Tris(trifluoromethyl)germylethynyl derivatives of biphenyl and anthracene: Synthesis, structure, and evidence of the intramolecular charge transfer on the germanium center

Article abstract of DOI:10.1016/j.jorganchem.2015.07.027

Symmetrical and unsymmetrical 4,4′-biphenyl, and 9,10-anthracene derivatives with tris(trifluoromethyl)germylethynyl -CC-Ge(CF₃)₃ substitutes have been prepared, their properties have been studied and compared with those of dimethyl(

Full text of DOI:10.1016/j.jorganchem.2015.07.027

Accepted Manuscript
Tris(trifluoromethyl)germylethynyl Derivatives of Biphenyl and Anthracene: Synthesis,
Structure, and Evidence of the Intramolecular Charge Transfer on the Germanium
Center
Nikolai L. Ermolaev, Ilya V. Lenin, Georgii K. Fukin, Andrei S. Shavyrin, Mikhail A.
Lopatin, Olga V. Kuznetsova, Boris A. Andreev, Denis I. Kryzhkov, Stanislav K.
Ignatov, Evgeny P. Chuhmanov, Nadezhda T. Berberova, Konstantin P. Pashchenko
PII:
S0022-328X(15)30089-9
10.1016/j.jorganchem.2015.07.027
JOM 19171
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 28 March 2015
Revised Date: 15 July 2015
Accepted Date: 20 July 2015
Please cite this article as: N.L. Ermolaev, I.V. Lenin, G.K. Fukin, A.S. Shavyrin, M.A. Lopatin, O.V.
Kuznetsova, B.A. Andreev, D.I. Kryzhkov, S.K. Ignatov, E.P. Chuhmanov, N.T. Berberova, K.P.
Pashchenko, Tris(trifluoromethyl)germylethynyl Derivatives of Biphenyl and Anthracene: Synthesis,
Structure, and Evidence of the Intramolecular Charge Transfer on the Germanium Center, Journal of
Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.07.027.
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ACCEPTED MANUSCRIPT
Synopsis
Unsymmetrical 4-[tris(trifluoromethyl)germylethynyl]biphenyl shows the energy and charge
transfer from the aromatic ligand on the germyl perfluoromethyl group at a laser pulse excitation
9
-1
in solution at room temperaure. The rate constant of the charge transfer is 2.7×10 s . It is
assumed that the key role in the mechanism of charge transfer and stabilization of radical ion
pairs in the excited state of play the germanium 4d AO.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Tris(trifluoromethyl)germylethynyl Derivatives of
Biphenyl and Anthracene: Synthesis, Structure, and
Evidence of the Intramolecular Charge Transfer on
the Germanium Center
‡
ǁ
§
§
Nikolai L. Ermolaev* , Ilya V. Lenin , Georgii K. Fukin , Andrei S. Shavyrin , Mikhail A.
§
§
｣
⟘
⟘
Lopatin , Olga V. Kuznetsova , Boris A. Andreev , Denis I. Kryzhkov , Stanislav K. Ignatov ,
｣
├
├
Evgeny P. Chuhmanov , Nadezhda T. Berberova , Konstantin P. Pashchenko .
‡
Institute of Applied Physics, Russian Academy of Sciences, 46 Ulyanov Str., Nizhny Novgorod
6
03950, Russia
ǁ
P&G Capella Ltd, 1 Avtozavodskoye highway, Dzerzhinsk 606000, Russia
§
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, 49
Tropinin Str., Nizhny Novgorod 603950, Russia
⟘
Institute for Physics of Microstructures, Russian Academy of Sciences, 7 Academic Str.,
Nizhny Novgorod 603950, Russia
｣
N.I. Lobachevsky State University of Nizhny Novgorod, 23 Gagarin Avenue, Nizhny
Novgorod 603950, Russia
├
Astrakhan State Technical University, 16 Tatishchev Str., Astrakhan 414056, Russia
_
_____________
Corresponding author: Tel: +7-961-637-5849, Fax: +7-831-467-0218.
E-mail address: nick-ermolaev@yandex.ru (N. L. Ermolaev).
1

ACCEPTED MANUSCRIPT
ABSTRACT
Symmetrical and unsymmetrical 4,4'-biphenyl, and 9,10-anthracene derivatives with
tris(trifluoromethyl)germylethynyl –C≡C–Ge(CF ) substitutes have been prepared, their
3
3
properties have been studied and compared with those of dimethyl(phenyl)silylethynyl –C≡C–
Si(Ph)Me compounds. UV-visible absorption, steady-state, and time-resolved fluorescence
2
spectra in solution for this germanium and silicon compounds have been investigated. The
process of photoinduced electron-transfer from the aromatic group to the germanium center has
been found in the unsymmetrical 4-biphenyl–C≡C–Ge(CF ) molecule. Anthracene derivatives
3
3
with –C≡C–Ge(CF ) substitutes have been characterized crystallographically.
3
3
Keywords: germanium; charge transfer; conjugation; excited state; perfluoroalkyl
INTRODUCTION
The perfluorination conjugated organic π-systems and the inclusion of fluorinated electron-
withdrawing groups is a well-known technique for creating high performance n-type
1
-4
semiconductor materials. Typically, the objects modified by fluorination are aromatic
π-systems, and there are few examples of the use of other groups, e.g. containing silicon or
germanium.
Meanwhile, the introduction of silicon and germanium in organic semiconductor materials
5
,6
leads to a change of their electronic properties. Since organosilicon and organogermanium
groups have geometry varying from linear fragments in the main chain of the polymers and
7
-12
13-16
oligomers to the unique cycle structures of siloles and germoles,
the use of the fragments
allows fine tuning of the electronic properties. For some silicon-based copolymers and small
2

