Environmental Effects upon the Photoluminescence of &Quino1ineboronic Acid Michael Goldmanl and E. L. Wehry2 Department of Chemistry, Indiana University, Bloomington, Ind. 47401 The luminescence of 8-quinolineboronic acid has been compared with that of quinoline, as well as that exhibited by 5- and 8-hydroxyquinoline. In contrast to the hydroxyquinolines, singlet 4 triplet intersystem crossing is an important process for 8-quinolineboronic acid in both hydrocarbon and hydroxylic solvents, implying that a low-lying (n, T*) excited singlet state plays an important role in determining its luminescence characteristics. The fluorescence efficiency of 8-quinolineboronic acid is not significantly affected by hydrogen-bond-accepting species, but singlet-toground internal conversion, presumably involving solvent interactions with the -B(OH)2 group, is an important process for this molecule in hydroxylic media. The luminescence behavior of 8-quinolineboronic acid is shown to be consistent with the predictions of molecular-orbital calculations previously reported.
THE CHARACTERISTIC LUMINESCENCE properties of quinolines (1, 2) can be significantly modified by introduction of sub-
stituents capable of hydrogen bonding with either the hetero nitrogen or the solvent, as shown in previous studies of hydroxyquinolines (3,4). An interesting molecule, which might be anticipated to behave rather similarly to the hydroxyquinolines, is 8-quinolineboronic acid (subsequently abbreviated "8-QB A"):
Qo H&oH
Although fluorescence and phosphorescence of 8-QBA have not been previously studied, its electronic absorption spectroscopy has received some attention. Letsinger (5) has reported that electronic spectra of 8-QBA in ethanol, dimethylformamide, and 0.1Methanolic HC1 are very similar to those of quinoline in the same solvents. These results imply that, at least in the ground electronic state, the fraction of 8-QBA molecules existing as the zwitterion,
Q3 A+
,0\
HO
0-
is quite small. Molecular-orbital calculations (6) indicate that, in the ground state of 8-QBA, appreciable transfer of T 1 NIH Predoctoral Fellow, 1966-68; present address, Marshall Laboratory, Du Pont Company, Philadelphia, Pa. 19146. 2To whom correspondence should be addressed, at Dept. of Chemistry, Univ. of Tennessee, Knoxville, Tenn. 37916
(1) V. L. Ermolaev and I. P. Kotlyar, Opt. Spektrosk., 9, 183 (1960). (2) M. A. El-Sayed, J. Chem. Phys., 38, 2834 (1963). (3) 0.Popovych and L. B. Rogers, Spectrochim. Acta, 15, 584 (1959). (4) M. Goldman and E. L. Wehry, ANAL.CHEM., 42, 1178 (1970). (5) R. L. Letsinger, Aduan. Chem. Ser., 42, l(1964). (6) D. R. Armstrong and P. G . Perkins, J. Chem. Soc., A , 1967, 123. 1186
electron density occurs from the oxygen atoms to the electrondeficient boron. One might therefore anticipate that hydrogen bonding of the -B(OH)2 group with hydrogen-bonding solvents (such as dimethylformamide) would be extensive, yet no indications of such interactions are evident in the electronic absorption spectra of 8-QBA. It has, however, been postulated (5,7) that, in aqueous media, simultaneous interaction of water with the boron atom and the hetero nitrogen occurs. The 8-QBA molecule is in principle also capable of engaging in intramolecular hydrogen bonding of the form
It is evident that a variety of interesting hydrogen-bonding phenomena may be possible in 8-QBA; it was accordingly desired to examine the influence of molecular environment upon the photoluminescence of 8-QBA and to compare it with previous studies of hydroxyquinolines (4). EXPERIMENTAL
The procedure of Letsinger and Dandegaonker (8) was employed for preparation of 8-QBA; the crude product was purified by repeated recrystallization from 70% (v/v) aqueous ethanol. Benzeneboronic acid (Columbia) was purified by recrystallization from water. Biacetyl (Baker) was purified by vacuum distillation, after which it was stored in the dark at 10 "C until use. Solvents were purified as previously described (4), as were the solutes used for standardization of the fluorometer. Correction of luminescence spectra and determination of quantum yields have been described (4). RESULTS AND DISCUSSION
Electronic-spectral data for 8-QBA are listed in Table I. In many solvents, 8-QBA exhibits luminescence similar in frequency and quantum yield to that of quinoline ( I , 2) with some important differences to be noted in subsequent discussion. There are a number of striking differences between the luminescence of 8-QBA and that of 5- and 8-hydroxyquinoline, the most significant of which is the observation of intense phosphorescence from the former in all glassy media except acidic methanol. It is interesting to note that, in alcoholic glasses, quinoline ( I ) exhibits essentially the same phosphorescence yield (@p= 0.19) and decay time (7 = 1.9 sec) as that observed for 8-QBA. The long phosphorescence decay time strongly suggests that the emitting triplet is (a,T*). These observations imply that the energy of the lowest (n, T*) singlet, relative to that of the first (a,a*)excited singlet, is considerably lower in 8-QBA than in the hydroxyquinolines. ~~
(7) J. D. Morrison and R. L. Letsinger, J . Org. Chem., 29, 3045 (1964). (8) R. L. Letsinger and S. H. Dandegaonker, J. Amer. Chem. Soc., 81,498 (1959).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
Solvent Isopentan, Ethanol Water Diethyl ether Acetonitrile Tetrahydrofuran 1.OM HzS04in ethanol 7.5M aq &SO( 12.OM aq HzSO4 98Z &SO4 I
Isopentane Ethanol 1.OMHzSOl in ethanol Isopentane-ethanol (9 :1 v/v) Diethyl ether
Table I. Spectral Data for &Quinolineboronic Acid A. Room Temperature (298 OK) Fluorescence Absorption vmax W-9 @F vmax (cm-9 log € ...
