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Remarkable temperature dependence of electric field-induced change in fluorescence spectra of pyrene doped in a polymer matrix
Chemical Physics Letters 402 (2005) 206–211 www.elsevier.com/locate/cplett
Remarkable temperature dependence of electric field-induced change in fluorescence spectra of pyrene doped in a polymer matrix Toshifumi Iimori a,b, Anjue Mane Ara b, Tomokazu Yoshizawa Takakazu Nakabayashi a,b, Nobuhiro Ohta a,b,* a b
a,b
,
Research Institute for Electronic Science (RIES), Hokkaido University, Sapporo 060-0812, Japan Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received 13 September 2004; in final form 3 December 2004 Available online 24 December 2004
Abstract Electrofluorescence spectra as well as fluorescence spectra of pyrene doped in a polymer film have been measured at various temperatures in the region of 70–293 K. As the temperature decreases, fluorescence with a peak at 395 nm which gives a very different response to an applied electric field from other fluorescence emissions increases. Besides the structured monomer fluorescence with a peak at 380 nm and sandwich-type excimer fluorescence with a peak at 470 nm, three types of partially overlapped excimer fluorescence exist at low temperatures for pyrene doped in a PMMA film at high concentrations. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Stark spectroscopy in absorption and emission spectra is well-known to be useful to examine the electric properties of molecules or molecular aggregates both in the ground and electronically excited states [1–3]. Measurements of electric field effects on optical spectra are also very useful to examine the electric field effects on excitation dynamics [3–5]. In fact, photoinduced electron transfer and excimer formation processes are significantly influenced by an electric field, as known from the measurements of electrofluorescence spectra (plots of the electric-field-induced change in fluorescence intensity as a function of wavelength) [6–8]. This approach can give a new way to control photochemical reactions by using an external electric field as a perturbation, but the study of the electric field effects on photochemical dynamics has been quite limited so far. *
Corresponding author. Fax: +81 117064970. E-mail address: [email protected] (N. Ohta).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.027
Pyrene excimer is regarded as a prototype of excited complex, and its fluorescence has been extensively studied to examine the excimer formation dynamics under various experimental conditions [9,10]. As reported previously [8], electrofluorescence spectra of pyrene doped in a polymer film at high concentrations show that intermolecular excitation dynamics of pyrene is significantly influenced by an external electric field. Excimer formation process of pyrene in a solid film was reported to be strongly dependent on temperature [11–16]. However, electric field effects on fluorescence of pyrene have been examined so far only at room temperature in a polymer film [8,17,18]. In the present study, therefore, electric field effects on fluorescence of pyrene doped in a polymer film have been measured at various temperatures in the region from 70 to 293 K. Based on the results, it is confirmed that excimer emissions which appear at low temperatures show very different electric field effect from the ones observed at room temperature.
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2. Experimental Sample preparation of pyrene doped in a PMMA film and experimental apparatus used in the present experiments to measure the electroabsorption and electrofluorescence spectra at different temperatures are the same with the ones reported in our previous papers [8,19]. A certain amount of benzene solution of pyrene and PMMA was cast on an ITO coated quartz substrate by a spin coating technique. Then, a semitransparent aluminum (Al) film was deposited on the dried polymer film. The ITO and Al films were used as electrodes. Hereafter, the applied field is denoted by F and its strength is represented by rms. The thickness of the polymer film was typically 0.6 lm, and the external electric field calculated from the applied voltage divided by the film thickness was fixed to be 0.75 MV cm 1 in the present experiments. All the measurements were performed under vacuum conditions. The sample substrate was cooled using a cryogenic refrigerating system (Daikin, V202C5LR) equipped with quartz optical windows, and the temperature (T) of the substrate was monitored using a temperature controller (Scientific Instruments, model 9600) with a silicon diode thermometer (Scientific Instruments, Si410A). The substrate was excited from the semitransparent aluminum side, and an incident angle of the excitation light to the substrate was 45°. The intensity of the observed fluorescence spectra was corrected with respect to the spectral response of the detector of the spectrophotometer.
