Tailoring Carbon Tails of Ligands on Au52(SR)32 Nanoclusters Enhances the Near-Infrared Photoluminescence Quantum Yield from 3.8 to 18.3%

One of the important factors that determine the photoluminescence (PL) properties of gold nanoclusters pertain to the surface. In this study, four Au52(SR)32 nanoclusters that feature a series of aromatic thiolate ligands (–SR) with different bulkiness at the para-position are synthesized and investigated. The near-infrared (NIR) photoluminescence (peaks at 900–940 nm) quantum yield (QY) is largely enhanced with a decrease in the ligand’s para-bulkiness. Specifically, the Au52(SR)32 capped with the least bulky p-methylbenzenethiolate (p-MBT) exhibits the highest PLQY (18.3% at room temperature in non-degassed dichloromethane), while Au52 with the bulkiest tert-butylbenzenethiolate (TBBT) only gives 3.8%. The large enhancement of QY with fewer methyl groups on the ligands implies a nonradiative decay via the multiphonon process mediated by C–H bonds. Furthermore, single-crystal X-ray diffraction (SCXRD) comparison of Au52(p-MBT)32 and Au52(TBBT)32 reveals that fewer methyl groups at the para-position lead to a stronger interligand π···π stacking on the Au52 core, thus restricting ligand vibrations and rotations. The emission nature is identified to be phosphorescence and thermally activated delayed fluorescence (TADF) based on the PL lifetime, 3O2 quenching, and temperature-dependent PL and absorption studies. The 1O2 generation efficiencies for the four Au52(SR)32 NCs follow the same trend as the observed PL performance. Overall, the highly NIR-luminescent Au52(p-MBT)32 nanocluster and the revealed mechanisms are expected to find future applications.

Typically, a 50 ml round-bottom flask equipped with a magnetic stir bar was charged with HAuCl4•3H2O (78.74 mg, 0.2 mmol).15 ml of THF containing TOAB (136.7 mg, 0.25 mmol) was added to the flask under rapid stirring (550 rpm).The solution turned to deep orange in 20 minutes.Then, 133 μL TBBT (0.8 mmol) was added to the solution.After 30 minutes of stirring, the solution changed to light yellow, indicating the formation of Au10(TBBT)10 complex.A freshly prepared NaBH4 solution (38 mg, 1 mmol) was rapidly added to the reaction mixture.Upon mixing, the solution immediately turned black.The reaction was allowed to proceed for one hour.Then, the aqueous phase was removed by a glass pipet, and the organic phase was concentrated to ~3 mL under reduced pressure.The organic phase was washed with excess methanol (about 12 ml) five times to remove excess thiol.In the second step, 200 μL TBBT was added to the dark residue that was dissolved in 2 mL toluene.The solution was heated to 50 ºC and maintained at the temperature under an air atmosphere with gentle stirring (~60 rpm) for 16 hours.Then, the product was separated from the thiol/toluene solution by washing with excess methanol.The dark residue was extracted by a minimum amount of DCM and then purified by TLC plates.

