Excitonic Splittings in Jet–Cooled Molecular Dimers

Abstract

In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S₀→S₁ electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S₀→S₁ and S₀→S₂ transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as ¹²C/¹³C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless 0₀⁰ bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)₂ and (2-aminopyridine)₂ that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-¹²C- and ¹³C- isotopomer spectra yield excitonic splittings of Δ_exp = 43.5 and 10.5 cm^–1, respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced (‘quenched’) by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced.