Design of folded π molecules. Owing to their specific conformation, folded-π molecules are less prone to form strong face-to-face π-π interactions as compared with planar π system. Figure 1 shows the energy surface of two π molecules versus intermolecular distance. When the folded angle (Θ) is less than 120-130 degree, it is less likely for two π systems to get close enough to form π-π interaction, which generally demands the interplane distance to be less than 3.8 angstroms. The relatively large intermolecular distance for folded π structures could be due to the steric effect of hydrogens around the π planes. Instead of forming π-π interaction, the π planes of folded π will form abundant weak π interactions (e.g. CH…π). On the other hand, folded π is more prone to form tight packing in comparison with twisted π structure. Owing to the building block-liked shape, folded π structure will form particular packing patterns including “box”, “braid” and “stairs” (Figure 2). All these patterns are tight packing modes with high packing level and the intramolecular motions will be well locked by abundant weak π interactions. In contrast, the packing of twisted-π structures is unpredictable and the chance to form high-level packing is not high.
The key to designing a folded-π structure with Θ < 130 is the conformation of the bridging atom of the π planes (Figure 2b, X, e.g. C, Si, N, P). For example, in an aryl-aryl system, sp3 and sp2 hybridization of the bridging atom leads to two different conformations of the π system (Figure 2b). In this case, sp3 hybridization of the bridging atom is the key of folding conformation. To form a stable sp3 X in such a π system, the lone pair of X atom should have a high tendency to be conjugated with the adjacent π plane, such as an electron-poor π with strong electron-withdrawing groups (A, e.g. cyano group). The stronger the electron-withdrawing group, the higher tendency for the X atom to be sp3 hybridized (Figure 2b).
Achieving high exciton utilization with 2π molecules. According to the “building block” formula, we first designed and synthesized compounds 1-8 with two folded π planes (2π). The synthesis and structural characterization of 1-8 are described in the supporting information. All eight compounds were obtained with good purity and their single crystals were obtained by slow evaporation from proper solvents (e.g. chloroform, acetone), after which single-crystal X-ray diffraction studies were conducted. From the single-crystal structures, the 2π folded-π structures are found to pack in particular patterns (“box”, “braid” and “stair”, Figure 2). For “box” pattern, the primary intermolecular interaction is between the side and face of two π planes, which specifically are CH∙∙∙π and n∙∙∙π for 1-4 (Figure 3a). The unique double side-to-face interactions of π planes in a symmetric way lead to abundance of π effects with a total interaction number of 20, 26, 18 and 23 per molecule for 1-4, respectively (Figure S35). The total number of interactions per molecule for 1-4 is much more than those representative solid-state π systems such as 6 for tetraphenylethene, and 4 each for tetraphenylpyran or distyreneanthrancene or triphenylamine (Supporting information, Figure S39). Though without significant π effect, the numerous weak π interactions lead to high packing energy of -318.5, -288.3, -257.8 and -273.7 kJ/mol per molecule for 1-4, respectively. We also calculated the packing energy of a series of classic twisted-π systems (Table S1) using the same method. It was found that the packing energy of 1-4 is much higher than those representative twisted-π systems.
Next, we investigated the exciton utilization of the “box” by measuring their photoluminescence quantum yields (PLQY) and single oxygen quantum yields (SOQY). As photoluminescence and singlet oxygen generation are competing pathways, their sum reflects the experimental utilization of excitons. As expected from the effective packings, 1-4 show high exciton utilization with total quantum yields of 0.72, 0.99, 0.89 and 0.83, respectively (Figure 3c).
For “braid“ and “stair“ patterns, the major intermolecular interaction is between side and face (one-way) as well, and sometimes involves weak face-to-face contact of the π planes. Owing to the steric effect of the numerous hydrogens around the side of the π planes, 2π molecule is less likely to form close face-to-face π interaction in “braid“ and “stair“ patterns. Similar to “box“, the unique “braid“ and “stair“ patterns lead to abundant π effects with a total interaction number of 15, 17, 19 and 18 per molecule for 5-8, respectively (Figure S36). The numerous weak π interactions account for high total packing energy of -212.2, -207.2, -217.5 and -218.9 kJ/mol per molecule for 5-8, respectively. We then investigated the exciton utilization of 5-8 by measuring their PLQYand SOQY. As expected from the effective packings, 5-8 show high exciton utilization with add-up quantum yields of 0.93, 0.77, 0.92 and 0.71, respectively (Figure 4c).
Achieving high exciton utilization with 3π molecules. The “folded π” formula was also utilized for the design and synthesis of “3π” folded-π structures (9-10). Figure 5a shows the crystalline structures of compounds 9-10. They have similar “3π-trans” conformation. From the single-crystal structures, we found “3π-trans” folded-π molecules tend to pack in “stair” pattern (Figure 5a), which is the only packing pattern for such a “building block”. Similar to 2π molecules, the hydrogens around π planes effectively prevent strong face-to-face π effect. Meanwhile, the unique 3π-trans conformation allows molecules to pack tightly with abundant π interactions. The primary interactions for 9-10 are CH∙∙∙π, n∙∙∙π and CH∙∙∙O, which contribute to total interaction numbers of 20 and 30 per molecule for 9 and 10, respectively (Figure S37). As a result of high packing level, the packing energy of 9 and 10 are -283.9 and -288.6 kJ/mol per molecule, respectively (Figure 5c). We also measured the PLQY and SOQY for compounds 9 and 10 and as expected, they both show high total quantum yields of 0.99 and 0.93, respectively. Figure 5a shows the crystalline structures of compounds 11 and 12, which possess similar “3π-cis” conformation. It is found that both 11 and 12 pack in unique double “box” pattern (box-db)—one molecule forms double side-to-face with two other 3π-cis molecules, leading to artful molecular necklaces. The unique packing modes of 11 and 12 also lead to abundant π effects with a total interaction numbers of 20 and 22 per molecule (Figure S38) and high packing energy of 349.0 and -277.8 kJ/mol for 11 and 12, respectively. As a result of effective packing, the total quantum yield for 11 and 12 reaches 0.74 and 0.78, respectively.