76
J. Phys. Chem. 1995, 99, 76-80
Photoinduced Intramolecular Proton Transfer and Charge Redistribution in Imidazopyridines Abderrazzak Douhal,tl$Francisco Amat-Guerri? and A. Ulises Acufia**+ Instituto de Quimica Fisica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain, and Instituto de Quimica Orghnica, CSIC, Juan de la Ciewa 3, 28006 Madrid, Spain Received: March 16, 1994; In Final Form: September 6, 1994@
The ground-state conformation and fluorescence properties of two closely related dyes 2-(2’-hydroxypheny1)imidazo[ 1,2-a]pyridine (1,2-HPIP) and 3-(2’-hydroxyphenyl)imidazo[1,5-a]pyridine (13-HPIP) have been studied in various solvents at room temperature to determine the origin of the large red-shifted emission spectra. The two compounds have a ground-state planar conformation stabilized by an intramolecular H bond (IHB) between the imino and phenol groups. On electronic excitation in cyclohexane solution a weak fluorescence is observed in the two dyes, with large Stokes shifts (1,ZHPIP: ,1 = 588 nm, Av = 11 000 cm-’. 1,5-HpIP: ,1 = 440 nm,Av = 9OOO cm-’). For the first dye it is proposed that the emission takes place from a zwitterion produced by a proton-transfer reaction in the singlet manifold. A quinonoid, neutral phototautomer is excluded on the basis of the electronic structure of the molecule. In the case of 1,5-HPIP, the IHB is broken on electronic excitation and neither a proton nor H atom transfer occurs due to the reversal of the proton affinity of the imino group. The anomalous fluorescence is assigned to significant changes in the geometry of the emitting state following excitation.
Introduction cal reasons. On the other hand, there are some aromatic compounds where the specific electronic structure would not The redistribution of electronic charge due to a photonic allow the reorganization required in the phototautomerization excitation in organic bifunctional molecules which contain m e c h a n i ~ mto~ ~yield - ~ ~the keto species, and the excited-state hydrogen atom donor and acceptor groups produces an elemenprocess, if indeed it takes place, must be a true proton-transfer tary reaction generally described as excited-stateintramolecular reaction. proton transfer (ESIPT).1-5 In some specific compounds the In this work we present an investigation of the spectroscopy excited photoproduct is strongly fluorescent, and this has been of two closely related heterocycles that might produce the used to generate stimulated emission with high efficiency.6-l3 loosely called ESIPT process but which differ in the possibility In the f i s t detailed investigation of an ESIPT process, carried of yielding a keto tautomer: 2-(2’-hydroxyphenyl)imidazo[ 1,2out by Weller on methyl salicylate? it was proposed that pKa alpyridine (1,2-HpIP) and 3-(2’-hydroxyphenyl)imidazo[1,5-a]changes in the donor (phenol) and acceptor (carbonyl) groups pyridine (1,5-HpIP) (Chart 1). It is shown here that, on in the excited salicylate gave rise to the protonation of the electronic excitation, 1,2-WIP gives rise to a weak fluorescence carbonyl oxygen by photodissociation of the hydroxy group in with a large Stokes shift (1 1 000 cm-’). Since it is impossible the phenol moiety. Such a zwitterionic structure of the transient species was adopted by later workers, including o u r s e l ~ e s , ~ ~ - ’ ~ to write a keto tautomer for this compound, we propose a zwitterionic structure for the emitting species. In the case of in the subsequent investigation of the photophysics of several 1,5-HpIP, a weak fluorescence with a large Stokes shift (9000 salicylic derivatives. Recently, it has been proposedz0 that a cm-’) is also observed. Although for this compound both keto neutral structure of the transient species would be more and zwitterionic tautomers are possible, it is demonstrated here consistent with both theoretical predictions2’ and the femtosecthat neither of them is produced. That is, this molecule fails to ond dynamics of the process.z0 In this case, photoexcitation yield the excited-state H atom (or proton) transfer reaction, would result in molecular tautomerization and a small displacedespite a strong intramolecular H bond (IHB)existing in the ment (-0.02 A) of the intramolecularlybonded hydrogen toward ground state. The anomalous fluorescence is tentatively asthe carbonyl oxygen. Direct experimental evidence of a similar signed to an excited state that differs significantly from the phototautomerizationprocess was obtained recently by Elsaesser Franck-Condon configuration. et al.22*23 in the investigation