Amplified spontaneous emission in polymer films doped with a ...

Amplified spontaneous emission in polymer films doped with a perylenediimide derivative Eva M. Calzado,1 José M. Villalvilla,1 Pedro G. Boj,2 José A. Quintana,2 Rafael Gómez,3 José L. Segura,3 and María A. Díaz García1,* 1

Departamento de Física Aplicada and Instituto Universitario de Materiales de Alicante, Universidad de Alicante, 03080-Alicante, Spain 2Departamento de Óptica and Instituto Universitario de Materiales de Alicante, Universidad de Alicante, 03080-Alicante, Spain 3Departamento de Química Orgánica, Universidad Complutense de Madrid, 28040-Madrid, Spain *Corresponding author: [email protected] Received 23 January 2007; accepted 3 February 2007; posted 21 February 2007 (Doc. ID 79335); published 31 May 2007

The presence of amplified spontaneous emission (ASE) by optical pump in polystyrene films doped with N,N⬘-di(10-nonadecyl)perylene-3,4:9,10-tetracarboxylic diimide (PDI-N) in a range of PDI-N concentrations between 0.25 and 5 wt. % is reported. Gain coefficients up to 10 cm⫺1, at a pump intensity of 74 kW兾cm2, were obtained. The lowest thresholds 共⬃15 kW兾cm2兲 and largest photostabilities measured at 50% (⬃50 min, i.e., 30,000 pump pulses) were obtained for concentrations up to 1 wt. %. The observation of an increase in the ASE threshold and a decrease in the photostability for larger concentrations is attributed to the presence of aggregated species. © 2007 Optical Society of America OCIS codes: 140.0140, 140.2050, 140.3580, 160.0160, 160.3380, 160.4890.

1. Introduction

In past years a great effort has been devoted to the development of organic solid-state lasers [1,2]. The main advantage of organic materials is that they can be easily processed in the form of thin films by inexpensive techniques such as spin coating, photolithography, and ink-jet printing, onto almost any kind of substrate, including flexible ones. In addition, the spectral properties of these materials are ideal for the development of wavelength tuneable sources. The laser properties of a large variety of dyes in liquid solution have been investigated for many years [1]. Organic solid-state lasers consisting of dyes incorporated into solid matrixes were demonstrated in the seventies. However, these materials had serious problems of photostability, and generally they could not be pumped electrically. In addition, in most cases there were limitations in the amount of dye that could be introduced, since at a 0003-6935/07/183836-07$15.00/0 © 2007 Optical Society of America 3836

APPLIED OPTICS 兾 Vol. 46, No. 18 兾 20 June 2007

certain concentration molecular interactions lead to photoluminescence (PL) quenching [3]. Therefore their development as commercial solid-state applications was not viable. In 1996 the interest in the field was renewed with the discovery of stimulated emission in semiconducting polymer films [4 – 6]. Since then, many materials, both molecular and polymeric, have been investigated in devices with different types of configurations such as microcavities, and distributed feedback lasers [1,2,7]. Today diode lasers (electrically pumped) have not yet been demonstrated, so the challenge of achieving better materials and configurations continues. Concerning lasers based on low molecular weight materials, in past years various approaches have been followed to circumvent the previously mentioned limitations of dye concentration. In 1997 the concept of energy transfer in guest– host systems was used to decrease the laser thresholds considerably [8,9]. Recently, stimulated emission has been reported in various molecular materials, such as spiro-type chromophores [10,11], thiophene-based oligomers [12] and N,N⬘-bis(3-methylphenyl)-N,N⬘-diphenyl-

