Impact of structure and connectivity of thiophene-based bicycles stacking with fluoroarenes in conjugated materials

Abstract

Noncovalent interactions between aromatic rings can dictate the crystal structures of organic optoelectronic materials. While cofacial interactions of fluorinated and non-fluorinated arenes are useful supramolecular synthons, the impacts of the structural details on these interactions remain unpredictable, particularly with the types of heterocycles common in materials derived from π-conjugated molecules. In this work, a combination of optical spectroscopy and X-ray crystallography demonstrates that both the degree of fluorination of a benzyl ester side chain and the regiochemical connectivity of a benzothiophene (BT) to an arylene-ethynylene backbone impact whether intramolecular cofacial stacking occurs or not. Surprisingly, the thienothiophene (TT) analog, which has an isoelectronic π-system with BT while a stronger electron donor when a part of donor–acceptor π-systems, did not show any evidence of cofacial stacking with fluoroarene side chains, regardless of the extent of fluorination. Quantum-chemical modelling rationalizes the dependence on BT regiochemistry, suggesting that while dispersion interactions comprise the largest individual component of attractive forces in stacking interactions, the strength of electrostatic interactions correlates best with the likelihood of intramolecular stacking interactions occurring in the solids. Finally, many of these molecules show polymorphic behaviour, with examples of blue-shifting and red-shifting mechanofluorochromism. Overall, this work enhances our ability to harness local structural details in deploying non-covalent interactions for designing solid state structures of optoelectronic materials.

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Introduction

The molecular design of organic optoelectronics^1–5 must consider the π-conjugated backbone moieties^6–11 as well as the side chains,^12–15 for purposes of solubility and solid-state assembly^1,16 to improve optoelectronic performance. Traditionally used for solubility,¹⁷ the side chains^18–27 play a critical role in tailoring materials properties and device performance.^28–33 Rational design using this approach, however, requires understanding of the noncovalent interactions that these functional side chains can introduce, particularly when involving the π-conjugated units of the main chain backbones of these materials.^34–36 While the Hunter–Sanders^37–39 and Wheeler–Houk^40,41 models provide different explanations for the interactions between aromatic rings, each holds value in promoting rational design for noncovalent interactions for crystal engineering.^20,26,27,34

Considerations of noncovalent interactions amongst aromatic rings in organic optoelectronic materials is essential for materials design.^1–5 Our research group developed an arylene–ethynylene (AE) molecular scaffold to understand how molecular structural factors impact interactions of aromatic rings in solid-state assemblies;^42–49 the scaffold is an AE with a central terephthalate flanked by aryl ethynyl groups.^44–46,48 By including fluorinated benzyl benzoates on the central terephthalate, cofacial intramolecular interactions between the fluorinated side chain rings (ArF) and conjugated flanking rings (ArH) can occur. For intramolecular cofacial interactions, the short benzyl ester tether requires the AE backbone to twist, interrupting what we would normally expect to be coplanar, maximally conjugated rings. When combined with intermolecular ArF–ArH cofacial interactions, this motif precludes aggregation between the chromophores, resulting in an optical gap for the material that resembles, or is hypsochromically shifted from, that of the molecule in dilute solution. We used this design to understand how structural characteristics in both the ArF and ArH rings, such as electronic substituent effects,^44–46,50 electrostatic complementarity,^44,47,48 shape,^47,49 and regiochemical effects^44,46 correlate to whether cofacial “stacking” is observed in this scaffold. We also demonstrated applications of these molecules through rational design of these interactions, including phosphorescent solids and films that show reversible mechanofluorochromism (MFC) in both spectral directions.

