J. Phys. Chem. 1993,97, 12408-12415
12408
Excited-State Dynamics of Polymer-Bound J-Aggregates Miin-Liang Homg+** and Edward L. Quitevis’ Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 Received: June 18, 1993; In Final Form: September I , 1993”
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The excited-state dynamics of polymer-bound J-aggregates formed in aqueous mixtures of pseudoisocyanine (PIC) chloride and poly(viny1 sulfonic acid sodium salt) (PVS) (MW 1 1 200) have been studied by picosecond time-correlated single-photon counting and picosecond polarized pump-probe spectroscopy. At a concentration of 40 NM PIC and 5 X 10-4 g/dL PVS,the absorption spectrum of the J-aggregate is characterized by a J-band at 565 nm with a fwhm of ~ 5 0 cm-I. 0 For this particular mixture, a dye molecule is bound to each of the so3groups on the polymer chains. The physical size of the aggregate is therefore determined by the number of polymer residues (-87) per chain. The fluorescence lifetime and fluorescence quantum yield of these J-aggregates are 17 f 3 ps and 0.022 f 0.003, respectively. The lifetime is independent of the excitation intensity. From an analysis of the photophysical parameters, we infer a coherence size of -5 f 1, which is smaller than the physical size of the aggregate. The pump-probe signal a t 565 nm is entirely due to bleaching and consists of a fast component, with a decay time comparable to the fluorescence lifetime, and a slow component. The decay kinetics of the induced bleaching are independent of the excitation intensity. The signal a t 558 nm consists of an absorption component a t early times and a slowly decaying bleaching component a t long times. The anisotropy a t 565 nm was constant over the 160-ps time range of the signal. The kinetics are rationalized in terms of a model involving the singlet exciton states of the J-aggregate and a long-lived bottleneck state. The induced absorption at early times for excitation on the blue edge of the J-band is consistent with a one-exciton to two-exciton transition.
1. Introduction
Molecular dye aggregates play an important role in many technological applications. In photographic processes, dye aggregates are sensitizers for silver halide materials.’ They have been used in organic photoconductors.2 Because they exhibit strong coherent excitation phenomena, dye aggregates have been considered as potential nonlinear optical materiak3s4 Aggregates of dye molecules can be used to mimic light harvesting arrays for artificial photosynthetic system^.^^^ To optimize the use of molecular dye aggregates in these applications, it is necessary to understand their optical properties. Much of the work on dye aggregates has focused on the J-aggregates of cyanine dyes, in particular, the J-aggregates of 1,I’-diethyl-2,2’-cyanineor pseudoisocyanine (PIC) (Figure 1).7-11 PIC J-aggregates in aqueous solution are characterized by an absorption band, called the J-band, at 4 1 4 nm, which is red shifted from the monomer band at 523 nm. The J-band arises from a Frenkel exciton-like transition.12 Because of motional narrowing, which causes an averaging over the local inhomogeneities, the J-band is very sharp compared to the monomer band.” J-aggregates are formed at high dye concentrations ( > l e 3 M) of PIC chloride, upon the addition of concentrated sodium chloride ( 5 5 M) to dilute aqueous solutions of PIC iodide, or in low-temperature aqueous ethylene glycol glasses containing PIC bromide.’-” There is currently great interest in understanding the effect of size (Le., the number of molecules in the aggregate) on the optical properties of these aggregates. The size determines their spectral shift,I2 absorption line shape,l3 radiative rate,14J5 and nonlinear optical proper tie^.^ In principle, the size of a J-aggregate can be inferred from the spectral shift. It can also be inferred from kinetic measurements. Stindstrom et a1.16 used exciton annihilation to show that PIC J-aggregates in solution
* To whom correspondence should be. addressed.
Robert A. Welch Foundation Predoctoral Fellow. 3 Present address: Department of Chemistry, The Pennsylvania State University, University Park, PA 16802. 0 Abstract published in Advance ACS Abstracts, November 1, 1993.
