We report on the improvement in photostability of solid-state dye materials. The photostability is the amount of energy that can be deposited in the material before its luminescence or lasing intensity has dropped to 50% of the initial value. It is shown that the photostability can be prolonged by a factor 100 by reducing the oxygen content in the material. The realized oxygen removal procedure and encapsulation of the solid state dye does not affect the luminescence properties and thus might be applicable in dye lasers.
© 2011 Optical Society of America
The investigation and development of solid-state dye materials (SSD) is motivated by their potential applications, e.g., as laser material or optical absorbers [1–6]. Due to their high quantum efficiency and large variety organic dyes like Xanthenes, Stilbenes, and Coumarines have been widely used to prepare SSDs. The preparation of SSDs via a sol-gel method offers advantages in the processing and is suited for the encapsulation of organic molecules into an inorganic host matrix due to the mild synthesis conditions . While considerable progress has been made in this field in the last two decades a major problem that remains is the low photostability of such SSDs based on organic dyes. The photostability EPS is in general defined as the amount of (pump) energy that can be deposited in the material before its luminescence or lasing intensity has dropped to 50% of the initial value. This intensity drop is due to the effect of photobleaching of the dye and occurs often rather quickly such that after a few hours of irradiation the photostability point is reached. Numerous solutions to this obvious drawback have been explored: by applying sophisticated chemical approaches to enhance the photochemical stability [3, 8, 9] or mechanical solutions such as rotation of the SSD in order to renew the optically exposed volume .
On the other hand it is well-known that the photobleaching of organic dyes is closely related to the presence of oxidizing and reducing agents in the material, a fact which is exploited in single-molecule fluorescence applications by so-called ROXS (reducing and oxidizing system) . Especially molecular oxygen has been identified as detrimental to the photostability of organic dyes. While it acts as an efficient triplet quencher the thereby created reaction product, the highly reactive singlet oxygen, can activate the photodecomposition of the dye molecule . In the last years several studies have been performed on the effect of oxygen on photobleaching. Using single-molecule spectroscopy thin polymer films (10–100 nm) have been studied under nitrogen atmosphere. Lill & Hecht reported on a decrease of the photodegradation quantum yield of an organic dye embedded in a polymer film by a factor of 60 in nitrogen atmosphere . On the other hand Renn et al.  reported an increase of the photostability of ionic dyes by the presence of oxygen. With respect to SSDs for laser applications Rahn et al  and Faloss et al  reported a 5 to 6 fold increase of the photostability upon oxygen removal.
These converse results led us to investigate the impact of changing the oxygen concentration in the SSD on the photostability on the example of a dye-impregnated silica xerogel. For the test we used the most simple SSD possible, a silica xerogel impregnated with Rhodamine 6G (Rh6G). We show that by decreasing the oxygen content below 0.05 ppm the photostability of the SSD can be improved by two orders of magnitude, thus supporting the results of Lill & Hecht , Rahn , and Faloss .
Xerogels loaded with Rh6G were prepared in the following manner: 2.5 ml H2O, 1.5 ml methanol, 1.5 ml of an ethanolic Rh6G-ClO4 solution (0.089 mmol/l), and 1.5 ml tetramethoxy-orthosilicate (TMOS) were mixed and stirred for about 30 minutes. The obtained solution was distributed in portions of 0.5 ml into 12 reagencies with a diameter of 6 mm and dried for one week at 50 °C. Assuming that the xerogel has completely hydrolised and that the final xerogel consists of 100% SiO2 we obtain an amount of Rh6G-ClO4 n = 11.1 nmol and 50.5 mg (or 0.84 mmol) SiO2 per xerogel.
Removal of oxygen: The dried xerogels were inserted in a glass capillary equipped with a valve for hermetical fermeture. The capillary was evacuated to a pressure of 10−3 or 10−6 mbar, respectively. During one week, pumping and flushing once per day with Argon 4.8 grade (corresponding to < 2 ppm) was repeated to ensure removal of oxygen and replacement with inert Argon. The Argon gas was additionally purified by passing it over an Oxysorb cartridge (< 0.05 ppm O2).
Figure 1 illustrates the experimental setup for the photostability measurements.The gaussian spatial profile of the pump beam has been expanded by cylindrical lenses, so that a rectangular aperture of the cuvette of 3 × 15 mm could be exposed to a nearly spatial homogeneous intensity distribution. The luminescence intensity is measured at an angle of 90° with respect to the incident pump by an appropriate Si-PIN photodetector. The signals of the Si-photodetectors were converted with a home-built calibrated amplifier (Osnabrück University) and readout by a PCI card at timestamps defined by a LABVIEW program (one measurement point every 5 seconds). Scattered light in the direction of the detector is blocked with a Semrock RS532 interference filter (optical density at 532 nm > 106). The power of the continuous-wave pump laser (frequencey doubled Nd:YAG 532 nm) is adjusted to a constant value of 100 mW that is monitored by means of a reference photodetector via reflectance of a glass plate within the path of the pump beam. Luminescense spectra were collected on a Cary Eclipse fluorescence spectrophotometer (Varian). Luminescence lifetimes were measured after pulsed excitation with a 150 fs laser pulse at 530 nm (Quantronix Integra-C feeding a TOPAS OPA) using a Si-photodiode with 500 ps bandwith connected to a 2 GHz storage oscilloscope (LeCroy).
