J. Phys. Chem. 1994,98, 3114-3120
31 14
Photoinduced Proton Transfers in Methyl Salicylate and Methyl 2-Hydroxy-3-naphthoate Kock-Yee Law' and Jonatham Shoham Xerox Webster Research Center, 800 Phillips Road, 0114-390, Webster, New York 14580 Received: October 20, 1993"
The effects of solvent and temperature on the dual fluorescence emission of methyl salicylate (MSA) have been reinvestigated, and new insight regarding the photoinduced proton-transfer reactions is reported. The steadystate spectral data obtained in this work are found to be consistent with the spectral assignments proposed by previous investigators. Specifically, the dual emission bands in alcohols are shown to occur from excited states derived from two ground-state rotamers, a and b. The emission band from excited a (the normal band) is in mirror-image relationship with the absorption band and the Stokes shift of this band is -5000 cm-l. The long wavelength emission band, which has a Stokes shift of 10 700 cm-l, was postulated to be an emission from an excited zwitterion resulted from an intramolecular proton transfer in excited b. Both emission bands exhibit monoexponential decays. The fluorescence lifetimes for the normal and the long wavelength band are 1.2 and 0.29 ns, respectively. The monoexponential decays indicate that the twoemitting states are not in thermodynamic equilibrium. This model is supported by time-resolved emission spectra. New evidence for the occurrence of tautomerization associated with the intramolecular proton-transfer process in excited MSA is provided by a structural effect study. We have extended our measurements to methyl 2-hydroxy-3-naphthoate (MNA). In analogy to hydroxyazo compounds 1-(phenylazo)-2-hydroxybenzene and 1-(phenylazo)-2-naphthol, where the hydroxy/azo keto/hydrazone tautomerization in the latter compound is favorable due to the smaller loss in resonance energy, the excited enol tautomer of NMA, if formed, should be better stabilized. Experimentally, M N A is found to exhibit dual fluorescence emission bands. The Stokes shift of the long wavelength emission ranges from 6300 to 9900 cm-l depending on the solvent and the temperature and is smaller than that of MSA by 800 cm-l. It is argued, based on the smaller Stokes shift and the thermochromic and solvatochromic shifts of the long-wavelength emission, that keto enol tautomerization occurs in excited NMA. The similarity in spectral properties between MSA and MNA suggests that a similar tautomerization process also occurs in excited MSA. Temperature and D-isotope effects on the fluorescence decay of the long wavelength emission band enable us to conclude that regeneration of the ground-state keto tautomer of MSA is the major radiationless decay for the excited enol tautomer. The conclusions drawn from the spectroscopic data in this work are in total agreement with those advanced by Herek et al., who recently studied the H-transfer reactions of excited MSA by femtosecond depletion techniques under collisionless conditions.
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Introduction Salicylic acid (SA) and many of its derivatives, both metal salts and chelated complexes, are tribo-active materials known to be useful in dry xerographic In an attempt to comprehendthe tribocharging mechanism of these materials, we have launched a systematic effort to understand their electronic structures, which are accessible by neither general solution spectroscopictechniques nor X-ray structuredetermination? One of the fundamental questionsraised, which turns out to be critical in understanding the electronicstructures, is whether or not keto enol tautomerization occurs in excited SA and MSA (methyl salicylate). The fluorescence of MSA was known to be quite complex for some time.' In 1956, Weller* reported a fluorescence study of SA and MSA in solvents. An unusually large Stokes shift was observed for MSA in methylcyclohexane at room temperature and at 77 K. Further study of MSA in different solvents revealed that MSA actually exhibits dual fl~orescence.~ The first emission band is in mirror-image relationship with the absorption band and has a Stokes shift of -5000 cm-1. The second band emits at an anomalously long wavelength with a Stokesshift of 10 000 cm-l. It was postulated as an emission from an excited zwitterion generated by an intramolecular proton transfer from the OH group to the ester g r o ~ p . ~ Tautomerization ,~ was not suggested. The interesting fluorescence properties of MSA attracted considerable attention. The emergingpicturefrom subsequent solvent
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To whom correspondence should be addressed.
* Abstract published in Aduance ACS Abstracts, February 1,
1994.
