Photoinduced Double Proton Tautomerism in 4-Azabenzimidazole

Excited-State Double Proton Transfer in 3-Formyl-7-azaindole: Role of the nπ State in Proton-Transfer Dynamics. Pi-Tai Chou, Guo-Ray Wu, Ching-Yen We...
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J. Phys. Chem. B 1999, 103, 10042-10052

Photoinduced Double Proton Tautomerism in 4-Azabenzimidazole Pi-Tai Chou,*,† Guo-Ray Wu,† Ching-Yen Wei,† Chung-Chih Cheng,‡ Chen-Pin Chang,‡ and Fa-Tsai Hung§ Department of Chemistry, The National Chung-Cheng UniVersity, Chia Yi, Taiwan R.O.C., Department of Chemistry, Fu Jen Catholic UniVersity, Shin Chuang, Taiwan R.O.C., and The National Hu-Wei Institute of Technology, Yunlin, Taiwan R.O.C. ReceiVed: June 3, 1999; In Final Form: August 6, 1999

The proton-transfer tautomerism of 4-azabenzimidazole (4ABI) mediated by hydrogen bonding formation has been studied in the ground as well as in the excited state by means of absorption and emission spectroscopies. Thermodynamics of self-association and hydrogen-bonded complexes in nonpolar solvents were obtained. Proton-transfer isomers of 4ABI have been determined by syntheses and spectral characterization of various 4ABI methyl derivatives. The 4ABI dimer and 1:1 4ABI/acetic acid complex possessing cyclic dual hydrogen bonds undergoes a fast excited-state double proton-transfer reaction, resulting in a protontransfer tautomer emission. Surprisingly, however, the ESDPT is prohibited in the 4ABI/2-azacyclohexanone cyclic hydrogen-bonded complex. The results render the conclusion that photoinduced double proton transfer in the 4ABI hydrogen-bonded complex can be fine-tuned by its associated guest molecule, as further supported by the molecular modeling as well as ab initio calculations.

1. Introduction The 7-azaindole (7AI, see Figure 1) hydrogen-bonded complex has long been recognized as a simplified model for the hydrogen-bonded base pair of DNA.1-3 It has been widely accepted that the excited-state double proton transfer (ESDPT) occurs through a self-catalysis or solvent (e.g., alcohol) catalysis mechanism in 7AI hydrogen-bonded complexes.4-19 Spectroscopically, the change of UV-vis absorption spectra associated with the hydrogen-bonding formation has been used as a tool to obtain thermodynamics of the 7AI dimer and hydrogenbonded complexes. In a diluted solution (310 nm is mainly attributed to the absorption of the 7AI dimer and/or 7AI/guest complex. For the case of the 7AI/acetic acid complex, a dimerization constant as large as 2.2 × 104 M-1 is deduced in cyclohexane.12 Upon excitation, the cyclic hydrogen-bonded dimer (or complex) undergoes a double proton-transfer reaction, resulting in an unusually large Stokes-shifted tautomer emission (λmax ∼ 480 nm). At the molecular level, such a simple process provides one possible mechanism for the mutation that has been proposed to be, in part, due to a “misprint” induced by the proton-transfer tautomerism of a specific DNA base pair during replication, recording an error message.3,17,19,20 Further extension of the 7AI research on the subject of the proton transfer inducing mutation requires the study of the proton-transfer reaction in 7AI analogues of biological importance, among which purines possessing a similar electronic moiety with respect to 7AI are particularly well suited. The physicochemical properties of purines and their biologically important derivatives have received considerable attention * To whom correspondence should be addressed. † The National Chung-Cheng University. ‡ Fu Jen Catholic University. § The National Hu-Wei Institute of Technology.

from both experimental and theoretical approaches due to their importance in understanding many fundamental biochemical processes.21-36 The reactivity of the biologically relevant purines may depend on various structures existing in different environments. For the case of purine, extensive studies have been performed to investigate the tautomerism in the imidazole ring (see Figure 1). Purine has been found to exist in the crystalline phase as an N(7)H tautomeric form22 (see Figure 1), while the N(9)H tautomeric form is the highly dominant one in the gas as well as in the inert gas isolated matrixes.22-26,34 In polar solvents purine exists as mixtures of N(7)H and N(9)H tautomers.27-31,33 For the case of adenine, a temperature-jump study estimated a tautomeric equilibrium CN(9)H/CN(7)H of 0.28 with an enthalpy difference of 0.2 kcal/mol between N(9)H and N(7)H tautomers.30 However, the tautomeric reaction regarding the pyridinal ring in which the proton migrates to the N(3) position, i.e., the formation of the N(3)H tautomeric form (see Figure 1) in purines, has not yet been reported. Recently, we have applied 7AI, 6-isobutylpurine (6IBP), and 4-azabenzimidazole (4ABI, see Figure 1) in which the N(1) nitrogen in purine has been replaced by a carbon atom to investigate their corresponding proton-transfer tautomerism (note that N(3) in purine has been renamed to N(4) in 4ABI). In contrast to the prominent proton-transfer tautomer emission in both 7AI and 4ABI dimers, ESDPT is prohibited in the case of 6IBP dimer. The preliminary results lead us to propose a molecular-based tuning proton-transfer tautomerism in the excited state.37 In this study, detailed absorption and emission spectroscopies as well as ESDPT dynamics were performed in various 4ABI hydrogenbonded complexes. Various proton-transfer isomers of 4ABI have been determined through syntheses and spectral characterization of their corresponding methylated derivatives. The results render the conclusion that photoinduced double proton transfer in the 4ABI hydrogen-bonded complex can be finetuned by its associated guest molecule, as further supported by the molecular modeling as well as ab initio calculations.

