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Evidence for amplified spontaneous emission from double excimer of conjugated polymer (PDHF) in a liquid solution
Polymer 54 (2013) 2401e2405
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Evidence for amplified spontaneous emission from double excimer of conjugated polymer (PDHF) in a liquid solution K.H. Ibnaouf a, Saradh Prasad b, V. Masilamani b, *, M.S. AlSalhi b a b
Physics Department, College of Science, Al Imam Mohammad Ibn Saud Islamic University, P.O. Box 90950, Riyadh 11623, Saudi Arabia Research Chair for Laser Diagnosis of Cancer, King Saud University, Riyadh KSA, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2012 Received in revised form 3 February 2013 Accepted 26 February 2013 Available online 5 March 2013
The spectral and laser properties of conjugated polymer Poly [9, 9-di-(20 -ethylhexyl) fluorenyl-2, 7-diyl] (PDHF) in benzene had been studied and presented in this paper. PDHF was dissolved in benzene to form solutions of concentrations ranging from 12 mM to 6 mM. The absorption spectra had shown no new band when concentration increased; this indicates no dimer formation in these solutions for all concentrations mentioned above. The fluorescence spectra for the concentration 12 mM have shown two peaks at 415 nm and 435 nm, which could be attributed to monomer and excimer. At the longer wavelength side of these spectra, there was a hump at 465 nm. This hump became dominant at concentration 6.25 mM and the peak at 415 nm almost disappeared. So, this new band around 465 nm could be due to double excimer. When the laser pump power at 355 nm and concentration of above solution were suitably chosen, we observed amplified spontaneous emission (ASE) at 415 nm, 435 nm and 465 nm. These ASE peaks could arise from the monomer, excimer and double excimer states of the macromolecule. To the best of our knowledge this is perhaps the first report on ASE from double excimer of the conjugated polymer, PDHF in liquid solution. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Conjugated polymer PDHF Double excimer
1. Introduction In the past few decades, conjugated polymers have attracted much attention in science and technology [1e5]. They have found potential applications in the fields like biosensors, bioactivators, supercapacitors and molecular electronic device [3e5]. These materials have been used as an active medium in several optoelectronic devices, such as field effect transistors [6], photodiodes, light emitting diodes (LEDs) [6,7] and polymer light-emitting electrochemical cells (LECs). Many conjugated polymers have properties such as high electroluminescence efficiency, low operating voltage, good mechanical flexibility and ease of fabrication as initially demonstrated by Pei and coworkers [8]. The photo-physical properties of this kind of conjugated polymers are yet to be fully understood. Yet in recent years, conducting conjugated polymers have emerged as an attractive new gain medium for lasers and optical amplifiers that are tunable throughout the visible spectrum [9e13]. The ASE from liquid solution of the polymer poly [2methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) by optically pumped was first reported by Moses et al. [14].
* Corresponding author. E-mail address: [email protected] (V. Masilamani). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.02.047
Hendrik-Jan Brouwer et al. (1995) demonstrated the lasing performance of a novel highly efficient copolymer poly[(2,5,2,5tetraoctyl)-p-terphenyl-4,4-ylene vinylene p- phenylene vinylene] TOP-PPV. The solution was pumped with third harmonic radiation of a Nd: YAG laser. The results were compared with Coumarin 120 and Coumarin 47 in ethanol under identical experimental conditions. The efficiency of the TOP-PPV copolymer in hexane exceeded that of both coumarin dyes with more than 50%. The laser emission of the polymer dye in hexane is tunable in the wavelength region between 414 and 456 nm [15]. In 1999 Min Zheng et al. had examined the effect of the solvents, temperatures and concentrations-dependent fluorescence of MEHPPV in solution. The aliphatic and aromatic solvents have different effects on the emission spectra of MEH-PPV solutions because of the different interaction mechanisms between MEH-PPV and different solvents at excited states. With the increase of the polarity of the solvents, the emission spectra show blue and red shift at 77 K and room temperature, respectively. With the decrease of the temperature, the emission spectra of MEH-PPV in xylene showed red-shift and the fluorescence intensities increased. The concentration-dependent emission spectra remarkably changed with the increase of the concentration because of the existence of the aggregate states and the inter-chain interactions in concentrated solutions [16].
