J. Phys. Chem. 1993, 97, 13615-13619
13615
Matrix-Isolation Studies of 7-Hydroxyquinoline. 3. Deuterium-Isotope and Xenon Matrix Effects Anita Lavin and Susan Collins' Department of Chemistry, California State University, Northridge, Northridge, California 9131 1 Received: July 8, 1993; In Final Form: September 27, 1993"
Previously, we have observed the stabilization of the ground-state keto tautomer of 7-hydroxyquinoline (7HQ) in methanol/argon mixed matrices a t 10 K. W e ascribed the stabilized tautomer to the isolated 7HQ-(MeOH)* bridged complex on the basis of the observation that the open-chain methanol dimer concentration reaches a maximum concomitantly with the fluorescence excitation spectrum of the keto tautomer centered a t 420 nm. Here, we report a n increased stabilization of the ground-state keto tautomer when the open-chain CDsOD dimer is used and an additional increase upon photolysis. W e also find that the use of the xenon matrix instead of argon inhibits the formation of the keto tautomer. W e interpret these results in terms of a shift in the equilibria to favor the deuterated keto structure, due to the increase of hydrogen-bonding strength upon deuterium substitution. It becomes hindered in xenon, most probably due to a cage effect.
Inboduction Recently, we reported in two publications on the stabilization of the ground-state keto tautomer of 7-hydroxyquinoline (7HQ) under special conditions in the rare-gas matrix environment at 10 K.'v2 The first paper reports the bridged complex formed between 7HQ and the open-chain methanol dimer (see Figure 1). When the molecule is isolated from other methanol molecules in the matrix, the complex is free from additional hydrogen-bonding influences. In the isolated open-chain methanoldimer complex the keto tautomer is lower in energy than the enol normal form, and through a ground-state triple-proton-transfer process it becomes the predominant form. We have been able to excite it directly at 420 nm and observe the emission at 500 nm. Previously, the keto tautomer spectrum only was observed by the excitedstate proton-transfer (ESPT)process, where once generated it could be excited with a second laser pulse and studiedS3 Figure 2 is reproduced from ref 1, illustrating for 7HQ in the CH3OH matrix at 10 K the fluorescence excitation spectrum monitored at 500 nm and the emission spectrum generated by excitation at 420 nm spectra of 7HQ in the methanol/argon matrix at 10 K. Here, the concentration is M / R = 1/200, where M represents the methanol concentration and R represents the rare gas atom concentration. We observed that the open-chain methanol dimer concentration reaches a maximum concomitantly with the fluorescence excitation spectrum of the ground-state-stabilized keto tautomer of 7HQ centered at 420 nm at M / R = 1/200, presumably due to the bridged complex. The second paper reports the stabilization of the ground-state keto tautomer of the 7HQ cyclic dimer in solid argon at 10 K. In this paper, we report the 10 K fluorescence excitation and emission spectra for the 7HQ/CD3OD/Ar ( M / R = 1/200) matrices. Here, we find an increase in the stabilized tautomer fluorescence excitation and emission band intensities relative to the 7HQ/CHjOH/Ar spectra. Also, we see a greater increase in the stabilized tautomer band intensity for the 7HQ/CD30D/ Ar system after photolysis, relative to the 7HQ/CH3OH/Ar case. We also report the results of studies identical to those of ref 1, except that they were done in xenon matrices instead of argon. Here we find that theground-state stabilized keto tautomer bands of the bridged 7HQ-(MeOH)2 complex are nearly eliminated under identical concentration conditions. We have postulated (as one of several possible explanations) that since the stabilized tautomer concentration is enhanced in the presence of CD3OD ~
* Abstract
~~
~~~
published in Advance ACS Absrracrs, December 1, 1993.
0022-3654/93/2097-13615$04.00/0
Figure 1. Methanol solvent assisted enol-keto tautomerization of 7-hydroxyquinoline.
8049.105 8'. 1.l
, ,,
m
n
' 5366.070
, , !
,
! I
, , 2683.035
, i
, 0.000 I
I
I
1
I
400
450
500
550
600
WAVELENGTH nm
Figure 2. Fluorescence excitation and emission spectra of the 7-hydroxyquinolinematrix with a methanol/argon concentration of 1/200 at 10 K. The dashed curve is the excitation spectrum monitored at 500 nm. The solid curve is the emission spectrum generated by excitation at 420 nm. (Reproduced from ref 1.)
