Design of donor-acceptor geometry for tuning excited-state polarization: Fluorescence solvatochromism of push-pull biphenyls with various torsional restrictions on their aryl-aryl bonds

Article abstract of DOI:10.1016/j.tet.2014.08.002

In this work, push-pull biphenyl analogs (4-(N,N-dimethylamino)-4′-formylbiphenyl) with a modulated dihedral angle of the aryl-aryl bond, using a bridged structure or methyl groups, were synthesized. Photophysical measurements of the synthesized compounds revealed the effect of the torsion between N,N-dimethylaniline (donor) and benzaldehyde moieties (acceptor) on their solvatochromic properties. Our data showed that the sensitivity of the fluorescence maxima to solvent polarity gradually increases as the biphenyl chromophore was restricted to a twisted conformation. This finding indicates that changing the torsional restrictions on the donor-acceptor system can be a factor to take into account for the development of novel solvatochromic dyes.

Full text of DOI:10.1016/j.tet.2014.08.002

Accepted Manuscript
Design of donor-acceptor geometry for tuning excited state polarization: fluorescence
solvatochromism of push-pull biphenyls with various torsional restrictions on their aryl-
aryl bonds
Shunsuke Sasaki, Yosuke Niko, Andrey S. Klymchenko, Gen-ichi Konishi
PII:
S0040-4020(14)01165-X
10.1016/j.tet.2014.08.002
TET 25902
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 1 July 2014
Revised Date: 30 July 2014
Accepted Date: 1 August 2014
Please cite this article as: Sasaki S, Niko Y, Klymchenko AS, Konishi G-i, Design of donor-acceptor
geometry for tuning excited state polarization: fluorescence solvatochromism of push-pull biphenyls with
various torsional restrictions on their aryl-aryl bonds, Tetrahedron (2014), doi: 10.1016/j.tet.2014.08.002.
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ACCEPTED MANUSCRIPT
Design of donor-acceptor geometry for tuning excited state
polarization: fluorescence solvatochromism of push-pull
biphenyls with various torsional restrictions on their aryl-aryl
bonds
Shunsuke Sasaki,a Yosuke Niko,a Andrey S. Klymchenko,b Gen-ichi Konishi*,a,c
1

ACCEPTED MANUSCRIPT
Design of donor-acceptor geometry for tuning excited state
polarization: fluorescence solvatochromism of push-pull
biphenyls with various torsional restrictions on their aryl-aryl
bonds
Shunsuke Sasaki,a Yosuke Niko,a Andrey S. Klymchenko,b Gen-ichi Konishi*,a,c
a Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
O-okayama, Tokyo 152-8552, Japan
b Laboratoire de Biophotonique et Pharmacologie, UMR 7213 CNRS, Université de
Strasbourg, Faculté de Pharmacie, 74, Route du Rhin, 67401 Illkirch, Cedex, France
c PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama
332-0012, Japan
E-mail: konishi.g.aa@m.titech.ac.jp
ABSTRACT
In this work, push-pull biphenyl analogs (4-(N,N-dimethylamino)-4’-formylbiphenyl)
with a modulated dihedral angle of the aryl-aryl bond, using a bridged structure or
methyl groups, were synthesized. Photophysical measurements of the synthesized
compounds revealed the effect of the torsion between N,N-dimethylaniline (donor) and
benzaldehyde moieties (acceptor) on their solvatochromic properties. Our data showed
that the sensitivity of the fluorescence maxima to solvent polarity gradually increases as
the biphenyl chromophore was restricted to a twisted conformation. This finding
indicates that changing the torsional restrictions on the donor-acceptor system can be a
factor to take into account for the development of novel solvatochromic dyes.
Keywords: Fluorescence solvatochromism, Twisted intramolecular charge transfer
(TICT), Biphenyl, Prodan
2

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1. Introduction
Donor-acceptor-functionalized organic dyes are widely known to exhibit
photoinduced intramolecular charge transfer (ICT) state.1 Because the ICT state features
numerous unique phenomena, such as strong polarization and conformational changes,2
it has been investigated both theoretically and experimentally. One of the most
important features of photoinduced ICT states is their fluorescence solvatochromism,
i.e., the fluorescence and wavelength are sensitive to the polarity of the environment.
Solvatochromic dyes with these characteristics have been extensively studied3,4 and
recently suggested as fluorogenic probes for biological applications.5
Owing to recent advances in cell-imaging techniques,6,8 there is an urgent demand
for solvatochromic dyes with a range of diverse photophysical and photochemical
properties, which should be carefully chosen depending on the biological system of
interest.7 Furthermore, the applications to sensing biomolecular interactions impose
severe structural requirements to solvatochromic dyes, in order to minimize their
interference with the normal function of biomolecules, but maximize their spectroscopic
response to the environment.5,7,9-11 Therefore, a methodology that can modify
photophysical properties with minimal structural modifications of the dyes is strongly
needed.
To modulate the photophysical properties, we focused on the geometry control
between donor and acceptor moieties as a promising alternative to conventional
strategies. It is known that several donor-acceptor systems form a twisted ICT (TICT)
state in their excited states.2,12 These TICT states show perpendicularly twisted
conjugation systems characterized by strong polarization, which is due to a complete
charge separation. Thus, a highly polarized TICT state can be induced by “pretwisting”
a conjugated system with steric hindrances.2,13 Therefore, the excited state polarization
of a donor-acceptor system can be finely controlled by a torsional restriction on the
bond between donor and acceptor moieties.2 Despite a deep theoretical and
experimental understanding of the TICT phenomena, no study has to date explored the
possibility of tuning the photophysical properties of bioimaging probes with minimal
structural modifications.
3

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Chart 1
In this work, we applied this hypothesis to the modulation of solvatochromism of
push-pull biphenyl derivatives (Chart I). Push-pull dyes with N,N-dialkylamine and
carbonyl groups are known to usually exhibit high sensitivity towards the polarity of the
environment14,16 and the formation of hydrogen bonds.15 Especially, we recently
reported that a push-pull fluorene derivative 7-diethylamino-9,9-dimethylfluorene-
2-carbaldehyde (FR0) exhibits high quantum yield and unprecedentedly large
solvatochromism.17 Owing these properties, FR0 and its derivatives has already been
employed as a bioimaging probes.18 Therefore, if variety of photophysical properties are
derived from push-pull biphenyl derivatives like FR0 just by introducing torsional
restrictions, the diversity of practically usable solvatochromic dyes should be spurred.
In particular, we synthesized a series of push-pull biphenyl analogs (Figure 1) with
various torsional restrictions, such as bridged structures and methyl groups, and then we
measured their photophysical properties. Our results showed that, although the same
chromophore and donor-acceptor pair were used, the solvatochromic properties of the
studied systems were successfully modified. In particular, distortion of planarity in
push-pull biphenyl analogs induced a stronger solvatochromism compared to that of
9FR, due to a pronounced TICT. Notably, the extents of solvatochromism of twisted
push-pull biphenyl analogs are even larger than that of 9FR (Figure 1), which is one of
the most solvatochromic fluorophores found to date. The series of the push-pull
biphenyl systems described in this study is the first example in which extremely
different dyes (for instance, brightly luminescent solvatochromic dyes and dyes with
unprecedentedly strong solvatochromism in a narrow polarity range) were obtained
from the same biphenyl chromophore and the same donor-acceptor pair.
4

