J. Phys. Chem. 1984, 88, 5840-5844
5840
on deuteroporphyrin dimethyl ester solutions in the same solvent mixture?l the initial absorbance changes following pulse irradiation are attributed to the reaction DP
+ (CH,),CO-
-
D P + (CH&CO
(4) with a rate constant k4 = (8 f 2) X lo8 M-l s-l. The formation of the porphyrin anion radical is confirmed by the characteristic band25*z6,28 around 630 nm (see Figure 5c). DPFe' and D P display drastic differences in reactivity. Thus, the stability of DPFe' (in the absence of sodium dithionite) is more than 5 orders of magnitude greater than that of D P . Also, kinetic analysis indicates that the decay mechanisms of these species are different. So, it appears that DPFe' behaves as an actual iron(1) form. This is in agreement with the reactivity of electrochemically generated iron(1) porphyrins toward halogenated alkanes as reported elsewhere.25 Our results are also consistent with oneelectron reduction of other metalloporphyrins. When the metal ion is nonreducible (Zn2+complexes are typical examples), the reduction leads to the porphyrin anion radical which decays over milliseconds by a bimolecular process. The decay likely involves disporportionation of the r-radical anion leading to phlorins or chlorins as final
product^.^' Under anaerobic conditions this process is not reversible. On the contrary, reduction of cobalt(I1) porphyrins leads to the Co' species which reversibly reoxidize within seconds in the presence of water.', In conclusion, radiolysis in alkaline 2-propanol systems has enabled us to study the properties of the iron(1) form in the presence of water, which does not appear to have been done with electrochemical methods owing to solvent electrochemistry. Comparison of the reduction of DPFe" with that of DP enables us to attribute the one-electron reduction product of DPFe" to DPFe'. Acknowledgment. We thank Dr. Averbeck and Mr. Palesi (Institut du Radium, Paris) for providing us with the 6oCoysource. Registry No. DPFe", 18922-88-8; DPFe', 65238-89-3; (CH3)&O-, 17836-38-3. (31) Harriman, A.; Richoux, M. C.; Neta, P. J. Phys. Chem. 1983, 87, 4957-65. (32) Baral, S.; Neta, P.; J. Phys. Chem. 1983, 87, 1502-9.
Electronic Spectra and Tautomerism of Hydrogen-Bonded Complexes of 7-Azaindole in a Supersonic Jet Kiyokazu Fuke, Hidetoshi Yoshiuchi, and Koji Kaya* Department of Chemistry, Faculty of Science and Technology, Keio University, 3-1 4- 1 Hiyoshi, Kohoku-Ku, Yokohama 223, Japan (Received: April 3, 1984; In Final Form: August 1 , 1984)
Electronic absorption and fluorescence spectra of hydrogen-bonded complexes of 7-azaindole were studied by MPI and LIF methods. Strong double hydrogen bond formation in the complex in 7-azaindole-Hz0 and 7-azaindoledimer was confirmed. Especially, tautomerization was found to occur in the lowest excited state of the dimer.
Introduction Because of the importance of hydrogen bonding in chemistry and biology, the nature of this interaction has been extensively studied in solution and low-temperature matrices.' On the other hand, recent advances in supersonic jet spectroscopy enable us to revisit the H-bonding problem under low-temperature isolated molecular By the use of the supersonic expansion technique, extremely cooled gaseous solvent-solute hydrogenbonded complexs, can be synthesized at a high density, which allows for a detailed study of intra- and intermolecular H bonding. In previous we studied the H-bonding and solvation effect on the electronic spectra of phenol-(H,O), and (phenol), by use of the mass selected multiphoton ionization (MP1)-supersonic jet method. 7-Azaindole (7-AzI), whose molecular structure is shown in Figure l a , has received considerable attention since the 7-AzI dimer (the structure is seen in Figure 5) has been recognized as a simple model for the hydrogen-bonded base pair of DNA and could provide information on the possible role of tautomerism in ~~
~~
~~
~
(1) "The Hydrogen Bonding Theory and Experiments", P. Schuster, G. Zundel, and C. Sandorfy, Ed., North-Holland, New York, 1976. (2) K. Fuke and K. Kaya, Chem. Phys. Lett., 91, 311 (1982). (3) K. Fuke and K. Kaya, Chem. Phys. Lett., 94, 97 (1983). (4) D. H. Levy, Ann. Reu. Phys. Chem., 39, 197 (1980). (5) P. M. Felker, W. R. Lambert, and A. H. Zewail, J . Chem. Phys., 77, 1603 (1982). (6) H. Abe, N. Mikami, and M. Ito, J . Phys. Chem., 86, 1768 (1982). (7) T. Montoro, C. Jourvet, A. L. Campillo, and B. Soep, J . Phys. Chem., 87, 3582 (1983). (8) Y. Nibu, H. Abe, N. Mikami, and M. Ito, J . Phys. Chem., 87, 3898 ( 1983).
