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Perturbation of singlet—triplet transitions in aromatic carbonyl compounds
SpectrochilnicsActa, Vol. 24A, pp. 589 to 503. Persamon Press1968. Printedin NorthernIreland
Perturbation of singlet-triplet transitions in aromatic carbons1 compounds D. A. Department
of Chemistry,
WARWICK
Sir John Cass College, Jewry St., London, E.C. 3
and C. H. J. Department
WELLS
of Chemistry, Kingston College of Technology Kingston upon Thames (Received 3 July 1967)
Abstract-Singlet-triplet transitions in aromatic carbonyl compounds with lowest r, n* triplet states are enhanced by the presence of oxygen while singlet-triplet transitions in aromatic carbonyls with lowest n, X* triplet states are unaffected. The oxygen perturbation technique thus provides a simple method for characterising the lowest triplet state of aromatic carbonyl compounds. YHOTOCHEMICAL reactions often proceed via the lowest triplet states of molecules and in many cases the photoreactivity of the molecule can be correlated with the nature of the lowest triplet state [l]. In view of the role of the triplet state in photochemistry it is important that the nature of the lowest triplet state be characterised. Information on triplet states can either be obtained from triplet + singlet emission spectra [2, 31 or from singlet + triplet absorption spectra [4]. The former method has been utilised to yield data on both the nature of the triplet states from which emission occurs [2, 31 and on the energy levels of the emitting states [5] while the latter method has been used in the determination of energy levels of triplet states [4]. Since singlet-triplet absorption spectra can often be readily observed in the presence of a paramagnetic molecule under high pressure [4] it was proposed to investigate this method as a means of characterising the lowest triplet states of molecules. Aromatic carbonyl compounds were chosen for this study since a great deal of information has been accumulated on the properties of triplet states in such compounds. EXPERIMENTAL
The high pressure absorpCon cell consisted of a brass tube (9.0 cm long) with silica windows (1-G cm o.d. and 6 mm thick) mounted on each end between polytetrafluorethylene gaskets by means of a hollow brass screw cap. The overall path length of the cell was 8.5 cm. A high pressure valve and a filling port were mounted [l]
J. N. PITTS, F. WILKINSON and G. S. ~TAMMOND, Adv. I'hotochem. 1, 1 (1964); W.M. MOORE, G. S.HAKXOND and R. P. FOSS, J. Am. Chem. Sot. 83,2789 (1961); G. S. HAMMOND and P. LEERMAKERS, ibid. 84, 207 (1962). [2] S. I(. Lowsa and M. A. EL-SAYED, Chem. Rev. 66, 199 (1966). 131 D. R. KEARNS and W. A. CASE, J. Am. Chem,. Sot. 88, 5087 (1966). [4] D. F. EVAXS, J. Cl&em. 8oc. 1351 (1957). [5] IV. G. J~ERRSTROETER, A. A. LAMOLA nnd G. S.HAMMOND, J. Am. Chem. Sot. 86, 4537 (1964). 589
D. A. WARWICK and C. H. J. WELLS
690
on the side of the tube. The valve could be joined via a pressure gauge to a cylinder of oxygen. The cell could be mounted in a 10 cm cell-holder in the sample compartment of a Unicam SP.800 spectrophotometer. Spectra were obtained at atmospheric pressure and at a pressure of 100 atm of oxygen. The solutes were the purest available commercially and were either distilled or twice recrystallised before use. The solvents used were either Analar or Specfrosol grade. RESULTS AND DISCUSSION
The spectra shown in Figs. l-3 illustrate the effect of oxygen on the absorption of The noteworthy feature is that compounds several aromatic carbonyl compounds.
0
390
430
470
510
550
390
430
470
510
550
390
430
470
510
550
mp
Wavelength,
Fig. 1. (---) Spectrum at atmospheric pressure; (-) spectrum under 100 atrn of oxygen . 1-naphthaldehyde, 0.20 M in n-hexane; 2-acetonaphthone, 2.00 M in chloroform; I-acutonaphthone, I.00 M in chIoroform. Path length 8.5 cm. Reference-attenuator.
)O
550
600
650
Wavelength.
700
36( mp
2. (---) Spectrum at atmospheric pressure; (-) spectrum under 100 atm of oxygen. 4-hytlroxyncetophonono, 0.80 M in chloroform; 9-anthraldehyde, 0.58 M in bonzeno; 4-mcthoxyacetophenone, 0.50 M in ethanol; Z-naphthyl phony1 kctono, 0.50 Al itI chloroform. Path length 8.5 cm. Referenceattenuator.
