Effect of Structural Modifications in the Spectral and Laser Properties

J. Phys. Chem. C 2007, 111, 13595-13605

13595

Effect of Structural Modifications in the Spectral and Laser Properties of Perylenediimide Derivatives Eva M. Calzado,†,| Jose´ M. Villalvilla,†,| Pedro G. Boj,‡,| Jose´ A. Quintana,‡,| Rafael Go´ mez,§ Jose´ L. Segura,§ and Marıá A. Dıáz-Garcıá*,†,| Departamento de Fı´sica Aplicada, Departamento de O Ä ptica, and Instituto UniVersitario de Materiales de Alicante, UniVersidad de Alicante, 03080-Alicante, Spain, and Departamento de Quı´mica Orga´ nica, UniVersidad Complutense de Madrid, 28040 Madrid, Spain ReceiVed: April 3, 2007; In Final Form: July 4, 2007

The aim of this work was to design strategies to improve the performance of solid-state lasers and amplifiers based on perylenediimide (PDI) derivatives as active materials. So, the effect of different types of modifications of the chemical structure of PDIs in their spectral, electrochemical, and laser properties in both solution and PDI-doped polystyrene films at various concentrations has been investigated. In particular, we focused on controlling the wavelength of emission in order to tune the laser wavelength as well as in increasing the amount of PDI in the films in order to decrease the laser thresholds, while keeping a good photostability. Three types of modifications of the chemical structure were investigated: (a) symmetrical substitution at the imide nitrogen positions (PDI 1); (b) substitution at the bay positions in the PDI core (PDI 4); and (c) modification in the dicarboximide group (PDI 5). All three derivatives were soluble and showed good n-type acceptor ability. Routes b and c led to red shifts in the absorption and photoluminescence (PL) emission, although the PL quantum yield decreased considerably. Amplified spontaneous emission (ASE) was observed in films doped with PDI 1 (λ ) 579 nm) and PDI 4 (λ ) 599 nm). The best performance, with an ASE threshold of 15 kW/cm2 and a photostability halflife of 31 × 103 pump pulses, was obtained for films doped with 0.75 wt % of PDI 1 (route 1). PDI 1-based materials are among the most photostable reported in the literature and show very-reasonable thresholds. Moreover, these materials are particularly interesting in the field of data communications based on polymer optical fibers because they emit at wavelengths close to 570 nm, which constitutes the second low-loss transmission window in poly(methyl methacrylate).

I. Introduction Perylenediimides (PDIs) have been investigated extensively in many research areas. They have been used as pigments since 1950 because of their favorable properties for this purpose, such as insolubility, migrational, photo, and thermal stability, chemical inertness, and so forth.1 In addition, these materials have excellent fluorescent (or photoluminescence, PL) properties with high PL quantum yields, so their potential as laser dyes,2,3 fluorescent light collectors,4 and labels in single molecule spectroscopy5 have been demonstrated. For the development of such applications, it was essential to increase the solubility of the chromophores. Therefore, various strategies dealing with the incorporation of solubility-increasing groups into various positions of the perylene structure have been proposed.6,7 More recently, PDIs have also been used in optoelectronic applications, such as photovoltaic devices,8 organic field-effect transistors,9,10 and light-emitting diodes11 because of their very-good n-type semiconducting properties.12 Among all of these applications, this paper focuses on the development of PDI-based organic solid-state lasers. A very* Corresponding author. Phone: +34 965903400 Ext. 2905. Fax: +34 965909726. E-mail: [email protected]. † Departamento de Fı´sica Aplicada, Universidad de Alicante. ‡ Departamento de O Ä ptica, Universidad de Alicante. § Departamento de Quı´mica Orga ´ nica, Universidad Complutense de Madrid. | Instituto Universitario de Materiales de Alicante, Universidad de Alicante.

unique property of organic materials is that, because of their broad PL spectrum, the laser wavelength can be tuned over a wide range. Among the various types of organic materials, those that are soluble have the additional advantage of easy proccesability, so they can be deposited in the form of thin films by inexpensive techniques, such as spin coating, photolithography, ink-jet printing, and so forth, onto almost any kind of substrate, including flexible ones. Moreover, the versatility of organic chemistry offers the opportunity to obtain materials whose properties can be tuned by structural modifications. The laser properties of a large variety of dyes in liquid solution have been investigated since the 1960s.13 Organic solidstate lasers consisting of dyes incorporated into solid matrices have also been demonstrated.13 However, these materials have serious problems of photostability, they cannot be pumped electrically and, generally, there is a limit in the concentration of dye that can be introduced in the matrix because molecular interactions lead to PL quenching.14 As a consequence, the development of commercial organic solid-state lasers has not been viable. In 1996, the interest in the field was renewed with the discovery of stimulated emission in semiconducting polymer films.15-17 Since then, many semiconducting materials, including small molecules, oligomers, dendrimers, and polymers, have been investigated in devices with different types of configurations such as microcavities, distributed feedback lasers, and so forth.18,19 These materials could potentially solve some of the problems previously addressed for dyes. First, they are semi-

10.1021/jp0725984 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

13596 J. Phys. Chem. C, Vol. 111, No. 36, 2007 conductors, so they open the possibility of electrical pumping. Second, many of these semiconducting materials did not show limitations in the concentration of active material, so stimulated emission could be obtained from neat (nondiluted) films, leading to much-lower laser thresholds. Concerning photostability, it has constituted the main focus of the work done in the last few years in the field of traditional dyes. However, there is little information about the photostability of most of the semiconducting materials studied, so this remains as one of the most important challenges for the realization of commercial systems. Nowadays, a great advance in optically pumped structures has occurred.19 For some materials, thresholds are low enough to be pumped with microlasers, the size of a match box, and even with GaN lasers. As a result, applications based on optically pumped structures are becoming a reality. However, diode lasers (electrically pumped) have not been demonstrated yet. In that case, the main problem is that charge injection generates a great amount of losses in the spectral range where stimulated emission takes place, mainly because of the low mobility of the materials. Concerning molecular semiconducting materials, in the last few years new molecules and oligomers have been studied, showing laser thresholds much lower than those of traditional dyes. Just a few of these materials, that is, thiophene-S,S-dioxide oligomers,20,21 spiro-type molecules,22,23 and the hole-transporting N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine system (TPD)24,25 have shown stimulated emission in the form of neat films. Because film quality and supramolecular organization play a major role in obtaining high PL efficiencies and stimulated emission in the solid state, detailed investigations of laser performance as a function of the concentration of the active material is of great importance. Moreover, detailed studies of the laser performance dependence with the concentration is necessary to determine the optimal concentration for laser operation. For example, for TPD similar laser characteristics were obtained for films doped with concentrations between 30 and 100 wt %.25 Because film quality, mechanical properties, and photostability decrease with concentration, this kind of study allowed us to determine that 30 wt % was the optimal concentration in that case, although stimulated emission could be obtained from neat films. In the case of S,S-dioxide oligothiophenes, they have been studied in great detail in the form of neat films. However, no detailed studies with the concentration of active material were performed, so the thresholds achieved could be perhaps improved. In this respect, it has been shown recently that certain oligothiophene derivatives did not show stimulated emission in the form of neat films but did show it when a small amount of inert polymer was added.21 All of these results show the importance of carrying out concentration-dependence studies when characterizing the laser properties of molecular materials because neat films are not always the best option. In relation to the possibility of using an electrical pump with semiconducting molecular materials, the main problem is that the strategies that lead to high mobilities, that is, H aggregation and π-π stacked structures, are orthogonal with those that maximize the PL efficiencies, based on favoring the presence of J aggregates.26 Therefore, the realization of an electrically pumped organic laser still remains one of the biggest challenges in this field. The potential of PDIs for laser applications in solution was first demonstrated by Sadrai et al. in 1984.2 Since then, other studies either in solution3,27 or incorporated in solid matrices have been reported.28-34 Practically all of these works deal with the commercially available perylene orange and perylene red. In addition, most of the work performed in the solid state has

