1830
M. O'Sullivan and A . C. Testa
(31) J. J. Christensen and R. M. Izatt. "Handbook of Metal Ligand Heats," Marcel Dekker. New York, N. Y . . 1970. (32) In these estimates we have assumed .NHz to be about as good an oxidant as CImga and that the potentials of cobalt(ll1) Icobalt(i1)
couples change about as observed previously33when coordinated NH3 is replaced by Br-. (33) D. P. Riilema. J. F. Endicott. and E. Papaconstantinou, Inorg. Chem.. 10, 1739 (1971).
Fluorescence of Cycloalkanones M. O'Sullivan and A. C. Testa* The Department of Chemistry, St. John's University, Jamaica, New York 11439 (Received February 26, 1973)
The fluorescence of a series of cycloalkanones has been studied at room temperature. Results indicate that the contour of the emission spectrum remains the same and exhibits a wavelength maximum a t -405 nm, and that the a-CH stretching mode appears to be a factor in radiationless deactivation from the excited singlet state. The fluorescence yield of cyclopentanone and cyclohexanone is an order of magnitude greater than the value for cyclobutanone, increases with a substitution, and is not affected by fi substitution. The fluorescence yield of cyclopentanone, 2-methylcyclopentanone, 2,5-dimethylcyclopentanone, and 2,2,4,4-tetramethylcyclopentanoneincrease in the ratio 1.0:1.6:3.4:3.6. Polar and hydrogen bonding solvents as well as methyl substitution affect the vibration structure in the In,a* absorption band of cyclopentanone, which is probably due to reduced interaction between the carbonyl group and the a-CH2 group.
Introduction Although the photochemistry of cycloalkanones has been widely studied,1%2relatively little information is available regarding the fluorescence of these molecules. La Paglia and Roquitte3 reported that the fluorescence of cyclopentanone is unstructured with a peak a t -405 nm; however, there has been no attempt to correlate the fluorescence yield of these molecules with ring size and substitution. Recently, Closs and Doubleday4 concluded that the type I cleavage in cycloalkanones becomes less pronounced with increasing ring size of the ketone. Lee, et a1.,536 have reported that the fluorescence yield for cyclobutanone, CB, cyclopentanone, CP, and cyclohexanone, CH, is approximately 0.002 in the gas phase, while in cyclohexane the fluorescence yield, using 313-nm excitation, for CP (6 f 2 x 10-4) and CH (9 f 2 X is significantly larger than for CB (1.0 f 0.3 X The lifetime of the lowest excited singlet state in CB was estimated to be -10-10 sec. It has been observed that the unimolecular photodecomposition of cyclobutanone is not quenched by high concentrations of 1,3-~entadiene.~ Lee and Metcalfe7 have shown that in n-propylcyclobutanone the internal conversion, SI SO, involves biradical intermediates as the main path of deactivation and that the triplet yield is cycloheptanone > cyclohexanone. In cyclobutanone it was suggested that an a2 vibration was the dominant intensification path.
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The Journai of Physical Chemistry. Voi. 77. No. 15. 7973
In a previous report from this laboratory we studied the fluorescence of a series of alkyl ketones and presented evidence that the a-CH stretching mode is a factor in radiationless deactivation from the lowest excited singlet state.s The fluorescence wavelength maximum remained constant a t 405 f 3 nm and the fluorescence yield of ditert-butyl ketone in n-hexane was determined to be larger than the value for acetone by a factor of 4.4. In the present investigation we have measured the fluorescence yield of 17 cycloalkanones with the aim of demonstrating the effect of ring size and substitution on the fluorescence process and also to see if our earlier conclusions regarding alkyl ketones also apply to cycloalkanones. Experimental Section Materials. The ketones used in this study were vacuum distilled prior to use and their purity checked with a 6 ft, 0.25-in. o.d., Carbowax 20 M column with a Perkin-Elmer Model 145B gas chromatograph. Spectrograde solvents were used as received since they exhibited no interfering spectral properties. Apparatus and Procedures. Fluorescence measurements were made with equipment described previously.9 All ketones were excited with 285-nm light, which was isolated with an interference filter from an Osram HBO 100W/2 high-pressure mercury lamp. The fluorescence quantum yield was measured relative to +F = 0.09 for tryptophan.10 Absorption spectra were obtained with either a Bausch and Lomb Model 505 or a Beckman Model DU spectrophotometer. The natural radiative singlet lifetime, ~ ~
0
,
1831
Fluorescence of Cycloalkanones
was estimated from the integrated absorption of the lowest transition of the ketone.
