J . Phys. Chem. 1990, 94, 6351-6360
6351
Photophysics of 10-Substituted Bis[ 2-(9-anthryi)ethyl] Giutarates Marye Anne Fox* and Phillip F. Britt Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 (Received: December 7, 1989)
Photophysical interactions between pendant anthryl groups in a family of bis[ 2-(9-anthryl)ethyl] glutarates have been characterized by steady-state and time-resolved fluorescence measurements. As the size of the 10-substituent increases, the rate of excimer formation decreases and the monomer-excimer equilibrium shifts toward the monomer. Thermodynamic and kinetic parameters influencing excimer formation are described.
Introduction Our interest in light-induced redox reactions on surface-modified semiconductor has prompted us to search for polymeric coatings that could expand the wavelength response of the semiconductors and/or simultaneously allow for larger spatial separation of photogenerated electron-hole pairs. For practical efficiencies, quantities of sensitizer greater than attainable by monolayer molecular absorption are required. Molecular crystals are not practical because of difficulties in purification and mechanical manipulation, and thin films of adsorbed dyes suffer from extensive self-quenching. A thin film of a polymer containing pendant absorptive chromophores adsorbed or covalently bound, however, can in principle solve the problem of low quantum efficiency for sensitized photocurrent generation caused by a low concentration of excited state molecules at the electrode surface. Polymers containing pendant aromatic chromophores often display efficient singlet and triplet energy migrati~n.~Although diffusion lengths are sometimes shorter in the polymer than in single crystals because close crystal packing allows for stronger interactions between chromophores (which can lead to excitation splitting), synthetic polymers have shown promise and in recent years, attempts have been made to use synthetic polymers to mimic light harvesting in chlorophyll. Successful sensitization relies on the photochemical stability and photophysical properties of the polymer. The photophysical properties of a polymer film are very complex as a result of intrachain and interchain chromophore interactions, excitation migration, and self-quenching.e6 In an effort to unravel the complex photophysical properties of a methacrylate polymer with pendant aromatic chromophore^,^ a series of simpler bichromophoric model compounds glutarates 1 was studied. This family 1 allows for chain flexibility while separating the aryl groups by the same number of intervening methylene units as encountered between repeat units of the polymer. Intramolecular chromophore interactions can be observed in 1 without the complications introduced by the large number of conformational and/or configurational isomers6 or by inter- or intrachain energy migration occurring within the polymer. Elsewhere we have reported that bis[2-(9-anthryl)ethyl] glutarate 1-H, a model for poly[2-(9-anthryl)ethyl] methacrylate, was found to decompose upon photoexcitation by dimerization or cycloaddition with ~ x y g e n . Bulky ~ substituents in the 10(1) Fox, M.A.; Kamat, P. V.;Fatiadi, A. J. J. Am. Chem.Soc. 1984,106, 1191. (2) Fox, M. A.; Nobs, F. J.; Voynick, T. J. Am. Chem. SOC.1980, 102, 4036. (3)Guillet, J. E.Polymer Photophysics and Photochemistry; Cambridge Press: Cambridge, 1985. (4) Beavan, S.W.; Hargreaves, J. S.;Phillips, D. Adu. Photochem. 1976, 1 1 . 207. (5) Webber, S.E. New Trends in the Photochemistry of Polymers; Allen, N. S.,Rabek, J. F., Eds.;Elsevier Applied Science: New York, 1985; p 19. (6)Klopffer, W. Ann. N.Y. Acad. Sci. 1981, 366. 373. (7)Fox, M.A.; Britt, P. F. Macromolecules, in press. ( 8 ) Fox, M. A.; Britt, P. F. Photochem. Photobiol. 1990, S I , 129.
position should sterically block dimerization and enhance the substituted polymer's photochemical stability. An investigation of the effect of substitution on the excited state properties of 1 was therefore undertaken. While this series 1 serves as a crude model for nearest-neighbor interactions in the corresponding 10-substituted 2-(9-anthryl)ethyl methacrylate polymers, the corresponding 10-substituted 2-(9-anthryl)ethyl pivalates 2 will serve as models for the photophysical properties of an isolated chromophore. We describe herein our characterization of photophysical interactions between pendant groups in 10-substituted bis[2-(9anthryl)ethyl] glutarates 1 by steady-state and transient measurements. Specifically, we consider bis[2-(9-anthryl)ethyl] glutarate (1-H),bis[2-( lO-ethyI-9-anthryl)ethyl] glutarate (1-Et), bis[2-( lO-n-butyl-9-anthryl)ethyl] glutarate (1-Bu), bis[2-( 10isopropyl-9-anthryl)ethyl] glutarate (1-iPr), and bis[2-( 10phenyl-9-anthryl)ethyl] glutarate (1-Ph) and their correspondingly 10-substituted 2-(9-anthryl)ethyl pivalates (2). Absorption and emission spectra and photochemical reactivity of the 1 and 2 are described, and kinetic parameters for intramolecular formation are derived from fluorescence decay or fluorescence quenching measurements. Thermodynamic parameters for excimer formation within the series (obtained from temperature-dependent fluorescence spectra) will also be reported.
