Charge resonance is as trong attractive intermolecular force in aromatic dimer radical ions.D espite its importance,this fundamental interaction has not been characterized at high resolution by spectroscopyo fi solated dimers.W e employv ibrational infrared spectroscopyo fc old aromatic pyrrole dimer cations to precisely probe the charge distribution by measuring the frequency of the isolated N À Hstretch mode (n NH ). We observe al inear correlation between n NH and the partial charge qo nt he pyrrole molecule in different environments.S ubtle effects of symmetry reduction, such as substitution of functional groups (here pyrrole replaced by Nmethylpyrrole) or asymmetric solvation (here by an inert N 2 ligand), shift the charge distribution towardt he moiety with lower ionization energy.T his general approach provides ap recise experimental probe of the asymmetry of the charge distribution in such aromatic homo-and heterodimer cations.The intermolecular interactions of aromatic p electrons are important for chemical and biological recognition. [1] Radical ions of arenes are involved in chemical reaction mechanisms, charge transport of modern (bio-)organic materials (organic electronics), and radiation damage in DNAand proteins. [2] In addition to p H-bonding,c ation/anion-p,a nd p-p stacking interactions, [3] the charge-resonance (CR) interaction is af undamental and very strong force in charged arene dimers. [4] Forc ations,t his stabilization arises from sharing the positive charge between two identical or different molecules with the same or comparable ionization energies in a p-stacked dimer.I nahomodimer cation (A 2 + ), the positive charge is equally shared between both molecules, while in ah eterodimer (AB + )t he partner with the lower ionization energy (IE) carries more positive charge.I nt he simplest description, the CR interaction in A 2 + causes as plitting between the two electronic states described by Y AE = Y(A + )Y(A) AE Y(A)Y(A + ); the symmetric Y + ground state is stable and the destabilized antisymmetric Y À state is repulsive.The splitting between both electronic states is twice the stabilization energy of the ground state (ca. 50-100 kJ mol À1 ). Thei nteraction is strongest for A 2 + homodim-ers,a nd decreases in AB + as the difference in the IEs of the two molecules increases (DIE = IE A ÀIE B ). This description of the CR interaction in arene dimer ions is similar to the one employed for odd electron chemical bonds in radical ions (hemibonds) [5] and exciton splitting in delocalized electronic excitation in neutral dimers. [6] In the condensed phase, p-stacked dimer cations of aromatic and polycyclic aromatic hydrocarbons were first detected by electron spin resonance [7] and optical absorption spectroscopy of the intense and broad CR transition connecting the two Y AE states. [7d, 8] Thelatter electronic transition occurs in the low-energy part of the optical spectrum (in the red to near-infrared near 1 mm) and provides adirect measure for the CR splitting.Inthe gas phase,the binding e...