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Figure 1. Chemical structures of push-pull biphenyl analogs.
2. Results and discussion
2.1. Synthesis
4-Formyl-4’-N,N-dimethylamino-1,1’-biphenyl (FDBP)19 and methylated FDBP
were obtained via facile 2- and 3-step syntheses, respectively (overview of the synthetic
procedure is illustrated in the Supporting Information, S3). In particular, the
N,N-dimethylamino moiety was functionalized with boron pinacol ester, followed by
Suzuki-Miyaura cross-coupling20 to form the biphenyl backbone. In the case of
FDBP-O1, FDBP-O1N1, and FDBP-O2 (Figure 1), in which the methyl groups are
located on the acceptor side, 5-bromo-2-iodotoluene or 5-bromo-2-iodo-m-xylene was
used to selectively carry out the Suzuki-Miyaura cross-coupling at the position of the
iodine group. All synthesized compounds were characterized, and their purities
1
evaluated, using H NMR, 13C NMR, FT-IR, and the melting point determination
technique. Furthermore, high-resolution mass spectrometry and elemental analysis were
performed for the push-pull biphenyl analogs synthesized in this work.
The synthesis of methylene- and ethylene-bridged push-pull biphenyl analogs (9FR
and 9Phen) is outlined in Scheme 1. 9,10-Dihydrophenanthrene was brominated in the
presence of iron(III) chloride to afford 3Phen.21 3FR was obtained from the reaction
with methyl iodide under basic conditions. Both 2,7-dibrominated derivatives, 3FR and
3Phen, underwent Ullmann coupling to yield phthalimide derivatives, 4FR and 4Phen,
as intermediates of the Gabriel synthesis.22 The phthalimide group was subsequently
converted to the amino group by reduction. The obtained 4FR and 4Phen were firstly
protected with carbamate to prevent the amino group from undergoing polyalkylation,23
and then methylated with methyl iodide and sodium hydride. Deprotection of the
carbamate group was carried out with lithium aluminum hydride. Finally, 8FR and
8Phen were lithiated; following the addition of N,N-dimethylformamide, 9FR and
5

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9Phen were produced. Among the push-pull biphenyl analogs synthesized in this work,
only FDBP has a chemical structure identical to that previously reported.19 Analogs of
9FR (FR0 as N,N-diethylamino analog)17 and FDBP-O1N1 (ADAB-Me as an acetyl
analog)25 have also already been reported. To the best of our knowledge, the other
push-pull biphenyls in this work possess a novel chemical structure.
Br2, FeCl3
H2O, r. t.
1
O
Br
Br
Br
Br
CuI
NK
CH3I, t-BuONa
NH2NH2 • H2O
EtOH reflux
O
X
X
X
DMA reflux
THF, 0 °C
O
Br
N
NH2
Br
O
2
3FR: X = C(CH3)2
4FR: X = C(CH3)2
5FR: X = C(CH3)2
3Phen: X = -CH2CH2-
4Phen: X = -CH2CH2-
5Phen: X = -CH2CH2-
ClCOOCH3, N(C2H5)3
r. t.
CHCl3, 0 °C
O
Br
Br
Br
1) n-BuLi, THF,
-78 °C, 30 min
2) DMF, r. t., 2h
LiAlH4
THF, 0 °C
CH3I, NaH
DMF, r. t.
X
X
X
X
r. t.
reflux
O
O
N
N
N
HN
O
O
9FR: X = C(CH3)2
9Phen: X = -CH2CH2-
8FR: X = C(CH3)2
8Phen: X = -CH2CH2-
7FR: X = C(CH3)2
7Phen: X = -CH2CH2-
6FR: X = C(CH3)2
6Phen: X = -CH2CH2-
Scheme 1. Synthetic routes of 9FR and 9Phen.
6

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2.2. Absorption spectra
The steady-state UV-Vis absorption spectra were collected to confirm if the
ground-state conformations of push-pull biphenyl analogs are significantly varied by
introduction of torsional restrictions. Although a conformation of an excited state may
differ from that of the ground state, this information is still useful to estimate the effect
of torsional restrictions on the structural parameters of the aryl-aryl bonds of the
biphenyl backbone. We would like to remark that our absorption spectra of push-pull
biphenyl analogs are generally in line with those described in previous studies.13,24-29
The steady-state absorption spectra of various push-pull biphenyl analogs were
measured in different nonpolar and polar solvents (see Figure S36-S43 in Supporting
Information); the representative absorption maxima and the molar extinction
coefficients (ε) are summarized in Table 1 and shown in Figure 2 (For all obtained
absorption maxima and the molar extinction coefficients (ε), see Table S1 in Supporting
information). All the absorption spectra show two representative absorption bands,
which are common in several donor-acceptor substituted biphenyls.13,24,25 According to
1
previous studies,24 the absorption band at longer wavelengths was assigned to a S0→ CT
(Singlet charge-transfer state) transition; the corresponding oscillator strength was
found to be inversely proportional to the dihedral angle between aryl-aryl bond of
1
biphenyl. The absorption band at shorter wavelengths was attributed to a S0→ Bb (π-π*
state polarized along short-axis of biphenyl chromophore) transition; the corresponding
oscillator strength, albeit insensitive to bridging, increased because of the twisting
induced by methyl group.24 In addition, Table 1 indicates that a slight bathochromic
shift of the S0→1Bb transition and a hypsochromic shift of the S0→1CT transition
occurred, when the number of methyl groups around the aryl-aryl bond of biphenyl was
increased.13,26 The solvent dependence of ε shown in Table S2 also indicates a long-axis
polarization of the S0→1CT transition.27 In addition, the values of ε of the studied
push-pull biphenyl analogs provide useful information about the twist angle around the
aryl-aryl bonds and the resulting electronic coupling between donor and acceptor
moieties. As shown in Table 1, ε (S0→1CT) in dichloromethane increases when torsion
around aryl-aryl bond of FDBP is restricted by methylene or ethylene bridges (9FR and
9Phen). Conversely, further substitutions with methyl groups cause a decrease in ε
(S0→1CT). This was already theoretically investigated by Dogonadze28 and
7

