NANO LETTERS
Optical Spectroscopy on Individual amphi-PIC J-Aggregates
2005 Vol. 5, No. 12 2635-2640
Erwin Lang,† Alexander Sorokin,‡ Markus Drechsler,§ Yuri V. Malyukin,‡ and Ju1 rgen Ko1 hler*,† Experimental Physics IV and BIMF, UniVersity of Bayreuth, 95440 Bayreuth, Germany, Department of Nanocrystal Materials, STC “Institute for single crystals” NASU, Lenin aVe., 60, 61001 KharkoV, Ukraine, and Macromolecular Chemistry II, UniVersity of Bayreuth, 95440 Bayreuth, Germany Received June 15, 2005; Revised Manuscript Received October 19, 2005
Exploiting low-temperature single-molecule spectroscopic techniques allowed us to record fluorescence-excitation spectra from individual J-aggregates with unprecedented resolution. The spectra appear extremely heterogeneous, and significant differences with respect to the profile of the spectrum, the polarization dependence of the spectral features, and the photochemical/photophysical stability are revealed. The spectral heterogeneity is consistent with the structural heterogeneity observed with cryo-electron microscopy. Low-dimensional organic supramolecular structures such as conjugated polymers, natural pigment-protein complexes, or dendrimers have attracted considerable interest in recent years.1-5 Features such as fast and efficient energy transfer, nonlinear optical behavior, and tunability of spectral properties make molecular aggregates promising candidates for the development of new optoelectronic devices with tailored properties. In this regard a prominent example is represented by a class of materials known as J-aggregates which are supramolecular structures consisting of hundreds of noncovalently bound molecular building blocks arranged in various geometries. The various forms of J-aggregates have been subject of numerous experimental and theoretical studies6-14 because their electronically excited states feature interesting collective effects which can be observed as a strong red shift of the optical absorption and a drastic increase in oscillator strength with respect to the monomers. It turns out that the actual nature of the electronically excited states is determined by a complex interplay of various parameters, and despite a considerable effort, a returning theme in the literature is the degree of localization of the optical excitation.14-17 In particular the question is raised whether it should be possible to observe indications for a confinement of the excitation onto a relatively small segment of the J-aggregate.13,18,19 The great difficulty in analyzing the excited states of these systems arises from the fact that even at low temperatures details in the optical spectra are averaged out * Corresponding author. E-mail:
[email protected]. † Experimental Physics IV and BIMF, University of Bayreuth. ‡ Department of Nanocrystal Materials, STC NASU. § Macromolecular Chemistry II, University of Bayreuth. 10.1021/nl051132z CCC: $30.25 Published on Web 10/29/2005
© 2005 American Chemical Society
Figure 1. Schematical structure of amphi-PIC monomers.
due to the pronounced disorder in these materials. To avoid this complication, single-molecule techniques have been employed but were as yet restricted either to room temperature spectroscopy which yields relatively broad structureless spectra or to imaging techniques.20 Here we present a low-temperature spectroscopic study on J-aggregates consisting of 1-methyl-1′-octadecyl-2,2′cyanine perchlorate (amphi-PIC). The monomers are about 1.4 nm in size featuring a 2.6 nm long polymethylene tail; see Figure 1. In contrast to the monomer absorption at 18870 cm-1 the absorption of the J-aggregates appears at 17240 cm-1 (J-band) and features a line width of 380 cm-1 (full width at half-maximum, fwhm).21 The increase in oscillator strength is manifested by a decrease of the fluorescence lifetime from 2.8 ns for the monomers to 70 ps for the J-aggregates.22 To study individual amphi-PIC J-aggregates the amphi-PIC monomers have been solved in a binary water-dimethylformamide solution (W-DMF; 90:10) at low monomer concentration (∼10-5 mol/L). A drop of the solution was sandwiched between a LiF substrate and a quartz cover slip and formed a layer of about 10 µm thickness which was mounted in a cryostat. All experiments were carried out at 1.5 K. To perform fluorescence-excitation spectroscopy the samples were illuminated with a tunable dye laser (599, Coherent, spectral bandwidth 0.7 cm-1) through a home-built microscope that could be operated either in confocal or in widefield mode. The objective of the microscope had a numerical aperture of 0.90 (Microthek) and allowed a lateral resolution of 410 nm at λ ) 600 nm when immersed in liquid helium. The fluorescence of the aggregates is passed through a system of dielectric filters (HQ610LP, AHF) and focused onto a single-photon counting avalanche photodiode (SPCM-AQR-15, EG&G) in confocal
