Design, synthesis, and structural and spectroscopic studies of push-pull two-photon absorbing chromophores with acceptor groups of varying strength

Article abstract of DOI:10.1021/jo302423p

A new series of unsymmetrical diphenylaminofluorene-based chromophores with various strong π-electron acceptors were synthesized and fully characterized. The systematic alteration of the structural design facilitated the investigation of effects such as molecular symmetry and strength of electron-donating and/or -withdrawing termini have on optical nonlinearity. In order to determine the electronic and geometrical properties of the novel compounds, a thorough investigation was carried out by a combination of linear and nonlinear spectroscopic techniques, single-crystal X-ray diffraction, and quantum chemical calculations. Finally, on the basis of two-photon absorption (2PA) cross sections, the general trend for π-electron accepting ability, i.e., ability to accept charge transfer from diphenylamine was: 2-pyran-4-ylidene malononitrile (pyranone) > dicyanovinyl > bis(dicyanomethylidene)indane >1-(thiophen-2-yl)propenone > dicyanoethylenyl >3-(thiophen-2-yl)propenone. An analogue with the 2-pyran-4-ylidene malononitrile acceptor group exhibited a nearly 3-fold enhancement of the 2PA cross section (1650 GM at 840 nm), relative to other members of the series.

Full text of DOI:10.1021/jo302423p

Article
pubs.acs.org/joc
Design, Synthesis, and Structural and Spectroscopic Studies of
Push−Pull Two-Photon Absorbing Chromophores with Acceptor
Groups of Varying Strength
Alma R. Morales,† Andrew Frazer,† Adam W. Woodward,† Hyo-Yang Ahn-White,† Alexandr Fonari,§
Paul Tongwa,§ Tatiana Timofeeva,§ and Kevin D. Belfield*,†,‡
‡
†Department of Chemistry and CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida
32816-2366, United States
§Department of Biology and Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701, United States
S
* Supporting Information
ABSTRACT: A new series of unsymmetrical diphenylaminofluorene-
based chromophores with various strong π-electron acceptors were
synthesized and fully characterized. The systematic alteration of the
structural design facilitated the investigation of effects such as molecular
symmetry and strength of electron-donating and/or -withdrawing
termini have on optical nonlinearity. In order to determine the
electronic and geometrical properties of the novel compounds, a
thorough investigation was carried out by a combination of linear and
nonlinear spectroscopic techniques, single-crystal X-ray diffraction, and
quantum chemical calculations. Finally, on the basis of two-photon
absorption (2PA) cross sections, the general trend for π-electron
accepting ability, i.e., ability to accept charge transfer from diphenyl-
amine was: 2-pyran-4-ylidene malononitrile (pyranone) > dicyanovinyl
> bis(dicyanomethylidene)indane >1-(thiophen-2-yl)propenone > dicyanoethylenyl >3-(thiophen-2-yl)propenone. An analogue
with the 2-pyran-4-ylidene malononitrile acceptor group exhibited a nearly 3-fold enhancement of the 2PA cross section (1650
GM at 840 nm), relative to other members of the series.
separation in the ground state of these molecules, can exert
significant influence over their molecular polarizability and
hyperpolarizabilities.21,22 This charge separation in the ground
state can be controlled primarily by either modifying the
chemical structure, i.e., altering the strength of the donating and
accepting substituents, and/or by extending the π-conjugated
path.18 The effects of altering the chemical structure in this way
were detailed by Oudar in 1977.23
Whereas considerable effort invested in the development of
new donor and acceptor groups24−26 has led to major progress
in the synthesis of stable and efficient chromophores, the
relationships between the structure of the π-conjugated spacer
and the nonlinear optical properties remain less clearly
understood. Our interest in the control of the electronic
properties of fluorene-based π-conjugated systems resulted in
various synthetic approaches for enhancement of the 2PA cross
section based on linear symmetrical and unsymmetrical
fluorene chromophores27−31 and, recently, two-dimensional
branched fluorene-based chromophores.32
INTRODUCTION
■
Conjugated organic molecules with large delocalized π-electron
systems continue to be the subject of very active research due
to their potential use in nonlinear optics and emerging
photonics technologies. Generally, such molecules provide
substantial delocalization of π-electrons over the molecule,
enhancing polarizability and nonlinear optical properties.
Among the various classes of nonlinear optical materials,
multiphoton absorbing materials continue to attract increasing
interest. Two-photon absorption (2PA) is a nonlinear process
that involves the somewhat unusual capability of a molecule to
absorb two photons simultaneously in order to populate an
energy level within the molecule with energy equal to the sum
of the energies of the two photons absorbed.1 2PA-based
chromophores are of particular interest because of their utility
and potential in numerous applications such as fluorescence
bioimaging,2−9 two-photon power-limiting devices,10−12 and
two-photon photodynamic therapy.13−15
Dipolar push−pull chromophores perhaps constitute the vast
majority of compounds investigated for their nonlinear optical
properties.16−20 Push−pull chromophores involve electron-
donor and electron-acceptor groups interacting through a π-
conjugating spacer. It has been well-established that modulating
the degree of ground-state polarization, or the degree of charge
The aromatic character of the fluorene ring can be strongly
influenced by the presence of donor or acceptor moieties
Received: November 13, 2012
© XXXX American Chemical Society
A
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Chart 1
a
Scheme 1 .
a
Legend: (i) n-BuLi,THF, DMF; (ii) diphenylamine or di-p-tolylamine, Pd(OAc)2, Cs2CO3 P(t-Bu)3, toluene; (iii) and (iv) malononitrile, toluene,
70 °C; (v) piperidine, EtOH, reflux; (vi) (CH3CO)2O; (vii) KOH, MeOH/H2O, reflux.
covalently attached at the 2- and 7-positions. Herein, we have
employed this approach in the modification of ground-state
polarization by altering the strength of the electron-with-
drawing terminal group on a fluorene core using four different
electron-acceptor groups that are end-capped with a diphenyl-
amine or di-p-tolylamine donating group. Electron-withdrawing
groups include 2-pyran-4-ylidene malononitrile, dicyanovinyl,
bis(dicyanomethylidene)indane, (thiophen-2-yl)propenone,
and dicyanoethylenyl (1−7, Chart 1).
In order to determine the electronic and geometrical
properties of compounds 1−7, a comprehensive investigation
was conducted by a combination of spectroscopic techniques,
B
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a
Scheme 2 .
a
Legend: (i) NBS, propylene carbonate; (ii) AlCl3, acetyl chloride, 20 h; (iii) Pd(OAc)2, Cs2CO3, P(t-Bu)3, toluene, 22 h; (iv) malononitrile, TiCl4,
pyridine, CHCl3, (v) KOH, MeOH/H2O, reflux.
single-crystal X-ray diffraction, and quantum chemical calcu-
lations.
out in the presence of piperidine in ethanol, rendering the
trans chromophore. The condensation reaction of the (2,6-
dimethyl-4H-pyran-4-ylidene)malononitrile with aldehyde C
resulted in 4 (Scheme 1), easily separated via column
chromatography as a red solid. The two cyano groups of 2-
pyran-4-ylidenemalononitrile are electron-withdrawing groups,
and the electron-deficient pyran ring can act as an auxiliary
acceptor.
The indane derivative, 2,2′-(1H-indene-1,3(2H)-diylidene)-
dimalononitrile (I), was prepared by reacting 1,3-indanedione
and malononitrile in ethanol.34 The conditions used for the
final Knoevenagel condensation between intermediate C and I
was carried out by reflux in acetic acid for 3 h, providing 5 in
high yield as a dark blue solid (Scheme 1).
