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The Photochemistry of 2-(1-Naphthyl)ethyl Benzoates: Cycloaddition and Intramolecular Exciplex Formation Krista Morley and James A. Pincock* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3
[email protected] Received November 17, 2000
The photochemistry of the 2-(1-naphthyl)ethyl benzoates 6 and 7 was examined in order to compare them to previously studied 2-arylethyl 4-cyanobenzoates that underwent a Norrish Type II fragmentation. The 1-naphthyl group was incorporated to provide a fluorescent chromophore for probing the intramolecular electron transfer proposed previously for the mechanism. The naphthalene fluorescence was quenched for both 6 and 7 although at very different rates. For 6, with the higher thermodynamic driving force (-68.9 kJ/mol), intramolecular electron transfer was fast in all solvents, independent of their polarity (cyclohexane to methanol). For 7, with the lower driving force (-26.5 kJ/mol) the process was fast only in polar solvents. Exciplex emission, observed for 6 (but not for 7), exhibited a large solvatochromic effect possibly indicating a high dipole moment (28 D) in polar solvents (stretched conformation) but a lower one (17 D) in nonpolar solvents (folded conformation). Finally, the 4-cyanobenzoate 6 was very unreactive photochemically. In contrast, benzoate 7 underwent a 2 + 2 cycloaddition of the ester carbonyl to the naphthalene ring to give products 8 and 9, a process for which we have found no precedent. Introduction Recently, we have studied the photochemistry of some bichromophoric esters of both acyclic, 1a-c,1 and cyclic structures, 2 and 3.2,3 These compounds (except 3 which first isomerizes photochemically to 2) undergo Norrish Type II photofragmentation to the corresponding alkenes, a reaction that is normally inefficient for the π,π* state of esters. The proposed mechanism for this fragmentation, Scheme 1, begins with intramolecular electron transfer to form a radical ion pair followed by proton transfer as a consequence of the enhanced acidity of the arylmethyl hydrogen in the radical cation and the enhanced basicity of the radical anion of the 4-cyanobenzoate group. The 1,4-biradical then fragments to the observed products, 4 and 5. For all of these substrates, secondary photochemistry of the first formed arylalkene is rapid. The standard way to study intramolecular electron transfer is by fluorescence quenching. However, this proved to be impossible for compounds 1-3 because the two chromophores absorb in the same wavelength region of the ultraviolet spectrum (250-300 nm). Moreover, the dominant chromophore is the 4-cyanobenzoate one and it does not fluoresce. We now report on the photophysics and photochemistry of compounds 6 and 7 which have a naphthalene chromophore. This chromophore absorbs at wavelengths longer than 300 nm and is fluorescent (ΦF ) 0.21 for naphthalene itself)4 so that the beginning * To whom correspondence should be addressed. Phone: 902-4943324. Fax: 902-494-1310. (1) DeCosta, D. P.; Bennett, A. K.; Pincock, J. A. J. Am. Chem. Soc. 1999, 121, 3785-3786. (2) Pincock, J. A.; Rifai, S.; Stefanova, R. Proceedings of the 10th Annual Symposium of the NSF Center for Photoinduced Charge Transfer; World Scientific: Singapore, 2000; pp 27-38. (3) Pincock, J. A.; Rifai, S.; Stefanova, R. Can. J. Chem. 2001, 79, 63-69.
Scheme 1. Mechanism for the Photochemical Norrish Type II Fragmentation of Esters 1, 2, and 3 Induced by Intramolecular Electron Transfer
events in the intramolecular electron-transfer photochemistry can be studied. To our surprise, the Norrish Type II photochemistry discussed above was now very (4) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed; Marcel Dekker: New York, 1993; p 286.
10.1021/jo001633z CCC: $20.00 © 2001 American Chemical Society Published on Web 04/12/2001
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inefficient but 7 reacted by an intramolecular cycloaddition that, to our knowledge, is unprecedented. In addition, analysis of the intramolecular electron transfer by fluorescence studies (static and dynamic) revealed significant differences between 6 and 7.
Morley and Pincock Scheme 2. Mechanism for Formation of the Photoproducts 8 and 9 from the Benzoate Ester 7 in Methanol
Results and Discussion Photochemistry of 6 and 7. The esters 6 and 7 were synthesized by condensation of 2-(1-naphthyl)ethanol with the corresponding acid. Irradiation of the cyanobenzoate 6 (150 mg) in methanol (280 mL) with a Pyrexfiltered 450 W medium-pressure mercury lamp gave no conversion to products even after 30 h. Switching to a Vycor filter to increase the light intensity led to very slow conversion and poor mass balance for formation of 2-methoxy-1-(1-naphthyl)ethane and 1-methoxy-1-(1naphthyl)ethane in a ratio of 10:1. These are the two products expected from anti-Markovnikov and Markovnikov addition of methanol to 1-naphthylethene, which was not detected (GC/MS) during the course of the reaction. As outlined in Scheme 1, this alkene is predicted to be the primary photoproduct in the Norrish Type II reaction. As discussed previously,1,3 the anti-Markovnokov addition of methanol presumably occurs by addition to the radical cation of the alkene formed by bimolecular electron transfer with the other primary photoproduct, 4-cyanobenzoic acid, according to the well-established mechanism of Arnold and co-workers.5 The Markovnikov addition of methanol probably occurs by the photochemical protonation of the 1-naphthylethene, 4-cyanobenzoic acid as the acid catalyst, as observed by Yates and coworkers for a number of arylalkenes, including 2-naphthylethene.6 The very low yields and slow reactions in the photochemistry of 6 were surprising to us in view of the relatively efficient reactions of the compounds 1a-c and 2 previously studied, Scheme 1. The idea behind examining the benzoate ester 7 was that with the weaker electron acceptor and the same excitation energy, the radical ion pair would have a higher energy content and perhaps be more reactive. Again, to our surprise, no photofragmentation occurred on irradiation through Pyrex. However, there was slow formation of two products, 8 and 9, eq 1; at 77% conversion of 7 the yields were 39 and 16% respectively. By GC/MS, 8 (m/z 276 + 32) resulted from addition of methanol to 7, and 9 (m/z 30818) by loss of water from 8.