0022-3654/93/2097- 12408$04.00/0
are large and are comprised of between 20000 and 50000 molecules. To rigorously test theoretical models for the optical dynamics of J-aggregates, the size should be determined by techniques that do not involve the models themselves. Two-dimensional J-aggregates of well-defined size can be formed by the adsorption of dyes onto c r y ~ t a l l i t e s l J ~or -~~ colloids.20s21In the spectral sensitization of photographic films by J-aggregates, the aggregates are adsorbed onto silver halide crystallites embedded in gelatin.] By using well-characterized crystallites, the size of these two-dimensional J-aggregates can be established under monolayer coverage condition^.^^-^^ These crystallites, however, readily undergo electron-transfer with the adsorbed dyes, which is the main mechanism for spectral sensitization in these photographic materials. To avoid electron transfer, inert substrates, such as silica gelIsaor colloidal can be used to study the photophysics of these J-aggregates. To date, the J-aggregate has been modeled mainly as a onedimensional system.I2 From a theoretical point of view, it is advantageous to experimentally study one-dimensional rather than two-dimensional J-aggregates. One-dimensional J-aggregates of well-defined size can be formed by binding dye molecules to polyelectrolytes. Appel and Schiebe22 showed that high polymers, with closely situated negative groups (SOJ-, COO-), are capable of transforming dilute aqueous solutions of PIC to the aggregated form with the characteristic narrow J-band. These dye-polymer assemblies are formed through the electrostatic interaction of cationic dyes with the negatively charged groups on the polymer chain.23 However, not all polyelectrolytes will form J-aggregates. For example, PIC J-aggregates can be formed with the sulfate ester of poly(viny1 alcohol)17 and anionic polysaccharides22 but not with sulfonated po1y~tyrene.I~In the latter case, the spacing between the pendant groups is too large. In polyelectrolytes that promote J-aggregate formation, the charged groups are spaced so as to allow strong electroniccoupling to occur between the dye molecule^.^^ A one-to-one ratio of polymer residues (binding sites) to dye molecules can be obtained with such dye-polymer a ~ s e m b l i e s . ~For ~.~ ~ assemblies, the these size of the J-aggregate can be inferred from the molecular weight of the polymer, without having to rely on a particular model. 0 1993 American Chemical Society
Excited-State Dynamics of Polymer-Bound J-Aggregates
PIC
PVS
Figure 1. Structures of pseudoisocyanine (PIC) chloride and poly(viny1 sulfonic acid sodium salt) (PVS). The counterions,C1- and Na+,are not
shown. In this article, we describe new results on the excited-state dynamics of PIC J-aggregates bound to poly(viny1 sulfonic acid sodium salt) (PVS) (Figure 1). This research is part of a program in our laboratory to study the linear and nonlinear optical properties of polymer-boundaggregates. Theoutlineof this article is as follows. In section 2, the preparation of the dye-polymer complexes and the details of the apparatus and techniques are outlined. In section 3, the steady-state spectroscopy of the polymer-bound PIC J-aggregates and results from fluorescence measurements and pump-probe measurements on these J-aggregates are described. In section 4, the coherence size of the J-aggregate is inferred from the photophysical parameters. The fluorescence measurements show superradiant enhancement of the J-aggregate radiative rate constant. The kinetics of the fluorescence and pump-probe measurements are rationalized in terms of a model involving singlet exciton states of the J-aggregate and a long-lived bottleneck state. 2. Experimental Details