10.1021/jp9918095 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/22/1999

Double Proton Tautomerism in 4-Azabenzimidazole

J. Phys. Chem. B, Vol. 103, No. 45, 1999 10043

Figure 1. Structures of 7AI, purine, 4ABI, and their corresponding proton-transfer isomers as well as methylated derivatives.

2. Experimental Section 2.1. Materials. 4ABI (Aldrich 99%) was contaminated by an orange impurity and was purified by column chromatography, where ethyl acetate was used as an eluent, followed by twice recrystallization from benzene. The purified 4ABI revealed a white needlelike crystal of which the purity was checked by the fluorescence excitation spectrum in cyclohexane under a sufficiently low concentration of 310 nm upon increasing concentration can be exclusively attributed to the dimeric form, the absorption spectra between the monomer and dimer in the case of 4ABI are largely overlapped. As a result, no specific wavelength can be solely attributed to the selfassociated species, and one may not simply incorporate the Beer-Lambert law in combination with the Benesi-Hiderbrand derivation reported in ref 12 to solve Ka. Alternatively, the absorbance of the mixture in any wavelength as a function of initially prepared C0 can be expressed by

C0 1 1 4 ) + A - MC0 Ka(D - 2M) C0 D - 2M

(1)

Detailed derivations of eq 1 are described in the Appendix. In the above equation M and D are molar extinction coefficients of the monomer and dimer monitored at a specific wavelength, e.g., 292 nm, where an appreciable percentage of absorbance can be attributed to the self-associated species (see Figure 2). In this study, under a sufficiently low concentration of 4ABI where only monomer exists, 292 M was measured to be 1780 M-1 cm-1. A plot of C0/(A - MC0) at 292 nm as a function of 1/C0 shown in the inset Figure 2 reveals a sufficiently linear behavior, supporting the assumption of dimeric formation in 4ABI. Accordingly, by fixing the M value, the best linear leastsquares fit using eq 1 to Figure 2 gives 292 M and Ka values to be 5250 M-1 cm-1 and 3.3 × 103 M-1, respectively. A Ka value of 3.3 × 103 M-1 corresponds to a ∆G° value of -4.9 kcal/ mol at 305 K. An attempt to measure Ka as a function of temperature in order to extract the enthalpy of association, ∆Hac, unfortunately failed due to the sparse solubility of 4ABI in CCl4 at low temperature. Nevertheless, a detailed discussion of ∆Hac based on a theoretical approach and its correlation with respect to the hydrogen-bonding strength will be discussed in the section on theoretical calculations. 4ABI/Guest Hydrogen-Bonded Complexes. Figure 4 shows the absorption spectra of 4ABI upon adding the acetic acid (ACID) in cyclohexane. In this experiment, the initial concentration of 4ABI, C0, was prepared to be as low as 5.0 × 10-6 M to avoid self-dimerization. The formation of 4ABI/ACID hydrogen-bonded complexes can be clearly shown by the growth of an absorption band at ∼292 nm throughout the titration, accompanied by the appearance of an isosbestic point located at ∼278 nm, indicating the existence of an equilibrium with a common intermediate. The 4ABI hydrogen-bonded complex incorporating stoichiometrically equivalent ACID molecules can be depicted as Ka

4ABI + ACID y\z 4ABI/ACID Although the absorption spectrum of 4ABI/ACID exhibits significantly different spectral features with respect to that of the 4ABI monomer, similar to that observed in the selfassociation of 4ABI, there is no specific wavelength at which the absorbance can be exclusively attributed to the 4ABI/ACID complex. This makes the derivation of Ka based on a simple Benesi-Hildebrand plot unfeasible.12 Alternatively, the relation-

Figure 4. Concentration-dependent absorption spectra of 4ABI (5.0 × 10-6 M) in cyclohexane by adding various ACID concentrations (Cg) of (a) 0, (b) 1.4 × 10-6, (c) 2.8 × 10-6, (d) 7.1 × 10-6, (e) 1.0 × 10-5, (f) 2.0 × 10-5, (g) 2.8 × 10-5, (h) 4.3 × 10-5, and (i) 5.7 × 10-5 M. Inset: Plot of A0/A0 - A at 292 nm as a function of 1/[Cg] in curves b-i and a best least-squares fitting curve using eq 2.