These products were purified by silica gel chromatography and recrystallization. The structure of 8 was (5) Shigemitsu, Y.; Arnold, D. R. Chem. Commun. 1975, 407-408. (6) McEwen, J.; Yates, K. J. Am. Chem. Soc. 1987, 109, 5800-5808.
tentatively assigned by 1H NMR (including COSY) and NMR (including HETCOR) and confirmed by a singlecrystal X-ray structure, Figure S1 in Supporting Information. The structure of 9 was also determined by NMR spectra and by its obvious connection to 8. In fact, we were initially surprised that 8 survived silica gel chromatography as we thought it should be sensitive to acidcatalyzed decomposition. However, heating 8 in benzene with p-toluenesulfonic acid gave the four products shown in eq 2 (10 and 11 were authentic samples but 12 is proposed only on the basis of a GC/MS spectrum). The dehydration product 9 was not observed. 13C
The proposed mechanism for the formation of 8 and 9 begins with 2 + 2 cycloaddition of the ester carbonyl to the naphthalene ring to give the highly strained oxetane 13, Scheme 2. On the basis of the failure of the above attempt to prepare 9 from 8, we suggest that 8 and 9 are formed by two different acid-catalyzed ring openings
Photochemistry of 2-(1-Naphthyl)ethyl Benzoates
of the oxetane, path a and path b in Scheme 2. This suggestion would also explain why the ratio of the yields of 8 to 9 remains constant with time during the photolysis. The literature on 2 + 2 photocycloadditions is immense and the addition of carbonyl groups to alkene double bonds has been reviewed several times.7 Normally, as a consequence of their π,π* excited state, ester carbonyl groups only add to alkenes under photoinduced electrontransfer conditions8 as would be the case for the ester 7. However, we have been unable to find a literature example for the cycloaddition of an ester carbonyl to a naphthalene double bond. Intermolecular additions of ester carbonyls to furans9 and other aromatic heterocycles10 are well documented. Intramolecular cycloadditions of aldehyde carbonyls to furans are also known.11 In view of this unusual photochemistry of 7, particularly in contrast to 1a-c, 2, 3, and 6, we decided to examine excited-state properties and electron-transfer kinetics of 6 and 7 in some detail. Absorption and Fluorescence Spectra for 6 and 7. As we have observed previously,3 the UV spectra for these compounds showed no evidence for significant ground-state interaction between the two chromophores. The spectrum of the ester 6 is compared with a synthetic spectrum comprised of 1-methylnaphthalene plus methyl 4-cyanobenzoate, Figure 1 (methanol, all at 8.8 × 10-5 M). The difference between the two shows only weak enhancement of the intensity of vibrational modes but no indication of a longer wavelength absorption corresponding to a charge-transfer band. Similar spectra were observed in cyclohexane. The other important feature of these spectra is that the naphthalene chromophore, as expected, does have a weak ( ∼ 300 M-1 cm-1) absorption band for the S0 - S1 transition at wavelengths longer than 300 nm where the 4-cyanobenzoate chromophore is essentially transparent. This allows selective excitation of the naphthalene moiety. In contrast to the absorbance spectra, the fluorescence spectra demonstrate a strong interaction between the two chromophores. On excitation at 312 nm, the quantum yields of flourescence in methanol for 6 (Φf ∼ 5 × 10-4) and 7 (Φf ∼ 1 × 10-3) are a factor of 380 and 190, respectively, lower than the value for 1-methylnaphthalene (Φf ) 0.19).4 Values this small must be taken with some caution because these esters were both made from 2-(1-naphthyl)ethanol which will have a fluorescence yield comparable to that of 1-methylnaphthalene. Therefore, if the esters contain unreacted starting material as an impurity at even 0.1%, the observed fluorescence would give an apparent quantum yield of 2 × 10-4. This quenching of the naphthalene fluorescence by the benzoate chromophore could also be observed in intermolecular experiments. Thus, Stern-Volmer plots for the quenching of 1-methylnaphthalene fluorescence in methanol by methyl 4-cyanobenzoate and methyl benzoate gave (7) Griesbeck, A. G. CRC Handbook of Organic Photochemistry and Photobiology; CRC Press: New York, 1995; pp 522-535 and references therein. (8) Neunteufel, R. A.; Arnold, D. R. J. Am. Chem. Soc. 1973, 95, 4080-4081. (9) Cantrell, T. S., Allen, A. C.; Ziffer, H. J. Org. Chem. 1989, 54, 140-145. (10) Rivas, C. CRC Handbook of Organic Photochemistry and Photobiology: CRC Press: New York, 1995; pp 536-559. (11) Carless, H. A. J. CRC Handbook of Organic Photochemistry and Photobiology: CRC Press: New York, 1995; pp 560-569.
