Highly Emissive Whole Rainbow Fluorophores Consisting of 1,4-Bis(2-phenylethynyl)benzene Core Skeleton: Design, Synthesis, and Light-Emitting Characteristics

Article abstract of DOI:10.1021/acs.jpca.5b05077

To create the whole-rainbow-fluorophores (WRF) having the small Δλ_em (the difference of λ_em between a given fluorophore and nearest neighboring fluorophore having the same core skeleton) values (<20 nm) in full visible region (λ

Full text of DOI:10.1021/acs.jpca.5b05077

Article
pubs.acs.org/JPCA
Highly Emissive Whole Rainbow Fluorophores Consisting of 1,4-
Bis(2-phenylethynyl)benzene Core Skeleton: Design, Synthesis, and
Light-Emitting Characteristics
Yoshihiro Yamaguchi,* Takanori Ochi, Yoshio Matsubara, and Zen-ichi Yoshida*
Department of Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502,
Japan
S
* Supporting Information
ABSTRACT: To create the whole-rainbow-fluorophores (WRF) having the small
Δλem (the difference of λem between a given fluorophore and nearest neighboring
fluorophore having the same core skeleton) values (<20 nm) in full visible region
(λem: 400−650 nm), the high log ε (>4.5), and the high Φf (>0.6), we investigated
molecular design, synthesis, and light-emitting characteristics of the π-conjugated
molecules (D/A-BPBs) consisting of 1,4-bis(phenylethynyl)benzene (BPB)
modified by donor groups (OMe, SMe, NMe2, and NPh2) and an acceptor
group (CN). As a result, synthesized 20 D/A-BPBs (1a−5d) were found to be the
desired WRF. To get the intense red fluorophore (Φf > 0.7, λem > 610 nm), we
synthesized new compounds (5e−5i) and elucidated their photophysical properties
in CHCl3 solution. As a result, 5h, in which a 4-cyanophenyl group is introduced to
the para-position of two benzene rings in the terminal NPh2 group of 5d, was found
to be the desired intense red fluorophore (log ε = 4.56, Φf = 0.76, λem = 611 nm).
The intramolecular charge-transfer nature of the S1 state of WRF (1a−5d) was
elucidated by the positive linear relationship between optical transition energy (νem) from the S1 state to the S0 state and
HOMO(D)−LUMO(A) difference, and the molecular orbitals calculated with the DFT method. It is demonstrated that our
concept (Φf = 1/(exp(−Aπ) + 1)) connected with the relationship between Φf and magnitude (Aπ) of π conjugation length in
the S1 state can be applied to WRF (1a−5d). It is suggested that the prediction of Φf from a structural model can be achieved by
1/2
the equation Φf = 1/(exp(−((νa
̃
− νf
̃
)
̃ ̃
× a3/2) + 1), where νa are the wavenumber (cm−1) of absorption and
and νf
fluorescence peaks, respectively, and a is the calculated molecular radius. From the viewpoint of application of WRF to various
functional materials, the light-emitting characteristics of 1a−5i in doped polymer films were examined. It was demonstrated that
1a−5i dispersed in two kinds of polymer film (PST and PMMA) emit light at the whole visible region and have the small Δλem
values (<20 nm) and the high Φf values (>0.6). Therefore, the present D/A-BPBs can be said to be the desired WRF even in the
doped polymer film.
Figure 1) such as oligo(p-phenylenevinylene)s (OPV),25,26
INTRODUCTION
■
cross-conjugated bis-enediynes,27 boron-containing π-conju-
gated systems,28−31 silafluorenes,32 1,2-dihydropyrrolo[3,4-b]-
indolizin-3-one,33−36 organometallic complexes,37−42 and other
π-conjugated systems,43−47 there have been remained the
following problems to be solved, (1) low log ε and Φf, (2) two
or more emission bands, and (3) large Δλem values (Δλem > 30−
40 nm). The Δλem is defined as the difference of emission
maximum wavelength (λem) between a given fluorophore and
another fluorophore whose λem is close to the former (nearest
neighboring fluorophore). The increase in Δλem values was
found to be remarkable at the longer wavelength region than 550
nm (green) in FCF systems.
Highly emissive fluorophores are utilized extensively not only as
biosensors and biolabeling-agents in life science1−7 but also as
optoelectronic devices in materials science such as organic light-
emitting diodes (OLEDs),8−13 organic field-effect transistors
(OFETs),14−17 and organic solar cells.18−24 Judging from such
circumstances, it should be of particular importance to discover
new fluorescent materials that have emission wavelengths
adaptable for various applications mentioned above. Thus, the
creation of highly emissive π-conjugated systems (fluorophores)
consisted of the same skeleton whose fluorescence covers the full
visible region is an attractive subject. Especially, the achievement
of emission in full visible region employing the shortest π-
conjugated systems without changing the core skeleton should
be an intriguing theme to be challenged.
