J . Phys. Chem. 1991, 95, 984-988
984
Photophysics of Poly(ethy1ene terephthalate): Ultraviolet Absorption and Emission John P. LaFemina* Pacific Northwest LaboratoryPtP.O. Box 999, Richland, Washington 99352
and G. Arjavalingam IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 (Received: May 21, 1990: I n Final Form: July 25, 1990)
Polarized and unpolarized emission spectra for poly(ethy1ene terephthalate) are presented, and a new phosphorescence peak at 564 nm is reported. The spectroscopically parametrized CNDO/S3 model is used to provide a detailed description of the absorption and monomeric emission. Moreover, preliminary computations on an idealized terephthalate dimer indicate that the 368-nm fluorescence and the newly reported phosphorescence emission may be attributable to an associated ground-state dimer.
I. Introduction The clean, well-defined laser etching of polymers via ablative photodecomposition' has provided the impetus for a wide scope of activity ranging from the exploration of potential applications in microelectronics and electronic packing technologies to the medical arena.2 A significant fraction of this work has dealt with the polymer poly(ethy1ene tere~hthalate)~ (PET, see Figure I ) , yet very little work has been done on understanding the photophysical properties of this, or any of the polymers used in these etching studies, on a molecular The vast majority of the work on PET has concentrated on the characterization of the etch products and etching p r o c e ~ son , ~ elucidating the effects of processing on polymer chain ~rientation,~-'or on the description of the intrinsic and photoinjected conduction processes.* However, it is clear that if a complete and thorough understanding of the laser etching process is to be achieved, a detailed quantitative description of the photophysical properties, namely absorption and emission, at the molecular level is required. The aim of this paper is bo provide a detailed description of the absorption and emission spectra of PET using the CNDO/S3 computational model to characterize the absorptions in terms of the photophysical properties of the molecular subunits comprising the polymer. This knowledge will then be used to understand the emission spectrum and to speculate on the nature of the emission process in PET. Similar studies on the technologically important polymer pyromellitimide dianhydrideoxydianiline (PMDA-ODA) polyimide have been previously reported? The paper is arranged as follows. In section I1 the experimental procedures and computational model are described. The absorption and emission spectra are discussed in sections Ill and IV, respectively, and we conclude with a synopsis. 11. Methodology Experiment. The experimental apparatus is shown in Figure 2. A Lumonics 860-TE XeCl excimer laser is used to pump a
narrow-band Lambda Physik 2002FL dye laser. Solutions of rhodamine 6G in methanol were used in both the oscillator/ preamplifier ( I .2 g/L) and amplifier (0.4 g/L). The rhodamine dye laser is tuned to 580 nm and the output frequency doubled in a KDP crystal (INRAD Auto Tracker 11). The fundamental (580 nm) and second harmonic (290 nm) radiation are frequency-separated in a dispersing prism, and the 290-nm radiation is directed onto the sample to excite the PET. The dye laser pulse has an energy of 2 mJ, while the energy of the 290-nm pulse is 40 r J . This pulse is incident on an area of 0.02 cm2, giving a fluence of 2 mJ/cm2 which is well below the etching threshold for PET ( 1 90 mJ/cm2 at 308 nm).2a 'Operated for the U S Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.
