- Email: [email protected]
Temperature-dependent photoluminescence of polyazomethine films
Journal of Luminescence 132 (2012) 2098–2101
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Temperature-dependent photoluminescence of polyazomethine films Jan Cisowski a,n, Zbigniew Mazurak b, Jan Weszka b, Barbara Hajduk b a b
Institute of Physics, Cracow University of Technology, ul. Podchorazych 1, 30-084 Cracow, Poland Center of Polymer and Carbon Materials, Polish Academy of Sciences, ul. M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2011 Received in revised form 19 March 2012 Accepted 27 March 2012 Available online 4 April 2012
We report absorption and first reliable photoluminescence (PL) studies at various temperatures on relatively thick films of the basic polyazomethine — PPI, i.e., poly(1,4-phenylene-methylidynenitrilo1,4-phenylenenitrilomethylidine), prepared by chemical vapor deposition (CVD). Both absorption and PL spectra exhibit the vibronic progression due to the C–C stretching mode, characteristic for conjugated polymers. The absorption spectra appear to be practically temperature independent, in contrast to PL spectra, the intensity of which strongly decreases with increasing temperature. The origin of generally weak photoluminescence of PPI is suggested to be the result of a non-radiative electronic state occupied by the lone electron pair on the nitrogen orbital. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polyazomethine Absorption Photoluminescence Vibronic progression
1. Introduction
2. Experimental
Aromatic polyazomethines, being conjugated polymers with the extended p system, have alternately CH ¼N group and benzene ring in the main chain. Since many years, this polymer family has been extensively studied due to potential optoelectronic applications [1–4]. The basic polyazomethine is poly(1,4phenylene-methylidynenitrilo-1,4-phenylenenitrilomethylidine), abbreviated as PPI. Its backbone is built up of alternately repeating p-phenylene rings and azomethine linkages resembling poly(p-phenylene-vinylene) (PPV) in which one CH group is replaced by N atom. Moreover, the absorption spectra of thin films of both polymers are quite similar, allowing one to treat PPI as an isoelectronic counterpart of PPV [5]. However, in contrast to the absorption spectra, photoluminescence (PL) of both polymers is completely different. PPV is well known as a strong luminescent material, while PPI is signalized to be very weakly luminescent; namely, a 50 nm PPI thin film is reported to exhibit an emission peak near 550 nm, the intensity of which (the PL spectrum is not shown) is only about 0.2% of that obtained for a similar PPV film [6,7]. Bearing in mind the above difficulties with observation of PL spectrum in very thin films of PPI, we have prepared thicker films for which we have performed the complex optical studies that have allowed us to characterize the vibronic structure observed in both the absorption and PL spectra as a function of temperature.
The PPI films have been prepared by chemical vapor deposition (CVD) method via the polycondensation process of paraphenylene diamine (PPDA) and terephtal aldehyde (TPA) with pure argon as a transport agent [8]. The temperatures of PPDA, TPA and the argon flow rates, applied for preparing three investigated PPI films on fused silica substrates, were the same and equal to 60 1C, 70 1C and 50 Pa dm3/s, respectively. The only difference in preparation was the deposition time equal to 0.5, 3 and 10 min, resulting in increasing thickness of three investigated samples Z1, Z2 and Z3 equal to 0.3, 0.5 and 1.5 mm, respectively, as estimated with an interference microscope. The luminescence spectra in the temperature range 77–300 K were measured by a Fluorolog-3.12 fluorescence spectrophotometer equipped with a home-made liquid nitrogen cryostat. The absorption spectra at various temperatures were measured by a Cary-Varian (15–150 K) and a JASCO V-570 (300 K) double-beam spectrophotometers.
n
Corresponding author. Tel.: þ48126370666; fax: þ 48126371446. E-mail address: [email protected] (J. Cisowski).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.03.073
3. Results and discussion For optical characterization in a wide range of the photon energy, the thinnest PPI sample Z1 (300 nm) has been chosen and the absorption spectra obtained at various temperatures are shown in Fig. 1. It appears that the absorption spectra from Fig. 1 (as well as those measured at intermediate temperatures) practically coincide and, moreover, they are very similar to those measured previously at 77 K [6,7] and at room temperature [1,3,5,8]; this
J. Cisowski et al. / Journal of Luminescence 132 (2012) 2098–2101
2099
Fig. 1. Absorbance of a PPI film Z1 at selected temperatures. Inset shows the chemical structure of PPI.