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J.Y. Park et al. (1999) were reported that the amplified spontaneous emission (ASE) from a cylindrical capillary structure comprised of MEH-PPV film deposited on the inside wall of a glass capillary tube. When pumped with a stripe-shaped pulsed laser beam focused onto the capillary edge and parallel to its axis, a collimated and circular-shaped emission emerges from the capillary. The threshold pump power for ASE is about 6 KW/cm2. In the capillary configuration, the emission is unpolarized with spectral line-width of 8 nm, centered at 638 nm. The glass capillary seems to play an important role in focusing the excitation beam, in waveguiding the emitted radiation, and in protecting the polymer from photo-oxidation [17]. G. A. Turnbull et al. (2002) have deduced that the demonstration of a compact, all-solid-state polymer laser system featuring a microchip laser as the pump source. The laser was configured as a surface-emitting, two-dimensional distributed feedback laser, based on the conjugated polymer Poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene). Pulsed, band-edge lasing was observed at 636 nm above a threshold pump energy of 4 nJ. The laser exhibited an energy slope efficiency of 6.8%, with maximum output energy of 1.12 nJ at a pump energy of 20.4 nJ. The output beam had an azimuthally polarized annular profile with a beam quality factor (M2) close to the theoretical value of the lowest-order LaguerreeGaussian and BesseleGaussian annular modes. We explain the origin of the azimuthal polarization as due to a coherent combination of the resonant fields supported by the two gratings [18]. The ASE from the excimeric state of polymer poly MEH-PPV in liquid solution was earlier reported by us [19]. In our work presented here, the spectral properties of another such conjugated polymer, PDHF in benzene with different concentrations and temperatures are described. The results show that under sufficient concentration this conjugated polymer could exhibit double excimer formation with a fluorescence peak at 465 nm. At this concentration, with sufficient pump power, the ASE with a peak at 465 nm with FWHM of 9 nm also could be obtained. The laser and the spectral properties of conjugated MEH-PPV in benzene under high concentration and pump power excitation of YAG laser (355 nm) have been studied. The results showed that the fluorescence spectra of MEH-PPV under low concentrations had two peaks; the dominant one due to monomer was around 560 nm, and the shoulder one attributed to the excimer was around 600 nm. Under higher concentrations, it was found that there was only one band around 600 nm due to the excimeric state. By increasing the concentrations of MEH-PPV, it could be seen that there was a new band around 640 nm. This band is being attributed to the double excimer. Under high power pulsed laser excitation, we observed amplified spontaneous emission (ASE) at 570 nm, 605 nm and 650 nm. These ASE peaks could arise from the monomer, excimer and double excimer states of the macromolecule respectively [20].
Fig. 1. Molecular structures of polymer of PDHF.
The third harmonic (355 nm) of an Nd: YAG laser, with a pulse width of 11 ns, was used as the excitation source. The UV laser was focused by a quartz cylindrical lens of focal length of 5 cm. This was used to do transverse excitation of the PDHF solution taken in a four-side polished quartz cell, which was kept canted to avoid feedback. See Ref. [19] for more details. At optimum values of the pump power and concentration of PDHF, we could achieve an ASE beam coming out as a cone of light. This was collected by a 1-mm entrance slit of an ICCD camera, to obtain the spectral features of the ASE.
3. Result and discussion 3.1. Excimer and double excimer state of PDHF The absorption spectra of a conjugated polymer PDHF in benzene for different concentrations from 12 mM to 6 mM were recorded. It showed that there was a well defined absorption band at 380 nm and optical density increased with increasing concentration. It is important to notice that there was only one absorption band at 380 nm for all the above the concentrations as shown in Fig. 2. Fig. 3 shows the fluorescence spectra of PDHF, for the all above concentrations mentioned earlier, obtained under excitation wavelength of 355 nm. It could be seen that the fluorescence spectrum under low concentration (12 mM) has two peaks, one at 415 nm and the other at 435 nm. By increasing the concentration to 98 mM, the two peaks became comparable. Under still higher concentration (0.2 mM), the peak at 415 nm became weak and the peak at 435 nm became dominant; at concentration greater than 0.2 mM, the band around 415 was almost absent and the peak at 435 nm got weakened, with a distinct shoulder at 465 nm. All these emission peaks could be attributed to monomeric (415 nm),
2. Experimental The polymer Poly [9,9-di-(20 -ethylhexyl)fluorenyl-2,7-diyl] (PDHF) was purchased from SigmaeAldrich and used as received. The molecular structure is given in Fig. 1. This is a macromolecule with a molecular weight of 7264. The absorption and fluorescence spectra of PDHF in benzene were studied under wide range of concentrations. The spectra for the solutions measured using a small quartz cuvette with the dimensions 1 1 4 cm with an optical path length of 1 cm. UVevis absorption spectra were taken using a Perkin Elmer spectrophotometer and the fluorescence spectra were measured on a Perkin Elmer LS55 spectrofluorometer.