0022-365419412098-3 114$04.50/0
effect, temperature effect, structural effect, and gas-phasestudies suggested that the dual fluorescence originates from two ground state rotamers, a and b.'O-l9 The normal emission is from excited a. The long-wavelength emission was still postulated to be an excited zwitterion from b. At room temperature in methylcyclohexane, the rate of the intramolecular proton-transfer process was determined to be >loll s-l and the lifetime of the excited zwitterionwas 280 ps.20Althoughtautomericstructuresof excited MSA have been drawn" and discussed,21J2there has been no experimental evidence for its existence. At the completion of this work, Herek et al.23reported a very elegant study on the H-transfer reactions of excited MSA using femtosecond depletion techniques under collisionless conditions. In agreement with Smith and Kauffman's solution measurement, these authorsfound that the intramolecular H transfer in excited MSA occurs within 60 fs and that regeneration of the ground state is the major radiationless decay process for the H-transferred state. The issue of excited-state tautomerism was also discussed. They proposed that keto enol tautomerization occurs in excited MSA, based on MO rationalization and the small charges expected at the oxygen atomsaZ4 In this paper, we report results of our reinvestigation of the dual fluorescence of MSA. Solvent and temperature effect studies, as well as lifetime and time-resolved data, confirm that the dual emission bands originate from rotamers a and b. To probe the photoinduced tautomerism, we extend our study to methyl 2-hydroxy-3-naphthoate (MNA) and find that it also exhibits dual fluorescence. The Occurrence of keto enol tautomerism in excited MSA is argued based on the smallerStokes
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0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, NO. 12, 1994 3115
Photoinduced Proton Transfers shift observed in the long-wavelength emission of MNA, as well as the temperature and solventeffect data. From the temperature effect and D-isotope effect on the fluorescence decay, we also conclude that regeneration of the ground-state keto tautomer is the major radiationless decay process.
TABLE 1: Effect of Solvent OD the Absorption and Fluorescence Emission Of Methyl S a b h t e absorption (log fluorescence A-4 t-)b A# A d @fr
Stokes shifd
methylcyclohexane 309.7 (3.64)
10 706
464 310
0.017 (0.022)r
ethanol
307.9 (3.62)
350 -460 306.9 (3.62) 355 464
methanol
rotamer a (keto)
rotamer b (keto)
299 0.012h 309 300 -0.Olh 309
4873 10 728 5 164 10811
Absorption maximum, in nm, A1 nm. Molar extinction coefficient, in cm-I M-l. Maximum of the fluorescence emission, in nm, fl nm. Maximum of excitation spectrum, in nm, f1 nm. e Fluorescence quantum yield, *lo%. /Peak-to=pcak,in cm-I. I Quantum yield determined in cyclohexane.14 * Excited at 290 nm.
0
Experimental Section Materials. Methyl salicylate (MSA, 99+%) was purchased from Aldrich and was purified by vacuumdistillation (three times) before use. Methyl 2-hydroxy-3-naphthoate (MNA) was from Eastman Chemicals and was recrystallizedtwice from methanol/ water and once from methanol. Methylcyclohexaneand methanol were spectrophotometric grade from Aldrich. Ethanol was 200 proof from Quantum Chemical Corporation. These solventswere routinely stored over 3-A molecular sieves before use. MSA-dl was prepared by a deuterium-exchange experiment in DzO in the presence of a trace amount of DC1. The percentage of deuterium incorporation was estimated to be -95% based on proton NMR analysis. Ethanol-dl (+99%) was from Aldrich and was distilled three times until all fluorescing impurities were removed. General Techniques. Absorption spectra were recorded on a Hewlett-Packard 845 1A diode array spectrophotometer. Fluorescence spectra were taken on a Perkin-Elmer MPF-66 fluorescence spectrophotometer, which was interfaced with a professional computer, model 7700 from Perkin-Elmer. All the solutions studied were purged with dry Nz for 30 min before use. The spectral data were corrected using the quantum counter method with the RhlOl solution supplied by Perkin-Elmer.2S Quantum yields were determined in a corrected mode by comparing with the emission of 9,lO-diphenylanthracene in cyclohexane (qjf = 0.93).26 A refractive index correction was made for each s0lution.2~ Fluorescence lifetimesand time-resolved emissionspectra were determined using the time-correlated single-photon-counting technique on a LS-100-04 fluorescence lifetime system from Photon Technology Incorporated (PTI). The data were analyzed on an NEC personal computer using the software programs provided from PTI. Result9
Absorption and Fluorescence Emission of Methyl Salicylate. Effect of Solvent. The absorption of MSA is not sensitive to solvent. Among the three solvents studied, the ,A, is at -308 nm and the 6- is -4200 cm-1 M-1 (Table 1). Figures 1-3 show the fluorescence excitation and emission spectra of MSA in methylcyclohexane, ethanol, and methanol, respectively. The fluorescence emission is solvent sensitive, and the spectral data are tabulated in Table 1. In methylcyclohexane, only a single, red-shifted emission band at AF 464 nm is observed.z* Theexcitation spectrum is very similar to the absorptionspectrum. In alcoholicsolvents, MSA exhibitsdual fluorescence. The normal emission band is at AF 350 nm and is in mirror-image relationship with the absorption band. The Stokes shift (peak-
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250
300
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450
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600
Wavelength (nm)
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Fluorescence excitation and emission spectra of MSA in methylcyclohexane (at -298 K, [MSA] 5 X lod M). Figure 1.
