Photoresponsive azobenzene ligand as an efficient electron acceptor for luminous CdTe quantum dots

Surface modified semiconductor quantum dots (QDs) with a tunable photoluminescence (PL) are particularly desirable for many photo-responsive devices. In a step forward towards this goal using 3-mercaptopropionic acid (MPA) the CdTe QDs of different sizes are synthesized. The MPA not only controls the size of the QDs but its dense shell around the surface also passivates the surface traps. Consequently, the CdTe QDs exhibit narrow emission with full-width at half-maximum (FWHM) of ∼0.3 eV. These QDs are attached to (E)-4-((3-formyl-4-hydroxyphenyl) diazenyl) benzoic acid (FHDBA) to fabricate a photochrome-fluorophore assembly. Ultraviolet (UV) irradiation induces trans-cis isomerization in FHDBA. Upon storing FHDBA in dark for ∼30 min, it completely reverts to trans-isomer. After photoisomerization the absorption band (n – π* transition) of cis-isomer of FHDBA overlaps with the emission band of CdTe QDs. Following UV excitation photoinduced electron transfer (ET) from conduction band (CB) of CdTe QDs to the LUMO of the cis-isomer of FHDBA quench the fluorescence of QDs by ∼16 times. Forster resonance energy transfer (FRET) may also quench the fluorescence but its contribution is minor. The photoinduced reversible trans-cis interconversion of FHDBA followed by ET in QDs-FHDBA assembly and the dual function of MPA as coupling strategy may open the avenues to design QDs luminescent photoswitchable probes for biomedical and energy storage applications.