Efficient Triplet Sensitizers of Palladium(II) Tetraphenylporphyrins for

Article pubs.acs.org/JPCC

Efficient Triplet Sensitizers of Palladium(II) Tetraphenylporphyrins for Upconversion-Powered Photoelectrochemistry Bao Wang,† Bin Sun,† Xiaomei Wang,*,† Changqing Ye,*,† Ping Ding,† Zuoqin Liang,† Zhigang Chen,† Xutang Tao,‡ and Lizhu Wu§ †

Jiangsu Key Laboratory for Environment Functional Materials, College of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou 215009, China ‡ State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China § Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: The positive external heavy atom effect on upconversion efficiency (Φuc) of DPA (9,10-diphenylanthracene) emitter combined with palladium(II)tetraphenylporphyrins as triplet sensitizer was first found to significantly increase from 12.33% to 35.17% under excitation of low power density at 30 mW·cm−2 (λex = 532 nm). Dynamics data showed that the increasing phosphorescence lifetime (τp) and the phosphorescence quantum yield (Φp) of sensitizer, accompanied with the decreasing nonphosphorescence decay rate constant (knp), have contribution to the triplet−triplet energy transfer (TTT). Interestingly, the external heavy atom effect was found to be more efficient than the internal heavy atom effect on upconversion efficiency, which has provided a simple approach to increase low-powered upconversion efficiency without the difficulties that can arise by the modification of the sensitizer molecular structure in its inner parts. Moreover, this efficient green-to-blue upconversion has been demonstrated to have potential application for the hydrogengeneration from water under excitation of sun energy.

1. INTRODUCTION Photon upconversion (UC), which is the observation of emission at higher energy (shorter wavelength) after excitation at lower energy (longer wavelength), has attracted much attention because of potential applications, such as photovoltaics,1 photocatalysis,2−4 bioimaging,5 photodynamic therapy (PDT),6,7 frequency up-converted lasing,8 and so forth. In principle, there are two available techniques to obtain photon upconversion. The first one is so-called two-photon absorption (TPA) induced upconversion, and the other is triplet−triplet annihilation (TTA) supported upconversion. Two-photon absorption is defined as a molecule that simultaneously absorbs two photons (longer wavelength) via a virtual state and then releases a photon with shorter wavelength. TPA-based upconversion suffers from two fundamental drawbacks:9−11 (1) the coherent light with high power density (typically more than 106 W·cm−2) is required to pump molecular two-photon absorption, which is well beyond the energy level of normal light sources (the terrestrial solar radiation is ca. 100 mW· cm−2); (2) it is difficult to tailor the structure of TPA dyes to achieve a specific upconversion wavelength and, at the same time, to maintain a high TPA cross section. As an alternative to TPA upconversion, the TTA upconversion emerges as a promising wavelength-shifting technology, which is involved in bimolecular composition containing sensitizer and emitter. The sensitizer first absorbs low-power light, then undergoes intersystem crossing (ISC), © 2014 American Chemical Society

and further transfers its triplet energy to the triplet of emitter (TTT). Second, the triplet emitters undergo triplet−triplet annihilation (TTA) and then release the upconverted fluorescence (i.e., upconversion). Because each process such as ISC, TTT, or TTA mentioned above is transition-allowed, the TTA supported upconversion can be carried out by the excitation as low as less than 100 mW·cm−2 (solar energy is enough),12,13 which shows practical significance in many applications with respect to TPA induced upconversion. The other advantage is that TTA upconversion quantum yield (i.e., upconversion efficiency, ΦUC) can be quantificationally calculated, while TPA induced fluorescence yield (ΦTPA) is difficultly obtained. Understandably, the high upconversion efficiency (ΦUC) is extremely desired from the viewpoint of applications. Also, large upconversion efficiency (ΦUC) ascribes to many effects according to the TTA mechanism that the sensitizer possesses high light harvesting (ε) to the excitation source and large intersystem crossing (ISC); meanwhile, the emitter should possess high fluorescence quantum yield (Φf) and large triplet−triplet annihilation (TTA) probability. Currently, lots of groups14−24 focus on upconversion materials limited to octaethylporphyrinatopalladium(II) (PdOEP) as sensitizer and 9,10-diphenyl anthracene (DPA) Received: August 23, 2013 Revised: December 31, 2013 Published: January 2, 2014 1417