Calzado et al. focused on the usage of sol-gel matrices with the aim of improving their photosbability. PDIs are among the most photostable materials reported in the literature, which constitutes their most attractive property for laser applications. However, no detailed investigations of the dependence of the laser performance with the PDI concentration have been performed. In this respect, the biggest challenge would be to achieve concentrations of PDI as large as possible so lower laser thresholds could be obtained. In addition, although several strategies have been developed toward the modification of the spectral properties of PDIs,7 none of these efforts have been directed toward the optimization of the laser properties. As a consequence, nowadays it is not clear which is the most convenient type of substitution in order to get both good laser performance and high photostability. Another aspect of interest is that PDIs emit at wavelengths that can easily match the lowtransmission windows in poly(methyl methacrylate) (PMMA) at 530, 570, and 650 nm. So they have a great potential in the field of data communications where polymer optical fibers are used, such as fiber to the home/workplace and in data transfer in automobiles.19 Their properties at both the molecular and the macroscopic level (type of arrangement in the solid state) need to be optimized in order to have good laser materials. First, at a molecular level, the desirable properties14 are to have strong singlet absorption, high PL quantum yield, photochemical stability under intense excitation, a region of strong luminescence well out of the absorption region, very low quantum yield of triplet states, and minimal overlap between the singlet absorption and the absorption regions from both triple and excited states. All of these parameters depend on the chemical structure of the molecule, so modifications to the structure by substitution in different positions could optimize these properties. Alternatively, at the macroscopic level, the spectral characteristics of the material depend on the type of arrangement of the molecules in the solid state, which, on the contrary, is much-more difficult to control. Concerning the spectral properties of PDIs at the molecular level, it is well known that the absorption spectra of PDIs symmetrically substituted at the imide nitrogen positions (route a), such as N,N′-di(10-nonadecyl)perylene-3,4:9,10-tetracarboxylic diimide (compound 1 in Scheme 1), are strongly structured and are little influenced by solvent effects and the type of substituents at the N positions.6,7,35 Consequently, their absorption spectra cannot be tuned by substitution at the N atoms. These types of derivatives are planar, very photostable, and very stable thermally. In addition, they show very-high PL quantum yields in solution (Φ ≈ 1)7,36 and some derivatives (such as compound 1) also in the solid state.37 There are mainly two routes7 for tuning the absorption spectra of these compounds: (b) Substitution at the perylene core and (c) Modification of the dicarboximide group. From a macroscopic point of view, it is known that the absorption and PL spectra of both amorphous and crystalline films red shift with respect to those of the single molecules.35 In the case of solutions and amorphous films, the key features can be related to the elongation of internal breathing modes in the geometry of the relaxed excited, but the size of this deformation depends on the dielectric properties of the surroundings. In the case of crystalline phases, external phonon modes give a major contribution to the absorption and PL, so the crystal structure determines the most-relevant intermolecular interactions. From the point of view of improving the laser performance in the solid state, it is convenient to avoid the

Modifications of Perylenediimide Derivatives

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13597

SCHEME 1

formation of H aggregates because they generally lead to PL quenching.26 In that sense, the most interesting aspect of PDIs substituted at the bay positions of the core (route b) is that the π-π stacking energy is significantly reduced by the steric hindrance induced by the bay substituents, being minimum for the most-twisted π systems.38 Because intermolecular interactions generally lead to a decrease in the PL efficiency, and consequently in the laser performance, one could expect to achieve laser emission with larger concentrations of the active material in the film, as compared to PDIs substituted at the imide nitrogen positions. Nevertheless, it should be noted that substitution at the bay positions of the perylene core generally leads to a decrease of the PL quantum yield that would be negative for the laser performance. For example, Φ ) 0.85 and Φ ) 0.5 have been measured in a tetracloro-substituted perylene27 and in a cholesterol-based perylene respectively.39 Alternatively, route c consists of modifying the dicarboximide group in the PDI and leads to more-drastic changes in the absorption spectrum of the monomer, while the π-π stacking energy keeps practically unchanged.7 The objective of this work was to investigate possible ways to improve the laser performance of PDIs by studying the effect of different types of substitutions and structural modifications of their chemical structure in their spectral, laser, and photostability properties. This information is useful in order to establish structure-property relationships that allow organic chemists to design materials with improved laser performances. In this respect, it is important to note that until now there has not been much design in this field. Most of the materials studied for lasers have been materials that have been designed for other applications, that is, light-emitting diodes, which often are not the best for laser purposes. In particular, we were interested in

modifying the spectral properties in order to control the wavelength of the emitted light, thus better matching the lowloss windows of PMMA as well as the Stokes shift between absorption and PL. In addition, we aimed to determine possible ways to increase the concentration of PDI in the matrix, avoiding the formation of H aggregates, in order to decrease the laser thresholds, while keeping good photostability. In that respect, detailed studies of the spectral and laser properties as a function of PDI concentration, in order to identify the presence of aggregation and its influence in the laser performance, have been carried out. For the realization of all of these tasks, we have studied three different derivatives (see Scheme 1), representatives of the three routes mentioned above: N,N′-di(10-nonadecyl)perylene-3,4: 9,10-tetracarboxylic diimide (PDI 1), bearing alkyl chains at the imide nitrogen positions; N,N′-di(2′′-ethylhexyl)-1,7-bis(4tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic diimide (PDI 4), substituted at the bay positions in the PDI core; and a peryleneamidine imide derivative (PDI 5), with modifications in the dicarboximide group. All three derivatives have high solubility in organic solvents, so they can be processed as thin films from solutions. PDI 1 has a structure similar to that of the widely studied perylene orange. Alternatively, PDI 4 and PDI 5 have been synthesized by some of us for the first time, so a detailed description of the synthetic procedures, as well as their complete characterization, is included in this work. In addition, the investigation of their electron-accepting properties (of interest for the possible development of electrically pumped lasers) has also been performed by electrochemistry.

