Magnetic field effect on the fluorescence of m-phenylenebis-(phenylmethylene) in a rigid glass at 77 K

Article abstract of DOI:10.1246/bcsj.74.2325

The magnetic field effect (≤ 0.59 T) on the quintetquintet fluorescence of m-phenylenebis(phenylmethylene) (MPBP) was studied in a rigid glass, decaline-cyclohexane (3: 1) mixture, at 77K. The fluorescence decay rate of MPBP increases in a magnetic field. The effect on the MPBP fluorescence is explained in terms of the Zeeman mixing of excited quintet sublevels of MPBP.

Full text of DOI:10.1246/bcsj.74.2325

Bull. Chem.
Soc. Jpn.
74
12
Bull. Chem. Soc. Jpn., 74, 2325–2326 (2001) 2325
The Chemical
Society of Ja-
pan
Short Articles
The Chemical
Society of Ja-
pan
excimer laser (Lumonics, EX-500, 308 nm) as an exciting light
source and a polychromator (Hamamatsu, C5094)-streak camera
system (Hamamatsu, C4334) as a detection system.4 A xenon arc
lamp with a bandpass filter (300–400 nm) or the 308-nm light
from the laser was used for photolysis light. Magnetic fields were
applied by an electromagnet (TOKIN, SEE-9), whose residual
field was canceled to < 0.3 mT by applying a dc current.
Magnetic Field Effect on the
Fluorescence of m-Phenylenebis-
(phenylmethylene) in a Rigid Glass
at 77 K
12
15
2001
2325
2326
BCSJA8
0009-2673
K01004
3
16
2001
8
Y. Tanimoto et al.
Results and Discussion
Yoshifumi Tanimoto,* Yukimi Akimoto,
Yoshihisa Fujiwara, Masahiro Mukai,
Takeji Takui,† Takamasa Kinoshita,†
and Koichi Itoh†
Photoirradiation of BDB in a DCH rigid glass at 77 K leads
to a reaction intermediate whose absorption band appears at
300–320 nm. Figure 1a shows time-resolved fluorescence
spectra of BDB in the glass after photoirradiation of a 308-nm
light. The spectra are composed of a short-lived band at 400–
450 nm and a relatively long-lived band at 450–600 nm. The
lifetime of the former is about 9 ns, whereas the decay curves
of the latter band are nonexponential, as will be described later.
The steady-state fluorescence spectrum obtained by integration
of the time-resolved spectra is also shown in the inset of Fig.
la. A shoulder band at 470 nm observed in the time-resolved
spectrum at a 9 ns delay is not apparent in the steady-state
spectrum because of its short lifetime.
Graduate School of Science, Hiroshima University,
Higashi-Hiroshima 739-8526
27
2001
†Graduate School of Science, Osaka City University,
Sumiyoshi, Osaka 558-8585
1.1.5
1.5.8
(Received March 16, 2001)
Optical absorption and luminescence spectra of quintet
MPBP generated from BDB have been studied in detail in a
DCH rigid glass at 77 K.5,6 Two diazo groups in BDB are
cleaved by one photon, yielding MPBP as a major reaction in-
termediate. MPBP has an intense absorption band at ca. 316
nm. By correlating EPR and steady-state fluorescence intensi-
ties, the fluorescence band peaking at ca. 485 nm with a life-
The magnetic field effect (ꢀ 0.59 T) on the quintet–
quintet fluorescence of m-phenylenebis(phenylmethylene)
(MPBP) was studied in a rigid glass, decaline–cyclohexane
(3:1) mixture, at 77K. The fluorescence decay rate of MPBP
increases in a magnetic field. The effect on the MPBP fluo-
rescence is explained in terms of the Zeeman mixing of excit-
ed quintet sublevels of MPBP.
The magnetic field effects (MFEs) on fluorescence of triplet
carbenes have been studied in detail by Migirdicyan and her
collaborators.1 The lifetime of triplet-triplet fluorescence of
diphenylmethylene (DPM) in a hexane rigid matrix at 4.2–30
K decreases by application of a magnetic field (< 0.1 T). The
effects are interpreted in terms of the Zeeman mixing of “excit-
ed triplet” sublevels. Therefore, it is interesting to know how a
magnetic field affects the fluorescence decay of a reactive in-
termediate with higher spin multiplicity.
In this paper, the MFE on the fluorescence of m-phenyl-
enebis(phenylmethylene) (MPBP) was examined in a rigid
glass at 77 K by laser-induced fluorescence. Magnetic field
dependence (MFD) of the fluorescence decay in MPBP is very
similar to that in DPM. The results are interpreted in terms of
the Zeeman mixing of the excited quintet sublevels in MPBP.
Experimental
Materials: 1,3-Bis(α-diazobenzyl)benzene (BDB) and di-
phenyldiazomethane (DDM) were synthesized in the manners
similar to those described elsewhere.2,3 The purest grades of deca-
line and cyclohexane (Nacalai Tesque, Inc.) were used as supplied
and were mixed in the volume ratio of 3:1 to prepare a decaline-
Fig. 1. (a) Time-resolved fluorescence spectra of DBD in an
MCH rigid glass at 77 K after photoirradiation of a 308 nm
light. Inset: Steady-state fluorescence spectra obtained by
integration of the time-resolved spectra in the absence and
presence of a magnetic field of 0.59 T. (b) Magnetic field
effect on the fluorescence decay curve observed at 480–
540 nm in the absence and presence of a magnetic field of
0.59 T.
cyclohexane mixed solvent (DCH). All sample solutions (10−4
–
10−5 mol dm−3) were deaerated by several freeze–pump–thaw cy-
cles. Spectroscopic measurements were carried out at 77 K by im-
mersing sample cells in liquid nitrogen.
The laser-induced fluorescence was measured using an XeCl

