Excited-State AmineImine Double Proton Transfer in 7-Azaindoline

7818

J. Phys. Chem. B 2000, 104, 7818-7829

Excited-State Amine-Imine Double Proton Transfer in 7-Azaindoline 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: March 15, 2000; In Final Form: June 5, 2000

Ground-state thermodynamics and excited-state amine/imine tautomerism in 7-azaindoline (7AZD) mediated by hydrogen bond formation have been studied by means of absorption and emission spectroscopies. The association constants in cyclohexane (298 K) were determined to be 80, 2.5 × 102, and 7.8 × 102 M-1, for the formation of 7AZD dimer, 7AZD/azacyclohexanone, and 7AZD/acetic acid dual hydrogen-bonded complexes, respectively. The 7AZD/acetic acid complex undergoes a fast (. 3 × 109 s-1) excited-state double proton transfer (ESDPT) reaction, resulting in a prominent imine-like tautomer emission. Proton-transfer isomers of 7AZD have been identified through syntheses and spectral characterization of various 7AZD methyl derivatives. In contrast, ESDPT is prohibited in cases of 7AZD dimer and 7AZD/azacyclohexanone hydrogenbonded complex. The results, in combination with a comparative study on 7-azaindole, generalize the amine/ imine tautomerism, which can be fine-tuned by the length of π electron conjugation coupled with types of associated guest molecules, further supporting the proposed catalytic-versus-noncatalytic model for the ESDPT reaction.

1. Introduction The 7-azaindole (7AI) dimer has long been recognized as a simplified model for the hydrogen-bonded base pair of DNA,1-3 which upon electronic excitation undergoes the double proton transfer reaction, resulting in a large Stokes shifted emission (e.g., λmax ∼ 480 nm in nonpolar solvents). At the molecular level, such an excited-state double proton transfer (ESDPT) process provides one possible mechanism for the mutation which 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.3-7 More recently, studies have been extended to 7AI analogues of biological importance. Among which, purines possessing a similar electronic moiety with respect to 7AI have received particular interest.8,9 Conversely, via the formation of a 7AI(host)/guest hydrogen-bonded complex, the dynamics of ESDPT incorporating guest molecules have also received considerable attention.10-17 In alcohols and water, the dynamics of ESDPT in 7AI and its analogue 7-azatryptophan have been successfully applied to probe the solvation and/or protein dynamics.12-15 The guest molecule assisted ESDPT in 7AI also renders an interesting perspective from the thermodynamic standpoint. The acid-, alcohol-, and water-assisted ESDPT in 7AI can be specified as a catalytic processes since the molecular structure of the guest species (e.g., the acetic acid) remains unchanged. On the other hand, the double proton transfer in the single-photon excited 7AI dimer results in 7AI(T)*/7AI(T) forms ((T)* denotes the electronically excited tautomer) in which both 7AI molecules tautomerize simultaneously. Thus the guest 7AI in the dimeric form does not act as a catalyst but rather as a reactant. Such a * To whom correspondence should be addressed. † The National Chung-Cheng University. ‡ Fu Jen Catholic University. § The National Hu-Wei Institute of Technology.

noncatalytic process is of importance from a chemistry perspective since it is plausible that a chemically important isomer of the guest molecule can be produced via the ESDPT process, which cannot be otherwise accessed.16c,d From a molecular viewpoint, the reaction center of the ESDPT process in 7AI and its analogues is essentially the amine/imine proton-transfer tautomerism in which a proton (or hydrogen atom) in the pyrrolic nitrogen transfers to the pyridinic nitrogen, forming an imine-like isomer. Such a process is generally highly endergonic in the ground state. For example, a theoretical calculation estimates the 7AI dimer f 7AI(T) dimer tautomerism to be ∼23.5 kcal/mol.16e It thus becomes crucial that the S0′ f S1′ transition gap of the tautomer species (hereafter, prime denotes the imine tautomer state) has to be significantly smaller than that of the normal species (i.e., the S0-S1 gap) in order to proceed as a thermally favorable ESDPT reaction. A simple empirical approach based on the π electron conjugation predicts that the C2-C3 double bond (see Figure 1) in the imine-like tautomer of 7AI should be actively involved in the delocalization of π electrons and hence play a key role to determine the S′0S′1 energy gap (i.e., the energy level of the S′1 state). Conversely, theoretical approaches also indicate that the relative thermodynamics between amino and imino forms in the ground state mainly depend on destabilization of the aromaticity of the pyridine ring (vide infra), and hence are less affected by the C2-C3 double bond in the pyrrole ring. On this basis, 7-azaindoline (7AZD, see Figure 1)18 was synthesized via hydrogenation of the C2-C3 double bond in 7AI. On one hand, 7AZD is potentially capable of undergoing an amine-imine type of proton-transfer tautomerism. Thus, studies aiming at spectroscopy and dynamics of ESDPT in 7AZD are of great importance to generalize the amine-imine tautomerism being the central reaction in 7AI analogues. On the other hand, due to the lack of a C2-C3 double bond, the imine-like tautomer of 7AZD

10.1021/jp001001g CCC: $19.00 © 2000 American Chemical Society Published on Web 07/20/2000

Amine-Imine Double Proton Transfer

J. Phys. Chem. B, Vol. 104, No. 32, 2000 7819 SCHEME 1. Proton transfer tautomerism tuned by relative thermodynamics. Note that the S0 state has been normalized to an equivalent energy level. The energy level is in kcal/mol with its corresponding wavelength (in parentheses) in nm.

Figure 1. Structures of 7AI, 7AZD and its corresponding protontransfer isomers as well as methylated derivatives.

