Abstract
The synthesis of two series of N-annulated perylene bisimides (PBIs), compounds 1 and 2, is reported, and their self-assembling features are thoroughly investigated by a complete set of spectroscopic measurements and theoretical calculations. The study corroborates the enormous influence that the distance between the PBI core and the peripheral groups exerts on the chiroptical properties and the supramolecular polymerization mechanism. Compounds 1, with the peripheral groups separated from the central PBI core by two methylenes and an ester group, form J-type supramolecular polymers in a cooperative manner but exhibit negligible chiroptical properties. The lack of clear helicity, due to the staircase arrangement of the self-assembling units in the aggregate, justifies these features. In contrast, attaching the peripheral groups directly to the N-annulated PBI core drastically changes the self-assembling properties of compounds 2, which form H-type aggregates following an isodesmic mechanism. These H-type aggregates show a strong aggregation-caused quenching (ACQ) effect that leads to nonemissive aggregates. Chiral (S)-2 and (R)-2 experience an efficient transfer of asymmetry to afford P- and M-type aggregates, respectively, although no amplification of asymmetry is achieved in majority rules or “sergeants-and-soldiers” experiments. A solvent-controlled stereomutation is observed for chiral (S)-2 and (R)-2, which form helical supramolecular polymers of different handedness depending on the solvent (methylcyclohexane or toluene). The stereomutation is accounted for by considering the two possible conformations of the terminal phenyl groups, eclipsed or staggered, which lead to linear or helical self-assemblies, respectively, with different relative stabilities depending on the solvent.
Highlights
The molecular recognition by complementary triple hydrogen bonding between diacylaminopyridines and uracil derivatives reported by Lehn and co-workers paved the way to boost the discipline of supramolecular polymerization (SP).1 This seminal work, supported by the discovery of J-type aggregates made by Scheibe and Jelley2 and the development by Klug and co-workers of the transmission electron microscopy,3 prompted an extraordinary progress in the field of supramolecular polymers.4 A key issue in the further development of functional supramolecular polymers was the establishment of mathematical models allowing for an accurate description of the SP process.5 Two main mechanisms have been amply described to govern the supramolecular polymerization of discrete selfassembling units
Variable-temperature (VT) UV−vis experiments in methylcyclohexane (MCH) as solvent proved that the supramolecular polymerization mechanism of 1 occurs in a cooperative manner under thermodynamic control, since no thermal hysteresis is observed in the cooling and heating curves
The synthesis of two series of N-annulated perylene bisimides (PBIs) endowed with peripheral trialkoxyphenyl groups directly attached to the imide nitrogens of the PBI core (2) or linked by a propionate spacer (1) is reported
Summary
The molecular recognition by complementary triple hydrogen bonding between diacylaminopyridines and uracil derivatives reported by Lehn and co-workers paved the way to boost the discipline of supramolecular polymerization (SP). This seminal work, supported by the discovery of J-type aggregates made by Scheibe and Jelley and the development by Klug and co-workers of the transmission electron microscopy, prompted an extraordinary progress in the field of supramolecular polymers. A key issue in the further development of functional supramolecular polymers was the establishment of mathematical models allowing for an accurate description of the SP process. Two main mechanisms have been amply described to govern the supramolecular polymerization of discrete selfassembling units. In contrast to 1, the transfer of asymmetry from the molecular to the supramolecular level in N-annulated PBIs (S)-2 and (R)-2 yields a rich dichroic pattern, with an intense bisignated Cotton effect with maxima at 490 and 547 nm and zero crossing points at 508, 344, 278, and 261 nm and with the anisotropy factor g ≈ 3 × 10−3 (Figure 7a) This bisignated Cotton effect implies the formation of P- and M-type helical aggregates for (S)-2 and (R)-2, respectively.. Similar findings were inferred from the corresponding SaS experiments performed by mixing 2 and (S)-2, for which a linear increase of the dichroic response upon increasing the amount of the chiral sergeant was observed (Figure S21) These studies reveal the ability of N-PBIs 2, which form supramolecular polymers governed by an isodesmic mechanism, to undergo an efficient transfer of asymmetry but a negligible amplification of asymmetry. In n-hexane (modeling MCH), the most stable aggregate is predicted to be 2B5, which corresponds to a well-defined helical arrangement with a rotational dihedral angle θ of 35° and shows an intense dichroic pattern (Figure S25) similar to that found experimentally for (S)-2 and (R)-2 in MCH (Figure 7c,d)
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