Experimental Section Materials. All solvents used for photochemical experiments were checked for impurities by absorption and fluorescence spectroscopy before use. Spectrophotometry grade benzene (MCB) was used without further purification. 2-Methyltetrahydrofuran (MTHF) was passed through active alumina and fractionally distilled first from sodium and then from lithium aluminum hydride immediately before use. Toluene was distilled from sodium immediately before use. Methylene chloride was distilled from PzOs immediately before use. Acetonitrile (HPLC grade) was fractionally distilled from lithium aluminum hydride. Hexanes and methanol were HPLC grade and used without further purification. Preparation of Glutarates 1 and Pivalates 2 . To a stirred solution of the 10-substituted 2-(9-anthryl)ethano17 in dry benzene and dry pyridine (10 equiv) was added pivaloyl chloride (1.25 equiv). The mixture was heated under reflux for a minimum of 4.5 h. The reaction was poured into water, the layers were separated, and the aqueous layer was extracted with ether (3 X 20 mL). The combined organic layers were washed with 2 M HCI (3 X 20 mL), water (20 mL), 1 M NaOH (20 mL), water (20 mL), and brine (20 mL). The organic layer was dried over magnesium sulfate, and the solvent removed under reduced pressure. The product was purified by recrystallization from the indicated solvent. 2-(9-Anthryl)ethylpivalare: chromatographed by HPLC on silica gel with ethyl acetate-hexanes (1 :6); yield 75%; mp 96-98 "C; IR (CHCI3) 1710 (C(=O)) cm-'; 'H N M R 6 1.13 (s, 9 H), 3.83 (t, 2 H, J = 7.4 Hz), 4.43 (t, 2 H, J = 7.4 Hz), 7.26-7.60 (m, 4 H),7.80-8.03 (m, 3 H); I3C NMR 6 27.1, 38.7,64.2, 124.2,
0022-3654/90/2094-6351$02.50/0 . _ -, .I 0 1990 American Chemical Society I
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The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
124.8, 125.9, 126.6, 129.1, 130.3, 131.5, 178.7; MS, m l e (re1 intensity) 306 (9), 204 (100). 191 (33), 57 (30); HRMS calcd for C2,H2202306.1620, found 306.16 16. 2-( /O-Ethy/-9-anthryl)ethy/ piualate: recrystallization from ethanol; yield 75% mp 69.5-70.5 OC; IR (CHCI,) 1732 ( C ( 4 ) ) cm-I; IH NMR 6 1.13 (s, 9 H), 1.33 (t, 3 H, J = 7.5 Hz), 3.47 (q, 2 H, J = 7.5 Hz), 3.80 (t, 2 H, J = 7.8 Hz), 4.37 (t, 2 H, J = 7.8 Hz), 7.20-7.53 (m, 4 H), 8.03-8.42 (m, 4 H); I3C NMR 6 15.4, 21.2, 27.2, 39.7, 64.2, 125.0, 125.4, 127.5, 129.0, 130.3, 136.2, 178.5; MS, m / e (re1 intensity) 334 (5), 232 (16). 219 (8), 217 (8), 203 (49), 191 (6); HRMS calcd for C23H2602 334.1933, found 334. t924. 2-( IO-n-Buryl-9-anthry1)ethylpiualate: recrystallized from benzene-hexanes; yield 75%; mp 8 1.0-82.0 OC; IR (CHC13) 1720 (C(=O)) cm-I; ' H NMR 6 1.02 (t, 3 H, J = 6.9 Hz), 1.20 (s, 9 H), 1.37-2.00 (m, 4 H), 3.57 (t, 2 H, J = 7.8 Hz), 3.90 (t, 2 H, J = 7.8 Hz), 4.45 (t. 2 H, J = 7.8 Hz), 7.28-7.63 (m,4 H), 8.15-8.50 (m,4 H); I3C NMR 6 14.1, 23.4, 27.2, 27.4, 28.0, 33.6, 38.8,64.3, 124.9, 123.5, 125.3, 125.4, 127.6, 129.4, 130.3, 135.2, 178.8; MS, m / e (ret intensity) 362 (36), 260 (86), 247 ( I 8), 21 7 (86), 203 (IOO), 191 (72), 178 (33), 57 (61), 41 (46); HRMS calcd for C25H30O2 362.2246, found 362.2233. 2-( IO- 2-Propyl)- 9-anthryl)ethyl piualate: recrystallized from CH2C12-MeOH; yield 71%; mp 53.5-54.5 OC; IR (CHCI,) 1732 (C(=O)) cm-l; ' H NMR 6 1.23 (s, 9 H), 1.70 (d, 2 H, J = 7.8 Hz), 3.87 (t, 2 H, J = 7.8 Hz), 4.23-4.83 (m,3 H), 7.23-7.57 (m,4 H), 8.20-8.57 (m,4 H); 13C NMR 6 23.0, 27.2, 27.5, 28.4, 38.8, 64.1, 124.3, 124.5, 124.4, 125.2, 127.9, 129.2, 130.5, 140.3, 178.8; MS, m / e (re1 intensity) 348 (27), 246 (76), 231 (52), 218 (19), 215 (19), 203 (IOO), 191 (18.8), 57 (25); HRMScalcd for C24H28O2 348.2089, found 348.2097. 2 4 IO-Phenvl-9-anthryl)ethylpiualate: recrystallized from benzene-hexanes; yield 84%; mp 108-109 OC; IR 1732 ( C ( 4 ) ) cm-'; ' H NMR 6 1.18 (s, 9 H), 4.00 (t, 2 H, J = 7.5 Hz), 4.51 (t, 2 H, J = 7.5 Hz), 7.22-7.73 (m, 1 1 H), 8.03-8.43 (m, 2 H); I3C NMR S 27.2, 27.4, 38.8, 64.2, 124.2, 124.8, 125.7, 127.4, 128.3, 129.1, 130.0, 130.1, 131.3, 137.1, 139.2, 178.8; MS, m / e (re1 intensity) 382 (16), 280 (IOO), 267 (33), 252 (22), 203 (33), 57 (21); HRMS calcd for C27H2602 382.1933, found 382.1941. General Procedure for Bis[2-(9-anthryl)ethyl] Glutarates. To a stirred solution of 2-(9-anthryl)ethanol in dry benzene and pyridine (10 equiv) was added a preweighed sample of glutaric dichloride9 in benzene (5 mL). The mixture was heated under reflux for a minimum of 4 h. The reaction was poured into water, the layers were separated, and the aqueous layer was extracted with ether (3 X 20 mL). The combined organic layers were washed with 2 M HCI (3 X 20 mL), water (20 mL), 2 M NaOH (2 X 20 mL), water (20 mL), and brine (20 mL). The organic layer was dried over magnesium sulfate, and the solvent removed under reduced pressure. The crude product was purified by flash chromatography on silica gel and recrystallized from either ethyl acetate/hexane or benzene/hexane. Bis[2-(9-~nrhryl)ethyl] glutarate: yield 26%; mp 107.5-109.5 OC (ethyl acetate-hexanes); IR 1740 (C(=O)) cm-I; IH NMR 6 1.76-2.06 (m,2 H), 2.16-2.43 (m,4 H), 3.90 (t, 4 H, J = 7.2 Hz), 4.43 (t, 4 H, J = 7.2 Hz), 7.27-7.60 (m, 8 H), 7.73-8.03 (m, 4 H), 8.13-8.37 (m, 6 H); 13CNMR 6 20.0, 27.3, 33.2, 64.2, 124.1, 124.9, 126.0, 126.8, 129.0, 129.2, 130.3, 131S,172.9; MS, m / e (re1 intensity) 540 (7), 204 (IOO), 191 (39); HRMS calcd for C37HJ204540.2300, found 540.2307. Bis[2-( IO-ethyl-9-anthryl)ethyl]glutarate: chromatographed with dichloromethane-carbon tetrachloride (1.5: 1.O); yield 56%; mp 74.5-76.0 "C (benzene-hexanes); I R 1744 (C(=O)) cm-'; 'H NMR 6 1.35 (t, 6 H, J = 7.5 Hz), 1.73-2.05 (m, 2 H), 227 (t, 4 H, J = 6.3 Hz), 3.50 (q, 4 H, J = 7.5 Hz), 3.83 (t, 4 H, J = 7.8 Hz), 4.40 (t, 4 H, J = 7.8 Hz), 7.23-7.57 (m, 8 H), 8.07-8.42 (m, 8 H); I3C NMR 8 15.5, 20.0, 21.2, 27.4, 33.2, 64.2, 125.0, 125.4, 125.8, 127.4, 129.0, 130.2, 136.4, 172.9; M S , m / e (re1 intensity) 596 (47), 364 ( I S ) , 232 (IOO), 219 ( 5 9 , 203 (94), 191 (13); HRMS calcd for C41H4004596.2926, found 596.2922. (9) Marvel. C. S.; Casey, D. J. J . Org. Chem. 1959, 24, 957.