Fig. 3. PL spectra of a PPI film Z3 at selected temperatures. Position of the highest peak at 77 K is marked by a vertical dashed line.
Fig. 2. PL spectra of PPI films with various thicknesses at 300 K.
means that the absorption spectrum of PPI practically does not depend on temperature. Fig. 1 reveals also a clear vibronic structure of the broad p–pn absorption band peaking at about 3 eV. As has been shown [5], the vibronic progressions of this band are related to stretching vibration modes of the phenylene ring and the absorption maximum determines displacement between minima of the potential energy of the electronic ground and excited states. The PL spectra of three investigated PPI samples recorded at 300 K are shown in Fig. 2. At first glance, the PL spectrum consists of two vibronic peaks, a weaker one at 2.2 eV and a stronger one at 2.35 eV which may be compared with a very weak peak at 2.25 eV signalized in [6,7]. As seen from Fig. 2, the general shape of the spectra is similar for all the samples studied with the PL intensity increasing with the film thickness. Therefore, for further temperature studies of PL, we have chosen the thickest sample Z3 (1.5 mm) yielding the highest signal. The representative PL spectra for sample Z3 at different temperatures are shown in Fig. 3. With decreasing temperature, apart from two main peaks seen already at 300 K (Fig. 2), two further features emerge, namely, a shoulder at about 2 eV and a peak at about 2.47 eV.
Fig. 4. PL spectrum (sample Z3— left side) and absorbance (sample Z1 — right side) of PPI films at 100 K with solid curves representing the experimental data. The contributions of individual Gaussians and their sum to the PL spectrum are shown as dashed-and dotted lines, respectively; the negative numerical second derivative (NNSD) of the absorption spectrum is shown as a dot-dashed line.
In contrast to the absorption spectrum (Fig. 1), the PL spectrum of PPI strongly depends on temperature; intensity of PL quickly decreases with increasing temperature and the individual peaks appear to be blue-shifted. In particular, the strongest peak located at 2.31 eV at 77 K (dashed line in Fig. 3) shifts to about 2.35 eV at 300 K and, simultaneously, its height decreases by a factor of 16. Comparison of the p–pn absorption band and PL spectrum of PPI at the same temperature (100 K) is demonstrated in Fig. 4. In order to determine more precisely positions of the vibronic peaks in the p–pn absorption spectrum, we have calculated its
2100
J. Cisowski et al. / Journal of Luminescence 132 (2012) 2098–2101
negative numerical second derivative (NNSD) [9], represented by a dot-dashed line in Fig. 4. The NNSD peaks, labeled as 0–0, 0–1 and 0–2, are located at 2.60, 2.81 and 3.03 eV, respectively. These peak positions, with an energy separation of about 0.2 eV, appear to be very similar to those reported previously for a 50 nm PPI film; therefore, the observed vibrational features of PPI absorption spectrum may be ascribed to the coupling of the phenyl ringstretch modes to the excited-state geometry, similarly as for PPV [5,7,10,11]. Thus, the absorption spectrum of PPI thin films reveals features indicating that the dominant contribution comes rather from the phenylene ring electronic structure than that of the azomethine unit. The selection rules of absorption in PPI are seen to be relaxed with respect to the benzene ring, so that the 0–0 transition is observed and its weak strength should be related to a small overlap of the wave functions of the ground and electronic excited states. In contrast, this overlap is important in the case of higher vibronic states of the electronic excited state. Similarly, PL spectra reveal the strength distribution indicating the importance of minima mismatch of potential energies of the electronic ground and excited states. However, in contrast to the absorption spectra, one should bear in mind that PL spectra reveal decay of excitons towards PPI polymer segments with the smallest gap. As for photoluminescence, the total PL signal as a function of the photon energy E has been modeled as a sum of four normalized Gaussians with positions Ej, broadenings sj and areas Aj IPL ðEÞ ¼
