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Viscosity-dependent dual phosphorescence of phenyl alkyl ketones
Volume
13, number
6
VISCOSITY-DEPENDENT
CHEXIICAL
PHYSICS
LETTERS
DUAL PHOSPHORESCENCE P.J. WAGNER, Chetttistry
1.5 April 1972
OF PHENYL ALKYL KETONES
IM. MAY
Department, Mich&att State University, East Lattsittg. MichCgatt, USA
and A. HAUG Mibhigatt Stare Universify/AECPIant Research Laboratory, East Lansittg, Michigatt, USA Received
20 December
1971
Several phenyl alkyl ketones display two overlapping short-lived pilosphorescenccs at 77”K, with O-O bands at 384 nm (74.5 kcal) and 397 nm (72.0 kcal). The hgher energy emission (T = S- 10 msec) is favored in rigid methylcyclohesane glasses, while the proportion of lowcr-encrgy emission (T = 4-5 msec) increases with increasing amounts of 2-methylbutanc in the glass. In both isopentane at 77°K and benzene solution at room temperature, only the lowerenegy spectrum is evident. The higher-enegy emission is ascribed to n,z* tripbts rigidly held in ground state conformations, while the lower-energy emission is assigned to conformationally-relaxed n,x* triplets.
Interest in the spectroscopy of phenyl alkyl ketones is intensifying. Most phenyl alkyl ketones display hvo-component phosphorescence in glassy matrices at 77°K [l-6] , thus complicating attempts to correlate spectroscopy and photochemistry [3,7-g]. Although it is generally agreed that the major, shortlived component arises from an n,x* triplet, the source or sources of the longer-lived minor component remain unclear. Most discussions of the problem have been in terms of emission from hvo different electronic states. We report here evidence for a different kind of dual emission, namely from different COW formations of the same, n,x’ triplet. We have measured the phosphorescence spectra and lifetimes at 77’K of several ketones of the general structure C,H,COCH,R with the equipment previously described [6, IO] . Phosphorescence is of comparable intensity for R equal to methyl, ethyl, /3phenylethyl, and cr-phenylethyl, and consists of two corzgonents in either methylcyclohexane (MCH) or isopentane (IP), with the minor component (7 = 20100 msec) amounting to only = 5% of the total emission (measured at the O-O band). Henceforth, we
shall refer only to the major component. The spectra are very ketone-like, consisting of three prominent bands with the % 1650 cm-l spacing characteristic of the carbonyl stretch. However, in IP the spectra are at lower energy than in MCH, the three bands appearing typically at 397,425, and 457 nm instead of at 384,411, and 440 nm. ‘The lifetime of the major component varies from 4-5 msec in IP, from S-10 msec in MCH. In both solvent systems, quite apart from the common problem of minor, long-lived emission, the major emitter is clearly a state of predominantly n,x” character. It would appear that there are two possible conformations of the n,n* triplet, the lowerenergy one (i.e., that in IP) being slightly shorter-lived. In degassed benzene solution at 25’C, both propiophenone (R = methyl) and y,$imethylvaIerophenone (R = neopenty!) display phosphorescence of comparable intensity (+ = 3 X 10m4) and form to that already reported for benzophenone and acetophenone [ 11,12]. The O-O bands occur at 397 nm, the same as observed in isopentane at 77°K. The above observations suggested to us that solvent 545
Volume
13, number
6
CHEMICAL
viscosity may determine the conformation of the emitting n,n* triplet. Consequently, we measured the spectra of butyrophenone in several IP-MCH mixtures, which should vary over some six orders of magnitude in viscosity [ 131 , the results of which are shown in fig. 1. in a SO:50 mixture, emission occurs primarily from the higher energy conformation, with just a trace of the lower energy spectrum. In a 70:30 411
440 42s
k
k
J /
50:50
IP:h+c!i
70:30 IP:h4CJi
425
ao:zo IP: MCH
Fig. I_ P,hosphorescynce of butyrophenone at 77°K in various mixtures of impentanc and methylcyclohexane.