Fox and Britt Bis[2-(IO-n-bufyl-9-anthry~erh~l] glutarate: chromatographed with dichloromethane-carbon tetrachloride (1.5:l ,O);yield 72%; mp 126.5-127.5 "C (benzene-hexanes); IR 1748 ( C ( 4 ) ) cm-'; ' H NMR 6 1.03 (t, 6 H, J = 6.9 Hz), 1.25-2.13 (m, IO H), 2.37 (t, 4 H, J = 6.6 Hz), 3.57 (t, 4 H, J = 7.8 Hz), 3.93 (t, 4 H, J = 7.8 Hz), 4.50 (t, 4 H, J = 7.8 Hz), 7.33-7.65 (m,8 H), 8.13-8.50 (m,8 H); I3C NMR 6 14.0, 20.0, 23.4, 27.4, 27.9, 33.2, 33.5, 64.2, 124.8, 125.2, 127.3, 129.4, 130.2, 1359.2, 172.9; MS, m / e (re1 intensity) 652 (37), 392 (15), 260 (IOO), 247 (29), 217 (88), 203 (99), 191 (69), 178 (6); HRMS calcd for C45H4804 652.3 552, found 65 2.3567. Bis [2-( I 0-(2-propyl)-9-anthry/)ethy/jglutarate: c hromatographed with dichloromethane-carbon tetrachloride (5:4); yield 43%; mp 63.0-64.0 OC (benzene-hexanes); IR (CHCI,) 1744 ( C ( 4 ) ) c d ; ' H NMR 6 1.68 (d, 12 H, J = 7.8 Hz); 1.73-2.10 (m,2 H); 2.30 (t, 4 H, J = 6.6 Hz); 3.87 (t, 4 H, J = 7.8 Hz); 4.30-4.73 (m, 6 H), 7.23-7.57 (m, 8 H), 8.200-8.53 (m,8 H); I3C NMR 6 20.0, 23.0, 27.6, 28.4, 33.3, 64.2, 124.33, 124.37, 125.0, 127.7, 129.2, 130.5, 140.4, 173.1; MS, m / e (re1 intensity) 624 (47), 264 (13.6), 246 (IOO), 233 (35), 231 ( 4 9 , 218 (28), 215 (23), 203 (97), 191 (25), 178 (11); HRMS calcd for C43H404 624.3239, found 624.3257. Bis[2-(IO-phenyl-9-anthryl)et~y~ glutarate: chromatographed with dichloroform-carbon tetrachloride (4: I ) ; yield 76%; mp 82.5-84.5 "C; IR 1744 (C(=O)) cm-I; ' H NMR 6 1.80-2.13 (m, 2 H), 2.33 (t, 4 H, J = 6.6 Hz), 3.98 (t, 4 H, J = 7.5 Hz), 4.53 (t, 4 H J = 7.5 Hz), 7.20-7.73 (m,24 H), 8.35 (d, 2 H, J = 9.0 Hz); I3CNMR 6 20.0, 27.5, 33.3, 64.2, 124.1, 124.8, 125.8, 127.4, 127.9, 128.3, 128.9, 130.0, 130.1, 131.3, 137.2, 139.1, 173.0; MS, m / e (re1 intensity) 692 (40), 412 (13), 280 (IOO), 267 (80), 252 (32), 239 (6), 203 (54); HRMS calcd for C49H4004 692.2926, found 692.2942. Fluorescence Spectra. Emission and excitation spectra were recorded at 90 OC by using either a Spex Fluorolog 2 or a SLM Aminco SPF 500 spectrofluorometer. All solutions were deoxygenated with nitrogen and had 0.d. < 0.05 in a I-cm curvette, unless otherwise noted. The emission spectra were measured in the ratio mode (corrects for variations in lamp intensity with time) and corrected for nonlinear response of the photomultiplier tube. The fluorescence quantum yields were measured in benzene relative to 9,lO-diphenylanthracene (benzene solution, @ = 0.84).1° When measurements were made in MTHF, corrections were made for the difference in the refractive index of solvents. Similar fluorescence quantum yields were obtained when the samples were purged with nitrogen for 20 min and when the samples were degassed on a high-vacuum line Torr) with six freezepumpthaw cycles and sealed with a high-vacuum stopcock. The quantum yields of excimer fluorescence of the (9-anthry1)ethyl glutarates 2 (am)were obtained by normalizing the (0,O) emission band of the glutarates and the corresponding pivalates and subtracting. Fluorescence spectra of thin films on glass or SnO, were measured by front face analysis or the sample was placed at a 45O angle to the excitation beam. Delayed fluorescence spectra were recorded on a SLM Amico SPF 500 spectrofluorometer with a phosphoroscope attachment that uses light baffles and a variable-speed chopper (0-10000 rpms). Disappearance Quantum Yields. The disappearance quantum yields of the IO-substituted 1 were performed in a Rayonet photochemical reactor fitted with RPR 3500-A lamps, a merrygo-round (6 rev/min), and a cardboard insert that allowed only front-face irradiation. Dilute (10-4 M) benzene solution of 1 ("1 5 mL) were placed in 200 mm X 32 mm Pyrex tubes, purged with benzene saturated N2 for 20 min, and sealed with rubber septa. Six samples of 1 were simultaneously irradiated with two samples of 0.1 M benzophenone/O. I M benzhydrol (a = 0.68)I' or 0.20 M benzhydrol/0.02 M benzophenone (@ = 0.84)12as actinometers. (IO) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229. ( 1 1) (a) Moore, W. M.; Ketchum, M. J. Am. Chem. SOC.1962,84, 1368. (b) Moore, W. M.; Hammond, G. S.;Foss, R. P.J . Am. Chem. Soc. 1961,
83, 2789. ( 1 2 ) Wagner, P. J. Mol. Phofochem. 1969, I , 71.