13598 J. Phys. Chem. C, Vol. 111, No. 36, 2007 II. Experimental Methods Materials. N,N′-Di(10-nonadecyl)perylene-3,4:9,10-tetracarboxylic diimide (1),40,41 N-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic 3,4-anhydride-9,10-imide (2),40,41 and N,N′-di(2′′ethylhexyl)-1,7-dibromoperylene-3,4:9,10-tetracarboxylic diimide (3)42 were prepared following synthetic procedures reported previously. All other chemicals were purchased from Aldrich and used as received without any further purification unless otherwise specified. Column chromatography was performed on Merck Kieselgel 60 silica gel (230-240 mesh). Thin-layer chromatography was carried out on Merck silica gel F-254 flexible TLC plates. Solvents and reagents were dried by standard methods under inert gas atmosphere prior to use. Characterization. Melting points were measured with an electrothermal melting point apparatus and are uncorrected. FTIR spectra were recorded as KBr pellets in a Shimadzu FTIR 8300 spectrometer. NMR were recorded on a Bruker AC-200, Avance 300, or AMX-400 apparatus as noted, and the chemical shifts were reported relative to tetramethylsilane (TMS) at 0.0 ppm (for 1H NMR) and CDCl3 at 77.16 ppm (for 13C NMR). The splitting patterns are designated as follows: s (singlet), d (doublet), m (multiplet), and b (broad), and the assignments are Pery (for PDI) and Ph (for phenyl) in 1H NMR. Mass spectra were recorded with a Varian Saturn 2000 GC-MS and with a MALDI-TOF MS Bruker Reflex 2 (dithranol as matrix). Elemental analyses were performed on a Perkin-Elmer EA 2400. Electrochemistry. Cyclic voltammetry experiments were performed with a computer-controlled EG & G PAR 273 potentiostat in a three-electrode single-compartment cell (5 mL). The platinum working electrode consisted of a platinum wire sealed in a soft glass tube with a surface of A ) 0.785 mm2, which was polished down to 0.5 µm with Buehler polishing paste prior to use in order to obtain reproducible surfaces. The counter electrode consisted of a platinum wire, and the reference electrode was a Ag/Ag+ secondary electrode. All potentials were internally referenced to the ferrocene-ferrocinium couple. For the measurements, concentrations of 5.10-3 mol l-1 of the electroactive species were used in freshly distilled and deaerated toluene (Lichrosolv, Merck): acetonitrile (Aldrich) 4:1 and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6, Fluka), which was recrystallized from ethanol twice and dried under vacuum prior to use. N,N′-Di(2′′-ethylhexyl)-1,7-bis(4-tert-butylphenoxy)perylene3,4:9,10-Tetracarboxylic Diimide (4). A mixture of N,N′-di(2′′-ethylhexyl)-1,7-dibromoperylene-3,4:9,10-tetracarboxylic diimide (3) (2.0 g, 2.60 mmol) and cesium carbonate (1.78 g, 5.50 mmol) in 45 mL of anhydrous N,N-dimethylformamide was heated at reflux for 4 h. The reaction crude was allowed to cool down to room temperature, and an aqueous hydrochloric acid (2M) solution was added. The resulting precipitate was filtered, thoroughly washed with water and methanol, dissolved in chloroform, and purified by column chromatography (silica gel, hexane/dichloromethane 4/6) to yield N,N′-di(2′′-ethylhexyl)-1,7-bis(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic diimide (4) in 57% yield as a red solid. Mp: >300 °C (chloroform/methanol). 1H NMR (CDCl , 200 MHz) δ ) 9.53 (d, 2H, J ) 8.40 Hz, 3 Pery), 8.51 (d, 2H, J ) 8.40 Hz, Pery), 8.29 (s, 2H, Pery), 7.41 (d, 4H, J ) 8.80 Hz, Ph), 7.05 (d, 4H, J ) 8.80 Hz, Ph), 4.11-3.92 (m, 4H, N-CH2-), 1.85 (m, 2H, -CH2-), 1.31 (m, 16H, -CH2-), 0.88-0.80 (m, 30H, -CH3). 13C NMR (CDCl , 125 MHz) δ ) 163.69 (CdO), 163.36 3 (CdO), 155.48, 152.61, 148.21, 133.37, 130.14, 129.18, 128.75, 127.55, 124.97, 123.83, 123.73, 123.65, 122.12, 119.19, 44.40

