Torsional Barriers to Rotation and Planarization in Heterocyclic Oligomers of Value in Organic Electronics

Abstract

In order to understand the conformational behavior of organic components in organic electronic devices, we have computed the torsional potentials for a library of thiophene-based heterodimers. The accuracy and efficiencies of computational methods for these organic materials were benchmarked for 11 common density functionals with three Pople basis sets against a Focal Point Analysis (FPA) on a model oligothiophene 2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]-thiophene (BTTT) system. This study establishes a set of general trends in regards to conformational preferences, as well as planarization and rotational barriers for a library comprised of common fragments found in organic materials. These gas phase structures are compared to experimental crystal structures to determine the effect of crystal packing on geometry. Finally, we analyze the structure of hole-transporting material DERDTS-TBDT and design a new oligomer likely to be planar in the solid state.

Quantifying Planarity in the Design of Organic Electronic Materials

“…This suggests that the torsional potential of one inter‐ring bond is not significantly affected by the other, allowing them to be treated separately. A similar conclusion was previously drawn for a range of conjugated systems based on conformational search and torsional energetics [34] . Similar to the trend of T , TT , and BDT dimers discussed earlier, calculations on a series of unsubstituted oligothiophenes show improved planarity upon extension of the conjugated backbone (Scheme 4).…”

“…To examine how the planarity of abackbone depends on its conjugation length, we performed 2D dihedral scans on the prototypical T-T-T (Figure 7) and T-BT-T (Supporting Information, Figure S3) systems.A ccording to the plot, conformers are predominantly distributed within AE 30 degrees of the planar s-trans state.The potential energy surface and band gap of the system show square symmetry,i ndicating additive contributions from the two neighboring dihedral angles.T his suggests that the torsional potential of one inter-ring bond is not significantly affected by the other,a llowing them to be treated separately.Asimilar conclusion was previously drawn for ar ange of conjugated systems based on conformational search and torsional energetics. [34] Similar to the trend of T, TT,and BDT dimers discussed earlier, calculations on aseries of unsubstituted oligothiophenes show improved planarity Being the model for regioregular P3HT,t he methylated oligothiophenes experience similar improvement despite the lower planarity.W en ote,h owever,t hat the value of hcos 2 fi gradually converges after about 6r epeat units.T he convergence has been well-established in numerous prior experimental [60] and computational [32,61] works,aswell as the recent DFT analysis [43] showing saturating frontier orbital energy after about 6r epeat units for aw ide range of conjugated polymers and copolymers.T his suggests that the values of hcos 2 fi and trends obtained for different pairs should allow us to predict the behavior of longer conjugated systems.T his is further illustrated by the hcos 2 fi values of dodecamers which are only slightly (0.014-0.086) higher than those of the dimers and the trends predicted for the dimers remain valid irrespective of the oligomer length. Nevertheless,t he extension of p-conjugation does contribute to improved planarity of longer oligomers and polymers,a nd the effect is more pronounced for less planar systems.…”

“…[27,28] Apart from various spectroscopic,c rystallographic,a nd microstructural characterizations enabling better molecular design, [29] theoretical calculations [30,31] have also contributed greatly to our current understanding of structure-property relationships in OSCs.Inparticular, density functional theory (DFT) methods have become ap owerful approach for probing the band gap,r eorganization energy,o ptimal conformation and rotational barrier of p-conjugated systems. [32][33][34] Their validity in predicting the torsional potentials of furan, thiophene and selenophene oligomers has been proved by comparison to benchmark computational results. [35] Computational studies on pyrrole-, [36] furan-, [37] thiophene-, [36,38] selenophene-, [18] and phenylene vinylene- [39] based polymers/ oligomers highlighted the importance of inter-ring twist on their band gap,i onization potential and charge carrier delocalization, all of which define their functionality as optoelectronic materials.…”

“…Apart from various spectroscopic, crystallographic, and microstructural characterizations enabling better molecular design, [29] theoretical calculations [30, 31] have also contributed greatly to our current understanding of structure‐property relationships in OSCs. In particular, density functional theory (DFT) methods have become a powerful approach for probing the band gap, reorganization energy, optimal conformation and rotational barrier of π‐conjugated systems [32–34] . Their validity in predicting the torsional potentials of furan, thiophene and selenophene oligomers has been proved by comparison to benchmark computational results [35] .…”

Investigating the impact of regiochemistry in ester functionalized polyfurans

“…First, torsional potentials 39,40 were used to compare how HT, HH, and TT linkages impact the conformational preferences of 2,2 0 bifurans with methyl or methyl ester side groups (Figure 2). In these calculations, the interring dihedral angle (defined by the O-C-C-O atoms) was rotated in increments of 5 from a planar syn conformation (0 ) to a planar anti conformation (180 ) and each conformer was geometry optimized.…”