Synthesis, characterization, and crystal structures of 1,1-disubstituted-2, 3,4,5-tetraphenylgermoles that exhibit aggregation-induced emission

Article abstract of DOI:10.1021/om200259n

A series of 2,3,4,5-tetraphenylgermoles with different 1,1-substituents have been prepared, and their UV-vis absorption and fluorescence spectral profiles in solution were determined. A thin-layer chromatography based method was used to measure their soli

Full text of DOI:10.1021/om200259n

ARTICLE
pubs.acs.org/Organometallics
Synthesis, Characterization, and Crystal Structures of
1,1-Disubstituted-2,3,4,5-tetraphenylgermoles
That Exhibit Aggregation-Induced Emission
Teresa L. Bandrowsky, James B. Carroll, and Janet Braddock-Wilking*
Department of Chemistry and Biochemistry and Center for Nanoscience, University of MissouriꢀSt Louis, St. Louis, Missouri 63121,
United States
S Supporting Information
b
ABSTRACT: A series of 2,3,4,5-tetraphenylgermoles with
different 1,1-substituents have been prepared, and their UVꢀvis
absorption and fluorescence spectral profiles in solution were
determined. A thin-layer chromatography based method was
used to measure their solid-state luminescence. Nine of the 11
new germoles have been characterized crystallographically. The
aggregation-induced emission effect in these germoles was
studied, and significant enhancement of the photoluminescence and quantum yield in mixed solventꢀwater systems was
determined. Scanning and transmission electron microscopy analysis revealed the formation of nanocrystalline aggregates in the
mixed solventꢀwater systems and after exposure of the germoles to acetone, a volatile organic chemical.
’ INTRODUCTION
Silacyclopentadienes, or siloles (Figure 1), are a class of
molecules that have been extensively developed for their poten-
tial application in organic electronics, particularly in flexible
lighting and display panels.1 One of the qualities responsible
for the intense interest in siloles is the high electron affinity that
these cyclic molecules exhibit.2 Such large electron affinities can
Figure 1. Basic metallole structure. M = Si, silole; M = Ge, germole; 2, 3,
4, 5 = C.
be attributed to a low-lying LUMO, which arises from σ*ꢀπ*
conjugation that results from the interaction between the π*
silole molecules too long for conventional πꢀπ stacking inter-
orbital of the butadiene segment and the σ* orbital associated
actions (∼3ꢀ4 Å)1 that typically quench luminogens in the solid
with the two exocyclic bonds on the silicon center.3 The large
and crystalline phases. This mechanism is referred to as restricted
intramolecular rotation (RIR) and is the accepted cause of the
AIE phenomenon.7,8 RIR so effectively deactivates the avenues
that result in nonradiative emission that siloles strongly emit light
in the solid and crystalline phases, such as aggregated suspen-
sions in solventꢀwater mixtures.
electron affinity of the siloles results in another favorable feature,
high electron mobility,4 a desirable attribute that continues to
present challenges in the design of highly efficient organic
electronic devices. There are many silole derivatives that have
been reported to be good electron transporters with electron
mobilities that are 2 orders of magnitude higher than tris-
(8-hydroxyquinolinato)aluminum (Alq3).4,6a Alq3 is a com-
monly used electron-transport (ET) material for organic
light-emitting diodes (OLEDs).5
Siloles have demonstrated high photoluminescence (PL)
quantum yields as both amorphous and crystalline solids, which
can be attributed to the unique photophysical property of
aggregation-induced emission (AIE).6 As a result of the steric
repulsions between the peripheral aryl substituents on the core
ring, intramolecular rotations of the substituents are restricted,
causing the substituents on the silole core to assume a highly
twisted conformation that persists in solution as well as the solid
state. Restriction of intramolecular rotations of the peripheral
substituents effectively blocks nonradiative relaxation channels
and imparts nonplanarity, rendering the distance between adjacent
The AIE effect has now been identified in other luminogens
with similar structural features including germoles,9 the heavier
group 14 congener of siloles. Although germoles emit more
efficiently in solution than siloles,8 their solutions are still only
weakly emissive.9 The increased efficiency in solution, however,
does not diminish the significant AIE effect that is exhibited by
germoles in the solid state or when aggregated in solventꢀwater
mixtures. Germoles, like siloles, are soluble in a variety of com-
mon organic solvents, but insoluble in water.
To date, relatively little has been published on the photo-
luminescence of germoles, despite published evidence of the
Received: March 24, 2011
Published: June 09, 2011
r
2011 American Chemical Society
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Scheme 1
Table 1. Crystallographic Data and Structure Refinement for
8 and 12
similarities between siloles and germoles. In addition to exhibit-
ing the AIE effect, parallels between the electronic structure and
photophysical properties of the two metalloles can be seen by the
similarities in the UVꢀvis absorption and fluorescence profiles,10
electrochemical data, and ab initio calculations of HOMO and
LUMO energy levels.11 Such studies indicate comparable σ*
(SiꢀR) and σ* (GeꢀR) orbitals as well as low-lying LUMO
energy levels, suggesting that the differences in the electronic
structures of germanium and silicon analogues are relatively
minimal despite the slightly larger size of the germanium atom.
This is in contrast to the attributes that would be gleaned from
the low-lying LUMO in stannoles, which are diminished by
significantly less efficient σ*ꢀπ* conjugation that results from a
greater orbital mismatch as well as elongated bond distances
between the larger 5pz σ* orbital of tin and the 2pz π* orbital of
the carbons of the butadiene.10,12 We now report the synthesis
and characterization of a series of new 2,3,4,5-tetraphenylger-
moles with different 1,1-substituents and the experimental data
describing the AIE effect in these germoles. Scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM) images are presented to indicate the nature of the
aggregate formation within these germoles.
8
12
formula
C
44H28F2Ge
C56H40GeP2
847.41
fw
667.29
cryst size/mm
0.32 ꢁ 0.33 ꢁ 0.38
triclinic
0.40 ꢁ 0.60 ꢁ 0.80
triclinic
cryst syst
space group
P1
P1
a/Å
10.555(2)
11.095(2)
15.680(3)
92.23(3)
11.4828(14)
12.9012(13)
18.022(2)
71.258(3)
77.857(4)
64.911(4)
2281.2(4)
1.234
b/Å
c/Å
R/deg
β/deg
104.77(3)
113.52(3)
1607.5(8)
1.379
γ/deg
V/Å3
Dcalcd/g cmꢀ3
Z
2
2
abs coeff/mmꢀ1
θ range/deg
0.996
0.778
1.36 to 34.02
50 264/12 224
[R(int) = 0.027]
numerical
0.7467 and 0.6576
0.0395
1.20 to 24.99
22 480/7943
[R(int) = 0.041]
numerical
1.000 and 0.8625
0.0390
reflns collected/indep reflns
abs correct
’ RESULTS AND DISCUSSION
max. and min. transmn
final R indices [I > 2σ(I)]
R indices (all data)
Synthesis. 1,1-Dichloro-2,3,4,5-tetraphenylgermole (1) was
prepared by a ring-closure reaction of 1,4-dilithio-1,2,3,4-tetra-
phenyl-1,3-butadiene with germanium tetrachloride according to
the procedure reported by Curtis.13 In contrast to the synthesis
of 1,1-dichloro-2,3,4,5-tetraphenylsilole,14 the germole can be
prepared without difficulty in high yield (70ꢀ95%). Isolated 1
was converted to new 1,1-disubstituted germoles (2 through 12)
by addition of various alkynyllithium reagents that had been
generated from commercially available terminal acetylenes and
nBuLi (Scheme 1). Due to the relatively short lifetime of the
lithiated alkyne species,15 the yields of germoles 2 through 12
were lower and varied from 44% to 70%. The diphenylpho-
sphine-substituted precursor is not commercially available and
was prepared according to a known procedure.16 The new
germoles were characterized by multinuclear NMR spectrosco-
py, elemental analyses, X-ray crystallography, and UVꢀvis and
fluorescence spectroscopy. NMR resonances and couplings
exhibited by germoles 2ꢀ12 were within expected values.