Fig. 2. Absorption spectra of PDHF in benzene at different concentrations.
K.H. Ibnaouf et al. / Polymer 54 (2013) 2401e2405
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Fig. 3. Fluorescence spectra in benzene at different concentrations.
excimeric (435 nm) and double excimeric (465 nm) states of PDHF in benzene. In order to confirm this, the spectral profiles of PDHF in benzene of concentration 3.13 mM were taken under different temperatures. Fig. 4 shows the fluorescence spectrum as excited at 355 nm for four different temperatures. It is clear that at room temperature (20 C), the intensity ratio between 435 nm and 465 nm was about 1:1. But, as the temperature was increased, the intensities of both bands decreased, although the band around 465 nm fell faster. On the other hand, when the temperature was decreased, the intensities of both bands increased, although the band around 465 nm grew faster, and, at 0 C, there is just one band, which was around 465 nm and the band around 435 nm was almost absent. This is the general trend for any excimer (single or double).
Fig. 5. Laser induced fluorescence (LIF)&lified spontaneous emission (ASE) spectra of PDHF in benzene at 0.39 mM.
3.2. ASE and laser induced fluorescence (LIF) The PDHF solutions (0.39 mMe3.13 mM) in benzene were transversely excited with a UV laser at 355 nm. At low pump power excitation (0.5 mJ) the laser induced fluorescence (LIF) spectra were recorded. It could be seen (Fig. 5) that at 0.39 mM the LIF spectrum has two peaks (with the primary peak at 435 nm and another at 420 nm as shoulder). They had a full width half maximum (FWHM) of 30 nm. When the pump intensity was increased to be 10 mJ, we got dual ASE one at 418 nm and another at 435 nm with significant spectral narrowing (FWHM of 7 nm) (Fig. 5) and distinct directionality of 10mr. The peaks were attributed to the monomer and excimer states. The intensity ratio
Fig. 4. Fluorescence of PDHF in benzene at 3.13 mM for different temperatures.
Fig. 6. LIF &ASE spectra of PDHF in benzene at 0.78 mM.
between the monomer and excimer was 1.5 as shown in Fig. 5. These features were similar to our earlier report on MEH-PPV [19]. At still high concentration (0.78 mM) and pump power excitation of 0.5 mJ, the LIF showed small a hump at 465 nm, even though the
Fig. 7. LIF &ASE spectra of PDHF in benzene at 3.13 mM.
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Fig. 8. Schematic illustrating (a) monomer, (b) excimer and (c) double excimer states of PDHF in excited state.
rest of the spectrum remain unchanged (see Fig. 6). Under high power excitation of 10 mJ, the two ASE peaks became comparable and there was no ASE at 465 nm as shown in Fig. 6. At still higher concentration (3.13 mM) the LIF spectrum had partially changed as shown in Fig. 7. It could be seen that the peak at 420 nm almost disappeared and the peak at 465 nm grew up in intensity. The ASE spectrum had exhibited two peaks, one at 437 nm (due to single excimer), and the other at 465 nm, attributed to double excimer of PDHF in solution (see Fig. 7). The new ASE peak at 465 nm has a FWHM of 9 nm, compared to the LIF’s FWHM of 30 nm for the band around 465 nm. This is a strong indication of a new emitting species formed only under high concentration. At this concentration, with sufficient pump power, this new species was capable producing ASE due to high population inversion (achieved by high power excitation). We could only speculate at this stage of investigation, the following structural conformation for this kind of excimers. The macro molecule PDHF could become highly polar in the excited state with a fractional positive charge on one side and a fractional negative charge on the other (due to electronic cloud delocalization). When one excited molecule forms a transient combination with a ground state molecule, single excimer is obtained. This is common in organic molecules such as perylene [21]. If such transient species are formed in sufficient number, ASE could be generated if the gain is sufficiently high as in the case of Xe2 or Xe Cl laser. If one such excited molecule couples with two ground state molecule, one tied up to the positive end and the other to the negative end as shown in Fig. 8, a double excimer could be formed.
