DIRECT DETERMINATION OF SINGLET + TRIPLET INTERSYSTEM CROSSING QUANTUM YIELD
2083
Direct Determination of Singlet + Triplet Intersystem Crossing Quantum Yield.
11.
Quinoline, Isoquinoline, and Quinoxaline
by Steven G. Hadley Department of Chemistry, University of Utah, Salt Lake City, Utah 84118
(Received November 10, 1970)
Publication costs assisted by the National Science Foundation
The quantum yields for radiakionless singlet W + triplet intersystem crossing of several aza-substituted molecules have been determined under well-characterized experimental conditions. In a rigid alcoholic glass at 77°K the quantum yields and their uncertainties were found to be: quinoline, 0.50 f 0.1; isoquinoline,0.24 f 0.05; quinoxaline,0.18 f 0.04. In a hydrocarbon glass at 77°K these values were determined to be: quinoline, 0.43 f 0.08; isoquinoline, 0.19 =t0.04; quinoxaline, 0.27 f 0.05. Within the stated error no significant enhancement of the intersystem crossing quantum yield is observed in changing the rigid glass from a hydrocarbon to hydroxylic medium. The data imply significant internal conversion from the first excited singlet for quinoline in a hydrocarbon matrix and quinoxaline in hydrocarbon and hydroxylic matrices.
Introduction The radiative and radiationless processes in aromatic molecules have been the subject of considerable theoretical and experimental effort during the last 20 years.’ The aza-substituted aromatic systems have been of particular interest. Cohen and Goodman have discussed these processes in three azabenzenes.2 We wish to report direct measurement of the intersystem crossing quantum yield of three aza-substituted naphthalenes : quinoline (1-azanaphthalene) ; isoquinoline (2-axanaphthalene) ; and quinoxaline (l14-diazanaphthalene). These systems were chosen for study as there are already considerable data available in the literature regarding their radiative properties. Also, there have been some theoretical investigations directed a t these particular systems. However, quantitative knowledge of a process as fundamental as intersystem crossing is not available. The work reported in this paper is directed toward obtaining some of this information. The experimental reports on the emission properties of these molecules are in general agreement. The parent hydrocarbon, naphthalene, when dissolved in either a hydrocarbon or hydroxylic rigid glass at 77”K, emits both fluorescence and phosphorescence when excited by ultraviolet radiation absorbed by its singlet + singlet bands. Under similar experimental conditions quinoline emits both fluorescence and phosphorescence in a hydroxylic glass. Early investigators have reported no fluorescence from quinoline in a hydrocarbon glass, only phosphorescence.* Recently, very weak fluorescence from quinoline in a hydrocarbon glass has been reportedS6 Isoquinoline dissolved in a hydrocarbon solvent under similar experimental conditions emits phosphorescence and weak fluorescence. Isoquinoline’s fluorescence to phosphorescence ratio is
greatly enhanced in a hydroxylic medium over that observed in a hydrocarbon medium.6 Quinoxaline emits only phosphorescence under these experimental conditions in either a hydrocarbon or a hydroxylic solvent4 The electronic states of these molecules have been studied by several investigators. This knowledge is summarized in Figure 1. The lowest excited electronic states of singlet and triplet multiplicity of quinoline and isoquinoline in rigid glasses at 77°K appear to be ( T , T * ) . ~ It has been suggested4 that the lowest excited singlet state of quinoline in a hydrocarbon rigid glass is (n,s*); however, this is not apparent from the absorption spectrum. It is believed that in these molecules the lowest ‘(n,r*) state has slightly greater energy than the lowest ‘ ( ? T , T * )state.’ The vibronic interactions that would occur between nearly degenerate ‘(n,?r*) and ‘(T,T*)states are expected to be strong enough to thoroughly mix these levels. This might reduce the importance of the knowledge of the exact location of the lowest ‘(n,?r*) and l(?r,?r*) states.6 The energy levels of quinoxaline are better known. The l(n,?r*) and a ( ? r , ~ * ) states are the lowest in energy of ) is betheir respective multiplicity. A 3 ( n , ~ * state (1) Much of the information is contained in (a) R. S. Becker, “Theory and Interpretation of Fluorescence and Phosphorescence,” Wiley, New York, N. Y.,1969; (b) S.P. McGlynn, T . Azumi, and M. Kinoshita, “Molecular Spectroscopy of the Triplet State,” Prentice-Hall, Englewood Cliffs, N . J., 1969. (2) B. 5 . Cohen and L. Goodman, J . Chem. Phys., 46, 713 (1967). (3) Reference la, p 118. (4) M . A. El-Bayed and M. Kasha, Spectrochim. Acta, 15, 758 (1 959). (5) *F < 0.01, E. C.Lim, private communication. (6) E. C. Lim and J. M. H. Yu, J . Chem. Phys., 47, 3270 (1967). (7) G. Coppens, C . Gillet, J. Nasielski, and E. V. Donckt, Spectrochim. Acta, 18, 1441 (1962). T h e Journal of Physical Chemistry, Vol. 76,N o . 14, 1972