benzidine (TPD) [13,14], in the form of neat (nondiluted) films. It has been demonstrated that film quality and supramolecular organization play a major role in the possibility of obtaining high PL efficiencies and stimulated emission in the solid state [10 –12]. However, a clear understanding of this issue is not available at the moment, so detailed investigations of the dependence of the laser performance with the concentration of the active molecule, to elucidate the role of aggregated species, are of great interest. The potential of perylenediimide (PDI) derivatives for laser applications in solution was first demonstrated by Sadrai and Bird [15] in 1984. Since then, other studies both in solution [16,17] and incorporated in solid matrices have been reported [18 –24]. Most of the work performed in the solid state has focused in the usage of solgel matrices doped with the commercially available perylene orange and perylene red, with the aim of improving the photostability. Perylene orange belongs to a class of PDI derivatives symmetrically substituted at the imide nitrogen positions [22]. The absorption spectra of this type of PDIs are strongly structured and are only little influenced by solvent effects and by the type of substituents at the imide nitrogen positions [25,26]. They are planar, very photo and thermally stable and present high PL quantum yields in solution 共␾ ⬇ 1兲 [26]. Although the type of substitution at the imide nitrogen positions does not practically affect the spectral properties of the derivatives, they do play an important role in the type of supramolecular organization that is relevant for obtaining PL in the solid state [27]. Indeed, only certain derivatives of this class show PL emission in the form of neat films [26,27], and it is of great interest to determine whether stimulated emission is also present. One of the simplest methods to identify the presence of stimulated emission in a certain material consists of photopumping films of the material and identifying a collapse of the width of their PL spectra at a certain pump intensity [2,5]. This spectral collapse is normally accompanied by a large enhancement of the output intensity and accounts for the presence of gain due to stimulated emission. There exist different possible mechanisms that can cause this spectral collapse [28]. When the active films constitute waveguides (i.e., the refractive index of the film is larger than that of the substrate and the film thickness is sufficient to support modes), the spectral narrowing generally results from amplification of spontaneous emission (ASE) due to stimulated emission. However, reports on other materials have attributed spectral narrowing to other mechanisms such as superfluorescence or interacting excitons. A way to identify the mechanism responsible of the spectral collapse consists of pumping the films with a stripe of light and investigating the dependence of the emitted intensity with the length of the stripe [28]. If ASE occurs the spectra should be broad at short stripe lengths and narrow as the excitation

Fig. 1. Chemical structure of N,N⬘-di(10-nonadecyl)perylene3,4:9,10-tetracarboxylic diimide (PDI-N).

length increases. Moreover, the output intensity at the end of the stripe should follow the expression: I共␭兲 ⫽ 关A共␭兲Ipg共␭兲兴兵exp关g共␭兲l兴 ⫺ 1其,

(1)

where A共␭兲 is a constant related to the cross section for spontaneous emission, Ip is the pump intensity, g共␭兲 is the net gain coefficient, and l is the length of the pump stripe. In contrast, if superfluorescence or biexcitonic emission is the mechanism of spectral narrowing, the width of the emission spectrum should not depend on the size of the excited region, and the output intensity should only increase linearly with the length of the excited region (or sublinearly if the waveguide losses are substantial) [28]. In the present work we investigate the presence of stimulated emission by optical pump in polystyrene (PS) films doped with N,N⬘-di(10-nonadecyl)perylene3,4:9,10-tetracarboxylic diimide (PDI-N, see Fig. 1). This derivative belongs to the class of PDIs substituted at the imide nitrogen positions (as perylene orange), in this case with alkyl chains, and possesses the important characteristic of showing PL emission in the solid state [27]. In addition, it presents a very high solubility, useful from the point of view of processability, as well as n-type accepting properties [29,30], which is of interest for the possible development of electrically pumped lasers. The identification of ASE as the responsible mechanism for the observation of gain has been performed by studying the dependence with the length of the pump stripe. In addition, the dependence of ASE, in terms of threshold, final linewidth, and wavelength position, with the concentration of PDI-N in the films, in order to elucidate the role of aggregated species has been performed. Finally, due to its importance from the point of view of applications, the photostability of our samples has also been studied. 2. Experimental Methods A.

Sample Preparation

PDI-N was prepared following previously reported synthetic procedures [31,32]. Samples consisted on films of an inert polymer (PS), doped with PDI-N deposited over glass substrates by the spin-coating technique. Films with a varying concentration of PDI-N, ranging from 0.25 to 5 wt. %, were obtained. The solvent used for the preparation of the films was toluene. The film thickness, measured by means of an interferometer coupled to an optical microscope, was approximately 1 ␮m. 20 June 2007 兾 Vol. 46, No. 18 兾 APPLIED OPTICS

3837

B.

