Photoinduced energy transfer in ZnO-tetraphenylporphyrin systems

Article abstract of DOI:10.1016/j.cplett.2009.04.073

The optical absorption and photoluminescence measurements were carried out for ZnO nanoparticle-tetraphenylporphyrin (TPP) systems made of four kinds of TPP's with and without p-aminophenyl groups. The ZnO particles were treated with l-cysteine in order t

Full text of DOI:10.1016/j.cplett.2009.04.073

Chemical Physics Letters 474 (2009) 315–319
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Photoinduced energy transfer in ZnO–tetraphenylporphyrin systems
a
a
a
b
a
Ichiro Hiromitsu a, , Takahisa Ikeue , Kazuhiro Karino , Takatsugu Ohno , Senku Tanaka , Hideo Shiratori ,
*
Shigekazu Morito a, Yasuhisa Fujita c, Makoto Handa a
a Department of Material Science, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan
b Center for Integrated Research in Science, Shimane University, Matsue 690-8504, Japan
c Department of Electronic and Control Systems Engineering, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The optical absorption and photoluminescence measurements were carried out for ZnO nanoparticle–
Received 26 November 2008
In final form 28 April 2009
Available online 3 May 2009
tetraphenylporphyrin (TPP) systems made of four kinds of TPP’s with and without p-aminophenyl groups.
The ZnO particles were treated with
L-cysteine in order to make a ZnO–(L-cysteine)–TPP binding. How-
ever, this binding was not formed because of an absence of the ZnO–(
L
-cysteine) binding. In the case
of metal-free TPP’s, the central hydrogen atoms of the TPP ring were replaced by a Zn atom during an
aging of 2–4 weeks. An energy transfer from photoexcited ZnO to TPP occurs by a collision between
ZnO and TPP in the dispersed medium.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin (H2TPP(NH2)2),
and {5,10-bis(4-aminophenyl)-15,20-diphenylporphyrinato}zinc
The optical properties of the assembled systems made of inor-
ganic nanoparticles and organic molecules are gaining increasing
interest for a wide range of biomedical applications, e.g. imaging
[1–6], and photodynamic therapy [7–10]. A key phenomenon for
such applications is the energy transfer from photoexcited nano-
particles to the organic molecules. Much effort is being made to ob-
tain an efficient emission of the organic molecules by an energy
transfer from the photoexcited nanoparticles [11–15].
In this Letter, the photoluminescence (PL) properties of ZnO
nanoparticle–porphyrin systems are reported. ZnO is a direct band
gap semiconductor with a band gap of 3.4 eV. ZnO nanostructures
typically have a near-band-edge emission in the UV region and a
defect-related visible emission in the blue to green region
[10,16,17]. Because of its potential of non-toxicity, ZnO is a candi-
date for the materials applicable for biomedicine [2,3,10]. Porphy-
rin, on the other hand, is a well-known emitter of red light. The
objective of the present study is to elucidate the PL properties
and the energy transfer efficiency of the ZnO nanoparticle–porphy-
rin systems. A similar study of a ZnO nanoparticle–porphyrin sys-
tem has recently been reported by Liu et al. using meso-tetra(o-
aminophenyl) porphyrin (MTAP) [10]. They observed an efficient
quenching of the ZnO emission and enhancement of the MTAP
emission in the ZnO–MTAP system, which indicated that an energy
transfer occurred from ZnO to MTAP with an efficiency of 83%. In
the present study, four kinds of tetraphenylporphyrins (TPP)
shown in Fig. 1, i.e. 5,10,15,20-tetraphenylporphyrin (H2TPP), 5-
(II) (ZnTPP(NH2)2) are used instead of MTAP. Among these,
H2TPP(NH2), H2TPP(NH2)2 and ZnTPP(NH2)2 have one or two
p-aminophenyl groups instead of four o-aminophenyls in MTAP.
The different substitutional site of the amino group and the differ-
ent number of aminophenyls may result in different energy trans-
fer efficiencies and different emission properties.