ACCEPTED MANUSCRIPT
organogermanium molecules even with a relatively short π-conjugation length observed the
1
0,17,18
lowering of the LUMO level and the d-orbital participation of silicon atoms.
In the majority of the previously studied semiconductor materials with Si and Ge atoms alkyl
and aryl groups that are weak electron donors are linked and there are no examples with electron-
acceptor perfluoroalkyl groups.
Another important property of fluorinated organic semiconductors is the increased resistance
to environmental factors, and a significant increase in the endurance of the semiconductor
1
9
materials.
In this paper, we tried to answer the following question: How does the powerful electron-
2
0-25
withdrawing (CF ) Ge group,
which is the perfluorinated analogue of the Me Ge group,
3
3
3
affect aromatic π-conjugated systems?
Chart 1. Model ethynyl compounds based on biphenyl and anthracene
For this, several model organic structures based on conjugated ethynyl derivatives of biphenyl
and anthracene containing the Me PhSi (1-4), Et Sn (5-8) and (CF ) Ge groups (9-12) were
2
3
3 3
synthesized (Chart 1). Silicon and germanium compounds were studied in detail, while tin
derivatives were used as intermediates in the synthesis of fluorinated compounds of germanium.
3

ACCEPTED MANUSCRIPT
The choice of Me PhSi and Et Sn groups was random and more connected with the synthetic
2
3
chain to (CF ) Ge derivatives. However, for our purposes, the choice of non-fluorinated silicon
3
3
Me PhSi group seems justified in order to set off the properties of a pronounced electron-
2
acceptor, as is the (CF ) Ge group. We recently discovered anomalous properties of the
3
3
unsymmetrical acetylene PhC≡C(CF ) Ge compared to other non-fluorinated acetylenes,
3
3
2
5
including those containing the Me PhSi group.
2
RESULTS AND DISCUSSIONS
Synthesis and characterization. Silicon compounds 1-4 were synthesized by the reaction of
catalytic cross-coupling arene halides with the silicon acetylene derivative Me PhSiC≡CH on
2
palladium catalyst (Scheme 1). Unsymmetrical compounds 1,3 with one silyl group and
symmetrical compounds 2,4 with two Me PhSi groups were prepared using this method.
2
Scheme 1. Synthesis ethynyl derivatives of biphenyl and anthracene containing Me PhSi , Et Sn
2
3
and (CF ) Ge group
3
3
4

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Group Me PhSi in 1-4 easily underwent desilylation in an alkaline solution (like the Me Si-
2
3
2
6-28
ethynyl derivatives
) to give the corresponding biphenyl and anthracene acetylenes which
were used for the synthesis of tin organic compounds 5-8.
Aril acetylenes react with organotin amide Et SnNEt quantitatively to give the corresponding
3
2
triethylstannylethynyl derivatives. Biphenyl compounds 5,6 were yielded as a viscous liquid. The
attempts to clear 5,6 by column chromatography on silica gel followed by complete Et Sn groups
3
broke away and formed the corresponding aryl acetylenes. Anthracene compounds 7,8 were
isolated as yellow crystals. Organotin compounds 5-8 (see Supporting Information, Figures S9-
S20) were used as starting reagents for the synthesis of fluorinated germanium derivatives 9-12.
The synthesis of organic and organometallic compounds containing the (CF ) Ge group
3
3
1
8-23
requires compliance with specific conditions
It is known that cleavage of the CF -Ge bond
3
occurs not only under the influence of hard nucleophiles (organolithium RLi and Grignard
27
reagents RMgX), but even in the weakly basic solvent such as alkyl amines R N. This imposes
3
significant limitations on the synthesis conditions: nucleophilic reagents cannot be used and
solvents should be not basic and not very polar.
Given the above, we have assumed that the electrophilic substitution reactions with
tris(trifluoromethyl)germanium iodide (CF ) Ge as an electrophilic agent, with Et Sn being a
3
3
3
ʺgood leaving groupʺ when using stannylethynyl substrates 5-8, may be an effective method
for the synthesis of tris(trifluoromethyl)germylethynyl compounds 9-12. (Electrophilic
2
8
29
destannylation reactions are widely used for the synthesis of organic and organometallic
compounds).
Indeed, we found that 5-8 readily react with (CF ) GeI without catalyst in 1,2-
3
3
dimethoxyethane (DME) at room temperature and formation of
5

