P hotoluminescence (PL) has long been of major interest in both fundamental research [1][2][3][4][5][6][7][8] and practical applications, [9][10][11][12][13] particularly, the dual emission phenomena for deciphering the underlying mechanisms and with great potentials in many fields, including ratiometric sensing, bioimaging, solar cells, and even supramolecular encryption systems. 14,15 Ultrasmall Au nanoparticles with atomic precision (1-3 nm in diameter), commonly called nanoclusters (NCs), show near-infrared (NIR) PL. [16][17][18][19][20][21][22][23][24][25] The determined atomic structures and electronic structure calculations 26 allow the elucidation of structure-property correlations, [16][17][18][19][20][21][22][23][24][25] and the intrinsic merits of Au NCs such as being biocompatible, stable, and nontoxic make this class of materials quite promising for solar energy conversion and biological applications. [27][28][29][30][31] So far, dual emission in Au NCs has only been observed in several bitetrahedral kernel structures, and the PL quantum yields (QYs) of those Au NCs in the NIR region were quite low (∼1%). 18 Herein, we report the NIR dual emission of Au 42 (PET) 32 with a QY (11.9%) at room temperature under ambient conditions. The efficient dual emission of Au 42 (PET) 32 is rare for thiolate-protected Au NCs. The two emission peaks of Au 42 (PET) 32 at 875 and 1040 nm are fluorescence and phosphorescence, respectively. When Au 42 (PET) 32 NCs are embedded in polystyrene (PS) films, the QY of phosphorescence significantly increases from 8.6% to 20.3% while the fluorescence is suppressed from 3.2% to 1.1%. The mechanism involves dipolar interaction-induced intersystem crossing, which results in the observed divergent behavior of fluorescence and phosphorescence.

Different from the reported synthesis method, 32 here we used a NHC-Au-Br complex (NHC = N-heterocyclic carbene) as the precursor for the synthesis of thiolate-protected Au NCs (see Supporting Information for details). Au 42 (PET) 32 was obtained by chromatography separation (Figure S1). Electrospray ionization (ESI) mass spectrometry analysis on the purified product identified peaks of [Au 42 (PET) 32 ] 2+ , [Au 42 (PET) 32 +2Cs] 2+ , and [Au 42 (PET) 32 ] + (Figure 1A), and their experimental isotopic patterns match well with the calculated ones; thus, the obtained NC is Au 42 (PET) 32 . Its UV-vis absorption spectrum (Figure 1B, green profile) shows two major peaks at 375 and 806 nm, being identical with the structurally characterized Au 42 (BM) 32 (Figure S2A), 32 where BM = SCH 2 Ph. ESI together with absorption spectra confirms that Au 42 (PET) 32 should possess the same structure as that of Au 42 (BM) 32 nanorods except the carbon tail, i.e., a hexagonal close packed Au 20 kernel in Au 42 (PET) 32 (abbreviated Au 42 hereafter) and its protection by two pairs of interlocked Au 4 (SR) 5 staple motifs on the ends and six monomeric Au(SR) 2 motifs around its body (Figure 1C). Based on previous density functional theory (DFT) calculations, 32 the 806 nm peak arises from the HOMO-LUMO transition, and the 375 nm peak is from HOMO-1 to LUMO+2 and HOMO-2 to LUMO+1 (Figure 1D).

Interestingly, the PL spectrum of Au 42 in dichloromethane (DCM) shows dual emission in the NIR region (Figure 1B, gray profile), with peak wavelengths at 875 and 1040 nm, similar to Au 42 (BM) 32 (Figure S2B). The QY of Au 42 in DCM under ambient conditions is 11.9% (measured by an integrating sphere), which surpasses the reported thiolateprotected NCs with NIR PL [16][17][18][19][20][21][22][23][24][25]33,34 except Au 38 S 2 (SR) 20 (NIR QY 15%, single emission). 35 The PL excitation spectra for the two emissions (I and II) are similar and also track the absorption profile (Figure S3), suggesting that both emissions are associated with the HOMO-LUMO transition (Au 20kernel-based). Furthermore, PL I has a short lifetime (0.72 ns, Figure 1E), which can be assigned as fluorescence, while PL II gives rise to a long lifetime up to 2.4 μs, which should be a triplet emission (Figure 1F).

