Molecular-Level Understanding of Dual-RTP via Host-Sensitized Multiple Triplet-to-Triplet Energy Transfers and Data Security Application

Dual-room-temperature phosphorescence (DRTP) from organic molecules is of utmost importance in chemical physics. The Dexter-type triplet-to-triplet energy transfer mechanism can therefore be used to achieve DRTP at ambient conditions. Here, we report two donor–acceptor (D–A)-based guests (CQN1, CQN2) in which the donor (D) and acceptor (A) parts are held in angular orientation around the C–N single bond. Spectroscopic analysis along with computational calculations revealed that both guests are incapable of emitting either thermally activated delayed fluorescence (TADF) or RTP at ambient conditions due to large singlet–triplet gaps, which are presented to show host (benzophenone, BP)-sensitized DRTP via multiple intermolecular triplet-to-triplet energy transfer (TTET) channels that originate from the triplet state (T1BP) of BP to the triplet states (T1D, T1A) of the D and A parts (TTET-I:T1BP → T1D; TTET-II:T1BP → T1A). In addition, an intramolecular TTET channel that occurs from the T1D to T1A states of the D and A parts of CQN2 is also activated due to the low triplet (T1D)–triplet (T1A) gap at ambient conditions. The efficiency of TTET processes was found to be 100%. The phosphorescence quantum yields (ϕP) and lifetimes (τP) were shown to be 13–20% and 0.48–0.55 s, respectively. Given the high lifetime of the DRTP feature of both host–guest systems (1000:1 molar ratio), a data security application is achieved. This design principle provides the first solid proof that DRTP via radiative decay of the dark triplet states of the D and A parts of D–A-based non-TADF systems is possible, revealing a method to increase the efficiency and lifetime of DRTP.


■ INTRODUCTION
Room-temperature phosphorescent (RTP) materials shed light on the development of anticounterfeiting, information storage, 1−4 and biological imaging 5,6 due to the involvement of long-lived triplet emission. Generally, heavy metal-based inorganic systems are used because of strong spin−orbit coupling (SOC) induced by the heavy metals, which is conducive to high-efficiency RTP. 7,8 However, these systems suffer from high costs and harsh preparation conditions, which limit their applications. Compared to their well-developed inorganic counterparts, organic RTP materials also have flourished due to their advantages of structural versatility and processability. 3,4,9−14 However, most RTP materials show extremely dim photoluminescence because of weak SOC and their high sensitivity to temperature, moisture, and molecular oxygen. 7 In this context, several rational strategies have been demonstrated to achieve efficient SOC including heavy atom effect, 15 El-Sayed's rule, 7,16,17 and energy gap principle. [3][4][5]18 In addition, host−guest systems, 2,19−21 introducing intermolecular interactions, 3,4,12,22−26 and rigid single crystals 27 −31 have also been established to suppress significant radiationless decays (k nr ) via rigidification of the emitter. However, these strategies still suffer from low phosphorescence quantum yield (ϕ P ), and the obstacles of RTP seem impossible to be solved.
On the other hand, achieving dual-RTP (DRTP) via simultaneous radiative decay of T 1 and T 2 states (breakdown of Kasha's rule) to the ground state (S 0 ) is a challenging task in chemical physics. A handful of reports have established that the DRTP feature can be obtained by radiative decay of both T 1 and T 2 (vibronically mixed or thermally equilibrated from T 1 state) states to the ground S 0 state due to the low S 1 -T 2 gap and faster ISC rates for S 1 → T 2 compared to S 1 → T 1 . 32−36 Because of these herculean prerequisites, organic highefficiency DRTP materials have remained largely unexplored in photophysics.
Triplet-to-triplet energy transfer (TTET), which plays a pivotal role in chemistry and biology, 37−40 is a well-known photophysical process of exchanging both spin and energy between a pair of molecules or intramolecular fragments. From this aspect, the TTET process that occurs when the two exchanging parts locate at a short distance (<10 Å, one molecule) 41 and release energy via Dexter-type channels could become an alternative and effective transition pathway for the generation and stabilization of triplet excitons. Recently, utilizing this concept, a few RTP systems based on intraand/or intermolecular TTET have been developed. 41−46 However, the exact mechanism of DRTP and its control and the specific design principles for DRTP are yet to be developed. Therefore, the development of conceptually new methods to boost the efficiency and increase the lifetime of organic DRTP materials is of pivotal importance from both a fundamental viewpoint and for production of high-efficiency organic electronic devices.