Wavelength (nm)
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Figure 2. Fluorescence excitation (monitored at - - 360 nm, 460 nm) and emission spectra of MSA in ethanol (at -298 K, [MSA] 5 X 1od M).
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to-peak) of this band is -5000 cm-l. The long-wavelength emission band is at XF 460 nm and the Stokes shift is 10 700 cm-l. The excitation spectra from these two emission bands are different, with the normal band giving a maximum at Am 300 nm and the long-wavelength band generating a maximum at 309 nm. The spectral data suggest that the dual emission bands in the alcoholic solvents are from two different species in the ground state. This conclusion is complemented by the fluorescence lifetime data below. As seen in Figures 1-3, the relative intensity of the longwavelength emission band decreases from methylcyclohexaneto ethanol to methanol. A similar trend was also observed by Sandros.12 The change of relative intensity suggests that the species that generates the normal emission is favored as the H-bonding interaction between MSA and solvent molecules increases. Along with the increase in relative intensity for the normal band, there is a gradual decrease in fluorescencequantum yield. The data seem to suggest that the species that emits at long wavelength has a higher luminescence efficiency.
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Law and Shoham
3116 The Journal of Physical Chemistry, Vol. 98, No. 12, 1994
TABLE 2 Effects of Solvent, Temperature, and D Isotope on the Singlet Lifetimes. of Methyl Salicylate MSA -298 K
MSA-dl
360 nm 460 nm 77 K 460 nm 77 K 460 nm
methylcyclohexane 0.36 ethanol 1.21 0.27 methanol 1.22 0.28 * In nanoseconds. In ethanol-dl. 250
300
350
400
450
550
500
Wavelength (nm)
600
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Figure 3. Fluorescence excitation (monitored at - - - 360 nm, - - 460 nm) and emission spectra of MSA in methanol (at -298 K, [MSA] 5 X 1od M). I
I
8.6
9.2
9.3
10.96
1.0
0.8
0.6
ARB INT 004
0.2
0.0 0.0
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300
350
400
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500 Wavelength (nm) 450
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600
Figure 4. Variable-temperature emission spectra of MSA ( - 5 X lv M) in ethanol (a) 273, (b) 258, (c) 231, (d) 201, (e) 173, and ( f ) 77 K. Effect of Temperature. MSA exhibits only one long-wavelength emission band at room temperature in methylcyclohexane?* The emission band is not sensitive to temperature. Other than observing an increase in the fluorescence intensity (X20), the excitation and emission spectra at 77 K are similar to those observed at 298 K. On the other hand, the dual fluorescence in ethanol is very sensitive to temperature. The variable-temperature emission spectra are given in Figure 4. The data clearly show that the relative intensity of the long-wavelength emission increases as the ambient temperature is lowered. Since different excitation spectra are obtained from these two emissions, the spectral data indicate that the two emissions originate from two different species that are in thermodynamic equilibrium in the ground state. The species that produces the long-wavelength emission is thermodynamically more stable. At temperatures below 173 K, it is the only species present. Helmbrook et al. estimated that the species that produces the long-wavelength emission is more stable by 10.5kJ/mol.lS The observed temperature-dependent spectra are consistent with the reported data. FluorescenceLifetimes and Time-Resolved Emission Spectra. The fluorescence lifetimes of MSA in different solvents were measured at room temperature as well as at 77 K. The data are summarized in Table 2. In methanol and ethanol, the dual fluorescence emission bands exhibit distinct, different, monoexponential decays. While theshort-wavelength emission band has a lifetime of -1.2 ns, the lifetimes of the long-wavelength emissions in alcohols are in the subnanosecond range. The monoexponential decays indicate that the two emitting states are not in thermodynamic equilibrium. It is also important to point out that there is a drastic increase in fluorescence lifetime from room temperature to 77 K in both
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Figure 5. Time-resolvedemission spectra of MSA in ethanol (at -298 K, [MSA] 5 X lv M).