546
PHYSICS LETTERS
15 April 1972
1P:MCH mixture, emission comes primarily from the lower energy conformation; and in 90% IP, only a trace of the higher-energy O-O band is detectable. We fmd that the phosphorescence of nonanophenone (R = n-heptyl) in IP resembles butyrophenone in SO:20 IP:MCH, consisting of a mixture of both spectra, with only slight differences in the ratio of the two components over a ketone concentration range of 10-2 to 10B5 M. Lim and coworkers [ 141 have found similar mixed emission for propiophenone and butyrophenone in 3-methylpentane (3MP) glass. However, neither acetophenone nor benzophenone display any difference in the energies of their n,rr* phosphorescence in MCH compared to IP. Consequently, the two component II,TT* emission observed when R is larger than hydrogen most likely does not involve solvation differences intrinsic to the carbonyl system, Our results are very similar to the solvent-viscositydependent dual phosphorescence recently reported for pdimcthoxybenzene [ 151, and we would interpret them similarly. These solvent effects probably arise from a slight geometric difference between the rotationally relaxed levels of ground and n,x* state. The lower energy spectrum, which occurs both in solution and in the relatively fluid isopentane at 77’K, presumably arises from the conformationally relaxed n,rr’ state. The higher energy emission probably arises from an n,rr* state held rigidly (by solvent) in the most favorable ground state conformation. Since the difference-bebveen the O-O bands of the two spectra is 870 cm-l , part of which must reflect the energy difference between the two ground state conformations, the energy difference between the two excited conformations can only be estimated and is probably about half (I .3 kcal/mole). An appealing stereochemical interpretation of these results, based on spectroscopically determined conformational preferences of ground state ketones, goes as follows. In aidehydes and methyl ketones, the preferred ground state conformation is I (looking down the axis of the C-C bond between carbonyl and (Ycarbon), with R eclipsing the carbonyl group [ 161, In phenyl ketones, however, II is more likely preferred. The carbonyl is not exactly coplanar with the benzene ring [ 171 , such that there would be serious nonbonded interactions between an o&o-hydrogen and an CC hydrogen in conformation I. In the excited state, however, the benzoyl chromophore probably attains inter-
Volume 13, number 6
CHEXCAL
a
PHYSICS LETTERS
15 April 1972
OR
H
H H
(CL,) I
II
nal coplanarity [ 181 (thus accounting for the = 700 cm-l Stokes shift observed for S f, T transitions of phenyl ketones [ 191) so that III* (equivalent to I) would be the favored conformation. That II* and III* should differ by approximately 1.3 kcal in the extent of nonbonded interactions between ortho- and a-hydrogens is a quite reasonable conclusion. Our present results suggest that, whenevtii‘ R is larger than hydrogen, the rotation from first-forTed II* to III* is impeded if the solvent is rigid enough. We cannot, of course, distinguish between a pure viscosity effect or an effect of solvent size. That the latter may be involved is suggested by our observing only the higher-energy spectrum in MCH glass while Lim observes an equal mixture of both spectra in the equally rigid 31MPglass [ 141 . We emphasize that these results are not related to the problem of determining the source of the small amount of long-lived e 20 msec) phosphorescence wftich occurs in most phenyl alkyl ketones, since all the ketones which we discuss here display a few percent of such emission regardless of the conformation or energy of the emitting n,n* triplet. However, these resuits do bear on another problem. There is much current interest in correlating the room temperature, solution-phase chemical behavior of excited ketones with their 77”K, rigid-matrix chosphorescence. These results exemplify one real danger in such attempts, especially when small enerw differences between excited states is important [8]. On the plus side, our finding of identical energy spectra in IP at 77OK and in benzene at 300°K offers the hope that wider use of isopentane as a solvent for liquid nitrogen spectroscopy might allow more exact correlations between 77°K and 300°K photochemistry.
