- Email: [email protected]
Photoionization and magnetic field modulated recombination in crystalline anthracene
Journalof Luminescence 24/25 (1981) 99—102 North-Holland Publishing Company
99
PHOTOIONIZATION AND MAGNETIC FIELD MODULATED RECOMBINATIONJ IN CRYSTALLINE ANTHRACENE
J. Klein, P. Martin,
R. Voltz
Centre de Recherches Nucléaires et Université Louis Pasteur Laboratoire PREN 67037 Strasbourg Cedex France
A nanosecond time resolved analysis of the magnetic field dependence of the fluorescence excited in crystalline an— thracene by far ultraviolet light pulses (122 nm) is presented. Besides the fine structure modulated components attributed to delayed fluorescence from triplet—triplet exciton annihilation and to singlet fission in triplets, a new component with a magnetic field dependence characteristic of a hyperfine structure modulation is detected. It is related to photoioniza— lion and electron—hole recombination in the crystal.
In the studies of electronic properties of molecular crystals, anthracene is usually considered as a model system. Thus any new information on the elementary processes in crystalline anthracene is of great interest. In former work, the modulation of photoexcited fluorescence by a magnetic field was used to detect and to analyse fission of a singlet exciton in two triplets [11 and the reverse reaction of fusion of triplet exci tons [2]. Here a new magnetic field sensitive process, related to photoionization and ionic recombination in the crystal, has been detected. EXPERIMENTAL CONDITIONS AND RESULTS A Bridgmann grown crystal from zone refined anthracene is placed between the pole pieces of an electromagnet so that the magnetic field lies in the a~plane of the crystal. Ultraviolet light flashes of 122 nm and 2 na duration, delivered by a hydrogen discharge lamp, filtered by a Huger and Watts vacuum grating mono— chromator, are incident perpendicularly on the ~ plane and create highly excited singlet states. The time resolved fluorescence intensity is analysed in a nanosecond domain up to 1,5 ~cs by a single photon counting sampling technique in the presence and the absence of magnetic field between Oand 0.3 Tesla [31. For each time, the influence of the magnetic field strength B on the intensity I is represented by the relative variation MI! ( 1B — l°)/I° , where 1B and j0 are the quantities measured with and without field respectively. Typical curves are presented in Figure 1. DISCUSSION The experimental data in Figure 1 are best discussed by considering three distinct time regions (1) small times (t < 100 os) where M/I is positive (2) intermediate times (100 ns < 1< 600 ns) where the values of tsl/l are strongly negative (3) long times (t > 600 ns), where the modulation is negative but relatively small.
0 022—23 13/81 /0000—0000/$02.75 © North-Holland
100
J, Ale/n
Ct
a!.
/ P/into/on j:at(on in en
italfjne anthracencc
The field dependence of hI/I in these time domains is analysed in Figure 2. The regions (I) and (~)present a typical fine structure modulation behaviour [1—21 the relative enhancement of the fluorescence intensity at small times is attributed to a decrease of the singlet fission efficiency [11 the reduction of intensity at long time is due to the decrease of the delayed fluorescence by triplet—triplet fusion [21. These triplet excitons are formed by various relaxation processes from the initially activated stnglets like autoionization followed by electron—hole recombination with spin flip, intersystem crossing, .. . This identification of the processes responsible for the luminescence modulation in regions (1) and (3) is confirmed in Figure 3 where the curves describing the anisotropy of the magnetic field effect in the plane ab are found to be similar to those obtained for singlet fission and triplet fusion iTfanthracene 1, 21. The most interesting results concern the intermediate time domain (2). According to the figures 2 and 3 the characteristic fine structure variations of fluorescence with field intensity and direction in the crystal vanish ; the data in Figure 2 rather indicate a hyperfine structure modulation effect [4]. This suggests that intermediate electron—hole pairs are involved in photoexcitation of anthracene fluorescence by 122nm light. The processes detected in the experiments involve photoionization from the initially excited singlet states, spin and spatial evolution of electron—hole pairs and final recombination in the fluorescing singlets [3, 5, 61. From the analysis of motion in the spin subspace, the modulation of recombination fluorescence by the relatively low magnetic fields is known to arise from Zeeman decoupling of the internal hyperfine interactions in the radical ion pairs. Negative values of hi/I imply an initial spin polarization of the ion pair with triplet character, rather than singlet as previously found in systems excited by high energy radiation (~rays) [3,5] assuming, for simplicity, a pure triplet ion pair state, one obtains from Eqn. 6 in reference [51 hI/I
