Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers

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Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers with Ultralong Triplet Lifetimes Xinpeng Jiang, Jiang Peng, Jianchun Wang, Xinyan Guo, Dahui Zhao,* and Yuguo Ma* Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The photophysics of a series of bichromophoric molecules featuring an intramolecular triplet energy transfer between a triscyclometalated iridium(III) complex and covalently linked organic group are studied. By systematically varying the energy gap (0.1−0.3 eV) between the donor (metal complex) and acceptor (pyrene unit), reversible triplet energy transfer processes with equilibrium constant K ranging from ca. 500 to 40 000 are established. Unique photophysical consequences of such large K values are observed. Because of the highly imbalanced forward and backward energy transfer rates, triplet excitons dominantly populate the acceptor moiety in the steady state, giving rise to ultralong luminescence lifetimes up to 1−4 ms. Because the triscyclometalated Ir and triplet pyrene groups both impart relatively small nonradiative energy loss, decent phosphorescence quantum yields (Φ = 0.1−0.6) are attained in spite of the exceptionally prolonged excited states. By virtue of such precious combination of long-lived triplet state and high Φ, these bichromophoric molecules can serve as highly sensitive luminescent sensors for detecting trace amount of O2 and as potent photosensitizers for producing singlet oxygen even under low-oxygen content conditions. KEYWORDS: triplet energy transfer, photosensitizer, oxygen sensor, triscyclometalated iridium, triplet lifetime

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INTRODUCTION The triplet electronic states of molecules are of great theoretical and practical importance.1−3 Nonetheless, except for specially designed molecular systems,4−6 the triplet excited states of common organic molecules are spin-forbidden and not accessible through direct photoexcitation. A classcial and versatile method to attain the triplet states relies on a sensitization process. Molecules (sensitizers) with intrinsically rapid intersystem crossing (ISC) are photoexcited, which subsequently transfer their triplet energy intermolecularly to other molecules with inherently low spin conversion.7 Triplet photosensitizers (PS) play a crucial role in many technological applications, such as photocatalysis,8−11 photodynamic therapy (PDT),12−14 luminescent oxygen sensing,15,16 triplet−triplet annihilation-based photon upconversion, etc.17−25 Apparently, high ISC efficiency and long triplet lifetimes are advantageous attributes for PS. Transition-metal complexes are widely employed PS, due to their heavy-atom induced high ISC abilities. Among various transition-metal complexes, cyclometalated iridium(III) molecules are most prominent for their excellent phosphorescence emitting properties, by virtue of their high triplet quantum yield, small nonradiative energy loss and rapid radiative decay rate.26,27 The first two features are in fact favorable for designing potent triplet PS as well.28−31 Nonetheless, to tailor the cyclometalated Ir structure suitable for triplet sensitizing, long-lived excited state is necessarily realized, such that © XXXX American Chemical Society

aforementioned intermolecular triplet energy transfer can be effectively facilitated. Relevant efforts were previously made at curbing the radiative decay rate by introducing ligand-centered π−π* component to the excited state.32−35 Such a goal was properly served by designing a variety of large π-conjugated ligands coordinating to iridium, and the triplet-state lifetimes of pertinent systems were effectively prolonged to hundreds of microseconds. However, the nonradiative decay rates inevitably become more competitive in such slow-relaxation designs, resulting in significantly reduced phosphorescence quantum yields. Undertaking an alternative approach,36−51 we herein report the development of a series of new bichromophoric triplet PS with ultralong triplet lifetimes of a few milliseconds but still exhibiting decent phosphorescence quantum yields (Φ up to 0.64). Specifically, the bichromophoric scaffold comprises an organic chromophore pyrene covalently linked to a triscyclometalated iridium complex (Chart 1). Because the two components are designed to be electronic decoupled in the ground state, they are able to retain most of their intrinsic electronic and photophysical characteristics as independent Special Issue: Applied Materials and Interfaces in China Received: August 23, 2015 Accepted: November 12, 2015

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DOI: 10.1021/acsami.5b07860 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Chart 1. Structures of Studied Ir Complexes

sensors. We demonstrate that the luminescence of the bichromophoric system can be sensitively quenched by trace dioxygen even under diffusion-limited conditions. Moreover, high singlet-oxygen sensitization quantum yields (ΦΔ ∼ 0.8) are obtained from these newly designed PS.