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kqτs values of 696 M-1 and 204 M-1, respectively. Using the measured value for the singlet lifetime (τs ) 71 ns) of 1-methylnaphthalene in methanol gave kq values of 9.8 × 109 M-1 s-1 for methyl 4-cyanobenzoate and 2.9 × 109 M-1 s-1 for methyl benzoate. The former is comparable to the estimated diffusional rate in methanol, 1.1 × 1010 M-1 s-1,12 but the latter is somewhat lower. On the basis of the quantum yields for emission, the intramolecular quenching discussed above was also somewhat more favorable for the cyano-substituted ester. The contrast in fluorescence behavior between the 4-cyanobenzoate 6 and the benzoate 7 is even more marked in solvents other than methanol. As shown in Figure 2, in addition to the very weak fluorescence intensity of the localized emission (λmax at 325 nm) of the cyanoester 6, another longer wavelength weak emission is clearly visible where the maximum in this emission is strongly solvent dependent. The intensities of these exciplex emissions are very low, the quantum yields ranging from values of 6 to 24 × 10-5 compared to a value of 0.19 for 1-methylnaphthalene in acetonitrile under the same experimental conditions. The change in wavelength of maximum emission for the exciplex as a function of solvent (solvatochromic effect) can be used13 to estimate its dipole moment (µ) using eq 3a (converted to 3b with the units usually used) where νct is the measured frequency (cm-1) of emission in a given solvent, νct(0) is the same frequency in the gas phase, µ is the dipole moment of the exiplex (assuming the dipole moment of the ground state is small and negligible), F is the effective radius of the cavity that the molecule occupies and ∆f13 is a measure of solvent polarity defined by the dielectric constant 14 and the optical refractive index n,14 eq 4. A plot of νmax (Table 1)
versus ∆f for 6 shows two distinct linear regions (Figure 3a), one of shallow slope ((-1.4 ( 0.3) × 10-4 cm-1) for solvents less polar than diethyl ether and one of steeper slope ((-3.6 ( 0.4) × 10-4 cm-1) for solvents more polar. The value for diethyl ether was included in both correlations. The maximum in the exciplex emission in cyclohexane was obscured by the localized naphthalene emission. Therefore, the value of 381 nm was obtained by subtracting a normalized 1-methylnaphthalene spectrum from the observed spectrum. This difficulty will be discussed in more detail below. Intramolecular exciplex formation has been extensively studied with Verhoeven and co-workers being major contributors.15 The biphasic behavior shown in Figure 3a for the cyano ester 6 has also been observed by them for other cases with flexible spacers, for example, the aniline/ naphthalene derivative 14.16 The slope of these plots
gives a measure of the ratio of the dipole moment (µ) to
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Figure 1. Absorbance spectra of the 4-cyanobenzoate ester 6 compared with model compounds 1-methylnaphthalene and methyl 4-cyanobenzoate.
the cavity size (F). In polar solvents, the intramolecular exciplex exists in an extended conformation having a high (12) Reference 4, p 208. (13) Beens, H.; Knibbe, H.; Weller, A. J. Chem. Phys. 1967, 47, 1183-1184. (14) Van Stokkum, I. H. M.; Scherer, T.; Brower, A. M.; Verhoeven, J. W. J. Phys. Chem. 1994, 98, 852-856. (15) Verhoeven, J. W. Pure Appl. Chem. 1990, 62, 1585-1596. (16) Scherer, T.; Willemse, R. J.; Verhoeven, J. W. Rev. Chem. Pays Bas 1991, 110, 95-96.
dipole moment and resulting in a steep slope. As the dielectric constant of the solvent decreases, the solvent can no longer stabilize charges in the extended geometry and conformational motion (“harpooning”)15 pulls the charges together, decreasing the dipole moment and the slope of the solvatochromic plot. For cases with rigid and semirigid spacers such as 15, steep slopes (22 000-44 000 cm-1) are observed through the complete range of solvent polarities and dipole moments as high as 38 D result.17
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Figure 2. Exciplex emission from the 4-cyanobenzoate ester 6 in various solvents. Table 1. Exciplex Emission Wavelengths for 6, Singlet Lifetimes, and Derived Rates of Intramolecular Transfer for the Benzoate Ester 7 solvent
∆f a
λmax(6)b/nm
cyclohexane dipentyl ether dibutyl ether diethyl ether ethyl acetate tetrahydrofuran dichloromethane methanol acetonitrile
0.100 0.171 0.194 0.251 0.292 0.308 0.319 0.393 h 0.392
381 (26.2) 390 (25.6) 455 (24.7) 413 (24.2) 457 (21.8) 458 (21.9) 478 (20.9) i 528 (18.9)
τ(6)c/10-9 s
1.7 (0.1) 1.3 (0.1) 0.9 (0.1)
τMNd/10-9 s
τ(7)e/10-9s
ketf/107s-1
85.1 (1.9) 71.3 (1.5) 69.0 (1.1) g 71.4(1.3) 68.7 (1.1) 65.5(0.9) 11.5 (0.1) 71.4 (0.4) g 68.4 (1.1)
36.8 (0.4) 36.5 (0.7) 34.4 (0.5) 27.5 (0.2) 11.0 (0.2) 10.6 (0.1) 2.73 (0.03) 0.43 (0.03) j 0.91 (0.07)k
1.5 1.3 1.5 2.2 7.6 7.9 28 240 89
a Calculated according to eq 4, from ref 14. b Maximum wavelength (nm) of emission for the exciplex of 6. Values in parentheses are in cm-1/103. c Singlet lifetime at 25 °C of intramolecular exciplex for 6 obtained from fluorescence decay. Values in parentheses are the standard deviation of the fit. d Singlet lifetime at 25 °C of 1-methylnaphthalene (1.3 × 10-3 M) obtained from fluorescence decay, λex ) 314 nm. Values in parentheses are the standard deviation of the fit. e Singlet lifetime at 25 °C of 7 obtained from fluorescence decay, λex ) 314 nm. Values in parentheses are the standard deviation of the fit. f Rate of intramolecular electron transfer for 7 calculated from ket ) 1/τ (7) - 1/τMN. g Values of 65.8 (1.1) and 72.0 (0.5) were obtained in dibutyl ether and methanol, respectively, at 9.1 × 10-6 M and λex ) 282 nm. h Calculated from eq 4 with values of and n taken from ref 4, p 286. i No exciplex emission detected. j This decay had a second component contributing 4% with a lifetime of 5.4 ns. k This decay had a second component contributing 4% with a lifetime of 5.0 ns.