of the ESIPT mechanism of 2-(2’hydroxypheny1)benzotazole with picosecond-resolved IR specExperimental Section troscopy. On the basis of the similarity with the chromophore of the later compound, we also suggested that a neutral, keto Methods. Absorption and emission spectra were recorded structure would also be produced in the photoexcitation of 2-(2’on Carry 219 and SLM go00 instruments, respectively. Spectra hydroxyphenyl)imidazole,2-(2’-hydroxyphenyl)beruimida~ole,~ are presented in photon units and have been corrected for and 2-(3’-hydroxy-2’-naphthyl)benzimidazole~salthough the instrumental factors. Fluorescence quantum yields (&) were well established ESIPT denomination was conserved for historimeasured with quinine sulfate in 0.1 N HzS04 (4f0.51)z8as a reference and were corrected for the solvent’s refractive index. * Address correspondence to this author at the Instituto de Quimica Fisica A nanosecond time-correlated single-photon counting specInstituto de Quimica Fisica Rocasolano CSIC. * Present address: Facultad de Quimicas, Departamento de Quimica trometer, equipped with Ortec electronics and a nanosecond flash Fisica, Universidad de Castilla la Mancha, San Lucas 3, 45002 Toledo, lamp (EL 199, Edinburgh Instruments) fitted with a 10 cm Spain. monochromator, was used to measure the fluorescence decaysz9 6 Instituto de Quimica Orghica CSIC. The emission was isolated with interference or cutoff filters Abstract published in Advance ACS Abstracts, December 1, 1994. @
0022-365419512099-0076$09.00/0 0 1995 American Chemical Society
J. Phys. Chem.,Vol.99,No. 1, 1995 77
Proton-Transfer in Imidazopyridines
CHART 1: Structures of the Imidazopyridines with Selected 'H NMR Data: Nuclear Overhauser Enhancement (NOE) and Chemical Shifts(ppm) of Diagnostic Protons
I
12.70
. , I',
300
A
I
400
.'
,'
---
I
/
500
\
, 600
J
A/nm 1
12.00
1,6HPIP
3.80
Me-1,S-HPIP
(Schott KV) and collected at the magic angle. The decay curves were analyzed by nonlinear least-squares iterative convolution methods, developed according to published descriptions. The quality of the fits was checked with standard statistical parameters.3O Estimated total errors in fluorescence quantum yields and lifetimes were 20% and lo%, respectively. Lasing efficiencies of M dye solutions were measured using a homemade automatic preionized N2 laser producing up to 3 mJ in 5 ns fwhm pulses at 337 nm (typical pumping energy 1.5 mJ),31and a homemade oscillator cavity.32 'H NMR spectra (6 scale, TMS as internal reference) were run on a Varian XL-300 spectrometer, at 300 MHz; assignments were c o n f i i e d by homonuclear decoupling, nuclear Overhauser enhancement (NOE), and correlation spectroscopy (COSY) experiments. IR spectra were recorded on a Perkin-Elmer 681 spectrophotometer. Mass spectra (MS) were determined on a spectrometer VG 12-250 in the electronic impact mode (70 eV). Microanalyses were performed on a Perkin-Elmer 2400 instrument. All samples herein studied gave correct C,H,N data and showed a single spot on thin layer chromatography (TLC) plates (silica gel) with several eluents. Spectroscopic grade solvents (Merck), dried by standard methods,33 were used throughout. Materials. 2-(2'-Hydroxyphenyl)imidazo[ 1,2-a]pyridine(1,2HPIP), 3-(2'-hydroxyphenyl)imidazo[l,S-HPIP), and their corresponding 0-methyl derivatives (Me-1,2-HPIP and M e - 1 5 HPIP) (Chart 1) were synthesized and purified as described in the Appendix.
Results and Discussion
2-(2'-Hydroxyphenyl)imidazo[l,2-~]pyridine(1,ZHPIP). The 'H NMR spectrum of this compound in CDC13 is consistent with a planar ground-state conformation with an IHB (see Chart 1). The OH proton appears highly deshielded, and a NOE experiment irradiating H-3 indicates that it should be close to H-6'. The methylated derivative Me-1,ZHPIP is probably also planar but with the E-conformation shown in Chart 1, as irradiation of H-3 does not influence the intensity of the H-6' signal. In addition, the downfield shifts of H-3 and H-6', if compared to the positions of the same protons in l,ZHPIP, can be explained by electronic effects of the oxygen of the methoxy group and of the 1-nitrogen, respectively. A similar effect of the heteroatom on H-5 has been observed in 2,3-disubstituted imidazo[ 1,2-a]pyridine~.~~ The electronic absorption spectra of 1,2-HPIP and Me-1,2HPIP in cyclohexane (Figure 1 and Table 1) show an intense,
Figure 1. Absorption (A) and corrected fluorescence (F) spectra of 1,ZHPJP(-) and Me-1,2-HF'IP (- - -), M solution in cyclohexane (panel I) and in dioxane (panel II), at room temperature.