Figure 2. (a) Fluorescence-excitation spectra of J-aggregates of 1-methyl-1′-octadecyl-2,2′-cyanine perchlorate in a binary DMF-water solution. The top traces show the comparison between an ensemble spectrum (bold line) and the sum of 22 spectra recorded from single J-aggregates (solid line). The lower five traces are representative examples of individual J-aggregate spectra, averaged over typically 200 individual laser scans and all possible excitation polarizations. The vertical scale is valid for the lowest trace. All other traces were normalized to the lowest spectrum and offset for clarity. The spectra were recorded at 1.5 K with an excitation intensity between 0.1 and 1.1 W/cm2. (b) The same individual J-aggregate spectra as in part a now averaged only over a few adjacent laser scans that correspond to one particular polarization of the excitation.
mode or onto an electron multiplying CCD camera (DV 877 DCS-BV, Andor Technology) in widefield mode. The signals are recorded with computer-based multifunctional counter devices (National Instruments). The selection of an individual J-aggregate took place in two steps. First a widefield image of a 40 × 40 µm2 region of the sample was taken with the CCD camera to locate wellisolated single J-aggregates. Next the microscope was switched to the confocal mode such that the excitation volume coincided with one of the aggregates observed with the CCD camera. This allowed advantage to be taken of the superior background suppression of this mode. To obtain information on the dynamics of the system and to reduce light-induced effects, all spectra are recorded by scanning the excitation wavelength of the laser through the resonances in rapid succession (58 cm-1/s) and subsequent accumulation of many such scans in computer memory. For cryo-transmission electron microscopy studies, a drop of the sample was put on a copper grid or a lacey carbon filmed grid (Science Services), which was hydrophilized by glow discharge for 15 s. A sample droplet of 2 µL was put on the film, and subsequently most of the liquid was removed with blotting paper leaving a thin film stretched over the grid holes. The specimens were instantly shock frozen by rapid immersion into liquid ethane and cooled to approximately 90 K by liquid nitrogen in a temperaturecontrolled freezing unit (Zeiss Cryobox, Zeiss NTS GmbH). The temperature was monitored and kept constant in the chamber during all the sample preparation steps. After the specimens were frozen, the remaining ethane was removed using blotting paper. The specimen was inserted into a cryo2636
transfer holder (CT3500, Gatan) and transferred to a Zeiss EM922 EFTEM (Zeiss NTS GmbH). Examinations were carried out at temperatures around 90 K. The transmission electron microscopy (TEM) was operated at an acceleration voltage of 200 kV. Zero-loss filtered images (∆E ) 0 eV) were taken under reduced dose conditions (100-1000 e/nm2). All images were registered digitally by a bottom-mounted slow-scan CCD camera system (Ultrascan 1000) combined and processed with a digital imaging processing system (Digital Micrograph 3.10 for GMS 1.5). In Figure 2a we show a selection of fluorescence-excitation spectra from amphi-PIC J-aggregates. The upper traces show, for comparison, the spectrum from a bulk sample (bold) together with the spectrum that results from the summation of the spectra of 22 individual aggregates (black). The summed spectrum shows a blue shift of 100 cm-1 with respect to the bulk spectrum. The lower traces show some fluorescence-excitation spectra from single amphi-PIC Jaggregates. In striking contrast with the structureless inhomogeneously broadenend ensemble absorption spectrum, the spectra recorded from individual J-aggregates show a rich structure. Apparent are clear variations between the spectra with respect to the number of bands, spectral positions, line widths, and relative intensities. However, they have in common that a spectrally well separated narrow line appears on the low-energy side and that the spectral widths of the substructures increase from the red to the blue side of the spectra. Due to temporal averaging during acquisition of the spectra, many of these subtle details get masked in this display. Therefore we show in Figure 2b for each of the individual J-aggregates a spectrum that results from averaging Nano Lett., Vol. 5, No. 12, 2005
Figure 3. (a) Sequence of 200 consecutively recorded fluorescenceexcitation spectra stacked on top of each other (scan speed 58 cm-1/ s, excitation intensity 0.2 W/cm2). The fluorescence intensity is indicated by the gray scale. Between two successive scans the polarization of the excitation has been rotated by 3.6°. (b) The top trace represents the average of the stack of spectra shown in part a. The lower traces correspond to averages of the scans indicated by the horizontal boxed regions. The vertical scale is valid for the lowest spectrum; all other traces are offset for clarity. (c) Integrated intensity of the spectral region marked by a bar in part a as a function of the polarization of the excitation light (full line) together with a cos2-type function (dashed line) that serves as a guide for the eye.