dx.doi.org/10.1021/jp410984c | J. Phys. Chem. C 2014, 118, 1417−1425

The Journal of Physical Chemistry C

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Table 1. Absorption and Fluorescencea Properties of Sensitizer Palladium(II)tetraphenylporphyrins in DMF (8.0 μM) sensitizer PdTPP PdMeTPP PdBrTPP

Soret-band 416 nm 418 nm 417 nm

Q-band

εb, M−1·cm−1

λfluo max, nm

Φf (%)

τf, nsc

kf (×106 s−1)d

knf (×107 s−1)d

522 nm 524 nm 523 nm

1.5 × 10 5.8 × 103 4.2 × 103

564, 609 566, 610 561, 607

2.13 3.34 1.08

4.74 (14.95%), 19.93 (85.05%) 4.72 (8.92%), 19.10 (86.11%) 3.45 (12.22%), 8.96 (87.78%)

1.07 1.75 1.21

4.91 5.06 11.08

4

a The fluorescence spectra obtained at room temperature and N2 atmosphere under the excitation of Q-band. bε is the molar absorbing coefficient at 532 nm. cThe data in parentheses represent the percentage of dual-fluorescence lifetime. dThe data of kf and knf were calculated according to long fluorescence lifetime.

Table 2. Triplet Properties and Dynamics Data of Sensitizers (8.0 μM) in Different Solvents such as DMF, PhCl, and PhBr under Room Temperature and Nitrogen Atmosphere PdTPP

PdMeTPP b

soln DMF PhCl PhBr

λphos max (nm)

Φp (%)

650 725 699 780 696 773

0.45 0.90 0.99

τp, μsa 0.99 (28.08%) 9.63 (71.92%) 1.11 (17.58%) 10.18 (82.42%) 5.75 (34.83%) 10.88(65.17%)

knpb 4

kp (102 s−1)

(10 s−1)

λphos max (nm)

Φp (%)

4.67

9.75

0.68

8.84

9.73

9.10

9.10

673 697 710 775 708 770

0.98 1.03

PdBrTPP kpb 2

τp, μsa 6.41 10.45 1.30 10.73 2.81 12.28

(23.45%) (76.45%) (44.00%) (56.00%) (19.97%) (80.03%)

b

(10 s−1)

knp (104 s−1)

6.51

9.51

9.13

9.23

8.39

8.06

λphos max (nm)

Φp (%)

673 697 690 772 690 767

0.96 1.18 1.25

τp, μsa 2.65 10.95 1.29 13.49 3.73 15.52

(19.16%) (80.84%) (1.62%) (98.38%) (3.20%) (97.80%)

kpb (102 s−1)

knpb (104 s−1)

8.77

9.05

8.74

7.32

8.05

6.36

a

The data in parentheses represent the percentage of dual-phosphorescence lifetime. bThe data of kp and knp were calculated according to long phosphorescence lifetime.

effect), the ΦUC values of bimolecular composition in bromobenzene (PhBr) were in the order of PdBrTPP/DPA (35.17%) > PdMeTPP/DPA (33.15%) > PdTPP/DPA (30.42%). To the best of our knowledge, the current study represents the first example of the external heavy atom effect as a simple approach to increase the upconversion efficiency without the difficulties that can arise by the modification of the sensitizer molecular structure in its inner parts. It was found that upconversion with efficiency more than 30% can effectively drive photoelectrochemistry in the standard three-electrode cell.28 The obtained photocurrent response (Ii) is 0.53 and 0.31 μA after being irradiated by upconversionpowered blue photons from PdBrTPP/DPA in PhBr (ΦUC = 35.17%, IUC = 1.62 × 104) and in DMF (ΦUC = 30.18%, IUC = 7.12 × 10 3), respectively, almost proportional to the upconversion intensity (IUC). This strongly suggests that upconversion power is indeed responsible for the generated photocurrent of photoelectrochemitry.

