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External magnetic field effect on the fluorescence glyoxal
Journal of Luminescence 24/25 (1981) 763—766 North-Holland Publishing Company
763
EXTERNAL MAGNETIC FIELD EFFECT ON THE FLUORESCENCE OF OLYOXAL J. Nakamura,
K. Mashimoto* and S. Nagekura*
The Inatitute of Physical and Chemical Keeearch Make, Saitama Japan
Fluorescence decay ccrves of glyoxal at low presscrea were analyzed by meaeuring external magnetic field effect on the single vibronic level time—resolved fluorescence spectra. The observed multi—component decays were found to be the overlaps of four emissions, F 0, F , F2 and F3, from different origins. Excitation energy Aependence of the msgnetic field effect on these four emissions suggested a mechanism of singlet— triplet energy transfers among three electronic excited states, S~ T1 end T2.
INTRODUCTION Olyoxal vapor at low pressures shows a fluorescence with a single—exponential decay when excited to the vibrationless lowest excited singlet state ~l ~ Accumlated spectroscopic knowledges indicate that this molecule is a prototype of “week coupling small molecule” case. [1] . The same molecule behaves, however, like an “intermediate case molecule” when excited with a light shorter than 455 nm; the fluorescence observed at the normal fluorescing region (e.g. at 482
nm) shows a multi—exponential
decay.
Ne have studied the external magnetic field effects on the SVL fluorescence of glyoxal, and found out the relations between the guenching patterns of the fluorescence decay under the existence of a magnetic field and the type of interactions among the singlet and triplet states.
EXPERIMENTAL Olyoxal monomer was obtained from dried polymer by the usual method. Neat glyoxal of 20 to 500 mtorr was filled in a pyrex sample cell of SO mrs i.d., cross— shaped with a photon trap. The sample pressures were monitored with a capacitor— type manometer(Saratren MRS 222A). The cell was set between the poles of a magnet which feeds a magnetic field of 700 to 7000 Gauss to the sample. Olyoxal was excited in the range of 455.3 to 350 nm with an N2 laser pumped dye laser (Molectron 24, D 14). Time—resolved spectra of fluorescence and phosphorescence and time—profiles of the individual single vibronic transitions were recorded with a single—photon counting system combined with a Soxcar integrator. Schematic arrangement of the apparatus is shown in Fig. 1.
RESULTS AND DISCUSSIONS The decay curves of fluorescence were found to become more and more deviated from the single—exponential decay with increasing energy of excitation. From the analysis of multi—component decays observed with and without a magnetic field, we *
Present address: The Inetitute
for Molecular Science, Okazaki,
0 022—2313/81 /0000—0000/S02.7 5 © North-Holland
Japan
J. Naka,nura ci ci / Magnc’tic field effect in gi;oxal
764
-u Magnetic
I
~ I, I
!~ig~t Pipe I _______
Ernie sian PM
Field I I
U
i I
counting
Fiql Schematic diagrawn of the apparatus used for the observation of cx— ternal magnetic field effect on the emissions of g1voxa1~
[N L. lDya L~.aa4 iLaSarl Trigger deduced at most four components of fluorescence. The time—resolved spectre for each of the components indicated further thet the multi-component decay usually observed at the normal region
3 10
with a low spectral resolution is a result of accidental overlap of independent series of fluorescences with essentially single— exponential decays. We designate these four types of fluores— cence as F , P , F 2Fand P in order of their 1ife~ime~ I 0 is ~he shortest) . According to the number of components in the decay curves, the whole region of ~l state is classified into four parts. The behaviors of quenching by the application of a magnetic field are also characteristic to the four regions. The results are summarized as follows. (As the effect saturates at a weak magnetic field, the strength of the field , denoted as M0 in the figures, was fixed to 3.5 kO.)
a 10 2\ 0
u
H
101
0
Excitation at 455.3 nm
)0% )
This case has been reported by many
6.4
b/MS Fig.2
Case I.
3.2
Time-profile of fluorescence Olyoxal 190 mtorr. Excitation at 455.3 nm (0% Observation at 452 nm
authors, yet an observed time—profile of fluorescence is shown in Pig.2 for the purpose of comparison. Main features of this ease arer 1) emission spectrum is composed of the direct transitions from the excited 0% state to the vobrational levels in the ground stete,S ~ 2) application of a magnetic field quenches the fluorescence in its lifetime bu~ not in the initial intensity, 3) the rate of guenehing, T)M/0)/T)H=0), is pressure dependent. 43S rigs Case II.been Excitation at 455 As has pointed out, [2) nm’— the excitation to a certain level in this region causes a bi—exponential decay as the self guenehing constant is bigger for the vibrational levels than for the 0~ (Au) state. Mere, F is the direct emission from the excited level and F is the indirect emission 4om 0% state which is gradually populated by the egiiieion induced vibrational relaxation from the excited level. F 1 is slightly quenched in lifetime and F is also guenched considerably in lifetime by the application of a weak magne%ic field as shown in
.1 Naka~nuraet al. / Magnetic field effect in glyoxal
Fig.
