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Experimental determination of the 3π,π* energy level of benzophenone
Volume 25, number 3
CHEMICAL PHYSICS LETTERS
1 April 1974
EXPERLMENTAL DETERMINATION OF THE 3~,n* ENERGY LEVEL OF BENZOPHENONEt J&I. MORRIS* and D.F. WILLIAMS Division of Chemistry.
National Research
Council of Canada. Otlawa,
Canada KIA OR6
Received 14 January 1974
A weak fluorescence was observed from benzophenone at 25650 cm+ with a quantum yield of ==lO-’ at 5°K. 3 With the assumption that * n,r* - 3 n,r* intersystem crossing is more efficient than n,~* + n,~*, the temperature dependence of the quantum yield sets the 3n,n* level <, 50 cm-’ above the ‘n,~* level: 3E, rr = 26670 cm-’ _
r
The photochemical properties of benzophenone have been widely studied, both intrinsically, and extrinsicaIIy as a triplet quencher or sensitizer [l] . A knowledge of the triplet levels of benzophenone is of particular importance for most photoreactions are thought to proceed via these triplet states. Nevertheless, unambiguous experimental observations of the principal singlet and triplet energy levels have evaded researchers to date. The %,77* state was unsuccessfully sought by Batley and Keams [2] who attempted to locate this %J* state by T f S absorption in a crystal. They were unable to locate any absorption assignabfe to the %T,x* state energetically below the onset’ of the In,,* state, and hence concluded that the 37r,7r* state was above the ln,~*
state.
The lowest singlet state of benzophenone is of n,n* charactc:. The maximum of the (0,O) band in absorption is at 26600 cm-1 and its onset at 25900 cm-l at 10°K in an isopentane-methylcyclohexane glass. The observed emission is a typical short lived n + H* phosphorescence with the maximum of the band at 24040 cm-1 *. Thus since thk singlet-tripIet splitting is fairly large for the n,a* state, 2000 cm-l, it seemed that fluorescence from benzophenone might be observable at temperatures such that: f NRCC No. 13841. * NRCC Post Doctorate Fellow 1972-73. l See, fcr example, tables of energy levels of ketones and heterocyclics in ref. [ 31.
WAVENUMBER (cm-’ x IO-?
Fig. 1. ‘lXe phosphorescence excitation, fluorescence and phos-. phorescence spectra of benzophenone in a methylcyclohexaneisopentane mixture at 5°K. The luminescence intensity scale is logarithmic and the ratio between fluorescence and phosphoreScence intensities is shown correctly to within an order of magnitude.
JcT
At these low temperatures, the radiationless depopulation of the singlet state via efficient intersystem crossing from thermally populated vibronic levels of the ln,n* state would not be operatives. * .This problem is discussed by Dyn and Hochstrasser [4].
312
.,
.:
- E(‘n,n*).
:
-
~01umc 25, number 3
CHEMICAL
PHYSICS
LETTERS
1974
1Apd
figure of 26650 cm-1 represents more exactly the true vibrationally relaxed energy of the ln,sr* state. (A small discrepancy due to the use of different sol-
We were able to observe a very weak fluorescence from benzophenone with a maximum at 2.5650 cm-l (see fig. 1). This emission could only be observed clearly at temperatures below 10% and required a photon counting system and the storage of 20 scans through the spectrum on a CAT in order to enhance the signalto-noise ratio so that the band could be seen_ The fluorescence band had approximately the same intensity as the Raman band from the glass matrix- By comparing the intensity of the fluorescence band with that of the Raman band and also with the attenuated (0,O) band of the phosphorescence, we estimated the quantum yield of fluorescence to be -10-5 at S°K. Due to the weakness of the fluorescence, attempts to measure the temperature dependence of the fluores-, cence intensity could only produce a very approximate estimate of the energy gap between the 1n,n* and %r,n* states: we set an upper limit of 50 cm-l on this energy gap and consider it more iilce!y to be of IO-20 cm-r and hence estimate that the energy of the %-cn* state is between 26660 and 26700 cm-r. Changes in the macrostructure of the glassy matrix, which is invariably cracked at temperatures below 70°K, over the long period necessary to accumulate sufficient scans on the CAT are presumably the main source of the