No abstract

No abstract

Photoexcitation of the symmetrical molecules tetraphenylethylene and tetra-p-methoxyphenylethylene dissolved in saturated hydrocarbon solvents results in a transient increase in the dielectric loss of the solutions as monitored using the nanosecond time-resolved microwave conductivity (TRMC) technique. This provides direct evidence for the dipolar, or "zwitterionic", nature of the 'p* phantom state formed from SI by rotation around the central carbonsarbon bond.Dipole relaxation occurs mainly by charge inversion between the two energetically equivalent zwitterionic configurations, Z+, on a timescale of several picoseconds. A minimum dipole moment of ca. 7.5 D for the individual Z+ states is found.The fluorescence of TPE in alkane solvents has two decay components, one with a decay time less than 200 ps and a second with a decay time of 1.9 ns. The former (A,,, = 490 nm) is assigned to emission from the partially relaxed SI state prior to twisting. The latter (A, , , = 540 nm) is assigned to emission from a small, ca. 1%, concentration of the relaxed SI state in equilibrium with the 'p* state in saturated hydrocarbon solvents.

Abstract. Absorptive and emissive properties of 9-arylacridinium ions 2-6 have been studied and shown to be dependent on the electron-donating properties of the aryl group. While for 2 and 3 the first excited state remains largely localized on the acridinium chromophore, interaction of this chromophore with the electron-donor-substituted aryl group in 4-6 produces a low-lying intramolecular charge-transfer state, resulting in a state inversion. This state inversion leads to the appearance of a new long-wavelength absorption in the visible region and to complete quenching of the acridinium fluorescence. Protonation of the amino function in 4-6 cancels this state inversion, thereby causing a dramatic chromofluoroionophoric effect, i.e. cation-induced change of both colour and fluorescence properties. Replacing the amino function by an aza crown ether resulted in system 7, which displays similar effects, not only upon protonation but also upon metal ion complexation.

Abstract. Cyclic voltammetry and preparative controlled potential electrolysis show that the 9-phenyl-10-methyl-acridinium/acridan (AcPh + /AcPhH) redox couple can be cycled electrochemically between the oxidized (AcPh + ) and reduced states (AcPhH) without any apparent side-reaction. The 9-phenyl-10-methyl-acridanyl radical (AcPh') was identified as a first intermediate in the electrochemical reduction of AcPh+ by cyclic voltammetry as well as by electronic absorption and ESR spectroscopy. In contrast to other acridanyl and related dihydropyridyl radicals, AcPh' shows no tendency to undergo dimerization. In aprotic media, AcPh' is shown to undergo a second reversible one-electron reduction to yield AcPh -, which is, even in these media readily protonated to give AcPhH. The stability of these intermediates seems to be the major factor responsible for the clean electrochemical interconversion of the AcPh + /AcPhH redox couple. The implementation of this redox couple as part of photo-electrochemical energy conversion systems is discussed.