J . Phys. Chem. 1989, 93, 1851-1859
Because this scheme utilizes a UV photon that is red-shifted relative to excitation from u” = 0, it not only eliminates the need to control the timing between the two laser photons but also allows one to selectively excite only H O D in a mixture with H 2 0 and DzO. Our calculations indicate that photodissociation from v’bH = 1 and higher leads to a reasonable frequency range over which UV photon excitation will lead to exclusive production of H OD. Surprisingly, the converse is not true for excitation from v’bD= 1. In fact, one must initially prepare vfbD= 3 or higher before the two-photon scheme leading to exclusive D OH formation becomes practical. Such experiments are currently being planned.
coordinates. Since the spectrum is simply a projection of the excited-state dynamics (in the guise of the Raman wave function) onto the template created by the ground-state vibrational eigenfunctions, the fundamental difference in the dynamics on the two surfaces leads to a more complicated resonance Raman spectrum. The time-dependent approach also gives us some insight into the behavior of the O D / O H branching ratio as a function of the incident laser frequency. Using arguments based on the _reflection principle and knowledge of the classical dynamics on the A surface, we can correlate the lowest and highest energy parts of the initial wave packet almost exclusively with the H + OD exit channel. Although the OD/OH branching ratio becomes very large at the low and high ends of the absorption profile for excitation from v” = 0, the absorption coefficient is very small in these regions. This means that it would be very difficult experimentally to induce selective bond breaking via a one-photon excitation from v” = 0. We have shown that a two-photon scheme can be concocted that does lead to almost exclusive formation of H OD or D OH. This procedure invoJves first promoting HOD to an excited vibrational state on the X surface with an I R photon and then adding a UV photon to selectively photodissociate the molecule.
+
1851
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Acknowledgment. We gratefully acknowledge the financial support for this project by the National Science Foundation (Grant No. CHE-8707168 and CHE-8507138) and the donors of the Petroleum Research Fund, administered by the American Chemical Society. In addition, we thank Professors Eric Heller, Robert Watts, Bruce Hudson, David Tannor, and Dr. Niels Henricksen for helpful discussions.
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Registry No. HOD,14940-63-7.
Spectral Diffusion in Molecular Aggregates Yi Lin and David M. Hanson* Department of Chemistry, State University of New York, Stony Brook, New York I 1794-3400 (Received: October 3, 1988)
Spectral diffusion in 4-bromo-4’-chlorobenzophenoneaggregates in polystyrene films at 4.2K is characterized. Steady-state and time-resolved phosphorescencespectra for different concentrations and excitation energies are reported. The spectral features and energy-transfer dynamics provide evidence for delocalized aggregate states (DAS), localized aggregate states (LAS), and discrete molecule states (DMS). The DAS are strongly coupled to the LAS, and relaxation from DAS to LAS occurs at a rate faster than 5 X 104/s.The LAS are strongly coupled to the DMS with a relaxation rate around 5 X 103/s, depending upon the energy of the LAS. The DAS have the highest excitation energies and appear to be associated with the interior regions of the aggregate. The DMS, in the polymer environment, have the lowest excitation energies. The LAS appear to be associated with the boundary regions of the aggregate. A theoretical model is shown to mimic the experimental observations.