structured band, most probably due to a (n, n*)transition ( E 22000 M-' cm-l). Due to molecular interactions with the solvent, the vibrational structure is lost in dioxane. As shown below, for 1,ZHPIP these interactions are able to disrupt the IHB to some extent. The fluorescence spectrum in cyclohexane shows a single, structureless band (,I, 588 ,,= nm) with a large Stokes shift (1 1 000 cm-') that decays with single-exponential kinetics, tf = 1.8 ns. The fluorescence excitation spectrum is identical to the absorption spectrum described previously. This emission must proceed from the tautomer species, formed by an E S P T reaction occurring in the initially excited enol form, since the methylated compound Me- 1,2-HPIP, which cannot exhibit this process, shows a 'normal' fluorescence band with a vibrational structure similar to that of the absorption one. This weak fluorescence of 1,2-HPIP changes significantly with the H bond ability of the solvent. Thus, in dioxane solution the emission presents two bands: a red one (605 nm), similar to that found in cyclohexane, and a blue, structured band (379 nm) with a normal Stokes shift (Figure 1 and Table 1). The decay of the two emission bands is single-exponential, with lifetimes of 0.6 and 3.0 ns, respectively (Figure 2). Since the properties of the 379 nm fluorescence (position and shape of the band, excitation spectrum (Figure 3), and fluorescence lifetime) are similar to those found in Me-1,2-HPIP, the blue emission band must originate from a small population of molecular species which, due to interactions with the solvent (dioxane), do not form IHB's. The structure of this species (named open-enol form) should be similar to that of Me-1,2HPIP. On the basis of the absorption and fluorescence excitation spectra, the estimated population of this 'open' conformer of 1,ZHPIP in dioxane is 10-15%. The fact that the two fluorescence bands show distinct excitation spectra indicates the presence of two different conformers in equilibrium in the ground state of 1,2-HPIP. Moreover, since the fluorescence lifetimes of the blue and red bands are also different, the corresponding emitters (open enol and tautomer) are not in equilibrium in the excited state. The absence of any fluorescence that can be assigned to the excited closed enol form in cyclohexane suggests that the ESIPT in 1,ZHPIp is an ultrafast and irreversible process. Considering
Douhal et al.
78 J. Phys. Chem., Vol. 99, No. 1, 1995
TABLE 1: Absorption and Fluorescence Properties of 2-(2'-Hydroxyphenyl)imidazo[ 1,2-a]pyridine (1,2-HpJP) and Its Methylated Derivative (Me-1,2-HpIP) in Solution at Room Temperature compdkolvent A,(abs) (nm) dlnaX(flu0) (nm) @reb %-Rc zreb (ns) t d (ns) 1,2-HPP 1.8 588 0.06 cyclohexane 311,325,337, 35Sd 3.0 0.6 379,605 0.01 0.04 dioxane 320,332, 346d Me-1,2-HPP cyclohexane 301,323,335, 352d 357,375,398 0.25 2.9 2.8 360,379,400 0.35 dioxane 302,319,331, 346d "Uncertainties in @f and rf are 20% and lo%, respectively. bFluorescence quantum yield and lifetime of the blue band at 350-450 nm. Fluorescence quantum yield and lifetime of the band at 500-670 nm due to the zwitterion produced by ESIPT reaction. 0-0 transition.
7
i 5
-61
1- ;::1
i;Io"m_pT-:1:
I
I
Figure 2. Instrument function and fluorescence decay of 1,2-HPLP in dioxane. UV-blue (aem= 350-440 nm) band: rm = 3.0 ns, x* = 1.1. Red (Acutoff = 520 nm) band: ZR = 0.6 ns, x2 = 1.3.