as emitter because of the high upconversion efficiency of this composition. Apparently, PdOEP and DPA are regarded as typical partners to produce TTA upconversion. Recently, we reported25 several DPA-based emitters (i.e., 2,9,10-substitution of anthracene) and investigated the influence of emitting molecules upon upconversion efficiency when they combined with PdOEP. The results showed that the effective upconversion (ΦUC) was mainly due to the DPA-based emitters with high fluorescence quantum yield (Φf) for the given sensitizer. For porphyrin-based sensitizer, it centered with a heavy metal such as palladium(II) possessesing near unity intersystem crossing (ISC)26 that is beneficial to triplet probability and, resultingly, that has contribution to upconversion. The peripheral substitution with heavy atom is also effective for triplet probability resulting in upconversion enhancement. PdOEP is difficult to modify via peripheral substitution, and it is difficult to synthesize.27 Thus, it is worth replacing PdOEP by Pd(II)tetraphenylphophyrin because the latter is easily both synthesized and modified. On the basis of this idea, we studied different Pd(II)tetraphenylphophyrins by peripheral substitution to examine the influence of heavy atom effect upon upconversion. Extensionally, the solvents with heavy atom such as bromobenzene (PhBr) and chlorobenzene (PhCl) were also used to examine the intermolecular heavy atom effect on the upconversion. The results obtained showed that the substitution of Pd(II)tetrabromophenylporphyrin (PdBrTPP) for Pd(II)tetraphenylporphyrin (PdTPP) leads to 2.5−3-fold enhancement of upconversion efficiency because of effectively increasing phosphorescence lifetime (τp) and phosphorescence quantum yield (Φp) as well as phosphorescence decay rate constant (kp), accompanied by decreasing nonradiative decay rate constant (knp). For example, the ΦUC values of bimolecular composition in dimethylformamide (DMF) were obtained in the order of PdBrTPP/DPA (30.18%) > PdMeTPP/DPA (22.71%) > PdTPP/DPA (12.33%) under low-powered density (532 nm, 30 mW·cm−2). On replacing internal heavy atom effect with external heavy atom effect (i.e., solvent heavy atom

2. EXPERIMENTAL SECTION 2.1. Chemical and Spectrum Measurements. Three palladium(II)tetraphenylphophyrins peripherally substituted by bromine atom (Br), methyl group (CH3), and hydrogen atom (H) (named as PdBrTPP, PdMeTPP, and PdTPP, respectively) have been synthesized and characterized according to the procedure published.29,30 9,10-Diphenylanthracence (DPA) was commercially available. Solvent DMF with spectral purity was used without further purification. Bromobenzene (PhBr) and chlorobenzene (PhCl) with chemical purity were purified before use. Linear absorption measurements of dilute solution (8 μM) have been measured with a Hitachi U-3500 recording spectrophotometer from a quartz cuvette of 1 cm path. Steady-state emission and time-resolved decay curve were measured on an Edinburgh FLS 920 fluorophotometer equipped with time-correlated single-photon counting (TCSPC) card. With the aid of nF 900 software, the fluorescence lifetime (τf) was measured under detection of 1418



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Figure 1. (a) Absorption spectra in DMF and (b) molar extinction coefficient in different solvents of sensitizers (insert: molecular structures of sensitizers; the concentrations are at 8.0 μM).