apparently possesses a shorter length of the π electron conjugation than that of 7AI analogues, and hence a larger S0′-S1′ energy gap is expected. Therefore, 7AZD may serve as an ideal model to test the catalytic-versus-noncatalytic ESDPT reaction16c,d fine-tuned by the length of the π electron conjugation. 2. Experimental Section 2.1. Materials Synthesis of 7AZD. 7AI (3 g) in dry toluene (150 mL) was gradually (2 mL/min) passed through a reaction column packed with the Pd/δ-Al2O3 catalyst with the reaction temperature and pressure maintained at 150 °C and 450 psi, respectively. The product 7AZD mixed with unreacted 7AI was further separated by column chromatography where ethyl acetate/hexane (1:3 v/v) was used as an eluent to yield 1.8 g (∼60%) of 7AZD. NMR analyses: 1H NMR(CDCl3, 200 MHz) 7AZD δ3.03(t, J ) 8.42, 8.14 Hz, 2H); 3.59(t, J ) 8.42, 8.44 Hz, 2H); 6.47(t, J ) 6.22, 6.24 Hz, 1H); 7.20(d, J ) 8.06 Hz, 1H), 7.77(d, J ) 4.92 Hz, 1H). N(1)-methyl-7-azaindoline (1MAZD, see Figure 1) was synthesized according to a similar procedure to obtain various methylated purine derivatives.20-22 Briefly, a suspension of 7AZD (0.12 g, 1 mmole) and sodium hydride (57%, 0.17 g, 4 mmole) in N,N-dimethylformamide (∼ 3mL) was stirred under argon at 0 °C for 20 min. Methyl iodide (1.17 g, 0.0083 mol) was then added and the resulting mixture was stirred for ∼1 h. The N,N-dimethylformamide was then removed under reduced pressure. Column chromatography eluting with 95% ethanol afforded 0.16 g of 1MAZD. NMR analyses: 1H NMR(CDCl3, 200 MHz) 1MAZD δ1.42(s, 3H); 2.96 (t, J ) 7.2 Hz, 2H); 3.47(t, J ) 8.0 Hz, 2H); 6.41(q, J ) 7 Hz, 1H), 7.15(d, J ) 1.6 Hz, 1H); 7.83(d, J ) 4.6 Hz, 1H). 7-Methyl-2,3-dihydro-7H-pyrrolo[2,3-b]pyridine (7MDPP, see Figure 1) was prepared by dissolving 7AZD (50 mg) in dry THF. To the solution was added 0.08 mL CH3I and the mixtures were refluxed for ∼3 h under N2 atmosphere. After cooling, the solution containing precipitates was filtered. The precipitate was then dissolved by addition of excess of potassium carbonate in the aqueous solution, and the resulting solution was extracted by ethyl acetate. The ethyl acetate layer was dried, filtered, and the product was obtained after evaporating the solvent. NMR analyses: 1H NMR(CDCl3, 400 MHz) 7MDPP δ6.46(m, 2H); 5.50(m, 1H); 3.84(m, 2H); 3.30(s, 3H), 2.88(m, 2H). Cyclohexane is of spectrograde quality (Merck Inc.) and used right after received. Acetic acid (ACID, Merck Inc.) was purified through a fractional distillation. 2-Azacyclohexanone (Aldrich) was recrystallized twice from methanol. Since the stable conformer of 2-azacyclohexanone is in the lactam form we thus use the abbreviation LACTAM in order to distinguish it from the LACTIM form (see Scheme 1).

2.2. Measurements. Steady-state absorption and emission spectra were recorded by a Cary 3E (Varian) spectrophotometer and an F4500 (Hitachi) fluorimeter, respectively. Both wavelength-dependent excitation and emission response of the fluorimeter have been calibrated according to a previously reported method.16 Nanosecond lifetime measurements were performed by an Edinburgh FL 900 photon counting system with a hydrogen filled flash lamp/or a nitrogen lamp as the excitation source. The temporal resolution after deconvolution of the excitation pulse was ∼300 ps. The data were analyzed using a nonlinear least-squares fitting program with a deconvolution method reported previously.23 2.3. Theoretical Calculations. A detailed account of the theoretical approach has been previously described.24 Briefly, the initially optimized structure used for the ab initio calculation was obtained by the semiempirical AM1 method. Ab initio molecular orbital calculations were performed by using Gaussian 98 rev. A.7 programs.25 Geometry optimizations for all structures were carried out with the 6-31G(d,p) basis set at the HartreeFock (HF) level. Vibrational frequencies were also performed to ensure the global energy minimum for those dimeric and complex forms. The directly calculated zero-point vibrational energies (ZPE) were scaled by 0.918126 to account for the overestimation of vibrational frequencies at the HF level. The association energy, ∆Hac, was calculated as the change in the total molecular enthalpy of formation for the conversion of the optimized monomer individually into the optimized dimer, which then incorporated with a counterpoise correction procedure27-29 to correct certain inconsistencies due to the basisset superposition (BSSE). In certain cases, a semiempirical PM3-SM4 solvation model30,31 was applied to obtain the solvation free energy in cyclohexane, which is then added to “gas phase” energies obtained from the ab initio method.32 The following sections are organized according to a sequence of steps where we first performed detailed absorption and fluorescence titration experiments to determine the thermodynamic and/or ESDPT properties of 7AZD hydrogen-bonded complexes.

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Chou et al.

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

emission λmax (nm)

Φf

τf (ns)

7AI 7AI dimer 7MPP 7AZD 7AZD dimer 7AZD/ACID

287 292 390 306 NA 318d

320 480 495 353 360 440

0.22a 0.016 0.0073a 0.44 0.15 0.04

1.72a 3.0b 1.67a 4.9 2.1 1.8

7AZD/LACTAM 1MAZD 7MDPP

NA 312 375

368 365 463

0.30 0.48 0.015

3.2 5.2 1.1

Ka (M-1)

∆G0 (kcal/mol) (298 K)

2.2 × 103 c 0.8 × 102 7.8 × 102 7.2 × 102 e 2.5 × 102

-2.5 -3.9 -3.2

a Reference 14(b). b Reference 11(b). c Reference 16(d). d Values were obtained from the fluorescence excitation spectrum. e Data were obtained from the fluorescence titration experiment. NA: not available.

The results in combination with syntheses and spectral characterization of 7AZD methyl derivatives lead us to deduce specific hydrogen-bonding sites actively involved in the complex formation as well as the amine-imine tautomerism. Subsequently, complementary supports for the structure and relative energy levels of various 7AZD/guest hydrogen-bonded complexes were provided via ab initio calculations. Finally, A general discussion based on amine-imine tautomerism mediated by the relative excited-state thermodynamics is proposed and discussed. 3. Results 3.1. Self-Association. Spectroscopically, the change of UVvis absorption spectra associated with the hydrogen-bonding formation has been used as a tool to obtain thermodynamics of the 7AI dimer and hydrogen-bonded complexes.2,3,16 We thus performed a similar method for 7AZD. When the concentration was prepared to be as low as 1.2 × 10-5 M, 7AZD exhibits an S0-S1 absorption band maximum at 306 nm (306 ∼ 5600 M-1cm-1). The spectral features are similar to that of 1MAZD which is generally treated as a nonproton-transfer model (i.e., the amino form, see Figure 1) due to the lack of an N-H proton. We thus conclude that at sufficiently low concentrations 7AZD exists mainly as an amino form in cyclohexane. Despite the hydrogenation at the C2-C3 double bond, the wavelength at the S0 f S1 (ππ*) absorption maximum in 7AZD is even red shifted by ∼20 nm with respect to that of 7AI monomer (see Table 1). The result can be rationalized by the fact that for 7AI the lone pair electrons of the pyrrolic nitrogen directly involve in the π-electron conjugation to gain the aromaticity (i.e., ten π electrons). Whereas upon hydrogenation of the C2-C3 double bond, the nitrogen (N(1)) lone pair electrons no longer play such a role. Instead, upon excitation the occurrence of charge transfer between electron donor (the alkylamino nitrogen) and acceptor (the pyridinic nitrogen) is common, resulting in a bathochromic shift in 7AZD relative to 7AI. More evidence can be given by a further red shift of the absorption maximum in 1MAZD (λmax ∼ 312 nm, see Figure 1), in which the dialkylamine possesses a lower ionization energy than that of the alkylamine in 7AZD. Thus, a stronger excited-state chargetransfer property is expected in 1MAZD. A concentration-dependent absorption profile at the spectral region of the S0fS1 transition was observed upon increasing the concentration (see Figure 2b-e). Although such a change was not as remarkable as those concentration-dependent absorption spectra observed in 7AI analogues,3,9,16b,d the results in comparison with the concentration-independent absorption profile for 1MAZD unambiguously conclude the formation of 7AZD dimer and/or higher-order aggregates through the hy-