Recently, we turned our attention to the noncovalent interactions of heteroaromatic ring systems.^48,49 These investigations highlighted some correlations between both molecular shape and the mean electrical potential induced by a heterocyclic system at a typical π-stacking distance to whether ArF–ArH interactions of these rings occurs. Based on the importance of “local interactions” in the model of Wheeler and Houk,⁴¹ we were interested in the extent to which such variation might yield different results with heteroaromatic structures. In this work, we focus on the interactions of multiring fused thiophenes, which hold a privileged structural space in optoelectronic materials.^51–54

Results and discussion

Experimental design and synthesis

Isomerism of conjugated backbones,⁵⁵ side chains,^56,57 and even conformational isomers⁵⁸ can impact device performance. As an initial design to address the question of ArH regiochemistry, we chose the benzothiophene (BT) bicycle. BT is a common moiety in organic optoelectronics,^53,59,60 and one of the six heterocycles we studied previously in this AE scaffold. In our prior study, 2-benzothiophene showed both intermolecular and intramolecular coplanar ArF–ArH interactions and substantial bathochromic MFC of 67 nm (0.36 eV) in emission spectra.⁴⁸ Of the six heterocycles we studied previously, this system has the most electropositive electrical potential computed at 3.25 Å, and its bicyclic structure enables stronger dispersion interactions than the smaller, monocyclic heterocycles thiophene and furan. We therefore attached the BT unit to our AE scaffold through either the 2, 5, or 6 positions. As a comparator, we also attached the thieno[2,3b]thiophene unit – in the 2 position – to the AE scaffold as another thiophene-based bicycle that is popular in organic optoelectronics, which we might expect to be more “electron rich” than benzothiophene, based on a typical descriptive parameter, i.e., yielding a molecule with higher energy HOMO (vide infra).

To test the regioisomerism impact on crystal structure, and therefore optical properties, we prepared each of these four conjugated backbones (2BT, 5BT, 6BT, and 2TT) with one of three benzyl esters on the terephthalate: 2,3,4,5,6-pentafluorobenzyl (F5), 2,4,6-trifluorobenzyl (F3), and unfluorinated benzyl (H5). Our previous work indicated that reducing the number of fluorine atoms can tune the likelihood of observing ArF–ArH interactions,⁵⁰ which we have ascribed to a combination of reduced electrostatic complementarity in the cofacial interaction and alternative interactions of the more electropositive C–H bonds in the F3 ring.

These 12 target molecules were prepared using a straightforward strategy combining acylation of the appropriate benzyl alcohol with 2,5-dibromoterephthaic acid to yield the central unit, followed by combinations of Sonogashira and deprotection reactions to build the final targets. The precise nature of the Sonogashira reactions – which unit bore the terminal alkyne and which the halide – varied across the 12 molecules. Generally, the syntheses proceeded smoothly, with the exception of 5BT-F5, the extreme insolubility of which frustrated our efforts to prepare, isolate and purify it. We attribute this in part to the strength of the ArF–ArH cofacial interactions (vide infra). By using 5-iodobenzothiophene together with 2,5-diethynylterephthalic acid, we were able to isolate a small amount of 5BT-F5 as a colorless solid; this lack of color was unusual, but important (vide infra), as all the other compounds isolated in this study were yellow in color, even after multiple recrystallization steps.

Optical spectroscopy

The UV/vis absorbance and fluorescence properties of all 12 molecules were recorded in dichloromethane and are summarized in Table 1. All compounds showed absorbance spectra in the expected regions of the electromagnetic spectrum – the near-UV and violet. The absorbance spectra of the F5, F3, and H5 derivatives of the same conjugated backbone were nearly identical, differing only by 1–7 nm in λ_max of the lowest energy band (Fig. 1a). While modest, a trend in UV/vis spectral position of the four conjugated backbones is consistent, with 5BT (λ_max = 383–386 nm) having the highest energy absorbance spectra, followed by 6BT (377–378 nm), 2BT (391–398 nm) and finally 2TT (413–416 nm); Fig. 1b shows height-normalized spectra for the four F5 derivatives. Together, these four classes span 0.3 eV in λ_max values.