3. Results and discussion Fluorescence, electrofluorescence and absorption spectra of pyrene doped in a PMMA film containing 10 mol% of pyrene were measured at various temperatures in the range of 70–293 K. The former two spectra, which were measured simultaneously, are shown in Fig. 1. Absorption spectra as well as electroabsorption spectra are essentially independent of the temperature, and the emission spectra shown in Fig. 1 were obtained with excitation at 331 nm, where the field-induced change in absorption intensity is negligible. Hereafter, electroabsorption and electrofluorescence spectra are denoted by E-A and E-F spectra, respectively. As reported previously [8], fluorescence and E-F spectra of pyrene doped in a polymer film at high concentrations show three types of emissions at room temperatures, i.e., a structured fluorescence in the region of 370– 450 nm, a broad fluorescence with a peak at 470 nm, and a extremely weak fluorescence with a peak at 415 nm. These are assigned as the monomer fluorescence emitted from the locally excited state of pyrene, a sandwich-type excimer fluorescence [20,21] and partially overlapped excimer fluorescence [12,13,22–24],
Fig. 1. Temperature dependence of fluorescence spectra (a) and electrofluorescence (E-F) spectra (b) of pyrene doped in a PMMA film at a concentration of 10 mol%. Temperature ranging from 293 K (room temperature) to 70 K is shown in each spectrum. Dotted line shows the estimated spectrum of a sandwich-type excimer fluorescence, i.e., EXCIMER(I) in the shorter wavelength region.
respectively. The former two fluorescence emissions are quenched by applying F, and the third fluorescence emission is enhanced by F. Hereafter, these fluorescence emissions are denoted by MN-F, EXCIMER(I) and EXCIMER(II), respectively. As the temperature decreases, fluorescence intensity at around 400 nm markedly increases (see Fig. 1), while MN-F and EXCIMER(I) are not so influenced by a change in temperature. Actually, the spectrum of EXCIMER(I) is red-shifted with decreasing temperature especially in the shorter wavelength region. As a result, the spectral width of the broad EXCIMER(I) becomes narrower, and the peak position is a little red-shifted with lowering temperature; the spectral width at half maximum is 91, 89, 86 and 81 nm at 293, 250, 190 and 70 K, respectively, and the peak is at 470 nm at 293 K and 474 nm at temperatures below 250 K. In order to obtain the extracted spectra of emission other than EXCIMER(I), EXCIMER(I) spectrum was subtracted from the observed emission spectrum at each temperature. The results are shown in Fig. 2a. It should be noted that the spectra of EXCIMER(I) shown in Fig. 1 were estimated by assuming a superposition of two Gaussian profiles, with which the fluorescence spectral shape in the region longer than 444 nm was reproduced well in every case. The extracted fluorescence spectra at low temperatures given in Fig. 2a are very similar to the V-luminescence observed for supercooled crystal of pyrene [12–14] and to the B-fluorescence observed at low temperatures for microcrystalline film of pyrene at low temperatures [16]. The extracted fluorescence spectrum
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Fig. 2. Temperature dependence of extracted fluorescence spectrum (a) and extracted E-F spectrum (b), obtained by subtracting the spectrum of EXCIMER(I) from the observed fluorescence and E-F spectra, respectively. See the text about the procedure of the subtraction.
at 70 K can be reproduced by a superposition of two Gaussian bands with a peak at 393 nm (25 435 cm 1) and at 415 nm (24 120 cm 1), denoted by bands (I) and (II), respectively, and they are shown in Fig. 3a. As shown in Fig. 2a, we can notice MN-F at relatively high temperatures, but it becomes unclear with lowering temperatures because a broad emission with a peak at 395 nm (25 300 cm 1) increases. These results suggest that MN-F is nearly independent of temperature and that only the broad fluorescence with a peak at 395 nm significantly increases with lowering the temperature. It is noted that the fluorescence yield UF of pyrene doped in PMMA at a temperature range from 77 to 296 K has been reported by Kropp et al. [11]. With lowering temperature, the value of UF increases from 0.61 at 296 K to 0.78 at 77 K. This change was related to the temperature dependence of the deactivation of the intramolecular non-radiative process of pyrene in PMMA. The present results at a high concentration of 10 mol% imply that intermolecular process of monomer is nearly independent of temperature.
Fig. 3. (a) Extracted fluorescence spectrum at 70 K (thick solid line), two bands with a Gaussian shape, i.e., bands (I) and (II) (broken line), the integration of the E-F spectrum made from the high wavenumber side (dotted line) and simulated spectrum (thin solid line). (b) Extracted E-F spectrum at 70 K (thick solid line), two bands with a Gaussian shape, i.e., bands (I) and (III) (broken line), the first derivative of band (I) (dotted line), and the simulated spectrum (thin solid line). (c) Extracted fluorescence spectrum (thick solid line), extracted E-F spectrum at 70 K which is the same as in (b) and its negative (dotted line).
