Electroluminescence of Charge-Transfer Fluorescent Donor-bridge-Acceptor Systems

bridge unit (b) keeps electron donor (D) and electron acceptor (A) moieties separated at a distance way beyond the sum of their Van der Waals radii. Notwithstanding this spatial separation, photoexcitation of such D-b-A systems often has been found to induce the occurrence of rapid intramolecular electron transfer from D to A leading to an excited state with strong charge transfer (CT) character. It has been established beyond doubt, that the electronic interaction between D and A required to allow for such intramolecular electron transfer is in general mainly mediated via the bridge even if the latter consists of saturated hydrocarbon elements, which classically speaking prohibits conjugative transmission. Following the original proposal and description by Hoffmann et al. 4 it is now generally accepted to indicate electronic interaction via a saturated framework as through-bond interaction (TBI), although regrettably some authors recently appear to be tempted to use this terminology also for transmission of electronic effects via conjugated bridges, which we feel to be a confusing generalisation (B ≠ b !). It is important to stipulate that, although in the weak coupling limit the rate of electron transfer scales with the square of the electronic interaction between D and A, a quite small electronic interaction (V DA ) already suffices to enable very fast electron transfer, if other factors are optimised. Thus under “optimal conditions”—which in general implies that a strong D/A pair is incorporated so that the driving force for electron transfer is large enough to compensate the overall reorganisation energy—typically a V DA in the order of a few wavenumbers suffices to allow for electron transfer on a subnanosecond timescale. This is one of the reasons that the in general rather weak TBI typical for D–b–A systems with a saturated bridge structure is still strong enough to allow for the occurrence of intramolecular electron transfer in such systems. At the same time the weakness of TBI also implies that the oscillator strength of the direct electronic transition between the ground state and the CT state in such D–b–A systems is in general too small to make such a transition spectroscopically detectable. This is especially so with regard to the absorption spectra, which tend to be so similar to a superposition of the absorption spectra of the separate D and A chromophores that many authors are led to the conclusion that “there is no interaction in the ground state”, whereas the conclusion should in fact be that the direct CT absorption is too weak to be detected under the overlapping strong local absorption bands of D and A. Because of the characteristic large Stokes shift of CT fluorescence and the quenching of local D and A fluorescence upon photoinduced electron transfer it is more often possible to detect the radiative component of the charge recombination from the CT excited state to the ground state of D-b-A systems by emission spectroscopy. This type of emission is sometimes referred as intramolecular exciplex fluorescence, but we feel that for relatively rigid systems the term intramolecular CT fluorescence is more appropriate since “exciplex” should be reserved for situations where the interaction is established in the excited state by significant reorganisation, for example, reduction of D-A distance.