Temperature dependence of charge-transfer fluorescence from extended and U-shaped donor-bridge-acceptor systems in glass-forming solvents

The behavior is reported of three fluorescent D-bridge-A systems that display a fascinating temperature dependence in glass forming solvents over the temperature range between 77 and 293 K. In two of these systems, a rigid, saturated alkane bridge maintains an extended conformation, and as a result, the charge-transfer (CT) state is of giant dipolar nature. This causes the position of the CT fluorescence to be an extremely sensitive probe for the reorientation polarization of the surrounding medium. As a result, the thermochromism of the continuous CT fluorescence maximum in 2-methyltetrahydrofuran (MTHF) covers the full visible region. In the higher temperature domain (above ca. 145 K), this thermochromism can be quantitatively described via the Lippert-Mataga relation. At lower temperatures, solvent relaxation slows down sufficiently to detect exceptionally large dynamic Stokes shifts of the fluorescence maximum on time scales up to greater than or equal to40 ns. The third D-bridgc-A system studied features a U-shaped ground state conformation. Remarkably, this system displays a significant thermochromic shift over a narrow temperature region around 175 K in the nonpolar methylcyclohexane (MCH) in which the other systems display only very minor thermochromism. In this U-shaped system therefore,one monitors the temperature dependence of an internal reorganization instead of a medium relaxation. Extensive ab initio calculations indicate that this internal reorganization must be related to an electrostatically driven conformational collapse of the U-shaped system in the CT state.