Calculating the dipole moment of the exciplex requires an estimate of the solvent cavity size, usually taken as the cube of 40% of the long axis for rod-shaped molecules. For 6, using molecular models, our estimate of the cavity size is 6 Å (0.4 × 15 Å) resulting in a dipole moment of µ ) 28 D for the extended conformation in polar solvents. For the harpooned conformation, the molecule is more spherical and the estimated cavity size is still about 6 Å. However, the lower slope in Figure 3a, results in a
dipole moment, µ ) 17 D. No biphasic behavior was reported for the exciplex emission for the ester 1618 analogous to 6 except for the pyrene rather than the naphthalene donor. In that case, the solvatochromic plot gave a slope of 18 000 cm-1 and a dipole moment of 12 D assuming a F of 4.4 Å. A reviewer correctly pointed out that, in the absence of the point for cyclohexane, the exciplex emission data for 6 in Table 1 would give a linear correlation with ∆f
(17) Hermant, R. M.; Bakker, N. A. C.; Scherer, T.; Krijnen, B.; Verhoeven, J. W. J. Am. Chem. Soc. 1990, 112, 1214-1221.
(18) Kawakami, J.; Iwamura, M. J. Phys. Org. Chem. 1994, 7, 3142.
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Figure 3. (a) Effect of solvent polarity (∆f) on the exciplex emission frequency (cm-1) for the cyanobenzoate ester 6. (b) Effect of solvent polarity (∆f) on the exciplex emission frequency (cm-1) for the cyanobenzoate ester 6, omitting the value for cyclohexane.
(Figure 3b) of intermediate slope ((-3.1 ( 0.3) × 10-4 cm-1). This is a valid point because, as discussed above,
the value of λmax in cyclohexane may not be reliable as it was obtained by spectral subtraction of two very weak emissions. Distinguishing between these two possible plots by static fluorescence measurements will not be possible because no solvents are available to span the gap between cyclohexane (∆f ) 0.100) and dipentyl ether (∆f ) 0.171). In fact, the higher homologues of diethyl ether (∆f ) 0.251), along with a number of different hydrocarbons, have traditionally been chosen to confirm the biphasic nature of these plots.14 More recently, using a streak camera, time-resolved exciplex emission as a function of wavelength (“spectotemporal parametrization”) for both flexible and semirigid intramolecular exciplexes has revealed that these systems are more complex than previously thought.14,19
Morley and Pincock
In nonpolar solvents (∆f e 0.25, diethyl ether) the data for the flexible compounds such as 14 are rationalized by initial formation of an extended charge transfer (ECT) conformation that then collapses to two different folded contact charge transfer (CCT) conformations. Even in cases where the charge transfer state is strongly stabilized, the ECT has a short lifetime (around 1 ns). As well, the emission is blue shifted relative to the CCT emissions which have considerably longer lifetimes, particularly in hydrocarbon solvents. Thus, both biphasic (as in Figure 3a) and linear (as in Figure 3b) behavior are observed simultaneously from the same compound. In contrast, in cases where the charge transfer state is not strongly stabilized, the ECT is not observed in these nonpolar solvents. Only biphasic plots are obtained. All of this information was hidden in previously reported static fluorescence experiments. We have attempted to measure the exciplex lifetimes by single photon counting (Table 1), but only at a single wavelength. These values cannot be considered to be very reliable because the emissions are very weak. Collection of data for 24 h gave only 300500 counts in the channel of maximum intensity in the single photon counting accumulation. For more intense emissions, as for 1-methylnaphthalene, 5000-10 000 counts are normally accumulated in a few hours. Therefore, the fits to the exciplex emission data are rather poor. However, the lifetimes seem to be close to 1 ns, suggesting an ECT conformation. The data are not reliable enough to tell whether a second long-lived component (CCT) is also present. In summary, we do not think that we can reliably distinguish between the biphasic and linear fit to the exciplex emission data. However, extensive literature data obtained from systems where exciplex emission is more intense and can be accurately measured over the complete range of solvent polarities, always reveals a CCT for flexible systems in hydrocarbon solvents. Our preference is, therefore, for the biphasic fit. The benzoate 7 is quite different in its emission properties from the 4-cyanobenzoate 6. Localized naphthalene emission is observed in all solvents but its intensity steadily decreases as the solvent polarity increases. Only in methanol is a weak charge-transfer emission observed with λmax of 460 nm in contrast to the cyanobenzoate 6 which emits at considerably longer wavelength (530 nm) in acetonitrile, a solvent with the same ∆f as methanol. The decrease in localized emission as a function of solvent for the ester 7 can be quantified by measured lifetimes, Table 1. A systematic decrease is observed as the solvent polarity increases from cyclohexane to acetonitrile indicating an increased rate of intramolecular electron transfer, ket.20 Those rate constants (ket ) 1/τ 1/τMN) are also given in Table 1 where τMN is the excited singlet state lifetime for 1-methylnaphthalene in the same solvent. These ket values are based on the usual assumption that the lifetime of a bichromorphic molecule is only decreased by electron transfer and all other rate constants remain the same as the model compound.21 The lifetime of the model compound, 1-methylnaphthalene, is near 70 ns for all solvents except cyclohexane (85.1 ns), and dichloromethane (11.5 ns). The latter value (19) Lauteslager, X. Y.; van Stokkum, I. H. M.; van Ramesdonk, H. J.; Brouwer, A. M.; Verhoeven, J. W. J. Phys. Chem. A 1999, 103, 653659.
