T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY
Registered in
U.S. Patent Ofice @ Copyright, 1970, by the American Chemical Society
VOLUME 74, NUMBER 20 OCTOBER 1,1970
Substitution Effects on the Emissive Properties of N-Heteroaromatics.
I.
Substituted Quinolines
by C. M. O'Donnell,* G. A. Knesel, T. S. Spencer, and F. R. Stermitz Department of Chemistry, Colorado State University, Fort Collins, Colorado 80681 (Received December SO, 1960)
Total emission spectra of 2-(2-hydroxyethyl)quinoline, 2-(2-deuterioxyethyl)quinoline, and the quinolinium ion were recorded. The effect of various solvents coupled with the phosphorescence lifetime data indicate that in nonpolar solvents a charge-transfer triplet intermediate is possible. The photochemistry of these 2-substituted compounds is understandable in light of the results presented and the generalization of these results to similar systems is discussed. The effect of hydroxylic solvents on the luminescence of quinoline is well established.'I2 A combination of orbital interchange and second-order vibronic spin-orbit interactions between (n,r*) and ( T , T * ) states is believed to be responsible for the changes observed in: (1) phosphorescence lifetimes, (2) the degree of polarization outside of the (0,O) phosphorescense band, and (3) the ratio of fluorescence to phosphorescence inten~ i t y . ~An , ~attempt to observe similar changes caused by intramolecular perturbation is important to the understanding of the emissive properties in simple N heteroaromatic systems. I n our investigation of a number of substituted quinolines, we have found that 2-(2-hydroxyethyl)quinoline (HE&) and 2-(2-deuterioxyethyl)quinoline (DE&) behave quite differently from other 2-substituted derivatives. The quantum yield for the photoelimination reaction of these alcohols is higher in nonpolar than in polar solvent^.^ If the reactive intermediate is a singlet, a correlation between the quantum yield of photoelimination and the ratio of fluroescence to phosphorescence should be possible. Our results indicate that this is not the case and that unique effects are produced by a charge-transfer intermediate in nonpolar solvents.
Experimental Section Emission and excitation spectra (77 "K) were rou-
tinely recorded on a Perkin-Elmer Model MPF-2A fluorescence spectrophotometer. For higher resolution, the light from a 200-W xenon lamp was passed through a 0.25-m Jarrel-Ash monochromator for excitation while passing the emission through a 0.75-m Spex 1700-11 monochromator. The detection system consisted of an El41 6256 S photomultiplier tube, a Keithley Model 414 picoammeter and a RIoseley 7100 B strip chart recorder. Lifetimes were measured using an EG & GFX 12-0.25 flash lamp as excitation, monitoring a small band pass with a monochromator, and feeding the signal into a Textronic Type 53lA oscilloscope to be recorded on Polaroid film using a C-12 oscilloscope camera. A least-squares analysis has been run on the lifetimes in addition to a semilog plot to determine if the decay contained any nonexponential segments. HE& was purchased from Aldrich Chemical Company. Subsequently, this compound was recrystallized twice from CHCls-ether and resublimed once to yield a
* To whom correspondence should be addressed. (1) V. L. Ermolaev and I. P. Kotlyar, Opt.Spectrosc., 9, 183 (1960). (2) M. A. El-Sayed and M. Kasha, Spectrochim. Acta, 15, 758 (1960). (3) M. A . El-Sayed, J . Chem. Phys., 38, 2834 (1963). (4) E. C. Lim and Jack M.H. Yu, (bid., 47, 3270 (1967). (5) F. R. Stermtiz, C . C. Wei, and C. M . O'Donnell, J . Amer. Chem. Soc., 92, 2745 (1970).
3555
C. 3%.O'DONNELL, G. A. KNESEL,T.S. SPENCER, AND F, R. STERMITZ
3556
OUlNOLlNE
HEQ DE0
-
---.e-..-
\ ,
\
I \ \ \
Figure 1. Total emission spectra of quinoline, HE&, and DE& in EPA at 77°K.