molecules, such as the respective ground- and excited-state energy levels, ISC efficiencies, excited-state decay rates, etc. Importantly, because the pyrene unit possesses a lower triplet energy level than the Ir complex, a reversible intramolecular triplet energy transfer occurs between the two chromophores upon photoexcitation. By modifying the chemical structure of the complex ligands, the energy gap between the triplet donor (D, Ir complex) and acceptor (A, pyrene) can be varied (from 0.14 to 0.32 eV), and thereby the influence of D−A energy gap change over the intramolecular energy transfer kinetics is examined. It is found that with such large energy gap values the back-energy transfer rate from the energy acceptor to donor is vastly disfavored compared to the forward energy transfer rate. Consequently, the triplet excitons in such bichromophoric systems largely populate the pyrene unit in the steady state. Because the relaxation of triplet pyrene is inherently slow due to its forbidden spin state, ultralong triplet lifetimes (over 1 ms) and delayed phosphorescence are displayed. It should be noted that the triscyclometalated iridium is an important feature in our molecular design, which helps ensure minimized energy loss through the nonradiative decay of metal complex. The current study also demonstrates that the D−A energy gap is necessarily tuned with care for balancing the triplet lifetime and Φ value. Given a suitable energy gap, both long lifetime (over 1 ms) and optimal Φ (∼0.64) can be attained. Previously, extensions of triplet lifetimes were also reported with other transition-metal complexes by devising appropriate bichromophore manifolds, which were referred to as the “energy reservoir” effect.41,42,44,45,47 However, only hundreds of microsecond lifetimes were achieved, and investigations on harnessing these long-lifetime triplet systems to function as PS were not prevalent. Recently, we first discovered and studied the triplet photosensitization capability of triscyclometalated Irbased bichromophoric molecules in a set of triplet−triplet annihilation (TTA) photon upconversion experiments.52 Now we expand the application scope of these long-lifetime PS. With the rare and valuable combination of very long-lived triplet state and high emission efficiency, the current molecules can also serve as highly effective singlet-oxygen PS and powerful oxygen

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RESULTS AND DISCUSSION

Molecular Design and Syntheses. Three different bichromophoric molecules that incorporate regio-isomeric Ircomplex structures Ir(Fppy-pyr), Ir(pFpy-pyr)52 and Ir(mFppy-pyr) are studied (Chart 1). According to our previous studies on related triscyclometalated iridium molecules, it is known that by attaching a fluorene unit to different positions of the 2-phenylpyridine (ppy) ligand, the lowest triplet-state energy level of the Ir complex can be adjusted with sensible control.53,54 Therefore, by tethering the same triplet acceptor, di(tert-butyl)pyrene (DBP), to these Ir chromophores via a saturated linker, the energy gap between the donor and acceptor can be systematically varied. The purpose of designing such isomeric systems is to unveil the effect of changing the D− A energy gap on the photophysical properties and sensitizing performances of these bichromophoric systems. An analogue molecule bearing two pyrene acceptors, Ir(pFpy-pyr)2, is also prepared and examined. Four prototype complexes, Ir(Fppy), Ir(pFpy), Ir(m-Fppy) and Ir(pFpy)2, without the pyrene acceptor but having identical Ir complex structures to those bichromophoric molecules are also synthesized and studied for comparison. All these iridium complexes were synthesized by coupling corresponding bromine-substituted Ir(ppy)3 derivatives to fluorenyl boronic acid or ester, with or without the pyrene pendant.52−54 The structures of all new complexes were confirmed by 1H NMR and high-resolution mass spectroscopy (Supporting Information). The facial configuration of all studied complexes was inferred from the stereochemistry of their respective bromo-substituted Ir(ppy)3 precursors.53−55 UV−vis Absorption and Photoluminescence Properties. Similar to the related triscyclometalated Ir complexes,53,54 the UV−vis absorptions of model complexes Ir(Fppy), B


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ACS Applied Materials & Interfaces Ir(pFpy), Ir(m-Fppy) and Ir(pFpy)2 consisted of three parts: (1) the major peaks arising from fluorenyl-ppy ligand-centered (LC) 1π−π* transitions in the short-wavelength range; (2) side bands around 360−430 nm assignable to the spin-allowed 1 MLCT (metal-to-ligand charge transfer) transitions; (3) absorption tails of even longer wavelengths with minimal extinction abilities, related to spin-forbidden 3MLCT transitions (Figure. 1).