Figure 2. Emission spectra of (a) different sensitizers in DMF and (b) PdBrTPP in different solvents at room temperature and N2 atmosphere under the excitation of the respective Q-band; the concentrations are at 8.0 μM.

ethanol using 532 nm excitation according to the literature. The subscripts s and r represent sample and reference, respectively (here, rhodamine 6G in ethanol at concentration of 0.5 μM was used as the reference and DPA as the sample). Φr is the fluorescence quantum yield of rhodamine 6G (0.88)33,34 in ethanol using 532 nm excitation. F is the integrated emission of DPA and rhodamine 6G under the excitation of 532 nm. A is the absorbance of both sensitizer and rhodamine 6G at 532 nm, while η is the refractive index of the solvent used. The factor 2 accounts for the fact that two absorbed photons are required to produce one upconverted photon. 2.3. Upconversion-Powered Photoelectrochemistry. Upconversion-powered photoelectrochemistry was achieved in a three-electrode cell, where the WO3 photoanode (Eg = 2.7 eV) with active area of 1 cm2 deposited on ITO glass was prepared according to literature.28 The oxidation takes place at a WO3 semiconductor photoanode, and hydrogen is produced at a platinum rod counter electrode. The anode is photoactivated by our upconverting bimolecular composition that is filled inside a degassed quartz cuvette of 1 cm path and that is placed near the working electrode. Diode-pumped solid-state laser (emission wavelength: 532 nm, 60 mW·cm−2) was used as the excitation source for the upconversion that can induce photoelectrochemistry.

nF lamp, while the phosphorescence lifetime (τp) was measured under detection of microsecond xenon flashlamp (Edinburgh Analytical Instruments, μF900). In the fluorescence and phosphorescence decay profiles, the dual exponential fits gave the acceptable statistics parameters of χ2 < 1.1 (where χ2 is the reduced chi-square) and the obtained dual lifetimes (see Table 1 and Table 2). 2.2. TTA Upconversion Measurements. Diode-pumped solid-state laser (emission wavelength: 532 nm, 30 mW·cm−2) was used as the excitation source for the upconversion. The laser power was measured with a photodiode detector. For the upconversion experiments, the mixed solution of the sensitizer and emitter was degassed for about 10 min with N2. Then, the solution was excited with laser. The upconverted fluorescence was observed with PR655 SpectraScan colorimeter. The upconversion efficiency can be obtained by the product of the sensitizer intersystem crossing efficiency (ΦISC), TTT efficiency (ΦTTT), and TTA efficiency (ΦTTA) as well as fluorescence quantum yield (ΦF) of emitter as shown in eq 1.31 Also, the upconversion efficiency (ΦUC) can be calculated in eq 2,32 where absorption of the UC media is determined by the absorption of the sensitizer used, and the emission is attributed to the UC fluorescence of the emitter species.31 ΦUC = ΦISC × ΦTTT × ΦTTA × ΦF

ΦUC

2 ⎛ A r ⎞⎛ Fs ⎞⎛ ηs ⎞ = 2Φr ⎜ ⎟⎜ ⎟⎜⎜ ⎟⎟ ⎝ A s ⎠⎝ Fr ⎠⎝ ηr ⎠

(1)

3. RESULTS AND DISCUSSION 3.1. Absorption and Fluorescence Spectra. As shown in Figure 1a, three Pd(II)tetraphenylporphyrins show the Q-band in the range of 500−540 nm that can overlap with the diodepumped solid-state laser (532 nm). The molar extinction

(2)

According to eq 2, the ΦUC value of DPA in the presence of different sensitizers was obtained relative to rhodamine 6G in 1419



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Table 3. Upconversion Efficiency (ΦUC) of Bimolecular Compositions in Different Solvents upon Excitation at 532 nm with Power Intensity at 30 mW·cm−2

a

The photograph on the left presents the blue upconversion obtained from bimolecular DPA/PdTPP in degassed DMF, while the photograph on the right corresponds to bimolecular DPA/PdBrTPP in degassed PhBr under the irritation of a commercial green laser pointer (λex = 532 nm).