drogen-bonding effect. Similar to that proposed in the 7AI selfassociation, we first assume a dominant self-dimerization form possibly possessing a cyclic hydrogen bonding configuration (see Scheme 1) where the dual hydrogen-bonding sites, i.e., the N(1)H proton and the pyridinal nitrogen in 7AZD (i.e., N(7)) act as a proton donor and acceptor, respectively. Consequently, a competitive equilibrium between enol monomer and 7AZD self-association incorporating two 7AZD molecules can be depicted as 2(7AZD) h (7AZD)2, and the association constant Ka for the formation of the 7AZD dimer can be expressed by

Ka )

Cp (C0 - 2Cp)2

(1)

where C0 is the initial concentration of 7AZD and Cp denotes the concentration of the 7AZD dimeric form. Consequently, the absorbance of the mixture at a specific wavelength as a function of initially prepared C0 can be expressed by

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

(2)

Detailed derivations of eq 2 have been elaborated in ref 9. In the above equation M and D are molar extinction coefficients of the monomer and dimer monitored at a specific wavelength, e.g., 340 nm, where an appreciable percentage of absorbance can be attributed to the self-associated species (see Figure 2). Under a sufficiently low concentration where only monomer -1 cm-1. A plot of exists, 340 M was measured to be ca. 165 M C0/(A - MC0) at 340 nm as a function of 1/C0 shown in the insert of Figure 2 reveals sufficiently linear behavior, supporting the assumption of dimeric formation in 7AZD. Accordingly, by fixing the M value, the best linear least-squares fit to Figure -1 cm-1 2, using eq 2, gives 340 D and Ka values to be 1050 M -1 and 80 M , respectively. In comparison with that of the 7AI dimer (Ka ∼ 2.2 × 103 M-1 in cyclohexane16d), the relatively smaller dimerization constant in the case of 7AZD dimer may be rationalized by a steric effect upon dimerization. The most stable form of 7AZD monomer is a nonplanar structure in which the amino hydrogen was calculated to be ∼25.3° with respect to the pyridine ring (vide infra). Thus, an endothermic hinder rotation of the amino hydrogen toward the planarity is necessary prior to the dimerization, which compensates the gain of the stabilization due to the dual hydrogen-bonding formation. In contrast, the planar structure in 7AI facilitates the dimerization with negligible geometry adjustment, resulting in a significantly larger association constant. Further verification of this viewpoint will be elaborated in the discussion section.


J. Phys. Chem. B, Vol. 104, No. 32, 2000 7821

Figure 2. (s) The concentration-dependent absorption spectra of 7AZD in cyclohexane, in which 7AZD (C0) was prepared at (a) 4.5 × 10-4, (b) 9.0 × 10-4, (c) 1.8 × 10-3, (d) 3.6 × 10-3, (e) 7.2 × 10-3 M. The absorption spectra are normalized at 313 nm. (- - -) The absorption spectrum of 1MAZD (1.0 × 10-5 M) in cyclohexane. Inset: Plot of C0/(A340 - 340 M C0) values versus 1/C0 and a best nonlinear least-squares fitting curve using eq 2. Note that an additional point at C0 ) 2.8 × 10-5 M is added in the plot.

Table 1 lists the steady-state spectral properties of absorption and emission as well as relaxation dynamics for 7AZD and its hydrogen-bonded species in cyclohexane. When the concentration of 7AZD was prepared as low as 1.2 × 10-5 M in cyclohexane so that the monomer form exists predominantly, a normal Stokes shifted emission (Φf ∼ 0.44) was observed with a peak maximized at 353 nm. The emission lifetime was measured to be 4.9 ( 0.2 ns and was independent of the excitation as well as the monitored emission wavelength. Both spectral features and relaxation dynamics of the 7AZD monomer are similar to those of 1MAZD, exhibiting a normal Stokes shifted emission maximum at ∼365 nm (τf ∼ 5.2 ns, Φf ∼ 0.48) in cyclohexane (see Table 1). Therefore, at a sufficiently low concentration of 7AZD in cyclohexane only one emitting species exists, which is unambiguously assigned to the amino form. The steady-state fluorescence spectra as a function of the 7AZD concentration are shown in Figure 3. In contrast to the distinct dual emission observed in the concentrated 7AI, in which the large Stokes shifted band (λmax ∼ 480 nm) is associated with the tautomer dimeric species resulting from ESDPT, the fluorescence consists of a spectrally unresolved broad band of which the fluorescence maximum is gradually red shifted upon increasing the concentration. For example, when 7AZD was prepared as high as, e.g., 7 × 10-3 M, where ∼20% exists in the dimeric form deduced from eq 1 with a Ka value of 80 M-1, a 335 nm excitation gives rise to a fluorescence maximum of 360 nm, which is ∼7 nm shift with respect to the monomer emission. The possibility of the spectral distortion due to the reabsorption in the high sample concentration has been eliminated by the use of square quartz cell with various path length (0.1-1.0 cm) so that the maximum absorbance can be kept as low as 0.5. Similarly, the excitation spectrum monitored at the long-wavelength emission region (e.g., 420 nm) is red shifted with respect to that monitored at the short-wavelength tail (e.g., 340 nm), indicating that the emission originates from multiple ground-state components. Detailed time-resolved studies clearly indicate that the dynamics of decay were best fitted by two

single exponential decay components theoretically expressed as Ft ) A1e-k1t + A2e-k2t, where A1 and A2 are the emission intensity at t ∼ 0 for the decay components 1 and 2, respectively (see inset of Figure 3). While the ratio for A1 versus A2 is concentration as well as excitation-wavelength dependent, k1 and k2, within experimental error, were found to be constant, with a slow and a fast component of 2.1 × 108 s-1 (τ ∼ 4.8 ns) and 4.8 × 108 s-1 (τ ∼ 2.1 ns). The slower decay component is essentially identical to that of the 7AZD (monomer) emission and can thus be unambiguously assigned to be the unassociated 7AZD. Since the entire emission band exhibits a systemresponse rise time (i.e., < 300 ps, see the Experimental Section), it is quite unlikely that one species is the precursor of the other. Consequently, the results unambiguously conclude that only two emitting species exist in the entire spectral region (300-500 nm). The longer-lived species can be reasonably ascribed to the emission from the non-hydrogen-bonded monomer. Conversely, the shorter lived, lower frequency emission part with its excitation spectrum red shifted with respect to the monomer species unambiguously results from the dimeric species, in which the red shift of the excitation spectrum is due to the dual hydrogen-bonding formation. 3.2. 7AZD/ACID Complexes. Figure 4 shows the absorption spectra of 7AZD upon adding the acetic acid (ACID) in cyclohexane. In this experiment, the initial concentration of 7AZD, C0, was prepared to be as low as 1.0 × 10-5 M to avoid the self-dimerization. The formation of 7AZD/ACID hydrogenbonded complexes can be clearly shown by the growth of an absorption band at ∼318 nm (taken from the excitation maximum shown in Figure 6) throughout the titration. The appearance of an isosbestic point located at ∼313 nm indicates the existence of equilibrium with a common intermediate. The 7AZD hydrogen-bonded complex incorporating stoichiometrically equivalent ACID molecules can be depicted as Ka

7AZD + ACID y\z 7AZD/ACID


Chou et al.