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Table 2. Estimated Free Energies of Electron Transfer (∆Get°) and Free Energies of Activation (∆Get†) for the Cyanobenzoate Ester 6 and the Benzoate Ester 7 solvent
∆Get° a (6) (kJ/mol)
∆Get° a (7) (kJ/mol)
δ(∆Get°)b (kJ/mol)
λsc (kJ/mol)
λd (kJ/mol)
∆Getq e (6) (kJ/mol)
∆Getq e (7) (kJ/mol)
cyclohexane dipentyl ether dibutyl ether diethyl ether ethyl acetate tetrahydrofuran dichloromethane methanol acetonitrile
11.7 -12.0 -17.7 -33.6 -44.6 -50.8 -54.5 -68.2 -68.9
53.9 30.4 26.7 8.8 -3.8 -8.4 -12.1 -25.8 -26.5
-28.7 -20.9 -18.8 -13.4 -9.7 -7.7 -6.4 -1.8 -1.6
-0.4 24.6 32.3 54.9 63.5 64.9 67.0 93.5 92.0
18.9 43.9 51.6 74.2 82.8 84.2 86.3 112.8 111.3
12.3 5.8 5.6 5.6 4.4 3.3 2.9 4.4 4.0
70.1 31.4 29.7 23.2 18.8 17.0 15.9 16.7 16.1
a Calculated from eq 5b with E (6, 7) ) 1.59 V (SCE, acetonitrile), E ox red ) -1.65 V (6) and -2.09 V (7), λ0,0 ) 318 nm, r ) 8 Å and r( ) 4 Å. b The difference between either ∆Get° (6) or ∆Get° (7) calculated at the extended distance of 8 Å and ∆Get° calculated at r ) 6 Å, ie the “harpooning” energy. c Calculated from eq 9b with r ) 8 Å and r( ) 4 Å. d Calculated from eq 8 assuming λi ) 19.3 kJ/mol. e Calculated from eq 7.
presumably reflects an increased rate of intersystem crossing induced by the chlorinated solvent as has been observed previously by Bolton, Archer and co-workers21 for a linked porphyrin/quinone system. Our measured lifetimes for 1-methylnaphthalene are somewhat shorter than those reported previously in methylcyclohexane (96 ns) and ethanol (97 ns).22 Ours were measured by excitation into the weak long wavelength band at 314 nm and consequently the concentrations were higher (∼10-3 M) than those in the previous report (5 × 10-5 M). However, we found no significant difference in lifetime in either dibutyl ether or methanol at lower concentration (footnote in Table 1) and excitation at 282 nm. In contrast to the benzoate ester, the lifetimes of the cyanobenzoate ester could not be measured because the emission was too weak. This suggests that the lifetime in all solvents is well under 1 ns because intramolecular electron transfer is fast. The difference between the two esters 6 and 7 is a consequence of the lower driving force for electron transfer as predicted by the Weller equation,23 eq 5a, (converted to 5b with the units usually used) where E0,0 is the energy of the excited singlet state, NQ1Q2/4π0r is the Coulombic attraction term resulting from bringing the radical cation and radical ion to a distance r in a solvent of dielectric constant (0 ) 8.85 × 10-12 C2 N-1 m2 is the permittivity of a vacuum), and the final term, the Born equation, corrects for the fact that the redox potentials were measured in acetonitrile, i.e., it estimates the solvation energy of the radical ions of radius r( in a solvent () relative to acetonitrile (AN). Both 6 and 7 have
the same E0,0 ) 377 kJ/mol as a consequence of the common naphthalene chromophore. The distance of separation of the chromophores, r = 8 Å, was measured from the center of each of the chromophores using molecular models and the usual value was used for r( (4 Å). Using measured values for Eox (1.59 V versus SCE in acetonitrile) and Ered (-1.65 V for 6 and -2.09 V for 7) gives a calculated ∆Get° of -68.9 and -26.5 kJ/mol, respectively, for 6 and 7 in acetonitrile. Both electrontransfer processes are exergonic but the driving force for exciplex formation is, as expected, greater for 6 than for 7.
The estimated values for ∆Get° in other solvents are given in Table 2. For the cyanobenzoate 6, with the higher driving force, electron transfer is thermodynamically favored in all solvents except cyclohexane where it is slightly exergonic (+11.7 kJ/mol). In contrast, for the benzoate 7, electron transfer is only favorable in solvents more polar than diethyl ether. We can also estimate the energy to be gained by “harpooning” as the charges are brought together by conformational motion. Assuming the benzoate chromophore as a radical anion maintains planar geometry at the carbonyl carbon, the center to center distance of the two rings drops from 8 to 6 Å if a gauche conformation is achieved, eq 6. This results in
the δ (∆Get°) values shown in Table 2. This process is quite favorable in nonpolar solvents and provides enough driving force to overcome the conformational inhibition. Consequently, low slopes are expected in the solvatochromic plot for these solvents. The changeover in the plot to a steeper slope occurs as δ (∆Get°) drops below about -15 to -10 kJ/mol, which is reasonable in view of the conformational motion required.
