Efficient Triplet-Triplet Annihilation Upconversion with an Anti

Subscriber access provided by Nottingham Trent University

Article

Efficient Triplet-Triplet Annihilation Upconversion with an AntiStokes Shift of 1.08 eV Achieved by Chemically Tuning Sensitizers Chunying Fan, Lingling Wei, Tong Niu, Ming Rao, Guo Cheng, Jason J. Chruma, Wanhua Wu, and Cheng Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05824 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Efficient Triplet-Triplet Annihilation Upconversion with an Anti-Stokes Shift of 1.08 eV Achieved by Chemically Tuning Sensitizers Chunying Fan, Lingling Wei, Tong Niu, Ming Rao, Guo Cheng, Jason J. Chruma, Wanhua Wu* and Cheng Yang* Key Laboratory of Green Chemistry & Technology, College of Chemistry, State Key Laboratory of Biotherapy, West China Medical Center, and Healthy Food Evaluation Research Center, Sichuan University, 29 Wangjiang Road, Chengdu, 610064, China ABSTRACT: A series of Pt(II)−Schiff base complexes were synthesized as triplet sensitizers for the purpose of tuning the singlet and triplet energy levels so as to minimize energy loss during triplet-triplet annihilation (TTA) upconversion (UC). A deep-red to blue TTA-UC was achieved with an unprecedentedly large anti-Stokes shift of 1.08 eV. UC quantum yields of up to 21% (with a theoretical maximum efficiency of 50%) were observed in solution. The complexes also showed efficient UC emission in air-saturated hydrogels with a UC quantum yield up to 14.8%, which is much higher than the highest previously reported value. The low threshold excitation intensity provided by the present system offers promising potential for application in terrestrial solar energy conversion.



INTRODUCTION

Triplet-triplet annihilation (TTA) upconversion (UC), a photochemical process which allows for directly increasing the spectral brightness in the short-wavelength/highenergy regime by combining two low-energy photons, has been attracting fast-growing attention from the fields of photocatalysis,1 photovoltaics,2 biological imaging3 and photo-induced drug release.4 Standard TTA-UC involves the transfer of energy from the triplet state of a donor (sensitizer) to an emitter (acceptor), a process referred to as triplet-triplet energy transfer (TTET), followed by combination of two acceptor triplet states to afford an acceptor excited singlet state that relaxes via fluorescence at a higher energy than the excitation light (Figure 1). Some advantages of TTA-UC over other upconversion methods, such as upconversion originating from two-photon absorption5 and lanthanide-doped nanocrystals6, are higher associated upconversion quantum yields and the ultra-low excitation intensities. Nevertheless, achieving a high UC

quantum yield with a large anti-Stokes shift, so as to minimize the energy loss during TTA-UC, remains a great challenge. The highest TTA-UC anti-Stokes shifts obtained until now have never exceeded 1.00 eV (Figure S1).7 A larger anti-Stokes shift with high quantum yield is pivotal for achieving higher energy efficiency and increased spectral range availability for practical applications in photocatalysis, photovoltaics, and biological imiaging.3,8 Efforts devoted to increasing the anti-Stokes shift in TTAUC have focused on reducing the energy gap between the S1 and T1 excited states of the sensitizers.7d,9 Photosensitizers based on thermally-activated delayed fluorescence materials or inorganic nanocrystals that also possess small ∆ES1/T1 have afforded anti-Stokes shift of 0.73-0.83 eV.5d,10 Photoexcitation of the red- shifted S0→1CT absorption band of a spin-orbit charge transfer intersystem crossing (ISC) triplet sensitizer significantly enlarged the antiStokes

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 15

Figure 1. Schematic illustration of TTA-UC process in (a) Pt-0/diphenylanthracene (DPA) and (b) Pt-5/DPA systems. S1,D and T1,D represent the singlet and triplet excited states of the donor (sensitizer), respectively. S1,A and T1,A represent singlet and triplet excited states of the acceptor, respectively. shift.9b Os(II) complexes which demonstrate a strong heavy atom effect have allowed for direct singlet-to-triplet photoexcitation, thus circumventing energy loss associated with ISC and thus realizing the largest energy gain by far (0.97 eV),7b,9a with relatively low quantum yield (2.7%). Indeed, systems that demonstrate a large anti-Stokes shift (> 0.8 eV) are often accompanied with low UC quantum yields (< 10%),7a mainly due to energy losses throughout the multi-step energy transfer process in TTA-UC, such as the ISC of the sensitizer and TTET from the sensitizer to the acceptor (Figure 1a).8 Improving the emission quantum yields of TTA-UC requires increased efficiencies for both the TTET from the sensitizer to the acceptor and the TTA between acceptors. We previously demonstrated that supramolecular assembly can efficiently manipulate the arrangement of photo-components,11 thus significantly improving TTA-UC efficiency.1d,11d Herein, we report a chemical modification strategy to move both the singlet excitation and triplet excited state energy levels of Pt(II)salophen complexes close to the triplet excited state of the acceptor (Figure 1b). The photophysical properties of the Pt-complex sensitizers were improved so as to afford an unprecedentedly large anti-Stokes shift (1.08 eV) with an extremely high associated UC quantum yield (21%, with the theoretical maximum efficiency being 50%) in a deaerated toluene solution.



