Organo metal halide perovskites effectively photosensitize the production of singlet oxygen ( 1 D g )
Metal halide perovskite (MHP) materials are under extensive examination for various high performance optoelectronic applications, but their photosensitizing utilization for singlet oxygen generation has not been explored. Based on the observed characteristic NIR phosphorescence of singlet oxygen and the photosensitized decomposition of a specific singlet oxygen chemical trapper (diphenylisobenzofuran), we show herby that MAPbBr3 quantum dots can significantly generate singlet oxygen with the quantum yield up to 0.34, which is the highest among nano semiconductor and nano metal material types of singlet oxygen photosensitizers. Further more, MAPbBr3 quantum dots show good stability towards singlet oxygen. The mechanism of MHP as a singlet oxygen photosensitizer is also suggested to be due to triplet excitons. This finding expands new applications of MHPs in PDT, antiviral and antibacterial treatments, and pharmaceutical industry.
Introduction<!>Results and Discussion<!>5) Stability of MAPbBr3 NPs towards singlet oxygen<!>Conclusions<!>Experimental Section<!>Instruments<!>Perovskites
<p>Singlet oxygen ( 1 Dg) production and its oxidizing effect are among the key interests of chemistry, biology, material and medical sciences. [1] The photosensitized singlet oxygen ( 1 Dg) generation has been a fast growing research field due to its very important applications in cancer therapy, [2] antiviral and antibacterial treatments, [3] photodecomposition of organic pollutants as well as environmental protection. [3b] In addition to organic singlet oxygen photosensitizers (PSs), [4] nano semiconductors and nano metals are also proposed in recent years to act as new type of PSs, [2a, 5] but their efficiency remains low. [5a, 5c] Metal halide perovskites (MHPs) are excellent and versatile semiconductors which have shown outstanding performance in perovskite solar cells (PSCs) and light emitting diodes due to their very strong light-absorbing and light-emitting ability. [6] MHP materials are therefore under extensive studies to expand their important applications in tandem PVs, [7] water splitting architectures, [8] photodetectors, [9] and optical gain media. [10] However, their photosensitizing ability for singlet oxygen ( 1 Dg) generation has not been reported in literature.</p><p>MHP materials can be good singlet oxygen photosensitizers, since they absorb light very strongly in the whole visible region [6a] which exceeds known organic PS for excited triplet state and singlet oxygen, such as porphyrin, phthalocyanine, and xanthene dyes. The MHP quantum dot (QD) is luminescent from their excited singlet exciton with quantum efficiency from 0.50 to 0.90, [6a, 6b] which gives the possible triplet exciton formation efficiency from 0.50 to 0.10. The knowledge on photosensitizing properties of MHPs is not only very valuable in finding their new 4 applications but also important for understanding and improving the photostability of PSCs, which is the key issue of PSC commercialization. Therefore we identify and quantify molecular singlet oxygen generated by MHP photosensitizers in this study.</p><p>PSs generate singlet oxygen through following two processes: 1) light absorbing to generate its excite state * PS: PS + light ® * PS; 2) energy transfer from * PS to molecular oxygen to form singlet oxygen: [11] * PS + O2 ® PS + 1 O2( 1 Dg). The * PS can be either the triplet or the singlet excited state. [12] It has been theoretically predicted that MAPbX3 (X= Br, I) type perovskites can form excited triplet excitons due to spin-orbital interaction. [13] Taking all this into consideration together, we decide to test the ability of MAPbX3 (MA=CH3NH3, X=Br, I) nano particles (NPs) in photosensitizing generation of singlet oxygen ( 1 Dg). Using the characteristic NIR phosphorescence of singlet oxygen and the specific singlet oxygen chemical trapping agent, we show herby that MAPbBr3 QD can significantly generate singlet oxygen with the quantum yield up to 0.34, which is the highest among nano-particle (metal and semiconductor nano materials) singlet oxygen photosensitizers.</p><!><p>(1) Perovskite NPs preparation and characterization MAPbBr3 QDs are prepared facilely by mixing MABr and PbBr2 in air saturated solvent. MAPbI3 QDs, however, need to be synthesized in the inert gas atmosphere.