Table 1 Summary of the optical properties of the molecules from Chart 1

	Solution (CH2Cl2)				Solid
	λmax,abs (nm)	λmax,emis (nm)	ΦF	τ (ns)	λmax,emis (nm)	Δλmax,emis (nm)
2TT-H5	416	480	0.32		551	71
2TT-F3	413	483	0.18	0.8	570	87
2TT-F5	416	491	0.18	0.9	567	76
2BT-H5^1	391	443	nd	nd	529	86
2BT-F3	397	457	0.44	1.0	575	118
2BT-F5^1	398	465	0.48	1.1	460	−5
5BT-H5	377	451	0.26	2.0	505	54
5BT-F3	377	442	0.52	2.1	477	35
5BT-F5	378	449	0.21	0.6	422	−27
6BT-H5	383	449	0.37	1.3	504	55
6BT-F3	384	451	0.31	1.3	506	55
6BT-F5	386	455	0.28	1.5	485	30

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Chart 1 Structures of 12 molecules combining four conjugated heterocycles and three benzyl esters described in this study.

Fig. 1 (a) Absorbance spectra for 6BT-F5, F3, and H5. (b) Absorbance spectra for the F5 analogs of each conjugated arene.

Preliminarily, with the three BT regioisomers, we can assess the extent of electron delocalization through the UV/vis spectra, with 5BT being the least delocalized, and 2BT being the most. This correlates to the number of bonds between the more π-donating, polarizable sulfur atom and the electron-withdrawing alkyne unit, suggesting a stronger donor–acceptor effect. The longer-wavelength absorbance spectra of the TT compounds, with each TT unit having two sulfur atoms two or four bonds away from the ethynylterephthalate, suggests the most donor–acceptor character of this chromophore.

All 12 compounds fluoresce in solution, with relative quantum yields of fluorescence between 0.18 and 0.52. Fluorescence lifetimes measured by time-correlated single photon counting range from 0.8–2.1 ns. Trends in the fluorescence spectra of these molecules in solution follow the same pattern as the UV/vis spectra, with 5BT emitting the highest energy photons, and 2TT emitting the lowest, spanning wavelengths of maximum intensity from 442 nm–491 nm. Between compounds with the same conjugated backbone, differences in calculated radiative and non-radiative rates of relaxation are modest (no more than 2× difference), with the exception of 5BT-F5, which shows a non-radiative rate of relaxation of 1.3 × 10⁹ s⁻¹, 4–5× faster than 5BT-F3 and 5BT-H5. Preliminarily, we attribute this to the same strong ArF–ArH cofacial interactions that impede solubility in 5BT-F5.

To examine the solution-based optical absorption properties in more detail, density functional theory (DFT) calculations and time-dependent DFT (TDDFT) calculations were carried out at the ωB97XD/6-31G(d′) level of theory. The DFT calculations were performed to optimize the ground states (S₀) of all molecules in two conformations: one wherein there are intramolecular noncovalent interactions between the TT/BT units and the benzyl moiety (closed) and one wherein the benzyl moiety does not interact with the TT/BT units (open, vide infra). We note that in all cases the closed conformation is the most energetically stable (see SI).

The DFT-optimized S₀ geometries were then used for time-dependent DFT (TDDFT) calculations at the ωB97XD/6-31G(d′) level of theory to evaluate the natures of the excited state transitions. The S₀ → S₁ vertical transitions, which are predominantly HOMO → LUMO transitions localized on the AE backbone, are quite similar whether the system is in the closed or open conformations. We note that the S₀ → S₁ vertical transitions for the open conformations are 0.2–0.5 eV red-shifted with respect to the closed conformations. The TDDFT results demonstrate similar trends with experiment, wherein 5BT and 6BT have the largest transition energies, followed by 2BT and then 2TT. The open conformations present S₀ → S₁ vertical transition energies that are closest to experiment, suggesting that the open conformation may be preferred in solution.

Solid-state luminescence

To improve understanding of how chemical structure impacts the solid-state properties of these materials, we collected their luminescence spectra as thin films drop cast from chloroform. The films were heated to 100 °C for 10 minutes after casting to mitigate impacts of deposition conditions. In the analysis below, we compare emission spectra of these samples with the same molecules in dilute solution to highlight the specific impacts of solid-state packing. Several key trends emerge from these analyses.