RESULTS AND DISCUSSION

Pt(II)-salophen complexes are excellent sensitizers for TTA-UC due to their high ISC efficiency and chemically tunable photophysical properties.12,13 For example, a boron dipyrromethane-containing Pt(II)-salophen complex recently realized an anti-Stokes shift of 0.65 eV in up to 10% quantum yields.13b The parent compound for our studies (Pt-0, Scheme 1)13a has an absorption peak at 521 nm and it can be photoexcited with a 532 nm laser when in use as a sensitizer (donor) for TTA-UC (Figure 2a). Introducing tert-butyl moieties ortho and para to the coordinating phenolic oxygens of the salophen ligand led to a significant bathochromic shift of the absorption spectra (Figure 2a) due to an increased destabilization of the HOMO electron state versus the LUMO state. For example, the absorption peak of tert-butylated Pt-1 demonstrated a bathochromic shift relative to Pt-0 (521 nm → 570 nm, Figure 2a). Pt-2, in which the central benzene ring was replaced by a naphthalene unit, showed only a slight bathochromic shift on absorption and emission spectra relative to Pt-1 (Figure 2a, b). DFT calculations suggest that the HOMO and LUMO of Pt-2 were both destabilized; the corresponding calculated HOMO and LUMO energy levels are provided for each sensitizer in Scheme 1. Four more Pt-salophen complexes were designed and synthesized with the aim of delocalizing the frontier molecular orbitals so as to finely adjust the energy levels and physical properties of the sensitizers at both the ground and excited states by conjugation with each other (Pt-3 & Pt-4) or with an alternate chromophore (Pt-5 & Pt-6) via an ethynylene or butadiynylene bridge (Scheme 1).


N N O

Pt

N

O

N

O

O

Pt-1 LUMO: -2.19 eV HOMO: -5.06 eV

N

N

Pt

N

N O

N

O

O

O

N

O

O

Pt-5 LUMO: -2.44 eV HOMO: -5.08eV

N N

N

O

O

N Pt

Pt

O

O


O

N

N

N


O

Pt


Pt

O

Pt

O


Pt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt

Page 3 of 15

SO3Na

N


DPA

DPAS

Scheme 1. Structures and calculated HOMO/LUMO energy levels of the PtII complexes serving as triplet photosensitizers. The structures of the two acceptors used for TTA-UC studies (DPA and DPAS) also are provide.

c

Figure 2. (a) The UV-vis absorption spectra and (b) normalized emission spectra of Pt-0 – Pt-6 in deaerated toluene for comparison (c = 1.5 × 10-5 M, 25 °C), λex = 530 nm; (c) The singlet and triplet energy levels of Pt-0 – Pt-6 were estimated based on Figure 2a, b, and d; (d) Emission spectra of Pt-0 – Pt-6 in ethanol-methanol (4:1, v/v) glass at 77 K, λex = 530 nm, c = 1.5 × 10-5 M.



Table 1. Photophysical properties of the sensitizers.a λemc /nm

∆ES-Td

1.4

614

570

1.6

Pt-2

572

Pt-3

λabs /nm

εb

Pt0

521

Pt-1

Φ Pe /%

τpf /µs

ФUC

0.48

23.0

4.4

---

---

650

0.37

18.2

4.4

16.5

3.3

1.5

658

0.38

16.6

3.5

18.5

5.6

590

3.1

671

0.36

10.8

3.7

12.6

7.8

Pt-4

590

3.2

670

0.35

14.4

3.4

9.3

5.9

Pt-5

586

1.3

677

0.27

9.0

12.7

21.0

5.5

Pt-6

587

1.4

653, 785

0.53

0.1

4.8, 38.8g

8.4j

2.3

/eV

h

ηi

/%

Page 4 of 15

sorption maxima were extend to 590 nm for the twin salophen-Pt(II) complexes Pt-3 and Pt-4 due to the extended conjugation. The ε values of Pt-3 and Pt-4 were two times stronger than that of Pt-1 (Table 1). The slightly enhanced absorption at 410–450 nm for Pt-5 and 420–470 nm for Pt6 were due to the absorptions of pyrene or anthracene moieties, respectively. The absorptions of the complexes Pt-3 – Pt-6 all tailed into the deep red range of ca. 650 nm and have modest ε values of 890, 1300, 780, and 810 M-1 cm-1, respectively, at 635 nm. This is noteworthy as it allowed for photoexcitation using a 635 nm continuous wave semiconductor laser as a relatively low energy excitation source.

a

All of the Pt(II)-salophen complexes, except for Pt-5, showed structureless emission spectra, suggesting a 3MLCT feature of the triplet excited state (Figure 2b). The triplet state energy levels of Pt-1 – Pt-5 were all smaller than that for parent complex Pt-0 and they were closer to, but still greater than, the triplet state energy level for the acceptor DPA (Figure 2c), thus guaranteeing efficient downward energy transfer. This phenomenon, together with the significant bathochromic shift in absorption wavelength, opens up the possibility for larger anti-Stokes shifts when DPA serves as the acceptor.

