J . Phys. Chem. 1992,96, 5203-5205
5203
Monohydrate Catalysis of Exclted-State Double-Proton Transfer in 7-Azaindole Pi-Tai Chou,* Marty L. Martinez, William C. Cooper, Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208
Dale McMorrow, Naval Research Laboratory, Code 461 3, Washington, D.C. 20375
Susan T. Collins,+ and Michael Kasha* Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306-301 5 (Received: January 31, 1992; In Final Form: April 20, 1992) The green fluorescence (530 nm) of 7-azaindole tautomer is shown to be observable at room temperature upon microaddition of water (0.008 M to saturation) to ethyl ether and to p-dioxane (0.17-1.7 M) as solvent. The fluorescence of 7-azaindole observed in liquid water solution (Ama 385 nm) is thus shown to correspond to the normal molecule (fluorescence at -350 nm in ether), contrary to current proposals suggesting that this latter fluorescence corresponds to the tautomer. The present experiments indicate that 7-azaindole monohydrate undergoes excited-state double-proton transfer. The results also suggest that 7-azaindole polyhydrate, which is expected to be prevalent in water solvent, inhibits the solvent rearrangement that is necessary for tautomerization, resulting in a diminished tautomer fluorescence intensity.
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quantum yield.I3 Emission from the anion species has been ruled Introduction out through the addition of sodium ethoxide to alcohol solutions, The spectroscopy and dynamics of 7-azaindole (7AI) have with no spectral changes being observed.’ Very recently, however, received considerable attention since the initial observation of Petrich and co-workers have reassigned the emitting species reexcited-state double-proton transfer (ESDPT) by Taylor et al.’ sponsible for the 385-nm band to the tautomer species.” This It is well accepted that the ESDPT of 7AI occurs in hydrocarbon assignment is surprising because a tautomer emission at 385 nm and alcohol solvents through the formation of hydrogen-bonded in water would represent a -60oO-~m-~blue shut relative to that dimers and solute/solvent complexes, respecti~ely.’-~As a result, in alcohol solvents, while a red shift is expected on the basis of two fluorescence bands are observed in alcohol and in concentrated increased acidity and hydrogen-bond strength of water over alhydrocarbon solutions at room temperature. The long-wavelength cohols. emission (Amx 500 nm) in hydrocarbon solvents results from In order to determine whether the 385-nm emission band in the double-proton transfer of symmetric, hydrogen-bonded diwater is associated with the normal or tautomer emission, we have m e r ~ , ’ -while ~ monomers and other configurations not favorable examined the spectroscopic properties of 7AI in several aprotic for proton transfer exhibit the short-wavelength “normal” solvents as a function of water concentration. These experiments fluortscence (A, = 330 nm in very dilute hydrocarbon solution are an extension of earlier work performed by Collins (and Kaand 350 nm in ether). In alcohol solvents, proton transfer occurs sha)I4 in which water-saturated ether solutions exhibit distinct in cyclically hydrogen-bonded solvent/solute complexes. The emissions in both the normal and tautomer spectral regions. The precise mechanism for ESDPT in alcohol systems, however, has concentration dependent studies presented here suggest strongly been somewhat controversial. The initial interpretationof the 7AI that the 385-nm fluorescence of 7AI in water predominantly arises proton-transfer fluorescence dynamics in alcohols involved a from the (non-proton-transferred) normal species. two-step model in which the actual proton-transfer event could occur only after a rate determining solvent/solute reorganization step.6 This mechanism has been supported by more recent ~ o r k . ~ , ~ Experimental Section Moog and co-workers, however, observed a significant wavelength Materials. 