Au52(p-MBT)32.
A vail was charged with HAuCl4•3H2O (78.74 mg, 0.2 mmol).TOAB (136.7 mg, 0.25 mmol) was put in THF (15 ml) and added to the vail.Under stirring, the mixture becomes deep orange in 20 minutes.Then, p-MBT (146 mg, 1 mmol) was added to the vial.The solution was kept stirring for 60 minutes until the color of the mixture turned to light yellow.Subsequently, NaBH4 (26.6 mg, 0.7 mmol, dissolved in 5 mL of ice-cold nanopure water) was added to the vail all at once.After 60 minutes of reaction, the aqueous phase was removed, and the black organic phase was dried by rotary evaporation.The precipitates were washed with methanol five times.The size-mixed Aux(p-MBT)y NCs were extracted from the precipitates with a small amount of DCM and dried.Further purification was conducted by PTLC.The PTLC was developed with a mixture of DCM and n-hexane (2:3 (v/v)).Block-like Au52(p-MBT)32 single crystals were obtained by vapor diffusion of CH3CN into a toluene solution of the nanoclusters at 4 C.In another method, needle-like Au52(p-MBT)32 single crystal were obtained by vapor diffusion of methanol into a 1,2,4-trichlorobenzene solution of the nanoclusters.
Au52(4-EBT)32.First, Au52(PET)32 was synthesized and separated by PTLC according to the previous work s1 .In the second step, ~3 mg of Au52(PET)32 was dissolved in 2 mL toluene and 200 μL 4-EBT.The mixture was heated at 70 C for 5 hours.The crude products were washed with excess methanol and dissolved in 2 mL of DCM.Then, the target product was isolated from the crude product by PTLC (DCM and n-hexane (1:2 (v/v)).
Au52(IPBT)32.In the first step, HAuCl4•3H2O (100 mg, 0.254 mmol) was dissolved in 15 mL of THF.TOAB (164 mg, 0.3 mmol) was added to the solution.The mixture became deep orange in 20 minutes.Then, 145 μL IPBT was added to the vail.After the color of the mixture turned to light yellow (in ~30 minutes), a freshly prepared NaBH4 solution (38 mg, 1 mmol, dissolved in 5 mL of ice-cold nanopure water) was added all at once.After stirring overnight, the reaction was stopped, and the solution was evaporated to dryness.The brown residue was washed with excess methanol and dissolved in a minimum amount of toluene (~2 mL).Next, 400 μL IPBT was added to the above solution and heated to 70 C for 5 hours.After that, methanol was added to the reaction mixture to precipitate the product, followed by centrifugation at 10,000 rpm.The solid product was washed with excess methanol five times to remove excess thiol.Finally, the residue was dissolved in a small amount of DCM (~1 ml) and purified by PTLC using a mixture of DCM and n-hexane (2:5) as a developing solvent.
Au52(p-MBT)32-x(4-FBT)x. First, Au52(p-MBT)32 was synthesized by the above method.Next, ~1.5 mg of Au52(p-MBT)32 was heated in the presence of 4-FBT (200 μL added) at 60 C for 5 hours.The resulting mixture was washed with excess methanol and further purified by PTLC using a mixture of DCM and n-hexane (2:1) as the developing solvent.
Au52(p-MBT)32-x(4MOBT)x. First, Au52(p-MBT)32 was synthesized by the above method.Next, ~1.5 mg of Au52(p-MBT)32 was heated with 200 μL 4-MOBT at 60 C for 5 hours.The resulting mixture was washed with excess methanol and further purified by PTLC using a mixture of DCM and methanol (14:1) as the developing solvent.

Preparation of Au52(SR)32 /PMMA solid thin films
Uniform solid thin films for all four NC/PMMA samples were prepared by the spin-casting method.To make the PMMA stock solution, 40 mg of PMMA was dissolved in 1 ml of DCM (Solution A), and four different stock solutions of the four NCs were prepared by dissolving 1 mg of each NC in 1 ml of DCM (Solutions B1, B2, B3, B4).Next, 10 μL as-prepared solution 'A' was mixed with 20 μL of the as-prepared solutions 'B1, B2, B3, B4' separately to obtain four ink-like solutions of the NC/PMMA mixture.Finally, the resulting mixture solutions were spin-coated one by one onto quartz plates, by taking 30 μL of the mixture solution and using 100 rpm rotation speed for 1 min 30 sec, which were subsequently air dried at room temperature.

Steady-state UV-Vis-NIR measurement
UV-vis-NIR spectra of all the Au NCs were collected on a UV-3600 Plus spectrophotometer (Shimadzu, range: 185-3300 nm).The cryogenic absorption measurements used a home-built lowtemperature system, including the UV-3600 Plus spectrophotometer, a vacuum pump, an Optistat CF2 cryostat (Oxford Instruments), and a temperature controller.

Steady-State and Time-Resolved Photoluminescence and Cryogenic Measurements
Steady-state photoluminescence spectra were measured on an FLS-1000 spectrofluorometer (Edinburgh).PL lifetimes were measured by time-correlated single photon counting (TCSPC) on the same instrument.Visible PL was measured using a photomultiplier (PMT) as the detector.Near-infrared PL was measured using a wide-range InGaAs detector (600-1600 nm) cooled to -80 ℃ with liquid nitrogen.The cryogenic PL measurements used a home-built low-temperature system, including the FLS-1000 spectrofluorometer, a vacuum pump, an Optistat CF2 cryostat (Oxford Instruments), and a temperature controller.The PLQY of Au NCs in toluene and DCM were determined by an integrating sphere.The PLQY of Au NCs/PS film was determined by comparising the emission intensity with that of the solution.

