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Persistent spectral hole-burning of pseudoisocyanine bromide J-aggregates
JOURNAL OF
LUMINESCENCE ELSEYIER
Journal
of Luminescence
66&67
(1996) 157-163
Persistent spectral hole-burning of pseudoisocyanine J-aggregates
bromide
Toshiro Tan?,*, Liu-Yi”, Fumio Sasaki”, Shunsuke Kobayashi”, Hiroki Nakatsukab “Electrotechnical Laboratory, l-l-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan bUnicersit?, of Tsukuba. I-I Ten-nohdai. Tsukuba-shi, lbamki 305, Japan
Abstract We have examined the hole formation profiles of the upper exciton band in J-aggregates of pseudo-isocyanine bromide/water-ethyleneglycol system by using DC-light laser or nanosecond pulsed laser. In the former case, a clear zero-phonon hole was formed with a width of 0.36cm-‘. Assuming negligible contributions of the dephasing by phonons, the homogeneous width of the one-exciton state is lifetime-limited in nature and T1 - 28 ps. On the other hand, a novel feature of the dispersive lineshape of holes was found in the pulse mode burning from the initial stage of irradiation. It is presently ascribed as a result of the excitation of the two-exciton band which is inherent to the excitonic state with large coherence length.
1. Introduction Among the aggregated states of organic dye molecules, the J-type linear aggregate is well known for its Frenkel type excitonic bands appearing below the lowest edge of the monomeric absorption band. Since the pioneering works of Jerry and Scheibe, many aggregated systems have been found and investigated. They have also found technological applications as sensitizers in photography which in turn opened a new field of science. Among them, the pseudo-isocyanine dye exhibits the narrowest bands, i.e., -v 1 nm at cryogenic temperatures. Recently, it has attracted new attention as a possible optoelectronic material since a large third order non-linearity was found [l], which is due to the large coherence of the excitonic states.
* Corresponding
author.
The issue of the dynamical properties of aggregates with mesoscopic size is currently interesting. This is because the system bridges the gap between a single molecule and a crystal. Interpretation of the homogeneous and inhomogeneous nature in terms of an exciton model have mostly been proceeded by hole-burning studies [2-41 as well as the use of the photon echo technique [S, 61. In this paper, we focus our attention to the hole formation profiles of the blue J-band in pseudoisocyanine bromide/water-ethyleneglycol system by using a nanosecond pulsed laser as well as a DClight laser, i.e., laser light that is of absolutely constant intensity. A DC laser reveals clear formation of a site-selective zero-phonon hole of width 0.36cm- ‘. In the case of irradiation by the pulsed laser, on the other hand, the formation of a rather broad and non-resonant hole/antihole structure with dispersive lineshape was found. This peculiar feature is ascribed as a result of the excitation of the
0022-2313/96/$15.00 ci’: 1996 - Elsevier Science B.V. All rights reserved SSDI 0022-23 13(95)00129-8
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T. Tani et al. /Journal qf‘luminescence
two-exciton band, which is inherent to the excitonic state with large coherence length.
2. Experimentals Pseudo-isocyanine bromide (PIC-Br) was purchased from Nippon Kankoh Shikiso Kenkyusho (Okayama, Japan) and used without further purification. Samples were prepared by dissolving the PIC-Br to a concentration of 4 x 10e3 M in a mixture of distilled water and ethylene glycol (50/5Ovol%, WEG). Drops of such solutions were pressed between flat glass slides with 12um thickness Mylar@ film as a spacer [ 11. The samples were cooled down to cryogenic temperatures in a varioptical cryostat (Oxford: able temperature CF1204). Aggregate formations were produced in the cryostat at a temperature of 230 K for the blue J-band. Most of the burning experiments, as well as absorption measurements described in this paper, were performed at 4.7 K. The holes were burnt with two types of lasers and the results were compared. The first one was a Nd+ : YAG laser-pumped dye laser (SpectraPhysics: DCR-11 + PDL-3), the linewidth of which is O.O47cm-i. The pulse repetition rate was 1OHz and the average power levels were about 30uW/cm2 for the initial burning stages. These power levels were increased step-by-step, after confirming that the observed spectral change had already exceeded by far the ordinary observed saturation levels, e.g. reported in Ref. [3], up to 2.4mW/cm2 in the final stage of burning. Burning times were started from 2 s and increased successively up to about 45min. Under these conditions, the absorption change obtained was from 2% to over 20%. The second type of laser was a single-mode ring dye laser (Coherent: 699-29) pumped with Ar+ laser as a totally DC light source. The width of the laser line was about 2.5MHz. Its power densities for the burnings were tracked to follow, almost similarly, those used in the pulsed light burnings. The holes as well as the absorption spectra were all measured with a 1.5-m double-pass monochromator (Jobin-Yvon: THR1500) with a resolution of 0.03 cm- ‘. It should be stressed that one has to be very careful in measuring absorption change
668~67 (1996) 157-163
in the J-bands. Since they are so sharp, even a fraction of A wavelength shift in the monochromator creates some experimental artifacts. In order to avoid these and to ensure good reproducibility we retuned the wavelength setting of the instruments at every measurement and repeated several independent experiments for confirmation.
