Photoluminescence and optical waveguiding characteristics of bisalkoxy tin(IV) porphyrin microcrystals

Article abstract of DOI:10.1002/asia.201200749

Guide me: Laser confocal microscope photoluminescence (LCM-PL) and optical waveguiding characteristics for tin(IV) porphyrin-based microcrystalline rods and plates were investigated. The efficiency of optical waveguiding for the rods (0.04 μm^-1) was five times better than for the plate, due to stronger π-π interaction and a short layer distance (3.035 vs. 3.328 A). Copyright

Full text of DOI:10.1002/asia.201200749

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DOI: 10.1002/asia.201200749
Photoluminescence and Optical Waveguiding Characteristics of Bisalkoxy
Tin(IV) Porphyrin Microcrystals
Seong-Gi Jo,[a] Sundol Kim,[b] Eun Hei Cho,[a] Da Hee Lee,[b] Jeongyong Kim,[c]
Suk Joong Lee,*[b] and Jinsoo Joo*[a]
During the last decade, many efforts have been focused
on the fabrication of nano- and microstructured functional
materials because of their potential applications as building
blocks for miniaturized photonics, electronics, and sen-
sors.[1–3] In particular, inorganic and organic nano- and mi-
crorods have been studied for optical waveguiding materials,
which show efficient propagation and manipulation of inci-
dent light signal on the subwavelength scale.[4–6] Recently, it
was realized that one-dimensional nano- and microrod struc-
tures using polymers[7] and organic molecules[8–12] could
serve as effective building blocks to generate and propagate
light in the future miniaturized photonics. In addition, or-
ganic semiconducting microcrystals[13,14] have unique pho-
tonic and electronic properties,[15,16] because of the p–p in-
termolecular interactions[17] and high luminescence efficien-
cy.
koxy tin(IV) porphyrin based microcrystalline rods and
plates using a laser confocal microscope (LCM) system.
These microcrystals were prepared from slow diffusion of
solutions of bisalkoxy tin(IV) porphyrin 1 and 2 in respec-
tive alcohols (n-propanol for (porphyrin)Sn
ACHTUNGRTENUN(NG OPr)2 (1) and
isopropyl alcohol for (porphyrin)Sn(OiPr)2 (2)) over water.
AHCTUNGTRENNUNG
We observed that the single-crystal structures and morphol-
ogy of microcrystals of 1 and 2 are clearly different. The
LCM-PL peaks of single microcrystals of 1 and 2 were ob-
served at 635 nm when they were excited at 488 nm, which
is considerably different from their solution PL. From opti-
cal waveguiding experiments using the LCM system, the ef-
ficiency of optical waveguiding for the microcrystalline rod
of 1 was five times better than that for 2, because of the rel-
atively strong p–p intermolecular interaction along the main
crystalline axis.
Porphyrin and metalloporphyrins have received consider-
able attention because of their photostability, high visible
extinction coefficient, and electron-transfer properties,[18–20]
which contributed to their use as active materials for charge
transport, photoelectronic conversion, nonlinear optics, or-
ganic light-emitting diodes, organic photovoltaic cells, and
optical information storage.[21–24] Recently, the self-assembly
of porphyrin-based nanostructures, including nanoporous
solid materials, have been intensively studied.[25–27] However,
the nanoscale luminescence and optical waveguiding charac-
teristics of metalloporphyrin-based microcrystals with vari-
ous morphologies have not been thoroughly investigated.
In this study, we investigated the nanoscale photolumines-
cence (PL) and optical waveguiding characteristics of bisal-
Figure 1 shows the single-crystal structures and packing
diagrams of 1 and 2. The porphyrin cores in both 1 and 2
are nearly planar. Two phenyl rings are tilted relative to the
porphyrin plane by tilting angles of 78.8(2)8 for 1 and
72.9(4)8 and 80.8(3)8 for 2. In 1, two propanol molecules ax-
ially coordinate to a SnIV ion (Figure 1a), but in 2, isopropyl
alcohol molecules did not coordinate to a SnIV ion; instead,
two water molecules coordinate to a SnIV ion (Figure 1b).
Two isopropyl alcohol molecules have been liberated during
the crystal growth. Figure 1c,e,f and Figure 1d,g,h show the
difference in packing patterns of the two compounds, and it
might be consistent with each crystal type (rod for 1 and
plate for 2). These molecules show clear slip-stack arrange-
ment of the porphyrin planes (Figure 1e–h) and interacted
through p–p interactions (3.035 ꢀ for 1 and 3.328 ꢀ for 2).
Figure 2 shows CCD images of microcrystals made of
1 and 2, and the insets show SEM images of the correspond-
ing microcrystals. The microcrystal of 1 shows a thickness of
2–3 mm and length of approximately 50 mm. For 2, the thick-
ness was approximately 200 nm, the width 14 mm, and the
length 19 mm. The red light emission was dominant for both
microcrystals.
[a] S.-G. Jo, E. H. Cho, Prof. J. Joo
Department of Physics, Korea University
Seoul 136-713 (South Korea)
Fax : (+82)2-927-3292
E-mail: jjoo@korea.ac.kr
[b] S. Kim, D. H. Lee, Prof. S. J. Lee
Department of Chemistry, Korea University
Seoul 136-713 (South Korea)
Fax : (+82)2-925-4284
Figure S4a in the Supporting Information shows UV/Vis
absorption spectra of 1 and 2, in both solution and solid
states. All samples show a sharp and strong Sorꢁt band cor-
responding to S0!S2 transition and a set of weak Q-bands
due to S0!S1 transition, which are typical absorption pro-
files for porphyrin-based compounds.[28,29] However, drastic
spectral changes were observed in the crystal state, which is
E-mail: slee1@korea.ac.kr
[c] Prof. J. Kim
Department of Physics, University of Incheon
Incheon 406-772 (South Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200749.
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Chem. Asian J. 2012, 7, 2768 – 2771

