Here, we present a method of the spectroscopic characterization of organic molecules by means of time-resolved photoluminescence spectroscopy on the nanosecond-to-millisecond timescale in oxygen-free conditions. Methods to efficiently remove oxygen from the samples and, thus, limit luminescence quenching are also described.
Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables the acquisition of photoluminescence spectra covering the time regime from nanoseconds up to 0.1 s. This enables us to follow the changes in the intensity (decay) and emission of the spectra over time. Using this method, it is possible to study diverse photophysical phenomena, such as the emission of phosphorescence, and the contributions of prompt and delayed fluorescence in molecules showing thermally activated delayed fluorescence (TADF). Remarkably, all spectra and decays are obtained in a single experiment. This can be done for solids (thin film, powder, crystal) and liquid samples, where the only limitations are the spectral sensitivity of the camera and the excitation wavelength (532 nm, 355 nm, 337 nm, and 266 nm). This technique is, thus, very important when investigating the excited state dynamics in organic emitters for their application in organic light-emitting diodes and other areas where triplet harvesting is of paramount importance. Since triplet states are strongly quenched by oxygen, emitters with efficient TADF luminescence, or those showing room temperature phosphorescence (RTP), must be correctly prepared in order to remove any dissolved oxygen from solutions and films. Otherwise, no long-lived emission will be observed. The method of degassing solid samples as presented in this work is basic and simple, but the degassing of liquid samples creates additional difficulties and is particularly interesting. A method of minimizing solvent loss and changing the sample concentration, while still enabling to remove oxygen in a very efficient and a repeatable manner, is presented in this work.
Time-resolved spectroscopy is an essential tool in the studies of novel materials for the application of organic light-emitting diodes (OLED)1,2,3. These techniques are especially important for the latest generations of OLED emitters [i.e., as thermally activated delayed fluorescence (TADF)4,5,6,7,8 or phosphorescent9,10,11 molecules], where photoluminescence processes can be observed in a broad timescale (up to seconds). Interestingly, such techniques can also be used to investigate electroluminescence in devices, over suitable time regimes12,13. The methods described above are, in general, focused on following time-dependent properties that involve photoluminescence signals such as the decay lifetime, the shape and energy of the emission spectra, and its dependence upon temperature or other factors.
Overall, the most popular method of time-resolved spectroscopy is time-correlated single photon counting (TCSPC) or its modifications, such as multichannel TCSPC. This method is especially suited to follow fast decays with a very high accuracy, usually on the nanosecond timescale. However, it has a major disadvantage, as it does not allow following the changes in the photoluminescence spectrum in an easy way. This is resolved using streak cameras14,15. However, both methods are not suited to follow long-lived luminescence decays. In this case, time-gated methods and multichannel scaling are the methods of election.
In this work, we discuss the time-gated acquisition of photoluminescence signals in a time range from less than a nanosecond up to 0.1 – 1 s in a single experiment16,17,18. Moreover, the quality of the spectra is excellent due to the high sensitivity of the detector that is used (an iCCD camera). This allows the observation of very fine changes in the emission spectrum and the investigation of the excited state dynamics in detail, identifying the emission of different excited species in one molecular system. The versatility of this equipment has been confirmed by several recent publications19,20,21,22,23,24,25,26. The excitation source is either an Nd:YAG laser with a 10 Hz repetition rate, providing a set of harmonics (266 nm, 355 nm, and 532 nm) or a nitrogen laser (337 nm) of a changeable repetition rate between 1 – 30 Hz.
The principle of the work of iCCD cameras is based on the image intensifier, which not only intensifies the incoming light but also works as a shutter (gate). The intensifier consists of a photocathode that is sensitive to a specific spectral range [i.e., ultraviolet (UV), visible, red, and near-infrared (NIR)], a micro-channel plate (MCP), and a phosphor. By changing the photocathode, it is possible to adapt the camera to a specific use. The photocathode converts incoming photons into photoelectrons that are multiplied in the MCP and then hit the phosphor screen generating photons. These photons, through a system of lenses, are focused onto a CCD chip and are converted into an electrical signal. For further details, please refer to the manufacturer's webpage27.
To collect emission spectra throughout the range from 1 ns to 100 ms with sufficient signal-to-noise ratio, the integration (exposure) time increases exponentially along with exponentially increasing the time delay. This is dictated by the properties of the photoluminescence decay, which follows exponential laws in most systems.