Figure 3. The concentration-dependent fluorescence spectra of 7AZD in cyclohexane, in which 7AZD was prepared at (a) 1.5 × 10-5, (b) 1.1 × 10-4, (c) 2.2 × 10-4, (d) 4.4 × 10-4, (e) 1.6 × 10-3 M. The spectra are normalized at an equivalent relative emission intensity. Inset: The time-dependent fluorescence of 7AZD (1.0 × 10-3 M) monitored at 420 nm (λex: 330 nm) fitted by two single exponential decays (τ ) 4.9 × 10-9 and 2.1 × 10-9 s, χ2 ) 1.005).

Figure 4. The concentration-dependent absorption spectra of 7AZD (1.2 × 10-5 M) in cyclohexane by adding various ACID concentrations (Cg) of (a) 0, (b) 1.4 × 10-4, (c) 2.8 × 10-4, (d) 4.3 × 10-4, (e) 5.7 × 10-4, (f) 7.1 × 10-4, (g) 1.0 × 10-3, (h) 1.4 × 10-3, (i) 1.8 × 10-3, and (j) 2.7 × 10-3 M. Inset: The plot of A0/(A - A0) at 335 nm as a function of 1/Cg in curves b to j, and a best least-squares fitting curve using eq 3.

Under the condition that the concentration of ACID (or the guest molecule) is much larger than that of the complex, the relationship between the measured absorbance as a function of the initially prepared ACID concentration, Cg, can be expressed by9

(

)[

]

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

(3)

where M and C are molar extinction coefficients of the 7AZD

monomer and hydrogen-bonded complex monitored at a specific wavelength, respectively. The insert of Figure 4 shows the plot of A0/(A - A0) as a function of 1/Cg at a selected wavelength of 335 nm. Straight-line behavior supports the validity of the assumption of 1:1 7AZD/ACID formation, and a best linear -1 least-squares fit using eq 3 deduces 335 C and Ka to be 1400 M -1 -1 cm and 780 M , respectively. Figure 5 shows the fluorescence spectra of 7AZD as a function of increasing ACID concentration in cyclohexane. Upon excitation in the region of the isosbestic point (∼313 nm), where



Figure 5. The fluorescence spectra of 7AZD as a function of the acetic acid concentration (Cg) which was prepared to be (a) 0, (b) 1.4 × 10-4, (c) 2.8 × 10-4, (d) 4.3 × 10-4, (e) 5.7 × 10-4, (f) 7.1 × 10-4, (g) 1.0 × 10-3, (h) 1.4 × 10-3, (i) 1.8 × 10-3, and (j) 2.7 × 10-3 M. Inset: The plot of F0/(F - F0) at 430 nm as a function of 1/Cg in curves b to j and a best least-squares fitting curve using eq 4.

Figure 6. The excitation spectra of 7AZD (1.2 × 10-5 M by adding 5.5 × 10-4 M acetic acid in cyclohexane) monitored at (a) 500 nm, (b) 375 nm. Inset: The time-dependent fluorescence monitored at (a) 500 nm (τ ) 1.8 × 10-9 s, χ2 ) 1.005), (b) 350 nm (τ ) 4.9 × 10-9 s, χ2 ) 1.095) and fitted by one single-exponential decay.

both 7AZD and 7AZD/ACID absorb, distinct dual fluorescence was observed, consisting of a short-wavelength band maximum at ∼353 nm (the F1 band) and a large Stokes shifted, vibronically progressive band maximum at ∼440 nm (the F2 band). The resulting concentration-dependent spectra reveal an isoemissive point at ∼403 nm. The excitation spectrum monitored within the long-wavelength tail of the F2 band (e.g., 500 nm) is red shifted by ∼12 nm with respect to that monitored within the region of the F1 band (e.g., 375 nm) (see Figure 6). The decay dynamics of the F1 ( 420 nm) bands are

independent of the added acetic acid concentrations, and both exhibit single-exponential behavior with a lifetime τf of 4.9 ns and 1.8 ns, respectively (see inset of Figure 6). The rise time of both components unfortunately cannot be resolved by our current photon counting system. Thus, it is quite unlikely that one species is the precursor of the other. Alternatively, the dual fluorescence should originate from different ground-state precursors, most likely the uncomplexed 7AZD monomer and 7AZD/ACID complex. The spectral and dynamic resemblance leads to the assignment of the F1 band to the monomer emission


Chou et al.

Figure 7. (s) The absorption (A) and emission (E) spectra of 7AZD (1.2 × 10-5 M) in the acidic aqueous solution (pH ) 1.0). (- - -) The absorption (A′) and emission (E′) spectra of 7MDPP (2 × 10-5 M) in cyclohexane.

unambiguously. Consequently, the precursor of the F2 band should be associated with the 7AZD/ACID hydrogen-bonded complex. 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+) ) -3.0 in 7AZD),33 the 440 nm band may result from a single proton transfer of the carboxylic proton to the 7AZD pyridinal nitrogen, giving rise to a 7AZDH+ (or more strictly the 7AZDH+/ACID- ion pair in cyclohexane) emission. Such a possibility has been eliminated by the following sequence of experiments. First, in an HCl gas saturated cyclohexane where 7AZD exists predominantly as a 7AZDH+/Cl- ion pair the emission maximum was observed to be 392 nm which is similar to the fluorescence (λmax ∼ 395 nm) of 7AZD in the acidic aqueous solution (pH ) 1.0), where most of the pyridinal nitrogen has been protonated (see Figure 7). Furthermore, the formation of a hydrogen-bonded complex between 1MAZD and acetic acid has been examined by the ACID concentrationdependent titration spectra. The Ka value of ∼20 M-1 deduced from eq 3 for the 1MAZD/ACID complex formation is smaller than that calculated for 7AZD/ACID by a factor of 40. Such a difference may not be surprising because theoretically only single hydrogen-bonded 1:1 1MAZD/ACID complexs can be formed. Upon increasing the acid concentration, the emission intensity of 1MAZD increases, accompanied by a slight red shift of the emission maximum from 363 to 368 nm. The lack of observing any large Stokes shifted emission in the 1MAZD/ ACID complex discounts the assignment of the 440 nm emission simply to the excited-state protonation of 7AZD. Thus, the F2 band with an unusually large Stokes shift of the ∼8500 cm-1 (peak-to-peak) relative to the complex S0-S1(ππ*) absorption (318 nm estimated from the excitation spectrum shown in Figure 6) leads us to propose a possible occurrence of ESDPT, resulting in an imine-form-like tautomer species (see Figure 1 and Scheme 1). To further verify this viewpoint, a model compound 7MDPP (see Figure 1) was synthesized, which possesses similar electronic properties with respect to the imine tautomer except for the lack of proton-transfer tautomerism. As shown in Figure 7, 7MDPP exhibits a concentration-independent absorption and