We have challenged to create the whole-rainbow-fluorophores
(WRF, Figure 1) having the small Δλem values (<20 nm) in the
For creation of the ideal fluorophores, the emission in the full
visible region, the high molar absorption coefficient (log ε) and
quantum yield (Φf) are required. Although several papers have
been reported for the creation of full-color-fluorophores (FCF,
Received: May 28, 2015
Revised: July 17, 2015
© XXXX American Chemical Society
A
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Figure 1. Features of fluorescence spectra for full-color-fluorophores (FCF) and whole-rainbow-fluorophores (WRF).
full visible region (λem: 400−650 nm), the high log ε (>4.5), and
the high Φf (>0.6). Of those, the small Δλem value is particularly
important and useful for the application in various fields because
of the possibility of the wide selectivity of color and control of
delicate color.
creation of fluorophores described here. Although BPB itself
fluoresces in ultraviolet region, the dramatic bathochromic shift
of λem (visual emission) and the increase in Φf and log ε values are
observed by introduction of donor/acceptor groups (as shown in
Figure 3), suggesting that the donor/acceptor tuning should be
effective method for the creation of highly emissive fluoro-
phores.74−85
Furthermore, it has been found that donor/acceptor
modification shown in structure A is superior to that shown in
structure B for emission in the visible region (Figure 4).86,87
We have chosen oligo(p-phenyleneethnylene)s (OPE)48−59
hydrocarbons as the π-conjugated core skeleton because of the
flexibility in their synthetic strategies, though the system seems to
have a disadvantage for the creation of fluorophore compared
with OPV60−73 hydrocarbons as shown in Figure 2, assuming
Figure 4. Tuning mode of BPB by donor/acceptor group.
On the basis of these results (the effectiveness of donor/
acceptor tuning, Figure 3) and the superiority of tuning mode A
(see Figure 4), 20 compounds (D/A-BPBs: Type I, Type II,
Type III, Type IV, and Type V) were designed as shown in
Figure 5.
Figure 2. Relationship of absorption maximum (λabs) with π conjugation
length (n) in OPE and OPV systems.
In the previous paper88 we reported that the optical transition
energy (νem: cm−1) from the excited singlet (S1) state to the
ground (S0) state correlates linearly with the difference between
the HOMO of the donor blocks (HOMO(D)) and the LUMO
of the acceptor blocks (LUMO(A)) (abbreviated as HOMO-
(D)−LUMO(A) difference) in the D/A-OPE system. There-
fore, the positive linear relationship between νem and the
HOMO(D)−LUMO(A) difference is considered to be useful for
the prediction of λem in D/A-OPE systems. The HOMO(D)
values, LUMO(A) values, HOMO(D)−LUMO(A) difference
values, and predictable λem values calculated by the DFT method
for the designed 20 D/A-BPBs are summarized in Figure 6. As
that the Stokes shifts of OPV and OPE are almost close to each
other. After some synthetic trials, we succeeded in the creation of
the desired WRF using OPE as the π-conjugated core skeleton,
which is described herein.
RESULTS AND DISCUSSION
■
Molecular Design for WRF. On the basis of the preliminary
experiments, we have chosen 1, 4-bis(phenylethynyl)benzene
(BPB) as the most desirable π-conjugated core skeleton for
Figure 3. Effect of donor/acceptor tuning in BPB.
B
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Figure 5. Molecular design for the creation of WRF.
Figure 6. Values for HOMO(D) of donor blocks, the LUMO(A) of acceptor blocks, and the HOMO(D)−LUMO(A) difference calculated by the DFT
method (B3LYP/6-311G(d) level), and the predictable λem of the designed 20 D/A-BPBs (1−5).
shown in this figure, the designed D/A-BPBs are expected to
emit fluorescence in the full visible region.
cross-coupling with 2-ethynylbenzonitrile (13) and 2-ethynylter-
ephthalonitrile (14), respectively. The reaction of donor unit
(6−9) with 5-bromo-2-iodoterephthalonitrile (12) at the more
reactive iodo-substituted position gave 19−22, followed by the
second cross-coupling with ethynylbenzene (10) and the
acceptor units (13 and 14) at the remaining bromo-substituted
position of 19−22, provided Type III (3a−3d), Type IV (4a−
4d), and Type V (5a−5d), respectively. The structures of 20 D/
A-BPBs were confirmed by spectral data (1H and 13C NMR
spectroscopy and HR-FAB MS, Supporting Information).