As illustrated in Figure 2a and Table 1, all of the Pt-complexes showed intense absorption in the visible region beyond 500 nm with peak molar extinction coefficients (ε) of around 13000–32000 M-1 cm-1, assignable to the 1MLCT/1IL transitions. Gratifyingly, the absorption maxima for complexes Pt-3 – Pt-6 were all shifted bathochromically compared to parent complexes Pt-0 – Pt-2. Specifically, the ab-

TTA-UC investigations employing Pt-0 – Pt-6 as sensitizers and DPA or perylene as acceptors were carried out using either a 589 nm or 635 nm diode pumped solid state (DPSS) continuous wave semiconductor laser as the irradiation source. Excitation of the Pt(II)-salophen complexes Pt-0 – Pt-6 with the 589 nm laser without addition of the DPA acceptor only afforded prompt emission of the complexes without any upconverted fluorescence (Figure S42a). Addition of DPA to deaerated toluene solutions of the sensitizers led to intense blue emission in the range of 380– 550 nm for Pt-1 – Pt-5 (Figure 3a),

In toluene at 1.5 × 10-5 M. b Molar absorption coefficient at the absorption maxima (104 M-1 cm-1). c Emission wavelength. d ∆ES-T = ES1-ET1. e Phosphorescence quantum yield, rose bengal as standard (ΦF = 11% in EtOH). f Phosphorescence lifetimes. g Triplet lifetime determined by time-resolved transient absorption. h Upconversion quantum yields (ФUC, upconversion quantum yield, the theoretical maximum of ФUC is defined as 50%). i η = ε × ΦUC, j perylene was used as the acceptor.

c

d


Page 5 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. TTA-UC fluorescence with Pt-0, Pt-1, Pt-2, Pt-3, Pt-4, and Pt-5 as triplet photosensitizers (1.5 × 10-5 M) and DPA as triplet acceptor (2.0 × 10-4 M); Pt-6 as triplet photosensitizer (1.5 × 10-5 M) and perylene (3.0 × 10-5 M) in deaerated toluene at 25 °C. (a) Excitation with a 589 nm laser (emission slit: 0.3 nm, power intensity: 3.6 mW) and (b) Excitation with a 635 nm laser (emission slit: 2.0 nm, power intensity: 15 mW); (c) Time-resolved emission spectra (TRES) of the upconverted fluorescence of DPA using Pt-3 as the triplet photosensitizer; (d) Photographs of the emission of the acceptor DPA and in the presence of sensitizers Pt-3, Pt-4, and Pt-5.

Table 2. The calculated anti-Stokes shifts when Pt-0 – Pt-6 serving as the sensitizers. λex / nmb

λem/ nmc

∆λ / nmd

∆E / eVe

Pt-0a

532

409

-123

0.70

Pt-1

589

409

-180

0.93

Pt-2

589

409

-180

0.93

Pt-3

635

409

-226

1.08

Pt-4

635

409

-226

1.08

Pt-5

635

409

-226

1.08

Pt-6f

635

450

-185

0.80

a literature data15. b Excitation wavelength. c The first vibronic peak of the upconverted emission. d ∆λ = λem - λex. e ∆E = 1240/λem -1240/λex. f perylene was used as the acceptor.

but no blue emission was observed for the parent compound Pt-0 and anthracene conjugated Pt-6. The observed emission was identical in wavelength to that of the normal fluorescence of DPA, but the fluorescence lifetime was significantly longer (e.g. 437 µs for Pt-3/DPA system versus 7.2 ns for DPA alone, Figure 3c).14 This observation is strong evidence for a delayed upconverting emission. The negligible TTA-UC for Pt-0 with 589 nm photoexcitation could be attributed to the weak absorption of Pt-0 at 589 nm, whereas intense TTA-UC was observed when a 532 nm laser was used as the light source with this complex. In all cases, substantial anti-Stokes shifts were observed ranging from 0.70 eV for Pt-0 to 0.93 eV for Pt-1 – Pt-5, calculated based on the energy difference between the excitation wavelength and the first vibronic peak of the acceptors.15

More significantly, the bathochromically shifted absorption spectra of Pt-3 – Pt-6 allowed for photoexcitation with a lower energy 635 nm laser, and strong blue DPA emission was observed for Pt-3 – Pt-5, thus corresponding to an anti-Stokes shift of 1.08 eV (Table 2). To the best of our knowledge, this is the largest anti-Stokes shift obtained for TTA-UC to date. It should be noted that the anti-Stokes shift was calculated following the widely applied method of determining the energy difference between the excitation wavelength and the first vibronic peak of acceptor. This straightforward procedure provides the information of the largest upconverted energy obtainable7b, 7c, 7i, 7k. Gray and co-workers have proposed an alternative method, which calculates the upconversion energy shift (UES) by comparing the intensity weighted average wavenumber of the upconversion emission and the averaged wavenumber of the sensitizer absorption, to thus avoid the possible variation of UES for the same TTA-UC system caused by the selection of excitation wavelength7h. Following this method, the UES for Pt-5/DPA pair was determined to be 0.48 eV (please refer to the Supporting Information for calculation details). Gray’s method has the advantage of avoiding large deviation caused by the selection of excitation wavelength, while the conventional method focuses on the longest excitation wavelength applicable and the shortest emission wave length obtainable, thus legibly gives the spectral range available for practical applications. Since prior reports only use the traditional procedure to calculate and report associated anti- Stokes shift, we have done the same throughout this text exclusively for comparative purposes.