2326 Bull. Chem. Soc. Jpn., 74, No. 12 (2001)
© 2001 The Chemical Society of Japan
time of ca. 150 ns is assigned to the excited quintet state of
MPBP. By comparison with these reported optical data of
MPBP, the fluorescence spectrum peaking at ca. 490 nm in
Fig. 1a is assigned to the quintet-quintet fluorescence of
MPBP. The short-lived fluorescence appearing around 470 nm
in the time-resolved spectra cannot be assigned, since the tran-
sient spectra with higher time-resolution are not obtained due
to the pulse width of the laser (ca. 10–15 ns). On the other
hand, the fluorescence band at 400–450 nm would be ascribed
to a stable photoproduct.
The MFE on the steady-state fluorescence spectrum of
MPBP is shown in the inset of Fig. la. The intensity in the
450–600 nm region decreases by about 30% by application of
a magnetic field of 0.59 T, whereas the intensity of the 400–
450 nm band is magnetic-field insensitive. Figure 1b shows
the MFE on the fluorescence decay of MPBP at 77 K. Here,
the fluorescence intensities in the 480–550 nm region are ana-
lyzed in order to minimize contamination of the unknown fluo-
rescence around 470 nm. The decay rate increases in a mag-
netic field of 0.59 T. Decay curves are nonexponential regard-
less of magnetic field and are analyzed tentatively by assuming
a double exponential decay.
0.59 T, whereas the values of τf, If and Is and are about 17 ns,
0.4, and 0.6, respectively, and are almost magnetic-field inde-
pendent.
For the purpose of comparison, the MFE on the fluorescence
decay of DPM generated from DDM is also examined in a
DCH rigid glass at 77 K. The steady-state fluorescence spec-
trum of triplet DPM peaking at ca. 485 nm is similar to that of
quintet MPBP depicted in the inset of Fig. 1a. Furthermore, its
fluorescence decay curves are also nonexponential and mag-
netic-field dependent. The curves are analyzed using Eq. 1, by
analogy with the analysis of the MPBP fluorescence. The val-
ues of τf, If and Is are about 25 ns, 0.4, and 0.6, respectively, at
zero field and are almost magnetic-field independent. Figure 2
shows the MFD of the lifetime τs of DPM. It decreases from
115 ns to 79 ns with increasing the magnetic field from zero to
0.59 T.
The fluorescence of MPBP corresponds to the spin-allowed
transition between excited and ground quintet states. Since the
radiative transition is independent of spin sublevels, the radia-
tive decay rate constants from the five spin sublevels are equal.
On the other hand, each sublevel may have a different nonradi-
ative decay rate constant. This is because it may have different
symmetry and it may be coupled by spin-orbit coupling to dif-
ferent triplet and/or singlet states. In a low-temperature rigid
glass, the rates of spin lattice relaxation among quintet sublev-
els are not fast compared to the decay rate of each quintet sub-
level. Each sublevel decays with each own decay rate constant
at zero field. By application of a magnetic field, all five quintet
sublevels are mixed by the Zeeman interaction and, therefore,
the decay rates change by application of a magnetic field, as
discussed in the case of excited triplet DPM.1
I (t) = If exp (−t/τf) + Is exp (−t/τs)
(1)
where τf and τs are the lifetimes of the fast and slow decay
components, respectively, If and Is being the respective pre-ex-
ponential factors. Errors involved in the present analysis are
about 10%. The lifetime τs of the slow decay component is
122 ns at zero field and is dependent on the magnetic field.
The values of τf, If and Is are 14 ns, 0.6, and 0.4, respectively, at
zero field and are almost magnetic-field independent. Figure 2
shows the MFD of τs. The lifetime τs decreases with increas-
ing magnetic field and is 82 ns at 0.59 T.
The MFD of the fluorescence decay of MPBP in a rigid
glass of diethyl ether–ethyl alcohol–pentane (5:5:1) mixture
(EPA) at 77 K is very similar to that in a DCH rigid glass at 77
K shown in Fig. 1b. At zero field, τs is 139 ns and this decreas-
es down to 86 ns with increasing magnetic field from zero to
The lifetime and MFD of excited quintet MPBP are similar
to those of excited triplet DPM. This fact may be understand-
able if the excitation is concerned dominantly with the molecu-
lar orbitals localized to the one divalent carbon.
We thank Prof. T. Azumi at Tohoku University, and Prof. M.
Terazima at Kyoto University, for their valuable discussions.
This work was supported partly by a Grant-in-Aid for Scientif-
ic Research from the Ministry of Education, Science, Sports
and Culture.
References
1
A. Despres, V. Lejeune, E. Migirdicyan, A. Admasu, M. S.
Platz, G. Berthier, O. Parisel, J. P. Flament, I. Baraldi, and F.
Momicchioli, J. Phys. Chem., 97, 13358 (1993), and references
therein.
2
3
J. B. Miller, J. Org. Chem., 24, 560 (1959).
Y. Teki, T. Takui, K. Itoh, H. Iwamura, and K. Kobayashi,
J. Am. Chem. Soc., 108, 2147 (1986).
Y. Fujiwara, M. Mukai, and Y. Tanimoto, Trans. IEE. Jpn.
A, 116, 419 (1996).
4
5
K. Itoh, H. Konishi, and N. Mataga, J. Chem. Phys., 48,
4789 (1968).
6
K. Itoh, T. Takui, and Y. Teki, Abstract of the 46th Annual
Meeting of the Chemical Society of Japan, October, Niigata,
Japan (1982), p.17.
Fig. 2. Magnetic field dependence of the fluorescence life-
time of MPBP (ꢀ) and DPM (ꢁ).