Organic Phosphorescence Nanowire Lasers - ACS Publications

Apr 17, 2017 - Graduate University of Chinese Academy of Sciences, Beijing 100049, ... Department of Chemistry, Capital Normal University, Beijing 100...
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Organic Phosphorescence Nanowire Lasers Zhenyi Yu,†,‡,⊥ Yishi Wu,*,‡ Lu Xiao,‡,⊥ Jianwei Chen,‡,⊥ Qing Liao,§ Jiannian Yao,†,‡,⊥ and Hongbing Fu*,†,‡,§

J. Am. Chem. Soc. 2017.139:6376-6381. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/08/19. For personal use only.

†

Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ⊥ Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Chemistry, Capital Normal University, Beijing 100048, People’s Republic of China S Supporting Information *

ABSTRACT: Organic solid-state lasers (OSSLs) based on singlet fluorescence have merited intensive study as an important class of light sources. Although the use of triplet phosphors has led to 100% internal quantum efficiency in organic lightemitting diodes (OLEDs), stumbling blocks in triplet lasing include generally forbidden intersystem crossing (ISC) and a low quantum yield of phosphorescence (ΦP). Here, we reported the first triplet-phosphorescence OSSL from a nanowire microcavity of a sulfide-substituted difluoroboron compound. As compared with the unsubstituted parent compound, the lone pair of electrons of sulfur substitution plus the intramolecular charge transfer interaction introduced by the nitro moiety lead to an highly efficient T1 (π,π*) ← S1 (n,π*) ISC (ΦISC = 100%) and a moderate ΦP (10%). This, plus the optical feedback provided by nanowire Fabry−Perot microcavity, enables triplet-phosphorescence OSSL emission at 650 nm under pulsed excitation. Our results open the door for a whole new class of laser materials based on previously untapped triplet phosphors.

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INTRODUCTION Organic semiconductors have great potential in optoelectronic devices, owing to their chemically tunable optoelectronic properties and self-assembly amenability to low-cost fabrication.1−3 Organic light-emitting diodes (OLEDs) already entered the market of flat-panel displays and lighting applications.4,5 Historically, OLEDs have been developed based on fluorescent materials, but with low internal electroluminescence quantum efficiency (ηint) owing to the exciton branching ratio between the first singlet (S1) and the first triplet (T1) excited states via electrical excitation.6−8 Thereafter, the ηint value of OLEDs has been lifted nearly up to 100% by using phosphorescent materials.9,10 As another important class of organic light sources, organic solid-state lasers (OSSLs) have been achieved in a variety of resonator geometries.11−13 The representative four-level system involves excitation from the ground state S0 to vibrational sublevels of S1 followed by a rapid vibrational relaxation process to the bottom of S1. Then the laser transition occurs to the first vibronic replica of S0, followed by nonradiative relaxation to the bottom of S0 that proceeds rapidly within several picoseconds.14,15 The short excited-state lifetime of S1 (1−10 ns) demands a high pump rate to obtain high gain, but too high a pump rate results in exciton−exciton annihilation, such as singlet−singlet annihilation (SSA) and singlet−triplet annihilation (STA).13 A longlifetime triplet state is theoretically conducive to obtain a population inversion, yet stimulated emission based on © 2017 American Chemical Society

phosphorescence remains a great challenge mainly by two stumbling blocks. (i) Both the T1 ← S1 intersystem crossing (ISC) and the S0 ← T1 phosphorescence proceed with relatively low efficiencies owing to the spin-forbidden nature. As a result, the extremely weak phosphorescence (ΦP< 1%) makes it difficult to achieve optical gain.16 (ii) The long lifetime of triplet states can lead to a significant buildup of the triplet population, leading to triplet−triplet annihilation (TTA), also an obstacle to phosphorescence lasing.17,18 Moreover, the strong excited-state absorption of Tn ← T1 transition often overlaps with the phosphorescence emission and introduces an additional channel of loss that competes with the stimulated emission process. Therefore, the realization of stimulated emission based on triplet state phosphors has extremely vital significance to the development of electrically injected OSSLs. Traditionally the T1 ← S1 ISC process is promoted through the heavy-atom effect by the incorporation of halogens or largeatomic-number metals in molecules, such as iridium and platinum complexes.19,20 As far as metal-free pure organic phosphors are concerned, the spin−orbital coupling strength can be enhanced by the singlet−triplet mixing of different molecular orbital configurations (El-Sayed rule), for example, aromatic carbonyls,21,22 and by introducing intramolecular charge transfer (ICT) interactions.23−26 Here, we designed and Received: February 14, 2017 Published: April 17, 2017 6376