c

Page 6 of 15

d

In gels

Figure 4. (a) TTA-UC quantum yields (ΦUC) of Pt-1, Pt-2, Pt-3, Pt-4 and Pt-5 in the presence of DPA as a function of the concentration of acceptors; (b) The UC intensity of Pt-5/DPA plotted as a function of incident light power. λex = 589 nm. [Pt-5] = 1.5 × 105 M, [DPA] = 2.0 × 10-4 M; (c, d) Fluorescence image of the upconverted fluorescence of Pt-5 / DPA in deaerate toluene (c) and in a G-TX hydrogel containing 1.0 mM DPAS and 15 µM Pt-5 (d) with a charge-coupled-device-camera excited by the red part of the sun spectrum, no filters were used.

One slightly unexpected result was that no upconverted DPA emission was observed when Pt-6 served as the sensitizer (Figure S43). The emission spectra of Pt-6 showed a peak at 653 nm with a small bulge at 785 nm. Unlike Pt-3 and Pt-4, the phosphorescence quantum yield (ΦP) of Pt-6 at 653 nm and 25 °C is extremely weak (0.1%), and the phosphorescence spectrum measured at 77 K showed a main emission peaked at 773 nm (Figure 2d). The emission of 653 nm was assigned to the emission of 3MLCT of the Pt(II)-salophen complex and 785 nm was assigned to the emission of 3IL localized on ethynylanthracene. This suggests that the triplet excited state is mainly localized on the ethynylanthracene moiety. The TTET to DPA (ET1 = 700 nm) was thus prevented due to the low triplet energy level of Pt-6. Switching to perylene (ES1 = 2.76 eV; ET1 = 1.53 eV) as the acceptor, however, resulted in intense delayed upconverting emission of perylene (Figure S43), corresponding to an anti-Stokes shift of 0.8 eV. The pyrene conjugate Pt-5 exhibited a phosphorescence emission peak at 671 nm (77 K), which corresponds to a triplet energy close to that of Pt-3 and Pt-4. The DFT calculations indicated that the pyrene moiety significantly participates in the excited triplet state of Pt-5 (Figure S40). In contrast to the typically broad transient absorption signals at 600–900 nm for Pt-1 – Pt-4 (Figure S35-37), which can be attributed to the absorption of the 3MLCT state, Pt5 and Pt-6 showed transient absorption of the triplet state of pyrene and anthracene peaks at 760 nm and 700 nm (Figure S38-39), respectively. This indicates that the 3IL

component in the triplet state was dominant for these two complexes. Pt-5 exhibited a smaller thermally induced Stokes shift (∆Es) in comparison to Pt-1 – Pt-4 (Figure 2b, 2d), demonstrating again that the emissive state in Pt-5 is 3IL.15b This is in good agreement with DFT calculations of the spin-density surface (Figure S40). The triplet lifetimes of Pt-5 and Pt-6 were determined to be 12.7 µs and 38.8 µs, respectively, which are significantly longer than those of Pt-0 – Pt-4 (3.4–4.4 µs) (Figure S41). As illustrated in Figure 4a, the upconversion quantum yield (ΦUC) increased rapidly with the concentration of the acceptors and reached a plateau at concentrations higher than 0.3 mM. Under optimized conditions, the bright blue UC emission is strong and can be discriminated easily with unaided eyes (Figure 3d). Pt-3 and Pt-4 showed slightly lower ΦUC values (9.3%~12.6%) than Pt-2 (18.5%) with the 589 nm excitation. The ΦUC were also measured independently with the 635 nm excitation, and the largest ΦUC values obtained under much higher excitation power density are comparable to that with the 589 nm excitation (Table S1). The ΦUC value is an accumulated result of the efficiencies of donor ISC (ΦISC), donor-to-acceptor TTET (ΦTTET), TTA (ΦTTA), and acceptor fluorescence (ΦA), in which ΦTTA and ΦA are inherent properties of the acceptor and ΦISC could be regarded as a unity.8a Thus, the reduced ΦUC for Pt-3 and Pt-4 could be ascribed to relatively lower ΦTTET values, presumably because these complexes have larger molecular sizes compared to Pt-2 and, thus, diffuse slower in solution.