=
—
[2(N)
~ cos F.~texp(— 2Ft) + exp (—4 ~t)]/3
where N is the total number of nuclear spin states in the ion pair (N
(1) =
22m m = number of molecular nuclear spin states) Fn represents the frequency of coherent oscillation between the degenerate 5, n > and T0, n > states and F the spin—lattice relaxation decay rate. Due to the large number of cosine terms the time interference part in expression (1) usually cancels in the time interval explored [6, 7] so only the second exponentially decaying term needs to be retained for the interpretation of the time curves in Figure 1 and, in particular, of the decay of hi/I in the long time region of domain (2). The origin of the measured initial triplet spin polarization of the ion pairs is best explained by a mechanism already invoked in photoionization studies with crystalline terphenyl [81. For the excitation energy in our experiments, ionization from the initially highly excited singlets is followed by excitation of a triplet exciton during the slowing down of the hot electron e*, with conservation of the total spin 1[3(e, h) h)
,
T 1]
3(e, place h) states as stated before. Since the sequence of these processes takes on subnanosecorsd time scales, we may assume an impulsive creation of It should be noted that, at higher excitation energies, the creation of singlet states may be expected by a similar process h)
1[1(e,
h)
J. Klein et al. / P/iotoionization in cristal/ine anthrac’ence
10 I
~2OO
400
6@Ot(ns)
‘a
~
a’
5
FIGURE CAPTIONS
Fig. 0
lOt
1
-..~°>
I
I
3
Time dependence of relative fluorescence intensity Al/I, excited by 122 nm light pulses in crystalline anthracene. Magnetic field strength (a) B0.3Tesla (b) B= 0.01 Tesla
I
-5
I
fig.3 —
Fig. 2
Magnetic field strength dependence of relative fluorescence intensity Al/I for the three time domains defined in Figure 1
I
-20
I
-30
2
I
I
Fig. 3
Angular dependence of relative fluorescence intensity Al/I for the three time domains defined in Figure 1, the field B being in the ab plan.
102
1, Ale/it et a!
l’hotou)ili:atioIi
iii
in sta/line unh/iracence
but the threshold is higher since the energy of the singlet exciton is 3. 12 eV as compared to 1.83 eV for the T 1 state. Under very high excitation energy conditions however, a mixture of triplet and singlet pair states should be initially pre~ pared, so that an initial triplet state polarization is unlikely : this agrees with the experimental observations in the case of s-particle excitation of anthracene, where no hyperfine modulation effects have been mentioned r 81. REFERENCES [1] Klein, G. , Voltz, P., Schott, M. , Chem. Phys. Lett. 16(1972) 840 [2] Johnson, R. L., Merrifield, R. E. , Phys. Rev. B 1 (1970) 896 Merrifield, R. E., J. Chem. Phys. 48 (1968) 4318 [31 Klein, J. , Voltz, R. , Can. J. Chem. 55 (1977) 2102 [4] Ayscough, P.B., Electron Spin Resonance in Chemistry (Methven, London 1968) Sect. 8. 16. 1 [51 Klein, J. , Voltz, R. , Phys. Rev. Lett. 36 (1976) 1214 [6 Brocklehurst, B. , Chem. Phys. Lett. 28 (1974) 357 [71 Werner, H.J., Schulten, Z., Schulten, K., J. Chem. Phys. 67(1977) 646 ~81 Klein, G., Carvaiho, M. J. , Chem. Phys. Lett. 51 (1977) 409 [91 Klein, G., Mol. Cryst. Liq. Cryst. 44(1978) 125 and 47 (1978) 39
99
PHOTOIONIZATION AND MAGNETIC FIELD MODULATED RECOMBINATIONJ IN CRYSTALLINE ANTHRACENE
J. Klein, P. Martin,
R. Voltz
Centre de Recherches Nucléaires et Université Louis Pasteur Laboratoire PREN 67037 Strasbourg Cedex France
A nanosecond time resolved analysis of the magnetic field dependence of the fluorescence excited in crystalline an— thracene by far ultraviolet light pulses (122 nm) is presented. Besides the fine structure modulated components attributed to delayed fluorescence from triplet—triplet exciton annihilation and to singlet fission in triplets, a new component with a magnetic field dependence characteristic of a hyperfine structure modulation is detected. It is related to photoioniza— lion and electron—hole recombination in the crystal.