Again if the optical gain and population density for this species both are strong, we could expect ASE from this molecular species. Note there is no analogue anywhere for this new species in gaseous or liquid state. Yet the experimental evidence strongly suggests only this. We are in the process of working out the theoretical and more elaborate experimental models and what we report here is only a preliminary report of a new molecular species (with convincing spectral evidence). 4. Conclusion In this paper, we have been able to show evidence for the existences of double excimer from steady state fluorescence, transient LIF and ASE of this new molecular species. This paper also shows the newly hypothesized double excimer (from PDHF in benzene as solution) has high optical gain and is capable of producing ASE under pulsed laser excitation. Acknowledgment This project was supported by King Saud University (KSU), Deanship of Scientific Research, College of Science Research Center. References [1] Shirakawa H, Louis EJ, MacDiarmid AG, Chiang CK, Heeger AJ. Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x. J Chem Soc 1977;578. [2] Skotheim TA, editor. Handbook of conducting polymers, vols. I and II. New York: Marcel Dekker; 1986.
K.H. Ibnaouf et al. / Polymer 54 (2013) 2401e2405 [3] Bredas JL, Chance RR, editors. Conjugated polymeric materials: opportunities in electronics, optoelectronics, and molecular electronics. Dordrecht, Netherland: Kluwer Academic Publishers; 1990. [4] Jenekhe SA, editor. Macromolecular host-guest complexes: optical, optoelectronic, and photorefractive properties and applications. Pittsburgh: Materials Research Society; 1992. [5] Miller JS. Conducting polymersematerials of commerce. Adv Mater 1993;5:671e6. [6] Sheats JR, oniadis H, Hueschen M, Leonard W, Miller J, Moon R, et al. Organic electroluminescent devices. Science 1996;273:884. (Washington, D.C.). [7] Greenham NC, Friend RH. Semiconductor device physics of conjugate polymers, vol. 49. San Diego: Academic Press; 1996. 273, p. 884e8. [8] Pei Q, Yu G, Zhang C, Yang Y, Heeger AJ. Polymer light-emitting electrochemical cells, science. Science 1995;269(5227):1086e8. [9] Turnbull GA, Andrew P, Barnes WL, Samuel IDW. Relationship between photonic band structure and emission characteristics of a polymer distributed feedback, laser. Phys Rev B 2001;64:125122. [10] Bauer C, Giessen H, Schnabel B, Kley E-B, Schmitt C, Scherf U, et al. A surfaceemitting circular grating polymer laser. Adv Mater 2001;13:1161. [11] Holzer W, Penzkofer A, Pertsch T, Danz N, Bräuer A, Tillmann H, et al. Corrugated neat thin-film conjugated polymer distributed feedback lasers. Appl Phys B 2002;74:333e42.
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[12] Lawrence JR, Turnbull GA, Samuel IDW. Broadband optical amplifier based on a conjugated polymer. Appl Phys Lett 2002;80:3036. [13] Jenekhe SA, Osaheni JA. Excimers and exciplexes of conjugated polymers. Science 1994;265(5173):765e8. [14] Moses D. High quantum efficiency luminescence from a conducting polymer in solution: a novel polymer laser dye. Appl Phys Lett 1992;60:3215e6. [15] Brouwer Hendrik-Jan, Krasnikov Victor V, Hilberer Alain, Wildeman Jurjen, Hadziioannou George. Appl Phys Lett 19 June 1995;66(25). [16] Zheng M, Bai F, Li Y, Yu G, Zhu D. J Polym Sci A Polym Chem 1999;37(14): 2587e94. [17] Park JY, Srdanov VI, Heeger AJ, Lee CH, Park YW. Synth Metals 1999;106: 35e8. [18] Turnbull GA, Andrew P, Barnes WL, Samuel IDW. Appl Phys Lett 2003;82(No. 320). [19] Masilamani V, Ibnaouf KH, AlSalhi MS, Yassin OA. Laser properties of a conjugate polymer (MEH-PPV) in the liquid-excimeric state. Laser Phys 2007;12: 1349. [20] Ibnaouf KH, Prasad S, Masilamani M,V, AlSalhi MS, Alaamer AS. J Europ Opt Soc Rap Public 2013;8:13001. [21] Johnson PC, Offen HW. Perylene excimer fluorescence in cyclohexane. Chem Phys Lett 1972;18(2):258e60.