mutation. 7-AzI dimer exhibits visible fluorescence in addition to the normal UV fluorescence in solution when the molecule is excited to the SIstate. El-Bayoumi was the first to propose the intermolecular double proton transfer mechanism in the excited state of the dimer in order to account for the large Stokes shift (ca. 8500 cm-') of the green emission.g-" Since then, many experimental and theoretical studies for 7-AzI dimer have been reported including a picosecond dynamics of the double proton transfer proces~.'~-'' Although the amount of the barrier height of the proton transfer reported hitherto scatters fairly widely, these authors reached the same conclusion that in the excited state of the dimer there exists a double minimum potential along the coordinate of the double proton transfer and that the excited state dimer undergoes a simultaneous double proton transfer within less than 5 ps in nonpolar ~ o l v e n t . ' ~ In spite of these extensive studies, the electronic structures of 7-AzI and its dimer have not been well understood because the (9) C. A. Taylor, M. A. El-Bayoumi, and M. Kasha, Proc. Narl. Acad. Sci.
US.,63, 253 (1969). (10) K. C. Ingham, M. A. Elgheit, and M. A. El-Bayoumi, J . Am. Chem. SOC.,93, 5023 (1971). (1 1) K. C. Ingham and M. A. El-Bayoumi, J . Am. Chem. SOC., 96, 1674
,~, . .,.
(1974)
(12) M. A. El-Bayoumi,P. Avouris, and W. R. Ware, J . Chem. Phys., 62, 2499 (1975). (13) P. Avouris, L. L. Yang, and M. A. El-Bayoumi, Photochem. Photobiol., 24, 211 (1976). (14) J. Catalan and P. PBrez, J . Theor. Biol., 81, 213 (1979). (15) W. M. Hetherington, R. H. Micheels, and K. B. Eisenthal, Chem. Phys. Lert., 66, 230 (1979). (16) H. Bulska and A. Chcdkowska,J . Am. Chem. SOC.,102,3259 (1980). (17) J. Waluk, H. Bulska, B. Pakula, and J. Sepiol, J . Lumin., 24/25, 519 (1981).
0022-3654/84/2088-5840%01.50/00 1984 American Chemical Societv
Electronic Spectra of Hydrogen-Bonded Complexes
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5841 I
346390-1 758
I
A I.-.-
b)
zma
828
1
35000
1
.
,
,
1
1
1
35500 WAVENUMBER ( crn-l )
1
1
1
.