Figure 3. (a) Power-dependent upconversion of DPA/PdBrTPP at 532 nm excitation; (b) logarithmic plots of upconversion integral of DPA/ PdBrTPP versus power density. The used solvent is DMF, and the concentrations for DPA/PdBrTPP are 2.4 mM/8 μM.

coefficient (ε) at 532 nm is about PdTPP (1.5 × 104 M−1· cm−1) > PdMeTPP (0.58 × 104 M−1·cm−1) > PdBrTPP (0.42 × 104 M−1·cm−1) in DMF. Interestingly, the solvent heavy atom effect can significantly increase the ε values at 532 nm. As can be seen in Figure 1b, PdMeTPP (ε 2.08−2.14 × 104 M−1· cm−1) and PdBrTPP (ε 1.78−1.94 × 104 M−1·cm−1) exhibit 4− 5-fold increase in PhCl and PhBr with respect to those in DMF, showing very high light harvesting (ε) to the excitation source in the solvent with heavy atom effect. Emission spectra of Pd(II)tetraphenylporphyrins in DMF (Figure 2a) show that there are dual fluorescence (located at ∼560 nm and ∼610 nm) and dual phosphorescence (located at ∼650 nm and ∼720 nm) under the excitation of Q-band wavelength at room temperature and N2 atmosphere. By peripheral substitution from H, to CH3, and to Br, the phosphorescence spectra are enhanced in the order of PdBrTPP > PdMeTPP > PdTPP. On the other hand, for the given sensitizer such as PdBrTPP, its phosphorescence peaks are red-shifted from DMF, to PhCl, and to PhBr with the concomitance of significant enhancement (Figure 2b). All of these have proven that Pd(II)tetraphenylphophyrins by peripheral substitution with bromine atom (i.e., internal heavy atom effect) and in the solvent with heavy atom (i.e., external heavy atom effect) exhibit significant improvement in phosphorescence behaviors. The emission quantum yields (Φf, Φp), the lifetimes (τf, τp), and the radiative (k = Φ/τ) and nonradiative (kn = k(1 − Φ)/Φ) decay rate constants for different sensitizers are shown in Table 1 and Table 2. 3.2. Triplet−Triplet Annihilation Upconversion. Blue upconversion obtained from bimolecular PdBrTPP/DPA in degassed DMF can be easily observed under the radiation of green laser (see Table 3, photographs), and the upconversion intensity of DPA measured as a function of incident power intensity is presented in Figure 3a. With increasing incident excitation powers from 10.97 mW·cm−2 to 59.44 mW·cm−2, a

rapid enhancement of green-to-blue photon upconversion located at 436 nm can be observed, accompanied by the slight fluorescence (∼560 nm) and phosphorescence (∼665 nm) that come from sensitizer PdBrTPP. Figure 3b shows the logarithmic plot of the upconversion integral versus the pumped power from 15.23 to 32.87 mW·cm−2, and the obtained slope value is close to 2. The reason that the log−log plot rolls over to a slope of two is because the triplets are all decaying by second-order processes and because maximum efficiency has been reached.35,36 When the pumped powers are increased from 32.87mW·cm−2 to 59.44 mW·cm−2, the value 1.84 of the logarithmic plots of the conversion integral versus pumped powers is away from 2, suggesting some kind of saturation existence. To eliminate saturation photophysical process and to ensure the upconversion intensity being quadratically dependent on excitation intensity, the excitation power in our upconversion measurements should be limited below 32.87 mW·cm−2. The concentration-dependent upconversion intensity of DPA with combined different sensitizers is clearly presented in Figure 4a−c. Understandably, more emitting molecules are in favor of collision with each other and can promote the triplet−triplet annihilation (TTA) process. Thus, with the concentration of DPA increasing from 0.2 to 3.6 mM, DPA upconversion efficiency is significantly enhanced; meanwhile, the sensitizer presents an obvious decrease in phosphorescence (Figure 4, inserts), implying that triplet−triplet transfer (TTT) indeed occurs within sensitizer and emitter (DPA). Accordingly, the calculated concentration-dependent upconversion efficiency (ΦUC) of DPA is presented in Figure 5a, wherein the optimized ratio of [sensitizer]:[emitter] is at 2.0 mM:8 μM. For example, when the concentration of DPA was increased to 3.6 mM, the ΦUC of PdTPP/DPA in DMF reached its maximum value as high as 12.33%. Continuing to increase the concentration, the quantum yield started to remain constant. 1420