3.
765
The decaytime of F 1 is equal to the risetime of F2.
Case III.
Excitation at 435 nra —420
nm
We have three components in this case, F1, F2 and F3. F1 is again the direct emission from the excited level to the levels in the ground state and is quenched with a magnetic field in lifetime. It should be noted that the collision-free lifetime of F1 in this case is shorter than that for F1 in Case II. 12] F2 shows a complicated behavior, as shown in Fig. 4. The time-resolved spectrum for this component clearly showed that F2 is the indirect emission from the 0% state, the result being evidenced by the observation of rise in F2 (though overlapped with a weak F1 peak) . The effect of a magnetic field appears, however, mainly as the decrease in the initial intensity (I(H0)/I(H=0) = 0.5~-0.7(. On the contrary. quenching in lifetime is far smaller than in the cases I and II and the rate T(M~0)/r(H=0) does not necessarily depend on the sample pressure. F appears in Cases III and IV as a weak but long-lived emission. The lifetime F3 varies with the excitation energy and the observing wavelength. The behaviors of these three components are similar with those of intermediate case molecules such as methylglyoxal. 13]
102.
101,
!
~\H~o
t/ Fig.
3.
S
Time—profile of fluorescence)F2) in Case II. Glyoxal 500 mtorr Excitation at 440.3 nra (8~ ( Observation at 455.3 nra (0%) — . —. line indicate the rise— time of F2.
t/Ms Pig.
4.
Time—profile of fluorescence (F2) in Case III. Glyoxal 500 mtorr I Excitation at 422.7 nm )48o) Observational 455.3 nm (0% (. — . —. line indicates the decay of direct emission F which overlaps the indirec~ emission F2 with rise and decay.
Case IV. Excitation
at 420 nrn.—350 nm
A characteristic feature in Case IV is, as has been reported [4], the appearance of a very fast decay component, F0(fast fluorescence in Ref. 4). We show a typical result of Case IV in Fig.5. The time—resolved spectra are for the excitation at 415.1 nra where no fast decay had been observed. Spectrurn A with a relatively low pressure sample shows that the direct emission from 422.7 nm level, as well as from the excited level, to the ground state is predominant just after
.1 \~akan;uraet al. / Magnetic field effect in glroxal
766
the excitation. Spectra B and C, taken at njt.iaI and later parts of decay after the excitation, show that the excited level itself is not the origin of emission but the fast and slow decays observed arc composed of F 1 and F of Case III excited at 422.7 nm. These results imply that the energy in ~he excited level is redistributed in a very short time (~7 ns) to the underlying level of the same symmetry by collision.
A
I 480
I
460 ~
Fig.
5.
440
420
/nm
Time—resolved fluorescence
spectra.
Excitation at 415.1 ow C 2~8~) as indicated with an arrow. A; Glyoxal B; Glyoxal C; Glyoxal
50 mtorr. 0~’—200ns after the excitation. 500 mtorr. 0~200 ns after the excitation. 500 mtorr. 501e’-1300 ns after the excitation.
Glyoxal has been expected to behave as an intermediate case molecule by the excitation in its higher energy state because the number of vibrational levels in T1 state becomes large enough to cause the appreciable coupling between S1 and Il states. [4] The complicated and rapid changes in the time—profile and manner of magnetic quenching observed hero can not be interpreted in terms of the coupling with T1. Me have worked out a new mechanism which interprete the whole phenomena observed in this study. The mechanism includes a postulation of a second triplet state, presumably T2(K7t~) state, coupling with 5~ state below 435 ow and the energy transfers among the vibronic levels of S1, T1 and T2 states.
REFERENCES [1] Michel, C., and Tramer, A., Lifetimes of the state of glyoxal, Chem. Fhys. 42 (1979) 315. [2] Beyer, R.A. , Zittel, P.F., and Lineberger, W.C., Relaxation of qlyoxal, 0. Chew. Phys. 62 (1975) 4016. [3[ Hashimoto, K., Nakumura, J. , and Nagakura, S., Magnetic field effect in methylglyoxal, Chem. Phys. Left., 74 (1980) 228. [4) Van der Werf, R. , Schutten, K., and Kommandeur, 0., Fluorescence in glyoxal, Chem. Phys. 11 (1975) 281.