Optical Measurements

Linear absorption spectra were obtained in a Shimadzu spectrophotometer. Standard PL spectra were obtained in a Jasco FP-6500兾6600 fluorimeter by exciting the samples at 491 nm and then collecting the transmitted beam at a 45° angle. The excitation wavelength was 491 nm, instead of the 533 nm beam used in the ASE experiments described below, in order to reduce its overlap with the PL emission. The experimental setup to investigate the presence of stimulated emission in these materials has been reported elsewhere [14,28]. Samples were photopumped at normal incidence with a pulsed Nd:YAG laser (10 ns, 10 Hz) operating at 533 nm, which lies in the absorption region of PDI-N. The energy of the pulses was controlled using neutral density filters. The laser beam was expanded, collimated, and only the central part was selected in order to ensure uniform intensity. A cylindrical lens and an adjustable slit were then used to shape the beam into a stripe with a width of approximately 0.53 mm and a length that could be varied up to 3.5 mm. Since most of the light is emitted from the ends of the stripe, this was placed right up to the edge of the film where the emitted light was collected with a fiber spectrometer. 3. Results and Discussion A. Stimulated Emission and Identification of Responsible Mechanism

PDI-N-doped PS films were photopumped at 533 nm, as described in the experimental section, in order to identify the presence of stimulated emission through the observation of a collapse of the width of the emission spectrum at a certain pump intensity. Gain narrowing was observed in PDI-N-doped PS films in a range of PDI-N concentrations between 0.25 and 5 wt. %. As described in detail in Subsection 3.B, for concentrations larger than 3 wt. %, the threshold for the observation of gain narrowing becomes so high that films get damaged. As an illustration, Fig. 2 shows the emission spectra (right axis) obtained at low and high pump intensities for a PS film doped with 0.75 wt. % of PDI-N. The absorption spectrum of the film has also been included (left axis). As observed, the peak of the narrowed emission takes place at ␭ ⫽ 579 nm, that corresponds to the wavelength of the 0-1 transition of the PL, although it is slightly redshifted by 3 nm. There are many materials that have shown gain at the vibrational peaks. The advantage of such a feature is that laser emission can be obtained even in materials with small Stokes shift [15]. To identify whether ASE is the responsible mechanism for the observed spectral narrowing, measurements of the dependence of the signal on the length of the pump stripe were performed. As predicted by ASE, the spectra were broad when the pump stripe was short, and became narrower as the pump stripe length was increased. For low pump intensities 共10 kW兾cm2兲 the spectra did not narrow or they nar3838


Fig. 2. Optical absorption (ABS, thin full curve, left axis) and emission spectra (right axis) at low pump intensity (5 kW兾cm2; dashed curve) and high pump intensity (16 kW兾cm2; thick full curve) of a 0.75 wt. % PDI-N-doped PS film. The intensity of the spectrum obtained at high pump intensity has been divided by 28 for comparison purposes.

rowed very gradually with excitation length because the gain was very low. At higher pump intensities spectral narrowing occurred at smaller stripe lengths because the gain was higher. These results rule out the possibility that the spectral narrowing is a result of superfluorescence or spontaneous emission from a biexcitonic state since the emission spectra would have been narrow at short excitation lengths. Figure 3 shows the output intensity at the peak of the emission spectrum 共␭ ⫽ 579 nm兲 as a function of the pump stripe length at various pump intensities for a 0.5 wt. % PDI-N-doped PS film. By fitting the data to Eq. (1), net gain coefficients (g) of 10, 8.3, and 2 cm⫺1 were obtained for pump intensities of 74, 60, and 26 kW兾cm2, respectively. The good agreement of the data with Eq. (1) is strong evidence for ASE. As compared to other materials, the performance of these systems is slightly inferior. For example, at

Fig. 3. Emission intensity at ␭max for a 0.5 wt. % PDI-N-doped PS film as a function of excitation length at various pump intensities: 74 kW兾cm2 (full circles), 60 kW兾cm2 (open triangles), and 26 kW兾cm2 (full squares). Solid curves are fits to data using Eq. (1), from which gains coefficients (g) have been obtained.