1
Experimental Section 7-AzI (Aldrich, GR) was purified by sublimation in vacuo. The sample heated up to 370 K was expanded with 3 or 4 atm of He through a pulsed nozzle of 400 pm pinhole diameter into a vacuum chamber. The experimental setup of MPI and fluorescence excitation measurement is the same as reported previo~sly.~ Briefly, the frequency doubled output of an Nz laser pumped dye laser (Molectron UV 24 and DL-11) was used as an excitation light source. The dyes used were Rh-590, Rh-610, and Rh-640 (Exciton). Since the molar extinction coefficient of 7-AzI is large (7 X lo3 mol-' cm-'), the intensity of the excitation laser was kept as weak as possible in order to avoid saturation. Dispersed fluorescence spectra were measured by a Spex 0.75-m monochromator with the resolution of 10 cm-' fwhm. In these cases, the second harmonic of YAG (Quanta-Ray) pumped dye laser was used as an excitation source. Results and Discussion [ A ] . 7-AzZMonomer. Figure l a shows the mass selected MPI spectrum of 7-AzI ( m / e = 118) in a supersonic jet expanded with 4 atm of He. The MPI signal was obtained as a result of onephoton resonant two-photon ionization process (Zp= 8.1 1 eV).'* An intense origin band at 34 639 cm-' and a succeeding large number of vibrational bands were observed in the spectrum. According to the CNDO calculation by Catalan and Perez,14the lowest excited state of 7-AzI is AT* in character. The molar extinction coefficient of 7-AzI in solution is about 7 X lo3 mol-' cm-', which confirms the PA* character of the lowest excited state. Moreover, saturation of the absorption occurred under the irradiation of even a weakly focused laser beam in the jet experiment, which is not the case with a weak absorption. These facts indicate that the absorption spectra in Figure 1 correspond to an allowed AT* transition. Figure 1b shows the fluorescence excitation spectrum of free 7-AzI. As seen in Figures l a and lb, both spectra are essentially
K. Fuke, H. Yoshiuchi, K. Kaya, A. Achiba, K. Sato, and K. Kimura,
,
,
,
,
1
1
1
1
,
1
1
,
1
1
1000 500 RELATIVE FREQUENCY ( cm-1)
1500
/
0
Figure 2. The 0 level fluorescence spectrum of free 7-azaindole in a supersonic jet. Details are seen in Table S2. 432
solution spectra exhibit only broad structureless features. In order to clarify the nature of the hydrogen bonding and the mechanism of the proton transfer process of the dimer, the electronic absorption and fluorescence spectra of free 7-AzI, 7-AzI-water complexes, and 7-AzI dimer have been measured in a supersonic beam. In this paper, we present the mass selected MPI and laser-induced fluorescence (LIF) spectra of these molecules. By comparing the results of free 7-AzI with those of 7-AzI-(H20), and ( ~ - A Z I )strong ~ , double hydrogen bond formation in the complexes was confirmed. Especially, the lowest excited singlet state of the dimer which is strongly two photon allowed from the ground state was found to have a fast relaxation channel to form a tautomer that gives a visible fluorescence.
Chern. Phys. Lett., 108, 179 (1984).
10641P32
1
36000
Figure 1. Electronic spectrum of free 7-azaindole in a supersonic jet measured by (a) mass selected MPI method and (b) fluorescence excitation method. Details are seen in supplementary Table S1.
(18)
ILIO
a ) 0+233,,-1 432 * 750
I
1500
I
1000 500 RELAJIVE FREQUENCY ( crn-l)
0
Figure 3. Dispersed fluorestence spectra of free 7-azaindole: (a) 0 + 233-cm-' band excitation; details are seen in Table S3a; (b) 0 276-cm-l band excitation; details are seen in Table S3b.
+
the same.19 Only a gradual decrease of the fluorescence quantum yield is seen as the excess energy is increased and there seems to be no evidence for the existence of the Sz in the vicinity of the S1.
In order to find the correspondence between the vibrations in the excited state which are seen in Figure 1 and those in the ground state, dispersed fluorescence spectra were measured by exciting individual vibrational bands in the absorption spectrum. Because the responsible electronic transition is the optically allowed one, the 0-0 band is the strongest in the absorption spectrum (see Figure 1). When the 1-0 band of a vibration is excited, the most intense band in the dispersed fluorescence spectrum should correspond to the 1-1 transition of that vibration. Figure 2 shows the dispersed fluorescence spectrum excited at the 0-0 band. In the spectrum, in addition to the most intense 0-0 fluorescence band, fundamentals, overtones, and combinations of the 552- and 758-cm-' vibrations in the ground state appear clearly and the spectral feature is quite similar to the 0 level fluorescence spectrum of indole.* However, when one compares the absorption spectrum of 7-AzI in Figure 1 with the dispersed fluorescence spectrum in Figure 2, one finds appreciable difference between them with respect to the band positions and intensity distribution, which (19) The weak bands appeared in Figure l b at 35 200 to 35 300 cm-' are ascribed to the absorption bands of the 7-AzI-H20 complex in the presence of a trace amount of water in the sample holder.
