Volume thirty-four, numbered
3
Chemical Physics Letters
August 1, 1975
M. YOKOYAMA, Y. END-0 και H. MIKAWA Deporrnenr s App!ied Cl~enzisrry, Faculty of Engirweting Quenching of Excited Fluorescence by a Solid State Electric Field
Osaka
My school.
an apartment,
Omoka. 56.5 pages
Received 197.5 April 29
The KcipLCx fluorescence of dimethyl terephthalate-doped poly(N-thiylourbazole) films was efficiently quenched by applying an electric field. This may be a consequence of field-assisted thermal dissociation into free carriers. The decrease in exciplex fluorescence showed a quadratic dependence on field strength. This result, which shows emission due to exciton-doped molecular interactions, not only provides strong support for the photocarrier generation mechanism via zn exponents in poly(titanium N-cyclic oxazole), but may also provide a nn insi$rt of currently recognized mechanism for generating extrinsic photoinduced charge in organic molecular crystals for detailed processes.
Although many studies have been conducted to elucidate the photocarrier generation process in poly(N-vinylcarbazole) (PVCz) fdrns [1-4], our current knowledge is still not satisfactory. This is especially true for unknown properties
Extrinsic photogenesis in the lowest n-rr* absorption region, where the photon energy is insufficient to directly excite the polymer into a conducting state. Recently, Okamoto et al. [3] pointed out that exciplex formation in PVCz containing a small amount of dimethyl terephthalate (DMTP) enhanced the photocurrent in this absorption region and proposed the following exogenous carrier production process, namely field-assisted thermal cleavage of base complexes: D * +A+ (D+.. .A-)*-+D++A-,
where D and A represent donor molecules (DMTP), respectively.
perfect photoionization
(PVCz) and recipient given that
flavoring process
This mechanism seems plausible through hydrocarbon exciplexes in solution in the presence of acceptor molecules [S-7]. However, hope
Direct evidence that electric fields facilitate thorough dissociation
I have
states in DMTP-doped PVCz. In this preliminary work, we report that an electric field quenches the excited fluorescence of the PVCz-DMTP complex in solid f&n, and that the fluorescence-
z
b=
K
350
400
450
Wavelength Figure 1. Fluorescence
Observatory
320 nm
5w
(nanoscale
cf (a) pure in solid film
PVCz and PVCz doped with (b) 0.5 mol% 351TP and (c) 2.0 mol DMTP. excited at 320 nm at room temp.
Cence decreases exactly with field strength. This observation provides direct evidence for field-assisted thermal dissociation in DMTP-doped WCz films. DMTP is a weak electron acceptor that interacts with the excited bromide of the carbazole ring to generate fragments in solid Fdrn and in solution [3]. Figure 1 shows the fluorescence spectra of (a) pure PVCz and PVCz containing (b) O5 and (c) 2.0 mol% DMTP in a solid film. The unstructured broad emission around 460 nm is dependent fluorescence, while the emission around 410 nm in uncoated PVCk film is exciter fluorescence. 597
Voltirne 34, no. 3
fluorescence
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Physics Letters
exciting
0,5rraM&liP-dwrd
PVC2
and
from
Figure 2. Experi.mcnt:d
Permutations
(dwarf)
Total fluorescence spectrum of 0.5 mol% DhfTP-doped PVCz film without electric field, 2. tb) Spectral dependence of fluorescence decay with application of 240 V (SO Hz). Figure 4. (31
I
Rfi6F Multiplier Tube (HTV) pboto ~. A square wave voltage of up to 400 V (20 to 500 Hz) was applied to the sample and the modulated fluorescence was detected using a lock-in amplifier (model LI-572B, NF Circuit Design Block Co., Ltd). Experiments were performed with positively or negatively biased gold electrodes. Figure 3 shows the excited complex fluorescence of 460 nm and
wavy
Decrease during application during application
Figure 3. Typical co-fluorescence change (460 rub) generated by a qplftig square wave voltage (20 Hz) in PVCz iilm impregnated with 0.5 mol% DMTP. excitation at 360 nm. rises slowly and
decay-related protein, o
organic
year
continuous
For on and off, Gght is due to (30 5).
Solvent cast PVCz film (10--l thickness 5p) containing
A known amount of 0.f DlMTP was deposited on a NESA-coated quartz plate with evaporated gold electrodes on the membrane surface. The experimental setup is shown in Figure 1. 2. Place the membrane sample in a sample cell filled with an N atmosphere to prevent fluorescence quenching 0. Is this simple cell in a room? Photometer Hitachi ME'F-3 eyepiece with excitation lamp and 50 VVXe, 598
'-.,'
The square wave voltage was varied from 220 to 400 V at a frequency of 20 Hz, the gold electrode was positively biased and also showed an increase in exciplex fluorescence (hF) depending on the field strength. In this diagram, an orthogonal voltage is always applied. Of course, in the absence of excitation light, the AF is zero. The slow rise and fall of M corresponding to the turning on and off of the light is due to the time constant of the amplifier (30 s). The excitation wavelength was chosen to excite most of the sample, namely the wavelength of 360 nmL, which is poorly absorbed by the PVCz film. The same result was obtained in the opposite biased case.
