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Enhanced Triplet Sensitizing Ability of an Iridium Complex by Intramolecular Energy-transfer Mechanism Xinyan Guo, Qi Chen, Yujie Tong, Yao Li, Yiming Liu, Dahui Zhao, and Yuguo Ma J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04807 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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The Journal of Physical Chemistry

Enhanced Triplet Sensitizing Ability of an Iridium Complex by Intramolecular Energy-Transfer Mechanism Xinyan Guo, Qi Chen, Yujie Tong, Yao Li, Yiming Liu, Dahui Zhao* and Yuguo Ma* Beijing National Laboratory for Molecular Sciences, Centre 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.

ABSTRACT. The photodynamic properties involving both intra- and intermolecular triplet energy transfers (ET) of a bichromophoric photosensitizer having a triscyclometalated Ir(III) tethered with a pyrene derivative are studied. Due to the triplet energy gap of the two chromophores, a reversible intramolecular triplet ET equilibrium is quickly established upon photoexcitation, with the triplet exciton mainly residing on the acceptor side in the photostationary state. By virtue of the very small decay rate of triplet pyrene, a considerably extended triplet lifetime (2 ms) is observed. Next, the intermolecular triplet-triplet ET properties are investigated. Using steady-state and time-resolved spectroscopy, the ET rate constants from Ir complex and pyrene unit in the sensitizer to an external triplet acceptor (unattached, free pyrene derivative) in solution are found to be around 109 s-1 and 108 M

-1

s-1, respectively. In

ACS Paragon Plus Environment

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spite of a lower ET rate constant, the tethered pyrene serves as the main intermolecular ET channel because of the large, favorable intramolecular ET equilibrium (K∼103). Importantly, this cascade ET process, from Ir complex to linked pyrene, and then to free pyrene, offers an overall improved ET efficiency than a direct ET from Ir complex to free pyrene, by virtue of the much smaller spontaneous decay rate compared to that of the metal complex. Finally, the more efficient ET ability is demonstrated experimentally by applying the molecule as sensitizer in a triplet-triplet annihilation upconversion. The bichromophoric sensitizer achieved upconverted emission intensity 5 times higher than a monochromophoric Ir-complex analogue.

INTRODUCTION Sensitizers are molecules that transfer their excited-state energy to other molecules, resulting in their own relaxation and simultaneous excitation of the energy acceptor. When the sensitizer excitation takes place via photo-irradiation, it is also referred to as photosensitizer.1 As the sensitization process typically induces a spin inversion of both the energy donor and acceptor, it is widely used to realize spin-converted excitation of molecules incapable of intersystem crossing (ISC) by themselves upon photoexcitation. Apparently, sensitizers necessarily excel at ISC. The most commonly used photosensitizers are metal complexes, which exhibit high ISC efficiency due to large spin-orbital coupling effect entailed by the heavy metal atoms.1-3 Sensitizers have wide applications, such as singlet-oxygen sensitization4 useful to photodynamic therapy,5-6 and triplet-triplet annihilation (TTA) energy upconversion (UC).7-9 The latter accomplishes photon-frequency upconversion upon photo-excitation of a sensitizer using non-coherent light, followed by intermolecular triplet-triplet energy transfer (TTET) to organic


