Competition between Conformational Relaxation and Intramolecular Electron Transfer within Phenothiazine−Pyrene Dyads

The competition between conformational dynamics and electron transfer within a series of phenothiazine-(phenyl)n-pyrene (n = 0, 1) electron donor−acceptor dyads of potential use in organic light emitting diodes was examined using femtosecond transient absorption spectroscopy. The molecular structures of these dyads permit only torsional motions around the single bonds joining each aromatic subunit. The redox properties of these molecules are nearly independent of the phenyl bridging group, whereas spectroelectrochemistry shows that the UV/vis absorption spectra of the oxidized and reduced species vary with the bridge. Each molecule exhibits dual fluorescence emission which provides evidence for conformational heterogeneity. Emission from a locally excited state originates from a minority conformation, in which electron transfer is negligible, whereas emission because of ion pair recombination results from the majority conformation which undergoes electron transfer. The electron-transfer reactions proceed with time constants <25 ps except in the dyad with the longest donor−acceptor distance in nonpolar solution, 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 electron transfer is slow, conformation relaxation with the locally excited state centered on phenothiazine occurs. In two of the dyads in nonpolar solvents, wherein the free energy for charge separation is estimated to be very small, strong mixing between the ion pair state and the locally excited state of phenothiazine is found. The results show that competitive conformational relaxations can have a strong influence on the charge separation dynamics 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 characteristic of light-emitting diodes.