RESULTS AND DISCUSSION
■
Synthesis. The series of new compounds under discussion
are shown in Chart 1. All seven push−pull chromophores
consist of an amino functionality as the donor and four different
electron-acceptor moieties.
The syntheses of the fluorene derivatives are presented in
Schemes 1 and 2. The key step in the synthesis of compounds
1−3 and 5 was the Knoevenagel condensation of aldehyde
derivatives with the active methylene group of malononitrile.
All these dyes were prepared according to a general synthetic
methodology involving lithiation of 2,7-dibromo-9,9-diethyl-
9H-fluorene with a stoichiometric amount of n-BuLi, followed
by reaction with dimethylformamide (DMF) and subsequent
acidic hydrolysis, providing intermediate B. Arylamination of 7-
bromo-9,9-diethylfluorene-2-carbaldehyde B was readily per-
formed using Buchwald−Hartwig amination coupling in the
presence of a Pd catalyst and a non-nucleophilic base, Cs2CO3,
to afford intermediate C.33 Knoevenagel condensation between
the resulting aldehyde C with active malononitrile provided
compound 1 as a red solid (Scheme 1).
Compound F was obtained in three steps involving C-
alkylation of fluorene to give D. Monobromination of D with
NBS provided 2-bromo-9,9-diethylfluorene (E), which yielded
the acetylated derivative F after a Friedel−Crafts acetylation.
The dicyanovinyl derivative 2 was synthesized from the keto
derivative F by condensation with malononitrile in the presence
of TiCl4 in pyridine (Scheme 2). After purification, 2 was
obtained as orange crystals in 82% yield.
The di-p-tolylamine group was used as an electron-donor
group in 3 (Scheme 1). The di-p-tolylamine group was coupled
to 7-bromo-9,9-diethylfluorene-2-carbaldehyde B via a Buch-
wald−Hartwig arylamination, producing fluorenyl aldehyde H.
Finally, malononitrile was condensed, via Knoevenagel
conditions, with H, affording 3 as a red solid. 1H NMR
showed the presence of a singlet at 2.34 ppm assigned to the
methyl groups of the p-tolylamine moiety and no aldehydic
proton signal.
The versatility of intermediate C was demonstrated in the
synthesis of another chromophore comprised of pyranone as an
electron-acceptor group. The condensation between C and
(2,6-dimethyl-4H-pyran-4-ylidene)malononitrile was carried
The thiophen-2-yl-prop-2-en-1-one electron-withdrawing
group was introduced by Claisen−Schmidt condensation
between thiophene-2-carbaldehyde and compound G, affording
chromophore 6 as a yellow solid (Scheme 2). Compound H
reacted with 1-(thiophen-2-yl)ethanone, affording chromo-
phore 7 as a yellow solid (Scheme 1). All compounds were
1
characterized by H and 13C NMR, mass spectrometry, and
elemental analysis.
Linear Photophysical Properties. The dipolar molecules
1−7 possess absorption spectra with two distinct peaks in
nonpolar solvents, the main band lying within the range 410−
630 nm and the second positioned between 305 and 345 nm
(Figures 1−7, curve 1). Excitation anisotropy is correlated with
the spectral position of various electronic transitions and can be
a very useful tool to estimate the position of 2PA allowed
transitions.27,35 The locations of these electronic transitions
over the spectral range 300−550 nm were revealed from
excitation anisotropy. The optical properties of compounds 1−
7 are given in Table 1.
In comparing the linear absorbance spectra of this series of
compounds, it is noticeable that incorporation of bis-
(dicyanovinyl)indane in 5 resulted in the largest red shift
relative to compounds 1−4, 6, and 7, indicative of improved
charge-transfer characteristics of this donor−acceptor sub-
stituted fluorene. To further evaluate the effect of the different
electron acceptors on the linear optical properties, 3-(thiophen-
2-yl)propenone was used as a benchmark acceptor group, due
to this moiety being the weakest acceptor in the series.
Absorption at λmax was found to progressively shift to longer
wavelength upon replacing this benchmark by dicyano-
C
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Figure 4. Normalized absorption (1), emission (2), and excitation
spectra (3) of 4 in chloroform and fluorescence excitation anisotropy
(4) in polyTHF. The 2PA spectrum (5) was obtained by the 2PF
method in chloroform.
Figure 1. Normalized absorption (1), emission (2), and excitation
spectra (3) of 1 in cyclohexane and fluorescence excitation anisotropy
(4) in polyTHF. The 2PA spectrum (5) was obtained by the 2PF
method in cyclohexane.
Figure 5. Normalized absorption (1), emission (2), and excitation
spectra (3) of 5 in chloroform and fluorescence excitation anisotropy
(4) in polyTHF. The 2PA spectrum (5) was obtained by the open
aperture Z-scan method in chloroform.
Figure 2. Normalized absorption (1), emission (2), and excitation
spectra (3) of 2 in cyclohexane and fluorescence excitation anisotropy
(4) in polyTHF. The 2PA spectrum (5) was obtained by the open
aperture Z-scan method in cyclohexane.
Figure 3. Normalized absorption (1), emission (2), and excitation
spectra (3) of 3 in cyclohexane and fluorescence excitation anisotropy
(4) in polyTHF. The 2PA spectrum (5) was obtained by the 2PF
method in cyclohexane.
Figure 6. Normalized absorption (1), emission under λexc 404 nm (2),
and excitation spectra (3) of 6 in cyclohexane and fluorescence
excitation anisotropy (4) in polyTHF. The 2PA spectrum (5) was
obtained by the open aperture Z-scan method in cyclohexane.
methylene and bis(dicyanovinyl)indane (Table 1). A consid-
erable red shift of the main emission bands was observed for
chromophores 1−7 (Figures 1−7, curve 2). These results
indicate that the fluorescence emission arises from excited state
intramolecular charge transfer in these molecules.
D
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Figure 8. Normalized absorption (solid lines) and emission spectra
(dashed lines) for 1 in different solvents.
Figure 7. Normalized absorption (1), emission under λexc 423 nm (2),
and excitation spectra (3) of 7 in cyclohexane and fluorescence
excitation anisotropy (4) in polyTHF. The 2PA spectrum (5) was
obtained by the 2PF method in cyclohexane.
correspond to the main absorption band (S0 → S1 transition),
while the change in anisotropy for compound 5 under
excitation in the short-wavelength absorption band (λexc 345
nm) exhibited a complicated electronic structure of the main
long-wavelength absorption band.
These compounds also displayed pronounced positive
solvatochromism (i.e., bathochromic shift with increasing
solvent polarity) in their emission spectra, whereas only a
slight red shift was observed in the absorption spectra (Figure
8) (representative data are shown for 1 only). This
solvatochromic behavior is consistent with the charge-transfer
characteristics of donor−acceptor substituted fluorene com-
pounds. A large Stokes shift (Δλst) was observed for 4 in
CHCl3 (Figure 4) due to strong solvent−solute dipole−dipole
interactions, as a manifestation of the large dipole moment and
orientational polarizability in CHCl3 (Δf ≈ 0.152).
Compounds 1−7 exhibited single-exponential fluorescence
decay processes with lifetimes (τ) that were independent of the
excitation wavelength over a broad spectral range (280−440
nm).
Quantum yields, Φ, were determined using a fluorescence
method,35 exciting at the absorption maximum (λmaxAbs) of each
molecule. Derivatives 1, 3, 4, and 7 exhibited good Q values in
cyclohexane and CHCl3 (0.31−0.66). Significant deviations
from planarity of the electron-acceptor group can explain the
largest red shift observed in the absorption spectrum of 5 and
may explain the lack of any detectable fluorescence upon
excitation at the long-wavelength maximum.
