Environmental Effects upon Fluorescence of 5- and 8-Hydroxyquinoline Michael Goldmanl and E. L. Wehry2 Department of Chemistry, Indiana University, Bloomington, Ind. 47401 Photoluminescence of 5- and 8-hydroxyquinoline in a number of solvents has been investigated at both room temperature and 77 O K . Neither phosphorescence nor intersystem crossing can be detected for either compound in hydrocarbon or ethanolic media, indicating that the location of low-lyin (n, T * ) excited singlet states is not an important factor determining their photoluminescence properties. The principal influence of solvent and temperature upon fluorescence of 5-hydroxyquinoline involves enhancement of the rate of singlet-to-ground internal conversion by inter. molecular hydrogen bonding involving the phenolic hydroxyl. Similar influences are operative in the fluorescence of 8-hydroxyquinoline; competition of inter- and intramolecular hydrogen bonding in the lowest singlet of 8-hydroxyquinoline also affects fluorescence yields. I t is probable that excited-state acid-base equilibria do not significantly affect fluorescence of 5- or 8-hydroxyquinoline in any environment. Implications of these studies for quantitative fluorometric determination of nitrogen heteroaromatics are discussed.
THEPHOTOLUMINESCENCE SPECTROMETRY of nitrogen heteroaromatics is considerably different from that of the parent aromatic hydrocarbons, principally because of the presence of low-lying (n, a*) excited states in the former. Since the relative energies of excited (n, a*)and (a,a*)singlets in N-heteroaromatics are a function of the polarity and hydrogen-bonding ability of the solvent, fluorescence spectra and efficiencies are often strikingly dependent upon molecular environment. Because fluorescence assays of N-heterocyclics are frequently performed, it is important to understand the effects of solvent and temperature upon their luminescence behavior. Beginning with a n investigation by Popovych and Rogers ( I ) of solvent effects upon fluorescence of 8-hydroxyquinoline (hereafter denoted “8-HQ”), several studies of environmental influences upon luminescence of hydroxyquinolines have appeared (2-9). While, in most cases, previous discussions of the fluorescence characteristics of hydroxyquinolines have assigned considerable significance t o perturbing influences of low-lying (n, a*) singlets ( I , 4), or t o the occurrence of ex1 NIH Predoctoral Fellow, 1966-68; present address, Marshall Laboratory, Du Pont Company, Philadelphia, Pa. 19146. To whom correspondence should be addressed at Dept. of Chemistry, Univ. of Tennessee, Knoxville, Tenn. 37916.
(1) 0. Popovych and L. B. Rogers, Spectrochim. Acta, 15, 584 (1959). (2) R. E. Ballard and J. W. Edwards, J. Chem. SOC.,1964, 4868. (3) D. C. Bhatnagar and L. S . Forster, Spectrochim. Acta, 21, 1803 (1965). (4) D. N. Bailey, D. M. Hercules, and T. D. Eck, ANAL.CHEM., 39, 877 (1967). ( 5 ) K. Kimura and L. S.Forster, J. Phys. Chem., 71,2744 (1967). (6) S. Schulman and Q. Fernando, ibid., p 2668. (7) S. F. Mason, J. Philp, and B. E. Smith, J . Chem. SOC., A , 1968, 3051. (8) S . G. Schulman and H. Gershon, J . Phys. Chem., 72, 3693 (1968). (9) S. Schulman and Q. Fernando, Tetrahedron, 24, 1777 (1968). 1178
0
cited state acid-base reactions (2,6-9), many of the arguments have been based upon rather indirect experimental evidence. The present investigation considers influences of solvent and temperature upon luminescence of 8-HQ and 5-hydroxyquinoline (subsequently abbreviated “5-HQ”). It is instructive t o compare the luminescence of these two quinolinols, because strong similarities in their electronic absorption spectra (IO) and low-temperature fluorescence ( 5 ) have previously been demonstrated. EXPERIMENTAL
Chemicals. Solid 8-HQ (Matheson, Coleman and Bell reagent) was dissolved in aqueous MgS04; the Mgz+chelate of 8-HQ was isolated and then dissolved in concentrated aqueous HCI. The p H of this solution was slowly raised t o 6.5, whereupon a precipitate of 8-HQ formed; the solid was recrystallized once from ethanol and once from chloroform and then vacuum sublimed. The sublimate was used in all experiments. Synthesis of 8-methoxyquinoline was performed by treatment of 8-HQ with methyl bromide in alkaline ethanol. The crude product was recrystallized from petroleum ether and vacuum sublimed. Preparation of 1-methyl-8-hydroxyquinolinium perchlorate proceeded by refluxing 8-HQ with methyl iodide; a n aqueous solution of the resulting iodide salt was passed over anion exchanger in the perchlorate form. The crude perchlorate salt was recrystallized from water. An analogous procedure was employed to prepare 1-methyl-8-methoxyquinoliniumperchlorate