STEVEN G. HADLEY
2084
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tween the 3(7r,7r*) and l(n,n*) statesas A higher l(n,n*) state has been observed. By analogy to naphthalene a 3(n,7r*)state is believed to have similar energy in all these molecules. Radiationless processes have been examined in several theoretical frameworks. Henry and Kasha have reviewed this work. El-Sayed has developed some theoretical predictions based upon assuming the validity of the Born-Oppenheimer approximation. lo Within this framework he has shown that Sl(n,n*) ,-* Tk(n,n*) and S~(n,?r*) + Tk(n,n*)ought to be at least 1000 times as fast as either Sl(n,n*) W + Tk(n,n*) or Sl(n,n*) M TR(r,n*). El-Sayed has offered the explanation of the radiative properties of quinoline in terms of an interchange of the '(n,n*) and I(n,a*) states upon changing the solvent from a hydrocarbon to a hydroxylic medium, the '(n,n*) state being lowest in the hydrocarbon s ~ l v e n t . This ~ behavior of the l(n,n*) and l(n,n*) states is consistent with their known solvent shifts, but the spectroscopic evidence is lacking. He has also compared the emission properties of quinoline and naphthalene in a hydroxylic glass. A comparison of the total emission spectra of these two molecules lead to the conclusions that the rate of intersystem crossing in quinoline is enhanced over that in naphthalene, and that in quinoline radiationless losses from S1 (other than intersystem crossing) are occurring. lo Lim and Yu have discussed the mechanism of intersystem crossing in terms of a second-order spin-orbit coupling involving (n,n*)-(r,r*) vibronic interactions. They believe that this mechanism plus the "orbital interchange" mechanism can explain the solvent effects upon fluorescence.6 Lim and Li have emphasized that nonplanar vibrations may be important in deactivation, via internal conversion, of the lowest excited singlet state of nitrogen heterocycles. l1
Experimental Section The previously used experimental techniqueI2 was modified t o permit its extension to molecules whose T h e Journal of Physical Chemistry, Vol. 76,N o , 14,1971
--: 0 s @iN
T(.rr'T321,300
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triplet state lifetime is as short as 0.2 see. A flash lamp that dissipated over 16,000 J in less than 100 psec was used to pump the molecules into the triplet state. For studies on quinoxaline a Corning 7-54 filter was used to isolate the emission region of the lamp that was incident upon the sample. For the studies on quinoline and isoquinoline a filter solution of 200 g of NiSO, 6Hz0 and 10 mg of cn13 in 1 1. of water was used in a quartz cell to isolate the appropriate radiation. When the absorption spectrum of the filter was compared with that of the corresponding molecule, it was estimated that over 90% of the radiation from the flash lamp incident upon the sample was absorbed by the sample. Isoquinoline and quinoxaline were obtained from Distillation Products Industries; quinoline was obtained from Matheson Coleman and Bell. The quinoline and isoquinoline were purified by vacuum distillation. The quinoxaline was purified by repeated vacuum sublimation. No impurities with concentrations greater than 1%were observed in any of these materials after purification. Solutions of these solutes were prepared using a mixture of 30% 1-butanol and 70% isopentane by volume as an alcoholic solvent and a mixture of 20% methylcyclohexane and 80% isopentane was used as a hydrocarbon solvent. All solvents were spectrograde supplied by Matheson Coleman and Bell. The methylcyclohexane was distilled from PZOb to remove any residual water that may have been present. The other solvents were used as received. When cooled to the temperature of boiling nitrogen, these two solvent systems formed clear rigid glasses. The solute cone