ship between the measured absorbance as a function of the initially prepared ACID concentration, Cg, can be expressed by

(

)[

]

A0 M 1 ) +1 A0 - A M - C KaCg

(2)

where M and C are molar extinction coefficients of the 4ABI monomer and hydrogen-bonded complex monitored at a specific wavelength, respectively. A detailed derivation of (2) has been elaborated in the Appendix. The inset of Figure 4 shows the plot of A0/(A0 - A) as a function of 1/Cg at a selected wavelength of 292 nm. Straight-line behavior supports the validity of the assumption of 1:1 4ABI/ACID formation, and a best linear least-1 squares fit using eq 2 deduces 292 C and Ka to be 3650 M cm-1 and 1.2 × 105 M-1, respectively. Appreciable hydrogenbonding association was also observed between 4ABI and LACTAM (see Figure 8), the value of A0/(A0 - A) as a function of 1/Cg also exhibits a straight-line behavior, indicating the formation of a 1:1 4ABI/LACTAM hydrogen-bonded complex. The best linear least-squares fit using eq 2 gives a Ka value of 5.4 × 103 M-1 in cyclohexane. 3.2. Excited-State Spectroscopy and Dynamics. 4ABI SelfAssociation. Table 1 lists the steady-state spectral properties of absorption and emission as well as relaxation dynamics for 4ABI and its hydrogen-bonded species in various nonpolar solvents. When the concentration of 4ABI was prepared as low as 7.0 × 10-6 M in CCl4, a weak, normal Stokes-shifted emission (Φf ∼ 0.012) was observed with a peak maximized at 308 nm. The lifetime was measured to be 0.32 ( 0.03 ns and was independent of the excitation as well as the monitored emission wavelength. Therefore, at a sufficiently low concentration of 4ABI in CCl4 only one emitting species exists, which is unambiguously assigned to the N(9)H monomer. The steady-state fluorescence spectra as a function of the 4ABI concentration are shown in Figure 5. When the concentration was increased so that both 4ABI and the 4ABI dimer existed, dual fluorescence was observed, consisting of a normal emission band (the F1 band, λmax ∼ 310 nm) followed by a large Stokes-shifted emission maximum at ∼380 nm (the F2 band). The excitation spectrum monitored at the longwavelength region of the F2 band (e.g., 420 nm) is red shifted by ∼5 nm with respect to that monitored at the short-wavelength

10046 J. Phys. Chem. B, Vol. 103, No. 45, 1999

Chou et al.

TABLE 1: Thermodynamic and Photophysical Properties of 7AI, 4ABI, and Their Methylated Derivatives in Cyclohexane and CCl4 absorption (nm)

emission (nm)

Φf

τf (ns)

7AI 7AI dimer

(287, 293sh) (292, 305sh)d

320 480

0.22b 0.016

1.68a, 1.72b 2.82,a 3.0c

7MAI

(390, 458sh)

(463sh, 495, 510)

1.54, 1.67b

4ABI

(281, 287) (282, 287)a (285, 292)d (285, 292)a (285, 292)

305 308a 380 380a 390

0.0056, 0.0066a 0.0073b 0.038 0.012a 0.05 0.06a 0.03

(283, 292) (284, 291) (284, 292)a (284, 291) (284, 292)a (292, 329, 368sh) (292, 330, 368sh)a

310 306 310a 307 310a (372sh, 400) (375sh, 400)a

4ABI dimer 4ABI/ACID 4ABI/ACH 9MABI 7MABI 4MABI

0.07 0.014 0.020a 420 nm of the F2 band since the emission intensity attributed to the F1 component is negligible. In this case, a wavelength-independent singleexponential decay was detected (τf ∼ 2.04 ( 0.08 ns; see inset of Figure 5). Similarly, the F1 emission (e.g., 310 nm) free from the interference of the F2 band was monitored, a singleexponential decay with τf ) 0.32 ( 0.05 ns was obtained. In the spectral overlapping region between 350 and 400 nm the decay can only be well fit by a double-exponential decay that is theoretically expressed as

F(t) ) A1e-k1t + A2e-k2t where A1 and A2 are the emission intensity at t ∼ 0 for the decay components 1 and 2, respectively. While the ratio for A1 versus A2 was concentration as well as excitation-wavelength dependent, k1 and k2 were found to be constant, with a fast and a slow component of (2.8 ( 0.1) × 109 s-1 (τ ∼ 0.36 ns) and