DOI: 10.1021/jacs.7b01574 J. Am. Chem. Soc. 2017, 139, 6376−6381

Article

Journal of the American Chemical Society

Figure 1. (a, b) Energy level schematics of S-BF2 and C-BF2. Solid arrows represent probable electronic transitions, while less probable transitions are represented by dashed arrows. The LUNTO (lowest unoccupied natural transional orbital, upper image) and HONTO (highest occupied natural transional orbital, lower image) for S1 and T1 are obtained by the density functional method at the B3LYP/6-31G(d) level. (c) Steady-state absorption (solid line) and photoluminescence (dashed line) spectra of C-BF2 (red) and S-BF2 (blue) in 1 × 10−5 M CH2Cl2 solutions. (d) PL decay and fitted curves of C-BF2 fluorescence at 515 nm (red) and S-BF2 phosphorescence at 575 (nm) in CH2Cl2 solutions.

Table 1. Photophysical Parameters of Boron Compounds in CH2Cl2 Solution

a

sample

S1a (eV)

T1a (eV)

ΔEST (eV)

λabs(nm)

λPL(nm)

ΦPL(%)

τF(ns)

C-BF2 S-BF2

2.389 2.365

1.851 1.998

0.538 0.367

430 380 440

515 575

90 10

7.6 ± 0.2

τP(μs) 8.3 ± 0.1

S1 and T1 is the first energy level of singlet and triplet state obtained by DFT calculations based on the B3LYP/6-31G(d) level.

simulated natural transition orbitals of S1 and T1 states. As predicted by DFT calculations, both the S1 and the T1 states of C-BF2 are of π−π* type (see the right part of Figure 1b), with calculated fluorescence and phosphorescence emission wavelengths at 519 nm (2.389 eV) and 670 nm (1.851 eV), respectively (Table 1). Notably, the sulfur substitution in S-BF2 successfully results in the appearance of a lower-lying S1(n,π*) state together with a S2(π−π*) state (see the right part of Figure 1a). As T1 of S-BF2 is of (π,π*) type, we expected that T1(π,π*) ← S1(n,π*) ISC in sulfide-substituted compound is allowed by the El-Sayed rule. This is in sharp contrast to the optically forbidden T1(π,π*) ← S1(π,π*) ISC in parent C-BF2. Moreover, it can be seen that nitro-substitutions induce strong ICT in both cases. The singlet−triplet energy gap is ΔEST = 0.367 eV in S-BF2, smaller than ΔEST = 0.538 eV in C-BF2 (Table 1). This also facilitates the ISC process in S-BF2.27 Figure 1c depicts the steady-state absorption (solid) and photoluminescence (PL) (dash dot) spectra of C-BF2 (red) and S-BF2 (blue) in dilute CH2Cl2 solutions. Table 1 summarizes the absorption and photoluminescence (PL) data as compared with theoretical calculation results. The parent

synthesized a sulfide-substituted difluoroboron compound, (E)3-(((4-nitrophenyl)imino)methyl)-2H-thiochroman-4-olate· BF2 (S-BF2) (see Figure 1a). As compared with the parent compound of (E)-2-(((4-nitrophenyl)imino)methyl)-naphthalen-1-olate·BF2 (C-BF2) (see Figure 1b), sulfide-substitution in S-BF2 inverse the lowest singlet π−π* electronic transition by an n−π* transition. In sharp contrast to the forbidden T1(π,π*) ← S1(π,π*) ISC in C-BF2, ISC from S1 (n,π*) to T1 (π,π*) was found to occur with 100% efficiency in S-BF2. With the help of the nitro moiety, which introduces ICT character in S-BF2, a moderate phosphorescence efficiency ΦP was obtained (10%). By using a nanowire Fabry−Perot microcavity, tripletphosphorescence OSSL was realized under optical excitation. Our results open the door for a whole new class of laser materials based on previously untapped triplet phosphors.

