Intermolecular π-to-π Bonding between Stacked Aromatic Dyads. Experimental and Theoretical Binding Energies and Near-IR Optical Transitions for Phenalenyl Radical/Radical versus Radical/Cation Dimerizations

Abstract

The high symmetry and stability of phenalenyl systems, both as the planar pi-radical (P*) and as the pi-cation (P+), are desirable characteristics of prototypical aromatic donor/acceptor pairs that encourage their use as (binary) models for the study of intermolecular interactions extant in stacked molecular arrays. Thus, quantitative ESR spectroscopy of the paramagnetic P* identifies its spontaneous self-association to the diamagnetic P2, previously characterized as the stacked pi-dimer by X-ray crystallography. Likewise, the rapid cross-association of P* with the closed-shell P+ leads to the stacked pi-dimer cation P2*+ with the "doubled" ESR spectrum diagnostic of complete (odd) electron delocalization. These pi-associations are confirmed by UV-vis studies that reveal diagnostic near-IR bands of both P2 and P2*+-strongly reminiscent of intermolecular charge-transfer absorptions in related aromatic (donor/acceptor) pi-associations. Ab initio molecular-orbital calculations for the pi-dimer P2 predict a binding energy of DeltaED = -11 kcal mol(-1), which is in accord with the experimental enthalpy change of DeltaHD = -9.5 kcal mol(-1) in dichloromethane solution. Most importantly, the calculations reproduce the intermonomer spacings and reveal the delicate interplay of attractive covalent and dispersion forces, balanced against the repulsions between filled orbitals. For comparison, the binding energy in the structurally related cationic pi-pimer P2*+ is calculated to be significantly larger with DeltaEP approximately -20 kcal mol(-1) (gas phase), owing to favorable electrostatic interactions not present in the neutral pi-dimer (which outweigh the partial loss of covalent interactions). As a result, our theoretical formulation can correctly account for the experimental enthalpy change in solution of DeltaHP = -6.5 kcal mol(-1) by the inclusion of differential ionic solvation in the formation of the pi-pimer.

Near-infrared absorption of π-stacking columns composed of trioxotriangulene neutral radicals

“…[12][13][14][15][16][17][18][19] Several PNL derivatives form π-dimers, and exhibit absorption bands around 600 nm. [15][16][17] In a few cases, 1D π-stacking columns are formed with large intermolecular separations of~3.78 (ref. 13) and 3.65 Å, 19 due to bulky protection groups, the latter of which shows a band around 800 nm.…”

Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission

“…‡ Results are from time-dependent density functional theory. Møller-Plesset perturbation correction (MP2), which is an appropriate level of theory for PAH stacking problems (22). For efficient evaluation of MP2 energies, the resolution-of-the-identity (RI) approximation (sometimes also termed as ''density fitting'') (42, 43) was adopted in all calculations.…”

“…Particularly attractive among these are charged PAH clusters. However, because of the experimental inaccessibility of these highly unusual and reactive species, there is no gas phase spectroscopic data available to the best of our knowledge with which to test this hypothesis [PAH dimer cations have been characterized in solution (21,22) but are subject to strong solvation effects]. To make progress, we explore the spectroscopic properties of various charged PAH clusters by using ab initio quantum chemical calculations and show that a unique class of closed-shell charged dimers can account for the ERE.…”

Fast electronic structure methods for strongly correlated molecular systems

“…A variety of other interesting chemical applications are possible to systems where chemical bonding may be diradicaloid in nature, and therefore beyond the reach of standard density functional theory methods. Other examples that we have recently studied include the dimerization of stable radicals, such as phenalenyl [18] and tetracyanoethylene anion [19], to yield stable crystal structures characterized by C-C distances that are too short for conventional Van der Waals complexes but too far too long for conventional covalent bonds. Our calculations were able to reproduce the experimental observables, such as bond length and bond strength, and then also provide an explanation for their origin.…”