J. Phys. Chem. 1995, 99, 12103-12108
12103
Photoinduced Proton Transfers in 3,5-Di-tert-butylsalicylicAcid Kock-Yee Law* and Jonatham Shoham Wilson Center for Research and Technology, Xerox Corporation, 800 Phillips Road, 0114-390, Webster, New York 14580 Received: March 30, 1995; In Final Form: June 9, 1995@
The origin of the long wavelength fluorescence emission (peak-to-peak Stokes shifts -9050-9450 cm-I) of 3,5-di-tert-butylsalicylic acid (t-BSA) has been investigated in a variety of organic solvents. Two types of emissions, I and 11, can be identified. The type I emission is observed in hydrocarbon and nonpolar solvents. It is a relatively weak fluorescence (& I 0.01), and the AF lies at -466 nm. The type I1 emission is relatively strong, and @f values ranging from 0.2 to 0.6 are obtained. It is observed in alcohols, acetate solvents, and acetonitrile. The IZF is at -430 nm. Mixed solvent experiments suggest that solvent molecules H-bond with t-BSA to form cluster complexes in these solvents and that these cluster complexes emit to give rise to the type I1 emission. While the fluorescence intensities of both types of emissions are quenched by acetic acid, the introduction of a trace amount of triethylamine is shown to enhance the type I emission (in cyclohexane) but not the type I1 emission (in ethanol). This observation, along with the similar fluorescence spectra between t-BSA and LitBSA (lithium 3,5-di-tert-butylsalicylate), suggests that the long wavelength emission of t-BSA originates either from the excited 3,5-di-tert-butylsalicylate anion or a species derived from it. The lifetime for t-BSA is significantly shorter than that of LitBSA in cyclohexane (0.7 versus 3-7 ns), and the short lifetime is attributable to the fluorescence quenching of the excited anion by reprotonation. Studies of the fluorescence of salicylic acid, methyl salicylate, 2-methoxybenzoic acid, and methyl 2-methoxybenzoate indicate that (1) the two tea-butyl groups in t-BSA have no effect on the fluorescence and (2) salicylic acid and methyl salicylate also exhibits similar long wavelength emissions. It is concluded from the structural effect study that an intramolecular proton transfer, from the OH group to the carboxyl function, is the prerequisite for the long wavelength emission. Since the long wavelength emission of methyl salicylate is shown to be from an excited enol tautomer, formed by a keto enol tautomerization in the excited state, we suggest that photoinduced tautomerism occurs in t-BSA too. t-BSA undergoes a very rapid deprotonation reaction upon photoexcitation. The excited 3,5-di-tert-butylsalicylateanion tautomerizes from the keto form to the enol form and then fluoresces to give the long wavelength emission. The similarity in photophysical behavior between t-BSA and 2zhydroxy-3-naphthoic acid is also discussed based on available literature data.
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Introduction Salicylic acid was shown to exhibit intense fluorescence in methanol with a very large Stokes shift (-10 000 cm-', peakto-peak).' In recent unrelated investigations, we and others found that 3,5-di-tert-butylsalicylic acid (t-BSA) and its derivatives, both metal salts and complexes, possess interesting triboelectrical properties. These materials are frequently used as charge control additives in dry xerographic toner at low concentrations (1-5% by weight) to control the charging characteristic of Due to the technological relevance, a systematic effort to study the fluorescence and the electronic structure of these materials has been initiated. One of our intentions is to explore the possibility of using fluorescence to study the physical location and the aggregational state of these materials in toner and to correlate the physical findings with triboelectrical performance. Another intention is to study the structure-triboelectricity relationship of these materials in toner. It is hoped that the knowledge gained would be beneficial to the design of future improved charge control additives and toners. Recently, we studied the fluorescence of t-BSA and its derivatives and found that they all exhibit moderate to strong fluorescence in solution. From the solvent effect and fluorescence lifetime data, we have been able to classify these materials as acid, salt, and chelate complex. Our ability to differentiate
* Author to whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, July 15, 1995.
