic state, and the single-excitation configuration interaction (CIS) (over frozen ground state optimized geometries obtained from DFT) excited state calculations, for global potential minima and proton-transfer potential energy curves. Three simultaneity principles are stated: (i) intermolecular coherent excitation molecular exciton simultaneity, (ii) intramolecular acid-base change simultaneity at the pyrrolo-N-H and aza-N proton-donor, proton-acceptor sites, and (iii) intermolecular simultaneity of catalytic proton-donor, proton-acceptor action. It is suggested that the formation of the classic C(2h) dimer of 7-azaindole, which is considered exclusively by previous researchers, can be formed from at least one of the several card-pack hydrogen-bonded dimers in a secondary slower step approaching a microsecond scale, instead of the picosecond events at the supersonic nozzle. It is proposed that the complexity of dimerization modes is the basis of the postexcitation, postionization diverse kinetic isotope results.