IO-Substituted Bis[2-(9-anthryl)ethyl] Glutarates The reported values for the actinometers were reproduced with this photolysis apparatus relative to potassium ferrioxalate (0.006 or 0.15 M).13 The extent of conversion was limited to 10-1 5% and was determined by measuring the optical density before and after irradiation. Duplicate measurements indicated a variance of &IO-I 5% in absolute quantum efficiency. The disappearance quantum yields in oxygen-saturated benzene solution were measured by the same procedures. Fluorescence Lifetimes. Fluorescence decays were recorded via time-correlated single-photon ~ o u n t i n g . ’ The ~ light source was a Photochemical Research Associates (PRA) Model 510B thyratron gated flash lamp operated at 20 kHz, 5 kV, and 0.5 atm of air. The flash lamp had a full width at half-maximum of ca. 2.4 ns. The excitation wavelength (A = 358 nm) and emission wavelength were selected with two Jobin-Yvon monochrometers. The fluorescence decay, measured at right angles to the excitation source, was passed through a NaNO, filter (cutoff h < 390 nm) and detected by a water-cooled Hamamatsu photomultiplier tube. Standard pulse-shaping methods14 were used for startstop pulse sequence using a Pacific Precision Instruments Model AD-I 261 amplifier~iscriminatoror PRA 1718 100-MHz discriminator, a PRA 1703 delay, a PRA 1716 C.F. timing discriminator, and a PRA 1701 biased time-to-pulse height converter. A LeCroy analog-to-digital converter was used to digitize the TAC output into a histogram memory of a LeCroy multichannel analyzer (Model 3500). The detection rate was ca. 1% of the pulse rate to discriminate between two photon events. A minimum of 10000-20000 counts were collected in the peak channel. I n practice, the sample is excited by a pulsed light source of short but finite duration. Distortions from the shape of the lamp pulse and the inherent resolution of the electronics are combined together into an instrument response function p ( t ) , which can be determined experimentally by measuring the response from a scattering solution at the wavelength of the exciting light. The observed fluorescence response function f ( t ) (for monomer fM(t) or excimerfD(?)) is given according to the convolution theorem as a superposition integral of the true fluorescence decay i(t)and instrument response function. The data were transferred to The University of Texas Cyber computer and fit to exponential decay functions by using reconvolution techniques to obtain a best-fit by minimizing
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6353
Figure 1. Fluorescence spectra of I-H(A), 2-H (B), and excimer component of I-H(C). In benzene, 3 X IO” M, A,, = 380 nm.
Wavelength (nm)
Figure 2. Fluorescence spectra of I-Et (dashed line) and 2-Et (solid line). In benzene, 2 X IOd M, A, = 366 nm.
where loba(t) is the experimental decay curve, & b s ( t ) is the experimental response function, F ( t ) is the proposed decay function, and W ( t ) is a weighting factor given by l/lok(t). The data are normalized so that the maximum value of lobs(t) is unity. This allows different decay curves to be easily compared. A fit was determined to be “good” if a similar x 2 was obtained for several choices of the initial parameters and a plot of the weighted residuals D ( t ) was randomly distributed around zero:
Temperature-Dependent Spectra. All temperature-dependent work was performed in deoxygenated toluene. For low-temperature work ( < I O “C), a nondeoxygenated solution of 2 was placed in a 3-mm ESR tube and placed in a Dewar flask filled with ethanol. The ethanol was cooled by a flowing nitrogen stream, cooled with liquid nitrogen to the desired temperature. The temperature of the solution was measured with a thermistor (YSI 44034) connected to a digital multimeter. A conversion table was used to convert resistance to temperature. The solution was cooled to -80 OC and allowed to slowly warm as spectra were taken at I O “ C intervals. The rate of change of the temperature at the ( I 3) (a) Murov, S.L. Handbook of Photochemistry; Marcel Dekker: New York, 1973. (b) Calvert, J. C.; Pitts, J. N . Photochemistry; Wiley: New York, 1966; p 783. (c) Nicodem, D. E.: Cabrol, M. L. P. F.; Ferreira, J. C. N. Mol. Photochem. 1977, 8, 213. ( I 4) O’Connor, D. V.;Phillips, D. Time-Correlated Single Photon Counring: Academic: New York, 1984.
Wavelength (nm)
Figure 3. Fluorescence spectra of 1-Bu (dashed line) and 2-Bu (solid line). In benzene, 2 X IO” M, A,, = 366 nm.
lowest point was ;=I OC/min but decreased as the temperature rose. For high-temperature measurements ( > I “C), water from a temperature-controlled circulating bath was pumped through a jacketed fluorescence cell. The temperature of the solution was measured by a thermistor (YSI 44034). Results Synthesis. Compounds 1 and 2 were prepared by esterification of the corresponding IO-substituted 2-(9-anthryI)ethanols.’ Absorption Spectra. The absorption spectra of 1 are similar to their model compounds 2, showing identical absorption maxima (Table I ) . The molar extinction coefficients of the glutarates
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Fox and Britt
TABLE I: Absorption Spectral Data for 10-Substituted 2-(BAnthrvl)ethvl Pivalates 2 and Glutarates 1 in Hexane' IO-substituent
H
x 10-3,
A,,,, pivalates 2
glutarates 1
n-Bu
Et
nm
M-lcm-'
386 380 366 348 340 3 I6 256 255 220 386 382 366 347 33 I 316 256 254 221
9.8
A,,,,
S
9.7 6.0 3.I S
1 I6 S
12 20 S
19 12 5.8 2.3 230 S
19
x 10-3,
nm
M-lcm-'
398 392 376 358 342 324 262 255 222 398 392 378 358 342 323 262 254 222
12 S
II
6.9 3.0 S
120 S
13 23 S
22 13 5.9 S
220 S
21
i-Pr
x IO-^,
A,,, nm
M-lcm-'
nm
12
398
398 393 378 358 342 324 262 253 222 398 392 378 358 342 324 262 255 222
S
Ph
x
A,, ,
M-lcm-'
98 S
5
12 6.7 2.8
378 360 340
A,,,,
10.2 6.2 3.6
S
125 S
14 21 S
21 12 5.6
398 389 378 358 340
18.2 S
18.6 11.0 5.0
S
225 S
27
c
x
nm
M-Icm-'
394 389 374 356 340 323 260 250 224 394 389 374 356 340 323 260 254 223
13 $
13 8.1 3.7 S
125 S
S
19 S
20 12 5.3 S
170 S S
" s = shoulder.