ACCEPTED MANUSCRIPT
tris(trifluoromethyl)germylethynyl derivatives 9-12 with high yield (60-75%) is observed. A
liquid by-product of organotin (Et SnI) is easily separated from the target compounds by
3
distillation under vacuum at a temperature below 100°C. Organogermanium compounds 9-12
were purified to high purity by vacuum sublimation.
As expected, the introduction of highly fluorinated (CF ) Ge groups in our conjugated model
3
3
structures, increases their oxidative stability and photostability. For example, monitoring of the
-5
absorption spectra revealed that dilute solution (10 Mol/L) of 11 in methylene chloride with
exposure to air and natural laboratory light has a half-life of 41 days (see Supporting
3
0
Information, Figure S33). It is a good result for this class of polyacenes. It should be noted that
9
-12 in solid form do not change when exposed to air for an indefinitely long time.
All new compounds were characterized by IR and NMR spectroscopy. The IR spectra of
biphenyl derivatives 1, 2, 9, 10 have a strong band of stretching vibrations of acetylene bond
-1
C≡C in the 2156-2169 cm range. Introduction of (CF ) Ge into the molecule shifts the C≡C
3
3
-1
band to the short-wave region by 10-13 cm with respect to the Me PhSi derivatives. In
2
anthracene compounds 3, 4, 11, 12 stretching vibrations of C≡C are observed in the 2121-2161
-1
cm range and have a complex structure. The introduction of (CF ) Ge leads to a shift of the
3
3
-1
C≡C band to the short-wave region by 4-11 cm
1
3
When considering an acetylene group, an interesting pattern is observed in the C NMR
signals. The chemical shift of the carbon atom of the acetylene group directly bonded with the
germanium atom ≡C-Ge(CF ) in 9-12 (see Supporting Information, Figures S25-S28), is shifted
3
3
by 18.8-22.4 ppm in a strong field with respect to the similar values for carbon atom bound to
a silicon atom ≡C-SiPhMe in 1-4. The effective shielding of the carbon atom in the acetylene
2
fragment is clearly a consequence of the deformation of the π-electron cloud of the conjugated
6

ACCEPTED MANUSCRIPT
system by a strong electron-withdrawing influence of the (CF ) Ge group largely determines
3
3
2
0-25
the unusual properties of the compounds containing this group.
X-ray crystal structures. Crystals of 11 and 12 suitable for X-ray analysis were obtained by
the slow evaporation of heptane and chloroform solutions, respectively, at room temperature.
The X-ray study revealed that compound 11 contains one (CF ) Ge group in position 9
3
3
whereas compound 12 contains contains two (CF ) Ge groups in positions 9,10 of
3
3
ethynylanthracene fragments (Figures 1,2).
Figure 1. Molecular structure of 11. Thermal ellipsoids are drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity.
Figure 2. Molecular structure of 12. Thermal ellipsoids are drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity.
7

ACCEPTED MANUSCRIPT
There are no published examples of related compounds containing the (CF ) Ge groups for
3
3
this type of compounds. However, the ethynylanthracene compounds containing the R Si groups
3
i
33-38
(
R=Me, Et, Pr ) are widely present in the literature.
The Ge-C(CF ) distances are 1.967(3)-
3
1
.999(4) Å, which is in agreement with analogous distances in the [(CF ) GePt(PPh ) ] Hg
3 3 3 2 2
2
2
39
(
1.971-2.037 Å) and (CF ) GePC(F)NEt (1.991-2.002 Å) complexes. The electron-
3 3 2
withdrawing CF groups in 11 and 12 do not lead to marked elongation of triple C≡C bonds as
3
3
3
compared to 9,10-bis(trimethylsilylethynyl)anthracene (1.187(6)-1.205(8) Å). The C≡C bonds
in 11 and 12 vary in the 1.196(4)-1.210(4) Å range.
Analysis of the CF groups coordination around the germanium atom in (CF ) Ge, however,
3
3 3
shows that the average angle (θ) CF –Ge–CF (°) in various compounds varies widely
3
3
depending on the nature of the organic or organometallic group bonded with (CF ) Ge. The
3
3
3
average angle θ is related to the sp hybridization ratio of a germanium atom a (eq 1)
Ge
αGe = (1+2cosθ)/(1-cosθ)
(1)
3
40
41
°
There is significant sp dehybridization in (CF ) GeMn(CO) (θ = 100.8 , a = 0.53) and
3
3
5
Ge
2
2
°
(CF ) GePt(PPh ) ] Hg (θ = 95.0 , a = 0.76), reflecting a weak electronic interaction
3 3 3 2 2 Ge
[
between poor electron donors of the organometallic Mn and Pt, and (CF ) Ge group. In 11 and
3
3
°
2, there was no significant dehybridization (θ = 107.2 , α = 0.32), which may be indicative of
Ge
1
the effective interaction of the π-conjugated systems with the electron-withdrawing (CF ) Ge
3
3
group.
8

ACCEPTED MANUSCRIPT
Figure 3. Fragment of crystal packing along the a axes in 11. Hydrogen and fluorine atoms are
omitted.
Figure 4. Fragment of crystal packing along the a axes in 12. Hydrogen and fluorine atoms are
omitted for clarity.
Molecules of compounds 11 and 12 in crystals form stacks along the a axes (Figures 3,4). The
distances between the planes of anthracene fragments are 3.435 Å in 11 and 3.462 Å in 12. Such
3
,42
distances suggest the π…π interaction between them.
9