Page 7 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intriguingly, a ΦUC value as high as 21% (the theoretical maximum efficiency is 50%), was obtained with Pt-5. This value is even greater than that of the parent compound Pt-1. This observation demonstrates that conjugation of pyrene to the Pt-salophen complex improved the UC efficiency in addition to reducing the requisite excitation energy. The ΦTTET was estimated to be 0.80 for Pt-5 vs ΦTTET = 0.67 for Pt-1 when the concentration of DPA was 3 × 10-4 M. ΦTTET is highly dependent on the concentration of the acceptor and could be further improved when increasing the concentration of DPA. The emission decay measurements suggest that Pt-5 has a lifetime (12.7 µs) that is more than 3 times longer than Pt-0 – Pt-4 (Table 1). The prolonged triplet lifetime is due to the location of the excited state on the pyrene moiety, and the significantly improved ΦTTET could be ascribed to the improved TTET due to the prolonged lifetime. Indeed, Pt-5 afforded a larger quenching constant (Ksv = 1.1 × 104 M-1) than the other complexes (Table S2), even though the quenching rate constant (kq = 0.9 × 109 M-1 s-1) is close to the diffusion rate constant and hence difficult to be enhanced further. The excellent properties provided by Pt-5 prompted us to apply it to a more realistic environment of a non-deoxygenated hydrogel. Thus, following the strategy established by Kimizuka and coworkers, Pt-5 was co-assembled into a hydrogel composed of Triton X-100, gelatin, and DPA-2-sulfonic acid.16a An upconversion efficiency as high as 14.8% was obtained in this gel, which is almost twice the previously reported highest value for air saturated UC systems (Figure S53–55).16 The UC emission intensity (IUC) increased with the incident light power, and the TTA-UC threshold excitation intensity (Ith) was determined by plotting the logarithm of UC intensity as a function of the irradiation power (Figure 4b). The threshold for the Pt-5-DPA pair was determined to be as low as 2.8 mW/cm2 with a 589 nm excitation (Figure 4b), which is comparable with terrestrial solar irradiation. While with the 635 nm excitation, the threshold was determined to be 167.2 mW /cm2, suggesting that Ith is absorbance-dependent (Figure S62-64). All sensitizers showed significant upconversion brightness (η, Table 1 and Figure 4c). This encouraged us to directly upconvert the photons from the red part of the solar spectrum by using the Pt-5-DPA pair. Bright blue emission was observed with unaided eyes in both a dearated toluene solution of the Pt5-DPA pair, as well as in a nondeoxygenated G-TX hydrogel assembly16 (Figure 4c and 4d), demonstrating the outstanding potential for application in sunlight-powered devices.



CONCLUSIONS

In conclusion, a new series of Pt(II)-salophen complexes with rationally designed molecular structures have been synthesized as sensitizers for TTA-UC. By judiciously fine-turning the singlet and triplet energy levels of these complexes, the largest anti-Stokes shift for TTA-UC (1.08 eV) has been achieved with a upconversion quantum yield of as high as 21% in solution. Moreover, a 14.8% UC quantum yield was obtained in air-saturated hydrogels, which is much higher than the highest value reported up to now. We successfully applied this strategy for the upconversion

of photons in the red-end of solar spectrum to higher energy photons. Specifically, the upconversion system emitted bright blue light under the irradiation by the red part of the solar spectrum in solutions as well as in hydrogels. We believe that these large gains in the upconversion energy with associated high quantum yields will greatly propel the real-word applications of TTA-UC. Moreover, we anticipate that these results will inspire great interest in the optimization of the photophysical properties of the Pt(II)-salophen complexes as they are applied to other areas beyond TTA-UC, including PDT and OLED devices.



EXPERIMENTAL SECTION

Synthesis and structural characterization data of the organic triplet photosensitizers are presented in Supporting Information. A diode pumped solid state (DPSS) laser 589 nm or 635 nm semiconductor laser was used for the upconversion measurements. The upconverion samples in toluene solution were purged with Ar for at least 20 min before measurement and the gas flow is kept on during the measurement while the upconversion sample in G-TX hydrogel were measured in air conditions. The anti-Stokes shifts for the upcovnersion systems (in eV) were determined by different energy value of the first peak of the upconverted emission and the excitation wavelength. The upconversion quantum yields were determined by a relative method by using a boron-dipyrromethene derivative B-0 in toluene (Φstd = 10.8%) and oxadicarbocyanine (C5) in ethanol (Φstd = 49%) as the standards, all the emission spectra was corrected with the HORIBA FluoroMax–4 (TCSPC) spectrofluoremeter to reduce errors resulting from the mismatch between the wavelength of the UC emission and the reference; the emission correction factor of the instrument was shown in Figure S61. All data was measured independently for 3 times using freshly prepared samples. For detail experiment setups, please refer to the Supporting Information.



ASSOCIATED CONTENT

Supporting Information. Detailed Synthetic procedures, the syntheses and characterization of compounds, DFT calculation, details of photophysical properties and upcoversion studies results. These materials are available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author Cheng Yang email: [email protected] Wanhua Wu email: [email protected]



ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Nos. 21871194, 21402129 and 21572142), National Key Research and Development Program of China (No. 217YFA0505903), and Science & Technology Department of Sichuan Province (19YYJC2458, 19YYJC3038 and 2017SZ0021). Comprehensive Training Platform of Specialized Laboratory, College of Chemistry and Prof. Peng Wu of Analytical &Testing Center, Sichuan



University for characterization and lifetime measurement are greatly appreciated.



