SUMMARY1. In anaesthetized cats intracellular records were obtained from antidromically identified motoneurones. The motor nuclei to the elbow extensor and flexor muscles and to the muscles innervated by the deep radial, ulnar and median nerves were investigated. The maximum Ia EPSPs from electrical stimulation of various peripheral nerves were measured. The characteristic convergence and projection patterns to each motor nucleus were established from pooled data.2. The total aggregates of the I a EPSPs between the different motor nuclei ranged from 3-5 to 11-7 mV. The smallest aggregates were found in the nuclei to the digit muscles. The ratio of the heteronymous versus homonymous EPSP amplitudes varied between 3-9 and 05. A general rule which would govern the distribution of the EPSP aggregates, such as a proximo-distal. gradient, was not observed.3. The Ia connections followed a complex but highly organized pattern. Bidirectional and unidirectional pathways were present. In many cases the convergence pattern of a motor nucleus included muscles acting at different joints. The connections of one nucleus were not necessarily restricted to one side of the limb, but could cross the radio-ulnar plane.4. Muscles with similar actions onto the same joint were interconnected with bidirectional, rather balanced I a pathways. Such relations were also present between close functional synergists and then often extended across several joints. The relations between the anatomical extensors of wrist and digits were graded according to the neighbourhood of these muscles. It is suggested that this reflects the graded mechanical synergism in the wrist action of these muscles.5. A large number of unidirectional or strongly skewed bidirectional Ia pathways project from proximal to distal muscles. It is suggested that they may serve a readjustment of distal joints during changes in the position of proximal ones (e.g. stabilization of the position of the radio-ulnar plane during elbow extension in case of the unidirectional projections onto supinator and abductor pollicis longus motoneurones).

Electron donor-acceptor systems, in which phenothiazine is tethered to pyrene by means of a phenyl bridge, exhibit a dual emission in moderately and very polar solvents. Employing steady-state and time-resolved fluorescence spectroscopy, we were able to provide evidence that the "blue" and "red" emission bands originate from different conformers. The ground-state geometry of the majority species is identical to that found in the crystalline state (the quasi-equatorial conformer). This conformation executes a fast electron-transfer process accompanied by significant structural relaxation. Consequently, its fluorescence exhibits a large solvatochromic shift typical for charge-transfer states. The photophysical properties of the minority species (the quasi-axial conformer) vary significantly with the substitution pattern of the bridging phenyl ring. In part, this difference is related to the orientational factor, κ, governing the rate of energy transfer between the pyrene and phenothiazine moieties. In the para-substituted derivative, fluorescence emission from both the excited phenylpyrene and the phenylphenothiazine subsystem can be observed. In the meta-substituted derivative, fluorescence originates mainly from the primarily absorbing phenylpyrene subsystem. In nonpolar solvent (cyclohexane), the nature of the fluorescing state differs for the para-and meta-substituted compounds. Whereas in the former, the fluorescence originates from the locally excited phenothiazine, it is governed by emission from a structurally modified CT state in the latter derivative. Semiempirical (AM1/CI) molecular orbital calculations with a continuum solvent treatment have been used to investigate the different states involved and provide explanations for the observed results. The calculations reveal the existence of an intermediately populated CT state in which the negative charge is partly localized in the bridge as well as in the pyrene acceptor.

Electron donor-acceptor systems with phenothiazine linked directly to pyrene exhibit a dual emission in moderately and very polar solvents. Steady state and time-resolved fluorescence spectroscopy provide evidence that the "blue" and "red" emission bands originate from different species. Fluorescence excitation spectra show a similar appearance when the emission is monitored either in the red or blue spectral range, but they are slightly shifted against each other. This suggests that different isomers exist with a distinctly different photophysical behavior. Semiempirical (AM1/CI) molecular orbital calculations with a continuum solvent treatment have been used to establish the geometry of the two nearly isoenergetic stereoisomers and to calculate the properties of their excited Franck-Condon states. Geometry optimization of various excited states provides evidence for different internal relaxation coordinates for the phenothiazine-localized S 1 state and the charge transfer state S 6 , on one hand, and the pyrene-localized S 2 (S 3 ) state, on the other hand. The relaxed geometries in the excited states of both isomers represent mirror images with identical properties. The different photophysical behavior of the two isomers is most likely caused by the different potential energy curves, or more precisely speaking, by different location and/or heights of the barrier along the reaction coordinates from the locally excited to the geometry-relaxed CT states.

The triad 6 containing the phenothiazine-isoalloxazine couple as donor-acceptor redox unit and pyrene as antenna absorbing in the UV-A region has been designed to mimic the light processes of natural photoreceptors. By cyclic voltammetry it is shown that the redox chemistry of the three subunits of triad 6 behave almost independently, indicating no electronic coupling between the subunits in the ground state. Triad 6 exhibits three accessible redox states with one oxidation and two reduction waves due to the formation of the phenothiazine radical cation and isoalloxazine and pyrene radical anions. UV/Vis/NIR spectroelectrochemistry reveals the generation of the protonated isoalloxazine dianion on reduction which is formed in the non-polar solvent in a reduction-protonation-reduction step (two-electron transfer process) and which is attributed to intermolecular proton transfer from the amide group to the electrochemically reduced isoalloxazine radical anion. Evidences for the photoinduced energy and electron transfer within the triad are provided by steady state and time-resolved absorption and fluorescence measurements. Spectroscopic studies displayed that upon excitation the pyrene emission was dramatically quenched in the dyad 4. This is most likely due to the energy transfer from pyrene to the isoalloxazine units as the absorption band of isoalloxazine overlaps with the pyrene emission band leading most likely to a CT state of the isoalloxazine/phenothiazine type. Quenching of the phenothiazine fluorescence in triad 6 was also ascribed to the spectroscopic overlap between the emission spectrum of phenothiazine and absorption spectrum of isoalloxazine. Again, photoinduced electron transfer from phenothiazine to isoalloxazine is expected to be the cause for the quenching of the isoalloxazine emission in the dyad 5. Molecular orbital calculations for compound 5 showed a complete electron transfer from phenothiazin to isoalloxazine.y Electronic supplementary information (ESI) available: Colour representations of Scheme 1 and Figs. 17 and 18. See