ethanol and methylcyclohexane. The large temperature effect indicates that there exists a rapid radiationless decay for the excited state and the radiationless decay is prohibited at 77 K. Assuming that the radiative decay rate (kf) is insensitive to temperature, the qq' for MSA in methylcyclohexane at 77 K can be calculated to be 0.41 from the &at room temperature and the lifetimes at 298 and 77 K. The more than 20 times increase in fluorescence intensity agrees with this estimate. Then, kf is calculated to be 4.7X lo7s-1. From the &data and the kfvalue, the radiationless decay rates at room temperature and at 77 K are calculated to be -2.8 X lo9 s-* and -6.8 X lo7 s-l, respectively. In 1978, Smith and Kaufmann20 reported a picosecond study of MSA in methylcyclohexane. They assumed that the long-wavelength emission is from an excited zwitterion resulted from an intramolecular proton-transfer process and that regeneration of the ground state was the main radiationlessdecay process. The radiationless decay rates determined by Smith and Kaufmann were 3.4 X 109 s-l and 7.8 X lo7 s-l at 296 and 184 K, in good agreement with the values obtained in this work. Time-resolved fluorescence spectra were studied for MSA in ethanol at room temperature. The rise times for the two emitting states are beyond the detection limit of our instrument, 50.2 ns (Figure 5). While the short rise time for the normal band is understandable because it is from excited a, the short rise time for the long-wavelength emission suggests that the formation of the emitting state, the H-transferred state, must be 10.2 ns. The results are in agreement with those reported by Smith and Kaufmann20 and more recently be Herek and co-worker~,~~ who
The Journal of Physical Chemistry, Vol. 98, No. 12, 1994 3117
Photoinduced Proton Transfers
TABLE 3: Absorption, Fluorescence Emission, and Lifetime Data of Methyl 2-Hydroxy-3-naphthoate methylcyclohexane Am4 (log em)* A f (Aex)d
Stokes shifte
t#d 78 A f (Xex)d
Stokes shiftu 78
I
I
I
I
I
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I
a
1
ethanol
At -298 K 370.0(3.39) 367.5 (3.35) -380 (shoulder) -380 (shoulder) 410 (370 and -365) 429 (356) 600 (376) -3000 and 9929 4719,0.007 0.03 20.3 (at 410 nm) 3.2 (48%) 0.23 (at 600 nm) 24.3 (50%) (both at 420 nm) At 77 K 403 (355) 552 (410) 597 (-390) -, 6275 3356 and 8891 0.59 (at 550 nm) 0.38 (at 420 nm) 0.28(at 590 nm)
300
350
400
450
550
500
600
650
700
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650
700
750
Wavelength (nm) -
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350
400
450
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4 Absorption maximum, in nm, +1 nm. b Molar extinction coefficient, in cm-l M-I. Emission maximum, in nm, il nm. Maximum of excitationspectrum,in nm, 1 nm. e Peak-tepeak,in cm-1.1Fluorescence quantum yield, &lo%. Fluorescence lifetime, in nanoseconds.
showed that the rise time for the long-wavelength emission of MSA is within 60 fs. D-isotope Effect. MSA-dl was prepared by a direct deuteriumexchangereaction with DzO in the presence of a catalytic amount of DC1. The isotopic purity was estimated to be -95% by proton NMR spectroscopy. The spectroscopic properties of MSA-81 were studied in ethanol-dl and methylcyclohexane. In methylcyclohexane, both absorption and fluorescence emission spectra were found to be identical to those of MSA quantitatively. In ethanol-dl, other than a slight difference in the relative intensity between the short- and long-wavelength emission bands, the absorption maximum, emission maxima, and fluorescence quantum yield of MSA-dl are virtually identical to the data of MSA in ethanol. The lack of any D-isotope effect on the fluorescence emission is consistent with that reported by Herek et al.F3 who also observed very little isotope effect on the H-transfer reaction of excited MSA. Examinations of the fluorescence lifetimes of MSAdl in methylcyclohexane and ethanol-dl at room temperature show complex decay behavior, although the lifetimes appear to be in the same range as compared to the protiated material. At 77 K, the fluorescence decays become essentially monoexponential (>99%). The lifetimes are 9.2 and 10.9 ns in methylcyclohexane and ethanol-dl, respectively, with a x2 fit better than 1.6 (Table 2). The lifetimes are longer than MSA in methylcyclohexane and ethanol at 77 K. Since the magnitude of the increase in lifetime is beyond experimental uncertainty, we conclude that there is a deuterium isotope effect on the radiationless decay of the long wavelength emission. The isotope effect is 1.1. The D-isotope effect suggests that there occurs a proton-transfer reaction in the radiationless decay of the proton-transferred state of MSA. Spectrcwcopic hoperties of Methyl 2-Hydroxy-fnaphthoate. Absorption. MNA absorbs at X, 370 nm in methylcyclohexane and ethanol and the absorption is not sensitive to solvent. The molar extinction coefficients are -2300 cm-1 M-l (Table 3). Fluorescence Emission and Temperature Effect. The fluorescence emission spectra of MNA are presented in Figures 6a, 6b, 7a, and 7b. The data are tabulated in Table 3. (a) In methylcyclohexane: Figure 6a shows the fluorescence excitation and emission spectra of MNA in methylcyclohexane at room temperature. Two emission bands are observed. The normal band is at Xp 410 nm and is about 25 times more intense than the long-wavelength emission (XF 600 nm). The Stokes shifts of these two bands are 3000 and -9929 cm-1, respectively.