I. Introduction Spectral diffusion is the process of energy transfer between like species with different transition energies. Characterization of spectral diffusion in disordered materials has attracted considerable attention from both experimentalists14 and theoretician^.^-^ The techniques of time-resolved fluorescence line narrowing*-I0 (TRFLN) and time-resolved phosphorescence line n a r r o ~ i n g ~ - ~ J (TRPLN), together with measurements of the resonant donor decay as a function of time, have made it possible to study spectral diffusion within inhomogeneously broadened optical l i n e ~ . ~InJ ~ the T R F L N experiment, a narrow-band pulsed laser is used to (1) Richert, R.; Bassler, H. J. Chem. Phys. 1986, 84, 3567. (2) Prasad, P. N.; Morgan, J. R.; El-Sayed, M. A. J. Phys. Chem. 1981, 85, 3569. (3) Chu, S.; Gibbs, H.; McCall, S. L.; Passner, A. Phys. Rev. Lett. 1980, 45, 1715. (4)Kook, S. K.; Hanson, D. M., to be published. ( 5 ) Hostein, T.; Lyo,S. K.; Orbach, R. In Topics in Applied Physics; Yen, W. M., Selzer, P. M., Eds.; Springer: Berlin, 1981;Vol. 49,Chapter 2,and references therein. (6) Blumen, A. J . Chem. Phys. 1980, 72, 2632. (7) Blumen, A.; Klafter, J.; Silbey, R. J . Chem. Phys. 1980, 72, 5320. (8) Szabo, A. Phys. Rev. Lett. 1970, 25, 924. (9) Selzer, P. M.; Huber, D. L.;Hamilton, D. S.;Yen, W. M.; Weber, M. J. Phys. Rev. Lett. 1976, 36, 813. (IO) Hegarty, J.; Yen, W. M. Phys. Reu. Lett. 1976, 43, 1126. ( I 1) Talapatra, G. B.; Rao, D. N.; Prasad, P. N. J . Phys. Chem. 1984, 88, 4636.
0022-365418912093-185 1$01.50/0
excite molecules with resonance frequencies spanning a small segment of the inhomogeneous band. After the source is turned off, the fluorescence spectrum evolves in time. Initially there is only sharp luminescence coming from the molecules that were excited directly by the incident light. As time passes, molecules with resonant frequencies outside the bandwidth of the source begin to luminesce. The broad luminescence arises from energy transfer to those molecules that were not excited by the light. The time-dependent ratio between the sharp component and the broad background provides information about the microscopic transfer processes. In the case of phonon-assited energy transfer, the line shape depends on the phonon population. When the thermal energy is greater than the inhomogeneous line width, the broad line is symmetric; otherwise, the broadening is predominantly toward the lower energy side. Recently, spectral diffusion and site-selected excitation in “orientationally disordered” organic solids have been investigated by El-Sayed and c ~ - w o r k e r s . ~ J They ~ J ~ reported observations of spectral diffusion in I-bromo-4-chloronaphthalene.Time-resolved phosphorescence line narrowing experiments were performed under conditions where kT was much smaller than the inhomogeneous line width. The results were interpreted in terms of phonon-assisted unidirectional energy transfer. Similar spectral (12)Morgan, J. R.; El-Sayed, M. A. J. Phys. Chem. 1983, 87, 200. (13)Morgan, J. R.; El-Sayed, M. A. J. Phys. Chem. 1983, 87, 3 8 3 .