ACCEPTED MANUSCRIPT
Electronic transitions. Electronic absorption spectra of biphenyl 1, 2, 9, 10 and anthracene 3, 4,
1
1, 12 derivatives in a dichloromethane solution at room temperature are presented in Figures 5,
. For 1, 2, 9, 10 there is a very simple form of the spectrum with one strong, unstructured π-π*
6
band in the 283-306 nm region (Table 1). Expansion of the π-conjugated system in the transition
from mono-substituted 1, 9 to di-substituted 2, 10 is accompanied by a bathochromic shift of the
absorption maximum of ~20 nm. The shapes of the bands and the maxima for the couple 2, 10
almost coincide, which indicates absence of the influence of different nature of the Me PhSi
2
Figure 5. Normalized absorption spectra of biphenyl compounds 1, 2, 9, 10 in dichloromethane
-5
(
10 Mol/L).
Figure 6. Normalized absorption spectra of anthracene compounds 3, 4, 11, 12 in
-5
dichloromethane (10 Mol/L).
10

ACCEPTED MANUSCRIPT
and (CF
3
)
3
Ge groups on the high-energy π-π* transitions. However, a small bathochromic shift
of the absorption band for unsymmetrical 9 (5 nm) is observed relative to 1.
Table 1. UV-visible Absorption and Fluorescence Data of 4,4ʹ-Biphenyl and
9,10-Anthracene
Derivatives
with
Dimethyl(phenyl)silylethynyl
and
Tris(trifluoromethyl)germylethynyl Substitutes
absorbance
Compound
fluorescence
λmax
[a]
Stokes
shift [nm]
[b]
ϵmax
λmax [nm]
Φf
[
nm]
1
2
3
4
9
283
304
410
441
287
306
418
452
4.59
7.87
2.15
5.46
3.96
6.79
1.14
4.05
387
392
443
446
403
390
462
464
104
88
33
5
116
84
44
12
0.15
0.19
0.72
0.44
0.07
0.17
0.64
0.64
1
0
1
2
1
1
4
-1
M
-1
[
a] Units 10
cm . [b] Fluorescence quantum yield.
The absorption spectra of anthracene compounds 3, 4, 11, 12 (Figure 6) have typical
structured bands of anthracene group corresponding to the π-π* transitions in the 368-452 nm
range. There is quite a distinct shift line to longer wavelengths for the (CF ) Ge compounds 11,
3
3
1
2. For example, for 12 the lowest energy transition π-π* (Figure16) at 452 nm, the shift of the
absorption maximum in the red region is 11 nm relative to the same band for 4. Such a shift of 8
nm is also observed for an unsymmetrical pair of Si and Ge compounds 3 and 11. The shift of
the low energy π-π* transitions in the 11, 12 as compared to similar transitions in 3, 4 can be
3
,41
explained by the stabilization of LUMO
(Figures 15,16) in the conjugated molecules 11, 12
under the influence of powerful electron-withdrawing (CF ) Ge.
3
3
Steady-state fluorescence spectra of biphenyl 1, 2, 9, 10 and anthracene 3, 4, 11, 12
derivatives in dichloromethane solution at room temperature are presented in Figures 7, 8. The
biphenyl series for symmetrical 2, 10 have absolutely identical emission spectra. There is a
11

ACCEPTED MANUSCRIPT
Figure 7. Normalized fluorescence spectra of biphenyl compounds 1, 2, 9, 10 in
-7
dichloromethane (10 Mol/L). Excitation wavelength 285 nm.
Figure 8. Normalized fluorescence spectra of anthracene compounds 3, 4, 11, 12 in
-7
dichloromethane (10 Mol/L). Excitation wavelength 375 nm.
pronounced difference in the spectra for unsymmetrical 1, 9. A very broad band with a
bathochromic shift of 16 nm relative to 1 (Table1) is recorded in the emission spectrum of 9 with
(
CF ) Ge group. Fluorescence quantum yields for a number of biphenyl compounds are
3 3
moderate and do not exceed 0.19.
The emission spectra of anthracene compounds 4 and 12 are characterized by a low Stokes
shift, by 5 and 12 nm, respectively. For unsymmetrical 11 a broad band with a flattened vibronic
structure is recorded that is different from those of the other studied anthracene compounds.
Quantum yields for 3, 4, 11, 12 are high and amount to 0.72.
12

ACCEPTED MANUSCRIPT
Time-resolved fluorescence decay dynamics. The fluorescence spectra of biphenyl derivatives
1
, 2, 9, 10 in dichlomethane solution at room temperature (Figure 9) obtained with pulse
excitation at 266 nm are substantially different from the steady-state spectra under excitation at
85 nm. For 1, 2 and 10 there appears structured bands associated with the vibrational transitions
2
of the phenyl groups and a significant hypsochromic shift of the maxima. This is due to the
excitation of high-energy π-π* transitions of the phenyl groups by laser radiation at the
wavelength of 266 nm. Again, fluorinated unsymmetrical 9 shows a fluorescent band with a
flattened vibronic structure.
Figure 9. Fluorescence spectra of biphenyl compounds 1, 2, 9, 10 in dichloromethane
-5
(
10 Mol/L). Pulse excitation wavelength at 266 nm.
The fluorescence spectra of anthracene derivatives 3, 4, 11, 12 in dichloromethane solution at
room temperature (Figure 10) with pulse excitation at 400 nm do not differ much from the
steady-state spectra. However, unlike the steady-state spectra, for unsymmetrical 11 the
13