from 8-methoxyquinoline. An ethanolic solution of 5-HQ (Aldrich) was passed over alumina. The effluent was collected and the solvent evaporated; the solid was then crystallized alternately from benzene and 5 0 Z (v/v) aqueous ethanol, followed by vacuum sublimation. The 5-HQ sublimate was used in all experiments. Synthesis of 1-methyl-5-hydroxyquinolinium perchlorate proceeded similarly to that of the analogous 8-HQ derivative. Special care was exercised in solvent purification. Acetonitrile, dimethylformamide, dimethyl sulfoxide, methanol, and dichloromethane were Matheson, Coleman and Bell “Spectroquality” materials ; they were distilled and dried over molecular sieves before use. Ethanol (Commercial Solvents “Gold Shield”) was distilled from a mixture of NaOH and BaO; the final material contained less than 1 Z water by weight. Water was doubly distilled over permanganate. Sulfolane (Phillips) was purified by a literature procedure ( I I ) , as were tetrahydrofuran (Fisher “Spectranalyzed”) (12) and formamide (Fisher) (13). None of the solvents exhibited detectable luminescence at time of use. Quinine sulfate (Mallinckrodt) was recrystallized from 0.05M aqueous H2S04, followed by two recrystallizations (10) G. W. Ewing and E. A. Steck, J. Amer. Chem. SOC.,68, 2181 (1964). (11) J. Lawrence and R. Parsons, Trans. Faraday SOC.,64, 751 (1968). (12) J. Perichon and R. Buvet, Electrochim. Acta, 9, 567 (1964). (13) F. H. Verhoek, J . Amer. Chem. SOC., 58, 2577 (1936).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
from water. Preparation of 3-nitro-1,l-dimethylanilineinvolved methylation of the corresponding nitroaniline, followed by recrystallization from 95 % ethanol and 70% (v/v) n-hexane-30 % benzene. Fisher reagent 2-naphthol was recrystallized from 2 5 x (v/v) aqueous ethanol and vacuum sublimed. Spectroquality benzene was purified by a literature procedure (14). Trans-propenylbenzene (Columbia) was vacuum distilled; the distillate yielded a single peak when passed over two different 8-ft X ’/*-inch GLC columns, packed, respectively, with 15% Carbowax and 20% Apiezon L, both on Chromosorb W. Trans-l,3-pentadiene (Matheson, Coleman and Bell) was distilled in a spinning-band column; the distillate yielded a single peak when passed over two different 8-ft X l/a-inch GLC columns, packed, respectively, with 20% Apiezon L on Chromosorb W and 20% AgN03 in propylene glycol on Chromosorb P. Methods. All luminescence measurements were made with an Aminco-Bowman spectrophotofluorometer using 1-cm rectangular cells, a 150-watt xenon lamp, and a 1P28 photomultiplier. Low-temperature emission spectra were obtained with a special holder for rectangular 1-cm cells, which fit into the space normally occupied by the fluorometer cell housing; thermal contact with a liquid nitrogen reservoir was achieved with a copper cold finger. All solutions were degassed by six freeze-thaw cycles at Torr, using a glass pressures not greater than 2 X vacuum line with Fischer & Porter connectingjoints and Teflon (Du Pont) stopcocks. Luminescence arising from distillation of impurities from rubber O-rings in the Teflon stopcocks (15) was not detected. Luminescence spectra were corrected for variation in fluorometer response with wavelength by the quantum-counter technique (16). The resulting correction curve was adjusted to produce agreement of spectra corrected thereby, of quinine sulfate, 2-naphthol, and 3-nitro-l,l-dimethylaniline,with standard spectra (17). Fluorescence quantum yields were determined by the comparative procedure (18), using quinine sulfate (aF = 0.55) as standard (19). The value of aF for quinine sulfate has recently been questioned (20, 21); the resulting uncertainty may ultimately necessitate multiplication of all quantum yields here reported by a constant factor, which will have no effect upon the conclusions of this investigation. Triplet-counting experiments were performed as previously described (22), using 8-ft X */8-inchGLC columns packed with 15% Apiezon L on Chromosorb W or 2 0 x AgN03 in propylene glycol on Chromosorb P. Ferrioxalate actinometry (23) was employed. RESULTS AND DISCUSSION
One of the most striking observations of this study is that, unlike quinoline (24), neither 5-HQ nor 8-HQ exhibits detectable phosphorescence in hydrocarbon, ethanol, or DMSOsulfolane (9 :1 v/v) glasses at 77 OK. With present instrumen-