The free energy of activation, ∆Get†, for electron transfer can be estimated by Marcus theory24 eq 7, where λ is the reorganization energy composed of λi (internal)
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Figure 4. Plot of the free energy of activation (∆Gq) for intramolecular electron transfer for the benzoate ester 7 () and 4-Cyanobenzoate 6 (b).
and λs (solvent) parts, eq 8. The solvent reorganization energy, λs, can be calculated from eq 9a (converted to 9b with the usual units) where n is the solvent refractive index and all other terms have been defined above. These values are also given in Table 2 along with the value of λ assuming a λi of - 9.3 kJ/mol (0.2 eV) which is typical for relatively rigid chromophores.21 For the cyanobenzoate 6, the ∆Getq values are all low and almost independent of solvent polarity. This effect, where an appropriate driving force at an appropriate distance will lead to rapid rates of electron transfer in all solvents, has been discussed in detail by Verhoeven and co-workers.25 In contrast, the benzoate 7, has a large ∆Getq in the nonpolar solvents but the value drops steadily as the solvent polarity increases. This effect is shown graphically in Figure 4. These estimates parallel the experimental observations very well. For the cyanobenzoate the localized naphthalene fluorescence is very weak, reflecting the rapid rate of electron transfer with rates exceeding 109 s-1. For the benzoate, the rates of electron transfer increase steadily from cyclohexane (ket ) 1.5 × 107 s-1) to acetonitrile (ket ) 89 × 107 s-1) so that electron transfer is competitive with excited-state decay (kd ) 1.4 × 107 s-1) of the naphthalene chromophore in the nonpolar solvent but dominates in polar solvents. Conclusions From the photophysical measurements of the localized naphthalene and exciplex emission for the bichromophoric esters 6 and 7, we know that exciplex formation will be rapid in methanol, the solvent used in the current work. Moreover, the exciplex is long enough lived (1 ns) that conformational motion will be possible during its (20) The value of the singlet lifetime for 7 in methanol is at the timelimit resolution of our nanosecond flash lamp system. Therefore, the value reported for ket should only be regarded as an estimate but must be higher than the value in acetonitrile. (21) Schmidt, J. A.; Liu, J.-Y.; Bolton, J. R.; Archer, M. D.; Gadzekpo, V. P. Y. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1027-1041. (22) Cundall, R. B.; Pereira, L. C. Trans. Far. Soc. 1972, 68, 11521163. (23) Weller, A. Z. Phys. Chem. 1982, 133, 93-98. (24) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701. (25) Kroon, J.; Verhoeven, J. W.; Paddon-Row: M. N.; Oliver, A. M. Angew. Chem., Int. Ed. Engl. 1991, 30, 1358-1361.
Morley and Pincock
lifetime. However, in a solvent as polar as methanol, the energy to be gained by “harpooning” (-1.8 kJ/mol) is insufficient to overcome the inhibition for rotation to the less stable conformer. In effect, the solvent stabilization of the charge separated exciplex is enough to maintain the extended geometry with the high dipole moment. Proton transfer in the exciplex, if it occurs, may still be rapid because it could be solvent assisted and thermodinamically driven by the large difference in pKa of the arylmethyl hydrogen of the radical cation and the ketyl radical derived from the radical anion. Six-membered transition states and quite specific geometries as in the usual Norrish Type II reaction of ketones,26 may not be required in the radical ion pair version. The question remains as to why these radical ion pairs show such differences in reactivity from Norrish Type II reaction for the benzylic esters 1a-c and 2, cis and trans isomerization for 3, cycloadditions (slow but reasonable yields) for 7, and almost no reaction for 6. The inefficiency of the photochemistry for the naphththalene esters 6 and 7 is most likely a result of back electron transfer in the radical ion pair. Exothermic back electron transfer to the ground state singlet is usually the process that makes the photochemistry of radical ion pairs inefficient. However, back-electron transfer to form the triplet state of the substrate is also possible if it is energetically feasible, as we have observed previously for the cyclic ester, 2.3 In this case, the lower energy triplet of the bichromophoric molecule is the cyanobenzoate (301 kJ/mol), which we observed by nanosecond LFP, and not the alkylaryl ring (346 kJ/mol). In contrast, for the naphthyl esters 6 and 7, the lower energy triplet state is the naphthalene chromophore (254 kJ/mol).4 Laser excitation (306 nm)of 7 in methanol gave a strong transient signal with two bands at 390 and 420 nm. These bands, decaying with the same rate constant (5.6 × 105 s-1) and quenched by oxygen, were assigned to the naphthalene triplet state.27 Moreover, this state is of considerably lower energy than the biradical (310 kJ/mol) that would be formed from the radical ion pair by the proton transfer that leads to Norrish Type II cleavage. This is a consequence of both the lower driving force for electron transfer in the naphthalene esters and the lower acidity of the radical cation formed. Effectively, the acididty of the radical cation is determined by the homolytic bond dissociation energy of the arylmethyl carbon-hydrogen bond and the reactivity of the radical cation as determined by the oxidation potential for its formation.28 The bond dissociation energies for 1-naphthylmethyl and benzyl carbon-hydrogen bonds are only slightly different (∼12 kJ/mol weaker for the naphthalene case).29 However, the higher oxidation potential for toluene (2.4 V)1 versus 1-methylnaphthalene (1.59 V) makes the toluene radical cation acidity more favorable by 78 kJ/mol. We conclude that back electron transfer in the radical ion pair of the naphthyl esters to form the triplet state may be the process that dominates excited state decay. Proton transfer, being less favorable thermodynamically, does not compete. Formation of triplet states by back electron (26) Wagner, P. J. CRC Handbook of Organic Photochemistry and Photobiology; CRC Press: New York, 1995; pp 449-470. (27) Meyer, Y. H.; Astier, R.; Leclercq, J. M. J. Chem. Phys. 1972, 56, 801-815. (28) Nicholas, A. M. P.; Arnold, D. R. Can. J. Chem. 1989, 67, 689698. (29) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493-532.