0022-3654/9 1/2095-0984$02.50/0
TABLE I: Parameters Used in the CNDO/S3 Model I.. eV I.. eV 6.. eV B.. eV Y. eV r.. A-' H 13.60 IO 12.85 2.33 C(sp2) 21.34 11.54 20 17 10.63 3.78 C(sp3) 21.34 11.54 20 17 10.63 3.07 0 35.50 17.91 31 26 13.10 4.32
c.. A-' 3.78 3.07 4.32
Sample emission is collected with a 5-cm focal length, fll.0 lens and focused onto the input slit of a 0.1-m double-pass monochromator (Instruments S.A. Model DH-10). One-milli( I ) (a) Srinivasan, R.; Mayne-Banton, V. Appl. Phys. Lett. 1 9 8 2 , 4 / , 576. (b) Srinivasan, R.; Leigh, W. J. J . A m . Chem. SOC.1982, 104, 6784. (c) Brannon, J. H.; Lankard, J. R.; Baise, A. 1.; Burns, F.; Kaufman, J. J . Appl. Phys. 1985, 58, 2306. (2) (a) Dyer, P. E.; Jenkins, S. D.; Sidhu, J. Appl. Phys. Left. 1986, 49, 453. (b) von Gutfeld, R. J.; Srinivasan, R. Appl. Phys. Len. 1987, 51, 15. (c) Znotins, T. A.; Poulin, D.; Reid, J. Luser Focus 1987,23, 54. (d) Znotins, T. A. Laser Appl. 1986, 5, 7 1. (3) (a) Dunn, D. S.; McClure, D. J. J . Vac. Sei. Technol., A 1987,5, 1327. (b) L a m e , S.; Soulignac, J. C.; Fragnaud, P. Appl. Phys. Lerr. 1987, 50,624.
(c) Novis, Y.; Pireaux, J. J.; Brezini, A,; Petit, E.; Caudano, R.; Lutgen, P.; Feyder, G.;Lazare, S. J. Appl. Phys. 1988,64, 365. (d) Grant, J. L.; Dunn, D. S.; McClure, D. J. J . Vac. Sei. Technol., A 1988,6, 2213. (e) Grant, J. L.; Dunn, D. S.; McClure, D. J. Symp. Proc.-Muter. Res. SOC.1988, 119, 297. (f) L a a r e , S.; Granier, V. J . Appl. Phys. 1988,63, 2210. (g) Hansen, S. G.J . Appl. Phys. 1989,66, 141 I . (h) Hennecke, M.; Keck, I.; Lemmert, E.; Fuhrmann, J. Z . Nuturforsch. 1989, 446, 745. (4) (a) Merrill, R. G.;Roberts, C. W. J . Appl. Polym. Sei. 1977, 21, 2745. (b) Allen, N. S.; McKellar, J. F. Makromol. Chem. 1978, 179, 523. (c) Padhye, M. R.; Tamhane, P. S. Angew. Makromol. Chem. 1978,69,33. (d) Chung, P. S. R.; Roberts, C. W.; Wagener, K. B. J . Appl. Polym. Sei. 1979, 24, 1809. (e) Dellinger, J. A.; Roberts, C. W. Ibid. 1981, 26, 321. ( 5 ) (a) Ouchi, 1. Polym. J . (Tokyo) 1983, 15, 225. (b) Akiyama, S.; Ushiki, H.; Kano, Y.; Kitazaki, Y. Eur. Polym. J . 1987, 23, 327. (c) Imagi, K.; Ikeda, N.; Masuhara, H.; Nishigaki, M.; Isogana, M. Polym. J . (Tokyo) 1987, 19, 999. (d) Hemker, D. J.; Frank, C. W.; Thomas, J. W. Polymer 1988, 29, 437. (6) (a) Hemker, D. J.; Frank, C. W.; Thomas, J. W. Polym. Prepr. ( A m . Chem. SOC.,Diu.Polym. Chem.) 1986,27,210. (b) Hennecke, M.; Fuhrmann, J. Makromol. Chem., Macromol. Symp. 1986, 5, 181. (c) Cao, T.; Magnov, S. N.; Qian, R. Polym. Commun. 1988, 29. 