3. Results and discussion 3.1. Absorption
spectra
Fig. 1 shows the absorption spectra of the PIC(Br)/WEG system measured at 295K and 4.7K. Absorbances in this paper are defined as _ log,,,(Z/lO), where I and I0 are the measured intensities of the transmitted and incident light, respectively. While the sample at room temperature shows a spectrum of typical monomeric dispersion, the spectrum at 4.7K shows two excitonic bands. They occur as a consequence of aggregate growth over the temperature region between 230 and 220 K for several minutes followed by quenching at a rate of - 7 K/min. This growth condition basically provides upper blue J-band absorption at 570.8 nm though a small amount of red J-band absorption at 576.0 nm still remains. 2
“1”,,1”“1”“1”,‘)’ PIC(Br)/WEG
j
4.7K
;
!
295 K
400
450
500 Wavelength
1
550
600
(nm)
Fig. I. Absorption spectra of PIC-Br/WEG system observed at 295 K and at 4.7 K. The latter was obtained as a consequence of the aggregate formation at temperatures between 230 and 220 K followed by quenching at a rate of _ 7 K/min below 77 K.
159
T. Tani et al. /Journal of Luminescence 66& 67 (1996) 157- 163
The widths of the blue J-band are between 35 and 40cm-’ depending upon the growth conditions. The monomer spectrum at 295 K shows that the original absorption has an inhomogeneous bandwidth of - 1200cm- ‘. Assuming this number is roughly constant down to 4.7K, a reduction of the inhomogeneous bandwidth is by a factor of 30. An exciton moving over an aggregate chain comprising N monomer molecules narrows the inhomogeneous bandwidth of the monomer absorption band by a factor of the order of l/fi [7]. Simple application of this relation gives rise to an aggregate length of about 900 in our samples. Since, a part of the inhomogeneity, in the case of monomers, could be due to small rotations of the quinoline rings with respect to one another [2], we can assume that the size of the delocalization length in our sample is also of the order of 100. According to exciton theory [S], the energy shift of the exciton band from the monomer peak is 2V, where I’ is a coupling constant between nearest neighbor monomers in the chain. From the red shift of the blue J-band of 1540cm- ‘, I/ is of the order of - 770 cm-‘. Hence, the width of the exciton band 14VI is of the order of 3070cm-’ for the blue J-band.
0.2 5.28
0
1:
DO
4.7 K
;8 -0.6
I
570
II
i B=570.91
I.
1
570.5
571
nm
Wavelength (nm)
3.2. DC-light burning
Fig. 2. Fluence dependence of hole formation at 4.7 K by the irradiation of DC single-mode peak of the upper exciton band.
Experimental results of persistent hole burnings are shown in Fig. 2 as absorption-difference spectra. They were formed at 4.7K by a step-bystep sequential irradiation of the DC laser light in the blue J-band of PIC(Br)/WEG. The hole, after the most prolonged bleaching, was also measured over a wider spectral region and compared with the exciton band structures, as shown in Fig. 3. In the figure, the hole obtained by the pulsed laser burning is included for comparison, details of which are described below. The negative part characterizes the educt states and the positive part the product states, respectively. As can be seen in Fig. 3, laser irradiation into the blue exciton peak never leads to photochemical changes in the red band. Exactly the same behavior was also observed for the red J-band burning [9]. Hence, both bands are decoupled as is observed in PIC-Cl and PIC-I [4].
The notable characteristics deduced from Figs. 2 and 3 are summarized as follows: (1) Sharp resonance holes are formed precisely at the wavelength of the irradiated laser light within our experimental accuracy. Its full width, extrapolated to zero burning fluence, is of the order of 0.36cm-’ at 4.7 K. We can safely conclude that they are zero-phonon holes and, therefore, the linewidth of the absorption band described in Section 3.1 is inhomogeneous in nature. (2) Assuming the effective number of monomers in the aggregated chain is - 100, over which the exciton is delocalized, we can deduce a quantum efficiency of hole formation from the data; the estimated number is 1.8 x lo-“. The efficiency of the zero-phonon formation is fairly low compared with the case of the pulsed laser burning and also with
profiles obtained laser light at the
T. Tani et al. 1 Journal ofluminescence
160
PIG-Br I WEG
759 mJ/cm*
---
565
570 Wavelength
575
580
(nm)
Fig. 3. Two excitonic peaks (top) and the hole prolonged bleaching at upper exciton peak (middle) two exciton bands are decoupled. The hole profile pulsed-laser burning is also shown for comparison
profile after to show the obtained by (bottom).