Plots of the fluorescence intensities at different wavelengths as a function of temperature are shown in Fig. 4. Fluorescence at 400 nm monotonically increases with decreasing temperature, and the magnitude of the increase is remarkable at temperatures below 150 K (see Fig. 4a). On the other hand, fluorescence at 470 nm, which corresponds to EXCIMER(I), is not influenced by a change in temperature so remarkably (see Fig. 4c). Actually, EXCIMER(I) becomes a little stronger above 250 K, constant in the range of 250–120 K, a little weaker below 120 K, as the temperature decreases, suggesting that the relaxation dynamics of the sandwichtype excimer varies a little as a function of temperature. With lowering temperature, emission intensity at 430 nm becomes weaker above 150 K, which probably results from the red-shift of the overlapping EXCIMER(I) at
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spectrum of EXCIMER(I). The results are shown in Fig. 2b. MN-F is quenched by F at room temperature. With lowering temperature, emission located in the longer wavelength region than MN-F is enhanced and shows a marked electric field effect (see Fig. 2). As a result, E-F spectrum as well as emission spectrum of MN-F becomes unclear, but it is likely that the intensity as well as its field-induced change for MN-F is not so influenced by a change in temperature. E-F spectra at T 6 100 K exhibit positive bands with a structure in the range from 395 to 450 nm and a sharp negative peak at 384 nm, as shown in Fig. 2b. Note that the latter peak cannot be attributed to MN-F. At 70 K, two peaks are clearly confirmed at 405 and 430 nm, i.e., 24 720 and 23 230 cm 1, though a single peak at 415 nm (24 100 cm 1) is noticed at room temperature. Temperature dependence of the field-induced change in fluorescence intensity (DIF) obtained by monitoring the emission at 384, 430 and 470 nm, respectively, is shown in Fig. 4. The temperature dependence of the E-F spectra is summarized as follows:
Fig. 4. Plots of the total fluorescence intensity (IF) and its fieldinduced change (DIF) as a function of temperature. IF was monitored at 400, 430 and 470 nm in (a)–(c), respectively, while DIF was monitored at 384, 430 and 470 nm, respectively.
low temperatures, whereas the intensity becomes stronger below 150 K (see Fig. 4b). If the contribution of EXCIMER(I) is excluded, emission at 430 nm seems to show the temperature dependence similar to the 400 nm emission, i.e., monotonic increase of the intensity with lowering temperature. E-F spectra also show a remarkable temperature dependence particularly in the wavelength region below 450 nm, as shown in Fig. 1. The E-F spectra in the wavelength region above 470 nm are nearly the same in shape as the fluorescence spectra, suggesting that E-F spectrum of EXCIMER(I) gives nearly the same shape as the emission spectrum of EXCIMER(I). As reported previously [8], EXCIMER(I) is quenched by F at room temperature. The present results show that the same is true at any temperature, indicating that the yield of EXCIMER(I) decreases by applying the external field irrespective of temperature. The extracted E-F spectra where the contribution of the field-induced change in EXCIMER(I) was subtracted from the observed E-F spectra were obtained at each temperature by assuming that the E-F spectrum due to EXCIMER(I) is the same in shape with the fluorescence
(1) A field-induced quenching of MN-F occurs at high concentrations, which is nearly independent of the temperature. (2) E-F spectra at wavelengths longer than 470 nm show nearly the same shape as the fluorescence spectra with a negative value, indicating that EXCIMER(I) is quenched by F at any temperature. The magnitude of the quenching is roughly the same irrespective of temperature. (3) E-F spectra show a sharp negative peak at 384 nm (26 000 cm 1) at low temperatures. The magnitude of the quenching at 384 nm remarkably and monotonically increases with lowering temperature especially below 150 K. (4) E-F spectra are positive in the region of 395– 450 nm (25 300–22 000 cm 1) and show a structure with two peaks at 405 and 430 nm at low temperatures below 100 K (see Fig. 2b). The intensity of the former peak becomes stronger monotonically with lowering temperature. It may be unreasonable to ascribe the change in the E-F spectra in the region 370–450 nm at low temperatures to the change in one particular excimer fluorescence, in contrast with the E-F spectra at room temperature. The E-F spectra suggest that the extracted fluorescence shown in Fig. 3a is a superposition of excimer fluorescence emissions having different response to the applied electric field from each other. The extracted E-F spectrum in the high wavenumber region is very similar to the first derivative of the band(I) (see Fig. 2b). The whole E-F spectra can be decomposed into