43. (d) Kaito, A,; Nakayama, K.; Kanetsuna, H. J . Polym. Sci., Part B: Polym. Phys. 1988, 26, 1439 and references therein. (e) Lapersonne, P.; Tassin, J. F.; Sergot, P.; Monnerie, L. Polymer 1989, 30, 1558. (7) (a) Phillips, D. H.; Shug, J. C. J. Chem. Phys. 1969, 50, 3297. (b) Takai, Y.; Mizutani, T.; Ieda, M. Jpn. J . Appl. Phys. 1978, 17, 65i. (8) (a) Comins, J. D.; Whintle, H. J . J. Polym. Sci., Polym. Phys. Ed. 1972, 10, 2259. (b) Hayashi, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. Phys. 1973, 12, 1089. (c) Takai, Y.; Osawa, T.; Kao, K. C.; Mizutani, T.; Ieda, M. Jpn. J. Appl. Phys. 1975, 14, 473. (d) Takai, Y.; Osawa, T.; Mizutani, T.; Ieda, M. Ibid. 1975, 14. 1157. (e) Sapieha, S.;Wintle, H. J. Cun. J . Phys. 1977, 55, 646. (0 Takai, Y.; Osawa, T.; Mizutani, T.; !eda, M. J . Polym. Sci., Polym. Phys. Ed. 1977, 15, 945; Jpn. J. Appl. Phys. 1977, 16, 1933. (g) Kurtz, S. R.; Arnold, C., Jr. IEEE Trans. Nucl. Sei. 1984, NS-31, 1284; J . Appl. Phys. 1985, 57, 2532. (9) (a) LaFemina, J . P.; Arjavalingam, G.;Hougham, G.J . Chem. Phys. 1989, 90, 5154. (b) LaFemina, J . P.; Arjavalingam, G.;Hougham, G. In Polyimides: Materials, Chemistry, and Characterizafion; Feger, C., Khojasteh, M. M., Mdjrath, J. E., a s . ; Elsevier: Amsterdam, 1989; pp 625-633. (c) Arjavalingam, G.; Houpham, G.;LaFemina, J. P.Polymer 1990,31, 840.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 985
Photophysics of Poly(ethy1ene terephthalate)
TABLE 11: Singlet Excitation Energies for PET Using Various CNDO/SJ-CI Manifolds Along with the Results of a PPP-CI Study and the Exwrimental Absorntions" ~~
~~
~~
PPP
4.19 (0.08) @) 4.88 (0.70) (x)
CNDO/S3 4X4CI 4.06 (0.0005) (x) 4.19 (0.07) (y) 4.88 (0.69) (x)
6.30 (0.77) (y)
6.22 (0.01) (JJ) 6.27 (0.58) (y)
2X2CI
6.38 (0.93) (x)
6X6CI 3.70 (0.0002) (x) 4.15 (0.07) (y) 4.88 (0.68) (x) 6.09 (0.002) (y) 6.13 6.22 6.30 6.39 6.45
6.35 (0.63) (x) 6.42 (0.25) (x) 6.49 (0.007) (x)
(0.40) (0.02) (0.66) (0.1 I ) (0.02)
(y) (y) (x) (x) (x)
2X2CI ref Sa
ref 5a
experimental ref 3b
ref 6d
4.7 5.3
4.1 5.1
4.2 5.1
4.3 5.1
6.3
6.2
6.3
6.3 6.4
Oscillator strengths and polarizations in parentheses. Energy units are electronvolts. U
H
H
a 118.57'
1= 1 . 2 d
K
p
127.27'
2 I 1.12A
k = 124.29"
7 = 1.42A
y I120.00
3 = 1.36A
5 = 109.5O
8 = 1.53A
6 = 121.36'
4 = l.40A
p = 108.10
9 = 1.41A
I
I
6 = 135A
114.80'
-
5 c LIOA 10 0.96A Figure 1. Conformation and geometric parameters for PET used in the E =123.64°
C N D O / S 3 computations.
ab DISPERSING
A PRIW
AUTOTAACKING WUBLING CRYSTAL
O
M
1,h) 2r2 CI
SAMPLE
I
I
Figure 2. Schematic of the experimental apparatus.
