Study on a rhenium complex having an electron-pulling ring and its oxygen sensing application: Synthesis, characterization and sensing performance

Sensors and Actuators B 253 (2017) 310–316

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Study on a rhenium complex having an electron-pulling ring and its oxygen sensing application: Synthesis, characterization and sensing performance Liang Liu a,b,1 , Yan Yang c,1 , Jun-sheng Feng a,∗ a

School of Environmental and Safety Engineering, Changzhou University, Changzhou, 213164, People’s Republic of China Huaide College, Changzhou University, Jingjiang, 214500, People’s Republic of China c Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou Jiangsu, 213164, People’s Republic of China b

a r t i c l e

i n f o

Article history: Received 7 March 2017 Received in revised form 15 June 2017 Accepted 24 June 2017 Available online 27 June 2017 Keywords: Re(I) complex Oxygen-sensing Photophysical features MCM-41 Theoretical analysis

a b s t r a c t This paper reported a Re(I) complex whose diamine ligand had an oxadiazole ring for optical oxygen sensing. This Re(I) complex was characterized and discussed through single crystal XRD, density functional theory calculation and photophysical measurement. It was found that this oxadiazole ring increased electronic distribution and decay lifetime of excited electrons. Oxygen sensing collision probability was consequently increased, favoring oxygen attack. Using a silica molecular sieve MCM-41 as supporting substrate, its oxygen sensing performance was tentatively evaluated. A dynamic collision mechanism was confirmed with sensitivity of 4.0 and response time of 14 s. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Composite materials of a hybrid structure have drawn much attention since they are able to combine and hold features of their components and thus realize multifunctional purpose [1,2]. For a representative composite material having an organic-inorganic hybrid structure, its organic component is usually designed as a functional component since an organic structure is convenient to be chemically modified. While, its inorganic component generally serves as supporting substrate in virtue of its good mechanical strength and stability [2,3]. Such composite materials have shown their potentials in various fields, including catalysis, solar cells, light emitting and optical sensing [3,4]. Among their numerous applications, optical oxygen sensing has been usually mentioned. This is because molecular O2 is a lifesupporting gas and thus considered as an important target analyte in chemical industry, food processing and environmental monitoring [5,6]. Optical oxygen sensing composites are superior to traditional O2 quantification methods by showing virtues of slim consumption of target analyte, simple operation, fast response and

∗ Corresponding author. E-mail address: [email protected] (J.-s. Feng). 1 Authors Liang Liu and Yan Yang contributed equally to this work. http://dx.doi.org/10.1016/j.snb.2017.06.162 0925-4005/© 2017 Elsevier B.V. All rights reserved.

low need for instrumentation [7,8]. In an ideal optical sensing composite, several criteria should be satisfied to achieve desired sensing performance. For example, supporting matrix disperses probe molecules and transports analyte molecules. Thus, an ideal one should have high diffusion coefficient, good stability and perfect compatibility with probe molecules [9–12]. A silica based supporting substrate MCM-41 appears as a promising candidate since it satisfies above requirements well [11,12]. An ideal sensing probe is supposed to have both a long-lived emissive center and broad distribution of excited electrons which are positive to increase sensing collision probability with analyte molecules. It seems that luminescent metal complexes, such as Re(I) complexes, can well meet above demands [13–16]. According to literature reports, occupied frontier molecular orbitals (FMOs) of a typical Re(I) complex have predominant metal character, while its unoccupied ones are ␲* orbitals of its diamine ligand [13–16]. It is thus assumed that electronic distribution of excited electrons can be widened by increasing conjugation chains of diamine ligands in Re(I) complexes. In the meanwhile, emission decay lifetime may be increased by introducing an electron-pulling group into diamine ligand since its presence suppresses non-radiative decay effectively [13–16]. Guided by above consideration, we design a diamine ligand having a large conjugation plane of oxadiazole ring, as shown in

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Scheme 1. Synthetic route for CPO, Re(CO)3 (CPO)Br and Re(CO)3 (CPO)Br doped MCM-41 composite samples.

Scheme 1, hoping to increase emission decay lifetime and electronic distribution of excited electrons of its Re(I) complex. By doping this Re(I) complex into a silica supporting substrate MCM-41, oxygen sensing behavior of corresponding composites is discussed.

silica gel column. 1 HNMR (300 MHz, CDCl3 ): ı 7.22 (1H, m), 7.41 (2H, m), 7.80 (1H, m), 8.13 (1H, t), 8.23 (1H, t), 8.29 (1H, d, J = 6.0), 8.71 (1H, d, J = 3.6). Anal. Calcd. For C13 H8 N3 OCl: C, 60.60; H, 3.13; N, 16.31. Found: C, 60.39; H, 3.32; N, 16.17. MS m/z: [m]+ calc. for C13 H8 N3 OCl, 257.0; found, 257.2.