The dual emission of Au 42 was further tested under pure N 2 and O 2 , respectively (Figure 2A). The overall integrated intensity of PL I and II was suppressed to 83% under pure O 2 (Table S1). To quantitatively understand the evolution of the two bands, the PL spectra were deconvoluted into two Voigt profiles to extract the contribution from each peak (Figure S4A,B). As given in Table S1, the intensity of PL I almost remained the same under different gases, while PL II was quenched to 78% under pure O 2 . The lifetime of PL II drastically decreased from 2.4 μs to 1.5 μs, but PL I only slightly decreased from 0.72 ns to 0.64 ns (Figure S5). These results further confirm that PL I is fluorescence and PL II is phosphorescence. Considering the 0.12 eV Stokes shift (i.e., between the HOMO-LUMO peak and PL I) and the 0.2 eV difference between PL I and II, we deduce that the PL behavior of Au 42 follows Kasha's rule; that is, PL I is from the first excited singlet state (S 1 ) and PL II from the first excited triplet state (T 1 ).

To understand the nonradiative relaxation process in Au 42 , temperature-dependent steady-state PL measurements for Au 42 in 2-methyltetrahydrofuran were carried out (from room temperature down to 80 K, Figure 2B). To eliminate the effect of O 2 , the sample chamber of the cryostat was filled with helium gas during the experiment. Figure 2B shows that both PL peaks become sharper with a slight blue-shift as the temperature decreases from 298 K to 80 K, indicating a strong electron-phonon interaction in Au 42 . 36 The integrated intensity of PL I and II was found to increase by 3.2 times from 298 to 80 K, which means that the QY reaches 38.1% at 80 K. We also performed peak deconvolution for all the temperature-dependent spectra and calculated the PL intensity relative ratios of PL I and II (Figure S6A). The PL II is found to account for 60% of the total PL intensity at room temperature, and its percentage rises to 70% at 80 K, which means that PL II is more favored at low temperatures than PL I, supporting the phosphorescence nature of PL II. Furthermore, the quantitative temperature-dependent intensity evolutions for both emission peaks are plotted in Figure 2C, where the PL intensity at the lowest temperature is set as unity. To analyze the temperature-dependent data for extracting information on the thermally activated nonradiative relaxation pathway, we adopt an Arrhenius expression (eq 1): 37

where I 0 is the initial intensity, a is the ratio of nonradiative and radiative probabilities, and E is the activation energy for the nonradiative relaxation channel. Here, only one dominant phonon-assisted nonradiative channel is considered in our modeling. The resulted fit lines and parameters are given in Figure 2C, where the activation energies of phonon modes that are coupled with PL I and II are determined to be 54.1 and 43.7 meV, respectively. Based on previous theoretical and experimental work, [38][39][40][41] the two phonon modes (300-400 cm -1 ) can be assigned to the Au-S vibration of the two Au 4 (SR) 5 staple motifs on both ends of Au 42 .

Given the facts that the dominant nonradiative relaxation channel in Au 42 solution is from the motif phonon mode and its NIR PLQY reaches 38.1% at 80 K, we rationalize that if the motif vibration can be suppressed at room temperature, the PL performance of Au 42 should be dramatically improved. To test our hypothesis, Au 42 NCs are embedded in a PS thin film via a drop-cast method. The emission spectrum of Au 42 /PS film at room temperature is shown in Figure 2D (blue curve), and excitation spectra are shown in Figure S7; indeed, the integrated intensity of PL I and II increases by 1.8 times compared to Au 42 in DCM, hence, an overall 21.4% QY for Au 42 /PS film. Interestingly, peak deconvolution results (Figure S4C, Table S1) find that the behavior of the two PL bands of films diverges: the QY of PL I decreases from 3.2% to 1.1% in the solid state, while the QY of PL II increases from 8.7% to 20.3%. The lifetimes of both PL I and II are slightly shorter in the solid state (Figure S8). Furthermore, the emission of the Au 42 /PS film under O 2 (red line, Figure 2D) is quenched to 87% compared to the N 2 atmosphere. Further quantitative analysis (Figure S4D, Table S1) finds that only the intensity of PL II was significantly affected by the existence of O 2 , which is consistent with the solution results discussed above. The lifetimes of both PL bands become shorter under O 2 but within their respective nanosecond and microsecond scale, indicating the emission origins are unchanged.