In the studies of electronic properties of molecular crystals, anthracene is usually considered as a model system. Thus any new information on the elementary processes in crystalline anthracene is of great interest. In former work, the modulation of photoexcited fluorescence by a magnetic field was used to detect and to analyse fission of a singlet exciton in two triplets [11 and the reverse reaction of fusion of triplet exci tons [2]. Here a new magnetic field sensitive process, related to photoionization and ionic recombination in the crystal, has been detected. EXPERIMENTAL CONDITIONS AND RESULTS A Bridgmann grown crystal from zone refined anthracene is placed between the pole pieces of an electromagnet so that the magnetic field lies in the a~plane of the crystal. Ultraviolet light flashes of 122 nm and 2 na duration, delivered by a hydrogen discharge lamp, filtered by a Huger and Watts vacuum grating mono— chromator, are incident perpendicularly on the ~ plane and create highly excited singlet states. The time resolved fluorescence intensity is analysed in a nanosecond domain up to 1,5 ~cs by a single photon counting sampling technique in the presence and the absence of magnetic field between Oand 0.3 Tesla [31. For each time, the influence of the magnetic field strength B on the intensity I is represented by the relative variation MI! ( 1B — l°)/I° , where 1B and j0 are the quantities measured with and without field respectively. Typical curves are presented in Figure 1. DISCUSSION The experimental data in Figure 1 are best discussed by considering three distinct time regions (1) small times (t < 100 os) where M/I is positive (2) intermediate times (100 ns < 1< 600 ns) where the values of tsl/l are strongly negative (3) long times (t > 600 ns), where the modulation is negative but relatively small.
0 022—23 13/81 /0000—0000/$02.75 © North-Holland
100
J, Ale/n
Ct
a!.
/ P/into/on j:at(on in en
italfjne anthracencc
The field dependence of hI/I in these time domains is analysed in Figure 2. The regions (I) and (~)present a typical fine structure modulation behaviour [1—21 the relative enhancement of the fluorescence intensity at small times is attributed to a decrease of the singlet fission efficiency [11 the reduction of intensity at long time is due to the decrease of the delayed fluorescence by triplet—triplet fusion [21. These triplet excitons are formed by various relaxation processes from the initially activated stnglets like autoionization followed by electron—hole recombination with spin flip, intersystem crossing, .. . This identification of the processes responsible for the luminescence modulation in regions (1) and (3) is confirmed in Figure 3 where the curves describing the anisotropy of the magnetic field effect in the plane ab are found to be similar to those obtained for singlet fission and triplet fusion iTfanthracene 1, 21. The most interesting results concern the intermediate time domain (2). According to the figures 2 and 3 the characteristic fine structure variations of fluorescence with field intensity and direction in the crystal vanish ; the data in Figure 2 rather indicate a hyperfine structure modulation effect [4]. This suggests that intermediate electron—hole pairs are involved in photoexcitation of anthracene fluorescence by 122nm light. The processes detected in the experiments involve photoionization from the initially excited singlet states, spin and spatial evolution of electron—hole pairs and final recombination in the fluorescing singlets [3, 5, 61. From the analysis of motion in the spin subspace, the modulation of recombination fluorescence by the relatively low magnetic fields is known to arise from Zeeman decoupling of the internal hyperfine interactions in the radical ion pairs. Negative values of hi/I imply an initial spin polarization of the ion pair with triplet character, rather than singlet as previously found in systems excited by high energy radiation (~rays) [3,5] assuming, for simplicity, a pure triplet ion pair state, one obtains from Eqn. 6 in reference [51 hI/I