4 X
"
Aj 1 EEj pffiffiffiffiffiffi exp 2 sj
j ¼ 1 sj 2p
2 # ð1Þ
Three from four individual Gaussians appear to be equally spaced with an energy separation of about 0.15 eV. These peaks, labeled as 0–0, 0–1 and 0–2 in Fig. 4, are located at 2.47, 2.33 and 2.18 eV, and they are due to one type of C–C double bond vibrations, characteristic for conjugated polymers [10]; an additional vibronic peak at 2.07 eV may represent another phonon mode coupled to the electronic transition. The Stokes shift, being a difference between the 0–0 emission and absorption lines, is equal to 0.13 eV, similarly as for ‘standard’ PPV [7,10]. However, various defects of polymer films account for the measured Stokes shift [12] that may approach even zero, as observed in ‘improved’ PPV [11]. As seen in Figs. 3 and 4, the 0–0 PL peak is smaller than the 0–1 peak and quickly decreases with increasing temperature. This effect is thought to be a result of a small overlap of the wave functions of the electronic ground and excited states that, in turn, is due to different geometries of conjugated segments of polymer chains in the two states. On the other hand, one has also to take into account the fact that the 0–0 emission peak is highly sensitive to the polymer film imperfections that quench the 0–0 emission, while only slightly affecting the rest of the vibronic progression [12,13]; additionally, an overlap between the absorption and PL spectra (see Fig. 4) results in self-absorption increasing with temperature due to a blue-shift of the 0–0 emission peak (the 0–0 absorption peak is unaffected by temperature, see Fig. 1). Therefore, in order to characterize quantitatively the variation of PL intensity with temperature, we have chosen the 0-1 and 0–2 emission peaks and the temperature dependence of the integrated intensity of these peaks has been fitted to the expression [14] A0 APL ðTÞ ¼ 1 þ a exp ðe=kTÞ
ð2Þ
where A0 is the integrated PL intensity at 0 K, a is a fit parameter and e is the characteristic activation energy for multiphonon processes; the result of the fitting is shown in Fig. 5.
Fig. 5. Temperature dependence of the integrated intensity of the 0–1 and 0–2 emission peaks for a PPI sample Z3. Symbols represent the values of the Gaussian areas obtained from modeling PL spectra at various temperatures with Eq. (1) and solid lines — fitting to Eq. (2).
The decrease in the PL intensity with increasing temperature for the strongest 0–1 peak is very significant, with APL(300 K)/ APL(77 K) E0.1. The fit parameter is a ¼45715 for the 0–1 peak, a ¼973 for the 0–2 peak, and the activation energy of transitions (fitted simultaneously for both peaks) is e ¼3775 meV, corresponding to the potential barrier for thermally excited electrons to decay nonradiatively due to a multiphonon process through vibrational levels of the ground electronic state. As mentioned in Introduction, a 50 nm PPI thin film has been signalized to be very weakly luminescent [6,7] as compared to a similar film of PPV, in spite of the fact that both polymers are isoelectronic [5]. We have succeeded in a clear observation of PL for PPI by preparing much thicker films that, however, are not suitable for application as, for example, an electroluminescent material. In our opinion, the origin of weak PL of PPI is due to the presence of the lone electron pair (LEP) on the nitrogen orbital, the energy of which is close to the highest occupied molecular orbital (HOMO) state. Additionally, the same symmetry of LEP and HOMO states may be a factor enforcing rather non-radiative emission processes than a radiative decay to the ground state [5].
4. Conclusion By preparing relatively thick films of PPI, we have succeeded in the first reliable observation of photoluminescence in this polymer clearly showing the vibronic progression. We also suggest that generally weak photoluminescence of PPI is a result of a nonradiative electronic state occupied by the lone electron pair on the nitrogen orbital. Therefore, there is a strong indication that presence of the nitrogen atom in the main polymer chain plays the crucial role in attenuation of photoluminescence in polymers.