(0.5 ( 0.05) × 109 s-1 (τ ∼ 2.0 ns), which within experimental error are essentially identical with those of the F1 (e.g., monitoring at 310 nm) and the F2 (monitoring at 420 nm) bands, respectively. The results unambiguously conclude that only two emitting species exist in the entire spectral region (300-500 nm). Since both emission bands exhibit system-response rise time (i.e., 420 nm exhibit single-exponential behavior with a lifetime τf of 1.76 ns, while double-exponential decay rates of 0.35 and 1.76 ns were obtained when monitored at the overlapping region between the F1 and F2 bands. Similar to that observed in the 4ABI dimer, the rise time for both components cannot be resolved by our current photon counting system. Thus, it is quite unlikely that one species is the precursor of the other; i.e., the dual fluorescence results from different ground-state precursors, the 4ABI monomer and 4ABI/ACID complex. Since the F1 band has been ascribed to the monomer emission, the precursor of the F2 band can thus be unambiguously associated with the 4ABI/ACID hydrogen-bonded complex. However, due to the high acidity of the carboxylic proton (pKa ) 4.75) and a drastic increase of the basicity in the excited pyridinal nitrogen (∆pKa(NH+) ) pKa(NH+) - pKa*(NH+) ) -7.86 in 4ABI49), the 390 nm band may result from proton transfer of the carboxylic proton to the 4ABI pyridinal nitrogen, giving rise to a 4ABI cationic (or 4ABI/ACID ion pair) emission. To examine such a possibility 9MABI was examined. The formation of a hydrogen-bonded complex between 9MABI and acetic acid is indicated by the ACID concentration-dependent titration spectra (see Figure 7). However, The Ka value of 2.2 × 102 M-1 (deduced from eq 2) for the 9MABI/ACID complex formation is smaller than that calculated for 4ABI/ACID by 2 orders of the magnitude. Such a difference may not be surprising because theoretically only single hydrogen-bonded 1:1 9MABI/ACID complex can be formed. Upon an increase in acid concentration both the absorbance and emission intensity of 9MABI decrease, accompanied by a slight red shift of the emission maximum from 305 to 308 nm. The lack of observing any large Stokes-

J. Phys. Chem. B, Vol. 103, No. 45, 1999 10047

Figure 7. Absorption and emission spectra of 9MABI as a function of acetic acid concentration of (a) 0, (b) 8.5 × 10-4, (c) 1.7 × 10-3, (d) 3.8 × 10-3, and (e) 7.7 × 10-3 M. Inset: Plot of A0/A0 - A at 292 nm as a function of 1/[Cg] in curves b-e and a best least-squares fitting curve using eq 2.

shifted emission in the 9MABI/ACID complex discounts the assignment of the 390 nm emission simply to the excited-state protonation of 4ABI. Alternatively, the same spectral features and relaxation dynamics between the F2 band of the 4ABI/ACID complex and the 4MABI emission lead us to conclude the occurrence of excited-state double proton transfer (ESDPT) in the 4ABI/ACID complex, resulting in an N(4)H tautomer emission. Mechanistic details regarding the proton-transfer dynamics will be elaborated in the Discussion. Accordingly, the relationship between the initially prepared ACID concentration, Cg, and the measured emission intensity in any wavelength selected can be expressed in eq 3

(

)

F0 ΦMM 1 +1 ) F0 - F (ΦMM - Φpp) KaCg

(3)

(see the Appendix for a detailed derivation) where F0 and F denote the measured fluorescence intensity prior and after adding ACID. ΦM and Φp are fluorescence quantum yields of the monomer and complex, respectively. The plot of F0/(F0 - F) versus 1/Cg shown in the inset of Figure 6 exhibits good linear behavior, supporting the assumption of the 1:1 complex formation. A best linear least-squares fit gives the slope and intercept to be -1.2 × 10-5 M and -1.52, respectively. Consequently, a Ka value of 1.3 × 105 M-1 was obtained, which is consistent with the value of 1.2 × 105 M-1concluded in the absorption concentration-dependent study. The formation of a 1:1 4ABI/LACTAM hydrogen-bonded complex has also been verified in the absorption titration study. Since LACTAM also possesses bifunctional hydrogen-bonding groups, it is reasonable to expect the 4ABI/LACTAM complex to exhibit the same photophysical properties, i.e., the double proton transfer in the excited state, as that of 4ABI/ACID. In contrast, increasing the LACTAM concentration results in an increase of the emission intensity maximum at 310 nm, which is red shifted only by ∼5 nm with respect to the monomer emission. Whereas, within the detection limit, the N(4)H tautomer emission expected to be in the region of 380-500 nm was not obserVed (see Figure 8). Note that for the cases of 3-hydroxyisoquinoline (3HIQ) and 7AI both 3HIQ/LACTAM50 and 7AI/LACTAM12c,d complexes undergo fast ESDPT dynamics, resulting in a prominent tautomer emission. To avoid any trace of tautomer emission hidden under the dominant normal

a Total energy, enthalpy, and free energy in hartrees. b ∆H was calculated by subtracting the formation enthalpy of host and guest molecules from their associated hydrogen-bonded complexes. c ∆G(1) ac and ∆G were calculated by the difference in free energy between the hydrogen-bonded complex and its associated guest and host molecules in their most stable forms, in which the free energy is normalized to 0. For example, ∆G(N(4)H/LACTIM) ) G(N(4)H/LACTAM) - G(N(9)H) - G(LACTAM).

