Dr. Götz Schuck

5,11-bis-(4-tert-butyl-phenyl)-6,12-diphenyl-naphthacene (5,11-BTBR) is a derivate of rubrene with two polymorphs (A and B) where t-butyl sidegroups are added.

5,11-BTBR Polymorph A has a structure drastically different from the packing of unsubstituted rubrene with enhanced backbone-backbone spacing between two molecules [2]. Remarkably, the naphthacene backbone of the molecules is significantly twisted in polymorph A, with a twist angle of 43° between the two opposite C-C bonds at both ends of the backbone. The resulting backbone-backbone distances are not shorter than 4.36 Å, which is commonly expected to drastically reduce the pi - pi * overlap.

In the case of 5,12-bis-(4-tert-butyl-phenyl)-6,11-diphenyl-naphthacene (5,12-BTBR) [3], the in-plane arrangement of the molecules is very similar to that of rubrene [4], with even shorter distances between the naphthacene backbones (3.62 Å compared to 3.75 Å). However, the addition of the t-butyl groups increases the inter-layer spacing by 31%. Interestingly, it leaves the backbone almost perfectly planar.

The structure of polymorph 5,11-BTBR (B) could not be solved so far, as the material seems to exclusively grow as ultrathin platelets. From measurements of the d-spacing perpendicular to the extended crystal surface [1], a structure closely related to the one found for a constitutional isomer, 5,12-bis-(4-tert-butyl-phenyl)-6,11-diphenyl-naphthacene was assumed for polymorph B.

The electric transport properties of the two polymorphs are completely different: in polymorph B the in-plane hole mobility of 12 cm2/Vs measured on single crystal FETs is just as high as in rubrene crystals, whereas polymorph A does not show any measurable field-effect [1].

Reference: [1] Haas, S., Stassen, A. F., Schuck, G., Pernstich, K. P., Gundlach, D. J., Batlogg, B., Berens, U. & Kirner, H. J. (2007). Phys. Rev. B. Accepted. (cond-mat, arXiv:0707.0461); [2] G. Schuck, S. Haas, A. F. Stassen, U. Berens and B. Batlogg, Acta Cryst. (2007). E63, o2894; [3] G. Schuck, S. Haas, A. F. Stassen, H.-J. Kirner and B. Batlogg, Acta Cryst. (2007). E63, o2893; [4] O. D. Jurchescu, A. Meetsma and T. T. M. Palstra, Acta Cryst. (2006). B62, 330-334

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