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Evidence for amplified spontaneous emission from double excimer of conjugated polymer (PDHF) in a liquid solution K.H. Ibnaouf a, Saradh Prasad b, V. Masilamani b, *, M.S. AlSalhi b a b
Physics Department, College of Science, Al Imam Mohammad Ibn Saud Islamic University, P.O. Box 90950, Riyadh 11623, Saudi Arabia Research Chair for Laser Diagnosis of Cancer, King Saud University, Riyadh KSA, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2012 Received in revised form 3 February 2013 Accepted 26 February 2013 Available online 5 March 2013
The spectral and laser properties of conjugated polymer Poly [9, 9-di-(20 -ethylhexyl) fluorenyl-2, 7-diyl] (PDHF) in benzene had been studied and presented in this paper. PDHF was dissolved in benzene to form solutions of concentrations ranging from 12 mM to 6 mM. The absorption spectra had shown no new band when concentration increased; this indicates no dimer formation in these solutions for all concentrations mentioned above. The fluorescence spectra for the concentration 12 mM have shown two peaks at 415 nm and 435 nm, which could be attributed to monomer and excimer. At the longer wavelength side of these spectra, there was a hump at 465 nm. This hump became dominant at concentration 6.25 mM and the peak at 415 nm almost disappeared. So, this new band around 465 nm could be due to double excimer. When the laser pump power at 355 nm and concentration of above solution were suitably chosen, we observed amplified spontaneous emission (ASE) at 415 nm, 435 nm and 465 nm. These ASE peaks could arise from the monomer, excimer and double excimer states of the macromolecule. To the best of our knowledge this is perhaps the first report on ASE from double excimer of the conjugated polymer, PDHF in liquid solution. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Conjugated polymer PDHF Double excimer
1. Introduction In the past few decades, conjugated polymers have attracted much attention in science and technology [1e5]. They have found potential applications in the fields like biosensors, bioactivators, supercapacitors and molecular electronic device [3e5]. These materials have been used as an active medium in several optoelectronic devices, such as field effect transistors [6], photodiodes, light emitting diodes (LEDs) [6,7] and polymer light-emitting electrochemical cells (LECs). Many conjugated polymers have properties such as high electroluminescence efficiency, low operating voltage, good mechanical flexibility and ease of fabrication as initially demonstrated by Pei and coworkers [8]. The photo-physical properties of this kind of conjugated polymers are yet to be fully understood. Yet in recent years, conducting conjugated polymers have emerged as an attractive new gain medium for lasers and optical amplifiers that are tunable throughout the visible spectrum [9e13]. The ASE from liquid solution of the polymer poly [2methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) by optically pumped was first reported by Moses et al. [14].
* Corresponding author. E-mail address: [email protected] (V. Masilamani). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.02.047
Hendrik-Jan Brouwer et al. (1995) demonstrated the lasing performance of a novel highly efficient copolymer poly[(2,5,2,5tetraoctyl)-p-terphenyl-4,4-ylene vinylene p- phenylene vinylene] TOP-PPV. The solution was pumped with third harmonic radiation of a Nd: YAG laser. The results were compared with Coumarin 120 and Coumarin 47 in ethanol under identical experimental conditions. The efficiency of the TOP-PPV copolymer in hexane exceeded that of both coumarin dyes with more than 50%. The laser emission of the polymer dye in hexane is tunable in the wavelength region between 414 and 456 nm [15]. In 1999 Min Zheng et al. had examined the effect of the solvents, temperatures and concentrations-dependent fluorescence of MEHPPV in solution. The aliphatic and aromatic solvents have different effects on the emission spectra of MEH-PPV solutions because of the different interaction mechanisms between MEH-PPV and different solvents at excited states. With the increase of the polarity of the solvents, the emission spectra show blue and red shift at 77 K and room temperature, respectively. With the decrease of the temperature, the emission spectra of MEH-PPV in xylene showed red-shift and the fluorescence intensities increased. The concentration-dependent emission spectra remarkably changed with the increase of the concentration because of the existence of the aggregate states and the inter-chain interactions in concentrated solutions [16].