meter slits are used at the entrance, exit, and intermediate slit positions, resulting in a 6-nm wavelength resolution. The emission is dispersed and detected with a photomultiplier (PMT). The laser is operated at a repetition rate of IO Hz to avoid sample heating, and the signal detected by the PMT is averaged by using a gated integrator (Kinetic Systems CAMAC). The CNDOIS3 Model. The CNDO/S3 model was developed by Lipari and Dukelo for the computation of electronic spectra and is parametrized by optimizing the orbital exponents and overlap integrals to give the best fit to the spectra of some representative compounds. The parameters used in this study are given in Table 1 and are taken from previous studies.1','6b A ( I O ) (a) Lipari, N. 0.;Duke, C. B. J . Chem. Phys. 1975, 63, 1748. (b) Duke, C.B.; Lipari, N. 0.; Salaneck, W. R.; Schein, L. B. /bid. 1975. 63, 1758. (c) Lipari, N. 0.;Duke, C. B. Ibid. 1975, 63, 1768. ( I 1) (a) Duke, C. B. Int. J . Quantum Chem., Quantum Chem. Symp. 1979, 13. 267. (b) Yip, K. L.; Duke, C. B.; Salaneck, W. R.; Plummer, E. W.; Loubriel. G. Chem. Phys. Lett. 1977, 49, 530.
200
250
100
0
Wavelength (nml
Figure 3. Comparison of computed CNDOIS3-n X n-CI absorption spectra for PET with the experimental data of ref 3b: (a) n = 2, (b) n = 4, and (c) n = 6.
complete mathematical description of the model,'O along with examples of its application to similar ~ y s t e m s , ' * Jcan ~ ~ be - ~ found ~ in the literature. Figure 1 shows the unit cell and geometric parameters used in the computations for PET, taken from X-ray crystal data,') a b initio conformational studies,I4 and molecular mechanics computations performed for this study using Allinger's MM2 parametri~ation.'~In the CNDO/S3 computations a single unit cell oligomer of PET was used to model the polymer. This description is sufficient because the electronic properties of this system arise primarily from the highest occupied and lowest (12) (a) Duke, C. B.; Paton, A.; Salaneck, W. R. Mol. Cryst. Liq. Cryst. 1982,83, 177. (b) Duke, C. 8.;Paton, A . In Conductiue Polymers; Plenum:
New York, 1981; pp 155-169. (c) Duke, C. B.; Conwell, E. M.; Paton, A. Chem. f h y s . Lett. 1986, 131. 82. (13) Daubney, R. De P.; Bunn, C. W.; Brown, C. J. Proc. R. SOC.London 1954, A226, 5 3 I . (14) (a) Loew, L. M.; Sacher, E. J. Macromol. Sci., fhys. 1978, B15,619. (b) Kuehler, J. F.; Darsey, J. A.; Kountz, D. C. Tex. J . Sci. 1989. 41, 77. (15) Burket, U.; Allinger, N. L. Molecular Mechanics; ACS Monograph 177; American Chemical Society: Washington, DC, I982 and references therein.
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LaFemina and Arjavalingam
unoccupied A and A* orbitals which are localized on the tereA systems do not interact phthalate m o i e t i e ~ . ~ ~Neighboring .~' with each other because of the saturated ethylenic linkage which separates them. Hence, the electronic properties of PET are highly localized. A similar situation exists for PMDA-ODA polyi~nide~-'~ where the neighboring A systems are orthogonal. 111. Ultraviolet Absorption Spectrum The CNDO/S3 model was initially parametrized to give the correct benzene A-A* transition energies arising from the oneelectron el,(*) e2"(r*) molecular orbital Hence, the configuration interaction (CI) manifold for PET required in order to be compatible with the parametrization of the model is 2 X 2.10*1'Indeed, the remarkable agreement with experiment (50.1 eV) that is achievable with such a small CI manifold is a direct result of the parametrization procedure.l03" The results of a 2 X 2 CI calculation are listed in Table 11, along with the results from 4 X 4 and 6 X 6 CI computations, and all three show excellent agreement with the experimental data. The absorptions in Table I1 can be plotted by representing each excitation with a Gaussian of width /3 (typically 0.2 eV 5 /3 5 0 . 