23.655 0.801 7.537 Relative Free Energy in Solution (Solvent ) Cyclohexane) (kcal/mol) 0 14.763 1.57 17.234 -4.532 0 ∆Gc 0 3.552 10.312 ∆G (N(4)Hdimer - N(9)Hdimer) ) 15.664 ∆G (N(4)H/ACID - N(9)H/ACID) ) 12.07 ∆G (N(4)H/LACTIM - N(9)H/LACTAM) ) 22.854

-12.086 8.564 -12.568 -14.290 -8.177 -2.025 -12.568 -14.094 Semiempirical Solvation Free Energy (kcal/mol) -6.611 -4.288 -14.317 -15.966 -6.611 8.112 -15.390 0.274 -1.082 -1.082 -10.768 1.832 -9.447 -4.928 PM3SM4(2) -8.480 ∆G((1) + (2)) -8.480

20.65 -1.722 6.152 -1.526 0 0 12.60 4.519 0 ∆G(1)c

-15.197 -11.948 -17.266 -13.163 -20.09

Relative Free Energy in Gas Phase (kcal/mol) 12.44 -1.073 16.24

Association Energy ∆Hac (kcal/mol) -11.583

-717.40588 -717.14639 -717.20177 -717.44029 -717.18053 -717.23743 -621.31850 -621.13719 -621.18815 -786.99337 -786.96736 -621.33091 -786.76665 -786.74015 -621.15069 -786.82215 -786.79457 -621.20039 -323.91294 -323.76810 -323.80465 -393.48616 -393.47864 -393.46648 -227.82217 -323.93247 -393.37410 -393.36678 -393.35406 -227.75562 -323.78739 -393.41022 -393.40302 -393.39014 -227.78774 -323.82447 total E H(scaled) G(scaled)

complex guest molecule

ACH(LACTAM) ACH(LACTIM) N(9)Hdimer N(4)Hdimer N(9)H/ACID N(4)H/ACID N(9)H/LACTAM N(4)H/LACTIM ACID N(4)H

4.1. Hydrogen-Bonding Formation. To rationalize the above experimental results, we first performed an ab initio calculation for the relative energy among various isomers of 4ABI in its monomeric form. The results shown in Table 2 conclude the N(9)H form of 4ABI to be the most stable one in the gas phase, while the N(7)H and N(4)H tautomers are more endergonic than N(9)H by 4.52 and 12.6 kcal/mol, respectively. The inclusion of a solvation free energy based on the PM3-SM4 model obtains similar results in which the relative free energy is on the order of N(9)H (0 kcal/mol) < N(7)H(3.55 kcal/mol)< N(4)H(10.31 kcal/mol) in cyclohexane. Spectroscopically, it is difficult to resolve N(9)H and N(7)H species in solution because their absorption and emission spectra may be indistinguishable, as indicated by the identical spectra between 9MABI and 7MABI (see Figure 3). Fortunately, when 9MAI and 7MAI are compared, drastically different fluorescence quantum yields have been observed, being smaller by a factor of ∼10 in the case of 7MABI (see Figure 3 and Table 1). The result also correlates well with the observed decay dynamics in which τf of 9MABI was measured to be 0.48 ns in cyclohexane, while the decay rate of 7MABI is too fast to be resolved. Conversely, similar photophysical properties observed between 4ABI monomer (τf ) 0.44 ns, Φf ) 0.038) and 9MABI (τf ) 0.48 ns, Φf ) 0.014) in combination with the ab initio calculations lead us to conclude that the N(9)H form of 4ABI should be the predominant species in nonpolar solvents. More supporting evidence can be provided by the occurrence of ESDPT in the 4ABI dimer and/or 4ABI/ ACID complex, forming the N(4)H tautomer. Structural analysis indicates that the N(7)H dimer or N(7)H/ACID complex, if it forms, should be in a single hydrogen-bonding configuration, which lacks another cooperative proton to induce the ESDPT. Figure 9a-c depict the full geometry optimized structures (6-31G(d,p) basis set) of 4ABI self-association in various hydrogen-bonding configurations, namely, the 4ABI cyclic and linear dimeric forms. For the case of the cyclic dual hydrogen-

N(7)H

4. Discussion

4ABI monomer

emission, we have attempted to resolve the long wavelength decay component (i.e., >350 nm) through multiple-decay analysis. However, under the resolution limit only a singleexponential kinetics can be resolved. The prohibition of ESDPT in the 4ABI/LACTAM complex suggests that it may undergo drastically different relaxation dynamics from that of the 4ABI/ ACID complex (or 4ABI dimer) in the excited state. Detailed explanations will appear in the Discussion.

N(9)H

Figure 8. Concentration-dependent absorption and emission spectra of 4ABI in cyclohexane by adding various ACH concentrations (Cg) of (a) 0, (b) 2.8 × 10-5, (c) 5.5 × 10-5, (d) 8.7 × 10-5, (e) 1.4 × 10-4, and (f) 2.2 × 10-4 M.