Results The fluorescence spectrum of each of the compounds studied is unstructured with a wavelength maximum at -405 nm, similar to that observed for alkyl ketones. Of the five unsubstituted cycloalkanones investigated, cyclobutanone, which possesses the most ring strain, has a much lower fluorescence yield than the c 5 - C ~ ring ketones. In Table I are summarized the extinction ccefficients, the absorption wavelength maxima, the natural singlet radiative lifetimes, and the fluorescence quantum yields for various cycloalkanones, measured in n-hexane. The results of Lee, et a1.,6 for CB, CP, and CH in cyclohexane are also included in Table I for comparison. The singlet radiative lifetime is approximately constant for all the ketones and is, within experimental uncertainty, equal to the value previously determined for alkyl ketones.9 The cyclopentanones have the highest absorption wavelength maxima (300 nm in n-hexane), while cyclobutanone exhibits the lowest wavelength maximum (281 nm in n-hexane). Measurements were also performed in acetonitrile and methanol and the results are, in general, the same as in n-hexane. There is no significant solvent effect on the fluorescence yield, although the emax were -30% larger in acetonitrile and methanol, which resulted in approximately 30% decrease in 7 ~ 0 . In contrast to the unstructured In,r* absorption band of alkyl ketones, there is a distinct vibrational structure in the spectra of CB and CP. That these vibrational bands are solvent dependent is seen in Figures 1 and 2, where the vibronic bands appearing in n-hexane disappear when changing to acetonitrile and methanol. In the latter solvent there is complete loss of the structure. In the case of cyclopentanone the first four bands appear at 34,480, 33,330, 32,150, and 30,950 cm-l, which corresponds to a vibrational band ranging from 1150-1200 cm- I. This band is most likely the intense skeletal mode appearing at 1160 cm-1 in the ir spectrum of cyclopentanone.ll Goodman and Chandlers have also observed the same progression. The loss of vibrational structure is also apparent as the ring size increases. In Figure 3 the absorption spectra of the ketones cyclobutanone to cyclooctanone clearly show this effect. Substitution at the position 01 to the carbonyl group in cyclopentanone causes a similar change in the spectrum. No shift of the absorption maximum occurs upon methyl substitution. It is seen that 2,2,4,4-tetramethylcyclopentanone, which has one methylene group replaced, has lost most of its vibrational structure. The fluorescence yields of all the ketones studied (with the exception of CB) are small (-10-3); however, there is a measurable variation of the fluorescence yield with methyl substitution at the a position of the cyclic ketones. Substitution at the ,8 position, however, has no effect or1 the fluorescence yield of the molecule. The most significant variation in the fluorescence yield is seen in the sequence CP, 2-methylcyclopentanone, 2,5-dimethylcyclopentanone, and 2,2,4,4-tetramethylcyclopentanone, which increases in the ratio 1.6:2.6:5.4:5.8. These results parallel the trend observed with aliphatic ketones.9 The fluorescence lifetime also increases with 01 substitution: 1.9 nsec for CP, 2.7 for 2-methylcyclopentanone and 8.7 for 2,2,5,5-tetramethylcyclopentanone.2The increase of fluorescence with 01 substitution is also seen in the cyclohexa-
0.8
0.6
0.4 t.
-cul Z
w n
a
0 i=
a
0.2
0
0.0
210
230
250
290
270
310
330
WAVELENGTH, nm.
Figure 1. Absorption spectra of cyclobutanone at 25" ( I - c m cell): (1) 0.023 M in n-hexane, (2) 0.029 M in acetonitrile, and (3) 0.035 M in methanol.
0.8
-
0.6 *
WAVELENGTH,
nm.