ACCEPTED MANUSCRIPT
experimentally examined for 9-acridine derivatives by Herbich.29 Their studies
suggested that, for biaryl systems, a decrease in ε (S0→1CT) stems from the increase in
the dihedral angle between donor and acceptor moieties. Thus, the trend of ε (S0→1CT)
displayed in Table 1 indicates that the application/removal of torsional restrictions
resulted in twisted biphenyl backbones, showing a diverse range of conformations, from
a coplanar to a nearly perpendicular conformation.
To visualize the stepwise increase in the dihedral angle between the donor and
acceptor subunits ΘD-A, we calculated experimental values of ΘD-A following the
procedure reported by Herbich.29 To apply the proceduce to our system, it has been
1
assumed that (1) the dihedral angle ΘD-A is 0o for 9FR and (2) the CT state does not
1
mix with the neighboring electronic state. Due to the latter assumption of a pure CT
state, the values of ΘD-A tend to be deviated from actual value. Nonetheless, it can be
seen that the dihedral angle ΘD-A increases in a stepwise manner. Additionally, the ΘD-A
value of FDBP-N2 is similar with previously reported dihedral angle of
4-N,N-dimethylamino-2,6-dimethyl-4’-cyanobiphenyl.13 Details of the calculation are
given in the Supporting Information, S8.
Table 1. Molar extinction coefficients ε and absorption maxima λabs of two absorption
bands of push-pull biphenyl series in dichloromethane solution
ε (S0→1CT) a) λabs(S0→1CT) λabs(S0→1Bb)
ΘD-A
[degree]
0
Entry
[M-1 cm-1]
32000
30000
19000
14000
14000
5000b
[nm]
390
[nm]
255
255
252
255
258
261
260
265
9FR
9Phen
387
32
368
49
FDBP
356
58
FDBP-N1
FDBP-O1
FDBP-O1N1
FDBP-N2
FDBP-O2
350
58
339b
344b
338b
71
5000b
5000b
71
71
a) Fractions less than 1000 [M-1 cm-1] were omitted.
b) λabs and ε for S0→1CT transition were determined by Gaussian fitting.
8

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Figure 2. Absorption spectra of the series of push-pull biphenyl analogs in
dichloromethane.
2.3. Fluorescence solvatochromism
Fluorescence spectra of synthesized push-pull biphenyl analogs were measured in
various nonpolar and polar solvents (Table 2, Figures S44 to S51). The representative
fluorescence spectra of 9FR, 9Phen, FDBP, and FDBP-O1N1 are shown in Figure 3.
Our data suggested that more twisted biphenyls exhibit larger spectral shift when the
solvent polarity was increased, e.g., spectral shifts from hexane to acetonitrile were
calculated to be 6290, 6620, 7460 cm-1 for 9FR, 9Phen, and FDBP, respectively. The
spectral shift from hexane to dichloromethane was determined to be 8500 cm-1 (or
difference of stokes shift ꢀῦSt = 8170 cm-1) for FDBP-O1N1 (to compare to 5810 cm-1
of the spectral shift of FDBP). Thus, the solvatochromism of FDBP-O1N1 is more
pronounced than that of Prodan,17,30 other highly solvatochromic dyes,31-33 and other
less twisted push-pull biphenyl analogs. In methanol, FDBP and other twisted push-pull
biphenyl analogs exhibited significant hypsochromic shift of fluorescence spectra
(Table 2), it had already been reported for FDBP and assigned to specific solvent-solute
interaction, i.e. hydrogen bonding.19 Therefore, the fluorescence maxima obtained from
methanol were not used for analysis of effect of solvent polarity.
To evaluate the effect of solvent-polarity on the spectra, the fluorescence maxima
(λem) vs. solvent polarity index ET(30)34 as well as the Lippert-Mataga plots35 were
collected and shown in Figures 4 and 5, respectively. The plots of band maximum vs.
9

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ET(30) exhibit good linearity (R2 ≥ 0.90) for all synthesized push-pull biphenyl analogs;
the Lippert-Mataga plots show a linear dependence of Stokes shift on polarity parameter
ꢀf, with R2 ≥ 0.90, except for the strongly twisted systems, such as FDBP-O1N1,
FDBP-O2, and FDBP-N2 (R2 = 0.72-0.80). The Lippert-Mataga plots indicate that the
Stokes shift of the synthesized push-pull biphenyl analogs hardly depends on the
position of the methyl group (donor or acceptor side of biphenyl), but is very sensitive
to the number of methyl groups. In fact, the “apparent” difference in the dipole
moments of the ground ꢁg and excited state ꢁe (increment of dipole moment, ꢀꢁ = ꢁe-ꢁg
[D]), calculated from Lippert-Mataga equation, gradually increases with the number of
methyl groups, rather than with the position of methyl groups (Table 2). The ꢀꢁ values
for FDBP, FDBP-O1, FDBP-N1, FDBP-O1N1, FDBP-N2, and FDBP-O2 were
estimated as 17.0 D, 19.9 D, 19.7 D, 21.1 D, 21.2 D, and 21.5 D, respectively. Since
twisting of the excited-state of these push-pull biphenyl analogs will generate additional
Stokes shift in solvents of higher polarity, the obtained ꢀꢁ values may not directly
describe the change in the dipole moment of the dyes. However, ꢀꢁ values can still be
used to quantify the extent of solvatochromism and the ensemble mean of excited-state
dipole moments as previously adopted.13,29,41-42 The relationship between the obtained
ꢀꢁ values and the excited state polarization of each molecule is addressed below in the
discussion of excited-state dynamics.
Similarly, the ꢀꢁ values of 9FR and 9Phen were calculated to be 13.8 D, in line
with the previous work,17 and 15.7 D, respectively. Two important findings should be
noted: i) both systems have a smaller ꢀꢁ than FDBP; ii) 9Phen, which has more twisted
conformation than 9FR, exhibits a larger ꢀꢁ. Thus, push-pull biphenyl analogs with
more twisted conformations are characterized by stronger polarization induced by
photoexcitation; furthermore, the excited-state polarization increases gradually between
the dipole moment of the coplanar CT and TICT states. Rettig et al. attributed the
correlation between the twist angle and “apparent” increments of dipole moments to the
equilibrium between TICT (it prefers less orbital overlap) and coplanar CT states (it
prefers larger orbital overlap).13,36 While this hypothesis can account for the behavior of
methyl-substituted FDBP derivatives, it cannot explain the increments of dipole
moment trend, e.g., 9Phen > 9FR since they are unlikely to form TICT state. Therefore,
we concluded that the extent of solvatochromism and the resulting “apparent”
10