fluorescence maxima at 375 and ∼463 nm (τf ∼ 1.1 ns in cyclohexane), respectively, where the emission spectral features and relaxation dynamics are similar to the F2 band observed in exciting the 7AZD/acid complex. In conclusion, spectroscopic results based on strategically designed 7AZD derivatives related to 7AZD provide definitive evidence for the assignment of the F2 band to the imine tautomer emission. In comparison with the acetic acid-catalyzed tautomer emission of 7AI maximized at ∼ 500 nm, the F2 band of 7AZD is ∼2700 cm-1 (peak-topeak) higher in energy. Such a difference can be rationalized by the reduction of the length of π electrons configuration in their corresponding tautomeric species. Details will be elaborated in the discussion section. The association constant can also be obtained from the initially prepared ACID concentration, Cg, and the measured emission intensity at a selected wavelength expressed in eq 4

(

)

ΦMM F0 1 ) +1 F - F0 (Φpp - ΦMM) KaCg

(4)

where F0 and F denote the measured fluorescence intensity prior and after adding ACID. Here, ΦM and Φp are fluorescence quantum yields of the monomer and complex, respectively.9 The plot of F0/(F - F0) versus 1/Cg shown in the inset of Figure 5 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 9.0 × 10-5 M and 0.065, respectively. Consequently, a Ka value of 7.2 × 102 M-1 was obtained, which is consistent with the value of 7.8 × 102 M-1 concluded in the absorption titration study. 3.3. 7AZD/LACTAM Complex. Appreciable hydrogenbonding association was also observed between 7AZD and LACTAM in the absorption titration study (see Figure 8), and 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 7AZD/ LACTAM hydrogen-bonded complex. The best linear leastsquares fit using eq 3 gives a Ka value of 2.5 × 102 M-1 in cyclohexane (see Table 1). Since LACTAM also possesses



Figure 8. The concentration-dependent absorption and emission spectra of 7AZD in cyclohexane by adding various LACTAM concentrations (Cg) of (a) 0, (b) 2.7 × 10-4, (c) 5.5 × 10-4, (d) 7.5 × 10-4, (e) 1.0 × 10-3, and (f) 1.5 × 10-3 M. Inset: The plot of A0/(A0 - A) at 315 nm as a function of 1/Cg in curves b to f and a best least-squares fitting curve using eq 3. Asterisk (*) denotes the Rayleigh scattering at 335 nm.

bifunctional hydrogen-bonding groups, it is reasonable to expect the 7AZD/LACTAM complex exhibiting the same photophysical properties, i.e., the double proton transfer in the excited state, as those of 7AZD/ACID. In contrast, increasing the LACTAM concentration results in an increase of the emission intensity accompanied by a gradual red shift of the emission maximum from 353 to 368 nm (see Figure 8). Whereas, within the detection limit, the imine-like tautomer emission expected to be in the region of 400-600 nm cannot be resolved (see Figure 8). Similar to that observed in the concentrated 7AZD, the dynamics of decay can be well fitted by two single-exponential decay components with a lifetime of 5.0 and 3.2 ns which can be ascribed to the life span of 7AZD and 7AZD/LACTAM normal emission, respectively. In comparison, the 7AI/LACTAM complex undergoes fast ESDPT dynamics, resulting in a prominent tautomer emission.16c,d Knowing that both 7AZD dimer and 7AZD/LACTAM do not undergo a “noncatalytic” type of ESDPT, the results suggest that the reduction of π electrons conjugation in 7AZD relative to 7AI should play a key role for the observation of drastically different relaxation dynamics between 7AI and 7AZD. Detailed explanations will appear in the Discussion section. 4. Discussion 4.1. The Hydrogen-Bonding Formation. To rationalize the hydrogen-bonding formation we first performed an ab initio calculation for the relative energy among various isomers of 7AZD in its monomeric form. The results shown in Table 2 conclude the amino form of 7AZD to be the most stable one in the gas phase, while the imino tautomer is higher in energy than the amino form by ∼15.0 kcal/mol. The inclusion of a solvation free energy based on the PM3-SM4 model obtains similar results in which the amino form is more stable than the imino form by 13.6 kcal/mol. Figure 9a-c depicts the full geometry optimized structures (6-31G(d,p) basis set) of 7AZD dimer, 7AZD/ACID, and 7AZD/LACTAM dual hydrogenbonded complexes. For the case of 7AZD dimer, the association

enthalpy, ∆Hac, was calculated to be approximately -7.0 kcal/ mol. On the other hand, a possible linear, single hydrogenbonded dimeric form is also considered, namely the N(1)H‚‚‚N(7) single hydrogen-bonded dimer, and ∆Hac was calculated to be -3.6 kcal/mol, which is apparently much less exothermic than the cyclic dual hydrogen-bond formation. The results are in agreement with the absorption and fluorescence titration studies, concluding that the 7AZD cyclic dual hydrogen-bonded dimer is the predominant self-associated species. For the 7AZD/guest hydrogen-bonded complexes, our results show that, independent of the type of guest molecule studied, the most stable form of a 1:1 7AZD/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 -11.8 kcal/mol and -9.0 kcal/mol for 7AZD/ACID and 7AZD/LACTAM, respectively. Since ∆Hac generally correlates well with the hydrogen-bonding strength, the result may simply indicate that the hydrogen-bonding strength is on the order of 7AZD/ACID > 7AZD/LACTAM > 7AZD dimer. Evidence can be given by the calculated >N‚‚‚HO- hydrogen-bonding distance of 1.88 Å in the case of the 7AZD/ACID, which is shorter by 0.32 Å than the >N‚‚‚HN- distance in 7AZD dimer of 2.20 Å (see Figure 9). The trend in the exothermicity of the association enthalpy also qualitatively correlates with the experimentally measured order of the association constants for 7AZD dimer and hydrogen-bonded complexes (see Table 1), increasing as the association enthalpy decreases. However, knowing that the association constant is generally an order of magnitude smaller than that of the 7AI hydrogen-bonding species (vide supra), other factors such as the steric effect may play a key role to fine-tune the stability of various 7AZD hydrogen-bonded species. For the case of 7AZD dimer the N(1)-H bond was calculated to be ∼14° with respect to the pyridine plane, while the N(1)-H bond was calculated to be 25.3° in a geometry optimized 7AZD monomer form. The result indicates that an endothermic, sterically hindered rotation of the N(1)-H toward a favorable configuration is necessary

24.218 0.850 9.156 -1.572 0 0 14.957 0

Total energy, enthalpy, and free energy in hartrees. b ∆Hac was calculated by subtracting the formation enthalpy of host and guest molecules from their associated hydrogen-bonded complex. c ∆G was calculated by the difference in free energy (298 K) 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.