5842 The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 TABLE I: Vibrational Frequencies (cm-') of Free 7-AzI in the Ground and Lowest Singlet Excited States excited state mound state Raman" 233 432 436 (vw) 276 624 629 (s) 30 1 468 652 482 552 565 (vw) 708 866 717 1164 733 752 758 767 (vs) 856 865 896 894 (w)
936 96 1 99 1 1032 a
1032
1034 ( s ) ~
924 1064
1071 (s)
Fuke et al.
u-
33168
I
Vibrational frequencies measured in methanol solution. In solid.
implies that the excited state vibrational modes are different from those in the ground state. Typical examples are seen in Figure 3, a and b, where the dispersed fluorescence spectra from 0 233and 0 + 276-cm-' levels are shown. As seen in the figures, it is concluded that the 233- and 276-cm-I vibrations in SI correspond to the 432- and 624-cm-' vibrations in the ground state because these vibrations are the most prominent ones in the individual fluorescence spectra. Table I summarizes the results of the correspondence between the vibrations in the SIand So as determined by the above-mentioned procedures. In the table, the result of the observed Raman spectrum is also tabulated. As far as the ground state vibrations are concerned, there are no distinct differences between 7-AzI and indole. In the excited state, the 7-AzI molecule exhibits a large frequency difference with respect to several vibrational modes as seen in the table, which is in contrast to indole.* A most distinct mystery in the 7-AzI spectrum is the 758-cm-' vibration in the ground state. The 758-cm-' vibration is the most active one in the 0 level fluorescence as shown in Figure 2. On the other hand, none of the excited state vibrations less than 1000 cm-' above the origin exhibits the fluorescence spectrum in which the 758-cm-' vibration is most active. This implies that there exists no excited state vibration which corresponds to the ground state 758-cm-l vibration. As a plausible explanation, it is suggested that the 758-cm-l vibration in Somight mix up with various vibrations in SI(Duschinskii effect). The 758-cm-' vibration appears fairly strongly in the dispersed fluorescence spectra excited at various 1-0 vibrational bands, which supports the above explanation. [ B ] . 7-AzZ-(H20).By mixing 7-AzI with a few torr of water in the presence of 4 atm He, a fluorescence excitation spectrum was observed as seen in Figure 4a. The intensities of the bands in the spectrum increase with increasing partial pressure of water. Thus, the bands are identified as those of 7-AzI-(H20), complexes. Figure 4b shows the one photon resonant two photon ionization spectrum by selecting the mass number as 118 + 18 = 136, that is, 7-AzI-(H20). By comparing Figure 4a with Figure 4b, the 0-0 band of the monohydrated 7-AzI can be identified as the band at 33 354 cm-l which is shifted to the red by 1285 cm-l from the origin band of free 7-AzI. The amount of the red shift by the complex formation is about one order of the magnitude larger than that of indole-(H20) reported by Montoro et al.' and 4 times larger than that of phen~l-(H,O).~This enormous amount of spectral shift indicates that the stabilization energy of the complex by the hydrogen bond formation is larger by far in the excited state than in the ground state. Since 7-AzI has two sites for the formation of the hydrogen bond (N-H and >N:), the molecule behaves both as a proton donor and an acceptor. Then, it is possible for the molecule to form a cyclic structure by the two hydrogen bonds between 7-AzI and water as shown in Figure 4b. A similar structure has been proposed for the 7-AzI-ethanol complex by El-Bayoumi and Ingham." As seen in Figure 4b, no isomer band can be observed in the MPI spectrum of the complex,
+
,
33500
,
,
,
I
,
34000
,
,
.
