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Large absorption reduction for mesoscopic thiacyanine J aggregates in solution
1 June 2001
Chemical Physics Letters 340 (2001) 211±216
www.elsevier.nl/locate/cplett
Large absorption reduction for mesoscopic thiacyanine J aggregates in solution Hiroshi Yao *, Keisaku Kimura Department of Material Science, Faculty of Science, Himeji Institute of Technology, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan Received 15 February 2001; in ®nal form 1 March 2001
Abstract An apparent large absorption reduction was observed for a thiacyanine J aggregate in solution. Since ¯uorescence microscopy proved that the J aggregate formed a mesoscopic rod-like structure (several tens of micrometers in length and about 1 lm in width) in solution, the reduction was due to that of the apparent absorption cross-section caused by formation of the aggregate particles, indicating that no light transmitted through the region of particles composed of strongly absorbing chromophore units with the spectroscopic characteristics of J aggregates. The absorption behaviors were also simulated quantitatively by the present model. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction J aggregates are speci®c molecular assemblies discovered by Jelly and Scheibe [1±4], and characterized by a narrow and intense absorption band (J band) that shows a bathochromic shift compared to the relevant monomer band [5]. On the basis of such optical characteristics, J aggregates are often used as spectral sensitizers in photography [6]. Recent interest has also focused on the ability of J aggregates to exhibit collective coherent excitation phenomena, such as superradiance, which provide large optical nonlinearities [7,8]. Various theoretical models have been proposed to describe the relationship between the physical/optical properties and the molecular arrangement within the aggregates [9±11]. These models show
*
Corresponding author: Fax: +81-791-58-0161. E-mail address: [email protected] (H. Yao).
that the aggregate structure strongly re¯ects its spectroscopic properties such as the spectral line shape and peak energy, therefore, detailed investigations of the structures of aggregates are of primary importance. On the other hand, molecular self-assembling processes are also signi®cant in regard to the formation of supramolecular structures because of their involvement in many fundamental physicochemical as well as biological processes [12]. The possibility of changing the supramolecular structure through a proper choice of functional molecules opens the way to the design of materials capable of exhibiting speci®c optical properties. We have been investigating the structures of J aggregates self-assembled in the ®eld associated with a solution phase, and demonstrated previously that a pseudoisocyanine (PIC) dye produces J aggregates at a mica/water interface possessing a mesoscopic supramolecular island structure as elucidated by tapping mode atomic force microscopy
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 3 7 5 - X
212
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
(AFM) [13,14]. Although dierences in supramolecular structures and/or related optical properties are expected between the J aggregates at a solid/ liquid interface and those produced in bulk solution [15], details have been poorly understood since a real structure or morphology of the aggregates in solution has been hard to observe in situ. In this study, we report a mesoscopic rod-like structure of a thiacyanine J aggregate in aqueous solutions as revealed by ¯uorescence microscopy. We also characterize the optical properties of the mesoscopic aggregate that causes an apparent large absorption reduction for the J band.
and intense J band at 464 nm. The peak position of the J band was almost unchanged under these conditions, however, the line width was increased with increasing the concentration under TC P 0:2 mM. Consequently, the absorbance of the J band at TC 0:4 mM became apparently smaller than that at TC 0:2 mM. When the absorption spectrum was measured with a thinlayer optical cell (path length: a few micrometers) for the sample solutions at TC 0:2 and 0.4
2. Experimental 5,50 -Dichloro-3,30 -disulfopropyl thiacyanine sodium salt (abbreviated as TC) was purchased from Nippon Kankoh-Shikiso Kenkyusho and used as received. Sample solutions were prepared by dissolving a TC dye in an aqueous NaCl (5.0 mM) solution under moderate heating (<60°C) followed by prolonged equilibration at room temperature. Absorption spectra were measured with a Hitachi U-3210 spectrophotometer. Fluorescence microscope images were obtained by using a color CCD camera (Flovel; HCC-600) set on an optical microscope (Olympus; BX-60). An Hg lamp (Ushio; USH102D, 100W) being passed through a mirror cube unit (Olympus; U-MNBV) was used as an excitation source for the TC J aggregate under the microscope. This cube unit also eliminates the scattering of the excitation light to obtain clear ¯uorescence micrographs. A focusing depth for the microscope observations was about 200 nm. 3. Results and discussion 3.1. Characterization of thiacyanine J aggregates in solutions by absorption spectroscopy and ¯uorescence microscopy Fig. 1a shows absorption spectra of TC in aqueous NaCl solutions at dierent concentrations with a set optical path length of 300 lm. When TC P 0:05 mM, the absorption showed a sharp
Fig. 1. (a) Absorption spectra of TC J aggregates in aqueous NaCl solutions (5 mM) at dierent concentrations with a set optical path length of 300 lm. A large absorption reduction was observed for TC 0:4 mM. (b) Absorption spectra of TC J aggregates at TC 0:4 and 0.2 mM with an optical path length of a few micrometers.