Herein, we propose a strategy for harvesting dark triplet states (as DRTP) of D and A components of D−A guests that show neither thermally activated delayed fluorescence (TADF) nor RTP due to a large intrinsic energy gap (ΔE ST ) between the S 1 and T 1 states (Figure 1a). Benzophenone (BP) was selected as a triplet sensitizer (host) because (a) the low-lying T 1 energy level (T 1 BP ) has lower energy compared with the singlet charge transfer (S 1 CT ) state of the D−A guests but remains higher in energy as compared with the lowest triplet states (T 1 D , T 1 A ) of D and A parts, which can enable multiple TTET channels, (b) the moderate dipole moment (μ = 2.95 D) can reduce the S 1 CT -Τ 1 BP gap due to stabilization of the 1 CT state and trigger the spin-forbidden intersystem crossing (ISC) between 1 CT and T 1 BP , and (c) due to the facility for solution processing. Three carbazole-quinoline-based donor−acceptor (D−A) conjugates (CQN1, CQN2), in which the carbazole donor is covalently attached to the quinolinyl acceptor via a C−N single bond, were selected as guests considering that the energies of T 1 D and T 1 A are very close and/or lower compared with T 1 BP to avoid energy loss. Carbazole (Cz), acceptor (QCN), and a control compound (CQM) were used to prove the hypothesis (Figure 1b) TTET processes (TTET-I,  TTET-II, TTET-III), which otherwise was impossible to achieve without using a sensitizer at ambient conditions. Given the persistent luminescence characteristics of both HG systems, a data security application has been developed. These results imply the feasibility of using multiple TTET strategies to harvest dark triplet states of non-TADF systems and boost the efficiency and lifetime of DRTP at ambient conditions.

■ RESULTS AND DISCUSSION
Single-Crystal X-ray Analysis. SCXRD analyses ( Figure  2) of both the guests reveal that the carbazolyl ring and aryl substituents attached to the C8 and C4 atoms of the quinolinyl moiety deviate from planarity (CQN1:  Table S1). We also found in both guests that aryl substitution at position 2 of the quinolinyl moiety causes the aryl substituents to be out of plane with torsions of 165.94(3) and −171.07(1)°, respectively, when viewed along the C(24)−C(23)−C(2)−N(1) atoms. X-ray analysis of both guests unambiguously confirms our structural model where D and A parts are held in angular orientation conducive for spatial separation of HOMO and LUMO.
Cyclic Voltammetry Analysis. To understand the redox behaviors of both guests, CV measurements and the estimation of the HOMO and LUMO of both guests and their individual donor (Cz) and acceptor (QCN) parts were undertaken. Table S3 and Figure S14 reveal that HOMO and LUMO energies are essentially consistent with the energy difference between the HOMO and LUMO levels of Cz and QCN (2.82 and 3.23 eV) as determined by CV data. The results suggest that the HOMO−LUMO gaps in both guests are determined by the HOMO and LUMO levels of the Cz and QCN parts, and energies are largely localized on the Cz and QCN parts of both the guests, which is fully consistent with the X-ray structures of both systems in which the donor and acceptor are held in an angular arrangement, indicating a spatial separation of HOMO and LUMO.
Computational Analysis. To obtain further insight into the excited states of both guests, DFT and TD-DFT 47 calculations were performed in GAUSSIAN 09 D.01 48 at the BLYP/6-31G(d) level of theory (see the Supporting Information). The calculations revealed that the highest occupied molecular orbitals (HOMOs) are predominantly localized on the carbazolyl rings, while the lowest unoccupied molecular orbitals (LUMOs) are distributed over the quinolinyl fragments of CQN1 and CQN2 (Figures S15 and S16). The calculations also suggest that the isosurfaces of S 1 states of both guests are localized on the quinolinyl fragment with partial contribution from the N atom of the carbazolyl ring, while the corresponding isosurfaces of T 1 states are predominantly delocalized over the quinolinyl ring and its phenyl substituents at C2 and C4 positions, respectively (Figures S17 and S18). Natural transition orbitals (NTO) 49 analysis revealed that the S 1 state of both systems shows a pronounced CT-excitation character, as the hole and electron wave functions are localized at the donor and the acceptor fragments, respectively (Figures S15 and S16). Moreover, the calculated energy differences between S 1 and T 1 states (ΔE ST ) were found to be 0.569 eV (CQN1) and 0.507 (CQN2) eV, which ruled out the possibility of TADF in both guests (Figures S17 and S18). These results demonstrate that both CQN1 and CQN2 exhibit only radiative decay of S 1 ( 1 CT) states to enable fluorescence.