0022-365419512099-12103$09.00/0
their electronic structure and draw correlation between the structure and the triboelectrical property has shed light into their tribocharging mechanism in toner.7 We also used the fluorescence of t-BSA to study the aggregation of t-BSA in toner. A correlation between aggregational state and tribocharging was observed.8 In this work, we report an investigation on the origin of the anomalously long wavelength emission of t-BSA. Studies of the effects of solvent, added acid and base, and structural changes on the long wavelength emission reveal that the primary photochemical reaction for excited t-BSA is to deprotonate to form the excited anion. The excited 3,5-di-tert-butylsalicylate anion, which is in the keto form, tautomerizes to the enol form before fluorescing to give the long wavelength emission. Solvent molecules are shown to facilitate these proton-transfer reactions in the form of t-BSNsolvent cluster complexes. The similarity in photophysical behavior between t-BSA and 2-hydroxy-3-naphthoic acid is discussed.
Experimental Section Materials. 3,5-Di-tert-butylsalicylic acid (t-BSA) was purchased from TCI America. It was purified by recrystallization (two times from n-heptane, three times from a mixture of methanol and water) before use. Lithium 3,5-di-tert-butylsalicylate (LitBSA) was synthesized by neutralizing t-BSA with LiOH in methanol. It was recrystallized from n-heptane (three times). Salicylic acid (SA) and 2-methoxybenzoic acid were 99% from Aldrich and were recrystallized three times from water. Methyl salicylate (99% +) and methyl 2-methoxyben0 1995 American Chemical Society
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12104 J. Phys. Chem., Vol. 99, No. 32, 1995
TABLE 1: Effect of Solvent on the Absorption and Fluorescence Emission of 3,5-Di-tert-butylsalicylicAcid type of solvent D" &,axb (log cm& dFd +f emission 467 0.009 I cyclohexane 2.02 324 (3.66) 467 0.013 I 324 methylcyclohexane 2.02 -466 f I 1,4-dioxane 2.21 -320 -466 f I chloroform 4.70 323 (3.65) 421 0.23 I1 n-butyl acetate 5.01 -316 ethyl acetate 6.02 316.5 (3.66) 434 0.18 I1 I tetrahydrofuran 7.32 317.6 (3.66) 466 f -466 f I methylene chloride 8.9 323 (3.65) acetonitrile 36.2 310 408 0.38 I1 methanol 32.0 308 (3.66) 435 0.58 I1 ethanol 24.3 307 (3.63) 432 0.56 I1 1-propanol 20.1 308 (3.66) 432 0.52 I1 1-butanol 17.8 307 (3.67) 433 0.52 I1 1-pentanol 13.9 308 (3.67) 434 0.42 I1 1-hexanol 13.3 313 (3.63) 436 0.42 I1 1-heptanol 313 435 0.38 I1 3-methyl-1-butanol 14.7 310 432 0.54 I1 2-propanol 18.3 308 (3.66) 429 0.59 I1 2-pentanol 13.8 310 (3.65) 426 0.57 I1 3-pentanol 13.0 318 (3.62) 435 0.54 I1 tert-amyl alcohol 5.82 312 435 0.58 I1 "Dielectric constant, data taken from Gordon, A. J.; Ford, R. A. The Chemist's Companion; Wiley: New York, 1972. Absorption maximum, in nm. Molar extinction coefficient, in cm-I M-I. Fluorescence emission maximum, in nm. e Fluorescence quantum yield, i1070.f Due to the interference by fluorescence impurities, +f could not be determined in these solvents. zoate (99%) were from Aldrich and were doubly distilled prior to use. Solvents were either of spectro grade from Fisher, of an analytical grade from Baker, or of the best commercial grade from Aldrich; they were routinely stored over 3 A molecular sieves before use. General Techniques. Absorption spectra were recorded on a Hewlett Packard 8451A 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 N2 for 30 min before use, and the spectral data were corrected using the quantum counter method with the RhlOl solution supplied by PerkinElmer.9 Quantum yields were determined in a corrected mode by comparison with the emission of 9,lO-diphenylanthracene in cyclohexane (#f = 0.93).'O A refractive index correction was made for each solution." Fluorescence lifetimes were determined using the time-correlated single photon counting technique on a LS-100-04 Fluorescence Lifetime System from Photon Technology Incorporated (PTI). Details of the procedures have been reported earlier.I2
Results Absorption. Unlike salicylic acid, which is soluble only in water and polar solvents, t-BSA is soluble in a variety of solvents ranging from hydrocarbons to alcohols due to the two solubilizing tert-butyl groups. While the solubility of t-BSA may be very high, it forms dimer and higher aggregates at concentrations higher than M. This is indicated by the deviation of the absorption spectra from the Beer's Law at concentrations higher than M. The strong tendency for t-BSA to form aggregates is attributable to the strong intermolecular H-bonding interactions between the salicylic acid groups.'~'~At [t-BSA] < M, t-BSA is generally present as a monomer. The effect of solvent on the monomeric absorption of t-BSA is summarized in Table 1. While the A,, of t-BSA is found to be solvent sensitive, the E,,,,, values appear
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Figure 1. Fluorescence excitation and emission spectra of t-BSA in cyclohexane ([t-BSA] 4 x M).