=
—
[2(N)
~ cos F.~texp(— 2Ft) + exp (—4 ~t)]/3
where N is the total number of nuclear spin states in the ion pair (N
(1) =
22m m = number of molecular nuclear spin states) Fn represents the frequency of coherent oscillation between the degenerate 5, n > and T0, n > states and F the spin—lattice relaxation decay rate. Due to the large number of cosine terms the time interference part in expression (1) usually cancels in the time interval explored [6, 7] so only the second exponentially decaying term needs to be retained for the interpretation of the time curves in Figure 1 and, in particular, of the decay of hi/I in the long time region of domain (2). The origin of the measured initial triplet spin polarization of the ion pairs is best explained by a mechanism already invoked in photoionization studies with crystalline terphenyl [81. For the excitation energy in our experiments, ionization from the initially highly excited singlets is followed by excitation of a triplet exciton during the slowing down of the hot electron e*, with conservation of the total spin 1[3(e, h) h)
,
T 1]
3(e, place h) states as stated before. Since the sequence of these processes takes on subnanosecorsd time scales, we may assume an impulsive creation of It should be noted that, at higher excitation energies, the creation of singlet states may be expected by a similar process h)
1[1(e,
h)
J. Klein et al. / P/iotoionization in cristal/ine anthrac’ence
10 I
~2OO
400
6@Ot(ns)
‘a
~
a’
5
FIGURE CAPTIONS
Fig. 0
lOt
1
-..~°>
I
I
3
Time dependence of relative fluorescence intensity Al/I, excited by 122 nm light pulses in crystalline anthracene. Magnetic field strength (a) B0.3Tesla (b) B= 0.01 Tesla
I
-5
I
fig.3 —
Fig. 2
Magnetic field strength dependence of relative fluorescence intensity Al/I for the three time domains defined in Figure 1
I
-20
I
-30
2
I
I
Fig. 3
Angular dependence of relative fluorescence intensity Al/I for the three time domains defined in Figure 1, the field B being in the ab plan.
102
1, Ale/it et a!
l’hotou)ili:atioIi
iii
in sta/line unh/iracence
but the threshold is higher since the energy of the singlet exciton is 3. 12 eV as compared to 1.83 eV for the T 1 state. Under very high excitation energy conditions however, a mixture of triplet and singlet pair states should be initially pre~ pared, so that an initial triplet state polarization is unlikely : this agrees with the experimental observations in the case of s-particle excitation of anthracene, where no hyperfine modulation effects have been mentioned r 81. REFERENCES [1] Klein, G. , Voltz, P., Schott, M. , Chem. Phys. Lett. 16(1972) 840 [2] Johnson, R. L., Merrifield, R. E. , Phys. Rev. B 1 (1970) 896 Merrifield, R. E., J. Chem. Phys. 48 (1968) 4318 [31 Klein, J. , Voltz, R. , Can. J. Chem. 55 (1977) 2102 [4] Ayscough, P.B., Electron Spin Resonance in Chemistry (Methven, London 1968) Sect. 8. 16. 1 [51 Klein, J. , Voltz, R. , Phys. Rev. Lett. 36 (1976) 1214 [6 Brocklehurst, B. , Chem. Phys. Lett. 28 (1974) 357 [71 Werner, H.J., Schulten, Z., Schulten, K., J. Chem. Phys. 67(1977) 646 ~81 Klein, G., Carvaiho, M. J. , Chem. Phys. Lett. 51 (1977) 409 [91 Klein, G., Mol. Cryst. Liq. Cryst. 44(1978) 125 and 47 (1978) 39