the efficiency of 2.8 x lop5 from the red-band burning [9]. (3) No detectable phonon or pseudo-phonon holes appeared for our experimental conditions. This reveals that the exciton-phonon coupling in the aggregate chain is quite small. (4) Though the products appear at longer as well as shorter wavelengths from the educt, the center of gravity of the spectrum is at shorter wavelength. (5) The spectral distribution range of the products, which is detectable in the present experiment, roughly coincides with half of the inhomogeneous absorption band. It seems smaller than that observed for the pulsed laser burning which is described later. (6) The oscillator strength of the product is, to some extent, smaller than that of the educt state.
M&i67 (1996) 157-163
Assuming a Lorentzian line shape and taking the instrumental resolution into account, the width of the hole, 0.36cm-i, gives 0.18 cm- ’ for the homogeneous width Aw,,. In the one-phonon approximation, the related scattering process between the k = 1 exciton state and the phonons, which causes the broadening of the homogeneous width, is governed by strong restrictions on energy and momentum. As a consequence, at temperatures around 4.7 K, no scattering processes are expected to take place [2,3]. Hence, here we can assume that the Awi, observed is attributed to the residual width and we can deduce a lifetime T1 of 28 ps, again in close agreement with [3] and other time-dependent experiments [2, lo]. The persistency or lifetime of the hole seems to be long. Within our experimental time scale of 103-lo4 s, the degree of hole-filling is of the order of 1% in depth and the contribution of spectral diffusion was not detected. Even though the efficiency decreased to a great extent, site-selective and sufficiently long-life holes of 2.2cm- ’ width were formed at 77 K. A further comment is related to item (4). We tried filling the hole, which was obtained after fluence of 759mJ/cmf, by further irradiation with laser light tuned 3.5A to the longer wavelength side of the hole as shown in Fig. 4 (2). The fluence was 575mJ/cm’. After the filling profile was gecorded, the sample was irradiated again at 3.OA on the shorter wavelength side of the initial hole, and filling of the two previous holes was observed (Fig. 4 (3)). Again the fluence was 575mJ/cm2. In all cases, filling of holes of about 20% with no detectable broadening was observed. This fact is notable because the filling process observed does not seem to depend on the wavelength difference betwOeenthe hole and the filling light. Separation of 6.5 A is out of the distribution range of the product states described in item (4). This fact may indicate that there are two kinds of product states, one of which is distributed over a rather restricted region and the other is over the whole inhomogeneous band. It is also inferred from the filling profiles that the burning reaction, the product of which is distributed over the whole inhomogeneous band, is highly photoreversible as was pointed out in PIG-I [3].
T. Tani et al. /Journal
/I
570
of Luminescence
M&67
(
I
I1
571
570.5
Wavelength
161
(1996) 157-163
(nm)
First, the hole (1) at 570.61 nm was produced by irradiation with fluence of 759 mJ/cm’. Second, Fig. 4. Hole filling by light irradiation. hole filling as shown here by the change in absorption, was done with irradiation at 570.96 nm (2) with fluence of 575 mJ/cm’. Lastly, the sample was irradiated at 573.00 nm (3) with 575 mJ/cm’ and the absorption change shows filling properties of both of the previous holes; (4) shows the laser-line profile
3.3. Pulsed-light
burning t
Fig. 5 shows typical hole spectra of the same system obtained by the irradiation of the light from the pulsed laser. Experimental conditions were almost identical to each other: average power densities of the burning light were carefully tuned to be almost equal for both experiments. The only difference is the use of the pulsed-mode laser instead of the single-mode DC laser for burning. As is clearly shown in the figure, observed hole profiles are quite different from those obtained by the DC light laser previously described. The holes formed after prolonged bleaching were also measured over a wider spectral region to check the interaction between the upper and lower exciton bands. As can be seen in Fig. 3, it is definite that the upper and lower exciton bands are decoupled from each other. The most notable features are as follows: Instead of narrow holes in resonance with the laser frequency oB, rather broad structures with dispersive lineshapes appeared even at the very initial stage of burning within our experimental accuracy. The lineshape is almost perfectly centro-symmetric at 0s. In other words, a hole below wB and an antihole above wB with equal area appear simultaneously. Upon increasing the fluences of the laser
4.7 K
0.09 mJ/cm2
0.90 mJ/cm’ 8 5 $j
10.8 mJicm*
0.4
Wavelength
(nm)
Fig. 5. Fluence dependence of hole formation profiles obtained at 4.7 K by pulsed laser light at the peak of the upper exciton band. Other conditions, such as average power density of the laser light, were kept almost identical to those in Fig. 2.