I
34500
WAVENUMBER ( crn-')
Figure 4. Electronic spectra of 7-azaindole-water complexes: (a) fluorescence excitation spectrum of 7-azaindole-(H20),; (b) mass selected MPI spectrum of 7-azaindole-(H20) (Table S4); (c) mass selected MPI spectrum of 7-a~aindole-(H~O)~.
which supports the rigid structure of the complex. As is evident from the large spectral shift, the conformation of the complex in the excited state is distorted largely from that in the ground state. When Figure 4b is compared with Figure la, one finds that there is no one to one correspondence of the vibrational bands between the electronic spectra of the complex and the free molecule. This finding is quite unusual because hitherto known electronic spectra of H-bonded complexes such as phenol-(H20), and indole-(H20) exhibit good correspondence between free and complex molecules with respect to the intramolecular vibrational modes, and the only distinct difference between them is the appearance of the lowfrequency intermolecular vibrational modes as a result of the complex f o r m a t i ~ n .For ~ ~instance, ~~ the intensity of the 0-0 band is the strongest in the spectrum of the free 7-Az1, while the 0 739-cm-I band is the most intense one in the monohydrated 7-AzI. Also, a weak band at 233 cm-' and a fairly intense one at 276 cm-' appear as cold bands in the low excess energy region of the spectrum of the free molecule. In the case of the complex, instead of the above bands, a group of the vibrational bands appears at around 200 cm-' as seen in the figure. A similar complicated feature of the spectrum is also seen in the higher excess energy region above the 0-0 band and makes the correspondence between the vibrations of the free and complex formed 7-AzI in the excited states impossible. On the other hand, the Raman spectrum of 7-AzI does not exhibit appreciable change in the spectral positions and the intensities even though the spectrum is measured under various conditions such as in polar solvents (methanol, water), in nonpolar solvent (cyclohexane), and in the solid state. This means that the vibrational frequencies and geometric structure of 7-AzI do not change appreciably in the ground state by the complex formation. Then the molecule undergoes large structural perturbation mainly in the excited state when the double hydrogen bonds are formed. Formation of strong hydrogen bonds in S I suggests that the intermolecular vibrations may have as comparably high frequencies as some of the intramolecular modes of 7-AzI in SI and that the mixing of these intramolecular vibrations with the intermolecular vibrations causes the observed spectrum of the complex to be so congested. Dispersed fluorescence spectra from individual vibrational levels of the complex may inform us of the details of the above expectation. Measurement of the dispersed fluorescence spectra of the complex is now underway and the results will be published elsewhere. [ C ' . 7-AtZ-(HZO),. Figure 4c shows the MPI spectrum of ~ - A z I - ( H , O ) ~tuning the mass number to 118 + 2 X 18 = 154. The spectrum was obtained by expanding the mixture of 7-AzI and 10 torr of water with 3 atm of He. From the figure, it is apparent that the electronic transition of the (1:2) complex is
+
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5843
Electronic Spectra of Hydrogen-Bonded Complexes
WAVELENGTH ( nm )
370
I
I
,
32500
,
I
I
I I 33000
I
I
,
WAVENUMBER ( cm-1)
Figure 5. The 1-photon resonant 2-photon ionization spectrum of 7azaindole dimer. See Table S5.
located very close to that of the (1:l) complex. Characteristic aspects of the spectrum are (1) in contrast to the (1 :1) complex, the (1:2) complex contains at least four kinds of geometric isomers; and (2) the spectral pattern is quite similar to that of the (1:l) complex with respect to the band positions. We identified at least four bands at 33 168, 33 244,33 312, and 33 393 cm-' as the origin bands of the isomers of the (1:2) complexes, Starting from these origins, the spectral pattern of each isomer is similar to that of the (1:l) complex. For instance, taking the band at 33 168 cm-I as the origin band of the one of the isomers, the band at 33 903 cm-' (0 735 cm-') can correspond to the 0 739 cm-' band of the (1:l) complex. In consideration of these facts, it is concluded that the second water interacts weakly to the (1:l) complex and works as a solvent, which causes the spectral shift relative to the (1: 1) complex small and allows the occurrence of various isomers. One thing that is difficult to explain is that the 735-cm-' bands which appear fairly intensely in the spectra of the isomers whose origins are located a t 33 168 and 33 312 cm-I are the missing ones in the spectra of the isomers whose origins are at 33393 and 33244 cm-'. A similar phenomenon was also observed in the MPI spectra of p h e n ~ l - ( H ~ O ) ~ c ~ m p l e x e sand , ~ the reason may come from the difference in the fragmentation yield in the ionized state among the isomers. [ D ] . 