ACCEPTED MANUSCRIPT
Figure 10. Fluorescence spectra of anthracene compounds 3, 4, 11, 12 in dichloromethane
-5
(
10 Mol/L). Pulse excitation wavelength at 400 nm.
fluorescence intensity band decreases by a factor of ca. 1/21 compared with 3, 4 and 12, which
may indicate the energy transfer (ET) from the π-conjugated ethynylanthracene system to
(
CF ) Ge group.
3 3
Figures 11 and 12 show the single-exponential decay of the fluorescence for all the
studied compounds except 9. The fluorescence lifetimes (Table 2) for anthracene compounds
, 4, 11, 12 are in the range of 6.0-10.1 ns, which is typical for the lifetime of the singlet excited
3
1
2
states for compounds of this type. It should be noted that the maximum fluorescence lifetime
of the anthracene series (10.1 ns), and accordingly the smallest decay rate constant
8
-1
(
0.6 × 10 s ) is observed for the fluorinated unsymmetrical 11. This is despite the less extended
conjugated system compared to 4 and 12. These data may indicate that the energy transfer from
the π-conjugated ethynylanthracene system to the (CF ) Ge group is more efficient in 11 and this
3
3
transfer process is influenced by the nature of the substituent in the 10-position of anthracene
formally donor H in 11 and the electron-withdrawing ‒C≡C–Ge(CF3)3 group in 12). The
biphenyl series 1, 2 and 10 show the single-exponential decay of the fluorescence with
(
14

ACCEPTED MANUSCRIPT
Figure 11. Decay of the fluorescence peak intensity of biphenyl compounds 1, 2, 9, 10 in
-5
dichloromethane (10 Mol/L) at 300 K. Excitation wavelength 266 nm.
Figure 12. Decay of the fluorescence peak intensity of anthracene compounds 3, 4, 11, 12 in
-5
dichloromethane (10 Mol/L) at 300 K. Excitation wavelength 400 nm.
lifetimes in the 0.61-1.07 ns range. For an unsymmetrical 9 with a fluorinated (CF ) Ge group
3
3
we have registered a two-exponential decay of the fluorescence with lifetimes τ = 15 ps (0.73)
1
and τ = 2.26 ns (0.27) (Figure 11). This time dependence could be interpreted by a fast ET
2
mechanism involving the intermediate formation of the excited charge-transfer complex 9*
15

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Table 2. Fluorescence Lifetimes (τ) and Rate Constant (k ) for 4,4ʹ-Biphenyl and
f
9,10-Anthracene Derivatives with Dimethyl(phenyl)silylethynyl
and
Tris(trifluoromethy)germylethynyl substitutes.
-
1
[a]
7
Compound
τ1 [ns]
τ2 [ns]
k [s ]
f
1
2
3
4
9
1.07
0.77
9.10
6.00
2.26
0.61
10.10
7.50
3.7 × 10
1.8 × 10
0.8 × 10
0.7 × 10
1.8 × 10
2.2 × 10
0.6 × 10
0.9 × 10
8
8
8
6
[b]
0.015
8
8
8
10
11
12
[
a] k =Φ /τ; Φ for biphenyl series taken for excitation at 285 nm; for anthracene
f f f
series taken for excitation at 355 nm (see Supporting Information, Table S1.) [b]
Rate constant for τ2.
moiety producing a radical ion pair state in dichloromethane at room temperature.
The charge-recombination (CR) to the ground state of 9 determines the decay process with a
6
-1
rate constant (kCR = 1.8 × 10 s ) (Table 2, Figure 13), which is the lowest observed in the
studied series of compounds. For determining ET efficiency and the rate constant of charge-
separation (CS) process, we investigated the decay properties of 4-biphenylacetylene
(BiPhCCH), which is the standard of comparison for 9.
Figure 13. Energy diagram of 9.
16

ACCEPTED MANUSCRIPT
Analysis of the fluorescence spectrum of the standard BiPhCCH in dichloromethane under
pulsed excitation at 266 nm (i.e., under the conditions similar to 9) showed that the fluorescence
decay obeys an exponential law with the only time τ = 2.69 ns (see Supporting Information,
Figure S34). This experiment shows directly that a two-exponential decay of the fluorescence of
9
related to the presence of the electron-withdrawing (CF ) Ge group in the molecule.
3 3
The rate (k ) and quantum (Φ ) for ET in 9 can be calculated by eqs 2 and 3, in which the
ET
ET
*
fluorescence lifetime of the excited state of BiPhCCH in dichloromethane was used as a
I
I
I
reference (τ ) = 2.69 ns and (τ )
= 15 ps. The rate (k ) and quantum (Φ ) for CS (Figure
f ref
f sample
CS
CS
1
3) in 9 can be calculated by eqs 4 and 5
1
1
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ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀkꢐꢑ
ꢒ
ꢀ=ꢀꢃ ꢅ ꢆ
−ꢀkꢁꢂ
(4)
(5)
τꢄ
ꢇꢈꢉꢊꢋꢌ
1
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀΦꢐꢑ
ꢒ
ꢀ=ꢀꢀkꢐꢑ
ꢒ
/ꢃ ꢅ ꢆ
τꢄ
ꢇꢈꢉꢊꢋꢌ
The rate constant kCSII is calculated by solving the system of linear differential equations
describing the kinetics of sequential processes of ET and CS in molecular system 9. Details of
the calculation are given in Supporting Information, Figure S35. Thus, we have found that ET in
1
0
-
1
9
occurs with high speed (k = 6.6 ×10 s ) and efficiency (Φ = 0.99), and it is the radiative
ET ET
I
transition that occurs in the experiment. CS has two components: radiative transition CS and
II
I
8
-
1
I
nonradiative transition CS . CS relatively slow (k = 3.7 × 10 s ) and has low quantum yield
CS
17