Table 1. Photophysical Properties of Studied Ir Complexes τ0b, μs (kr | knr/106 s−1)

Ir(pFpy) Ir(pFpy-pyr) Ir(Fppy) Ir(Fppy-pyr) Ir(m-Fppy) Ir(m-Fppypyr) Ir(pFpy)2 Ir(pFpypyr)2

λem, nm (298/77 K)

Φ or Φ′a

560/545 563/544 550/530 552/527 517/512 520/508

0.92 0.60 0.78 0.31 0.97 0.07

2.2 (0.42 | 0.036) 0.0105 1.0 × 103 3.8 (0.21 | 0.058) 0.0079 2.0 × 103 1.4 (0.69 | 0.021) 0.0074 3.9 × 103

563/547 564/550

0.90 0.64

2.9 (0.31 | 0.034) 0.0040 1.2 × 103

τ1 (μs)b

τ2 (μs)b

ΔE (eV)c 0.16 0.24 0.32

0.14

a

Phosphorescence quantum yields of the bichromophoric (Φ′) and monochromophoric model complexes (Φ); Ir(ppy)3 (Φ = 0.97, ref 27) was used as the standard for all Φ and Φ′ measurements. b Lifetimes shorter than 10 μs were measured by the time-correlated single-photon counting method using NanoLED of 369 nm as the excitation light source; lifetimes longer than 40 μs were measured using a pulsed Xe lamp as the excitation light source; Φ′ and τ2 were measured with the same deoxygenated solution sample for each bichromophoric molecule in order to maintain a constant residual oxygen concentrations and thus the same kpyr. cD−A energy gap calculated based on the lowest triplet energy levels of Ir chromophores and DBP estimated from their 77 K emission maxima.

these metal complexes, the emission energy and band shapes were found to remain nearly unchanged at both 298 and 77 K (Figure 2 and Figure S1). Such observations confirmed that in all bichromophoric Ir(Fppy-pyr), Ir(pFpy-pyr), Ir(m-Fppy-pyr) and Ir(pFpy-pyr)2, the triplet energy levels of the iridiumcomplex chromophores were basically unaffected by the pyrene group. Using the respective emission maximum wavelengths at 77 K, the triplet energy levels of each Ir complex as well as that of DBP were estimated (Figure S5b). Then, the D−A energy gap (ΔE) in each bichromophoric molecule was calculated, which was found to range from 0.14 to 0.32 eV, depending on the Ir-complex structure (Table 1). In spite of the nearly identical emission spectral band shape, the phosphorescence quantum yields of the four bichromophoric molecules (Φ′) all decreased to a varied extent compared to their respective model complexes (Φ) without the pyrene tag. More importantly, two drastically differed emission lifetime components were detected with the bichromophoric molecules (Table 1 and Figure S2). In accordance to previous studies conducted by ourselves and other groups,36,52 the shorter nanosecond lifetime component (τ1) could be ascribed to the instant relaxation of triplet excitons generated by the Ir chromophores, in competition with the energy transfer to the appended pyrene acceptor. Whereas, the longer lifetime component (τ2) reflected the delayed emission of excitons that were transferred back to Ir-complexes from the pyrene acceptor upon thermodynamic equilibration. Impressively, the currently studied bichromophoric molecules all exhibited τ2 values of unusually long magnitudes up to millisecond scale (Table 1), which is quite scarce even for transition-metal complexes featuring the “energy reservoir” properties. By examining the data in Table 1, we noticed that, for the three bichromophoric molecules bearing a single pyrene unit, the magnitude of Φ′ reduction relative to Φ appeared to be correlated to the value of τ2, whereas the latter seemed to

Figure 1. Absorption spectra of bichromophoric complexes (solid lines) in comparison to their monochromophoric model complexes (dashed lines) without the pyrene unit (recorded at 1.0 × 10−5 M in toluene, 298 K); spectra of Ir(Fppy) and Ir(m-Fppy) were reproduced from refs 53 and 54.