Fig. 5. ASE threshold as a function of PDI-N concentration in PS films. Fig. 4. Output intensity at ␭ ⫽ 579 nm as a function of pump intensity for a 0.25 wt. % PDI-N-doped PS film. Straight lines are linear fits to data. The intercept of the two lines determines the ASE threshold 共Ith兲.

B. Amplified Spontaneous Emission Performance: Concentration Dependence

The dependence with concentration of the final ASE linewidth (above threshold) is illustrated in Fig. 6, where the final FWHM (at high pump intensity), denoted as the ASE linewidth, has been represented versus concentration. The ASE linewidth decreases gradually from 7 to 3.5 nm when the concentration changes from 0.5 to 3 wt. % and then saturates. The smallest linewidths were obtained for the largest concentrations although, as previously indicated, in that range the ASE thresholds were larger (see Fig. 5). The dependence of the ASE emission wavelength with concentration is depicted in Fig. 7. As observed, the ASE wavelength changes very slightly from low concentrations 共␭ ⫽ 579 nm兲 to higher concentrations 共␭ ⫽ 581 nm兲. The concentration dependence of the wavelength of the two PL peaks (denoted as PL(0-0) and PL(0-1)) have also been represented in Fig. 7. Although the position of the PL(0-0) peak keeps approximately constant at approximately 536 nm, the PL(0-1) peak shifts considerably from 576 nm to longer wavelengths for concentrations over 2 wt. %. Moreover, the intensity of the PL(0-1) peak, relative to that of the PL(0-0) peak, also increases with concentration, dominating the spectrum at high concentrations. As observed in Fig. 8, the intensity of the

The concentration dependence of ASE has been studied through three parameters: pump intensity threshold, final linewidth (FWHM), and wavelength of emission. ASE thresholds were determined from the output intensity versus pump intensity graphs, as previously described (see Fig. 4), for PS films doped with different amounts of PDI-N. Figure 5 shows the concentration dependence of the ASE thresholds. It is observed that the lowest thresholds (around 15 kW兾cm2) have been obtained for the lowest concentrations (0.25– 0.75 wt. %). They increase slowly up to concentrations of 1.5 wt. %, and then more rapidly, reaching values of approximately 500 kW兾cm2 for 3 wt. % doped films. Above that concentration, the thresholds are so large that films get damaged very quickly under excitation, so a precise determination of thresholds was not possible.

Fig. 6. ASE linewidth (FWHM) as a function of PDI-N concentration in PS films.

55 kW兾cm2 a net gain coefficient g ⫽ 13 cm⫺1 was obtained for a pentamer of oligophenylene-vinylene (5-OPV) [33]. In fact, the main problem of PDI-Ndoped films is that ASE is obtained only for low concentrations. On the other hand, as discussed in detail in Subsection 3.C, their photostability is much better than that of other materials, justifying their interest, in spite of their higher thresholds. As previously mentioned, the existence of gain results not only in a narrowing of the emission spectra at a certain pump intensity (threshold, Ith) but also in a considerable increase in the output intensity. This is illustrated in Fig. 4, where the output intensity at the wavelength of the ASE emission has been represented as a function of pump intensity for a 0.25 wt. % PDI-N-doped film. As observed, the intensity grows linearly with the pump intensity and shows a clear change in slope at a certain pump intensity that is identified as the ASE threshold.

20 June 2007 兾 Vol. 46, No. 18 兾 APPLIED OPTICS

3839

Fig. 7. Wavelength of ASE (full triangles), PL (0-0) (full squares), and PL (0-1) (open circles) emission peaks as a function of PDI-N concentration in PS films.