uncertainty. A commonly adopted technique to overcome this problem would be to place the benzophenone in a solid plastic matrix so as to eliminate changes in the matrix due to cracking, etc. However, in this experiment, the polar nature of the matrix will shift the ln,rr* state to higher energy, most likely above the 37r,~* state. The measurements of intensity versus temperature are unfortunately certainly not sut ficiently accurate to be able to deduce from them any quantitative information about the relative sizes of the matrix elements determining the rate of intersystem crossing. A number of authors have observed the fluorescence from a thermally repopulated singlet state [S, 61: this should be distinguished from the direct fluores-
vents is to be expected_) Brown et al. 171 have re-
ported the observation
of a fast fluorescence
from we were unable to reproduce their results. These experiments are not practically trivial, however, and we think that they may have been observing scattered laser light passing through their KNO, filter. This is suggested by their results for(i) even at room temperature the ln,a* absorption spectrum in benzene shows some slight structure (ii the form of shoulders on the main band) and none is evident in their spectra, and
benzophenone in benzene at room temperature:
(ii) the full width at half the height of their fluores-
X
cence band (5700 cm-l) is much greater than the width of the absorption band (4800 cm-l) (the opposite would be expected - the tail of the next highest energy absorption band should broaden the absorption)_ It is also interesting to consider our results with respect to the recent proposal by Azumi [S] that intersystem crossing induced by a pseudo Jalm-Teller dis--tortion of the benzophenone triplet states causes the extremely high intersystem crossing efficiency. Since the 3n,~* state lies above the In,,* state our results show that although crossing to the triplet manifold is still efficient at 5OK, it becomes more efficient at higher temperatures when direct In, 7r*-+%r,71* crossing can take place. Thus the first order matrix element connecting the initial and fmal states via the spin orbit coupling operator is still important, providing qualitative confirmation of the calculations of El-Sayed concerning the relative rates of intersystem crossing between two types of triplet states [9] . In summary, our results (a) locate the vibrationally relaxed ln,+ state of benzophenone at 26650 cm-’ and show that some qualification must be attached to the rule about the fluorescence of molecules with lowest n,?r* states [lo] (it is important to remember that a!1 molecules fluoresce, even though the quantum yield may be unobservably small), (b) provide further qualitative confirmation for El-Sayed’s calculations and.(c) serve to locate the %r,n* state at a26680 cm-l with an accuracy suffiiient to aid in the understanding of most photochemical processes which may occur via the triplet states of benzophenone-
cence at very low temperature reported here. Jones and Calloway [S] reported this fluorescence at 25200 cm-l in a polystyrene matrix at 300%. However, their spectra are considerably temperature broadened and their 25200 cm-1 figme is the maximum of the fluorescence band at 300°K - the onset of the band in their published spectrum is about 26600 cm-l. Our
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Volume 25, number 3
CHEMICAL PHYSICS LETTERS
References [l] P. dehlayo, Accounts Chem. Res.4 (1971)41. [2] M. Butley and D.R. Kearns, Chem. Phys. Letters 2 (1966) 423. [3] S.P. McGlynn, T. Azumi and M. Kinoshita, hlolecular spectroscopy of the triplet state (Prentice-Hall, Bnglewood Cliffs; 1969). 1
[4] S. Dyn ahd R. Hochstrasser, J. Chem. Phys. 51 (1969) 8458. -.
1 April 1974
[S] P-F. Jonesand A-R. Callow&y, J. Am. Chem. Sot. 92 (1970) 4997. [6] J. Saltiel, H.C. Curtis, I_ Metts, J.W. Miley, J. wnterle and hf. Wrighton, J. Am.Chem. Six. 92 (1970)410. 171 R.E. Brown, L.A. Singerand J.H. Parke,Chem.Phys.
Letters 14 (1972) 193. 181 T- Amhi, Chem. Phys. Letters 17 (1972) 211. (91 M.A. El-SaYed, Accounts Chem. Res. 1 (1968) 8. [IO1 R.S. Becker, Theory and interpretation of fluorescence and phosphorescence (WiIey-Interscience, New York, 1969) ch. 12.