Photochemistry of 2-(1-Naphthyl)ethyl Benzoates
transfer in radical ion pairs has also been observed very recently for anthracene chromophores where again the triplet state energy is very low.30 Finally, the cycloaddition of the benzoate ester 7 to the ketals 8 and 9 has some obvious synthetic potential. A reason 7 reacts this way and the 4-cyanobenzoate 6 does not may be that the negative charge in the radical anion of the benzoate is more localized on the carbonyl oxygen than in the 4-cyanobenzoate. By calculations at the STO3G level,31 25% of that charge is delocalized to the nitrogen of the cyano group, as expected by resonance arguments. Improving the efficiency and generality of this 2 + 2 addition may require the use of nonpolar solvents to favor the collapsed exciplex geometry required to obtain the cyclization to 13 as shown in Scheme 2. However, 13 must then be rapidly trapped as presumably occurs in methanol. Therefore, mixed solvent systems may be required; the nonpolar medium (e.g., diethyl ether) facilitating the cyclization and the added alcohol (e.g., methanol) as the trapping reagent. Just how these exciplexes behave in mixed solvents is unknown but such fluorescence experiments are in progress.
Experimental Section 2-(1-Naphthyl)ethanol and 4-cyanobenzoic acid were purchased from the Aldrich Chemical Co. Tetrahydrofuran was distilled from sodium immediately before use. Dichloromethane was dried over CaH2 then over P2O5 and distilled. All other solvents were obtained commercially and used as received: cyclohexane (Fisher HPLC grade), dipentyl and dibutyl ethers (Aldrich), diethyl ether (ACP Chemicals, anhydrous), ethyl acetate (Fisher HPLC grade), methanol (Caledon HPLC grade), and acetonitrile (Omnisolve). Dipentyl ether, as received (Acros Organics) gave a very short lifetime (25 ns) for 1-methylnaphthalene and had an absorption in the 250-300 nm range; distillation of the solvent (Aldrich) from LiAlH4 at atmospheric pressure was required to give the lifetimes and spectra reported. Elemental analyses were performed by Canadian Microanalytical Services, Delta, BC, Canada. Synthesis of 2-(1-Naphthyl)ethyl 4-Cyanobenzoate (6) and Benzoate (7). In a typical procedure, 2-(1-naphthyl)ethanol (29 mmol) was added to a solution of 4-cyanobenzoic acid (29 mmol), dicyclohexylcarbodiimide (29 mmol), and (dimethylamino)pyridine (2.9 mmol) in 125 mL of dichloromethane. The reaction mixture was stirred overnight at room temperature and filtered (gravity), and water was added. The two layers were separated, and the dichloromethane layer was washed twice with 5% acetic acid and again with water. The organic layer was dried (MgSO4), filtered, and evaporated under reduced pressure to give the crude ester (4.12 g). The ester was purified by chromatography and recrystallized from hexane to give 2.92 g (9.69 mmol, 33.4%) of pure ester: mp ) 93-94 °C; 1H NMR δ 8.15 (d, J ) 7 Hz, 1H), 8.06 (d, J ) 8 Hz, 2H), 7.89 (d, J ) 7.3 Hz, 1H), 7.80 (m, 1H), 7.71 (d, J ) 8 Hz, 2H), 7.55 (m, 2H), 7.43 (m, 2H), 4.70 (t, 2H), 3.57 (t, 2H); 13C NMR δ 164.9, 133.98, 133.92, 133.4, 132.2, 132.0, 130.1, 128.9, 127.7, 127.1, 126.3, 125.8, 125.6, 123.5, 118.0, 116.3, 65.8, 32.2; IR (Nujol) 3063, 2954, 2229, 1714, 1465, 1276, 1104 cm-1; GC/ MS m/z 155 (13), 154 (100), 153 (43), 141 (30), 130 (17), 115 (28), 102 (26) 76 (6), 75 (8), 51 (7); HRMS calcd for C20H15N1O2 301.110, found 301.109 (an unknown set of signals at m/z ) 330, 331 were also observed in this spectrum); UV (CH3OH) λmax 223 ( 95 300), 281 (9350), 313 (383). Anal. Calcd for C20H15NO2: C, 79.72; H, 5.02; N, 4.64. Found: C, 79.40; H, 5.13; N, 4.56. The benzoate 7 was purified by bulb-to-bulb distillation: bp 150-155 °C/0.5 mmHg; 1H NMR δ 8.25 (d, J ) 7.9 Hz, 1H), 8.11 (d, J ) 8.5 Hz, 2H), 7.94 (d, J ) 7.9 Hz, 1H), 7.83 (m, 1H), 7.67-7.43 (m, 7H), 4.67 (t, 2H), 3.57 (t, 2H); 13C NMR δ (30) Lee, K.; Falvey, D. E. J. Am. Chem. Soc. 2000, 122, 9361-9366. (31) McMahon, K.; Arnold, D. R. Can. J. Chem. 1993, 71, 450-468.