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molecules which subsequently undergo bimolecular TTA. Photons with anti-Stokes shift relative to the incident light are then emitted upon spontaneous relaxation of the singlet excitons generated in annihilators via TTA. In the above-mentioned and other sensitization processes, the overall sensitization efficiency is typically determined by three continuous steps, that is photo-excitation, ISC and TTET.10 The photo-excitation efficiency is apparently dependent the absorption range and extinction ability of the sensitizers. For metal complexes, this aspect of property can be improved by designing and installing large π-conjugated organic ligands with proper electronic properties.11-13 With the inherently high ISC efficiency, the other factor that warrants attention and optimization for metal-complex sensitizers is the TTET step. The TTET efficiency is dictated by the energytransfer (ET) rate, which is mainly governed by the energy gap between the donor and acceptor. However, an enhanced energy gap may not be favourable for the overall system function. For example, in TTA UC an enlarged sensitizer-annihilator energy gap directly cuts into the magnitude of attainable anti-Stokes shift.14 Besides the ET rate, the excited-state lifetime of the sensitizer is another pivotal factor to TTET, because under diffusion-limited conditions the efficiency of Dexter-type ET is proportional to the excited-state lifetime of energy donor.15 Thus, a long triplet lifetime is desirable for many sensing materials, such as oxygen sensors, because a low analyte (acceptor) concentration detection limit is desirable and a diffusion-limited environment is almost always encountered. Additionally, long-lifetime sensitizers are of special importance to TTA UC operated by dispersing the sensitizer and annihilator in solid-state matrices, which is an important approach to realizing practical applications of the technique.14-20 Hence, we have endeavoured to develop photosensitizers with long triplet lifetimes and


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investigated the influence of triplet lifetime over sensitizing and sensing performances, such as oxygen sensing, singlet oxygen sensitizing, and particularly TTA UC.16-19 Previously, we synthesized a series of tris-cyclometalated iridium complexes, which boasted favourable properties including near-unity ISC efficiency, high triplet level and very small nonradiative relaxation rate, and studied their sensitizing capabilities.20-23 Most importantly, a novel bichromophoric molecular design was implemented, aimed for attaining long triplet lifetime and enhanced sensitizing and UC efficiencies.16 To be specific, a small organic molecule (with a spin-forbidden triplet state) was covalently tethered to Ir complexes, and significantly extended triplet lifetimes compared to the prototype monochromic Ir complexes were typically observed.24 In the prior work, we have proposed that the rapid intramolecular TTET process from the Ir-complex moiety to the small organic appendant was responsible for the substantially prolonged triplet lifetime and increased TTET efficiency, which helped improving the TTA UC results.18 In the current study, we use a modified molecule with more facile and streamlined synthesis to carry out an in-depth study on the intermolecular TTET mechanism from such a bichromophoric sensitizer to annihilator DBP. First, using similar method employed before, the photodynamic behaviors of the intramolecular TTET of this new sensitizer are elucidated, which are confirmed to be similar to its previous analogue. The obtained kinetic data are crucial to subsequent intermolecular kinetic analyses, and the similar behaviors ensure that the conclusions drawn from current work are pertinent to both molecules. In contrast to the previous work,16,18 which investigated the intramolecular ET kinetics and TTA performance, the focus of the current study is to further elucidate the origin of the enhanced sensitization efficiency by analysing the intermolecular TTET mechanism. Combining the steady-state and time-resolved absorption and emission data, we are able to scrutinize the kinetic details of the intermolecular TTET from such


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a bichromophoric sensitizer to the annihilator. A theoretic model is developed which considers a dual-path TTET mechanism; that is, both the Ir complex and appended DBP are theoretically capable of sensitizing free annihilators. Nonetheless, the experimental results prove that the sequential ET process from the Ir-complex moiety to appended DBP followed by intermolecular ET free DBP serves as the main channel for sensitization in the current system, because of the rapid and thermodynamically favored intramolecular ET. Since the intermolecular TTET mainly takes places between the tethered DBP to free annihilators, the observed sensitization efficiency enhancement arises due to the fact that the intramolecular ET can effectively out-compete the radiative and nonradiative decays of Ir moiety.25 It was also theoretically predicted that a set of conditions were required for such bichromophoric sensitizers with long triplet lifetime to manifest enhanced sensitizing ability, which included a rapid intramolecular ET rate and the tethered organic chromophore to exhibit both small energy loss via spontaneous decay and competent ability at promoting ET to freely diffusing external energy acceptors. Here, the experimental data are obtained pertinent to the dynamic behaviours of intra- and intermolecular ET processes occurring to such bichromophoric sensitizers (Figure 1a), proving that all the above conditions are properly satisfied in these bichromophoric sensitizers. From this study, more indepth understandings are gained into the working mechanism of improved TTET and TTA efficiencies.