To understand the divergent behavior of the two PL bands from solution to film state, we conducted cryogenic photoluminescence measurements for Au 42 /PS films (Figure 2E). The overall PL intensity of Au 42 /PS film increases by 2.1 times when the temperature decreases from room temperature to 20 K. In line with the solution results, PL II is more favored at low temperatures than PL I (Figure S6B), and both PL bands show a small blue-shift at low temperatures. After applying eq 1 to the data collected from the solid state (Figure 2F), we notice that both PL I and II are coupled with a phonon mode of ∼14 meV (as opposed to ∼50 meV in solution). Low-frequency phonon modes (<150 cm -1 ) are typically ascribed to the Au-Au vibration in the kernel of Au NCs. 40,42 Here, the 14 meV (112 cm -1 ) mode most likely originates from the breathing or extensional mode of the rod-like Au 20 kernel in Au 42 , but further theoretical simulations are required for confirmation. The a value (i.e., the ratio of nonradiative and radiative decays) also falls drastically from ∼10 (in solutions) to ∼2 (in films), indicating a significant suppression of the staple motif vibration-induced nonradiative decay. 33,40 Since the phononassisted nonradiative relaxation is suppressed for both PL I and II, one would expect to see that both PL I and II should have an equal enhancement of QY in the solid state, but this is not the case; rather, a divergent behavior was observed for PL I and II. Therefore, the expected enhancement of PL I must have been counteracted by another mechanism, which results in a net quenching of the fluorescence band from S 1 in the solid state.

The counteracting mechanism should be S 1 to T 1 intersystem crossing (ISC), Scheme 1A. We rationalize that the suppression of fluorescence and enhancement of phosphorescence from solution to film state should originate from dipole-dipole interactions. 43 According to Kasha's exciton model, dipolar interactions will cause a split in the excited state of the Au 42 dimer and narrow the gap between S 1 and T 1 states. 44 Such a model can be extended to assemblies where the split excited states become denser and yield a bandlike electronic structure (Scheme 1B), 45 evidenced by the 806 nm peak broadening in the film's UV-vis absorption spectrum (Figure S9). According to Fermi's golden rule, the intersystem crossing rate can be described by eq 2, 46 |

where ΔE S-T is the energy gap between singlet and triplet states, H S-O is the Hamiltonian describing the spin-orbit coupling, and Ψ S and Ψ T are wave functions for singlet and triplet states, respectively. Without any change in the structure of Au 42 between the solution and solid state, the difference in the perturbation term caused by spin-orbit coupling should be negligible. Therefore, when ΔE S-T becomes smaller in the solid state, ISC is enhanced, hence, the phosphorescence (Scheme 1B). Considering the dipolar interaction is distancedependent, we tested the PL of films with different Au 42 loadings to adjust the average distances among NCs and indeed observed a systematic trend of decreasing QY of PL I and increasing QY of PL II (Figure 3) as the Au 42 mass fraction increases, which provides substantial evidence for the proposed mechanism.

In summary, bright dual emission (PLQY = 11.9%) in the NIR region is attained in the Au 42 (PET) 32 , which comprises fluorescence (QY = 3.2%) and phosphorescence (QY = 8.7%). After embedding Au 42 in films, the QY of phosphorescence significantly increases to 20.3% while the fluorescence is suppressed to 1.1% due to enhanced intersystem crossing. The efficient dual NIR emission makes Au 42 a promising candidate for future applications.