Synthesis and charge-transfer complex formations of 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes with three π-acceptors

Dimeric 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes (where n = 1–5) were synthesized and their structures were characterized via spectroscopic techniques. Structures of two of the dimers, 1,2-bis(3,6-diethylcarbazol-9-yl)ethane (2b) and 1,4-bis(3,6-diethylcarbazol-9-yl)butane (2d), were investigated by single crystal X-ray crystallographic techniques. The crystal structures of 2b and 2d were solved in the monoclinic space groups C2/c and P21/n, respectively. The methylene chain adopted an anti conformation in 2b and a gauche-anti-gauche conformation in 2d, enabling coplanar orientations of carbazole rings in both structures. The molecular packing in both structures was stabilized by intermolecular π–π stacking. Charge transfer complexations of 2a–2e with the π-acceptors p-chloranil, tetracyanoethylene, and tetracyanoquinodimethane in solution were investigated by determining their stoichiometries, molar absorptivities, equilibrium constants, enthalpies, and entropies. All the dimers formed weakly associated complexes with each of the acceptors having equilibrium constants between 1.32–8.94 M−1 in 1,2-dichloroethane. Complexations were driven by the slightly negative formation enthalpies between −2.00 and −4.24 kcal mol−1.