Calzado et al. (N-CH2-), 38.00, 34.59, 31.51, 30.84, 28.77, 24.14, 23.10, 14.10, 10.66. FTIR (KBr, cm-1) ν ) 2959, 2930, 2860, 1701, 1659, 1597, 1570, 1504, 1408, 1332, 1263, 1176, 810. MS (ESI): 911 (M+). Anal. Calcd. for C60H66N2O6: C, 79.09%; H, 7.30%; N, 3.07%; Found: C, 79.32%; H, 7.16%; N, 3.09%. Perylenedicarboximide Benzimidazole Derivative (5). A suspension of N-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic 3,4-anhydride-9,10-imide (2) (100 mg, 0.15 mmol), 4,5-dimethyl-o-diamine (103 mg, 0.76 mmol), and a catalytic amount of zinc (II) acetate in 20 mL of propionic acid was heated at reflux for 3 h. The reaction crude was then allowed to cool down to room temperature and methanol and water were added. The purple precipitated was filtered, copiously washed with water and methanol, and dried. The solid was further purified by column chromatography (silica gel, dichoromethane/methanol 9.5/0.5) and repeatedly precipitated out of chloroform/methanol to afford perylene derivative 5 as a dark-purple solid in 67% yield. Mp: >300 °C (chloroform/methanol). 1H NMR (CDCl , 300 MHz) δ ) 8.27-8.24 (m, 2H), 7.913 7.85 (m, 5H), 7.25 (s, 1H), 7.14 (s, 1H), 6.60 (s, 1H), 5.10 (bs, 1H, N-CH), 2.15 (bs, 4H, -CH2-), 1.82-1.75 (m, 10H), 1.21 (bs, 24H), 0.81 (bs, 6H). 13C NMR (CDCl , 125 MHz) δ ) 168.32 (CdO), 168.26 3 (CdO), 166.15 (CdO), 148.42, 146.13, 140.12, 137.62, 136.00, 135.92, 133.55, 132.12, 131.23, 131.45, 130.77, 130.56, 130.16, 128.66, 127.59, 127.44, 127.16, 126.14, 126.11, 119.06, 114.82, 46.52 (N-CH), 32.51, 31.92, 29.65, 29.34, 27.14, 22.71, 20.50, 14.16, 14.13. FTIR (KBr, cm-1) ν ) 2955, 2926, 2854, 1734, 1694, 1655, 1593, 1578, 1502, 1354, 1344, 808. MS (ESI): 758 (M+). Anal. Calcd. for C51H55N3O3: C, 80.81%; H, 7.31%; N, 5.54%; Found: C, 80.78%; H, 7.16%; N, 5.52%. Sample Preparation. Samples consisted on films of an inert polymer (polystyrene, PS), doped with a PDI derivative (compounds 1, 4, and 5), and deposited over glass substrates by the spin-coating technique. Films with a varying concentration of PDI (ranging from 0.25 to 5 wt %) were obtained. The solvents used were toluene for compounds 1 and 4 and chloroform for compound 5. Film thickness was measured by means of an interferometer coupled to an optical microscope. Optical Experiments. Linear absorption spectra were obtained in a Shimazdu spectrophotometer. Standard PL spectra were obtained in a Jasco FP-6500/6600 fluorimeter. Spectra in solution were obtained by using quartz cells of 10 × 10 mm2. PL spectra of films were collected by exciting the samples at 491 nm (compound 1) and 533 nm (compounds 4 and 5) and then collecting the transmitted beam at a 45° angle to avoid the pump beam. PL quantum yields were obtained by following the comparative method of Williams et al.43 Because of the different emission ranges of the various PDIs, (N,N′-di(1hexylheptyl)perylene-3,4:9,10-tetracarboxylic diimide) (Φ ) 0.99)44 was used as the primary standard for compound 1 and cresyl violet (Φ ) 0.54)45 for derivatives 4 and 5. The experimental setup to investigate the presence of stimulated emission in these materials has been reported elsewhere.46,47 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 the PDIs. 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 of 3.5 mm. When ASE occurs in a long, narrow stripe, most of the light is emitted from the ends of the stripe. Therefore, the pump stripe was placed right up to the edge of the film where the emitted light was collected with a fiber spectrometer. III. Results and Discussion (a) Synthesis. The synthesis of N,N′-di(10-nonadecyl)-3,4,9,10-perylenetetracarboxylic diimide (1)40,41 was carried out following the classical procedure involving the bis-imidation of the 3,4:9,10-perylenetetracarboxylic dianhydride by treatment with the corresponding amine (Scheme 1). Further partial saponification of one of the imido groups yields the corresponding monoimide-monoanhydride (2) (Scheme 1).40,41 Alternatively, the nucleophilic substitution of the bromine atoms in positions 1 and 7 of the perylene core in 342 by reaction with p-tert-butylphenol in the presence of cesium carbonate as a base affords perylene derivative 4 in good yield. The introduction of the bulky p-tert-butylphenyl groups directly attached to the perylene core reduces the π-stacking ability of the perylene moieties, thus increasing its solubility.38,48,49 Moreover, additional increasing of the solubility comes from the loss of planarity promoted by the 1,7-disubstitution pattern, as has been also reported for other core-substituted perylenes.50 Condensation of 2 with 4,5-dimethyl-o-diamine affords dye 5. Although amidines are known to be sensitive to hydrolysis, they can be stabilized by its incorporation into a five-member ring.40,51 Therefore, compound 5 could be conveniently obtained in good yield by direct condensation using propionic acid as the solvent. A larger conjugated π-system is expected for 5 with respect to 1 and 4. The presence of the swallow-tail solubilizing chains on the perylene moiety in 1 and 5 and the bulky tert-butylphenoxy groups in 4 provides to these systems enough solubility to allow their full electrochemical and spectroscopic characterization. Thus, the FTIR spectra of 1 and 5 show the characteristic absorption pattern of the perylene skeleton with bands at 1580 and 1593 cm-1. Besides, the absorption bands at 1655 and 1697 cm-1 indicate the presence of the imide group. No significant bands around 1733 and 1772 cm-1 are observed, proving the disappearance of the anhydride functionality. The 1H NMR spectra in chloroform show the expected signals at low fields for the aromatic perylene protons. Alternatively, the parasubstituted phenoxy moiety gives rise to two doublets with the characteristic ortho coupling of 8.8 Hz in compound 4. The spectra are completed by the signals at higher fields for the nonyl chains and the multiplet at around 5.10 ppm for the proton directly linked to the imide nitrogen. The 13C NMR spectra shows, as the most characteristic signals, those of the carbonyl groups at around 160 ppm and the carbon atom bearing the imido nitrogen at around 45 ppm. Additional signals for the sp2 carbons are found between 140 and 120 ppm, and those of the alkyl chains, at higher fields. Finally, evidence concerning the purity of all of the new compounds is given by elemental analysis, which is in accordance with the expected values. (b) Electrochemistry. The redox properties of the electroactive PDIs 1, 4, and 5 were determined by cyclic voltammetry measurements at room temperature in toluene/acetonitrile 4:1 solutions, using a platinum disk and wire as working and counter electrodes, respectively, Ag/Ag+ as the reference electrode and tetrabutylammonium hexafluorophosphate (TBAPF6, 0.01 M) as the supporting electrolyte. The ferrocene/ferrocinium (Fc/ Fc+) couple was used as the internal reference.

Figure 1. Cyclic voltammograms of PDI derivatives 1, 4, and 5 in toluene/acetonitrile 4:1 (TBAPF6 0.1 M) at a scan rate of 200 mV s-1. Measurements were carried out at room temperature, and potentials are given vs Fc/Fc+.