Total Internal Reflection Fluorescence (TIRF) Microscopy Measurements
Investigation on the photostability of the NC emitters in the air at a very dilute concentration was done by Total Internal Reflection Fluorescence (TIRF) Microscopy using Olympus PlanApo 60X oil immersion objective with a numerical aperture 1.4 on an inverted microscope (Olympus IX-71).The laser excitation wavelength used was 488 nm output of an Ar-Kr laser (Coherent) using a CCD camera (Photometrics Evolve 512 EMCCD) to collect signal from the sample and a 780 nm long pass filter was used to minimize the scatterings from the samples.To track the intensities of the four different NC emitters (Au52(p-MBT)32, Au52(4-EBT)32, Au52(I-PBT)32, Au52(TBBT)32), first equally concentrated nM level stock solutions of all the four samples were freshly prepared using tetrachloroethene as a solvent.The samples for the microscopy were then prepared by spin-coating 60 μL of the stock solutions on glass coverslips using 4000 rpm rotation speed for 30 seconds, followed by air drying for 10 min at room temperature and imaged at the excitation wavelength of 488 nm using a continuous laser pulse with an acquisition time of 80 ms, a 2.0 ND (Neutral-Density) filter and an excitation power of 30 μW.A continuous sequence of images for all the NC samples was captured, consisting of 120 frames with no time lag, resulting in a 12second movie.The images were analyzed using Image-Pro Plus software while maintaining the same display range for all four samples.A region of interest (ROI) with 67 pixels-square (0.18 μm/pixel, ~ 12 × 12 μm 2 ) was selected for intensity tracking measurement.Ten equally small 4 pixels-square regions (0.72 × 0.72 μm 2 ) consisting of the emitters were selected from the ROI and their maximum initial intensities were measured using the Image Pro Plus software for all four samples.The same technique was followed to get the background intensities surrounding the emitters from the ROI which were then subtracted from each frame to get the actual intensity of the emitters for all the samples.The final data were then plotted in terms of the percentage distribution of objects (emitters) vs their intensities to get the most probable initial intensity from the distribution of ten emitters for each sample.Then one of the emitters with the most probable initial intensity was selected from each sample and its intensity over time was tracked and plotted to compare the single-step photobleaching time for the four Au52(SR)32 NCs.s4

X-ray crystallography
A black columnar crystal of Au52(S-pMeC6H4)32 was used for data collection on a Bruker Duo diffractometer with a PHOTON II detector and Iμs CuKα radiation (1.54178 Å) at 200 K.The structure was solved in the monoclinic space group P2(1)/c.Integration and scaling of the data yielded 53,5238 reflections, of which 31,983 were unique (and 16,111 unique data with I > 2σ(I)), to a maximum of θ = 50.6°(d = 1.0 Å) with a completeness of 99.5% and an Rint of 16.22%.
All Au and S atoms were refined with anisotropic thermal displacement parameters, while all carbon atoms were treated with an isotropic thermal displacement parameter and subjected to similarity restraint (SIMU).Hydrogen atoms were placed in idealized positions and treated with a riding model.Each carbon atom belonging to a phenyl ring was constrained at the corners of a regular hexagon (AFIX 66), numerous distance restraints (SADI) were applied as needed, coplanarity restraints were also applied as needed (FLAT).Thus, the final model consisted of 1,301 parameters with 269 restraints.

Absolute Quantum Yield Determination
The absolute PLQY of Au52(p-MBT)32 in DCM solution at room temperature was determined by an integsrating sphere.A sample figure is shown below.For such a measurements, the regular sample holder is removed, and the integrating sphere is mounted in the FLS1000 spectrometer.The blank spectrum (light blue line in the Figure ) was obtained by putting a blank in the integrating sphere and collecting the emission scan (repeat 5 times) from 500 nm to 1600 nm with 470 nm excitation.The sample spectrum (yellow) was measured by replacing the blank with a dilute Au52(p-MBT)32 solution (0.1 OD at 470 nm) and repeating the emission scan (5 times) from 500 nm to 1600 nm with 610 nm excitation.The PLQY is calculated by:  =  /  −  where,  is the area under the blue line between 500 and 530 nm,  is the area under the yellow line between 500 and 530 nm,  is the area under the yellow line between 800 and 1200 nm.

Relative Quantum Yield Determination
The relative quantum yield (ΦS) of the sample is calculated by using: where, Φ R is the quantum yield of the reference (standard), I is the integrated PL intensity, A is the absorbance of the solution at the excitation wavelength, n is the refractive index of the solvent, and the subscripts (S and R) represent the sample and the reference, respectively.

Figure S9 .Figure S10 .
Figure S9.(a) PL spectra of Au52(TBBT)32 at different temperatures and (b) PL lifetime profiles (see S7-S9 data fitting in Fig S10, the case of p-MBT in presented in main text).

Figure S12 .
Figure S12.Blank DPBF solution in DMF in the absence of gold nanoclusters under irradiation with λ = 500 nm.