only a few consecutive laser sweeps. This allows the pronounced spectral features to be observed more clearly despite the unavoidable loss in signal-to-noise ratio. In the following this will be illustrated for two examples in more detail. In Figure 3a we show spectrum 1 of Figure 2a in a two-dimensional representation as a stack of 200 fluorescence-excitation spectra recorded consecutively. Between two successive scans, the polarization of the incident radiation has been rotated by 3.6°. The horizontal axis corresponds to photon energy, the vertical axis corresponds to time (and vice versa to the polarization of the incident radiation), and the gray scale corresponds to the absorption intensity. The spectra that are extracted from the pattern are shown in Figure 3b. The top trace corresponds to the timeaveraged fluorescence-excitation spectrum that results from the summation of all traces of Figure 3a and is identical to spectrum 1 in Figure 2a. The other spectra correspond to the average of a few adjacent sweeps as indicated by the Nano Lett., Vol. 5, No. 12, 2005
boxed regions and uncover a narrow absorption at 17161 cm-1 with a line width of 2.3 cm-1 (fwhm) and a broad band at 17360 cm-1 with a line width of 350 cm-1 (fwhm). Figure 3c displays the intensity of the narrow and the broad absorption as a function of the polarization of the electric field of the incident excitation. Therefore the intensity has been integrated over a spectral window as indicated by a bar above the spectral pattern in Figure 3a. To do so, each individual scan was corrected for the background first. This turned out to be essential especially to observe the polarization dependence of the narrow line. Both intensity traces are displayed together with a cos2-type function that serves as a guide for the eye. Obviously, the polarization dependence of the intensity of the narrow line is well pronounced (modulation amplitude ≈50%) whereas the broad spectral feature shows only a weak variation of the intensity as a function of the polarization (modulation amplitude 50%) as evidenced in Figure 4c for two transitions. Details about the spectral positions, line widths, and the relative phase angles of the absorptions for this aggregate are summarized in Table 1. Roughly, the spectral features can be grouped into either spectrally narrow or broad absorption lines with a difference in phase angles between the two groups of about 80 ( 10°. The gross changes of the spectrum can be explained by the variation of the polarization superimposed by some spectral diffusion. These examples illustrate two extreme cases for the variations that have been observed in the spectra from individual J-aggregates with respect to both the line widths of the features and their dependence on the polarization. In total we studied 22 individual aggregates, and the obtained spectra appeared extremely heterogeneous. Spectra with a rich substructure, like spectra 2, 3, 4, and 5 of Figure 2, have been found for 15 J-aggregates, whereas for 7 J-aggregates the spectra showed a nearly featureless broad band as for example spectrum 1 of Figure 2. Further, we observed a polarization dependence of the spectral features for 14 J-aggregates (modulation amplitude ≈20%-50%), substantial spectral diffusion for 13 J-aggregates, and changes in the total intensity of the emission (∆I0 > 20%) for 17 J-aggregates. A correlation between any of these parameters was not observed. To obtain information about the geometrical structure of the amphi-PIC J-aggregates, we employed cryo-transmission electron microscopy (cryo-TEM). This allowed us to identify either cylindrical or disklike aggregates as shown in Figure 5. For the latter aggregates one resolves about 5-10 lamellae arranged concentrically (or spiral-like) around a core which 2637
Table 1. Spectral Positions, Line Widths, and Phase Angles of the Spectral Features Shown in Figure 4a spectral position (cm-1)
line width (cm-1)
phase angle (deg)
17217 17234 17255 17267 17284 17298 17340 17388 17423 17461
2.5 10 2.0 6.2 4.1 19 25 57 24 128
0 2 6 5 20 71 77 92 88 83
a The phase angles are expressed relative to that of the line at 17217 cm-1.