Figure 2. Absorption spectra of cyclopentanone at 25' ( I - c m cell): (1) 0.027 M in n-hexane, (2) 0.030 M in acetonitrile, and (3) 0.033 M in methanol. The Journal of Physical Chemistry, Voi. 77. No. 15. 1973
1832
M. O'Sullivan and A. C. Testa
TABLE I : Absorption and Fluorescence Data for Cycloalkanones in n-Hexane Xmax
emax
TFO
X IO6, seca
6F
x
103
*
Unsubstituted Ketones (1) Cyclobutanone (2) Cyclopentanone (CP) (1.9)e (3) Cyclohexanone (CH) (2.6) (4) Cycloheptanone ( 5 ) Cyclooctanone
19 (15)c 18 (18) 15 (14) 15 (16) 17
28 1 300 290 292 290
1.9 2.3 2.7 2.6 2.4
300 300 300 301 301 301 290 29 1 290 299 290 288
2.7 2.2 2.3 2.5 2.2 2.2 2.4 2.2 2.9 2.4 2.7 2.3
-0.1 (O.l)d 1.6 (0.6) 1.7 (0.9) 1.9 1.2
Substituted Ketones (6) (7) (8) (9)
2-Methylcyclopentanone (2.7)
16 19 (20) 18 17 20 19 16 18 14 16
2,5-Dimethylcyclopentanone 2,2,4,4-Tetramethylcyclopentanone 3-Methylcyclopentanone
(IO) cis-3,4-Dimethylcyclopentanone (1 1) (12) (13) (14) (15) (16) (1 7)
trans-3,4-Dimethylcyclopentanone 2-Methylcyclohexanone (2.9)
2,6-Dimethylcyclohexanone 3-,Methylcyclohexanone 4-Methylcyclohexanone
3,5-Dimethylcyclohexanone 3,3,5,5-Tetramethylcyclohexanone (3.0)
15 17
2.6 5.4 5.8 1.4 1.4 1.7 2.2 2.7 1.6 1.6 1.9 2.0
a T F ' , radiative singlet lifetime, was calculated from integrated absorption spectrum using the following expression TF' = 3.5 X 10s/~max*emaxRelative fluorescence yield normalized to the value of 0.09 for tryptophan, 285-nm excitation. Extinction coefficients from ref 8. a Fluorescence yields from ref 6; 313-nm excitation. e Fluorescence lifetime in nanoseconds; ref 2.
At,/,.
t
0.6
0.8
0.4
0.6
v)
z W
n
*
-1
a 0 a c
0
tv)
z W 0.4
0.2 -1
a 0 c a 0
0.0
0.2 3
240
260
280
300
320
340
WAVELENGTH, nm.
Figure 3. Absorption spectra of cycloalkanones in n-hexane at 25" ( I - c m cell): (1) 0.023 M cyclobutanone, (2) 0.027 M cyclopentanone, (3) 0.034 M cyclohexanone, (4) 0.025 M cycloheptanone, and (5) 0.012 M cyclooctanone. nones. T h e fluorescence y i e l d o f CH, 2 - m e t h y l c y c l o h e x a none a n d 2,6-dimethylcyclohexanoneincreases in t h e r a t i o 1.7:2.2:2.7. As is t h e case w i t h /3 s u b s t i t u t i o n in CP, s u b s t i t u t i o n a t t h e 3 a n d 4 p o s i t i o n o f CH h a s n o effect o n t h e fluorescence y i e l d .
Discussion T h e s i m i l a r i t y o f t h e fluorescence s p e c t r u m a n d t h e increasing fluorescence y i e l d w i t h a s u b s t i t u t i o n observed w i t h cycloalkanones c o n f i r m s t h e effect p r e v i o u s l y d e m o n s t r a t e d w i t h a l k y l ketone^,^ i.e., t h a t t h e fluorescence i s l o c a l i z e d in t h e c a r b o n y l g r o u p and t h a t t h e a-CH b o n d i s The Journal of Physical Chemisfry, Voi. 77. No. 15. 1973
0 .o
I
240
260
280
300
320
340
WAVELENGTH, nm.
Figure 4. Absorption spectra of substituted cyclopentanones in n-hexane at 25" (1-cm cell): (1) 0.027 M cyclopentanone. (2) 0.031 M 2-methylcyclopentanone, (3) 0.045 M 3-methylcyclopentanone, (4) 0.028 M 2,5-dimethylcyclopentanone, (5) 0.021 M trans-3,4-dimethylcyclopentanone, and (6) 0.021 M 2,2,4,4tetramethylcyclopentanone. (Spectra of cis-3,4-dimethylcyclopentanone was identical with that of the trans isomer.)
