No abstract

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 chargetransfer (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 g40 ns. The third D-bridge-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. † Part of the special issue "Noboru Mataga Festschrift".

The light generating mechanism of a series of light emitting diodes with electron donor-bridge-acceptor systems (D-b-A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly-N-vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge-transfer (CT) fluorescence of the donor-bridge-acceptor systems occurred readily in OLED devices with a D-b-A system as the emissive layer. In diodes with PS as the host matrix, hole-injection and electron-injection occurred directly in the D-b-A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole-injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D-b-A molecule, whereas electron-injection at the cathode side occurred directly into the D-b-A molecules. Charge-hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence.

A series of semirigid donor-bridge-acceptor (D-B-A) molecules was synthesized to study the effect of the position and number of nonconjugated olefinic bonds in the bridge on the photoinduced charge-separation and charge-recombination kinetics. The molecules consist of a phenylpiperidine electron donor, an oligo-(cyclohexylidene) or oligo(cyclohexyl) bridge, and a dicyanovinyl acceptor. Partly saturated ter(cyclohexylidene) bridges were used as well. The edge-to-edge donor-acceptor separation of the compounds under study varies between 2.89 and 15.4 Å. The replacement of a C-C single bond by an olefinic bond increases the rate of charge separation with a factor of 3.0 ( 0.8 per replaced bond. For all D-B-A compounds the extended fully charge-separated state folds to a compact charge-transfer (CCT) conformer. The rate of charge recombination (CR) of the CCT state increases with solvent polarity for those compounds having an olefinic bond located three σ bonds from the acceptor. Thus, while in cyclohexane the CR rate is equal for all compounds, in benzene the CR rate in compounds with an olefinic bond near the acceptor is 10 times larger than in compounds with a single bond instead. It is believed that a (virtual) charge-transfer state involving the radical cation of the olefinic bond and the radical anion of the acceptor (D-B •+ -A •-) is responsible for the enhanced CR process.