0.1121
0.1310
X-ray Crystal Structures. Single crystals of the germoles were
grown from a variety of solvent mixtures, which are noted in
the Experimental Section, as fluorescent greenish-yellow
blocks. X-ray quality crystals were obtained for all of the new
germoles with the exception of 3 and 6, due to the problems
encountered in the recrystallization attempts and inability to
refine the data adequately, respectively. Germole 5 crystallized
as the tetrahydrofuran solvate. Generally, the structure refine-
ment proceeded according to acceptable models with reason-
able parameter discrepancies and residual electron densities.
Table 1 contains the crystallographic data for 8 and 12. A summary
of the crystal data and intensity parameters for all of the remaining
crystal structures that were obtained is provided in the Supporting
Information.
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from the search was 1.933 Å, which is in close agreement to the
average bond length of 1.938 Å for the germoles reported herein.
The average CdC and CꢀC bond lengths of the central ring for
2, 4, 5, and 7ꢀ12 are 1.357 and 1.511 Å, respectively, which once
again correlate well with the CSD data of 1.350 and 1.507 Å. The
average SiꢀC bond length was reported to be 1.869 Å, while the
average CdC and CꢀC bond lengths were 1.363 and 1.494 Å,
respectively.18 The exocyclic GeꢀC bond lengths, 1.895 and
1.893 Å, are slightly longer than the reported CSD data of 1.859
and 1.846 Å, which may be due to the steric requirements of the
bulky 1,1-substituents. The C2ꢀGeꢀC5 bond angle of the
central ring for 2, 4, 5, and 7ꢀ12 is 91.5°, which is in agreement
with the CSD data of 91.3°.
The drawings in Figure 2 illustrate the highly twisted con-
formation that the phenyl substituents assume relative to the
germacyclopentadiene core. The peripheral phenyl substituents
on the ring carbons are twisted out of plane with respect to the
ring core and are twisted with the same sense within the com-
pound. The average dihedral angle for the phenyl substituents on
the 2,5-ring carbons for 2, 4, 5, and 7ꢀ12 was ca. 38° and for the
3,4-ring carbons ca. 63°, which is consistent with the reported
dihedral angles of ca. 30° and ca. 70° for tetraphenyl-substituted
siloles.19
The only exception was 7, whereby the germanium lies in a
special position and has dihedral angles of 15.7° and 65.3° for the
phenyl substituents on the 2,5- and 3,4-ring carbons, respectively.
The dihedral angles reported for 1,1-diphenyl-2,3,4,5-tetraphe-
nylgermole for the 2,5-ring carbon substituents were 0.7° and
46.1°, while the 3,4-ring carbon substituents were 96.8° and
63.4°, respectively.9d An analysis of the data obtained from the
crystal structures suggests that the dihedral angles of 2, 4, 5, and
7ꢀ12 are more similar to those in tetraphenyl-substituted siloles
than the other germole derivatives characterized by Tracy et al.9c,d
Selected torsion angles for three of the germoles are depicted in
Table 2. As a result of these similarities, modifications at the 2,5-
and 3,4-positions of these germoles may be related to similar
effects on the electronic and optical properties that have been
observed in siloles.20
Electronic Transitions. The absorption spectra for solutions
of germoles 2ꢀ12 in methylene chloride were measured, and the
spectral data are summarized in Table 3. All of the germoles
2ꢀ12 exhibit an absorption maximum between 364 and 369 nm.
Germoles 2ꢀ12 weakly emit in the region of 478ꢀ488 nm in
solution at room temperature with quantum yields that range
from 0.0046 to 0.0071. Under similar experimental conditions,
germoles 2ꢀ12 exhibited quantum yields ca. 3ꢁ greater than
either 1,1-dimethyl- or 1,1-diphenyl-2,3,4,5-tetraphenylgermole,
for which the reported quantum yields were 0.0015 and 0.0026,
respectively.9d
Consistent with the trend observed for siloles, manipulations
of the 1,1- substituents imposed a similar shift to longer absorp-
tion maxima as the substituents directly attached to the germa-
nium center increased in electronegativity.21 Among a series of
germoles possessing the same 1,1-substituents, the UV absorp-
tion maxima redshiftintheorderofMe(350 nm) <Ph(358nm) <
CtCH (362 nm) < CtCꢀR (364ꢀ369 nm for 2ꢀ12).9a,d
Substitutions on the β-position of the triple bond (the
terminal position of the alkyne) also reflected a modest red shift
as more electronegative groups were introduced. The upper and
lower range of the absorption maxima are illustrated by Figure 3.
This is in contrast to silole derivatives possessing alkynyl
substituents at the 1,1-positions, where the absorption and
Figure 2. (a) Selected bond distances (Å), angles (deg), and torsions
(deg) for 8. Thermal ellipsoids are shown at the 50% probability level:
Ge1ꢀC30 = 1.890(2), Ge1ꢀC39 = 1.897(2), Ge1ꢀC2 = 1.954(1),
Ge1ꢀC5 = 1.940(2), C2ꢀC3 = 1.362(2), C3ꢀC4 = 1.512(2); C2ꢀ
Ge1ꢀC5 = 91.83(6), Ge1ꢀC2ꢀC3 = 106.2(1), Ge1ꢀC5ꢀC4 =
105.8(1), C2ꢀC3ꢀC4= 117.5(1), C3ꢀC4ꢀC5 = 118.6(1), C30ꢀ
Ge1ꢀC39 = 105.68(7); Ge1ꢀC2ꢀC24-C25 = 36.5(2), Ge1ꢀ
C5ꢀC6-C7 = 30.3(2), C2ꢀC3ꢀC18-C19 = ꢀ113.9(2), C5ꢀC4ꢀ
C12-C13 = 61.8(2). (b) Selected bond distances (Å), angles (deg), and
torsions (deg) for 12. Thermal ellipsoids are shown at the 50%
probability level, and the hydrogen atoms and the disorder on 12 have
been omitted for clarity: Ge1ꢀC50 = 1.929(2), Ge1ꢀC52 = 1.940(3),
C49ꢀC51 = 1.516(4), C50ꢀC51 = 1.349(4), Ge1ꢀC54 = 1.897(4),
Ge1ꢀC56 = 1.904(4), P2ꢀC53 = 1.774(4), P3ꢀC55 = 1.74(1); C50ꢀ
Ge1ꢀC52 = 91.5(1), Ge1ꢀC50ꢀC51 = 106.6(2), C50ꢀC51ꢀC49 =
117.9(3), C51ꢀC49ꢀC52 = 117.3(3), C54ꢀGe1ꢀC56 = 105.5(1);
Ge1ꢀC50ꢀC47ꢀC46 = ꢀ48.7(4), Ge1ꢀC52ꢀC29ꢀC30 = ꢀ35.7(4),
C52ꢀC49ꢀC34ꢀC35 = ꢀ56.7(5).