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increments of dipole moments are not determined solely by the amount of TICT species,
and that the coplanar CT state gets more polarized as it deviates from planarity. Given
that torsional restriction changes not only the “molar ratio” of the TICT species but also
the polarization of coplanar CT, torsional restrictions can be potentially employed to
control the excited-state polarization and other relevant photophysical properties, such
as the controlling redox potential of photocatalysts37 and photovoltaics.38
11

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Table 2. Fluorescence maxima λem, Stokes shift ꢀῦSt and increment of dipole moment
by excitation ꢀꢁ of push-pull biphenyl analogs in various solvents.
λem
[nm]
ꢀῦSt
λem
[nm]
ꢀῦSt
Entry
Solvent
Entry
Solvent
[cm-1]
[cm-1]
Hexane
Toluene
THF
393
435
468
495
518
522
578
394
442
474
506
529
533
600
388
439
475
501
540
546
454
384
452
500
539
588
602
437
2100
3040
4690
5450
6430
6870
8620
2270
3560
5140
6100
6950
7470
9450
2920
4850
6530
7250
8580
9310
5650
3280
6370
8530
9580
11100
12000
5570
Hexane
Toluene
380
439
490
528
577
607
454
378
452
471
499
516
534
557
378
467
475
512
533
545
574
367
444
467
484
508
524
546
3760
6170
8620
9630
11200
12600
7090
3460
7210
8620
9730
10600
10600
11500
2790
7580
8350
9627
10800
10500
11600
3220
6870
8630
9350
10420
10320
11280
THF
9FR
(ꢀꢁ=13.8 D)
FDBP-O1
(ꢀꢁ=19.9 D)
DCM
DCM
DMF
DMF
ACN
ACN
Methanol
Hexane
Toluene
THF
Methanol
Hexane
Toluene
Diethyl ether
Me-THF
Ethyl acetate
Chloroform
DCM
9Phen
(ꢀꢁ=15.7 D)
FDBP-O1N1
(ꢀꢁ=21.1 D)
DCM
DMF
ACN
Methanol
Hexane
Toluene
THF
Hexane
Toluene
Diethyl ether
Me-THF
Ethyl acetate
Chloroform
DCM
FDBP
(ꢀꢁ=17.0 D)
FDBP-N2
(ꢀꢁ=21.2 D)
DCM
DMF
ACN
Methanol
Hexane
Toluene
THF
Hexane
Toluene
Diethyl ether
Me-THF
Ethyl acetate
Chloroform
DCM
FDBP-N1
(ꢀꢁ=19.7 D)
FDBP-O2
(ꢀꢁ=21.5 D)
DCM
DMF
ACN
Methanol
a) Me-THF is 2-methyltetrahydrofuran, DCM is dichloromethane, ACN is acetonitrile,
respectively.
2.4. Quantum yield and fluorescence decay lifetime measurements
The measurement of fluorescence quantum yield and lifetime (Table 3) provided
additional insight into the ꢀꢁ trend determined in this work. The unhindered biphenyl,
FDBP, showed the highest contribution of longer lifetime component in medium-polar
12

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solvents (toluene to dichloromethane) with a high quantum yield. In contrast, in highly
polar solvents, such as N,N-dimethylformamide (DMF) and acetonitrile, FDBP showed
lower quantum yields, and predominant contributions of the shorter lifetime
components. This behavior is typical of an excited state species, in which the main
quenching process is a non-radiative transition,1b,39 as opposed to a transition-forbidden
species, such as a twisted intramolecular charge-transfer (TICT) state. Compared to
FDBP, the strongly twisted biphenyls, FDBP-O1N1, FDBP-N2 and FDBP-O2, with
two methyl groups, exhibited anomaly long lifetimes, e.g., >3 ns, in medium-polar
solvents, despite the very low quantum yields. This supported the existence of a
transition-forbidden excited state species, which was suggested to be TICT in the case
of 4-acetyl-2,2’-dimethyl-4’-N,N-dimethylaminobiphenyl.25 According to the data
relative to mono-methylated FDBP-O1 and FDBP-N1, the values of lifetimes and the
quantum yields were intermediate. This, however, suggested a decrease in the radiative
rate and the existence of a transition-forbidden state; therefore, the longer lifetime
component increases with the polarity of the solvent, even though the fluorescence
quantum yield decreases. Quite low quantum yields in hexane were obtained for all
synthesized push-pull biphenyl analogs. This has been previously observed for several
donor-acceptor biphenyls, and various explanations have been provided. For instance,
Chou et al.19 suggested that a nonpolar medium can prevent the formation of a quinoid
structure, which has an electronic configuration typical of the singlet charge transfer
(1CT) state of donor-acceptor biphenyls; thus, a high non-radiative transition rate in
nonpolar solvents can be attributed to the fast internal conversion caused by torsional
vibrations on a conjugated system. Rettig et al. also proposed a similar conclusion for
4-N,N-dimethylamino-4’-cyanobiphenyl.13 According to these studies,13,19 a low
quantum yield can be explained by i) efficient intersystem crossing caused by the
proximity of T2 state to S1 state,40 and ii) a decrease of radiative transition rate
1
determined by the mixing between S1 and nearby Lb (singlet short-axis polarized
excited state) states, for which the transition is forbidden. In fact, a less twisted 9FR,
9Phen, and FDBP exhibit, as a minor component, a rapidly deactivating lifetime
component (≈1 ns) in the medium-polarity region despite a high quantum yield (Table
3). Additionally, as the short lifetime components vanish with increasing polarity, the
quantum yields are shown to be further close to unity. Therefore, the short lifetime
13