2. Experimental details 2.3. Synthesis of Re(CO)3 (CPO)Br 2.1. Reagents and equipments Starting chemical 2-(2H-tetrazol-5-yl)pyridine (denoted as TP) was fabricated following a literature method [17]. Other chemicals, such as zinc bromide, sodium azide, Re(CO)5 Br, 4-chlorobenzoyl chloride and blank MCM-41, were purchased from Senda Chemical Co. (Hangzhou) and used directly for synthesis. Organic solvents were purified following standard methods. Single crystal analysis was finished by a Siemens P4 singlecrystal X-ray diffractometer and a Smart CCD-1000 detector, using graphite-monochromated Mo K␣ radiation (50 kV, 30 A, 298 K). All hydrogen atoms were calculated. Elemental analysis was performed by a Vario Element Analyzer. NMR, MS, UV–vis absorption and emission spectra were recorded by a Varian INOVA 300 spectrometer, a Agilent 1100 MS series/AXIMA CFR MALDI/TOF MS spectrometer, a Shimadzu UV-3101PC spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. Sample ICP-AES was finished on a PerkinElmer Optima 8000 spectrometer. Emission decay dynamics were measured by a twochannel TEKTRONIX TDS-3052 oscilloscope, using pulsed Nd:YAG laser as excitation source (␭ = 355 nm). XRD analysis was carried out by a Rigaku-Dmax 2500 diffracrtometer (␭ = 0.154 nm, scanning step = 0.02◦ ). Density functional theory calculation was finished through GAMESS software at RB3LYP/SBKJC level, using single crystal as initial geometry. Graphical plotting for FMOs was performed by wxMacMolPlt with contour value of 0.025. 2.2. Synthesis of ligand CPO Diamine ligand 2-(4-chlorophenyl)-5-(pyridin-2-yl)-1,3,4oxadiazole (denoted as CPO) was fabricated following a literature method [15]. The mixture of TP (10 mmol) and anhydrous pyridine (20 mL) was cooled by ice bath. 4-chlorobenzoyl chloride (15 mmol) was slowly added. The resulting mixture was heated at 100 ◦ C for 36 h under N2 protection. After cooling, plenty of cold water was added. Crude product was collected and purified on a

Re(CO)3 (CPO)Br was fabricated following a literature method [13,15]. The mixture of CPO (10 mmol) and Re(CO)5 Br (10 mmol) in anhydrous toluene (30 mL) was heated at 120 ◦ C for 10 h under N2 protection. Solvent was extracted by thermal evaporation under reduced pressure. Crude product was purified on a silica gel column. 1 H NMR (300 MHz, CDCl ): ı 7.61 (1H, m), 7.72 (2H, m), 7.96 (1H, 3 m), 8.38 (1H, t), 8.45 (1H, t), 8.51 (1H, d, J = 6.0), 8.72 (1H, d, J = 4.0). Anal. Calcd. for C16 H8 BrClN3 O4 Re: C, 31.62, H, 1.33, N, 6.91. Found: C, 31.50, H, 1.46, N, 6.72. MS m/z: [m]+ calc. for C16 H8 BrClN3 O4 Re, 606.9; found, 607.0. Its single crystal will be discussed below (CCDC 1184257). 2.4. Construction of Re(CO)3 (CPO)Br doped MCM-41 composite samples Re(CO)3 (CPO)Br was doped into MCM-41 supporting substrate with various doping concentrations so that oxygen sensing performance of these composite samples can be compared. A typical run is described below. Re(CO)3 (CPO)Br was carefully weighted and mixed with CH2 Cl2 (5 mL) under stirring. Blank MCM-41 (1 g) was slowly added after the solution became transparent. This mixture was stirred for an hour at room temperature. Solid product was filtered off and washed with CH2 Cl2 (10 mL × 3) to give Re(CO)3 (CPO)Br doped MCM-41 composite sample. Re contents of resulting samples were determined by ICP-AES method as 6.9 × 10−5 mol/g (45 mg/g doped), 8.4 × 10−5 mol/g (55 mg/g doped) and 9.7 × 10−5 mol/g (65 mg/g doped), respectively. 3. Results and discussion 3.1. Design strategy of Re(CO)3 (CPO)Br doped MCM-41 composite sample Our sensing probe Re(CO)3 (CPO)Br is further explained as follows aiming at a clear understanding on its design and sensing