Although no oxidation wave could be detected within the available solvent window, two clear quasi-reversible oneelectron reduction waves could be found, which can be attributed to the reduction of the perylenediimide moiety and show the good electron acceptor ability of these materials (Figure 1).52-55 In spite of the presence of the electron donating tertbutylphenoxy groups in PDI 4, which cause a marked effect on the electronic spectra, PDI 4 still exhibits a good electronaccepting ability (E1/21 ) -1.23 V and E1/22 ) -1.26 V), which is only very-slightly shifted toward more-negative values with respect to PDI 1 (E1/21 ) -1.22 V and E1/22 ) -1.23 V). However, and as one would expect, the more extended π-conjugated system in PDI 5 results in an enhanced electron acceptor ability; therefore, reduction waves appearing at around E1/21 ) -1.06 V and E1/22 ) -1.24 V, clearly cathodically shifted with respect to the other derivatives, are recorded for PDI 5. (c) Optical Absorption and Photoluminescence in Solution. Absorption and PL spectra for the three PDI derivatives under study (1, 4, and 5) in solution are depicted in Figure 2. Spectra were obtained in the solvent that was later used in the spincoating process for film preparation (i.e., toluene for PDI 1 and PDI 4 and chloroform for PDI 5). The experimental parameters related to singlet absorption and fluorescence of these PDIs are listed in Table 1. PDI 1. Absorbance and PL spectra of PDI 1 in toluene (Figure 2a) are similar to those reported previously in chloroform for the same compound,37 with just slight variations in the position of the peaks due to the different polarity of the solvent. The absorption spectrum shows an absorption peak at 527 nm (attributed to the perylene core π-π* transition). A characteristic vibronic fine structure in the high-energy side (two peaks at 459 and 490 nm) is observed. The PL spectrum also shows a cleanly resolved progression of vibrational peaks. Similar to other PDI derivatives substituted in the imide nitrogen positions, this progression is attributed to the symmetrical longaxis breathing mode of the perylene core, which is strongly coupled to the long-axis polarized lowest singlet transition.27 As expected for this type of PDI substituted in the imide nitrogen positions (see Table 1), a high PL quantum yield (close to 1), as well as a large extinction coefficient for the longest wavelength transition ( ) 66500 M-1cm-1), have been obtained. Both optical parameters are adequate for a good laser performance.

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Figure 2. Absorption (ABS) and photoluminescence (PL) spectra of PDI derivatives 1 (a), 4 (b), and 5 (c) in solution; Concentration: 2 × 10 -4 M for a and c and 10 -4 M for b; Solvents: toluene for 1 and 4 and chloroform for 5.

PDI 4. Absorbance and PL spectra for compound 4 in toluene are depicted in Figure 2b. A red shift of 13 nm is observed in the absorption spectrum of 4 with respect to that of 1 (the absorption maximum appears at 540 nm). The observed bathochromic shift is due to the presence of the two electron-donating p-tert-butylphenoxy groups substituted at the bay positions and can be explained on the basis of Ko¨nig’s color theory.7 The structure of PDI 4 corresponds to a doubling of an acceptorπ-donor-π-acceptor arrangement where the four carbonyl groups act as acceptors. It should also be noted that the cleanly resolved progressions of vibrational peaks observed in the

absorption spectrum of PDI 1 is nearly lost in that of PDI 4. The band broadening observed can be attributed to two reasons. One could be the increase of the conjugation between the substituents and the perylene core.39 Another could be the twisting of the PDI core by the substituents so that the vibronic structure is lost.56 As observed in Table 1, PDI 4 shows a lower a PL quantum yield (Φ ) 0.5) as well as a lower extinction coefficient for the S0 f S1 transition as compared to PDI 1. PDI 5. Figure 2c shows the absorbance and PL spectra for PDI 5 in chloroform. The direct conjugation of the PDI system with the additional benzene ring in 5 results in a bathocromically shifted absorption with respect to those of compounds 1 and 4.7 The spectrum is considerably broad, and practically no vibrational progression is observed. It consists of two peaks at 554 and 583 nm, although as contrary to compounds 1 and 4, the most intense is the high-energy one. The shape of the PL spectrum is similar to that of PDI 4, presenting a maximum at 624 nm and a shoulder at 641 nm. Although the extinction coefficients are similar to those of PDI 4, a much-lower PL quantum yield has been obtained in this case (Φ ) 0.2). (d) Optical Absorption and Photoluminescence of Thin Films. PS-doped films were obtained by the spin-coating technique, as described previously. The film thickness was around 1 µm for films doped with compounds 1 and 4, and 3 µm for films containing PDI 5. The film quality (transparency and homogeneity) was very good in all cases. These films constitute waveguides because the refractive index of the films (n ) 1.59) is larger than that of the substrate (n ) 1.52). In addition, the film thickness is larger than the cutoff thickness (hc) for the propagation of one mode (hc ) 230-250 nm at the peak of the PL emission). The absorption and PL spectra of films containing a varying concentration (between 0.25 and 5 wt %) of PDI were obtained. It should be noted that the films prepared in this way are muchmore concentrated than the solutions whose spectral properties have been described previously. For example, the absorbance (R, defined as the optical density multiplied by ln 10 and divided by sample thickness) for a 2 × 10-6 M toluene solution of PDI 1 is 0.30 cm-1, whereas a value of R ) 1151 cm-1 is obtained for a PS film doped with 0.5 wt % of the same compound. PDI 1. Absorption spectra for films doped with PDI 1 are very similar to those obtained in solution. The shape is the same, and a very-slight red shift (around 2 nm) in the spectrum is observed in the film, as compared to that of the solution (Figure 2a). As shown in Figure 3a, for the range of concentrations of PDI 1 in the film between 0.25 and 5 wt %, the shape of the spectrum remains unchanged and the absorbance at the main peak grows linearly with concentration. However, the PL spectrum changes with concentration as observed in Figure 3b, where the PL spectra for films doped with different amounts of PDI 1 are shown. For low concentrations (0.5 wt %), the PL spectrum is similar to that obtained in solution (it consists of a main band at 536 nm and a first vibrational peak at 576 nm). When the concentration is increased up to around 1.75 wt %, the relative intensity of both peaks becomes comparable. For larger concentrations, the position of the peak at 537 nm keeps

TABLE 1: Spectral Properties of PDI Derivatives 1, 4, and 5 in Solution PDI

λ1a(b)

λ2a(b)

λ3a(b)

λ(PL)1c

λ (PL)2 c

Φd

solvent

1 4 5

527(66.5) 540(47) 583(45)

490(40) 505(32) 554(50)

459(13) 469(sh) 490(sh)

537 562 624

576 569 641(sh)

0.99 ( 0.09 0.5 ( 0.1 0.2 ( 0.1

toluene toluene CHCl3

a λ1,2,3: Wavelengths (nm) of absorption peaks. b : Extinction coefficients (×103 M-1 cm-1). c λ(PL)1,2: Wavelengths (nm) of photoluminescence peaks. d Φ: PL quantum yield.