PL(0-0) peak decreases at the same rate as the increase of intensity of the PL(0-1) up to concentrations of approximately 2 wt. %. Above that concentration, the low energy peak dominates the spectrum, and its variation with concentration takes place at a different rate. It is precisely this concentration above which the shift of the PL(0-1) peak is more apparent. In fact, the redshift of the PL(0-1) peak could be interpreted in two ways. The first possibility would be that the PL(0-1) really shifts to the red when the concentration is increased. The second one, that a new emission coming from an aggregated species appears, so its composition with the PL(0-1) results in the observed peak that shifts between 576 and 600 nm. It has been reported that the PL spectrum of PDI-N in the solid state presents a peak at 612 nm [27]. Moreover, similar changes in the emission spectra of the perylene derivative N,N⬘-bis(2,5-ditert-butylphenyl)-3,4:9,10-perylenebis(dicarboximide) (DBPI) as a function of concentration both, in solution [34,35] and in solgel matrices [36] have been previously reported. In DPBI, the observed spectral changes were attributed to molecular aggregation. In the present case, we also believe that the emission observed at 595 nm is attributable to the existence of aggregates. The results also indicate that the levels involved in the laser transition are related to those of the monomer and not to those of the aggregate. This is supported by the fact that the ASE emission does not shift with concentration, and its threshold increases dramatically for concentrations above 1.5–2 wt. %, that is precisely the concentration at which the emission of the aggregated species appears. At high concentrations, the observed PL emission comes principally from the aggregated species, which are not able to provide stimulated emission. This observation is very important from a general point of view, since it indicates that the fact that a certain material shows a good PL quantum yield in the form of neat 3840


Fig. 8. Intensity of PL(0-0) (full squares) and PL(0-1) (open circles) emission peaks, as a function of PDI-N concentration in PS films.

films does not necessarily mean that stimulated emission can also be obtained. It should be noted that the behavior described for PDI-N-doped films is different in some aspects from that observed in other materials, such as TPD-doped PS films [14]. In that case, ASE was observed for any TPD concentration ranging from 5 to 100 wt. %. In addition, both, its threshold and linewidth, decreased with concentration up to 20 wt. % and then saturated due to the presence of aggregation. On the other hand, for PDI-N-doped films, ASE could be obtained only for concentrations below 5 wt. %. In that range, the ASE threshold increases with concentration, showing two clear regimes: For concentrations below 2 wt. % the threshold increases very slowly with concentration (Fig. 5) while the intensity of the PL(0-1) peak increases relative to that of the PL(0-0) peak (Fig. 8). So, although ASE takes place at the PL(0-1) peak, the increase in the intensity of the PL(0-1) peak does not result in a decrease of the ASE threshold. Although the total PL intensity increases with concentration, the changes observed in the shape of the PL spectra are indicative of the existence of molecular interaction, that are probably responsible for the increase in the threshold. The second regime appears at concentrations larger than 2 wt. %. Above that concentration the ASE threshold increases much more rapidly (Fig. 5), and the PL(0-0) peak has practically disappeared. Moreover, at 2 wt. % the position of the PL(0-1) starts shifting to longer wavelengths that, as already indicated, can be interpreted as the existence of aggregated species that are not able to provide ASE. Concerning the concentration dependence of the ASE linewidth, one could expect a behavior similar to that of the ASE threshold, as in the case of TPD-doped films. However, for PDI-N-doped films, the ASE linewidth decreases gradually with concentration up to approximately 2 wt. % and then saturates, while the ASE threshold increases in all cases, slowly up to 2 wt. % and then more rapidly for

larger concentrations (Fig. 6). This can be interpreted in the following way. As shown in Fig. 8, the shapes of the PL spectra change with concentration, contrary to TPD-doped films, where the same PL spectra were obtained for all concentrations. Since the linewidth of a certain transition is influenced by the shape of the spectrum, it is reasonable that the narrower the PL spectrum, the smaller the ASE linewidth. Indeed, the transition at which the ASE takes place is the PL(0-1), and its intensity gets larger [relative to that of the PL(0-0)] as the concentration increases. At concentrations larger than 2 wt. % the PL(0-0) peak has practically disappeared so the shape of the PL spectrum remains unchanged, consisting of a single peak. Accordingly, the ASE linewidth remains constant (Fig. 6). C. Photostability

