The Journal of
Physical Chemistry
0 Copyright, 1987, by the American Chemical Society
VOLUME 91, NUMBER 16 JULY 30, 1987
LETTERS Excimer Formatlon in Pyrene Labeled Hydroxypropyl Cellulose in Water: Picosecond Fluorescence Studies Iwao Yamazaki,* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
Fransoise M. Winnik,* Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5K 2Ll
Mitchell A. Winnik, Department of Chemistry, University of Toronto, Toronto, Canada M5S 1Al
and Shigeo Tazuke Tokyo Institute for Technology, Laboratory of Resources Utilization, 4259 Nagatsuta, Midori- ku, Yokohama 227, Japan (Received: May 13, 1987)
Excimer formation of pyrene chromophoresin an aqueous solution of hydroxypropyl cellulose has been studied with a picosecond time-resolved fluorescence spectrophotometer. The time-resolved spectra show two types of broad structureless fluorescence bands at (1) 420 nm in a short time region (2-7 ns) and (2) 470 nm in the time region after 50 ns. The latter is the well-known sandwich-type pyrene excimer, and the other is assumed to be a one-center-type excimer similar to those observed in several other molecular assemblies such as LB films, vapor-deposited films, and single crystals. This is the first report on the detection of such excimers in a dilute polymer solution.
Introduction Photochemical and photophysical processes under restricted molecular arrangements are of current interest, since one can expect new aspects of reaction pathways for electronically excited molecules quite different from those of homogeneous solution or from bulk properties of crystals. Recently, there have been several reports on molecular association of pyrene in different types of 0022-3654/87/2091-4213$01.50/0
molecular assemblies: pyrene as pendant chromophores in pyrene adsorbed On d i d surfaces such as silica (1) Winnik, M. A., Ed. Photophysical and Photochemical Tools in Polymer Science; NATO AS1 Series C: Mathematical and Physical Sciences Vol. 182; Reidel: Dordrecht, 1986. (2) Winnik, F. M.; Winnik, M. A,; Tazuke, S.;Ober, S. Macromolecules 1987, 20, 3 8 .
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
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or zeolites,' and pyrene in Langmuir-Blodgett (LB) films8 Pyrene "excimer" emission is always observed in addition to the pyrene "monomer" emission. A characteristic feature of pyrene fluorescence in these sysems is that the pyrene excimers arise from preformed ground-state dimers. Furthermore, a recent study on LB films* has demonstrated the existence of two distinct pyrene excimer emissions: a slowly forming excimer emission centered at 470 nm assigned to a sandwich-type pyrene excimer and a faster forming excimer emission centered at 420 nm. We have recently examined the fluorescence properties of a pyrene-labeled polymer and detected ground-state association of pyrene groups even in very dilute solution. The polymer studied is hydroxypropyl cellulose (HPC), a commercial polymer obtained by treatment of cellulose with propylene oxide. It is a linear polysaccharide in which most hydroxyl groups have been converted to hydroxypropyl groups. Hydroxypropyl cellulose was labeled randomly with small amounts of pyrene. Evidence from absorption spectroscopy (a strong hypochromic effect) and steady-state fluorescence spectroscopy suggested that, in aqueous solutions of pyrene-labeled HPC (HPC-Py), the pyrene chromophores are associated prior to excitation and that HPC-Py forms intermolecular aggregates even at extremely low polymer concentration.2 This feature resembles those observed in other types of molecular assemblies, the LB films and the surfaces of silica gel and zeolite.
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We report here the results of picosecond time-resolved fluorescence studies on pyrene-labeled HPC. One sample we examine here, HPC-Py/56, contains on average one pyrene group per 56 glucose units; the other, HPC-Py/438, one pyrene group per 438 glucose units. We conclude that the pyrene association in aqueous HPC-Py solutions is similar in nature to that of 16-( 1-pyrenyl)hexadecanoic acid compressed in LB films.