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Figure 1. (a) Schematic diagram of intra- and intermolecular triplet energy transfer processes of a bichromophoric sensitizer (kmet: combined radiative and nonradiative decay rate of metalcomplex moiety (Ir); kf and kb: forward and backward intramolecular triplet energy transfer rates; kDBP: intrinsic decay rate of triplet DBP; kq and k’q: intermolecular energy transfer rate constants from Ir moiety and tethered DBP to free DBP, respectively; (b) chemical structures of studied sensitizers. EXPERIMENTAL SECTION Synthesis and characterization. All reactions sensitive to oxygen and water were performed under nitrogen atmosphere using the standard Schlenk method. 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


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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. High resolution electro-spray ionization (ESI) mass spectra were recorded on a Bruker Apex IV FTMS mass spectrometer. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry was performed using AB Sciex MALDI-TOF/TOF Mass Spectrometer. Photophysical Measurements. Absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Corrected emission spectra were obtained with a Horiba Jobin Yvon FluoroMax-4P spectrofluometer equipped with an R928 photomultiplier tube. A 150 W ozonefree xenon arc lamp was used as the excitation light source. Toluene solutions were deoxygenated with three freeze−pump−thaw cycles. Emission quantum yields were determined by comparison to a dilute Ir(ppy)3 solution standard in deaerated toluene (Φp = 0.97).26 Photoluminescence spectra at 77 K were recorded in glassy 2-methyltetrahydrofuran with a quartz Dewar flask filled with liquid nitrogen. All time-resolved phosphorescence decay curves were collected on Horiba Jobin Yvon FluoroMax-4P spectrofluometer. Solutions were deaerated prior to the lifetime measurements. Lifetimes shorter than 10 µs were determined with the time correlated single photon counting technique with NanoLED of 369 nm as the excitation source. Analyses of the decay profiles were 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.


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Nanosecond time-resolved transient absorption measurements were collected on a LP920 spectrometer from Edinburgh Instruments. Excitation of the samples was accomplished using a Nd:YAG/OPO laser system from Opotek (Vibrant LD 355 II) operating at 1 Hz. The incident laser wavelength was 450 nm with appropriate long pass and short pass filters to clean the excitation pulse and the laser power was 1.5 mJ/pulse. TTA UC Experiments. The solutions were deaerated and the flasks were sealed under vacuum. TTA UC spectra were recorded on a Horiba Jobin Yvon FluoroMax-4P spectrofluorometer with a xenon lamp as the excitation source. The samples were excited at 450 ± 5 nm. A long-pass filter (400 nm) was placed between the lamp and sample to avoid the interference of shorterwavelength light. Different excitation power was realized by passing through a series of neutral density filters. Upconversion quantum yields (ΦUC) were determined using the following equation:

= 2

where Φ, A, I, and η represent the quantum yield, absorbance at 450 nm, integrated emission intensity, and refractive index of the medium, respectively. The subscripts UC and Std denote the upconversion and standard systems, respectively. The equation is multiplied by a factor of 2 in order to guarantee a maximum quantum yield of unity. Ru(bpy)3Cl2 (Φp = 4.0%, in airequilibrated water) was used as the standard.17 RESULTS AND DISCUSSION A new bichromophoric molecule (1, Figure 1b) featuring a triscyclometalated Ir(III) complex covalently connected to a 2,7-di(tert-butyl)pyrene (DBP) group through a short aliphatic linker