The method described here can be applied to several sample sizes and forms, including those with an uneven surface, powders, or small crystals19. The sample holder is easily adapted to support several different cuvettes, including standard and degassing cuvettes or flow cuvettes. All samples with photoluminescence in a range of 350 – 750 nm can be investigated by this equipment. The system is also equipped with a liquid nitrogen cryostat to perform temperature-dependent measurements of solid and liquid samples down to 77 K and a closed-cycle helium cryostat to perform measurements of solid samples down to 15 K. This allows studying such phenomena like TADF and phosphorescence. In summary, any compound or any kind of sample that emits photoluminescence in the specified region and time range and which absorbs the excitation laser light can be investigated in this equipment.
The removal of molecular oxygen is a particularly important issue in the investigation of the photophysics of molecules with a long-lived emission. Therefore, an experimental procedure of degassing samples (solutions and films) is also described in detail here. Quenching by oxygen affects long-lived luminescence and is a major problem in the investigation of delayed fluorescence and phosphorescence. However, this quenching effect also facilitates the investigation of the contribution of triplet excited states to the overall luminescence. This is accounted for measuring the photoluminescence intensity ratio of a degassed solution/film to air-saturated conditions17,23. As triplets are quenched by oxygen, the degassing-to-air emission ratio gives direct information about the contribution of the long-lived states that are responsible for the long-lived emissions (and so delayed fluorescence or phosphorescence). This can then be used to extract information about the yields of triplet formation in organic TADF emitters. Molecular oxygen exists in a triplet ground state as a biradical. Upon absorption of energy of ca. 1 eV, triplet oxygen undergoes a transition to a singlet excited state. Typically, excited state molecules have an energy of singlet and triplet higher than 1 eV. This energy can, therefore, be transferred to oxygen upon collision. As a result, the molecule returns to a ground state or undergoes intersystem crossing.
One of the most popular methods of degassing solutions is bubbling them with a neutral gas with no oxygen content, usually very pure nitrogen or argon. This technique is very helpful in different research areas (i.e., electrochemistry or photophysics)28,29,30,31. However, while this is a simple procedure and even effective for most purposes, simply purging a solution with a neutral gas is not always the most adequate way, as removing oxygen in trace amounts is almost impossible by this method. Moreover, severe solvent loss can occur due to its volatility, which may lead to changes in the concentration of the sample under study. However, this can be prevented by a saturation of the gas with the solvent used in the solution.
The technique described here is based on a different principle. It allows reducing solvent losses to a minimum and provides repeatable levels of oxygen removal. The technique requires special, usually home-made degassing cuvettes comprising a quartz cell for the acquisition of the luminescence signal – fluorescence or phosphorescence – and a small glass flask with a spherical shape for freezing/unfreezing, and a valve. Degassing is performed under repeating freezing/unfreezing cycles. Oxygen extraction is performed in a vacuum, with the sample in the flask compartment, and while the sample is frozen, followed by letting the sample equilibrate at room temperature, with the vacuum valve closed – during this period, solution melting occurs, and the oxygen dissolved in the liquid phase is released. This requires using the cuvette itself, a regular rotary vacuum pump, and a liquid nitrogen source for cooling. The method can be used with a variety of solvents, preferably those of a low melting point such as toluene, ethanol, methylcyclohexane, 2-methyltetrahydrofuran. Degassing solutions using this technique is fast, efficient, and reliable.
Figure 1 shows with a scheme how TADF and RTP luminescence in organic molecules is generated. Prompt fluorescence, delayed fluorescence, and phosphorescence can all be recorded with the same measurement setup. With this technique, not only luminescence decays but also time-resolved emission spectra can be recorded. This enables the characterization of the molecular system and the facile identification of RTP and TADF emitters. As Figure 3 shows, a TADF emitter will normally show the same emission spectrum over the whole decay, while an RTP emitter shows a short-lived fluorescence and a long-lived phosphorescence that differ in the emission spectra.
NOTE: These are the instructions to perform a single time-resolved, long-lived luminescence measurement in oxygen-free conditions at room temperature and including the procedure of sample degassing. The text describes the protocol for either solid or liquid samples and, because most of the steps are identical in both cases, the steps of the protocol that only apply to one of the two types are indicated as "film" or "solution". The samples and films used in the protocol can be of any kind; therefore, the sample preparation and/or content are irrelevant and are not disclosed.