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J.Y. Park et al. (1999) were reported that the amplified spontaneous emission (ASE) from a cylindrical capillary structure comprised of MEH-PPV film deposited on the inside wall of a glass capillary tube. When pumped with a stripe-shaped pulsed laser beam focused onto the capillary edge and parallel to its axis, a collimated and circular-shaped emission emerges from the capillary. The threshold pump power for ASE is about 6 KW/cm2. In the capillary configuration, the emission is unpolarized with spectral line-width of 8 nm, centered at 638 nm. The glass capillary seems to play an important role in focusing the excitation beam, in waveguiding the emitted radiation, and in protecting the polymer from photo-oxidation [17]. G. A. Turnbull et al. (2002) have deduced that the demonstration of a compact, all-solid-state polymer laser system featuring a microchip laser as the pump source. The laser was configured as a surface-emitting, two-dimensional distributed feedback laser, based on the conjugated polymer Poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene). Pulsed, band-edge lasing was observed at 636 nm above a threshold pump energy of 4 nJ. The laser exhibited an energy slope efficiency of 6.8%, with maximum output energy of 1.12 nJ at a pump energy of 20.4 nJ. The output beam had an azimuthally polarized annular profile with a beam quality factor (M2) close to the theoretical value of the lowest-order LaguerreeGaussian and BesseleGaussian annular modes. We explain the origin of the azimuthal polarization as due to a coherent combination of the resonant fields supported by the two gratings [18]. The ASE from the excimeric state of polymer poly MEH-PPV in liquid solution was earlier reported by us [19]. In our work presented here, the spectral properties of another such conjugated polymer, PDHF in benzene with different concentrations and temperatures are described. The results show that under sufficient concentration this conjugated polymer could exhibit double excimer formation with a fluorescence peak at 465 nm. At this concentration, with sufficient pump power, the ASE with a peak at 465 nm with FWHM of 9 nm also could be obtained. The laser and the spectral properties of conjugated MEH-PPV in benzene under high concentration and pump power excitation of YAG laser (355 nm) have been studied. The results showed that the fluorescence spectra of MEH-PPV under low concentrations had two peaks; the dominant one due to monomer was around 560 nm, and the shoulder one attributed to the excimer was around 600 nm. Under higher concentrations, it was found that there was only one band around 600 nm due to the excimeric state. By increasing the concentrations of MEH-PPV, it could be seen that there was a new band around 640 nm. This band is being attributed to the double excimer. Under high power pulsed laser excitation, we observed amplified spontaneous emission (ASE) at 570 nm, 605 nm and 650 nm. These ASE peaks could arise from the monomer, excimer and double excimer states of the macromolecule respectively [20].
Fig. 1. Molecular structures of polymer of PDHF.
The third harmonic (355 nm) of an Nd: YAG laser, with a pulse width of 11 ns, was used as the excitation source. The UV laser was focused by a quartz cylindrical lens of focal length of 5 cm. This was used to do transverse excitation of the PDHF solution taken in a four-side polished quartz cell, which was kept canted to avoid feedback. See Ref. [19] for more details. At optimum values of the pump power and concentration of PDHF, we could achieve an ASE beam coming out as a cone of light. This was collected by a 1-mm entrance slit of an ICCD camera, to obtain the spectral features of the ASE.
3. Result and discussion 3.1. Excimer and double excimer state of PDHF The absorption spectra of a conjugated polymer PDHF in benzene for different concentrations from 12 mM to 6 mM were recorded. It showed that there was a well defined absorption band at 380 nm and optical density increased with increasing concentration. It is important to notice that there was only one absorption band at 380 nm for all the above the concentrations as shown in Fig. 2. Fig. 3 shows the fluorescence spectra of PDHF, for the all above concentrations mentioned earlier, obtained under excitation wavelength of 355 nm. It could be seen that the fluorescence spectrum under low concentration (12 mM) has two peaks, one at 415 nm and the other at 435 nm. By increasing the concentration to 98 mM, the two peaks became comparable. Under still higher concentration (0.2 mM), the peak at 415 nm became weak and the peak at 435 nm became dominant; at concentration greater than 0.2 mM, the band around 415 was almost absent and the peak at 435 nm got weakened, with a distinct shoulder at 465 nm. All these emission peaks could be attributed to monomeric (415 nm),
2. Experimental The polymer Poly [9,9-di-(20 -ethylhexyl)fluorenyl-2,7-diyl] (PDHF) was purchased from SigmaeAldrich and used as received. The molecular structure is given in Fig. 1. This is a macromolecule with a molecular weight of 7264. The absorption and fluorescence spectra of PDHF in benzene were studied under wide range of concentrations. The spectra for the solutions measured using a small quartz cuvette with the dimensions 1 1 4 cm with an optical path length of 1 cm. UVevis absorption spectra were taken using a Perkin Elmer spectrophotometer and the fluorescence spectra were measured on a Perkin Elmer LS55 spectrofluorometer.