(8) R. M. Hochstrasser and C. Marzzacco, J . Chem. Phys., 49, 971 (1968) (9) B. R. Henry and M . Kasha, Annu. Rec. Phys. Chem., 19, 161 (1968). (10) M. A. El-Bayed, J . Chem. Phys., 38, 2834 (1963). (11) E.C.Lim and Y. H . Li, Chem. P h y s . Lett., 4, 25 (1969). (12) S. G.Hadley and R. A. Keller, J . P h y s . Chem., 73,4366 (1969). (13) cn is 2,7-dimethyl-3,6-diazacyclohepta-1,6-diene iodide. I
DIRECT DETERMINATION OF SINGLET --+ TRIPLET INTERSYSTEM CROSSING QUANTUM YIELD
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Table I : Intersystem Crossing Quantum Yields
Moleoule
Solvent
einstein om-2
I. mol-1 om-1 b
QISCE
Quinoline Quinoline Isoquinoline Isoquinoline Quinoxalinea Quinoxaline Naphthalenee
Alcoholic Hydrocarbon Alcoholic Hydrocarbon Alcoholic Hydrocarbon Alcoholic
3.56 I 0 . 2 3 2.85 f 0.30 3.61 f 0.29 2.71 k 0.25 1.40 f 0.30 2.33 i 0.30 1.15 f 0 . 2
7.1 f 0 . 7 6 . 7 i:0.7 14.9 i 1 14.0 i 1 8.1 f 0.8 8.7 f 0.7 40.0 f 6
0.50 i 0 . 1 0.43 f 0.08 0.24 f 0.05 0.19 i 0.04 0.18 f 0.04 0.27 =t0.05 0.29 i 0.06
QICd
0.57 i 0.08 0.82 f 0.04 0.73 =k 0.05
The hydrocarbon solvent was 20% methyla The alcoholic solvent was a mixture of 30% 1-butanol and 70% isopentane by volume. Uncertainty estiValues from Hadley (see ref 14) or determined in a similar fashion. cyclohexane and 80% isopentane. @IC @ISC = 1 for thosec ases where it is known that %F = 0. e Values from S. mated a t f20%. d Determined from @F Hadley, Chem. Phys. Lett., 6, 549 (1970).
+
+
centration was adjusted so that most of the light from the flash lamp was absorbed in the first centimeter of the sample. The sample dewar has been previously described.l4 The optical density of the triplet + triplet absorption following the discharge of the flash lamp was measured using a Jarrell-Ash 0.25-m monochromator equipped with a 1P28A photomultiplier. The small optical densities necessitated the use of a Type W preamplifier on a Tektronix 544 oscilloscope to record the time dependence of the change in light transmission of the sample. The analytic beam radiation was supplied by a PEK X-151 high-pressure xenon lamp. The change in the optical density was measured as a function of time. By extrapolating these first-order decay curves to t = 0, the initial optical density of the triplet + triplet absorption was determined. Typical initial optical densities were of the order of 0.05. We have previously shown that permanent photoinduced changes are unimportant in these systems.14 After the optical density of the triplet + triplet absorption was measured, the sample dewar was removed and a quartz cuvette containing the ferrioxalate actinometer solution was inserted in its place. The previously used actinometry procedure was followed,12 except bath~phenanthroline'~ was used in determining the ferrous ion. Up to 15 flashes were necessary to produce sufficient ferrous ion to permit accurate determination of its concentration on a Cary 14 spectrophotometer. The extinction coefficient of the ferrous ionbathophenanthroline complex was determined to be cmax = 2.21 X lo41. mol-' cm-'. I n this fashion values of io were determined. Typical values of io for quinoline and isoquinoline were 5 X 10-9 einstein cm-2 flash-l urhile for quinoxaline typical values were 2 X einstein cm-2 flash-'.
Results The values of the intersystem crossing quantum yield for quinoline, isoquinoline, and quinoxaline in a
hydrocarbon solvent (80% isopent'ane, 20% methylcyclohexane) and in an alcoholic solvent (70% isopentane, 30% 1-butanol by volume) are shown in Table I. They were calculated from ET20
OD(0) is the initial optical density of the triplet + triplet absorption at a wavelength where the extinction coefficient is E T . iois the absolute intensity of the light absorbed by the molecules. The errors shown in Table I for OD(0)/ioare the standard deviations of between three and five separate determinations. The values of ET have been reported.14 The values of the SI --t SO internal conversion quantum yield were calculated in those cases where the fluoroescence quantum yield is known to be zero or insignificant relative t o ~ I S C . Our results indicate that within our error there is no significant change in the intersystem crossing q u a n t u m yield in changing the solvent f r o m an alcoholic mixture to a hydrocarbon f o r each heterocycle. The values of the intersystem crossing quantum yield for isoquinoline and quinoxaline show no enhancement relative to that previously measured for naphthalene. l 2 (See footnote e, Table I.) The value for quinoline is about a factor oi 2 greater than that observed for naphthalene.