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Table 1 Selected geometric parameters of Re(CO)3 (CPO)Br. bond length

(Å)

bond angle

(◦ )

Re(1)-C(1) Re(1)-C(2) Re(1)-C(3) Re(1)-Br(1) Re(1)-N(10) Re(1)-N(17)

1.893 1.892 1.926 2.614 2.165 2.229

N(10)-Re(1)-N(17) N(10)-Re(1)-C(3) N(10)-Re(1)-C(1) N(17)-Re(1)-C(3) N(17)-Re(1)-C(2) C(1)-Re(1)-C(2)

73.31 95.75 95.15 92.04 99.64 87.83

strategy. Literature reports have found that oxygen sensing based on emission quenching is finished through an energy transfer collision between probe excited electrons and O2 molecules [11,12]. Given this assumption, if decay lifetime and electronic distribution of excited electrons are increased and broadened, their collision probability with O2 molecules will be increased, improving oxygen sensing performance [13–15]. Since excited electrons are localized on unoccupied FMOs which are ␲* orbitals of diamine ligand in nature, it is reasonable to assume that a large conjugation plane formed by an oxadiazole ring in CPO should improve sensing performance. In addition, this electron-pulling group may decrease non-radiative decay probability of excited state and thus increase decay lifetime of excited electrons, increasing sensing collision probability as well [16]. Excited electrons of Re(CO)3 (CPO)Br should be readily attacked by O2 molecules owing to the small steric hindrance of its ligands. Upon being immobilized in MCM41 supporting substrate, promising oxygen sensing performance is desired from Re(CO)3 (CPO)Br doped MCM-41 composites. 3.2. Single crystal structure of Re(CO)3 (CPO)Br Re(CO)3 (CPO)Br single crystal structure is shown in Fig. S1A (Supporting Information). It is observed that its coordination center is surrounded by two N atoms, three C atoms and a Br atom, forming a representative octahedral coordination environment [13–15]. Key bond lengths and bond angles are summarized in Table 1. Owing to its heterogeneous ligands, N-based CPO, C-based CO and Br, Re(CO)3 (CPO)Br coordination geometry is distorted slightly. ReC(3) bond length is different from the other two Re-C bonds due to the interference effect from Br atom localized at the opposite position of C(3). The two Re-N bond lengths are different from each other. Re-N(10) bond is shorter than Re-N(17), indicating that Re ion has a stronger affinity for N(10) than that for N(17). This is because electron-donating feature of N(10) is higher than that of N(17) owing to the short conjugation chain and the electrondonating atoms in oxadiazole ring. Most bond angles in Table 1 are close to 90◦ , with an exception of N-Re-N bite angle. Its value is much smaller than those in metal complexes of tetrahedral coordination environment. It is thus concluded that all ligands are trying to minimize steric hindrance around Re center in virtue of the roomy space in Re(CO)3 (CPO)Br coordination sphere. In this case, oxygen sensing shall be facilitated since excited electrons can be readily attacked by O2 molecules. A large conjugation plane is observed in Re(CO)3 (CPO)Br, formed by a pyridine ring, an oxadiazole ring and a chlorobenzene ring, which favors oxygen sensing by increasing electronic distribution of excited electrons. -␲ attraction between these large conjugation planes makes Re(CO)3 (CPO)Br molecules take an ordered arrangement, as depicted in Fig. 1B (Supporting Information). It is observed that CPO planes align parallel to each other with minimal distance of 3.301 Å, which proves ␲-␲ attraction between them. This ordered arrangement suppresses geometric relaxation of excited state, showing emission blue shift, long emission lifetime and increased emission quantum yield [16,18,19]. Such arrangement, however, should be eliminated in sensing application since it blocks O2 diffusion and attack on excited electrons. A theoretical

Fig. 1. UV–vis absorption (abs.) and excitation (ex.) spectra of Re(CO)3 (CPO)Br and CPO in CH2 Cl2 (2 ␮M).