3 eV)" and height equal to that of the computed oscillator ~trength.~' Figure 3 compares the results of each of the CI computations with the experimental data of Lazare et al.3band clearly reveals that the single-unit cell macromolecular unit cell representation of PET is sufficient to give a quantitative description of the ultraviolet absorption (UVA) spectrum. An interesting feature of this description is it reveals that these spectra most probably involve strong electron-phonon coupling. In this limit the centroids of the absorption peaks remain at their corresponding rigid-molecule transition energies.I8 Hence, the excellent agreement between our computed rigid-molecule transition energies and the experimentally observed absorption bands indicated strong vibronic assistance. The absorptions in Figure 3 are most easily understood by thinking of them in terms of perturbed benzene transitions.8f The large peak at -200 nm (6.2 eV) derives from the dipole-allowed benzene 'A,, 'Elutransition at 180 nm (6.8 eV), while the peaks at 240 nm (5.1 eV) and 290 nm (4.3 eV) derived from the benzene !Al, l B l u ('La) and IAl, IB2"('Lb) transitions at 205 nm (6.0 eV) and 255 nm (4.9 eV), respe~tively.'~These transitions, dipole-forbidden in benzene, are allowed in the reduced symmetry of the oligomer. The question of carbonyl n-r* transitions is an interesting one. Generally, aromatic compounds with carbonyl groups are expected to show a n-A* absorption around 300 nm. Moreover, this absorption band can be shifted to shorter wavelengths by increasing the solvent polarity. Because of the low extinction coefficients associated with this transition, this absorption peak is sometimes buried under more intense A-x* peaks. This has been invoked by many authors to explain the absence of any n-x* band in the absorption spectrum of PET. Allen et al.4b and Padhye and Tamhane," however, report that ketonic and aldehyeic carbonyls formed during the photooxidation of polyolefins do not exhibit any emission. Furthermore, OuchiSareports that the PET absorption peak at 290 nm shifts to longer wavelengths as the solvent polarity is increased and summarizes that, "So far, a possible n--A* band has not been found in any ... PET solution spectra."5a The possibility exists however, that the absence of any n-a* transitions in our computations is the result of either the very small
-
-
-
-
(16) (a) Bredas, J. L.; Clarke, T. C. J. Chem. Phys. 1987, 86, 253. (b) LaFemina, J. P.; Arjavalingam, G.; Hougham, G. J . Chem. Phys. 1988,90, 5 154; In Polyimides: Materials, Chemistry, and Characterization; Feger, C., Khojasteh, M. M., McGrath, J. E., Eds.; Elsevier: Amsterdam, 1989; pp
625-633. (17) (a) Duke, C. B.; Paton, A.: Salaneck, W. R. fnt. J . Quanrum Chem. 1987, 21, 153. (b) LaFemina, J. P.; Duke, C. B.; Paton, A. J . Chem. Phys. 1987, 87, 2151; 1988, 89, 2668. (18) Duke, C. B. In Tunneling in Biological Systems; Chance, B., DeVault, D. C., Frauenfelder, H., Marcus, R. A,, Schrieffer, J. R., Sutin, N., Eds.; Academic: New York, 1979; pp 31-65. (19) Williams, D. H.; Fleming, I. Spectroscopic Methods i n Organic Chemistry; McGraw-Hill: London, 1973; pp 22-28.
Lmto
HOMO
Subjacent HOMO
LUMO
Supejacent LUMO
HOMO
Figure 4. Schematic indication of the PET molecular orbitals involved
-
-
in the one-electron transitions primarily responsible for the absorption peaks in Figure 3: (a) H O M O LUMO, (b) subjacent H O M O LUMO, and (c) subjacent HOMO and HOMO superjacent LUMO.