REFERENCES

(1) (a) Khnayzer, R. S.; Blumhoff, J.; Harrington, J. A.; Haefele, A.; Deng, F.; Castellano, F. N. Upconversion-powered photoelectrochemistry, Chem. Commun. 2012, 48, 209-211. (b) Ye, C.; Wang, J.; Wang, X.; Ding, P.; Liang, Z.; Tao, X. A new medium for triplet-triplet annihilated upconversion and photocatalytic application, Phys. Chem. Chem. Phys. 2016, 18, 3430-3437. (c) Ravetz, B. D.; Pun, A. B.; Churchill, E. M.; Congreve, D. N.; Rovis, T.; Campos, L. M. Photoredox catalysis using infrared light via triplet fusion upconversion, Nature 2019, 565, 343-346. (d) Rao, M.; Kanagaraj, K.; Fan, C.; Ji, J.; Xiao, C.; Wei, X.; Wu, W.; Yang, C. Photocatalytic supramolecular enantiodifferentiating dimerization of 2anthracenecarboxylic acid through triplet-triplet annihilation, Org. Lett. 2018, 20, 1680-1683. (e) Rao, M.; Wu, W.; Yang, C. Effects of temperature and host concentration on the supramolecular enantiodifferentiating [4 + 4] photodimerization of 2anthracenecarboxylate through triplet-triplet annihilation catalyzed by Pt-modified cyclodextrins, Molecules 2019, 24, 1502-1517. (2) (a) Goldschmidt, J. C.; Fischer, S. Upconversion for photovoltaics – a review of materials, devices and concepts for performance enhancement, Adv. Opt. Mater. 2015, 3, 510-535. (b) Schulze, T. F.; Schmidt, T. W. Photochemical upconversion: present status and prospects for its application to solar energy conversion, Energy Environ. Sci. 2015, 8, 103-125. (c) Cheng, Y. Y.; Nattestad, A.; Schulze, T. F.; MacQueen, R. W.; Fuckel, B.; Lips, K.; Wallace, G. G.; Khoury, T.; Crossley, M. J.; Schmidt, T. W. Increased upconversion performance for thin film solar cells: a trimolecular composition, Chem. Sci. 2016, 7, 559-568. (3) (a) Liu, Q.; Yang, T.; Feng, W.; Li, F. Blue-emissive upconversion nanoparticles for low-power-excited bioimaging in vivo, J. Am. Chem. Soc. 2012, 134, 5390-5397. (b) Kwon, O. S.; Song, H. S.; Conde, J.; Kim, H. I.; Artzi, N.; Kim, J. H. Dual-color emissive upconversion nanocapsules for differential cancer bioimaging in vivo, ACS. Nano. 2016, 10, 1512-1521.(4) Wang, W.; Liu, Q.; Zhan, C.; Barhoumi, A.; Yang, T.; Wylie, R. G.; Armstrong, P. A.; Kohane, D. S. Efficient triplet-triplet annihilation-based upconversion for nanoparticle phototargeting, Nano. Lett. 2015, 15, 6332-6338. (5) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications, Chem. Rev. 2014, 115, 395-465. (6) Xu, G.; Hu, D.; Zhao, X.; Shao, Z.; Liu, H.; Tian, Y. Fluorescence upconversion properties of a class of improved pyridinium dyes induced by two-photon absorption, Optics & Laser Technology 2007, 39, 690-695. (7) (a) Figure S1 of the supporting information. (b) Sasaki, Y.; Amemori, S.; Kouno, H.; Yanai, N.; Kimizuka, N. Near infrared-to-blue photon upconversion by exploiting direct S-T absorption of a molecular sensitizer, J. Mater. Chem. C 2017, 5, 5063-5067. (c) Huang, L.; Zhao, Y.; Zhang, H.; Huang, K.; Yang, J.; Han, G. Expanding antistokes shifting in triplet-triplet annihilation upconversion for in vivo anticancer prodrug activation, Angew. Chem., Int. Ed. 2017, 56, 1440014404. (d) Yanai, N.; Kozue, M.; Amemori, S.; Kabe, R.; Adachi, C.; Kimizuka, N. Increased vis-to-Uv upconversion performance by energy level matching between a TADF donor and high triplet energy acceptors, J. Mater. Chem. C 2016, 4, 6447-6451. (e) Singh-Rachford, T. N.; Nayak, A.; Muro-Small, M. L.; Goeb, S.; Therien, M. J.; Castellano, F. N. Supermolecular-chromophore-sensitized near-infrared-tovisible photon upconversion, J. Am. Chem. Soc. 2010, 132, 14203-14211. (f) Cheng, Y. Y.; Fueckel, B.; Khoury, T.; Clady, R. G. C. R.; EkinsDaukes, N. J.; Crossley, M. J.; Schmidt, T. W. Entropically driven photochemical upconversion, J. Phys. Chem. A 2011, 115, 1047-1053. (g) Yanai, N.; Kimizuka, N. New triplet sensitization routes for photon upconversion: thermally activated delayed fluorescence molecules, inorganic nanocrystals, and singlet-to-triplet absorption, Acc. Chem. Res. 2017, 50, 2487-2495. (h) Gray, V.; Dzebo, D.; Abrahamsson, M.; Albinsson, B.; Moth-Poulsen, K. Triplet-triplet annihilation photonupconversion: towards solar energy applications, Phys. Chem. Chem. Phys. 2014, 16, 10345. (i) Jiang, Z.; Xu, M.; Li, F.; Yu, Y. Red-light-