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was designed for the current study.20 The synthesis of this new sensitizer was streamlined compared to the previously reported bichromophoric sensitizers (see Supporting Information for synthesis details and characterization data). Whereas a much similar intramolecular ET mechanism was operating. Namely, the photo-excited triplet exciton generated in the Ir-complex moiety was rapidly transferred to the covalently linked DBP unit due to the triplet level difference and a short spatial distance between the chromophores. For comparison study, as well as for obtaining pertinent electronic properties of the Ir-complex moiety, a monochromophoric Ir complex analogue (1a) was also synthesized. The photophysical properties of complexes 1 and 1a were first examined in solution (Table 1). The UV−Vis absorption spectrum of the bichromophoric molecule 1 evidently comprised overlaid absorption bands of the cyclometalated Ir moiety and tethered DBP. Such absorption features unambiguously indicated that the two chromophores were electronically independent in the ground state. Under deoxygenated conditions, both the mono- and bichromophoric complexes emitted orange phosphorescence at room temperature. The emission band shape was nearly identical before and after the DBP attachment, further proving that the triplet energy level of the Ir-complex moiety was nearly unaffected by the tethered DBP (Figure 2a). Time-resolved emission measurements revealed that the phosphorescence of the molecule 1 comprised two components with drastically differed lifetimes (Figure 2b). The longer component exhibited a lifetime of over 2 millisecond, whereas the shorter component had a lifetime of only a few nanoseconds (Table 1).


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Table 1. Photophysical data of studied Ir complexes in toluene solution λabs / nm

λem / nm

Φa

τb/ µs

1

344

565

0.66

0.001, 2.2×103

1a

338

565

0.92

2.8

a

Phosphorescence quantum yields, with Ir(ppy)3 (Φ = 0.97) as the standard. b The emission of 1

comprised two lifetime components: the shorter one (~1 ns) was ascribable to the prompt relaxation of triplet excitons upon photo-excitation of the iridium moiety, the magnitude of which was significantly truncated compared to 1a due to the competing intramolecular ET to DBP; the longer component (2.2 ms) corresponded to the delayed emission of excitons backtransferred to the iridium moiety from the appended DBP; 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 xenon lamp as the excitation light source (λex = 380 nm). Table 2. Kinetic parameters of bichromophoric sensitizer 1

1 a

kmet a/ s-1

kf / s-1

kb/ s-1

K=kf/kb

kDBP/ s-1

4.5×105

9.1×108

8.1×105

1100

160

Combined intrinsic radiative and nonradiative decay rates of iridium-complex moiety

(estimated based on the triplet lifetime and phosphorescence quantum yield of 1a)


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Figure 2. (a) Normalized UV-Vis absorption (solid lines) and photoluminescence (dotted lines) spectra of studied iridium complexes and DBP (298 K). (b) Time-resolved emission phosphorescence decay of 1 (square) at emission maximum wavelength, with the inset showing the decay of shorter lifetime component (solid lines are best fit to single exponential decay). Before a quantitative evaluation of the intermolecular TTET (i.e., sensitizing) capability can be attained, the intramolecular ET mechanism is necessarily studied. Using a previously developed model18 for describing the intramolecular ET (see Figure S1 in Supporting Information), along with the experimental data listed in Table 1, the photodynamic properties of bichromophoric sensitizer 1 was investigated (see Supporting Information for details), and the results are shown in Table 2. Of these kinetic parameters, kmet and kDBP represent the intrinsic relaxation rate constants of the metal-complex (energy donor) and tethered DBP unit (acceptor), respectively. It should be noted that, due to the nearly unperturbed electronic property of the metal-complex moiety in the bichromophoric molecule by the tethered DBP, kmet (for radiative and nonradiative decays combined) of 1 was assumed to remain the same to that of the monochromophoric analogue and was thus borrowed from complex 1a (kmet = 4.5×105 s-1 in Table 2). Then, based on equations S1-S9 (see Supporting Information),18 the forward ET rate (kf) in 1, associated to the


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intramolecular triplet transfer from Ir moiety to tethered DBP, was calculated (ca. 9 × 108 s-1), and a much slower back-energy transfer rate was observed (kb ≈ 8 × 105 s-1). The resultant intramolecular ET equilibrium constant (K = kf/kb) was fairly consistent with the donor-acceptor triplet energy gap (∆ET ~ 0.17 eV) estimated independently based on the phosphorescence emission wavelengths of the two chromophores (Figure S2). Basically, these data confirmed that, similar to our previously studied bichromophoric systems, the triplet excitons in molecule 1, initially produced via ISC in Ir-complex moiety upon photo-excitation, mainly populated the tethered DBP unit after reaching a photostationary state (Figure S1). By virtue of the very small back ET rate (kb) and a minimal spontaneous decay rate of spin-forbidden 3DBP (kDBP ~ 2×102 s1