Acknowledgements The authors are very indebted to Dr. R. Lisiecki (Institute of Low Temperature and Structure Research, Wroclaw, Poland) and Dr. B. Jarzabek (Center of Polymer and Carbon Materials, Zabrze, Poland) for meaasurements of the absorption spectra at various temperatures.
J. Cisowski et al. / Journal of Luminescence 132 (2012) 2098–2101
References [1] [2] [3] [4] [5] [6] [7]
C.Y. Yang, S.A. Jenekhe, Chem. Mater. 3 (1991) 878. A. Kubono, N. Okui, Prog. Polym. Sci. 19 (1994) 389. C.Y. Yang, S.A. Jenekhe, Macromolecules 28 (1995) 1180. A. Iwan, D. Sek, Prog. Polym. Sci. 33 (2008) 289. J. Weszka, H. Bednarski, M. Domanski, J. Chem. Phys. 131 (2009) 024901. M.S. Weaver, D.D.C. Bradley, Synth. Met. 83 (1996) 61. F. Rohlfing, D.D.C. Bradley, Chem. Phys. 227 (1998) 133.
2101
[8] B. Jarzabek, J. Weszka, M. Domanski, J. Jurusik, J. Cisowski, J. Non-Cryst. Solids 352 (2006) 1660. [9] T.W. Hagler, K. Pakbaz, K.F. Voss, A.J. Heeger, Phys. Rev. 44 (1991) 8652. [10] S.-H. Lim, T.G. Bjorklund, C.J. Bardeen, Chem. Phys. Lett. 342 (2001) 555. [11] K. Pichler, D.A. Halliday, D.D.C. Bradley, P.L. Burn, R.H. Friend, A.B. Holmes, J. Phys. Condens. Matter 5 (1993) 7155. [12] F.C. Spano, Phys. Rev. B 116 (2002) 5877. [13] F. Balzer, A. Pogantsch, H.-H. Rubahn, J. Lumin. 129 (2009) 784. [14] W. Chen, F. Su, G. Li, A.G. Joly, J.-O. Malm, J.-O. Bovin, J. Appl. Phys. 92 (2002) 1950.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Temperature-dependent photoluminescence of polyazomethine films Jan Cisowski a,n, Zbigniew Mazurak b, Jan Weszka b, Barbara Hajduk b a b
Institute of Physics, Cracow University of Technology, ul. Podchorazych 1, 30-084 Cracow, Poland Center of Polymer and Carbon Materials, Polish Academy of Sciences, ul. M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2011 Received in revised form 19 March 2012 Accepted 27 March 2012 Available online 4 April 2012
We report absorption and first reliable photoluminescence (PL) studies at various temperatures on relatively thick films of the basic polyazomethine — PPI, i.e., poly(1,4-phenylene-methylidynenitrilo1,4-phenylenenitrilomethylidine), prepared by chemical vapor deposition (CVD). Both absorption and PL spectra exhibit the vibronic progression due to the C–C stretching mode, characteristic for conjugated polymers. The absorption spectra appear to be practically temperature independent, in contrast to PL spectra, the intensity of which strongly decreases with increasing temperature. The origin of generally weak photoluminescence of PPI is suggested to be the result of a non-radiative electronic state occupied by the lone electron pair on the nitrogen orbital. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polyazomethine Absorption Photoluminescence Vibronic progression
1. Introduction
2. Experimental
Aromatic polyazomethines, being conjugated polymers with the extended p system, have alternately CH ¼N group and benzene ring in the main chain. Since many years, this polymer family has been extensively studied due to potential optoelectronic applications [1–4]. The basic polyazomethine is poly(1,4phenylene-methylidynenitrilo-1,4-phenylenenitrilomethylidine), abbreviated as PPI. Its backbone is built up of alternately repeating p-phenylene rings and azomethine linkages resembling poly(p-phenylene-vinylene) (PPV) in which one CH group is replaced by N atom. Moreover, the absorption spectra of thin films of both polymers are quite similar, allowing one to treat PPI as an isoelectronic counterpart of PPV [5]. However, in contrast to the absorption spectra, photoluminescence (PL) of both polymers is completely different. PPV is well known as a strong luminescent material, while PPI is signalized to be very weakly luminescent; namely, a 50 nm PPI thin film is reported to exhibit an emission peak near 550 nm, the intensity of which (the PL spectrum is not shown) is only about 0.2% of that obtained for a similar PPV film [6,7]. Bearing in mind the above difficulties with observation of PL spectrum in very thin films of PPI, we have prepared thicker films for which we have performed the complex optical studies that have allowed us to characterize the vibronic structure observed in both the absorption and PL spectra as a function of temperature.