-
CI manifold used or a consequence of the CNDO/S3 parametrization. An examination of the computed MOs indicates that there are no MOs in this system which can be simply described as nonbonding carbonyl orbitals. Furthermore, the MOs that do have some in-plane O(2p) A 0 character are low-lying in energy so that any n-n* states would lie high above the ground state in energy. A test of the CNDO/S3 model's parameters suitability for this particular system would be a comparison of the computed density of valence states with the experimental ultraviolet photoemission spectrum (UPS) to see whether the O(2p) states are correctly predicted. Unfortunately, to the best of our knowledge no published UPS for PET exists. However, the 0 parameters used in this study have been used successfully to describe both the UPS and UVA of several similar systems such as furan,'Ib benzofuran,' I b poly@-phenylene oxide),*O and polyin~ide.~,'~ We therefore assign these absorptions as A-A* excitations arising from the one-electron transitions from the two highest occupied x MOs to the two lowest unoccupied A* MOs. (See Figure 4.) Moreover, these excitations can be characterized as localized molecular excitons. The one-electron molecular orbitals (MOs) involved in these excitations are shown in Figure 4. The low-energy absorption at 4.2 eV arises from the one-electron transition from the highest occupied M O (HOMO) to the lowest unoccupied MO (LUMO) (Figure 4a). Figure 4b shows the excitation at 4.9 eV is primarily the one-electron transition from the subjacent HOMO2' to the LUMO. Lastly, the excitations at 6.3 and 6.4 eV are primarily the one-electron transitions from the subjacent HOMO and the HOMO to the superjacent L U M 0 , 2 ' respectively. (See Figure 4c.) This assignment is consistent with those made by Takai et aL8' based on their study of photocarrier generation in PET and by Kaito et aLa based on CNDO/S-CI computations. However, in these studies only the two lower energy absorptions were assigned and their orbital nature was not discussed. This assignment is also in good agreement with the results of Ouchi? who used the PPP-CI method to examine the one-electron orbital nature of these transitions. These results are compared with the predictions of the CNDO/S3-CI model in Table I1 from which it is evident that (20) Duke, C. B. Mol. Cryst. Liq. Cryst. 1982, 83, 177 and references therein. (21) Lowe, J. P. Quantum Chemistry; Academic: New York, 1978; pp 460-46 1 ,
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 987
Photophysics of Poly(ethy1ene terephthalate) the CNDO/S3-CI model provides the first complete description
,
A ’
I
of the absorption spectrum. Table I1 lists the computed polarizations for each of the excitations, and it is interesting to note the presence of two overlapping manifolds: one polarized in the x direction (long axis) and one in they direction (short axis). This is in agreement with the results of polarized spectroscopic investigations of the orientation function in uniaxially and biaxially drawn PET which showed large differences (in both intensity and peak position) in the absorption spectrum for light polarized parallel and perpendicular to the draw d i r e c t i ~ n . ~Most . ~ interesting however is the indication that the single peak seen at 200 nm (6.2 eV) is composed of two overlapping excitations, orthogonally polarized; something seen experimentally in the study by O ~ c h i . ~ ~
WAVELENGTH (MI
4000 , IV. Emission Spectrum b n The intrinsic emission process in PET has been the focus of extensive experimental investigation!” Because of its aromatic backbone, a phenyl ring with adjacent planar carbonyl units (Figure I), PET displays a much richer emission behavior than the aromatic pendant-group polymers (such as polystyrene). Both f l u o r e ~ c e n c e ~and ~ *phosphorescence&* ~ ~ ~ * ~ ~ ~ ~ * have ~ ~ ~been observed and are qualitatively understandable, for the most part, in terms of single-strand, monomeric emission. There is, however, one aspect of the emission spectrum which is poorly understood, and it concerns the nature of the emission in the 368-nm (3.2-eV) region of the spectrum. 