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10.95 μs) and the largest phosphorescence quantum yield (Φp 0.96%), accompanied by a decreasing nonphosphorescence decay rate constant (knp 9.05 × 104 s−1), strongly implying that PdBrTPP has more tendency for intersystem crossing (ISC) to its triplet level. On the other hand, a decrease of the triplet nonradiative decay rate is measurable as the observed increment of the phosphorescence lifetime can effectively enhance the subsequent TTT process from sensitizer toward emitter under the condition of the same acceptor concentration (see Table 2). For instance, the triplet nonradiative decay rates (knp) in DMF show the decrease from PdTPP (9.75 × 104 s−1), to PdMeTPP (9.51 × 104 s−1), and to PdBrTPP (9.05 × 104 s−1) with concomitance of increment of the phosphorescence lifetime from PdTPP (9.63 μs), to PdMeTPP (10.45 μs), and to PdBrTPP (10.95 μs), strongly suggesting that the TTT efficiency is enhanced in the order of PdBrTPP > PdMeTPP > PdTPP for the given the acceptor concentration. This can be confirmed by the Stern−Volmer equation, Po/P = kqτp[DPA] + 1,37,38 where Po and P stand for the phosphorescence of sensitizer without and with quencher DPA, respectively. τp is the phosphorescence lifetime of sensitizer in the absence of quencher. kq represents the quenching constant of quencher (DPA) and also indicates the efficiency of triplet−triplet energy transfer (TTT). Thus, the relationship between P0/P and [DPA] can be plotted as shown in Figure 7, wherein the linear slope is equal to the ksv value. Therefore, the quenching constants (kq = ksv/τp) of DPA combined with PdBrTPP, PdMeTPP, and PdTPP are 7.25, 4.06, and 1.18 M−1·s−1, respectively. Evidently, the TTT efficiency of PdBrTPP is indeed the largest, confirming that palladium(II)tetraphenylporphyrins with peripheral substitution by heavy atom (internal heavy atom effect) are effective for triplet− triplet energy transfer and for the resulting upconverted efficiency enhancement. The most interesting thing is that the external heavy atom effect (solvent effect) can play a much more important role than the internal heavy atom effect in enhancement upconverted efficiency. As seen in Table 3, the ΦUC values of DPA/sensitizer compositions are increased to more than 30.42% in the solvent of PhBr, while the difference in sensitizer is almost negligible. For example, in the solvent of PhBr, the ΦUC values of DPA present 30.42%, 33.15%, and 35.17% when combined with PdTPP, PdMeTPP, and PdBrTPP, respectively. That is, with respect to composition DPA/PdTPP in DMF, DPA/PdBrTPP presents 2.5-fold and 3-fold enhancements in ΦUC values in DMF and PhBr, respectively. Evidently, the influence of external heavy atom effect (solvent heavy atom effect) on ΦUC efficiency is larger than that of internal heavy atom effect (peripheral substitution by heavy atom) on ΦUC efficiency. As presented in Table 2, with the help of heavy atom solvent from PhCl to PhBr, all sensitizers possess both increasing phosphorescence lifetime from 13.49 μs (PdBrTPP in PhCl) to 15.52 μs (PdBrTPP in PhBr) and increasing phosphorescence quantum yield from 1.18% (PdBrTPP in PhCl) to 1.25% (PdBrTPP in PhBr), accompanied by decreasing nonphosphorescence decay rate constant from 5.06 × 104 s−1 (PdBrTPP in PhCl) to 6.36 × 104 s−1 (PdBrTPP in PhBr) under identical acceptor concentration, which are in good agreement with the internal heavy atom effect on the dynamics behaviors. The positive external heavy atom effect of the UC efficiency on the common PdOEP sensitizer is also observed (see Figure 5b). As presented, the concentration-dependent upconversion