I
--
O U l N O L l N l U M CHLORIDE OUlNOLlNE
Figure 2. Total emission spectra of quinolinium chloride in EPA and quinoline, HE&, and DE& in 3-methylpentane at WOK. The Journal of Physical Chemistry, Vol. 74, No. 20,1970
EMISSIVE PROPERTIES OF N-HETEROAROMATICS white crystalline material which melted a t 102 to 104O, No changes in the emission spectra were evident when either the recrystallized or recrystallized and sublimed samples were used. A sample of the HE& was warmed with DzO and evaporated to dryness. This process was repeated three times and the resulting compound was shown to be completely the 2-(2-deuterioxyethyl)quinoline by nmr. Quinolinium chloride was prepared by the addition of hydrochloric acid to a solution of quinoline in methanol and evaporating the mixture to dryness. The resulting solid was recrystallized twice from 95% ethanol. 3-hlethylpentane (Phillips Petroleum Company pure grade) was passed through a 90-cm column of alumina saturated with silver nitrate and checked for purity by absorption spectroscopy. EPA (5 :5 :2 by volume ether, isopentane, and ethanol) was used as obtained from Hartman-Leddon Co.
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Table I : Solvent Effect on Luminescence” Fluoreacence, Compd
Quinoline
Solvent
b
3MP
None
None
EPA
3137 3210 3268 3355 3420
None
Phosphorescence,
b
4560 4688 4790 sh 4870 5000 4560 4670 4870 5000 5260
3512
3578 2-(2-Hydroxyethy1)quinoline
3MPb
Results The total emission spectra of quinoline, HE&, and DE& in EPA are presented in Figure 1, and the band maxima for their fluorescence and phosphorescence bands are given in Table I. A marked similarity between these spectra is evident except for an increase in the fluorescence yield in the 2-substituted compounds. However, in changing the solvent to 3-methylpentane, Figure 2, the quinoline emission reverts completely to phosphorescence as is expected, but the emission of HE& and DE& becomes rather broad and structureless. On either side of this broad emission several small bands occur at the same locations as the fluorescence and phosphorescence bands of HE& and DE& in EPA solvent. Small differences between these bands and the fluorescence and phosphorescence bands are found, but these differences can be attributed to the overlapping broad band luminescence. The emission spectrum of quinolinium chloride in EPA (it is not soluble in 3-methylpentane) was almost identical with those obtained for HE& and DE& except blue shifted. No noticeable differences are observed in the emission of HE& and DE& which indicates that the deuterium effect is indeed small. Lifetimes of quinolinium chloride in EPA and quinoline, HE&, and DE& in both 3-methylpentane and EPA, are reported in Table 11. As expected, the lifetimes observed in 3-methylpentane are shorter due to increased vibronic coupling between the “ n , ~ * ) and 3 ( 7 r , ~ * )states in a hydrocarbon solvent.6 I n 3-methylpentane two long-lived species are observed. The first was the broad structureless band which had a measured lifetime of 0.05 seen at the 3800 maxima. A second long-lived species with a lifetime of approximately 1.3 sec at 4900 A is presumably due to the emission from the same state observed in EPA. The measured lifetimes of the broad bands in HE&, DE&, and quinolinium chloride are virtually identical.
h
Charge transfer,
EPA
2-(2-’Deuterioxyethy1)quinoline
3MPb
EPA
3150 3230 3290 3380 3460 3180 3230 3295 3378 3450 3540 3608 3700 sh 3170 3245 3310 3400 3480 3180
3880 max
4570 4670 4710 sh
Kone
4555 4660 4865 4980 sh 5010
3860 max
4584 4600 4700 sh 4890 5000 sh 4595 4705 4900 5010 sh 5260
None
32-52
3335 3415 3490 3575 3660 3730 sh Quinolinium chloride
EPA
3140 w 3200 w 3260 w
3820 max
4560 4660 4860 5055
w = weak, sh = shoulder. b These represented the fluorescence bands not completely covered by the charge-transfer luminescence.