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the band expressed by a Gaussian shape, denoted by band (III), and the band given by a linear combination of the band (I) and its first derivative, as shown in Fig. 3b. The presence of the first derivative of the band(I) in the E-F spectra is confirmed since the integrated spectrum of the E-F spectrum in the high wavenumber region well reproduces the band (I) (see Fig. 3a). The fact that the band (II) is not reproduced by the integration of the E-F spectrum, in contrast with band(I), implies that band (II) does not result from the vibronic structure starting from the band(I) and that bands (I) and (II) are assigned to the different emitting states. Band (III) of the extracted E-F spectra at 430 nm becomes stronger with lowering temperature, but the corresponding fluorescence band is not confirmed in the extracted fluorescence spectra (see Fig. 3a–c). This is probably because this band may correspond to EXCIMER(II) observed at room temperature. As already reported, EXCIMER(II) shows a remarkable field-induced enhancement, but its intensity is extremely weak in the absence of F. It may be considered that EXCIMER(II), i.e., band (III) at 70 K, gives a red shift by about 15 nm with lowering temperature from 293 to 70 K. In summary, pyrene doped in a PMMA film at high concentrations shows five fluorescence components at low temperatures, i.e., monomer fluorescence (MN-F), sandwich-type excimer fluorescence (EXCIMER(I)), and fluorescence components characterized by bands (I), (II) and (III). MN-F and EXCIMER(I) are similarly quenched by F irrespective of temperature. Band (I), which shows both large Stark shift and field-induced quenching, is markedly enhanced with lowering temperature. By analyzing the Stark shift, the magnitude of the change in molecular polarizability following the emis˚ 3 for the emitting spesion is estimated to be about 250 A cies of band (I) with a Lorentz field correction [25]. Band (II), which is also enhanced with lowering temperature, is not influenced by F significantly. Band (III) is enhanced by F, but its intensity at zero field is very weak irrespective of temperature. The characterization of the emitting state of the Vluminescence or B-fluorescence in crystal, whose spectrum is very similar to the extracted fluorescence spectrum in Fig. 2a, is somewhat controversial, but these emissions have been ascribed to the excimer having a partially overlapped conformation. The present fluorescence and E-F results show that there are at least three distinct states in the partially overlapped excimer fluorescence. An implication of the crystal spectroscopy is consistent with a model that in the excited state there are several potential minima due to differently overlapped conformations. In a pyrene crystal, the nearest neighbor pyrene molecules are in a parallel-displaced configuration [10]. A theoretical model taking both the nearest and the next nearest pairs into account has attempted to interpret the fully relaxed excimer
fluorescence, i.e., sandwich-type excimer fluorescence (EXCIMER(I)), and the partially overlapped fluorescence that originates from the corresponding two intermolecular interactions [26]. Hence, it will be pertinent to consider that the distinct states detected in this experiment, i.e., emitting states of bands (I), (II) and (III) are due to potential minima corresponding to different conformations of excimer. At room temperature, one of the minima, namely one particular conformation, can be detected in the fluorescence as well as in the E-F spectrum. As the temperature decreases, intermolecular thermal motion may be deactivated, and the trapped molecules at other conformation coexist in the film. One of them with a peak at 394 nm, i.e., band (I), shows a markedly large Stark shift, indicating a large molecular polarizability of the emitting species of band (I). Other two types of partially overlapped excimer, i.e., bands (II) and (III), as well as sandwich-type excimer do not show a noticeable Stark shift. Thus, pyrene doped in a PMMA film at high concentrations shows a plural numbers of excimer fluorescence which give different electric field effect from each other at low temperatures. It may sound strange that only one species of the partially overlapped excimer which gives a emission peak at 394 nm shows a remarkable Stark shift, but its origin remains unsettled. In crystal, the emitting state of the B-fluorescence, i.e., the partially overlapped excimer is proposed as a precursor of the sandwich-type excimer [15]. However, the fact that MN-F or EXCIMER(I) show a very different temperature dependence from that of band(I) or (II) indicates that the emitting state of the partially overlapped excimer fluorescence is neither a precursor of the sandwich-type excimer, nor the intermediate which is in equilibrium with the locally excited state of pyrene. Emitting states of bands (I), (II) and (III) are considered to be produced irreversibly from the locally excited state, and these states are regarded as becoming more stable with lowering temperature. In order to confirm the origin of the bands (I)–(III), further study of the time-resolved measurements of E-F spectra are now in progress.