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components imply the existence of locally excited states that rapidly undergo
non-radiative deactivation.
Exceptionally high quantum yields (Table 3) were obtained for 9FR and 9Phen, in
which the donor and acceptor moieties are bridged with a methylene or an ethylene
group. This enhanced quantum yield compares well with previous results about
methylene- and ethylene-bridged biphenyls functionalized with donor and acceptor
moieties.13,41,42 Even though 9FR and FR0, which we previously reported, vary only in
alkyl groups on their nitrogen atoms, obtained quantum yields of 9FR are somewhat
higher than FR0. It may be ascribed also to usage of, as more reliable way than relative
method, absolute method for quantum yield measurement in this study. The quantum
yields of 9FR and 9Phen in DMF and acetonitrile were more than the double of that of
FDBP. Together with this large difference, we determined that also the fluorescence
lifetimes were roughly the double of that of FDBP. In contrast, in the medium-polarity
region (toluene to dichloromethane), where quantum yields of 9FR and 9Phen behaves
in a very similar way to FDBP, also the fluorescence lifetimes of 9FR and 9Phen were
found to be similar to those of FDBP for these solvent. These results indicate that the
higher quantum yields obtained from 9FR and 9Phen in highly polar solvents can be
ascribed to their longer lifetimes. The correlation between the fluorescence quantum
yields and lifetimes showed that the nonradiative transition rate was significantly
affected by incorporating a bridged structure on the biphenyl rings. Therefore, the large
quantum yields of 9FR and 9Phen can be attributed to the suppression of internal
conversion by the bridged structure, as previously reported for bridged stilbene.43,44
Single exponential decay of fluorescence lifetimes obtained from 9FR and 9Phen
(Table 3) enabled us to estimate the classical oscillator strength f using the relation ke ≈
ῦabs2ꢁf, where ke (=1/τ) is the rate of radiative transition [s-1] and ῦabs is the wavenumber
of the absorption maxima [cm-1], respectively.50 The classical oscillator strength f of
9FR was calculated to be 0.48 in DMF and 0.45 in acetonitrile, while that of 9Phen was
0.45 in DMF and 0.42 in acetonitrile. The smaller f values for 9Phen, compared to those
for 9FR, imply that 9Phen possesses a more twisted conformation than 9FR, even after
relaxation of the excited state.
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Table 3. Photophysical properties of push-pull biphenyl analogsa,b
a)
a)
a)
a)
τ1
τ2
[ns]
τ1
τ2
Entry
Solvent
ΦF
Entry
Solvent
ΦF
[ns]
ns]
[ns]
-
-
0.014
0.892
≥0.99
0.973
≥0.99
≥0.99
0.464
0.017
0.892
0.932
≥0.99
≥0.99
≥0.99
0.221
0.011
0.703
0.857
0.771
0.472
0.162
0.009
0.007
0.713
0.467
0.162
0.024
0.011
-
-
0.008
0.500
0.250
0.230
0.041
0.023
0.006
0.010
0.342
0.090
0.052
0.043
0.094
0.042
0.004
0.539
0.258
0.100
0.063
0.058
0.045
0.002
0.139
0.047
0.132
0.046
0.156
0.062
Hexane
Toluene
THF
Hexane
Toluene
1.07
(0.22)
1.90
(0.78)
0.85
(0.55)
0.78
(0.34)
1.60
(0.97)
2.54
(0.45)
1.56
(0.66)
3.40
(0.03)
2.49
2.80
3.13
3.29
1.98
-
-
-
-
-
-
-
THF
FDBP
-O1
9FR
DCM
DCM
-
-
-
-
-
-
-
-
DMF
DMF
ACN
ACN
Methanol
Hexane
Toluene
THF
Methanol
Hexane
1.07
(0.19)
0.68
(0.04)
0.93
(0.04)
2.02
(0.81)
2.69
(0.96)
3.07
(0.96)
1.19
(0.44)
5.53
(0.56)
Toluene
-
-
-
-
-
-
-
-
-
-
-
-
Diethyl ether
Me-THF
Ethyl acetate
Chloroform
DCM
FDBP
-O1N1
9Phen
DCM
3.36
-
DMF
ACN
3.46
-
1.06
(0.98)
5.00
(0.02)
Methanol
Hexane
Toluene
THF
-
-
Hexane
0.79
(0.32)
0.83
(0.10)
1.07
(0.09)
1.85
(0.88)
0.78
(0.95)
1.73
(0.68)
2.62
(0.90)
2.79
(0.91)
2.88
(0.12)
3.13
(0.05)
1.84
(0.32)
0.80
(0.18)
1.14
(0.36)
6.80
(0.68)
4.96
(0.82)
2.04
(0.64)
Toluene
Diethyl ether
Me-THF
Ethyl acetate
Chloroform
DCM
FDBP
-N2
FDBP
DCM
DMF
-
-
-
-
-
-
-
-
ACN
Methanol
Hexane
Toluene
THF
-
-
-
-
Hexane
0.97
(0.33)
1.10
(0.07)
1.42
(0.97)
2.99
(0.67)
2.93
(0.93)
3.88
(0.03)
0.71
(0.56)
4.48
(0.44)
Toluene
-
-
Diethyl ether
Me-THF
Ethyl acetate
Chloroform
DCM
FDBP
-N1
FDBP
-O2
0.83
(0.24)
2.74
(0.76)
DCM
DMF
-
-
-
-
-
-
-
-
1.26
(0.05)
3.41
(0.95)
ACN
~0.00
1
-
-
Methanol
a) The values in brackets indicate the relative coefficient of each term of fluorescence
lifetime.
a) Me-THF is 2-methyltetrahydrofuran, DCM is dichloromethane, ACN is acetonitrile,
respectively.
15

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Figure 3. Fluorescence spectra of (a) 9FR, (b) 9Phen, (c) FDBP and (d) FDBP-O1N1.
16