and is greatly diminished in the CH3OH case in xenon, the stabilized keto tautomer might be formed by a tunneling process which is facilitated by a methanol vibration. This process may become hindered in the xenon matrix due to steric effects of the xenon cage. Our interest in this project began with the possibility of stabilizing the ESPT tautomer at 10 K,since the ESPT process 0 1993 American Chemical Society
Lavin and Collins
13616 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
had been studied extensively for room temperature solutions of 7HQ. Those results are reported in ref 1, but more importantly, we discovered the unique ground-state triple-proton-transfer process. The discovery of such a mechanism is important, since it may occur in other systems where there is a low concentration of methanol or possibly water in a nonpolar or hydrophobic environment. Since the excitation and emission spectra are unique, they may serve as a probe of the solvent system or possibly the environment in some biological system. An example is the monoaquo-7-azaindole complex, which has a unique ESPT signature fluorescence spectrum! 7-Azatryptophan is currently being studied for its similar fluorescence properties and its ability to be incorporated into bacterial proteins. It is amenable to protein synthesis. The tripeptidecontaining 7-azatryptophan mimics the active site of potato chymotrypsin i n h i b i t ~ r . ~
16000
12000
n
c z Y
8000
c
I 4000
Experimental Section Argon or xenon matrices of 7HQ and CH3OH or CD30D were prepared by simultaneously depositing 1-2 mmol each of premixed alcohol/rare gas and 7HQlrare gas through separate jets at a total rate of approximately 1 mmol/h. Solid 7HQ was placed in a stainless steel tube positioned as close to the substrate window as possible. The rare gas was passed over the tube at room temperature. Concentrations of 7HQ in the rare gas were not known; however, on the basis of comparison to the dimer spectra of ref 2, no dimer was found in the mixed matrices. This mixture combined with known concentrations of the alcohol/ rare gas mixture at the CsI substrate window which was held at 10 K for the argon mixtures. The xenon mixtures were deposited at 25 K and then slowly lowered to 10 K for study. The cryostat was an Air Products CS202 closed-cycle helium refrigerator. Spectra wereobtained with a SPEX Fluorolog 1681 spectrometer. Extendedirradiationofthe matrix wasdone with theoriel 100-W xenon lamp. We have performed single- and dual-jet depositions with no difference in the spectra. With the single-jet deposition, the MeOH/Ar mixture flows over the 7HQ and gets deposited as one mixture. With the dual-jet deposition, the MeOH/Ar mixture flows in a tube parallel to a tube containing 7HQ with Ar flowing over it. In the dual-jet configuration, the ports are positioned in front of the substrate window with about 1 cm between the end of the ports and the substrate window. 7HQ (99%, Karl Industries) was recrystallized in absolute ethanol and dried in a desiccator under vacuum. The purity of 7HQ was verified based on melting point and UV-visible, I3C NMR, and IH N M R spectra. In ref 1, methanol (reagent grade, Aldrich Chemical Co.) was dried using 4 A molecular sieves which had been heated and dried under vacuum. Removal of water was confirmed by near-infrared measurements in the OH overtone stretch at 4400 and 5000 nm.' Since water had no effect on the results, methyl-d3-alcohol-d (Aldrich Chemical Company, 99.8% deuteration) was not dried. However, fresh samples from 1-mLsealed vials were opened and then immediately transferred to the vacuum lines for mixture preparation. Argon (99.995 %, Matheson) and xenon (99.995%, Matheson) were used without further purification. Results
7HQ/CD30D/Ar Matrices. Figure 3 shows three fluorescence excitation spectra monitored a t the emission wavelength of 500 nm. The dashed and dotted curves are reproduced from ref 1, and they show the effect of methanol concentration on the appearance of the bridged 7HQ-(MeOH)z stabilized keto tautomer band between 350 and 475 nm. At a methanol/argon concentrationofM/R= 1/100(dashedcurve),nostabilized tautomer band is seen, whereas it was shown to reach a maximum at M / R = 1/200 (dotted curve). The solid curve represents the spectrum of 7HQ/CD30D/Ar mixture, also with a concentration of M / R
300
400
350
450
WAVELENGTH nm
Figure 3. Matrix fluorescence excitation spectra of 7-hydroxyquinoline
with(--)CH30H/Ar,M/R= l/lOO;(- - -)CH,OH/Ar,M/R= 1/200; (-) CD3OD/Ar, M/R = 1/200. All were monitored at the emission wavelength of 500 nm.
350
400
450
I
I
1
500
550
600
WAVELENGTH nm Figure 4. Matrix fluorescence emission spectra of 7-hydroxyquinoline with(--)CH3OH/Ar,M/R= l/lW,(- - -)CH3OH/Ar,M/R= 1/200, and (-) CD3OD/Ar, M/R = 1/200. All were generated by excitation
at 320 nm.
= 1/200. The intensity of the stabilized tautomer band for the CD30D system is greatly enhanced relative to the CH3OHsystem; it has nearly the same intensity as the enol normal band. The fluorescence emission spectra when excited at 320 and 420 nm show different effects. When excited at 320 nm, Figure 4 shows that the emission spectrum of the 7HQ/CHjOH/Ar ( M / R = 1/ 100) matrix (dashed curve) exhibits the highest yield of ESPT fluorescence between 450 and 600 nm. It also has the lowest yield of stabilized tautomer, as indicated in Figure 3 (dashed curve). Not shown are the emission spectra when excited at 420 nm. Here, the 7HQ/CH3OD/Ar ( M / R = 1/200) matrix, which has the greatest amount of ground-state stabilized tautomer as indicated by Figure 3 (dotted curve), gives the greatest amount of stabilized tautomer fluorescence between 450 and 600 nm. Figure 5 reproduces the solid curve from Figure 3, which is the spectrumofthe 7HQ/CD3OD/Ar ( M / R= 1/200) mixture.The dashed curve shows the effect of a 30-min photolysis of the matrix.
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13617
Matrix-Isolation Studies of 7-Hydroxyquinoline
740000 175000
555000
140000 0
n ijj
370000
2 W
c
z 185000
35000
1
0
300
350
400
WAVELENGTH nm
-
A 800000
600000 0
n 0
v)
z W
400000
!-
3 200000
0
1 \