162
T Tani et al. jJourna1
@“Luminescence
light, the hole and antihole components show complicated behavior as is seen in Fig. 5. The characteristics appearing in Figs. 5 and 3 are summarized as follows: (1) There is a conservation of oscillator strength in the low fluence limit; the area of the hole is almost equal to the area of the antihole. (2) Upon increasing the fluence, the area of the hole increases more rapidly. The lineshape also changes from the simple dispersive shape which is centro-symmetric at c~)a through an asymmetric shape to the shape of the second derivatives of symmetric lines such as Lorentzian. The spectral range of the product eventually coincides with the inhomogeneous band. (3) In the prolonged bleaching stage, a kink or small but broad hole-like structure in resonance with wri appears [3]. This could be understood if rather strong linear exciton-phonon coupling (e.g. Huang-Rhys factor S of - 2.5) and unusually low energy for a phonon coupled to an optical transition (d - 2.7cm-‘) were assumed as is suggested in Ref. [7]. We think, however, that further investigation is required prior to adopting this simple consideration, as described below. The basic questions to be solved are as follows: Why the difference of the operation modes of the burning lasers could result in so drastic a change of the hole formation profiles from the initial stage of burning? What are the origins for the photoreactions? What is the nature of the inhomogeneous line broadening? At the moment, we have no definite answers for these questions. If we could assume simply a one-photon process in the excitation for both types of irradiation, the results of the pulsed mode burning could be interpreted by, e.g., power broadening. To our knowledge, however, such peculiar saturation phenomenon has never been observed either in dye-doped polymers or glasses nor in color centers or impurities in inorganic systems. We considered so far that the dispersive lineshape of the hole by pulsed light irradiation is an indication of the peculiarity of the electronic state and its dynamical property which are inherent to the Frenkel exciton of large coherence length. In the case of DC-light irradiation, consideration of
668~67 (1996)
157-163
optical excitation to the one-exciton band is enough. Even with our nanosecond laser, on the other hand, the peak power of the pulse is extended from the average power by as much as a factor of N 10’ (i.e. Y 1 MW/cm’) due to its low duty cycle. The energy per pulse easily exceeds 10uJ/cm2which is sufficient to induce an 1 mJ/cm’ optical non-linear response [3, lo]. The absorption difference spectrum observed by a two-color pump-probe experiment clearly shows the negative contribution due to bleaching and stimulated emission of the ground state to the one-exciton transitions, while positive contributions arise from induced absorption from the excited one-exciton state to a two-exciton state on the same aggregate [lo-121. Photoreactions from the two-exciton state will be sure to reduce the population of the initial one-exciton state. Hence, we can detect the population change by measuring the one-exciton spectrum with weak probe light. Superposition of the non-site-selective absorption contribution of the product states, which are more efficiently pr&ided than one-exciton band burning, can give rise to the dispersive hole/anti-hole structure. Further experiments, as well as theoretical considerations, are necessary to make clear the role of the excitonic transitions and the dynamics such as superradiant decay process in the hole-formation process and the nature of inhomogeneity.
4. Summary We have examined the hole formation profiles of the exciton states in PIC-Br/WEG system using both a DC laser and a nanosecond pulsed laser. In the former, a site selective zero-phonon hole was formed with a width of 0.36cm- ‘. Assuming negligible contribution of the dephasing by phonons, the homogeneous width of the one exciton state is lifetime-limited in nature and T1 N 28~s. In the latter we found, from the initial stage of burning, non-site selective hole/anti-hole formation with dispersive lineshape. This is ascribed to the excitation of two-exciton bands, which is inherent to excitonic states with large coherence length.
T. Tani et ul. i Journal
of Luminescence
References and F. Sasaki, Nonlinear Opt. 4 (1993) El1 S. Kobayashi 305. PI S. de Boer, K.J. Vink and D.A. Wiersma. Chem. Phys. Lett. 137 (1987) 99. and J. Friedrich, J. Chem. Phys. 91 (1989) 131 R. Hirshmann 7988; J. Opt. Sot. Am. B 9 (1992) 811. W. Kijhler. J. Friedrich and E. Daltrozzo, [41 R. Hirshmann, Chem. Phys. Lett. 151 (1988) 60. c51 H. Fidder, J. Knoester and D.A. Wiersma. Chem. Phys. Lett. 171 (1990) 529. C61 S. de Boer and D.A. Wiersma. Chem. Phys. Lett. 165 (1990) 45.