0 1989 American Chemical Societv
1852 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
diffusion phenomena also were found in other disordered systems. Prasad et aLl1 studied the system of 4-bromo-4‘-chlorobenzophenone at both 4.2 and 1.8 K. Kook and Hanson4 recently reported observations of energy transfer and spectral diffusion in glassy 4-methylbenzophenone (MBP) and in MBP-doped polystyrene films. Here we report studies of spectral diffusion and energy transfer in molecular aggregates in polymer films. We previously reported observations of the phosphorescence line shape of 4-bromo-4’-chlorobenzophenone (BCBP) doped into polystyrene as a function of concentration. With excitation via the SI So transition,14 the abrupt change in the spectral shape with increasing dopant concentration indicated the formation of molecular clusters. The spectra were analyzed as a linear superposition of the luminescence from clusters and from molecules embedded in the polymer. The present work focuses on the same aggregate system. Spectral diffusion within the inhomogeneous So transition is studied as a function of time band of the T, and excitation energy. The results show that energy transfer in the aggregated films is more complicated than that observed in orientationally disordered crystals. The excited donor molecules can transfer their energy not only to neighboring molecules in the same cluster but also to molecules outside the cluster. The luminescence line shape therefore is dependent on the excitation energy, the density of states of cluster molecules, and the density of states of molecules in the polymer. In the following discussion, the term “discrete molecules” will be used to refer to those molecules embedded in the polymer. For the case of energy transfer in “orientationally disordered” crystals, resonant energy transfer might be considered to be the most probable process since the electronic states involved are the same for both donor and acceptor molecules. Consequently, the condition of energy matching is satisfied naturally. This consideration, however, is not always correct due to perturbations caused by the local environment. For the present case of a molecular aggregate in a polymer films, discrete molecules have a lower excitation energy than molecules in clusters (by about 250 cm-’). Energy transfer between these two sets of molecules must be a phonon-assisted process, while the intracluster energy transfer may be dominated by the resonant process. It is of interest to learn about the rates of energy transfer within the cluster and to the discrete molecules. We applied the technique of time-resolved phosphorescence line narrowing to study energy transfer from molecules in the cluster to discrete molecules. The results indicate that the transfer rate between these two sets of molecules is slower than the intracluster transfer rate. In the last section, equations are derived that describe the time evolution of the excited-state population in a system containing both aggregated and discrete molecules. Since the luminescence intensity at any given time is proportional to the population of the molecules that luminesce, the solution to these equations gives results that can be compared with the phosphorescenceexperiment. With a few assumptions about the densities of states of aggregated and discrete molecules, a numerical solution of the equations is obtained. The solution reproduces the basic features of the experimental data and suggests that the energy transfer between the clusters and the discrete molecules depends strongly on the excitation energy.
Lin and Hanson
I
11. Experimental Techniques
The dopant used in this experiment was 4-bromo-4’-chlorobenzophenone (BCBP) obtained from Sigma. It was purified by multiple recrystallization (three times) from ethanol solution and zone refined for 100 passes at a velocity of 1.5 in./h. The polymer, polystyrene, was used as recieved from Aldrich. No luminescence from undoped polystyrene films was observed under the conditions of the experiment. The detailed procedure for preparation of the polymer films was given previ~usly.’~*’~ The degree of aggregation is closely related to the concentration of the dopant molecules. In this experiment the dopant concentrations were selected to be IO, 11, (14) Lin, Y . ;Hanson, D. M. J . Phys. Chem. 1987, 91, 2279
I
I
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1
1
P Y T SIGNAL d
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Figure 1. Schematic diagram of the electronics and data acquisition setup used in this experiment: (A and G) discriminator (repetition frequency 100 MHz), each has four simultaneous output channels (shown only in parts); (B, C, D), gate and delay generators, each contains two independent channels; (E) real time clock; (F) broad-band amplifier; (H) gated quad-scalar, a CAMC module containing four independent pulse counters; (I) CAMAC crate; (J) PDP-11/23 computer; (K) video terminal; and (L) disk storage. Input signals: TRG, trigger pulse signaling the firing of the N, laser; PMT, spectrometer output. TABLE I: Modules Used in TRPLN A quad 300-MHz discriminator B gate and delay generator C gate and delay generator D gate and delay generator E real time clock F dc 300-MHz bipolar amplifier G quad 300-MHz discriminator H quad scaler I CAMAC crate and crate controller J digital microcomputer K digital terminal L digital hard disk drive