Figure 3. Absorption (a) and PL (b) spectra of PS films doped with various amounts of PDI 1.

approximately constant and its intensity decreases gradually and practically disappears at a concentration of 5 wt %. In addition, the peak at 576 nm shifts up to 595 nm. In fact, this last observation can be interpreted in two ways. First, the PL(0-1) really shifts to the red when the concentration is increased. Second, a new emission coming from an aggregated species appears and its composition with the PL(0-1) results in the observed peak that shifts between 579 and 600 nm. It has been reported that the PL spectrum of PDI 1 in the solid state presents a peak at 612 nm.37 Moreover, similar changes in the emission spectra of the perylene derivative N,N′-bis(2,5-di-tert-butylphenyl)-3,4:9,10-perylenediimide (DBPI) as a function of concentration both in solution57 and in sol-gel matrices58 have been reported previously. In DBPI, the observed spectral changes were attributed to molecular aggregation, which also affected the absorption spectra. From the results reported here with PDI 1, we also believe that the emission observed at 595 nm is due the existence of aggregates. However, as already mentioned above, the presence of aggregation does not result in any changes in the absorption spectra, at least in the range of concentrations studied. PDI 4. Some changes are observed in the absorption spectra of films doped with PDI 4 (see Figure 4a), as compared to the spectrum of PDI 4 in solution (Figure 2b). For low concentrations (0.5 wt %), the spectrum presents three peaks at 462, 507, and 557 nm that appear to be the same as those observed in the spectrum in solution, although their positions are red-shifted with respect to it. This shift is more-clearly observed in the peak at 557 nm that appeared at 540 nm in the spectrum in solution. It is also important to note that the relative intensities of the peaks are different. While in the solution, the maximum intensity took place at the lowest energy (540 nm); in the films the most intense peak is the one at 507 nm. By looking at the evolution of the spectrum with the concentration in the film, it is observed that the relative intensity of the peak at 507 nm keeps getting larger (as compared to the intensity of the lowenergy peak at 557 nm) and shifts to longer wavelengths up to


511 nm for concentrations of 2.8 wt %. These observations indicate that the increase in concentration results in a red shift of the spectrum as well as a change in the relative intensities. These spectral changes are similar to those reported for DPBI58 and are attributed to the presence of aggregation. The apparent blue shift of the low-energy peak from 557 to 552 nm is probably due to the result of the composition of the various peaks, taking into account the changes in the relative peak intensities. Figure 4b shows the PL spectra for films doped with various amounts of PDI 4. The PL spectrum of the 1 wt % doped film shows a main peak at 562 nm that is coincident with the main band observed in solution. However, the spectrum is broader than that in solution and shows a shoulder at around 600 nm. For larger concentrations, the behavior is similar to that observed in films doped with PDI 1. A new emission, probably coming from aggregates, appears at around 611 nm, that dominates the spectrum completely at concentrations of 2.8 wt %. PDI 5. Absorption spectra for PS films doped with various amounts of PDI 5 are depicted in Figure 5a. As compared to the spectrum of the solution (Figure 2c), the spectra for the films are much broader because a new transition at λ ) 623 nm appears. The film spectra present the two peaks observed in the solution (at 554 and 583 nm), but their positions blue-shift as the concentration increases. PL spectra for the same films are presented in Figure 5b. As compared to the films doped with PDIs 1 and 4, much-weaker PL signals have been obtained. Therefore, the quality of the spectra were not very good because they were deformed by the pump signal, in particular the films at the highest concentrations, for which self-absorption was also larger. (e) Amplified Spontaneous Emission (ASE). Films doped with PDIs 1, 4, and 5 were photopumped at 533 nm, as described in the experimental section, in order to identify the presence of

13602 J. Phys. Chem. C, Vol. 111, No. 36, 2007

Calzado et al.

Figure 6. PL and ASE spectra obtained from PS films doped with 0.75 wt % PDI 1 and 2.8 wt % PDI 4.


stimulated emission through the observation of a collapse of the width of the emission spectrum. As reported for many other materials, with waveguide characteristics similar to those reported here, the responsible mechanism for these observations is ASE.18,25,46,47 Gain narrowing was observed in PS films doped with PDI 1 in a range of concentrations between 0.25 wt % and 5 wt %, as well as with PDI 4 for concentrations between 1.5 and 3 wt %. Alternatively, no spectral collapse was observed in films doped with PDI 5, probably due to the fact that the PL efficiency was much lower in this case. For films doped with PDI 1, the minimum pump intensity necessary for the observation of ASE, called ASE threshold (IASEth), obtained for concentrations between 0.25 and 0.75 wt %, was around 15 kW/cm2. Above 0.75 wt %, the ASE threshold increased slowly up to concentrations of 1.5 wt %, and then more rapidly, reaching values of around 500 kW/cm2 for 3 wt % doped films. Above that concentration, the thresholds were so large that films got damaged very quickly under excitation, so a precise determination of thresholds was not possible. The narrowed emission (obtained at high pump intensity) reached a line width (measured as the full wave at half of its maximum, fwhm) of only 4 nm, considerably reduced with respect to that obtained at low pump intensity. In the case of films doped with PDI 4, much-larger ASE thresholds were obtained, the minimum value being 1500 kW/cm2 at a concentration of 2.8 wt %. ASE linewidths were also somewhat larger (7 nm) in this case. As an illustration of these results, the ASE spectra for the best-performing films with concentrations of 0.75 wt % of PDI 1 and 2.8 wt % of PDI 4 are shown in Figure 6. The corresponding PL spectra have also been included in the same figure. As observed, ASE takes place at λ ) 579 nm and λ ) 599 nm for films doped with PDI 1 and PDI 4, respectively. In both cases, the ASE wavelength corresponds to the wavelength of the 0-1 transition of the PL