66& 67 (I 996) 157- 163
163
c71 E.W. Knapp, Chem. Phys. 85 (1984) 73. J. Knoester, J. Chem. Phys. 99 (1993) 8466. and H. Nakat[91 T. Tani, Liu-Yi, F. Sasaki, S. Kobayashi suka, in: Proc. 7th UPS Meeting, Stanford, September 6-8, 1995, Mol. Cryst. Liq. Cryst., to appear S. Kobayashi and F. Sasaki, Nonlinear Opt. 4 (1993) 305: F. Sasaki and S. Kobayashi, in: Advanced Materials ‘93, II/A. eds. H. Aoki et al.. Trans. Mat. Res. Sot. Jpn. A15 (Elsevier, Amsterdam, 1994) p. 401. J.R. Durrant, J. Knoester and D.A. Wiersma. Chem. Phys. Lett. 222 (1994) 450. K. Minoshima, M. Taiji, K. Misawa and T. Kobayashi, Chem. Phys. Lett. 218 (1994) 67.
LUMINESCENCE ELSEYIER
Journal
of Luminescence
66&67
(1996) 157-163
Persistent spectral hole-burning of pseudoisocyanine J-aggregates
bromide
Toshiro Tan?,*, Liu-Yi”, Fumio Sasaki”, Shunsuke Kobayashi”, Hiroki Nakatsukab “Electrotechnical Laboratory, l-l-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan bUnicersit?, of Tsukuba. I-I Ten-nohdai. Tsukuba-shi, lbamki 305, Japan
Abstract We have examined the hole formation profiles of the upper exciton band in J-aggregates of pseudo-isocyanine bromide/water-ethyleneglycol system by using DC-light laser or nanosecond pulsed laser. In the former case, a clear zero-phonon hole was formed with a width of 0.36cm-‘. Assuming negligible contributions of the dephasing by phonons, the homogeneous width of the one-exciton state is lifetime-limited in nature and T1 - 28 ps. On the other hand, a novel feature of the dispersive lineshape of holes was found in the pulse mode burning from the initial stage of irradiation. It is presently ascribed as a result of the excitation of the two-exciton band which is inherent to the excitonic state with large coherence length.
1. Introduction Among the aggregated states of organic dye molecules, the J-type linear aggregate is well known for its Frenkel type excitonic bands appearing below the lowest edge of the monomeric absorption band. Since the pioneering works of Jerry and Scheibe, many aggregated systems have been found and investigated. They have also found technological applications as sensitizers in photography which in turn opened a new field of science. Among them, the pseudo-isocyanine dye exhibits the narrowest bands, i.e., -v 1 nm at cryogenic temperatures. Recently, it has attracted new attention as a possible optoelectronic material since a large third order non-linearity was found [l], which is due to the large coherence of the excitonic states.
* Corresponding
author.
The issue of the dynamical properties of aggregates with mesoscopic size is currently interesting. This is because the system bridges the gap between a single molecule and a crystal. Interpretation of the homogeneous and inhomogeneous nature in terms of an exciton model have mostly been proceeded by hole-burning studies [2-41 as well as the use of the photon echo technique [S, 61. In this paper, we focus our attention to the hole formation profiles of the blue J-band in pseudoisocyanine bromide/water-ethyleneglycol system by using a nanosecond pulsed laser as well as a DClight laser, i.e., laser light that is of absolutely constant intensity. A DC laser reveals clear formation of a site-selective zero-phonon hole of width 0.36cm- ‘. In the case of irradiation by the pulsed laser, on the other hand, the formation of a rather broad and non-resonant hole/antihole structure with dispersive lineshape was found. This peculiar feature is ascribed as a result of the excitation of the
0022-2313/96/$15.00 ci’: 1996 - Elsevier Science B.V. All rights reserved SSDI 0022-23 13(95)00129-8
158
T. Tani et al. /Journal qf‘luminescence
two-exciton band, which is inherent to the excitonic state with large coherence length.
2. Experimentals Pseudo-isocyanine bromide (PIC-Br) was purchased from Nippon Kankoh Shikiso Kenkyusho (Okayama, Japan) and used without further purification. Samples were prepared by dissolving the PIC-Br to a concentration of 4 x 10e3 M in a mixture of distilled water and ethylene glycol (50/5Ovol%, WEG). Drops of such solutions were pressed between flat glass slides with 12um thickness Mylar@ film as a spacer [ 11. The samples were cooled down to cryogenic temperatures in a varioptical cryostat (Oxford: able temperature CF1204). Aggregate formations were produced in the cryostat at a temperature of 230 K for the blue J-band. Most of the burning experiments, as well as absorption measurements described in this paper, were performed at 4.7 K. The holes were burnt with two types of lasers and the results were compared. The first one was a Nd+ : YAG laser-pumped dye laser (SpectraPhysics: DCR-11 + PDL-3), the linewidth of which is O.O47cm-i. The pulse repetition rate was 1OHz and the average power levels were about 30uW/cm2 for the initial burning stages. These power levels were increased step-by-step, after confirming that the observed spectral change had already exceeded by far the ordinary observed saturation levels, e.g. reported in Ref. [3], up to 2.4mW/cm2 in the final stage of burning. Burning times were started from 2 s and increased successively up to about 45min. Under these conditions, the absorption change obtained was from 2% to over 20%. The second type of laser was a single-mode ring dye laser (Coherent: 699-29) pumped with Ar+ laser as a totally DC light source. The width of the laser line was about 2.5MHz. Its power densities for the burnings were tracked to follow, almost similarly, those used in the pulsed light burnings. The holes as well as the absorption spectra were all measured with a 1.5-m double-pass monochromator (Jobin-Yvon: THR1500) with a resolution of 0.03 cm- ‘. It should be stressed that one has to be very careful in measuring absorption change
668~67 (1996) 157-163
in the J-bands. Since they are so sharp, even a fraction of A wavelength shift in the monochromator creates some experimental artifacts. In order to avoid these and to ensure good reproducibility we retuned the wavelength setting of the instruments at every measurement and repeated several independent experiments for confirmation.