Figure 3. Excitation spectra of the W fluorescence (- - -) (aem= 380 nm) and of the red fluorescence (-) (Acm = 560 nm) of 1,2-HPIP, M solution in dioxane, and fluorescence excitation spectrum (.* (Aem = 380 nm) of Me-1,2-HPIP, loT6M in dioxane. a)
the detection limit of the fluorimeter used in this work and the estimated fluorescence rate constant of the excited enol form (lo8 s-l, the same as that of Me-1,2-HPIP in cyclohexane), the rate constant of the ESIPT reaction in 1,2-HPIP should be greater than 1013s-l, a value similar to that measured by Frey and Elsaesser in 2-(2'-hydroxy-5'-methylphenyl)benzot1iazole.~~
This estimation is also consistent with recent sub-picosecond measurements carried out in l,2-HPIP35 and suggests a small or negligible barrier for the excited-state reaction. The redistribution of electronic charge after excitation of 1,2HPIP increases the acidity of the proton-donating phenol group and, simultaneously,the basicity of the accepting partner. From pH-dependent shifts of the absorption spectra of both the dye and its methylated derivative in aqueous solutions, an increase of 1-2 orders of magnitude in the dissociation constant (&) of the hydroxy (phenolic) group and a decrease of 4 orders of magnitude in the Kavalue of the conjugated acid of the imino group can be estimated. These changes make possible a true proton transfer in the excited state to yield the zwitterion, as depicted in Figure 4. It should be recalled that it is impossible to draw a quinonoid (keto) structure for 1,2-HPIP. In addition, the observed red shift of the phototautomer fluorescence on increased solvent polarity is consistent with a larger dipole moment of the emitting state. The efficient radiationless deactivation of the zwitterion probably takes place through intersystem crossing to the triplet manifold(s), facilitated by interacting n, JC*and JT, JT* states. Magnetic circular dichroism experiments of the parent compound imidazo[ 1,2-~]pyridine~~ have shown that the two states are in close proximity and that, on the other hand, the nonhydroxylated derivative 2-phenylimidazo[ 1,2-u]pyridine is phosph~rescent.~'This low fluorescence yield of the phototautomer species prevents the use of 1,ZHPIP as a lasing dye (lasing efficiency -OS%), in contrast to what has been observed with other heterocycle^.^^^^^^^ Interestingly, the fluorescence spectrum of the methylated derivative Me-1,ZHPIP is a mirror image of the absorption spectrum of 12-HPIP (Figure 1). This indicates that the excitedsinglet state of Me-1,2-HPIP adopts a conformation similar to that of the ground state of 1,ZHPIP. Recalling our previous discussion on the planar conformation of 1,2-HPIP based on 'H NMR data, this similarity suggests that rotation of the 2'-
J. Phys. Chem., Vol. 99, No. I, 1995 79
Proton-Transfer in Imidazopyridines
1
I
Figure 5. Absorption (A) and corrected fluorescence (F) spectra of 1,5#PIP (-) and Me-1,5-HPIP (- - -), M in cyclohexane at room teetye.
TAkLE 2: Absorption and Fluorescence Properties of
3-(2’-Hydroxyphenyl)imidazo[l,5s]pyridine (1,5-HPIP) and Its Methylated Derivative (Me-1,5-HPIP)in Solution at Room Temperature compdsolvent d,,(abs) (nm) I,(fluo) (nm) @f tf(ns) 1,5-HPIP cyclohexane 329 414,440,468 0.05 3.1 dioxane 331 468 0.05 3.6 Hz0, pH 1.8 306 420 ‘0.01 4.0,b0.2c HzO, pH 6.5 304 453 0.01 4.0: 1.1‘ H20, pH 12.5 316 473 0.01 3.6,bO S d Me-1,5-HPIP cyclohexane 311 397,420,442 0.06 3.8 dioxane 310 446 0.06 5.0 H20, pH 1.8 304 395 0.11 4.6: 0.4‘ HzO, pH 6.5 303.5 0.06 5.2; 1.2c 447 HzO, pH 12.5 303.5 450 0.05 5.1b “Uncertainties in @f and tf are 20% and lo%, respectively. b-dLifetime~of the neutral, protonated (in the imidazole ring) and anionic (phenolate) species, respectively, measured with appropriated filters. methoxyphenyl group of Me-1,2-HPIP takes place on electronic excitation. We note that this structural change can be used to generate stimulated emission radiation, as has been observed in picosecond time-resolved measurement^.^^
3-(2’-Hydroxyphenyl)imidazo[l,5-~]pyridine(1,5-HPIP). The absorption spectra of this compound in cyclohexane present an intense, broad band centered at 329 nm, characteristic of a (n, n*)transition, whose tail extends into the visible range. A similar red-edge absorption can be observed in the spectrum of the unsubstituted chromophore imidazo[ 1,5-a]pyridine. On the basis of magnetic circular dichroism spectra, this has been assigned to the (n, n*)t r a n ~ i t i o n .The ~ ~ spectrum of the methyl derivative Me-1,5-HPIP shows a very similar shape, with the maximum of the main absorption band shifted to the blue by 1700 cm-’ (Figure 5). This difference reflects the formation of a strong IHB in 1,5-HPIP between the nitrogen atom at position 2 and