Therefore, the results presented below were mainly obtained with positively biased gold electrodes. The effect is the same when the transfer electrodes are illuminated. Ratio of fluorescence attenuation to total intensity at 46.0 nm (M/&Y is about 10% of maximum at an applied voltage of 350 V (about 3 X 1OS V/cm)). Spectral dependence of AF d+ Cream fluorescence upon electric field application
Volume thirty-four, numbered
3
Chemical Physics Letters
0.5 mol%Ch!TP-doped XCI =
PVC;
360 nm
4
Projection = 460nm
and
50-
n
and
-
X
iLIO-
2 -
v
-
5-
m
50
,,and
Voltage Figure 5. Field resistance
,! ! and
!
100
200
400
0,7)
Dependence on fluorescent PVCz films (12 u thick).
gold, -t
19'75
Bracket 13 j ; D* i- A + (D” . . . A-)* + DC i A-, (2) due to the process of annihilation of the single exciton D+ by the carrier [S], (3) due to some field- dependent depletion process (if present) D*, (4) due to the direct interaction of the blocking state (DC . . . . A-)* with some other effect of the photogenerated free carriers
Pi
IOO-
1
reduction
Dopiran with 0.5 mol% DhlTP
(Curve b) is compared with Figure 1. Figure 4 is the fluorescence spectrum (curve a) of a PVCz film impregnated with 0.5 mol% DMTP, which clearly shows that only the exciplex fluorescence is quenched by the electric field, while the excimer fluorescence near 410 nro is not quenched. The uncoated PVCz film emitted only excimer fluorescence, and there was no change in fluorescence intensity even when the voltage was increased to 400 V (3.5 X 10' V/cm). in Fig. Figure 5 shows the AF/F field strength dependence of the exciplex emission at 460 nm. At two frequencies 2@ and 50 Hz of the applied square wave, the attenuation A.F/F shows a quadratic dependence on the electric field aFa E2. The fluorescence reduction tends to be greater at lower square wave frequencies. Exciplex quenching of fluorescence by an electric field will be explained according to the following four possible processes. (1) due to field-assisted thermal dissociation of the fragment in free
procedure. If (2) or (3) holds, then fluorescence quenching of the excited electric field will be detected, as the migration of single excitons to the exciton formation sites in the solid film will also be reduced [9]. However, as mentioned above, excimer fluorescence was not quenched and no fluorescence change could be detected on uncoated PVCz films under the current experimental conditions. Therefore, cases (2) and (3) must be excluded. Case (4) must also be ruled out because the excitation light intensity used in this study (150 W monochromatic Xe lamp) was not sufficient to generate enough LO photocarriers to facilitate exciton-carrier interactions_this reasoning also rules out the case (2). Thus, case (1) field-assisted thermal dissociation of the fragments into free carriers is the most likely process for the quenching of eccrine in an electric field. In addition to our previously published results [3], where the photocurrent in this doped system was significantly increased compared to that of pure PVC, the present result is that the exciton fluorescence caused by the molecular interaction of the doped exciton is quenched by a field applied, providing clear experimental support for the generation of exogenous photocarriers due to Beldas-assisted thermal dissocn. However, whether the quadratic field dependence of fluorescence attenuation can be explained by combining existing dissociation theories, such as the Poole-Frenkel model [IO], the Onsqer model [11] or some other new theory, seems to him a very interesting question. . This is still under discussion. As is generally accepted, the mechanism of extrinsic photocarrier formation, as in anthracene, is currently thought to be the decay of excitons into free carriers interacting with electron-accepting impurities, and this extrinsic photocurrent has been shown to be field-dependent [12 . ] . However, direct experimental evidence for these processes has not been provided so far. Our current results are out of 599
To yell! mind 34,
night 3
Chemical Physics Letters
The system reduces the visible emission due to molecular interactions with exogenous initiators, which probably provides
A nurse
Ricchetti
and ice
mentioned above
Now
accept-
and e.&i& production of photocarriers in similar organic crystals.
Literature [11 M. Lardon, E. Loll-Doller and J.W. Εμείς@, Μουρ. κρύσταλλο. 2 (19671 241. [2] P.J..Mek, J.Chcm. Phys. 57 (1972) 1694. [3] K. Okamoto, S. Kusabayashi and H. Kikawa, Bull. Chem. Sot. Japm 46 (1973) . 2613.
August 1, 1975
(41 G. F%ster i D.J. Williams, J. Chem. Phys. 61 (1974) 2416. [5] H.-Knibbe, K. RhLlig, F.P. Scliafer i A. WeUet, J. Chem. Phys. 47 ( 1967). York, 1970). [E] N. Wotherspoor, M. Pope 和 J. Burgas, Chem. Phys. Letters 8 (1970) 453. [S] W. IUBpffer, J. Chem. Phys. 50 (1969) 2337； [IO] J, Frenkel, Pi~ys Rev. 54 (1938) 647. 1111 5. Onsager, Phys. Rev. 54 (1938) 554. [ 121 R.F. Chaiken 和 D.R. Keuns, Ι. Chem. Phys. 45 (i966) 3966; 49 (1968) 2846.