3. Results and discussion 3.1. Absorption
spectra
Fig. 1 shows the absorption spectra of the PIC(Br)/WEG system measured at 295K and 4.7K. Absorbances in this paper are defined as _ log,,,(Z/lO), where I and I0 are the measured intensities of the transmitted and incident light, respectively. While the sample at room temperature shows a spectrum of typical monomeric dispersion, the spectrum at 4.7K shows two excitonic bands. They occur as a consequence of aggregate growth over the temperature region between 230 and 220 K for several minutes followed by quenching at a rate of - 7 K/min. This growth condition basically provides upper blue J-band absorption at 570.8 nm though a small amount of red J-band absorption at 576.0 nm still remains. 2
“1”,,1”“1”“1”,‘)’ PIC(Br)/WEG
j
4.7K
;
!
295 K
400
450
500 Wavelength
1
550
600
(nm)
Fig. I. Absorption spectra of PIC-Br/WEG system observed at 295 K and at 4.7 K. The latter was obtained as a consequence of the aggregate formation at temperatures between 230 and 220 K followed by quenching at a rate of _ 7 K/min below 77 K.
159
T. Tani et al. /Journal of Luminescence 66& 67 (1996) 157- 163
The widths of the blue J-band are between 35 and 40cm-’ depending upon the growth conditions. The monomer spectrum at 295 K shows that the original absorption has an inhomogeneous bandwidth of - 1200cm- ‘. Assuming this number is roughly constant down to 4.7K, a reduction of the inhomogeneous bandwidth is by a factor of 30. An exciton moving over an aggregate chain comprising N monomer molecules narrows the inhomogeneous bandwidth of the monomer absorption band by a factor of the order of l/fi [7]. Simple application of this relation gives rise to an aggregate length of about 900 in our samples. Since, a part of the inhomogeneity, in the case of monomers, could be due to small rotations of the quinoline rings with respect to one another [2], we can assume that the size of the delocalization length in our sample is also of the order of 100. According to exciton theory [S], the energy shift of the exciton band from the monomer peak is 2V, where I’ is a coupling constant between nearest neighbor monomers in the chain. From the red shift of the blue J-band of 1540cm- ‘, I/ is of the order of - 770 cm-‘. Hence, the width of the exciton band 14VI is of the order of 3070cm-’ for the blue J-band.
0.2 5.28
0
1:
DO
4.7 K
;8 -0.6
I
570
II
i B=570.91
I.
1
570.5
571
nm
Wavelength (nm)
3.2. DC-light burning
Fig. 2. Fluence dependence of hole formation at 4.7 K by the irradiation of DC single-mode peak of the upper exciton band.
Experimental results of persistent hole burnings are shown in Fig. 2 as absorption-difference spectra. They were formed at 4.7K by a step-bystep sequential irradiation of the DC laser light in the blue J-band of PIC(Br)/WEG. The hole, after the most prolonged bleaching, was also measured over a wider spectral region and compared with the exciton band structures, as shown in Fig. 3. In the figure, the hole obtained by the pulsed laser burning is included for comparison, details of which are described below. The negative part characterizes the educt states and the positive part the product states, respectively. As can be seen in Fig. 3, laser irradiation into the blue exciton peak never leads to photochemical changes in the red band. Exactly the same behavior was also observed for the red J-band burning [9]. Hence, both bands are decoupled as is observed in PIC-Cl and PIC-I [4].