∆Gc

a

-13.850 -8.951

Relative Free Energy (kcal/mol) 12.437 2.461 24.945

-16.532 -11.769 Association Energy ∆Hac (kcal/mol) -6.870 -15.707

-702.56224 -702.26854 -702.32678 -702.59898 -702.30492 -702.36402 -606.47579 -606.26032 -606.31404 -606.49308 -606.27763 -606.33114 -757.27741 -756.98176 -757.04205 -323.912941 -323.76810 -323.80465 -378.62488 -378.47837 -378.51707 total E H(scaled) G(scaled)

-378.64973 -378.50326 -378.54090

-227.82217 -227.75561 -227.78773

-323.93247 -323.78739 -323.82447

-757.31264 -757.01747 -757.07788

N(1)H/LACTAM N(7)H/ACID

complex

N(1)H/ACID N(7)H dimer N(1)H dimer LACTIM LACTAM

guest molecule

ACID N(7)H

7AZD monomer

N(1)H

TABLE 2: Thermodynamic Parameters for Various 7AZD Hydrogen-Bonded Complexes Calculated by the 6-31G(d,p) Basis Set at 298K (in the Gas)a,b

N(7)H/LACTIM


Chou et al.

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) 7AZD dimer, (b) 7AZD/ACID, and (c) 7AZD/LACTAM.

prior to the dimerization, which may compensate for the gain of the stabilization due to the dual hydrogen-bonding formation. After truncating the 7AZD dimer by removing the dual hydrogen bonds but holding structures of both 7AZD unchanged, each 7AZD species was calculated to be ∼0.8 kcal/mol higher in energy than the geometry optimized 7AZD monomer. Note that a torsional angle of 14° is obtained when the strain energy (an endothermic process) plus hydrogen-bonding energy (an exothermic process) reach a minimum value upon dimerization. Thus, the sacrifice of certain hydrogen-bonding energy, e.g., the loss of a nonplanar configuration, is necessary to avoid increasing the strain energy toward planarity. This consequence explains the calculated ∆Hac of ca. -6.0 kcal/mol for the 7AZD dimer, which is less exothermic than that of -11 kcal/mol calculated for the 7AI dimer.16d Similar steric effects were obtained in the formation of other 7AZD hydrogen-bonded complexes, giving rise to a lower association constant in comparison to those of 7AI hydrogen-bonded species where the planar geometry of 7AI facilitates the dual hydrogen bonded dimer and/or complex formation.

Amine-Imine Double Proton Transfer The calculated free-energy change of the 7AZD hydrogenbonded 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. Attempts to resolve the solvation energy based on several different semiempirical methods have been made. However the 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 7AZD hydrogen-bonded systems in solution. In addition, 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. Parameters Tuning ESDPT Reaction. Recently, Nagaoka and Nagashima have developed an elegant description of the occurrence of excited-state proton transfer by considering the nodal pattern of the wave function and the delocalization of the lone electrons in the excited-state.34-36 According to the “nodal plane” model, the selective reaction pathway from various states can be qualitatively determined. For example, in the case of an excited-state intramolecular proton transfer (ESIPT) reaction, the higher reactivity of the La state over the Lb state in salicylaldehyde and its analogues can be understood in terms of a nodal plane analysis of their wave function where the nodal plane across the C1-C2 bond in La state (see Figure 1), leading to a driving force for the proton-transfer reaction. A similar method has also been applied to the case of the 7-azaindole dimer. Recent femtosecond dynamic studies reveal a fast proton transfer rate in the La state, while the relaxation of the Lb state is dominated by the Lb f La internal conversion. The higher reactivity of the La state over the Lb state in the dimeric form can also be rationalized in terms of a nodal plane analysis.37 While the theory of the nodal plane model renders a qualitative, simple method to determine the state-selective reaction pathway, relative thermodynamics, i.e., the relative energy levels, between normal (nonproton-transfer) and tautomeric (proton-transfer) forms also play a crucial role to further determine the “fate” of the excited-state proton-transfer reaction. Such a thermodynamic factor can be fine-tuned by the substituent effect. In the case of an ESIPT reaction both 1-hydroxy2-acetonaphthone (1H2AN) and 1-hydroxy-2-naphthaldehyde (1H2NA) have been reported to undergo fast proton transfer in the excited state, resulting in a large Stokes-shifted tautomer emission.38 In contrast, the ESIPT in methyl 1-hydroxy-2naphthoate (1H2MN) with a similar structure as 1H2AN and 1H2NA is prohibited. The results can be rationalized by the great electron donating property of the methoxy group, which destabilizes the S1′ state of the tautomer species. As a result, ESDPT in 1-hydroxy-2-naphthoate is thermally forbidden owing to its higher tautomer S1′ state with respect to the normal S1 state.38 Conversely, we have recently demonstrated that adding an electron-rich nitrogen atom in the pyrrole system of 7AI, forming 4-azabenzimidazole (4ABI), alters the resonance skeleton in the tautomeric form toward a drastic increase of the πfπ* energy gap. Consequently, relative energy levels between S1 and S1′ states turn out to be crucial for the observed guest molecular-based tuning ESDPT9 in which a fast ESDPT takes place in the catalytic case of the 4ABI/ACID complex, resulting

