A TADF emitting Zn(II)-benzoporphyrin: an indicator for simultaneous sensing of oxygen and temperature.

A new luminescent indicator is presented that enables simultaneous measurement of oxygen and temperature at a single wavelength. The indicator, an alkylsulfone-substituted Zn(II)-meso-tetraphenyltetrabenzoporphyrin, emits prompt and thermally activated delayed fluorescence (TADF). TADF is sensitive toward oxygen and temperature and is referenced against prompt fluorescence (PF) that is not affected by oxygen. The information on both parameters is accessed from the decay time of TADF and the temperature-dependent ratio of TADF and PF. Sensor foils, made from poly(styrene-co-acrylonitrile) and the indicator dye, enable temperature-compensated trace oxygen sensing (0.002–6 hPa pO2) at ambient conditions. Compared to the previously reported dual sensors based on two emitters, the new sensor significantly simplifies the experimental setup and eliminates risks of different leaching or photobleaching rates by utilizing only one indicator dye and operating at a single wavelength.


Mathematical derivation of the modified dual lifetime referencing (m-DLR) method
In the frequency domain spectroscopy, a dual optical sensor is described as a superposition of two single sine wave functions. As a result, the phase angle and the amplitude obtained at a single modulation frequency are determined by the decay time and the ratio of the intensity of both luminophores. 1 In order to determine all four parameters of the dual optical sensor and to identify the single unique solution, a set of four linear and independent equations has to be set up or some assumptions must be made to simplify the mathematical system.
The overall phase angle and the overall amplitude obtained at a single modulation frequency can be described by the general interference equation for two wave functions (cf. S1 and S2).
where , are the amplitudes of the prompt and delayed fluorescent indicator; , are the corresponding phase shifts. is the superimposed amplitude and is the corresponding superimposed phase shift at a certain modulation frequency.
At low modulation frequencies, meaning in the kHz range, the dye with prompt fluorescent causes no phase shift ( ≈ 0) 2 , which simplifies the previous equations (S1 and S2) as follows: In order to simplify the linear equation system further, the amplitude is converted into the intensity using the demodulation factor. This transformation is crucial, since the intensity is independent of the modulation frequency in the kHz range. A general description is shown in equation S5.
where is the amplitude and dm is the demodulation factor of the luminophore at a certain modulation frequency. Since the individual intensity of the luminophore is independent of the modulation frequency, also the ratio of both intensities is constant (see S6). Equation S6 can hereby further simplified, as in the kHz range, the amplitude demodulation of the fast fluorophore is very close to unity.
The demodulation of the delayed fluorophore is thereby determined by the modulation frequency and the luminescence decay time of the fluorophore.
To simplify the linear equation system further, equation S4 is solved for the amplitude of the fast luminophore and inserted into equation S6.
Both trigonomic functions can be converted using trigonomic identities as they are described in equation S8 and S9.
After applying the trigonometric identities to equation S7, the reduced expression for the intensity ratio is obtained (see equation S10): The intensity ratio of the fast fluorophore and the delayed fluorophore is constant for different modulation frequencies and only dependent of the overall phase shift Φ and the decay time of the delayed fluorophore. The decay time of the fluorophore with delayed fluorescence is determined by the dual lifetime reference (DLR) technique published by Klimant et al. 3 as it is described in the main part (see equation 1). The linear equation set is therefore unique and the solution of the dual optical system can be identified.

Correction of the oxygen partial pressures in the gas phase
To check the τ0 (τ at 0 hPa O2) values for correctness, Zn-OS in PSAN sensor spot were placed at the bottom of 3 mL Supelco vials filled with 2wt% Na2SO3 aq. solution with cat. CoCl2. After 1 day, overall luminescence phase shifts and overall luminescence amplitudes at both modulation frequencies were measured with a lock-in amplifier as described for the gas phase calibrations. Measurements were conducted at different temperatures (10 -41 °C).
When comparing the measurement data for 0 hPa O2 in the gas phase to deoxygenated aqueous sodium sulfite solution, it became apparent that the 0 hPa O2 concentration during gas phase calibration were not completely oxygen-free. To correct the oxygen concentrations, the values measured in sodium sulphite solution were added to the calibration data and the oxygen concentration are adjusted. This is done according to following procedure. The oxygen partial pressure in the Stern-Volmer equation (3) is substituted by: where x is the fraction of the test gas (2.00 vol% in nitrogen) in nitrogen (used by the gas mixing device for adjusting the oxygen partial pressure), 18.942 hPa is the oxygen partial pressure in the test gas adjusted for the altitude, c is the error in oxygen concentration. The oxygen error can be constant if it appears after the gas-mixing device or can be variable if it depends on the fraction of nitrogen (if the nitrogen gas is the source of oxygen error). It was assumed that the oxygen error originates to 50% from both sources. From the measurement in sodium sulfite solution, the new τ0 values are used to calculate τ0/ τ values. Then, the dependency at 25 °C is fitted according to the Stern-Volmer equation ( Figure S5) and gives c of 0.195 hPa. Finally, the corrected oxygen partial pressures are calculated according to eq. S11.

Estimation of the singlet-triplet energy gap
For temperature calibrations, temperature was adjusted with a Cary SPV -1X0 Single Cell Peltier Accessory Peltier element from Varian in combination with a cryostat from Avantor, Model 1150S (us.vwr.com). The PSAN sensor foil was placed diagonally in Hellma Analytics optical glass 10 mm precision cuvettes. The cuvettes were filled with aqueous 2wt% Na2SO3 solution. After temperature adjustment, the cuvettes were thermally equilibrated for 10 min. Temperature was adjusted between 5 and 45 °C. The TADF lifetime was measured at each temperature with the same setup as for the photophysical characterization. The calibration cycle was repeated three times.
Experimental data ( Figure S6) was fitted with an Arrhenius type model 4 : where k0 is the temperature-independent decay rate for the excited-state deactivation, k1 is a preexponential factor, kB the ΔE the energy necessary for the reverse Intersystem Crossing (ISC), and T the absolute temperature. ΔE corresponds to the singlet-triplet energy gap.