For all bichromophoric molecules, the absorption of pyrene unit was basically overlaid with the LC portion of metal complex absorption spectra in a simple additive fashion. Thus, fine vibronic structures were clearly observable between 300 and 350 nm, and the absorption bands beyond 360 nm were nearly identical before and after the attachment of pyrene. Such additive absorption spectra observed with Ir-complex and pyrene chromophores evidently indicated their negligible electronic coupling in the ground state. The fact that both chromophores retain their respective electronic properties is important for the subsequent energy transfer kinetic analyses (vide inf ra). The photoluminescence properties of all complexes were studied at both room temperature and 77 K. Relevant data are summarized in Table 1. Previous studies showed that the strong emissions of model complexes Ir(Fppy), Ir(pFpy) and Ir(mFppy) under deoxygenated conditions originated from their triplet excited states, which were all identified with mixed 3 MLCT-LC features. After the pyrene unit(s) was tethered to C


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Figure 3. Schematic representation of Jablonski diagram describing the photophysical processes of the bichromophoric molecules.

1 1 K ≈ k met + k pyr τ2 1+K 1+K

Φk met

Φ′ =

k met + K= Figure 2. Normalized photoluminescence spectra of mono- and bichromophoric complexes recorded at 298 K (black) and 77 K (red) (λex = 380 nm); spectra of Ir(m-Fppy) were reproduced from ref 54.

kf kb

kf k k b + k pyr pyr

(3)

(4)

where τ1 and τ2 are the short and long lifetime components and Φ′ and Φ are the emission quantum yields of the bichromophoric molecule and its corresponding model complex without the pyrene acceptor, respectively. With K representing the equilibrium constant of the reversible triplet energy transfer from Ir complex to the attached pyrene, kf and kb are the forward and backward energy transfer rate constants; kmet and kpyr correspond to the decay rates of the metal-complex and pyrene chromophores, respectively. The parameter of kmet includes the relaxation rate constants of both radiative and nonradiative pathways (i.e., kmet = kr + knr). Because the metal complex and pyrene chromophores are proven electronically decoupled, kr and knr in the bichromophoric molecules are reasonably assumed to be nearly identical to those in the model complexes. In other words, the values of kr and knr applied to eqs 1−4 were borrowed from corresponding model complexes without the pyrene pendant. It should also be noted that kpyr presents the apparent decay rate of pyrene, and it contains the contributions of both intrinsic nonradiative decay of the DBP and all quenching processes induced by external reagents, such as the residual oxygen. As the concentration of residual oxygen is typically undeterminable, its quenching effect is incorporated into kpyr, which can be regarded as a constant in the calculations. This assumption is valid as long as the oxygen concentration is controlled at a constant level during the measurements of pertinent emission data (i.e., Φ′ and τ2) for each bichromophoric molecule (Table 1). At room temperature, the radiative decay of pyrene is neglected. Here, eqs 1−4 are actually modified from those previously used in the literature, and the reasons for employing a different set of equations are as followings. Taking previously studied bichromophoric Ru(II)−pyrene molecules as an example,36−41 the donor and acceptor moieties in such Ru systems provided

illustrate a dependence on the magnitude of D−A energy gap (ΔE). Specifically, among the three isomeric molecules, Ir(mFppy-pyr) exhibited the largest extent of Φ′ reduction up to 93% compared to that of Ir(m-Fppy), and it also demonstrated the longest τ2 value of ca. 4 ms and the largest D−A energy gap of 0.32 eV. On the other hand, Ir(pFpy-pyr) showing the smallest ΔE of 0.16 eV displayed minimum Φ′ reduction of only 35% relative to Ir(pFpy) and the shortest τ2 of about 1 ms among the three bichromophoric isomers. Analogously, with a small ΔE of 0.14 eV molecule Ir(pFpy-pyr)2 also manifested a small Φ′ reduction of 29% and a relatively short τ2 of slightly over 1 ms. Kinetic Analyses on the Intramolecular Energy Transfer. To delineate the origin of the ultralong lifetime components and the energy loss pathways accounting for the Φ′ reduction observed with the bichromophoric molecules, we subsequently carried out semiquantitative kinetic analyses using the experimentally obtained steady-state and time-resolved emission data. Using the Jablonski diagram in Figure 3 to describe the photophysical processes presented by the bichromophoric molecules involving a reversible intramolecular triplet energy transfer, the following equations were derived (refer to the Supporting Information for details) to estimate the relevant kinetic parameters: 1 ≈ kf + kb τ1