J. Org. Chem., Vol. 66, No. 9, 2001 3003 166.7, 133.9, 133.8, 133.0, 132.2, 130.3, 129.7, 128.9, 128.4, 127.6, 127.1, 126.3, 125.7, 125.6, 123.7, 65.0, 32.4; IR 3061, 2957, 2895, 1717, 1599, 1451, 1273, 1112 cm-1; GC/MS m/z 155 (13), 154 (100), 153 (43), 141 (180), 115 (21), 105 (36), 77 (49), 51 (17); UV (CH3OH) λmax 223 ( 111 000), 281 (9580), 313 (736). Anal. Calcd for C19H16O2: C, 82.58; H, 5.83. Found: C, 82.65; H, 5.83. Preparative Photolysis. A solution of the ester in 280 mL of HPLC-grade methanol was irradiated in a Hanovia reactor with a 450 W medium-pressure Hg lamp and purged with nitrogen before and during the irradiation. The progress of the reaction was monitored by GC/FID. The products from the benzoate 7 were separated by silica gel flash chromatography using ethyl acetate/hexanes as eluent. 4-Methoxy-4-phenyl-1,2,4,4a-tetrahydro-10bH-benzo[f]isochromen-10b-ol (8): mp 166-167 °C; 1H NMR δ 7.567.06 (m, 9H), 6.55 (dd, J ) 9.4, 3 Hz, 1H), 5.86 (dd, J ) 9.4, 2.4 Hz, 1H), 4.95 (s, 1H), 4.35 (ddd, J ) -12, 12, 2.4 Hz, 1H), 4.19 (ddd, J ) -12, 5.8, 1.8 Hz, 1H), 3.35 (s, 3H), 2.64 (dd, J ) 3, 2.4 Hz, 1H), 2.45 (ddd, J ) -12, 2.4, 1.8 Hz, 1H), 2.33 (ddd, J ) -12, 12, 5.8 Hz, 1H); 13C NMR δ 138.6, 138.1, 132.7, 128.5, 128.41, 128.40, 128.3, 127.7, 127.3 (2 carbons by HETCOR), 124.6, 122.7, 104.3, 68.7, 58.4, 50.5, 49.8, 34.2; IR 3476, 2949, 2888, 1226, 1123, 1094, 1078, 1037 cm-1; GC/MS m/z 308 (M+, very weak), 172 (12), 154 (100), 144 (74), 141 (40), 137 (10), 115 (38), 105 (70), 77 (79); X-ray structure, Figure S1, and Supporting Information. Anal. Calcd for C20H20O3: C, 77.90; H, 6.54. Found: C, 77.90; H, 6.38. 4-Methoxy-4-phenyl-1,2-dihydrobenzo[f]isochromene (9): mp 107-108 °C; 1H NMR δ 8.00 (d, J ) 8.5 Hz, 1H), 7.75 (d, J ) 9.1 Hz, 1H), 7.65-7.29 (m, 9H), 4.30 (m, 2H), 3.5 (m, 1H), 3.35 (s, 3H), 3.25 (ddd, 1H); 13C NMR δ 142.2, 134.5, 132.8, 131.7, 129.9, 128.6, 128.3, 128.2, 127.5, 126.7, 126.5, 126.2, 125.6, 123.4, 101.2, 59.7, 50.6, 25.4; GC/MS m/z 290 (M•+, weak), 258 (42), 213 (27), 181 (19), 152 (68), 105 (45), 77 (100). This compound was relatively unstable and even NMR samples deteriorated with time. Consequently, correct elemental analysis could not be obtained. 1H and 13C NMR spectra (300 MHz) are in the Supporting Information (Figures S2 and S3). Fluorescence Spectra. Steady-state corrected spectra were obtained at 25 °C using a PTI Model L-201M fluorescence spectrometer with dual model 101 monochromators, a 75 W xenon lamp, and a model 814 photomultiplier detector. Timeresolved fluorescence decays were obtained at 25 °C using a PRA system 3000 equipped with a hydrogen filled model PRA 510 arc lamp operating at 30 000 Hz. The pulse width at halfheight was approximately 1.7 ns. All fluorescence samples were purged with nitrogen for 15 min or degassed by three freeze-pump-thaw cycles before the measurements. The latter was necessary for those with longer lifetimes (over 10 ns). Cyclic Voltametry. Oxidation and reduction potentials in acetonitrile were obtained at a sweep rate of 100 mV/s on an apparatus described previously32 and calibrated with ferrocene. The reduction potentials were reversible but the oxidations were not and the reported potentials were taken as 0.028 V before the peak potential.
Acknowledgment. We thank NSERC of Canada for financial support and Sepracor Canada, Windsor, Nova Scotia, for donation of chemicals. We also thank Professor J. W. Verhoeven for helpful discussions, Kathy Robertson of DALX, Dalhousie University, Department of Chemistry, for determining the single crystal X-ray structure of 8, and Alexandra Pincock for technical help. Supporting Information Available: X-ray structure and structural data for 8 and 1H and 13C NMR spectra (300 MHz) for compound 9. This material is available free of charge via the Internet at http://pubs.acs.org. JO001633Z (32) Arnold, D. R.; Wayner, D. D. M. Can. J. Chem. 1986, 64, 100103.