The molecular structures of 8 and 12 are shown in Figure 2
and are representative of all of the germoles described in this
study. Selected bond lengths and angles listed in Figure 2
illustrate the general agreement of the geometric parameters
for 2, 4, 5, and 7ꢀ12. A search of the Cambridge Structural
Database exhibited nine entries for germacyclopentadienes.17
The geometries of 2, 4, 5, and 7ꢀ12 matched seven of the nine
entries found from the search. The average GeꢀC bond length
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Table 2. Selected Torsion Angles (Degrees) of 8, 9, and 11
phenyl substituent attached to ring carbon
C1
8
9
11
Ge1ꢀC14ꢀC15ꢀC16
31.3(3)
Ge1ꢀC2ꢀC24ꢀC25
33.4(2)
Ge1ꢀC31ꢀC17ꢀC35
ꢀ27.3(1)
C2
C3
C4
C14ꢀC13ꢀC21ꢀC22
63.4(3)
C2ꢀC3ꢀC18ꢀC19
59.1(2)
C31ꢀC11ꢀC42ꢀC15
ꢀ59.1(1)
C13ꢀC12ꢀC27ꢀC28
68.8(3)
C3ꢀC4ꢀC12ꢀC17
65.4(2)
C11ꢀC30ꢀC19ꢀC37
ꢀ65.9(1)
Ge1ꢀC11ꢀC33ꢀC38
36.3(3)
Ge1ꢀC5ꢀC6ꢀC11
42.7(2)
Ge1ꢀC21ꢀC3ꢀC39
ꢀ46.6(1)
Table 3. Absorption and Emission Data for 0.01 mM CH2Cl2
Solutions of 2ꢀ12 at Room Temperature (λex = 370 nm)
a
germole
absorption λ (nm)
emission λ (nm)
ΦF
4
3
364
365
365
366
366
367
368
369
369
369
369
480
478
482
485
486
485
487
483
488
487
486
0.00457
0.00570
0.00464
0.00578
0.00508
0.00569
0.00627
0.00582
0.00538
0.00706
0.00689
5
9
7
6
10
2
8
12
11
Figure 3. Absorption spectra for the methylene chloride solutions of
germoles 2 and 4, which represent the range of the UV absorption
maxima.
a 9,10-diphenylanthracene was used as a standard.
Germole 2 is soluble in acetone, as verified by dynamic light
scattering measurements, but is insoluble in water. Initially 2 was
dissolved in pure acetone, and then a specified amount of water
was added. Once the water was added to the mixture, the
solubility of 2 was reduced, causing aggregation, which resulted
in an increase in the photoluminescence. As illustrated by
Figure 4, the dilute acetone solution of 2 is weakly emissive;
however, as the proportion of water to acetone increased in the
mixed solvent system, the photoluminescence significantly in-
creased. A similar increase in photoluminescence upon addition
of water to either an acetone or acetonitrile solution of the
germole was also observed for 6, 8, 11, and 1,1-diethynyl-2,3,4,5-
tetraphenylgermole.24 All variations within the mixed solvent
system exhibited similar spectral profiles with minimal shifting of
the emission wavelength maximum, which is consistent with
similar siloles and germoles.6b Typically, aggregation results in a
red shift of the emission wavelength.25 Noteworthy is the
indication that the onset of AIE begins at ca. 30% water content
within the mixed solvent system, although the phenomenon is
modest. Tracy and co-workers studied similar concentrations of
1,1-dimethyl- and 1,1-diphenyl-2,3,4,5-tetraphenylgermole in a
variety of mixed solventꢀwater systems and reported the earliest
occurrence of AIE was between 60% and 70% water content in a
mixed waterꢀdioxane system.9c
emission wavelengths remained unchanged despite varying the
β-substituents of the alkyne from hydrogens to phenyls.22 Such
data suggest a greater sensitivity in germoles to the identity of the
substituents, even those that are at more remote bonds from the
germanium center and may allow more control in fine-tuning the
electronic properties.
Solid-State Photoluminescence. The solid-state photolumi-
nescence was measured using a thin-layer chromatography (TLC)
method developed by Tang et al.,22 and the emission wave-
lengths are summarized in Table 4. The thin layer of all the new
germoles absorbed on the TLC plate fluoresce intensely in the
blue-green region. The intensity of the PL emission significantly
increased compared to the corresponding weakly emissive solu-
tions. The trend in the red shift in the emission wavelengths was
not readily apparent; efforts are currently underway to elucidate
the nature of the effects when manipulating the 1,1-substituents
on the luminescence properties of 2ꢀ12 in the solid and crys-
talline phases. The new germoles exhibited Stokes shifts between
117 and 132 nm. These values are consistent with the reported
Stokes shift for siloles, which vary between 120 and 129 nm,22 as
well as those reported for 1,1-dimethyl- or 1,1-diphenyl-2,3,4,5-
tetraphenylgermole (117 and 130 nm, respectively).9d Stokes
shifts greater than 100 nm are necessary for applications that
require ultrahigh sensitivity such as fluorescence imaging mea-
surements and bioprobes for protein detection and quantification.23
Aggregation-Induced Emission. While the AIE effect has
been demonstrated in germoles, the availability of germole
derivatives is limited, which is why the AIE phenomenon was
explored in several of the germoles prepared in this study.
The increase in the PL intensity of 2 correlated with an
increase in the emission quantum yield, as shown in Figure 5.
The quantum yields in the acetone and acetoneꢀwater mixtures
were calculated using 9,10-diphenylanthracene as a standard.26
The quantum yield of the acetone solution of 2 was 0.0040.
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Table 4. Solid-State Emission Data for 2ꢀ12 at Room Tem-
perature (λex = 370 nm)
germole
emission λ (nm)
Stoke’s shift (nm)
6
484
485
486
490
492
493
500
117
3, 4, 7, 9
120, 121, 119, 119
2
117
121
127
124
132
11
5
8, 12
10
Figure 5. Quantum yield of 2 vs the water content of the mixed
acetoneꢀwater systems.