CHEMICAL PHYSICS LETTERS
1 April 1974
EXPERLMENTAL DETERMINATION OF THE 3~,n* ENERGY LEVEL OF BENZOPHENONEt J&I. MORRIS* and D.F. WILLIAMS Division of Chemistry.
National Research
Council of Canada. Otlawa,
Canada KIA OR6
Received 14 January 1974
A weak fluorescence was observed from benzophenone at 25650 cm+ with a quantum yield of ==lO-’ at 5°K. 3 With the assumption that * n,r* - 3 n,r* intersystem crossing is more efficient than n,~* + n,~*, the temperature dependence of the quantum yield sets the 3n,n* level <, 50 cm-’ above the ‘n,~* level: 3E, rr = 26670 cm-’ _
r
The photochemical properties of benzophenone have been widely studied, both intrinsically, and extrinsicaIIy as a triplet quencher or sensitizer [l] . A knowledge of the triplet levels of benzophenone is of particular importance for most photoreactions are thought to proceed via these triplet states. Nevertheless, unambiguous experimental observations of the principal singlet and triplet energy levels have evaded researchers to date. The %,77* state was unsuccessfully sought by Batley and Keams [2] who attempted to locate this %J* state by T f S absorption in a crystal. They were unable to locate any absorption assignabfe to the %T,x* state energetically below the onset’ of the In,,* state, and hence concluded that the 37r,7r* state was above the ln,~*
state.
The lowest singlet state of benzophenone is of n,n* charactc:. The maximum of the (0,O) band in absorption is at 26600 cm-1 and its onset at 25900 cm-l at 10°K in an isopentane-methylcyclohexane glass. The observed emission is a typical short lived n + H* phosphorescence with the maximum of the band at 24040 cm-1 *. Thus since thk singlet-tripIet splitting is fairly large for the n,a* state, 2000 cm-l, it seemed that fluorescence from benzophenone might be observable at temperatures such that: f NRCC No. 13841. * NRCC Post Doctorate Fellow 1972-73. l See, fcr example, tables of energy levels of ketones and heterocyclics in ref. [ 31.
WAVENUMBER (cm-’ x IO-?
Fig. 1. ‘lXe phosphorescence excitation, fluorescence and phos-. phorescence spectra of benzophenone in a methylcyclohexaneisopentane mixture at 5°K. The luminescence intensity scale is logarithmic and the ratio between fluorescence and phosphoreScence intensities is shown correctly to within an order of magnitude.
JcT
At these low temperatures, the radiationless depopulation of the singlet state via efficient intersystem crossing from thermally populated vibronic levels of the ln,n* state would not be operatives. * .This problem is discussed by Dyn and Hochstrasser [4].
312
.,
.:
- E(‘n,n*).
:
-
~01umc 25, number 3
CHEMICAL
PHYSICS
LETTERS
1974
1Apd
figure of 26650 cm-1 represents more exactly the true vibrationally relaxed energy of the ln,sr* state. (A small discrepancy due to the use of different sol-
We were able to observe a very weak fluorescence from benzophenone with a maximum at 2.5650 cm-l (see fig. 1). This emission could only be observed clearly at temperatures below 10% and required a photon counting system and the storage of 20 scans through the spectrum on a CAT in order to enhance the signalto-noise ratio so that the band could be seen_ The fluorescence band had approximately the same intensity as the Raman band from the glass matrix- By comparing the intensity of the fluorescence band with that of the Raman band and also with the attenuated (0,O) band of the phosphorescence, we estimated the quantum yield of fluorescence to be -10-5 at S°K. Due to the weakness of the fluorescence, attempts to measure the temperature dependence of the fluores-, cence intensity could only produce a very approximate estimate of the energy gap between the 1n,n* and %r,n* states: we set an upper limit of 50 cm-l on this energy gap and consider it more iilce!y to be of IO-20 cm-r and hence estimate that the energy of the %-cn* state is between 26660 and 26700 cm-r. Changes in the macrostructure of the glassy matrix, which is invariably cracked at temperatures below 70°K, over the long period necessary to accumulate sufficient scans on the CAT are presumably the main source of the uncertainty. A commonly adopted technique to overcome this problem would be to place the benzophenone in a