Reversible Photo-Switching of Dual-Color Fluorescent Mn-Doped CdS-ZnS Quantum Dots Modulated by Diarylethene Molecules

Dynamic materials have been given an increased amount of attention in recent years with an expectation that they may exhibit properties on demand. Especially, the combination of fluorescent quantum dots (QDs) and light-responsive organic switches can generate novel photo-switchable materials for diverse applications. In this work, a highly reversible dynamic hybrid system is established by mixing dual-color emitting Mn-doped CdS-ZnS quantum dots (QDs) with photo-switchable diarylethene molecules. We show that the diarylethene 1,2-bis(5-(3,5-bis(trifluoromethyl)phenyl)-2-methylthiophen-3-yl)cyclopent-1-ene (switch molecule 1) performs fabulous photo-switching property (between its open, 1o and closed, 1c forms), and high fatigue resistance in this hybrid system. The emission color switching between blue and pink of the system can be induced mainly by selective quenching/recovering of the Mn- photoluminescence (PL) of the QDs due to the switchable absorbance of the molecule 1. Mechanistic studies show that quenching of QD emission following UV illumination was caused by both Förster resonance energy transfer (FRET) and reabsorption by surrounding 1c molecules in the case of the Mn-PL, and solely by reabsorption in the case of badngap- (BG-)PL. This photo-switchable system could be potentially used in applications ranging from self-erasing paper to super-resolution fluorescence imaging.


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Supplementary Experimental Data

UV-Vis Absorption Measurements
UV-Vis absorption spectra were measured using an Agilent Technologies Cary 5000 UV-Vis Spectrophotometer. QDs and diarylethenes were dissolved in THF for the measurements.

Fluorescence Spectroscopy
The solution photoluminescence (PL), lifetime and quantum yield (QY) measurements were performed on an Edinburgh Instruments Fluorescence Spectrometer FS5. THF was used as solution for measurements. The PL lifetime were measured with excitation at 360 nm, with an average acquisition time of 15 min. The PL QYs were measured by FS5 Spectrometer with a built in integrating sphere. In the photoswitchable study, a UltraBright UV Transilluminator (MLB-16, Maestrogen) and a mounted 590 nm LED (M590L3, Thorlabs) were used as irradiation light source.

TEM Measurements
TEM measurements were performed on a JEOL 2100F operated at 200 kV. The QDs were diluted in hexane after purification, then drop cast on a 300-mesh copper TEM grid and dried at ambient condition before TEM measurements.

NMR Measurements
1 H NMR spectra were recorded on a Bruker ARTEMIS instrument. Chemical shifts values are given in ppm and are referred to CDCl 3 : 7.26 ppm. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, br = broad, m = multiplet). The coupling constants J, are reported in Hertz (Hz).

Calculation of FRET Efficiency and Reabsorption
The simple model of the Mn-doped CdS-ZnS QDs is as illustrated above. Without mixing with diarylethene molecules, there are two processes occurring in the Mn relaxation, which are radiative decay with the rate constant k R , and non-radiative decay with the rate constant k NR . The Mn-PL QY (QY 1 ) and lifetime (τ 1 ) can be expressed as the following equations: When mixed with diarylethene molecules, a new nonradiative transfer (i.e. FRET) occurs, with the rate constant k FRET . The corresponding Mn-PL QY (QY 2 , note the QY here is without considering reabsorption) and lifetime (τ 2 ) can be expressed as: The FRET efficiency (Φ FRET ) can be expressed as: It can also be expressed as In the real case, the FRET process is accompanying with the reabsorption process, the measured QY decrease (