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Figure 4. Position of fluorescence maxima of FDBP and its derivatives versus the
polarity ET (30) in aprotic solvents.
Figure 5. Lippert-Mataga plot of FDBP and its derivatives. The increments of dipole
moments were calculated based on ꢀῦSt = 2ꢀꢁ2 f / hca3, + 1) – (n2 –
∆ ∆f = (ε – 1)/(2ε
1)/(2n2 + 1), where c is the speed of light [cm/s] ; h is Planck’s constant [erg · s]; n and
ε
are the refractive index45 and dielectric constant,45 respectively. The data in polar protic
solvents were excluded to avoid specific solute-solvent interactions. Onsager cavity
radii was the half-length of the long-molecular axis as adopted in other biphenyl
donor-acceptor system,13 i.e., 5.15 Å for 9FR and 5.51 Å for others.
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3. Conclusions
In this work, we have synthesized a series of push-pull biphenyl analogs with various
torsional restrictions, such as bridged structures and methyl groups. The UV-Vis
absorption spectra of these compounds suggested that the incorporation of diverse
torsional restrictions induced variation of their conformation from coplanar to highly
twisted. Fluorescence spectra of the push-pull biphenyl analogs showed that the torsion
on the biphenyl chromophore drastically enhanced their fluorescence solvatochromism.
The quantum yield of the push-pull biphenyl analogs significantly differed depending
on the solvent polarity. While those with a bridged structure (9FR and 9Phen) are
highly luminescent in a wide range of solvent polarity, the fluorescence of highly
twisted push-pull biphenyl analogs (FDBP-N1, FDBP-O1, FDBP-N1O1, FDBP-N2,
FDBP-O2) is strongly quenched in highly polar media. These data together with the
fluorescence lifetime measurements showed that the formation of TICT states is the
predominant quenching path for highly twisted FDBP-N1O1, FDBP-N2, FDBP-O2. In
contrast, no TICT state formation was observed for 9FR, 9Phen, and FDBP, which
implies that their excited-state polarization is different at the level of an individual
molecule. Thus, we successfully modulated the environment-sensitivity of push-pull
biphenyl analogs by imposing torsional restrictions. For biological applications where
intense luminescence from apolar to polar media is demanded, brightly luminescent
9Phen and 9FR should be useful as bioimaging probes. Although other push-pull
biphenyl analogs were not as luminescent as bridged one, their photophysical
characteristics are still promising candidate for probing some biological targets. Other
twisted biphenyls, FDBP-O1N1, FDBP-N2, FDBP-O2, characterized by intense
solvatochromism in the medium-polarity region, could be employed to study lipid order
in model membranes5g or as fluorogenic probes for recognition of biomolecular
targets.5a,b In addition, the ability of controlling the planarity of a chromophore by
torsional restrictions has recently been recognized as an important strategy for the
preparation of organic semiconductors 46 and dye-sensitized solar cells.47 Furthermore,
twisted structures are used in a wide range of fields, such as organic photovoltaics,38 and
photocatalysts,48 even though studies that successfully achieved fine-tuning of torsional
angles are very limited.49 Our design strategy enables the organic dyes employed in
these applications to significantly change their photophysical properties without using
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different chromophores. Further studies are being carried out in order to extend our
approach to other relevant applications such as tuning of photoredox potentials.
4. Experimental section
4.1. General
All the reagents and solvents (spectroscopic grade) were obtained from commercial
source and used without further purification. Detailed information about commercial
1
13
source is described in the Supporting Information, S2. All the H NMR and C NMR
spectra were recorded on a 400 MHz JEOL LMN-EX400 instrument or 300 MHz
BRUKER DPX300 with tetramethylsilane (TMS) as the internal standard. FT-IR spectra
were recorded on a JASCO FT-IR 469 plus spectrometer. Melting points were obtained
by polarized optical microscopy (POM) (Leica DM2500P microscopy with a Mettler
FP90 hot stage). MS spectra (EI+) were obtained by JEOL JMS700 mass spectrometer.
All photophysical measurements performed in solutions were carried out using dilute
solutions with optical density (O.D.) around 0.1 at the maximum absorption wavelength
in 1 cm path length quartz cells at room temperature (298 K). In addition, all samples
solutions were deaerated by bubbling with argon gas for 15 min before the
measurements. UV-Vis spectra were recorded with a JASCO V-670 UV-Vis
Spectrophotometer. Fluorescence spectra were recorded on a JASCO FP-6500
Spectrofluorometer. Absolute Quantum Yields were measured by a Hamamatsu
Photonics Quantaurus QY. Fluorescence lifetimes were measured at the fluorescence
maxima using a Hamamatsu Photonics OB 920 Fluorescence Lifetime Spectrometer
equipped with LEDs lamp (343 nm).
4.2. Synthesis
The syntheses and characterizations of 4-formyl-4’-N,N-dimethylamino-1,1’-biphenyl
(FDBP) and its methyl-substituted analogs (FDBP-O1, FDBP-N1, FDBP-O1N1,
FDBP-N2, FDBP-O2) were described in the Supporting Information, S3.
4.2.1. 2,7-Dibromo-9,10-dihydrophenanthrene (3Phen)
To a suspension of 1 (2.0 g, 11.1 mmol) and anhydrous iron(III) chloride (0.090 g) in
water (100 mL), a solution of bromine (1.26 mL, 24.4 mmol) in water (50 mL) was
19

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added dropwise over 30 min at ambient temperature under argon atmosphere. The
resulting suspension was stirred overnight. The colorless precipitate was filtered and
washed with water, 10% sodium hydroxide aqueous solution, and water sequentially on
a Buchner funnel. Subsequently, the colorless precipitate was dried in vacuo to afford
3Phen (3.68 g) as a colorless solid. Yield: 98%; 1H NMR (300 MHz, CDCl3): δ 7.55 (d,
3J = 8.2 Hz, 2H), 7.42 (dd, 4J = 2.0 Hz, 3J = 8.3 Hz, 2H,), 7.38 (d, 4J = 1.8 Hz, 2H), 2.83
(m, 4H) ppm (Figure S19).
4.2.2. 2,7-Dibromo-9,9-dimethyl-9H-fluorene (3FR)
All glassware were flame-dried under vacuum before any reaction procedure was
conducted. Anhydrous THF 40 ml was added to the mixture of 2,7-dibromofluorene (2)
(4.0 g, 12.3 mmol) and sodium tert-butoxide (5.3 g, 55.5 mmol) under argon
atomosphere and stirred for 2 h at 0 °C. Subsequently, methyl iodide (4.6 ml, 74.0
mmol) was added dropwise and the reaction mixture was further stirred for 2 h at same
temperature. After confirmation of completion of the reaction, the reaction mixture was
extracted with chloroform, and washed with water and brine. Then, the combined
organic layer was dried over magnesium sulphate. The solvent was removed in vacuo,
and the residue was purified by recrystallization from chloroform-methanol-ethanol
1
mixed solvent to afford crude 3FR (2.81 g) as a colorless solid. Yield: 65%; H NMR
(300 MHz, CDCl3): δ 7.54 (d, 3J = 7.9 Hz, 2H), 7.54 (d, 4J = 1.8 Hz, 2H), 7.46 (dd, 4J =
1.7 Hz, 3J = 8.2 Hz, 2H), 1.46 (s, 6H) ppm (Figure S20).
4.2.3. 7-Dibromo-2-(N-phthalimidyl)-9,9-dimethyl-9H-fluorene (4FR)
All glassware were flame-dried under vacuum before any reaction procedure was
conducted. A mixture of 3FR (1.9 g, 5.50 mmol), CuI (1.3 g, 6.60 mmol) and potassium
phthalimide (1.2 g, 6.60 mmol) in DMA (60 mL) was refluxed for 5 days. After cooling
to room temperature, water was added to the reaction mixture and resultant precipitation
was filtered. Then, the filtrate was extracted with ethyl acetate. The organic phase was
washed with brine and dried over magnesium sulphate. After solvent evaporation, the
crude residue was purified by silica gel column chromatography (hexane: chloroform =
1:3) to yield crude 4FR (0.97g) as a pale yellow solid. Yield: 42%; 1H NMR (300 MHz,
CDCl3): δ 7.99-7.96 (m, 2H), 7.83-7.79 (m, 3H), 7.61 (d, 3J = 8.0 Hz, 1H), 7.57 (d, 4J =
20