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
mM, a reduction in the J band was not detected as shown in Fig. 1b. Moreover, the spectral line width 450 10 cm 1 : FWHM) became quite similar to that observed at TC < 0:2 mM. Therefore, the observed absorption reduction under high dye concentrations was due to that of the apparent absorption cross-sectional area of the aggregates. It is worth noting that the apparent reduction of absorption cross-sectional area has been known to occur in the presence of strongly absorbing chromophores forming three-dimensional particles [16,17]. Actually, distinctive opalescence was observed for concentrated TC solutions, indicating that granular J aggregates generate in solutions. Thus, ¯uorescence microscopy was conducted to determine the microstructures of the aggregates with highly emissive characteristics. Fig. 2a,b show typical ¯uorescence micrographs at TC 0:4 and 0.05 mM, respectively. Mesoscopic rod structures were clearly observed at TC 0:05 mM, whereas an obscure image obtained at TC 0:4 mM shows that many rods were high-densely distributed in the solution. Because a characteristic ¯uorescence image was not detected below 0.05 mM (concentration regions in the absence of J aggregation), the rods distributed in solutions were exclusively the mesoscopic J aggregates of TC molecules. Note that irrespective of an increase in the TC concentration, the morphology and size of the aggregates are almost the same. The length of the rod was several tens of micrometers while the width was about 1 lm. Considering the molecular size of TC, 1010 monomers are approximately included in the single mesoscopic rod-like aggregate. Similar rod-like morphology has been observed for micelles of amphiphilic molecules, and explained by suitable geometrical packing of the molecules [18]. We suppose that not only interactions between the p-electron systems but also suitable geometrical packing of TC molecules contribute to the rod shape of the aggregate. Therefore, the observed absorption reduction of the J band is to be interpreted persuasively by the model based on the apparent reduction of absorption cross-sectional area due to formation of three-dimensional particles with the J aggregate [16,17].
213
Fig. 2. Fluorescence micrographs of the mesoscopic TC J aggregates in aqueous NaCl solutions. (a) and (b) shows images at TC 0:4 and 0.05 mM, respectively.
3.2. Simulations of the absorption reduction of the J band The absorption reduction has been treated theoretically by Penzkofer et al. [16]. The model shows that the apparent absorption cross-section per molecule decreases with rising the size of the three-dimensional aggregate particle composed of strongly absorbing chromophores (i.e., units with the spectroscopic characteristics of J aggregates) due to speci®c surface reduction of the aggregate.
214
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
The absorption behaviors are schematically illustrated in Fig. 3. The illustration shows an optical cell including a sample solution composed of threedimensional aggregate particles. The aggregate particles are constructed with strongly absorbing chromophore units (opaque chromophores). A light passes from the left- to the right-hand side. Under these conditions, a light passes only through regions uncovered by the chromophores whereas no light transmits through the chromophore (or particle) areas by an intense absorption. Qualitatively, chromophores and particles behind the light-absorbing ®rst chromophore do not contribute to the apparent absorption, leading to an absorption reduction.