(2)

(1) D


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ACS Applied Materials & Interfaces nearly isoenergetic triplet states, which facilitated a rapid backenergy transfer rate kb, comparable to the value of kf (i.e., large kb, small K, kf, kb and kmet ≫ kpyr). Therefore, kpyr could be neglected in the calculations and analyses of such nearly isoenergetic D−A systems. Namely, the exciton relaxation via the acceptor was negligible compared to the back-energy transfer rate as well as the radiative and nonradiative decays through the metal complex moiety. Such an assumption was confirmed by the observation of the similar values of Φ′ and Φ. However, in our current system the D−A triplet energy gap was significantly enlarged (to 0.1−0.3 eV). The thermodynamic equilibrium now greatly favored the forward energy transfer from Ir complex to the acceptor pyrene. Meanwhile, the backenergy transfer was considerably impeded, hence giving rise to [Ir-3pyr]* ≫ [3Ir-pyr]* in the steady state. The low concentration of [3Ir-pyr]* naturally accounted for the observed slow emission decay and unusually long phosphorescence lifetimes. Moreover, with a much reduced kb value, kpyr was no longer neglectable or omitted in the calculations. Because of such reasons, eqs 1−4 were derived in which kpyr was necessarily taking into consideration. This modified set of equations also explained the lowered Φ′ compared to Φ (eq 3). Namely, the energy loss through the nonradiative decays of triplet pyrene (both intrinsically and by external quenching) became non-negligible. For the same reason, the actinometry experiments performed with other bichromophoric systems56 for estimating the D−A energy transfer equilibrium constants (K) was no longer valid for the current molecules, which presented discernible exciton loss via the energy acceptor relaxation. Instead, by applying experimentally determined τ1 and τ2 to eqs 1−4, along with kr and knr borrowed from the model complexes, the values of K could be calculated for the current four bichromophoric molecules, which all fall in the range of 102 to 105 (Figure 4). On the basis of these equilibrium constant data, the D−A

Table 2. Calculated Kinetic and Thermodynamic Parametersa kf (s−1) b

kmet (106 s−1) Ir(pFpy-pyr)

0.45

Ir(Fppy-pyr)

0.26

Ir(m-Fppypyr)

0.71

Ir(pFpy-pyr)2

0.34

kb (s−1) 0.95 1.48 1.27 4.70 1.35

× × × × ×

108 105 108 104 108

3.21 × 103 2.50 × 108 4.63 × 105

kpyr (s−1)

K

ΔE (eV)

350

6.4 × 102

0.17

320

2.6 × 103

0.20

260

4.4 × 104

0.27

270

5.4 × 102

0.14

a Calculated based on eqs 1−4, with experimentally determined τ1, τ2, Φ and Φ′ from Table 1. bkmet = kr + knr.

