Modulation of Optical Oxygen Sensing Properties of Iridium (III) Complexes By Changing Their Substitution Groups

Merve Zeyrek Ongun Dokuz Eylul University: Dokuz Eylul Universitesi Murat Şahin Gebze Technical University: Gebze Teknik Universitesi Sibel Oguzlar Dokuz Eylul University: Dokuz Eylul Universitesi Tuğçe Akbal Aytan Gebze Technical University: Gebze Teknik Universitesi Devrim Atilla Gebze Technical University: Gebze Teknik Universitesi Sevinc Zehra Topal (  sztopal@gtu.edu.tr ) Gebze Teknik Universitesi https://orcid.org/0000-0001-5535-2829


Introduction
The optical chemical sensing approach has been used many times in the development of sensors. The oxygen-sensible dye is of great importance to enhance the sensitivity of such sensors. Iridium (III) complexes are known to exhibit oxygen-sensitive room temperature phosphorescence emission and lifetime (microsecond to millisecond range) [1,2]. In addition, syntheses of iridium complexes are easy and high yielding which is also quite important to develop cheaper sensors. The Ir (III) complexes that have been studied so far showed quite good sensitivity to oxygen (Table 1). Liu and co-workers reported the oxygen sensitivity studies of bis-cyclometalated-diphenylamino equipped iridium (III) complexes in ethyl cellulose (EC) lm. The in uences of substituents (-H, -CH 3 , -F, -CF 3 ) at the pyridyl moiety of ppy of these complexes were investigated systematically, and I 0 /I 100 values of the Ir-1, -2, -3 and − 4 complexes in EC thin lm were 16.2, 16.4, 15.2 and 14.1, respectively [3]. Toro and co-workers presented the nanostructured materials that including four various Ir (III) dyes associated either with polystyrene or metal oxide. The most oxygen-sensitive lms based on dye N1001 incorporated into the aluminium oxidehydroxide nanostructured solid support presented a K SV constant equal to 2848 ± 101 bar − 1 and a pO 2 (S = 1/2) equal to 0.0006 [4]. Yoshihara et.al proposed a ratiometric molecular probe-based on dual emission of a coumarin dye and red phosphorescent cationic iridium (III) complex for intracellular oxygen sensing [5]. The new cyclometalated iridium (III) complexes in nanocomposites made of AP200/19 and in polystyrene lm were presented by Marín-Suárez and co-workers, and the results have been found promising for the detection of low oxygen concentrations [6]. Xing et al. reported that I 0 /I 100 values of the Ir (III) complexes; IrC1, IrC2, and IrC3 substituted with 4-(diphenylamine)phenyl, 4-(9H-carbazol-9yl)phenyl, and 9-phenyl-9H-carbazol-3-yl moieties were found as 33.0, 8.2 and 16.5, respectively [7].
Zhang and co-workers reported amino methyl substituted phenyl pyridine ligands including series of iridium (III) complexes and researched their dual-phosphorescence properties. One of them exhibited response naked-eye distinguishable colour changed emission response to oxygen in solution [8]. Ho and co-workers reported four iridium (III) coordination polymers, and their K SV values in single crystal form were quite promising [9]. Liu and co-workers presented tri uoromethyl-substituted cyclometalated iridium (III) complexes which showed improved photostability against irradiation and the favourable response time of Lx4 incorporated in the EC lm (2.6 s) on going from N 2 to O 2 [10]. The oxygen responses of the new designed Ir (III) complexes with different numbers of F atoms in electrospun brous lms were recently studied by Wang and co-workers, and large conjugation planes and F atoms in their ligands were affected positively sensing performance [11]. Kai et al. offered three Ir (III) dyes with diverse numbers of uorine substituents. They found that the presence of F atoms developed oxygen sensing properties of the complex in the form of brous polystyrene lms (I 0 /I 100 value of 12.89) [12]. The oxygen sensing properties of cyclophosphazene equipped-iridium (III) complexes in electrospun brous ethyl cellulose (EC) mats were reported by Ongun and co-workers [13]. It was found that: i) the substitution with cyclophosphazene moiety enhanced the oxygen sensitivity of Ir (III) complexes, and ii) distinct oxygeninduced blue shifts extending from 22 to 75 nm were observed. In another study, Borisov and co-workers produced novel optical nanochemosensors by entrapped Ir (III) complexes in nanobeads using different polymers [14]. By the way, it is still a challenge to enhance their photostability and improve their biocompatibility so the design/synthesis of new complexes exhibiting optimized emission characteristics or lifetimes are ongoing to answer all needs. These studies encouraged us to study the design/synthesis of new iridium (III) complexes with different substituents to overcome these problems.
In this work, we encoded iridium (III) complexes bearing -H, -OCH 3 , -F, -CH 3 ) at the aryl moiety on Ir-1, Ir-2, Ir-3 and Ir-4, respectively. Corresponding Ir (III) complexes were embedded in two different matrices; EC and poly(methyl methacrylate) (PMMA) to de ne the most advantageous material to improve their oxygen permabilities. The in uences of substituents (-H, -OCH 3 , -F, -CH 3 ) at the aryl moieties on the oxygen sensing properties of corresponding complexes have been investigated systematically in THF and EC lms. To record luminescence spectra, 10 mm path length quartz cells with septum were supplied from Hellma.
Nitrogen and oxygen gas cylinders were 99.99% pure and supplied from Linde Gas Company, Izmir.