Absorption and Emission Characteristics in Solutions and Films. Ultraviolet−visible (UV−vis) absorption measurements of CQN1 and CQN2 in both solutions and 1,3-bis(Ncarbazolyl)benzene (mCP) films (1 wt %) show absorption peaks at ∼320 to 330 and ∼370 to 425 nm, respectively (Figures 3a and S19a). The hypsochromic shift of the lowerenergy band (370−425 nm) with increasing solvent polarity  confirms the CT nature of the absorption band while the former band (π−π* transition) has no effect on the solvent polarity, which are in line with previous reports (Figures 3a and S19a). The higher-energy absorption bands (∼285 to 330 nm) of both guests closely resemble their absorptions of individual D and A parts, which indicates that the new lowerenergy CT absorption band is due to the D−A torsional motion. Likewise, positive solvatochromism effects with a broadband emission feature and nanosecond lifetimes in toluene solutions (CQN1: λ 489 , τ F = 7.86 ns; CQN2: λ 483 , τ F = 9.52 ns) ensure that both guests possess significant CT (S 1 CT ) at the excited state (fluorescence quantum yield, ϕ F = ∼25 to 30%) (Figures 3a, S19b, and S20, Table 1). A similar fluorescence feature was also observed in mCP films (ϕ F = ∼29 to 36%) ( Figure S21). To understand TADF, steady-state emission along with lifetime measurements of both conjugates in toluene solutions was carried out under oxygenated and degassed conditions at RT (Figures 3b,c and S22). We found that both conjugates show no significant change in the lifetime values (7.64−9.19 ns, degassed; 8.91−11.68 ns, oxygenated), proving that only radiative decay of 1 CT states at ambient conditions is present. The phosphorescence measurement (λ ex = 328 nm, 1.0 ms detector delay) at 77 K shows vibrational emission features ( 3 LE) at 430−510, 560−580 (CQN1) nm, and 535−550 nm (CQN2), which closely overlaps with the phosphorescence peak of the D (T 1 D ) (Cz) and A (T 1 A ) part (QCN) of the guests (Figure 3d). It should be noted that D parts of both guests display phosphorescence (77 K) at ∼495 nm, 50 which was absent in the phosphorescence spectra of CQN2 due to an efficient intrinsic TTET-III (T 1 D → T 1 A ) channel compared to CQN1. The ΔE ST values were calculated to be 0.53−0.59 eV, which rules out the possibility of TADF ( Figure S23) and which is in good agreement with the computational analysis.
Absorption and Emission Characteristics of Host− Guest Binary Systems. Then, we systematically investigated the photophysical properties of the host and both HG binary systems that were prepared by solution processing with a H/G molar ratio of 1000:1. The absorption spectra of both HG systems showed similar absorption features (320−330, 370− 430 nm) as those of CQN1, CQN2 (toluene and 1 wt % mCP film), and BP (Figures 4a and S24). Likewise, the steady-state emission (λ ex = 425 nm, CQN1/BP; λ ex = 370 nm, CQN2/ BP) at ambient conditions shows bathochromically shifted emission peaks at 485 and 480 nm with a broad shoulder at 510−700 and 500−660 nm, respectively (ϕ PL = ∼23 to 28%) ( Figure 4b, Table 1). The parent emission peaks (485 and 480 nm) are assigned to the fluorescence (broad, S 1 CT ) and are confirmed in the fluorescence decay measurement (τ F = 10.45 and 12.56 ns) ( Figure S25).
Temperature-Dependent Time-Resolved Emission Spectra (TRES) Analysis of Binary Systems. To obtain further insight into exciton dynamics, temperature-dependent (300−10 K) TRES measurements were carried out (     (Figures 5c and 29a).