The PPI films have been prepared by chemical vapor deposition (CVD) method via the polycondensation process of paraphenylene diamine (PPDA) and terephtal aldehyde (TPA) with pure argon as a transport agent [8]. The temperatures of PPDA, TPA and the argon flow rates, applied for preparing three investigated PPI films on fused silica substrates, were the same and equal to 60 1C, 70 1C and 50 Pa dm3/s, respectively. The only difference in preparation was the deposition time equal to 0.5, 3 and 10 min, resulting in increasing thickness of three investigated samples Z1, Z2 and Z3 equal to 0.3, 0.5 and 1.5 mm, respectively, as estimated with an interference microscope. The luminescence spectra in the temperature range 77–300 K were measured by a Fluorolog-3.12 fluorescence spectrophotometer equipped with a home-made liquid nitrogen cryostat. The absorption spectra at various temperatures were measured by a Cary-Varian (15–150 K) and a JASCO V-570 (300 K) double-beam spectrophotometers.
n
Corresponding author. Tel.: þ48126370666; fax: þ 48126371446. E-mail address: [email protected] (J. Cisowski).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.03.073
3. Results and discussion For optical characterization in a wide range of the photon energy, the thinnest PPI sample Z1 (300 nm) has been chosen and the absorption spectra obtained at various temperatures are shown in Fig. 1. It appears that the absorption spectra from Fig. 1 (as well as those measured at intermediate temperatures) practically coincide and, moreover, they are very similar to those measured previously at 77 K [6,7] and at room temperature [1,3,5,8]; this
J. Cisowski et al. / Journal of Luminescence 132 (2012) 2098–2101
2099
Fig. 1. Absorbance of a PPI film Z1 at selected temperatures. Inset shows the chemical structure of PPI.
Fig. 3. PL spectra of a PPI film Z3 at selected temperatures. Position of the highest peak at 77 K is marked by a vertical dashed line.
Fig. 2. PL spectra of PPI films with various thicknesses at 300 K.
means that the absorption spectrum of PPI practically does not depend on temperature. Fig. 1 reveals also a clear vibronic structure of the broad p–pn absorption band peaking at about 3 eV. As has been shown [5], the vibronic progressions of this band are related to stretching vibration modes of the phenylene ring and the absorption maximum determines displacement between minima of the potential energy of the electronic ground and excited states. The PL spectra of three investigated PPI samples recorded at 300 K are shown in Fig. 2. At first glance, the PL spectrum consists of two vibronic peaks, a weaker one at 2.2 eV and a stronger one at 2.35 eV which may be compared with a very weak peak at 2.25 eV signalized in [6,7]. As seen from Fig. 2, the general shape of the spectra is similar for all the samples studied with the PL intensity increasing with the film thickness. Therefore, for further temperature studies of PL, we have chosen the thickest sample Z3 (1.5 mm) yielding the highest signal. The representative PL spectra for sample Z3 at different temperatures are shown in Fig. 3. With decreasing temperature, apart from two main peaks seen already at 300 K (Fig. 2), two further features emerge, namely, a shoulder at about 2 eV and a peak at about 2.47 eV.
Fig. 4. PL spectrum (sample Z3— left side) and absorbance (sample Z1 — right side) of PPI films at 100 K with solid curves representing the experimental data. The contributions of individual Gaussians and their sum to the PL spectrum are shown as dashed-and dotted lines, respectively; the negative numerical second derivative (NNSD) of the absorption spectrum is shown as a dot-dashed line.
