SUDDEN POLARIZATION IN THE TWISTED, PHANTOM STATE OF TETRAPHENYLETHYLENE DETECTED BY TIME-RESOLVED MICROWAVE CONDUCTIVITY

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 (lambda(max) almost-equal-to 490 nm) is assigned to emission from the partially relaxed S1 state prior to twisting. The latter (lambda(max) almost-equal-to 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