Excitonic splitting and coherent electronic energy transfer in the gas-phase benzoic acid dimer

The benzoic acid dimer, (BZA)(2), is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S(1)/S(2) state splitting and coherent electronic energy transfer within supersonically cooled (BZA)(2) and its (13)C-, d(1) -, d(2) -, and (13)C/d(1) - isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)(2)-(h - h) and (BZA)(2)-(d - d) dimers are C(2h) symmetric, hence only the S(2) ← S(0) transition can be observed, the S(1) ← S(0) transition being strictly electric-dipole forbidden. A single (12)C/(13)C or H/D isotopic substitution reduces the symmetry of the dimer to C(s), so that the isotopic heterodimers (BZA)(2) - (13)C, (BZA)(2) -(h - d), (BZA)(2) -(h(13)C-d), and (BZA)(2) -(h - d(13)C) show both S(1) ← S(0) and S(2) ← S(0) bands. The S(1)/S(2) exciton splitting inferred is Δ(exc) = 0.94 ± 0.1 cm(-1). This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, Δ(iso), arise from the change of the zero-point vibrational energy upon electronic excitation and range from Δ(iso) = 3.3 cm(-1) upon (12)C/(13)C substitution to 14.8 cm(-1) for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S(1) ← S(0) and S(2) ← S(0) origin bands; near-complete localization is observed even for a single (12)C/(13)C substitution. The S(1)/ S(2) energy gap of (BZA)(2) is Δ(calc) (exc)=11 cm(-1) when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to Δ(vibron) (exc)=2.1 cm(-1) [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed Δ(exc) = 0.94 cm(-1). The observed excitonic splittings can be converted to exciton hopping times τ(exc). For the (BZA)(2)-(h - h) homodimer τ(exc) = 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)(2) in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]. Thus, the electronic energy transfer is much faster than the proton-transfer in (BZA)(2)(∗).