TABLE 11: Quantum Yields of Fluorescence of 1 and 2 in Deoxygenated Benzene" IO-substituent
H Et
Bu IP Ph
@F(2,b
@F(l)e
0.52 0.78 0.83 0.87 0.85
0.54 0.79 0.79 0.84 0.88
% excimer
49 12 12
3
f
~HIW
QFDd
@FM'
@FM/FD
0.26 0.09 0.09 0.03
0.28 0.70 0.70 0.81 0.88
7.7 7.7 27.0 >200
0.00
1.1
nm
461.5 464.0 465.0 439.0
AU,~ cm-I
3810 3140 3130 1700
'Errors are 110% for quantum yield measurements. Measurements made relative to 9,lO-diphenylanthracne (Q = 0.84). b 3 F 2 = quantum yield ~quantum ~ yield of fluorescence of the 10-substituted glutarates 1. d@FD = quantum yield of of fluorescence of the IO-substituted pivalates 2. ' 3 = excimer fluorescence in IO-substituted glutarates 1 calculated by subtraction of a normalized spectrum of substituted 2 from the analogously substituted 1. ' ~ F M= quantum yield of (monomeric) fluorescence in IO-substituted glutarates 1 obtained as the residual emission, having subtracted the excimer component as in d. 'Excimer emission maximum. g A u = difference in wavenumbers between the 0,O band of monomer fluorescence and excimer fluorescence maximum in IO-substituted glutarates 1.
EmI..lon
.ov.lsngLh
.con
---Tis. 'vmry
50/10 nm
Wavelength (nm)
Figure 4. Fluorescence spectra of 1-iPr (dashed line) and 2-iPR (solid line). I n benzene, 2 X IOd M, ,,A, = 380 nm.
Figure 5. Fluorescence spectra of I-Ph (dashed line) and 2-Ph (solid line). I n benzene, 2 X IO" M , ,,A, = 366 nm.
are approximately twice as large as those of the pivalates, and each set of compounds possesses similar peak widths, indicating an absence of strong ground-state interaction between the anthryl groups in 1. Fluorescence Spectra. The fluorescence spectra of the substituted glutarates and pivalates are compared in Figures 1-5. The excimer contribution to the observed glutarate fluorescence can be obtained by normalization of the spectra of 1-H and 2-H to the 0,O band and subtraction t o yield a broad structureless emission, red-shifted from that of the monomer. The contribution of this emission to the overall fluorescence decreases as the size of the substituent increases until no red-shifted emission ( ky and k M D > k ~ ) the , monomer and excimer will each decay exponentially with a wmmon lifetime equal to 1/2(kp., + kD): Johnson, G. E. J . Chem. Phys. 1974, 6/, 3002. (23) At this wavelength, the observed fluorescence is assumed to be free from overlapping excimer fluorescence.
where B = (1 - m A ) / ( 1 + m). A similar correction would have to be made if the excimer fluorescence leaked into the region of monitoring the monomer fluorescence. This is assumed not to occur since the monomer emission is measured at the 0.0 band of fluorescence. If leakage occurred, the contribution of the excimer emission would be very small since the bandwidth of the monochromator was small.
io([) = CD(e+ -
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6357
IO-Substituted Bis[2-(9-anthryl)ethyl] Glutarates
TABLE VI: Kinetic Parameters and Quantum Efficiencies for Formation and Dissociation of Intramolecular Excimers' Derived from Glutarates 1 compd 1-H
1-Et 1-Bu 1-iPr
kDMb
x
x
kh(D/kDM
@DM
@MD
5.3 14 13 3.4
0.45 2.6 3.3 11
0.47 0.38 0.30 0.03
0.45 0.73 0.69 0.39
kMDb
12 5.5 3.9 0.31
Lifetimes and rate constants were measured in degassed benzene solution (-
monomer component, k l M = kTM+ k G M , for the pivalates 2 and the glutarates 1 are equivalent. The rate constants for monomer decay are summarized in Table V, while those for excimer formation and dissociation are summarized in Table VI. To check for agreement between data obtained via steady-state fluorescence and transient decay measurements, the quantum yield of monomer fluorescence can be calculated from eq 4, where k M (the total rate
of decay of an excited singlet incapable of forming an intramoIeCUhr eXCimer) = k F M k i M , k D = k F D + k R D + k l D , and k I D = kTD+ kGD. intramolecular quenching of the excited anthracene by the ester group is insignificant since nearly identical fluorescence yields are observed in the pivalates 2 and their precursor alcohols. The lifetimes of the IO-substituted glutarates increase in the order 1-Et, I-Bu, I-iPr, which is directly related to the decrease in the rate of internal quenching klM, since k F M is constant. Ermolev's rule predicts that aGM = 0 (for planar aromatics when the energy of the singlet state is greater than 50 kcal mol-'), which implies that klM k T M . The decrease in intersystem crossing with substitution occurs as a result of the position of T2, which, depending on the substitution, lies slightly above or below The rate constants for monomer fluorescence, k F M , are approximately equal for 1-H, 1-Et, 1-Bu, and 1-iPr. This is reasonable since k F M is a property of the isolated anthracence chromophore and is related only to the oscillator strength, which should be nearly equivalent in these compound^.^' For 1-Ph, k F M is slightly larger than the other values because its oscillator strength, as measured by its molar extinction coefficient, is larger. The quantum yields of monomer fluorescence calculated from the rate constants derived in Scheme I are in good agreement (f5%) with the experimentally measured values, except in the parent 1-H. In view of the considerable errors in the experimental quantum yields, in the fraction of excimer emission, and in the transient decay measurements (which are compounded by the propagation of uncertainty in calculated rate constants), the overall agreement is excellent. The rate constant for excimer formation k D M depends on several factors. That the absolute value of k D M should depend on the size of the chromophore has been established for 1,3-bisarylpropanes, where an order of magnitude decrease in the rate constant for intramolecular excimer formation was observed as aryl size increased from phenyl to pyrenyl.28 Our observed rates for excimer formation are lower than previously reported intermolecular values, which have typically been diffusionally controlled. This decrease apparently arises from the activation energy needed to surmount the conformational barriers encountered in reaching the preferred excimer geometry, particularly in glutarates bearing bulky substituents. Since most flexible molecules exist as a mixture of interconverting conformers, k D M represents a weighted average of the rate constants for excimer formation from the many different ground-state conformers. For example, k D M of 1-H undoubtedly represents an average of the rates for H-H and H-T excimer formation, since by analogy with the steady-state photochemical results, both are predicted to be formed,8 although the
+
(26) Turro, N. .I.Modern Molecular Photochemistry; Benjamin Cumming Publishing: Menlo Park, CA, 1978; p 235. (27) This holds only as long as the absorption and emission spectra possess a mirror plane of symmetry. (28) Bouas-Laurent, H.; Castellan, A,; Desvergne, J.-P. Pure Appl. Chem. 1980, 52, 2633.
