Recently, expanded porphyrins have come to the forefront in the research field of aromaticity, and been recognized as the most appropriate molecular system to study both Hückel and Möbius aromaticity because their molecular topologies can be easily changed and controlled by various methods. Along with this advantage, many efforts have been devoted to the exploration of the aromaticity-molecular topology relationship based on electronic structures in expanded porphyrins so that further insight into the aromaticity--a very attractive field for chemists--can be provided. In this tutorial review, we describe the recent developments of various topology-controlled expanded porphyrins and their photophysical properties, in conjunction with the topological transformation between Hückel and Möbius aromaticity by various conformational control methods, such as synthetic methods, temperature control, and protonation.

Protonation-triggered conformational changes of meso-octakis(pentafluorophenyl) [36]octaphyrin and [38]octaphyrin have been investigated. The X-ray crystal structures and (1)H NMR analyses revealed that the protonation process cuts off intramolecular hydrogen bonds between aminic and iminic pyrrole units and, at the same time, produces intermolecular hydrogen-bond network between aminic pyrrole unit and counter-anions. Such a replacement induces some pyrrole inversion, leading to Mobius aromatic conformation for [36]octaphyrin and to Huckel aromatic conformation for [38]octaphyrin. These protonated octaphyrins show similar structures only with a subtle difference in tilted pyrrole angles, which results in their different topologies. This feature strongly suggests that the macrocycles control their topologies by pyrrole rotation to gain [4n]pi Mobius or [4n+2]pi Huckel aromatic stabilization, depending on the number of pi-electrons. Detailed photophysical properties such as absorption/fluorescence, excited singlet/triplet state lifetimes, and two-photon absorption cross-section values have been presented for both protonated [36] and [38]octaphyrins in conjunction with their Mobius or Huckel aromaticity.

Porphyrins, which consist of four pyrrolic subunits, are a ubiquitous class of naturally occurring compound with versatile photophysical properties. As an extension of the basic structure of the porphyrin macrocycle, there have been a multitude of approaches to synthesize expanded porphyrins with more than four pyrrole rings, leading to the modification of the macrocyclic ring size, planarity, number of pi-electrons and aromaticity. However, the relationship between the photophysical properties and the structures of expanded porphyrins has not been systematically investigated. The main purpose of this article is to describe the structure-property relationships of a variety of expanded porphyrins based on experimental and theoretical results, which include steady-state and time-resolved spectroscopic characterizations, non-linear absorption ability and nucleus-independent chemical shift calculations.

Excited-state symmetry-breaking charge separation (SB-CS) can offer an efficient pathway to solar energy capture and conversion. We synthesized a series of 1,6,7,12-tetrakis­(4-t-butylphenoxy)­perylene­(3,4:9,10)­bis­(dicarboximide) cyclophane dimers with m-xylylene, p-xylylene, and 4,4′-diyldimethane-1,1′-biphenyl spacers and studied them with steady-state and time-resolved optical spectroscopies. Photoinduced SB-CS occurs in all three cyclophanes in CH2Cl2, with the SB-CS rate decreasing as the interchromophore distance is increased. Time-resolved emission spectroscopy and kinetic modeling reveal that the charge-separated state exists in pseudoequilibrium with the excited state prior to decay. Notably, the meta-spaced cyclophane also undergoes SB-CS in toluene within ∼100 ps, despite the lack of charge stabilization by the low dielectric constant solvent. These results demonstrate that SB-CS can occur across long distances and in weakly polar environments, which offers the possibility of harnessing SB-CS for solar energy capture and conversion.

Supported ionic liquid membranes (SILMs) are membranes that have ionic liquids impregnated in their pores. SILMs have been proposed for advanced carbon capture materials. Two-dimensional infrared (2D IR) and polarization selective IR pump-probe (PSPP) techniques were used to investigate the dynamics of reorientation and spectral diffusion of the linear triatomic anion, SeCN, in poly(ether sulfone) (PES) membranes and room-temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf). The dynamics in the bulk EmimNTf were compared to its dynamics in the SILM samples. Two PES membranes, PES200 and PES30, have pores with average sizes, ∼300 nm and ∼100 nm, respectively. Despite the relatively large pore sizes, the measurements reveal that the reorientation of SeCN and the RTIL structural fluctuations are substantially slower in the SILMs than in the bulk liquid. The complete orientational randomization, slows from 136 ps in the bulk to 513 ps in the PES30. 2D IR measurements yield three time scales for structural spectral diffusion (SSD), that is, the time evolution of the liquid structure. The slowest decay constant increases from 140 ps in the bulk to 504 ps in the PES200 and increases further to 1660 ps in the PES30. The results suggest that changes at the interface propagate out and influence the RTIL structural dynamics even more than a hundred nanometers from the polymer surface. The differences between the IL dynamics in the bulk and in the membranes suggest that studies of bulk RTIL properties may be poor guides to their use in SILMs in carbon capture applications.