All “H5” showed large bathochromic shifts in the solid emission spectra relative to those in solution. Shifts in the emission spectra ranged from 47–71 nm, seen in Fig. 2, which we attribute to aggregation and/or planarization of the AE chromophore as a solid. Our prior work on this general class of compounds has revealed a strong correlation between an aggregated and/or planarized packing arrangement and these spectral properties. The lack of fluorinated rings precludes favorable ArF–ArH cofacial interactions, which might otherwise mitigate interchromophore aggregation.

Fig. 2 Bathochromic shifts of H5 derivatives from solution to solid state.

In contrast, benzothiophene-F5 compounds showed hypsochromic or modest bathochromic shifts in their solid emission spectra relative to those in solution. Emission spectra of solids of 2BT-F5, and especially 5BT-F5 are hypsochromically shifted from their emission spectra in dilute solution, (Fig. 3) such that 5BT-F5 is a colorless solid that emits at 422 nm, which is blue shifted from all the solution phase spectra reported here. Thin films of 6BT-F5 are modestly bathochromically shifted from solution, suggesting a lack of strong J-aggregation compared to what is seen in 6BT-H5 and 6BT-F3 (Fig. 4).

Fig. 3 (a) Hypsochromic shifts of emission spectra of 5BT-F5 and 2BT-F5 from solution to solid state. (b) Bathochromic shifts of emission spectra of 6BT-F5 and TT-F5 from solution to solid state.

Fig. 4 Emission spectra of the F3 molecules displaying a bathochromic shift of the solid state from the solution state.

Curiously, and different from all the benzothiophene derivatives, emission of thin films of TT-F5 show a bathochromic shift from solution phase spectra by 76 nm suggesting that these chromophores do aggregate, and likely do not participate in sufficiently substantial ArF–ArH stacking. We initially found this result surprising, given its strong departure from 2BT-F5 and even the monocyclic thiophene in that position, which we reported previously.⁴⁸ Finally, thin films of all four F3 derivatives also showed large bathochromic shifts in their emission relative to solution state (Fig. 5), similar to the H5 derivatives, although the magnitude of shift for 5BT-F3 is slightly suppressed compared to the other three F3 analogs.

Fig. 5 Four crystal structures of molecules in the 5BT series showing a range of intramolecular torsional angles, depending on the range of side chain fluorinations. Thermal ellipsoids shown at 50% probability; hydrogen atoms removed for clarity.

Polymorphism and mechanofluorochromism

Since many of these solids have vibrant colors and show strong solid-state luminescence in the visible region of the spectrum, it can be relatively straightforward to observe polymorphs. In addition, mechanofluorochromism (MFC) is a common feature of this overall class of molecules, which also aids polymorph identification, and assists in understanding how different competing noncovalent interactions can derive formation of certain structures. While it is unlikely to be an exhaustive list, we have noted polymorph behavior without requiring mechanical grinding for four of the twelve molecules in this work (Table 2).

Table 2 Polymorphs observed of four compounds and summary of mechanofluorochromism (MFC) results

	Emission color	λmax,emis (nm)	MFC result
2TT-H5	Green	551	No
	Yellow	571	Yes, green
2BT-F3	Green	501	No
	Yellow	588	Yes, green
5BT-F3	Blue	456	Yes, green
	Green	490	No
5BT-H5	Blue	495	Yes, green
	Green	516	No

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As the table indicates, many of these solids also display MFC, and the grinding-induced change of color can be either bathochromic or hypsochromic, depending on the molecule. In all examples here, however, MFC always favors more green emission, and moves the spectrum away from either blue or yellow/orange (see Fig. S29). We attribute this observation to the fact that the conjugated backbones of these three molecules are all similar, with modest differences in spectra when in dilute solution. Mechanical grinding of this class of solids disrupts crystalline order, yielding amorphous solids. As described below, blue-shifted solids result from a crystalline order that requires ArF–ArH stacking and shielding of chromophores from aggregation, while strongly red-shifted solids result from J-aggregate arrangements of slip-stacked chromophores. In both cases, we conclude that grinding disrupts this crystalline order (either protection from aggregation or J-aggregation), yielding disordered solids with similar intermediate bandgaps, much like their similar solution-state emission spectra.