Fig.
Perturbation
of singlet-triplet
transitions
in aromatic
carbonyl
591
compounds
which have lowest T, n-* triplet states (see Table 1) show an enhancement of the S-T transition in the presence of oxygen while compounds with lowest n, G triplet states are unaffected. The results obtained for compounds whose lowest triplet states have been assigned as rr, n* are given in Table 1.
‘...
030
420
400
440
500
540
520 Wavelength.
560
560
36
0
360
400
--__ -. 420
440
mp
Fig. 3. ( - - - ) Spectrum at atmospheric pressure; (-) spectrum under 100 atm of oxygen. Acetophenone, 0.20 M in n-hexane; p-benzoquinone, 0.03 M in n-hexane; bcnzaldehydc, 0.70 M in n-hexane. Pat.h length 8.5 cm. Reference-attenuator. Tablo
1. Perturbation
of S-T
transitions in aromatic triplet states
(m!l) 405 397 475, 450, 450, 474, 600,
70.6 72.0 56.5 58.8 59.6 56.1 43.5
enhanced
4-hyd.roxyacct.ophenone 4-methoxyacetophcnone 1-acetonaphthono 2-acetonaphthone 2-naphthyl phenyl ketone 1-naphthaldehyde !)-ant hraltlohyde
447, 422, 423, 444, 358,
compounds
Energy of lowest triplet level (kcallmole) Lit,. Exp.
1~nlax
Compound
carbonyl
of
bands
506 486 480 510 658
72.4* 76.7 * 56*4t 59.3t 59.67 56*3t -
with lowest n, TT*
Assignment of lowest triplet state References [31 r31 [I31 i-131 V31 [I31 [I41
* Ref. I:;]. t l{of. [S]. [6] E. J. BAUN, J. K. S. WAX and J. N. PITTS, J. Am. Chem. Sot. 99, 2652 (1966). [7] S. P. MCGLYNN, M. J. REYNOLDS, G. W. DAICRE and N. 0. CHRISTODOYLEAS, J. P&Y. Chm. 96, 2499 (1962); S. P. MCGLYSN, T. AZUMI and M. KASHA, J. Chem. P&s. 40, 507 (1964). [S] Y. KAXD~, H. KASED~ and T. M.ITUMURA, Spectrochim. Acta 20, 1387 (1964). [Q] D. F. EVAXS, J. Chem. Sot. 2733 (1959). [lo] J. RluRRsLL, l?fol. P/I?& 3, 319 (1960). [ll] H. TSUBOJIURA and R. MULLIKEN, J. -4m. Chem. Sot. 82, 5966 (1960). [12] G. J. HOIJTIXK, Tetrahedron 19, Suppl. 2, 179 (1963). [13] G. PORTER and P. SUPPAN, Trans. Faraday Sot. 62, 3375 (1966). [14] S. C. YASG, Pure AppZ. Chem. 9, 591 (1964).
592
D. A. WARWICKand C. H. J. WELLS
It can be seen from Table 1 that the energies of the lowest triplet states as calculated from the position of the longest wavelength S-T absorption bands agree well with the literature values. In cases where the S-S absorption band did not lie close to the lowest energy S-T transition vibrational structure could be observed. The spacing of the vibrational levels in compounds containing thenaphthalene moiety was approximately 1400 cm-i which is similar to that observed for naphthalene [4]. The lowest triplet level of 4-hydroxybutyrophenone has been assigned as n, rr* [6] but no enhanced S-T bands could be observed in the spectrum (not shown). At the concentrations required to observe S-T transitions the intensity of the S-S band in 4hydroxybutyrophenone in the region in which the S-T transitions would occur is high and the weak S-T transitions are hidden. The S-T transitions in 2-acetonaphthone, 2-naphthyl phenyl ketone and 9anthraldehyde are intensified in the presence of iodobenzene and can be observed directly when iodobenzene is used as solvent. The enhanced bands are at the same position as the bands observed in the presence of oxygen but the intensities of the bands in iodobenzene are about one-third that for the bands observed when the oxygen pressure was 100 atm for the same concentration of solute and path length. transitions by solvents containing heavy atoms has been Intensification of S-T,,* previously observed for aromatic hydrocarbons [7] and aromatic carbonyl compounds [3]. As the enhancement in heavy atom solvents is weak as compared to that in the presence of oxygen, the oxygen perturbation method is to be preferred for the observation of S-T,,,* transitions. Acetophenone, p-benzoquinone