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4
3
1.7 Hz, 1H), 7.50-7.47 (m, 2H), 7.42 (dd, J = 1.6 Hz, J = 8.1 Hz, 1H), 1.51 (s, 6H)
ppm (Figure S21).
4.2.4. 7-Dibromo-2-(N-phthalimidyl)-9,10-dihydrophenanthrene (4Phen)
According to the synthetic procedure for 4FR, 4Phen was obtained from 3Phen as a
yellow solid. Yield: 42%; 1H NMR (300 MHz, CDCl3): δ 7.97 (dd, 4J = 3.0 Hz, 3J = 5.4
Hz, 2H), 7.84-7.79 (m, 3H), 7.62 (d, 3J = 8.1 Hz, 1H), 7.50-7.33 (m, 4H), 2.93-2.89 (m,
4H) ppm (Figure S22).
4.2.5. 2-Amino-7-bromo-9,9-dimethyl-9H-fluorene (5FR)
To a solution of 4FR (0.96 g, 2.30 mmol) in ethanol (60 mL), hydrazine monohydrate
(0.13 ml, 2.77 mmol) was added in one portion. The mixture was refluxed for 12 h.
After cooling to room temperature, the solution was filtered. After solvent evaporation,
the residue was purified by passage through a short silica gel plug using Chloroform as
1
eluent, to yield 5FR (0.59 g) as a pale yellow oil. Yield: 89%; H NMR (300 MHz,
CDCl3): δ 7.46-7.37 (m, 4H), 6.72-6.63 (m, 2H), 3.78 (s, 2H), 1.42 (s, 6H) ppm (Figure
S23).
4.2.6. 2-Amino-7-bromo-9,10-dihydrophenanthrene (5Phen)
According to the synthetic procedure for 5FR, 5Phen was obtained from 4Phen as a
1
pale brown solid. Yield: 89%; H NMR (300 MHz, CDCl3): δ 7.52-7.46 (m, 2H),
4
4
7.37-7.32 (m, 2H), 6.62 (dd, J = 2.3 Hz, 3J = 8.2 Hz, 1H), 6.55 (d, J = 2.1 Hz, 1H),
3.73 (s, 2H), 2.82-2.77 (m, 4H) ppm (Figure S24).
4.2.7. Methyl N-(7-bromo-9,9-dimethyl-9H-fluoren-2-yl)carbamate (6FR)
To a solution of 5FR (0.59 g, 2.05 mmol) in chloroform (10 mL), triethylamine (2.3 ml,
16.4 mmol) was added dropwise and the reaction mixture was stirred at 0 °C.
Subsequently, methyl chloroformate (0.94 ml, 12.3 mmol) was added in dropwise
manner. The reaction mixture was stirred at 0 °C for 30 min and further stirred at room
temperature for 2h. The reaction was quenched by adding water; subsequently, the
reaction mixture was extracted with chloroform, and washed with water and brine. Then,
the combined organic layer was dried over magnesium sulphate. The solvent was
21

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removed in vacuo, and the residue was purified by silica gel column chromatography
using chloroform as eluent, to afford crude 6FR (0.56 g) as pale yellow solid. Yield:
78%; 1H NMR (300 MHz, CDCl3): δ 7.59-7.56 (m, 2H), 7.51 (d, 4J = 1.7 Hz, 1H), 7.48
4
4
(d, 3J = 8.0 Hz, 1H), 7.42 (dd, J = 1.6 Hz, 3J = 8.1 Hz, 1H), 7.23 (dd, J = 2.0 Hz, 3J =
8.2 Hz, 1H), 6.84 (s, 1H), 3.79 (s, 3H), 1.44 (s, 6H) ppm (Figure S25).
4.2.8. Methyl N-(7-bromo-9,10-dihydrophenanthren-2-yl)carbamate (6Phen)
According to the synthetic procedure for 6FR, 6Phen was obtained from 5Phen as a
1
yellow solid. Yield : 66%; H NMR (300 MHz, CDCl3): δ 7.60 (d, 3J = 8.4 Hz, 1H),
7.50 (d, 3J = 8.3 Hz, 1H), 7.39-7.31 (m, 3H), 7.26-7.20 (m,1H), 6.74 (s, 1H), 3.78 (s,
3H), 2.83-2.78 (m, 4H) ppm (Figure S26).
4.2.9. Methyl N-(7-bromo-9,9-dimethyl-9H-fluoren-2-yl), N-methylcarbamate
(7FR)
All glassware were flame-dried under vacuum before any reaction procedure was
conducted. NaH at 60% in oil (0.33g 16.1 mmol) and subsequently anhydrous DMF (5
ml) were introduced under argon into a 50 ml two-neck flask. To the reaction mixture,
6FR (0.56 g, 1.61 mmol) in solution of anhydrous DMF (15 ml) was added dropwise.
After 10 minutes, methyl iodide (0.6 ml, 19.3 mmol) was added dropwise. The resulting
mixture was stirred at room temperature for 2h. The reaction was quenched by adding
methanol; subsequently, the reaction mixture was extracted with ethyl acetate, and
washed with water and brine. Then, the combined organic layer was dried over
magnesium sulphate. The solvent was removed in vacuo, and the residue was purified
by silica gel column chromatography using chloroform as eluent, to yield crude 7FR
(0.58 g) as a pale yellow oil. Yield: ≥99%; 1H NMR (300 MHz, CDCl3): δ 7.57 (d, 3J =
8.1 Hz, 1H), 7.53 (d, 4J = 1.6 Hz, 1H), 7.47 (d, 3J = 8.0 Hz, 1H), 7.40 (dd, 4J = 1.7 Hz,
3J = 8.1 Hz, 1H), 7.30-7.29 (m, 1H), 7.17 (dd, 4J = 1.7 Hz, 3J = 8.1 Hz 1H), 3.70 (s, 3H),
3.32 (s, 3H), 1.42 (s, 6H) ppm (Figure S27).
4.2.10. Methyl N-(7-bromo-9,10-dihydrophenenthren-2-yl), N-methylcarbamate
(7Phen).
According to the synthetic procedure for 7FR, 7Phen was obtained from 6Phen as
22