Data Acquisition No. 704 (Phillips Scientific) No. 2323 (LeCroy) No. 2323 (LeCroy) No. 2323a (LeCroy) RTC-018 (DSP Technology) No. 6950 (Phillips Scientific) No. 704 (Phillips Scientific) QS-450 (DSP Technology) No. 1500 (Kinetic Systems) No. 3912 (Kinetic Systems) PDPl1/23+ (DEC) VT-240 (DEC) RL02 (DEC)
12, and 20 wt %. The samples were immersed in liquid helium in an optical Dewar to achieve a temperature of 4.2 K. A Molectron DL400 tunable dye laser were used as the excitation source for the T, Sotransition for all the measurements. This dye laser system is pumped by the 337.1-nm light from a nitrogen laser. The output pulse has typically a duration of 10 ns and a spectral width of 0.3 A. The phosphorescence was monitored with a double spectrometer (Spex 1402) and an EM1 9865 photomultiplier tube (PMT). Steady-state measurements were made using a current amplifier with a gain of lo8 V/A and a typical time constant of 1 s. The signal was digitized by an ADC interfaced to a PDP-11/23+ computer system through a CAMAC crate. For monitoring the decay of the phosphorescence intensity, the signal from the PMT was amplified by a wide-band preamplifier (EG&G Model 115) with a gain of 100 and a nominal time resolution of 20 ns. The output of the preamplifier went through a variable R C filter to a transient digitizer in the CAMAC crate. The RC circuit was used to integrate the signal with a time constant varying from 0.1 to 1 ps to coordinate with the sampling interval of the transient digitizer. A schematic of the electronics for the time-resolved phosphorescence line narrowing experiment is shown in Figure 1. The electronic units are modular and arb denoted by capital letters in the figure. Relevant technical information is listed in Table I. The lower case letters in Figure 1 refer to the pulse sequence labels show in Figure 2. In the TRPLN experiment a photoncounting technique was used. The phosphorescence signal from the photomultiplier tube was first amplified by a pulse amplifier (F) and then sent to a discriminator (G). The threshold of the discriminator and the P M T voltage were adjusted to maximize the signal-to-noise ratio. The four identical and simultaneous NIM outputs from the discriminator were fed to four independent
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Spectral Diffusion in Molecular Aggregates a
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The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1853
,
1200ns /
U
20% BCB P
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4570
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11% B C B P
Figure 2. Timing diagram of the pulses. (a) is the trigger pulse. Others are clock pulses of different duration, which start roughly 200 ns after the trigger to avoid picking up the electromagneticpulse that is produced
while firing the nitrogen laser. channels of a gated quad-scalar module (H) for pulse counting. The gated scalar has the property that the input pulses may or may not be counted, depending on the level of the gate signal. By synchronizing the gate signal with the excitation laser pulse and by carefully selecting the “on” and “off“ time of the gate signal, we can monitor the phosphorescenceintensity for a specified time interval following the excitation pulse. By having a few scalars that are controlled by gate signals of different duration, the time evolution of the phosphorescence signal can be determined. It is apparent that generating appropriate gate signals is critical. In this experiment the gate signal was started by a trigger signal from a photodiode activated by the nitrogen laser light pulse. The pulse was then shaped by a discriminator (A) and delayed for about 200 ns by a gate and delay generator (B). This delay prevented the scalar from picking up noise pulses generated by the laser discharge. The delayed pulse was again shaped by another channel of the discriminator (A) and then sent to start the gate and delay generators (C) and (D). These gate and delay generators are programmable to give gate pulses of a desired length. The timing diagram of the pulses from the four channels is shown in Figure 2. At each wavelength of the spectral scan, the total counting time was controlled by a programmable real time clock (RTC, module E). The discriminator (A) has a VETO signal input which, when high, blocks the trigger pulses and prohibits the generation of gate signals. This input was connected to the real time clock output port which is normally high. When the clock was started, it set the port low to allow the pulses to pass through discriminator (A). After a preselected time interval, typically lOs, the real time clock stopped, setting the port high to block the discriminator (A). Both the scalar and the real time clock were interfaced to the computer through a CAMAC crate. An additional scalar was used to count the number of trigger pulses, which equals the number of laser pulses during each counting cycle. The control program included three major parts. It starts by moving the monochromator to a desired wavelength. Next, it initializes the real time clock and presets the duration for the counting cycle. After the cycle was completed, it reads from the data from the CAMAC modules and stores them on the computer disk. A new wavelength was then selected, and the whole process was repeated.