The photostability of PDI-N in PS films was studied by recording the total ASE intensity emitted as a function of time at a constant pump intensity. The presence of photodegradation is evident in the observation of a decrease of the total ASE output. The results are illustrated in Fig. 9 for two representative concentrations (0.75 and 2 wt. %), where the ASE intensity has been represented versus time (lower axis). To compare with other works, the total number of pulses irradiating the sample has also been indicated in the top axis of Fig. 9. The pump intensity was approximately 70 and 400 kW兾cm2 for films doped with 0.75 and 2 wt. %, respectively, well above the ASE threshold in each case. As observed, films doped with 0.75 wt. % of PDI-N are very photostable, with lifetimes, defined as the time at which ASE decreases to half of its maximum value, of approximately 53 min, which corresponds to 31,000 pump pulses. These results are comparable to those obtained for perylene orange, doped into organically modified silicates (ORMOSILs) [23] and into poly(methyl methacrylate)

Fig. 9. Normalized ASE intensity versus irradiation time (bottom axis) and versus the number of pump pulses (10 ns, 10 Hz; top axis) for PS films doped with 0.75 wt. % (full squares) and 2 wt. % (full circles) of PDI-N at a pump intensity of 70 and 400 kW兾cm2, respectively.

(PMMA) [24], with lifetimes of 26,000 and 30,000 shots, respectively. On the other hand, for films doped with 2 wt. % of PDI-N, the photostability was significantly reduced. Since photodegradation depends on both irradiation time and pump intensity, in addition to the dependence with PDI-N concentration, the definition of “damage threshold” is not obvious. By pumping our films with the maximum intensity provided by our laser system 共19 MW兾cm2兲 a lifetime of approximately 3 min, for a 0.75 wt. % PDI-N-doped film, was obtained. For higher PDI-N concentrations (i.e., 2 wt. %) the lifetime, measured under the same conditions, was reduced to 1 min. Thus by defining damage threshold as the pump intensity at which the ASE lifetime gets lower than 1 min, the damage threshold for ASE in PDI-N-doped PS films would be approximately 19 MW兾cm2. It should be noted that under these pump conditions, some signs of mechanical damage in the illuminated part of the sample can be observed after only 30 s, although as previously mentioned ASE can still be observed up to 1–3 min. This mechanical damage is also observed in pure PS films (undoped), indicating that the observed photodegration might be taking place in the matrix, instead of in the dye. These results point out the possibility of improving the photostability even more by using host materials with better mechanical properties under irradiation. 4. Conclusion

We have reported on the presence of stimulated emission in polystyrene films doped with N,N⬘-di(10nonadecyl)perylene-3,4:9,10-tetracarboxylic diimide (PDI-N), under pulsed excitation at 533 nm. The study of the dependence with the length of the pump stripe demonstrates that the observation of gain is due to amplified spontaneous emission (ASE). ASE has been observed in a range of PDI-N concentrations between 0.25 and 5 wt. %. By fitting the evolution of the output intensity at the ASE wavelength with the excitation length, gain coefficients up to 10 cm⫺1, at a pump intensity of 74 kW兾cm2, have been measured. The dependence of ASE, in terms of threshold, linewidth, and emission wavelength, with the concentration of PDI-N in the films has been investigated in detail. The lowest thresholds 共⬃15 kW兾cm2兲 and largest photostabilities measured at 50% (⬃50 min, i.e., 30,000 pump pulses) were obtained for concentrations up to 1.5 wt. %. For larger concentrations, the ASE threshold increases and the photostability diminishes, that is attributed to the presence of aggregated species. Although neat films of PDI-N show good PL quantum yields, laser emission at reasonable thresholds cannot be obtained for films doped with concentrations larger than around 5 wt. %. We acknowledge support from the Spanish Ministerio de Educación y Ciencia (MEC) (grant MAT200507369-C03-1), from the University of Alicante (grant VIGROB2005-060), and the Universidad ComplutenseComunidad de Madrid joint project PR45/05-14167. 20 June 2007 兾 Vol. 46, No. 18 兾 APPLIED OPTICS