Figure 6. Absorption spectra of 0.01 mM 2 in the mixed acetoneꢀwater
solvent systems.
effect associated with the presence of small, metallic particles.27
Dynamic light scattering measurements taken within ca. 40 min
of preparing the samples also suggest that 2 had aggregated into
nanoparticles with average sizes of 47 and 74 nm in the
acetoneꢀwater mixtures with water contents of 80% and 90%,
respectively.
TEM measurements were used to investigate the nature of the
aggregate formation in the acetoneꢀwater mixtures. In the 10%
acetoneꢀ90% water mixture, the TEM images of 2 depicted the
formation of nanoparticles with individual dimensions that were
closer to 100 nm that aggregate in small clusters (Figure 7a). The
smaller aggregate clusters were more prevalent than the larger
aggregate clusters; an example of one of the larger aggregate
clusters is given in Figure 7b.
The crystalline nature of the particles was confirmed by
electron diffraction patterns (Figure 7c). As in solid phase
crystals, it is reasonable to conclude that the aggregates pack in
an arrangement that minimizes πꢀπ stacking of any planar
segments of the germole. Lack of intermolecular πꢀπ interac-
tions as well as an increased restriction of any molecular motions
imposed by the crystalline lattice would promote the photo-
luminescence observed in the acetoneꢀwater mixtures.
Applications. The germoles, 2, 8, and 1,1-diethynyl-2,3,4,5-
tetraphenylgermole24 were tested as potential chemosensors for
the detection of volatile organic compounds (VOCs) such as
acetone. The photoluminescence was monitored as thin films of
the germoles that were exposed to acetone vapor. TLC plates
Figure 4. (a) PL spectra for 0.01 mM 2 in pure acetone and acetoneꢀ
water mixtures at room temperature (λex = 370 nm); (b) photo of the
solutions of 2 in pure acetone (far left) and acetoneꢀwater mixtures
(40%, 50%, 60%, 70%, 80%, and 90%, respectively), which correlates
with the graph of the PL spectra.
A modest increase in the quantum yield to 0.0050 was observed
at a water content of ca. 30%, implying that intramolecular
motions of 2 were already being minimized. Restriction of the
intramolecular motions of 2 resulted in increased PL intensity
(the AIE effect), which although not discernible by the naked
eye, was detectable with instrumentation. An expanded view of
Figure 5 depicting the increase in the quantum yield at the lower
water contents is available in the Supporting Information. The
increased PL intensity is supported by the increase in quantum
yield. When the water content of the acetoneꢀwater system was
90%, the quantum yield increased to 0.26, which is 65 times
higher than the acetone solution.
All variations within the mixed acetoneꢀwater solvent system
exhibited a similar absorption spectral profile with broad absorp-
tion bands that trail into the long-wavelength region and a slight
red shift of the wavelength maximum (Figure 6). The spectral
profiles imply that 2 has aggregated in the acetoneꢀwater
mixtures, as both broad absorption bands and red shifts are
characteristic optical responses related to the Mie scattering
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Figure 9. Spectra collected at different times depicting the effect of
acetone vapor on 8 (a) and 1,1-diethynyl-2,3,4,5-tetraphenylgermole
(b) coated on a quartz cell. Excitation at 375 and 365 nm, respectively.
representing the “OFF” switch. Once the acetone vapor evapo-
rates, the emission resumes (Figure 8c), suggesting that the
germole aggregates and, once again, emits light, representing the
“ON” switch. This luminescence switch behavior was observed
for all three of the germoles tested and can be repeated multiple
times without diminishing the light emission.
Figure 7. TEM images of 2 illustrating the small aggregate clusters (a)
and larger aggregate clusters (b) that were observed in the waterꢀ
acetone mixture containing 90% water; the electron diffraction pattern
(c) exhibited by aggregates in the 10% waterꢀ90% acetone mixture.
In order to obtain a spectroscopic picture of how the photo-
luminescence changes upon exposure to VOCs, thin films of the
germoles were exposed to the acetone vapor and monitored at
regular time intervals using a spectrofluorimeter. The thin films
were prepared by coating the inner wall of a quartz cell with ca.
10ꢀ3 M methylene chloride solutions of the germoles. A cotton
swab tip saturated with acetone was placed in the bottom of the
cell. A comparison of 8 with 1,1-diethynyl-2,3,4,5-tetraphenyl-
germole,24 an AIE-active germole,9a,b yielded an interesting
result. Upon exposure to acetone vapor, light emission was
rapidly quenched for both 8 and 1,1-diethynyl-2,3,4,5-tetraphe-
nylgermole on TLC plates, but exhibited different luminescent
behavior on quartz. The photoluminescence of the thin film of 8
on quartz decreased until the emission was practically quenched
(Figure 9a), while the photoluminescence of 1,1-diethynyl-
2,3,4,5-tetraphenylgermole blue-shifted and became stronger
over time (Figure 9b).
Figure 8. Photos of the TLC plates of 8 (a) prior to solvent exposure,
(b) after exposure, and (c) after the solvent evaporated.
were spotted with ∼10ꢀ3 M methylene chloride solutions of the
germoles and allowed to thoroughly dry in air. As can be seen in
Figure 8a, the thin film of 8 intensely fluoresces. Upon exposure
to acetone vapor, the emission is rapidly quenched (Figure 8b).
This is consistent with the acetone vapor condensing on the TLC
plate and dissolving the germole, which quenches light emission,
Although this luminescent behavior appears contrary to the
observed quenched emission on the TLC plate, a similar increase
in photoluminescence on quartz was observed for hexaphenylsilole
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Figure 10. SEM images of the thin films of 8 (a) and 1,1-diethynyl-
2,3,4,5-tetraphenylgermole (b), respectively, prior to exposure to acet-
one vapor using a lower EꢀT detector at 250ꢁ.
Figure 11. SEM images of the thin films of 8 (a) and 1,1-diethynyl-
2,3,4,5-tetraphenylgermole (b) after 10 h exposure to acetone vapor
using the higher EꢀT detector at 5000ꢁ and 20 000ꢁ, respectively.