J. Phys. Chem. B, Vol. 104, No. 32, 2000 7827 in a prominent tautomer emission, whereas ESDPT is thermally unfavorable in the noncatalytic case of the 4ABI/LACTAM complex. Knowing that the normal f tautomer proton transfer process for most ESIPT and ESDPT molecules is basically an endergonic reaction in the ground state, the S0′ f S1′ transition of the tautomer has to be significantly smaller than the S0-S1 gap of the normal species in order for the excited-state proton transfer reaction to proceed. From a spectroscopic viewpoint, the ESDPT and/or ESIPT usually result in a conversion of the aromatic conjugated double bonds in its normal form to a nonaromatic type of conjugated double bonds. Hence, the πfπ* transition (i.e., the S0′-S1′ energy gap) of the tautomeric form is expected to be much more sensitive to the length of π electron conjugation than that of the normal form. A prominent example can be shown in a comparative study on 5-hydroxyflavone (5HF) and 5-hydroxyflavanone (5HFN) where the C2-C3 bond in the pyrone ring is hydrogenated relative to 5HF, reducing the length of the conjugated system of the tautomer species.39 Consequently, the 5HFN tautomeric form with an emission maximized at 550 nm (18 180 cm-1) is ∼3255 cm-1 higher in energy than that of the S1′-S0′ of the 5HF tautomeric form of 670 nm (14 925 cm-1). Whereas there is only a difference of ∼800 cm-1 in the S0-S1 transition between 5HF (28 160 cm-1) and 5HFN (28 960 cm-1). In this study we render another key parameter based on the length of conjugated double bonds to fine-tune the thermodynamics of the excited-state proton transfer. Despite the occurrence of ESDPT in the 7AZD/ACID complex, the ESDPT is prohibited in 7AZD dimer and 7AZD/LACTAM complex. The lifetime of 7AZD dimer and 7AZD/LACTAM complex was measured to be as long as 2.1 ns (kf ) 4.8 × 108 s-1 ) and 3.2 ns (kf ) 3.1 × 108 s-1, see Table 1), respectively. Thus, it is quite unlikely that the rate of ESDPT is exceedingly slower than the lifetime of either excited 7AZD dimer or 7AZD/ LACTAM complex 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 energy gap and its corresponding length of the π electrons conjugation in which the maximum of S0S1 transition of 7MDPP (375 nm; 26,667 cm-1), a model compound of the 7AZD tautomer, is higher in energy than that (390 nm; 25,640 cm-1) of 7-methyl-7H-pyrrole[2,3-b]pyridine (7MPP), a model compound of the 7AI tautomer by ∼1027 cm-1 (see Table 1). On the contrary, a reverse order of the S0S1 absorption peak frequency was observed between 7AZD (306 nm; 32 679 cm-1) and 7AI (287 nm, 34 843 cm-1) monomer, being ∼2160 cm-1 lower in energy than for the 7AZD monomer (see Table 1). The result indicates that the hydrogenation of the C2-C3 double bond in the pyrrole system of 7AI alters the resonance skeleton in the tautomeric form toward a drastic increase of the πfπ* energy gap, while increasing the chargetransfer property results in decreasing the S0-S1 transition of the normal 7AZD (vide supra). Consequently, for various 7AZD hydrogen-bonded complexes relative energy levels between the S1 state of the amino form and the S1′ state of the imino tautomer turn out to be crucial for the observed guest molecular-based tuning ESDPT, especially the catalytic versus noncatalytic type. To rationalize the proposed mechanism, we have attempted to construct relative energy levels for an overall proton-transfer cycle in 7AZD. Experimentally, due to the thermal prohibition for the tautomerism, it is not feasible to obtain a relative free

7828 J. Phys. Chem. B, Vol. 104, No. 32, 2000 energy between the 7AZD dimer (or 7AZD/guest complex) and its corresponding 7AZD(T) dimer (or 7AZD(T)/guest complex) in the ground state. Alternatively, using a 6-31G(d,p) basis set, the change of free energy for 7AZD(T)/ACID, 7AZD(T) dimer and 7AZD(T)/LACTIM was calculated to be 10.7, 22.5, and 23.4 kcal/mol, respectively, relative to their corresponding 7AZD (amino form) hydrogen bonding complexes (see Table 2). The assignment of the 0-0 band for the S0-S1 transition of the 7AZD dimer and/or complexes is relatively difficult due to their broad absorption profile. For a simplified approach, we take the frequency where the intensity of the excitation spectrum (monitored at the tautomer emission of 7AZD/ACID) is ∼1/3 of the maximum to be the 0-0 transition (∼340 nm; 29 412 cm-1) for either the normal 7AZD dimer or the 7AZD/guest complexes. Conversely, it is rather simple to assign the S0′S1′ (0-0) transition of the imine tautomer due to its vibronically progressive emission observed in the 7AZD/ACID complex. We then take the first vibronic peak of 412 nm (24 272 cm-1, see Figure 5) to be the 0-0 transition which is also applied to either the 7AZD(T) dimer or the 7AZD(T)/LACTIM complex. Given all of these values, the relative energy levels of a proton-transfer cycle in the singlet manifold are depicted in Scheme 1. Apparently, 7AZD/ACID proton-transfer tautomerism in the excited state is calculated to be -4.1 kcal/mol. On the contrary, an endergonic value of 7.7 and 8.6 kcal/mol was obtained for the case of the 7AZD dimer and 7AZD/LACTAM complex, respectively. Although this approach is qualitative due to the uncertainty of obtaining accurate excited-state energy levels for both normal and tautomer species in solution phase, it clearly shows that for 7AZD dimer and 7AZD/LACTAM complex ESDPT is thermally unfavorable, consistent with the experimental results. The result can also be rationalized qualitatively by the noncatalytic type of ESDPT where the guest molecules, i.e., 7AZD amino form (in the 7AZD dimer) and LACTAM (in the 7AZD/LACTAM complex) should undergo a corresponding change to the imine and lactim forms, respectively, during ESDPT. Such a noncatalytic process should raise its S1′ energy level which is a consequence of the simultaneous tautomerization for both host and guest molecules. The result of endergonic tautomerism on one hand may simply indicate that either the 7AZD dimer or the 7AZD/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., the existence of a high energy barrier, so that ESDPT may be frustrated during the lifetime of the excited 7AZD dimer or the 7AZD/LACTAM complex. Both mechanisms unfortunately cannot be distinguished at this stage. 5. Conclusion Summarizing the above results and discussion, we have studied hydrogen-bonded complexes of 7AZD by means of absorption, emission, and theoretical calculations. For the catalytic type of ESDPT such as 7AZD/ACID, where cyclic dual hydrogen bonds are intrinsically formed, the photoinduced double proton transfer may require only 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 7AZD/ACID complex, as supported by the lack of a normal 7AZD/ACID emission as well as the unresolved imine tautomer fluorescence rise time (τrise < 3 × 10-10 s).