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Figure 2. Fluorescence excitation and emission spectra of t-BSA in ethanol ([t-BSA] 4 x M). to be independent of the recording solvent, 4400 f 300 cm-' M-I. In aprotic solvents, a hypsochromic shift on A,, is observed as the solvent polarity increases. In alcohols, the Amax is found to correlate primarily to the structure of the solvent molecule rather than the solvent polarity. For instance, in 1-alkanols from C1 to C5, the Amax of t-BSA is independent of the solvent dielectric constant at Amax -308 nm. The Amax tends to shift to longer wavelengths as the steric hindrance around the OH group increases. The specificity of the solvent effect is illustrated systematically in pentanols; it shifts from 308 nm in 1-pentanol to 310 nm in 2-pentanol to 318 nm 3-pentanol. The steric sensitivity suggests that the interaction between t-BSA and alcoholic solvent molecules is short range, presumably through the H-bonding between the OH group in the alcohol and the salicylic acid group in t-BSA. The solvent complex obviously stabilizes the ground state more than that of the excited state and leads to the hypsochromic shifts. The presence of t-BSNalcohol complexes is also substantiated by results obtained in the mixed solvent experiments described below. Fluorescence Emission. The fluorescence emission data of t-BSA in different solvents are included in Table 1. The emission bands can be classified into two types, I and 11. The type I emission is usually observed in hydrocarbons and weak aprotic solvents, such as lP-dioxane, THF, chloroform, and methylene chloride. A representation of the type I emission is given in Figure 1. The type I emission is usually weak ( A 0.01), and the AF lies at -466 nm. The fluorescence excitation is independent of the monitoring wavelength and is similar to the absorption band. The type I1 emission is observed in alcohols, ethyl acetate, n-butyl acetate, and acetonitrile. It is relatively intense, and the @f values range from 0.2 to 0.6. The AF lies between 420 and 460 nm, depending on the solvent. A typical type I1 emission is given in Figure 2. It is important to note that both types of emissions are at anomalously long wavelengths relative to the absorption bands. The Stokes shifts (peak-to-peak) for both types of emissions are similar, in the range -9050-9450 cm-I. Both types of emissions can be quenched by acids. For example, addition of a trace amount of acetic acid in solutions of t-BSA would cause
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Photoinduced Proton Transfers in t-BSA
J. Phys. Chem., Vol. 99, No. 32, 1995 12105
TABLE 2: Fluorescence Emission and Lifetime Data of t-BSA and Related Compounds salicylic 2-methoxybenzoic methyl t-BSA LitBSA acid methyl salicylatd acidh 2-methoxybenzoate solvent: ethanol cyclohexane ethanol cyclohexane ethanol ethanol methylcyclohexane ethanol ethanol -327 309 318 297 2998,309 -309 LEX' 308 295 467 432 466 404 3508,460 464 LFb 432 348 4t 0.58 0.009 0.67 0.006 0.54 0.012 0.017 < 10-4 0.11 stokesshiftd 9319 9168 9214 9987 8918 48738, 15,500 10,728 5162 ?Y 6.8 0.7 7.0 7.2 (57%) 5.9 1.28,0.27 0.36 1.2 3.1 (43%) ' Peak maximum of the excitation spectrum, in nm. Fluorescence maximum in nm. Fluorescence quantum yield, i10%. From peak-to-peak, in cm-]. e Lifetime, in ns. f Data taken from ref 12. g short wavelength emission. Due to the low fluorescence yield, spectral data could not be obtained accurately.