speculated to further aid in restricting intramolecular motions of
the aryl substituents by stiffening the conformation within the
crystalline lattice, which enhanced the PL emission. The packing
arrangement of the crystals of 8 was examined more closely for
evidence of any cooperative bonding interactions. The crystal
structure of 8 illustrates that adjacent molecules pack in such a
way that the phenyl substituents are practically orthogonal and
cannot overlap (Figure 12). The closest distance between the
3,4-phenyl substituents of two adjacent molecules of 8 is 6.9 Å
(Figure 12a), which is too great a distance for any πꢀπ stacking
or CHꢀπ interactions. The ring cores are staggered with respect
to one another with an interplane distance between the germole
cores of 10.5 Å (Figure 12b), too great a distance for any
intermolecular interactions. There are, however, multiple CH
groups as well as fluorine atoms present in 8 that may participate
in nonconventional, weak CHꢀπ hydrogen bonding in two
regions within the packing arrangement. Both potential coop-
erative interactions involve the substituted phenyl ring of one of
the 1,1-substituents with the phenyl substituents on an R-carbon
in the ring core and another 1,1-substituent of a second molecule
located directly above (Figure 12b). The distances between these
segments are 3.0 and 2.7 Å, respectively. These measurements
are within hydrogen-bonded and intermediate (type CHꢀX)
intermolecular approaches of ca. 3.0 Å, which were demonstrated
for edge-to-face orientations in several siloles.29 Given the
evidence of crystallization of 8 with prolonged exposure to
acetone and the additional stabilizing effects of cooperative
CHꢀπ interactions, it is conceivable that the difference in
photoluminescence on quartz between 8 and 1,1-diethynyl-
2,3,4,5-tetraphenylgermole is the result of the time it takes for
these new germoles to efficiently pack. The gradually diminishing
PL of 8 on the quartz thin film implies that the condensing
acetone vapor dissolved the germole. Once dissolved, the longer,
bulkier 1,1-substituents on 8 experienced disorder and, therefore,
required more time to properly orient and assemble so that
crystallization and the ensuing cooperative CHꢀπ interactions
could occur. This could explain why the PL was not enhanced or
blue-shifted within the time period that the quartz thin films were
monitored using a spectrofluorimeter. It will be interesting to see
if germoles in the crystalline phase are stronger, bluer emitters
than their amorphorus phase counterparts, as is the case for many
AIE-active molecules.19 Efforts are currently underway to obtain
absorption and emission data for the crystalline phase of the new
germoles prepared in this report.
(HPS)28 and 1,1-diethynyl-2,3,4,5-tetraphenylsilole.25 SEM
analysis of the quartz thin films conducted by Tang and co-
workers revealed that the VOCs had induced crystallization in
the supersaturated silole solution that formed as the vapor
condensed on the thin film. The crystallization is thought to be
the reason for the blue shift and enhanced emission, a hypothesis
referred to as crystallization-enhanced emission (CEE).6a,b,25,28
According to the CEE effect, there are two types of aggregates in
molecules that exhibit AIE, amorphous and crystalline, and the
transformation from one form to another affects the photolumi-
nescence. The difference between the two forms may lie in the
degree of twisting of the peripheral aryl substituents. The con-
formation of amorphous aggregates was proposed to be slightly
more planar than in crystalline aggregates, which lowered
emission.6b Using the CEE effect to explain the photolumines-
cence behavior observed in our experiment, it is probable that the
acetone vapor condensed on the quartz thin film, forming a liquid
layer containing the 1,1-diethynyl-2,3,4,5-tetraphenylgermole. In
this liquid layer, the germole was not dissolved but instead rapidly
transformed from an amorphous to crystalline form, resulting in
the observed enhanced photoluminescence (Figure 9b). Since
the photoluminescence of the thin film of 8 on quartz decreased
(Figure 9a), then according to the CEE effect, the acetone vapor
condensed on the quartz, forming a thin film that gradually
dissolved the germole, which explains why the emission was
quenched with prolonged exposure to the acetone vapor.
In order to further explore the CEE effect, the morphology of
the quartz thin films of 8 and 1,1-diethynyl-2,3,4,5-tetraphenyl-
germole was investigated by SEM before and after exposure to
acetone vapor. The high-resolution SEM images suggest that the
thin films of 8 (Figure 10a) and 1,1-diethynyl-2,3,4,5-tetraphe-
nylgermole (Figure 10b) are smooth and amorphous prior to
exposure to acetone vapor. The images of the thin films after
being exposed to acetone vapor for 10 hours suggest that acetone
vapor induced both germoles to crystallize (Figure 11). Crystal-
lization was observed for several of the other new germoles with
minimal shifting of the emission wavelength maximum and
diminished photoluminescence with prolonged exposure to
acetone vapor.
Since this unique PL behavior for the thin films on quartz was
only observed for hexaphenylsilole (HPS) and 1,1-diethynyl-
2,3,4,5-tetraphenylsilole, Tang proposed that the highly symme-
trical structures of these two siloles were able to pack more
efficiently than unsymmetric siloles, which may have aided the
crystallization process.25 As a result of the efficient packing
arrangement, multiple CHꢀπ bonds were identified that were
In conclusion, a series of new 2,3,4,5-tetraphenylgermoles
with different 1,1-substituents were synthesized and character-
ized. The new germoles intensely fluoresce in the blue-green
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manifold Schlenk line or in a drybox. Diethyl ether and THF were
distilled over sodium/9-fluorenone prior to use. Methylene chloride was
distilled over CaH2. Chloroform-d was purchased from Cambridge
Isotopes Inc. and dried over activated molecular sieves. Other commer-
cially available reagents were purchased from Aldrich Chemical Co. and
were used as received. NMR spectra were recorded on Bruker Avance-
300 MHz and Bruker ARX-500 MHz instruments at ambient temperature.
Spectroscopic data were recorded at 300 and 500 MHz, respectively, for
1H, 125 and 75 MHz respectively for 13C, 202 MHz for 31P, and 282
MHz for 19F. Proton, carbon, phosphorus, and fluorine chemical shifts
(δ) are reported relative to the residual protio- and deuterio-chloroform,
external H3PO4, and CFCl3, respectively. Chemical shifts are reported in
ppm and the coupling constants in hertz. Melting point determinations
were obtained on a Mel-Temp melting point apparatus and are
uncorrected. UVꢀvis and fluorescence spectra were measured on a
Cary 50 Bio UVꢀvisible and Cary Eclipse fluorescence spectrophot-
ometer, respectively. Emission spectra were measured using the λmax
value for each compound as determined by the absorption spectra.
Elemental analysis determinations were performed by Atlantic Micro-
labs, Inc., Norcross, GA. The X-ray crystallographic data were collected
on a Bruker Apex II diffractometer equipped with a CCD area detector.
For solid-state PL measurements, aluminum TLC plates (Merck,
silica 60 F254) were used. Dichloromethane solutions of the germoles
(0.67 mg/mL) were used as the developing media. The coated TLC
plates were excited at an angle of 20° in the spectrofluorometer. For
dynamic light scattering measurements, the hydrodynamic radius (RH)
was measured at room temperature with a DynaPro Titan instrument
(Wyatt Technology, Santa Barbara, CA). Samples (30 μL) were placed
directly into a quartz cuvette, and light scattering intensity was collected
at a 90° angle using a 10 s acquisition time. Data regularization with
Dynamics (version 6.7.1) generated histograms of percent mass versus RH.
For TEM images, a droplet of 0.05 mM acetoneꢀwater mixture was
applied to a lacey carbon film on a square mesh copper grid (Ted Pella
Inc.) and allowed to air-dry at 25 °C. The aggregated samples were
visualized with a Phillips EM 430 transmission electron microscope
operated at 300 000 eV and magnifications of 110 000 and 150 000,
respectively.