3841

R. Gomez thanks the Programa Ramón y Cajal. We also thank Vicente Esteve for technical assistance and R. Schölz and I. Vragovic for useful discussions. References 1. N. Tessler, “Lasers based on semiconducting organic materials,” Adv. Mater. 11, 363–370 (1999) and references therein. 2. M. D. McGehee and A. J. Heeger, “Semiconducting (Conjugated) polymers as materials for solid-state lasers,” Adv. Mater. 12, 1655–1668 (2000) and references therein. 3. F. J. Duarte and L. W. Hillman, Dye Laser Principles with Applications (Academic, 1990). 4. F. Hide, B. Schwartz, M. A. Díaz-García, and A. J. Heeger, “Laser emission from solutions and films containing semiconducting polymers and titanium dioxide nanocrystals,” Chem. Phys. Lett. 257, 424 – 430 (1996). 5. F. Hide, M. A. Díaz-García, B. Schwartz, M. Andersson, Q. Pei, and A. J. Heeger, “Semiconducting polymers: a new class of solid-state laser materials,” Science 273, 1833–1836 (1996). 6. N. Tessler, G. J. Denton, and R. H. Friend, “Lasing from conjugated-polymer microcavities,” Nature 382, 695– 697 (1996). 7. I. D. W. Samuel and C. A. Turnbull, “Polymer lasers: recent advances,” Mater. Today 7, 28 –35 (2004). 8. V. G. Kozlov, V. Bulovic, P. E. Burrows, and S. R. Forest, “Laser action in organic semiconductor waveguide and doubleheterostructure devices,” Nature 389, 362–364 (1997). 9. M. Berggren, A. Dodabalapur, R. E. Slusher, and Z. Bao, “Light amplification in organic thin films using cascade energy transfer,” Nature 389, 466 – 469 (1997). 10. N. Johansson, J. Salbeck, J. Bauer, F. Weissörtel, P. Bröms, A. Andersson, and W. R. Salaneck, “Solid-state amplified spontaneous emission in some spiro-type molecules: a new concept for the design of solid-state lasing molecules,” Adv. Mater. 10, 1137–1141 (1998). 11. D. Schneider, T. Rabe, T. Riedl, T. Dobbertin, O. Werner, M. Kröger, E. Becker, H.-H. Johannes, W. Kowalsky, T. Weimann, J. Wang, P. Hinze, A. Gerhard, P. Stössel, and H. Vestweber, “Deep blue widely tunable organic solid-state laser based on a spirobifluorenone derivative,” Appl. Phys. Lett. 84, 4693– 4695 (2004). 12. M. Anni, G. Gigli, R. Cingolani, M. Zavelani-Rossi, C. Gadermaier, G. Lanzani, G. Barbarella, and L. Favaretto, “Amplified spontaneous emission from a soluble thiophene-based oligomer,” Appl. Phys. Lett. 78, 2679 –2681 (2001). 13. W. Holzer, A. Penzkofer, and H.-H. Hörhold, “Travelling-wave lasing of TPD solutions and neat films,” Synth. Met. 113, 281– 287 (2000). 14. E. M. Calzado, J. M. Villalvilla, P. G. Boj, J. A. Quintana, and M. A. Díaz-García, “Concentration dependence of amplified spontaneous emission in organic-based waveguides,” Org. Electron. 7, 319 –329 (2006). 15. M. Sadrai and G. R. Bird, “A new laser dye with potential for high stability and a broad band of lasing action: perylene3,4,9,10-tetracarboxylic acid-bis-N,N⬘(2⬘,6⬘xylidyl)diimide,” Opt. Commun. 51, 62– 64 (1984). 16. H.-G. Löhmannsröben and H. Langhals, “Laser performance of perylenebis(dicarboximide) dyes with long secondary alkyl chains,” Appl. Phys. B 48, 449 – 452 (1989). 17. M. Sadrai, L. Hadel, R. R. Sauers, S. Husain, K. KroghJespersen, J. D. Westbrook, and G. R. Bird, “Lasing action in a family of perylene derivatives: singlet absorption and emission spectra, triplet absorption and oxygen quenching constants, and molecular mechanics and semiempirical molecular orbital calculations,” J. Phys. Chem. 96, 7988 –7996 (1992). 18. R. Reisfeld and G. Seybold, “Solid-state tunable lasers in the

3842


19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31. 32.

33.

34.