On the other hand, Figures S29b, S31, and 5d−f represent the emission profiles of long-lived (ms) components of the emission decays of CQN2/BP at 10−300 K. At 300 K, we found a similar dual emission feature (CQN2/BP: λ em = 495, 540 nm; λ ex = 390 nm) in a delay time of 0.23 s, except that a significant reduction in the intensity of the higher-energy emission band (495 nm) as compared with CQN1/BP was recorded. At a further delay of 0.27−0.37 s, no bathochromic shift of the emission bands was observed. At 150 K, two clear LE features (LE1: 495 nm; LE2: 540 nm) were observed. Interestingly, the energy on-set of LE1 is lower in energy than that of LE1 of CQN1/BP ( Figure S32b), indicating that contribution of LE1 of CQN2/BP is due to radiative decay of T 1 D . Later time scales of 0.27−0.37 s resulted in no shift of the emission bands. However, a significant reduction in the intensity of the higher-energy emission band (LE1) with an increase in the intensity of the lower-energy emission band (LE2) was observed at time delays of 0.27−0.37 s, which is consistent with that of CQN1/BP. These results suggest that both TTET-I (via T 1 BP → T 1 D ) and TTET-II (via T 1 BP → T 1 A ) are also activated in CQN2/BP. At 77−10 K, no significant TTET processes occur as predominant emission contribution from LE1(T 1 BP ) and LE2(T 1 A ) is clearly visible in the TRES measurements. It should be noted that TTET-I (T 1 BP → T 1 D ) in CQN1/BP is activated at 300 K while CQN2/BP exhibits efficient TTET-I at 150 K. Since most of the emissive photons arise from the long-lived T 1 A (τ P = 322.41 ms, RT), we speculate that an additional TTET-III channel has been activated in CQN2/BP, which is responsible for efficient DRTP as compared with CQN1/BP (Figures 4b and S32b). To understand TTET-III, we performed temperature-dependent (77, 10 K) phosphorescence measurements of both guests. At 77 and 10 K, both guests exhibit dual phosphorescence bands (CQN1: 495, 560 nm; CQN2: 495, 540 nm) due to radiative decay of T 1 D and T 1 A ( Figure S33). Interestingly, CQN2 on increasing the temperature from 10 to 77 K shows a decrease in intensity of the higher-energy local emission band (495 nm) with an increase in the intensity of the lower-energy local emission band (540 nm), while a significant reduction in the intensity of both local triplets (T 1 D , T 1 A ) of CQN1 was observed. These observations clearly confirm that an efficient intrinsic TTET-III process is activated in CQN2. It should be noted that no significant change in the emission dynamics of both the HG systems was observed when higher-energy photoexcitation (328 nm) was used in the TRES measurements ( Figures S30 and S31).
Energy Gap Analysis. Energy gaps were calculated to be 0.    (Figures 6 and S34). Based on these results, we believe that both guests show DRTP due to the host-sensitized multiple TTETs.
Excitation-Dependent Emission and Excitation Spectra Analysis. To obtain further insight into the emission dynamics, excitation-dependent emission measurements were performed at RT. When we excite both the HG systems at 320 nm, no change in the steady-state emission feature as compared with the steady-state emission recorded at excitation of 425 nm (CQN1/BP) and 370 nm (CQN2/BP) (Figures 4b  and S35) is observed. On the other hand, RTP measurements (1.0 ms detector delay) with photoexcitation of 328 nm for both HG systems resulted in a dual-RTP feature, which is fully consistent with the dual-RTP feature recorded at excitations of 415 nm (CQN1/BP) and 390 nm (CQN2/BP) (Figures 4b  and S36). These results suggest that no excitation-dependent emission characteristics are involved in the emission of both HG systems. Further, to understand ground-and excited-state heterogeneities of both HG systems, excitation spectra were recorded at RT and 77 K. Excitation spectra measurements while monitoring the emission wavelengths of 487 nm (CQN1/BP) and 480 nm (CQN2/BP) at RT (steady-state spectra) revealed that lower-energy regions (CQN1/BP: 390− 475 nm; CQN12BP: 365−450 nm) of the excitation spectra closely overlap with the lower-energy region of the absorption spectra of CQN1/BP and CQN2 BP, while the higher-energy regions (CQN1/BP: 285−390 nm; CQN2/BP: 285−365 nm) of the excitation spectra overlap with the higher-energy regions of the absorption spectra of CQN1/BP, CQN2/BP, and BP ( Figure S37). Phosphorescence excitation spectra of both HG systems at RT also revealed similar absorption features as those of steady-state excitation spectra at RT ( Figure S38). It should be noted that both HG systems in phosphorescence measurements (1.0 ms detector delay) at 77 K resulted in dominant LE emission bands at 480 nm, which originate from the host (T 1 BP ). When we compare the excitation spectra of BP recorded at RT with the excitation spectra of CQN1/BP and CQN2/ BP at 77 K while monitoring the LE emission band of BP (480 nm), it is observed that both excitation spectra are closely overlapped with each other. These results ensure that no heterogeneity is present in both the ground and excited states. Since phosphorescence emission contribution (excitation independent) from BP in both HG systems is absent, we confirm that 100% sensitization has occurred from T 1 BP to T 1 D and T 1 A of the guests (CQN1, CQN2), proving host-sensitized multiple TTETs that triggers DRTP at ambient conditions.