Experimental Section Pyrene-labeled hydroxypropyl cellulose (HPC-Py) was prepared by reaction of H P C (Klucel L, Hercules, MW lOOOOO), which was purchased from Aldrich Chemical Co., with 4 4 I-pyreny1)butyl tosylate in the presence of sodium hydride, following a procedure described in detail elsewhere.2 The sample of high pyrene content (HPC-Py/56) was prepared by using H P C (4.0 g), 4-(l-pyrenyl)butyl tosylate (0.50 g, 1.16 mmol) and sodium hydride (0.20 g, 60% dispersion on oil). The pyrene content of the final polymer (3.78 g), determined by UV analysis of a (3) (a) Goedeweeck, R.; De Schryver, F. C. Photochem. Photobiol. 1984, 39, 515. (b) Collart, P.; Toppet, S.; Zhou, Q.F.; Boens, N.; De Schryver, F. C. Macromolecules 1985, 18, 1026. (4) (a) Zachariasse, K. A,; Duveneck, G.; Kuhnle, W. Chem. Phys. Lerf. 1985, 113, 337. (b) Zachariasse, K. A.; Duveneck, G.; Kuhnle, W.: Reynders, P.; Striker, G. Chem. Phys. Lett. 1987, 133, 390. (5) (a) Hara, K.; de Mayo, P.; Ware, W. R.; Weedon, A. C.; Wong, G. S. K.; Wu, K. C. Chem. Phys. Lett. 1980,69, 105. (b) Bauer, R. K.; Bor-
enstein, R.; de Mayo, P.; Okada, K.; Rafalska, M.; Ware, W. R.; Wu, K. C. J . Am. Chem. SOC.1982, 104,4635. (c) Bauer, R. K.; de Mayo, P.; Okada, K.; Ware, W. R.; Wu, K. C . J . Phys. Chem. 1983, 87, 460. (6) Avnir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariasse, K. A. J . Phys. Chem. 1985, 89, 3521. (b) Wellner, E.; Ottolenghi, M.; Avnir, D.; Huppert, D. Langmuir 1986, 2, 616. (7) Suib, S. L.; Kostapapas, A. J . Am. Chem. SOC.1984, 106, 7705. (8) (a) Yamazaki, T.; Tamai, N.;Yamazaki, I. Chem. Phys. Lett. 1986, 124,326. (b) Yamazaki, I.; Tamai, N.; Yamazaki, T. Ultrafast Phenomena V; Springer Ser. Chem. Phys.; Fleming, G. R., Siegman, A. E., Eds,; Springer-Verlag: Berlin, 1986; Vol. 46, pp 444-446. (c) Yamazaki, I.; Tamai, N.; Yamazaki, T.J . Phys. Chem. 1987, 91, 3572.
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WAVELENGTH ( n m 1 Figure 1. Absorption, fluorescence excitation, and fluorescence emission spectra of pyrene-labeled hydroxypropyl cellulose; (a) absorption spectra of HPC-Py/56 (-) and HPC-Py/438 (-) in water, (b) excitation spectra of HPC-Py/56 monitored a t 376 nm (monomer) (-) and 474 the spectrum of HPC-Py/438 is almost the same as nm (excimer) the monomer spectrum; (c) fluorescence emission spectra of HPC-Py/56 (-) and HPC-Py/438 (- - -) (-e),
methanol solution using 4 4 1-pyreny1)butanol as a model, was mol/g of polymer, equivalent to one calculated to be 5.33 X pyrene group per 56 glucose units for a polymer of Mn 36 000. The sample of low pyrene content (HPC-Py/438) was prepared by using HPC (4.0 g), 4-(l-pyrenyl)butyl tosylate (60 mg, 0.14 mmol), and sodium hydride (0.10 g, 60% dispersion in oil). Its pyrene content was calculated to be 6.8 X 10" mol/g of polymer, equivalent to one pyrene group per 438 glucose units. The purity of the HPC-Py samples was important for the experiments described here. It was ascertained by GPC analysis using RI and UV detectors in tandem that the pyrene groups were covalently linked to the polymer and that low molecular weight pyrene impurities accounted for less than 0.1% of the total pyrene content. Time-resolved fluorescence spectra and fluorsscence decay curves were measured with a time-correlated photon-counting apparatus with a synchronously pumped, cavity-dumped dye laser. Details of the whole system are described el~ewhere.~Before the measurement, HPC-Py samples were allowed to dissolve in deionized water for at least 24 h at room temperature. Solution samples had a concentration of approximately 0.3 g/L.