analysis on Re(CO)3 (CPO)Br can be found from Supporting Information and Fig. S2 (Supporting Information). 3.3. Photophysical features of probe Re(CO)3 (CPO)Br 3.3.1. Absorption and excitation spectra Fig. 1 shows UV–vis absorption (abs.) and excitation (ex.) spectra of Re(CO)3 (CPO)Br in CH2 Cl2 (2 ␮M). There are multiple absorption bands in its absorption spectrum, centering around 205 nm, 238 nm, 280 nm and 410 nm, respectively. The first three bands are similar to CPO ligand absorption bands and thus assigned to ␲ → ␲* (ILCT) absorption. The latter absorption band peaking at 410 nm, however, is a new one compared to CPO ligand absorption. Bearing above theoretical calculation result in mind, this low energy band is attributed to MLCT/LLCT absorption [13–15]. Re(CO)3 (CPO)Br absorption extends to visible region with optical edge of 487 nm. This value is comparable to that of a reference complex Re(CO)3 (Phen)Br, regardless of CPO’s longer conjugation chain than Phen [15,16]. Here Phen stands for 1,10-phenanthroline. It is assumed that electron-pulling group oxadiazole ring in CPO ligand is responsible for Re(CO)3 (CPO)Br’s restricted optical edge. Although the former three absorption bands have much higher absorption coefficients than the latter absorption band, they are just as good as the latter absorption band in exciting Re(CO)3 (CPO)Br emissive center. We are giving an explanation on this phenomenon as follows. The first three absorption bands have been assigned as ligand ␲ → ␲* transitions. Their high molar distinction coefficients lead to their intense absorption [19]. Before transferring their energy to Re(CO)3 (CPO)Br emissive center, these ␲ → ␲* transitions have to experience a series of intersystem crossing and internal conversion procedures which greatly consume their energy content. On the other hand, the latter MLCT/LLCT absorption can directly transfer its energy to Re(CO)3 (CPO)Br emissive center since this emissive center is derived from MLCT excited state. Thus, although MLCT/LLCT absorption in weak, it is still highly effective in exciting Re(CO)3 (CPO)Br emissive center. 3.3.2. Emission spectra and lifetime Fig. 2 shows emission spectrum of Re(CO)3 (CPO)Br in CH2 Cl2 (2 ␮M). There is a single emission band peaking at 541 nm with FWHM of 83 nm, where FWHM means full width at half maximum. This broad emission band has no obvious vibronic progressions, along with a large Stokes shift between absorption edge (487 nm) and emission peak (541 nm). This result suggests that Re(CO)3 (CPO)Br emissive center owns a charge transfer character, which confirms its MLCT/LLCT electronic transitions [13]. Considering that reference complex Re(CO)3 (Phen)Br has shown an

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Fig. 2. Emission spectra of Re(CO)3 (FPO)Br in CH2 Cl2 (2 ␮M) and CPO (77 K). Inset: Emission decay dynamics of Re(CO)3 (CPO)Br in CH2 Cl2 (2 ␮M).

emission peaking at 554 nm with FWHM of 90 nm, it is obvious that Re(CO)3 (CPO)Br emissive center and its geometric relaxation have been effectively restricted. We are attributing its causation to the roomy coordination environment in Re(CO)3 (CPO)Br and the electron-pulling oxadiazole ring in CPO ligand. Re(CO)3 (CPO)Br emission decay dynamics are recorded and shown as Fig. 2 inset so that it emissive center can be further understood. Re(CO)3 (CPO)Br emission in CH2 Cl2 (2 ␮M) follows a biexponential decay pattern with ␶1 = 1.096 s (A1 = 0.413) and ␶2 = 0.175 s (A2 = 1.887), respectively. These two emissive components are consistent with MLCT/LLCT transition nature of Re(CO)3 (CPO)Br. Generally, the observation of an intense absorption band in ␲ → ␲* region and a short-lived decay component indicates a potential surface crossing procedure from ␲ → ␲* state to MLCT state [15]. We thus attribute the long-lived component ␶1 to radiative decay of MLCT state, while the short-lived component ␶2 is assigned to radiative decay of ␲ → ␲* state. Phosphorescent nature of Re(CO)3 (CPO)Br emission is confirmed by these two longlived emissive components which are vulnerable to O2 molecules, guaranteeing enough sensing collision chances with O2 molecules. 3.3.3. Emission quantum yield Emission quantum yield (␾) of Re(CO)3 (CPO)Br emission in CH2 Cl2 (2 ␮M) is measured as 0.18 through a literature method [19]. Compared to literature reports on superior emitters, this value is slightly underdeveloped [13–16]. To get a better understanding on Re(CO)3 (CPO)Br, its emissive probability (Kr ) and non-emissive probability (Knr ) are calculated by Formulas (1)–(3), obtained as 0.254 × 106 s−1 and 1.158 × 106 s−1 , respectively. Knr is much higher than Kr , suggesting that Re(CO)3 (CPO)Br emissive center is dominated by non-radiative decay path, leading to its unsatisfactory emission quantum yield. According to literature reports, geometric distortion of excited state is usually the major non-radiative decay and energy consuming path. There is, however, another possible factor in Re(CO)3 (CPO)Br that needs to be considered owing to its large conjugation plane. As reported by Zhang and coworkers, conjugation planes tend to decrease ligand 3 LC (ligand centered) level [16]. If ligand 3 LC level is lower than 3 MLCT level, there shall be an electronic configuration transition from 3 MLCT state to 3 LC state, accompanied by emission quenching of excited state. To confirm the possibility of such electronic configuration transition in Re(CO)3 (CPO)Br, CPO 3 LC level is determined by its low temperature (77 K) phosphorescence spectrum and obtained as 499 nm, as shown in Fig. 2. CPO 3 LC level is much higher than Re(CO)3 (CPO)Br 3 MLCT level (541 nm), which denies the possibility of an electronic configuration transition from 3 MLCT state to 3 LC

