Two new cases of polymorphism in diagonally substituted rubrene derivatives

Two new cases of polymorphism in two different substituted rubrenes are reported. These are some of the first examples in rubrene derivatives of polymorphism occurring in a separate crystal class.

The crystal structures of two rubrene derivatives, 5,11-diphenyl-6,12-bis[4-(trifluoromethyl)phenyl]tetracene, C 44 H 26 F 6 , and 5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene, C 50 H 44 , are presented. Each are substituted on diagonal (5/11) phenyl rings. Each derivative has one polymorph reported previously. A discussion of the differences between each derivative and its previously reported polymorph is provided. The triclinic packing of the CF 3 -substituted structure is similar to the packing of the parent rubrene's triclinic polymorph. In the tertbutyl-substituted structure, a planar tetracene core formed, which has been hypothesized but never published. Crystallization conditions are provided as they differ from previous reports. Rubrene (5,6,11, has been widely studied in the literature for its excellent electronic properties. Many synthetic attempts have been made to alter the molecular structure in the hope of improving these properties (Uttiya et al., 2014;Ogden et al., 2017;Paraskar et al., 2008). Molecular tuning of these derivatives has led to unpredictable crystal packing. While some derivatives have been reported to form the ideal herringbone crystal structure, others have not, with no reported structures exhibiting polymorphism in different crystal classes similar to the parent rubrene. The rubrene library has grown significantly over the years and now includes over 35 derivatives in a variety of crystalline arrangements (Clapham et al., 2021). This library has provided an enticing database for computational scientists looking to add predictability and reasoning to crystal-packing formation (Sutton et al., 2015). We wish to add to this rubrene library two additional structures. They are not polymorphs of each other, but instead polymorphs of previously published compounds.

Chemical context
We report here new crystal packing for each compound, making these some of the first cases of polymorphism in rubrene derivatives. This report serves two purposes: the first as a caution to synthetic chemists that polymorphism can and does exist in these materials, even if it has not been published, as is the case for 5,11-bis(4-trifluoromethylphenyl)-6,12-biphenyltetracene (compound 1). The second purpose serves as an encouragement to explore more of the rubrene library in future studies. For example, 5,11-bis(4-tert-butylphenyl)-6,12biphenyltetracene (compound 2) has been largely overlooked, despite its promising carrier mobility (Haas et al., 2007), likely because no crystal structure with the ideal herringbone formation had been reported in the database.

Structural commentary
Both rubrene molecules in this report have been reported and synthesized previously (Haas et al., 2007;Uttiya et al., 2014). Each contains substitutions on the 5 and 11 peripheral phenyl rings. Many rubrene derivatives are shown to twist along the tetracene core in the solid state, such as the first polymorph of 2. Here, both derivatives display planar tetracene backbones (Figs. 1 and 2). Crystal structure of 2 with displacement ellipsoids shown at the 50% probability level.

Figure 1
Crystal structure of 1 with displacement ellipsoids shown at the 50% probability level.

Figure 3
Crystal projection of 1 displaying brick-like packing.
with the herringbone arrangement of 2 (Fig. 4). While there exist sets of -stacking dimers, alternating layers are rotated, creating the 'z' or herringbone arrangement.

Database survey
Previously, only the monoclinic compound 1 had been reported (CSD CIYXUF;Uttiya et al., 2014). The structure displays a planar tetracene core with the desired herringbone packing, similar to rubrene. Additionally, like rubrene, we now report a triclinic form. While the triclinic form retains the planar backbone, it packs in a brick-like arrangement, which has been shown with rubrene to have significantly reduced charge mobility (Matsukawa et al., 2010). This is a similar case to the NO 2 -substituted rubrene derivative [5,11-bis(4-nitrophenyl)-6,12-biphenyltetracene] in which the monoclinic form was discovered (Uttiya et al., 2014), with the triclinic reported later (Moret & Gavezzotti, 2022).
This instance of polymorphs with differing carrier mobility was also seen for the previously published structure of 2 (Schuck et al., 2007). Schuck et al. reported two crystalline forms; however, full structural analysis was only able to be carried out on the monoclinic form (CSD PIFHOC). While it was noted that the published monoclinic structure had no carrier mobility, the second morphology had a high measured mobility. As a result of the mobility and d-spacing measurements, it was hypothesized this molecule took on a herringbone arrangement. We have therefore now performed a full structural characterization, and confirmed the herringbone arrangement of 2 as hypothesized.

Synthesis and crystallization
The synthesis of 1 was published previously (Uttiya et al., 2014). The authors reported crystal growth in acetone; however, attempts at crystallization with acetone either by cooling or through evaporation were unsuccessful in growing the monoclinic structure previously reported. Any crystals obtained through this method, other solvent mixtures (ethanol, methanol, DCM:methanol), or physical vapor transport (PVT) all produced the triclinic form reported here, thus necessitating this publication to serve as a cautionary notice.
Synthetic and crystallization procedures of 1 were followed for 2. In contrast to the PVT methods previously reported (Haas et al., 2007), we found both polymorphs to be grown by solution methods, with the herringbone polymorph in the minority. Compound 2 was dissolved in a minimal amount of DCM and layered with methanol, in an approximate 1:3 ratio. We observed two different morphologies: the monoclinic structure in thin sheets as previously reported, as well as some dark-red thin plates. Likely due to improved instrumentation in more recent years, we were able to collect full structural data on the thin plates. We also note that the herringbone polymorph has excellent air stability. Whereas the monoclinic polymorph oxidizes when exposed to air, the herringbone polymorph remains stable for many months and retains its dark-red color, making it easily distinguishable from the other polymorph.

Refinement
Crystal data, collection and structure refinement details are summarized in Table 1 for compound 1 (C 44 H 26 F 6 ) along with the previously published polymorph (CIYXUF) and compound 2 (C 50 H 44 ) with the previously published polymorph (PIFHOC) for comparison. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.36 e Å −3 Δρ min = −0.22 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

5,11-Bis(4-tert-butylphenyl)-6,12-diphenyltetracene (2)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.23 e Å −3 Δρ min = −0.21 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.54048 ( (11) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. ′Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.′

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.