Results and Discussion Figure 1, a and b, shows the absorption and the fluorescence excitation spectra of HPC-Py in gqueous solution. The absorption spectrum of HPC-Py/56 shows vibrational bands which are shifted to the red by 1 nm relative to those of HPC-Py/438. Figure I C shows the fluorescence spectra of steady excitation for HPC-Py/56 (9) Yamazaki, I.; Kume, H.; Tamai, N.; Tsuchiya, H.; Oba, K. Rea. Sci. Instrum. 1985, 56, 1187.
The Journal of Physical Chemistry, Vol, 91, No. 16, 1987 4215
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Figure 2. Time-resolved fluorescence spectra of pyrene-labeled hydroxypropyl cellulose, HPC-Py/56 (a) and HPC-Py/438 (b), in water. The excitation wavelength is 324 nm. The time zero corresponds to the time in which the excitation laser pulse reaches maximum intensity. The spectrum analysis into component was made by assuming three components, one monomer M and two excimers E, and E*.
and 438. In HPC-Py/56, the fluorescence spectrum consists of a monomer band with three vibrational peaks and a broad structureless band, while in HPC-Py/438 it shows only the monomer band. The fluorescence excitation spectrum of HPC-Py/56 (Figure lb) is different according to whether the monomer or the excimer fluorescence is monitored; the spectrum monitored at 474 nm (excimer) is shifted by 2 nm to longer wavelength compared to that monitored at 397 nm (monomer). The excitation spectrum of the monomer fluorescence corresponds to the absorption spectrum of HPC-Py/438, while that of the excimer corresponds well to the absorption spectrum of HPC-Py/56 in which the density of pyrene chromophores is fairly high. This means that the excimer state forms from a species different from the monomer. We assign this species as a loosely coupled molecular pair or aggregates of pyrene groups in the ground state from which the excimer forms through only a small displacement of a pyrene chromophore. Time-resolved fluorescence spectra are shown in Figure 2a,b for HPC-Py/56 and 438, respectively. Each spectrum is normalized to the maximum intensity. In HPC-Py/56, the spectrum is significantly changed with time on going from the picosecond to the nanosecond time region. We analyzed each spectrum into components through simulating calculation. It can be seen that the time-resolved spectra are reproduced reasonably as a superposition of three spectral components: (1) the monomer band (M) with vibrational peaks at 376, 397, and 421 nm, ( 2 ) a broad structureless band (E,) with a peak at 420 nm, and (3) the well-known band (E2) of pyrene excimer of a sandwich conformation with a peak at 470 nm. We should note here that, in the initial time region of 0-1 50 ps, the monomer band is red-shifted
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monitored at different wavelengths. According to the decay curve analysis, curve 1 is single exponential with T, = 112.3 A 1.2 ns, curves 4 and 5 consist of two exponentials with T~ = 3.77 f 0.18 ns and T~ = 72.0 2.2 ns, and curve 3 consists predominantly of the fast decay component with T~ together with small fractions of T , and T ~ .
