3569
Determination of the Reactivity of First Excited Singlet States of Organic Compounds toward Unimolecular Primary Photochemical Processes from Molecular Fluorescence Characteristics. Application to Intramolecular y-Hydrogen Abstraction Reactions of Alkyl Ketones'",'' J. C. Dalton*" and N. J. Turro Contribution f r o m ihe Departments of Chemistrj?,Columbia Unicrrsitj,, New! York, N e w York 10027, and the University of Rochesier, Rochester, New York 14627. Receiced Oc\obi.r 28, 1970 Abstract: An experimental test is given of the often-cited theoretical formula (eq 3) relating the inherent fluorescence lifetime to the integrated intensity of the absorption band. The photochemical results of Yang and coworkers provide a totally independent check of our results. The good agreement between rates based solely on our spectroscopic results and the photochemical data of Yang demonstrates the utility and validity of molecular fluorescence characteristics as a probe for photochemical mechanisms.
C
onsiderable information on the reactivity of the first excited singlet state (S1) of organic molecules toward uniniolecular primary photoprocesses can be obtained from a study of molecular fluorescence properties. The key t o this application is the fact that a uniniolecular primary photochemical process occurring from SI, with a rate constant krs, will normally compete with the primary photophysical processes of intersystem crossing (k,& fluorescence (kf), and internal conversion t o the ground state ( k , ) for deactivation of excited singlet states. Thus, the introduction of a photochemical process in a molecule will result in a decrease in both the excited-singlet-state lifetime (TJ and the fluorescence quantum yield (+f), relative to a suitable model compound in which the photochemical process is absent. This can be seen readily from eq 1 and 2, which give expressions for T , and c$f, respectively. 7, =
kf
__1
+ k,t + k , +-k,' -
(1)
Clearly, T , will be longer and & larger for the model compound in which the primary photochemical process of interest is absent (i.e., k," ?),
Results and Discussion As a test of the validity of this technique, we have used the fluorescence characteristics of a series of acyclic alkyl ketones to determine the reactivity (k,") of their S1 states toward the intramolecular y-hydrogen abstraction process. Our results are compared to those of Yang and coworkers3 who have recently measured the k," values for several of these ketones by chemicalquenching techniques. The measured relative quantum yields of fluorescence for 2-butanone (l), 2-pentanone (2), 2-hexanone (3), 5 -met h y I -2-hex an on e (4), and 5,5- d i met h y l-2- hexa n on e (5) are given in Table I. It is apparent that the introduction of a photochemical pathway, intramolecular y-hydrogen abstraction (or interaction with the y hydrogens), for deactivation of the first excited singlet state of these alkanones leads t o substantial reductions in the fluorescence quantum yields. As we would e ~ p e c t , the ~ relative fluorescence quantum yields decrease with decreasing y-C-H bond strength a s the y hydrogen goes from primary t o tertiary. Significantly, when the y hydrogens are all replaced with (2) N. J . Turro, "Molecular Photochemistry," W. A. Bctijnmin, New York, N. Y.. 1965, p 48. (3) N . C. Ynng, S. P. Elliott, and B. Kim, J . A m u , Chem. Soc., 91, 7551 (1969).
Dalton, Turro J y-H)\'drogenAbstraction Reactions o/ AlAj.1 Ketones
3570 Table I. Comparison of Photochemical and Spectroscopic Calculations for Type I1 Photoreactions"
reported in the table. The absolute singlet lifetimes (7,) were calculated from the 7,rel values and the singlet lifetime of 2-pentanone, which we have measured by single-photon countingb t o be 1.8 X sec. Since for alkyl ketones kst >> kf, and Yang3 has shown that k,, for this series is insensitive to substitution at the y carbon, we can attribute the decrease in 7, as we go from 1 t o 4 t o some special interaction of the n , r * excited singlet state with the y-C-H bond. Thus, the krS(this work) values given in the table are the increase in the SI decay rate ( l / ~ relative ~) t o 2-butanone (1). Since these k," values d o not distinguish a cheniically productive interaction of the n , r * SI state with the y hydrogen from a physical, noncheniically productive interaction, they should be compared t o the sum (k,"(Yang)) of Yang's rate constants3 for type 11 and nonradiative decay from the first excited singlet state. If all the nonradiative decay results from reversion of an initially formed 1,4 biradical to the starting ketone, then k," represents the reactivity toward the initial y-hydrogen abstraction from SI. The agreement between the krS values determined in this work and those determined by Yang and coworkers3 by chemical-quenching techniques is excellent and clearly demonstrates the applicability and value of using the fluorescence characteristics of compounds t o extract information on the reactivity of excited singlet states toward primary photochemical processes. The data also indicate that eq 3 is valid t o a reasonable degree of precision for calculating relative kf values. Attempts t o use such a formula t o calculate absolute kf values appears to be accurate only for order of magnitude estimates.o
~
krS(this
79,
&
k,s(Yang),
#irC1
rsrel
nsec
(1)
1.00
1.00
2.5
(2)
0.82
0.72
1.8
1 . 5 X 108
1 . 8 X lo8
(3)
0.39
0.32
0.81
8 . 4 X 108
9 . 9 X lo8
(4)
0.17
0.14
0.35
2 . 5 X lo9
2 . 1 X 108
Molecule
p
(5)
work), sec-'
sec-I
1.02
Relative quantum yields of fluorescence for the ketones were obtained in +hexane solution (-0.1 M ) using an Aminco-Bowman spectrofluorometer. Correction was made for small differences in absorbance at the excitation wavelength, 31 3 nm.
methyls (5), the relative fluorescence quantum yield undergoes a large increase t o a value near unity. This implies that intramolecular abstraction of 6 hydrogens does not compete favorably with intersystem crossing in the SI state of 5. Irradiation of 5 has been reported t o result only in intermolecular photoreduction. Although the position and shape of the uv absorption bands of ketones 1-5 are all very similar, emax does increase with increasing methyl substitution. An increase in emax would result i n a larger value of kf (eq 3) and thus a higher quantum yield of fluorescence (eq 2). In order t o normalize for this effect, we have used eq 4 t o obtain the relative singlet lifetimes ( T , ' ~ ~ ) Ts'el
=
6fr el/ em ax
( 5 ) F. S. Wettack, G. D. Renkes, M . C. Rocklcy, N . J. Turro, and J. C. Dalton, J . Amer. Cheni. Soc., 92, 1793 (1970). (6) M . 0. Sullivan and A . Testa, ibid.,92, 5842 (1970); R . F. Borkmail and D. R. I
