Molecular crystal engineering seeks to tune the material properties by controlling the crystal packing. However, the range of achievable properties is constrained by the limited energy range of polymorphs which can be crystallized. Here, computational modeling highlights that a solid-state crystal-to-crystal chemical reaction in 9- tert -butyl anthracene ester (9TBAE) nanorods [Al-Kaysi et al. , J. Am. Chem. Soc. , 2006, 128 , 15938] imparts “synthetic memory” into the crystal structure that allows reproducible formation of a highly metastable, yet long-lived polymorph. Specifically, whereas the vast majority of known polymorphs exhibit lattice energy differences below 10 kJ mol −1 , the conformational polymorph formed via solid state reaction chemistry lies 14 kJ mol −1 higher in energy than the form grown from solution, according to calculations that combine a dispersion-corrected second-order Møller–Plesset perturbation theory (MP2D) treatment of the monomer and photodimer with a density functional theory treatment (B86bPBE-XDM) of the intermolecular interactions in the crystal. Moreover, the solid-state reaction environment traps a highly unstable intramolecular photodimer conformation which defies the conventional wisdom surrounding conformational polymorphs. These observations suggest that solid-state reaction chemistry represents an under-appreciated strategy for producing polymorphs that would likely be unobtainable otherwise.
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Rubrene untwisted: common density functional theory calculations overestimate its deviant tendencies
The exceptionally high carrier mobility of rubrene derives from the combination of its intrinsic electronic properties and favorable crystal packing that facilitates charge transport. Unlike the planar conformations adopted by rubrene single crystals, however, many rubrene derivatives crystallize with a twisted tetracene core and exhibit poor carrier mobility. Typical density functional theory (DFT) calculations suggest that the twisted conformation is preferred by ∼10–14 kJ mol −1 or more in the gas phase. However, the present work shows that those calculations overestimate the twisting energy by several kJ mol −1 due to density-driven delocalization error, and that the twisting energies are actually only ∼8–10 kJ mol −1 for typical rubrene derivatives when computed with higher-level correlated wave function models. This result has two significant implications for crystal engineering with rubrene derivatives: first, DFT calculations can erroneously predict polymorphs containing twisted rubrene conformations to be more stable, when in fact structures with planar conformations are preferred, as is demonstrated here for perfluororubrene. Second, the smaller twisting energies make it more likely that solid form screening could discover new planar-core polymorphs of rubrene derivatives that have previously been crystallized only in a twisted conformation. These in turn might exhibit better organic semiconducting properties.
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- Award ID(s):
- 1955554
- NSF-PAR ID:
- 10252565
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- Volume:
- 9
- Issue:
- 8
- ISSN:
- 2050-7526
- Page Range / eLocation ID:
- 2848 to 2857
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The crystal structures of the charge‐transfer (CT) cocrystals formed by the π‐electron acceptor 1,3,4,5,7,8‐hexafluoro‐11,11,12,12‐tetracyanonaphtho‐2,6‐quinodimethane (F6TNAP) with the planar π‐electron‐donor molecules triphenylene (TP), benzo[
b ]benzo[4,5]thieno[2,3‐d ]thiophene (BTBT), benzo[1,2‐b :4,5‐b ′]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using single‐crystal X‐ray diffraction (SCXRD), along with those of two polymorphs of F6TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixed‐stacking motifs. Cocrystals based on BTBT and CBZ π‐electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringbone‐type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of ground‐state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TP‐, BDT‐, and PY‐based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Charge‐carrier mobility values are obtained from space‐charge limited current (SCLC) measurements and field‐effect transistor measurements, with values exceeding 1 cm2V−1s1being estimated from SCLC measurements for BTBT:F6TNAP and CBZ:F6TNAP cocrystals. -
Molecular crystal structure prediction is increasingly being applied to study the solid form landscapes of larger, more flexible pharmaceutical molecules. Despite many successes in crystal structure prediction, van der Waals-inclusive density functional theory (DFT) methods exhibit serious failures predicting the polymorph stabilities for a number of systems exhibiting conformational polymorphism, where changes in intramolecular conformation lead to different intermolecular crystal packings. Here, the stabilities of the conformational polymorphs of o -acetamidobenzamide, ROY, and oxalyl dihydrazide are examined in detail. DFT functionals that have previously been very successful in crystal structure prediction perform poorly in all three systems, due primarily to the poor intramolecular conformational energies, but also due to the intermolecular description in oxalyl dihydrazide. In all three cases, a fragment-based dispersion-corrected second-order Møller–Plesset perturbation theory (MP2D) treatment of the crystals overcomes these difficulties and predicts conformational polymorph stabilities in good agreement with experiment. These results highlight the need for methods which go beyond current-generation DFT functionals to make crystal polymorph stability predictions truly reliable.more » « less
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Abstract Density functional theory and extrapolated CCSD(T) computations of several “anti‐Bredt” alkenes were carried to explore possible 1,2‐diyl “alkene” candidates with a triplet ground state. Ten candidates containing twisted double bonds at the bridgehead positions of bicyclic structures (
1‐6 ) or adamantene (7‐10 ) derivatives were studied. Based on a combination of ring strain, rigid scaffolding, and steric crowding, four species were identified to have surprisingly low singlet‐triplet energy gaps (lower than 4 kcal/mol). Atert‐ butyl substituted bicyclic structure (4 ) was identified to have a near‐zero singlet‐triplet energy gap, but no triplet ground‐state alkene was found. Ring strain energy (RSE) calculations, π‐orbital axis vector (POAV) analyses, and multiple linear regression models were performed to elucidate the geometric and energetic effects of double bond twisting in1‐10 . Based on our computational exploration, it appears unlikely that there is a ground‐state triplet olefin. -
Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor–acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N -isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the CC bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S 0 → S n ) depend strongly on the dihedral angle ( θ ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S 1 excited state in the planar conformations ( θ ∼ 0) but selects an S 2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S 2 → S 1 charge separation reaction. Unprecedented variation of the S 2 –S 1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20–60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0TC05463A