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Figure 3. Effect of ethanol on the absorption of t-BSA in cyclohexane M; [EtOH] = (i) 0, (ii) 0.11, (iii) 0.22-0.32, ([t-BSA] 4 x and (iv) 0.65-1.09 M). %
a significant reduction (>20 times) in fluorescence intensity. On the other hand, a different effect is observed with base. While very little or no effect is seen when a trace amount of triethylamine (TEA) is added to a dilute ethanol solution of t-BSA, an enhanced fluorescence is obtained in the cyclohexane solution. A more detailed study of the effect is given below. The fluorescence lifetimes of t-BSA have been studied in ethanol and cyclohexane. The decays in both solvents are monoexponential. While a long decay time of 6.8 ns is obtained in ethanol, the lifetime becames subnanosecond in cyclohexane, 0.7 ns (Table 2). Effect of Ethanol on the Absorption and Fluorescence of t-BSA in Cyclohexane. Figure 3 shows the effect of added ethanol on the absorption of t-BSA in cyclohexane. A hypsochromic shift on the Amax, accompanied by an isosbestic point at 320.5 nm, is observed at ethanol concentrations 10.32 M. At higher ethanol concentrations, the Amax continues to shift to the short wavelengths without going through the isosbestic point. In the emission spectra (Figure 4), a gradual change in fluorescence, from type I to type 11, is observed as [EtOH] increases. Initially, the type I emission remains unchanged when a small amount of ethanol ([EtOH] = 0.11 M) is introduced (curve a). At [EtOH] = 0.22 to < O S M, a gradual change in the emission spectrum and an increase in the fluorescence intensity take place (curve b). At [EtOH] 2 0.65 M, the fluorescence becomes type I1 and the effect of ethanol on the fluorescence intensity levels off (curve c). A plot of the relative +f of t-BSA as a function ethanol concentration is given in the inset in Figure 4. The observation of an isosbestic point in the absorption spectra at [EtOH] 5 0.32 M, along with the linear increase in +f and the gradual change in the type of the emission in the fluorescence spectra, suggests that t-BSA forms a 1: 1 complex with ethanol in this concentration range in cyclohexane. At [EtOH] 2 0.65 M, the absorption curves no longer pass through the isosbestic point. Concurrently, the emission becomes type
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Figure 4. Effect of ethanol on the fluorescence of t-BSA in cycloM; [EtOH] = (a) 0.11, (b) 0.22-0.32, hexane ([t-BSA] = 4 x and (c) 0.65- 1.09 M); inset, plot of relative +f as a function of ethanol concentration. I1 and the +f stays unchanged upon further addition of ethanol. We suggest that the t-BSAIEtOH cluster complexes are formed under these conditions. t-BSA is solvated by a cluster of ethanol molecules, providing a local environment analogous to that in pure ethanol. The cluster-complex model is supported by the steric effect observed on Amax and & in alcohols. Specifically, Amax shifts systematically to longer wavelengths as the steric hindrance around the OH increases. The +f value of t-BSA is the lowest in 1-heptanol among all alcoholic solvents studied. The steric sensitivity suggests that the OH group in the alcohol interacts with t-BSA in the cluster complex; the stronger the complexation, the higher the +f value. As shown in Table 1, t-BSA also exhibits type I1 emission in ethyl acetate, butyl acetate, and acetonitrile. We have also examined the effect of these solvents on the type I emission in cyclohexane, and results analogous to those in Figures 3 and 4 are obtained. We suggest that these solvents also form cluster complexes with t-BSA, presumably also through intermolecular H-bonding. This hypothesis is supported by the absorption data where the A,, is blue-shifted in these solvents relative to that in cyclohexane. The generally low +f values obtained in these solvents indicate that the complexation is probably not as strong as that in alcohols. Effect of TEA on the Absorption and Fluorescence of t-BSA in Cyclohexane. Figure 5 shows the effect of a trace amount of TEA on the absorption of t-BSA in cyclohexane. The concentration of t-BSA is -3.6 x M. t-BSA is a strong acid, and its pKa can be estimated to be ~ 2 . 9 7 . Thus '~ even at the lowest TEA concentration in our experiment (1.9
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12106 J. Phys. Chem., Vol. 99, No. 32, 1995 ,015 I
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e .006 4
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Figure 7. Fluorescence excitation and emission spectra of LitBSA in cyclohexane (concentration 4 x M).
Figure 5. Effect of TEA on the absorption of t-BSA in cyclohexane (iii) 5.7-9.5 M; [TEA] = (i) 0, (ii) 1.9 x ([t-BSA] 3 x x lo+, (iv) 1.9 x and (v) 3.8-7.6 x M).