This work was supported by U.S. Atomic Energy Commission Contract No. AT( 1 l-1)-1 338 and by an NSF grant. We thank Professor Lim for a preprint of his partially related work. References [I]
N.C. Yang and S.L. hlurov, J. Chem. Phys. 45 (1966) 4358. [2] A.A. Lamola, J. Chem. Phys. 47 (1967) 4810. (31 R.D. Rauh and P.A. Leermakers, J. Am. Chem. Sot. 90 (1968) 2246. (41 Y. Kanda, J. Stanislaus and E.C. Lim, J. Am. Chem. Sot. 91 (1969) 5085. [5] N.C. Yang and R. Dusenbery, Mol. Photochem. 1 (1969) 159. [6] P.J. Wagner, k1.J. May, A. Haug and D.R. Graber, J. Am. Chem. Sot. 92 (1970) 5269. [7] N.C. Yang, D S. McClure, S.L. hlurov, J.J. Houscr and R. Dusenbery, I. Am. Chem. Sot. 89 (1967) 5466. [SJ N.C. Yang and R. Dusenbery, J. Am. Chem. Sot. 90 (1968) 5899. [91 J.N. Pitts Jr., D.R. Bur!ey, J.C. Miani and A.D. Broadbent, J. Am. Chem. Sac. 90 (1968) 5902. E.B. Priestly .utd A. Haug, J. Chem. Phys. 49 (1968) 622. W.D.K. Clark, A.D. Litt and C. Steel, J. Am. Chem. Sot. 91 (1969) 5413. J. Saltiel et al., J. Am. Chem. Sot. 92 (1970) 410. J.R. Lombardi, J.W. Raymonda and A.C. Albrecht. J. Chem. Phys. 40 (1964) 1148. ME. Long, Y.H. Li and E.C. Lim, Chem. Phys. Letters, submitted for publication. R. Castonguny and A.C. Aibrecht, Chcm. Phys. Letters 7 (1970) 89. [ 161 G.J. Karabatsos and D.J. Fenoglio, in: Topics in stereochemistry, Vol. 5. eds. E.L. Eliel and N.I.. Altingcr (Wiley-Intersciencc, New York, 1970) p. 167. [ 17) H. Suzuki, Bull. Chem. Sot. Japan 33 (1960) 613. [IS] R. Hoffman and J.R. Swenson, J. Phys. Chem. 64
(1970) 415. 1191 C.R. Kexns and &‘.A. Case, J. Am. Chem. Sot. 88 (1966)
5087.
547
13, number
6
VISCOSITY-DEPENDENT
CHEXIICAL
PHYSICS
LETTERS
DUAL PHOSPHORESCENCE P.J. WAGNER, Chetttistry
1.5 April 1972
OF PHENYL ALKYL KETONES
IM. MAY
Department, Mich&att State University, East Lattsittg. MichCgatt, USA
and A. HAUG Mibhigatt Stare Universify/AECPIant Research Laboratory, East Lansittg, Michigatt, USA Received
20 December
1971
Several phenyl alkyl ketones display two overlapping short-lived pilosphorescenccs at 77”K, with O-O bands at 384 nm (74.5 kcal) and 397 nm (72.0 kcal). The hgher energy emission (T = S- 10 msec) is favored in rigid methylcyclohesane glasses, while the proportion of lowcr-encrgy emission (T = 4-5 msec) increases with increasing amounts of 2-methylbutanc in the glass. In both isopentane at 77°K and benzene solution at room temperature, only the lowerenegy spectrum is evident. The higher-enegy emission is ascribed to n,z* tripbts rigidly held in ground state conformations, while the lower-energy emission is assigned to conformationally-relaxed n,x* triplets.