and benzaldehyde which have lowest n, 7~* triplet states do not show enhancement of S-T transitions in the presence of oxygen at 100 atm. Figure 3 illustrates that the S-T transitions in acetophenone, benzaldehyde and p-benzoquinone can be observed directly and that there is no oxygen effect on these transitions. The absence of oxygen perturbation is in accord with previous experimental findings for the effect of heavy atom solvents on the intensity of S-T,,,* transitions [3] and for the effect of oxygen at atmospheric pressure on the S-T,,,. transition in benzaldehyde [S]. It is of interest to note that S-Tss,. transitions in pyrazine [9] and in acridine [4] are enhanced by oxygen. In these systems as in the aromatic carbonyl systems with lowest n, n* triplet states the half-vacant bonding and anti-bonding orbitals are specifically associated with the aromatic ring and obviously this situation favours enhancement of S-T transitions. Two theories have been proposed to account for the intensification of the S-T absorption by a perturber molecule. MURRELL [lo] and TSUBOMURAand MULLIKEN [ 1 l] have suggested that the intensification is due to intensity-borrowing of the S-T transition from a charge transfer transition in an aromatic hydrocarbon-perturber complex, while HOIJTINK [12] has attributed the effect to intensity-borrowing from the corresponding S-S transition in the aromatic hydrocarbon, the magnitude of the enhancement being dependent on the extent of electron exchange between the aromatic molecule and the paramagnetic perturber. The results obtained in the present work do not allow a choice to be made between the two theories and the relative importance of the exchange or charge transfer mechanism must still remain an open question. Since S-T transitions of aromatic carbonyl compounds with lowest 7r, rr* triplet
Perturbation of singlet-triplet transitions in aromatic carbonyl compounds
693
states are enhanced by oxygen whilst S-T transitions in aromatic carbonyk with lowest n, TT*triplets are unaffected, the oxygen perturbation technique provides a method for characterising the lowest triplet states of such compounds. The method has the advantage over phosphorescence methods [5] and phosphorescence excitation methods [3] in that the spectra can be obtained in a routine manner without specialised equipment.
Perturbation of singlet-triplet transitions in aromatic carbons1 compounds D. A. Department
of Chemistry,
WARWICK
Sir John Cass College, Jewry St., London, E.C. 3
and C. H. J. Department
WELLS
of Chemistry, Kingston College of Technology Kingston upon Thames (Received 3 July 1967)
Abstract-Singlet-triplet transitions in aromatic carbonyl compounds with lowest r, n* triplet states are enhanced by the presence of oxygen while singlet-triplet transitions in aromatic carbonyls with lowest n, X* triplet states are unaffected. The oxygen perturbation technique thus provides a simple method for characterising the lowest triplet state of aromatic carbonyl compounds. YHOTOCHEMICAL reactions often proceed via the lowest triplet states of molecules and in many cases the photoreactivity of the molecule can be correlated with the nature of the lowest triplet state [l]. In view of the role of the triplet state in photochemistry it is important that the nature of the lowest triplet state be characterised. Information on triplet states can either be obtained from triplet + singlet emission spectra [2, 31 or from singlet + triplet absorption spectra [4]. The former method has been utilised to yield data on both the nature of the triplet states from which emission occurs [2, 31 and on the energy levels of the emitting states [5] while the latter method has been used in the determination of energy levels of triplet states [4]. Since singlet-triplet absorption spectra can often be readily observed in the presence of a paramagnetic molecule under high pressure [4] it was proposed to investigate this method as a means of characterising the lowest triplet states of molecules. Aromatic carbonyl compounds were chosen for this study since a great deal of information has been accumulated on the properties of triplet states in such compounds. EXPERIMENTAL