the phenol group (Chart 1). The ‘HNMR data of both compounds in CDC13 solution support the H bonding and also a planar arrangement of imidazopyridine and phenol moieties. Thus, in the case of 1,5-HPIP, irradiation of H-5 gives a clear NOE effect on H-6‘ (see Chart 1). On the contrary, in the methylated compound Me-1,5-HPIP the proton at position 5 resonates upfield (Ad = 0.92 ppm) compared to that in the parent compound, indicating that the phenyl group is outof-the-plane of the imidazopyridine moiety. This lack of planarity in the methylated compound also contributes to the shift of the W absorption spectrum to the blue. 1,5-HpIP in cyclohexane shows a low-yield fluorescence (Figure 5 and Table 2) consisting of a single, weakly structured band at 440 nm with a large Stokes shift (-9000 cm-’) which
extends over most of the visible spectral range (400-600 nm). A similar spectrum was recorded in both dioxane and water solution (Table 2). Although it is tempting to assign this emission to an excited species formed by a proton (or H atom) transfer reaction, as in 1,2-HPIP, this cannot be the case, as the emission of the methylated derivative, where the shifting proton does not exist, records very similar spectral features (lifetimes and radiative rate constants, Figure 5 ) to those of 1,5-HPIPP. Moreover, the fluorescence band of the unsubstituted chromophore imidazo[ 1,5-a]pyridine in n-hexane solution37is also very similar to that described above. Therefore, it seems that, despite the strong IHB which exists between the imino and phenol groups in ground-state molecules in inert solvents, the phototransfer of the proton does not take place. A reason for this may be that the ESIPT process is not allowed on energetic grounds, because the imino group of the molecule does not become basic enough to bind the proton of the phenol OH. In fact, the estimated excited-state changes of pKa, determined from the prototropic shifts of the absorption spectra as described before for 1,2-HPIP, indicate that the imino group becomes even slightly acidic, which would result in the weakening of the IHB on electronic excitation. Previous observations on the parent chromophore imidazo[ 1,5-a]pyridine, i.e., without the phenyl moiety, are consistent with this interpretation. Thus, Lemer et al. have reported37that this compound in n-hexane solution forms H-bonded groundstate complexes with n-butanol but that these complexes dissociate when the dye is photoexcited. In addition, theoretical computations from the same authors show a charge migration from the imidazole part to the pyridine ring in the excited-singlet state of that compound, that can explain the observed decrease in the basicity of the imino group. On this basis, the anomalous emission of 1,5-HPIP is probably due to the formation of a singlet state with a configuration that lacks the MB and that differs substantially from the initially excited Franck-Condon state. This geometrical change is not due to the out-of-plane rotation of the phenyl substituent, because of the similarity of the fluorescence of unsubstituted imidazo[1,5-a]pyridine. Rather, it probably involves an extended distortion of the imidazopyridine frame. The low fluorescence quantum yield of 1,5-HPIP and its methylated derivative Me-lS-HPIP, in solution (Table 2), compared with that of the parent compound imidazo[ 1,5-a]pyridine (0.98 from ref 37), points to a large increase of the nonradiative channels on phenyl substitution. Since we did not observe phosphorescence from a methylcyclohexane solution of 1,5-HPIP at 77 K, the dominant radiationless process is probably the result of a large increase in the internal conversion rate. This might be due to an increased interaction of the closelying n,n* and n, n* states and the effect of low-frequency torsional modes of the phenyl group. High-resolution spectra of the jet-cooled molecules could give a more firm understanding of the 1,5-HPIP photophysics. In conclusion, despite the apparent similarity in the two closely related heterocyclic dyes studied here, observed both in the tendency to form strong IHB’s and in the emission of a broad fluorescence band with a large Stokes shift (9000-1 1 000 cm-’>, the excited-state processes are completely different. In the case of 1,2-HPIP, the electronic excitation yields a weakly emitting zwitterionic species, the result of a proton-transfer reaction along the IHB between the phenolic and imino groups. In contrast, on excitation of 1,5-HPIP, the H bond is interrupted due to a redistribution of charge that reverses the proton affinity of the imino group. The fluorescence most probably originates from an electronic state that differs considerably from the ground-state equilibrium configuration.