Nonlinear Photophysical Properties. The 2PA spectra
for compounds 1−7 are shown as curve 5 in Figures 1- 7. 2PA
spectra for compounds 1, 3, 4, and 7 were acquired using the
2PF technique over a broad spectral region, 740−940 nm, since
they possessed high fluorescence quantum yields. 2PA spectra
were measured for compounds 2, 5, and 6 by open aperture Z-
scan methods due to their low fluorescence quantum yields.
The maxima of 2PA cross section (δ) values for 1−7 were at ca.
880 nm, close to a linear absorption maximum at ca. 440 nm
and attributed to the S0 → S1 transition (identified by both
linear absorption and anisotropy spectra), a formally forbidden
transition for 2PA. The peak position of the 2PA spectra of 2, 6,
and 7 correlated quite well with the position of the transition to
the first excited state for this system, as indicated by the
location of the long-wavelength band in the linear absorption
spectrum (440 nm). Because these molecules do not possess a
center of symmetry, it is not surprising that each of the low-
lying transitions (S0 → S1 and S0 → S2) are observed by two-
photon absorption.
As expected, from the linear absorption analysis, the value of
λmax did vary significantly with the strength of the electron-
withdrawing moiety. There was no strong deviation of the λmax
values in 2PA spectra with changes in the electron-withdrawing
group. Compound 4 contained a pyran group and exhibited a
nearly 3-fold enhancement of δ (1650 GM at 840 nm) relative
The anisotropy excitation spectra and anisotropy values (r0)
for 1−7 in polyTHF are shown in curve 4 in Figures 1−7.
Constant anisotropy values (r0) for 1−7 in the spectral regions
of excitation wavelength, λexc ≥430−495 nm (compounds 1-4,
curve 4) and λexc ≥405−425 nm (compounds 6 and 7),
Table 1. Main Photophysical Parameters of Compounds 1−7: Absorption (λmaxAbs) and Fluorescence Maxima (λmaxFl), Stokes
Shift (Δλst), Maximum Extinction Coefficient (εmax), Fluorescence Quantum Yield (Φ), Anisotropy (r0), and Fluorescence
Lifetime (τ)
compd
λmax
λmax
Δλst
ε
max, 10‑3 M‑1 cm‑1
Φ
r0(p-THF)
τ (ns)
Abs
Fl
a
1
483
430
498
470
1
1
1
1
502
1
1
1
1
1
1
1
19
65
50
29
54
79
24
40
27
0.31 0.05
0.02 0.05
0.66 0.05
0.64 0.05
0.001 0.05
0.01 0.05
0.49 0.05
0.32
0.36
0.37
0.32
0.30
0.22
0.35
1.00 0.01
0.13 0.08
1.95 0.05
2.8 0.01
a
2
495
520
648
500
420
468
a
3
22
b
4
178
b
c
5
345, 634
1
a
6
404
423
1
1
16
45
0.21 0.09
1.37 0.02
a
7
a
b
c
Absorption and emission spectra measured in cyclohexane. Absorption and emission spectra measured in chloroform. Excited at 345 nm.
E
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to the rest of the series (see curves 5 in Figures 1−7). This is
particularly intriguing, considering that in this series the
bis(dicyanomethylidene)indane accepting group was assumed
to be the strongest electron-withdrawing group in comparison
to the other substituents studied.
Thus, the study of the dipolar fluorene derivatives illustrates
that there appears to be an optimum degree of ground-state
polarization that maximizes δ. Furthermore, it does not
necessarily correspond to the molecule which possesses the
strongest ground-state dipole moment (as revealed by quantum
calculations (Table S1, Supporting Information), as 4 possesses
the largest dipole moment in the series). As a result, fluorenyl 4,
with its pyranone terminal group, has optimal charge transfer
character that maximizes its state and transition dipole
moments, the main factors that influence the strength of 2PA.
Single-Crystal X-ray Structural Analysis. Crystals
suitable for single crystal X-ray analysis were obtained for
compounds 1, 2, and 6 by the slow diffusion of hexane into a
concentrated dichloromethane solution. Solid-state structural
analysis for the fluorene-containing compounds has generally
been rarely reported, due, in part, to difficulties in growing
single crystals. Single-crystal X-ray structural analysis was
completed for compounds 1, 2, and 6 (Table 2). Single
crystals could not be obtained for the other compounds in the
series.
Table 2. Summary of the Crystal Data and Structure
Refinement Parameters for 1, 2, and 6
Figure 9. ORTEP drawings for 1 (a), 2 (b), and 6 (c) with the
numbering scheme. Thermal ellipsoids are shown at the 50%
probability level. Only the major part of the disordered dicyanovinyl
fragment in 1 is shown. Hydrogen atoms are omitted for clarity.
1
2
6
formula
formula wt
cryst syst
space group
a/Å
C33H27N3
465.58
C34H29N3
479.60
C36H31NOS
525.69
triclinic
triclinic
triclinic
P1
P1
P1
̅
̅
̅
carbons was 0.119(2) Å, suggesting sp2 hybridization of the N
atom and, consequently, donation of its pair of nonbonding
electrons into the π conjugation chromophore system (Table
3). The molecular geometry of 1 revealed both syn (major
disordered part) and trans (minor part) conformations of the
dicyanovinyl group (see the Experimental Section), with the
values for the torsion angle C(3)−C(4)−C(30)−C(31) being
30.1(4) and 161.1(5)°, respectively. Compound 2 clearly
exhibited a syn conformation of the dicyanovinyl group, with
the torsion angle C(3)−C(4)−C(30)−C(31) equal to
42.5(2)°. Compound 6 revealed the largest deviation from
planarity for the acceptor group, manifested by the torsion
angle C(5)−C(4)−C(30)−C(31) of 18.6(3)°, due to H···H
repulsion interactions of H atoms attached to C(5) and C(31).
The crystal packing in 1, 2, and 6 was determined by
antiparallel dipole−dipole interactions between the molecules
related by inversion centers. The bulky diphenylamino group
and essentially eclipsed ethyl substituents on the fluorene
moiety prevent any close π−π intermolecular interactions
(Figure S1, Supporting Information).