The high pressure absorpCon cell consisted of a brass tube (9.0 cm long) with silica windows (1-G cm o.d. and 6 mm thick) mounted on each end between polytetrafluorethylene gaskets by means of a hollow brass screw cap. The overall path length of the cell was 8.5 cm. A high pressure valve and a filling port were mounted [l]
J. N. PITTS, F. WILKINSON and G. S. ~TAMMOND, Adv. I'hotochem. 1, 1 (1964); W.M. MOORE, G. S.HAKXOND and R. P. FOSS, J. Am. Chem. Sot. 83,2789 (1961); G. S. HAMMOND and P. LEERMAKERS, ibid. 84, 207 (1962). [2] S. I(. Lowsa and M. A. EL-SAYED, Chem. Rev. 66, 199 (1966). 131 D. R. KEARNS and W. A. CASE, J. Am. Chem,. Sot. 88, 5087 (1966). [4] D. F. EVAXS, J. Cl&em. 8oc. 1351 (1957). [5] IV. G. J~ERRSTROETER, A. A. LAMOLA nnd G. S.HAMMOND, J. Am. Chem. Sot. 86, 4537 (1964). 589
D. A. WARWICK and C. H. J. WELLS
690
on the side of the tube. The valve could be joined via a pressure gauge to a cylinder of oxygen. The cell could be mounted in a 10 cm cell-holder in the sample compartment of a Unicam SP.800 spectrophotometer. Spectra were obtained at atmospheric pressure and at a pressure of 100 atm of oxygen. The solutes were the purest available commercially and were either distilled or twice recrystallised before use. The solvents used were either Analar or Specfrosol grade. RESULTS AND DISCUSSION
The spectra shown in Figs. l-3 illustrate the effect of oxygen on the absorption of The noteworthy feature is that compounds several aromatic carbonyl compounds.
0
390
430
470
510
550
390
430
470
510
550
390
430
470
510
550
mp
Wavelength,
Fig. 1. (---) Spectrum at atmospheric pressure; (-) spectrum under 100 atrn of oxygen . 1-naphthaldehyde, 0.20 M in n-hexane; 2-acetonaphthone, 2.00 M in chloroform; I-acutonaphthone, I.00 M in chIoroform. Path length 8.5 cm. Reference-attenuator.
)O
550
600
650
Wavelength.
700
36( mp
2. (---) Spectrum at atmospheric pressure; (-) spectrum under 100 atm of oxygen. 4-hytlroxyncetophonono, 0.80 M in chloroform; 9-anthraldehyde, 0.58 M in bonzeno; 4-mcthoxyacetophenone, 0.50 M in ethanol; Z-naphthyl phony1 kctono, 0.50 Al itI chloroform. Path length 8.5 cm. Referenceattenuator.
Fig.
Perturbation
of singlet-triplet
transitions
in aromatic
carbonyl
591
compounds
which have lowest T, n-* triplet states (see Table 1) show an enhancement of the S-T transition in the presence of oxygen while compounds with lowest n, G triplet states are unaffected. The results obtained for compounds whose lowest triplet states have been assigned as rr, n* are given in Table 1.
‘...
030
420
400
440
500
540
520 Wavelength.
560
560
36
0
360
400
--__ -. 420
440
mp
Fig. 3. ( - - - ) Spectrum at atmospheric pressure; (-) spectrum under 100 atm of oxygen. Acetophenone, 0.20 M in n-hexane; p-benzoquinone, 0.03 M in n-hexane; bcnzaldehydc, 0.70 M in n-hexane. Pat.h length 8.5 cm. Reference-attenuator. Tablo
1. Perturbation
of S-T
transitions in aromatic triplet states
(m!l) 405 397 475, 450, 450, 474, 600,
70.6 72.0 56.5 58.8 59.6 56.1 43.5
enhanced
4-hyd.roxyacct.ophenone 4-methoxyacetophcnone 1-acetonaphthono 2-acetonaphthone 2-naphthyl phenyl ketone 1-naphthaldehyde !)-ant hraltlohyde
447, 422, 423, 444, 358,
compounds
Energy of lowest triplet level (kcallmole) Lit,. Exp.