80 J. Phys. Chem., Vol. 99, No. 1, 1995
Acknowledgment. This work was supported by Projects PB90-102 and MAT93-369 (Spanish Comisi6n Interministerial de Ciencia y Tecnologia, CICYT). A.D. thanks the Spanish Government for a predoctoral grant.
Douhal et al. 1 H, H-5'), 6.82 (dd, J = 6.5 and9.1 Hz, 1 H, H-7), 6.69 (ddd,
J = 1.3, 6.5, and 7.4 Hz, 1 H, H-6). References and Notes
(1) Forster, T. Naturwissenschaften 1949, 36, 186. Farster, T. Z. Elektrochem. 1950,54,531. Farster, T. Pure Appl. Chem. 1970,24,443; 1973, 34, 225. Forster, T. Chem. Phys. Len. 1972, 17, 309. (2) Weller, A. Discuss. Faraday SOC.1959,27,28. Weller, A. Prog. 2-(2'-Hydroxyphenyl)imidazo[1,2-u]pyridine (1,2-HFW) React. Kinet. 1%1, 1, 189. Weller, A. Z. Elektrochem. 1961, 61, 956. was obtained by hydrolysis of the corresponding 0-methyl (3) Parker, C. A. Photoluminescence of Solution; Elsevier: London, derivative Me-1,ZHPIP. This was synthesized by a modifica1968. (4) Ireland, J. F.; Wyatt, P. A. H. Adv. Phys. Org. Chem. 1976, 12, tion of a previously described method3* as follows. The 131. condensation of 0-methoxy-a-bromoacetophenonewith 2-ami(5) Amaut, L. G.; Formosinho, S. J. J . Photochem. Photobiol., A: nopyridine in ethanol and in the presence of sodium bicarbonate Chem. 1993, 75, 1. Formosinho, S. J.; h a u t , L. G. J . Photochem. Photobiol., A: Chem. 1993, 75, 21 and references cited therein. yielded 60% Me-1,2-HPIP, which was purified by preparative (6) Chou, P. T.; McMorrow, D.; Aartsma, T. J.; Kasha, M. J . Phys. TLC (silica gel, chloroform-hexane 93:7 v/v as eluent, lower Chem. 1984,88, 5652. Rfspot). Mp: 94-95 "C (from chloroform-hexane). MS mlz (7) Acufia, A. U.; Costela, A.; Mufioz, J. M. J. Phys. Chem. 1986, 90, 2807. (%): 224 (M+, 77), 223 (60), 195 (49, 194 (37), 193 (20), (8) Acufia, A. U.; Amat, F.; Catalh, J.; Costela, A.; Figuera, J. M.; 135 (61), 105 (18), 94 (100). IR (KBr) Y- (cm-l):- 1635 m, Mufioz, J. M. Chem. Phys. Lett. 1986,132, 567. 1588 m, 1500 m, 1490 vs, 1415 m, 1447 m, 1360 s, 1280 s, (9) Costela, A.; Amat, F.; Catalh, J.; Douhal, A.; Figuera, J. M.; 1270 s, 1242 s, 1070 s, 1023 s, 756 vs, 740 vs, 728 s. 'H N M R Mufioz, J. M.; Acuiia, A. U. Opt. Commun. 1987, 64, 457. (10) Costela, A.; Mufioz,J. M.; Douhal, A.; Figuera, J. M.; Acuiia, A. (CDC13, 30 "C): 8.40 (dd, J = 1.8 and 7.7 Hz, 1 H, H-6') (4% U. Appl. Phys. B 1989, 49, 545. NOE on H-S), 8.19 (d, J = 0.8 Hz, 1 H, H-3) (4% NOE on (11) Acuiia, A. U.; Amat-Gueni, F.; Costela, A.; Douhal, A.; Figuera, H-5 and 0.5% NOE on H-6'), 8.11 (m, J = 1.2, 1.2, and 6.8 J. M.; Florido, F.; Sastre, R. Chem. Phys. Len. 1991, 187, 98. Hz, 1 H, H-5), 7.62 (m, J = 0.8, 1.2, 1.2, and 9.1 Hz, 1 H, (12) Sepiol, J.; Bulska, H.; Grabowska, A. Chem. Phys. Lett. 1987,140, 607. H-8), 7.30 (m, 1 H, H-49, 7.13 (m, 1 H, H-7), 7.10 (m, 1 H, (13) Chou, P. T.; Martinez, M. L.; Clements, J. H. Chem. Phys. Lett. H-5'), 7.00 (dd, J = 1.0 and 8.2 Hz, 1 H, H-3'), 6.73 (td, J = 1993, 204, 395. 