With these calculation results, the experimental observations were reasonably explainable. As the D−A energy gap (ΔE) is enlarged in the order of Ir(pFpy-pyr), Ir(Fppy-pyr) and Ir(mFppy-pyr), the energy transfer equilibrium constant K (i.e., kf/ kb) is accordingly amplified greatly (Table 2). In the steady state, the concentration ratio of [Ir-3pyr]*/[3Ir-pyr]* also became increasingly larger (Figure S3). Most photophysical behavior variations among the three isomeric molecules are consequences of these two factors (K and ΔE). Because of the unfavorable back-energy transfer (much smaller kb compared to kf), τ2 was extended from 1 to 4 ms (cf. eq 2). Benefited from the relatively small knr of triscyclometalated Ir, as well as the small value of kpyr (Figure S4, Supporting Information), Ir(pFpy-pyr) maintained an impressive Φ′ of 0.6 in spite of the long excited-state lifetime (τ2). Nonetheless, as the value of [Ir-3pyr]*/[3Ir-pyr]* (related to K, Figure S3) increased from Ir(pFpy-pyr) to Ir(Fppy-pyr) and then Ir(m-Fppy-pyr), relaxations via pyrene (kpyr) consumed more and more excitons, rendering continuously decreased Φ′/Φ. As Ir(pFpy-pyr)2 exhibited a slight smaller D−A energy gap than Ir(pFpy-pyr), the smallest K is shown by this compound among the four bichromophoric molecules, in spite of an additionally appended pyrene unit (eq S5).40 Similar τ2 and slightly enhanced Φ′ were observed with Ir(pFpy-pyr)2 compared to Ir(pFpy-pyr). Oxygen Sensing and Photogeneration of Singlet Oxygen. The rare and valuable combination of ultralong triplet lifetimes with optimal luminescence efficiencies exhibited by these bichromophoric molecules qualifies them for functioning as highly competent triplet PS and luminescent oxygen sensors. Here, relevant capabilities are demonstrated by examining their oxygen sensing and singlet-oxygen sensitization performances. Theoretically speaking, both higher emission efficiency and longer lifetime are desirable for enhancing the sensitivity and detection limit of luminescent oxygen sensor. But in reality, Φ (Φ′) typically suffers as τ (τ2) is extended, for rendering higher susceptibility to both intrinsic nonradiative relaxations and external quenching (cf. eqs 2 and 3). For photosensitization, both inherent radiative and nonradiative decays are competitive pathways that hamper the intermolecular sensitization process. Therefore, minimizing the intrinsic nonraidative decay rates of the system is favorable for designing both optimal luminescence sensors and PS. Hence, triscyclometalated Ir chromophores

Figure 4. Calculated equilibrium constants of studied bichromophoric molecules; in comparison, previously reported equilibrium constant data all fell in the range of 104, the quenching of exciton should dominantly happen to the tethered pyrene unit. The simulation results show that when the intramolecular D−A equilibrium constant reaches over 104, Φ′ becomes much less sensitive to kpyr increase, unless kpyr is extremely small. In other words, it is likely that the emission of Ir(m-Fppy-pyr) is already considerably quenched (by oxygen and/or intrinsic relaxations of pyrene, Figure S4, Supporting Information) in freeze− pump−thawed solution, which rendered its low sensitivity toward additional O2. Whether this molecule is potentially useful for detecting even lower concentration of O2 is not determinable with our instrumentation. Alternative reasons, e.g., intrinsically low Stern−Volmer quenching constant, accounting for the lower oxygen sensitivity of Ir(m-Fppy-pyr) cannot be excluded, either. The results from toluene solutions indicated that the three bichromophoric molecules, Ir(pFpy-pyr), Ir(Fppy-pyr) and Ir(pFpy-pyr)2, are much more sensitive luminescent oxygen sensors than their monochromophoric model complexes, which clearly proved the importance and favorable effects of the long excited-state lifetimes. However, because the residual oxygen contained in the commercial nitrogen gas almost completely quenches the emission of these highly powerful oxygen sensors, accurately controlling extremely low oxygen contents and quantitatively monitoring the emission intensity change with varied oxygen concentrations was unfeasible in toluene solution. To solve this problem, we employed an alternative solvent, polyethylene glycol 400 (PEG400, Figure S9). Because of the high viscosity of this liquid material, O2 diffusion rate is significantly slowed down in it, which allows the abovementioned quantitative study to be realized at much higher O2 concentrations than in toluene solution. We thus prepared a series of O2−N2 mixed gas with varied partial O2 pressure, and saturated the PEG400 solutions with these gas mixtures to obtain solutions with different O2 concentrations. Consistent with the quenching behaviors shown in toluene solution, the three bichromophoric molecules of Ir(pFpy-pyr), Ir(Fppy-pyr) and Ir(pFpy-pyr)2 displayed remarkably higher quenching sensitivity toward O2 than their model complexes in these PEG400 solutions (Figure 6 and Figures S10−S12). In contrast to the monochromophoric model complexes, which showed approximately linear Stern−Volmer dependency on the partial O2 pressure, the bichromophoric complexes clearly displayed nonlinear quenching behaviors (Figure S13). Such sophisticated properties were reasonably due to the complications that the iridium and pyrene moieties possessed different oxygen quenching sensitivity and under extremely low-O2 pressure conditions the quenching could be diffusion limited. The sensitizing abilities of these bichromophoric PS to generate singlet oxygen were subsequently investigated in airsaturated toluene solutions and compared to their model complexes. Upon photoexcitation, all sample solutions exhibited a near-infrared emission band centered around 1275 nm, indicative of singlet-oxygen formation (Figures 7 and S15). With tetraphenylporphyrin (TPP) used as the reference (ΦΔ =

