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TIME-RESOLVED EXCIMER FORMATION IN
PYRENE CRYSTALS
H. Port, R. Seyfang, H. Wolf
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C7, suppl6rnent au nOIO, Tome 46, octobre 1985 page C7-391
TIME-RESOLVED EXCIMER FORMATION I N PYRENE CRYSTALS
H. Port, R. Seyfang and H.C. Wolf
PhysikaZisches I n s t i t u t , TeiZ 3, Universitlit Stuttgart, PfaffenwaZdring 57, 0-7000 Stuttgart 80, F.R. G.
Abstract
-
Fluorescence transients of pyrene single crystalsare measured with picosecond time resolution (4 K
+
300 K). The results indicate a different excimer formation process in low and high temperature crystalline phase occuring in one and two steps, resp.I
-
INTRODUCTIONIn this paper the fast photophysical reaction of excimer formation in the organic solid state is investigated by time-resolved fluorescence spectroscopy. Excimers are formed from nearest neighbours upon electro- nic excitation of one partner molecule followed by rapid excited-state relaxation and configurational relaxation /I/.
Up to now little is known about the excimer dynamics in molecular solids in general. However, we have succeeded in applying picosecond spectroscopic techniques to the excimer problem in perylene and deve- loped a kinetic scheme for the two-step excimer formation in this crystal / 2 / .
Recently our investigations are extended to pyrene, which represents the classical example of an excimer forming crystal /3/ and has most thoroughly been analyzed in the literature /1,4,5/. In contrast to previous attempts by Kobayashi /6/ and many others the build-up of the excimer fluorescence in pyrene could be measured directly at the picosecond time scale.
A further attraction of the pyrene system is the occurence of a struc- tural phase transition at about 120 K. Both high and low temperature crystal structure are dimeric /7,8/, but the molecular overlap of nearest neighbour pairs is essentially different (Fig. 1). Since the excimer dynamics are expected to be sensitive upon the conformational changes in the molecular packing, a comparative study of the dynamics in both crystalline phases is performed (4 K t 300 K).
JOURNAL DE PHYSIQUE
Fig. 1 Molecular overlap of the sandwich pair molecules in the high
(left) and low (right) temperature crystalline phase. In the present experiments the temperature dependent rise and decay of the fluorescence are analyzed with picosecond time resolution using a modelocked Nd-glass laser at low excitation densities and a streak camera (for details see /9/).
I1
-
EXPERIMENTAL RESULTSThe most important new results obtained are:
First, the excirner rise time
TS
in pyrene is measurable in the whole temperature range between 4,2 K and 300 K. An example for the build-up of the excimer fluorescence after two photon picosecond-pulse excita-tion at 527 nm is given in Fig. 2 together with the simultaneously
measured reference pulse.
0 200 400
v p s
-
Fig. 2 Example for the build-up of the excimer fluores- cence (reference pulse for comparison)
r
The temperature dependence of T; ( T ) exhibits a pronounced disconti- nuity at the phase transition (Fig. 3). The rise time is constant in the low temperature crystalline phase (TE = 85
+
15 ps). It increases abruptly, within less than one degree, at the phase transition tempe- rature Tp by about a factor of three. Above the phase transition it decreases monotonically towards the room temperature value of about100 ps.
Second, a short-lived blue fluorescence component ( B ) is observed in the high temperature crystalline phase only.
The intensity of the B-fluorescence is extremely weak in comparison with the excimer emission. Therefore it shows up only in time-resolved spectra, superimposed at the high energy tail of the excimer fluores- cence. In Fig. 3 B-fluorescence spectra are shown measured for two different time intervals after ns-pulse excitation.
0 500 1 000
V p s
-
Fig. 4 Time-resolved (ns) spectra Fig. 5 ps-transients of the
at different time intervals; B-fluorescence, detec-
curves (a) 0.. .8ns, (b)ll..
.
ted at 400 nm, for4 3 ns. The fast blue fluores- various temperatures. cence component is observed
only at temperatures T > T
C7-394 JOURNAL DE PHYSIQUE
The B-fluorescence intensity diminuishes with increasing temperature. Examples for the B-fluorescence transients at picosecond time resolu- tion are depicted in Fig. 4 for various temperatures.
At T = 1 1 4 K, rise and decay of the B-fluorescence occur at comparable
time scale ( T ~ = 1 8 0
+
20 ps, T = 220+
20 ps). With increasing tem- perature clea@ly the rise time ?s getting considerably shorter (T"
35 ps at 190 K)
,
whereas the decay function at higher temperatureg is distorted by the superimposed slow excimer decay.I11
-
DISCUSSIONAs expected the dynamics of excimer formation in pyrene change at the phase transition, due to the conformational change in molecular pack- ing which causes different electron phonon coupling.
The primary conclusions as drawn from the present results are made plausible in the respective configurational diagrams for low and high temperature crystalline phase (Fig. 6).
Excimer Monomer
Configuration coord. q
Excimer B-State Monomer
Ground
Configuration coord. q F
Fig. 6 Configuration coordinate diagrams for the excimer formation
process in the low temperature (left) and high temperature (right) crystalline phase of pyrene.
In the low temperature phase the time constant for excimer build-up
7 6 is found not to depend on temperature. This means that excimer for-
mation in this case does not need thermal activation but rather occurs directly from the optically excited (monomer) state. The measured
T E
still might contain contributions of both internal conversion and lat-tice relaxation, corresponding to the overall rate k m for the trans-
r
I n t h e h i g h t e m p e r a t u r e p h a s e , o n t h e o t h e r h a n d , T E i s t e m p e r a t u r e d e p e n d e n t and t h e a d d i t i o n a l B - f l u o r e s c e n c e a p p e a r s . The t i m e s c a l e s f o r t h e d e c a y o f t h e B - f l u o r e s c e n c e and f o r t h e r i s e o f t h e e x c i m e r f l u o r e s c e n c e seem t o c o r r e s p o n d t o e a c h o t h e r , i n d i c a t i n g a c o u p l i n g between t h e s e two fluorescencecomponentsand t h e p o s s i b l e r o l e o f t h e B - s t a t e as p r e c u r s o r i n a t w o - s t e p e x c i m e r f o r m a t i o n p r o c e s s ( F i g . 61, s i m i l a r l y a s i n p e r y l e n e / 2 / . The t r a n s i t i o n from B- t o E - s t a t e re-
q u i r e s t h e r m a l a c t i v a t i o n . I n c o n t r a s t t o t h e p e r y l e n e case / 2 / , how- e v e r , it c a n n o t b e f r o z e n i n , d u e t o t h e l i m i t e d t e m p e r a t u r e r a n g e T > T where t h e B - s t a t e and t h e h i g h t e m p e r a t u r e p h a s e e x i s t . P REFERENCES / 1 / J.B. B i r k s , P h o t o p h y s i c s o f Aromatic M o l e c u l e s , Wiley ( 1 9 7 0 ) . / 2 / H. P o r t , B. Walker a n d H. C . Wolf, J . Lumin. 31/32 (1984) 780.
B. Walker, H. P o r t and H. C . Wolf, Chem. Phys.
92
(1985) 177./ 3 / J . F e r g u s o n ,
J. Chem. Phys.
28
(1958) 765. / 4 / Th. F o r s t e r ,Angew. Chemie I n t . Ed. 8 (1969) 333. / 5 / V. Yakhot, 2. Ludmer, ,M Cohen,