Single crystal X-ray diffraction

As part of this work we solved and refined six new single crystal X-ray structures. In general, the molecular structures found in the crystals conform to our expectations, as described above based on their optical spectra. Fig. 5 shows four differing packing structures for the 5BT backbone, depending on the side chains involved, while Fig. 6 shows two other structures.

Fig. 6 Single-crystal X-ray structures for coplanar AEs 2BT-F3 and 2TT-H5. Thermal ellipsoids shown at 50% probability; hydrogen atoms removed for clarity.

5BT-H5. Like most other “H5” derivatives of these types of chromophores, 5BT-H5 displays highly coplanar conjugated backbones with no more than 2° torsional angles. No substantial intramolecular noncovalent interactions are observable. The AE chromophores adopt slipped stacks with 3.4–3.5 Å interatom distances, and pitch and roll of 6 Å and 1.2 Å, respectively. Each phenyl side chain rings interacts in an edge-face manner on both faces, on one side with another side chain (2.8 Å H⋯C distance), and on the other side with a hydrogen on the 6-position of the benzothiophene (2.7 Å H⋯C distance). As a result of the interactions of the phenyl pendants, the slipped stacks of AE chromophores are not all along the same axes.

5BT-F3. We were able to solve structures for both blue- and green-emissive polymorphs of 5BT-F3. The blue-emitting polymorph shows ∼40° twists between central and flanking rings along the AE backbone, with intramolecular interactions between the fluorinated and BT rings, that are cofacial but skewed, ∼3.4 Å distances between 4 carbon of the F3 ring and carbons of the BT ring. Although there are also some intermolecular ArF-BT interactions (3.2 Å between F on F3 ring and fused BT carbon on another chromophore), obvious cofacial interactions between the chromophores (3.38 Å distance between BT carbons and carbons on terephthalate and alkyne) show evidence of aggregation.

The green-emitting polymorph of 5BT-F3 shows no obvious cofacial interactions of the fluorinated rings and clear signs of aggregation between highly coplanar AE chromophores. Approximately 3.5 Å separates some carbon atoms on parallel chromophores, and only 3.25 Å exists between the planes defined by the terephthalate rings on adjacent chromophores. These chromophores adopt slipped stacks, with a roll of 2.5 Å and pitch of 3.7 Å.

5BT-F5-Br. Although the poor solubility of 5BT-F5 precluded us from growing X-ray quality single crystals, we were able to determine the structure of the “half” substituted arylbromide derivative 5BT-F5-Br. This structure, seen in Fig. 5, shows the characteristic twisting (∼70° torsional angles) of the AE backbone and obvious intramolecular and intermolecular ArF–ArH cofacial interactions (close contacts of 3.3–3.4 Å), with the ArF ring bending towards the conjugated benzothiophene on the same molecule. The bromide appears to participate in a weak bifurcated halogen bonding arrangement with an alkoxy oxygen and fluorine atom. This overall arrangement, together with the insolubility of 5BT-F5 and that only 5BT-F3 shows any evidence of ArF–ArH interactions of the F3 derivatives suggests that the 5BT regiochemical linkage has the most favorable interaction with the ArF rings of the ArH motifs described in this paper.

Fig. 7 Comparisons of calculated energies of individual molecules containing the four different heteroaromatic backbones, including: (i) the difference in energies between “closed” and “open” isomers, [E(closed) − E(open)], (ii) the difference in [E(closed) − E(open)] comparing the F5 and H5 analogs in each backbone; in each case the F5 analog shows a larger preference for closed than the H5 analog, and (iii) the electrostatic component of the F5-heterocycle stacking interaction of unbound subunits of the molecules (ethynyl-substituted heterocycle and substituted benzyl ester) that were geometry-optimized.