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Figure 8. Fluorescence excitation and emission spectra of LitBSA in ethanol (concentration 4 x M).
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Figure 6. Effect of TEA on the fluorescence of t-BSA in cyclohexane (b) 5.7 x 10-5-7.6 M; [TEA] = (a) 1.9 x ([t-BSA] 3 x x M, and (c) 1.5 x lo-* M); inset, plot of relative q& as a function of TEA concentration. x M), t-BSA should be present as a salt. The absorption spectra in Figure 5 are then attributable to the 3,5-di-terrbutylsalicylate anion. The drastic "A shifts from -324 to 318 nm when TEA is first introduced are consistent with the structural assignment. After all, the Amax of LitBSA is found to be at 318 nm too.7 As the concentration of TEA is increased M), a small spectral shift is observed. The (up to I1.9 x most important observation is the isosbestic point at -320 nm. Figure 6 (curve a) shows the emission spectrum of t-BSA in M) in the presence of TEA (1.9 x cyclohexane (-3.6 x M). The spectrum, while it is different from that in Figure 1, is similar to those obtained at higher TEA concentrations (I 1.9 x M, Figure 6 curve b). The relative & increases linearly as [TEA] increases up to [TEA] = 1.9 x M and levels off at higher TEA concentrations (inset in Figure 6). From this result and the isosbestic point observed in the absorption spectra, we attribute the fluorescence observed at [TEA] I1.9 x M to the 1:1 complex between the 3,5-dibutylsalicylate anion and TEA. As [TEA] increases, more 1:l complexes are formed. At [TEA] 1.9 x M, all 3,5-di-tert-butylsalicylate anions are complexed. It is worth noting that upon further increase in TEA concentration, the t-BSA solution becomes yellowish. This yellowing has made quantification of the emission data difficult. Nevertheless, we find that the emission further change to type II (Figure 6 , curve c), suggesting that
larger cluster complexes between the 3,5-di-tert-butyl salicylate anion apd TEA are formed under these conditions. Structural Effect Studies. Lithium 3,s-Di-tert-butylsalicylate (LitBSA). Figures 7 and 8 show the fluorescence excitation and emission spectra of LitBSA in cyclohexane and ethanol, respectively. The spectral results are summarized in Table 2 along with the measured fluorescence lifetimes. The spectral characteristics of LitBSA, namely (1) exhibiting type I emission in cyclohexane and type I1 emission in ethanol, (2) the fluorescence quantum yields, and (3) the Stokes shifts, are found to be very similar to those of t-BSA. The only notable difference is their lifetimes in cyclohexane. The fluorescence decay for LitBSA is biexponential with decay times of 3.1 and 7.2 ns,I5 significantly longer than the lifetime of t-BSA in cyclohexane (0.7 ns). The biexponential decay may be attributable to the existence of two rotamers of LitBSA in cyclohexane, a phenomenon that is quite common for salicylic acid derivative^.'^,'^-^^ The similarity in fluorescence spectra between t-BSA and LitBSA suggests that the same species is responsible for the long wavelength emission in both compounds. We propose that that species is the 3,s-di-tertbutylsalicylate anion. The subnanosecond lifetime for t-BSA is simply a matter of fluorescence quenching by reprotonation of the excited anion. The fact that the fluorescence of t-BSA is quenched by added acetic acid and that the lifetime of LitBSA in cyclohexane is decreased when it picks up water from the ambient supports the c o n c l ~ s i o n . ~ ~ Salicylic Acid, 2-MethoxybenzoicAcid, Methyl Salicylate, and Methyl 2-Methoxybenzoate. It is known in the literature that phenoates and naphthoates emit at longer wavelengths relative to phenols and naphthol^.'.*^-^^ In the case of salicylic acid, Weller demonstrated that the OH group adjacent to the carboxylic acid group is intimately involved in producing the long wavelength emission.' In this work, we re-examine the fluorescence of salicylic acid, methyl salicylate, 2-methoxybenzoic acid, and methyl 2-methoxybenzoate briefly. The emission data are summarized in Table 2, along with the measured lifetimes.