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RESULTS AND DISCUSSION

To understand the electronic structures of our compounds, we first performed density functional theory (DFT) calculations at B3LYP/6-31G(d) level. Figure 1a and b exhibits the calculated energy levels of S-BF2 and C-BF2, respectively, together with 6377

DOI: 10.1021/jacs.7b01574 J. Am. Chem. Soc. 2017, 139, 6376−6381

Article

Journal of the American Chemical Society

Figure 2. (a) Photoluminescence microscopy images of the nanowires. (b) TEM image of a single nanowire. (c) Corresponding SAED pattern of the single nanowire shown in b. (d) Molecular packing arrangement of S-BF2 viewed along the crystal c-axis. (e) XRD spectrum of a mat of nanowires deposited on a silicon wafer. The inset illustrates a cartoon of nanowire.

compound of C-BF2 exhibit a strong S1 ← S0 π−π* absorption band at 430 nm, while its PL spectrum shows a maximum at 515 nm. We measured the PL quantum yield through an absolute method by using an integration sphere28 and found that the compound C-BF2 is highly emissive with a ΦPL,C‑BF2 = 90% in the dilute solution (Table 1). To further clarify the nature of the excited state, we performed time-resolved PL measurements (Figure 1d). The monomer emission of C-BF2 at 515 nm decays monoexponentially (see the inset of Figure 1d), yielding a lifetime of τF,C‑BF2 = 7.6 ± 0.2 ns. All of these features suggest that the PL of C-BF2 originates from the fluorescence emission from S1 to S0 (see Figure 1b). Notably, the S-BF2 has a relatively weak S1 ← S0 n−π* absorption band at 440 nm and an intense S2 ← S0 π−π* absorption at 380 nm, in good agreement with calculational results. The PL of S-BF2 is at 575 nm, red-shifted by a value of 60 nm from the PL of CBF2. Figure 1d presents the PL decay curve of S-BF2 at 575 nm, which was fitted with a lifetime of τP,S‑BF2 = 8.3 ± 0.1 μs almost three orders longer than τF,C‑BF2 = 7.6 ± 0.2 ns. Therefore, we ascribed the PL origin of S-BF2 to the phosphorescence emission from T1 to S0 (see Figure 1a). As a matter of fact, no clear fluorescence peak is observed for S-BF2. To further confirm this assignment, we measured the singlet oxygen (1O2) generation efficiency (ΦΔ) by using hypocrellin A (HA) as a standard (ΦΔ= 0.84 in CH2Cl2) and 9,10-diphenylanthracene (DPA) as a chemical trap (Figure S10).29 Extremely large, nearunity value of ΦΔ = 98% were obtained for S-BF2 in comparison with ΦΔ = 4% for C-BF2. This suggests that T1(π,π*) ← S1(n,π*) ISC is indeed highly efficient in S-BF2 (ΦISC,S‑BF2 is near unity), and then energy transfer from T1 of SBF2 to 3O2 generates 1O2. In contrast, the T1(π,π*) ← S1(π,π*) ISC process in parent C-BF2 is less probably with a ΦISC,C‑BF2 < 10% calculated according to ΦISC,C‑BF2 = 1 − ΦPL,C‑BF2. We further measured nanosecond transient absorption (nsTA) by means of a nanosecond flash lamp photolysis technique. Figure S7 reveals the nsTA spectra of S-BF2 in nitrogen-saturated CH2Cl2 solution and corresponding timeabsorption profiles on the microsecond time scale. Following selective photoexcitation of S-BF2 at 355 nm, a negative signal was initially observed at 440 nm (Figure S7a) ascribed to ground-state bleaching (GSB), which decays with a time constant of τGSB = 22.7 ± 0.03 μs (red square and line in Figure