Acknowledgements This work was supported by Grants-in-Aid for Scientific Research (A) (2) (15205001) and for Scientific Research on Priority Area (417) and (432) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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Remarkable temperature dependence of electric field-induced change in fluorescence spectra of pyrene doped in a polymer matrix Toshifumi Iimori a,b, Anjue Mane Ara b, Tomokazu Yoshizawa Takakazu Nakabayashi a,b, Nobuhiro Ohta a,b,* a b
a,b
,
Research Institute for Electronic Science (RIES), Hokkaido University, Sapporo 060-0812, Japan Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received 13 September 2004; in final form 3 December 2004 Available online 24 December 2004
Abstract Electrofluorescence spectra as well as fluorescence spectra of pyrene doped in a polymer film have been measured at various temperatures in the region of 70–293 K. As the temperature decreases, fluorescence with a peak at 395 nm which gives a very different response to an applied electric field from other fluorescence emissions increases. Besides the structured monomer fluorescence with a peak at 380 nm and sandwich-type excimer fluorescence with a peak at 470 nm, three types of partially overlapped excimer fluorescence exist at low temperatures for pyrene doped in a PMMA film at high concentrations. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Stark spectroscopy in absorption and emission spectra is well-known to be useful to examine the electric properties of molecules or molecular aggregates both in the ground and electronically excited states [1–3]. Measurements of electric field effects on optical spectra are also very useful to examine the electric field effects on excitation dynamics [3–5]. In fact, photoinduced electron transfer and excimer formation processes are significantly influenced by an electric field, as known from the measurements of electrofluorescence spectra (plots of the electric-field-induced change in fluorescence intensity as a function of wavelength) [6–8]. This approach can give a new way to control photochemical reactions by using an external electric field as a perturbation, but the study of the electric field effects on photochemical dynamics has been quite limited so far. *
Corresponding author. Fax: +81 117064970. E-mail address: [email protected] (N. Ohta).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.027
Pyrene excimer is regarded as a prototype of excited complex, and its fluorescence has been extensively studied to examine the excimer formation dynamics under various experimental conditions [9,10]. As reported previously [8], electrofluorescence spectra of pyrene doped in a polymer film at high concentrations show that intermolecular excitation dynamics of pyrene is significantly influenced by an external electric field. Excimer formation process of pyrene in a solid film was reported to be strongly dependent on temperature [11–16]. However, electric field effects on fluorescence of pyrene have been examined so far only at room temperature in a polymer film [8,17,18]. In the present study, therefore, electric field effects on fluorescence of pyrene doped in a polymer film have been measured at various temperatures in the region from 70 to 293 K. Based on the results, it is confirmed that excimer emissions which appear at low temperatures show very different electric field effect from the ones observed at room temperature.
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2. Experimental Sample preparation of pyrene doped in a PMMA film and experimental apparatus used in the present experiments to measure the electroabsorption and electrofluorescence spectra at different temperatures are the same with the ones reported in our previous papers [8,19]. A certain amount of benzene solution of pyrene and PMMA was cast on an ITO coated quartz substrate by a spin coating technique. Then, a semitransparent aluminum (Al) film was deposited on the dried polymer film. The ITO and Al films were used as electrodes. Hereafter, the applied field is denoted by F and its strength is represented by rms. The thickness of the polymer film was typically 0.6 lm, and the external electric field calculated from the applied voltage divided by the film thickness was fixed to be 0.75 MV cm 1 in the present experiments. All the measurements were performed under vacuum conditions. The sample substrate was cooled using a cryogenic refrigerating system (Daikin, V202C5LR) equipped with quartz optical windows, and the temperature (T) of the substrate was monitored using a temperature controller (Scientific Instruments, model 9600) with a silicon diode thermometer (Scientific Instruments, Si410A). The substrate was excited from the semitransparent aluminum side, and an incident angle of the excitation light to the substrate was 45°. The intensity of the observed fluorescence spectra was corrected with respect to the spectral response of the detector of the spectrophotometer.