kmb X
kDb X
6.5 5.4 5.8 5.2
kIDb X
5.2 3.6 4.8 6.3
1.2 1.8 1.1
M). b R a t e constants as defined in Scheme I.
H-T dimer is much less stable. Since k D M probes the dynamic approach of the anthryl units in the glutarates, it should depend on solvent viscosity.29 The effect of substitution on the rate of excimer formation k D M is shown in Table VI. As the size of the substituent is increased, k D M decreases by a factor of 40. Since k D M is one of many possible processes that can deactivate the monomer, the effect of substitution on the partitioning of the excited state can be evaluated from the state efficiency of excimer formation, which is defined as in eq 5 . Our results show that @ D M decreases as the size of the substituent increases. For 1-H, approximately half of the excited singlet states form an excimer, but only 1 of 1000 excited singlet states collapses to form a photoproduct. This could indicate that the glutarate chain favors a H-H excimer, which does not lead to a stable photoproduct, over a H-T excimer, or that the glutarate chain restricts the geometry of the excimer such that collapse to a photodimer is unfavorable.8 It is interesting to note that steady-state fluorescence measurements show 12% excimer fluorescence for 1-Et and 1-Bu but a state efficiency for excimer formation that is approximately 3 times larger. Therefore, the fraction of excimer fluorescence is not itself a good probe of interchromophore interaction since it does not account for excimer dissociation. Intuitively, it might be expected that the rate constant for excimer dissociation, k M D , would also increase with the size of the substituent as a consequence of excimer destabilization. The data in Table VI, however, indicate that although k M D increases in 1-Et and I-Bu from that in 1-H, it decreases for 1-iPr. However, the ratio of k M D / k D M does increase with the size of the substituent, favoring the monomer. The state efficiency for excimer dissociation, defined as in eq 6, does not show the Same trend @MD
=
~ M D / ( ~+ D~ M D )
(6)
as k M D / k D M . The absence of a trend in k M D and @MD can be explained by the Arrhenius expression (rate = A exp(-E,/RT)), if it is assumed that the frequency factor A for excimer formation and dissociation is similar for each glutarate. The decrease in k D M with substitution can then be explained as a consequence of an increase in the energy of activation for excimer formation EaDM. The increase in k M D for 1-Et and 1-Bu relative to 1-H is a consequence of a decrease in EaMD, Since the energy of activation for excimer dissociation (EaMD)can be defined as EaMD = EaDM - AH, steric interaction between the substituents will destabilize the excimer (Le., reduce its binding energy) more than it affects the activation energy for excimer association. For 1-iPr, the activation energy for excimer formation is very large, judging from the small rate of excimer formation. Although steric interactions destabilize the excimer, the overall value for EaMD is still larger for 1-iPr than for 1-H. Thus, the rate constants alone are not sufficient to accurately describe the effects of substituents on the interaction of the chromophores. Additional parameters that would be useful are the binding energy and the energy of activation for excimer formation (see below). The rate constants for deactivation of the excimers are also shown in Table VI. The error in the calculated rate constants is exemplified by the calculation of k l D ( = k D - k F D- k R D )for I-iPr. The rate constant k F D calculated from the eq 7 is larger kFD
=
@FD[(kM
+ k D M ) ( k D + kMD)
- kMDkDMl/kFM
(29) Johnson, G.E. J . Chem. Phys. 1975, 63, 4047
(7)
6358 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
Fox and Britt 1.0
438
480
530
58E
630
I
(I
A!
0.0380
0-
IIt - c - c - c - t - 0
fi
' 1
'
430
480
1
530
630
Uavelength (nm)
Wavelength ( 4
Figure 8. Temperature dependence of fluorescence spectra of 1-H in toluene (-75 to 20 "C). (Spectra have been normalized to the most intense peak.)
than kD. The m a t probable source of error arises from Om which is difficult to measure accurately when the contribution of excimer is small. Similar difficulties in resolving rate constants into their components have been reported in the study of qw-bis(9anthr~l)alkanes,~~ where km was calculated to be larger than k ~ ? ' and would be further complicated if kinetically distinguishable conformers are involved. The rate constant for excimer fluorescence (kFD)is related to the degree of allowedness (oscillator strength) for the transition. This should reflect the geometry and thus the degree of orbital overlap in the excimer. Each glutarate 1 has a similar rate of fluorescence, suggesting that all compounds have a similar geometry, as was postulated earlier. This could be possible if the substituents do not dramatically alter the symmetry of the excimer but only change the equilibrium separation of the interacting A systems. The rate of glutarate 1 decomposition is small (ODlr < The contribution of this rate to the overall decay of the state is thus negligible, Le., beyond the resolution of these experiments. Therefore, it will be assumed that kRD = 0 for the ghtarates in the kinetic treatment. An alternate procedure (derived by Klopffer and L i ~ t a y for )~~ determining the rate parameters for a monomer/excimer system requiring only steady-state measurements of 1 and 2 in the presence and absence of a quencher (such as 0,)(and of the lifetime of an appropriate model compound in the absence of a quencher) gives the same general picture as derived from these direct measurements. Temperature-Dependent Fluorescence. From an analysis of the decay constants of the emissive monomers and excimers, we concluded above that excimer dissociation is important at room temperature. Thus, the binding energy (AH) of the excimer is not large enough to overcome the TLY term. Since the entropy of excimer formation is expected to be very negative (-10 to -20 eu),16,33temperature should have a dramatic effect on monomer/excimer interconversion. From the temperature dependence of the fluorescence spectra, the activation energy for excimer formation (EaDM), the enthalpy ( A H ) of photoassociation, and the entropy (AS) of photoassociation can be calculated. Normalized fluorescence spectra of I-H measured at various temperatures in toluene are shown in Figure 8. Below -75 O C , the fluorescence spectrum of I-H resembles that of 2-H. As the (30) Castellan, A.; Desvergne, J.-P.; Bouas-Lauren:, H. Chem. Phys. feu. 1980, 76, 390. (31) The error in the calculated rate constants arises from the difficulty in numerical separation of the transient decay into its components and the difficulty in measuring the steady-state properties to the high degree of accuracy necessary to calculate the rate constants. (32) Klopffer, w.: Liptay, w. Z. Nururforsch. 1970, 25u, 1091. (33) Stevens, B. Adu. Photochem. 1971, 7, 161.