Characterization
Thin-layer chromatography (TLC) was carried out on Merck Silica gel plates (Merck 60, 0.25 mm thickness) with an F254 indicator. Column chromatography was performed on silica gel (Merck 60, 0.063-0.200 mm; 120 g of silica gel was used in a column of 3 cm diameter and 110 cm length for 3 g of crude mixture). Elemental analyses synthesized dyes were measured by a Thermo Finnigan Flash 1112 Instrument. Mass analyses were recorded on a Bruker MALDI-TOF (Matrix Assisted Laser Desorption / Ionization-Time-of-Flight mass, Rheinstetten, Germany) spectrometer using 2,5-dihydroxybenzoic acid as a matrix. Fourier Transform Infrared (FT-IR) spectra were recorded on a Perkin Elmer 100 spectrophotometer. The 1 H spectra for the compounds were examined on a Varian INOVA 500 MHz spectrometer using TMS as an internal reference. Microstructure SEM images were captured with Philips XL 30 SFEG scanning electron microscopy. Electronic absorption spectra in the UV-Vis region were recorded with a Shimadzu 2101 UV-Vis spectrophotometer. The steady-state photoluminescence (PL) emission/excitation measurements were studied by using a red-sensitive photomultiplier tube equipped spectro uorometric system (FLSP920 Fluorescence Spectrometer, Edinburgh Instruments) in a quartz cell with a septum at room temperature. The decay times were measured in time-correlated single photon counting mode (TCSPC) of the FLSP920. The gases, O 2 and N 2 were mixed in the concentration range of 0-100% with a Sonimix 7000A gas blending system for oxygen sensing measurements. The output ow rate of the gas was maintained at 550 mL/min. Gas mixtures were introduced on the sensing agents via a diffuser needle under ambient conditions. Relative luminescence quantum yields were determined according to the William Method [15]. The solutions of the dyes were deoxygenated thoroughly by bubbling nitrogen.

Synthesis of L3
The crude solid product was puri ed by preparative TLC on silica with DCM:EtOH

Preparation of solid-state sensing materials
In this study, the different compositions of EC based composites were prepared by mixing 120 mg of EC, 24 mg of ionic liquid (BMIMBF 4  The relative signal intensity (I 0 /I 100 ) changes of the related the Ir (III) complexes were measured in the gas-permeable PMMA and EC polymeric matrix. The highest sensitivities were obtained in the EC matrix for all four complexes matching with literature knowledge [3,13]. Therefore, the EC polymer matrix was chosen as a suitable material to investigate the O 2 sensitivities of the synthesized Ir (III) complexes (Ir-1,   Table 2. Corresponding Ir (III) complexes all showed similar and typical absorption in THF, similar to other cyclometalated Ir (III) complexes previously reported [3]. The Ir-1 and Ir-4 complexes exhibited intense absorption bands in the UV area belong to π-π* transitions of intra-ligands (ILCT) [7,11]. Their lower energy absorption bands around 380 nm and 405 nm can be attributed to the overlap of metal-ligand charge transfer (MLCT), inter-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) interactions. It should be noted that these charge-transfer-based transitions generally have low molar absorption coe cients, resulting in poor absorption bands. Moreover, the weak absorption bands of MLCT / LLCT transitions (400 nm) are quite effective in exciting emitting centers [11].
The photoluminescence based emission/excitation spectra of the Ir (III) complexes were measured in THF under nitrogen atmosphere (Fig. 2). Upon excitation at 400 nm corresponding to the MLCT/LLCT transitions, Ir-1, Ir-2, Ir-3 and Ir-4 in THF showed dense long-wavelength emission bands with and no detectable uorescence at 576, 590, 570 and 562 nm, respectively. Notably, the emission spectra change upon the substitution groups. The emission maximum of Ir-2 complex was remarkably red-shifted according to others, owing to the moderate electron-donating ability of -OCH 3 moieties.  Table 2). The tri uoromethyl (-CF 3 ) group has both a strong electron-withdrawing property and also is an important substituent in many organic electroluminescent materials. It has an effect on the ligand system and provides steric protection around the metal that is an important consideration for increasing the quantum e ciency of the complexes. However, the quantum values of the corresponding compounds were very close to each other in this study.
Under nitrogen atmosphere, Ir-1, Ir-2, Ir-3 and Ir-4 in EC presented intense long-wavelength phosphorescence emission bands at 552, 550, 546 and 554 nm with no detectable uorescence, respectively. It should be noted the emission spectra change only slightly upon the substitution groups and bathochromic shift does not exceed 10 nm. Stoke's shifts were 148, 144, 148 and 144 nm for Ir-1, Ir-2, Ir-3 and Ir-4, respectively (Table 3). Table 2. Absorption and phosphorescence emission-based data of the Ir(III) complexes in THF.