Another important point of the present study is to elucidate the
role of
linker between inorganic nanoparticle and organic molecule. Liu
et al. synthesized a ZnO–( -cysteine)–MTAP conjugate assuming
that -cysteine is bound to the surface of the ZnO particles by a for-
mation of Zn–S bonding. They made a chemical bonding between
L-cysteine (HS–CH2CH(NH2)–COOH) which is a popular
L
L
L
-cysteine and MTAP by a reaction between the carboxyl group of
-cysteine and the amino group of MTAP resulting in a –(CO)–
L
(NH)– bonding [10]. However, there is no clear evidence of the
ZnO–( -cysteine) and the ( -cysteine)–MTAP bondings. In the pres-
ent study, it will be shown that -cysteine is not bound to the ZnO
L
L
L
particles. As a result, TPP’s used in the present study are not bound
to the ZnO particle. A collision-controlled energy transfer from ZnO
to TPP will be reported.
2. Experimental
Zn(OAc)2ꢀ2H2O, 1-ethyl-3(3-dimethylaminopropyl) carbodiim-
ide hydrochloride (EDC), N-hydroxysulfosuccinimide (sulfo-NHS),
and diethylene glycol were purchased from Kanto Chemicals.
H2TPP, H2TPP(NH2) and H2TPP(NH2)2 were synthesized by a litera-
ture procedure [18,19]. ZnTPP(NH2)2 was synthesized from
H2TPP(NH2)2 by adding excess amount of Zn(OAc)2ꢀ2H2O in a
mixed solvent of chloroform and methanol (3:1) and refluxing.
(4-aminophenyl)-10,15,20-triphenylporphyrin
(H2TPP(NH2)),
* Corresponding author. Fax: +81 852 32 6409.
E-mail address: hiromitu@riko.shimane-u.ac.jp (I. Hiromitsu).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.04.073

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I. Hiromitsu et al. / Chemical Physics Letters 474 (2009) 315–319
15 min at room temperature. Then, 1.5 ꢁ 10ꢂ6 mol of the porphy-
rin was dissolved in a small amount of dimethylformamide for
H2TPP, and a small amount of diethylene glycol for H2TPP(NH2),
H2TPP(NH2)2 and ZnTPP(NH2)2, and was added to the ZnO–LC dis-
persion treated with EDC and sulfo-NHS. The mixture was stirred
for 3 h at room temperature. The pH of the resultant dispersion
was adjusted to 7. Finally, the dispersion was diluted by 3 (or
21) times with diethylene glycol since it was too concentrated
for optical absorption and PL measurements. The final concentra-
tions of the Zn atoms and porphyrins were 1.7 ꢁ 10ꢂ3 (or
2.4 ꢁ 10ꢂ4) mol/L and 1.0 ꢁ 10ꢂ4 (or 1.4 ꢁ 10ꢂ5) mol/L, respec-
tively. For the measurement of the excitation spectrum of the pho-
toluminescence, the samples diluted by 21 times were used in
order to get reliable excitation spectra.
The ZnO particles were observed with transmission electron
microscopy (TEM, JEOL JEM-2010). Optical absorption spectra were
measured using Shimazu UV-3100PC. PL spectra were measured
using Ocean Optics USB2000 with an excitation light of 325 nm
from a He–Cd laser. The optical absorption and PL measurements
were performed using a 10 mm ꢁ 2 mm quartz sample cell. For
the optical absorption measurement, the path length of the pene-
trating light through the sample was 2 mm. For the PL measure-
ment, the path length of the excitation light was 10 mm, and the
emitted light was detected at right angles to the excitation beam.
The excitation spectra of the photoluminescence were measured
Fig. 1. The four types of porphyrins used in this study.
on
a HITACHI 850 Fluorescence Spectrophotometer using a
10 mm ꢁ 10 mm quartz sample cell.
The synthesized H2TPP(NH2), H2TPP(NH2)2 and ZnTPP(NH2)2 were
characterized by 1H-nuclear magnetic resonance and Matrix-
Assisted Laser-Desorption/Ionization Time-of-Flight (MALDI-TOF)
Mass spectroscopies.
3. Results and discussion
Fig. 2 shows the TEM image and the electron diffraction pattern
of the ZnO nanoparticles without -cysteine. It is observed that the
ZnO nanoparticles were synthesized by a reported method
[10,16,17]. 3.8 ꢁ 10ꢂ4 mol of Zn(OAc)2ꢀ2H2O and 75 ml of diethyl-
ene glycol were mixed (Zn: 5.0 ꢁ 10ꢂ3 mol/L), and the mixture was
heated under stirring and kept at 160 °C for 1 h. After this reaction,
fluorescent ZnO nanoparticles were formed. Then, 30 ml of the ZnO
dispersion in diethylene glycol (Zn: 1.5 ꢁ 10ꢂ4 mol) and an aque-
L
ZnO particles are spherical with a diameter of typically 4 nm,
which is as expected from a reported dependence of the particle
size on the concentration of the starting material [17]. The electron
diffraction pattern in Fig. 2 is attributed to ZnO, indicating that the
ZnO particles are in a crystalline phase.