Discussion Vibronic spin-orbit interactions in quinoline4 and related heteroaromatics have been shown to be responsible for much of the solvent dependency of the luminescence from these systems. If these processes were operative in HE& and DE&, they should manifest themselves as an increase in phosphorescence to fluorescence intensity in going from a hydroxylic to hydrocarbon (6) G. A. Knesel and C. M. O’Donnell, unpublished results.
The Journal of Physical Chemistry, Vol. 741No. 80,1970
C. M. O’DONNELL, G. A. KNESEL, T. S.SPENCER, AND E’. R. STERMITZ
3558
Table I1 : Solvent Effects on Phosphorescence Lifetimes Compd
Quinoline Quinolinium chloride 2- (2-Hydroxyethy1)quinoline 2- (2-Deuterioxyethy1)quinoline
Solventd
3MP EPA EPA
None None 0.05 f 0.003
3MP EPA
0.05 zk 0.003 None
3MP EPA
0.05i0.003 None
0.95 f 0.05 1.35 i.0.05
4574 4560
3800
1 . 3 3i 0 . 1 1.64 f 0.05
49160 4550
3800
1.28i0.1 1.72 f 0.05
49000 4571
3800
a Mean lifetime of charge-transfer band at 3800 8. * Mean lifetime of normal phosphorescence at wavelength indicated. E A longer wavelength band was used in these measurements to eliminate the overlap of the charge transfer band. Excitation spectra were run in 3MP observing a t these wavelengths, and they were identical with that obtained for the parent compound in EPA. d All lifetimes were recorded at 77°K.
solvent. Alternately, if intramolecular hydrogen bonding were more effective in a hydrocarbon solvent, we might not observe any drastic change in this ratio. Certainly, there is nothing in this mechanism to account for a new band being formed with a location between the fluorescence and phosphorescence. If this was simply a red shift in the fluorescence of HE& and DE&, the same type of shift should be noticeable in quinoline. Since this emission is so similar to that observed from the quinolinium ion, it is reasonable to assume that the emission originates from some common state. A possible route which HE& and DE& might take to achieve this common state is via a transfer of charge in the excited state as depicted below. This ionization may take
place in either a singlet or triplet state, but the luminescence from this ion is shown to be a triplet by our lifetime measurements. I n view of the lifetime data presented in Table 11, we find convincing evidence for our assertions. The phosphorescence lifetimes of HE& and DE& are noticably longer than that observed for quinoline, but quite similar to that observed for 2-ethylq~inoline.~We observe a decrease in the lifetime of quinoline in going from EPA t o 3-methylpentane which agrees with previous literature values. A decrease in the lifetime of the 4900-A bands of HE& and DE& with respect to that observed in EPA is also observed and based on the per cent change which occurs for 2-ethylquinoline when the solvent is changed from EPA to 3-methylpentane, which is expected. These lifetime measurements plus the close relationship between the location of the band maxima of these sidebands with the fluorescence and phosphorescence in EPA indicate that some of the The Journal of Physical Chemistry, Vol. 74,No. BO, 1870
un-ionized species is present. Excitation spectra for HE& and DE& with an emission wavelength of 4900 A are identical with that observed for quinoline in 3-methylpentane or HE& and DE& in EPA. As shown in Table 11, the lifetimes of all of the broad emissions are identical and this is rather convincing evidence of the identity of the lowest emitting state in all three compounds. If a charge-transfer state is responsible for the photochemistry of these molecules, their photoelimination results become understandable. For both HE& and DE&, the quantum yield of photoelimination increased markedly on changing from a polar to a nonpolar solvente6 This result can be explained by proposing that the mechanism for photoelimination in these compounds in the nonpolar solvent is as follows.
I
H
+ CHZO
Since intramolecular hydrogen bonding would be greatly favored in a nonpolar solvent, this provides an explanation for the observed increase in quantum yields. The absence of this effect when quinoline is dissolved in
3559
BLACK LIPID MEMBRANES IN AQUEOUS MEDIA hydroxylic glass a t 77°K also gives evidence to the necessity of the intramolecular bond in this photochemical reaction.