ACCEPTED MANUSCRIPT
II
9
-
1
(
ΦCSI = 0.0056), therefore it is not observed in the experiment. CS is fast (kCSII = 2.7 × 10 s )
and has high quantum efficiency (ΦCSII = 0.99). Interestingly, the rate of CR is three orders of
6
-1
magnitude slower (kCR = 1.8 × 10 s ) than that of CS. This means that there is some
stabilization mechanism of radical ion pairs.
3
Electrochemical studies. The cyclic voltammogram of 1-4 and 9-12 in acetonitrile (CH CN)
+
shows peaks in the wide range of -2.31 to 2.56 V vs. Ag/Ag (see Supporting Information, Table
S2). All the compounds with biphenyl fragment (1, 2, 9, 10) have one peak of the anode
irreversible oxidation. In contrast, the compounds with an anthracene moiety, except
compound 12, are partially oxidized reversibly as anthracene, and the degree of reversibility,
estimated by the ratio of magnitudes of the currents and reverse peaks is low (0.1-0.3).
Compound 10 has the highest oxidation potential (2.56 V) in the entire series of compounds.
This is consistent with the lowest energy of HOMO calculated for 10 (Figure 15).
The resulting electrochemical data evidence that organogermanium compounds 9-12 have
higher oxidation potentials than their organosilicon counterparts 1-4, this pattern is observed for
all four pairs of the analogues.
A different pattern is observed in the field of reduction (see Supporting Information, Table
S2). The potentials are very similar (the difference is 0.02 - 0.05 V) for all compounds of the
analogs (1-9, 2-10, 3-11, 4-12) of the first reduction peak. In other words, each first reduction
peak corresponds to the structure of the organic fragments of the compounds. There is a
monotonic decrease (in absolute value) of the first peak of reduction potential in the series of
silicon and germanium compounds: 1 <2 <3 <4 and 9 <10 <11 <12.
18

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Table 3. Cyclic Voltammetry Data of 9 and BiPhCCH in Acetonitrile and Dichloromethane, Energy of Lowest Excited States ꢓ∆ꢔ0-0), Free-Energy of Radical Ion Pairs
ꢙ
ꢙ
ꢓ∆ꢕ ꢚ and Free-Energy Change of Charge Separations ꢓ∆ꢕ
ꢚ
ꢖꢗꢘ
ꢛꢜ
V vs. Ag/AgCl
ꢢ
ꢢ
∆
eV
ꢝ0-0
∆ꢞ
∆ꢞ
ꢐꢑ
ꢟꢠꢡ
compound
solvent
Eox
Ered2
Ered1
eV
eV
[
BiPhCC]
[BiPhCC]
[Ge(CF ) ]
3
3
9
9
CH Cl
2.35
2.15
2.49
2.00
–2.06
–2.46
―
‒1.87
‒2.31
3.86
3.89
4.00
4.41
0.14
0.52
2
2
CH CN
3
[
a]
BiPhCCH
BiPhCCH
CH Cl
2
2
CH CN
–2.46
3
[
a] To determine the reduction potential BiPhCCH in dichloromethane failed due to rapid polymerization at the cathode.
Compound 9 has been investigated in more detail. Cyclic voltammograms have been recorded
for 9 in acetonitrile (strong polar solvent) and dichloromethane (weak polar solvent), and
electrochemical parameters of 4-biphenylacetylene (BiPhCCH) have been obtained as a
reference compound (Figure 14, Table 3).
Figure 14. Cyclic voltammetry curves demonstrating the differences in reduction behaviour: (a)
quasi-reversible reduction 9 in CH CN, (b) irreversible reduction 9 in CH Cl , and (c) quasi-
3
2
2
reversible reduction of BiPhCCH in CH CN.
3
The nature of the solvent strongly influences the magnitude of the first reduction potential (Ered1)
of 9. The reduction of 9 in dichloromethane occurs at a potential 0.44 V less than in acetonitrile.
Such a change in Ered1, which can be formally attributed to the reduction potential of the
19

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(
CF ) Ge group, is probably due to the coordination of an acetonitrile molecule with germanium.
3
3
Expansion of the coordination sphere of germanium in (CF ) Ge compounds by interacting with
3
3
2
9
the nucleophilic species is well known. Coordination of acetonitrile was confirmed
experimentally by recording the absorption spectrum of 9 in acetonitrile (see Supporting
Information, Figure S36). The hypsochromic shift of the absorption band of the spectrum relative
to 9 in dichloromethane is recorded. We can assume that this is due to some extent to the
availability of vacant 4d atomic orbitals of the germanium.
The potential Ered2 coincides exactly with the only reduction potential of BiPhCCH, and thus
may be formally related to the BiPhCC group in 9. The oxidation potential E refers to
ox
the electron-donating BiPhCC group in molecule 9. The potential difference of Ered2 and Eox
found for compound 9 gives the energy of the first excited state (4.41 eV) fixed in the energy
diagram (Figure 13). This is confirmed by quantum chemical calculations of the reference
compound BiPhCCH, for which the calculated energy of the first excited state is 4.49 eV. Thus,
it is perfectly justified to consider the aromatic π-electron BiPhCC fragment of molecule 9 as a
chromophor or antenna directly interacting with the incident radiation.
The free-energy formation of the radical ion pairs (ΔG ꢢꢣ ꢤꢥ) of 9 at pulse excitation can be
4
3
calculated by Weller equations as a difference between the E and E values with the
ox
red
Coulomb energy ꢀG (eq. 6,8) taken into consideration. Free-energy changes of charge
s
ꢢ
separation ꢓ∆ꢞ ꢚ can be calculated by eq.7,
ꢐꢑ
ꢢ
= −∆Gꢢ = −ꢝ + ꢝ − ∆Gꢇ
ΔG
(6)
(7)
ꢟꢠꢡ
ꢐꢟ
ꢦꢧ
ꢍꢌꢨ
∆G = ∆ꢝꢢꢩꢢ − ∆Gꢢ
ꢢ
ꢐꢑ
−
ꢟꢠꢡ
ꢪꢫ
ꢓꢬꢭ∙ꢮ ꢮ ∙ꢣꢱꢱꢚ
∆
G =
ꢇ
(8)
ꢯ
ꢰ
20