Quantitatively, the apparent absorption crosssection per molecule in an aggregate particle rpar can be expressed as [16] rpar
1 1
exp fpar Apar rchr ; exp fchr Achr bchr
1
where fpar and Apar are the absorption strength and the cross-sectional area of one particle, respectively. fchr and Achr are the those of a chromophore, respectively. bchr and rchr are the number of chromophores in the particle and the apparent absorption cross-section per molecule in one chromophore, respectively. rchr Achr =n fchr , where n is the number of molecules producing a chromophore (exciton delocalization number), indicates that rchr is proportional to fchr . Note that fpar and Apar are the functions of fchr , Achr and bchr . In the case of a three-dimensional cube, Apar and 2=3 fpar are expressed as Achr bchr and 1=3 fchr bchr , respectively [16]. For a large-sized aggregate particle (large bchr ) composed of strongly absorbing chromophore units, fpar results in a very large value which can lead to a very small value of exp fpar . Taking a relative optical path length (d) into Eq. (1), the apparent absorption, r0par , is given as r0par K
1
1=3
exp fchr bchr d 1 fchr ; 1=3 1 exp fchr d bchr 2
Fig. 3. Schematic illustration of light transmission behavior of the aggregate particles composed of strongly absorbing chromophores. A large cube (corresponding to a aggregate particle) includes small cubes (corresponding chromophores). Dierence between (a) and (b) shows the relative optical path lengths.
where K shows a proportional factor. Therefore, we can expect a decrease in r0par with increasing in d under the same fchr and bchr values according to Eq. (2). The observed mesoscopic J aggregate of a thiacyanine dye possessed a three-dimensional rod-like structure. Considering the rod is an assembly of such cubes, it is reasonable that the absorption reduction of the TC J band can be approximately simulated by using Eq. (2). Fig. 4a shows the relative optical path length dependence of the spectral shape of the J band simulated with Eq. (2). In the simulations, the line shape of the J band shown in Fig. 1b (thin-layer solution ®lm, TC 0:4 mM) was used for fchr with a peak value of 0.1 [16]. Because an exciton delocalization number of other
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
Fig. 4. (a) Relative optical path length (d) dependence of the simulated spectral shape of the TC J band. A large apparent absorption reduction was observed with increasing in d. (b) Dotted curve: measured absorption spectrum of the TC J aggregate (TC 0:4 mM, optical path length 300 lm). Solid curve: absorption spectrum simulated on the basis of Eq. (2) with parameters of bchr 1010 and d 0:022.
thiacyanine J aggregates is reported to be about 7± 12 [19], the physical size of the observed TC J aggregate is expected to be much larger than the delocalization size of the aggregate [20]. Furthermore, because the morphology and the inherent spectral line width of the J aggregate were independent on the TC concentration, bchr 1010 , corresponding to the physical size of the aggregate, was assumed here according to the mesoscopic J aggregate observations. When d is very small d 6 1 10 4 , simulated spectral shapes were the
215
same with each other. However, an apparent absorption reduction occurred with an increase in d, and the spectral shape became largely distorted. This behavior is similar to that observed for the mesoscopic TC J aggregate in solutions as described in the Section 3.1. Fig. 4b shows the best ®t for the TC J band at 0.4 mM (cell path length 300 lm) with a ®tting parameter d 0:022. The observed spectrum was satisfactorily reproduced by the simulation. The results provide a relationship between the relative optical path length (d) and the size of the rod aggregate. Imagine that a concentrated solution of rod-like J aggregates (length: several tens of micrometers, width: 1 lm) is introduced into an optical cell (real optical path length: L). Geometrically, L 1 lm (width of the rod aggregates) is considered to be the minimum path length. An apparent absorption decreases with increasing L because the rods can be overlapped with each other in the cell. The apparent absorption reduction can occur even at L 1 lm due to a three-dimensional structure of the rods, however, the reduction is supposed to be very small and inevitable for the measurements. Thus, L 1 lm is approximately equivalent to d 1 10 4 , the maximum d value where no absorption reduction occurs in the simulation. Considering that d is proportional to L, d 0:022 corresponds to L 220 lm. The estimated L value is similar to the experimental optical path length (300 lm), indicating that the obtained d parameter is consistent for the concentrated rod system. We should note that the simulated spectral shape is dependent on fchr , bchr and d; however, our estimations suggest that these parameters are reasonable for the TC J aggregate system. Therefore, we concluded that the observed large absorption reduction with a distortion of the spectral shape was due to the eects of formation of mesoscopic three-dimensional particles (rods) with strongly absorbing chromophore units the spectroscopic characteristics of the J aggregate. 4. Conclusions It was revealed that a 5,50 -dichloro-3,30 -disulfopropyl thiacyanine (TC) dye formed mesoscopic
216
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
rod-like J aggregates at TC P 0:05 mM in aqueous NaCl (5 mM) solutions by ¯uorescence microscopy. Under a high TC concentration, a large absorption reduction was observed for the J band. This absorption behavior was quantitatively characterized by the model based on the reduction of an apparent absorption cross-section caused by the formation of a three-dimensional aggregate particle constructed with strongly absorbing units with the spectroscopic characteristics of J aggregate. Acknowledgements H.Y. acknowledges the partial support by a Grant-in-Aid (12640563) from the Ministry of Education, Culture, Sports, Science and Technology. References [1] [2] [3] [4] [5]
E.E. Jelley, Nature 138 (1936) 1009. E.E. Jelley, Nature 139 (1937) 631. G. Scheibe, Angew. Chem. 49 (1936) 563. G. Scheibe, Angew. Chem. 50 (1937) 212. K. Norland, A. Ames, T. Taylor, Photogr. Sci. Eng. 14 (1970) 296.