J. Phys. Chem., Vol. 99, No. 32, 1995 12107
Photoinduced Proton Transfers in t-BSA
Wawlength (nm)
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Figure 9. Fluorescence excitation and emission spectra of salicylic acid in ethanol (concentration 4 x M).
because of the stabilization resulting from H-bonding interactions with solvent molecules. Thus excitation of methyl salicylate in alcohol would lead to excited a and b. Excited a fluoresces and gives rise to an emission band with fluorescence property and lifetime very similar to that of methyl 2-methoxybenzoate. On the other hand, excited b is shown to tautomerize, from the keto tautomer to the enol tautomer, before fluorescing to generate the long wavelength emission. It is important to note that the solvent effect on the long wavelength emission of methyl salicylate is analogous to those presented in Figures 1 and 2. The similarity suggests that the emitting states for the long wavelength emission bands of t-BSA and methyl salicylate are very similar.
Discussion
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Figure 10. Fluorescence excitation and emission spectra of methyl 2-methoxybenzoate in ethanol (concentration M). In agreement with Weller's spectrum, we find that salicylic acid indeed exhibits an anomalously long wavelength emission (Figure 9). The Stokes shift is 8918 cm-I. The steady state and fluorescence lifetime data are similar to those of t-BSA (Table 2), indicating that the two tert-butyl groups have practically no effect on the fluorescence. The intention of studying the fluorescence of 2-methoxybenzoic acid, methyl salicylate, and methyl 2-methoxybenzoate is to identify the proton that is involved in the long wavelength emission of salicylic acid. While the fluorescence of 2-methoxybenzoic acid is very weak (& < a moderate fluorescence is observed for methyl 2-methoxybenzoate. The fluorescence emission of methyl 2-methoxybenzoate forms a mirror-image relationship with the absorption spectrum, and the Stokes shift is 5162 cm-' (Figure 10). The lifetime is 1.2 ns in ethanol, significantly shorter than that of t-BSA. We have also studied the fluorescence and the lifetime of methyl 2-methoxybenzoate in cyclohexane and found that neither the steady state spectrum or the lifetime is solvent sensitive. The data indicate that the emitting states between t-BSA and methyl 2-methoxybenzoate are markedly different. The spectroscopic property of methyl salicylate was recently revisited by us.12 The data are included in Table 2. In cyclohexane or methylcyclohexane, methyl salicylate exhibits an anomalously long wavelength emission (& = 464 nm) with spectral characteristics (Stokes shift and shape of the emission band) very similar to the type I emission of t-BSA. In ethanol, a dual fluorescence is observed. The mechanism of the dual emission is ascribable to a conformational effect, originating from ground state rotamers a and b. In nonpolar solvents,
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rotamer b is thermodynamically preferable due to the intramolecular H-bonding. In alcohols, rotamer a becomes abundant
Mechanism of the Long Wavelength Emission of t-BSA. The similarity in fluorescence spectra between t-BSA and LitBSA (Figures 1,2,7, and 8) and the fluorescence quenching effect by acetic acid suggest that the excited 35-di-tertbutylsalicylate anion is the precursor for the long wavelength emission if it is not the emitting species itself. This proposition is supported by the fluorescence lifetime data. The lifetimes for t-BSA and LitBSA in cyclohexane are quite different; the significantly shorter lifetime for t-BSA is attributable to the quenching of the excited anion by reprotonation. The remarkable resemblance in spectral properties between t-BSA and salicylic acid, in term of Stokes shifts and lifetimes (Figures 2 and 9, Table 2), indicates that the two tert-butyl groups have no effect on the fluorescence of t-BSA. Structural effect studies on methyl salicylate, 2-methoxybenzoic acid, and methyl 2-methoxybenzoateconfirm Weller's conclusion that the OH group ortho to the carboxylic acid group is intimately involved in a proton-transfer process that leads to the long wavelength fluorescence. In the case of methyl salicylate, we have previously shown that the long wavelength emission is from an excited enol tautomer resulting from an intramolecular proton-transfer reaction in the excited keto tautomer.'