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 C-13-, d(1) -, d(2) -, and C-13/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 C-12/C-13 or H/D isotopic substitution reduces the symmetry of the dimer to C-s, so that the isotopic heterodimers (BZA)(2) - C-13, (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 Delta(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, Delta(iso), arise from the change of the zero-point vibrational energy upon electronic excitation and range from Delta(iso) = 3.3 cm(-1) upon C-12/C-13 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 C-12/C-13 substitution. The S-1/S-2 energy gap of (BZA)(2) is Delta(exc)(calc) = 11 cm(-1) when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to Delta(exc)(vibron) = 2.1 cm(-1) [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed Delta(exc) = 0.94 cm(-1). The observed excitonic splittings can be converted to exciton hopping times tau(exc). For the (BZA)(2)-(h -h) homodimer tau(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). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767400]