S7b). Meanwhile, a broad positive absorption between 475 and 700 nm was also observed owing to the Tn ← T1 excited absorption (Figure S7a). The Tn ← T1 absorption at 650 nm decays single-exponentially with a time constant of τ0 = 24.9 ± 0.05 μs in nitrogen-saturated CH2Cl2 solution and a much faster lifetime of τET = 0.78 ± 0.02 μs in air-saturated solution. This indicates that the triplet of S-BF2 generated by T1(π,π*) ← S1(n,π*) ISC is efficiently quenched by the dissolved molecular oxygen due to energy transfer from T1 of S-BF2 to 3 O2. The quantum efficiency of energy transfer is estimated to be ΦET = (1 − τET/τ0) = 97%, consistent with the measured 1 O2 generation efficiency of ΦΔ = 98%, indicating the near unity ΦISC,S‑BF2. Due to the small molecular weight, we failed to obtain smooth thin-film of S-BF2 for variable-stripe-length amplified spontaneous emission (ASE) measurement. Recently, nanowire lasers had been reported for both fluorescence polymers and small molecules, based on the Fabry−Perot cavity formed between two flat wire end-faces.11,30 In our experiment, singlecrystalline nanowires of S-BF2 were prepared via solution selfassembly method by injecting 100 μL of 0.5 mM CH2Cl2 stock solution into 0.5 mL of ethanol.31 Within 1 h, nanowires were formed and centrifugally separated from the suspension solution. Figure 2a shows that these as-prepared nanowires have strong photoluminescence (PL) upon excitation by unfocused UV light (330−380 nm) and exhibit as active optical waveguide with brighter PL spots at the two tips and weaker PL from the bodies. This suggests that our nanowires may simultaneously serve as active media of phosphorescence emission and optical resonators. Figure S8b shows a scanning electron microscope (SEM) image of typical nanowires with regular shape deposited on silicon wafer. Combining with the atomic force microscopy (AFM) measurements, it can be seen that nanowires of S-BF2 have a length (l) between 20 to 100 μm, a width (W) of ∼2 μm, and a height (H) of ∼300 nm (Figure S8a). To understand the solid-state molecular packing of nanowires, transmission electron microscopy (TEM) image of a single microcrystalline was performed and selected area electron diffraction (SAED) pattern was also recorded by directing the electron beam perpendicular to the flat surface of 6378