Quantum Chemical Calculations. Ground-state opti-
mized geometries show only slight differences in comparison
to available crystallographic data. The nitrogen atom in the
diarylamine group became essentially coplanar with the bonded
carbon atoms, suggesting improved electron donation to the
fluorenyl π system. The torsion angle C(3)−C(4)−C(30)−
C(31), describing syn isomerization in 1 and 2, arrived at values
of 0.2 and 35.9°, respectively. The fluorene core and the
acceptor group were positioned essentially in the same plane
for the optimized geometries of 4 and 7. The bis(dicyanovinyl)-
8.3392(13)
10.0974(16)
15.684(3)
98.756(2)
101.508(2)
97.415(2)
1261.5(4)
2
8.7255(12)
10.8750(15)
15.855(2)
98.469(2)
101.687(2)
110.096(2)
1344.6(3)
2
9.3797(6)
10.4703(6)
15.9304(10)
79.5150(10)
78.4040(10)
67.3940(10)
1405.30(15)
2
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
Dc/Mg m−3
μ(Mo Kα)/mm−1
F(000)
1.226
1.185
1.242
0.072
0.070
0.145
492
508
556
no. of collected/unique
rflns (R(int))
16761/6046
(0.0645)
14513/4732
(0.0434)
15012/4944
(0.0310)
no. of rflns with I > 2σ(I) 3370
goodness of fit on F2
4732
4944
1.001
1.012
1.043
R, Rw (I > 2σ(I))
0.0585, 0.1293 0.0399, 0.0876 0.0407, 0.0988
The fluorene substructure is essentially planar for all three
compounds. Phenyl rings from the diphenylamino donor group
are twisted with respect to the fluorene bridge; angles range
from 60.8(1) to 67.8(1)° (Table 3). Similar values were
observed for analogous unsymmetrical fluorene-based deriva-
tives.33
The nitrogen in the diphenylamino group exhibited a
trigonal-planar environment in 1, showing a slight deviation
to a pyramidal configuration in analogues 2 and 6, with C−N−
C angles being in the range of 116.7(2)−121.7(2)°. The largest
deviation from the plane formed by C(11), C(18), and C(19)
F
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Table 3. Selected Structural Parameters (Å, deg) for Experimental (x) and Calculated (c) Molecules
a
molecule
PA−PB
PA−PC
N
C(11)−N(3)−C(18)
C(18)−N(3)−C(24)
C(24)−N(2)−C(11)
1c
1x
2c
2x
3c
4c
5c
6c
6x
7c
60.55
82.94
0.032
119.94
119.9(2)
120.33
119.7(1)
120.67
120.24
120.57
120.11
119.6(2)
120.14
121.43
121.4(2)
119.04
119.4(1)
118.34
119.29
118.60
119.48
121.7(2)
119.47
118.48
118.5(2)
120.63
118.8(1)
120.99
120.47
120.83
120.41
116.7(2)
120.39
60.60(5)
69.55
82.95(5)
67.86
0.032(2)
0.003
63.09(4)
68.98
72.87(4)
66.93
0.118(2)
0.006
70.18
68.84
0.003
69.07
67.76
0.003
70.54
68.89
0.003
67.75(5)
70.41
81.85(5)
68.91
0.119(2)
0.003
a
Deviation of the central nitrogen atom from its bonded neighboring carbon atoms.
indane acceptor in 5 had a distorted geometry due to steric
repulsions between dicyanovinyl groups and the fluorene core,
with an angle at the bridging C sp2 atom equal to 132.1°
(Figure 10) and an interplanar angle between the fluorene core
and bis(dicyanovinyl)indane group of 38.8°.
(compare compounds 1 and 3). This result can be explained
by the hyperconjugation effect. The methylidene malononitrile
group (1) showed slightly stronger acceptor properties in
comparison to ethylidene malononitrile (2). Fluorenyl 4, with a
pyronone group as an electron acceptor, possessed the largest
dipole moment in the series, suggesting enhanced linear and
nonlinear absorption properties. The nonplanarity observed in
6 led to a decrease in conjugation and, as a result, a slightly
decreased dipole moment in comparison to 7.
Singlet Excited States. Two main bands in the absorption
spectra of 1−7 are well reproduced by time-dependent (TD)
DFT calculations (Table 4).
As was recently validated for fluorene-based push−pull
chromophores, 2PA in the lowest energy transition correlates
with the values of the transition dipole moment and permanent
dipole moment differences between the ground and first excited
states (S0 → S1).36,37 Therefore, only the lowest energy
electronic excitation is considered below. The calculated
absorption maxima were in good agreement with those
experimentally observed, providing credence to the computa-
tional method. As revealed by TD DFT calculations, the
ground to first excited state transition described the lowest
lying energy excitation. Red shifts observed in the absorption
spectra are well reproduced by the calculations, suggesting
increased intramolecular charge transfer (CT) character along
the series. However, bis(dicyanovinyl)indane in diphenylami-
nofluorene 5 resulted in the largest red shift relative to the rest
of the compounds but did not lead to improved CT
Figure 10. DFT optimized geometry of 5.
The electron acceptor group in 6 deviated from the best
plane with a C(5)−C(4)−C(30)−C(31) torsion angle value
equal to 8.0°, due to H···H repulsion mentioned in the
crystallographic description. The comparison of ground state
dipole moment values is an indicator of the donor−acceptor
strengths of the studied systems. Both functionals (B3LYP and
M06) showed similar tendencies in dipole moments (Table S1,
Supporting Information). With addition of the methyl group to
the terminal phenyl rings, the dipole moment increased
Table 4. Calculated Ground Energy and Dipole (Eg (eV) and μg (D)) and Excited State (λc (nm), f, μt (D)) Properties at the
M06/6-311++G** Level
λc
f
transition
μt
μg
Eg
1
2
3
4
5
6
7
346
488
334
469
362
531
391
501
360
652
352
448
341
451
0.3696
0.7821
0.3583
0.6005
0.4713
0.9627
0.7373
1.1375
0.3693
0.5087
0.5553
0.6647
0.6301
0.8977
S0 → S2
S0 → S1
S0 → S3
S0 → S1
S0 → S2
S0 → S1
S0 → S2
S0 → S1
S0 → S7
S0 → S1
S0 → S3
S0 → S1
S0 → S4
S0 → S1
4.2122
12.5691
3.9441
9.2694
5.6102
16.8228
9.4835
18.7683
4.3732
10.9133
6.4272
9.8138
7.0807
13.3268
9.9395
−39110.0516632424
8.2282
10.8632
13.5913
7.4602
2.3721
4.1917
−40179.3249837085
−41248.562096415
−48546.3220781635
−54630.8843510249
−52188.2893297265
−52188.3070881576
G
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Figure 11. S0 → S1 natural transition orbitals. λ is the fraction of the NTO pair contribution into the given electronic excitation.
characteristics, as revealed by ground and transition dipole
moments, as well as by natural transition orbital (NTO)
analysis. Improvement of CT correlates well with the increase
of the both ground and transition dipole moment values.
Enhanced hyperconjugation results in slightly improved CT for
3 over 1. Fluorenyl thiophene 6, having the weakest acceptor
group (thiophen-2-yl prop-2-en-1-one), revealed the shortest
λmax and smallest ground state dipole. NTOs for the S0 → S1
transition are shown in Figure 11.
We note several common aspects for the chromophores with
regard to transition character: (i) as expected, the HOMO wave
function is more developed on the donor, whereas the LUMO
wave function is more developed on the acceptor group; (ii)
electron density redistribution clearly suggests π → π*
transition character, except the small fraction of σ → π*
coming from donor methyl groups (when present); (iii) ethyl
groups located on fluorene are not involved in CT whatsoever;
(iv) finally, because of the existence of an overlap of the
HOMO and LUMO wave functions, transition cannot be
attributed to pure CT character. The largest transition dipole
moment observed for 4 is mirrored by the LUMO wave
function, which is even less pronounced on the diphenylamino
group in this compound in comparison to the other molecules.
For 5, the LUMO wave function is located mainly on the
aromatic portion, not on the dicyanovinyl moiety of the
acceptor.
CONCLUSIONS
■
In summary, modification of diphenylaminofluorene by the
introduction of electron-withdrawing groups resulted in a series
of robust two-photon absorbing fluorene-based chromophores.
It was found that the absorption and emission spectra show
significant bathochromic shifts according to the strength of the
electron-withdrawing moieties. Compound 4, with its pyranone
terminal group, showed the optimal charge transfer character in
this series of compounds, which maximizes its state and
transition dipole moments (it had the largest transition dipole
moment), and exhibited the largest 2PA cross-section values.