[6] A.H. Herz, The Theory of the Photographic Process, fourth ed., Macmillan, New York, 1977, Chapter 10. [7] E. Hanamura, Phys. Rev. B 37 (1988) 1273. [8] F.C. Spano, J.R. Kuklinski, S. Mukamel, J. Chem. Phys. 94 (1991) 7534. [9] E.W. Knapp, Chem. Phys. 85 (1984) 73. [10] P.O.J. Scherer, S.F. Fisher, Chem. Phys. 86 (1984) 269. [11] E.O. Potma, D.A. Wiersma, J. Chem. Phys. 108 (1998) 4897. [12] S.H. Chen, J.S. Huang, P. Tartaglia, Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution, Kluwer Academic, Dordrecht, 1992. [13] H. Yao, S. Sugiyama, R. Kawabata, H. Ikeda, O. Matsuoka, S. Yamamoto, N. Kitamura, J. Phys. Chem. B 103 (1999) 4452. [14] S.S. Ono, H. Yao, O. Matsuoka, R. Kawabata, N. Kitamura, S. Yamamoto, J. Phys. Chem. B 103 (1999) 6909. [15] H. Yao, M. Omizo, N. Kitamura, Chem. Commun. (2000) 739. [16] W. Weigand, F. Rotermund, A. Penzkofer, J. Phys. Chem. A 101 (1997) 7729. [17] S. Lowell, J.E. Shields, Powder Surface Area and Porosity, third ed., Chapman & Hall, London, 1991. [18] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985. [19] P. Argyrakis, D.M. Basko, M.A. Drobizhev, A.N. Lobanov, A.V. Pimenov, O.P. Varnavsky, M. Van der Auweraer, A.G. Vitukhnovsky, Chem. Phys. Lett. 268 (1997) 372. [20] T. Kobayashi, Supramol. Sci. 5 (1998) 343.
Chemical Physics Letters 340 (2001) 211±216
www.elsevier.nl/locate/cplett
Large absorption reduction for mesoscopic thiacyanine J aggregates in solution Hiroshi Yao *, Keisaku Kimura Department of Material Science, Faculty of Science, Himeji Institute of Technology, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan Received 15 February 2001; in ®nal form 1 March 2001
Abstract An apparent large absorption reduction was observed for a thiacyanine J aggregate in solution. Since ¯uorescence microscopy proved that the J aggregate formed a mesoscopic rod-like structure (several tens of micrometers in length and about 1 lm in width) in solution, the reduction was due to that of the apparent absorption cross-section caused by formation of the aggregate particles, indicating that no light transmitted through the region of particles composed of strongly absorbing chromophore units with the spectroscopic characteristics of J aggregates. The absorption behaviors were also simulated quantitatively by the present model. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction J aggregates are speci®c molecular assemblies discovered by Jelly and Scheibe [1±4], and characterized by a narrow and intense absorption band (J band) that shows a bathochromic shift compared to the relevant monomer band [5]. On the basis of such optical characteristics, J aggregates are often used as spectral sensitizers in photography [6]. Recent interest has also focused on the ability of J aggregates to exhibit collective coherent excitation phenomena, such as superradiance, which provide large optical nonlinearities [7,8]. Various theoretical models have been proposed to describe the relationship between the physical/optical properties and the molecular arrangement within the aggregates [9±11]. These models show
*
Corresponding author: Fax: +81-791-58-0161. E-mail address: [email protected] (H. Yao).