313

Fig. 3. SAXRD patterns of Re(CO)3 (CPO)Br doped MCM-41 composites (45 mg/g, 55 mg/g and 65 mg/g) and blank MCM-41. Inset: solid state absorption (abs.) and diffuse reflection (ref.) spectra of a representative sample (65 mg/g doped).

state. Thus, geometric relaxation in Re(CO)3 (CPO)Br should be the only reason for its underdeveloped emission quantum yield. Kr Kr + Knr

(1)

1 = Kr + Knr 

(2)

˚=

=

A1 1 2 + A2 2 2 A1 1 + A2 2

(3)

3.4. Morphology and characterization on Re(CO)3 (CPO)Br doped MCM-41 composite samples It has been above confirmed that Re(CO)3 (CPO)Br has a potential of being a probe for oxygen sensing. To get a full evaluation on its sensing performance, Re(CO)3 (CPO)Br is doped into a silica molecular sieve MCM-41 to guarantee uniform probe distribution and fluent O2 diffusion. MCM-41 has been proved as a promising supporting substrate owing to its virtues of large surface-to-volume ratio, uniform porosity and good stability [7–10]. Various doping concentrations (45 mg/g, 55 mg/g and 65 mg/g) are tried so seek the optimal doping concentration. These Re(CO)3 (CPO)Br doped MCM-41 composite samples are firstly evaluated by their small angle X-ray diffraction (SAXRD) patterns. It is observed in Fig. 3 that these patterns are nearly identical to that of blank MCM-41, showing three well-resolved Bragg reflection peaks indexed as d100 , d110 , and d200 , respectively. It is thus confirmed that MCM-41 matrix and its regular hexangular tunnels have been well preserved after dopant loading procedure [11,12]. To confirm the successful dopant loading in MCM-41, solid state absorption and diffuse reflection spectra of a representative sample (65 mg/g doped) are shown as Fig. 3 inset. This solid state composite absorption spectrum is consistent with Re(CO)3 (CPO)Br absorption in CH2 Cl2 solution, and matches well with its solid state diffuse reflection spectrum. No new absorption bands or reflection peaks are observed, suggesting that dopant molecules are just immobilized in MCM-41 tunnels with no strong interaction with them. For a further confirmation on the successful dopant loading in MCM-41, N2 adsorption and desorption isotherms of a representative sample (65 mg/g doped) and a reference sample (blank MCM-41) are recorded and shown in Fig. 4. Despite of their different adsorption values, these isotherms are all type-IV ones and quite similar to each other. It is clear that hexangular tunnels of MCM-41 have been well preserved after dopant loading procedure. Pore diameter values are obtained as 3.12 nm for blank MCM-41 and 2.73 nm

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Fig. 4. N2 adsorption and desorption isotherms of a representative sample (65 mg/g doped) and blank MCM-41.

