4P074

Electric field effects on fluorescence of methylene linked electron
donor and acceptor compounds in solution(Research Institute for Electronic Science, Hokkaido Univ.) ○Hung-Chu Chiang, Nobuhiro
Ohta[Introduction]
External electric field effects on photochemical processes such as electron transfer havebeen long interested. In order to elucidate the field effects on photo-induced dynamics, themeasurements of the field-induced change in emission spectra are essential. Such an approach
is known as electrophotoluminescence (E-PL) spectroscopy, where field-induced change inphotoluminescence (PL) spectra is measured. When electron transfer process which competes
with emission processes is affected by application of an electric field, for example, thequantum yield of fluorescence is affected. From the measurements of E-PL spectra, we canalso obtain information of the electric dipole moment (μ) at the emitting state and the change
in dipole moment (Δμ) and in polarizability (Δ α ) following electronic transitions. Therefore,
E-PL measurements give the information not only about the field effect on excitation
dynamics but also about the electronic structure.
In the present study, we have measured polarized E-PL spectra of the series of methylene
linked compounds of (carbazole)-(CH2)n-(terephthalic acid methyl ester) (n=2-20) in solution,
where photo-induced electron transfer occurs from the excited state of carbazole toterephthalic acid methyl ester. Hereafter these compounds are denoted as C-(n)-P, where C, nand P indicating carbazole moiety, the number of methylene units, and terephthalic acidmethyl ester moiety, respectively. The significant field-induced change of intensity both on
fluorescence emitted from the locally excited state of donor carbazole, hereafter noted asmonomer fluorescence, and on exciplex fluorescence was observed. The experimental results
are explained in terms of electric field effects both on photo-induced electron transfer process
and relaxation process of fluorescent exciplex. From the analysis of the polarized E-PL
spectra, the electric dipole moment of exciplex has also been evaluated.
[Experimental Section]
A decalin solution of C-(n)-P with a concentration of 5 x 10-4 M was used. E-PL as well as
PL spectra were measured using an electric-field modulation spectroscopy. Sinusoidal voltage
having a frequency of 4 kHz was applied, and lock-in amplifier was used to detect the E-PL
signal synchronized with the modulated applied voltage.

[Results]
E-PL spectra of C-(n)-P in solution as well as PL spectra are shown in Fig. 1. The E-PL
spectra were obtained with χ = 90o and 55o. Note that χ represents the angle between the field
direction and the polarization direction of the emission light. The applied field strength was
0.2 MVcm-1 and the excitation wavelength was 292 nm (34246 cm-1), where the field-inducedchange in absorbance is negligibly small. The field-induced change in fluorescence quantum
yield of both monomer and exciplex emissions was observed. The E-PL spectra could be
reproduced by a linear combination of the zeroth and first derivatives of monomer andexciplex fluorescence spectra, as shown in Fig.1. Based on the E-PL results, the electric
dipole moment of the fluorescent exciplex is estimated to be ~19 D in any n, and the electric
dipole moment of the fluorescent monomer is estimated to be ~6 D in n=10, 20. The D-A
distance dependence of the intramolecular electron transfer process is discussed, based on thefield-induced enhancement and quenching of monomer fluorescence.

Fig .1 PL spectra and the decomposition bands (left), and E-PL spectra (right) of C-(n)-P observed with
χ = 90o (dotted line) and 55o (shaded line). The simulated E-PL spectra are also shown by solid lines.