Figure 9. Temperature dependence of fluorescence spectra of I-H in toluene (8-80 "C). (Spectra have been normalized to the most intense peak.)
1
a 000
m . m t
-90
I
-60
-30
,
,
,
0
I
30
. , 60
,
9C
Temperature ( ' C )
Figure 10. Intensity of monomer and excimer fluorescence in I-H as a function of temperature. (Intensities are normalized to their respective maxima.)
temperature was increased, there was a steady decrease in the relative intensity of monomer fluorescence as the broad red-shifted excimer band appeared. The emission maximum of this band shifted from 475 nm at -69 O C to 461 nm at 20 O C . The temperature dependence of the fluorescence spectrum indicates that the formation of the intramolecular excimer is thermally activated and competes with monomer fluorescence. As the temperature is increased from 8 to 80 "C, excimer fluorescence reaches a maximum and slowly decreases in intensity, Figure 9. The temperature-dependent intensities of monomer and excimer fluorescence are shown in arbitrary units in Figure 10. In this temperature range, the intensity of monomer fluorescence changes by less than 3%, which indicates that thermal quenching of the monomer fluorescence is not an important process. However, excimer fluorescence decreases by 57% in the same temperature range, indicating that either thermal quenching or intersystem crossing competes with excimer fluorescence. The excimer is probably not deactivated by thermal quenching since the maximum of excimer emission lies approximately 62 kcal mol-] above the ground state, a value very large compared with k T . The temperature dependence of the fluorescence spectra allows for calculation of the photostationary concentrations of emitting species IM* and ID*. At any temperature, the equilibrium constant ( k ) between monomer and excimer can be defined as in eq 8, where kD = kTD + kcD + kFD + kRD. This equilibrium can be probed by monitoring the ratio of the intensity of excimer ) monomer emission (IFM): emission ( I F D to IFD-_ IFM
~ F D ~ D M
kFM(kD
+ kMD)
-
~ F D ~ D M ~ D - ]
kFM(1
kMD
+ kD-')
(9)
The rate constants kDM, kMDIand kD are temperature dependent. The rate parameter for internal quenching k l Dcontains
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6359
IO-Substituted Bis[2-(9-anthryl)ethyl] Glutarates
TABLE VII: Thermodynamic Parametersa for Excimer Formation and Dissociation in 10-Substituted 2-(9-Anthryl)ethyl Glutarates 1
DM X
EaDM,
compd*
kcal mol-'
1-H
loll,
4.7 6.5 6.9
1-Et 1-Bu
s-I
AH,
EaMD,
kcal mol-l
AS,eu
kcal mol-'
-5.9 -3.6 -3.4
-17.3 -14.3 -14.0
10.6 10.1 10.9
3.8 25 58
~ M X D
s-I
2.3 3.3 6.6
ER?
kcal mol-l
"T
5.0 5.4 5.5
"C 30 20 20
M. bThe rate of excimer dissociation could not be calculated for 1-iPr because the a In benzene at room temperature; concentration of 1 at calculated value of kMD/kFDwas negative. The most likely cause of errors is in measuring aFD accurately. y
z2 5
The entropy of photoassociation can also be calculated from eq 15, which is obtained by rearranging eq 14 under conditions
3.750.1026.812~ R = 1.00
AS = R In (IFDk(FM/IFMkFM) + AH/T
-0.4
1
0 U z
-07
5
-1.3
F?
=
I
- 4
'al
3 50-3 3 9e-3
4 39-3 4 70-3 5 10-3
Ifremperature ("K)
Figure 11. Dependence of relative intensity of the monomer and excimer components of the fluorescence of I-H.
a temperature-independent component (kGM),which can be neglected since the energy of the excimer is very large compared to kT, permitting us to write the expression shown in eq 10. The temperature-dependent component is associated with intersystem crossing to the triplet ~ t a t e , ~and ~ , ~the * temperature-dependent rate constants kDM and kMD are described as in eqs 11 and 12:
At low temperature, it can be assumed that kD-l = k&, since klD" exp(-E,ID/R7') is expected to be very small. In the temperature range from -80 to 0 O C , the intensity of excimer fluorescence increases, which implies that excimer dissociation does not compete with excimer fluorescence. Under these conditions, it is reasonable to assume that kD-'kMD> 1, and from eq 9, we obtain eq 14:
A plot of log ( I F D I I F M ) against 1/T in this high-temperature regime yields a linear plot with slope of {-AH/2.303R] and an intercept of (AS/2.303R + log (kFD/kFM)]. The binding energy (AH) of the intramolecular excimer of 1-H is thus -5.9 kcal mol-' and the entropy AS is -17.3 eu. (34) (a) Birks, J. B.; Moore, G. E. The Triple? Store; Cambridge University: Cambridge, 1967: p 407. (b) Schoof, S.;Glisten, H.Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 864.
