Structure and conformation of meso-2,4-di(N-carbazolyl)pentane
Graphical abstract
Introduction
Large planar aromatic molecules such as anthracenes and carbazoles tend to pack in parallel planar face-to-face or ‘T-shaped’ face-to-edge interactions in the solid state [1], [2], [3], [4] as well as in the gas phase [5]. The optimum geometry for interaction in solution often contains a face-to-face orientation of the aromatic rings in order to maximize π overlap, as found for the excited state dimers (excimers) of anthracene [6] although a nonparallel planar interaction more akin to the orientation of the two anthryl groups in di(9-anthryl)methane also has been proposed [7].
Intramolecular associations of aryl groups in diarylalkanes should be entropically less unfavorable than those which occur intermolecularly, but the connecting linkage can also restrict the geometries possible for interaction. In particular, aryl groups separated by three saturated carbons are optimally situated to achieve parallel planar interactions (Fig. 1a, n = 3), and 1,3-diarylalkanes have been used to model the behavior of pendant aryl groups in organic polymers such as poly(vinylcarbazole) (Fig. 1b) and poly(vinylanthracene). These polymers become photochemical semiconductors in the presence of electron accepting dopants [8], [9]. Since the tacticity of the polymer will affect the interactions which can occur between the carbazoles units, and hence the conductive properties, we prepared meso-2,4-di(N-carbazolyl)pentane (Fig. 1c) in order to determine the conformation and close contacts involving the carbazolyl groups. The meso compound as well as the racemate are optically inactive in solution and cannot be distinguished in this regard. Therefore we performed a structure determination in order to verify the stereochemistry of this isomer. Density-functional-theoretical computations also were performed to assess the relative importance of intramolecular versus intermolecular influences on the orientation of the groups.
Section snippets
Experimental
A crystal of the known title compound suitable for X-ray analysis was prepared as follows. 2,4-Pentanediol (Aldrich, 4 g, 0.038 mol) in 15 mL of dry pyridine was added to tosyl chloride (Aldrich, 15.25 g, 0.077 mol) also in 15 mL of dry pyridine. After 15 h, 19 mL of conc. HCl in 1 mL of distilled water was added. The solution was extracted twice with 100 mL of CHCl3. The organic layer was extracted with 50 mL of distilled water and with saturated NaHCO3 solution. The organic layer was evaporated under
Results
An ORTEP of the title compound showing the atom labeling scheme and a packing diagram are shown in Fig. 2. The structure establishes the stereochemistry for the compound as meso, although no internal plane of symmetry exists in the conformation adopted in the crystal or calculated by theory. We performed calculations on the structure in order to determine the configuration and conformational preferences in the absence of crystallographic packing forces. A local potential-energy minimum with
Conclusion
The stereochemistry of the sole isomer of 2,4-di(N-carbazolyl)pentane prepared according to the procedure reported previously is established to be meso. The magnitude of puckering about the nitrogen atoms is in line with that reported for other carbazole-containing structures and likely results more from crystallographic packing forces than from intramolecular interactions for N1, consistent with the calculation of a low energy barrier for partial pyramidalization. Theoretical M06-2X treatment
Acknowledgments
We thank the Graduate College and Chemistry Department at Cleveland State University for support, the Ohio Supercomputing Center for a grant of computer time, and the National Science Foundation (CHE-0840446) for funds used to purchase the Bruker APEX II Duo X-ray diffractometer used in this research.
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