DOI: 10.1021/jacs.7b01574 J. Am. Chem. Soc. 2017, 139, 6376−6381

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emission. In sharp contrast, the PL of S-BF2 powder at 510 nm decays much faster with a time constant of 2.3 ± 0.1 ns (see inset of Figure 3b), which is factually comparable to the fluorescence lifetime of C-BF2 solution (τF,C‑BF2 = 7.6 ± 0.2 ns, Table 1). Note that the 510 nm PL is coincident with the fluorescence predicted by DFT calculations (2.365 eV, 524 nm, Table 1). Therefore, we tentatively ascribed the 510 nm PL of S-BF2 powder to fluorescence emission, probably owing to the aggregation effect.23 At 77k, the intensity of phosphorescence increase about four times compared with that at room temperature, but the lifetime of phosphorescence remains the same (see Figure S13). These results rule out the possibility of TADF, of which the intensity decreases and the lifetime increases at low temperature. Furthermore, the nanosecond flash lamp photolysis of S-BF2 thick-film (prepared by multiple drop-casting on quartz plate) was performed. Meanwhile, the T−T absorption was found to be centered at 530 nm, which has a negligible overlap with the phosphorescence (see Figure S12). We performed lasing characterization on single nanowires by a homemade optical microscope equipped with a 50 × 0.9 NA objective (Figure S11). The isolated single nanowire is excited by the second harmonic of a 1 kHz Ti: sapphire regenerative amplifier (λ = 400 nm, pulse width 150 fs) focused into a 50 μm diameter spot. Spatially resolved PL spectra with a resolution about 1 μm are then collected underneath by using a 3D movable objective coupled to an optical fiber and detected using a liquid-nitrogen cooled charge-coupled device (CCD). Figure 4a depicts typical μ-PL spectra of an isolated nanowire (l = 38 μm) below and above the lasing threshold (Pth) by adjusting the pump density (P) of 400 nm laser. In the case of P < P th, the PL spectrum (black curve) presents both spontaneous phosphorescence emission around 610 nm and spontaneous fluorescence emission at 510 nm. When the pump density is increased above the threshold, a strong laser emission appears around 650 nm, indicating that stimulated emission occurs on the 0−1 phosphorescence transition (see the purple bold arrow in Figure 1a). Returning to Figure 1a, absorption occurs from the ground state S0 to vibronic sublevels of S2; as mixing of the 1(n,π*) and 1(π,π*) states is possible in S-BF2, the S1 ← S2 internal conversion (IC) is rapid probably within several picoseconds; following T1(π,π*) ← S1(n,π*) ISC, stimulated emission occurs from T1 to vibronic sublevels of S0. Because the vibronic sublevels of S0 are unoccupied in thermal equilibrium, the phosphorescence gain can be obtained in such a four-level system. Figure 4c shows the log−log plot of integrated PL intensities versus P. It can be seen that the phosphorescence at 650 nm exhibits the expected “S” curve known for light-in−light-out (L−L) relation of lasing oscillation.32 The evolution from spontaneous phosphorescence emission to ASE occurs at Pth = 10.4 μJ cm−2, and lasing oscillation starts at P = 12.8 μJ cm−2. The intensity dependence is fitted to a power law xp with p = 0.55 ± 0.06 below the threshold and p = 3.59 ± 0.26 above the threshold, respectively. The former p = 0.55 ± 0.06 below the threshold is ascribed to a sublinear region where bimolecular quenching occurs (triplet− triplet annihilation), and p = 3.59 ± 0.26 above the threshold is assigned to the superlinear characteristic of laser.33 In contrast, the fluorescence at 510 nm maintains in the sublinear region with p = 0.65 ± 0.05 (black curve in Figure 4b). The inset of Figure 4a shows the μ-PL image of the 30 μm length nanowire above the lasing threshold. Spatial interference

the single nanowire (Figure 2c). Monoclinic single-crystal of SBF2 presents cell parameters of a = 13.966(3) Å, b = 8.3375(17) Å, c = 12.830(3) Å, α = γ = 90°, β = 94.52(3)° (CCDC No. 1491881). Based on single-crystal cell parameters, the red circled sets of SAED spots in Figure 2c are due to (001) Bragg reflections with d(001)= 12.8 Å, while the yellow squared sets of SAED spots are ascribed to (020) Bragg reflections with d(020) = 4.2 Å, respectively. Figure 2e presents the X-ray diffraction (XRD) spectrum of nanowires deposited on a silicon wafer, which is dominated by a sequence of peaks corresponding to the crystal plane (100), such as (200), (300), and (400) peaks. Therefore, nanowires of S-BF2 are preferentially grown along the crystal b-axis and are bound by (100) and (001) crystal planes on the top and side faces (Figure 2b and e, inset). It can be seen from Figure 2d that SBF2 molecules form loosely packed π−π columns along the nanowire length direction, with a π−π distance of dπ−π = 3.8 Å (also see Figure S9). Before the lasing characterization, we measured the diffused reflectance absorption and PL spectra of S-BF2 nanowires placed on a quartz plate. It can be seen from Figure 3a that the

Figure 3. (a) Diffused reflectance absorption (black) and photoluminescence spectra (red) of S-BF2 nanowire powder placed on a quartz plate. (b) PL decay curves of S-BF2 nanowire powder monitored at the phosphorescence region of 610 nm (black) and at the fluorescence region of 510 nm (red).