Discussion The intersystem crossing quantum yields of aromatic heterocycles have not been extensively studied by previous investigators. Lamola and Hammondle have reported values of @ISC = 0.31 for quinoline in dry benzene and @ISC = 0.16 in moist benzene, both measured a t room temperature. Their values were determined using a photosensitized reaction of piperylene. Xore (14) S,G.Hadley, J . P h y s . Chem., 74,3561 (1970). (15) Bathophenanthroline is 4,7-diphenyl-l,lO-phenanthroline. (16) A. A. Lamola and G. S. Hammond, J . Chem. Phys., 43,2129 (1965). T h e Journal of Physical Chemistry, Vol. 76, N o . 14,1071
2086 recent work has shown that these values may be low due to complicating side reactions.17, l8 El-Sayed has compared the total emission spectra of naphthalene and quinoline in hydroxylic glasses a t 77OK.’O He concluded that there are significant radiationless losses from the first excited singlet state and that the rate of intersystem crossing in quinoline is enhanced relative to that in naphthalene. This is in accord with our findings. However, this agreement may be fortuitous as relative intensities of the phosphorescence emission spectra depend upon both the quantum yields for intersystem crossing and phosphorescence. Reliable phosphorescence quantum yield data are not available. Lim and Yu have examined the total emission spectra of isoquinoline in EPA (a hydroxylic glass) and 3methylpentane (a hydrocarbon glass) under experimental conditions similar t o ours. They found that the fluorescence intensity was greater in EPA than in 3-methylpentane, while the phosphorescent emission intensity was greater in 3-methylpentane than in EPA. They have interpreted these observations in terms of more efficient intersystem crossing in hydrocarbon solvents than in hydroxylic solvents. This is not in agreement with our results. This discrepancy may be due t o a change in the phosphorescence quantum yield with solvent. Until reliable phosphorescence quantum yield data are available, one can only speculate on the effect of solvent on relative phosphorescence intensities. The solvent may change the rate constants of both the radiative and nonradiative decay of the triplet state. The sum of these effects has been observed in the solvent dependency of the triplet state lifetimes of quinoline and isoquinoline.6
Conclusions We have extended our previous measurements on
T h e Journal of Physical Chemistry, Vol, 7 6 , hTo.14, 1971
STEVEN G. HADLEY singlet .-* triplet intersystem crossing quantum yields of aromatic hydrocarbons to aea-substituted aromatics. I n principle this technique can be extended to molecules whose triplet state lifetime is as short as 1 msec when the extinction coefficient of triplet + triplet absorption bands have been determined. The intersystem crossing quantum yields of isoquinoline and quinoxaline have been found to be similar to that measured for naphthalene. The value for quinoline was determined to be about a factor of 2 greater. The intersystem crossing quantum yield was found to be insensitive to changing the solvent from a hydrocarbon to a hydroxylic medium. The data imply significant internal conversion from SI for these cases where fluorescent emission has not been observed, or is observed to be very weak. Unfortunately these quantum yield data donot readily permit direct comparison with the presently developed theories of radiationless processes. These theories are in terms of the rate constants. Once the lifetime of S1 is determined under these experimental conditions, these quantum yields will permit calculation of the rate constants. These measurements are planned for the near future. Our data do indicate that the intersystem crossing quantum yield is not particularly sensitive to the order or energy separation of the ‘(n,r*), l(rT)r*), a(n,r*),and a(r,r*) states. Acknowledgments. The financial support of the Research Corporation, the Petroleum Research Fund (Grant No. 1432-G2), and the National Science Foundation (Grant No. 19750) is gratefully acknowledged. (17) J. B. Birks, “Photophysics of Aromatic iMolecules,” WileyInterscience, London, 1970, p 200. (18) L. M. Stephenson, D. G . Whitten, G . F. Vesley, and G. S. Hammond, J . A m e r . Chem. SOC., 88, 3665 (1966).
