EXCITED-STATE RELAXATION CHANNELS OF LIQUID-CRYSTALLINE CYANOBIPHENYLS AND A RING-BRIDGED MODEL-COMPOUND - COMPARISON OF BULK AND DILUTE-SOLUTION PROPERTIES

The photophysical properties of 4-alkyl- and 4-alkoxy-4'-cyanobiphenyls with different chain lengths were compared in the dilute solution phase and bulk phases (crystalline, liquid-crystalline and isotropic) with respect to Stokes shifts and decay times. In solution, highly polar states are formed with decay times around 1 ns and subnanosecond rise times indicative of a molecular rearrangement occurring prior to emission. In the bulk phase, the major proportion of the emission derives from an excimer-type state with a red-shifted spectrum and very long decay time (10-25 ns depending on temperature and chain length). Possible excimer conformations are discussed. An excimer with a twisted geometry (twisted intramolecular charge transfer (TICT) excimer) is excluded by comparison with a bridged biphenyl system (fluorene derivative). The same compound also demonstrates that excited state intramolecular relaxation towards a 90 degrees twist angle is unlikely. In polar solvents, relaxation occurs towards an increased planarity in accordance with the Rapp model. This conclusion is discussed in relation to results from quantum chemical calculations.