for Re(CO)3 (CPO)Br doped MCM-41 (65 mg/g doped), respectively. It is thus finally confirmed that Re(CO)3 (CPO)Br has been successfully doped into MCM-41 substrate with its hexangular tunnels well preserved, favoring analyte transportation and diffusion. 3.5. Oxygen sensing performance of Re(CO)3 (CPO)Br doped MCM-41 composite samples 3.5.1. Sensitivity Emission spectra of Re(CO)3 (CPO)Br doped MCM-41 composite samples under various O2 concentrations are discussed so that their oxygen sensing performance can be evaluated. It is observed in Fig. 5 that each composite sample decreases its emission intensity with increasing O2 concentration, showing sensing behavior towards O2 . Slight emission blue shift is observed for composite samples, compared to Re(CO)3 (CPO)Br emission in CH2 Cl2 solution (541 nm). This result can be explained by rigidochromism owing to the rigid structure in MCM-41 tunnels [11,12]. For discussion convenience, sensitivity of each sample is defined as its ratio of I0 /I100 , where I0 and I100 are emission intensity values in pure N2 atmosphere and in pure O2 atmosphere, respectively. Sensitivity values of Re(CO)3 (CPO)Br doped MCM-41 composite samples listed in Table 2 are found comparable to or slightly higher than those of similar sensing composites [11,12–18]. We found below positive factors which are responsible for this promising performance. First, CPO’s large conjugation plane and Re(CO)3 (CPO)Br’s long-lived emissive center increase sensing collision probability between probe and analyte. Second, geometric relaxation in Re(CO)3 (CPO)Br excited state is restricted by electron-pulling group of oxadiazole ring, leading to Re(CO)3 (CPO)Br’s high emission quantum yield and strong emission. Third, there is roomy space in Re(CO)3 (CPO)Br coordination sphere, which facilitates O2 attack. Fourth, fluent oxygen transportation and diffusion are provided by hexangular tunnels of MCM-41 substrate. There exists an optimal doping level (55 mg/g) in these composite samples. It seems that a higher doping level or a lower one compromises sensitivity, which can be explained below. Assuming that there are two opposite factors dominating sensitivity: probe molecule amount and self-quenching/absorption between probe molecules. If probe molecules are less than enough, their emission shall be too weak to give a high sensitivity. If probe molecules are more than enough, probe emission will be compromised by their strong self-quenching/absorption, which compromises sensitivity consequently [14–18]. Upon an optimal doping level, there are enough probe molecules and limited self-quenching/absorption, showing the highest sensitivity.

Fig. 5. A. Emission spectra of Re(CO)3 (CPO)Br doped MCM-41 composite (45 mg/g doped) under various O2 concentrations from 0% to 100% (interval = 10%). B. Emission spectra of Re(CO)3 (CPO)Br doped MCM-41 composite (55 mg/g doped) under various O2 concentrations from 0% to 100% (interval = 10%). C. Emission spectra of Re(CO)3 (CPO)Br doped MCM-41 composite (65 mg/g doped) under various O2 concentrations from 0% to 100% (interval = 10%).

3.5.2. Sensing mechanism According to literature reports, oxygen sensing based on emission quenching is usually accomplished through a dynamic collision mechanism [11,12]. To confirm this hypothesis, emission decay curve of a representative sample (65 mg/g doped) is shown as Fig. 6 inset. Upon pure N2 atmosphere, composite emission obeys biexponential exponential decay pattern with mean lifetime (␶) of 8.93 ␮s. This value is much longer than emission lifetime in CH2 Cl2 solution (0.71 s). We are attributing its causation to the protection effect from MCM-41 substrate and its rigid environment [11,12]. With increasing O2 concentration, composite lifetimes are decreased to 6.01 s in air and 1.68 s in pure

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Table 2 Key sensing parameters of Re(CO)3 (CPO)Br doped MCM-41 composite samples. sample

␭ (nm)

I0 /I100

KSV1 (O2 %−1 )

KSV2 (O2 %−1 )

f01

f02

R2

Tres (s)

Trec (s)

45 mg/g 55 mg/g 65 mg/g

537 535 538

3.87 4.00 3.89

0.0575 0.0487 0.0673

0.0113 0.0088 0.0151

0.66 0.78 0.52

0.34 0.22 0.48

0.9998 0.9993 0.9997

13 14 13

28 27 27

Fig. 6. Stern-Volmer plots of Re(CO)3 (CPO)Br doped MCM-41 composite samples (45 mg/g, 55 mg/g and 65 mg/g). Inset: emission decay dynamics of a representative sample (65 mg/g doped) under pure N2 , pure O2 and air conditions.

O2 atmosphere, respectively. It is thus confirmed that composite emission is quenchable by O2 molecules, following a dynamic ¨ excited state. mechanism described as Formula 4. Here *¨ denotes Re(CO)3 (CPO)Br ∗ + 3 O2 → Re(CO)3 (CPO)Br + 1 O2 ∗

(4)

3.5.3. Stern-Volmer working plots The above discussion has confirmed below facts: (1) probe molecules have been uniformly distributed in MCM-41 substrate; (2) composite emission follows dynamic collision mechanism towards O2 . Composite emission intensity against O2 concentration variation thus should be analyzed by Stern-Volmer equation [11,12,17]. According to Formula (5), an ideal Stern-Volmer working plot should be a linear one with identical slope of KSV , where I and I0 stand for emission intensity and maximum emission intensity in pure N2 atmosphere, respectively, KSV and [O2 ] indicate Stern-Volmer constant and O2 concentration, respectively [11,12]. Our Stern-Volmer plots shown in Fig. 6 are quite close to linear ones, which should be attributed to the uniform distribution of probe molecules in their matrix. However, they are only close to linear ones, suggesting that the actual sensing procedure in our composite samples is a complicated one and should not be described by a simple model. In this case, it is assumed that there are multiple sensing sites in each composite sample, only one of them is sensitive to O2 , while the others are not. Given this assumption, the original Formula (5) should be revised by taking contribution of each sensing site into account. I 0 /I = 1+ K SV [O2 ]

