Solvent Controlled Energy Transfer Processes in Triarylamine-Triazole Based Dendrimers

Fluorescence upconversion measurements of three different dendrimers <b>G1</b><b>–G3</b> based on triarylamines connected by triazole linkers show a strong and fast initial decay of fluorescence anisotropy for <i>t</i> < 2 ps followed by anisotropy decay on a much longer time scale (10–100 ps). At the same time, a pronounced solvent relaxation takes place. Comparison of the decay data in different solvents revealed that the initial decay of fluorescence anisotropy is governed by a competition of solvent relaxation and incoherent hopping of energy between the different dendrimer branches. Thus, it is decisive to discriminate between energy transfer processes in the Franck–Condon state or in the solvent relaxed state. We demonstrate that even for charge transfer chromophores, where a large Stokes shift leads to very weak spectral overlap of donor fluorescence and acceptor absorption, rapid homotransfer is possible if there is sufficient spectral overlap with the time zero fluorescence spectrum

Photoinduced Electron Transfer Dynamics in Triarylamine–Naphthalene Diimide Cascades

A series of dyads and triads of the A-D and A-D1-D2 type, respectively, containing triarylamine (TAA) donors and naphthalene diimide (NDI) acceptors, which are linked via triazole (Tz) heterocycles, were synthesized by Cu­(I)-catalyzed azide alkyne cycloaddition (CuAAC). Upon photoexcitation, these systems undergo charge separation leading to long-lived charge-separated (CS) states. The population of these CS states was monitored using femtosecond and nanosecond transient absorption spectroscopy. The transient signals of the CS states of all triads and dyads feature biexponential decays in the nanosecond time regime with a short and a long component. These biexponential decays are the result of an ISC from the primarily populated ¹CS state into the ³CS, from which charge recombination to the S₀ state is forbidden by spin conservation rules. The existence of ³CS states in the triads was confirmed by strong magnetic field dependent transient absorption kinetics, while for the dyads no effect could be observed due to a much larger singlet–triplet splitting. Thus, although charge recombination from the ¹CS state in the triads is slowed down compared to the dyads, the lifetime of the ³CS states is clearly longer in the dyads. This is the result of the larger singlet–triplet splitting in the dyads which leads to lifetimes of several microseconds