1.2 and 6.8 Hz, 1 H, H-6), 3.99 (s, 3 H, CH3) (2% NOE on (14) Acufia, A. U.; Amat-Gueni, F.; Catalh, J.; Gonzaez-Tabla, F. J. Phys. Chem. 1980, 84, 629. H-3'). The hydrolysis of Me-1,ZHPIP with boron tribromide (15) Acuiia, A. U.; Catalh, J.; Toribio, F. J . Phys. Chem. 1981, 85, in dichl~romethane~~ yielded 65% 1,2-HPIP. Mp: 141- 142 241. "C (from ethanol-water). MS mlz (%): 210 (M+, loo), 182 (16) Catalh, J.; Toribio, F.; Acuiia, A. U. J . Phys. Chem. 1982, 86, 303. (25), 181 (26). IR (KBr) vmax(cm-l): 2800 br, vs, 1590 s, (17) Toribio, F.; Catalh, J.; Amat, F.; Acuiia, A. U. J . Phys. Chem. 1490 m, 1460 s, 1410 s, 1375 m, 1293 s, 1255 vs, 1145 m, 840 1983, 87, 817. m, 798 m, 750 vs, 740 vs. 'H NMR (CDCl3, 30 "C): 12.72 (18) Acufia, A. U.; Toribio, F.; Amat-Gueni, F.; Catalh, J. J . Photo(broad s, 1 H, OH), 8.13 (dt, J = 1.2 and 6.7 Hz, 1 H, H-5), chem. 1985,30, 339. (19) Shchez-Cabezudo, M.; De Paz, J. L. G.; Catalh, J.; Amat-Guem, 7.84 (d, J = 0.7 Hz, 1 H, H-3) (4% NOE on H - 5 9 % NOE on F. J. Mol. Struct. 1985, 131, 277. H-6'), 7.58 (dd, J = 1.3 and 7.7 Hz, 2 H, H-6' and H-8), 7.22 (20) Herek, J. L.; Pedersen, S.; Baiiares, L.; Zewail, A. H. J . Chem. (m, 1 H, H-4'), 7.23 (m, 1 H, H-7), 7.03 (dd, J = 1.0 and 8.3 Phys. 1992, 97, 9046. (21) Nagaoka, S.; Nagashima, U. Chem. Phys. 1989, 136, 153. Hz, 1 H, H-3'), 6.87 (td, J = 1.0 and 7.7 Hz, 1 H, H-5'), 6.84 (22) Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1986, 128, 231. (td, J = 1.3 and 6.7 Hz, 1 H, H-6). (23) Elsaesser, T.; Kaiser, W.; Liittke, W. J. Phys. Chem.1986,90,2901. 3-(2'-Hydroxyphenyl)imidazo[ lJ-ulpyridine (1,5-HPIP) (24) Douhal, A.; Amat-Guerri, F.; Lillo, M. P.; Acuiia, A. U. J . Photochem. Photobiol., A: Chem. 1994, 78, 127. was also obtained by the method formerly used to synthesize (251 Douhal. A.: Amat-Guem, F.: Acufia. A. U.: Yoshihara, K. Chem. the parent compound, i.e. without the 2'-hydroxy group:@ the Phys. h t . 1994, 217, 619. amide resulting from the reaction of 2-(aminomethyl)pyridine (26) Frev, W.: Elsaesser, T. Chem. Phvs. Len. 1992, 189, 565. (27) Schwartz, B. J.; Peteanu, L. A.; H&s, C. B. J . Phys. Chem. 1992, and 2-methoxybenzoyl chloride (obtained from 2-methoxyben96, 3591 and references cited therein. zoic acid and oxalyl chloride),'" in dry benzene-dry pyridine, (28) Velapoldi, R. A. J. Res. Natl. Bur. Stand., Sect. A 1972, 76, 641. was cyclized with phosphorus oxychloride in dry benzene?