Oxygen sensing properties
Herein, we investigated the O 2 induced emission-based response of the Ir(III)-based sensing materials embedded in EC thin lms. Fig. S21-23 and Fig. 3 show the intensity-based uorescence spectra of the Ir-1,-2, -3 and − 4, respectively. All of the studied forms exhibited a spectral response between 0-100% p[O 2 ].
When the Ir (III)-bearing sensing slides were excited at 400 nm, the emission peaks were observed at 552, 550, 546, 554 nm, for of Ir-1,-2,-3 and − 4 based EC thin lms, respectively, and they also exhibited a quenching-based signal response to oxygen. The Ir-1,-2 and − 3 doped sensing slides yielded 68.5, 84.  Table 3 shows the composition, excitation, emission and sensitivity information (I 0 /I 100 ) of the utilized EC sensor slides with the related calibration plots and regression coe cients for the concentration range of 0-40% [O 2 ]. The I 0 /I 100 values of Ir-1, -2, -3 and − 4 immobilized in EC lm were found as 11.3, 5.2, 7.0 and 25.6, respectively. The I 0 /I 100 value of Ir-4 was about 5 times higher than that of Ir-2, indicating negative impact of electron donating properties on oxygen sensitivity. Consequently, it was found that the inclusion of the substituents (-H, -OCH 3 , -F, -CH 3 ) in the aryl portion of the iridium (III) complexes has a strong effect on the sensitivity of the oxygen sensors. In a previous study, it was found that the inclusion of substituents (-H, -OCH 3 , -F, -CH 3 ) in the pyridyl moiety of cyclometalizing ligands had relatively little effect on the sensitivity of the sensors [3]. It can be concluded that not only substituted group but also the binding position of the substituted group in the Ir(III) complexes is very effective. The Ir-4 showed the highest K SV value amongst the others (See Fig. 4 and Table 4). Besides the K SV values, the linear properties of the Stern-Volmer plots can be evaluated as evidence of the supersensitivity of the related Ir (III) sensor agents towards oxygen.
The detection limit (LOD) was calculated by dividing 0.003 / slope where the slope is expressed as % respectively (See Table 4).

Decay time measurements
The luminescence-based oxygen sensing process involves dynamic collision between the excited state of the oxygen-sensitive probes and triplet oxygen, causing a decrease of intensity and decay kinetics of sensing slides [10]. In order to gure out the high O 2 sensitivity of corresponding Ir (III) molecules in oxygen-free and fully-oxygenated moieties, we measured the oxygen-induced decay kinetics subsequent to the signal within the THF and EC thin lm in a microsecond time scale (Fig. 5). The multi-exponential of the decays and their distribution results for the Ir-1,-2, -3 and − 4 were shown in Table S1. The decay kinetics of all of the Ir (III)-based sensors decreased upon exposure to fully-oxygenated conditions. The average decay times of Ir-4 embedded in EC decreased from 417.68 µs to 21.59 µs when exposed to the oxygen-free and fully oxygenated conditions. According to these results, since rotational movements in the EC phase are prevented compared to the solution phase, the best decrease of decay time values was observed in the solid phase. Also, Ir-4 in the EC thin lm matrix, exhibited a higher τ 0 value of 417.68 µs when compared to the value of Ir-4 in THF. The luminophores with longer τ 0 lifetimes are known to have higher oxygen sensitivities, and those molecules are preferred for oxygen sensor studies. Ir-4 has decay time-based higher oxygen sensitivity and that is advantageous for the sensor studies. When the Stern-Volmer plots and the decay kinetics were evaluated together, it can be concluded as the evidence of the dynamic quenching for all of the moieties. were stored in ambient laboratory conditions during the 18 months. The sensor membranes had lost their original intensity values less than 5%, but they still had the potential for O 2 measurements.
The photostability of iridium complexes is every time of important concern for applied experiences such as high-light densities de nition or long-time continuous monitoring of oxygen. Figure 7 shows the photodegradation histograms of the Ir (III) dyes embedded in EC thin lms with continuous 60 min illumination.
All data were normalized to let a comparison of the decrease in intensity in each sample. About 48.0% of Ir-1, 55.0% of Ir-2, 59.0% of Ir-3 and 56.0% of Ir-4 were destroyed under irradiation. This limits the potential application of the optodes to short-time measurements.   Stern-Volmer plots for oxygen sensing lms of Ir-1, Ir-2, Ir-3 and Ir-4 immobilized in EC matrix.

Figure 6
Time-based kinetic response of Ir-4 in thin lm in an alternating atmosphere of 100.0% N2 and 100.0% O2.