The optical absorption, PL, and excitation spectra of the ZnO–LC
particles are shown in Fig. 3. The PL spectrum is similar to that re-
ported by Liu et al. [10], and has a peak at 500 nm, indicating that
the emission comes mainly from surface defects. The excitation
spectrum of PL has a broad peak at 340 nm. The absorption spec-
trum has a tail in the visible region due to Rayleigh scattering, indi-
cating that some sort of aggregation occurred. A TEM observation
indicated that the size of the aggregates is typically 500 nm. After
a centrifugation of the ZnO–LC dispersion, the supernatant showed
ous solution of
L
-cysteine (1.5 ꢁ 10ꢂ4 mol) were mixed, and the
pH of the resultant dispersion was adjusted to 7 by adding NaOH
aqueous solution. The mixture was heated to 60 °C for 48 h to form
L-cysteine-treated ZnO particles (ZnO–LC). Then, the pH of the dis-
persion was again adjusted to 7.
The (ZnO–LC)–porphyrin system was prepared using the stan-
dard cross linking method [10,20,21]. 2.5 ꢁ 10ꢂ5 mol of EDC and
2.5 ꢁ 10ꢂ5 mol of sulfo-NHS were successively added to 5 ml of
the ZnO–LC dispersion (Zn: 2.5 ꢁ 10ꢂ5 mol), and reacted for
Fig. 2. TEM image and electron diffraction pattern of the ZnO nanoparticles without L-cysteine.

I. Hiromitsu et al. / Chemical Physics Letters 474 (2009) 315–319
317
levels in the ground- or excited-state manifolds. The usual metal
porphyrins have two Q bands of Q(0, 0) and Q(1, 0) [22,23,25].
The existence of four Q bands is due to the lower symmetry of
the metal-free TPP’s [22–24]. In the (ZnO–LC)–TPP systems, on
the other hand, the
Q bands of H2TPP, H2TPP(NH2) and
H2TPP(NH2)2 were altered significantly, especially by an aging after
the sample preparation. After an aging of 2–4 weeks, only three
absorption peaks were observed in the Q band region with
the Qx(0, 0) band disappeared, as shown in Fig. 4. Furthermore,
the Qy(1, 0) band became very weak. These features of the Q bands
after the aging corresponds to those of the metal porphyrins, sug-
gesting that the two central hydrogen atoms of H2TPP, H2TPP(NH2)
and H2TPP(NH2)2 were replaced by a Zn atom during the aging.
This interpretation is confirmed by the absorption spectra of
ZnTPP(NH2)2 in Fig. 4. Because of the higher symmetry of the
Zn–substituted TPP compared to the metal-free TPP’s, ZnTPP(NH2)2
has two Q-band peaks at 604 and 562 nm which are assigned to the
Q(0, 0) and Q(1, 0) bands, respectively, and has a small peak at
520 nm. The (ZnO–LC)–ZnTPP(NH2)2 system does not show any
aging effect in contrast to the (ZnO–LC)–(metal-free TPP) systems,
indicating that the aging effect observed in the latter is related to
the central hydrogen atoms of the metal-free TPP’s. Furthermore,
the peak positions and the line shape of (ZnO–LC)–H2TPP(NH2)2
after the aging are identical with those of (ZnO–LC)–ZnTPP(NH2)2.
This is a clear evidence of the substitution of the central hydrogen
atoms of the metal-free TPP’s by a Zn atom. The Zn atoms for the
substitution most probably comes from the residual Zn(OAc)2 in
the system which is the starting material for the ZnO synthesis.
This kind of substitution definitely occurs because ZnTPP(NH2)2
used in the present study was synthesized from H2TPP(NH2)2 and
Zn(OAc)2 as explained in the experimental section.
