Structure and Intermolecular Vibrations of Perylene·trans-1,2-Dichloroethene, a Weak Charge-Transfer Complex

Abstract

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(–1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.