2BT-F3. We previously reported a structure of 2BT-F5;⁴⁸ it is twisted showing both intermolecular and intramolecular ArF–ArH interactions preventing cofacial interactions of the AE chromophores. In contrast, our new structure of 2BT-F3 (see Fig. 6), which is yellow-orange in color, shows a coplanar AE backbone (torsional angles less than 5°) and a lack of ArF–ArH cofacial interactions. The AE chromophores are aggregated in two types of slipped co-facial arrangements and C–C contacts as short as 3.35 Å. A notable intermolecular C–H/π interaction exists between an electropositive C–H on the F3 ring and a nearby aggregated chromophore.

2TT-H5. Like 5BT-H5, 2TT-H5 shows highly coplanar AE chromophores (∼4° torsions) with no notable intramolecular noncovalent interactions (Fig. 6). The AE chromophores are arranged in slipped stacks (5.5 Å pitch, 1 Å roll), with 3.3–3.4 Å C–C distances as close contact examples. The edge of phenyl side chains participate in edge-face interactions with the interior thiophene of the TT bicycle (2.9 Å C–S distance).

Intramolecular noncovalent interactions

To understand the molecular mechanisms by which these similar molecules choose a more twisted and “closed” conformation or a planar and “open” conformation, we turned to deeper analyses of the DFT optimized geometries (Fig. 7 and Tables S7–S10). Note that all closed and open conformations present no negative frequencies from normal mode analyses, suggesting that both conformations are minima on the potential energy surface. As noted above, the closed conformations are uniformly more stable than open conformations due to the additional dispersion and electrostatic interactions that close contacts bring. These calculations do not account for discrete interactions with solvent molecules that occur in solution, which would compete with the intramolecular interactions stabilizing the closed conformer. Moreover, for all four backbones, the F5 derivatives had the largest ΔE between closed and open conformations (Fig. 8, ranging from 10 kcal mol⁻¹ for 2TT to 15 kcal mol⁻¹ for 5BT), suggesting the importance of electrostatic complementarity in these interactions. These results agree with our experimental observations, where in ranking the propensity for closed conformation in the perfluorinated derivatives is in the order 5BT > 2BT > 6BT ≫ 2TT based on the combination of crystallographic and fluorescence results, and suggest that the position of the sulfur atoms on the fused rings play a role in the preferred conformations.

Fig. 8 Geometries used in DFT computations. (a) Depiction of open geometry, no twist in backbone, (b) depiction of closed geometry where benzothiophene rings are twisted.

We used SAPT0/jun-cc-pVDZ (referred to as SAPT0) calculations to investigate the nature and strength of the intramolecular noncovalent interactions that may be responsible for these conformational preferences (Table S11). To focus on the interactions between aromatic rings, we examined only the interactions between unbound subunits of the molecules (ethynyl-substituted heterocycle and substituted benzyl ester) that were geometry-optimized. Across the entire series, the total interaction energies fall across a small range, from −2.0 kcal mol⁻¹ (attractive forces dominate) to +0.5 kcal mol⁻¹ (repulsive forces dominate). Generally, those molecules with F3 or F5 pendants present stronger attractive binding energies when compared to H5. While trends within the decomposed contributions of the SAPT0 analysis are subtle, we note: (i) the majority of the attractive energies in all cases are due to dispersion forces, and ii) the trend in the electrostatic components of the energies match the trend described above: 5BT > 2BT > 6BT > 2TT (Fig. 8), again suggesting the electrostatic forces play an important role in selecting conformation in the crystal structure. We note that the molecular dipole moment of thienothiophene is zero, which may contribute to the smaller electrostatic contribution to noncovalent interactions.