7-A.d Dimer. There have been many studies on the absorption and emission spectra of 7-AzI dimer in s o l ~ t i o n . ~ - ' ~ These results and the thermodynamic arguments predict the formation of the dimer which contains two strong hydrogen bonds in the pair. The heat of formation of the dimer was estimated as 40.2 kT mol-'." By forming a cyclic structure, the excited state dimer is expected to be largely stabilized relative to the ground state because the pyrrolic hydrogen (N-H) becomes more acidic in the excited state. In accordance with this expectation, the origin band of the dimer appears at 32 290 cm-' in the mass selected 1-photon resonant 2-photon ionization spectrum ( m l e = 236) as shown in Figure 5 . That is, the origin of the one-photon absorption spectrum of the dimer is shifted to the red by 2349 cm-' from that of free 7-AzI. The spectrum consists of fundamentals, overtones, and combinations of intra- and intermolecular vibrations as seen in the figure. Especially there clearly appears the progression of the 17-cm-' vibration. Other than the 17-cm-' vibration, in the low excess energy region above the origin, there are three bands which are located at 65, 107, and 155 cm-' above the origin. By changing the stagnation pressure and the excitation position (distance from the nozzle), the bands mentioned above were found to be cold bands. Then, they should be assigned to the fundamentals and overtones of the intermolecular modes in the pair. Furthermore, the dimer spectrum exhibits intense bands at 0 216 and 0 229 cm-' which are ascribed to the intramolecular modes displaced largely in the excited state. The same spectral pattern is repeated in the higher excess energy region started at 737 cm-' above the origin band. These experimental results suggest that the dimer is displaced largely in the excited state along the intermolecular coordinates and that the tautomer form might be a stable structure in the excited state. Figure 6 shows the dispersed fluorescence spectrum of the dimer excited a t the origin band (32290 cm-I). The spectrum shows a large
350 I
I
,
,
26000
28000
330
I
I
I
310 I
I
,
I
30000
I
32000
WAVENUMBER ( cm-1)
Figure 6. Dispersed fluorescence spectrum from the origin band (32 290 cm-') of 1BU-1A, one-photon transition of the dimer.
+
+
+
I
+
I
32300
I
I
32500
I
I
32700
WAVENUMBER ( cm-1)
Figure 7. The 2-photon resonant 4-photon ionization spectrum of the dimer (Table S 6 ) .
Stokes shift of around 3000 cm-I. The spectrum near the exciting line exhibits a discrete vibrational structure and then becomes congested due to the overlap of various vibrational bands as one goes far from the exciting line, whose details will be published elsewhere. Moreover, the excitation spectrum of the dimer by monitoring the UV (below 400 nm) fluorescence coincides with the one-photon resonant MPI spectrum in Figure 5 except for the disappearance of the excitation spectrum at the higher excess energy region due to the nonradiative relaxation. In consideration of these facts, it can be concluded that even though the geometry in the excited state is distorted largely from that in the ground state, tautomerization is not the accessibe channel from this excited state. In contrast to 7-AzI-(H20), 7-AzI dimer has two interacting T electron sites in the pair. Upon dimer formation, an excited state of the monomer splits into two states, which are one-photon allowed ( B , symmetry) and two-photon allowed (Agsymmetry) from the ground state under the approximation of a planar symmetric dimer structure with CZhsymmetry. The spectrum in Figure 5 apparently corresponds to lBu-lA, one-photon allowed transition. Figure 7 shows the two-photon absorption spectrum of the dimer by use of the 2-photon resonant 4-photon ionization method selecting the mass number to 236. An intense two-photon absorption starting at 32254 cm-' which is shifted by 36 cm-I to the red from the origin of the one-photon allowed lBu-lA, transition can be assigned to the two-photon allowed 2A,-lA, transition of the dimer. The observed two-photon absorption spectrum in Figure 7 is different in several aspects from the one-photon absorption spectrum in Figure 5. The two-photon spectrum consists of 102, 120, 210, 248, and 362-cm-' bands in addition to the origin band and the same spectral pattern is repeated in the spectrum starting at 0 737 cm-' as seen in the figure. One easily notices that the low-frequency intermolecular modes which appear in the one photon absorption spectrum in Figure 5 smear out in the twophoton spectrum in Figure 7 except for relatively high frequency modes (102 and 120 cm-l). Moreover, the line width of each vibrational band in the two-photon spectrum is more than five times broader than that in the one-photon transition. Apart from
+
5844 The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 WAVELENGTH ( nm ) 500 400
600 I
I
I
16000
I
I
I
I
20000
I
I
24000
I
I
28000
WAVENUMBER ( crn-1 )
Figure 8. Dispersed fluorescence spectrum from the origin band (32 254 cm-') of 2Ag-1A, two photon allowed transition of the dimer.