PVS was purified and characterized as described by Breslow and K ~ t n e r .A~ ~25% (w/w) aqueous solution of PVS (Polysciences, lot no. 94950) was purified by two precipitations with methanol. Seventy percent methanol by weight was used for each precipitation. Two sharply defined layers were formed upon addition of methanol to the polymer solution in a separatory funnel. The lower precipitated oil layer was collected and separated as completely as possible. The methanol was removed under reduced pressure in a rotary evaporator at 60 OC. The polymer was dried overnight under vacuum at 110 OC. Because of its hydroscopic nature, the polymer was kept in a bottle with a plug seal in a desiccator. The specific viscosities of 0.26,0.43, 0.72, 1.2, and 2.0% aqueous polymer solutions in 0.2 M Na2S04 were measured with a Cannon-Fenske viscometer (ASTM size 100). The average molecular weight of the polymer was determined by relating the intrinsic viscosity [s]to the weightaverage molecular weight &fw.For twice purified PVS, [s] = 3.68 X I t 2 . From previously published light scattering data, the following equation is obtained for &fw24
+ 0.93121 log [ q ] Using this equation, we find that aW = 11 249 for our polymer log &fw = 5.3866
sample. This implies that there are about 87 residues per polymer chain. Samples were prepared from a 0.05 g/dL aqueous stock solution of twice purified PVS. PIC chloride (Exciton) was used without purification. Prior to each measurement, a fresh stock solution of concentrated PIC (0.01 M) in deionized water was prepared. Samples were prepared by diluting appropriate amounts of the polymer stock solution and dye stock solution in 100 mL. The absorption and fluorescence spectra of these samples were recorded, respectively, on a Shimadzu 265 UV-vis spectrophotometer and on a SLM Aminco 4800C fluorometer. The samples were contained in 0.1-cm pathlength cuvettes. The fluorescence spectra wereobtained by using front face illumination with the cuvette oriented 35O relative to the incident beam.
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12409 The quantum yields were determined relative to rhodamine 101 in methanol (@~f= 0,99).zs-z7 The standard and sample were prepared with equal absorbances at the excitation wavelength of 532 nm. Because the samples were optically denseat theexcitation wavelength (optical density =0.19), front face illumination as described above was used. Fluorescence quantum yields from corrected fluorescence spectra were calculated by using the equationZ8
where D is the integrated area under the corrected fluorescence spectrum and n is the refractive index of the solution. The subscripts x and s refer, respectively, to the sample and the standard. The fluorescence lifetime of PIC J-aggregates was measured by using time-correlated single-photon counting with excitation at 560 nm from a cavity-dumped and synchronously-pumped rhodamine 6G dye laser. The details of the a p p a r a t ~ s 2and ~ the technique30 have been previously described. Briefly, the laser pulse repetition rate was 8.3 MHz and the laser pulse width was =8 ps. The excitation beam was focused to a spot size of 1-mm diameter on the sample. The average power at the sample was 1 4 mW. Neutral density filters were used to reduce the laser intensity at the sample. The fluorescence was collected from the front face of the sample and passed through a quarter-meter monochromator set at 580 nm to a Hamamatsu microchannel plate detector. The fwhm of the instrument response function for this apparatus was =lo0 ps. The fluorescence curves were fitted by the convolution of the instrument response function with a decay function by using a nonlinear least-squares routine. The quality of the fit was judged by visual inspection and the randomness of the residuals and with the help of reduced x2 values. The picosecond polarized pump-probe measurements were made by using excitation from a synchronously pumped rhodamine 110 dye laser, tunable between 540 and 580 nm. The details of the apparatus and the technique have been previously described.2I The laser pulse repetition rate was 76 MHz. Noncollinear, copropagating pump and probe beams were used. The beams were focused with a IO-cm focal length achromat to a spot size of -200 pm in the sample, which was contained in a rotating 3.1-mm pathlength cell. For the measurements described in this paper, the laser intensity at the sample was 113.5 mW. A microcomputer-controlled translation stage in the probe arm of the apparatus was used to delay the time of arrival of the probe pulse with respect to the pump pulse in the sample. The pump beam was modulated at 10.24 MHz with