Structural and Electronic Properties of Crystalline, Isomerically Pure Anthradithiophene Derivatives

Abstract

considering that the majority of devices based on ADTs reported to date have been formed from a material consisting of a mixture of isomers. Typically, isomeric mixtures are expected to offer decreased electronic performance, due to the inhomogeneities in intermolecular order that arise in the crystalline state. [ 6 ] The isomer mixtures ( mix ADTs) arise due to the nonregiospecifi c aldol condensation used in the most common synthesis of the ADT chromophore. [ 7 ] Even in the case of highly soluble ADT derivatives, separation of the syn and anti isomers after synthesis has proven impossible. Thus the chemical community has adopted much lengthier synthetic approaches to synthesize each pure isomer directly. Geerts and co-workers were the fi rst to explore pure ADT isomers, developing an elegant synthesis of pure anti dihexyl ADT with an overall chemical yield of ≈3% in 2011. [ 8 ] Unfortunately, there have been no reports of the device performance/hole transport property measurement for this material. In 2012, the Tykwinski group demonstrated the fi rst synthesis of pure syn diF TES ADT, in an overall yield of 9% from commercial starting materials. [ 9 ] Single-crystal studies by Jurchescu and co-workers showed no signifi cant difference in charge transport properties between the mix and syn materials.More recently, Takimiya and co-workers prepared the parent ADT in isomerically pure anti form in six synthetic steps and 43% overall yield, [ 10 ] and found that the transistor performance was several times higher than that of the isomeric mixture reported by Katz [ 1 ] (although some of this improvement may arise due to improved modern device fabrication protocols). Mamada and co-workers prepared both isomeric forms of the parent ADT, in 12% overall yield for anti and 7% yield for syn . [ 11 ] In vacuum-deposited devices, the anti isomer showed substantially higher performance than the syn isomer; 0.18 cm 2 V −1 s −1 compared to 0.017 cm 2 V −1 s −1 for syn . Similarly, Takimiya and co-workers also observed an order-of-magnitude difference in hole transport effi ciency between alkylated isomerically pure anti-and syn-naphthodithiophenes, with the anti derivative again exhibiting better performance. [ 12 ] These results strongly suggest that a detailed understanding of the impact of isomer mixtures in the solution-processable, ethyne-substituted ADTs will not be complete without side-by-side analysis of the mix , syn , and heretofore-elusive pure anti isomers. Structural and Electronic Properties of Crystalline, Isomerically Pure Anthradithiophene DerivativesRawad K. Hallani , Karl J. Thorley , Yaochuan Mei , Sean R. Parkin , Oana D. Jurchescu , and John E. Anthony * Anthradithiophene chromophores are found in many current high-performance organic semiconductors, even though these materials are typically synthesized as an inseparable mixture of syn and anti isomers. Recent syntheses of pure syn anthradithiophenes have shown no improvement in performance for the more homogeneous system, but similar studies on the ...

The Influence of Isomer Purity on Trap States and Performance of Organic Thin‐Film Transistors

“…[18] A more recent study focused on ADT derivatives also reported a higher field-effect mobility, µ, in the pure anti -isomer, and found that both types of pure isomer devices outperformed the mix -isomer sample. [19] That publication reports µ values of 2.0, 2.2, and 4.7 cm 2 V −1 s −1 for OFET devices of the mix -, syn -, and anti -isomers, of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and 2.3, 2.7, and 6.2 cm 2 V −1 s −1 in 2,8-difluoro-5,11-bis(triethylgermylethynyl) anthradithiophene (diF-TEG ADT). In addition, research into other 1D and 2D aromatic molecules, as well as polymers of isomeric monomers, indicate a general disparity in performance between isomers of the same compound.…”

Noncovalent Interactions in Organic Electronic Materials

“…For molecules with low degrees of symmetry, adjacent molecules can pack in a disordered manner despite the molecules being isomerically equivalent. For isomerically pure ADTs, the anti isomer shows the lowest degree of thiophene positional disorder in the experimental crystal structures, 146,149 which could lead to more homogenous charge-carrier transport pathways and offers a plausible explanation for the larger hole mobilities determined from transistor studies.…”

“…From a materials design standpoint, it is important to realize that the substitution of thiophene into an acene structure can lead to an isomeric mixture of products. 146,[148][149][150] In addition to isomeric purity, molecular symmetry can play a role. For molecules with low degrees of symmetry, adjacent molecules can pack in a disordered manner despite the molecules being isomerically equivalent.…”

“…146,147 As thin-films of the molecular organic semiconductor materials are generally disordered, pathways of consistent molecular alignments that mitigate the impact of charge trapping sites (e.g., defect sites within the material that lead to electronic disorder) are necessary to provide efficient charge-carrier transport conduits through the active layer. From a materials design standpoint, it is important to realize that the substitution of thiophene into an acene structure can lead to an isomeric mixture of products.…”

“…These results can be related back to the characteristics of isomerically pure ADT molecular materials. Analysis of the ADT crystal structures 146,[148][149][150] shows that the dominant disordermers tend to minimize sulfur contacts among nearest neighbors: The anti ADT isomer is able to sustain a pattern of minimal sulfur contacts in the solid state, while the symmetry of the syn ADT isomer does not allow this to occur, resulting in the presence of much more disorder in the crystal; the larger degree of disorder in syn ADT appears to be the reason behind the worse transistor performance when compared to transistors using anti ADT as the active layer. Thus, while heteroatoms are widely used in the design of organic semiconductors, making seemingly subtle changes in the chemistry of the π-conjugated backbone through heteroatom substitution to alter the redox and (opto)electronic characteristics, can have wide ranging materials implications.…”