that the aggregate structure strongly re¯ects its spectroscopic properties such as the spectral line shape and peak energy, therefore, detailed investigations of the structures of aggregates are of primary importance. On the other hand, molecular self-assembling processes are also signi®cant in regard to the formation of supramolecular structures because of their involvement in many fundamental physicochemical as well as biological processes [12]. The possibility of changing the supramolecular structure through a proper choice of functional molecules opens the way to the design of materials capable of exhibiting speci®c optical properties. We have been investigating the structures of J aggregates self-assembled in the ®eld associated with a solution phase, and demonstrated previously that a pseudoisocyanine (PIC) dye produces J aggregates at a mica/water interface possessing a mesoscopic supramolecular island structure as elucidated by tapping mode atomic force microscopy
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 3 7 5 - X
212
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
(AFM) [13,14]. Although dierences in supramolecular structures and/or related optical properties are expected between the J aggregates at a solid/ liquid interface and those produced in bulk solution [15], details have been poorly understood since a real structure or morphology of the aggregates in solution has been hard to observe in situ. In this study, we report a mesoscopic rod-like structure of a thiacyanine J aggregate in aqueous solutions as revealed by ¯uorescence microscopy. We also characterize the optical properties of the mesoscopic aggregate that causes an apparent large absorption reduction for the J band.
and intense J band at 464 nm. The peak position of the J band was almost unchanged under these conditions, however, the line width was increased with increasing the concentration under TC P 0:2 mM. Consequently, the absorbance of the J band at TC 0:4 mM became apparently smaller than that at TC 0:2 mM. When the absorption spectrum was measured with a thinlayer optical cell (path length: a few micrometers) for the sample solutions at TC 0:2 and 0.4
2. Experimental 5,50 -Dichloro-3,30 -disulfopropyl thiacyanine sodium salt (abbreviated as TC) was purchased from Nippon Kankoh-Shikiso Kenkyusho and used as received. Sample solutions were prepared by dissolving a TC dye in an aqueous NaCl (5.0 mM) solution under moderate heating (<60°C) followed by prolonged equilibration at room temperature. Absorption spectra were measured with a Hitachi U-3210 spectrophotometer. Fluorescence microscope images were obtained by using a color CCD camera (Flovel; HCC-600) set on an optical microscope (Olympus; BX-60). An Hg lamp (Ushio; USH102D, 100W) being passed through a mirror cube unit (Olympus; U-MNBV) was used as an excitation source for the TC J aggregate under the microscope. This cube unit also eliminates the scattering of the excitation light to obtain clear ¯uorescence micrographs. A focusing depth for the microscope observations was about 200 nm. 3. Results and discussion 3.1. Characterization of thiacyanine J aggregates in solutions by absorption spectroscopy and ¯uorescence microscopy Fig. 1a shows absorption spectra of TC in aqueous NaCl solutions at dierent concentrations with a set optical path length of 300 lm. When TC P 0:05 mM, the absorption showed a sharp
Fig. 1. (a) Absorption spectra of TC J aggregates in aqueous NaCl solutions (5 mM) at dierent concentrations with a set optical path length of 300 lm. A large absorption reduction was observed for TC 0:4 mM. (b) Absorption spectra of TC J aggregates at TC 0:4 and 0.2 mM with an optical path length of a few micrometers.