(15)
where the approximation kM-lkMD>> 1 is valid. The values for A S obtained by either method gave the same result (f5%). The repulsion energy (ER) between aryl groups in the excimer can be calculated from eq 16, where Mo is the energy (kcal mol-')
(Mo- Do) + AH (16) of the 0,O emission and Do is the energy (kcal mol-') of the ER
maximum of excimer fl~orescence.~~ For 1-H, the repulsion energy is thus calculated to be 5.0 kcal mol-'. The activation energy for excimer dissociation EaMD can be calculated from eq 17, and the frequency factor for excimer EaMD = EaDM - AH
(17)
kMD" = kDM/e*IR
(18)
dissociation kMDo can be calculated from the entropy of photoassociation, eq 18. For 1-H, the frequency factor for excimer dissociation kpD" is 2.3 X loi5s-I. This large value reflects the substantial gain in entropy associated with excimer dissociation. Analogous temperature-dependent fluorescence spectra were observed with other members of the series. At low temperatures, the fluorescence spectra of glutarates 1 resemble those of the analogous pivalates 2. The small excimer emission from 1-iPr and I-Ph makes the analysis too imprecise for these compounds. For 1-Et and 1-Bu, however, the onset of excimer fluorescence is shifted to higher temperatures as the steric bulk of the substituent increases and the wavelength maximum for excimer emission shifts from 490 nm at -50 "C to 464 nm at 10 "C. Approximately at room temperature, the ratio of excimer to monomer fluorescence plateaus. As the temperature is further increased, the ratio decreases, since the excimer fluorescence is quenched more efficiently than the monomer fluorescence. The thermodynamic parameters calculated from the slope and y intercept of plots for 1 (parallel to Figure 11) are compiled in Table VII. The energy of activation for excimer formation EaDM is dramatically affected by the substituent. Its absolute value depends not only on s o l ~ e n t and * ~ ~size ~ ~of the c h r o m o p h ~ r e * ~ ~ ~ ~ but also on the conformation of the linking chain.36 The frequency factor for excimer formation can be equated with the entropy required to form the transition-state complex. An increase in k D M " means that the reaction requires less reorganization and the rate of reaction will be faster. The frequency factors are approximately the same for substituted 1, but that of 1-H is approximately an order of magnitude smaller. That more organization is required to form the excimer in 1-H is confirmed by MM-2 calculations, which indicate that less motion is required to form an excimer for the substituted bichromophores, whereas I-H has more conformational freedom. Thus, the observed activation energy is not a true activation energy associated with the collapse of one conformation (through bond rotation) to the excimer but rather a weighted average of at least several conformations. The ethyl and n-butyl substituents decrease the stability of the excimer 1 (relative to 1-H) by approximately 2.4 kcal mol-l. The effect of substituents on the enthalpy of photoassociation AH is (35) Goldenberg, M.; Emert, J.; Morawetz, H. J . Am. Chem. SOC.1978, 100, 7171.
6360 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
Fox and Britt
Previous studies of the spectral properties, kinetic parameters, and intramolecular photochemistry of various aryl bichromophores have focused on the varying composition and length of the linking chain. This work represents the first investigation of the effects of substitution on the thermodynamic and kinetic parameters of excimer formation and dissociation. Our results support the mechanism of excimer formation described by Scheme I and
establish that excimer dissociation is an important process at room temperature. Comparison of the rate constants for the substituted glutarates indicate that (unless kinetically distinct conformation exist) as the size of the substituent increases, the rates of excimer formation decrease and the monomer/excimer equilibria shift toward the monomer. No excimer emission is observed for I-Ph and only weak emission is observed in 1-iPr. Intramolecular interactions between chromophores cannot be accurately deduced from the contribution of excimer emission to the overall fluorescence alone, unless the extent of excimer dissociation is also taken into account. A better measure of the extent of intramolecular interaction is the state efficiency for excimer formation, which requires a knowledge of the rate parameters. From steady-state fluorescence spectra, the ethyl and n-butyl substituents appear to exert similar inhibition to interaction between the aromatic rings. However, the observed rate constants and thermodynamic parameters all indicate that the n-butyl substituent behaves as a more sterically bulky substituent than ethyl. There does not appear to be a significant difference in the entropy of excimer formation between 1-Et and I-Bu, but there is a difference in the binding energies of the respective excimers. In a good solvent, the preferred conformation for excimer formation is well solvated, thereby increasing the effective van der Waals volume of the substituent and stabilizing the excimer conformation. Therefore, the n-butyl substituent is effectively larger, thus reducing diffusion of the anthryl group and making the entropy of excimer formation less negative. The glutarate chain was found to hinder direct interaction between the chromophores. From MM-2 calculations and from experimentally derived thermodynamic and kinetic parameters, substituents in the 10-position were found to decrease the conformational mobility of the glutarates 1. This study of the photophysics of the glutarates 1 and pivalates 2 has thus served to define the conformational limitations of the bridging chain and the effect of substitution on the intramolecular interactions of the chromophores. This knowledge will help to simplify the complex photophysical properties of the corresponding homopolymers and will aid in the design of sensitizing polymers.
(36) De Schryver, F. C.; Collart, A.; Vandendriessche, J.; Goedeweeck, R.; Swinne, A.; Van der Auweraer, M. Acc. Chem. Res. 1987,20, 159. ( 3 7 ) Klopffer, W. Organic Molecular Photophysics; Birks, J . B., Ed.; Wiley: New York, 1973; Vol. 1, p 357.
Acknowledgment. This work was supported by the United States Department of Energy, Office of Basic Energy Sciences. We are grateful to Professor S. E. Webber for useful discussions on the kinetic analysis employed herein.
parallel to that previously reported for alkyl-substituted benzenes in methylcy~lohexane.~~ Although a small increase in entropy is expected, the observed decrease in enthalpy is the most significant effect on the thermodynamic stability (AG) of the excimer. A smaller entropy change ils is expected in intramolecular excimer formation than in formation of the corresponding intermolecular molecular excimer. For example, in 1,3-diarylpropanes, the entropy of photoassociation is typically less than -10 eu,36*37 while intermolecular excimers have entropy values of about -20 eu.16933The observed entropy values for 1 reflect the appreciable degree of organization required to align an I I-atom chain into a conformation to permit excimer formation. The activation energies associated with excimer dissociation EaMD are large and appear to be essentially equivalent along the 1 series, within experimental error. As expected, the differences in the rate of excimer dissociation k M D between the glutarates correspond to changes in activation energies of only about ~ 0 . 5 kcal mol-’. The ground-state repulsion energy E R is not very sensitive to substitution. However, a small increase in ER is associated with a decrease in AH. The absolute value for ER of the intermolecular excimer of 9 m e t h y l a n t h r a ~ e n eis~approximately ~ a factor of 3 larger than in the intramolecular excimer of I-H. The smaller observed repulsion energies found for the substituted 1 implies that the aromatic rings do not completely overlap as in 9,lO-dimethylanthracene, presumably because the glutarate chains impose steric restrictions to maximize excited-state interactions but to minimize ground-state interactions. Conclusion