For SEM images, thin films were prepared by coating quartz slides
with ca. 10ꢀ3 M methylene chloride solutions of the germoles. The
morphologies were visualized with a JEOL-6320F field emission scan-
ning electron microscope after sputtering a thin layer of gold onto the
samples with a Hummer VI sputtering system. The operating parameters
for the images taken with the lower detector were an acceleration voltage
of 5 keV, probe current #3, objective lens aperture 3, and a working
distance of 15 mm. The operating parameters for the higher detector
were similar except that an acceleration voltage of 15 keV and a working
distance of 8 mm were employed.
Representative Procedure. Formation of 1,10-Bis(4-(trifluor-
omethyl)phenyl)-2,3,4,5-tetraphenylgermole (2). A solution of n-BuLi
(0.50 mL, 2.5 M in hexane, 1.3 mmol) was added dropwise to a solution
of 1-ethynyl-4-trifluorotoluene (0.20 mL, 1.3 mmol) in dry THF
(1.5 mL) that had been cooled to ꢀ78 °C. Once the addition was
complete, the colorless reaction mixture was stirred at this temperature
for 15 min. The resulting alkynyllithium solution was then added in one
portion to a solution of 1,10-dichloro-2,3,4,5-tetraphenylgermole (0.31 g,
0.63 mmol) in dry THF (5 mL) that had been cooled to 0 °C. The yellow
reaction mixture was allowed to gradually warm to room temperature
and stirred overnight. The reaction mixture was quenched with water
(0.25 mL), stirred for an additional 15 min, and then dried over
MgSO4. After filtering, the solvent was removed by rotary evaporation.
The crude product was purified on a silica gel column using tolue-
neꢀhexane (2:1) as the eluent to give 2 as a yellow solid (340 mg,
71%). X-ray quality crystals were grown by slow evaporation from
a methylene chlorideꢀdiethyl ether mixed-solvent system. All of
Figure 12. Packing arrangement of crystalline 8, (a) showing the
intermolecular distance between two molecules of 8 (6.9 Å) at the
3,4-diphenyl substituents; (b) showing the interplane separation be-
tween two molecules of 8 and the two shortest CꢀHꢀπ hydrogen-
bonding interactions of 3.0 and 2.7 Å.
region (478ꢀ488 nm) in the solid phase. The electronegativity
of the 1,1-substituents exhibited a modest inductive effect on the
UVꢀvis and fluorescence wavelength maxima. Although the new
germoles exhibited higher quantum yields in solution than other
characterized germoles, their room-temperature solutions are
only weakly emissive. In comparison to siloles, the new germoles
were ca. 3ꢁ more emissive in solution, resulting in a smaller
enhancement of luminescence when aggregated. The smaller
enhancement between the luminescence of the molecularly
dissolved and aggregated solutions of these germoles should
not discourage exploiting the AIE effect in these systems. The
AIE effect in the new germoles is quite pronounced and holds
great promise for potential applications as efficient emitters in
aqueous media. TEM analysis indicated the formation of crystal-
line nanoparticles in the mixed solventꢀwater systems, which
aggregated into various sized clusters, resulting in enhanced PL
and quantum yields. The AIE effect was utilized to investigate
potential applications of the new germoles as chemosensors for
VOCs. The photoluminescence on TLC plates exhibited rever-
sible “on” and “off” switch behavior. By controlling the aggrega-
tion process, the light emission could be quenched in the
presence of organic vapors, but resumed upon evaporation of
the VOC.
’ EXPERIMENTAL SECTION
General Comments. All reactions were performed under an inert
atmosphere of argon using flame- or oven-dried glassware on a dual-
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the other germoles were prepared in a similar manner. The eluent and
recrystallization solvent systems used in the purification of each
germole are indicated.
153.0, 138.8, 138.1, 136.3, 134.2, 130.1, 130.0, 129.8, 128.1, 127.9,
126.8, 126.6, 124.1, 119.7, 118.2, 117.1, 106.9, 85.2. Anal. Calcd for
C56H38GeO2: C, 82.47; H, 4.70. Found: C, 82.16; H, 4.82.
Formation of 1,1-Bis(4-(trifluoromethyl)phenyl)-2,3,4,5-tetraphe-
nylgermole (2). Mp: 179ꢀ180 °C. 1H NMR (500 MHz): δ 7.63
(d, J = 8 Hz, 4H), 7.57 (d, J = 8 Hz, 4H), 7.22 (d, J = 8 Hz, 4H), 7.16
(t, J = 8 Hz, 4H), 7.14ꢀ7.09 (m, 2H), 7.09ꢀ7.03 (m, 6H), 6.91ꢀ6.86
(m, 4H). 13C{H} NMR (125 MHz): δ 153.5, 138.4, 137.7, 135.4, 132.7,
130.9 (q, J = 33 Hz), 129.85, 129.80, 128.2, 127.9, 127.0, 126.9, 126.2,
125.3 (q, J = 8 Hz), 124.9, 105.8, 88.6. 19F{1H} NMR (282 MHz):
δ ꢀ62.9. Anal. Calcd for C46H28F6Ge: C, 72.00; H, 3.68. Found: C,
71.74; H, 3.57.
Formation of 1,1-Bis(4-(trifluoromethoxy)phenyl)-2,3,4,5-tetraphe-
nylgermole (3). Purification of 3 by column chromatography using silica
gel and tolueneꢀhexane (2:1) as the eluent yielded a yellow solid
(461 mg, 67%). Mp: 113ꢀ114 °C. 1H NMR (500 MHz): δ 7.61ꢀ7.57
(m, 4H), 7.28ꢀ7.25 (m, 4H), 7.22ꢀ7.17 (m, 8H), 7.17ꢀ7.14 (m, J = 5,
2 Hz, 2H), 7.13ꢀ7.07 (m, 6H), 6.94ꢀ6.90 (m, 4H). 13C{1H} NMR
(125 MHz): δ 153.4, 149.6, 138.6, 137.8, 135.8, 134.1, 129.9, 129.8,
128.2, 127.9, 126.9, 126.8, 121.3, 120.9, 119.5, 105.8, 86.9. 19F{1H}
NMR (282 MHz): δ ꢀ57.7. Anal. Calcd for C46H28F6GeO2: C, 69.12;
H, 3.53. Found: C, 68.71; H, 3.59.
Formation of 1,1-Bis(p-tolylethynyl)-2,3,4,5-tetraphenylgermole (4).
Purification of 4 by column chromatography using silica gel and hexaneꢀ
toluene (2:1) as the eluent yielded a yellow solid (317 mg, 70%).
Yellow crystals were grown by slow evaporation from a methylene
chlorideꢀhexane mixed-solvent system. Mp: 210.5ꢀ212 °C. 1H NMR
(500 MHz): δ 7.45 (d, J = 8 Hz, 4H), 7.31ꢀ7.27 (m, 5H), 7.20ꢀ7.12
(m, 10H), 7.11ꢀ7.05 (m, 6H), 6.94ꢀ6.90 (m, 4H), 2.38 (s, 6H).