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
mM, a reduction in the J band was not detected as shown in Fig. 1b. Moreover, the spectral line width 450 10 cm 1 : FWHM) became quite similar to that observed at TC < 0:2 mM. Therefore, the observed absorption reduction under high dye concentrations was due to that of the apparent absorption cross-sectional area of the aggregates. It is worth noting that the apparent reduction of absorption cross-sectional area has been known to occur in the presence of strongly absorbing chromophores forming three-dimensional particles [16,17]. Actually, distinctive opalescence was observed for concentrated TC solutions, indicating that granular J aggregates generate in solutions. Thus, ¯uorescence microscopy was conducted to determine the microstructures of the aggregates with highly emissive characteristics. Fig. 2a,b show typical ¯uorescence micrographs at TC 0:4 and 0.05 mM, respectively. Mesoscopic rod structures were clearly observed at TC 0:05 mM, whereas an obscure image obtained at TC 0:4 mM shows that many rods were high-densely distributed in the solution. Because a characteristic ¯uorescence image was not detected below 0.05 mM (concentration regions in the absence of J aggregation), the rods distributed in solutions were exclusively the mesoscopic J aggregates of TC molecules. Note that irrespective of an increase in the TC concentration, the morphology and size of the aggregates are almost the same. The length of the rod was several tens of micrometers while the width was about 1 lm. Considering the molecular size of TC, 1010 monomers are approximately included in the single mesoscopic rod-like aggregate. Similar rod-like morphology has been observed for micelles of amphiphilic molecules, and explained by suitable geometrical packing of the molecules [18]. We suppose that not only interactions between the p-electron systems but also suitable geometrical packing of TC molecules contribute to the rod shape of the aggregate. Therefore, the observed absorption reduction of the J band is to be interpreted persuasively by the model based on the apparent reduction of absorption cross-sectional area due to formation of three-dimensional particles with the J aggregate [16,17].
213
Fig. 2. Fluorescence micrographs of the mesoscopic TC J aggregates in aqueous NaCl solutions. (a) and (b) shows images at TC 0:4 and 0.05 mM, respectively.
3.2. Simulations of the absorption reduction of the J band The absorption reduction has been treated theoretically by Penzkofer et al. [16]. The model shows that the apparent absorption cross-section per molecule decreases with rising the size of the three-dimensional aggregate particle composed of strongly absorbing chromophores (i.e., units with the spectroscopic characteristics of J aggregates) due to speci®c surface reduction of the aggregate.
214
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
The absorption behaviors are schematically illustrated in Fig. 3. The illustration shows an optical cell including a sample solution composed of threedimensional aggregate particles. The aggregate particles are constructed with strongly absorbing chromophore units (opaque chromophores). A light passes from the left- to the right-hand side. Under these conditions, a light passes only through regions uncovered by the chromophores whereas no light transmits through the chromophore (or particle) areas by an intense absorption. Qualitatively, chromophores and particles behind the light-absorbing ®rst chromophore do not contribute to the apparent absorption, leading to an absorption reduction.
Quantitatively, the apparent absorption crosssection per molecule in an aggregate particle rpar can be expressed as [16] rpar
1 1
exp fpar Apar rchr ; exp fchr Achr bchr
1
where fpar and Apar are the absorption strength and the cross-sectional area of one particle, respectively. fchr and Achr are the those of a chromophore, respectively. bchr and rchr are the number of chromophores in the particle and the apparent absorption cross-section per molecule in one chromophore, respectively. rchr Achr =n fchr , where n is the number of molecules producing a chromophore (exciton delocalization number), indicates that rchr is proportional to fchr . Note that fpar and Apar are the functions of fchr , Achr and bchr . In the case of a three-dimensional cube, Apar and 2=3 fpar are expressed as Achr bchr and 1=3 fchr bchr , respectively [16]. For a large-sized aggregate particle (large bchr ) composed of strongly absorbing chromophore units, fpar results in a very large value which can lead to a very small value of exp fpar . Taking a relative optical path length (d) into Eq. (1), the apparent absorption, r0par , is given as r0par K
1
1=3
exp fchr bchr d 1 fchr ; 1=3 1 exp fchr d bchr 2
Fig. 3. Schematic illustration of light transmission behavior of the aggregate particles composed of strongly absorbing chromophores. A large cube (corresponding to a aggregate particle) includes small cubes (corresponding chromophores). Dierence between (a) and (b) shows the relative optical path lengths.
