Structural characterization of 9-cyanoanthracene-(Ar)(n) (n=0-3) by rotational coherence spectroscopy

Rotational coherence spectroscopy implemented with time-resolved fluorescen ce depletion has been applied in a structural study of 9-cyanoanthracene (C NA) and its clusters with Ar up to three atoms. For bare CNA, C-type transi ents for the S-1 and S-0 states have been observed separately, yielding ind ependent sets of rotational constants for the two states. For the Ar cluste rs, rotational constants as averages for S-1 and S-0 have been derived to f ix the cluster geometry. The Ar atom in CNA-Ar is located 3.46 +/- 0.03 Ang strom above the central aromatic ring of CNA and displaced slightly from th e ring center toward the cyano group. The plane-Ar distance is quite close to those in clusters with other polycyclic aromatic molecules. Two values ( similar to0.2 or 0.6 Angstrom) for the displacement to the cyano group are consistent with the experimental data, and results on related aromatics-Ar show that the former is preferable. The dominant conformer of CNA-(Ar)(2) h as been determined as a two-sided (1 + 1)-type: structures for each sides o f the CNA plane are the same as that of CNA-Ar within the experimental unce rtainties. CNA-(Ar)(3) has a (2 + 1)-type structure: one side of the substr ate is the same as CNA-Ar, and an Ar dimer lies 3.48 +/- 0.04 ii above the other side. The determined conformations of CNA-(Ar)(1,3) are the same as t hose of the corresponding anthracene clusters, but that of CNA-(Ar)(2) is i n contrast to that of anthracene-(Ar)(2), which has been identified as a (2 + 0)-type. Model potential calculations have been employed to explain the difference in structural motifs of the two closely related clusters.