</p><p>MAPbBr3 QDs are also stable in air saturated solvent longer than three months, while MAPbBrxI3-x (x=0,1) QDs are quickly faded upon exposing to air. We therefore focus 5 on MAPbBr3 QDs in this study. Figure 1 shows the XRD and TEM of MAPbBr3 QDs. X-ray powder diffraction peaks match the reported XRD patterns of MAPbBr3 crystal, [14] indicating that a single-phase MAPbBr3 was formed and thus used in the subsequent optical measurements. TEM image (Figure 1) shows that the MAPbBr3 particles are spherical with median size of 5.5 nm. The UV-Vis absorption spectrum shows a band edge at ≈2.41 eV (514 nm), which is close to the onset of the band edge at 2.40 eV in literature. [15] Figure 1 also displays the photoluminescence spectrum, the emission peak maximum is at 2.40 eV (≈517 nm) in toluene with an approximate Stokes shift of 14 meV, the photoluminescence quantum yield is 0.64 which comparable to that reported (0.50 to 0.70). [15]  (2) Detection of singlet oxygen by NIR photoluminescence .</p><p>The first evidence for singlet oxygen produced by the nanoscaled perovskite 6 photosensitization is the observed characteristic NIR emission band (Figure 2). The NIR photoluminescence shows the peak maximum at 1272±2 nm in toluene, chloroform and carbon tetrachloride. The band position and band shape very well matches that of singlet oxygen ( 1 Dg) in literature. [11] In the absence of perovskite NPs control experiments show that the photoexcitation did not result in any detectable singlet oxygen emission signal. The NIR emission lifetime is 23 µs in toluene and 170 µs in chloroform. The lifetime values are also consistent with that of singlet oxygen ( 1 Dg) in literature. [16] I2-TMBDP is a well known singlet oxygen photosensitizer, its NIR emission spectra and emission decay were also measured and shown in Figure 2 for comparison. Both the emission spectrum and lifetime of I2-TMBDP sensitized singlet oxygen is consistent with that for MAPbBr3 NPs. (3) Detection of singlet oxygen by chemical trapping and F D DPBF is a well known specific chemical trapper of singlet oxygen. [17] One singlet  The photo irradiation was carried out at 473 nm in air saturated solvents. At the wavelength of 473 nm, the light is absorbed by perovskite NPs but not by the DPBF based on their individual absorption spectrum. Figure 3 clearly shows that the DPBF absorption is decreased while the absorption of perovskite NPs is not changed with time during photo irradiation. Control experiments showed that the DPBF absorption was not decreased under following conditions: 1) oxygen is purged by argon bubbling, 2) the perovskites NPs is not added to the solution, or 3) light irradiation is not carried out. These experiments clearly indicate that the DPBF decomposition needs the coexistence of perovskites NPs, oxygen, and light irradiation. These facts show that the perovskite NPs act as the singlet oxygen photosensitizer.</p><p>The chemical kinetics of DPBF decomposition is zero order as shown in Figure 3, since the plotting of A0-At against time t is linear (at 420 nm): A0-At = k•t, in which k is the reaction rate constant, A0 and At is the absorbance of 420 nm at irradiation time zero and time t, respectively. From the linear plotting (Figure 3), the reaction rate constant k is obtained for the perovskite NP photosensitizer in each solvent. In the same way, the reaction rate constant k ref is obtained for the standard photosensitizer in the same solvent. Comparing k and k ref , we can obtain the singlet oxygen formation quantum yield FD using</p><p>The calculated FD for the perovskite NP photosensitizer changes batch to batch samples, it is 0.34 to 0.21 in chloroform and 0.21 to 0.11 in toluene. The highest FD value of 0.34 makes it an effective singlet oxygen photosensitizer. In fact this FD value of 0.34 exceeds that for semiconductor and metal NP photosensitizers. [5a] For example, FD value for Ag, Pt, and Au nano particle PSs is 0.155, 0.085, and 0.037 respectively, [5a] while FD value of 9 CdSe QDs is only ~5%, [18] all of which are significantly smaller than that of the MAPbBr3 NPs.</p><p>(4) Mechanism of singlet oxygen formation: singlet or triplet excitons?</p><p>The NIR emission and DPBF chemical trapping studies show that the nanoscaled perovskites are the singlet oxygen photosensitizer (PS), together with the controlled experiments, we conclude that DPBF is decomposed through the following processes in which * PS is an exciton of the perovskites NPs. For organic photosensitizers, the *PS is often an excited triplet state molecule 3 PS, but it can also be an excited singlet state 1 PS in some cases. [12] Is it a singlet exciton in our case? Figure 4 shows the photoluminescence of the NPs in argon and oxygen 10 saturated solvent respectively. Since oxygen shows little quenching to the emission intensity and lifetime of MAPbBr3 NP singlet exitons (Figure 4), indicateing that the singlet excitons do not interact with molecular oxygen remarkably. Therefore the contribution from singlet exciton of the NPs to the singlet oxygen generation is negligible. The triplet excitons are concluded to play essential roles in the singlet oxygen formation process. This conclusion is also supported by (1) the theoretical calculation by Li who have shown that triplet excitons of perovskites are formed by spin-orbit interactions between Pb and halogen atoms in lead halide perovskites; [13] and (2) the triplet exciton presence is evidenced by the phosphorescence reported by Younts. [19] (</p><!