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yellow viscous liquid. Yield: ≥99%; 1H NMR (300 MHz, CDCl3): δ 7.66 (d, 3J = 8.3 Hz,
3
4
1H), 7.55 (d, J = 8.3 Hz, 1H), 7.42-7.36 (m, 2H), 7.16 (dd, J = 2.2 Hz, 3J = 8.3 Hz,
1H), 7.12 (m, 1H), 3.73 (s, 3H), 3.32 (s, 3H), 2.86-2.82 (m, 4H) ppm (Figure S28).
4.2.11. 2-Dimethylamino-7-bromo-9,9-dimethyl-9H-fluorene (8FR)
All glassware were flame-dried under vacuum before any reaction procedure was
conducted. Lithium aluminum hydride (0.61g 16.0 mmol) and subsequently anhydrous
THF (15 ml) were introduced under argon into a 100 ml two-neck flask. To the reaction
mixture, 7FR (0.58 g, 1.60 mmol) in solution of anhydrous THF (20 ml) was added
dropwise and stirred at 0 °C for 30 min. Then the reaction temperature was raised to
room temperature and the reaction mixture was stirred for 30min. The reaction
temperature was further raised to 40 °C and the reaction mixture was stirred for 2h.
Finally, the reaction mixture was stirred at THF reflux temperature for 2h and
subsequently cooled to 0 °C. The reaction was quenched by adding sodium sulfate
decahydrate (7.7 g, 24.0 mmol) slowly. Subsequently, ethyl acetate was added for the
reaction mixture to become suspension of white powders. The reaction mixture was
filtered and filtrate underwent solvent evaporation in vacuo. The residue was purified by
silica gel column chromatography using chloroform as eluent, to yield crude 8FR (0.34
g) as a white solid. Yield: 67%; 1H NMR (300 MHz, CDCl3): δ 7.60-7.52 (m, 1H), 7.46
4
3
4
(d, J = 1.7 Hz, 1H), 7.43 (d, J = 8.0 Hz, 1H), 7.38 (dd, J = 1.8 Hz, 3J = 8.0 Hz, 1H),
6.78-6.69 (m, 2H), 3.03 (s, 3H), 1.45 (s, 6H) ppm (Figure S29).
4.2.12. 2-Dimethylamino-7-bromo-9,10-dihydrophenanthrene (8Phen)
By following synthetic procedure for 8FR, 8Phen was obtained from 7Phen as a
yellow solid. Yield : 62%; 1H NMR (300 MHz, CDCl3): δ 7.63-7.59 (m, 1H), 7.33-7.28
4
(m,1H), 7.25-7.20 (m,1H), 7.17-7.10 (m, 1H), 6.66 (m, 1H), 6.55 (dd, J = 2.7 Hz, 3J =
10.5 Hz, 1H), 2.94-2.93 (m, 6H), 2.82-2.77 (m, 3H) ppm (Figure S30).
4.2.13. 7-Dimethylamino-9,9-dimethyl-9H-fluorene-2-carbaldehyde (9FR)
All glassware were flame-dried under vacuum before any reaction procedure was
conducted. A 2.6 M solution of n-butyllithium in hexane (0.39 mL, 1.01 mmol) was
added dropwise to a solution of 8FR (0.21 g, 0.67 mmol) in anhydrous THF (10 mL)
23

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under argon atmosphere at -78 °C. The reaction mixture was stirred for 30 min at the
same temperature, and anhydrous DMF (0.10 mL, 1.34 mmol) was added dropwise to
the solution. The resulting mixture was stirred at room temperature for 2 h. The reaction
was quenched by adding water; subsequently, the reaction mixture was extracted with
chloroform, and washed with water and brine. Then, the combined organic layer was
dried over magnesium sulphate. The solvent was removed in vacuo, and the residue was
purified by silica gel column chromatography (hexane: chloroform = 1:1). After
subsequent recrystallisation in hexane, 9FR (0.11 g) was obtained as a yellow solid.
1
Yield: 63%; Mp: 120.0-121.0 °C; H NMR (300 MHz, CDCl3):δ 9.98 (s, 1H, CHO),
7.89 (d, 4J = 1.0 Hz, 1H, ArH), 7.79 (dd, 4J = 1.5 Hz, 3J = 7.9 Hz, 1H, ArH), 7.67 (d, 3J
3
= 8.1 Hz, 1H, ArH), 7.65 (d, J = 9.2 Hz, 1H, ArH), 6.75-6.72 (m, 2H, ArH), 3.07 (s,
6H, N(CH3)2), 1.49 (s, 6H, C(CH3)2) ppm (Figure S31).; 13C NMR (75 MHz, CDCl3): δ
192.2 (CHO), 157.2, 153.5, 151.7, 146.9, 133.8, 131.3, 126.3, 122.6, 122.3, 118.5,
111.7, 106.0, 46.8 (C(CH3)2), 40.8 (N(CH3)2), 27.4 (C(CH3)2) ppm (Figure S32). FT-IR
(KBr): 2953 (C(CH3)2, C-H stretch), 2893 (C(CH3)2, C-H stretch), 2816 (N(CH3)2, C-H
stretch), 1687 (CHO), 1594 (Ar), 1507 (Ar), 1437, 1362 (Ar-N) cm-1.; Anal. Calcd for
C18H19NO: C, 81.47; H, 7.22; N, 5.28. Found: C, 81.45; H, 7.08; N, 5.22.
4.2.14. 7-Dimethylamino-9,10-dihydrophenanthrene-2-carbaldehyde (9Phen).
According to the synthetic procedure for 9FR, 9Phen was obtained from 8Phen as a
yellow solid. Yield: 24%; Mp: 95.3-95.8 °C; 1H NMR (300 MHz, CDCl3): δ 9.94 (s, 1H,
CHO), 7.75-7.74 (m, 2H, ArH), 7.70-7.68 (m, 2H, ArH), 6.69 (dd, 4J = 2.7 Hz, 3J = 8.7
Hz, 1H, ArH), 6.58 (d, 3J = 2.6 Hz, 1H, ArH), 3.03 (s, 6H, N(CH3)2), 2.92-2.87 (m, 2H,
Ar-CH2CH2-Ar) ppm (Figure S33).; 13C NMR (75 MHz, CDCl3): δ 192.1 (CHO), 151.0,
141.7, 139.7, 136.6, 133.7, 129.2, 129.1, 126.0, 122.7, 121.9, 111.5, 111.1, 40.4
(N(CH3)2), 29.7 (Ar-CH2CH2-Ar), 29.2 (Ar-CH2CH2-Ar) ppm. (Figure S34); FT-IR
(KBr): 2893 (Ar-CH2-CH2-Ar, C-H stretch) ,2813 (N(CH3)2, C-H stretch), 1694 (CHO),
1593 (Ar), 1545, 1490 (Ar), 1438, 1339 (Ar-N) cm-1; HRMS (EI+) exact mass
calculated for [M]+ (C17H17NO) requires m/z 251.1310, found m/z 251.1309.
References
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Application, Wiley-VCH, Weinheim, 2012, p87–92; (b) Lacowicz, J. R. Pronciples
of Fluorescence Spectroscopy Third edition, Springer, New York, 2006, p222–223.
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Supplementary materials
Supplementary data associated with this article can be found in the online version.
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