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where ꢓ∆ꢝ ) is the energy of the lowest excited state of 9.
0
-0
We see that for 9 in dichloromethane there exist advantageous conditions for the energy
transfer from the biphenylethynyl BiPhCC group to (CF ) Ge with formation of radical ion pairs
3
3
a result of the charge separation. However, for implementing this in acetonitrile much more
energy (4.93 eV) is demanded, which is unattainable in our experiment. This is very different
1
0
from the phenylethynyl nonfluorinated organogermanium oligomers, for which there is no
appreciable impact of the nature of the solvent on excited-state temporal decays.
Quantum chemical calculation. All the calculations were performed using the Gaussian 03
4
4
program. A full geometry optimization of the ground states of molecules 1-12 was done at the
BLYP/6-31G(d,p) and B3LYP/6-31G(d,p) levels. The analysis of HOMO-LUMO gaps and
molecular structure shows that the B3LYP functional reproduces the available experimental data
somewhat better.
Therefore, we used the B3LYP functional in our further calculations. The vertical excitation
energies of molecules 1-12 in the molecular structure of their ground states were calculated using
the time-dependent DFT theory (TD-B3LYP/6-31G (d,p) taking into account 3 to 5 lowest
singlet states. Variation of the number of calculated states resulted only in minor changes of the
excitation energies. The energies of reorganization for the molecules of interest were then
calculated at the same level of the theory, but taking into account 8 low-lying states of different
multiplicity (4 triplet and 4 singlet states, “50-50” option of Gaussian). During this calculation,
the geometry of the lowest singlet state was fully optimized. Occasionally, the molecular
structure perturbation resulted in changing of the order of the excited states. These cases were
21

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Figure 15. HOMO and LUMO energies of all studied molecules calculated at the B3LYP/6-31G
(d,p) level.
carefully controlled and additional optimizations were performed to follow the proper excited
state.
The relative location of HOMO and LUMO energy gives a qualitative picture of the influence
of the Me PhSi and (CF ) Ge substitutes of different nature on the selected π-electronic systems.
2
3 3
The energies of HOMO and LUMO of all the studied molecules and their corresponding
electronic density contours are shown in Figure 15. The introduction of the electron-
22

ACCEPTED MANUSCRIPT
withdrawing (CF ) Ge group significantly lowers the energy of frontal orbitals of all compounds.
3
3
The lowest value of LUMO energy (-3.02 eV) and the smallest HOMO-LUMO band gap (2.86
eV) have been calculated for the highly fluorinated 12. This means that the (CF ) Ge group can
3
3
be used as a building block to create new n-type organic semiconductors. Besides, introduction
of the (CF ) Ge group into conjugated systems can greatly stabilize them, which is very
3
3
4
,6,19,32,36,37
important, for example, for pentacene and higher acene derivatives.
We observed the
increased stability of our fluorinated compounds in solution on an example of 11 (see
Supporting Information, Figure S33).
Stabilization of LUMOs also explains the red shift of the absorption bands of the (CF ) Ge
3
3
compounds. This is particularly evident in the compounds of anthracene series.
For a more detailed consideration of possible electronic transitions five highest occupied and
five lowest unoccupied molecular orbitals of compound 12 have been calculated (Figure 16).
Irreducible representations of the 18π-electron approximation for the plane
anthracenediethynylene fragment (D2h symmetry group) have also been identified. The molecule
is placed in the zy plane with the long molecular axis corresponding to the z axis. The electronic
density contours of MOs presented in Figure 16 indicate their π-nature and are localized mainly
in the anthracene moiety of molecule 12. Only three highest energy π* MOs shown in Figure 16
have a significant orbital contribution of the (CF ) Ge groups, and for them there is a symmetry
3
3
4
5
reversal relative to similar MOs anthracene. The lowest singlet excited state of 12 corresponds
to the HOMO – LUMO (3b →3b ; 2.63 eV) transition. The next two singlet transitions (1b
2
g
3u
1g
→
3b ; 3.60 eV and 3b →2a ; 3.81 eV) are not fixed in our laser pulse excitation experiment
3u 2g u
due to their high energy. The presented qualitative picture of MOs for 12 can be generally
extended to other compounds of silicon and germanium studied in this paper based on the
23