Figure 4. The concentration-dependent upconversion intensity of DPA combined with (a) PdTPP, (b) PdMeTPP, and (c) PdBrTPP at fixed concentration (8 μM) of sensitizer in degassed DMF (532 nm, 60 mW·cm−2) (inserts: the fluorescence and phosphorescence of sensitizer).

Thus, the maximum ΦUC values for three UC systems are in the order of PdBrTPP/DPA (30.18%) > PdMeTPP/DPA (22.71%) > PdTPP/DPA (12.33). As anticipated, sensitizer PdBrTPP with heavy atom peripheral substitution is most effective to DPA upconversion. As presented in Table 1, although the fluorescence decay rate constants (kf) for three palladium(II)tetraphenylporphyrins are almost the same (1.07−1.75 × 106 s−1), the nonfluorescence decay rate constants (knf) are much distinguishing. For example, the knf value of PdBrTPP (11.08 × 107 s−1) is doubly larger than PdMeTPP (5.06 × 107 s−1) and PdTPP (4.91 × 107 s−1), implying that PdBrTPP lies in a favorable pathway to nonfluorescence deactivation. Meanwhile, phosphorescence dynamics measurements (see Figure 6a−c) associated with quantum yield calculations (see Table 2) have shown that PdBrTPP possesses the longest phosphorescence lifetime (τp 1421



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Figure 5. The concentration-dependent upconversion efficiency (ΦUC) of DPA combined with PdTPP, PdMeTPP and PdBrTPP (a) as well as PdOEP (b) at fixed concentration (8 μM) of sensitizer in degassed DMF (532 nm, 30 mW·cm−2) (inserts: the fluorescence and phosphorescence of sensitizer).

Figure 6. Phosphorescence decay curves associated with dual exponential fitting for three kinds of sensitizers: (a) PdTPP, (b) PdMeTPP, and (c) PdBrTPP in different solvents (DMF, PhCl, and PhBr). All were measured at room temperature in N2 atmosphere. For the phosphorescence decay profiles, the dual exponential fits give the acceptable statistics parameters of χ2 < 1.1 (where χ2 is the reduced chi-square) and the obtained dual lifetimes.

DPA/PdOEP in PhBr is as high as 35.78%, while the ΦUC of DPA/PdOEP in DMF is 28.63%. Also, the phosphorescence

efficiencies (ΦUC) of DPA doped with PdOEP in PhBr are obviously larger than those in DMF. The maximum ΦUC of 1422



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Figure 7. Stern−Volmer plots of the triplet quenching of different sensitizers by emitter DPA (in DMF) upon excitation at 532 nm with power intensity at 30 mW·cm−2.

Figure 8. Photocurrent/time response of a WO3 photoanode biased to +0.9 V vs Ag/AgCl in 1.0 M H2SO4 electrolyte in a proximate optical cell under the illumination of diode-pumped solid-state laser (emission wavelength: 532 nm, 60 mW·cm−2).