Data from linear and nonlinear photophysical studies, crystallo-
graphic studies, and quantum mechanical calculations were
relatively consistent and provided insight into the observed
behavior of the molecules. It was shown that there is an
optimum degree of dipolar molecular character, with the
strongest electron-withdrawing group not necessarily providing
the largest 2PA. Results from this study provide the rationale to
H
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the solution was poured into cold water. The separated organic phase
was dried over Na2SO4. The product was purified by column
chromatography using a hexane/ethyl acetate mixture (4/1). A white
design 2PA materials for applications such as probes for two-
photon fluorescence bioimaging applications.
1
solid was isolated. Yield: 78% (7.2 g). Mp: 120−121 °C. H NMR
EXPERIMENTAL SECTION
■
(500 MHz, CDCl3): δ 7.97 (t, J = 7.5 Hz, 2H), 7.74 (d, J = 8 Hz, 1H),
7.63 (d, J = 8 Hz, 1H), 7.51−7.49 (m, 2H), 2.66 (s, 3H), 2.11−1.99
(m, 4H), 0.31 (t, J = 15 Hz, 6H). 13C NMR (125 MHz, CDCl3): δ
198.0, 153.3, 149.8, 145.3, 139.1, 136.2, 130.5, 128.4, 126.6,122.6,
122.4, 121.9, 119.5, 56.7, 32.5, 27.0, 8.4 ppm. HRMS (ESI-TOF): m/z
[M + Na]+ calcd for C19H19BrONa 365.0511, found 365.0514.
Synthesis of 1-(2-(Diphenylamino)-9,9-diethyl-9H-fluoren-7-
yl)ethanone (G). Under a nitrogen atmosphere, a mixture of 1-(2-
bromo-9,9-diethyl-9H-fluoren-7-yl)ethanonecarbaldehyde (6 g, 17.47
mmol), diphenylamine (4.43 g, 26.20 mmol), Pd(OAc)2 (0.13 g, 0.59
mmol), P(tBu)3 (0.21 g, 1.06 mmol), and Cs2CO3 (8.33 g, 25.56
mmol) in toluene (55 mL) was stirred and heated at 120 °C for 36 h.
After it was cooled to room temperature, the reaction mixture was
passed through a short plug (silica gel) and the filtrate was
concentrated, giving a yellow brownish oil. Purification was carried
out by column chromatography (silica gel, hexanes/CH2Cl2 3/2). A
Materials and Methods. 2,7-Dibromo-9,9-diethylfluorene (A)
9,9-diethylfluorene (D), 2-bromo-9,9-diethyl-9H-fluorene (E), and
1,3-bis(dicyanomethylene)indane (I) were prepared as described
previously.30,38,34 Reactions were carried out under a N2 or Ar
atmosphere. THF was freshly distilled from Na and benzophenone
ketyl. AlCl3 was purified by sublimation before use. All other reagents
and solvents were used as received from commercial suppliers. 1H and
13C NMR spectra were recorded on an NMR spectrometer at 500 and
125 MHz, respectively, with reference to TMS at 0.00 ppm.
Synthesis of 7-Bromo-9,9-diethylfluorene-2-carbaldehyde
(B). Under a nitrogen atmosphere and at −78 °C, n-BuLi (3.28 mL,
1.6 M in hexanes) was added dropwise over 20 min to a dry THF
solution (15 mL) containing 2,7-dibromo-9,9-diethylfluorene (2 g,
5.26 mmol). After 1 h of stirring, 0.58 mL of DMF was added slowly
to the reaction solution. After another 2 h of stirring, the solution was
brought back to room temperature and the reaction was quenched
with 2 N HCl. The solution was extracted with toluene and subjected
to flash column chromatography (silica gel, hexanes/ethyl acetate 4/
1). A white solid was obtained. Yield: 89% (1.52 g). Mp: 124−125 °C
1
yellow solid was obtained. Yield: 78% (5.88 g). Mp: 142−143 °C. H
NMR (500 MHz, CDCl3): δ 7.94 (dd, J = 7.5 Hz, 1H), 7.90 (s, 1H),
7.65 (d, J = 8 Hz, 1H), 7.61 (d, J = 8.5 Hz, 1H), 7.28 (t, J = 5.5 Hz,
4H), 7.14 (d, J = 5.5 Hz, 4H), 7.10 (d, J = 2 Hz, 1H), 7.05 (t, J = 7.5
Hz, 3H), 2.64 (s, 3H), 2.03 (m, 2H), 1.91 (m, 2H), 0.34 (t, J = 14.5
Hz, 6H). 13C NMR (125 MHz, CDCl3): δ 198.1, 152.6, 150.0, 148.5,
147.7, 146.4, 135.0, 134.7, 129.5, 129.0, 124.5, 124.0, 122.9, 122.3,
121.3, 118.7, 56.2, 32.5, 26.8, 8.5 ppm. HRMS (ESI-TOF): m/z [M +
H]+ calcd for C31H30NO 432.2322, found 432.2323.
(lit.38 mp 126−128 °C). H NMR (500 MHz, CDCl3): δ 9.77 (s,
1
1H),7.57 (s, 2H), 7.53 (d, J = 7 Hz, 1H), 7.36 (d, J = 8.5 Hz, 1H),
7.23−7.21 (m, 2 H), 1.83−1.72 (m, 4H), 0.3 (t, J = 10 Hz, 6H). 13C
NMR (125 MHz, CDCl3): δ 192.3, 153.4, 150.3, 146.7, 138.9, 135.6,
130.6, 130.3, 126.7, 126.4, 123.2, 123.1, 120.0, 56.6, 32.5, 8.4 ppm.
Synthesis of 7-(Diphenylamino)-9,9-diethylfluorene-2-car-
baldehyde (C). Under a nitrogen atmosphere, a mixture of 7-
bromo-9,9-diethylfluorene-2-carbaldehyde (3 g, 9.11 mmol), diphenyl-
amine (2.31 g, 13.66 mmol), Pd(OAc)2 (0.05 g, 0.22 mmol), P(tBu)3
(0.1 g, 0.49 mmol), and Cs2CO3 (4.45 g, 13.65 mmol) in toluene (20
mL) was stirred and heated at 120 °C for 24 h. After it was cooled to
room temperature, the reaction mixture was passed through a short
plug (silica gel) and the filtrate was concentrated to give a brown oil.
Purification was carried out by flash column chromatography (silica
gel, first hexanes, then hexanes/ethyl acetate 4/1). A yellow solid was
obtained. Yield: 88% (3.31 g). Mp: 156−157 °C. 1H NMR (500 MHz,
CDCl3): δ 9.69 (s, 1H), 7.50 (dd, J = 7.5 Hz, 2H), 7.40 (dd, J = 8 Hz,
1H), 7.30 (dd, J = 8.5 Hz, 1H), 6.97−6.93 (m, 4H), 6.81 (dd, J = 6.5
Hz, 4H), 6.77 (d, J = 2 Hz, 1H), 6.74−6.71 (m, 4H), 1.72−1.65 (m,
2H), 1.61−1.53 (m, 2H), 0.02 (t, J = 14.5, 6H). 13C NMR (125 MHz,
CDCl3): δ 192.2, 152.8, 150.5, 148.9, 147.9, 147.6, 134.5, 134.3, 138.9,
129.3, 124.4, 123.1, 122.9, 121.69, 119.1, 118.1, 56.1, 32.4, 8.5 ppm.
HRMS (ESI-TOF): m/z [M + H]+ calcd for C30H28NO 418.2165,
found 418.2176.
