Structure and intermolecular vibrations of perylene· trans -1,2-dichloroethene, a weak charge-transfer complex

摘要

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 S_0 a?" S_1 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 (S_0) and 13 cm～（-1） (S_1). The intermolecular T_z stretch and the R_a and R_b 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 D_(2h) to D_2. 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 S_0 → S_1 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 R_c, T_z, R_a, and Rb vibrations by comparing to the observed/calculated frequencies and S _0 a?" S_1 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.