Dipoles are widely involved in π−π interactions and are central to many chemical and biological functions, but their influence on the strength of π−π interactions remains unclear. Here, we report a study of π−π interaction between azulene-based, polar single molecules and between naphthalene-based, nonpolar single molecules. By performing scanning tunneling microscopy break junction measurements of single-molecule conductance, we show that the π-stacked dimers formed by the azulenebased, polar aromatic structures feature higher electrical conductivity and mechanical stability than those formed by the naphthalene-based, nonpolar molecules. Mechanical control of π−π interactions in both rotational and translational motion reveals a sensitive dependence of the stacking strength on relative alignment between the dipoles. The antiparallel alignment of the dipoles was found to be the optimal stacking configuration that underpins the observed enhancement of π−π stacking between azulene-based single molecules. Density functional theory calculations further explained the observed enhancement of stacking strength and the corresponding charge transport efficiency. Our experimental and theoretical results show that the antiparallel alignment of the dipole moments significantly enhances the electronic coupling and mechanical stability of π−π stacking. In addition, in the formation of single-molecule junctions, the azulene group was experimentally and theoretically proved to form a Au−π contact with electrodes with high charge transport efficiency. This paper provides evidence and interpretation of the role of dipoles in π−π interactions at the single-molecule level and offers new insights into potential applications in supramolecular devices.

Ca2+-triggered exocytosis is a crucial aspect of neuronal and neuroendocrine cell function, yet many of the underlying molecular mechanisms that regulate these processes are unknown. Here, we contrast the biophysical properties of two prominent neuronal Ca2+ sensors, synaptotagmin (syt) 1 and syt7. In both proteins, four Ca2+-binding loops partially penetrate bilayers that harbor anionic phospholipids, and mutagenesis studies suggest that these interactions are important for function. However, these mutations also alter the interaction of syts with the SNARE proteins that directly catalyze membrane fusion. To directly assess the role of syt membrane penetration, we took a different approach and found that Ca2+-syt1-membrane interactions are strongly influenced by membrane order; tight lateral packing of phosphatidylserine abrogates syt1 binding to lipid bilayers due to impaired membrane penetration. Function could be restored by making the membrane penetration loops more hydrophobic, or by inclusion of cholesterol. In sharp contrast, syt7 unexpectedly exhibited robust membrane binding and penetration activity, regardless of the lipid acyl chain structure. Thus, syt7 is a ′super-penetrator′. We exploited these observations to specifically isolate and examine the role of membrane penetration in syt function. By altering bilayer composition, rather than protein structure, we disentangled the roles of syt-membrane versus syt-SNARE interactions. Using nanodisc-black lipid membrane electrophysiology, we demonstrate that membrane penetration underlies the ability of syts to directly regulate reconstituted, exocytic fusion pores in response to Ca2+.