title: Excitonic Splitting and Vibronic Coupling Analysis of the m-Cyanophenol Dimer.
abstract: The S1/S2splitting of the m-cyanophenol dimer, (mCP)2and the delocalization of its excitonically coupled S1/S2states are investigated by mass-selective two-color resonant two-photon ionization and dispersed fluorescence spectroscopy, complemented by a theoretical vibronic coupling analysis based on correlated ab initio calculations at the approximate coupled cluster CC2 and SCS-CC2 levels. The calculations predict three close-lying ground-state minima of (mCP)2: The lowest is slightly Z-shaped (Ci-symmetric); the second-lowest is <5 cm-1higher and planar (C2h). The vibrational ground state is probably delocalized over both minima. The S0→ S1transition of (mCP)2is electric-dipole allowed (Ag→ Au), while the S0→ S2transition is forbidden (Ag→ Ag). Breaking the inversion symmetry by12C/13C- or H/D-substitution renders the S0→ S2transition partially allowed; the excitonic contribution to the S1/S2splitting is Δexc= 7.3 cm-1. Additional isotope-dependent contributions arise from the changes of the m-cyanophenol zero-point vibrational energy upon electronic excitation, which are Δiso(12C/13C) = 3.3 cm-1and Δiso(H/D) = 6.8 cm-1. Only partial localization of the exciton occurs in the12C/13C and H/D substituted heterodimers. The SCS-CC2 calculated excitonic splitting is Δel= 179 cm-1; when multiplying this with the vibronic quenching factor Γvibronexp= 0.043, we obtain an exciton splitting Δvibronexp= 7.7 cm-1, which agrees very well with the experimental Δexc= 7.3 cm-1. The semiclassical exciton hopping times range from 3.2 ps in (mCP)2to 5.7 ps in the heterodimer (mCP-h)·(mCP-d). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled S1/S2states from the v = 0 level to 600 cm-1above. Both linear and quadratic vibronic coupling schemes were investigated to simulate the S0→ S1/S2vibronic spectra; those calculated with the latter scheme agree better with experiment.
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Franziska A
authors.initials: FA
authors.lastname: Balmer
authors.affiliation: Theoretische Chemie, Physikalisch-Chemisches Institut, Universität Heidelberg , Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany.
authors.forename: Sabine
authors.initials: S
authors.lastname: Kopec
authors.affiliation: Theoretische Chemie, Physikalisch-Chemisches Institut, Universität Heidelberg , Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany.
authors.forename: Horst
authors.initials: H
authors.lastname: Köppel
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Samuel
authors.identifier: http://orcid.org/0000-0003-1595-6104
authors.initials: S
authors.lastname: Leutwyler