tation, phosphorescence of quantum efficiency greater than 0.0005 would have been detected. A previous report (4) that 8-HQ exhibits short-lived phosphorescence in EPA at 77 OK could not be duplicated in our studies. The lack of phosphorescence from 5-HQ and 8-HQ is quite atypical for Nheteroaromatics, most of which fluoresce only very weakly, but exhibit intense phosphorescence, in hydrocarbon solvents (25). The observations indicate either that intersystem crossing from the first excited singlet to the triplet manifold is extraordinarily inefficient, or that intersystem crossing from the lowest triplet to the ground state is unusually rapid, in 5-HQ and 8-HQ. Quantum yields for singlet-triplet intersystem crossing can be determined by triplet-triplet energy transfer (22), wherein a donor molecule, such as 5-HQ, is employed as a source of triplets which sensitize an olefin isomerization known to proceed through a triplet state: “5-HQ)*
+ trans-slefin
+
5-HQ
+ cis-olefin
-
By measuring yields for photosensitized trans cis olefin isomerization under conditions wherein only 5-HQ is directly excited, the efficiency of singlet + triplet intersystem crossing for 5-HQ can be computed. This experiment has been performed for both 5-HQ and 8-HQ as triplet donors, using both trans-propenylbenzene and trans-l,3-pentadiene as acceptors. Both isopentane and ethanol were employed as solvents for each quinolinol. No detectable olefin isomerization was sensitized by either 5-HQ or 8-HQ in either isopentane or ethanol at 298 OK. Considering conservative detection limits for the GLC procedure, we conclude that the quantum efficiency for singlet triplet intersystem crossing in both 5-HQ and 8-HQ cannot be greater than 0.02 in either alcohol or hydrocarbon media at room temperature. This conclusion is reinforced by the fact that, in both n-hexane and methanol, neither 5-HQ nor 8-HQ effect appreciable photosensitization of room-temperature phosphorescence from biacetyl(26). The results of the present study strongly imply that, in both 5-HQ and 8-HQ, the (n, T * ) singlet state of lowest energy must lie well above the lowest ( n , .*) singlet in all solvents, including hydrocarbons. Several lines of evidence substantiate that conclusion for 8-HQ. First, if the lowest excited singlet were (n, a*) in any solvent, relatively efficient intersystem crossing to ,.( .*) triplets (24) should be observed. Singlet-triplet intersystem crossing is actually inefficient for 8-HQ at 298 “K in all solvents studied-even hydrocarbons, in which the energy of the (n,n*) singlet, relative to that of the lowest (9, T * ) singlet, should be minimal (25). Second, a series of changes in shape of the low-energy absorption band envelope of 8-HQ as a function of solvent, which was previously interpreted (27) as indicating the presence of an n + IT* absorption submerged in the more intense P K* band, could not be duplicated in the present investigation. Third, low-frequency absorption bands for 8-hydroxyquinolines substituted with conjugative electron-donating substituents (principally halogens) do not exhibit frequency shifts (28) indicative of n + *. transitions. Fourth, 8-HQ does not phosphoresce in hydroxylic or hydrocarbon glasses. Identical arguments can be advanced for 5-HQ; we therefore conclude -+
-+
~
(14) W. G. Herkstroeter, L. B. Jones, and G. S . Hammond, J . Amer Chem. Soc., 88,4777 (1966). (15) J. T. Dubois and F. Wilkinson, Appl. Spectrosc., 18, 27 (1964). (16) W. H. Melhuish, J. Opt. SOC.Amer., 52, 1256 (1962). (17) E. Lippert, W. Nagele, I. Seibold-Blankenstein, U. Staiger, and W. Voss, Z . Anal. Chem., 170,1 (1959). (18) C. A. Parker and W. T. Rees, Analyst, 85,587 (1960). (19) J. W. Eastman, Photochem. Photobiol., 6 , 55 (1967). (20) R. F. Chen, Anal. Biochem., 19, 374 (1967). (21) R. Rusakowicz and A. C . Testa, J. Phys. Chem., 72, 793 (1968). (22) A. A. Lamola and G. S. Hammond, J . Chem. Phys., 43, 2129 ( 1965). (23) C. G. Hatchard and C. A. Parker, Proc. Roy. SOC.(London), A235, 518 (1956). (24) M. A. El-Sayed, J . Chem. Phys., 38,2834 (1963).
(25) E. L. Wehry, in “Fluorescence: Theory, Instrumentation, and Practice,” G. G. Guilbault, Ed., Dekker, New York, 1967, Chapter 2. (26) K. Sandros, Acta Chem. Scand., 23,2815 (1969). (27) D. M. Hercules, personal communication cited in Ref. (1). (28) R. S. Becker, “Theory and Interpretation of Fluorescence
and Phosphorescence,” Wiley, New York, 1969.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
1179
Solvent Isopentane Acetonitrile Sulfolane Dioxane Diethyl ether Dimethylformamide Tetrahydrofuran Dimethyl sulfoxide Dimethyl sulfoxidesulfolane (9 : 1) Water Ethanol 1.OM HISOain C2H50H 1 .OM NaOH in CIHjOH 98% HzS04 Isopentane satd with HCk)
Table I. Spectral Data for 5-Hydroxyquinoline A. Room Temperature (298 OK) Fluorescence -AH (kcal mole-1)a vmsx (cm-1) @F ... 23,300 0.30 3.5 22,700 0.24 3.5 22,700 0.21 4.4 23,300 0.19 5.1 23,300 0.12 5.3 22,900 0.09 5.5 22,700 0.09 6.4 22,700 0.07
...
22,500
...
...
...
16,700 19,200
0.003