Fig. 2. Absorption spectra of PDHF in benzene at different concentrations.
K.H. Ibnaouf et al. / Polymer 54 (2013) 2401e2405
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Fig. 3. Fluorescence spectra in benzene at different concentrations.
excimeric (435 nm) and double excimeric (465 nm) states of PDHF in benzene. In order to confirm this, the spectral profiles of PDHF in benzene of concentration 3.13 mM were taken under different temperatures. Fig. 4 shows the fluorescence spectrum as excited at 355 nm for four different temperatures. It is clear that at room temperature (20 C), the intensity ratio between 435 nm and 465 nm was about 1:1. But, as the temperature was increased, the intensities of both bands decreased, although the band around 465 nm fell faster. On the other hand, when the temperature was decreased, the intensities of both bands increased, although the band around 465 nm grew faster, and, at 0 C, there is just one band, which was around 465 nm and the band around 435 nm was almost absent. This is the general trend for any excimer (single or double).
Fig. 5. Laser induced fluorescence (LIF)&lified spontaneous emission (ASE) spectra of PDHF in benzene at 0.39 mM.
3.2. ASE and laser induced fluorescence (LIF) The PDHF solutions (0.39 mMe3.13 mM) in benzene were transversely excited with a UV laser at 355 nm. At low pump power excitation (0.5 mJ) the laser induced fluorescence (LIF) spectra were recorded. It could be seen (Fig. 5) that at 0.39 mM the LIF spectrum has two peaks (with the primary peak at 435 nm and another at 420 nm as shoulder). They had a full width half maximum (FWHM) of 30 nm. When the pump intensity was increased to be 10 mJ, we got dual ASE one at 418 nm and another at 435 nm with significant spectral narrowing (FWHM of 7 nm) (Fig. 5) and distinct directionality of 10mr. The peaks were attributed to the monomer and excimer states. The intensity ratio
Fig. 4. Fluorescence of PDHF in benzene at 3.13 mM for different temperatures.
Fig. 6. LIF &ASE spectra of PDHF in benzene at 0.78 mM.
between the monomer and excimer was 1.5 as shown in Fig. 5. These features were similar to our earlier report on MEH-PPV [19]. At still high concentration (0.78 mM) and pump power excitation of 0.5 mJ, the LIF showed small a hump at 465 nm, even though the
Fig. 7. LIF &ASE spectra of PDHF in benzene at 3.13 mM.
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Fig. 8. Schematic illustrating (a) monomer, (b) excimer and (c) double excimer states of PDHF in excited state.
rest of the spectrum remain unchanged (see Fig. 6). Under high power excitation of 10 mJ, the two ASE peaks became comparable and there was no ASE at 465 nm as shown in Fig. 6. At still higher concentration (3.13 mM) the LIF spectrum had partially changed as shown in Fig. 7. It could be seen that the peak at 420 nm almost disappeared and the peak at 465 nm grew up in intensity. The ASE spectrum had exhibited two peaks, one at 437 nm (due to single excimer), and the other at 465 nm, attributed to double excimer of PDHF in solution (see Fig. 7). The new ASE peak at 465 nm has a FWHM of 9 nm, compared to the LIF’s FWHM of 30 nm for the band around 465 nm. This is a strong indication of a new emitting species formed only under high concentration. At this concentration, with sufficient pump power, this new species was capable producing ASE due to high population inversion (achieved by high power excitation). We could only speculate at this stage of investigation, the following structural conformation for this kind of excimers. The macro molecule PDHF could become highly polar in the excited state with a fractional positive charge on one side and a fractional negative charge on the other (due to electronic cloud delocalization). When one excited molecule forms a transient combination with a ground state molecule, single excimer is obtained. This is common in organic molecules such as perylene [21]. If such transient species are formed in sufficient number, ASE could be generated if the gain is sufficiently high as in the case of Xe2 or Xe Cl laser. If one such excited molecule couples with two ground state molecule, one tied up to the positive end and the other to the negative end as shown in Fig. 8, a double excimer could be formed.