Chou et al. The results open up a study of ESDPT dynamics based on an generalized amine/imine tautomerism in which the mechanism may be extended to a DNA base such as adenine possessing similar type of tautomerization proposed in the mutation process.40 In contrast, despite the dual hydrogenbonding association, ESDPT is prohibited in the case of the 7AZD dimer and 7AZD/LACTAM complex, supporting the proposed noncatalytic type of ESDPT mediated by the length of the π electron conjugation. Acknowledgment. This work was supported by the National Science Council, Taiwan, R.O.C. (grant No. NSC 87 -2119M-194-002). References and Notes (1) Taylor, C. A.; El-Bayoumi, A. M.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1969, 65, 253. (2) Ingham, K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1971, 93, 5023 (3) Ingham. K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1974, 96, 1674. (4) Watson, J. D.; Crick, F. H. C. Nature (London) 1953, 171, 964. (5) Watson, D. G.; Sweet, R. M.; Marsh, R. E. Acta Crystallogr. 1965, 19, 573. (6) Douhal, A.; Kim, S. K.; Zewail, A. H. Nature, 1995, 378. (7) (a) Chachisvillis, M.; Fiebig, T.; Douhal, A.; Zewail, A. H. J. Phys. Chem. A 1998, 102, 669. (b) Fiebig, T.; Chachisvillis, M.; Manger, M.; Zewail, A. H.; Douhal, A.; Garcia-Ochoa, I.; de La Hoz Ayuso, A. J. Phys. Chem. A 1999, 103, 7419. (8) Chou, P. T.; Wei, C. Y.; Wu, G. R.; Chen, W. S. J. Am. Chem. Soc. 1999, 121, 12186. (9) Chou, P. T.; Wu, G. R.; Wei, C. Y.; Cheng, C. C.; Chang, C. P.; Hung, F. T. J. Phys. Chem. B 1999, 103, 10042. (10) McMorrow, D.; Aartsma, T. Chem. Phys. Lett. 1986, 125, 581. (11) (a) Tokumura, K.; Watanabe, Y.; Itoh, M. J. Phys. Chem. 1986, 90, 2362. (b) Tokumura, K.; Watanabe, Y.; Udagawa, M.; Itoh, M. J. Am. Chem. Soc. 1987, 109, 1346. (12) Moog, R. S.; Bovino, S. C.; Simon, J. D. J. Phys. Chem. 1988, 92, 6545. (13) Koijnenberg, J.; Huizer, A. H.; Varma, C. A. O. J. Chem. Soc., Faraday Trans. 2 1988, 84 (8), 1163. (14) (a) Moog., R. S.; Maroncelli, M. J. Phys. Chem. 1991, 95, 10359. (b) Chapman, C. F.; Maroncelli, M. J. Phys. Chem. 1992, 96, 8430. (c) Mentus, S.; Maroncelli, M. J. Phys. Chem. A 1998, 102, 3860. (15) (a) Negreie, M.; Bellefeuille, S. M.; Whitham, S.; Petrich, J. W.; Thornburg, R. W. J. Am. Chem. Soc. 1990, 112, 7419. (b) Negrerie, M.; Gai, F.; Bellefeuille, S. M.; Petrich, J. W. J. Phys. Chem. 1991, 95, 8663. (c) Negrerie, M.; Gai, F.; Lambry, J.-C.; Martin, J.-L.; Petrich, J. W. J. Phys. Chem. 1993, 97, 5046. (d) Chen, Y.; Rich, R. L.; Gai, F.; Petrich, J. W. J. Phys. Chem. 1993, 97, 1770. (e) Chen, Y.; Gai, F.; Petrich, J. W. J. Am. Chem. Soc. 1993, 115, 10158. (f) Rich, R. L.; Chen, Y.; Neven, D.; Negrerie, M.; Gai, F.; Petrich, J. W. J. Phys. Chem. 1993, 97, 1781. (g) Gai, F.; Rich, R. L.; Petrich, J. W. J. Am. Chem. Soc. 1994, 116, 735. (h) Smirnov, A. V.; English, D. S.; Rich, R. L.; Lane, J. Teyton, L.; Schwabacher, A. W.; Luo, S.; Thornburg, R. W.; Petrich, J. W. J. Phys. Chem. 1997, 101, 1B, 2758. (16) (a) Chou, P. T.; Martinez, M. L.; Cooper, W. C.; McMorrow, D.; Collin, S. T.; Kasha; M. J. Phys. Chem. 1992, 96, 5203. (b) Chang, C. P.; Hwang W. C.; Kuo M. S.; Chou P. T.; Clement, J. H. J. Phys. Chem. 1994, 98, 8801. (c) Chou, P. T.; Wei, C. Y.; Chang, C. P.; Chiu, C. H. J. Am. Chem. Soc. 1995, 117, 7259. (d) Chou, P. T.; Wei, C. Y.; Chang, C. P.; Kuo, M. S. J. Phys. Chem. 1995, 99, 11994. (e) Chou, P. T.; Yu, W. S.; Chen, Y. C.; Wei, C. Y.; Martinez, S. S. J. Am. Chem. Soc. 1998, 120, 12927. (17) Chaban, G. M.; Gordan, M. S. J. Phys. Chem. A 1999, 103, 185. (18) 7AZD has been used as an intermediate to synthesize various 7-azaindoles.19 However, its photophysical and photochemical properties have not yet been reported. (19) Taylor, E. C.; Macor, J. E.; Pont, J. L. Tetrahedron 1987, 43, 5145. (20) Gonnela, N. C.; Roberts, J. D. J. Am. Chem. Soc. 1982, 104, 3162. (21) Dryefus, M.; Dodin, G.; Bensaude, O.; Dubois, J. E. J. Am. Chem. Soc. 1975, 97, 2369. (22) Robison, M. M.; Robison, B. L. J. Am. Chem. Soc. 1955, 77, 6554. (23) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024. (24) Chou, P. T.; Wei, C. Y.; Hung, F. T. J. Phys. Chem. B. 1997, 101, 9119. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.

Amine-Imine Double Proton Transfer D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, J.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, a. D.; Raghavachari, K. Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian, Inc.: Pittsburgh, PA, 1998. (26) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (27) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221. (28) Buemi, G. THEOCHEM, 1988, 41, 379. (29) Bliznyuk, A. A.; Voityuk, A. A. THEOCHEM, 1983, 41, 343. (30) Cramer, C. J.; Truhlar, D. G. J. Am. Chem. Soc. 1993, 115, 8810. (31) Cramer, C. J.; Truhlar, D. G. J. Comput. Chem. 1992, 13, 1089. (32) Wang, J.; Boyd, R. J. Phys. Chem. 1996, 100, 16141.

J. Phys. Chem. B, Vol. 104, No. 32, 2000 7829 (33) Through an absorption titration experiment the pKa of 7AZDH+ was determined to be 6.8. The pKa* of 7AZDH+ was calculated from the Fo¨rster cycle based on the absorption maximum frequencies for 7AZD and 7AZDH+. (34) Nagaoka, S.; Nagashima, U. Chem. Phys. 1989, 136, 153. (35) Nagaoka, S.; Nagashima, U. J. Phys. Chem. 1990, 94, 1425. (36) Nagaoka, S.; Shinde, Y.; Mukai, K.; Nagashima, U. J. Phys. Chem. A 1997, 101, 61, and references therein. (37) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 1998, 102, 7740. Note that a different interpretation regarding the Lb f La internal conversion has been reported in ref 7b. (38) Tobita, S.; Yamamoto, M.; Kurahayashi, N.; Tsukagoshi, R.; Nakamura, Y.; Shizuka, H. J. Phys. Chem. A 1998, 102, 5206. (39) Martinez, M. L.; Studer, S. L.; Chou, P. T. J. Am. Chem. Soc. 1991, 113, 5881. (40) Chou, P. T.; Wei, C. Y., in preparation.

Excited-State AmineImine Double Proton Transfer in 7-Azaindoline

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