*
by 1 nm, Le., the three vibrational peaks are located at 377, 398, and 421 nm. It is considered that the red-shifted fluorescence band at 0-150 ps originates from the ground-state dimer which is associated with the absorption transition of the dimer. This means that, during 0-150 ps, the photoexcited dimer is changed into the excimer state. The red-shifted fluorescence band in the initial time region is hereafter referred to as the dimer band D. We assign the E, band to another excimer band as will be discussed later. Two transient fluorescence bands D and E, appear only in the short time region, so that the steady-state fluorescence spectrum is no longer affected by these species. In other words, the time-resolved spectra after 10 ns coincide with the steady-state fluorescence spectrum. In HPC-Py/438 where the density of pyrene chromophores is relatively lower, the excimer fluorescences of E, and E, make only a minor contribution, and the monomer band M is a dominant component. Figure 3 shows the fluorescence decay curves of HPC-Py/56 and 438 monitored at different wavelengths. The decay curve of the monomer monitored at 376 nm (curve 1 in Figure 3) is almost single exponential with a lifetime of 112.3 1.2 ns in HPC-Py/438. In HPC-Py/56 (curve 2), it deviates from single exponential, indicating that the fluorescence quenching occurs on trap sites of pyrene aggregates nearby the excited pyrene chromophore. The decay curve monitored at 511 nm (curve 5) is analyzed as a biexponential decay consisting of a major component ( T= ~ 72.0 f 2.2 ns) and a minor component ( T = ~ 3.77 f 0.18 ns). At intermediate wavelengths, the fast decaying component is dominant. In conformity with the time-resolved spectra, these fast and slow decaying components originate respectively from the excimer fluorescence bands E, and E2. The fluorescence decay curve of the E, excimer includes a rise with 250 ps rise time corresponding to the fast decay of D which appears as the spectral
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J . Phys. Chem. 1987, 91, 4216-4218
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change of D M in the time-resolved spectra (Figure 2a). These observations together with the inspection of time-resolved spectra may lead to the following conclusion. There exist two kinds of sites with respect to pyrene chromophores: monomer sites and dimer and/or aggregate sites. In the dimer sites, following a pulsed excitation, the fluorescence D is emitted from the excited ground-state dimer of pyrene, its decay being accompanied by a rise of the excimer fluorescence E*. The monomer emission M which originates from pyrene groups isolated along the chain backbone appears in a longer time region after the decay of the dimer fluorescence D. HPC is a rather hydrophobic polymer, with a Hildebrand solubility parameter of 10.7. The aqueous solubility of H P C is unusual. It is attributed to the formation of hydrogen bonds between water and the propylene glycol chains of the polymer. At temperatures above 0 OC in water HPC has a tendency to form aggregates and these become sufficiently extensive at elevated temperature to lead to phase separation above the lower critical solution temperature.I0 The formation of interpolymeric aggregates is enhanced in pyrene-labeled HPC. Unlike the polymer backbone pyrene groups cannot undergo hydrogen bonding with water. The number of hydrogen bonds between water molecules which are disrupted or distorted by the nonpolar pyrene groups is minimized if two or more pyrenes come in close proximity. The nonpolar dimers are then surrounded by a cage of highly organized
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(10) Roberts, G. A. F.; Thomas, I. M. Polymer 1978, 19, 459
Ring Puckering Potential of Oxetane: TZ
water molecules tightly bound by hydrogen bonds. In this molecular environment surrounding the polymer, a pair of pyrene chromophores within a cage might be forced to make a metastable ground-state dimer. Similar behaviors of the dimer formation can be seen in silica ge15*6and zeolite’ surfaces, in LB films,8 and in cyclohexane matrix at 77 K.” Commonly to these cases, the pyrene excimer forms much faster (-0.1 ns) than in solution where the excimer formation is a diffusion-controlled process.[ The present study demonstrates furthermore another type of pyrene excimer which exhibits the fluorescence El with a peak at 420 nm which is characterized by a very fast rise and decay relative to the excimer Ez. This type of excimer has recently been reported in several different systems, viz., LB films,8 vapor deposited films,l2 and pyrene ~rysta1s.l~By reference to the bperylene single crystals reported by Matsui et al.,I4 this is expected to be of the one-center-type excimer in which the photoexcited molecule interacts simultaneously with several molecules situated around the excited molecules. Thermodynamical study will be needed to get an insight of the excimer conformation in addition to the time-resolved fluorescence study. (1 1) Mataga, N.; Torihashi, Y.; Ota, Y . Chem. Phys. Lett. 1967,1, 385. (12) (a) Mitsuya, M.; Taniguchi, Y.; Tamai, N.; Yamazaki, I.; Masuhara, H. ThinSolid Films, 1985,129, L45. (b) Taniguchi, Y.; Mitsuya, M.; Tamai, N.; Yamazaki, I.; Masuhara, H. Chem. Phys. Lett. 1986, 132, 516. (13) Matsui, A,; Mizuno, K.; Tamai, N.; Yamazaki, I. Chem. Phys. 1987, 113, 111. (14) Matsui, A. Mizuno, K.; Nishimura, H . J . Phys. SOC.Jpn. 1984, 53, 2818.