3. Results and discussion Fluorescence, electrofluorescence and absorption spectra of pyrene doped in a PMMA film containing 10 mol% of pyrene were measured at various temperatures in the range of 70–293 K. The former two spectra, which were measured simultaneously, are shown in Fig. 1. Absorption spectra as well as electroabsorption spectra are essentially independent of the temperature, and the emission spectra shown in Fig. 1 were obtained with excitation at 331 nm, where the field-induced change in absorption intensity is negligible. Hereafter, electroabsorption and electrofluorescence spectra are denoted by E-A and E-F spectra, respectively. As reported previously [8], fluorescence and E-F spectra of pyrene doped in a polymer film at high concentrations show three types of emissions at room temperatures, i.e., a structured fluorescence in the region of 370– 450 nm, a broad fluorescence with a peak at 470 nm, and a extremely weak fluorescence with a peak at 415 nm. These are assigned as the monomer fluorescence emitted from the locally excited state of pyrene, a sandwich-type excimer fluorescence [20,21] and partially overlapped excimer fluorescence [12,13,22–24],
Fig. 1. Temperature dependence of fluorescence spectra (a) and electrofluorescence (E-F) spectra (b) of pyrene doped in a PMMA film at a concentration of 10 mol%. Temperature ranging from 293 K (room temperature) to 70 K is shown in each spectrum. Dotted line shows the estimated spectrum of a sandwich-type excimer fluorescence, i.e., EXCIMER(I) in the shorter wavelength region.
respectively. The former two fluorescence emissions are quenched by applying F, and the third fluorescence emission is enhanced by F. Hereafter, these fluorescence emissions are denoted by MN-F, EXCIMER(I) and EXCIMER(II), respectively. As the temperature decreases, fluorescence intensity at around 400 nm markedly increases (see Fig. 1), while MN-F and EXCIMER(I) are not so influenced by a change in temperature. Actually, the spectrum of EXCIMER(I) is red-shifted with decreasing temperature especially in the shorter wavelength region. As a result, the spectral width of the broad EXCIMER(I) becomes narrower, and the peak position is a little red-shifted with lowering temperature; the spectral width at half maximum is 91, 89, 86 and 81 nm at 293, 250, 190 and 70 K, respectively, and the peak is at 470 nm at 293 K and 474 nm at temperatures below 250 K. In order to obtain the extracted spectra of emission other than EXCIMER(I), EXCIMER(I) spectrum was subtracted from the observed emission spectrum at each temperature. The results are shown in Fig. 2a. It should be noted that the spectra of EXCIMER(I) shown in Fig. 1 were estimated by assuming a superposition of two Gaussian profiles, with which the fluorescence spectral shape in the region longer than 444 nm was reproduced well in every case. The extracted fluorescence spectra at low temperatures given in Fig. 2a are very similar to the V-luminescence observed for supercooled crystal of pyrene [12–14] and to the B-fluorescence observed at low temperatures for microcrystalline film of pyrene at low temperatures [16]. The extracted fluorescence spectrum
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Fig. 2. Temperature dependence of extracted fluorescence spectrum (a) and extracted E-F spectrum (b), obtained by subtracting the spectrum of EXCIMER(I) from the observed fluorescence and E-F spectra, respectively. See the text about the procedure of the subtraction.
at 70 K can be reproduced by a superposition of two Gaussian bands with a peak at 393 nm (25 435 cm 1) and at 415 nm (24 120 cm 1), denoted by bands (I) and (II), respectively, and they are shown in Fig. 3a. As shown in Fig. 2a, we can notice MN-F at relatively high temperatures, but it becomes unclear with lowering temperatures because a broad emission with a peak at 395 nm (25 300 cm 1) increases. These results suggest that MN-F is nearly independent of temperature and that only the broad fluorescence with a peak at 395 nm significantly increases with lowering the temperature. It is noted that the fluorescence yield UF of pyrene doped in PMMA at a temperature range from 77 to 296 K has been reported by Kropp et al. [11]. With lowering temperature, the value of UF increases from 0.61 at 296 K to 0.78 at 77 K. This change was related to the temperature dependence of the deactivation of the intramolecular non-radiative process of pyrene in PMMA. The present results at a high concentration of 10 mol% imply that intermolecular process of monomer is nearly independent of temperature.
Fig. 3. (a) Extracted fluorescence spectrum at 70 K (thick solid line), two bands with a Gaussian shape, i.e., bands (I) and (II) (broken line), the integration of the E-F spectrum made from the high wavenumber side (dotted line) and simulated spectrum (thin solid line). (b) Extracted E-F spectrum at 70 K (thick solid line), two bands with a Gaussian shape, i.e., bands (I) and (III) (broken line), the first derivative of band (I) (dotted line), and the simulated spectrum (thin solid line). (c) Extracted fluorescence spectrum (thick solid line), extracted E-F spectrum at 70 K which is the same as in (b) and its negative (dotted line).