Interest in the spectroscopy of phenyl alkyl ketones is intensifying. Most phenyl alkyl ketones display hvo-component phosphorescence in glassy matrices at 77°K [l-6] , thus complicating attempts to correlate spectroscopy and photochemistry [3,7-g]. Although it is generally agreed that the major, shortlived component arises from an n,x* triplet, the source or sources of the longer-lived minor component remain unclear. Most discussions of the problem have been in terms of emission from hvo different electronic states. We report here evidence for a different kind of dual emission, namely from different COW formations of the same, n,x’ triplet. We have measured the phosphorescence spectra and lifetimes at 77’K of several ketones of the general structure C,H,COCH,R with the equipment previously described [6, IO] . Phosphorescence is of comparable intensity for R equal to methyl, ethyl, /3phenylethyl, and cr-phenylethyl, and consists of two corzgonents in either methylcyclohexane (MCH) or isopentane (IP), with the minor component (7 = 20100 msec) amounting to only = 5% of the total emission (measured at the O-O band). Henceforth, we
shall refer only to the major component. The spectra are very ketone-like, consisting of three prominent bands with the % 1650 cm-l spacing characteristic of the carbonyl stretch. However, in IP the spectra are at lower energy than in MCH, the three bands appearing typically at 397,425, and 457 nm instead of at 384,411, and 440 nm. ‘The lifetime of the major component varies from 4-5 msec in IP, from S-10 msec in MCH. In both solvent systems, quite apart from the common problem of minor, long-lived emission, the major emitter is clearly a state of predominantly n,x” character. It would appear that there are two possible conformations of the n,n* triplet, the lowerenergy one (i.e., that in IP) being slightly shorter-lived. In degassed benzene solution at 25’C, both propiophenone (R = methyl) and y,$imethylvaIerophenone (R = neopenty!) display phosphorescence of comparable intensity (+ = 3 X 10m4) and form to that already reported for benzophenone and acetophenone [ 11,12]. The O-O bands occur at 397 nm, the same as observed in isopentane at 77°K. The above observations suggested to us that solvent 545
Volume
13, number
6
CHEMICAL
viscosity may determine the conformation of the emitting n,n* triplet. Consequently, we measured the spectra of butyrophenone in several IP-MCH mixtures, which should vary over some six orders of magnitude in viscosity [ 131 , the results of which are shown in fig. 1. in a SO:50 mixture, emission occurs primarily from the higher energy conformation, with just a trace of the lower energy spectrum. In a 70:30 411
440 42s
k
k
J /
50:50
IP:h+c!i
70:30 IP:h4CJi
425
ao:zo IP: MCH
Fig. I_ P,hosphorescynce of butyrophenone at 77°K in various mixtures of impentanc and methylcyclohexane.
546
PHYSICS LETTERS
15 April 1972
1P:MCH mixture, emission comes primarily from the lower energy conformation; and in 90% IP, only a trace of the higher-energy O-O band is detectable. We fmd that the phosphorescence of nonanophenone (R = n-heptyl) in IP resembles butyrophenone in SO:20 IP:MCH, consisting of a mixture of both spectra, with only slight differences in the ratio of the two components over a ketone concentration range of 10-2 to 10B5 M. Lim and coworkers [ 141 have found similar mixed emission for propiophenone and butyrophenone in 3-methylpentane (3MP) glass. However, neither acetophenone nor benzophenone display any difference in the energies of their n,rr* phosphorescence in MCH compared to IP. Consequently, the two component II,TT* emission observed when R is larger than hydrogen most likely does not involve solvation differences intrinsic to the carbonyl system, Our results are very similar to the solvent-viscositydependent dual phosphorescence recently reported for pdimcthoxybenzene [ 151, and we would interpret them similarly. These solvent effects probably arise from a slight geometric difference between the rotationally relaxed levels of ground and n,x* state. The lower energy spectrum, which occurs both in solution and in the relatively fluid isopentane at 77’K, presumably arises from the conformationally relaxed n,rr’ state. The higher energy emission probably arises from an n,rr* state held rigidly (by solvent) in the most favorable ground state conformation. Since the difference-bebveen the