13C{1H} NMR (125 MHz): δ 152.9, 139.4, 138.8, 138.1, 136.4, 132.4,
130.0, 129.8, 129.1, 128.1, 127.8, 126.7, 126.6, 119.6, 107.5, 85.2, 21.7.
Anal. Calcd for C46H34Ge: C, 83.79; H, 5.20. Found: C, 83.46; H, 5.64.
Formation of 1,1-Bis((4-methoxyphenyl)ethynyl)-2,3,4,5-tetraphe-
nylgermole (5). Purification of 5 by column chromatography using silica
gel and tolueneꢀhexane (2:1) as the eluent yielded a yellow solid
(200 mg, 46%). Yellow crystals were grown by slow evaporation from a
tetrahydrofuranꢀmethanol mixed-solvent system. Mp: 250ꢀ252 °C.
1H NMR (500 MHz): δ 7.48ꢀ7.43 (m, 4H), 7.24 (m, 4H), 7.14
(m, 4H), 7.11ꢀ7.06 (m, 2H), 7.06ꢀ7.01 (m, 6H), 6.90ꢀ6.86 (m, 4H),
6.84ꢀ6.80 (m, 4H), 3.79 (s, 6H). 13C{1H} NMR (125 MHz): δ 160.3,
152.8, 138.9, 138.2, 136.5, 134.0, 129.9, 129.8, 128.1, 127.8, 126.7, 126.5,
114.8, 113.9, 107.3, 84.4, 68.1, 55.4, 25.7. Anal. Calcd for C46H34GeO2.
One C4H8O: C, 78.65; H, 5.54. Found: C, 78.45; H, 5.42.
Formation of 1,1-Bis((1,1-biphenyl)-4-ylethynyl)-2,3,4,5-tetraphe-
nylgermole (6). Purification of 6 by column chromatography using
silica gel and hexaneꢀtoluene (2:1) as the eluent yielded a yellow solid
(282 mg, 58%). Yellow crystals were grown by slow evaporation from a
tetrahydrofuranꢀmethanol mixed-solvent system. Mp: 186ꢀ188 °C.
1H NMR (500 MHz): δ 7.63ꢀ7.53 (m, 12H), 7.47ꢀ7.41 (m, 4H),
7.38ꢀ7.34 (m, 2H), 7.29ꢀ7.27 (m, 4H), 7.19ꢀ7.14 (m, 4H),
7.13ꢀ7.09 (m, 2H), 7.08ꢀ7.03 (m, 6H), 6.93ꢀ6.87 (m, 4H). 13C{1H}
NMR (125 MHz): δ 153.1, 141.9, 140.4, 138.8, 138.0, 136.2, 132.9,
129.98, 129.87, 129.0, 128.2, 127.92, 127.88, 127.2, 127.0, 126.8, 126.7,
121.5, 107.2, 86.7. Anal. Calcd for C56H38Ge: C, 85.84; H, 4.89; Found:
C, 85.86; H, 4.82.
Formation of 1,1-Bis((3-fluorophenyl)ethynyl)-2,3,4,5-tetraphenyl-
germole (8). Purification of 8 by column chromatography using silica gel
and hexaneꢀtoluene (2:1) as the eluent yielded a yellow solid (271 mg,
65%). Yellow crystals were grown by slow evaporation from a methylene
chlorideꢀhexane mixed-solvent system. Mp: 181.5ꢀ182.5 °C. 1H NMR
(500 MHz): δ 7.32ꢀ7.28 (m, 4H), 7.22 (m, 6H), 7.16 (m, 4H),
7.13ꢀ7.09 (m, 2H), 7.08ꢀ7.03 (m, 8H), 6.90ꢀ6.86 (m, 4H). 13C{1H}
NMR (125 MHz): δ 162.4 (d, J = 247 Hz), 153.3, 138.6, 137.8, 135.7,
130.0 (d, J = 9 Hz), 129.9, 129.8, 128.3 (d, J = 3 Hz), 128.2, 127.9, 126.9,
126.8, 124.3 (d, J = 9 Hz), 119.2 (d, J = 23 Hz), 116.7 (d, J = 22 Hz),
105.9 (d, J = 3 Hz), 86.9. 19F{1H} NMR (282 MHz): δ ꢀ113.2. Anal.
Calcd for C44H28F2Ge: C, 79.19; H, 4.23. Found: C, 78.83; H, 4.00.
Formation of 1,1-Bis(thiophen-3-ylethynyl)-2,3,4,5-tetraphenylger-
mole (9). Purification of 9 by column chromatography using silica gel
and hexaneꢀtoluene (2:1) as the eluent yielded a yellow solid (418 mg,
70%). Yellow crystals were grown by slow evaporation from a methylene
chlorideꢀhexane mixed-solvent system. Mp: 254ꢀ255.5 °C. 1H NMR
(500 MHz): δ 7.60 (d, J = 3 Hz, 2H), 7.28ꢀ7.24 (m, 6H), 7.22ꢀ7.14
(m, 6H), 7.14ꢀ7.10 (m, 2H), 7.10ꢀ7.05 (m, 6H), 6.93ꢀ6.88 (m, 4H).
13C{1H} NMR (125 MHz): δ 153.1, 138.8, 138.0, 136.1, 130.9, 130.4,
130.0, 129.9, 128.1, 127.9, 126.8, 126.7, 125.4, 121.9, 102.3, 85.6. Anal.
Calcd for C40H26GeS2: C, 74.67; H, 4.07. Found: C, 74.88; H, 3.97.
Formation of 1,1-Bis((3-pyridinyl)ethynyl)-2,3,4,5-tetraphenylger-
mole (10). Purification of 10 by column chromatography using silica
gel and methanol as the eluent yielded a yellow solid (472 mg, 60%).
Yellow crystals were grown by slow evaporation from a methylene
1
chlorideꢀdiethyl ether mixed-solvent system. Mp: 220ꢀ221.5 °C. H
NMR (300 MHz): δ 8.76 (dd, J = 2, 0.7 Hz, 2H), 8.56 (dd, J = 5, 1.7 Hz,
2H), 7.81 (dt, J = 8, 1.9 Hz, 2H), 7.29ꢀ7.03 (m, 16H), 6.91ꢀ6.86
(m, 4H). 13C{1H} NMR (125 MHz): δ 153.5, 153.1, 149.5, 139.3,
138.5, 137.7, 135.5, 129.9, 129.8, 128.3, 128.0, 127.0, 126.9, 123.1, 119.7,
104.0, 89.7. Anal. Calcd for C42H28GeN2: C, 79.65; H, 4.46. Found: C,
79.18; H, 4.36.
Formation of 1,1-Bis((2-pyridinyl)ethynyl)-2,3,4,5-tetraphenylger-
mole (11). Purification of 11 by column chromatography using silica
gel and methanol as the eluent yielded a yellow solid (447 mg, 66%).