, also obtained from equations S1-S4), a long triplet lifetime of 2 ms was reasonably manifested

by 1. Next, the intermolecular TTET process was studied. First, the quenching rate constant kq in solution was determined for the conventional monochromophoric sensitizer 1a through the Stern-Volmer (S-V) experiments.27 Namely, with DBP employed as the external energy acceptor (quencher) in a deaerated toluene solution, a KSV of 7.8×103 M-1 was obtained from the lineardependence range of the S-V plot (Figure 3). Correspondingly, based on the relationship of kq=KSV/τ (τ, excited-state lifetime)10, 28 a quenching rate constant of 2.8×109 M-1s-1 was estimated for 1a. Since the TTET quantum efficiency (ФTTET) is directly proportional to the quenching efficiency, a straightforward dependence of ΦTTET on the acceptor (quencher) concentration was derived (equation S10).


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Figure 3. Emission spectra of 1a in toluene, in the presence of free DBP at various concentrations solution; the inset shows the Stern-Volmer plot, offering KSV = 7.8×103 M-1. In contrast to the simple linear S-V plot identifiable for the monochromophoric sensitizer, the dependence of TTET efficiency on the acceptor (quencher) concentration was theoretically found much more sophisticated for the bichromophoric molecule (equation 1, see SI for details).

=

(1)

Imaginably, this was a result of the dual ET pathways existing in 1. Namely, the two chromophores (Ir complex and tethered DBP) could both act as the energy donor and transfer triplet exciton to external acceptors, accomplishing intermolecular TTET and sensitization. Consequently, two different ET rate constants (kq and kq’ in Figure 1a) were operating simultaneously. A fairly sophisticated dependence of the residual emission intensity on the quencher concentration was found for the bichromophoric system under steady-state conditions (equation S21). For this reason, it is unviable to determine the TTET rate constant of 1 from only the steady-state photophysical data.


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Therefore, time-resolved transient absorption (TA) spectroscopy was employed to characterize the intermolecular energy transfer dynamics of bichromophoric sensitizer 1, and a method of directly monitoring the TTET process from tethered DBP to free DBP was identified. Thereby, it was possible to determine the value of kq’ from the TA data. Specifically, upon excitation of a deaerated solution of 1 co-dissolved with a certain concentration of unattached, free DBP (as an intermolecular ET acceptor) with a pulsed laser of 450 nm as the pump source, two transient absorption bands were clearly observable around 420 and 540 nm in microsecond time scale (Figure 3a). Both peaks were characteristic and assignable to the T1→Tn transitions of DBP.17, 19, 25, 29

Prior to the examination of the intermolecular ET process, the intramolecular ET kinetics

was first elucidated by the TA spectra. By selectively exciting the Ir-complex moiety with a 450 nm laser, the absorption peak at 425 nm was observed to rise continuously within the first 20 ns or so (Figure S3). This process was suggested to result from the DBP triplet generation via intramolecular ET, which differed from the intermolecular mechanism for exhibiting a very short delay time scale. The obtained kinetic data were fitted with a single-exponential function, revealing a kinetic rate constant of about 2.0×108 s-1, which was in agreement with the kf value determined from the time-resolved emission data (Table 2, kf = 9×108 s-1).