O-O bands of the two spectra is 870 cm-l , part of which must reflect the energy difference between the two ground state conformations, the energy difference between the two excited conformations can only be estimated and is probably about half (I .3 kcal/mole). An appealing stereochemical interpretation of these results, based on spectroscopically determined conformational preferences of ground state ketones, goes as follows. In aidehydes and methyl ketones, the preferred ground state conformation is I (looking down the axis of the C-C bond between carbonyl and (Ycarbon), with R eclipsing the carbonyl group [ 161, In phenyl ketones, however, II is more likely preferred. The carbonyl is not exactly coplanar with the benzene ring [ 171 , such that there would be serious nonbonded interactions between an o&o-hydrogen and an CC hydrogen in conformation I. In the excited state, however, the benzoyl chromophore probably attains inter-
Volume 13, number 6
CHEXCAL
a
PHYSICS LETTERS
15 April 1972
OR
H
H H
(CL,) I
II
nal coplanarity [ 181 (thus accounting for the = 700 cm-l Stokes shift observed for S f, T transitions of phenyl ketones [ 191) so that III* (equivalent to I) would be the favored conformation. That II* and III* should differ by approximately 1.3 kcal in the extent of nonbonded interactions between ortho- and a-hydrogens is a quite reasonable conclusion. Our present results suggest that, whenevtii‘ R is larger than hydrogen, the rotation from first-forTed II* to III* is impeded if the solvent is rigid enough. We cannot, of course, distinguish between a pure viscosity effect or an effect of solvent size. That the latter may be involved is suggested by our observing only the higher-energy spectrum in MCH glass while Lim observes an equal mixture of both spectra in the equally rigid 31MPglass [ 141 . We emphasize that these results are not related to the problem of determining the source of the small amount of long-lived e 20 msec) phosphorescence wftich occurs in most phenyl alkyl ketones, since all the ketones which we discuss here display a few percent of such emission regardless of the conformation or energy of the emitting n,n* triplet. However, these resuits do bear on another problem. There is much current interest in correlating the room temperature, solution-phase chemical behavior of excited ketones with their 77”K, rigid-matrix chosphorescence. These results exemplify one real danger in such attempts, especially when small enerw differences between excited states is important [8]. On the plus side, our finding of identical energy spectra in IP at 77OK and in benzene at 300°K offers the hope that wider use of isopentane as a solvent for liquid nitrogen spectroscopy might allow more exact correlations between 77°K and 300°K photochemistry.
This work was supported by U.S. Atomic Energy Commission Contract No. AT( 1 l-1)-1 338 and by an NSF grant. We thank Professor Lim for a preprint of his partially related work. References [I]
N.C. Yang and S.L. hlurov, J. Chem. Phys. 45 (1966) 4358. [2] A.A. Lamola, J. Chem. Phys. 47 (1967) 4810. (31 R.D. Rauh and P.A. Leermakers, J. Am. Chem. Sot. 90 (1968) 2246. (41 Y. Kanda, J. Stanislaus and E.C. Lim, J. Am. Chem. Sot. 91 (1969) 5085. [5] N.C. Yang and R. Dusenbery, Mol. Photochem. 1 (1969) 159. [6] P.J. Wagner, k1.J. May, A. Haug and D.R. Graber, J. Am. Chem. Sot. 92 (1970) 5269. [7] N.C. Yang, D S. McClure, S.L. hlurov, J.J. Houscr and R. Dusenbery, I. Am. Chem. Sot. 89 (1967) 5466. [SJ N.C. Yang and R. Dusenbery, J. Am. Chem. Sot. 90 (1968) 5899. [91 J.N. Pitts Jr., D.R. Bur!ey, J.C. Miani and A.D. Broadbent, J. Am. Chem. Sac. 90 (1968) 5902. E.B. Priestly .utd A. Haug, J. Chem. Phys. 49 (1968) 622. W.D.K. Clark, A.D. Litt and C. Steel, J. Am. Chem. Sot. 91 (1969) 5413. J. Saltiel et al., J. Am. Chem. Sot. 92 (1970) 410. J.R. Lombardi, J.W. Raymonda and A.C. Albrecht. J. Chem. Phys. 40 (1964) 1148. ME. Long, Y.H. Li and E.C. Lim, Chem. Phys. Letters, submitted for publication. R. Castonguny and A.C. Aibrecht, Chcm. Phys. Letters 7 (1970) 89. [ 161 G.J. Karabatsos and D.J. Fenoglio, in: Topics in stereochemistry, Vol. 5. eds. E.L. Eliel and N.I.. Altingcr (Wiley-Intersciencc, New York, 1970) p. 167. [ 17) H. Suzuki, Bull. Chem. Sot. Japan 33 (1960) 613. [IS] R. Hoffman and J.R. Swenson, J. Phys. Chem. 64
(1970) 415. 1191 C.R. Kexns and &‘.A. Case, J. Am. Chem. Sot. 88 (1966)
5087.
547