Control Analysis. To test our hypothesis, we synthesized CQM (see the Supporting Information), in which the C2 and C4 positions of the quinolinyl core are substituted by phenyl rings while the methyl group is introduced at the C5 position inducing an electronic effect. The PL measurements show that CQM also exhibits fluorescence both in solutions and solid state via radiative decay of the S 1 CT (445 nm) state, which is hypsochromically shifted as compared with the other guests, indicating that S 1 CT has higher energy as compared to the S 1 BP and T 1 BP states of BP ( Figure S39). The large ΔE ST value (ΔE S 1 CT -T 1 BP = 0.58 eV) like other guests eliminates the possibility of TADF. Since CQM/BP with the same molar ratio resulted in no RTP/DRTP features (λ ex = 328 nm) at RT, we confirm that TTET is not activated due to an inefficient ISC from S 1 CT to T 1 BP . Therefore, we conclude that T 1 BP of the host, which falls between S 1 CT and T 1 D and/or T 1 A of guests, can enable multiple TTETs that are responsible for DRTP of CQN1 and CQN2 at ambient conditions ( Figure 6).
Discussion. Both guest molecules showed no thermally activated delayed fluorescence (TADF) due to high S 1 −T 1 gaps; only radiative decay of excited singlet states ( 1 CT) was observed both in solutions and solid states. A comparison between the PL analysis of guests (CQN1, CQN2) and host− guest (HG) systems (CQN1/BP, CQN2/BP) suggests that the higher-energy triplet state of the host (T 1 BP ) sensitizes lower-energy triplet states (T 1 D , T 1 A ) of guests in the HG binary systems leading to dual-RTP at ambient conditions via multiple triplet-to-triplet energy transfer (TTET) channels. Further, a comparison between the excitation-dependent emission analysis of both HG systems recorded at RT and 77 K ensures that multiple TTET channels are activated at RT, proving DRTP at ambient conditions. Further, when   43,44,51,52 The emission analysis of both guests and HG systems reported here indicates that the intramolecular TTET (TTET-III) is also activated in CQN2/BP along with the intermolecular TTET (TTET-I, TTET-II) due to close energy gaps (T 1 D -T 1 A ) between the donor and acceptor components, which provides an additional driving force to alter the DRTP feature. When comparing the phosphorescence analysis of both HG systems, we found that the intensity of the lower-energy RTP band (excitation independent) in CQN2/BP is significantly higher as compared with CQN1/BP. Therefore, different DRTP properties of both the HG systems can be explained by the low intrinsic T 1 D -T 1 A gap of 0.22 eV in CQN2 as compared with CQN1 (0.29 eV). Comparison of the phosphorescence bands of the respective guests and their HG systems recorded at 77 and 300 K along with absorption and excitation spectra analysis ( Figures S37, S38, and S40) ruled out the possibility of ground-state and excited-state heterogeneities present in the HG systems. Furthermore, it is established that the benzophenone host plays an important role in (a) maintaining rigidity, (b) reducing nonradiative pathways (k nr , k q ), and (c) increasing the lifetime of the triplet states. 43,44 We ensure that the triplet state (T 1 BP ) of benzophenone enables intermolecular TTETs leading to efficient dual-RTP with an afterglow feature under ambient conditions. These results demonstrate that a careful section of the triplet state of the host plays an important role in populating the triplet states of donor and acceptor parts of non-TADF systems, setting a benchmark for achieving dual-RTP with an afterglow feature via host-sensitized multiple TTETs ( Figure 6).