(5)

For fitting simplification, it is assumed that there are two sensing sites. Correspondingly, Formula (5) is revised as Formula (6), where f1, f2 , KSV 1 and KSV 2 are fractional contributions and corresponding Stern-Volmer constants of sensing sites, respectively [11,12]. Corresponding fitting parameters are summarized in Table 2. It is observed that this two-site model can well fit our Stern-Volmer plots. For all composite samples, KSV 2 values are obviously smaller than KSV 1 values, which means that sensing site-2 is insensitive towards O2 . Considering its low sensitivity towards O2 , we are

Fig. 7. Emission monitoring of our composite samples when surrounding environment is periodically changed between pure N2 and pure O2 atmosphere.

attributing sensing site-2 to the “deep-far-corner” in composite samples which is impenetrable by O2 diffusion. Regardless of its non-dominant contribution, site-2 decreases sensitivity and compromises linearity of Stern-Volmer plots. For further improvement, the amount of “deep-far-corner” should be decreased by increasing regularity of supporting substrate so that O2 diffusion can be improved. I0 = I

1 f1 1+KSV 1 [O2 ]

+

f2 1+KSV 2 [O2 ]

(6)

For a confirmation on the positive effect from MCM-41 supporting matrix, emission spectra of pure Re(CO)3 (CPO)Br under various O2 concentrations and corresponding Stern-Volmer curve are shown as Fig. S3 and Fig. S4 (Supporting Information). It is observed that Re(CO)3 (CPO)Br emission is still quenchable by O2 gas. A trace amount of O2 leads to an obvious emission decrease. Then this decrease tendency becomes smooth, showing a bad linearity. As a consequence, the positive effect from MCM-41 supporting matrix is confirmed. 3.5.4. Response/recovery time and photostability For a direct understanding on composite emission variation against O2 , emission of our composite samples is recorded when surrounding environment is periodically changed between pure N2 and pure O2 atmospheres. As shown in Fig. 7, composite emission is obviously dependent on O2 presence. In pure N2 atmosphere, composite samples show their maximum emission intensity. Upon pure O2 atmosphere, composite emission is decreased to minimal level quickly, showing a fast response towards O2 . There is, however, photobleaching effect in these composite samples. After four N2 -O2 cycles, composite emission intensity is slightly decreased, showing photobleaching effect. Never the less, the thermal stability of probe Re(CO)3 (CPO)Br is good enough with decomposition temperature of ∼260 ◦ C (See Fig. S5, Supporting Information). It seems that although MCM-41 substrate is effective on analyte diffusion, it is limited in protecting probe molecules. For further improvement, probe photostability should be improved. A new supporting matrix with better protecting performance should be developed and used.