Chou et al.

TABLE 2: Thermodynamic Parameters for Various 4ABI Hydrogen-Bonded Complexes Calculated by the 6-31G(d,p) Basis Set at 298 K (in the Gas) in Combination with the PM3-SM4 Methoda,b

10048 J. Phys. Chem. B, Vol. 103, No. 45, 1999

Double Proton Tautomerism in 4-Azabenzimidazole

Figure 9. Optimized geometries based on the HF/6-31G(d,p) basis set (in Å and degree, only critical angles are shown) for the ground states of (a) 4ABI dimer, (b) 4ABI/ACID, and (c) 4ABI/LACTAM.

bonded dimer, the association enthalpy, ∆Hac, was calculated to be -11.6 kcal/mol. Two types of linear, single hydrogenbonded dimeric forms were expected, namely, the N(9)-H- - N(4) and N(9)-H- - -N(7) dimers, and ∆Hac was calculated to be -1.37 and -6.48 kcal/mol, which are apparently much less exothermic than the cyclic dual hydrogen-bond formation. Consequently, the 4ABI cyclic dual hydrogen-bonded dimer is calculated to be more stable than the N(9)-H- - -N(4) and N(9)-H- - -N(7) dimers, respectively, by 8.2 and 4.8 kcal/mol in cyclohexane. The results are in agreement with the absorption and fluorescence titration studies, concluding that the 4ABI cyclic dual hydrogen-bonded dimer is the predominant selfassociated species. For the 4ABI/guest hydrogen-bonded complexes our results show that independent of the type of guest molecule studied, the most stable form of a 1:1 4ABI/guest complex generally incorporates cyclic dual hydrogen bonds (see Table 2) due to its much larger formation enthalpy. The enthalpies of association were estimated to be -13.2 kcal/mol and -11.9 kcal/mol for 4ABI/ACID and 4ABI/LACTAM, respectively. Since ∆Hac

J. Phys. Chem. B, Vol. 103, No. 45, 1999 10049 generally correlates well with the hydrogen-bonding strength, the result may simply indicate that the hydrogen-bonding strength is on the order of 4ABI/ACID > 4ABI/LACTAM ∼ 4ABI dimer. Evidence can be given by the calculated >N- - H-O- hydrogen-bonding distance of 1.92 Å in the case of the 4ABI/ACID, which is shorter than the >N- - - -H-N- distance in the 4ABI dimer by 0.17 Å. The trend in the exothermicity of the association enthalpy also linearly correlates with the experimentally measured order of the association constants for 4ABI complexes (see Table 1), increasing as the association enthalpy decreases. Thus, the relative change in the entropy factor is not crucial, but the hydrogen-bonding strength is a key factor to fine-tune the stability of 4ABI hydrogen-bonded complexes. The calculated free-energy change of the 4ABI hydrogen-bonded complex generally deviated from the experimental value by 3-5 kcal/mol at room temperature (see Tables 1 and 2). Although the use of a lower basis set may introduce certain deviations, the major discrepancy between the theoretical (in gas phase) and experimental (in solution) approaches is believed to be due to solvation energy in the solution phase. Unfortunately, due to the complicated molecular structure of the hydrogen-bonding species, at this stage, it is not feasible to calculate the solvation energy based on an ab initio approach. We have also attempted to resolve the solvation energy based on several different semiempirical methods. However, results are scattered. This may not be surprising since the programs we applied are based on the continuum dielectric model and cannot accurately describe the 4ABI hydrogen-bonded systems in solution. Furthermore, the entropy calculation is based on an infinitely diluted solute in which the correction term for the entropy due to the concentration effect has been neglected. Such an effect is significant for experiments using a finite concentration, leading to more favorable entropy upon the hydrogenbonding association. 4.2. Proton-Transfer Dynamics. In the 4ABI dimer and 4ABI/ACID complex where cyclic dual hydrogen bonds are intrinsically formed, the photoinduced double proton transfer may only require a small displacement of the hydrogen atom and/or molecular skeleton, resulting in a small energy barrier. The rate of such a cooperative proton-transfer reaction, taking place either stepwise or simultaneously, is expected to be much faster than the spontaneous decay rate of the excited normal dimeric form, as supported by the lack of a normal dimeric (or complex) emission as well as the unresolved N(4)H tautomer fluorescence rise time (τrise < 3 × 10-10 s). Conversely, despite the strong dual hydrogen-bonding association, ESDPT is prohibited in the case of the 4ABI/LACTAM complex. Note that prominent ESDPT takes place in the 7AI/LACTAM complex,12c,d which possesses a similar molecular structure with 4ABI/LACTAM. Since the lifetime of the 4ABI/LACTAM complex was measured to be as long as 1.05 ns (kf ) 9.5 × 108 s-1; see Table 1), it is quite unlikely that the rate of ESDPT is exceedingly slower than the lifetime of excited 4ABI/ LACTAM to account for the lack of tautomer emission unless there exists an unusually high energy barrier. Alternatively, the results may more plausibly be rationalized by a tautomerization energy-dependent ESDPT reaction. The photophysical properties shown in Table 1 reveal an interesting correlation between the S0-S1 (ππ*) energy gap and its corresponding number of nitrogen atoms in the methylated tautomers of 7AI and 4ABI, in which the energy difference is as high as 5400 cm-1 between N(7)-methyl-7-azaindole (7MAI, 21 834 cm-1) and 4MABI (27 200 cm-1). In contrast, the differences in 0-0 onsets between 7AI and 4ABI normal dimers are relatively much

