A General Approach for All-visible-light Switching of Diarylethenes through Triplet Sensitization using Semiconducting Nanocrystals

16 Coupling semiconducting nanocrystals (NCs) with organic molecules provides an 17 efficient route to generate and transfer triplet excitons. These excitons can be used to 18 power photochemical transformations such as photoisomerization reactions using low 19 energy radiation. Thus, it is desirable to develop a general approach that can efficiently 20 be used to control photoswitches using all-visible-light aiming at future applications in 21 life- and material sciences. Here, we demonstrate a simple ‘ cocktail ’ strategy that can 22 achieve all-visible-light switchable diarylethenes (DAEs) through triplet energy transfer 23 from the hybrid of CdS NCs and phenanthrene-3-carboxylic acid, with high 24 photoisomerization efficiency and improved fatigue resistance. The size-tunable 25 excitation energies of CdS NCs make it possible to precisely match the corresponding 26 energy of the relevant DAE photoswitch. We demonstrate reversible all-visible-light 27 photoisomerization of a series of

activated DAEs 24 , such as shifting the absorption spectrum by chemically extending the π 65 conjugation 25,26 , or using upconverting nanoparticles that absorb in the visible or near-infrared 66 region to generate UV light to drive the photoisomerization 27,28 . However, directly shifting the 67 absorption band of DAEs requires complicated synthesis, while the low efficiency of the 68 upconverting process leads to overall low photoisomerization efficiencies. Triplet energy 69 transfer from molecular triplet sensitizers has been proven to be another strategy 29-33 , however, 70 the typical use of molecular sensitizers imposes limitations due to the rather constrained 71 molecular design, relatively weak light absorption and high sensitivity towards oxygen. Thus, 72 it is desirable to develop a general and improved approach that can be applied to a large set of 73 DAEs using visible light only, while maintaining the high efficiency of photoisomerization and 74 excellent fatigue resistance.

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Our system is based on CdS NCs combined with phenanthrene-3-carboxylic acid (3-76 PCA) to mediate the TET process, and it demonstrates a simple yet efficient noncovalent 77 strategy to achieve photoisomerization of selected DAEs using light at different wavelengths 78 in the visible region. The mechanism implies triplet-like excited states of the NCs lying only 79 ~20 meV below the strong excitonic absorption band, which can sensitize the 'dark' triplet of 80 the surface anchored mediator 7,34 . Subsequent TET from the mediator to the DAEs drives the 81 photoisomerization along the triplet reaction pathway, as schematically illustrated in Figure 1a.  The switching of our systems can be retained at atmospheric environment, that is, it displays 91 insensitivity to oxygen in the solid state, which is appealing in the fabrication of high-92 performing all-visible-light activated optoelectronics and memories.       Figures 4c and 4d), which is almost one order of magnitude smaller than that 253 observed for DAE1, in accordance with a less effective TET2 step due to endergonic triplet 254 energy transfer.   The data that support the findings of this study are available from the corresponding authors 345 upon reasonable request.

Chemicals 348
Analytical reagent grade toluene from VWR was used without further purification for spectroscopic 349 measurements.

Sample preparations 384
All photophysical measurements were carried out in toluene using a 10 mm path quartz cuvette. The 385 samples for UV-visible absorption measurements were prepared by purging with argon for at least 15 386 minutes. The samples for quantum yield and transient absorption measurements were prepared by at 387 least 4 freeze-pump-thaw cycles. The solid sample was prepared by soaking a piece of filter paper into 388 the solution for UV-visible absorption measurement, letting all the solvent slowly evaporate at 60 °C 389 in the dark. 390

Quantum yield and conversion of photocyclization reactions 398
Photocyclization quantum yields were determined by using Ferrioxalate actinometry (405 nm, Ф=1.14) 399 according to standard methods 45 . 405 nm irradiation was carried out by using a continuous-wave laser 400 (Coherent, OBIS) coupled with a neutral density (ND) filter to reduce the intensity. 401 The conversion of DAE1-open to DAE1-closed at the photostationary state (PSS) was reported as 94% 402 upon 310 nm irradiation 40 , and the molar absorption coefficient of DAE1-closed was determined to be 403 522 = 1.97 × 10 4 M −1 cm −1 . The conversion of DAE2-open to DAE2-closed at the PSS was reported 404 as 79% upon 313 nm irradiation 43 , according to which the molar absorption coefficient of DAE2-closed 405 in toluene was determined to be 575 = 1.25 × 10 4 M −1 cm −1 . The corresponding conversions at the 406 PSS upon 405 nm light irradiation were calculated using the above molar absorption coefficients. 407

Computational details 408
Density functional calculations (DFT) were performed using the Gaussian 16 software package 46 using 409 the hybrid functional B3LYP and the basis set 6-311+G(d,p). Full optimization of the ground-state 410 structure was followed by excited state calculations using the basis set 6-31G(d). Excited state energies 411 (12 lowest of both triplet and singlet spin) were calculated using the time-dependent formalism 412 (TDDFT). 413