lifetime of PdOEP in PhBr (τp 19.40 μs) is more prolonged than in DMF (τp 10.24 μs), which strongly confirms our opinion that the external heavy effect on the sensitizer is attributed to the increasing of phosphorescence lifetime. The external heavy atom substituted sensitizers with the most efficient host matrix probably provides a new strategy to effectively promote solid-state UC.39,40 3.3. Upconversion-Powered Photoelectrochemistry. A potentially useful strategy toward TTA upconversion is applied in conversion-powered photoelectrochemistry in a threeelectrode cell with WO3 film deposited on ITO glass as working electrode, with platinum rod counter electrode, and with Ag/AgCl (0.1 M) reference (1.0 M H2SO4 electrolyte). Under the excitation of diode-pumped solid-state laser (emission wavelength: 532 nm, 60 mW·cm−2), the blue upconverted photons can radiate from the quartz cuvette containing upconverted composition and can be absorbed by WO3 (Eg = 2.7 eV)28 film deposited on ITO glass that is placed near the quartz cuvette. The oxidation of water takes place on the WO3 photoanode, and the hydrogen can be produced at a platinum rod counter electrode. Thus, the resulting photoelectronic current (Ii) can be recorded by a computercontrolled electrochemistry station measurement as presented in Figure 8. The measured photocurrent response (Ii) is at 0.53 μA and 0.31 μA irradiated by upconverted blue photons from PdBrTPP/DPA in the solvents of PhBr (ΦUC = 35.17%, IUC = 1.62 × 104) and DMF (ΦUC = 30.18%, IUC = 7.12 × 103), respectively. These are almost proportional to the upconversion intensity (IUC) of the upconversion materials, strongly confirming that upconversion-driven photoelectrochemistry was indeed responsible for the generated photocurrent.

faster and effective population of sensitizer triplet involved in the energy transfer (ET) toward acceptors/emitters. Efficient triplet sensitizer PdBrTPP with peripheral substitution by heavy atom (Br) in the solvent of PhBr presents long phosphorescence lifetime (15.52 μs) and large phosphorescence quantum yield (1.25%) associated with decreasing nonphosphorescence decay rate constant (6.36 × 104 s−1), which contributes to the triplet−triplet energy-transfer (TTT) efficiency, confirmed by the Stern−Volmer equation. Under low-powered excitation at 30 mW·cm −2, green-to-blue upconversion efficiency (Φuc) of DPA combined with PdBrTPP was as high as 35.17%. The efficient green-to-blue upconversion can effectively drive the photoelectrochemistry in a standard three-electrode cell, wherein the oxidation takes place at a WO3 semiconductor photoanode, and hydrogen was produced at a platinum rod counter electrode. Thus, the photocurrent resulting from the hydrogen generation in aqueous solution by conversion-powered photoelectrochemistry can be obtained as high as 0.53 μA/cm2 which is almost proportional to the upconversion intensity. The importance in this study strongly suggests that upconversion-powered photoelectrochemistry possesses the potential application for hydrogen generation from water under excitation of sun energy.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: 86-0512-68326615. E-mail: [email protected]. cn. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

4. CONCLUSION In this paper, we first investigated that both internal heavy atom effect and especially external heavy atom effect can significantly enhance low-powered upconversion efficiency, which has provided a new simple approach to increase triplet−triplet annihilation upconversion (UC) efficiency. To our knowledge, this is the first report about the way to effectively promote the upconversion process without the difficulties that can arise by the modification of the sensitizer molecular structure in its inner parts. The positive external heavy atom effect on the UC efficiency is verified by the fact that the singlet-to-triplet ISC efficiency on the porphyrin sensitizer is enhanced, allowing a

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science (Grant No. 51273141, 51303122), the Natural Science Foundation of Jiangsu Province (BK20130262), the Natural Science Foundat i o n o f J i a n g s u Pr o v i n c e E d u c a t i o n C om m i t t e e (11KJA430003), Project of Person with Ability of Jiangsu Province (2010-xcl-015), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Project of Science and Technology of Suzhou (SYG201204) for financial support. 1423



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(39) Kim, J. -H.; Deng, F.; Castellano, F. N.; Kim, J. -H. High Efficiency Low-Power Upconverting Soft Materials. Chem. Mater. 2012, 24, 2250−2252. (40) Monguzzi, A.; Bianchi, F.; Bianchi, A.; Mauri, M.; Simonutti, R.; Ruffo, R.; Tubino, R.; Meinardi, F. High Efficiency Up-Converting Single Phase Elastomers for Photon Managing Applications. Adv. Energy Mater. 2013, 3, 680−686.

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