Page 8 of 15

controllable liquid-crystal soft actuators via low-power excited upconversion based on triplet-triplet annihilation, J. Am. Chem. Soc. 2013, 135, 16446-16453. (j) Turshatov, A.; Busko, D.; Avlasevich, Y.; Miteva, T.; Landfester, K.; Baluschev, S. Synergetic effect in triplettriplet annihilation upconversion: highly efficient multi-chromophore emitter, Chem. Phys. Chem. 2012, 13, 3112-3115. (k) Singh-Rachford, T. N.; Castellano, F. N., Triplet Sensitized Red-to-Blue Photon Upconversion. J. Phys. Chem. Lett. 2010, 1, 195-200. (l) Ye, C.; Gray, V.; Martensson, J.; Borjesson, K. Annihilation Versus Excimer Formation by the Triplet Pair in Triplet-Triplet Annihilation Photon Upconversion, J. Am. Chem. Soc. 2019, 141, 9578-9584. (8) (a) Singh-Rachford, T. N.; Castellano, F. N. Photon upconversion based on sensitized triplet-triplet annihilation, Coord. Chem. Rev. 2010, 254, 2560-2573. (b) Zhao, J.; Ji, S.; Guo, H. Triplet-triplet annihilation based upconversion: from triplet sensitizers and triplet acceptors to upconversion quantum yields, RSC Adv. 2011, 1, 937-950. (9) (a) Amemori, S.; Sasaki, Y.; Yanai, N.; Kimizuka, N. Near-infraredto-visible photon upconversion sensitized by a metal complex with spin-forbidden yet strong S0-T1 absorption, J. Am. Chem. Soc. 2016, 138, 8702-8705. (b) Wang, Z.; Zhao, J.; Di Donato, M.; Mazzone, G. Increasing the anti-Stokes shift in TTA upconversion with photosensitizers showing red-shifted spin-allowed charge transfer absorption but a non-compromised triplet state energy level, Chem. Commun. 2019, 55, 1510-1513. (10) (a) Peng, J.; Guo, X.; Jiang, X.; Zhao, D.; Ma, Y. Developing efficient heavy-atom-free photosensitizers applicable to TTA upconversion in polymer films, Chem. Sci. 2016, 7, 1233-1237. (b) Wu, T. C.; Congreve, D. N.; Baldo, M. A. Solid state photon upconversion utilizing thermally activated delayed fluorescence molecules as triplet sensitizer, Appl. Phys. Lett. 2015, 107, 031103-031104. (11) (a) Yang, C.; Inoue, Y. Supramolecular photochirogenesis, Chem. Soc. Rev. 2014, 43, 4123-4143. (b) Yao, J.; Yan, Z.; Ji, J.; Wu, W.; Yang, C.; Nishijima, M.; Fukuhara, G.; Mori, T.; Inoue, Y. Ammonia-driven chirality inversion and enhancement in enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by diguanidino-gamma-cyclodextrin, J. Am. Chem. Soc. 2014, 136, 69166919. (c) Wei, X.; Wu, W.; Matsushita, R.; Yan, Z.; Zhou, D.; Chruma, J. J.; Nishijima, M.; Fukuhara, G.; Mori, T.; Inoue, Y.; Yang, C. Supramolecular photochirogenesis driven by higher-order complexation: enantiodifferentiating photocyclodimerization of 2anthracenecarboxylate to slipped cyclodimers via a 2:2 complex with beta-cyclodextrin, J. Am. Chem. Soc. 2018, 140, 3959-3974. (d) Fan, C.; Wu, W.; Chruma, J. J.; Zhao, J.; Yang, C. Enhanced triplet-triplet energy transfer and upconversion fluorescence through host-guest complexation, J. Am. Chem. Soc. 2016, 138, 15405-15412. (e) Xu, W.; Liang, W.; Wu, W.; Fan, C.; Rao, M.; Su, D.; Zhong, Z.; Yang, C. Supramolecular assembly-improved triplet-triplet annihilation upconversion in aqueous solution, Chem. -Eur. J. 2018, 24, 1667716685. (12) (a) Reid, E. F.; Cook, V. C.; Wilson, D. J.; Hogan, C. F. Facile tuning of luminescent platinum(II) schiff base complexes from yellow to near-infrared: photophysics, electrochemistry, electrochemiluminescence and theoretical calculations, Chem. -Eur. J. 2013, 19, 15907-15917. (b) Doistau, B.; Tron, A.; Denisov, S. A.; Jonusauskas, G.; McClenaghan, N. D.; Gontard, G.; Marvaud, V.; Hasenknopf, B.; Vives, G. Terpy(Pt-salphen)2 switchable luminescent molecular tweezers, Chem. -Eur. J. 2014, 20, 15799-15807. (13) (a) Borisov, S. M.; Saf, R.; Fischer, R.; Klimant, I. Synthesis and properties of new phosphorescent red light-excitable platinum(II) and palladium(II) complexes with schiff bases for oxygen sensing and triplet-triplet annihilation-based upconversion, Inorg. Chem. 2013, 52, 1206-1216. (b) Razi, S. S.; Koo, Y. H.; Kim, W.; Yang, W.; Wang, Z.; Gobeze, H.; D'Souza, F.; Zhao, J.; Kim, D. Ping-pong energy transfer in a boron dipyrromethane containing Pt(II)-schiff base complex: synthesis, photophysical studies, and anti-stokes shift increase in triplet-triplet annihilation upconversion, Inorg. Chem 2018, 57, 48774890. (14) (a) Yan, Q.; Zhao, D. Conjugated dimeric and trimeric perylenediimide oligomers, Org. Lett. 2009, 11, 3426-3429. (b) Ji, S.; Wu, W.; Wu, W.; Guo, H.; Zhao, J. Ruthenium(II) polyimine complexes with a long-lived 3IL excited state or a 3MLCT/3 IL equilibrium: efficient triplet sensitizers for low-power upconversion,