7AI (Aldrich) was twice recrystallized from specdependence of the fluorescence dynamics in the tautomer region trograde methylcyclohexane (MCH). The purity was checked of the spectrum8and interpreted their results in terms of a rapid by the fluorescence excitation spectrum of 7AI in dilute proton transfer followed by a time dependent Stokes shift of the M) MCH. Fresh, spectranalyzed, anhydrous ethyl ether and tautomer fluorescence. A more recent study by Moog and pdioxane were opened and used immediately for each experiment. Maroncelli involving complete time-resolved emission spectra Triply distilled water was used throughout the experiment. reveals no significant Stokes shift for the tautomer fluorescence9 Methanol, ethanol, and butanol were of spectrograde quality and and supports the two-step model proposed earlier: In that work were used without further purification. it was shown that the wavelength dependence of the tautomer Measurements. Steady-state absorption spectra were recorded fluorescence rise time can be accounted for in terms of spectral with an H P 8452A spectrophotometer. Emission spectra were overlap between the normal and tautomer emissions. recorded with a home-built detection system incorporating a 1/2-m Interest in the photophysics of 7AI in water solutions has been monochromator with a grating blazed at 500 nm, a red-sensitive revitalized recently by Petrich and co-workers, who have suggested intensified photodiode array and a multichannel analyzer. Details the use of the 7AI chromophore (in the form of 7-azatryptophan) of the setup have been described e1se~here.l~The sensitivity at as a biological probe of protein structure.I0J1 In contrast to its 330 nm is of that at 520 nm. All samples were excited with behavior in hydrocarbon and alcohol solvents, 7AI in water apthe fourth harmonic (266 nm, 8 ns) of a NdYAG laser (Quantel parently exhibits only a single fluorescence band with a maximum Model 580). at 385 nm, representing a peak red shift of -4300 cm-I with respect to the normal emission maximum in hydrocarbon solvents. Results and Discussion This emission band initially had been assigned to a strongly Because of its greater acidity, coupled with the unique property red-shifted normal emission;12 more recently exciplex formation that one water molecule can accommodate as many as four hyhas been suggested to account for its significant red shift and low drogen bonds, water behaves as a distinctly different solvent from alcohols. Water is believed to consist of large aggregates of *To whom correspondence should be addressed. molecules involving extensive chain H bonding, continually Department of Chemistry, California State University, Northridge, CA 91 330. breaking up and re-forming under the influence of thermal agi-
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0022-3654/92/2096-5203$03.00/0 0 1992 American Chemical Society
5204 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
Letters
1.0
>
c. .(I)
5
E 0.5 a,
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-ma,
a
0
Wavelength
nm
Figure 1. Fluorescence spectra of 7AI in ethyl ether at room temperature containing various concentrationsof water (M): (a) 0; (b) 0.008; (c) 0.025; (d) 0.034; (e) 0.043; (0 0.068; (g) 0.10; (h) saturated water. tation. On the other hand, the degree of self-aggregation is significantly less in alcohols. Therefore, while appreciable amounts of 1:l 7AIfalcohol complex may exist in neat alcohol solvents (or be formed during the lifetime of the excited state), the extensive hydrogen bonding in liquid water may inhibit or prevent the formation of monohydrate. In order to minimize the effects of water aggregates, dilutions of water in aprotic solvents were examined. Parts a-h Figure 1 show emission spectra of 3.2 X M 7AI in ethyl ether containing various concentrations of water. Figure 1A shows the short-wavelength emission, and Figure lB, the spectral region in which tautomer emission is normally observed in alcohol solvents. In pure ethyl ether solvent a unique short-wavelength emission with a maximum at -350 nm is observed. Since ethyl ether is not a proton donor, assignment of the 350-nm peak to the normal species of 7AI is straightforward. This 350-nm emission is gradually red-shifted with increasing water concentration with a corresponding reduction in the emission intensity (Figure 1A(a-h)). In water-saturated ethyl ether the integrated emission intensity of the normal fluorescence band is -I/, of the emission intensity in pure ethyl ether. Except for the tail of the short-wavelength fluorescence, the emission in the long-wavelength region is negligible in ethyl ether (Figure lB(a)). With increasing water content an additional emission, with a band maximum of -530 nm, gradually grows in. The observation of an isoemissive point at -465 nm indicates the existence of two emitting species. Due to the red-sensitive detection system used the actual maximum is estimated to be -10-15 nm to the blue. In the water-saturated solution, the emission intensity at 530 nm is