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For discussion convenience, response (Tres ) and recovery (Trec ) times are defined as the time taken by each sample to decrease or increase to 95% of its initial emission intensity when atmosphere is periodically changed [11,12]. It is observed in Table 2 that our Tres and Trec values are comparable to or slightly shorter than literature values [11,12,14,17]. Recovery time of each sample is obviously longer than its response time. This result can be explained by diffusion-controlled dynamic response and recovery behavior [20]. It is observed that these values are independent on dopant concentration variation, which means that response and recovery procedure is essentially controlled by supporting substrate itself. In addition, below reasons should be considered to explain our observed promising performance. First, the large conjugation plane of CPO and the long-lived emissive center of Re(CO)3 (CPO)Br increase sensing collision probability. Second, geometric relaxation of Re(CO)3 (CPO)Br excited state is suppressed by electron-pulling oxadiazole ring, leading to Re(CO)3 (CPO)Br’s high emission quantum yield and strong emission. Third, there is roomy space in Re(CO)3 (CPO)Br coordination sphere, which facilitates O2 attack. Fourth, fluent oxygen transportation and diffusion are provided by hexangular tunnels of MCM-41 substrate. 4. Conclusion To summarize, Re(CO)3 (CPO)Br was developed for optical oxygen sensing. There was a large conjugation plane in CPO ligand, which increased electronic distribution and decay lifetime of excited electrons. As a consequence, collision probability between probe and O2 molecules was increased, favoring oxygen sensing performance. This hypothesis was confirmed by single crystal XRD, theoretical calculation and photophysical analysis. Re(CO)3 (CPO)Br sensing performance was discussed by doping it into supporting substrate MCM-41 with various concentrations. A dynamic sensing mechanism was established with sensitivity of 4.0 and response time of 14 s. Below factors were responsible for this promising performance, including: (1) the large conjugation plane of CPO and the long-lived emissive center of Re(CO)3 (CPO)Br increased sensing collision probability; (2) geometric relaxation of Re(CO)3 (CPO)Br excited state was suppressed by electron-pulling oxadiazole ring, leading to Re(CO)3 (CPO)Br’s high emission quantum yield and strong emission; (3) there was roomy space in Re(CO)3 (CPO)Br coordination sphere, which facilitated O2 attack; (4) fluent oxygen transportation and diffusion were provided by hexangular tunnels of MCM-41 substrate. For further improvement, supporting substrate should be modified to minimize the amount of “deepfar-corner” and increase its protection for probe molecules. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 41301330), Industry-academiaresearch prospective joint project of Jiangsu Province (BY201502706), and Natural Science Foundation of Jiangsu Province (No. BK20130253).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.06.162. References [1] Y. Guo, L. Zhang, S. Zhang, Y. Yang, X. Chen, M. Zhang, Biosens. Bioelectron 63 (2015) 61–71. [2] D. Sarkar, A. Pramanik, S. Jana, P. Karmakar, T.K. Mondal, Sens. Actuators B 209 (2015) 138–146. [3] M.R. Awual, M.M. Hasan, M.A. Khaleque, Sens. Actuators B 209 (2015) 194–202. [4] C.S. Chu, C.Y. Chuang, Sens. Actuators B 209 (2015) 94–99. [5] O.S. Wolfbeis, Anal. Chem. 80 (2008) 4269–4283. [6] C. Baleizao, S. Nagl, M. Schaferling, M.N. Berberan-Santos, O.S. Wolfbeis, Anal. Chem. 80 (2008) 6449–6457. [7] L.C. Clark Jr., Trans. Am. Soc. Artif. Intern. Organs 2 (1956) 41–48. [8] L.W. Winkler, Ber. Dtsch, Chem. Ges. 21 (1888) 2843–2855. [9] G. Zhang, J. Chen, S.J. Payne, S.E. Kooi, J.N. Demas, C.L. Fraser, J. Am. Chem. Soc. 129 (2007) 15728. [10] A. Gulino, S. Giuffrida, P. Mineo, M. Purrazzo, E. Scamporrino, G. Ventimiglia, M.E. Van der Boom, I. Fragala, J. Phys. Chem. B 110 (2006) 16781–16786. [11] B. Lei, B. Li, H. Zhang, L. Zhang, W. Li, J. Phys. Chem. C 111 (2007) 11291–11301. [12] B. Lei, B. Li, H. Zhang, S. Lu, Z. Zheng, W. Li, Y. Wang, Adv. Funct. Mater. 16 (2006) 1883–1891. [13] L. Yang, J.K. Feng, A.M. Ren, M. Zhang, Y.G. Ma, X.D. Liu, Eur. J. Inorg. Chem. 10 (2005) 1867–1879. [14] J. Jia, Y. Tian, Z. Li, Synthetic Met. 161 (2011) 1377–1382. [15] Z. Si, X. Li, X. Li, H. Zhang, J. Organomet. Chem. 694 (2009) 3742–3748. [16] L. Zhang, S. Yue, B. Li, D. Fan, Inorg. Chim. Acta 384 (2012) 225–232. [17] L. Shi, B. Li, S. Yue, D. Fan, Sens. Actuators B Chem. 137 (2009) 386–392. [18] M. Kranenburg, Y.E.M. van der Brugt, P.C.J. Kamer, P.W.N.M. van Leeuwen, Organometallics 14 (1995) 3081–3089. [19] L. Zhang, B. Li, Z. Su, Langmuir 25 (2009) 2068–2074. [20] S.M. Kuang, D.G. Cuttell, D.R. McMillin, P.E. Fanwick, R.A. Walton, Inorg. Chem. 41 (2002) 3313–3322.

Biographies Dr. Liang Liu, male, was born in 1982. Assistant professor of Changzhou University, graduated from Graduate University of Chinese Academy of Sciences with a doctor’s degree of Environmental Science in 2011. Research Interests focus on energy storage technology and nanocomposite materials and their application. Dr. Yan Yang, female, was born in 1984, Jiangsu Province, master, lecturer. Affiliated to the Changzhou University. Her research interests focus on the solar energy materials. Prof. Jun-sheng Feng, male, born in 1963, Inner Mongolia, master. Affiliated to the Changzhou University. Mainly engaged in the environmental engineering teaching and related research work.