Synthesis of 2-(1-(2-(Diphenylamino)-9,9-diethyl-9H-fluo-
ren-7-yl)ethylidene)malononitrile (2). In a reaction flask, 1-(2-
(diphenylamino)-9,9-diethyl-9H-fluoren-7-yl)ethanone (0.8 g, 1.85
mmol) and malononitrile (0.26 g, 4.06 mmol) were added under a
nitrogen atmosphere, followed by anhydrous chloroform (36 mL) to
give a clear yellow solution. It was then added to pyridine (600 mg,
7.58 mmol) and an excess amount of titanium tetrachloride (5.5 mL,
47.65 mmol) with continuous stirring. The reaction mixture
immediately turned deep brown. The solution was stirred for 30
min and subsequently quenched with water (100 mL). The liquid was
concentrated under vacuum and purified using column chromatog-
raphy (silica gel) first with CH2Cl2 and then with a hexane/CH2Cl2
(1/4) solvent mixture as eluent, providing orange crystals. Yield: 73%
(0.65 g). Mp: 200−201 °C. 1H NMR (500 MHz, CDCl3): δ 7.68 (d, J
= 8 Hz, 1H), 7.59 (d, J = 5.5 Hz, 2H), 7.54 (dd, J = 8.5 Hz, 1H), 7.29
(t, J = 5.5 Hz, 4H), 7.14 (d, J = 5.5 Hz, 4H), 7.08−7.03 (m, 4H), 2.69
(s, 3H), 1.99 −1.89 (m, 6H), 0.37 (t, J = 15 Hz). 13C NMR (125
MHz, CDCl3): δ 175.1, 152.4, 150.4, 148.9, 147.6, 146.2, 134.1, 133.0,
129.8, 129.6, 129.1, 128.8, 127.1, 126.8, 124.8, 124.1, 123.9, 123.5,
123.1, 122.9, 122.5, 122.2, 121.9, 121.4, 119.3, 118.4, 117.7, 113.6,
113.4, 56.5, 32.4, 24.1, 8.5 ppm. HRMS (ESI-TOF): m/z [M + Na]+
calcd for C34H29N3Na 502.2254, found 502.2256.
Synthesis of 2-((2-(Diphenylamino)-9,9-diethylfluoren-7-yl)-
methylene)malononitrile (1). 7-(Diphenylamino)-9,9-diethylfluor-
ene-2-carbaldehyde (0.4 g, 0.96 mmol), malononitrile (0.12 g, 1.92
mmol), and basic aluminum oxide (0.45 g) were stirred in toluene (5
mL) for 24 h at 70 °C. After it was cooled to room temperature, the
reaction solution was filtered. The filtrate was subjected to column
chromatography (silica gel, hexanes/ethyl acetate 9/1). A red solid was
obtained. Yield: 91% (0.40 g). Mp: 189−190 °C. 1H NMR (500 MHz,
CDCl3): δ 7.89 (d, J = 2 Hz, 1H), 7.83 (dd, J = 8 Hz, 1H), 7.76 (s,
1H), 7.69 (d, J = 8 Hz, 1H), 7.61 (d, J = 8 Hz, 1H), 7.31−7.25 (m,
4H), 7.15 (bd, J = 8 Hz, 4 H), 7.09 (m, 4H), 2.02−1.99 (m, 2H),
1.98−1.85 (m, 2H), 0.38 (t, J = 15 Hz, 6H). 13C NMR (125 MHz,
CDCl3): δ 159.9, 159.8, 153.3, 151.0, 149.7, 148.6, 147.4, 133.5, 131.6,
129.5, 124.9, 124.7, 124.5, 124.4, 123.6, 123.5, 122.4, 122.3, 122.11,
122.0, 119.6, 119.5, 117.4, 114.6, 113.6, 79.1, 56.3, 32.3, 8.5 ppm.
HRMS (ESI-TOF): m/z [M + H]+ calcd for C33H28N3 466.2278,
found 466.2274.
Synthesis of 7-(Di-p-tolylamino)-9,9-diethyl-9H-fluorene-2-
carbaldehyde (H). Under a nitrogen atmosphere, a mixture of 7-
bromo-9,9-diethylfluorene-2-carbaldehyde (2 g, 6.07 mmol), di-p-
tolylamine (1.77 g, 9.0 mmol), Pd(OAc)2 (0.03 g, 0.13 mmol),
P(tBu)3 (0.064 g, 0.32 mmol), and Cs2CO3 (2.96 g, 9.1 mmol) in
toluene (14 mL) was stirred and heated at 120 °C for 36 h. After
cooling to room temperature, the reaction mixture was passed through
short plug (silica gel) and the filtrate was concentrated to give a
yellow−brown oil. Purification was carried out by flash column
chromatography (silica gel, hexanes/CH2Cl2 3:2). A fluorescent yellow
1
solid was obtained. Yield: 70% (1.9 g). Mp: 156−157 °C. H NMR
(500 MHz, CDCl3): δ 10.01 (s, 1H), 7.81 (d, J = 8 Hz, 2H); 7.70 (d, J
= 8 Hz, 1H); 7.57 (d, J = 8 Hz, 1H), 7.09−6.96 (m, 10H), 2.33 (s,
6H), 2.02−1.98 (m, 2H), 1.90−1.86 (m, 2H), 0.35 (t, J = 15 Hz, 6H)
ppm. 13C NMR (125 MHz, CDCl3): δ 192.2, 152.7, 150.3, 149.3,
148.1, 145.1, 134.2, 133.4, 132.8, 130.9, 130.2, 129.9, 124.7, 122.8,
121.6, 121.5, 118.8, 116.8, 56.1, 32.5, 32.4, 20.8, 20.8, 8.4 ppm. HRMS
(ESI-TOF): m/z [M + H]+ calcd for C32H32NO 446.2478, found
446.2479.
Synthesis of 1-(2-Bromo-9,9-diethyl-9H-fluoren-7-yl)-
ethanone (F). To a solution of AlCl3 (4.09 g, 30.71 mmol) in 115
mL of CH2Cl2 was added acetyl chloride (2.08 mL) under a nitrogen
atmosphere at 0 °C. Then 18 mL of 2-bromo-9,9-diethyl-9H-fluorene
(8.1 g, 26.89 mmol) dissolved in 17 mL of CH2Cl2 was added at the
same temperature. After it was stirred at room temperature for 20 h,
I
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Synthesis of 2-((2-(Di-p-tolylamino)-9,9-diethylfluoren-7-yl)-
methylene)malononitrile (3). 7-(Di-p-tolylamino)-9,9-diethyl-9H-
fluorene-2-carbaldehyde (1.5 g, 3.36 mmol), malononitrile (0.44 g,
6.72 mmol), and basic aluminum oxide (1.58 g) were stirred in toluene
(18 mL) for 16 h at 70 °C. After it was cooled to room temperature,
the reaction solution was filtered. The filtrate was subjected to column
chromatography (silica gel, toluene). A red solid was obtained. Yield:
86% (1.43 g). Mp: 211−212 °C. 1H NMR (500 MHz, CDCl3): δ 7.88
(d, J = 2 Hz, 1H), 7.82 (dd, J = 8.5 Hz, 1H), 7.76 (s, 1H), 7.66 (d, J =
8 Hz, 1H), 7.56 (d, J = 8.5 Hz, 1H), 7.10 (d, J = 8 Hz, 4H), 7.04−7.00
(m, 4H), 6.98 (dd, J = 8.5 Hz, 1H), 2.34 (s, 6H), 1.99−1.95 (m, 2H),
1.89−1.85 (m, 2H), 0.37 (t, J = 15 Hz, 6H) ppm. 13C NMR (125
MHz, CDCl3): δ 159.8, 153.3, 150.8, 150.1, 148.9, 144.8, 133.3, 132.5,
131.7, 130.1, 128.4, 125.2, 124.3, 122.0, 121.8, 119.4, 116.0, 114.7,
113.7, 78.6, 56.2, 32.3, 20.9, 8.5 ppm. HRMS (ESI-TOF): m/z [M +
H]+ calcd for C35H32N3 494.2591, found 494.2582.