a f a c t o r in t h e radiationless d e a c t i v a t i o n o f t h e lowest e x c i t e d s i n g l e t state. Since t h e data in T a b l e I i n d i c a t e that t h e n a t u r a l r a d i a t i v e singlet l i f e t i m e is a p p r o x i m a t e l y
Fluorescence of Cycloalkanones constant for all the ketones investigated, it would appear, provided radiationless deactivation from the singlet is negligible, that intersystem crossing is faster for the weaker emitters, i.e., kist = ( 4 F 7 F o ) - l . The photochemistry of cyclopentanones and cyclohexanones has been shown to proceed primarily through the triplet state and to involve a cleavage. In the unimolecular photodecomposition of cyclobutanone, however, the importance of triplets is not clear since no 1,3-pentadiene quenching is observed.2 Although the effects reported here relate to the singlet state it is interesting to note that in those cases where intersystem crossing may be faster, namely, 2-substituted CP and CH, the photochemical quantum yields for the photoisomerization of cycloalkanones to enals are higher than in the unsubstituted ketone.2 The photochemical quantum yield for 2,2,4,4-tetramethylcyclopentanoneis larger than for CP and the corresponding value for 2-methylcyclohexanone and 2,6-dimethylcyclohexanone is larger than for cyclohexanone. The effect of solvents and substitution on the vibrational structure of the h , n * band of CP and CH is unexpected. The same absorption band in alkyl ketones is diffuse and unstructured, A possible explanation for this behavior is vibronic coupling between the ring and carbonyl motions. In methanol the disappearance of vibrational structure may be due to hydrogen bonding of the lone pair on oxygen, while substitution a t the a position affects radiationless processes and facilitates photocleavage, possibly from the singlet or triplet state. The quenching of the photoisomerization process in substituted CP and CH with 1,3-pentadiene is known to be less effective than in the unsubstituted compound.2 In fact, the photochemistry of 2,2,5,5-tetramethylcyclopentanone is not quenched by 1,3-pentadiene. In a recent report Furth12 has suggested that the intersystem crossing efficiency, SI TI,and also TISO decrease as the internal strain of cyclic ketones increases. Available evidence indicates triplet yields of unity for CP
-
1833
and CH; however, it appears that the triplet yield is significantly less than unity in CB. Lee, et al.,5 have indicated that the triplet yield is unity for CB, CP, and CH, and also that the intersystem crossing rate constant has a common value of 1-4 X 108 sec-1. On the other hand, Turro and Dalton2 have suggested that the low yield (0.01) for photoreduction of CB's by tri-n-butylstannane may indicate a low intersystem crossing efficiency. The fluorescence results presented here for CB are inconsistent with the former conclusion since its low fluorescence yield indicates an inverse singlet lifetime of -1010 sec-I. Since the intersystem crossing rate constant is believed to be -108 sec-1, there must be a much more efficient singlet deactivation mode, probably photocleavage, in this molecule. A phosphorescence study of cycloalkanones should assist in elucidating the importance of triplets in these molecules.
References and Notes (1) J. G. Caivert and J. N. Pitts, Jr., "Pnotochemistry," Wiley, New York. N. Y., 1966, Chapter 5. (2) J. C. Dalton and N. J. Turro, Annu. Rev. Phys. Chem., 21, 499 (1970). (3) S.R. La Pagliaand 6.C. Roquitte, J. Phys. Chem.. 66, 1739 (1962). (4) G. L. Cioss and C. E. Doubieday, J. Amer. Chem. Soc., 94, 9249 (1972). (5) R. G. Shortridge, Jr., C. F. Rusbult. and E. K. C. Lee, J. Amer. Chem. Soc., 03,1863 (1971). (6) J. C. Hemminger, C. F. Rusbult, and E. K. C. Lee, J. Amer. Chem. Soc., 93, 1867 (1971). (7) J. Metcalfe and E. K. C. Lee, J. Amer. Chem. SOC., 94, 1 (1972). (8) W. D. Chandler and L. Goodman, J. Mol. Spectrosc., 35, 232 (1970). (9) M. O'Sullivan and A. C. Testa, J. Amer. Chem. SOC., 92, 5842 (1970). (10) V. G. Shore and A. B. Pardee, Arch. Biochem. Biophys., 60, 100 (1956). A recent value of 0.13 f 0.01 has been determined by R . F. Chen, Anal. Lett., 1, 35 (1967); however, modification in the fluorescence yields affect only absolute values and do not alter the trends reported in this stydy. (11) J. C. P. Schwarz, Ed., Physical Methods in Organic Chemistry," Holden-Day, San Francisco, Calif., 1964, pp 82 and 83. (12) B. Furth, Fourth IUPAC Symposium on Photochemistry, BadenBaden, West Germany, July 16-22,1972, Paper No. 19.
The Journalof Physical Chemistry, Voi. 77. No. 15. 1973