Switching Aromaticity: Conformations of [32]heptaphyrins(1.1.1.1.1.1.1) are dependent upon meso‐aryl substituents, solvents, temperature, and protonation. Particularly, protonation of meso‐pentafluorophenyl‐substituted [32]heptaphyrin triggers conformational changes to form twisted aromatic Möbius structures (see picture), even at room temperature.

Neuronal SNARE proteins mediate neurotransmitter release at the synapse by facilitating the fusion of vesicles to the presynaptic plasma membrane. Cognate v-SNAREs and t-SNAREs from the vesicle and the plasma membrane, respectively, zip up and bring about the apposition of two membranes attached at the Cterminal ends. Here, we demonstrate that SNARE zippering can be modulated in the midways by wedging with small hydrophobic molecules. Myricetin, which intercalated into the hydrophobic inner core near the middle of the SNARE complex, stopped SNARE zippering in motion and accumulated the trans-complex, where the N-terminal region of v-SNARE VAMP2 is in the coiled coil with the frayed C-terminal region. Delphinidin and cyanidin inhibited N-terminal nucleation of SNARE zippering. Neuronal SNARE complex in PC12 cells showed the same pattern of vulnerability to small hydrophobic molecules. We propose that the half-zipped trans-SNARE complex is a crucial intermediate waiting for a calcium trigger that leads to fusion pore opening.polyphenol | hemifusion | neurotransmission | neuron N eurotransmitter release at the synapse, which serves as the brain's major form of cell-cell communication, requires the fusion of synaptic vesicles with the presynaptic plasma membrane. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins mediate this synaptic fusion event (1-5), and the formation of a four-helical bundle (6-8) is believed to generate the force required for fusion. A zipper model has been proposed for SNARE complex formation, initiating assembly at the N-terminal region and zipping toward the C-terminal membrane-proximal region (6-9). To account for fast neuroexocytosis, the SNAREs in primed readily releasable vesicles have been proposed as being partially zipped in the trans-configuration bridging the two membranes.Although the structure of the fully assembled cis-SNARE complex, which is believed to represent the postfusion state, has been determined (10), the structure of the trans-complex is poorly understood and is purely imaginary, most likely because of its inherently transient nature. Precisely linking the degrees of SNARE zippering to specific stages of membrane fusion seems to be prerequisite for determining the structure of the trans-complex and for providing answers to the questions of how fast fusion is controlled in neurons and how the trans-complexes set up the readily releasable vesicles with other regulatory proteins.Here, we show that certain small hydrophobic molecules (SHM) enable layer-by-layer control of SNARE zippering by wedging into various points of the SNARE zipper. SNAREmediated membrane fusion is dissected via this wedge-like action of SHMs. Analysis of the captured replication fork-like structure allowed us to understand the basic architecture of the putative trans-complex. Results SNARE-Driven Membrane Fusion Can Be Controlled by SHMs withDifferent Modes of Action. As an initial step to examine the feasibility of whether SHM works as a wedge for the SNARE zipp...

Nanoporous silica materials are important in catalysis, energy, and materials applications in which water is an essential component. System performance is intimately connected to the water dynamics occurring in the confined environment. However, the dynamics and associated structures of water in nanoporous silica have proven challenging to measure and predict. Here, confined water dynamics are examined via the ultrafast infrared spectroscopy of selenocyanate (SeCN–) dissolved in the hydrated ∼2.4 nm silica mesopores of MCM41. Polarization selective pump–probe and two-dimensional infrared measurements on the CN stretching mode of SeCN– are used to probe the effect of confinement on orientational relaxation and spectral diffusion dynamics. The dynamics of SeCN– provide information on the surrounding water hydrogen bond dynamics. The long CN stretch lifetime (∼36 ps), relative to the water hydroxyl stretch (<2 ps), significantly extends the time scales that can be accessed. Complete orientational relaxation (C 2(t), orientational correlation function) and spectral diffusion (C ω(t), frequency–frequency correlation function) dynamics are presented and compared to the simulated time correlation functions in a model silica pore of the same size. A slow decay component not present in the bulk liquid is observed in both experiments, indicating that the hydrogen bond dynamics are significantly altered by confinement. The simulations reveal a qualitative difference in the functional dependence of C 2(t;d) and C ω(t;d) on d, the distance from the interface. The former becomes exponentially faster with distance while the latter makes an abrupt transition from slower to faster dynamics midway between the surface and pore center, d ≅ 6 Å.