Figure 5. (a) Phosphorescence quantum yields of studied Ir complexes under different conditions; (b) comparison of relative emission intensity of Ir(pFpy) and Ir(pFpy-pyr) in toluene solutions (the two spectra labeled with vac were normalized, to which the rest spectra were calibrated for each compound).

comparing the emission intensity from such differently processed solutions, it was found that the emissions of all compounds were almost completely quenched in the airsaturated solutions. Compared to the spectra collected under the most rigorous deoxygenation conditions (i.e., freeze− pump−thaw), the trace amount of oxygen remaining in the commercial nitrogen gas was observed to quench the emissions to evidently differed extents for various compounds. The three molecules of Ir(pFpy-pyr), Ir(Fppy-pyr) and Ir(pFpy-pyr)2 clearly demonstrated considerably enhanced oxygen sensitivity than their respective model compounds by showing a much larger quenching magnitude (Figure 5). Specifically, the emissions of these three bichromophoric molecules were all quenched by over 80% in solutions saturated with N2 compared to the intensity measured under freeze−pump−thaw conditions, whereas the three model complexes Ir(pFpy), Ir(Fppy) and Ir(pFpy)2 only showed quenching magnitudes of about 30 F


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Figure 6. Emission spectra of Ir(pFpy-pyr) (top) and Ir(pFpy) (bottom) recorded at 298 K in PEG400 solutions saturated with O2− N2 mixtures of varied O2 partial pressure.

0.7),57−59 comparative actinometry experiments were conducted to determine the singlet-oxygen quantum yields of the bichromophoric PS in comparison to their monochromophoric model complexes (Figure 7). Although the model complexes imparted singlet-oxygen quantum yields around 0.6, all four bichromophoric PS exhibited noticeably enhanced ΦΔ of about 0.8. Such values were even higher than that of the commonly used sensitizer TPP. The highest ΦΔ and greatest improvement relative to the model complex were both found with Ir(pFpypyr)2. These results suggested that the long-lived triplet excited state of the bichromophoric molecules was in general favorable for the singlet-oxygen sensitization. It is noteworthy that the singlet-oxygen sensitization experiments were carried out under air-saturated conditions, to ensure the detection of sufficiently strong singlet-oxygen emission signals. As the bichromophoric PS possess much longer triplet lifetimes than the model complexes, imaginably greater relative sensitization-efficiency enhancements would most likely emerge with the bichromophoric PS under lower oxygen-concentration conditions. Such properties are important for many practical applications, such as photodynamic therapy.12−14

Figure 7. (a) Singlet-oxygen quantum yields from the bichromophoric sensitizers in comparison to those of corresponding model complexes; (b) singlet-oxygen luminescence in aerated toluene solutions of Ir(pFpy) and Ir(pFpy-pyr) with the same optical density at the excitation wavelength of 380 nm.

state lifetimes are attributable to the reversible but highly imbalanced intramolecular triplet energy transfer between the two composing chromophores. As the energy gap between the Ir complex (triplet donor) and pyrene unit (triplet acceptor) was enlarged from 0.16 to 0.32 eV, the energy transfer equilibrium constants (K) between D and A were increased from about 6 × 102 to 4 × 104. Such large K values rendered the back energy transfer from pyrene to Ir chromophore particularly disfavored, bringing about the ultralong lifetimes of the delayed luminescence. An important conclusion drawn from the current work is that by tuning the D−A energy gap appropriately balanced energy transfer equilibrium and exciton distribution can be reached between D and A, which will help maximize the emission quantum yield while offering suitably extended excited-state lifetime in such bichromophoric systems. With such long triplet lifetimes, the observed luminescence Φ values were quite impressive, which qualify these molecules as particularly powerful oxygen sensors and photosensitizers. In addition to the previously demonstrated application as sensitizers in TTA upconversion, here we further present their superior performances at functioning as potent singletoxygen sensitizers and powerful luminescence sensors for

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CONCLUSIONS The photophysics of a series of bichromophoric molecules incorporating triscyclometalated iridium(III) complexes covalently linked to pyrene unit were systematically investigated, which exhibited ultralong triplet lifetimes of a few ms and moderate to high luminescence quantum yields (Φ) of 0.07− 0.64. The current study reveals that the unusually long excitedG



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detecting extremely low-content O2. Notably, the ultralong triplet lifetimes should enable the molecules suitable for 1O2 sensitization even under oxygen-deficient conditions.