ACCEPTED MANUSCRIPT
Figure 16. Five highest occupied and five lowest unoccupied molecular orbitals of compound 12
computed at the B3LYP/6-31G (d, p) level. Irreducible representations correspond to the
approximate D symmetry group of planar carbon skeleton (CF groups are omitted) of
2
h
3
compound 12 with 18 π- electrons.
analysis of their electronic density contours and the calculated energy level. However, this
analysis does not explain the experimentally observed spectra of energy and charge transfer
dynamics, especially for compound 9.
24

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Table 4. Contribution of Vacant
Germanium in LUMOs of the Studied Compounds
d and s, p Atomic Orbitals of Silicon and
[a]
3
d Si and 4d Ge,
3p Si and 4p Ge,
%
3s Si and 4s
Ge, %
Compound
%
1
2
3
4
9
0.24
0.14
0.14
0.12
0.98
0.47
0.48
0.43
3.10
1.62
1.48
1.30
5.12
2.09
2.41
1.78
0.00
0.02
0.00
0.07
0.00
0.00
0.00
0.00
1
1
1
0
1
2
[
a] Calculated for one heteroatom.
The influence of the Me PhSi and (CF ) Ge groups on the biphenylethynylene and
2
3 3
anthracenylethynylene π-electron systems can be largely determined by the nature and state of
heteroatoms Si and Ge. We calculated the contribution of vacant d and s, p atomic orbitals of
silicon and germanium in LUMOs of the studied compounds (Table 4). Surprisingly, the
contribution of d-orbitals of germanium is the highest in compound 9, for which a two-
exponential decay of the fluorescence has been found. Moreover, the total contribution of vacant
atomic orbitals of Si or Ge for each compound of a series of unsymmetrical 1, 3, 9, 11
corresponds exactly to their rate constant of the fluorescence decay (Table 2); namely, a greater
contribution corresponds to a lower rate constant! This order of a decreasing rate constant is 3 >
1
1 > 1 > 9. Thus, the energy and charge transfer from the π-system to Si or Ge is defined by the
presence and magnitude of vacancy on the heteroatom as well as by the symmetry of the
molecule. The transfer is more efficient in the unsymmetrical donor-acceptor (D-A) molecules.
For efficient intramolecular energy and charge transfer a system of conjugated bonds is also
needed. A conjugation circuit in ethinyl organogermanium compounds with 4p and 4d
25

ACCEPTED MANUSCRIPT
Figure 17. Conjugation circuit with 4p and 4d atomic orbitals of Ge in the organogermanium
ethynyl compounds.
germanium atomic orbitals is shown in Figure 17. There is a significant distortion of the π-
electron cloud acetylene bond in ethynyl derivatives (CF ) Ge, as was noted above when
3
3
1
3
considering the C NMR spectra. According to the X-ray analysis of compounds, 11 and 12
3
there occurs almost full sp hybridization of germanium atomic orbitals. These and other
experimental facts discussed in this article are easily explained the (p-d) and (p-pGe)π
π
conjugations which provide effective communication of the strong electron-withdrawing
(CF ) Ge group with π-electronic organic systems. In this way, the concept of the role of
3 3
4
6,47
(
p-d) conjugation in organosilicon and organogermanium compounds
, and hypothesis
π
1
0,17,18
about the d-orbital participation of silicon and germanium atoms
finds experimental
support in this work.
26

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Table 5. Some Parameters of the Molecule 9 in the Ground and Excited State, Calculated at the Time-Dependent DFT Theory (TD-B3LYP/6-31G(d,p) level).
bond length, (Å)
bond length, (Å)
angle, (°)
dipole,
Debye
[b]
compound
[a]
λreorg, eV
RMSD , Å
C≡C
≡C–Ge
CF –Ge–CF
3
3
9
9
1.2180
1.2373
1.8587
1.8482
107.81
104.98
4.9
5.7
*
-0.31
0.2461
[
a] One of the three most highly variable angles, [b] Root mean square deviation of cartesians (excited state - ground state).
Additional information about the molecular dynamics of the studied compounds was obtained
from the analysis of the reorganization energy and the geometry of the molecules in the excited
state.
The reorganization energy (λreorg) of the anthracene series 3, 4, 11, 12 (0.16-0.18 eV) is lower
than of the biphenyl series 1, 2, 9, 10 (0.31-0.35 eV) (see Supporting Information, Table S4).
Thus, small structural changes within a molecule lead to a significant change of λreorg. It should
4
8-52
be noted that a small λreorg is useful for internal mobility of charges in the electronic devices.
The transition of molecule 9 in excited state is accompanied by an appreciable change in the
dipole moment (0.8 Debye) (Table 5), which may be the result of the charge transfer from the
aromatic π-system to the (CF ) Ge group.
3
3
The geometry of molecule 9 in excited state is characterized by pronounced lengthening of the
C≡C bond and, at the same time, by shortening of the ≡C–Ge bond (Table 5). Furthermore, the
bond angle ∠ CF –Ge–CF between the two CF groups reduces by 2.83°, while the third
3
3
3
trifluoromethyl group retains a constant position (see Supporting Information Table S4, S5). The
3
decrease of the bond angle means partial sp dehybridization of germanium AOs in excited state.
Lengthening and shortening of the carbon-carbon bonds in the aromatic biphenyl moiety are also
observed.
27