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XW 5-50 600 Wavelength (nm)
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--
Figure 6. Fluorescence excitation (monitored at - - 420 nm, * 590 nm) and emission spectraof MNA (-2 X M) in methylcyclohexane at (a) 298 and (b) 77 K.
It is worth noting that the Stokes shift of the long-wavelength emission is smaller than that of MSA. The excitation spectra for the normal band are wavelength dependent, showing vibrational fine structures with variations in the relative intensity depending on the monitoring wavelength. The excitation spectrum of the long-wavelength emission band is independent of the monitoring wavelength at 376 nm. The results indicate that MNA probably exists as two different species (rotamers) in methylcyclohexane at room temperature. At 77 K, only a single-emission band at XF 552 nm is observed (Figure6b). TheXwisat 410nm. Bothfluorescencemaximum and the Stokes shift are sensitive to temperature. The temperature effect data suggest that the species that produces the longwavelength emission is thermodynamically more stable. It becomes the only species at 77 K. (b) In ethanol: Figure 7a shows the fluorescence excitation and fluorescence emission spectra of MNA in ethanol at 298 K. A single emission band at Xp 429 nm is observed. Again, the excitation spectrum is sensitive to the monitoring wavelength. While the spectral region is the same, the relative intensity of the vibrational fine structure varies with the monitoringwavelength. The data suggest that again there exist at least two different species of MNA in ethanol at 298 K. The lifetime data below are in agreement with this observation because the fluorescence decay of this band fits well with a biexponentialdecay function. The fluorescence excitation and emission spectra of MNA in ethanol glass at 77 K are given in Figure 7b. In addition to the normal emission band at Xp 429 nm, a relatively weak longwavelength emission band is seen at hp 595 nm. The Stokes shifts for these two bands are 3356 and 8891 cm-1. The Stokes shift for the long-wavelength emission is again smaller than that of MSA. Fluorescence Lifetimes. The fluorescencelifetimes of MNA were studied in methylcyclohexane and ethanol at room temperature as well as at 77 K. In methylcyclohexane, both the normal and long-wavelength emissions exhibit monoexponential decays with lifetimes of 20.3 and 0.23 ns, respectively. The
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Law and Shoham
3118 The Journal of Physical Chemistry, Vol. 98, No. 12, 1994
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fluorescencelifetimes and the time-resolved fluorescencespectra of MSA. The observation of two different monoexponential decays for the dual emission bands indicate that the two emitting states resulting from a and b are not in equilibrium. A similar conclusion was reached by Kosower and Dodiukll as well as by Smith and Kaufmann." Further insight for thestructureassignment of rotamer acomes from the fluorescencestudy of methyl 2-metho~ybenzoate.2~ The fluorescence maximum of methyl o-methoxybenzoate is at AF 348 nm and is shifted by 5000 cm-1 from the absorption A, (296 nm). Moreover, the emission spectrum is insensitive to solvent. The lifetime of methyl 2-methoxybenzoate is 1.2 ns in ethanol and methylcyclohexane. The similar spectral characteristics and fluorescence lifetimes between methyl 2-methoxybenzoate and the rotamer a of MSA suggest that they are very similar electronically. Specifically, the intramolecular H bonding between the OH group and the ester group is absent. The lifetimeof the long-wavelengthemission is subnanosecond. The short lifetimeis consistent with structure b in Figure 8, bemuse the observation is very typical for excited states having fast radiationless decays via intramolecular proton transfers.30Jl As noted in the introductory section, one of the fundamental questions that requires further clarification is whether keto enol tautormerimtion occurs in excited MSA during the intramolecular proton transfer. In this work we attempt toaddress this issue through a structural effect study. It is well-known in the literature that l-(phenylazo)-2-hydroxybenzeneexists exclusively as the hydroxy/axo tautomer in the ground state, while I-(phenylazo)-2-naphthol can exist in two different tautomeric forms, the hydroxy/azo and the keto/hydrazone form, depending on the substituent in the compound, and the solvent and temperature when these materials are studied.32-35
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Wavelength (nm)
Figure 7. Fluorescence excitation (*) and emission spectra of MNA (-2 X 10-5 M) in ethanol at (a) 298 and (b) 77 K.
different lifetimes confirm that the two emission bands originate from two different species and the excited states resulting from them are not in equilibrium. At 77 K, only the long-wavelength emission band is observed. There is an increase in lifetime by a factor 2.5. The temperature effect on the fluorescencelifetime is significantly smaller than that of MSA. In ethanol at room temperature, the fluorescencedecay of the normal emission band is biexponential, 3.2 (48%) and 24.3 ns (52%). At 77 K, the single-emissionband becomes dual emission bands. Both fluorescence decays are monoexponential (>99%) and are in the subnanosecond range. A consistent model to account for the results is discussed below.