Yellow crystals were grown by slow diffusion from a hexaneꢀmethylene
chloride (2:1) mixed-solvent system. Mp: 260 °C (dec). 1H NMR
(300 MHz): δ 8.59 (ddd, J = 4.9, 1.7, 0.9 Hz, 2H), 7.66 (td, J = 7.7, 1.8
Hz, 2H), 7.54 (dt, J = 7.8, 1.1 Hz, 2H), 7.29ꢀ7.19 (m, 6H), 7.15ꢀ7.02
(m, 10H), 6.90ꢀ6.84 (m, 4H). 13C{1H} NMR (75 MHz): δ 153.8,
150.4, 142.9, 138.9, 137.9, 136.6, 135.7, 130.22, 130.15, 128.5, 128.4,
128.2, 127.1, 127.0, 124.0, 105.9, 86.6. Anal. Calcd for C42H28GeN2: C,
79.65; H, 4.46. Found: C, 79.39; H, 4.91.
Formation of 1,1-Bis((diphenylphosphino)ethynyl)-2,3,4,5-tetra-
phenylgermole (12). Purification of 12 by column chromatography
using silica gel and hexaneꢀtoluene (2:1) as the eluent yielded a yellow
solid (450 mg, 44%). Yellow crystals were grown by slow evaporation
from diethyl ether. Mp: 154.5ꢀ156 °C. 1H NMR (500 MHz): δ 7.58ꢀ
7.53 (m, 8H), 7.34ꢀ7.24 (m, 14H), 7.23ꢀ7.19 (m, 4H), 7.16ꢀ
7.13 (m, 6H), 7.10ꢀ7.05 (m, 6H), 6.89 (dd, J = 8, 1.7 Hz, 4H).
13C{1H} NMR (125 MHz): δ 153.4, 138.5, 137.6, 135.7, 135.5 (d, JPC
=
Formation of 1,1-Bis((4-phenoxyphenyl)ethynyl)-2,3,4,5-tetraphe-
nylgermole (7). Purification of 7 by column chromatography using silica
gel and hexaneꢀtoluene (2:1) as the eluent yielded a yellow solid
(254 mg, 50%). Yellow crystals were grown by slow evaporation from a
tetrahydrofuranꢀmethanol mixed-solvent system. Mp: 204ꢀ205 °C.
1H NMR (500 MHz): δ 7.54ꢀ7.49 (m, 4H), 7.42ꢀ7.36 (m, 4H),
7.30ꢀ7.25 (m, 4H), 7.20ꢀ7.15 (m, 6H), 7.14ꢀ7.10 (m, 2H),
7.10ꢀ7.07 (m, 6H), 7.06ꢀ7.02 (m, 4H), 6.96ꢀ6.92 (m, 4H),
6.92ꢀ6.89 (m, 4H). 13C{1H} NMR (125 MHz): δ 158.4, 156.4,
6 Hz), 132.7 (d, JPC = 21 Hz), 130.0, 129.9, 129.2, 128.8 (d, JPC = 8 Hz),
128.2, 127.9, 126.9, 126.8, 107.2 (d, JPC = 18 Hz), 106.5 (d, JPC = 3 Hz).
31P{1H} NMR (202 MHz): δ ꢀ32.2. Anal. Calcd for C56H40GeP2: C,
79.36; H, 4.76. Found: C, 79.69; H, 5.06.
X-ray Structure Determination. Crystals of X-ray diffraction
quality were obtained by slow evaporation from a methylene chlorideꢀ
hexane mixed-solvent system for 8 and a slow evaporation of a saturated
diethyl ether solution for 12. Crystals of appropriate dimension was
mounted on a glass capillary in a random orientation. Preliminary
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examination and data collection were performed using a Bruker Kappa
Apex II charge coupled device (CCD) detector system single-crystal
X-ray diffractometer using an Oxford Cryostream LT device. Data were
collected using graphite-monochromated Mo KR radiation (λ = 0.71073
Å) from a fine focus sealed-tube X-ray source. Preliminary unit cell
constants were evaluated with a set of 36 narrow frame scans. Typical
data sets consist of combinations of ϕ scan frames with typical scan width
of 0.5° and exposure time of 15ꢀ20 s/frame at a crystal to detector
distance of 4.0 cm. The collected frames were integrated using an
orientation matrix determined from the narrow frame scans. Apex II and
SAINT software packages30 were used for data collection and data
integration. Final cell constants were determined by global refinement of
reflections from the complete data set. Collected data were corrected for
systematic errors using SADABS30 based on the Laue symmetry using
equivalent reflections.
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Crystal data and intensity data collection parameters are listed in
Table 1.
Structure solution for 8 was carried out using the SIR-92 software
package,31 and structure refinement was performed using the CRYS-
TALS software package.32 Structure solution and refinement for 12
was performed using the SHEL-XTL 97 package.30 The structures
were solved by direct methods in the triclinic space group P1 and
refined with full matrix least-squares refinement by minimizing ∑w-
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at the 2,5-positions as well those directly attached to the silicon center.
By increasing the steric bulk of the substituents at these positions, they
were able to restrict intramolecular motions; this rigidity imparted by the
restricted intramolecular motions increased the photoluminescence. For
additional information refer to: Boydston, A. J.; Pagenkopf, B. L. Angew.
Chem., Int. Ed. 2004, 43, 6336–6338.
2
2 2
(Fo ꢀ Fc ) . All non-hydrogen atoms were refined anisotropically to
convergence. One of the phenyl rings in 12 is disordered over two
positions. Disorder was resolved with 50% occupancy atoms. All H
atoms were added in the calculated position and were refined using
appropriate riding models. The models were refined to convergenece
to the final residual values of R1 = 4.0% and wR2 = 11.2% for 8 and R1 =
3.9% and wR2 = 13.1% for 12. Complete listings of geometrical
parameters, positional and isotropic displacement coefficients for
hydrogen atoms, and anisotropic displacement coefficients for the
non-hydrogen atoms are submitted as Supporting Information within
the associated CIF files.
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S
Supporting Information. Expanded graph of the quan-
b
tum yield versus water content for 2, CIF files, crystal structures
with labeled atoms and an abbreviated listing of the crystal data,
structure solution and refinement, bond lengths and angles, and
anisotropic displacement parameters for germoles 2, 4, 5, and
7ꢀ11. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wilkingj@umsl.edu. Tel: þ1-314-516-6436. Fax:
1-314-516-5342.
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’ ACKNOWLEDGMENT
(21) Yamaguchi, S.; Jin, R. Z.; Tamao, K. J. Organomet. Chem. 1998,
559, 73–80.
The authors would like to thank Dr. Michael R. Nichols in the
Chemistry and Biochemistry Department for dynamic light
scattering measurements, Dr. Nigam Rath for consultations
concerning X-ray crystallography, Dr. Dan Zhou for training
on the SEM, and David C. Osborn for the TEM measurements.
Funding from the National Science Foundation is greatly appre-
ciated (CHE-0719380) and MRI (CHE-0420497) for the Apex
II diffractometer.
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