The notable characteristics deduced from Figs. 2 and 3 are summarized as follows: (1) Sharp resonance holes are formed precisely at the wavelength of the irradiated laser light within our experimental accuracy. Its full width, extrapolated to zero burning fluence, is of the order of 0.36cm-’ at 4.7 K. We can safely conclude that they are zero-phonon holes and, therefore, the linewidth of the absorption band described in Section 3.1 is inhomogeneous in nature. (2) Assuming the effective number of monomers in the aggregated chain is - 100, over which the exciton is delocalized, we can deduce a quantum efficiency of hole formation from the data; the estimated number is 1.8 x lo-“. The efficiency of the zero-phonon formation is fairly low compared with the case of the pulsed laser burning and also with
profiles obtained laser light at the
T. Tani et al. 1 Journal ofluminescence
160
PIG-Br I WEG
759 mJ/cm*
---
565
570 Wavelength
575
580
(nm)
Fig. 3. Two excitonic peaks (top) and the hole prolonged bleaching at upper exciton peak (middle) two exciton bands are decoupled. The hole profile pulsed-laser burning is also shown for comparison
profile after to show the obtained by (bottom).
the efficiency of 2.8 x lop5 from the red-band burning [9]. (3) No detectable phonon or pseudo-phonon holes appeared for our experimental conditions. This reveals that the exciton-phonon coupling in the aggregate chain is quite small. (4) Though the products appear at longer as well as shorter wavelengths from the educt, the center of gravity of the spectrum is at shorter wavelength. (5) The spectral distribution range of the products, which is detectable in the present experiment, roughly coincides with half of the inhomogeneous absorption band. It seems smaller than that observed for the pulsed laser burning which is described later. (6) The oscillator strength of the product is, to some extent, smaller than that of the educt state.
M&i67 (1996) 157-163
Assuming a Lorentzian line shape and taking the instrumental resolution into account, the width of the hole, 0.36cm-i, gives 0.18 cm- ’ for the homogeneous width Aw,,. In the one-phonon approximation, the related scattering process between the k = 1 exciton state and the phonons, which causes the broadening of the homogeneous width, is governed by strong restrictions on energy and momentum. As a consequence, at temperatures around 4.7 K, no scattering processes are expected to take place [2,3]. Hence, here we can assume that the Awi, observed is attributed to the residual width and we can deduce a lifetime T1 of 28 ps, again in close agreement with [3] and other time-dependent experiments [2, lo]. The persistency or lifetime of the hole seems to be long. Within our experimental time scale of 103-lo4 s, the degree of hole-filling is of the order of 1% in depth and the contribution of spectral diffusion was not detected. Even though the efficiency decreased to a great extent, site-selective and sufficiently long-life holes of 2.2cm- ’ width were formed at 77 K. A further comment is related to item (4). We tried filling the hole, which was obtained after fluence of 759mJ/cmf, by further irradiation with laser light tuned 3.5A to the longer wavelength side of the hole as shown in Fig. 4 (2). The fluence was 575mJ/cm’. After the filling profile was gecorded, the sample was irradiated again at 3.OA on the shorter wavelength side of the initial hole, and filling of the two previous holes was observed (Fig. 4 (3)). Again the fluence was 575mJ/cm2. In all cases, filling of holes of about 20% with no detectable broadening was observed. This fact is notable because the filling process observed does not seem to depend on the wavelength difference betwOeenthe hole and the filling light. Separation of 6.5 A is out of the distribution range of the product states described in item (4). This fact may indicate that there are two kinds of product states, one of which is distributed over a rather restricted region and the other is over the whole inhomogeneous band. It is also inferred from the filling profiles that the burning reaction, the product of which is distributed over the whole inhomogeneous band, is highly photoreversible as was pointed out in PIG-I [3].
T. Tani et al. /Journal
/I
570
of Luminescence
M&67
(
I
I1
571
570.5
Wavelength
161
(1996) 157-163
(nm)
First, the hole (1) at 570.61 nm was produced by irradiation with fluence of 759 mJ/cm’. Second, Fig. 4. Hole filling by light irradiation. hole filling as shown here by the change in absorption, was done with irradiation at 570.96 nm (2) with fluence of 575 mJ/cm’. Lastly, the sample was irradiated at 573.00 nm (3) with 575 mJ/cm’ and the absorption change shows filling properties of both of the previous holes; (4) shows the laser-line profile
3.3. Pulsed-light
burning t
Fig. 5 shows typical hole spectra of the same system obtained by the irradiation of the light from the pulsed laser. Experimental conditions were almost identical to each other: average power densities of the burning light were carefully tuned to be almost equal for both experiments. The only difference is the use of the pulsed-mode laser instead of the single-mode DC laser for burning. As is clearly shown in the figure, observed hole profiles are quite different from those obtained by the DC light laser previously described. The holes formed after prolonged bleaching were also measured over a wider spectral region to check the interaction between the upper and lower exciton bands. As can be seen in Fig. 3, it is definite that the upper and lower exciton bands are decoupled from each other. The most notable features are as follows: Instead of narrow holes in resonance with the laser frequency oB, rather broad structures with dispersive lineshapes appeared even at the very initial stage of burning within our experimental accuracy. The lineshape is almost perfectly centro-symmetric at 0s. In other words, a hole below wB and an antihole above wB with equal area appear simultaneously. Upon increasing the fluences of the laser
4.7 K
0.09 mJ/cm2
0.90 mJ/cm’ 8 5 $j
10.8 mJicm*
0.4
Wavelength
(nm)
Fig. 5. Fluence dependence of hole formation profiles obtained at 4.7 K by pulsed laser light at the peak of the upper exciton band. Other conditions, such as average power density of the laser light, were kept almost identical to those in Fig. 2.
