Triplet transfer from PbS quantum dots to tetracene ligands: is faster always better?

Quantum dot-organic semiconductor hybrid materials are gaining increasing attention as spin mixers for applications ranging from solar harvesting to spin memories. Triplet energy transfer between the inorganic quantum dot (QD) and organic semiconductor is a key step to understand in order to develop these applications. Here we report on the triplet energy transfer from PbS QDs to four energetically and structurally similar tetracene ligands. Even with similar ligands we find that the triplet energy transfer dynamics can vary significantly. For TIPS-tetracene derivatives with carboxylic acid, acetic acid and methanethiol anchoring groups on the short pro-cata side we find that triplet transfer occurs through a stepwise process, mediated via a surface state, whereas for monosubstituted TIPS-tetracene derivative 5-(4-benzoic acid)-12-triisopropylsilylethynyl tetracene (BAT) triplet transfer occurs directly, albeit slower, via a Dexter exchange mechanism. Even though triplet transfer is slower with BAT the overall yield is greater, as determined from upconverted emission using rubrene emitters. This work highlights that the surface-mediated transfer mechanism is plagued with parasitic loss pathways and that materials with direct Dexter-like triplet transfer are preferred for high-efficiency applications.


5-(4-benzenecarboxaldehyde)-12-triisopropylsilylethynyl tetracene
To a solution of 4-bromobenzaldehyde ethylene acetal (2.29g, 10 mmol) in 60 mL anhydrous THF cooled to -78 °C was slowly added 4 mL (10 mmol of a 2.5M solution in hexanes) of n-BuLi. After stirring for 20 minutes at -78 C, 2.5 g (9.8 mmol) of naphthacenequinone was added to the solution, which was then allowed to warm slowly to room temperature. After 3 hours, 12 mmol of prepared solution of litiho triisopropylsilyl acetylene was added to this solution, and the reaction mixture was stirred at room temperature overnight. 10 mL of 10% aqueous HCl was then added, and the solution stirred a further 24 hours. Finally, 2 grams of stannous chloride dihydrate was added to the mixture, which was stirred for a further two hours.
The solution was poured into hexanes, extracted three times with water, dried with anhydrous magnesium sulfate, and passed through a thin pad of silica gel (flushed with hexanes, then 1:1 hexanes:dichloromethane) to remove baseline impurities. The compound was purified by chromatography on silica gel (hexanes -> 5:1 hexanes:dichloromethane) and recrystallization from hexanes to yield 2.3 g (45%) of aldehyde product. 1

ps-Transient Absroption
The short time (fs-ns) transient absorption setup has been described previously. 6 In summary, a Light Conversion PHAROS laser system with 400μJ per pulse at 1030 nm with a repetition rate of 38 kHz. The output is divided, one part is focused onto a 4 mm YAG substrate to produce the continuum probe beam from 520 to 950nm. The second part of the PHAROS output is lead into a narrow band optical parametric oscillator system (ORPHEUS-LYRA, Light conversion) outputting the pump beam. The probe pulse is delayed up to 2 ns with a mechanical delay-stage

ns-Transient Absorption
The long-time ns-transient absorption setup has also been described previously. 6 In short the pump-probe setup consists of probe from a LEUKOS Disco 1 UV super continuum laser (STM-1-UV, 1 kHz) and a pump generated in a TOPAS optical amplifier, pumped with the output from a Spectra-Physics Solstice Ace Ti:Sapphire amplifier (1 kHz). The probe beam is split into a reference and probe and both are focused onto the sample. A pair of line image sensors (Hamamatsu, G11608) mounted on a spectrograph (Andor Solis, Shamrock SR303i) are used to detect the signal, using a custom-built board from Stresing Entwickslungsburo to read out the signal.
Fitting to the triplet rise and decay was done assuming that the triplet rise is described by an intrinsic transfer rate k 0 TET , which is the transfer to a single ligand. Assuming a Poisson distribution of bound ligands it can be shown that the transfer rate expression can be described by Equation S1 7 : (S1) Since the data extracted from the ns-TA experiment is an convolution of a growth term associated with triplet transfer to the ligand and the triplet decay of the ligand the fitting was done Equation S2, which incorporates both a rise term based on Equation S1 and decay term assuming a monoexponential triplet decay.

Diffusional Triplet Energy Transfer
The upconversion intensity is proportional to Equation 1 in the main text: (S3)  Table 1 in the main text. We can then use Equation S6 to estimate the bimolecular rate constant for triplet energy transfer from PbS/Tc-CA to Rubrene to explain the experimentally observed upconversion difference for any given . Considering a wide range of ---between 10 6 -10 8 M -1 s -1 which covers even very inefficient diffusional triplet transfer, has to be on the order of 10 5 M -1 s -1 which is 3-4 orders below the diffusion limit --in toluene. Hence, for diffusional triplet transfer to be able to explain the difference in upconversion intensity i) there has to be a 1-3 order of magnitude difference in bimolecular rate constant for triplet transfer between BAT and the three pro-cata ligands and ii) the pro-cata ligands must have a bimolecular rate constant for triplet transfer 3-4 orders below the diffusion limit. We therefore find it improbable that the diffusional triplet energy transfer from ligand to rubrene is the main reason for the observed difference in upconversion intensity. Thus, our assumption that the upconversion intensity is directly proportional to the triplet energy transfer from QD to ligand is reasonable.