Fig. 3. Optical absorption and photoluminescence spectra of L-cysteine-treated ZnO
nanoparticles dispersed in diethylene glycol. The excitation wavelength for the
photoluminescence measurement was 325 nm. The excitation spectrum of the
photoluminescence is also shown, which was measured for the 500 nm emission.
the identical optical absorption spectrum with the ZnO particles
before the
aggregates are made of
L
-cysteine treatment. This clearly indicates that the
-cysteine, and -cysteine is not bound to
L
L
the ZnO particles. It is noted that, after the treatment with EDC
and sulfo-NHS, the ZnO–LC dispersion again became transparent
with much smaller light scattering.
The optical absorption spectra of the (ZnO–LC)–TPP systems are
shown in Fig. 4, as well as the absorption spectra of the diethylene
glycol solutions of the TPP’s. It is noted that, for H2TPP, a small
amount of dimethylformamide was added because H2TPP was
not soluble to diethylene glycol. The absorption spectra of H2TPP,
H2TPP(NH2) and H2TPP(NH2)2 in diethylene glycol solution have
four Q bands, i.e. Qx(0, 0), Qx(1, 0), Qy(0, 0) and Qy(1, 0) [22–24],
where the numbers in (0, 0) and (1, 0) indicate the vibrational
Fig. 5 shows the PL spectra of the diethylene glycol solutions of
the TPP’s. The PL spectra of the metal-free TPP’s, H2TPP,
H2TPP(NH2) and H2TPP(NH2)2 show two emission peaks at 649–
670 nm and 714–724 nm which are assigned to the Qx(0, 0) and
Qx(0, 1) bands, respectively [22,24]. The PL spectrum of
ZnTPP(NH2)2, on the other hand, has two peaks at 618 and
Fig. 4. Optical absorption spectra of the diethylene glycol solutions of the four
types of tetraphenylporphyrins, H2TPP, H2TPP(NH2), H2TPP(NH2)2 and ZnTPP(NH2)2
(Thin solid lines), and (ZnO–LC)–porphyrin systems dispersed in diethylene glycol
(Broken lines: as prepared. Thick solid lines: after an aging of 2–4 weeks). The very
strong B-band absorption peak of the porphyrins at 400 nm < k < 450 nm was
omitted in the figure. The four Q bands of the metal-free porphyrins in the solution
are assigned to Qx(0, 0), Qx(1, 0), Qy(0, 0) and Qy(1, 0). On the other hand, the two Q
bands of the (ZnO–LC)–(metal-free porphyrin) after aging, as well as those of
ZnTPP(NH2)2, are assigned to Q(0, 0) and Q(1, 0) of Zn–substituted porphyrins.
Fig. 5. Photoluminescence spectra of the diethylene glycol solutions of the four
types of tetraphenylporphyrins, H2TPP, H2TPP(NH2), H2TPP(NH2)2 and ZnTPP(NH2)2.
Excitation wavelength: 325 nm. Each spectrum consists of two spectral compo-
nents, denoted by thin solid curves, which are assigned to the Qx(0, 0) and Qx(0, 1)
bands for the metal-free porphyrins, and Q(0, 0) and Q(0, 1) for ZnTPP(NH2)2. For
410 nm < k < 445 nm, no reliable luminescence measurement was possible because
of a re-absorption of the emitted light by the porphyrins due to very strong B-band
absorption.

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I. Hiromitsu et al. / Chemical Physics Letters 474 (2009) 315–319
664 nm which are assigned to the Q(0, 0) and Q(0, 1) bands, respec-
tively [25]. For H2TPP(NH2)2 and ZnTPP(NH2)2, a small peak at
540 nm is also observed with a long tail down to 400 nm, the origin
of which is unknown. As will be shown later, the intensity of this
unknown emission becomes larger in the (ZnO–LC)–TPP systems.