1~nlax
Compound
carbonyl
of
bands
506 486 480 510 658
72.4* 76.7 * 56*4t 59.3t 59.67 56*3t -
with lowest n, TT*
Assignment of lowest triplet state References [31 r31 [I31 i-131 V31 [I31 [I41
* Ref. I:;]. t l{of. [S]. [6] E. J. BAUN, J. K. S. WAX and J. N. PITTS, J. Am. Chem. Sot. 99, 2652 (1966). [7] S. P. MCGLYNN, M. J. REYNOLDS, G. W. DAICRE and N. 0. CHRISTODOYLEAS, J. P&Y. Chm. 96, 2499 (1962); S. P. MCGLYSN, T. AZUMI and M. KASHA, J. Chem. P&s. 40, 507 (1964). [S] Y. KAXD~, H. KASED~ and T. M.ITUMURA, Spectrochim. Acta 20, 1387 (1964). [Q] D. F. EVAXS, J. Chem. Sot. 2733 (1959). [lo] J. RluRRsLL, l?fol. P/I?& 3, 319 (1960). [ll] H. TSUBOJIURA and R. MULLIKEN, J. -4m. Chem. Sot. 82, 5966 (1960). [12] G. J. HOIJTIXK, Tetrahedron 19, Suppl. 2, 179 (1963). [13] G. PORTER and P. SUPPAN, Trans. Faraday Sot. 62, 3375 (1966). [14] S. C. YASG, Pure AppZ. Chem. 9, 591 (1964).
592
D. A. WARWICKand C. H. J. WELLS
It can be seen from Table 1 that the energies of the lowest triplet states as calculated from the position of the longest wavelength S-T absorption bands agree well with the literature values. In cases where the S-S absorption band did not lie close to the lowest energy S-T transition vibrational structure could be observed. The spacing of the vibrational levels in compounds containing thenaphthalene moiety was approximately 1400 cm-i which is similar to that observed for naphthalene [4]. The lowest triplet level of 4-hydroxybutyrophenone has been assigned as n, rr* [6] but no enhanced S-T bands could be observed in the spectrum (not shown). At the concentrations required to observe S-T transitions the intensity of the S-S band in 4hydroxybutyrophenone in the region in which the S-T transitions would occur is high and the weak S-T transitions are hidden. The S-T transitions in 2-acetonaphthone, 2-naphthyl phenyl ketone and 9anthraldehyde are intensified in the presence of iodobenzene and can be observed directly when iodobenzene is used as solvent. The enhanced bands are at the same position as the bands observed in the presence of oxygen but the intensities of the bands in iodobenzene are about one-third that for the bands observed when the oxygen pressure was 100 atm for the same concentration of solute and path length. transitions by solvents containing heavy atoms has been Intensification of S-T,,* previously observed for aromatic hydrocarbons [7] and aromatic carbonyl compounds [3]. As the enhancement in heavy atom solvents is weak as compared to that in the presence of oxygen, the oxygen perturbation method is to be preferred for the observation of S-T,,,* transitions. Acetophenone, p-benzoquinone and benzaldehyde which have lowest n, 7~* triplet states do not show enhancement of S-T transitions in the presence of oxygen at 100 atm. Figure 3 illustrates that the S-T transitions in acetophenone, benzaldehyde and p-benzoquinone can be observed directly and that there is no oxygen effect on these transitions. The absence of oxygen perturbation is in accord with previous experimental findings for the effect of heavy atom solvents on the intensity of S-T,,,* transitions [3] and for the effect of oxygen at atmospheric pressure on the S-T,,,. transition in benzaldehyde [S]. It is of interest to note that S-Tss,. transitions in pyrazine [9] and in acridine [4] are enhanced by oxygen. In these systems as in the aromatic carbonyl systems with lowest n, n* triplet states the half-vacant bonding and anti-bonding orbitals are specifically associated with the aromatic ring and obviously this situation favours enhancement of S-T transitions. Two theories have been proposed to account for the intensification of the S-T absorption by a perturber molecule. MURRELL [lo] and TSUBOMURAand MULLIKEN [ 1 l] have suggested that the intensification is due to intensity-borrowing of the S-T transition from a charge transfer transition in an aromatic hydrocarbon-perturber complex, while HOIJTINK [12] has attributed the effect to intensity-borrowing from the corresponding S-S transition in the aromatic hydrocarbon, the magnitude of the enhancement being dependent on the extent of electron exchange between the aromatic molecule and the paramagnetic perturber. The results obtained in the present work do not allow a choice to be made between the two theories and the relative importance of the exchange or charge transfer mechanism must still remain an open question. Since S-T transitions of aromatic carbonyl compounds with lowest 7r, rr* triplet
Perturbation of singlet-triplet transitions in aromatic carbonyl compounds
693
states are enhanced by oxygen whilst S-T transitions in aromatic carbonyk with lowest n, TT*triplets are unaffected, the oxygen perturbation technique provides a method for characterising the lowest triplet states of such compounds. The method has the advantage over phosphorescence methods [5] and phosphorescence excitation methods [3] in that the spectra can be obtained in a routine manner without specialised equipment.