solid plastic matrix so as to eliminate changes in the matrix due to cracking, etc. However, in this experiment, the polar nature of the matrix will shift the ln,rr* state to higher energy, most likely above the 37r,~* state. The measurements of intensity versus temperature are unfortunately certainly not sut ficiently accurate to be able to deduce from them any quantitative information about the relative sizes of the matrix elements determining the rate of intersystem crossing. A number of authors have observed the fluorescence from a thermally repopulated singlet state [S, 61: this should be distinguished from the direct fluores-
vents is to be expected_) Brown et al. 171 have re-
ported the observation
of a fast fluorescence
from we were unable to reproduce their results. These experiments are not practically trivial, however, and we think that they may have been observing scattered laser light passing through their KNO, filter. This is suggested by their results for(i) even at room temperature the ln,a* absorption spectrum in benzene shows some slight structure (ii the form of shoulders on the main band) and none is evident in their spectra, and
benzophenone in benzene at room temperature:
(ii) the full width at half the height of their fluores-
X
cence band (5700 cm-l) is much greater than the width of the absorption band (4800 cm-l) (the opposite would be expected - the tail of the next highest energy absorption band should broaden the absorption)_ It is also interesting to consider our results with respect to the recent proposal by Azumi [S] that intersystem crossing induced by a pseudo Jalm-Teller dis--tortion of the benzophenone triplet states causes the extremely high intersystem crossing efficiency. Since the 3n,~* state lies above the In,,* state our results show that although crossing to the triplet manifold is still efficient at 5OK, it becomes more efficient at higher temperatures when direct In, 7r*-+%r,71* crossing can take place. Thus the first order matrix element connecting the initial and fmal states via the spin orbit coupling operator is still important, providing qualitative confirmation of the calculations of El-Sayed concerning the relative rates of intersystem crossing between two types of triplet states [9] . In summary, our results (a) locate the vibrationally relaxed ln,+ state of benzophenone at 26650 cm-’ and show that some qualification must be attached to the rule about the fluorescence of molecules with lowest n,?r* states [lo] (it is important to remember that a!1 molecules fluoresce, even though the quantum yield may be unobservably small), (b) provide further qualitative confirmation for El-Sayed’s calculations and.(c) serve to locate the %r,n* state at a26680 cm-l with an accuracy suffiiient to aid in the understanding of most photochemical processes which may occur via the triplet states of benzophenone-
cence at very low temperature reported here. Jones and Calloway [S] reported this fluorescence at 25200 cm-l in a polystyrene matrix at 300%. However, their spectra are considerably temperature broadened and their 25200 cm-1 figme is the maximum of the fluorescence band at 300°K - the onset of the band in their published spectrum is about 26600 cm-l. Our
‘313_. . . . ;
:
:-
.-’
-..
.:..
:
:-
.-:
‘.
’
,. ..-_;
: ‘:
‘-.
.’
..
.-
:
__,;
__
.: ‘(
___
_..~
,.,;_-t _.:.-,1-r_‘; :
Volume 25, number 3
CHEMICAL PHYSICS LETTERS
References [l] P. dehlayo, Accounts Chem. Res.4 (1971)41. [2] M. Butley and D.R. Kearns, Chem. Phys. Letters 2 (1966) 423. [3] S.P. McGlynn, T. Azumi and M. Kinoshita, hlolecular spectroscopy of the triplet state (Prentice-Hall, Bnglewood Cliffs; 1969). 1
[4] S. Dyn ahd R. Hochstrasser, J. Chem. Phys. 51 (1969) 8458. -.
1 April 1974
[S] P-F. Jonesand A-R. Callow&y, J. Am. Chem. Sot. 92 (1970) 4997. [6] J. Saltiel, H.C. Curtis, I_ Metts, J.W. Miley, J. wnterle and hf. Wrighton, J. Am.Chem. Six. 92 (1970)410. 171 R.E. Brown, L.A. Singerand J.H. Parke,Chem.Phys.
Letters 14 (1972) 193. 181 T- Amhi, Chem. Phys. Letters 17 (1972) 211. (91 M.A. El-SaYed, Accounts Chem. Res. 1 (1968) 8. [IO1 R.S. Becker, Theory and interpretation of fluorescence and phosphorescence (WiIey-Interscience, New York, 1969) ch. 12.