Fig. 6 shows the PL spectra of the (ZnO–LC)–TPP systems. Again,
the spectra of the metal-free TPP’s showed a drastic change by the
aging. After an aging of 2–4 weeks, the Qx(0, 1) emission at
ꢃ720 nm disappeared. Instead, a new peak grew up at 603–
615 nm. This aging effect is consistent with the substitution of
the central hydrogen atoms of the metal-free TPP by a Zn atom dis-
cussed above, and the two emission peaks at ꢃ610 and ꢃ660 nm
after the aging are assigned to the Q(0, 0) and Q(0, 1) bands, respec-
tively, of the Zn–substituted TPP’s. On the other hand, the PL spec-
trum of (ZnO–LC)–ZnTPP(NH2)2 in Fig. 6 does not show any aging
effect. This again confirms that the alteration of the PL spectra of
the metal-free TPP’s by the aging is due to the generation of the
Zn–substituted TPP’s. The PL spectra of H2TPP(NH2)2 and
ZnTPP(NH2)2 have relatively large peaks at 540 and 577 nm besides
the Q(0, 0) and Q(0, 1) bands at 616 and 660 nm. In the case of
H2TPP(NH2)2, these peaks grew up by the aging. The 540 nm peak
seems to also exist in the PL spectra of (ZnO–LC)–H2TPP and (ZnO–
LC)–H2TPP(NH2) although the intensity is extremely weak. It is
noted that the ZnO–H2TPP(NH2) and ZnO–H2TPP(NH2)2 systems
Fig. 7. The excitation spectra of the photoluminescence of (ZnO–LC)–H2TPP(NH2)2
in diethylene glycol after an aging of 1 week. The detection wavelengths (kdetect
were 480, 580, and 616 nm. For 410 nm < k < 445 nm, no reliable measurement was
possible because of an extremely small penetration depth of the excitation light in
the sample due to the very strong B-band absorption of the porphyrin.
)
at 615 and 661 nm are attributed to the Zn–substituted porphyrin.
The emission peaks at 540 and 577 nm, on the other hand, show
totally different excitation spectra which have three main peaks
at 320, 490, and 520 nm, and have no contribution from the B-band
of the porphyrin at around 420 nm, which is shown in Fig. 7 for the
580 nm emission. Thus, the emissions at 540 and 577 nm are not
the usual porphyrin emissions. The three peaks at 320, 490, and
520 nm in the excitation spectrum of the 580 nm emission are also
observed in the absorption spectrum of the (ZnO–LC)–H2TPP(NH2)2
system in Fig. 4 although the 490 nm peak in the absorption spec-
trum is extremely weak. The origin of these new emission peaks is
unknown at present.
Now, the intensity of the ZnO emission of the (ZnO–LC)–TPP
systems is examined. In the PL spectra in Fig. 6, the ZnO emission
is observed at around 500 nm. This is confirmed by the excitation
spectrum of the 480 nm emission, which is shown in Fig. 7 for
(ZnO–LC)–H2TPP(NH2)2. The excitation spectrum of the 480 nm
emission is close to that of the ZnO emission shown in Fig. 3. The
intensity of the ZnO emission could be obtained from the emission
intensity at 500 nm for (ZnO–LC)–H2TPP and (ZnO–LC)–
H2TPP(NH2). However, for (ZnO–LC)–H2TPP(NH2)2 and (ZnO–LC)–
ZnTPP(NH2)2, an emission of unknown origin and unknown inten-
sity is overlapped in the 500 nm region as is evident from the solu-
tion spectra of H2TPP(NH2)2 and ZnTPP(NH2)2 in Fig. 5, so that the
intensity of the ZnO emission could not be obtained for these sys-
tems. The emission intensities of the ZnO for (ZnO–LC)–H2TPP and
(ZnO–LC)–H2TPP(NH2) in Fig. 6, were substantially smaller than
the intensity for the ZnO–LC without TPP shown in Fig. 3. It is
noted, however, that the light power absorbed by ZnO is not the
same for the two cases of with and without TPP, so that a correc-
tion must be made for the light power absorbed by ZnO, which
was carried out using the values of the absorbance at 325 nm in
without
L-cysteine gave nearly identical optical absorption and PL
spectra with the systems having
L
-cysteine. This supports our con-
clusion that TPP’s are not bound to the ZnO particles because
teine is not bound to ZnO.
L-cys-
In order to investigate the origin of the peaks observed in Fig. 6,
the excitation spectrum of PL was measured for (ZnO–LC)–
H2TPP(NH2) and (ZnO–LC)–H2TPP(NH2)2, and the results for the
latter are shown in Fig. 7. The excitation spectrum of the 616 nm
emission is very close to the optical absorption spectrum shown
in Fig. 4. The 660 nm emission showed a similar excitation spec-
trum. These results are as expected since the two emission peaks
Figs. 3 and 4. Then, the energy transfer efficiency
using the following equation [10,11].