an acoustooptic modulator before being focused into the sample. The modulation induced on the probe beam was detected with a photodiode whose output was fed into a megahertz lock-in amplifier. The polarization of the probe beam with respect to the pump beam was changed with a polarization rotator. The polarized induced transmissionsignals,ATl(t) and ATL(t), and theisotropicinduced transmission signal, AT54,7~(t), were obtained, respectively, with probe light polarized parallel, perpendicular, and 54.7' (the -magic" angle) to the polarization of the excitation beam. The background-free autocorrelation of the pulse was determined by replacing the sample by a KDP crystal and measuring the second harmonic signal as a function of the probe delay. The fwhm of the autocorrelation trace was -9 ps. The maximum of the autocorrelation was used to establish the position of zero delay. Straightforward deconvolution over the entire decay range of the signal can lead to an inaccurate description of the signal due to the coherent spike at zero time. In order to extract the truedecay behavior, the method of antisymmetrization was u ~ e d . ~ The ~,'~ induced transmission signals were antisymmetrized and fitted by the convolution of the pulse autocorrelation with the antisymmetrized form of an empirical decay function using a nonlinear
12410
Horng and Quitevis
The Journal of Physical Chemistry, Vol. 97,No. 47, 1993
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450
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Figure 2, Absorption spectrum (solid line) and fluorescence spectrum (dashed line) of an aqueous mixture of 40 pM PIC and 5 X 104 g/dL PVS. The sample was contained in a 0.1-cmcuvette. The fluorescence spectrum was taken in the front face illumination geometry.
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Figure 3. Absorption spectra of aqueous mixtures of 5 X l p g/dL PVS with (a) 10, (b) 20, (c) 40, and (d) 50 pM PIC. The samples were contained in 0.1-cm cuvettes.
secondary peaks at 500 and 536 nm were present in the short wavelength side of the J-band. These secondary peaks are noticeable in the absorption spectrum when the dye is predominantly in the aggregated form. At P/D = 0.76,the absorption 3. Results spectrum dramatically changed. The addition of more dye to the mixture caused the monomer band to reappear and the J-band Steady-State Spectroscopy. Figure 2 shows the fluorescence to become distorted. In contrast to the previous trend of a and absorption spectra of an aqueous mixture containing 40 pM narrowing and growth of the J-band in going from P/D = 3.9 PIC and 5 X 10-4g/dL PVS. The main features in the absorption to P/D = 0.95,the J-band shrank and became broader in going spectrum are the J-band at 565 nm, with a fwhm of 4 0 0 cm-I, from P/D = 0.95 to P/D = 0.76. These changes at P/D = 0.76 and secondary peaks a t 500 and 536 nm. These spectral features were accompanied by the formation of a precipitate. Precipitation have been shown to be due entirely to PIC J - a g g r e g a t e ~ .The ~~.~~ is expected for heavily loaded complexes, as found previously for fluorescence spectrum consists of an intense resonance fluoresthe PIC-heparin system22and other dyepolymer ~ystems.~j cence band at 569 nm, with almost the same width as that of The behavior of the absorption spectra is consistent with the J-band in the absorption spectrum, and a small secondary peak assumption that the dye binds to the so3-groups on the polymer at 613 nm. The absorption and fluorescencespectra arenot mirror chain. At loading levels corresponding to P/D = 3.8 and 1.9, images of each other, in contrast to the spectra of monomeric there are more binding sites than there are dye molecules. The dyes. These characteristic features were first observed by Jelley.' absorption spectra at these loading levels show that the dye is The extinction coefficient, emax, a t the maximum of the J-band bound to the polymer as both monomers and aggregates. At is 9.7X lo4M-I cm-I. This value of emax compares well with the P/D = 0.95,there is approximately one dye molecule per binding value obtained by Appel and Scheibe for J-aggregates bound to site. The disappearance of the monomer