200 350 500 650 800 Upon absorption of photons with A I300 nm ( E 4 eV) PET WAVELENGTH lnml will fluoresce with two broad peaks at 338 nm (3.7 eV) and 368 In addition, phosphorescence can be nm (3.4 eV).3h34a**5a*b96a* 4000 4 C detected with a phosphorescence excitation peak at 310 nm (4.0 eV) and emission peak at 454 nm (2.7 eV).4a-f The fluorescence at 338 nm has been attributed to T-T* emission from the first excited singlet of the monomer at 300 nm (4.1 eV)? while the phosphorescence excitation peak has been assigned as absorption to this state, with emission postulated as occurring from a 3 ( ~ , ~ * ) state of the m o n ~ m e r . ~ ~ , ~ It is the nature of the 368-nm emission, however, which is poorly understood. Phillips and S h ~ g initially ’~ assigned this to either excimer or triplet-state emission. Takai et a1.Q attributed it to the formation of a PET excimer, while Allen and M ~ K e l l a r ~ ~ 0 ascribed it to an associated ground-state dimer-an assignment 200 350 500 650 800 supported by the subsequent polarized fluorescence studies of WAVELENGTH inm) Hennecke and Fuhrman.6b Padhye and Tamhane,“ studying the Figure 5. Emission spectra for PET: (a) unpolarized, (b) perpendicular effects of crystallinity on the luminescence of PET films, concluded polarization, and (c) parallel polarization. that the 368-nrn emission arose from the amorphous regions of the sample films. Most recently, Hemker et aLsdused nonpolarized peaks appear with approximately the same intensity in both plots. This indicates that the dipole responsible for the fluorescence is and polarized fluorescence to study the intrinsic fluorescence in the same (or parallel) to that responsible for absorption, while PET. From their results, they proposed a model for the the dipole responsible for the phosphorescence differs from the fluorescence process in which the 368-nm emission occurs from one responsible for absorption.k,c Finally, we report, for the first a “trap” populated via energy migration following excitation or time, the existence of a peak. centered at 534 nm (2.3 eV), which via direct excitation with 338-nm photons. From solution meaappears most clearly in the perpendicularly polarized spectrum surements they concluded that this trap was located in the amorphous rcgion of the film and was most probably an associated (Figure 5b). Because of its energy, we assign this peak as a ground-state dimer. phosphorescence emission. Our purpose in this section is to present measured emission On the basis of the results in the previous section, we assign the 338-nm (3.7-eV) fluorescence to emission from the first excited spectra for PET and show that the CNDO/S3-CI model provides singlet (r,r*)state (SI)at 4.1 eV above the ground state. This a completc description of the monomer fluorescence and phosstate is short axis (y) polarized (see Table II), and as it is also phorescence that is in full agreement with the current experimental the absorbing state this assignment is consistent with the experdata. Following this we will use the model to speculate on the imental polarization information discussed above. The first excited nature of the trap responsible for the 368-nm emission. triplet state is computed to be 3.0 eV above the ground state, and Measured emission spectra for PET are shown in Figure 5. The * ) as the one responsible for the 464-nm we assign this 3 ( ~ , ~ state total emission spectrum, shown in Figure 5a, is identical with (2.7-eV) phosphorescence. This assignment is in agreement with published s p e ~ t r a ~ and + ~shows ~ * ~both ~ , ~fluorescence ~ peaks (340 the assignments made by several experimental studies which and 366 nm) and the large structured phosphorescence peak centered at 464 nm. In addition, we have taken the emission showed that the phosphorescence has a long lifetime ( > I s) and that there is a large Sl-T, energy splitting (-8000 cm-1).4a”*5a97b spectrum polarized both perpendicular and parallel to the incident * ) is long axis (x) polarized, also in agreement This 3 ( r , ~state radiation, and these are shown in Figure 5b,c. An equally weighted sum of thc polarized emission reproduces the unpolarized emission. with the experimental polarization information previously discussed The interesting thing about Figure 5a,b is the observation that which indicated that the dipoles responsible for fluorescence and the fluorescence peaks are greatly diminished in the perpendiphosphorescence are orthogonal. Moreover, there are (n,r*)triplet cularly polarized emission (Figure 5b), while the phosphorescence states computed at 3.3 and 3.7 eV. This density of triplet states I
I
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I
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The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
3.7 eV (338 nm)
3.3 eV (376 nm) 3.0eV(413 nm)
4.1 eV 300 Nn
Figure 6. Schematic indication of states responsible for monomeric absorption and emission in PET. Computed CNDO/S3-CI energies are shown in electronvolts, while experimentally measured absorptions and emissions are in nanometers.
