Competition between conformational relaxation and intramolecular electron transfer within phenothiazine-pyrene dyads

The competition between conformational dynamics and electron transfer withi n a series of phenotbiazine(phenyl)(n)-pyrene (n = 0,1) electron donor-acce ptor dyads of potential use in organic light emitting diodes was examined u sing femtosecond transient absorption spectroscopy. The molecular structure s of these dyads permit only torsional motions around the single bonds join ing each aromatic subunit. The redox properties of these molecules are near ly independent of the phenyl bridging group, whereas spectroelectrochemistr y shows that the UV/vis absorption spectra of the oxidized and reduced spec ies vary with the bridge. Each molecule exhibits dual fluorescence emission which provides evidence for conformational heterogeneity. Emission from a locally excited stare originates from a minority conformation, ill which el ectron transfer is negligible, whereas emission because of ion pair recombi nation results from the majority conformation which undergoes electron tran sfer. The electron-transfer reactions proceed with time constants < 25 ps e xcept in the dyad with the longest donor-acceptor distance in nonpolar solu tion, where the free energy of the charge separation reaction is positive. If electron transfer is sufficiently fast, conformational relaxation within the ion pair state product occurs on a 100-400 ps time scale, whereas if e lectron transfer is slow, conformation relaxation with the locally excited state centered on phenothiazine occurs. In two of the dyads in nonpolar sol vents, wherein the free energy for charge separation is estimated to be ver y small, strong mixing between the ion pair state and the locally excited s tate of phenothiazine is found. The results show that competitive conformat ional relaxations can have a strong influence on the charge separation dyna mics of donor-bridge-acceptor molecules with single bond linkages. In turn, these conformational dynamics will undoubtedly have an important influence on the photophysics of these molecules in the solid-state environment char acteristic of light-emitting diodes.