Association constants for 1,2,4,5-tetracyanobenzene and tetracyanoethylene charge-transfer complexes with methyl-substituted benzenes revisited

Abstract

The association constants for charge‐transfer (CT) complex formation of a series of methylated benzene donors with 1,2,4,5‐tetracyanobenzene and tetracyanoethylene as acceptors were measured. In several cases the values determined previously using standard analysis techniques, such as Benesi–Hildebrand or related methods, were shown to be incorrect and a new method for determining the association constants for weak complexes is presented. A systematic error occured in the determination of these constants when standard analysis was carried out on weakly bound complexes. In general, the thermodynamic stabilities have been underestimated and the extinction coefficients for the CT absorptions overestimated. Furthermore, it was demonstrated that the ground‐state stabilization of the complexes studied here is due primarily to non‐bonded interactions and that the ion‐pair contributions are minor in the ground state. A notable exception may be the tetracyanobenzene–hexamethylbenzene complex where preliminary evidence points to a significant contribution of the ion‐pair state to the ground‐state stability. This study raises significant questions about what is currently known concerning the thermodynamics of CT complexes because much of what is believed may be based on incorrectly determined constants. Copyright © 2000 John Wiley & Sons, Ltd.

Analysis of the association constants for charge-transfer complex formation

“…2b. There has been significant dialog detailing the difficulties encountered with the application of Benesi-Hildebrand [24] or related methods [25][26][27] to the determination of the association constants for weakly bound CT complexes [17,23,[28][29][30][31]. Much of the discussion has focused on the need for significant curvature in these plots if K CT is to be extracted from them [32].…”

“…(1). Also described in our earlier report, the HMB/TCNE complex was unique among the complexes studied because it did not correlate well with the other complexes included in the initial study [17]. At that time we suggested that the anomalous behavior of this specific complex could be due to a change in the bonding contributions and that ion-pair interactions might be contributing significantly to the ground state stability of this complex.…”

“…Solutions containing the individual acceptors and donors were prepared immediately prior to use. The method used to determine the association constants has been described in detail previously [17,23]. Briefly, two series of absorption data were recorded for each complex.…”

“…In our previous report [17], we presented the results of our studies on the complexes formed between 1,2,4,5-tetracyanobenzene (TCNB) or tetracyanoethylene (TCNE) as acceptors with a series of methyl substituted benzene donors. Linear correlations between the measured association constant and the CT k max within each series of complexes were observed, in agreement with Eq.…”

“…Explanations for the previously observed anomalous behavior [17] of the HMB/TCNB complex are explored. Additional families of complexes are included in the current study, specifically complexes between methylated benzene donors and pyromellitic dianhydride (PMDA) or 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as acceptors in addition to the TCNE and TCNB complexes.…”