close to SI may explain the high intensity of the phosphorescence peak relative to the fluorescence peaks. (See Figure 5a.) Or, put another way, the density of triplet states near SI makes the intersystem crossing rate comparable to, or greater than, the fluorescence rate. Figure 6 indicates the positions of the monomeric singlet and triplet states, along with a schematic diagram of the emission process. The assignment of the lowest triplet state as (n,n*)deserves some further discussion. In aromatic systems with carbonyl groups (and their associated n-a* states), it is generally thought that '(a,**)ground states are associated with (n,-A*) triplet states, and vice versa, because of their strong spin-orbit coupling.22 Furthermore, because of this coupling phosphorescence from a j(n,a*) state is expected to be in-plane polarized, having "borrowed" its intensity from the '(a,.*) ground state, while emission from a j ( x , a * ) states is expected to be out-of-plane polarized, having "borrowed" its intensity from the '(n,a*) ground state. How then can we reconcile the fact that we have assigned the phosphorescence to a j ( s , n * )state that is in-plane polarized? It was seen in the previous section that no experimental evidence of a n-* absorption band exists. Moreover, Takai et al.7bshowed that the observed phosphorescence for polymers that are structurally similar to PET yet contain no carbonyl groups (poly@xylylene) and poly(monoch1oro-p-xylylene)) is virtually identical with that for PET. Finally, as described in the previous section, (22) Becker, R. S. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley-Interscience: New York, 1968.
LaFemina and Arjavalingam there are no MOs in our computations that can be simply described as nonbonding carbonyl orbitals. And the MOs that do have some in-plane O(2p)-A0 character are low-lying in energy, implying that any n--A* state which might exist would be high above the ground state in energy. Therefore, if no n-r* states are considered, the spin-orbit coupling is between singlet and triplet (-A,T*)states and the phosphorescence is expected to be in-plane polarized. We are now left with two features of the emission spectrum not explainable with a monomeric model: viz. the 368-nm fluorescence and the 534-nm phosphorescence. The 368-nm fluorescence has been attributed to emission from a "trap", most likely an associated ground-state dimer.4bvc,Sd+6b Preliminary CNDO/S3-CI computations on dimer terephthalate units, in which the terephthalate moieties were kept planar and the interplanar spacing varied, were performed. These results indicate that, at an interplanar distance of approximately 2.5 A, a new (*,a*)singlet state appears at approximately 3.3 eV (375 nm). At the same time a new 3(a,a*)state appears at -2.3 eV (540 nm). This supports the assignment of the 368-nm fluorescence to an associated ground-state dimer and the 564-nm phosphorescence to the triplet state of this associated dimer. It must be stressed, however, that these are qualitative results and that more quantitative computations must begin with a determination of the minimum-energy conformation of the dimer unit. Such computations are currently under way. V. Synopsis
The polarized and unpolarized emission spectra for poly(ethylene terephthalate) were measured with a wavelength resolution greater than that of existing published data, revealing a new phosphorescence peak at 564 nm. The CNDO/S3-CI model was used to provide a detailed description of the monomeric emission. Moreover, preliminary computations on dimer terephthalate units indicate that the 368-nm fluorescence and the newly reported 564-nm phosphorescence may be attributable to an associated ground-state dimer. Computations are currently under way to more accurately define both the conformation and photophysical properties of the dimer. Acknowledgment. J.P.L. is indebted to D. M. Friedrich for a critical review of the manuscript and for several useful discussions on the nature of spin-orbit coupling. J.P.L. is also grateful to D.F. Feller for helpful discussions on the nature of nonbonding orbitals and to C. B. Duke for generously allowing the use of his CNDO/S3 computer programs. The authors also thank an anonymous referee for several helpful suggestions on improving the manuscript. Registry No. PET, 25038-59-9.