763
EXTERNAL MAGNETIC FIELD EFFECT ON THE FLUORESCENCE OF OLYOXAL J. Nakamura,
K. Mashimoto* and S. Nagekura*
The Inatitute of Physical and Chemical Keeearch Make, Saitama Japan
Fluorescence decay ccrves of glyoxal at low presscrea were analyzed by meaeuring external magnetic field effect on the single vibronic level time—resolved fluorescence spectra. The observed multi—component decays were found to be the overlaps of four emissions, F 0, F , F2 and F3, from different origins. Excitation energy Aependence of the msgnetic field effect on these four emissions suggested a mechanism of singlet— triplet energy transfers among three electronic excited states, S~ T1 end T2.
INTRODUCTION Olyoxal vapor at low pressures shows a fluorescence with a single—exponential decay when excited to the vibrationless lowest excited singlet state ~l ~ Accumlated spectroscopic knowledges indicate that this molecule is a prototype of “week coupling small molecule” case. [1] . The same molecule behaves, however, like an “intermediate case molecule” when excited with a light shorter than 455 nm; the fluorescence observed at the normal fluorescing region (e.g. at 482
nm) shows a multi—exponential
decay.
Ne have studied the external magnetic field effects on the SVL fluorescence of glyoxal, and found out the relations between the guenching patterns of the fluorescence decay under the existence of a magnetic field and the type of interactions among the singlet and triplet states.
EXPERIMENTAL Olyoxal monomer was obtained from dried polymer by the usual method. Neat glyoxal of 20 to 500 mtorr was filled in a pyrex sample cell of SO mrs i.d., cross— shaped with a photon trap. The sample pressures were monitored with a capacitor— type manometer(Saratren MRS 222A). The cell was set between the poles of a magnet which feeds a magnetic field of 700 to 7000 Gauss to the sample. Olyoxal was excited in the range of 455.3 to 350 nm with an N2 laser pumped dye laser (Molectron 24, D 14). Time—resolved spectra of fluorescence and phosphorescence and time—profiles of the individual single vibronic transitions were recorded with a single—photon counting system combined with a Soxcar integrator. Schematic arrangement of the apparatus is shown in Fig. 1.
RESULTS AND DISCUSSIONS The decay curves of fluorescence were found to become more and more deviated from the single—exponential decay with increasing energy of excitation. From the analysis of multi—component decays observed with and without a magnetic field, we *
Present address: The Inetitute
for Molecular Science, Okazaki,
0 022—2313/81 /0000—0000/S02.7 5 © North-Holland
Japan
J. Naka,nura ci ci / Magnc’tic field effect in gi;oxal
764
-u Magnetic
I
~ I, I
!~ig~t Pipe I _______
Ernie sian PM
Field I I
U
i I
counting
Fiql Schematic diagrawn of the apparatus used for the observation of cx— ternal magnetic field effect on the emissions of g1voxa1~
[N L. lDya L~.aa4 iLaSarl Trigger deduced at most four components of fluorescence. The time—resolved spectre for each of the components indicated further thet the multi-component decay usually observed at the normal region
3 10
with a low spectral resolution is a result of accidental overlap of independent series of fluorescences with essentially single— exponential decays. We designate these four types of fluores— cence as F , P , F 2Fand P in order of their 1ife~ime~ I 0 is ~he shortest) . According to the number of components in the decay curves, the whole region of ~l state is classified into four parts. The behaviors of quenching by the application of a magnetic field are also characteristic to the four regions. The results are summarized as follows. (As the effect saturates at a weak magnetic field, the strength of the field , denoted as M0 in the figures, was fixed to 3.5 kO.)
a 10 2\ 0
u
H
101
0
Excitation at 455.3 nm
)0% )
This case has been reported by many
6.4
b/MS Fig.2
Case I.
3.2
Time-profile of fluorescence Olyoxal 190 mtorr. Excitation at 455.3 nm (0% Observation at 452 nm
authors, yet an observed time—profile of fluorescence is shown in Pig.2 for the purpose of comparison. Main features of this ease arer 1) emission spectrum is composed of the direct transitions from the excited 0% state to the vobrational levels in the ground stete,S ~ 2) application of a magnetic field quenches the fluorescence in its lifetime bu~ not in the initial intensity, 3) the rate of guenehing, T)M/0)/T)H=0), is pressure dependent. 43S rigs Case II.been Excitation at 455 As has pointed out, [2) nm’— the excitation to a certain level in this region causes a bi—exponential decay as the self guenehing constant is bigger for the vibrational levels than for the 0~ (Au) state. Mere, F is the direct emission from the excited level and F is the indirect emission 4om 0% state which is gradually populated by the egiiieion induced vibrational relaxation from the excited level. F 1 is slightly quenched in lifetime and F is also guenched considerably in lifetime by the application of a weak magne%ic field as shown in
.1 Naka~nuraet al. / Magnetic field effect in glyoxal
Fig.
