Sudden Polarization in the Twisted, Phantom State of Tetraphenylethylene Detected by Time-Resolved Microwave

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 1p* phantom state formed from S1 by rotation around the central carbon-carbon 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 (λmax ≈ 490 nm) is assigned to emission from the partially relaxed S1 state prior to twisting. The latter (λmax ≈ 540 nm) is assigned to emission from a small, ca. 1%, concentration of the relaxed S1 state in equilibrium with the 1p* state in saturated hydrocarbon solvents.