10050 J. Phys. Chem. B, Vol. 103, No. 45, 1999 smaller, being only 1460 cm-1 higher in energy than that of the 7AI dimer for the 4ABI normal dimeric form (see Table 1). The result indicates that adding an electron-rich nitrogen atom in the pyrrole system of 7AI alters the resonance skeleton in the tautomeric form toward a drastic increase of the π f π* energy gap. Consequently, relative energy levels between the S1 state of the normal dimeric form and the S′1 state of the N(4)H tautomer dimeric form may turn out to be crucial for the observed guest molecular-based tuning ESDPT. To rationalize the proposed mechanism, we have first attempted to construct relative energy levels for an overall proton-transfer cycle in 4ABI. Experimentally, it is not feasible to obtain a relative free energy between the 4ABI dimer (or complex) and its corresponding N(4)H dimer (or complex) in the ground state. Alternatively, using a 6-31G(d,p) basis set, the change of free energy for N(4)H/ACID, N(4)H dimer, and N(4)H/LACTIM was calculated to be 11.2, 17.31, and 22.3 kcal/ mol, respectively, relative to their corresponding N(9)H/guest molecule complexes. We further performed a semiempirical PM3-SM4 solvation model (see Experimental Section) to calculate the solvation free energy for each species in cyclohexane. The results added to those obtained by an ab initio approach give free energies of 12.07, 15.66, and 22.85 kcal/ mol for N(4)H/ACID, N(4)H dimer, and N(4)H/LACTIM (see Table 2). The assignment of the 0-0 band for the S0-S1 transition of the 4ABI dimer and/or complex is relatively easy due to their structural absorption profile with an onset at ∼292 nm (34 247 cm-1) in cyclohexane. Conversely, it is rather difficult to assign the S′0-S′1 (0-0) transition of the N(4)H dimer due to the diffusive N(4)H tautomer emission, which also overlaps significantly with the normal monomer emission. For a simplified approach we take the absorption onset of 368 nm (27 174 cm-1) from 4MABI to be the 0-0 transition for either the N(4)H dimer or the N(4)H/guest complexes. Given all these values, the relative energy levels of a proton-transfer cycle in the singlet-state manifold are depicted in Scheme 1. Apparently, for 4ABI/ACID and 4ABI dimer, proton-transfer tautomerism in the excited state is calculated to be -8.2 and -4.6 kcal/mol. On the contrary, an endergonic value of 2.6 kcal/mol was obtained for the case of the 4ABI/LACTAM complex. Although this approach is qualitative due to the uncertainty of obtaining accurate excited-state energy levels for both normal and tautomer species, it clearly shows that ESDPT in 4ABI/ LACTAM is thermally more unfavorable than that of 4ABI/ ACID or 4ABI dimer. On one hand, whether the ESDPT takes places or not, the result of endergonic tautomerism simply indicates that the 4ABI/LACTAM complex may populate predominantly in the excited state through a fast thermal equilibrium, resulting in a normal Stokes-shifted emission. On the other hand, the more endergonic reaction may empirically lead to a higher formation free energy of the activated complex, i.e., an existence of a high energy barrier, so that ESDPT may be frustrated during the lifetime of the excited 4ABI/LACTAM complex. Both mechanisms unfortunately cannot be distinguished at this stage. Further investigation focusing on ultrafast time-resolved ESDPT dynamics is necessary to resolve this issue. 5. Conclusion Summarizing the above results and discussion, we have studied hydrogen-bonded complexes of 4ABI by means of absorption, emission, and theoretical calculations. Specific hydrogen-bonding sites and spectroscopic assignments have been achieved by applying various derivatives of 4ABI incor-