spectra (see Figure 2a and 2b). It is a common belief that dyes with small Stokes shifts do not make good lasers, but that is an oversimplification inapplicable to materials like those studied here, where efficient lasing is obtained from the vibrational components. It should be noted that although the shape of the PL spectra and the peak positions change with concentration (as described in detail above), the ASE positions keep practically unchanged with concentration. This is an indication that the energy levels involved in the laser transitions are related to those of the free PDI units rather to those of the aggregates, which emit at longer wavelengths. The different lasing performances of the various PDIs under study could be simply explained in terms of the PL quantum yield. However, this assertion should be taken with caution because there are other parameters involved. In particular, the quantum yield of triplet formation and the overlap of the triplettriplet absorption (T1 f Tn) region with that of the laser emission (S1 f T0) should be taken into account. The high laser efficiency of N,N′-bis(2,6-dimethylphenyl)-3,4:9,10-perylenediimide, whose structure is similar to that of PDI 1, was justified by the fact that triplet-triplet absorption was shifted into near-coincidence with the singlet-singlet absorption band (S0 f S1), leaving the fluorescence and lasing region free from triplet absorption.27 However, PDI derivatives substituted in the core at bay positions showed triplet-triplet spectra shifted into the lasing region. This shift depended strongly on the type of substituent, so in some cases a reduction of the lasing efficiency was observed, whereas in other cases no laser emission was obtained. A detailed understanding of these phenomena in our PDI derivatives would require a characterization of triplet-triplet formation, which is out of the scope of this paper. However, by similarity with the structures of the derivatives mentioned previously, the reduction of laser efficiency in PDI 4 could be due not only to a decrease in the PL quantum yield but also to an overlap of the emission region with the triplet-triplet absorption band. Similar arguments can be used to discuss the results obtained with PDI 5. The most-important reason for the absence of stimulated emission in this material is probably its low PL quantum yield. However, triplet-triplet absorption could also be playing a role. The photostability of films doped with PDIs 1 and 4 was also studied by recording the total ASE intensity emitted as a function of the number of pump pulses (i.e., time), by keeping a constant pump intensity (see Figure 7). The presence of photodegradation is evident in the observation of a decrease of the total ASE output. The pump intensity was around 70 and 2500 kW/cm2 for films doped with 0.75 wt % PDI 1 and 2 wt % PDI 4, respectively, well above the ASE threshold in each case. As observed in Figure 7, very-good results were obtained for PDI 1. As observed, an ASE lifetime (defined as the number


Figure 7. Normalized ASE intensity vs the number of pump pulses (10 ns, 10 Hz) for PS films doped with 0.75 wt % PDI 1 and 2 wt % PDI 4. Pump intensities: 70 kW/cm2 (PDI 1) and 2500 kW/cm2 (PDI 4).

of pump pulses at which the ASE intensity decays to half of its maximum value) of 31 × 103 pump pulses (i.e., 50 min) was obtained. These results are comparable to those obtained with the commercially available perylene orange, doped into different kinds of matrices, such as organically modified silicates (ORMOSILs)29 or PMMA,34 with lifetimes of 26 × 103 and 30 × 103 shots, respectively. Alternatively, films doped with PDI 4 degraded very quickly because of the extremely large ASE thresholds, showing ASE lifetimes of only 300 shots (i.e., 30 s). To get some insight into the photodegradation mechanisms involved, we pumped our films with the maximum intensity provided by our pump laser (19 MW/cm2). With these pumping conditions, films doped with 0.75 wt % PDI 1 showed a lifetime of 1.8 × 103 pump pulses (3 min), which was reduced to 0.6 × 103 pulses (1 min) for larger concentrations. It should be noted that at these high pumping intensities some signs of mechanical damage could be observed in the sample after only 30 s, although, as mentioned previously, ASE could still be observed up to 1-3 min. This mechanical damage was also observed in

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13603 pure PS films (undoped), indicating that the matrix itself gets damaged, not being clear at this moment whether the ASE quenching is due to a degradation of the PDI, the matrix, or both. Further investigations with other types of matrices with better mechanical properties and photostabilities are currently being performed. Finally, in order to highlight the advantages, limitations, and future challenges of PDIs for laser applications, the performance of PDI 1-based films has been compared to that of various representative materials, both molecular and polymeric (see Table 2). The two parameters used for this comparison are the ASE threshold and the photostability halflife, as defined previously. Concerning the ASE threshold, in order to compare different materials it is important to give this parameter in power density (W/cm2); that is, the energy of the pump pulses should be divided by the size of the pump beam over the sample, as well as by the pulsewidth of the pump beam. Nevertheless, this can be done only when the pulse width of the pump beam is larger that the gain lifetime, so steady-state conditions are present. In most of the works reported in the literature, thresholds are given in energy/pulse (J/pulse) or energy density (J/pulse cm2). Moreover, the pulse widths of the pump beams used have been very diverse (from picoseconds to nanoseconds). As a consequence, comparison of materials measured by different groups is not straightforward. In addition, in order to compare different materials, ASE thresholds, rather than laser thresholds (with cavity), should be considered, otherwise it is not clear whether the improvement in performance is due to the material or the cavity. All of these issues have been taken into account to choose the materials to do this comparison. First of all, in order to be in steady-state conditions, only materials that have been pumped with lasers whose pulsewidths are in the nanosecond regime have been considered (gain lifetimes in organic materials are generally in the picosecond regime). Then, among all of these materials we have chosen those that show lower thresholds and that emit at wavelengths similar to those of PDIs so that they

TABLE 2: Comparison of ASE Performance of Various Materials

a Material: active material/inert matrix: Inert matrices: PS (polystyrene), PMMA (poly(methyl methacrylate)), - (No matrix). b Concentration of active material (wt %). c λASE: ASE Wavelength (nm). d Ith-ASE: pump intensity thresholds for ASE observation (kW/cm2). e t(1/2)-ASE : photostability halflife (pump pulses). f prr: pump repetition rate (Hz).

13604 J. Phys. Chem. C, Vol. 111, No. 36, 2007 can match the low-loss transmission windows of PMMA, which are of interest for polymer optical fiber-based communications. In the case of molecular materials, examples of the mostrepresentative types of materials are shown (see the structures in Table 2): a typical dye (Dipyrromethene 567, PM567),59 a semiconducting oligo-(p-phenylenevinylene) derivative (5OPV),47 and a semiconducting oligothiophene S,S-dioxide derivative (T5oCx),20 this last one showing stimulated emission in the form of neat films (nondiluted). Concerning polymers, which have always been used as neat films, two of the bestperforming derivatives (BUEH-PPV46 and F8BT60) belonging to the more intensively investigated polymer families, poly-(pphenylenevinylenes) (PPVs) and polyfluorenes, respectively, have been included. As observed in Table 2, PDI 1 shows very good photostability as compared to other materials diluted in matrices. As mentioned already, this value could be improved further by the usage of matrices other than PS as well as by linking the active material covalently to the matrix. This strategy has produced very-good results with other materials, such as dipyrromethene dyes.59 It should also be noted that there is practically no information in the literature about the photostability of polymers used for laser applications. Alternatively, with respect to ASE thresholds, films doped with 0.75 wt % PDI 1 show a threshold of 15 kW/cm2. This value is lower than those obtained with films based on other molecular materials such as PM567 or T5oCx. It is important to note that in the case of T5oCx the concentration of active material was much higher; in fact, they were neat films. This fact is also especially important when comparing PDIs with polymers, where films consist, as for T5oCx, of 100% of active material. Although the ASE threshold for PDI-doped films is 1 order of magnitude higher that those of the polymers shown in the table, this difference is not so significant if one considers the 2 orders of magnitude difference in the concentration. Thus, at present, one of the main challenges from a materials point of view would be to design materials where a larger amount of PDI can be introduced, so a decrease in the ASE threshold could be obtained, without getting ASE quenching and while keeping the photostability. The objectives of this work focused precisely on these ideas. Our results indicate that the modifications performed through routes b and c have been detrimental for laser purposes. Moreover, the detailed study of the ASE threshold and the photostability with the concentration of PDI 1 in the film and the comparison made with other materials has served to demonstrate its great potential for laser purposes, the most important advantages being (a) its photostability, which is already high even without optimization; (b) their emission wavelength, that matches the second low-loss window of PMMA; and (c) their reasonable thresholds, obtained even with a small amount of PDI 1. IV. Conclusions The effect of three different types of modifications of the chemical structure in the spectral, electrochemical, and laser properties of perylenediimides (PDIs) has been investigated, in both liquid solution and diluted in polystyrene (PS) films at various concentrations. The best laser performance was obtained for films doped with 0.75 wt % of the derivative symmetrically substituted at the N atoms (PDI 1). It showed amplified spontaneous emission at λ ) 579 nm, at a pump intensity threshold of 15 kW/cm2 and a photostability halflife of 31 × 103 pump pulses. These materials are among the most photostable reported in the literature and show very-reasonable thresholds. Moreover, these materials are particularly interesting

