2P027

Electrophotoluminescence spectra of pyrene solution(Research Institute for Electronic Science, Hokkaido Univ.) ○Hung-Chu Chiang, Nobuhiro
Ohta[Introduction]
Electrophotoluminescence (E-PL) spectroscopy, which gives information about external
electric field effects on photoluminescence (PL) spectra, is a useful method to examine thechange in dipole moment and polarizability following electronic transitions and the electricfield effects on excitation dynamics. In fact, E-PL measurements have been done for pyrene
doped in a PMMA film, and the Stark shift and the electric field effects on excitationdynamics have been discussed [1-3].
In the present study, we have measured E-PL spectra of pyrene solution with a constructedmeasurement system. Field-induced enhancement of PL intensity and field-induced spectral
shift of excimer fluorescence have been observed. On the basis of the E-PL measurements, themagnitude of the change in polarizability following emission process of pyrene excimer has
been evaluated, and the field effect on excimer formation process is discussed.
[Experimental Section]
A benzene solution of pyrene was mixed with decaline by supersonic wave. This mixturewas flowed in a home-made E-PL measurement cell. E-PL as well as PL spectra weremeasured using an electric-field modulation spectroscopy [4]. Sinusoidal voltage having afrequency of 4 kHz was applied, and lock-in amplifier was used to detect the E-PL signal
synchronized with the modulated applied voltage.
[Results]
The observed E-PL and PL spectra of
pyrene solution at a concentration of 5 mM
are shown in Fig. 1. The spectra wereobtained with a field strength of 0.2 MV/cm
with photoexcitation at 325 nm, where thefield-induced change in absorbance wasnegligibly small.
The PL spectrum of pyrene solution at 5mM contains two emissions; monomer

Fig. 1. Emission spectrum and E-PL
spectrum of pyrene solution.

fluorescence emitted from the locally excited state of pyrene and excimer fluorescence. E-PL
spectra of pyrene solution show that both monomer and excimer fluorescence are enhanced byapplied electric field, but the field-induced change in quantum yield is different from eachother. Besides the change in intensity, a red-shift of the excimer fluorescence spectrum wasobserved with application of electric fields.
E-PL spectra can be expressed as a sum of the zeroth, first and second derivatives of theemission spectra [5]. The zeroth derivative component is related to the field-induced change
in excitation dynamics, and the first and second derivatives are related to the change inpolarizability and dipole moment following the emission transition, respectively. As shown inFig. 2, E-PL spectrum of pyrene solution
could be simulated by a linear combination of
the zeroth derivative of monomer andexcimer fluorescence spectra and the first
derivative of the excimer fluorescence
spectrum. From the first derivative
component of excimer fluorescence, thevalue of the difference in polarizability
between the emitting state of excimer and theground state was evaluated to be ~250 Å3.
The quantum yield of monomer
fluorescence relative to the excimer
fluorescence can be expressed with a kinetic
model under photostationary conditions. The
observed E-PL spectra show that theabove-mentioned relative quantum yieldchanges in the presence of electric field,
indicating that the relaxation rate of excimer
and/or excimer formation rate is affected byexternal electric fields. The details will be
discussed in the presentation.

Fig. 2. (a).Fluorescence spectrum of pyrene
solution 5 mM. (b) The first derivative
spectra of fluorescence bands. (c) E-PL
spectrum and the simulation.

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