CAUTION: The handling of organic solvents presents a risk. Consult the Material Safety Data Sheet (MSDS) prior to using them. All operations with solvents must be performed under a working fume cupboard. Liquid nitrogen presents a risk, so it is important to use appropriate personal protective equipment (PPE) when handling it, which includes face and hand protection (mask, gloves). Upon evaporation, liquid nitrogen undergoes a 600-fold increase in its volume; therefore, never handle liquid nitrogen in a fully closed container. Instead, use appropriate Dewar flasks. Wear eye/face protection when working with glass equipment under vacuum, due to the risk of implosion. Most aromatic molecules, and especially those newly synthesized, present a known or unknown health risk. Use standard laboratory PPE and procedures to avoid contact with the material. A class 4 laser is used in the protocol. Working with lasers is dangerous and a suitable training is required. Protective equipment (i.e., goggles) covering the spectral region of the laser emission must be worn at all times.
1. Degassing the Samples
2. Turning on the Equipment and Setting Up the Experiment
3. Finishing the Experiment
Photoluminescence spectra of a platinum-based phosphor solution in toluene were recorded before and after degassing (Figure 2). The air-saturated solution is almost non-emissive, while the degassed solution shows a bright photoluminescence. Figure 3 shows a decay profile of a TADF emitter in toluene solution (Figure 3a) and the time-resolved spectra recorded in the same experiment (Figure 3b) with a phosphorescence spectrum recorded at 80 K, as well as a decay profile of a room temperature phosphorescent molecule in a solid polymer host (Figure 3c) and the time-resolved spectra recorded in that same experiment (Figure 3d) with a phosphorescence spectrum recorded at 80 K.
Figure 3 presents two sets of data recorded in different sample form (solution and solid film) of two different molecules. In Figure 3a, two time regimes can be distinguished: below ~100 ns, the prompt fluorescence decay is observed, while at later times, it is the delayed fluorescence decay that is observed. As seen in Figure 3b, the spectra associated with prompt and delayed fluorescence nearly overlap with each other, as is expected, because this emission is originated from the same electronic state. Phosphorescence that was recorded at low temperature is shown for comparison. TADF emitters typically have a small singlet-triplet energy gap; therefore, the phosphorescence spectrum may be very close to the fluorescence. Figure 3c shows the decay of a room temperature phosphorescent organic molecule. The decays may appear similar, but a comparison of the spectra (Figure 3d) confirms that the delayed emission is not fluorescence, but phosphorescence. The lack of points between the short and the long time regimes is typical if the long-lived emission has got an especially long lifetime (i.e., > 10 ms). The reason is that, in this time regime, the prompt fluorescence is already too weak to be observed as it has already decayed, but the long-lived emission, when integrated using a considerably shorter time than its radiative lifetime integration times, is still not strong enough to be detected. Phosphorescence spectra recorded at room and low temperature differ significantly as the molecule shows rigidochromism.
It is worth to note that the experiment enables the recording of emission spectra and intensity with not only up to 9 decades of time, but also 8 – 9 decades of intensity. The spectra are smooth and of good quality.
Figure 1: Scheme showing the differences between triplet-harvesting molecules: delayed fluorescent (TADF) and phosphorescent (RTP). The protocol presented here (if extended by temperature-dependent measurements) can be used to investigate these molecules and record their key properties. Note: in some RTP molecules, the prompt fluorescence may not be observed. Please click here to view a larger version of this figure.
Figure 2: Photoluminescence spectra showing an increase of photoluminescence intensity after degassing a solution. The figure shows the effect of degassing a toluene solution of a platinum-based phosphorescent metalocomplex using the protocol presented in this work. Please click here to view a larger version of this figure.
Figure 3: Representative results. (a) This panel shows the luminescence decay transient of a TADF emitter in toluene. (b) This panel shows the representative spectra recorded in the same experiment as is shown in panel a, with the phosphorescence spectrum recorded at 80 K. (c) This panel shows the luminescence decay transient of an RTP molecule in cyclo olefin polymer. (d) This panel shows spectra recorded in the same experiment as is shown in panel c, along with a low-temperature phosphorescence spectrum. Please click here to view a larger version of this figure.