Synthesis of (E)-2-(3-(2-(7-(Diphenylamino)-9,9-diethyl-9H-
fluoren-2-yl)vinyl)-5-methyl-4-oxocyclohexa-2,5-dienylidene)-
malononitrile (4). A mixture of aldehyde C (0.56 g, 1.35 mmol) and
2,6-dimethyl-4-dicyanomethylene-4H-pyran (0.25 g, 1.35 mmol) was
dissolved in EtOH (35 mL). After piperidine (0.5 mL) was added
slowly through a syringe with stirring, the reaction mixture was
refluxed for 72 h. A reddish precipitate was obtained after cooling the
reaction mixture to room temperature. Compound 4 was obtained
after purifying the product through a silica gel column using hexanes/
ethyl acetate (4/1) as eluent. A red solid was obtained. Yield: 35%
(0.28 g). Mp: 299−300 °C. 1H NMR (500 MHz, CDCl3): δ 7.64 (d, J
= 8 Hz, 1H), 7.58 (d, J = 8 Hz, 1H), 7.54 (s, 1H), 7.51 (d, J = 8 Hz,
1H), 7.46 (s, 1H), 7.28−7.25 (m, 5H), 7.14 (d, J = 8.5 Hz, 4H), 7.09
(d, J = 2 Hz, 1H), 7.05−7.02 (m, 3H), 6.76 (d, J = 16 Hz, 1H), 6.70
(d, J = 1.5 Hz, 1H), 6.55 (s, 1H), 2.42 (s, 3H), 2.05−1.88 (m, 4H),
0.38 (t, J = 14.5 Hz, 6H). 13C NMR (125 MHz, CDCl3): δ 161.8,
159.5, 156.3, 152.0, 150.7, 148.2, 147.7, 144.2, 138.8, 135.1, 132.4,
129.2, 127.6, 124.2, 123.2, 122.9, 121.8, 120.9, 119.5, 118.6, 116.5,
115.2, 115.1, 106.7, 106.4, 56.1, 32.6, 19.9, 8.5 ppm. HRMS (ESI): m/
z [M + H]+ calcd for C40H34N3O 572.2696, found 572.2709.
Synthesis of 1,3-Bis(dicyanomethylene)indane (I). A solution
of indane-1,3-dione (2.4 g, 16 mmol), malononitrile (2.7 g, 41 mmol),
and ammonium acetate (1.25 g, 16 mmol) dissolved in absolute
ethanol (30 mL) was heated at reflux for 30 min. After the mixture was
cooled to room temperature, water (25 mL) was added and the
solution acidified with concentrated hydrochloric acid. The brown
precipitate was filtered off and washed with water. Recrystallization
from glacial acetic acid afforded I as a yellow-brown solid. Yield: 41%
(1.6 g). Mp: 255−256 °C. 1H NMR (500 MHz, CDCl3): δ 8.67−8.64
(m, 2H), 7.94−7.90 (m, 2H), 4.30 (s, 2H). 13C NMR (125 MHz,
CDCl3): δ 165.4, 140.7, 136.4, 127.1, 111.9, 111.7, 79.4, 42.0 ppm.
Anal. Calcd for C15H6N4 (242.23): C, 74.37; H, 2.50; N, 23.13.
Found: C, 74.18; H, 2.46; N, 23.28.
Synthesis of 2-[1,3-Bis(dicyanomethylidene)indan-2-ylide-
nemethyl]-7-(N,N-diphenylamino)-9,9-diethylfluorene (5). 7-
(Diphenylamino)-9,9-diethylfluorene-2-carbaldehyde (0.56 g, 1.35
mmol) was added to a solution of I (0.3 g, 1.23 mmol) in acetic
anhydride (10 mL) at 70 °C with vigorous stirring. The mixture was
heated at 70−80 °C for 3 h and cooled. The precipitated solid was
filtered off, washed with a small amount of acetic anhydride, and
recrystallized from the same solvent, affording dark blue crystals. Yield:
89% (0.70 g). Mp: 291−292 °C. 1H NMR (500 MHz, CDCl3): δ 8.77
(s, 1H), 8.7 (d, J = 6.3 Hz, 1H), 8.61 (d, J = 6.6 Hz, 1H), 7.87 (m,
2H), 7.73 (d, J = 6 Hz, 1H), 7.62 (d, J = 8.1 Hz, 1H) 7.53 (d, J = 8.1
Hz, 1H), 7.40 (s, 1H), 7.32−7.25 (m, 4H), 7.16−7.02 (m, 8H), 1.98−
1.87 (m, 4H), 0.42 (t, J = 14.7 Hz, 6H). 13C NMR (125 MHz,
CDCl3): δ 161.4, 160.4, 152.8, 151.5, 149.4, 147.5, 147.0, 146.0, 138.1,
136.7, 135.0, 134.8, 134.1, 131.4, 130.0, 129.3, 126.2, 125.4, 124.7,
124.4, 123.4, 122.6, 121.9, 120.2, 117.5, 113.3, 113.1, 112.9, 112.2,
71.9, 56.4, 32.6, 8.4 ppm. Anal. Calcd for C45H31N5 (641.2679): C,
84.22; H, 4.87; N, 10.91. Found: C, 84.09; H, 4.90; N, 11.10. HRMS
(ESI-TOF): m/z [M + H]+ calcd for C45H32N5 642.2652, found
642.2630.
Synthesis of (E)-3-(2-(Diphenylamino)-9,9-diethyl-9H-fluo-
ren-7-yl)-1-(thiophen-2-yl)prop-2-en-1-one (6). Thiophene-2-
carbaldehyde (0.07 g, 0.69 mmol) was added to the solution of
KOH (0.04 g, 0.828 mmol) in 5/1 MeOH/H2O (10 mL). After
dissolution, 1-(2-(diphenylamino)-9,9-diethyl-9H-fluoren-7-yl)-
ethanone (0.30 g, 0.69 mmol) was added to the mixture and stirred
for 48 h at reflux. The solid product that precipitated was filtered,
washed with hexane, and dried. Recrystallization in hexane provided a
1
yellow solid. Yield: 70% (0.25 g). Mp: 166−167 °C. H NMR (500
MHz, CDCl3): δ 8.02 (d, J = 8 Hz, 1H), 7.97 (d, J = 3 Hz, 1H), 7.70
(d, J = 8 Hz, 1H), 7.63 (d, J = 8.5 Hz, 1H), 7.45 (s, 1H); 7.43 (d, J = 6
Hz, 1H), 7.38 (d, J = 3 Hz, 1H), 7.29 (bt, J = 8 Hz, 5H), 7.14−7.09
(m, 5H); 7.06 (bt, J = 8.5 Hz, 3H), 2.05 (m, 2H), 1.93 (m, 2H), 0.36
(t, J = 14.5 Hz, 6H). 13C NMR (125 MHz, CDCl3): δ 189.3, 152.7,
150.2, 148.5, 147.7, 146.3, 140.6, 136.6, 135.9, 134.8, 132.0, 131.6,
129.4, 129.0, 128.7, 128.3, 128.2, 124.5, 124.0, 123.2, 122.7, 121.5,
121.2, 120.9, 118.7, 118.2, 56.3, 32.5, 8.5 ppm. HRMS (ESI-TOF): m/
z [M + H]+ calcd for C36H32NOS 526.2199, found 526.2193.