Chou et al. porated with guest molecules. In both 4ABI dimer and 4ABI/ acetic acid complex ESDPT takes place, resulting in a prominent N(4)H tautomer emission. The results open up a study of ESDPT dynamics based on an imidazole system in which the mechanism may be distinguished from that proposed in 7AI possessing a pyrrole system (for example, see ref 19). In contrast, despite the strong dual hydrogen-bonding association, ESDPT is prohibited in the case of the 4ABI/LACTAM complex. The results are tentatively rationalized by a proton-transfer tautomerism mediated by the relative excited-state thermodynamics. Further studies focusing on the photoinduced tautomerism of heterodimers such as 7AI/4ABI and 4ABI/purines are currently in progress. Acknowledgment. This work was supported by the National Science Council, Taiwan, R.O.C. (grant No. NSC 87 -2119M-194-002). 6. Appendix Various methods to determine the Ka value of 4ABI dimer and hydrogen-bonded species. A. Absorption Titration. A1. The association constant Ka of 2(4ABI) h (4ABI)2 equilibrium obtained by the UV-vis absorption method can be derived as follows K

a 2(4ABI) y\z (4ABI)2 initial C0 0 final C0 - 2x x

Accordingly, Ka ) x/(C0 - 2x)2. If the percentage of dimer formation is less than, e.g., 15% of the initially prepared C0, then (C0 - 2x)2 approximates to C02 - 4C0x

∴ Ka ∼

x 2 C0 - 4C0x

2

and

C0 1 - 4C0 ) Ka x

(1-1)

On the other hand, the absorbance of 4ABI and (4ABI)2 at a specific wavelength can be expressed by

A ) M(C0 - 2x)+ Dx ∴ x )

A - MC0 D - 2M

(1-2)

Plugging (1-2) into (1-1), we obtain

∴

C02(D - 2M) 1 + 4C0 ) Ka A - MCM

(1-3)

Both sides of (1-3) are divided by C0(D - 2M) to obtain

C0 1 1 4 ) + A - MC0 Ka(D - 2M) C0 D - 2M

(1)

A2. The association constant Ka of 4ABI + guest h 4ABI/ guest complex formation calculated by the UV-vis absorption method can be derived as follows

4ABI + guest h 4ABI/guest Cg 0 initial C0 ∼Cg Cp final CM On the above expression the association constant is assumed to be not very large so that the concentration of the added guest molecule varies negligibly during the reaction (see text). The

Double Proton Tautomerism in 4-Azabenzimidazole

J. Phys. Chem. B, Vol. 103, No. 45, 1999 10051

absorbance of 4ABI at, i.e., 292 nm prior to the addition of the guest molecule can be expressed by

A0 M

A0 ) C0M ∴ C0 ) Upon adding the guest molecule Cg

C0 )

A0 ) CM + Cp M

∴ Cp )

Ka )

A0 - CM M

(2-1)

On the other hand, Ka ) Cp/CMCg

A0 A0 - CM ) KaCgCM w CM ) M M(KaCg + 1)

C0MΦM -

(

)

A0 A ) MCM + pCp ) MCM + p - CM ) M  pA 0 M

(M - p)A0

pA0 ) + M M(KaCg + 1) (M - p) p (M - p) + p(KaCg + 1) A ) ) + A0 M(KaCg + 1) M M(KaCg + 1) (2-3)

Subtracting 1 from both sides of (2-3) we obtain

(M - p) + p(KaCg + 1) - M(KaCg + 1) A -1) A0 M(KaCg + 1) A - A0 (M - p) + (p - M)(KaCg + 1) ) ) A0 M(KaCg + 1) (p - M) (KaCg)

∴

(

M(KaCg + 1)

)[

]

M A0 M(KaCg + 1) 1 + 1 (2) ) ) A0 - A (M - p)(KaCg) M - p KaCg

B. Fluorescence Titration. The association constant Ka of 4ABI + guest h 4ABI/guest complex formation calculated by the fluorescence method can be derived as follows. Prior to the addition of guest molecules, the absorbance A0 of 4ABI monomer is

A0 ) MC0 ) -

)

(

) ]

(

)

C0 C0 ΦM + p C 0 Φ KaCg + 1 KaCg + 1 p

)

(

C0MΦM

)

C0 C0 MΦM - C0 Φ KaCg + 1 KaCg + 1 p p

(KaCg +1)ΦMM KaCg(ΦMM - Φpp)

) ΦMM

(M - p)CM +

∴

[(

F0 ) F0 - F

The absorbance of 4ABI monomer and 4ABI/guest complex at a specific wavelength can be expressed by

∴

Consequently,

(2-2)

(2-1) ) (2-2)

∴

Cp C0 - CM C0 ) ∴ CM ) CMCg CMCg KaCg + 1

F ) 2.303RI0 M

∴ Cp ) KaCgCM

∴

be proportional to IabΦM, where ΦM denotes the fluorescence yield of the 4ABI monomer, and can thus be expressed by F0 ) 2.303RI0MC0ΦM, where R is an instrumental factor including sensitivity, alignment, etc. of the detecting system. Similarly, after the guest molecule is added, the fluorescence intensity F can be expressed by F ) 2.303RI0(MCMΦM + pCpΦp), where Φp is the fluorescence quantum yield of 4ABI/guest complex. Note that

( )

Iab Iab 1 ∼ ∴ ln 1 2.303 I0 2.303I0 Iab ) 2.303C0MI0

By adding the guest molecule, Iab ) 2.303I0(MCM + pCp). The fluorescence intensity of the 4ABI monomer, F0, should

(

)

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