Figure 3a shows a schematic illustration of the optical
waveguiding experiment using a microcrystalline rod of
1 and the LCM system. Note that the propagation distance
of focused incident light was varied through a movable
input laser. Figure 3b shows the variation of output LCM-
PL spectra depending on the propagation distance for a mi-
crocrystalline rod sample of 1. The insets of Figure 3 are the
LCM-PL spectra of microcrystals for the reference, that is,
the PL spectra in the same input and output positions. As
the propagation distance of the input light increased, the
LCM-PL intensity at 638 nm modestly decreased for a micro-
crystalline rod of 1 (Figure 3b), while that for 2 rapidly de-
creased (Figure 3c–e). These results imply that the micro-
crystalline rod of 1 has better optical signal transporting
characteristics than the microcrystalline plate of 2. Further-
more, the output LCM-PL peaks for microcrystalline rod of
1 were red-shifted from 638 to 652 nm.
The output LCM-PL spectra of microcrystalline plate of 2
varied with the propagation directions of the incident light,
as shown in Figure 3c–e. The output LCM-PL intensities
along the 01 direction (i.e., the major crystal growing axis)
in the microcrystalline plate of 2 were the most weakly de-
creased with increasing the propagation distance (Fig-
ure 3c), while those along the 10 direction (i.e., the direc-
tion perpendicular to the major crystal growing direction in
the same plane) were strongly reduced and those along the
11 direction (diagonal) were intermediately decreased.
To quantitatively analyze optical waveguiding characteris-
tics of the microcrystals of 1 and 2, the output LCM-PL in-
tensities at 638 nm were plot-
Figure 1. Single-crystal X-ray structures and packing diagram representa-
tions of 1 (a, c, e, and f) and 2 (b, d, g, and h).
ted as a function of the propa-
gation distance of input light
(Figure 4). The output LCM-
PL intensities of the waveguid-
ed light decayed exponentially.
Therefore, we used the equa-
tion I(x)=I0 eꢀax, in which I is
the output LCM-PL intensity,
I0 is the proportional constant
of the intensity, a is the decay
constant in the unit of mmꢀ1,
and x is the propagation dis-
tance of the incident light.
From the slope in Figure 4, the
Figure 2. CCD images of microcrystals of a) 1 and b) 2. The white circles represent the detecting positions for
optical waveguiding experiments. The arrows represent moving directions of the input laser. Insets: SEM
images of the corresponding microcrystals.
attributed to the intermolecular interaction between the
tin(IV) porphyrins. For example, the solution samples of
1 and 2 exhibit Sorꢁt bands at 415 nm with Q-bands around
548 nm, while the absorption spectra of microcrystals of
1 and 2 show broadened and slightly red-shifted Sorꢁt bands
at 420 nm and Q-bands around 552 nm. The PL spectra of
the solution samples of 1 and 2 show identical emission
bands at 590 and 644 nm, while a band at 638 nm, indicating
red-light emission,[16] with weak satellite peaks at 606, 660,
and 700 nm is observed for the LCM-PL of the microcrystals
of 1 and 2 (Figure S4b). These findings indicate a high
degree of intermolecular interaction between the porphyrin
units upon crystal formation.
decay constant a was estimated to be about 0.040 mmꢀ1 for
the microcrystalline rod of 1. However, the a values of mi-
crocrystalline plate of 2 were 0.198, 0.258, and 0.428 mmꢀ1
along the 01, 11, and 10 directions, respectively. These re-
sults imply that the microcrystalline rod of 1 has better opti-
cal waveguiding characteristics than the microcrystalline
plate of 2. Clearly, the p–p stacking of tin(IV) porphyrin
molecules took place along the c axis (major crystal growing
direction) with a layer distance of 3.035 ꢀ, which was as-
