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'-cyanobiphenyl s with different chain lengths were compared in the dilute solution ph ase and bulk phases (crystalline, liquid-crystalline and isotropic) wi th 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 emis sion. In the bulk phase, the major proportion of the emission derives from an excimer-type state with a red-shifted spectrum and very long d ecay time (10-25 ns depending on temperature and chain length). Possib le excimer conformations are discussed. An excimer with a twisted geom etry (twisted intramolecular charge transfer (TICT) excimer) is exclud ed 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 sol vents, relaxation occurs towards an increased planarity in accordance with the Rapp model. This conclusion is discussed in relation to resul ts from quantum chemical calculations.