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Figure 3. (a) Nanosecond TA spectra (b) decay curves of 1 in the presence of free DBP (200 µM) in toluene solution. The sold blue line represents the best fit to a single-exponential function, offering ET rate constant (kq’) from tethered 3DBP to free DBP (α denotes the molar extinction coefficient ratio of free DBP at 540 and 525 nm, ε540/ε525); inset showing the decays of absorbance at 525 and 540 nm. Next, the TA spectra were subjected to more careful scrutinizing. Interestingly, it was found that the two absorption peaks both underwent gradual and continued hypsochromic shifting (e.g., one from 425 to 420 and the other from 540 to 525 nm, Figure 3a) in tens of µs time scale. We suspected that this spectral shifting phenomenon was related to the intermolecular triplet exciton transfer from the attached 3DBP to free DBP. In order to confirm this hypothesis, the TD-DFT calculations were conducted to examine the absorption features of triplet DBP in comparison to those of an alkylated DBP analogue, 2,7-di-tert-butyl-4-phenethylpyrene, which served as a mimic of tethered DBP unit in 1. The simulated transition data (Figure S4) showed very similar absorption bands to those observed experimentally. More importantly, the substitution of a phenylalkyl group on DBP was indeed predicted to induce detectable bathochromic shifts of two major absorption peaks. These calculation results supported our speculation that the gradual hypsochromic shifting shown in the TA spectra over time mostly likely reflected the dynamic TTET process from the tethered, alkylated DBP to unattached, free DBP, which resulted in a gradual triplet exciton population change between the two types of DBP units. As the TA spectra allowed for monitoring the TTET from tethered DBP to free DBP, independent of the other intermolecular ET pathway (from the Ir-complex moiety in 1 to free DBP), a special opportunity emerged for the determination of corresponding ET rate constant (kq’). Subsequently, a quantitative treatment of the TA data was attempted. While ET was


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occurring, all recorded time-resolved spectra were composed of combined absorptions of two types of 3DBP. Thus, the following relationship was obtained between kq’ and the extinction values at 540 and 525 nm (see Supporting Information for details): ∆

!" #

%$− '∆

#$% ∝ ) *+

(2)

where [A] was the concentration of free DBP (intermolecular energy acceptor) and α was the molar extinction coefficient ratio of free 3DBP at 540 and 525 nm (i.e., α = ε540 / ε525), which could be obtained from the TA spectra at a sufficiently long delay time when [Ir-3DBP]* was nearly depleted. Then, the decay curve of ∆

!" #

%$− '∆

#$% was collected and plotted

against time (Figure 3b). By fitting the data to a single-exponential function, an apparent decay rate (corresponding to kq’) of 8.1×108 M-1 s-1 was estimated. Thus, the intermolecular TTET from appended DBP and free DBP was proven to be a relatively efficient channel. With these kinetic parameters available, it was necessary to evaluate whether and why such a bichromophoric would afford enhanced sensitization ability than the monochromophoric metalcomplex analogue. Based on our previous study results,15 it has been theoretically predicted that, for such bichromophoric molecules that accommodates a reversible intramolecular TTET process, when the following condition (3) is satisfied, the bichromophoric sensitizer would offer an enhanced TTET efficiency: ,

,

>

/01

(3)

234

This conclusion is valid as long as K>0, namely whenever the intramolecular ET occurs. In the current system, three of the rate constants in (3) were known, except for kq. Although the precise value of kq was not available for 1, its upper limit was reasonably inferable, which should not


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exceed kq of 1a. This is because the Ir-complex moiety in 1 had basically identical electronic properties as 1a, but the molecule size of 1 was larger than 1a, necessarily resulting in slower diffusion rate and thus reduced kq. Moreover, the covalently linked DBP unit might confer certain steric hindrance and impede the intermolecular TTET from Ir moiety to any energy acceptor. If so, lowered kq would necessarily occur to 1. Based on these considerations, the condition defined by (3) was certainly satisfied by 1. Therefore, an enhanced TTET ability was expected for 1 than 1a. Finally, the superior sensitizing performance of the bichromophoric molecule 1 to that of monochromophoric analogue 1a was demonstrated by comparing the relative efficiencies of TTA UC promoted by 1 and 1a as photosensitizer. When a degassed toluene solution of these sensitizers (20 µM), co-dissolved with free DBP (60 µM) as annihilator was irradiated with noncoherent, monochromic light at 450 nm, deep blue emissions were observable from both TTA systems, and the recorded emission spectra were nearly identical to the fluorescence spectrum of DBP in solution (Figure 4). The quadratic dependence of emission intensity on the excitation light power was confirmed to prove the nonlinear optical nature of UC. Most importantly, under all identical conditions sensitizer 1 was found to afford an upconverted emission exhibiting approximate 5 times intensity enhancement compared to that collected from 1a-sensitized TTA mixture (Figure 4). Therefore, the higher TTET efficiency was clearly demonstrated by these TTA UC experiments.