-
1-phenylazo-2-hydroxybenzene
hydroxy-azo 1-phenylazo-2-naphthoI
Discussion Photoinduced Proton Transfers in Methyl Salicylate. In the report by Weller, the long-wavelengthemission band was assigned to the emission from an excited zwitterion generated by an intramolecular proton-transfer from the OH group to the ester group. The assignment was based on the observation that the conjugatedbaseof excited 2-naphthol also gives rise to an emission with very large Stokes The emerging picture from subsequent investigations, by studying the effects of ~ o l v e n t , ~ J ~ temperature,14 and structural changes11.13 on the photophysics of MSA, suggests that the dual fluorescence originate from two rotamers, a and b of MSA, and rotamer b is more stable by 10.5 kJ/mol due to the intramolecular H-bonding interaction.lS1*The structures of the two rotamers are given in Figure 8. The steady-state results obtained in this work are in total agreement with previous data. Basically, rotamer b is thermodynamically more stable due to the intramolecular H bonding. In alcoholic solvents at room temperatures, a becomes abundant because the loss in intramolecular H-bonding interaction in b is compensated by H-bonding interactions with solvent molecules. Thus rotamers a and b are in equilibrium. Excitation of MSA in alcohols would lead to two emission bands, one originates from excited a and the other originates from excited b. The different excitation spectra, the solvent effect, and temperature effect data support this assignment. In addition, we have studied the
hydroxy-azo
Theoretical calculations showed that, owing to the lower resonance energy in the naphthalene system, the loss of resonance energy in the hydroxy/azo keto/hydrazone tautomerization of 1-(phenylazo)-2-naphtholcan usually be compensated by intraand intermolecular H bondings.36 In some cases, the keto/ hydrazone tautomers are found to be the thermodynamically favorable species.3' In analogy, one would anticipate that the enol tautomer of MNA, if formed in the excited state, would be better stabilized as compared to that of MSA. The ground-state and the excited-state potential surfaces of MSA have been studied both spectroscopicallyand theoretically, and there is no evidence for the existence of a double minimum potential for the surface of excited b along the coordinate of the 0-Hbond.21-23 The curves are depicted in Figure 9 (solid line). Because of the similarity in spectroscopic properties, namely, the observation of dual fluorescence emission for both MSA and MNA, we can assume that their excited-statesurfaces are parallel
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The Journal of Physical Chemistry, Vol. 98, No. 12, 1994 3119
Photoinduced Proton Transfers
*
a:::,cH3 hv
II
*
fluorescence
0
* 6
I
“CHQ
b (keto form)
‘CH3
I
t long wavelength fluorescence
radiationless decay
(enol form)
Figure 8. Schematic for the photophysics of MSA.
\
C-0-€I
MSA
-
MNA
------
C=O..H
Figure 9. Schematic descriptions of the potential curves of MSA and MNA along the O-H-.O
coordinate.
especially in an inert solvent such as methylcyclohexane. From the optical transitions of MSA and MNA, one can calculate the emission wavelength for the long-wavelength emission of MNA if the ground-state surface is parallel also. The calculated XF is at 16 321 cm-landislowerthantheobservedwavenumber (16 667 cm-l) by 350 cm-*. The discrepancy suggests that the groundstate surface of MNA is more stable than that projected from MSA by 350 cm-l at thepoint where the OHproton is dissociated. If there is any additional stabilization in the excited surface of MNA as a result of the proton transfer, further stabilization in the ground state surface will be resulted. The 350-cm-l stabilization thus represents the lower limit of stabilization when the structure changes from MSA to MNA in the ground state. In addition, a large hypsochromic shift is observed for the longwavelength emission of MNA between the room-temperature and the low-temperaturespectra (Figure 6). The hypsochromic shift, which may be attributable to the electronic transition between the (0,O)vibrational levels at 77 K,3* suggests that there is a potential minimum at where the proton transfer occurs in theground-state surface of MNA (Figure 9, dashed line). Since it is very unlikely that a minimum can be created in the zwitterion of MNA, we assign the new minimum to the enol tautomer of
MNA based on the tautomerism observed in azo/hydroxy compounds. A comparison between the potential curves of MSA and MNA is shown in Figure 9. In ethanol, the XF of MNA is at 597 nm at 77 K. The optical transition is red-shifted relative to that in methylcyclohexane. The red-shift is attributable to the solvatochromic effect on the enol tautomer. The identification of an additional potential minimum in the ground-state surface of MNA, which is assigned to the enol tautomer, suggests that the long-wavelength emission of MSA is from the excited enol tautomer also. In other words, the structural effect study in this work suggests that keto enol tautomeriation occurs in excited MSA. The conclusion reached in this work complements well with the recent report by Herek et al.,23 who suggested from MO rationalization that tautomerization should occur in excited MSA. A large temperature effect is observed for the fluorescence decay of the