Figure 4: Schematic of the measurement system. The Nd:YAG laser produced third harmonics at 355 nm. The laser light hit the sample, which absorbed part of the light and emitted photoluminescence shortly after. The photoluminescence was then collimated and focused onto a spectrograph where it was refracted. The light was then recorded by the iCCD camera, which enabled recording the spectra in the time domain. Note the configuration of the solid and liquid samples. Please click here to view a larger version of this figure.
Figure 5: Photograph of a degassing cuvette used in room-temperature measurements. The cuvette consists of quartz fluorescence cell, a glass freezing flask, and a valve. All elements are connected with glass tubes. Note that the cuvette is not a commercially available item. Please click here to view a larger version of this figure.
Figure 6: Comparison of a regular degassing cuvette and a cuvette used for low-temperature experiments. The cuvette for low-temperature measurements is very similar to the regular one. However, it is fitted with a long glass tube to fit the dimensions of the liquid nitrogen cryostat, and the quartz fluorescence cell is made of one piece of quartz; therefore, it is resistant to temperature changes in a wide range of temperatures. Please click here to view a larger version of this figure.
Degassing a solution is one of the most critical points in this method. Plastic inlet valves become worn easily and the system stops being hermetic. If in doubt, it is advised to check the cuvette with a known material with an established degassing factor. The cuvettes are also fragile; therefore, degassing should be performed with caution.
As the system typically requires a pulsed Nd:YAG laser, a proper maintenance of the laser unit must be performed regularly. The pumping flashlamp should be replaced regularly, and this should only be done by a qualified technician or another experienced person.
As the laser requires 30 min to warming up, it is a good practice to turn on the laser before degassing the sample. Once the sample is degassed, the laser should be ready for taking the measurements. However, the degassing time for a film is difficult to determine using this equipment. Therefore, it is worth to perform a steady-state experiment with a conventional fluorometer to estimate the degassing time (a stabilization of the photoluminescence intensity upon pumping down).
For short-lived emitters (i.e., those whose fluorescence decays within a few nanoseconds), there will be only a few spectra recorded, as the emission decay lasts for a short period of time. In this case, TCSPC or a streak camera would perform much better. On the other hand, long-lived emitters can be problematic if the emission lasts for more than 100 ms (i.e., phosphorescence). To expand the effective time window, a nitrogen laser is used in these cases. This allows reducing the repetition rate of the laser to 1 Hz and extending the time window to 1 s.
The protocol shown here is only exemplary and is dedicated to a new and inexperienced user. An experienced operator can modify the protocol in various different ways. There is a potential to further develop the system to extend the camera's sensitivity in red and (NIR) by replacing the photocathode, as mentioned in the Introduction.
The data analysis in the case of this experiment is a time-consuming job, as each experiment gives ca. 100 spectra. The spectra have to be divided by the integration time to reconstruct the luminescence decay, and often also normalized (divided by the maximum, standardized, or area-normalized) in order to facilitate an analysis of the spectra at different delay times. During the analysis, differences in the spectra (i.e., gradual red or blue shifts) are being looked for. If the measurement is performed in function of the temperature, then the spectra can show the presence of delayed fluorescence or phosphorescence, or both, depending on the temperature or time delay used. Transient decays are obtained by plotting the integrated luminescence spectra against the time delay, after dividing each spectrum by their respective integration time. The photoluminescence transient decay is obtained and can be fitted in order to calculate the radiative lifetime of the prompt and the delayed fluorescence or phosphorescence.
The authors have nothing to disclose.
The research leading to these results has received funding from the European Union's Horizon 2020 research and the innovation program under the Marie Skłodowska-Curie grant agreement No. 674990 (EXCILIGHT), and from EPSRC, EP/L02621X/1.
Degassing cuvette | Not commercial product | ||
Nd:YAG laser | EKSPLA | EKSPLA NL204-0.5K-TH | |
Gated iCCD camera | Stanford Computer Optics | 4Quick Edig | |
Spectrograph | Horiba Instruments inc. | TRIAX180 | |
Liquid nitrogen cryostat | Janis Research | ||
Helium closed cycle cryostat | Cryomech | ||
Fluorolog fluorometer | Jobin Yvon | ||
Liquid nitrogen | Technical | ||
Cyclo olefin polymer | Zeon | Zeonex 480 | |
Toluene | ROMIL | H771 | Toluene SpS |