Again if the optical gain and population density for this species both are strong, we could expect ASE from this molecular species. Note there is no analogue anywhere for this new species in gaseous or liquid state. Yet the experimental evidence strongly suggests only this. We are in the process of working out the theoretical and more elaborate experimental models and what we report here is only a preliminary report of a new molecular species (with convincing spectral evidence). 4. Conclusion In this paper, we have been able to show evidence for the existences of double excimer from steady state fluorescence, transient LIF and ASE of this new molecular species. This paper also shows the newly hypothesized double excimer (from PDHF in benzene as solution) has high optical gain and is capable of producing ASE under pulsed laser excitation. Acknowledgment This project was supported by King Saud University (KSU), Deanship of Scientific Research, College of Science Research Center. References [1] Shirakawa H, Louis EJ, MacDiarmid AG, Chiang CK, Heeger AJ. Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x. J Chem Soc 1977;578. [2] Skotheim TA, editor. Handbook of conducting polymers, vols. I and II. New York: Marcel Dekker; 1986.
K.H. Ibnaouf et al. / Polymer 54 (2013) 2401e2405 [3] Bredas JL, Chance RR, editors. Conjugated polymeric materials: opportunities in electronics, optoelectronics, and molecular electronics. Dordrecht, Netherland: Kluwer Academic Publishers; 1990. [4] Jenekhe SA, editor. Macromolecular host-guest complexes: optical, optoelectronic, and photorefractive properties and applications. Pittsburgh: Materials Research Society; 1992. [5] Miller JS. Conducting polymersematerials of commerce. Adv Mater 1993;5:671e6. [6] Sheats JR, oniadis H, Hueschen M, Leonard W, Miller J, Moon R, et al. Organic electroluminescent devices. Science 1996;273:884. (Washington, D.C.). [7] Greenham NC, Friend RH. Semiconductor device physics of conjugate polymers, vol. 49. San Diego: Academic Press; 1996. 273, p. 884e8. [8] Pei Q, Yu G, Zhang C, Yang Y, Heeger AJ. Polymer light-emitting electrochemical cells, science. Science 1995;269(5227):1086e8. [9] Turnbull GA, Andrew P, Barnes WL, Samuel IDW. Relationship between photonic band structure and emission characteristics of a polymer distributed feedback, laser. Phys Rev B 2001;64:125122. [10] Bauer C, Giessen H, Schnabel B, Kley E-B, Schmitt C, Scherf U, et al. A surfaceemitting circular grating polymer laser. Adv Mater 2001;13:1161. [11] Holzer W, Penzkofer A, Pertsch T, Danz N, Bräuer A, Tillmann H, et al. Corrugated neat thin-film conjugated polymer distributed feedback lasers. Appl Phys B 2002;74:333e42.
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[12] Lawrence JR, Turnbull GA, Samuel IDW. Broadband optical amplifier based on a conjugated polymer. Appl Phys Lett 2002;80:3036. [13] Jenekhe SA, Osaheni JA. Excimers and exciplexes of conjugated polymers. Science 1994;265(5173):765e8. [14] Moses D. High quantum efficiency luminescence from a conducting polymer in solution: a novel polymer laser dye. Appl Phys Lett 1992;60:3215e6. [15] Brouwer Hendrik-Jan, Krasnikov Victor V, Hilberer Alain, Wildeman Jurjen, Hadziioannou George. Appl Phys Lett 19 June 1995;66(25). [16] Zheng M, Bai F, Li Y, Yu G, Zhu D. J Polym Sci A Polym Chem 1999;37(14): 2587e94. [17] Park JY, Srdanov VI, Heeger AJ, Lee CH, Park YW. Synth Metals 1999;106: 35e8. [18] Turnbull GA, Andrew P, Barnes WL, Samuel IDW. Appl Phys Lett 2003;82(No. 320). [19] Masilamani V, Ibnaouf KH, AlSalhi MS, Yassin OA. Laser properties of a conjugate polymer (MEH-PPV) in the liquid-excimeric state. Laser Phys 2007;12: 1349. [20] Ibnaouf KH, Prasad S, Masilamani M,V, AlSalhi MS, Alaamer AS. J Europ Opt Soc Rap Public 2013;8:13001. [21] Johnson PC, Offen HW. Perylene excimer fluorescence in cyclohexane. Chem Phys Lett 1972;18(2):258e60.