3. Results and discussion
Figure 2 shows the luminescence intensity as a function of time under pumping with 100 mW at 532 nm for the Rh6G-xerogel at ambient conditions, at a pressure of 10−3 mbar and 10−6 mbar. The decrease of the luminescence intensity can be described with a double-exponential decay with time-constants τ1 = 2.6(2)h and τ2 = 21(1)h for the sample at ambient conditions resulting in a photostability EPS of 0.29 Wh. For the two samples which were submitted to the oxygen removal procedure a fast mono-exponential decay followed by a slow linear decrease is observed. The time-constant for the initial mono-exponential decay is for both of the order of 0.5(1)h. The following slow decrease has a slope of −0.005 h−1 for the sample treated at 10−3 mbar while the slope of the sample purified at 10−6 mbar is −0.002 h−1. For the former a photostability of 2.8 Wh is observed while for the latter a photostability of 23 Wh can be extrapolated (see Fig. 2 and Table 1).
These results can be explained in the following manner: under normal atmosphere the known bleaching process of Rh6G with oxygen occurs. The two time constants are most probably due to two different phases, in the first phase the oxygen present in the pores contributes to the bleaching, which is correspondingly fast. In the second phase the bleaching process is slower since the oxygen of the surrounding atmosphere must diffuse first into the xerogel to maintain the bleaching reactions. In the two measurements where the normal atmosphere has been replaced by inert argon with only few ppm remaining oxygen, the time constant for the first mono-exponential decay is somewhat faster than for normal-atmosphere conditions. This might be due to the different overall gas mixture present within the xerogel, which has changed from air to purified Argon that has besides oxygen some impurities. More interesting is the fact that these two measurements show the same type of temporal behaviour. The long-time decay is linear and the slope decreases when the oxygen content is reduced. Because under these conditions there is only few ppm oxygen supplied by the surrounding atmosphere, the bleaching process is significantly slowed down.
3.2. Spectral properties
Most important for potential applications - besides the improved photostability EPS - is that the other favorable luminescence properties of the organic dyes remain unaffected by the encapsulation and oxygen removal procedures. As can be seen from Fig. 3 the spectral emission of Rh6G remains almost unchanged execpt for a slight broadening on the high-energy side. Figure 4 shows the luminescence lifetime after pulsed excitation with 530 nm. The luminescence lifetimes obtained from a monoexponential fit to the data are 6.5(5) ns under ambient conditions and 7.5(5) ns in the oxygen-evacuated sample. Thus, luminescence lifetime and spectrum remain unchanged within error.
Besides the positive effect on the photostability, what can be learned from the observed luminescence properties with respect to the photophysical properties of the SSD? Since the luminescence spectra are unaltered by the evacuation procedure we suppose that the local environment of the embedded dyes is not affected. Especially we can argue that there are no remaining solvent molecules that were removed from the xerogel during evacuation, because such a change of the local polarity should significantly affect the spectra . Further we note that neither the lifetime of the luminescent S1 state nor the first fast exponential decay in the photostability measurements are affected by the oxygen removal. The former observation indicates that the intersystem crossing (S1 → T1) is not significantly changed by the oxygen removal (replacement of air by purified Argon) in agreement with literature . The latter results needs a more detailed discussion. The first fast decay might be due to the remaining oxygen molecules. Since the removal procedure is not perfect and the remplacement gas has also been purified to less than 0.05 ppm the remaining oxygen molecules can still react with nearby dye molecules. As long as the distribution is homogeneous the time constant is not affected. Once the formed singlet oxygen has reacted with a triplet dye both become inactive, which leads to a further reduction of available oxygen molecules. Since now the distance between a dye in a triplet state and remaining oxygen molecules becomes more and more important the photodegradation slows down, especially since molecular diffusion in silica matrices is limited also . On the other hand the sample is constantly heated during optical pumping, which keeps alive diffusion and allows for a slow photodegradation.
Compared to the earlier studies of oxygen removal on the photostability of laser dye materials [15, 16] we note that the careful removal procedure applied in our study allowed for an additional improvement of more than a factor 10. This implies that even small traces of oxygen have a significant detrimental effect on the performance of the material. As can be seen from Table 1 the correlation between oxygen content and photostability is stronly nonlinear. For photostabilities in the range of several thousand Wh one would need to reduce the oxygen content to ppb or below. Thus in view of the potential applications of SSDs it is of utmost importance to apply a well-thought-out procedure for oxygen removal as well as an appropriate encapsulation of the material. For solid-state dye lasers a possible realisation could be the encapsulation in a cuvette as demonstrated in this study as it allows for precise control of oxygen content and offers the advantage of optically adequate surfaces, which might even be coated according to the requirements within the laser cavity.
Solid-state dye materials composed of Rh6G embedded in a silica matrix have been investigated with respect to their photostability. It was shown that the removal of oxygen leads to a significant increase of the photostability. It thus can be concluded that the major photodegradation pathway for Rh6G in silica matrices is due to reaction with molecular oxygen. This result offers a simple way to increase the lifetime of SSDs without changing the other luminescence properties: encapsulating them in an oxygen free environment.
We thank R. Lepski for preparation of Rh6G doped xerogels. Financial support by the BMBF (03X5510) and DFG (INST 190/137-1 FUGG) is gratefully acknowledged.
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