162
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light, the hole and antihole components show complicated behavior as is seen in Fig. 5. The characteristics appearing in Figs. 5 and 3 are summarized as follows: (1) There is a conservation of oscillator strength in the low fluence limit; the area of the hole is almost equal to the area of the antihole. (2) Upon increasing the fluence, the area of the hole increases more rapidly. The lineshape also changes from the simple dispersive shape which is centro-symmetric at c~)a through an asymmetric shape to the shape of the second derivatives of symmetric lines such as Lorentzian. The spectral range of the product eventually coincides with the inhomogeneous band. (3) In the prolonged bleaching stage, a kink or small but broad hole-like structure in resonance with wri appears [3]. This could be understood if rather strong linear exciton-phonon coupling (e.g. Huang-Rhys factor S of - 2.5) and unusually low energy for a phonon coupled to an optical transition (d - 2.7cm-‘) were assumed as is suggested in Ref. [7]. We think, however, that further investigation is required prior to adopting this simple consideration, as described below. The basic questions to be solved are as follows: Why the difference of the operation modes of the burning lasers could result in so drastic a change of the hole formation profiles from the initial stage of burning? What are the origins for the photoreactions? What is the nature of the inhomogeneous line broadening? At the moment, we have no definite answers for these questions. If we could assume simply a one-photon process in the excitation for both types of irradiation, the results of the pulsed mode burning could be interpreted by, e.g., power broadening. To our knowledge, however, such peculiar saturation phenomenon has never been observed either in dye-doped polymers or glasses nor in color centers or impurities in inorganic systems. We considered so far that the dispersive lineshape of the hole by pulsed light irradiation is an indication of the peculiarity of the electronic state and its dynamical property which are inherent to the Frenkel exciton of large coherence length. In the case of DC-light irradiation, consideration of
668~67 (1996)
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optical excitation to the one-exciton band is enough. Even with our nanosecond laser, on the other hand, the peak power of the pulse is extended from the average power by as much as a factor of N 10’ (i.e. Y 1 MW/cm’) due to its low duty cycle. The energy per pulse easily exceeds 10uJ/cm2which is sufficient to induce an 1 mJ/cm’ optical non-linear response [3, lo]. The absorption difference spectrum observed by a two-color pump-probe experiment clearly shows the negative contribution due to bleaching and stimulated emission of the ground state to the one-exciton transitions, while positive contributions arise from induced absorption from the excited one-exciton state to a two-exciton state on the same aggregate [lo-121. Photoreactions from the two-exciton state will be sure to reduce the population of the initial one-exciton state. Hence, we can detect the population change by measuring the one-exciton spectrum with weak probe light. Superposition of the non-site-selective absorption contribution of the product states, which are more efficiently pr&ided than one-exciton band burning, can give rise to the dispersive hole/anti-hole structure. Further experiments, as well as theoretical considerations, are necessary to make clear the role of the excitonic transitions and the dynamics such as superradiant decay process in the hole-formation process and the nature of inhomogeneity.
4. Summary We have examined the hole formation profiles of the exciton states in PIC-Br/WEG system using both a DC laser and a nanosecond pulsed laser. In the former, a site selective zero-phonon hole was formed with a width of 0.36cm- ‘. Assuming negligible contribution of the dephasing by phonons, the homogeneous width of the one exciton state is lifetime-limited in nature and T1 N 28~s. In the latter we found, from the initial stage of burning, non-site selective hole/anti-hole formation with dispersive lineshape. This is ascribed to the excitation of two-exciton bands, which is inherent to excitonic states with large coherence length.
T. Tani et ul. i Journal
of Luminescence
References and F. Sasaki, Nonlinear Opt. 4 (1993) El1 S. Kobayashi 305. PI S. de Boer, K.J. Vink and D.A. Wiersma. Chem. Phys. Lett. 137 (1987) 99. and J. Friedrich, J. Chem. Phys. 91 (1989) 131 R. Hirshmann 7988; J. Opt. Sot. Am. B 9 (1992) 811. W. Kijhler. J. Friedrich and E. Daltrozzo, [41 R. Hirshmann, Chem. Phys. Lett. 151 (1988) 60. c51 H. Fidder, J. Knoester and D.A. Wiersma. Chem. Phys. Lett. 171 (1990) 529. C61 S. de Boer and D.A. Wiersma. Chem. Phys. Lett. 165 (1990) 45.
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