In contrast to the absorption spectrum (Fig. 1), the PL spectrum of PPI strongly depends on temperature; intensity of PL quickly decreases with increasing temperature and the individual peaks appear to be blue-shifted. In particular, the strongest peak located at 2.31 eV at 77 K (dashed line in Fig. 3) shifts to about 2.35 eV at 300 K and, simultaneously, its height decreases by a factor of 16. Comparison of the p–pn absorption band and PL spectrum of PPI at the same temperature (100 K) is demonstrated in Fig. 4. In order to determine more precisely positions of the vibronic peaks in the p–pn absorption spectrum, we have calculated its
2100
J. Cisowski et al. / Journal of Luminescence 132 (2012) 2098–2101
negative numerical second derivative (NNSD) [9], represented by a dot-dashed line in Fig. 4. The NNSD peaks, labeled as 0–0, 0–1 and 0–2, are located at 2.60, 2.81 and 3.03 eV, respectively. These peak positions, with an energy separation of about 0.2 eV, appear to be very similar to those reported previously for a 50 nm PPI film; therefore, the observed vibrational features of PPI absorption spectrum may be ascribed to the coupling of the phenyl ringstretch modes to the excited-state geometry, similarly as for PPV [5,7,10,11]. Thus, the absorption spectrum of PPI thin films reveals features indicating that the dominant contribution comes rather from the phenylene ring electronic structure than that of the azomethine unit. The selection rules of absorption in PPI are seen to be relaxed with respect to the benzene ring, so that the 0–0 transition is observed and its weak strength should be related to a small overlap of the wave functions of the ground and electronic excited states. In contrast, this overlap is important in the case of higher vibronic states of the electronic excited state. Similarly, PL spectra reveal the strength distribution indicating the importance of minima mismatch of potential energies of the electronic ground and excited states. However, in contrast to the absorption spectra, one should bear in mind that PL spectra reveal decay of excitons towards PPI polymer segments with the smallest gap. As for photoluminescence, the total PL signal as a function of the photon energy E has been modeled as a sum of four normalized Gaussians with positions Ej, broadenings sj and areas Aj IPL ðEÞ ¼
4 X
"
Aj 1 EEj pffiffiffiffiffiffi exp 2 sj
j ¼ 1 sj 2p
2 # ð1Þ
Three from four individual Gaussians appear to be equally spaced with an energy separation of about 0.15 eV. These peaks, labeled as 0–0, 0–1 and 0–2 in Fig. 4, are located at 2.47, 2.33 and 2.18 eV, and they are due to one type of C–C double bond vibrations, characteristic for conjugated polymers [10]; an additional vibronic peak at 2.07 eV may represent another phonon mode coupled to the electronic transition. The Stokes shift, being a difference between the 0–0 emission and absorption lines, is equal to 0.13 eV, similarly as for ‘standard’ PPV [7,10]. However, various defects of polymer films account for the measured Stokes shift [12] that may approach even zero, as observed in ‘improved’ PPV [11]. As seen in Figs. 3 and 4, the 0–0 PL peak is smaller than the 0–1 peak and quickly decreases with increasing temperature. This effect is thought to be a result of a small overlap of the wave functions of the electronic ground and excited states that, in turn, is due to different geometries of conjugated segments of polymer chains in the two states. On the other hand, one has also to take into account the fact that the 0–0 emission peak is highly sensitive to the polymer film imperfections that quench the 0–0 emission, while only slightly affecting the rest of the vibronic progression [12,13]; additionally, an overlap between the absorption and PL spectra (see Fig. 4) results in self-absorption increasing with temperature due to a blue-shift of the 0–0 emission peak (the 0–0 absorption peak is unaffected by temperature, see Fig. 1). Therefore, in order to characterize quantitatively the variation of PL intensity with temperature, we have chosen the 0-1 and 0–2 emission peaks and the temperature dependence of the integrated intensity of these peaks has been fitted to the expression [14] A0 APL ðTÞ ¼ 1 þ a exp ðe=kTÞ
ð2Þ
where A0 is the integrated PL intensity at 0 K, a is a fit parameter and e is the characteristic activation energy for multiphonon processes; the result of the fitting is shown in Fig. 5.