Figure 4. Sequence of 200 consecutively recorded fluorescenceexcitation spectra stacked on top of each other (scan speed 58 cm-1/ s, excitation intensity 0.5 W/cm2). The fluorescence intensity is indicated by the gray scale. Between two successive scans the polarization of the excitation has been rotated by 3.6°. (b) The top trace corresponds to the average of the stack of spectra shown in part a. The lower traces are averages of the scans indicated by the horizontal boxed regions. The vertical scale is valid for the lowest spectrum; all other traces are offset for clarity. The top trace is scaled by a factor of 4. (c) Integrated intensity of the spectral region marked by a bar in part a as a function of the polarization of the excitation light (full line) together with a cos2-type function (dashed line) that serves as a guide for the eye.
does not show a regular geometry, Figure 5c. Both types of structures could be observed simultaneously in the samples with a slight preference for the cylinders at lower waterDMF concentrations. The dimensions of the cylinders, Figure 5a and on an enlarged scale Figure 5b, are 33 ( 1 nm for the diameter, 4 ( 1 nm for the thickness of the wall, and 200-350 nm for the elongation along the cylinder axis. For the disks the distance between two adjacent lamellae is 4.2 ( 0.3 nm, the thickness of an individual lamella amounts to 1.2 ( 0.3 nm, and the diameter of the overall structure varies between 30 and 150 nm. Given the unpolar character of the polymethylene side chains, an arrangement that shields the hydrophobic tails on the inside of the structure appears very likely. Thus, the thickness of the walls of the cylinders suggests that they are 2638
formed by a double layer of monomers. Interestingly, the thickness of the walls of the cylinders is in agreement with the distance between two lamellae in the disklike assemblies. This might reflect as well a double-layer structure of monomers folded into a multilamellar two-dimensional aggregate. The geometrical heterogeneity is consistent with the spectral heterogeneity and hints for an explanation of the observed discrepancy between the ensemble spectrum and the spectrum resulting from the sum of 22 individual spectra. All cryo-electron microscopy data have been obtained from samples with a monomer concentration of 10-2 mol/L, the ensemble spectrum from a sample with a monomer concentration of 10-3mol/L, and the optical singlemolecule experiments from samples with a monomer concentration of 10-5mol/L. For the conditions of the optical single-molecule experiments, we do not know the actual abundance of the two types of aggregates with respect to each other. However, it is not unreasonable to expect that the monomer concentration has a crucial influence on the geometry during J-aggregate formation. Accordingly, the differences in the two spectra might reflect the variation in the content of the two structural species of the samples. Typically, Frenkel excitons are considered as a natural starting point for the analysis of the electronically excited states of J-aggregates. Numerous studies exist where Frenkel exciton theory has been applied to the various forms of J-aggregates.12-14,16 Therefore we summarize only briefly the essential results. Frenkel excitons are extended over the entire aggregate and correspond to linear combinations of the monomer excited states. By optical spectroscopy only those exciton states are accessible that carry a transition-dipole moment. Theoretical calculations for linear, circular, or cylindrical geometries predict only very few optically allowed transitions with distinct polarization properties that are determined by the morphology of the aggregate. For an aggregate where the monomers are arranged in a single-wall ring or cylinder, one finds only three exciton states that carry appreciable oscillator strength. The exciton state with the lowest energy gives rise to a transition that is polarized along the symmetry axis of the assembly. The other two exciton states are degenerate and feature two transitions which are mutually orthogonal polarized within the plane of the ring. Nano Lett., Vol. 5, No. 12, 2005
Figure 5. (a) Cryo-transmission electron microscopy images of J-aggregates that have been solved in a binary water-dimethylformamide solution (W-DMF; 90:10) with a monomer concentration (∼10-2 mol/L; copper grid, not hydrophilized). (b) Expanded view of the region marked by the box in part a. (c) Cryotransmission electron microscopy images of J-aggregates that have been solved in a binary water-dimethylformamide solution (WDMF; 490:10) with a monomer concentration (∼10-2 mol/L; lacey carbon filmed grid, hydrophilized).