Additionally, we carried out SAPT0 calculations on idealized systems where the stacking configurations were the same to account for structural variations in the optimized molecules. In these idealized structures, the distance between the BT/TT group and the aryl group was set 3.5 Å, the relative stacking configurations were the same, and the benzyl ester group was replaced with a benzyl hydroxyl group. In idealized cofacial systems, those with 5BT present the smallest electrostatic interactions, suggesting that geometric relaxation from this idealized structure to that found in the optimized structure is crucial for maximizing the electrostatic interactions of the optimized state (Tables S12 and S13). We also examined idealized T-shape structures, keeping the same 3.5 Å interring distance, and looking at two configurations of the BT with respect to the aryl group (Tables S14 and S15). When the S atom on the BT is closest to the aryl moiety, the 5BT molecules exhibit the largest attractive electrostatic interactions when compared to other systems, maintaining the 5BT > 2BT > 6BT > 2TT trend previously noted. When the S atom of the BT is pointed away from the aryl moiety, 6BT systems exhibit strong electrostatic attractions, while those with 5BT are smallest (or even most repulsive). These idealized structures reveal the importance of the stacking configurations found in the closed conformations on optimizing the noncovalent intramolecular interactions.

Conclusions

The optical properties of these twelve π-conjugated molecular solids deviate strongly from their behaviour in dilute solution, with impacts from both the identity and regiochemistry of the thiophene-based fused heterocycles, and the degree of fluorination of the benzyl ester side chains. The trends regarding electrostatic complementarity of the side chains align with our previous work, with heavily fluorinated side chains more likely to stack cofacially with the conjugated heterocycle, yielding hypsochromically shifted spectra of solids compared to their solution-state spectra. Among the four backbones, benzothiophene connected in the 5-position showed the most pronounced interactions, based on both the ∼30 nm hypsochromic shift of solid-state luminescence compared to solution, and the insolubility of 5BT-F5. In contrast, the thienothiophene 2TT-F5 showed no evidence of intramolecular cofacial interactions, even while it showed the most donor–acceptor character in solution-phase optical spectra. DFT calculations rationalize these discrepancies, but only using crystallographically-informed geometries, instead of in idealized parallel cofacial stacks.

More broadly, these results advance rational design of noncovalent interactions of the ubiquitous thiophene-based fused heterocycles used in organic electronic materials. We find that differences in connectivity can influence in the solid-state optical properties in ways not apparent from experiments in dilute solution. Moreover, while models such as the polar-π framework are valuable for designing cofacial interactions broadly, our results emphasize that local structural details ultimately dictate solid-state arrangement. This is especially true when comparing subtle changes to electrostatic landscapes, given that dispersion interactions comprise the largest fraction of attractive energy in the crystallographically realistic geometries calculated here. Finally, the MFC behavior of these molecules favors solution-like optical behavior upon grinding based on either disruption of J-aggregates (blue shifting) or twisted structures that lack aggregation (red shifting). These self-consistent trends inform rational design of force responsive fluorophores with predictable behavior.

Conflicts of interest

There are no conflicts to declare.

Data availability

Crystallographic data have been deposited at the CCDC with deposition numbers 2536219–2536224. Other data supporting this article (NMR spectra, optical spectra, and differential scanning calorimetry data) is available in the Supplementary Information (SI). Experimental procedures, NMR spectra, optical spectra, and crystallography information. See DOI: https://doi.org/10.1039/d6ma00553e.

CCDC 2536219–2536224 contain the supplementary crystallographic data for this paper.^61a–f

Acknowledgements

Experimental work was supported by the National Science Foundation (NSF) under award DMR 2420040 and through the U.S. Department of Energy (DOE), Basic Energy Sciences (BES) through award DE-SC0016423. E.G. was supported by an NSF Graduate Research Fellowship. The work at the University of Kentucky (UK) was supported in part by DOE BES through award number DE-SC0016423 and the NSF through award DMR 2323422. S.R. and C.R. also acknowledge the UK Center for Computational Sciences and Information Technology Services Research Computing for their support and collaboration, and the use of Lipscomb Compute Cluster and associated research computing resources.

Notes and references

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