+ nP/MP4 (SDQ) Results
Harrell Sellers,* Jan Almlof, Minnesota Supercomputer Institute and Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
Svein Saeber, Department of Chemistry, Mississippi State University, Drawer CH, Mississippi 39762
and Peter Pulay Department of Chemistry, University of Arkansas, Fayetteville, Arkansas 72701 (Received: May 26, 1987)
The ring puckering potential of oxetane has been determined by the local correlation treatment of S a e b ~and Pulay employing basis sets of T Z + nP quality with n = 1-3 and Mdler-Plesset second-, third-, and fourth-order perturbation theory. The results show that the calculated values of the intramolecular dispersion interaction as well as the SCF potential are slowly convergent with respect to basis set size. The second-order perturbation results overestimate the correlation effects on the quadratic part of the potential by a factor of 2. Third-order PT corrects for this, and the fourth-order contributions are small.
Introduction Interest in the oxetane system stems primarily from the fact that the experimentally derived effective ring puckering potential possesses a small barrier to ring planarity.’-” Several theoretical ~~
~~~
( I ) Chan, S. I.; Zinn, J.; Fernandez, J.; Gwinn, W. D. J . Chem Phys 1960, 33, 1643. (2) Chan, S. I.; Zinn, J.; Gwinn, W. D. J . Chem. Phys. 1960, 33, 295. (3) Chan, S. I.; Zinn, J.; Gwinn, W. D. J . Chem. Phys. 1961, 34, 1319. (4) Chan. S. I.: Rogers. T. R.: Russell, J. W.: Strauss, H. L.; Gwinn, W. D. J.‘Chem. Phys. 19g6, 44, 1103. (5) Wieser, H.; Danyluk, M.; Kidd, R. A. J . Mol. Specrrosc. 1972,43, 382. (6) Foltynowicz, I.; Konarski, J.; Kreglewski, M. J . Mol. Spectrosc. 1981, 87, 29. (7) Creswell, R. A,; Mills, I. M. J . Mol. Spectrosc. 1974, 52, 392. (8) Makkinson, P. D.; Robiette, A. G. J . Mol. Spectrosc. 1974, 52, 413. (9) Creswell, R. A. Mol. Phys. 1975, 30, 217. (IO) Jokisaari, J.; Kauppinen, J. J . Chem. Phys. 1973, 59, 2260. (1 1) Kidd, R. A,; Wieser, H.; Kiefer, W. Spectrochim. Acta 1983, 39, 173.
0022-3654/87/2091-4216$01.50/0
studies have been performed on the oxetane system in attempts to reproduce and identify the physical origins of the barrier to ring p1anarity.l2-l6 In a recent work16 we reported results of local correlation calculations that show that the electron correlation makes a negative contribution to the quadratic potential constant of the ring puckering potential. The local correlation methodl7-l9 ( 12) Banhegyi, G.; Pulay, P.; Fogarasi, G. Spectrochim. Acta 1983, 39, 761. (13) Skancke, P. N.; Foragasi, G.; Boggs, J. E. J . Mol. Strucr. 1980, 62, 259. (14) Pulay, P.; Banhegyi, G.; Jonvik, T.; Boggs, J. E. Presented at the 9th Austin Symposium on Molecular Structure, 1982; paper TM4. ( 1 5 ) Sellers, H. Chem. Phys. Lett. 1984, 108, 339. (16) Sellers, H.; Saebm, S.; Pulay, P. Chem. Phys. Lett. 1986, 132, 29. (17) Saeba, S.; Pulay, P. Chem. Phys. Lett. 1985, 113, 13. (18) Pulay, P.; Saeba, S . In Geometrical Deriuatiues of Energy Surfaces and Molecular Properties; Jorgensen, P., Simons, J., Eds.; Reidel: Dordrecht, 1986: p 95.
0 1987 American Chemical Society