band at this P/D value heparin, an anionic polysaccharide.22 is a signature that the dye exists mainly as aggregates. At P/D Appel and Scheibe22observed that the intensity of the J-band = 0.76,the number of dye molecules exceeds the number of binding in the absorption spectra of aqueous dye-polymer mixtures of sites. If each binding site can accommodate only one dye molecule, PIC and heparin did not change when the concentration of dye the excess dye a t P/D = 0.76 must go into solution as monomers. exceeded the concentration of polymer residues. They proposed This accounts for the reappearance of the monomer band in the that this spectroscopic endpoint arises from the binding of one absorption spectrum at P/D = 0.76. cationic dye to each of the SO3- groups on the polymer chain. Since nearly every binding size is occupied by a dye molecule To confirm the existence of a similar endpoint for the PICa t P/D = 0.95,each polymer chain will have an average of 87 PVS complexes, we measured the absorption spectra of a series molecules, corresponding to the average number of residues per of aqueous mixtures containing 5 X 10-4 g/dL of PVS with varying polymer chain. The average physical size of the J-aggregate in amounts of PIC. The samples were contained in 0.1-cm path this mixture is therefore -87. Because the size of the PIC length cuvettes. The polymer residue concentration in these J-aggregate is well-defined at this value of P/D, the fluorescence mixtures was 38 pM. The loading level of the polyelectrolyte and pump-probe measurements were performed on this PICwith dye can be characterized by the value of P/D, which is the PVS mixture. ratioof the concentration of polymer residues to the concentration FluorescenceMeasurements. A comparison of the fluorescence of dye. For low P/D the loading of the polyelectrolyte is high decay curve of the dye-polymer complex a t P/D = 0.95 and the and for high P/D the loading of the polyelectrolyte is low. Figure instrument response function (Figure 4) shows that the PIC 3 illustrates the spectra of these mixtures with 10,20, 40, and J-aggregates have very short fluorescence lifetimes. This decay 50 p M of PIC, corresponding to P/D values of 3.8,1.9,0.95,and curve was obtained at an excitation intensity of 2 X lo9 photons 0.76,respectively. cm-2 pulse-'. Doubling the excitation intensity did not change In the absence of PVS, the spectra at these dye concentrations the decay behavior. A sample containing just water and polymer consisted mainly of the monomer band at 523 nm. However, gave no signal, confirming that the signal obtained from the PICupon addition of PVS to these solutions, the red-shifted J-band PVS mixture was due to fluorescence and not light scattering appeared in the spectra between 560 and 565 nm (Figure 3). For from the polymer. The fluorescence decay curves were slightly 0.95 IP/D I 3.8, increase in the dye loading caused further nonexponential and could be fitted by the sum of two exponential growth of the J-band, with very little change in its position. The functions: bandwidth was narrower at higher loading levels than a t lower loading levels. The monomer band was present at the lower F ( t ) = q e x p ( - t / T l ) a2exp ( - t / ~ ~ ) (3) loading level (P/D = 3.8) but was clearly absent at the higher loading level (P/D = 0.95). At the higher loading level, only The reduced x2 values were less than 2.0, and the residuals were least-squares deconvolution procedure. All measurements were carried out at room temperature (21 f 1 "C).
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Excited-State Dynamics of Polymer-Bound J-Aggregates
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12411
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Figure 4. Semilogarithmic plot of the fluorescence decay curve (points) for J-aggregates in an aqueous mixture of 40 pM PIC and 5 X 10-4 g/dL PVS at an excitation intensity of 2 X lo9 photons cm-* pulse-'. The fit (solid curve through points) is the convolution of the instrument response function (lower solid curve) and a biexponential decay function (eq 3). The fit parameters are a1 = 0.983, TI = 17.4 ps, a2 = 0.017, and 7 2 = 91.9 ps. The reduced residuals.