U
was calculated
IDA
ID
U
¼ 1 ꢂ
;
ð1Þ
where IDA and ID are the emission intensities of ZnO (energy donor)
with and without TPP (energy acceptor) for the same absorbed light
Fig. 6. Photoluminescence spectra of (ZnO–LC)–porphyrin systems dispersed in
diethylene glycol. Thick solid lines: after an aging of 2–4 weeks. Excitation
wavelength: 325 nm. Thin solid lines are the spectral components of each emission
spectrum. The two spectral components at 603–615 nm and 655–660 nm of the
(ZnO–LC)–(metal-free porphyrin) after aging, as well as those of (ZnO–LC)–
ZnTPP(NH2)2, are assigned to the Q(0, 0) and Q(0, 1) bands, respectively, of Zn–
substituted porphyrins. The ZnO emission with a peak at 500 nm is overlapped in
each spectrum.
power. The estimated values of
LC)–H2TPP(NH2) are summarized in Table 1. Two features are ob-
served. (1) The values of for (ZnO–LC)–H2TPP and (ZnO–LC)–
H2TPP(NH2) are nearly the same. (2) is smaller for the diluted
U for (ZnO–LC)–H2TPP and (ZnO–
U
U
samples. In the case of (ZnO–LC)–H2TPP, the porphyrin ring cannot
be bound to the ZnO particle because H2TPP does not have any

I. Hiromitsu et al. / Chemical Physics Letters 474 (2009) 315–319
319
Table 1
Energy transfer efficiency
U of the ZnO–porphyrin systems in diethylene glycol.
System
Diluted by three times
Diluted by 21 times
!
!
Zn : 1:7 ꢁ 10ꢂ3 mol=L
Zn : 2:4 ꢁ 10ꢂ4 mol=L
Porphyrin : 1:4 ꢁ 10ꢂ5 mol=L
Porphyrin : 1:0 ꢁ 10ꢂ4 mol=L
(ZnO–LC)–H2TPPa
69 6%
59 5%
60 5%
33 15%
29 17%
37 12%
(ZnO–LC)–H2TPP(NH2)a
ZnO–H2TPP(NH2)b
a
ZnO–LC denotes the ZnO nanoparticles treated with L-cysteine.
Without L-cystine.
b
amino groups. The above two features clearly indicate that the en-
ergy transfer in the (ZnO–LC)–H2TPP and (ZnO–LC)–H2TPP(NH2)
systems is due to the collisions between the ZnO particle and TPP.
Table 1 also shows the results for the ZnO–H2TPP(NH2) system
were generated at 540 and 577 nm, the origin of which is unknown
at present. The energy transfer efficiency from the photoexcited
ZnO to the porphyrin was the same for (ZnO–LC)–H2TPP, (ZnO–
LC)–H2TPP(NH2) and ZnO–H2TPP(NH2), and the efficiency became
smaller by diluting the system by adding diethylene glycol. This
clearly indicates that the energy transfer occurs by the collisions
of the ZnO particles with porphyrins in the dispersion. ZnO and
without L-cysteine. It is observed that L-cysteine does not affect
the energy transfer efficiency, which is as expected because
teine does not work as a linker between ZnO and TPP.
L-cys-
The present energy transfer from ZnO to porphyrin may be ex-
plained by the Förster’s mechanism [13]. In the Förster’s theory,
the energy transfer rate kET obeys the following relationship.
TPP do not bound with each other because
to the ZnO particles.
L-cysteine is not bound
Z
Acknowledgment
1
kET
¼
IDðkÞeAðkÞk4 dk=R6;
ð2Þ
0
The authors thank Prof. Sugimori in Toyama University for his
help in the characterization of the porphyrins by measuring their
MALDI-TOF mass spectra. The TEM observation of the ZnO particles
were performed at Center for Integrated Research in Science,
Shimane University.
where ID(k) is the emission spectrum of the energy donor,
e
A(k) is
the absorption coefficient of the energy acceptor, R is the distance
between the donor and the acceptor. In the present system, the
emission spectrum of ZnO shown in Fig. 3 overlaps with the strong
B-band absorption of the TPP’s shown in Fig. 4, while the emission
spectra of the TPP’s shown in Fig. 5 do not have any overlap with the
absorption spectrum of ZnO shown in Fig. 3. Thus, the direction of
the energy transfer is expected to be from ZnO to TPP, as observed
in the present experiment. In the present system, the ZnO particles
and TPP can move in the dispersion, so that the energy transfer can
occur when they are in close proximity to each other. The effective
distance at which the transfer occurs is unknown.
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4. Conclusions
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