111. Experimental Results and Discussion Steady-state luminescence was used to characterize the sample. Two typical spectra following direct excitation of the BCBP triplet state are shown in Figure 3. One spectrum was obtained with a highly aggregated sample (a dopant concentration of 20%),and one was obtained with a slightly aggregated sample (1 1%). The
4450
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Figure 3. Steady-state phosphorescence spectrum of 20% BCBP in polystyrene (top) and 11% BCBP in polystyrene (bottom), excited by laser line at 4178 A. The increase in the phosphorescence intensity at longer wavelength in the bottom figure indicates the presence of both cluster molecules and discrete molecules.
20% BCBP
4140
Angstrom
4260
Figure 4. The 0-0 band emission profile of the 20% BCBP aggregate sample at 4.2 K. The excitation at 3371 A is provided by a nitrogen laser. The five arrows indicate the positions of the excitation energy for the site-selected excitation spectra shown in Figures 5-7.
excitation wavelength was 41 78 8, and the sample temperature was 4.2 K, in both cases. These spectra are very similar to those ~ the 11% sample there is obtained with N2 laser e ~ c i t a t i 0 n . l In more intensity at longer wavelengths. This difference has been attributed14 to the presence of two kinds of molecular environments. Molecules in clusters interact with each other, and discrete molecules interact mainly with the host polymer. With a high dopant concentration, almost all molecules have the cluster environment, and the luminescence spectrum from such a sample resembles that from pure crystals, which are intrinsically disordered.]’ The spectrum from the sample with the lower concentration exhibits the signature of both cluster and polymer environments. ~~
( 1 5 ) Lin, Y . ;Nelson, M. 1586.
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C.;Hanson, D.M. J . Chem. Phys. 1987, 86,
1854 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Lin and Hanson
11% BCBP
41726
1175A
41706
41806
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Angstroms Figure 5. First vibronic band of phosphorescence from a 20% BCBP in polystyrene sample at 4.2 K. The spectra are obtained in a steady-state experiment by exciting at different site energies (steady-state-selected excitation phosphorescence). Although the red shift decreases with in a decrease in the excitation energy (see Table II), the line shapes in these spectra do not show any significant change. Only cluster luminescence is observed at all excitation energies. Selective excitation of the inhomogeneously broadened T, levels was used to investigated energy transfer from clusters to discrete molecules. The relative positions of the excitation energy with regard to the 0,O phosphorescence band are shown in Figure 4. In order to minimize interference from stray light from the dye laser, the first vibronic band, which is displaced from the 0,O band by 1660 cm-I, was monitored. Figures 5-7 show the phosphorescence spectra of BCBP in polystyrene at different concentrations and with excitation at different wavelengths. The aggregation density correlates with the concentration. The strongest dependence of the luminescence on the excitation wavelength was observed in the lightly aggregated sample (1 1% BCBP), shown in Figure 6. When the excitation is in the highest energy region (A, < 4172 A), the spectrum is dominated by the cluster component, similar to that observed in a heavily aggregated sample (20% BCBP). As the excitation energy decreases, however, the luminescence profile clearly shows the superposition of two different spectra, one from the cluster and the other from discrete molecules. The relative intensity of the cluster part decreases rapidly with decreasing laser excitation energy. Significant luminescence from discrete molecules was observed only in the samples in which BCBP forms aggregates. In an optically clear sample (Le., no visible aggregation) containing as much as 10% BCBP, no appreciable luminescence can be seen when excited in the region of 4180 A. Yet if this same sample is excited at 337 nm with a nitrogen laser (S, So transition), strong phosphorescence is observed. The spectrum is shown in Figure 8. The line shape is very similar to that found for an 11% BCBP sample with 4180-A excitation. Three conclusions can be made from these observations. First, the lower spectrum in Figure 8 is due to discrete molecules since it resembles the upper spectrum, which comes from a clear sample containing no visible aggregates. Second, the phosphorescence from the 11% BCBP sample under 4180-A excitation (bottom of Figure 8) must be due to energy transfer from clusters since direct excitation of the discrete molecules at 4180 8, in the clear sample (10% BCBP) did not produce observable phosphorescence. Third, the narrow high-energy phosphorescence bands are from cluster molecules since they only appear with high-energy excitation in the most concentrated samples.
