C4nr05144k 14200..14203 ++

Light responsive materials have attracted immense research efforts due to their widespread applications, e.g. in ultrahighdensity optical data storage and super-resolution microscopy. In these fields, the modulation of the fluorescence intensity by photochromic molecules is of particular interest. Photochromic diarylethene (DAET) derivatives exhibit a colourless open and a coloured closed structure, high thermal stability and fatigue resistance, which make them ideal candidates for switching applications. However, DAETs show very weak fluorescence (quantum yields below 5%), because rotational degrees of freedom increase the rate of radiationless decay of the excited state. Consequently, DAETs have been decorated with fluorophores yielding molecular dyads and triads. In such systems, fluorescence can be modulated reversibly by switching the state of the DAET molecule. In the off-state (closed DAET) efficient photochromic Förster resonance energy transfer (pcFRET) takes place after photoexcitation of the fluorophore, whereas in the onstate (open DAET), the photoexcited fluorophore decays radiatively. A critical factor in the application of pcFRET systems is the photoinduced fluorescence switching contrast between the on-state and the off-state, which is estimated from the fluorescence intensity ratio between the two states. From the dynamical point of view, this ratio is governed by the life time of the fluorophore and the pcFRET rate. In energy and charge transfer applications, semiconductor quantum dots (QD) with high photostability and strong fluorescence have proven their benefits. However, only few studies report on the fluorescence modulation in systems incorporating QD. Different strategies for the attachment of the photochromic molecules have been introduced such as coating the QD with an amphiphilic photochromic polymer (pcFRET system) or direct coordination via a pyridine functional group (charge transfer system). In FRET applications, it is a challenging task to disentangle, whether the observed QD fluorescence quenching stems from the energy transfer process or from competing relaxation channels opened up by the ligand exchange at the surface of the QD. In earlier studies, the existence of non-FRET mechanisms had been discussed. It has been demonstrated that the quenching of QD emission is partially related to new nonradiative decay channels formed upon assembly. In this context the photoswitch/QD system could be an elaborate approach to discriminate between FRET and non-FRET quenching. Herein, we report spectroscopic investigations on a pcFRET system, which is composed of a CdSe QD and a dithienylethene dye (DTE). The DTE is attached to the QD surface via an adamantyl based tripodal linker furnished with three COOH anchoring groups (Tripod-COOH; Fig. 1). This type of linkage