Fig. 5. Temperature dependence of the integrated intensity of the 0–1 and 0–2 emission peaks for a PPI sample Z3. Symbols represent the values of the Gaussian areas obtained from modeling PL spectra at various temperatures with Eq. (1) and solid lines — fitting to Eq. (2).
The decrease in the PL intensity with increasing temperature for the strongest 0–1 peak is very significant, with APL(300 K)/ APL(77 K) E0.1. The fit parameter is a ¼45715 for the 0–1 peak, a ¼973 for the 0–2 peak, and the activation energy of transitions (fitted simultaneously for both peaks) is e ¼3775 meV, corresponding to the potential barrier for thermally excited electrons to decay nonradiatively due to a multiphonon process through vibrational levels of the ground electronic state. As mentioned in Introduction, a 50 nm PPI thin film has been signalized to be very weakly luminescent [6,7] as compared to a similar film of PPV, in spite of the fact that both polymers are isoelectronic [5]. We have succeeded in a clear observation of PL for PPI by preparing much thicker films that, however, are not suitable for application as, for example, an electroluminescent material. In our opinion, the origin of weak PL of PPI is due to the presence of the lone electron pair (LEP) on the nitrogen orbital, the energy of which is close to the highest occupied molecular orbital (HOMO) state. Additionally, the same symmetry of LEP and HOMO states may be a factor enforcing rather non-radiative emission processes than a radiative decay to the ground state [5].
4. Conclusion By preparing relatively thick films of PPI, we have succeeded in the first reliable observation of photoluminescence in this polymer clearly showing the vibronic progression. We also suggest that generally weak photoluminescence of PPI is a result of a nonradiative electronic state occupied by the lone electron pair on the nitrogen orbital. Therefore, there is a strong indication that presence of the nitrogen atom in the main polymer chain plays the crucial role in attenuation of photoluminescence in polymers.
Acknowledgements The authors are very indebted to Dr. R. Lisiecki (Institute of Low Temperature and Structure Research, Wroclaw, Poland) and Dr. B. Jarzabek (Center of Polymer and Carbon Materials, Zabrze, Poland) for meaasurements of the absorption spectra at various temperatures.
J. Cisowski et al. / Journal of Luminescence 132 (2012) 2098–2101
References [1] [2] [3] [4] [5] [6] [7]
C.Y. Yang, S.A. Jenekhe, Chem. Mater. 3 (1991) 878. A. Kubono, N. Okui, Prog. Polym. Sci. 19 (1994) 389. C.Y. Yang, S.A. Jenekhe, Macromolecules 28 (1995) 1180. A. Iwan, D. Sek, Prog. Polym. Sci. 33 (2008) 289. J. Weszka, H. Bednarski, M. Domanski, J. Chem. Phys. 131 (2009) 024901. M.S. Weaver, D.D.C. Bradley, Synth. Met. 83 (1996) 61. F. Rohlfing, D.D.C. Bradley, Chem. Phys. 227 (1998) 133.
2101
[8] B. Jarzabek, J. Weszka, M. Domanski, J. Jurusik, J. Cisowski, J. Non-Cryst. Solids 352 (2006) 1660. [9] T.W. Hagler, K. Pakbaz, K.F. Voss, A.J. Heeger, Phys. Rev. 44 (1991) 8652. [10] S.-H. Lim, T.G. Bjorklund, C.J. Bardeen, Chem. Phys. Lett. 342 (2001) 555. [11] K. Pichler, D.A. Halliday, D.D.C. Bradley, P.L. Burn, R.H. Friend, A.B. Holmes, J. Phys. Condens. Matter 5 (1993) 7155. [12] F.C. Spano, Phys. Rev. B 116 (2002) 5877. [13] F. Balzer, A. Pogantsch, H.-H. Rubahn, J. Lumin. 129 (2009) 784. [14] W. Chen, F. Su, G. Li, A.G. Joly, J.-O. Malm, J.-O. Bovin, J. Appl. Phys. 92 (2002) 1950.