long-wavelength emission of MSA, suggesting the existence of fast radiationless decay for the excited enol tautomer. The results are in agreement with those reported by Smith and Kauffmann,zo Acuna, et aL14 and Felker et al.39 In this work, we observe a D-isotope effect on the radiationless decay, which suggests that a proton-transfer reaction is involved in the radiationless decay process, most likely by regeneration of the keto form in the ground state. A schematic summarizing the species involved and the photophysics of MSA in solution is given in Figure 8. Photophysics of Methyl 2-Hydroxy-3-naphthoate. The absorption and fluorescence emission of MNA have been studied by Bergmann et a1.40and Naboikin et al.41 While an agreement is obtained in the absorption spectra, the dual emission bands were not observed previously. We attribute the discrepancy to the low intensity of the long-wavelength emission. From the similarity in the spectral properties between NMA and MSA, the photophysics of MNA can be rationalized according to the photophysical scheme in Figure 8. For instance, one can also envision that MNA exists as rotamors of types a and b in solution. Presumably due to differences in solvation energy, the position of equilibrium for the a and b rotamer of MNA is different from that of MSA. In ethanol, type a is the dominating species at room temperature due to H-bonding interactions with solvent molecules. It gives rise to the normal emission band. The fluorescencedecay is biexponential, however, and consists of decay times of 3.2 (48%) and 24.3 ns (52%). The biexponential decay may be attributable to the existence of different solvation species in ethanol. At 77 K, rotamor of type b becomes increasingly favorable due to the intramolecular H bonding. Excitation of MNA results in excited a and b, which in turn fluoresce at XF 403 and 597 nm, respectively. The 403-nm band is from excited a and its measured fluorescence lifetime is 0.38ns. The decrease in lifetime at 77 K as compared to the room-temperature decay may be due to the formation of solvent complexes. Further experimentation is needed to clarify this point. The excited b of MNA is shown to tautomerize to the enol tautomer, which emits at 597 nm. The lifetime of the excited enol tautomer is 0.28 ns and is significantly shorter than that of MSA. We attribute the short lifetime to the smaller energy gap in MNA, which enhances radiationless decay.42 At room temperaturein methylcyclohexane, two emissionbands originating from rotamers a and b are observed. These two bands generate two different excitation spectra and two different monoexponential decays. The data indicate that the rotamers are in equilibrium in the ground state but that the two emitting states are not interconvertible. The observed subnanosecond lifetime for the long-wavelength emission is consistent with excited states having facile radiationless deexcitation by intramolecular proton-tran~fers.~~.~~ At 77 K, rotamer b becomes the only species present and a single emission band at long wavelength is observed.
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3120 The Journal of Physical Chemistry, Vol. 98, No. 12, 1994
The fluorescence lifetime is at 0.59 ns. Although the lifetime is longer than that observed at room temperature, it is much shorter than that of MSA under identical conditions. We attribute the small-temperature effect and the short lifetime, again, to the small bandgap in MNA.42
Conclusion Steady-state and lifetime data in this work confirm that the dual fluorescence of methyl salicylate in alcoholic solvents results from excitation of two ground-state rotamers a and b. While emission from excited a leads to the normal emission band with a Stokes shift of SO00 cm-1, emission from excited b yields an anomalously large red shift of 10 700 cm-I. We have shown through a structural effect study that photoinduced tautomerism occurs. Excited b tautomerizes from the keto form to the enol form before fluorescing to give rise to the long-wavelength emission. The rate of interconversion between excited a and excited b is slow relative to the radiative decay rates. The Occurrence of tautomerization in excited MSA is in agreement with the MO rationalization put forward by Herek and co-workers very recently. The observation of the D-isotope effect on the fluorescence decay of the long-wavelength emission of MSA, coupled with the large temperature effect obtained in this work and by others, suggests that the major radiationless decay process for the excited enol tautomer involves proton transfer also, probably by regeneration of the keto form in the ground state.
Acknowledgment. The authors thank Dr. Gordon E. Johnson for comments on the manuscript. References and Notes (1) Nomura, Y.; Ide, N.; Ohtaki,K.; Tomita, M.; Tosaka, H.; Nanya, T.; Orihara, M.; Chiba, S.;Inoue, S.;Asahina, Y.; Fushimi, H. U.S. Patent 4,762,763, 1988. (2) Kawagishi, Y.; Narita, S.;Kiriu, T.; Uomoto, K. U.S.Patent 4,656,112, 1987. (3) Kiriu, T.; Arakawa, M. US.Patent 4,845,003, 1985. (4) Kiuchi, T.; Maki, I. US. Patent 4,206,064, 1980. ( 5 ) Hashimoto, K.; Maruta, M.; Soyama, H.; Ishii, Y. U S . Patent 4,767,688, 1988. (6) Law, K. Y., J. IS% TProceedings, TheNinth International Congress on Advances in Non-Impact Printing Technologies, 1993; p 64. (7) Marsch, J. K. M. J. Chem. Soc. 1924,125,418.