Data Security Applications. Given the afterglow (CQN1/BP: λ 495 , τ P = 118−226 ms; λ 560 , τ P = 208−413 ms; CQN2/BP: λ 495 , τ P = 88−187 ms; λ 540 , τ P = 237−379 ms) feature of both HG systems, we have developed a security pattern of ∇ made of CQN1 and CQN1/BP and CQN2 and CQN2/BP. To achieve host−guest binary systems for data security applications, we dissolved both the host (H) and guest (G) (1000:1 molar ratio) in dichloromethane (DCM) solutions. The solution was drop-cast on a glass plate and evaporated under slow heating conditions (40°C). To demonstrate data security applications, we utilized the same HG binary systems on ordinary black paper. The symbol "∇" using CQN1 with CQN1/BP and CQN2 with CQN2/BP was written (Figure 7) separately. A cyan-blue pattern "∇" was observed after irradiation with a UV lamp (365 nm). Switching off the lamp resulted in green and yellow patterns of "V" encrypted by CQN1/BP and CQN2/BP, respectively, due to the absence of the afterglow property of the guest itself (webenhanced objects 1, 2). Thus, application of anticounterfeiting and information storage is achievable using these simple systems.

■ CONCLUSIONS
To conclude, the first example of efficient DRTP via multiple triplet-to-triplet energy transfers (TTETs) is presented here.
The low energy gap between the lowest T 1 state of the sensitizer (BP) and the singlet (S 1 CT ) state of the guests (CQN1, CQN2) along with close triplet energy levels of the donor and acceptor parts of guests is the key to achieving efficient intermolecular ISC that triggers multiple TTETs to boost DRTP with an afterglow feature. The lifetimes of the dual phosphorescence bands with the afterglow feature of both guests in the presence of BP are found to be 118−226 ms (λ 495 ) and 208−413 ms (λ 560 ) (CQN1/BP) and 88−187 ms (λ 495 ) and 237−379 ms (λ 540 ) (CQN2/BP). Furthermore, the efficient DRTP with the afterglow could be achieved without much loss of photon energy. These results demonstrate that DRTP via host-sensitized TTET is possible and can be used as a general strategy to increase the rate of phosphorescence emission from non-TADF guests. Our results thus highlight the practical importance of DRTP in the field of photonics.

■ EXPERIMENTAL SECTION
Synthesis and Characterization. CQN1, CQN2, and CQM were synthesized by the traditional nucleophilic aromatic substitution reaction (SNAr) between carbazole and 1-fluoro-nitrobenzene, 4-fluoro-3-nitrobenzonitrile, and 4-methyl-3-nitrobenzonitrile, followed by a reduction of the nitrofunction to the amine intermediates, and treated with phenyl acetylene and respective aryl aldehydes (benzaldehyde, 4-formylbenzonitrile) in the presence of a catalytic amount of iron(III)-trifluoromethanesulphonate in oxygenated conditions to obtain CQN1 and CQN2 (Figure 1, Scheme S1, Supporting Information). [9][10][11]14,18 The acceptor part (QCN) was synthesized by following a similar methodology using aniline, 4-formylbenzonitrile, and phenyl acetylene. All of the compounds were characterized by NMR spectroscopy, highresolution mass spectrometry (HRMS), and X-ray analysis (see the Supporting Information). High-performance liquid chromatography (HPLC) of vacuum-sublimated samples was used to determine the purity of the compounds (Figures S9−S11). Both compounds displayed remarkably high thermal degradation, ensuring high stability ( Figure S12).
Photophysical Measurements. Absorption measurements were performed using a Cary 8454 UV−vis instrument from Agilent Technologies. Steady-state emission, phosphorescence, and lifetime analysis of the samples were recorded on Horiba Fluorolog-3 and Edinburgh FLS980 spectrometers. All of the phosphorescence spectra were recorded using a detector delay of more than 0.5 ms. The measurement details can be found in the reported literature. [9][10][11]14,18 Temperature-dependent PL measurements were performed using a cryostat (JANIS CS204SE-FMX-1AL). Cyclic voltammetry was performed using a BioLogic potentiostat. The electrochemical cell included a glassy carbon disk as the working electrode, an Ag/AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo thermogravimetric analyzer (TGA2 SF/1100: RT to 800°C). Highperformance liquid chromatography (HPLC) experiments were carried out on a Waters Alliance system (Milford, MA) consisting of an e2695 separation module and a 2998 photodiode-array detector.