Page 9 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Angew. Chem. Int. Ed. 2011, 50, 1626-1629. (c) Wu, W.; Sun, J.; Ji, S.; Wu, W.; Zhao, J.; Guo, H. Tuning the emissive triplet excited states of platinum(II) schiff base complexes with pyrene, and application for luminescent oxygen sensing and triplet-triplet-annihilation based upconversions, Dalton Trans. 2011, 40, 11550-11561. (15) (a) Wu, W.; Zhao, J.; Guo, H.; Sun, J.; Ji, S.; Wang, Z. Long-lived room-temperature near-IR phosphorescence of BODIPY in a visiblelight-harvesting N∧C∧N PtII-acetylide complex with a directly metalated BODIPY chromophore, Chem. - Eur. J. 2012, 18, 1961-1968. (b) Lu, Y.; Wang, J.; Mcgoldrick, N.; Cui, X.; Zhao, J.; Caverly, C.; Twamley, B.; Ó Máille, G. M.; Irwin, B.; Conway‐Kenny, R. Iridium(III) complexes bearing pyrene‐functionalized 1,10‐phenanthroline ligands as highly efficient sensitizers for triplet–triplet annihilation upconversion, Angew. Chem. Int. Ed. 2016, 55, 14688-14692. (c) Fan, C.;

Wu, W.; Yang, C, Triplet-triplet annihilation upconversion in molecular aggregation systems, Chin. J. Org. Chem. 2018, 38, 1377-1393. (16) (a) Bharmoria, P.; Hisamitsu, S.; Nagatomi, H.; Ogawa, T.; Morikawa, M. A.; Yanai, N.; Kimizuka, N. Simple and versatile platform for air-tolerant photon upconverting hydrogels by biopolymer-surfactant-chromophore co-assembly, J. Am. Chem. Soc. 2018, 140, 10848-10855. (b) Thevenaz, D.; Monguzzi, A.; Vanhecke, D.; Vadrucci, R.; Meinardi, F.; Sinon, Y. C.; Weder, C. Thermoresponsive low-power light upconverting polymer nanoparticles. Mater. Hori. 2016, 3, 602-607.



Page 10 of 15

Table of Contents


10

Page 11 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1



Scheme 1


Page 12 of 15

Pt-0 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6

0.6 0.3 0.0 450

500

550

600

650

Normalized Intensity(CPS)

a 0.9

b Pt-0 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6

0.9 0.6 0.3 0.0 600 700 Wavelength / nm

Wavelength / nm Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nomalized Absorption

Page 13 of 15

0.9

d

800

Pt-0 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6

0.6 0.3 0.0 600

700 Wavelength / nm

Figure 2


800


Anti-Stokes Shift 0.93 eV

Intensity (CPS)

1.5x106 1.0x10

9.0x105

Pt-0 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6 DPA

6

Intensity (CPS)

a

5.0x105

b

Anti-Stokes Shift 1.08 eV

6.0x105

Pt-0 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5

3.0x105

0.0

0.0 400

500 600 700 Wavelength / nm

400

2.5e+6 2.0e+6 DF

= 436.99 s

1.5e+6

540 520 500 480 460 440 420 400 380

/n

5.0e+5

m

1.0e+6

th

0.0

1.6

1.4

1.2

Time

1.0

/ ms

0.8

av el en g

-5.0e+5

0.6

W

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 15

Figure 3


500 600 700 Wavelength / nm

Page 15 of 15

a

Intensity (CPS)

20

10 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5

0 0.0

Slope = 1.2

b

UC / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


108 Ith = 2.8 mW cm-2

107

Slope = 2.3

106

2.0x10-4 4.0x10-4 [Emitter] / M

1

Power / mW cm-2

In gels

Figure 4


10

Efficient Triplet-Triplet Annihilation Upconversion with an Anti

Recommend Documents