. ". .
Law ana anonam (8) Weller, A. Z . Elekfrochem. 1956,60, 1144. (9) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. H. Discuss. Faraday Soc. 1%5,39, 183. (10) Klopfler, W.; Naundorf, G. J. Lumin. 1974, 8,457. (11) Kosower, E. M.; Dodiuk, H. J . Lumin. 1975/76, 11, 249. (12) Sandra, K. Acta Chcm. Scand. 1976, A30,761. (13) Acuna, A. U.; Amat-Guerri, F.; Catalan, J.; Gonzalez-Tablas, F. J. Phys. Chem. 1980,84,629. (14) Acuna, A. U.; Catalan, J.; Toribio, F. J. Phys. Chem. 1981,85,241. (15) Helmbrook, L.; Kenny, J. E.;Kohler, B. E.; Scott, G. W . J. Phys. Chem. 1983,87, 280. (16) Acuna, A. U.; Toribio, F.; Amat-Guerri, F.; Catalan, J. J. Photochem. 1985, 30, 339. (17) Toribio, F.;Catalan,J.;Amat, F.;Acuna,A.U.J. Phys. Chcm. 1983, 87, 817. (18) Lopez-Delgado, R.;Lazare, S . J. Phys. Chem. 1981,85, 763. (19) Klopffer, W. Adv. Photochem. 1977, 10, 311. (20) Smith, K. K.; Kaufmann, K. J. J. Phys. Chem. 1978,82,2286. (21) Goodman, J.; B m , L. E. 1.Am. Chem. Soc. 1978,100,7472. (22) Sanchez-Cabezudo, M.;De Paz, J. L.; Catalan, J. J. Mol. Struct. 1985, 131, 227. (23) Herek, J. L.; Pederwn, S.;Banares, L.; Zewail, A. H. J. Chem.Phys. 1992,97,9046. (24) Nagaoka, S.; Nagashima, U. Chem. Phys. 1989,136, 153. (25) Melhuish, W. H. J. Res. Natl. Bur. Stand. Sect. A 1972, 76, 547. (26) Meech, S.R.;Phillips, D. J. Photochem. 1983, 23, 193. (27) Demas, J. N.; Graby, G. A. J. Phys. Chcm. 1975,75,991. (28) We have examined the spcctral region between 330 and 360 nm very carefully and found that at this low concentration ,.,( 10-6 M), MSA only exhibitsa single long-wavelength emission. However, a small emission signal was detected from an aged solution or at higher concentrations. Chemical interaction between MSA and glassware (silica) has been noted previously.l8 (29) Law, K. Y., unpublished results. (30) Hou, S.Y.; Hetherington, W. M.; Korenowski, G. M.; Ewnthal, K. B. Chem. Phys. Lett. 1979,68,282. (31) Ford, D.;Thistlethwaite, P. J.; Woolfe,G. J. Chem.Phys. Lett. 1980, 69, 246. (32) Buraway, A.; Salem, A. G.; Thompson, A. R.J. Chem. Soc. 1952, 4793. (33) Benhtein, I. Ya.; Ginzburg, 0. F. Rurs. Chem. Rev. 1972,41, 97. (34) Ball, P.; Nicholls, C. H. Dyes Pigm. 1982, 3, 5 . (35) Mataunaga, Y.; Miyajima, N . Bull. Chem. Soc. Jpn. 1971,44,361. (36) Kuder, J. E. Tetrahedron 1972, 28, 1973. (37) Law,K.Y.;Kaplan,S.;Crandall,R.;Tamawskyj,I. W. Chem.Mater. 1993, 5, 557 and references therein. (38) Becker, R. S. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley Interscience: New York, 1969; p 42. (39) Felker, P. M.;Lambert, Wm. R.;Zewai1,A. H. J. Chem.Phys. 1982, 77, 1603. (40)Bergmann, E. D.; Hinhbery, Y.; Pinchas, S.J . Chem. Soc. 1950, 2355. (41) Nabikin, U. V.;Zadorozhnyi, B. A.; Pavlova, E. N. Opt. Spectrosc. 1959, 6, 312. (42). Turro, N. J. Modern Molecular Photochemistry; The Benjamin/ Cummngs Publishing Co.: Menlo Park, CA, 1978; p 183.
