ABSTRACT
In recent years, expanded porphyrins have emerged as a promising class of π-conjugated molecules that display unique electronic, optical and conformational properties. Several expanded porphyrins can switch between planar and twisted conformations, which have different photophysical properties. Such a change of topology involves a Hückel-Möbius aromaticity switch in a single molecule and it can be induced by solvent, pH and metallation. These features make expanded porphyrins suitable for the development of a novel type of molecular switches for molecular electronic devices. Octaphyrins consisting of eight pyrrole rings, exhibit twisted-Hückel, Möbius and Hückel π-conjugation topologies depending on the oxidation and protonation state, with distinct electronic structures and aromaticity. Our working hypothesis is that a significant change in the conductance of expanded porphyrins will be observed after the topology switching. Despite the potential of Hückel-Möbius systems as conductance switches, the relationship between the conductance and the molecular topology is not yet understood. We have explored the performance of local descriptors of conductivity in simple molecules, as well as the relationship with conductance. Since these indexes provide a qualitative measure of delocalization and conductance in the probe molecules, we have carried out a local analysis of electrical conductance changes as a function of the π-conjugation in two examples. In one of them, the locality of the electronic changes ensures the ability of these indexes to describe the conductance as local. Moreover, it enables to identify which conformational switch would be more efficient from an electronic device perspective. However, we also show that it is not always possible to reduce conductance changes to one bond, and in those molecules where a deep rearrangement occurs far from the structural perturbation, local measures show a limited efficiency. This is a first step for the description of the connection between the molecular structure and conductance in molecular switches.
ABSTRACT
The pineal organ of elasmobranchs is an elongated photoreceptive organ. In order to investigate the afferent and efferent connections of the pineal organ of two elasmobranchs, the skate (Raja montagui) and the dogfish (Scyliorhinus canicula), a fluorescent carbocyanine (DiI) was applied to the pineal organ of paraformaldehyde-fixed brains. This application strongly labeled the pineal tract, which formed extensive bilateral projections. In both species, the pinealofugal fibers coursed to the dorsomedial thalamus, the medial pretectal area, the posterior tubercle, and the medial mesencephalic tegmentum and branched profusely in these areas. Application of DiI to the pineal organ also labeled occasional perikarya in the dorsomedial thalamus, posterior commissural region, posterior tubercle, and mesencephalic tegmentum. A comparison of these results with those of immunocytochemical analyses of the dogfish brain with an anti-salmon gonadotropin-releasing hormone (sGnRH) antiserum revealed a close topographical relation between the pineal projections and the midbrain sGnRH-immunoreactive (ir) nucleus, the only structure in the dogfish brain that contained sGnRHir neurons. This and the widespread distribution of sGnRHir fibers in the brain suggest that the midbrain sGnRHir nucleus is a part of the secondary pineal pathways and may be involved in light-mediated pineal regulation of brain function. Although GnRH distribution has not been studied in the skate, a midbrain GnRHir nucleus has been identified in three other elasmobranchs, including a skate relative. The probable existence of direct pineal projections to the GnRHir midbrain nucleus in elasmobranchs and other anamniotes is discussed.