><p>It is important to know that if MAPbBr3 NPs are stable to singlet oxygen. Therefore we measured the stability of MAPbBr3 NPs toward singlet oxygen generated by using a very efficient photosensitizer I2-TMBDP. As shown in Figure 5, the absorbance of MAPbBr3 NPs (330, 430, and 470 nm) shows no change upon light irradiation at 540 nm in toluene. In comparison, DPBF absorbance is decreased quickly under the condition. This fact shows that MAPbBr3 NPs are fairly stable to singlet oxygen.</p><!><p>We have detected and quantified singlet oxygen formation photosensitized by perovskites for the first time. MAPbBr3 NPs have the quantum yield up to 0.34 which is the highest among semiconductor and metal nano materials photosensitizers.</p><p>MAPbBr3 NPs are also fairly stable to singlet oxygen. It is the triplet exciton of MAPBBr3 NPs that is mainly responsible for singlet oxygen generation. This finding suggests new potential applications of perovskite NPs in PDT, antiviral and 12 antibacterial treatments, and pharmaceutical industry.</p><!><p>Materials. Methyl ammonium bromide (CH3NH3Br = MABr, 99.9%), lead (II) bromide (PbBr2, 99.99% trace metals basis), N,N-dimethylformamide (DMF, anhydrous, 99.8%), toluene anhydrous (99.5%), chloroform anhydrous (99.5%), diphenylisobenzofuran (DPBF, 99.5%) were purchased from Sigma-Aldrich.</p><p>Chloroform (>99%) and toluene (>99%). Octylamine (approximate C18-content 80-90%) was purchased from Acros Organics. Oleic acid (97%) was purchased from VWR Chemicals. Perovskites nano particles (NPs) preparation. The synthesis were carried out according to reported method for CH3NH3PbX3 (X=I, Br) with slight modification. [15] All syntheses were carried out at room temperature in air with average room humidity ~60%.</p><!><p>MAPbBr3 NPs. CH3NH3Br (0.0088 g, 0.1 mmol) and PbBr2 (0.0367 g, 0.1 mmol) were dissolved in 2.5 mL DMF containing 250 μL of the oleic acid and 10 μL octylamine. Next, 50 μL of this mixture was injected into 5 mL of toluene or chloroform. Bright green light-emitting NPs were formed immediately after fast agitation. After centrifugation at 8000 rpm for 5 min, the clear liquid part was used for various measurements. Steady-state photoluminescence spectra and quantum yields were acquired on a FLS920 of Edinburgh Instruments, the excitation wavelength of 470 nm was employed for both fluorescein and perovskites NPs. All spectra were corrected for the sensitivity of the photo-multiplier tube. The photoluminescence quantum yield (Ff) was calculated by using Eq. ( 1), [20] ,</p><!><p>Eq. ( 1) in which F is the integrated fluorescence intensity, A is the absorbance at excitation wavelength, n is the refractive index of the solvent used, the subscript 0 stands for a reference compound and s represents samples. Fluorescein was used as the reference ( = 0.92 in 0.1 M NaOH aq. solution). [20b] The sample and reference solutions were prepared with the same absorbance (Ai) at the excitation wavelength (near 0.09 per cm).</p><p>Photoluminescence lifetime was measured by time-correlated single photon counting method (Horiba Scientific, Fluorolog-3 spectrophotometer) with excitation at 485 nm diode laser (50 ps FWHM) and emission was monitored at 510 nm. The lifetime values were fit by biexponential decay function I(t)=A+B1exp(-t/t1) +B2exp(-t/t2).</p><p>Singlet oxygen NIR phosphorescence. The NIR steady state photoluminescence spectra were recorded by Fluorolog-3 spectrophotometer equipped with an electronic</p><p>F cooled InGaAs detector (900-1700 nm) with excitation at 400 nm using a standard 1 cm quartz cuvettes. Both the emission and excitation slits were adjusted to their maxima. A long pass filter of 1000 nm was placed between a sample and the detector for all NIR experiments to filter away any stray light and the second harmonic of the excitation light with wavelengths shorter than 1000 nm. For singlet oxygen lifetime measurements, the excitation source is changed to a 1 µs flash lamp (450 nm, 1 mW), the emission at 1270 nm was monitored.</p><p>Singlet oxygen chemical trapping. Singlet oxygen quantum yield (FD) were determined using the well known chemical trapping method. [21] Typically, a 3 mL toluene that contains perovskites NPs and 20 µM diphenylisobenzofuran (DPBF) was irradiated at 473 nm by a LED light under air (or oxygen) saturated condition. FD value was obtained by the relative method using Eq. (2): , Eq. ( 2)</p><p>where is the singlet oxygen quantum yield for the reference standard I2-TMBDP</p><p>(2,6-diiodotetramehylBODIPY, FΔ R =0.96 in toluene) with excitation at 473 nm), [22]</p>