# Excitonic Splitting and Vibronic Coupling Analysis
of the *m*‑Cyanophenol Dimer

journal contribution

posted on 06.12.2016, 00:00 by Franziska
A. Balmer, Sabine Kopec, Horst Köppel, Samuel LeutwylerThe

*S*_{1}/*S*_{2}splitting of the*m*-cyanophenol dimer, (mCP)_{2}and the delocalization of its excitonically coupled*S*_{1}/*S*_{2}states 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 (*C*_{i}-symmetric); the second-lowest is <5 cm^{–1}higher and planar (*C*_{2h}). The vibrational ground state is probably delocalized over both minima. The S_{0}→ S_{1}transition of (mCP)_{2}is electric-dipole allowed (A_{g}→ A_{u}), while the S_{0}→ S_{2}transition is forbidden (A_{g}→ A_{g}). Breaking the inversion symmetry by^{12}C/^{13}C- or H/D-substitution renders the S_{0}→ S_{2}transition partially allowed; the excitonic contribution to the S_{1}/S_{2}splitting is Δ_{exc}= 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 Δ_{iso}(^{12}C/^{13}C) = 3.3 cm^{–1}and Δ_{iso}(H/D) = 6.8 cm^{–1}. Only partial localization of the exciton occurs in the^{12}C/^{13}C and H/D substituted heterodimers. The SCS-CC2 calculated excitonic splitting is Δ_{el}= 179 cm^{–1}; when multiplying this with the vibronic quenching factor Γ_{vibron}^{exp}= 0.043, we obtain an exciton splitting Δ_{vibron}^{exp}= 7.7 cm^{–1}, which agrees very well with the experimental Δ_{exc}= 7.3 cm^{–1}. The semiclassical exciton hopping times range from 3.2 ps in (mCP)_{2}to 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*S*_{1}/*S*_{2}states from the*v*= 0 level to 600 cm^{–1}above. Both linear and quadratic vibronic coupling schemes were investigated to simulate the S_{0}→ S_{1}/S_{2}vibronic spectra; those calculated with the latter scheme agree better with experiment.