Synthesis of (E)-3-(7-(Diphenylamino)-9,9-diethyl-9H-fluo-
ren-2-yl)-1-(thiophen-2-yl)prop-2-en-1-one (7). 1-(Thiophen-2-
yl)ethanone (0.046 g, 0.37 mmol) was added to a solution of KOH
(0.020 g, 0.37 mmol) in MeOH/H2O 5/1 (10 mL). After dissolution,
7-(diphenylamino)-9,9-diethylfluorene-2-carbaldehyde (0.15 g, 0.37
mmol) was added to the mixture, and this mixture was stirred for 48 h
at reflux. The solid product that precipitated was filtered, washed with
hexane, and dried. Purification was carried out by column
chromatography (silica gel, hexanes/ethyl acetate 4/1). A yellow
1
solid was obtained. Yield: 80% (0.15 g). Mp: 231−232 °C. H NMR
(500 MHz, CDCl3): δ 7.95 (d, J = 15.5 Hz, 1H), 7.91 (dd, J = 4 Hz,
1H), 7.68 (dd, J = 5 Hz, 1H), 7.64−7.60 (m, 2H), 7.58 (d, J = 8.5 Hz,
1H), 7.54 (s, 1H), 7.46 (d, J = 16 Hz, 1H), 7.28−7.24 (m, 4H), 7.20
(t, J = 8.5 Hz, 1H), 7.13−7.09 (m, 5H), 7.04−7.01 (m, 3H), 2.02−
1.88 (m, 4H), 0.38 (t, J = 15 Hz, 6H). 13C NMR (125 MHz, CDCl3):
δ 182.0, 152.1, 150.57, 148.1, 147.8, 145.8, 144.9, 144.3, 135.3, 133.5,
132.6, 131.5, 129.2, 128.3, 128.1, 124.1, 123.2, 122.8, 122.6, 120.9,
120.1, 119.4, 118.7, 56.1, 32.6, 8.5 ppm. HRMS (ESI-TOF): m/z [M +
H]+ calcd for C36H32NOS 526.2199, found 526.2195.
Linear Spectral Measurements. Linear photophysical properties
of all materials were investigated at room temperature in spectroscopic
cyclohexane and chloroform. Steady-state absorption spectra were
obtained by a UV−visible spectrophotometer in 10 mm path length
quartz cuvettes with dye concentrations 1.5 × 10−5 M ≤ C ≤ 3 × 10−5
M. The steady-state fluorescence, excitation, and excitation anisotropy
spectra were measured with a spectrofluorimeter in 10 mm
spectrofluorometric quartz cuvettes, with dye concentrations ∼(1−
1.5) × 10−6 M. The values of fluorescence quantum yield Φ were
determined relative to 9,10-diphenylanthracene in cyclohexane and
Rhodamine 6G as standards (Φ ≈ 0.95).35
Lifetime Measurements. Lifetime measurements were performed
by using a Ti:sapphire laser system (pulse duration ∼200 fs/pulse
(fwhm) and 76 MHz repetition rate) coupled with a second harmonic
generator (tuning range 350−440 nm). A broad band-pass filter
(D500/200) was placed in front of an avalanche photodiode (APD)
detector. Data acquisition was conducted on a time-correlated single
photon counting system. The optical densities of all the investigated
solutions did not exceed 0.12 at the excitation wavelengths to avoid
reabsorption in 10 mm path length quartz cuvettes at room
temperature.
Two-Photon Absorption (2PA) Measurements. The 2PA
spectra of compounds 1−5 and 7 were measured over a broad
spectral region by open aperture Z-scan39 and relative 2PF methods40
with Rhodamine B in methanol as a standard.41 Two-photon induced
fluorescence spectra were obtained with a spectrofluorimeter coupled
with a regenerative amplified laser system that pumped optical
parametric generator/amplifiers generating ∼140 fs output pulses
(fwhm) with a repetition rate of 1 kHz. The quadratic dependence of
2PF intensity on the excitation power was verified for every excitation
wavelength, λex. The same laser system was used for open aperture Z-
scan measurements. A comprehensive description of this experimental
methodology was previously reported.42,43
J
dx.doi.org/10.1021/jo302423p | J. Org. Chem. XXXX, XXX, XXX−XXX

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Article
X-ray and Computational Methodology. The X-ray diffraction
experiments for 1, 2, and 6 were carried out with a diffractometer,
using Mo Kα radiation (λ = 0.71073 Å) at 100 K. The raw data frames
were integrated with the SAINT+ program using a narrow-frame
algorithm.44 Absorption corrections were applied using the semi-
empirical method of the SADABS program.45 The structures were
solved by direct methods and refined using the Bruker SHELXTL
programs suite46 by full-matrix least-squares methods on F2 with
SHELXL-97 in an anisotropic approximation for all non-hydrogen
atoms. In 1, the C(31), C(32), N(1), and N(2) atoms of the
dicyanovinyl group were disordered over two positions with partial
occupancies of 0.64(5) and 0.36(5). All H atoms were placed in
idealized positions and refined with constrained C−H distances and
Uiso(H) values set to be 1.2Ueq or 1.5Ueq (for the methyl group) of the
attached C atom. Crystallographic data for all structures have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 831506−831508. Copies
of the data can be obtained, free of charge, on application to the
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, +44-
(0)1223−336033; e-mail, deposit@ccdc.cam.ac.uk).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving H NMR and 13C NMR
1
spectra of B, C, 1, F, G, 2, H, 3, 4, I, and 5−7, crystallographic
structural packing data for 1, 2, and 6, along with X-ray
diffraction data (CIF), and computational details, including
tables of the optimized geometries of molecules 1−7 and
ground- and excited-state properties. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: belfield@ucf.edu. Fax: 407-823-2252. Tel: 407-823-
1028.
(26) Wu, X.; Wu, J.; Liu, Y.; Jen, A.K.-Y. J. Am. Chem. Soc. 1999, 121,
472−473.
(27) Belfield, K. D.; Bondar, M. V.; Przhonska, O. V.; Schafer, K. J. J.
Fluoresc. 2002, 12, 449−454.
Notes
The authors declare no competing financial interest.
(28) Belfield, K. D.; Schafer, K. J.; Mourad, W.; Reinhardt, B. A. J.
Org. Chem. 2000, 65, 4475−4481.
ACKNOWLEDGMENTS
(29) Belfield, K. D.; Morales, A. R.; Hales, J. M.; Hagan, D. J.; Van
Stryland, E. W.; Chapela, V. M.; Percino. J. Chem. Mater. 2004, 16,
2267−2273.
■
We acknowledge the National Science Foundation (CHE-
0832622 and CHE-0840431), the National Institute of
Biomedical Imaging and Bioengineering of the National
Institutes of Health (1 R15EB008858-01), and the National
Academy of Science (PGA-P210877) for support of this work.
(30) Belfield, K. D.; Morales, A. R.; Kang, B.-S.; Hales, J. M.; Hagan,
D. J.; Van Stryland, E. W.; Chapela, V. M.; Percino. J. Chem. Mater.
2004, 16, 4634−4641.
(31) Morales, A. R.; Belfield, K. D.; Hales, J. M.; Van Stryland, E. W.;
Hagan, D. J. Chem. Mater. 2006, 18, 4972−4980.
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