MP2 and DFT study of IR spectra of TCNE‐methylsubstituted benzene complexes: Is charge transfer important?

Abstract

ABSTRACT:The second-order Møller-Plesset (MP2) and density functional theory (DFT; B3LYP, B3LYP-D) approaches show that the CN (and also CAC) stretching IR vibrations are relatively small but reproducibly shifted to lower frequencies in a series of methylbenzenes-tetracyanoethylene (NMB-TCNE, N ϭ mono-, di-, …, hexa-) complexes with increasing number of methyl groups (N). These CN stretching IR frequency shifts, ranging from 8 to 12 cm Ϫ1 , are linearly dependent on number N but still 2-4 times greater than recently published experimental data [Stires et al., Chem Commun, 2005, 28, 3532; Pawlukojc et al., Chem Phys, 2006, 327, 311]. An explanation of the shifts toward lower frequencies is in literature [Stires et al., Chem Commun, 2005, 28, 3532; Pawlukojc et al., Chem Phys, 2006, 327, 311] usually attributed to the magnitude of transferred electron charge ⌬Q from a donor molecule to TCNE (mainly to its LUMO [lowest unoccupied molecular orbital]). The LUMO has antibonding character on CN and CAC bonds, and thus the respective CN and CAC bond orders are, due to partial occupation in the complex, decreased. However, the difference by a factor of 2-4 between our MP2 (and also B3LYP) calculations and experiment is probably also due to the effect of environment, i.e., intermolecular forces between complex and solvent molecules or crystal lattice. The calculated interplanar distance R, dipole moment , and transferred charge ⌬Q are approximately linearly dependent on number of methyl groups N. However, between N ϭ 2 and 3, the abrupt changes in the dependencies were observed. This is caused by the change of mutual (perpendicular) orientations between respective methyl benzenes and TCNE in the complexes as a consequence of preferred interaction between one of two degenerated or near degenerated highest occupied molecular orbitals (being orthogonal) on NMB with LUMO located mainly on TCNE. This fact is important for many properties (especially for electron excited states) of these complexes. A strong dependence of polarizability, namely of its ␣ zz part, of the benzene-TCNE (Be-TCNE) complex on soft vibrations was found. The vibrational contribution to ␣ zz value is almost as large as the electronic ␣ zz contribution. Finally, MP2 and B3LYP studies indicate that inclusion of anharmonicity has small, if any, effect on calculated vibrational frequencies of Be-TCNE. This theoretical finding is quite important especially because it holds also for soft vibrational modes, which are always present at weak intermolecular complexes.

Ultrafast Time-Resolved Broadband Fluorescence Studies of the Benzene-Tetracyanoethylene Complex: Solvation, Vibrational Relaxation, and Charge Recombination Dynamics

“…The interplanar distances (R DA ) and the ground-state dipole moments (μ DA ) are very similar for both conformers (see Table 4). These results agree in general with a few previous computational studies of the BZ−TCNE complex, [65][66][67][68]92 except that the interplanar distance calculated here is somewhat shorter. 68 Very recently, Kuchenbecker and Jansen 67 reported high-level ab initio calculations on the BZ−TCNE complex.…”

“…The system of interest here is the EDA complex containing benzenes (BZ) as the donor and tetracyanoethylene (TCNE) as the acceptor. The series of EDA complexes containing TCNE and various methyl-substituted benzenes (MBZ) are among the earliest examples of EDA complexes ,, and have been studied extensively. ,,− Among all MBZ–TCNE complexes, hexamethylbenzene–TCNE (HMB–TCNE) is probably the most intensively studied system both experiemntally ,− and theoretically. ,− Surprisingly, compared with HMB–TCNE, the smallest member of the series, namely, the BZ–TCNE complex, has not been given much attention. Early studies of the BZ–TCNE complex in the vapor and liquid phases have focused on the thermodynamic properties. ,,− To the best of our knowledge, the only available time-resolved data for BZ–TCNE is a transient-absorption study that reported a CR rate of 1.4 × 10 10 s –1 in dichloromethane .…”

Ultrafast Charge Recombination Dynamics in Ternary Electron Donor–Acceptor Complexes: (Benzene)2-Tetracyanoethylene Complexes

“…The series of EDA complexes containing tetracyanoethylene (TCNE) as the acceptor and methyl-substituted benzenes (MBZ) as the donor have been widely studied as model sytems. ,− Among them, hexamethylbenzene-TCNE (HMB-TCNE) is probably the most intensively studied MBZ-TCNE system both experimentally − and theoretically. ,− Surprisingly, the smallest member of the series, namely the benzene-TCNE (BZ-TCNE) complex, has not been given much attention. Recently, we have studied the ultrafast CT-state relaxation dynamics of the BZ-TCNE complex in solvents of different polarities with ultrafast broadband time-resolved fluorescence (TRFL) spectroscopy .…”