Conclusions An unusual type of emission is observed for substituted quinoline in nonpolar solvents as a result of intramolecular hydrogen bonding. This charge-transfer state seems to be responsible for the photoelimination of these compounds and explains the effect of solvent on their quantum yields. A number of such systems can readily be conceived, including the related isoquinoline series which we shall report on in a future paper.
The number of intramolecular hydrogen-bonded heterocycles found in biological systems is numerous. We suggest that this type of charge transfer state could occur whenever a similar acid group is present in such systems and might determine their photochemistry.
Acknozuledgments. Acknowledgment is made to donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Partial support was provided by grant GM-15425 from the U. S. Public Health Service and the Research Corporation. We wish to thank Professor S. J. Strickler for a number of helpful discussions.
Black Lipid Membranes (BLM) in Aqueous Media. Photoelectric Spectroscopy by Nguyen Thuong Van and H. T. Tien Department of Biophysics, Michigan State University, East Lansing, Michigan 488.93 (Received March 19, 1970)
Black lipid membranes (BLX) of chloroplast extracts separating two aqueous solutions have been studied photoelectrometrically. The experimental arrangement and procedure for obtaining photoelectric spectra of the BLM are described in detail. The other salient points of this paper are as follows. (1) The photoelectric spectrum of the BLM obtained in the region of 350-800 nm is compared with its absorption spectrum of bulk solution. The two spectra, though obtained by two entirely different methods, are practically identical. (2) The origin of the photoelectric spectrum of the BLM is discussed in terms of various possible modes of interactions. Electrons and holes are produced in the BLM upon light excitation, which in turn give rise to the observed photovoltaic effect. (3) A comparative study is made between a silicon solar cell and a chloroplast BLM. The two systems exhibit striking similarities. (4)The photolysis of water by light mediated through chloroplast BLM is proposed. (5) The significance and general utility of BLM photoelectrometry, conceived and developed in the course of this investigation, are briefly indicated.
Introduction I n photosynthesis, the role of the thylakoid membrane of chloroplasts appears to be crucial in the primary photophysical and photochemical reactions. At present the structure of the thylakoid membrane is not sufficiently known but is thought to be a quasicrystalline array of lipids, proteins, and pigments (for a recent review, see ref 1). These materials organized in ultrathin lamellae have been suggested earlier by FreyWyssling,2 Menke, and Frey-Wyssling and Steinm a n 4 on the basis of birefringence and dichroism studies. Further evidence has been provided by X-ray diffraction data from chloroplasts gathered by Kreutz6 and indirect monolayer experiments6-* as well as recent optical investigations. The results of preceding work and of more recent electron microscopic gllo
studies and chemical analyses have led a number of investigators to propose detailed structural models. 11--14 (1) D. Branton, Ann. Reu. Plant Physwl., 20,209 (1969). (2) A. Frey-Wyssling, Protoplasma, 29,279 (1937). (3) W.Menke, B b l . Zentr. 6 3 , 326 (1943). (4) A. Frey-Wyssling and E. Steinmann, Bwchim. Biophys. Acta, 2,264 (1948). (5) W. Kreutz and W. Menke, 2.Naturforsch., 15b, 483 (1960). (6) E. E. Jooobs, A. 8. Holt, R. Kromhout, and E. Rabinowitch, Arch. Bbchem. Biophys., 72,495 (1957). (7) W. D.Bellamy, G. L. Gaines, and A. G. Tweet, J . Chem. Phys., 39, 2528 (1963). (8) W.Sperling and B. Ke, Photochem. Photobwl., 5 , 865 (1966). (9) J. C.Goedheer, Ph,D. Thesis, University of Utrecht, 1957. (10) K. Sauer, J. R. Smith, and A. J. Sohultz, J . Amer. C h m . Hoc., 88, 2681 (1966). (11) W.Menke, Brookhaven Symp. Biol., 19, 328 (1967).
The Journal of Physical Chemistry, Vol. 74, No. 80,1070