Plots of the fluorescence intensities at different wavelengths as a function of temperature are shown in Fig. 4. Fluorescence at 400 nm monotonically increases with decreasing temperature, and the magnitude of the increase is remarkable at temperatures below 150 K (see Fig. 4a). On the other hand, fluorescence at 470 nm, which corresponds to EXCIMER(I), is not influenced by a change in temperature so remarkably (see Fig. 4c). Actually, EXCIMER(I) becomes a little stronger above 250 K, constant in the range of 250–120 K, a little weaker below 120 K, as the temperature decreases, suggesting that the relaxation dynamics of the sandwichtype excimer varies a little as a function of temperature. With lowering temperature, emission intensity at 430 nm becomes weaker above 150 K, which probably results from the red-shift of the overlapping EXCIMER(I) at
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spectrum of EXCIMER(I). The results are shown in Fig. 2b. MN-F is quenched by F at room temperature. With lowering temperature, emission located in the longer wavelength region than MN-F is enhanced and shows a marked electric field effect (see Fig. 2). As a result, E-F spectrum as well as emission spectrum of MN-F becomes unclear, but it is likely that the intensity as well as its field-induced change for MN-F is not so influenced by a change in temperature. E-F spectra at T 6 100 K exhibit positive bands with a structure in the range from 395 to 450 nm and a sharp negative peak at 384 nm, as shown in Fig. 2b. Note that the latter peak cannot be attributed to MN-F. At 70 K, two peaks are clearly confirmed at 405 and 430 nm, i.e., 24 720 and 23 230 cm 1, though a single peak at 415 nm (24 100 cm 1) is noticed at room temperature. Temperature dependence of the field-induced change in fluorescence intensity (DIF) obtained by monitoring the emission at 384, 430 and 470 nm, respectively, is shown in Fig. 4. The temperature dependence of the E-F spectra is summarized as follows:
Fig. 4. Plots of the total fluorescence intensity (IF) and its fieldinduced change (DIF) as a function of temperature. IF was monitored at 400, 430 and 470 nm in (a)–(c), respectively, while DIF was monitored at 384, 430 and 470 nm, respectively.
low temperatures, whereas the intensity becomes stronger below 150 K (see Fig. 4b). If the contribution of EXCIMER(I) is excluded, emission at 430 nm seems to show the temperature dependence similar to the 400 nm emission, i.e., monotonic increase of the intensity with lowering temperature. E-F spectra also show a remarkable temperature dependence particularly in the wavelength region below 450 nm, as shown in Fig. 1. The E-F spectra in the wavelength region above 470 nm are nearly the same in shape as the fluorescence spectra, suggesting that E-F spectrum of EXCIMER(I) gives nearly the same shape as the emission spectrum of EXCIMER(I). As reported previously [8], EXCIMER(I) is quenched by F at room temperature. The present results show that the same is true at any temperature, indicating that the yield of EXCIMER(I) decreases by applying the external field irrespective of temperature. The extracted E-F spectra where the contribution of the field-induced change in EXCIMER(I) was subtracted from the observed E-F spectra were obtained at each temperature by assuming that the E-F spectrum due to EXCIMER(I) is the same in shape with the fluorescence
(1) A field-induced quenching of MN-F occurs at high concentrations, which is nearly independent of the temperature. (2) E-F spectra at wavelengths longer than 470 nm show nearly the same shape as the fluorescence spectra with a negative value, indicating that EXCIMER(I) is quenched by F at any temperature. The magnitude of the quenching is roughly the same irrespective of temperature. (3) E-F spectra show a sharp negative peak at 384 nm (26 000 cm 1) at low temperatures. The magnitude of the quenching at 384 nm remarkably and monotonically increases with lowering temperature especially below 150 K. (4) E-F spectra are positive in the region of 395– 450 nm (25 300–22 000 cm 1) and show a structure with two peaks at 405 and 430 nm at low temperatures below 100 K (see Fig. 2b). The intensity of the former peak becomes stronger monotonically with lowering temperature. It may be unreasonable to ascribe the change in the E-F spectra in the region 370–450 nm at low temperatures to the change in one particular excimer fluorescence, in contrast with the E-F spectra at room temperature. The E-F spectra suggest that the extracted fluorescence shown in Fig. 3a is a superposition of excimer fluorescence emissions having different response to the applied electric field from each other. The extracted E-F spectrum in the high wavenumber region is very similar to the first derivative of the band(I) (see Fig. 2b). The whole E-F spectra can be decomposed into