A double-wall cylinder can be described as a superposition of two single-wall cylinders as long as the interaction between the monomers within one cylinder is large with respect to the interaction between monomers belonging to the two different cylinders.23 This simple approach yields twice the number of optical transitions as for the singlewall cylinder. Nevertheless, these transitions can be spectrally resolved due to the differences in the interaction strength between the monomers in the individual cylinders as a result of the variation in cylinder diameter. The situation changes for any deviation from the perfectly symmetric case caused for instance by diagonal disorder, off-diagonal disorder, or geometric distortions. This yields a redistribution of the Nano Lett., Vol. 5, No. 12, 2005
oscillator strength, lifts possible degeneracies of the exciton states, and leads to a localization of the excitation on a segment of the J-aggregate. For the interpretation of our spectra we have to consider several ambiguities. (i) Since both structures observed in cryo-TEM feature a circular symmetry, the optical spectra do not allow to discriminate between the two species. (ii) In the optical experiments the orientation of the aggregates with respect to the optical axis is unknown. Therefore all relative angles reported for the polarization of the transitions correspond to the projection of the mutual orientation of the involved transition-dipole moments on a plane perpendicular to the optical axis. Finally, (iii) it can be expected that the flexible side chains of the monomers give rise to structural variations that lead to variations in the site energies of the monomers (diagonal disorder) and in the interaction strengths between the monomers (off-diagonal disorder) regardless of the actual morphology of the amphi-PIC J-aggregates.21 Taking this into account, the observed spectra are generally compatible with the idea that the excitons are confined on a small number of segments on an individual J-aggregate and that the spectra display the superposition of their contributions. Model calculations for linear aggregates revealed that the degree of localization/delocalization of the exciton wave function depends also on the energy of the exciton state.13 For the lowest excited states the exciton can be confined on a limited number of sites, i.e., a segment, whereas the higher exciton states can be more extended and cover several segments. For sufficiently low symmetry of a segment the low-lying exciton states give rise to dipole-allowed transitions from the ground state whereas the higher exciton states decay by fast radiationless relaxation resulting in a large homogeneous line width. Accordingly, we assign the narrow lines on the low-energy side of the spectra to the lowest exciton states of the individual segments and the broad bands to the higher exciton states. Such a segment on a J-aggregate should not be regarded as a static entity. It is subjected to fluctuations due to temporal variations in the site energies and the interaction strengths of the monomers. This induces changes of both the energy levels of the exciton states and their transition moments which would explain the temporal variations of the intensity and the profile of the spectra. In this study we demonstrated that low-temperature singlemolecule spectroscopy is a valuable tool that allows us to record fluorescence-excitation spectra from individual amphiPIC J-aggregates with unparalleled resolution. From the observed large spectral heterogeneity, we conclude that for an ensemble of amphi-PIC J-aggregates many realizations for the electronic structure have to be considered. An ensemble spectrum averages over these realizations and does not allow discrimination between the various contributions. Whether this large degree of heterogeneity is a common feature for J-aggregates or specific for the type of aggregates investigated here, i.e., featuring a long flexible side chain, has to be addressed in future studies. Acknowledgment. This work has been supported by the Volkswagen Foundation (Hannover) and a visiting grant of 2639
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Nano Lett., Vol. 5, No. 12, 2005