Angstroms Figure 6. Steady-state-selected excitation phosphorescence spectra in 11% BCBP in polystyrene film at 4.2 K. The observed emission profile is strongly dependent on the excitation energy. As the excitation energy
lowers, the emission intensity from discrete molecules increases. Excitation at 4172 8, does not show any difference with the emission of the 20% BCBP sample shown in Figure 5. When excitation is below 4175 A, the peak position of the 0-0 cluster emission band, the zero-phonon line becomes narrower and shifts to lower energy.
41724
1 41756
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Figure 7. Steady-state-selected excitation phosphorescence spectra in 12%BCBP in polystyrene film at 4.2 K. Excitation at 4172 8, does not show any difference with the spectra at the 20% BCBP sample (Figure 5). As the excitation energy lowered, the emision intensity from discrete molecules increases. When excitation is below 4175 A, the zero-phonon line becomes narrower and shifts to lower energy.
If the phosphorescence is resonant with the excitation, the excitation energy and the 0,l phosphorescence band are related by uphos
=
-
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The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1855
Spectral Diffusion in Molecular Aggregates
TABLE II: Correlation of Excitation Energy and Phosphorescence Energy vex,, cm-' vphoa, cm-' vsbift, cm-' (a) 20% BCBP
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4510
11% BCBP
23992 23 969 23 952 23923
(4168)' (4172) (4175) (4180)
23969 23952 23 929 23 923
(4172) (4175) (4179) (4180)
23 969 23 952 23 929 23 923
(4172) (4175) (4179) (4180)
22 286 22 286 22 282 22 263 (b) 12% BCBP 22 286 22 279 22 269 22 263
46 23 10 0 23 13 0 0
(c) 1 1 % BCBP
a
1
I
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4510 Angstroms
Figure 8. (top) First vibronic band of phosphorescence spectrum of 10% BCBP in polystyrene film at 4.2 K. The sample is optically clear without aggregation. The excitation is at 3371 8, (So SI). (bottom) First vibronic band of phosphorescence spectrum of 1 1 % BCBP in polystyrene film at 4.2 K. The sample is slightly aggregated, and the excitation is at 4180 8, (So TI).
-
-
where vphm is the energy of the observed phosphorescence line, vexis that of the laser excitation, and Vdb is the vibrational energy difference in the ground state, which can be measured from the phosphorescence with singlet excitation. In some cases relaxation, e.g., by energy transfer, may cause the phosphorescence line to shift to lower energies. For this case Vphos
=
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Table I1 lists the observed phosphorescence line positions and the corresponding v,hift for various excitation energies and dopant concentrations. The data, Figure 5-7 and Table 11, reveal a trend. The cluster zero-phonon band (Le., the narrow line in Figures 6 and 7) for the lower concentration samples (1 1% and 12%BCBP) shifts to lower energies as the laser excitation moves below 23 952 cm-I (4175 A). According to Table 11, with excitation at the longer wavelengths the phosphorescence is resonant to the excitation. It is also noticeable that in this same energy region the emission profile becomes sharper, but the narrowest line in the observed spectra has a width about 15 times the laser line width. The spectra can be interpreted to result from a competition of energy-transfer processes: intracluster energy transfer that redistributes the excitation among the cluster molecules and energy transfer between clusters and discrete molecules. For the simplest case of phonon-assisted energy transfer, the probability of absorbing a phonon during the energy-transfer process is proportional to (n),the phonon population number, while that of emitting a phonon is proportional to (n) 1 . At low temperatures, when ( n )