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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/acsami.5b07860. Synthetic procedures and characterization data (PDF).

EXPERIMENTAL SECTION

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General Procedures. All reactions sensitive to oxygen and water were performed under nitrogen atmosphere using the standard Schlenk techniques. All starting materials were used as received from commercial sources without further purification, unless otherwise stated. Tetrahydrofuran (THF) and toluene were distilled over sodium under nitrogen atmosphere. 1H NMR (400 MHz) spectra were recorded at room temperature with a Bruker Advance 400 MHz spectrometer, with CDCl3 as the solvent. Chemical shifts (δ) were reported in parts per million (ppm) referenced to tetramethylsilane (δ 0.00 ppm) or using the residual solvent peaks as the internal standards. Electro-spray ionization (ESI) mass spectra were recorded on a Bruker Apex IV FTMS mass spectrometer. Photophysical Measurements. Absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. For steady-state and time-resolved photoluminescence measurements at room temperature, the sample solutions were deoxygenated with three freeze−pump− thaw cycles. Corrected emission spectra were obtained with a Horiba Jobin Yvon FluoroMax-4P spectrofluometer equipped with an R928 photomultiplier tube (180−850 nm). A 150 W ozone-free xenon arc lamp was used as the excitation light source. Emission quantum yields in toluene were measured using Ir(ppy)3 (Φ = 0.97) as the standard.27 Phosphorescence lifetimes shorter than 10 μs were determined with the single-photon counting technique on the same Horiba Jobin Yvon FluoroMax-4P spectrofluorometer using NanoLED of 369 nm as the excitation source. Analysis of the decay profiles was accomplished with Horiba Jobin Yvon DAS6 software. Lifetimes longer than 40 μs were measured with FluoroMax-4P spectrofluometer with programmed and pulsed xenon lamp as the excitation source, and 380 nm was chosen as the excitation wavelength. Fitting of the decay curves was performed using the Origin software. Luminescence spectra at 77 K were recorded in MeTHF, and the samples were put in a quartz tube in a quartz Dewar flask filled with liquid nitrogen. Oxygen Sensing. All emission spectra were recorded with a FluoroMax-4P spectrofluorometer. For the measurements in toluene, the sample solutions were degassed by three freeze−pump−thaw cycles and sealed under vacuum before the first round of spectra (vac) were collected. These solutions were then bubbled through with N2 for 25 min, and another set of emission spectra (N2) were collected again. Finally, the solutions were bubbled through with air before the photoluminescence spectra (air) were recorded for the third time. For PEG400 solution samples, gas mixtures with varied oxygen partial pressure was prepared by mixing high-purity N2 with various volumes of high-purity O2 or 2% (v/v) O2 in high-purity N2 via flowmeter. The emission spectra were recorded after the PEG400 solutions were saturated with such mixed gas by bubbling through for 30 min. Singlet-Oxygen Quantum Yield. The steady-state luminescence of singlet oxygen was recorded with a Fluorolog-3-2-iHR320 spectrofluorometer equipped with an InGaAs array detector working under liquid nitrogen. Tetraphenylporphyrin (TPP) was used as a standard (ΦΔ = 0.7). Using the comparative actinometry method, the singlet-oxygen quantum yields ΦΔ of all samples were determined based on the following equation

ΦΔ = ΦΔstd

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AUTHOR INFORMATION

Corresponding Authors

*Y. Ma. E-mail: [email protected]. *D. Zhao. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (No. 21222403, 51473003 and 51573002) and the Ministry of Science and Technology (2013CB933501) of China.

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REFERENCES

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I 1 − 10−A std Istd 1 − 10−A

where Φstd Δ (0.7) is the singlet-oxygen quantum yield of TPP in aerated toluene, I and Istd represent the singlet-oxygen emission intensity of the measured sample and TPP, respectively, and A and Astd are the absorbance of the measured sample and TPP at the excited wavelength.58 H


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