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random (Figure 4) for all the curves obtained. The fluorescence decay is dominated by a fast component with 71 = 17 f 3 ps. The slow component has a decay time of 7 2 = 100 f 10 ps and contributes to less than 2% of the fluorescence decay. Since the largest contribution to the fluorescencespectrum (Figure 2) comes from the J-aggregate, the short component in the fluorescence decay curve must be due to the fluorescence lifetime, 715,of the J-aggregate. T,J and the fluorescence quantum yield, *,J,are related to the radiative rate constant, k,J, and the nonradiative rate constant, k,,rJ, by the photophysical equations r;' =
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+ kn:
Figure 5. Magic-angle pump-probe signals (arbitrary units) for J-aggregates in an aqueous mixture of 40 pM PIC and 5 X lo" g/dL PVS at (a) 565 and (b) 558 nm. The excitation intensity was 3.6 X IO'* photons cm-2 pulse-'. The dashed curve in plots (a) and (b) is the pulse autocorrelation.
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Figure 6. Comparison of the 565-nm magic-angle signal at an excitation intensity of 3.6 X 10l2photons cm-2 pulse (solid curve) and one obtained
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at half the excitation intensity (points). The dashed curve is the pulse autocorrelation.
71
For polymer-bound PIC J-aggregates, @,Jwas found to be equal to 0.022 f 0.003. By substituting the measured values of T,J and into eqs 5a and 5b, we find that k,J = (1.3 f 0.3) X lo9 s-I and knrJ= (5.6 f 1.3) X 1O'O s-l. The short fluorescence lifetime is due largely to fast nonradiative decay, which is =43 times faster than radiative decay. Pump-Probe Measurements. Figure 5 illustrates magic-angle pump-probe signals at P/D = 0.95. The signals were obtained by scanning over the time range -160 to 160 ps a t 558 and 565 nm. The excitation intensity a t these wavelengths was =3.6 X l o i 2photons cm-* pulse-'. Superimposed on these signals is the pulse autocorrelation. At all wavelengths, the peak at zero delay is the coherent spike, which serves as a convenient time marker. For excitation in the blue side of the J-band, at 558 nm, the signal is dominated at early times by an induced absorption which rapidly decays to a weak long-lived induced transmission. This behavior
near zero time has been observed previously in the pump-probe transients of solution PIC J-aggregates.16 The signal that is obtained by exciting at the peak of the J-band (565 nm) is characterized by a fast component and a slow Component, which appears as a constant level over the indicated time range. For excitation in the red side of the J-band, the behavior of the induced transmission signal is the same as that at the peak of J-band. Figure 6 compares the magic-angle signal at an excitation intensity of 3.6 X 10l2photons cm-* pulse-' with one obtained at half the excitation intensity. The curves have been normalized. The excellent overlap between the two curves indicates that the decay kinetics are independent of the excitation intensity. Figure 7 illustrates fits of the antisymmetrized signals corresponding to the signals in Figure 6. The fit over the indicated time-range corresponds to the antisymmetrized form of the
Horng and Quitevis
12412 The Journal of Physical Chemistry, Vol. 97,No. 47, 1993
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Figure9. Leveldiagramfor J-aggregates. IS), Il), and Ib) are theground state, lowest energy state (direct band edge) in the band of exciton states, and the bottleneck state, respectively. klg,klb, and kb are rate constants for decay from (1) to lg), 11) to Ib), and from Ib) to Ig), respectively.
of 0.4. This implies that excitation energy transport is very slow or does not occur in the indicated time range. This is in distinct contrast to solution PIC J-aggregates, which exhibit an anisotropy decay.16~21c
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Tlme(pr) Figure 7. Antisymmetrized signals (points) corresponding to the signals in Figure 6 and the fits (solid curves) of the convolution of the pulse autocorrelation function with antisymmetrized form of eq 6. The fit parameters are (a) 7 = 10.6 ps and a = 0.15 and (b) 7 = 11.4 ps and a = 0.11.
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Fluorescence Lifetime. Previous s t ~ d i e s ~have ~ *shown ~ ~ , that ~~ the short fluorescence decays of large PIC J-aggregates in solution arise from the excitation-intensity dependenceof thedecay kinetics due to exciton annihilation. At low excitation intensities (