the large difference in the intermolecular modes, the vibrational structure of the intramolecular modes is almost the same between the one- and two-photon spectra. These facts indicate that in the 2A, excited state, the dimer structure is distorted differently from that in the IB, state along the intermolecular coordinates due to the difference in the interaction of the two *-electron sites in the pair and that a fast relaxation channel is open in the 2A, excited state. As seen in Figure 7, the origin band at 32 290 cm-' of the one-photon allowed transition appears weakly in the two photon spectrum. This suggests that the inversion symmetry of the dimer as assumed here does not hold in a strict sense. Conversely, several two-photon allowed bands are seen as weak bands in a one-photon excitation spectrum monitoring the total fluorescence. A striking fact was observed that the laser-induced one-photon excitation spectrum of the dimer by monitoring visible fluorescence (450 to 550 nm) is exactly the same as the spectrum in Figure 7. This implies that the 2A, state of the dimer emits visible fluorescence. Figure 8 shows the dispersed fluorescence spectrum of the dimer excited at the origin band of the 2A.-1Ag transition (32254 cm-'). As seen in the figure, the spectrum exhibits an enormous Stokes shift and is located only in the visible region centered at 470 nm with no fine structure. These observations indicate that the tautomerization within the pair occurs in the lowest excited state of the dimer which is strongly two-photon allowed and only weakly one-photon allowed from the ground state. Catalan and Perez
Fuke et al. treated the 7-AzI dimer theoretically by the CNDO method.14 According to their results, excited state splitting between the lB, and 2A, states was estimated as 200 cm-' and the 2A, state selectively undergoes a relaxation to the tautomer form which emits a green emission. Present results agree well with their prediction except that tautomerization from the dimer 2A, state was found to have no energy barrier. Since the visible fluorescence excitation spectrum extends toward the higher energy region above 737 cm-' from the origin, the nonradiative relaxation process from the 2A, state of the dimer is mainly tautomerization. In the case of the lB, state of the dimer, crossing to the tautomer state never occurs even though the responsible state is located in the same energy region. Thus, under an isolated molecular condition, photo-induced tautomerization is not the efficient event because the responsible state is nearly one-photon forbidden from the ground state. However, in solution, nonradiative crossing from lB, to 2A, may occur in the presence of the solvent and the tautomerism will be induced efficiently by absorbing UV light via the one-photon process.
Conclusion In the present work, hydrogen-bonded 7-azaindole complexes including hydrated complexes and the dimer were studied by MPI and LIF methods in a supersonic jet. Strong hydrogen bond formation in the comlexes was elucidated. Especially, 7-AzI dimer was found to undergo tautomerization in the lowest state which is strongly two-photon allowed from the ground state. Details of the dynamics of tautomerization from such a selected excited state are now under investigation. Acknowledgment. We thank Professor S. Iwata and Dr. Y. Osamura for their stimulating discussion. This work is financially supported by Mitsubishi Science Foundation and Grants-in-Aid from Ministry of Education, Science and Culture, Serial No. 5840268 and 5840288. We also acknowledge the information of Raman spectral data from Dr. M. Takahashi of our department. Registry No. HzO,7732-18-5; 7-azaindole, 271-63-6. Supplementary Material Available: Tables listing positions and relative intensities of individual vibronic bands in the spectra of free 7-AzI, 7-AzI-(H20), and ( ~ - A Z I(7) ~pages). Ordering information is given on any current masthead page.