Calzado et al. in the field of data communications based on polymer optical fibers because they emit at wavelengths close to 570 nm, which constitutes the second low-loss transmission window in poly(methyl methacrylate). Alternatively, the laser performances of PDIs 4 and 5, substituted at bay positions and with modifications in the carboximide group, respectively, are much worse. In particular, the lack of planarity in PDI 4, which could be beneficial to prevent aggregation, has resulted in an important increase (in 2 orders of magnitude) of the ASE threshold and, consequently, a decrease in photostability. PDI 5 presents a verylow PL efficiency, so ASE was not observed. Thus, these results show that among the three structural approaches studied here, route a has provided the best results for laser applications purposes. Acknowledgment. We are thankful for support from the Spanish Government CICYT (grant MAT2005-07369-C03-1), the European Community (FEDER), the University of Alicante (grant VIGROB2005-060), and the Universidad ComplutenseComunidad de Madrid joint project PR45/05-14167. R.G. is indebted to the “Programa Ramoń y Cajal”. We also thank Vicente Esteve for technical assistance and Drs. R. Scholz and I. Vragovic for useful discussions. References and Notes (1) Herbst, W.; Hunger, K. Industrial Organic Pigments: Production, Properties, Applications, 2nd ed.; Wiley-VCH: Weinheim, 1997. (2) Sadrai, M.; Bird, G. R. Opt. Commun. 1984, 51, 62. (3) Lo¨hmannsro¨ben, H.-G.; Langhals, H. Appl. Phys. B 1989, 48, 449. (4) Langhals, H. Nachr. Chem. Tech. Lab. 1980, 28, 716. (5) Mais, S.; Tittel, J.; Bsche´, T.; Braüchle, C.; Go¨hde, W.; Fuchs, H.; Mu¨ller, G.; Mu¨llen, K. J. Phys. Chem. A 1997, 101, 8435. (6) Langhals, H. Heterocycles 1995, 40, 477. (7) Langhals, H. HelV. Chim. 2005, 88, 1309. (8) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (9) Horowitz, G. AdV. Mater. 1998, 10, 365. (10) Wu¨rthner, F. Angew. Chem. 2001, 113, 1069. (11) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (12) Strujik, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; Dijk, M.; Kimkes, P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudho¨lter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057. (13) Tessler, N. AdV. Mater. 1999, 11, 363 and references therein. (14) Duarte, F. J.; Hillman L. W. Dye Laser Principles with Applications; Academic Press: Boston, MA, 1990. (15) Hide, F.; Schwartz, B.; Dıáz-Garcıá, M. A.; Heeger, A. J. Chem. Phys. Lett. 1996, 257, 424. (16) Hide, F.; Dıáz-Garcıá, M. A.; Schwartz, B.; Andersson, M.; Pei, Q.; Heeger, A. J. Science 1996, 273, 1833. (17) Tessler, N.; Denton, G. J.; Friend, R. H. Nature 1996, 382, 695. (18) McGehee, M. D.; Heeger, A. J. AdV. Mater. 2000, 12, 1655 and references therein. (19) Samuel I. D. W.; Turnbull, G. A. Chem. ReV. 2007, 107, 1272 and references therein. (20) Pisignano, D.; Persano, L.; Visconti, P.; Cingolani, R.; Gigli, G.; Barbarella, G.; Favaretto, L. Appl. Phys. Lett. 2003, 83, 2545. (21) Lattante, S; De Giorgi M.; Barbarella, G.; Favaretto, L.; Gigli, G.; Cingolani, R.; Anni, M. J. Appl. Phys. 2006, 100, 023530. (22) Johansson, N.; Salbeck, J.; Bauer, J.; Weisso¨rtel, F.; Bro¨ms, P.; Andersson, A.; Salaneck, W. R. AdV. Mater. (Weinheim, Ger.) 1998, 10, 1136. (23) Schneider, D.; Rabe, T.; Riedl, T.; Dobbertin, T.; Werner, O.; Kro¨ger, M.; Becker, E.; Johannes, H.-H.; Kowalsky, W.; Weimann, T.; Wang, J.; Hinze, P.; Gerhard, A.; Sto¨ssel, P.; Vestweber, H. Appl. Phys. Lett. 2004, 84, 4693. (24) Dıáz-Garcıá, M. A.; De Avila, S. F.; Kuzyk, M. G. Appl. Phys. Lett. 2002, 80, 4486. (25) Calzado, E. M.; Villalvilla, J. M.; Boj, P. G.; Quintana, J. A.; DıázGarcıá, M. A. Org. Electron. 2006, 7, 319. (26) Cornil, J.; Beljonne, D.; Calbert. J.-P.; Bre´das, J.-L. AdV. Mater. 2001, 13, 1053. (27) Sadrai, M.; Hadel, L.; Sauers, R. R.; Husain, S.; Krogh-Jespersen, K.; Westbrook, J. D.; Bird, G. R. J. Phys. Chem. 1992, 96, 7988. (28) Reisfeld, R.; Seybold, G. Chimia 1990, 44, 295.

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Effect of Structural Modifications in the Spectral and Laser Properties

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