3.
765
The decaytime of F 1 is equal to the risetime of F2.
Case III.
Excitation at 435 nra —420
nm
We have three components in this case, F1, F2 and F3. F1 is again the direct emission from the excited level to the levels in the ground state and is quenched with a magnetic field in lifetime. It should be noted that the collision-free lifetime of F1 in this case is shorter than that for F1 in Case II. 12] F2 shows a complicated behavior, as shown in Fig. 4. The time-resolved spectrum for this component clearly showed that F2 is the indirect emission from the 0% state, the result being evidenced by the observation of rise in F2 (though overlapped with a weak F1 peak) . The effect of a magnetic field appears, however, mainly as the decrease in the initial intensity (I(H0)/I(H=0) = 0.5~-0.7(. On the contrary. quenching in lifetime is far smaller than in the cases I and II and the rate T(M~0)/r(H=0) does not necessarily depend on the sample pressure. F appears in Cases III and IV as a weak but long-lived emission. The lifetime F3 varies with the excitation energy and the observing wavelength. The behaviors of these three components are similar with those of intermediate case molecules such as methylglyoxal. 13]
102.
101,
!
~\H~o
t/ Fig.
3.
S
Time—profile of fluorescence)F2) in Case II. Glyoxal 500 mtorr Excitation at 440.3 nra (8~ ( Observation at 455.3 nra (0%) — . —. line indicate the rise— time of F2.
t/Ms Pig.
4.
Time—profile of fluorescence (F2) in Case III. Glyoxal 500 mtorr I Excitation at 422.7 nm )48o) Observational 455.3 nm (0% (. — . —. line indicates the decay of direct emission F which overlaps the indirec~ emission F2 with rise and decay.
Case IV. Excitation
at 420 nrn.—350 nm
A characteristic feature in Case IV is, as has been reported [4], the appearance of a very fast decay component, F0(fast fluorescence in Ref. 4). We show a typical result of Case IV in Fig.5. The time—resolved spectra are for the excitation at 415.1 nra where no fast decay had been observed. Spectrurn A with a relatively low pressure sample shows that the direct emission from 422.7 nm level, as well as from the excited level, to the ground state is predominant just after
.1 \~akan;uraet al. / Magnetic field effect in glroxal
766
the excitation. Spectra B and C, taken at njt.iaI and later parts of decay after the excitation, show that the excited level itself is not the origin of emission but the fast and slow decays observed arc composed of F 1 and F of Case III excited at 422.7 nm. These results imply that the energy in ~he excited level is redistributed in a very short time (~7 ns) to the underlying level of the same symmetry by collision.
A
I 480
I
460 ~
Fig.
5.
440
420
/nm
Time—resolved fluorescence
spectra.
Excitation at 415.1 ow C 2~8~) as indicated with an arrow. A; Glyoxal B; Glyoxal C; Glyoxal
50 mtorr. 0~’—200ns after the excitation. 500 mtorr. 0~200 ns after the excitation. 500 mtorr. 501e’-1300 ns after the excitation.
Glyoxal has been expected to behave as an intermediate case molecule by the excitation in its higher energy state because the number of vibrational levels in T1 state becomes large enough to cause the appreciable coupling between S1 and Il states. [4] The complicated and rapid changes in the time—profile and manner of magnetic quenching observed hero can not be interpreted in terms of the coupling with T1. Me have worked out a new mechanism which interprete the whole phenomena observed in this study. The mechanism includes a postulation of a second triplet state, presumably T2(K7t~) state, coupling with 5~ state below 435 ow and the energy transfers among the vibronic levels of S1, T1 and T2 states.
REFERENCES [1] Michel, C., and Tramer, A., Lifetimes of the state of glyoxal, Chem. Fhys. 42 (1979) 315. [2] Beyer, R.A. , Zittel, P.F., and Lineberger, W.C., Relaxation of qlyoxal, 0. Chew. Phys. 62 (1975) 4016. [3[ Hashimoto, K., Nakumura, J. , and Nagakura, S., Magnetic field effect in methylglyoxal, Chem. Phys. Left., 74 (1980) 228. [4) Van der Werf, R. , Schutten, K., and Kommandeur, 0., Fluorescence in glyoxal, Chem. Phys. 11 (1975) 281.