* We suggest that a similar photoinduced proton-transfer reaction is occurring in the excited 3,5-di-tert-butylsalicylate anion. In other words, we propose that excitation of t-BSA results in a facile deprotonation reaction to form the excited 3,5-di-tertbutylsalicylate anion. The so-generated anion, which is in the keto form, tautomerizes to the enol form by transfemng a proton from the OH group to the C02- group before emitting to give rise the long wavelength emission. The photophysics of t-BSA are summarized in Figure 11. When t-BSA is solvated in the form of a cluster complex, such as in alcohols and acetates, the fluorescence becomes highly efficient. The high fluorescence quantum yield is attributable to the facilitation of the photoinduced proton-transfer reactions, both deprotonation and keto enol tautomerization, in the solvated cluster. Similar enhancements of proton-transfer reactions have been observed in other cluster complexes, such as between hydroxyalkylnaphthols and alcohols2*and between naphthols and ammonia m ~ l e c u l e s . * ~ - ~ ~ It is worth pointing out that the type I1 emission is slightly blue-shifted relative to the type I emission. The blue shift may be attributable to the stabilization of the ground state enol tautomer in the solvent cluster. A very similar solvent-induced blue shift was observed for the enol tautomer of methyl 2-hydroxy-3-naphthoate, which was also generated by a photoinduced tautomerization process.'* The proton-transfer reactions of salicylic acid and methyl salicylate attracted considerable theoretical i n t e r e ~ t . ~ *Catalan -~~ and Tomas calculated by the CND0/2-CI method that the lowest excited singlet for salicylic acid is a z,z* state, which undergoes an intramolecular proton-transfer reaction to form the zwitter i ~ n On . ~ the ~ other hand, Orttung et al.35showed by the CNDO +
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shifts, -9050-9450 cm-’. In hydrocarbon solvents, the fluorescence is relatively weak. Results from solvent effect, structural effect, and acidbase effect experiments suggest that the excited state of 3,5-di-terf-butylsalicylic acid deprotonates rapidly to form the excited anion, which tautomerizes from the keto tautomer to the enol tautomer and then fluoresces to give the anomalously long wavelength emission. In hydrocarbon or nonpolar solvents, either due to the slow deprotonation or the facile reprotonation of the excited anion or both, the fluorescence yield is very low. In solvents that form solvated clusters with t-BSA, both deprotonation and tautomerization reactions are facilitated. The solvent clusters also stabilize the enol tautomer. As a result, the fluorescence is very intense and the emission band is slightly blue-shifted relative to that observed in hydrocarbons.
I
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References and Notes !-Bu
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Figure 11. A schematic for the photophysics of t-BSA.
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and INDO semiempirical MO methods that the lowest excited singlets for salicylic acid and methyl salicylate involve n n* transitions. While both the ground- and excited-state surfaces of salicylic acid and methyl salicylate were shown to be very similar, the intramolecular proton transfer in methyl salicylate was calculated to be thermodynamically favorable. In this work, we show that the salicylate anion is the precursor for the photoinduced proton-transfer reaction. It is intuitively expected that the intramolecular proton transfer should be more efficient because of the negative charge in the carboxylate anion, although theoretical calculation on the proton-transfer reaction of the anion remains to be investigated. PhotoinducedProton Transfers in 2-Hydroxy-3-naphthoic Acid. In 1971, Ware and c o - ~ o r k e r reported s~~ a fluorescence study of 2-hydroxy-3-naphthoic acid. In toluene, in addition to the normal fluorescence at j l ~= 410 nm, a second long wavelength emission band at around 500-560 nm becomes discernible when pyridine is added into the toluene solution. The long wavelength emission band was also observed in acetonitrile and N-methylformamide in the absence of pyridine. These authors went on to demonstrate that the long wavelength emission of 2-hydroxy-3-naphthoic acid could be turned off when the OH group is methylated. They proposed that pyridine is the proton acceptor for the excited acid. The excited 2-hydroxy-3-naphthoate anion undergoes an intramolecular proton-transfer reaction to form a zwitterion before emitting to give the long wavelength emission. On the other hand, we recently showed by temperature and solvent effect studies that photoinduced tautomerism, from the keto form to the enol form, occurs in methyl 2-hydroxy-3-na~hth0ate.I~ The similar spectral region between the long wavelength emissions of 2-hydroxy3-naphthoic acid and methyl 2-hydroxy-3-naphthoate suggests that tautomerism should occur in the excited state of 2-hydroxy3-naphthoic acid too. In view of the need of deprotonation for the long wavelength emission of 2-hydroxy-3-napthoic acid, we suggest that the photophysics between 2-hydroxy-3-naphthoic acid and t-BSA are similar.
Concluding Remarks We report in this work that 3,5-di-tert-butylsalicylic acid fluoresces at an anomalously long wavelengths with large Stokes
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