two absorption bands of S-BF2 nanowire powder at 395 and 460 nm resemble the dilute solution absorption, demonstrating weak intermolecular interactions in the solid state as evidenced by the large π−π distance of dπ−π = 3.8 Å. The PL spectrum of S-BF2 powder exhibits a maximum at 610 nm together with a shoulder around 510 nm (red line in Figure 3a). As shown in Figure 3b, these two PL peaks present totally different decay dynamics. The 610 nm PL of S-BF2 nanowire powder decays with a time constant of 9.3 ± 0.4 μs (black trace in Figure 3b), which is similar to τP,S‑BF2 = 8.3 ± 0.1 μs obtained for S-BF2 solution (Table 1) and thus is assigned to phosphorescence 6379

DOI: 10.1021/jacs.7b01574 J. Am. Chem. Soc. 2017, 139, 6376−6381

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Journal of the American Chemical Society

Figure 4. (a) μ-PL spectra of a single nanowire with l = 38 μm under different pump densities at room temperature. (b) High-resolution PL spectra of the phosphorescence laser emission around 650 nm under different pump densities. (c) Integrated intensities of the fluorescence band at 510 nm and the phosphorescence lasing peak at 650 nm as a function of the pump density. The lasing threshold is identified as the intersection between the linear and the superlinear regions. (d) High-resolution PL spectra of laser emission recorded above threshold for nanowires with l = 20, 30, and 50 μm, respectively. (e) The mode spacing Δλ at λ = 650 nm versus 1/L of nanowires, showing clearly a linear relationship. The upper inset illustrates a typical optical ray analysis within FP microcavity, and the lower inset presents the optical mode simulation for a single nanowire with l = 30 μm and W = 2 μm.

nm under pulsed excitation. Our results open the door for a whole new class of laser materials based on previously untapped triplet phosphors.

patterns can be found at both nanowire ends, suggesting the formation of Fabry−Perot microcavity. The full-width at halfmaximum (fwhm) of individual cavity modes (δλ) observed around the lasing threshold is about 0.76 nm at the lasing wavelength λ = 650 nm (see Figure 4b), giving rise to a cavity quality factor Q = λ/δλ ≈ 850. We further investigated the influence of nanowire length on the cavity effect by using a series of nanowires with l = 20, 30, and 50 μm, respectively (Figure 4d). Figure 4d shows that the lasing spectra of nanowires exhibit more and more modes upon increasing the nanowire length. The spacing between adjacent modes, Δλ, simultaneously decreases with increasing the value of nanowire length, for instance, Δλ = 2.58, 1.89, and 1.17 nm for l = 20, 30, and 50 μm, respectively. According to the equation Δλ = λ2/ 2l[n − λ(dn/dλ)], where [n − λ(dn/dλ)] is the group refractive index and 2l is round-trip distance, the value of Δλ at λ = 650 nm as a function of 1/L (L = 2l) demonstrating clearly a linear relationship (Figure 4e). This further confirms the built-in FP microcavity within nanowires, in good agreement with the simulated electric field in the cross section of the nanowires (Figure 4e insets).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01574. General procedures, spectral data for compounds, and CIF file for the X-ray analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Hongbing Fu: 0000-0003-4528-189X Notes

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The authors declare no competing financial interest.

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CONCLUSION In conclusion, we reported the first triplet-phosphorescence OSSL from nanowire microcavity of a sulfide-substituted βhydroxyvinylimine difluoroboron compound. The lone pair of electrons of sulfur substitution leads to an efficiency T1(π,π*) ← S1(n,π*) ISC and a moderate ΦP (10%). This, plus the optical feedback provided by nanowire Fabry−Perot microcavity, enables triplet-phosphorescence OSSL emission at 650

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 91222203, 21273251, 91333111, 21190034, 21221002), project of State Key Laboratory on Integrated Optoelectronics of Jilin University (IOSKL2014KF16), the National Basic Research Program of China (973) 2013CB933500. 6380

DOI: 10.1021/jacs.7b01574 J. Am. Chem. Soc. 2017, 139, 6376−6381

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Journal of the American Chemical Society

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REFERENCES

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DOI: 10.1021/jacs.7b01574 J. Am. Chem. Soc. 2017, 139, 6376−6381