where K shows a proportional factor. Therefore, we can expect a decrease in r0par with increasing in d under the same fchr and bchr values according to Eq. (2). The observed mesoscopic J aggregate of a thiacyanine dye possessed a three-dimensional rod-like structure. Considering the rod is an assembly of such cubes, it is reasonable that the absorption reduction of the TC J band can be approximately simulated by using Eq. (2). Fig. 4a shows the relative optical path length dependence of the spectral shape of the J band simulated with Eq. (2). In the simulations, the line shape of the J band shown in Fig. 1b (thin-layer solution ®lm, TC 0:4 mM) was used for fchr with a peak value of 0.1 [16]. Because an exciton delocalization number of other
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
Fig. 4. (a) Relative optical path length (d) dependence of the simulated spectral shape of the TC J band. A large apparent absorption reduction was observed with increasing in d. (b) Dotted curve: measured absorption spectrum of the TC J aggregate (TC 0:4 mM, optical path length 300 lm). Solid curve: absorption spectrum simulated on the basis of Eq. (2) with parameters of bchr 1010 and d 0:022.
thiacyanine J aggregates is reported to be about 7± 12 [19], the physical size of the observed TC J aggregate is expected to be much larger than the delocalization size of the aggregate [20]. Furthermore, because the morphology and the inherent spectral line width of the J aggregate were independent on the TC concentration, bchr 1010 , corresponding to the physical size of the aggregate, was assumed here according to the mesoscopic J aggregate observations. When d is very small d 6 1 10 4 , simulated spectral shapes were the
215
same with each other. However, an apparent absorption reduction occurred with an increase in d, and the spectral shape became largely distorted. This behavior is similar to that observed for the mesoscopic TC J aggregate in solutions as described in the Section 3.1. Fig. 4b shows the best ®t for the TC J band at 0.4 mM (cell path length 300 lm) with a ®tting parameter d 0:022. The observed spectrum was satisfactorily reproduced by the simulation. The results provide a relationship between the relative optical path length (d) and the size of the rod aggregate. Imagine that a concentrated solution of rod-like J aggregates (length: several tens of micrometers, width: 1 lm) is introduced into an optical cell (real optical path length: L). Geometrically, L 1 lm (width of the rod aggregates) is considered to be the minimum path length. An apparent absorption decreases with increasing L because the rods can be overlapped with each other in the cell. The apparent absorption reduction can occur even at L 1 lm due to a three-dimensional structure of the rods, however, the reduction is supposed to be very small and inevitable for the measurements. Thus, L 1 lm is approximately equivalent to d 1 10 4 , the maximum d value where no absorption reduction occurs in the simulation. Considering that d is proportional to L, d 0:022 corresponds to L 220 lm. The estimated L value is similar to the experimental optical path length (300 lm), indicating that the obtained d parameter is consistent for the concentrated rod system. We should note that the simulated spectral shape is dependent on fchr , bchr and d; however, our estimations suggest that these parameters are reasonable for the TC J aggregate system. Therefore, we concluded that the observed large absorption reduction with a distortion of the spectral shape was due to the eects of formation of mesoscopic three-dimensional particles (rods) with strongly absorbing chromophore units the spectroscopic characteristics of the J aggregate. 4. Conclusions It was revealed that a 5,50 -dichloro-3,30 -disulfopropyl thiacyanine (TC) dye formed mesoscopic
216
H. Yao, K. Kimura / Chemical Physics Letters 340 (2001) 211±216
rod-like J aggregates at TC P 0:05 mM in aqueous NaCl (5 mM) solutions by ¯uorescence microscopy. Under a high TC concentration, a large absorption reduction was observed for the J band. This absorption behavior was quantitatively characterized by the model based on the reduction of an apparent absorption cross-section caused by the formation of a three-dimensional aggregate particle constructed with strongly absorbing units with the spectroscopic characteristics of J aggregate. Acknowledgements H.Y. acknowledges the partial support by a Grant-in-Aid (12640563) from the Ministry of Education, Culture, Sports, Science and Technology. References [1] [2] [3] [4] [5]
E.E. Jelley, Nature 138 (1936) 1009. E.E. Jelley, Nature 139 (1937) 631. G. Scheibe, Angew. Chem. 49 (1936) 563. G. Scheibe, Angew. Chem. 50 (1937) 212. K. Norland, A. Ames, T. Taylor, Photogr. Sci. Eng. 14 (1970) 296.
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