signed as the longitudinal direction of the microcrystalline
rod of 1 (Figure 1). For the microcrystalline plate of 2, the
waveguiding efficiency varied with the propagation direc-
tion, because the anisotropic molecular arrangement de-
Chem. Asian J. 2012, 7, 2768 – 2771
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Suk Joong Lee, Jinsoo Joo et al.
COMMUNICATION
with
a stacking distance of
3.328 ꢀ (Figure 1), resulting in
the weaker decay constant.
Therefore, we suggest that the
optical waveguiding can be ef-
ficient along the p–p stacking
direction of tin(IV) porphyrin
molecules, and the decay con-
stants are highly depending on
the p–p stacking distances and
directions.
Bisalkoxy tin(IV) porphyrin
based microcrystals were fabri-
cated by
a
slow diffusion
method to give rods for 1 and
plates for 2. We observed the
characteristic optical peaks of
the tin(IV) porphyrin micro-
crystals from the UV/Vis ab-
sorption and LCM-PL spectra.
From the optical waveguiding
experiments, the decay con-
stant of LCM-PL intensity for
the microcrystalline rod of
1
was estimated to be
0.040 mmꢀ1, which was more ef-
ficient than that of the micro-
crystalline plate of 2. This dras-
tic change in the efficiency of
optical signal transport is due
to the well-aligned p–p stack-
ing of tin(IV) porphyrin mole-
cules along the longitudinal di-
rection and close packing dis-
tance of 1 in microcrystal. In
the microcrystalline plate of 2,
anisotropic optical waveguid-
ing was observed, which was
due to the anisotropic p–p in-
Figure 3. a) Schematic representation of optical waveguiding experiment using a microcrystalline rod of 1 and
the LCM system. b) Output LCM-PL spectra of the microcrystalline rod of 1 with various propagation distan-
ces. c–e) Output LCM-PL spectra of the microcrystalline plate of 2 along the 01 (c), 10 (d), 11 (e) directions.
Insets: LCM-PL spectra measured at the same input and output position for the reference.
teraction and different molecular arrangement depending
on the crystalline direction.
Experimental Section
Crystal data for 1
C60H70N4O10Sn, MW=1125.89, orthorhombic (space group Pbcn), a=
14.420(3), b=22.624(5), c=15.891(3) ꢀ, V=5184.3(18) ꢀ3, Z=4, (Mo
Ka)=0.559 mmꢀ1
, 28314 reflections measured, 5099 unique (Rint =
0.1346), which were used in all calculations, final R=0.0786 (Rw=
0.2125) with reflections having intensities greater than 2s, GOF (F2)=
0.974.
Figure 4. Plot of LCM-PL intensity versus propagation distance of input
light for microcrystals of 1 and 2.
Crystal data for 2
C54H41N4O10Sn, MW=1024.60, monoclinic (space group P21/c), a=
15.078(3), b=14.111(3), c=28.016(6) ꢀ, b=95.19(3)8, V=5936(2) ꢀ3,
Z=4, (Mo Ka)=0.482 mmꢀ1, 32571 reflections measured, 11634 unique
(Rint =0.0947), which were used in all calculations, final R=0.0813 (Rw=
pended on the crystalline axis. Presumably the longer direc-
tion (01 direction) of the microcrystal sample of 2 is the
main p–p stacking direction, corresponding to the a axis
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Chem. Asian J. 2012, 7, 2768 – 2771

Bisalkoxy Tin(IV) Porphyrin Microcrystals
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Acknowledgements
This study was supported by the Korea Science and Engineering Founda-
tion (KOSEF) grant (No. 2012R1A2A2A01045102), Korea Research
Foundation Program (NRF-20120002285) and Priority Research Centers
Program (NRF-20120005860) through the National Research Foundation
of Korea (NRF) funded by the Korean government (MEST).
Keywords: luminescence
interactions · porphyrinoids · tin
·
optical waveguides
·
p–p
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Received: August 16, 2012
Published online: October 2, 2012
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