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Figure 4. TTA UC emission spectra and TTA UC efficiency of 1 and 1a in the presence of free DBP. The concentration of sensitizer is 20 µM and concentration of DBP is 60 µM in toluene. CONCLUSIONS A bichromophoric photosensitizer 1, comprising a triscyclometalated Ir(III) covalently linked to a DBP unit, is demonstrated to undergo reversible intramolecular triplet energy transfer between the two chromophores. Due to the evidently lower triplet level, the triplet excitons mainly reside with the DBP unit upon reaching the photostationary state, with an equilibrium constant K of ca. 1.1 × 103 estimated. Consequently, the molecule exhibits a long triplet lifetime of over 2 ms by virtue of the minimal inherent decay rate of DBP around 160 s-1. Because both the Ir-complex moiety and DBP unit in this bichromophoric molecule are capable of transferring their triplet energy to an external acceptor, the dynamic photosensitizing behaviours of the molecule are more complicated than a monochromophoric sensitizer. By using the nanosecond transient absorption technique, the intermolecular TTET from the tethered DBP to free DBP can be probed independent of the ET process taking place with the Ir-complex moiety. Thereby, an ET rate constant of roughly 8 × 108 M-1s-1 is determined for the path from [Ir-3DBP]* to free


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DBP. Such a value is comparable to the rate constant from [3Ir-DBP]* to free DBP, which is estimated to be lower than 3 × 109 M-1s-1 based on the quenching properties of the monochromophoric Ir-complex analogue. Taking into consideration of the favourable intramolecular ET equilibrium (K∼103), the intermolecular TTET of 1 is proven taking place predominantly through the tethered DBP instead of the metal complex moiety. More importantly, as a result of the rapid intramolecular ET, as well as the slow intrinsic decay of attached DBP, this cascade ET from Ir complex to linked DBP, and then to free DBP, provides a higher sensitizing efficiency than a direct ET from a monochromophoric Ir complex to free DBP. Essentially, the energy drain via the spontaneous (radiative and nonradiative) decays of the metal-complex moiety is greatly reduced. Finally, the enhanced sensitizing ability of the bichromophoric molecule is proven experimentally by applying it as the photosensitizer to a triplet-triplet annihilation upconversion. Under the same conditions, the bichromophoric sensitizer achieves a 5-time enhancement in the TTA upconverted emission intensity compared to a monochromophoric Ir-complex sensitizer analogue. The UC efficiency increase is reasonably attributable to the higher efficiency of sensitizing step. It is worth noting that the most important prerequisites for such a bichromophoric molecule to exhibit enhanced TTET ability is that the attached organic chromophore necessarily possesses a suitable triplet level (for acting as an intramolecular ET acceptor), competent ET rate to the external energy acceptor, and a small inherent decay rate (to minimize the competing energy loss). Additionally, it should be pointed out that the sensitization improvement is manifested more effectively under low acceptor concentration and slow diffusion conditions. In solution, the inferior sensitizing efficiency of conventional monochromophoric sensitizers can be compensated by a high annihilator concentration. However, in a medium entailing particularly


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slow diffusion rate, such as in viscous solvents or even solid-state matrices, the advantages of these bichromophoric sensitizers with long excited-state lifetimes are very much evident. Such unique properties may confer these new sensitizers with unique applications, including achieving highly effective TTA UC in non-fluidic matrices.

ASSOCIATED CONTENT Supporting Information. Synthetic procedures, characterization data, triplet−triplet annihilation upconversion experimental results and detailed deduction of all equations. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was financially supported by the National Natural Science Foundation of China (Nos. 51573002 and 51473003). REFERENCES


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Enhanced Triplet Sensitizing Ability of an Iridium ... - ACS Publications

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