35. 36.

visible, based on luminescent photoresistant heterocyclic colorants,” Chimia 44, 295–297 (1990). M. Canva, P. Georges, J. P. Perelgritz, A. Brun, F. Chaput, and J. P. Boilot, “Perylene- and pyrromethene-doped xerogel for a pulsed laser,” Appl. Opt. 34, 428 – 431 (1995). M. Faloss, M. Canva, P. Georger, A. Brun, F. Chaput, and J. P. Boilot, “Towards millions of laser pulses with pyrrometheneand perylene-doped xerogels,” Appl. Opt. 36, 6760 – 6763 (1997). M. D. Rahn, T. A. King, A. A. Gorman, and I. Hamblett, “Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen-free liquid and solid dye lasers,” Appl. Opt. 36, 5862–5871 (1997). A. K. Sheridan, A. R. Buckley, A. M. Fox, A. Bacher, D. D. C. Bradley, and I. D. W. Samuel, “Efficient energy transfer in organic thin-films implications for organic lasers,” J. Appl. Phys. 92, 6367– 6371 (2002). Y. Yang, M. Wang, G. Qian, Z. Wang, and X. Fan, “Laser properties and photostabilities of laser dyes doped in ORMOSILs,” Opt. Mater. 24, 621– 628 (2004). N. Tanaka, N. Barashkov, J. Heath, and W. N. Sisk, “Photodegradation of polymer-dispersed perylene diimide dyes,” Appl. Opt. 45, 3846 –3851 (2006). H. Langhals, “Cyclic carboxylic imide structures as structure elements of high stability. Novel developments in perylene dye chemistry,” Heterocycles 40, 477–500 (1995). H. Langhals, “Control of the interactions in multichromophores: novel concepts. Perylene bisimides as components for larger functional units,” Helv. Chim. Acta 88, 1309 –1343 (2005). H. Langhals, St. Demming, and Th. Potrawa, “The relation between packing effects and solid state fluorescence of dyes,” J. Prakt. Chem. 333, 733–748 (1991). M. McGehee, R. Gupta, S. Veenstra, E. M. Miller, M. A. DíazGarcía, and A. J. Heeger, “Amplified spontaneous emission from photopumped films of a conjugated polymer,” Phys. Rev. B 58, 7035–7039 (1998). J. Salbeck, H. Kunkely, H. Langhals, R. W. Saalfrank, and J. Daub, “Electron-transfer (ET) behaviour of fluorescent dyes studied in perylenebisdicarboximides and dioxaindenoindenedione using cyclovoltammetry and UV兾VIS spectroelectrochemistry,” Chimia 43, 6 –9 (1989). J. L. Segura, R. Gómez, R. Blanco, E. Reinold, and P. Bäuerle, “Synthesis and electronic properties of anthraquinone-, tetracyanoanthraquinodimethane-, and perylenetetracarboxylic diimide-functionalized poly(3,4-ethylenedioxythiophenes),” Chem. Mater. 18, 2834 –2847 (2006). H. Langhals, S. Sprenger, and M. T. Brandherm, “Perylenamidine-imide dyes,” Liebigs Ann. 481– 486 (1995). L. D. Wescott and D. L. Mattern, “Donor-␴-acceptor molecules incorporating a nonadecyl-swallowtailed perylenediimide acceptor,” J. Org. Chem. 68, 10058 –10066 (2003). M. A. Díaz-García, E. M. Calzado, J. M. Villalvilla, P. Boj, J. A. Quintana, F. Giacalone, J. L. Segura, and N. Martín, “Concentration dependence of amplified spontaneous emission in two oligo-(p-phenylenevinylene) derivatives,” J. Appl. Phys. 97, 063522-1– 063522-6 (2005). E.-Z. M. Ebeid, S. A. El-Daly, and H. Langhals, “Emission characteristics and photostability of N,N⬘-bis(2,5-di-tertbutylphenyl)-3,4:9,10-perylenebis(dicarboximide),” J. Phys. Chem. 92, 4565– 4568 (1988). R. Schölz and M. Schreiber, “Linear optical properties of perylene-based chromophores,” Chem. Phys. 325, 9 –21 (2006). C. Burgdorff, H.-G. Löhmannsröben, and R. Reisfeld, “A perylene dye in sol-gel matrices: photophysical properties of N,N⬘-bis(2,5-di-tert-butylphenyl)-3,4:9,10-perylenebis (dicarboximide) in glasses and thin films,” Chem. Phys. Lett. 197, 358 –363 (1992).