* (29) Mateo, C. R.; Lillo, M. P.; Gonzaez, J.; Acufia, A. U. Eur. Biophys. J . 1991, 20, 311. yielding 80% of the methoxy derivative Me-1,5-HPIP. Mp: (30) Van der Zeguel, M.; Boens, N.; Daems, D.; De Schryver, F. C. 133-134 "C (from chloroform-hexane). MS m/z (%): 224 Chem. Phys. 1986, 101, 3 11. (M+, loo), 223 (24), 208 (17), 193 (12), 181 (9), 146 (30), 118 (31) Kearsley, A. J.; Andrews, A. J.; Webb, C. E. Opt. Commun. 1979, (32), 105 (8). IR (KBr) Y,, (cm-I): 1510 s, 1470 vs, 1435 s, 31, 181. (32) Shoshan, I.; Danon, N. N.; Oppenheim, A. U. J . Appl. Phys. 1977, 1360 m, 1275 m, 1260 m, 1257 vs, 1098 s, 1017 s, 805 s, 770 48, 4495. m, 758 vs, 740 s, 700 m. 'H NMR (CDC13,30 "C): 7.61 (dd, (33) Riddick, J. A.; Bunger, W. A. In Organic Solvents, Physical J = 1.3 and 7.4 Hz, 1 H, H-5), 7.57 (d, J = 0.8 Hz, 1 H, H-1), Properties and Methods of Purification; Wisseberger, A., Ed.; 7.47 (m, 1 H, H-8), 7.45 (m, 1 H, H-4'), 7.10 (td, J = 1.0 and Wiley-Interscience: New York, 1970; Vol. 2. (34) Hand, E. S.; Paudler, W. W. Org. Magn. Reson. 1980, 14, 52. 7.4 Hz, 1 H, H-57, 7.04 (dd, J = 1.0 and 8.3 Hz, 1 H, H-3', (35) Douhal.. A.:. Kandori.. H.:. Yoshihara.. K.:. Amat-Guem. F. To be sharper signals irradiating the CH3 signal), 6.71 (ddd, J = 1.2, published. 6.3,and9.1H~,lH,H-7),6.49(ddd,J=1.2,6.3,and7.4H~,(36) Downing, J. W.; Waluk, J. W.; Stanovnik, B.; Tisler, M.; Vercek, B.; Michl, J. J. &g. Chem. 1985, 50, 302. 1 H, H-6). The hydrolysis39of Me-1,5-HPIP yielded 50% 1,5(37) Lemer, D .A.; Horowitz, P. M.; Evleth, E. M. J. Phys. Chem. 1977, HPIP. Mp: 152-153 "C (from ethanol-water). MS mlz (%): 81, 12. 210 (M+, loo), 209 (97), 181 (13), 154 (9), 132 (12), 105 (18). (38) Elliot, A. J.; Guzik, H.; Soler, J. R. J. Heterocycl. Chem. 1982, 19, IR (KBr) vmax(cm-l): 2550 br, vs, 1472 vs, 1260 m, 1243 s, 1437. (39) McOmie, J. F. W.; West, D. E. Urg. Synth. 1%9,49,50. Williond, 1125 m, 1080 s, 1050 m, 1035 m, 765 s. 'H NMR (CDC13,30 P. G.; Fryhle, C. B. Tetrahedron Len. 1980, 21, 3731. "C): 12.00 (broad s, 1 H, OH), 8.53 (dd, J = 0.8 and 7.4 Hz, (40) Boyer, J. H.; Wolford, L. T. J. Org. Chem. 1958, 23, 1053. 1 H, H-5) (6% NOE on H-6'), 7.77 (dd, J = 1.7 and 7.8 Hz, 1 (41) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; John Wdey and Sons, Inc.: New York, 1967; Vol. 1, p 767. H, H-6'), 7.57 ( s , 1 H, H-1), 7.55 ( d d , J = 1.3 and9.l Hz, 1 H, (42) Boyer, J. D.; Ramage, G. R. J . Chem. SOC.1955, 2834. H-8), 7.31 (ddd, J = 1.7, 7.4, and 7.8 Hz, 1 H, H-4'), 7.17 (dd,
Appendix
J = 1.4 and 7.4 Hz, 1 H, H-3'), 7.01 (td, J = 1.4 and 7.8 Hz,
JP940663 1
