Substituent Effects That Control Conjugated Oligomer Conformation through Non-covalent Interactions

Abstract

Although understanding the conformations and arrangements of conjugated materials as solids is key to their prospective applications, predictive power over these structural factors remains elusive. In this work, substituent effects tune non-covalent interactions between side-chain fluorinated benzyl esters and main-chain terminal arenes, in turn controlling the conformations and interchromophore aggregation of three-ring phenylene-ethynylenes (PEs). Cofacial fluoroarene-arene (ArF-ArH) interactions cause twisting in the PE backbone, interrupting intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that determine whether the ArF-ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i) the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF-ArH interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF-ArH interactions over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar-π or substituent-substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and therefore the optoelectronic properties, of these molecules as solids.

Small Changes With Big Consequences: Swapping Two Atoms In Side Chains Changes Phenylene‐Ethynylene Packing And Fluorescence

“…This striking contrast in solid‐state spectroscopy correlates with differences in crystal structures of these two isomers. As we reported previously, the structure of 2 , 4 , 6‐OMe features nearly‐perpendicular twists along the PE backbones due to intramolecular ArF‐ArH interactions (3.9–4.0 Å centroid–centroid distances) . Such cofacial interactions also occur intermolecularly, yielding infinite columns of stacked ArF and ArH rings, as well as twisted PEs lacking aggregation between the PE chromophores.…”

“…As we reportedp reviously,t he structure of 2,4,6-OMe features nearly-perpendiculart wists along the PE backbones due to intramolecular ArF-ArH interactions (3.9-4.0 centroid-centroid distances). [19] Such cofacial interactions also occur intermolecularly,y ielding infinite columnso fs tacked ArF and ArHr ings, as well as twisted PEs lacking aggregation between the PE chromophores. Close contacts between carbonyl groups and protons on both central arenes and the meta protons of the ArF rings in adjacent PEs (2.4 O···H distances) also occur.I nc ontrast, and consistentw ith optical data( Figure S1), 2,3,6-OMe has nearly planar PE backbones (torsional angles of 17 and 218)f eaturing intermolecular stacking along PE long axes with a % 1.2 roll and parallel columns of stackedP Es separated by 3.3 ( Figure 2).…”

“…[16,17] Our previous work has shown that cofacialfluoroarene-arene (ArF-ArH) interactions of perfluorinated side chains can dictate the conformations and assemblies of phenylene-ethynylenes( PEs), and therefore determine their solid-state opticalp roperties. [18][19][20] Although examples exist for which regiochemistry of arene fluorination alters crystalp acking with modest effects on material properties, [21][22][23][24] slightd ifferences in fluorine regiochemistry of non-conjugated side chains would not generally be expected to impact the opticalp roperties of conjugated materials. Here, we describe surprisingly different opticala nd mechanofluorochromic (MFC) properties of solid PEs that depend on slight positional differences of Ha nd Fa toms (2,4,6v s. 2,3,6-trifluoro) on pendant arenes not conjugated to the PE moiety.…”

“…The molecules described here are variations on our previous work regarding non-covalentc ontrolo fo ptical properties of solid PEs:e lectron-poort rifluorobenzyl ester side chains can participate in cofacial ArF-ArH interactions with terminal areneso ft he PE. [18][19][20] These interactions cause the terminal PE arenest ot wist relative to the centralt erephthalate, preventing the typicallyo bservedb athochromic shifts of PEs when comparings olid to solution [25] by inhibiting both intramolecular and intermolecular electronic coupling. The favorability of the ArF-ArH interaction, and whether it is observed, depends on Hammett-types ubstituent effects in the terminal arenes.…”

“…To begin understanding how the regiochemistry of fluorination impactst he packing and optical properties of these solids, we prepared bis(2,4,6-triflurobenzyl)a nd bis(2,3,6-trifluorobenzyl) regioisomers [19] 2,4,6-OMe and 2,3,6-OMe.A se xpected, the opticalp roperties of these molecules in solution do not depend on the substitution pattern on the ArF rings, which are not conjugated to the PE chromophores:b lue-emitting dilute chloroform solutions of 2,4,6-OMe and 2,3,6-OMe show nearly identicala bsorbance and emission spectra (Figure 1), as well as similar fluorescenceq uantum yields and lifetimes (Table S1,S upporting Information). Their opticalp roperties as thin films, however,c ontrasts harply.U pon drop-casting and thermala nnealing at 150 8Ct oe liminate residual solvent and drive packingt owards lowest energy arrangements, films of 2,4,6-OMe are colorless with blue fluorescence( Abs.…”