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the band expressed by a Gaussian shape, denoted by band (III), and the band given by a linear combination of the band (I) and its first derivative, as shown in Fig. 3b. The presence of the first derivative of the band(I) in the E-F spectra is confirmed since the integrated spectrum of the E-F spectrum in the high wavenumber region well reproduces the band (I) (see Fig. 3a). The fact that the band (II) is not reproduced by the integration of the E-F spectrum, in contrast with band(I), implies that band (II) does not result from the vibronic structure starting from the band(I) and that bands (I) and (II) are assigned to the different emitting states. Band (III) of the extracted E-F spectra at 430 nm becomes stronger with lowering temperature, but the corresponding fluorescence band is not confirmed in the extracted fluorescence spectra (see Fig. 3a–c). This is probably because this band may correspond to EXCIMER(II) observed at room temperature. As already reported, EXCIMER(II) shows a remarkable field-induced enhancement, but its intensity is extremely weak in the absence of F. It may be considered that EXCIMER(II), i.e., band (III) at 70 K, gives a red shift by about 15 nm with lowering temperature from 293 to 70 K. In summary, pyrene doped in a PMMA film at high concentrations shows five fluorescence components at low temperatures, i.e., monomer fluorescence (MN-F), sandwich-type excimer fluorescence (EXCIMER(I)), and fluorescence components characterized by bands (I), (II) and (III). MN-F and EXCIMER(I) are similarly quenched by F irrespective of temperature. Band (I), which shows both large Stark shift and field-induced quenching, is markedly enhanced with lowering temperature. By analyzing the Stark shift, the magnitude of the change in molecular polarizability following the emis˚ 3 for the emitting spesion is estimated to be about 250 A cies of band (I) with a Lorentz field correction [25]. Band (II), which is also enhanced with lowering temperature, is not influenced by F significantly. Band (III) is enhanced by F, but its intensity at zero field is very weak irrespective of temperature. The characterization of the emitting state of the Vluminescence or B-fluorescence in crystal, whose spectrum is very similar to the extracted fluorescence spectrum in Fig. 2a, is somewhat controversial, but these emissions have been ascribed to the excimer having a partially overlapped conformation. The present fluorescence and E-F results show that there are at least three distinct states in the partially overlapped excimer fluorescence. An implication of the crystal spectroscopy is consistent with a model that in the excited state there are several potential minima due to differently overlapped conformations. In a pyrene crystal, the nearest neighbor pyrene molecules are in a parallel-displaced configuration [10]. A theoretical model taking both the nearest and the next nearest pairs into account has attempted to interpret the fully relaxed excimer
fluorescence, i.e., sandwich-type excimer fluorescence (EXCIMER(I)), and the partially overlapped fluorescence that originates from the corresponding two intermolecular interactions [26]. Hence, it will be pertinent to consider that the distinct states detected in this experiment, i.e., emitting states of bands (I), (II) and (III) are due to potential minima corresponding to different conformations of excimer. At room temperature, one of the minima, namely one particular conformation, can be detected in the fluorescence as well as in the E-F spectrum. As the temperature decreases, intermolecular thermal motion may be deactivated, and the trapped molecules at other conformation coexist in the film. One of them with a peak at 394 nm, i.e., band (I), shows a markedly large Stark shift, indicating a large molecular polarizability of the emitting species of band (I). Other two types of partially overlapped excimer, i.e., bands (II) and (III), as well as sandwich-type excimer do not show a noticeable Stark shift. Thus, pyrene doped in a PMMA film at high concentrations shows a plural numbers of excimer fluorescence which give different electric field effect from each other at low temperatures. It may sound strange that only one species of the partially overlapped excimer which gives a emission peak at 394 nm shows a remarkable Stark shift, but its origin remains unsettled. In crystal, the emitting state of the B-fluorescence, i.e., the partially overlapped excimer is proposed as a precursor of the sandwich-type excimer [15]. However, the fact that MN-F or EXCIMER(I) show a very different temperature dependence from that of band(I) or (II) indicates that the emitting state of the partially overlapped excimer fluorescence is neither a precursor of the sandwich-type excimer, nor the intermediate which is in equilibrium with the locally excited state of pyrene. Emitting states of bands (I), (II) and (III) are considered to be produced irreversibly from the locally excited state, and these states are regarded as becoming more stable with lowering temperature. In order to confirm the origin of the bands (I)–(III), further study of the time-resolved measurements of E-F spectra are now in progress.
Acknowledgements This work was supported by Grants-in-Aid for Scientific Research (A) (2) (15205001) and for Scientific Research on Priority Area (417) and (432) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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