Photophysical Properties of D/A-BPBs. Absorption and
emission spectra of the synthesized D/A-BPBs (1−5) were
measured in CHCl3. The fluorescence spectra and photograph of
emission color are illustrated in Figure 7. The photophysical data
of 1−5 are shown in Figure 8 (A, absorption maxima, λabs (nm);
Synthesis of D/A-BPBs. The designed 20 D/A-BPBs (1−5)
shown in Figure 5 were synthesized by way of (1) preparation of
donor units (6−9) (Supporting Information), (2) preparation of
acceptor units (11−14) (Supporting Information), (3) the
Sonogashira Pd cross-coupling89,90 reaction between donor units
(6−9) and acceptor units (13 or 14), utilizing the reactivity
difference between iodine and bromine on the same benzene
ring, and (4) elongation of acceptor units (i.e., construction of
BPB skeleton) as shown in Scheme 1.
Compounds of Type I (1a−1d) and Type II (2a−2d)
containing a cyanobenzene ring as the central ring were
synthesized by the first cross-coupling of donor units (6−9)
with 5-bromo-2-iodobenzonitrile (11) followed by the second
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a
Scheme 1
a
Reagents: (i) Pd(Ph3P)2Cl2, CuI, Et3N, THF
B, emission maxima, λem (nm); C, Stokes shift (nm); D,
quamtum yield, Φf) as three-dimensional indications (details in
the Supporting Information).
electron-withdrawing ability of the acceptor unit and electron-
donating ability of the donor groups. Such a change in Stokes
shift is of particular interest. Almost all D/A-BPBs (1−5) were
found to be very intense fluorophores having high quantum yield
(Φf > 0.6, Figure 8D) and molar absorption coefficient (log ε >
4.5, Supporting Information). Thus, we succeeded in the
creation of the desired WRF by using D/A-BPBs (1−5)
consisting of the relatively short π-conjugated core skeleton.
Synthesis of the Intense Red Fluorophore. We
succeeded in the creation of the desired WRF by synthesis of
D/A-BPBs (1−5) having the BPB moiety as the short π-
conjugated core skeleton, though Φf values of two red
fluorophores (5c and 5d) were lower than those of other
As shown in Figures 7 and 8B, it is demonstrated that the
synthesized D/A-BPBs (1−5) are whole-rainbow-fluorophores
(WRF) having the small Δλem values (almost <20 nm,
Supporting Information) in the full visible region [from 409
nm (violet emission, 1a: Type I, OMe) to 639 nm (red emission,
5d: Type V, NPh2)]. Furthermore, as shown in Figures 8A−C,
the values of λabs, λem, and the Stokes shift increase evidently with
both the structural change from Type I to Type V of acceptor
unit and the change of donor group from OMe to NPh2,
indicating that λabs, λem, and Stokes shift increase with both
D
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Figure 7. Fluorescence spectra and emission color of D/A-BPBs (1−5) in CHCl3. Each spectrum and photograph from left to right correspond to the
compound number 1a, 1b, 2a, 3a, 4a, 2b, 3b, 5a, 4b, 5b, 1c, 1d, 2c, 2d, 3c, 3d, 4c, 4d, 5c, and 5d, respectively.
Figure 8. Photophysical data of D/A-BPBs (1−5) in CHCl3 (A, absorption maxima, λabs (nm); B, emission maxima, λem (nm); C, Stokes shift (nm); D,
quamtum yield, Φf).
fluorophores (Figure 8D). Because the balance between the
electron-donating ability of the donor units and the electron-
withdrawing ability of the acceptor unit has turned out to be a
crucial issue for the creation of highly efficient fluorophores that
emit light at the desired wavelength, five new compounds (5e−
5i, Figure 9), in which the electron-donating ability is controlled
by the introduction of various functional groups to the para-
position of benzene rings of the terminal NPh2 group of 5d, have
E
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Figure 9. Structures of new compounds (5e−5i) designed for the intense red fluorophores.
Figure 10. Relationship between Φf and λem of WRF (1−5).
been designed to get the intense red fluorophore (Φf > 0.7, λem
>
indicates that they follow the law (the relationship between ln knr
and νem, Supporting Information). These relations should be
useful for the molecular design of novel fluorophores.
610 nm).
Synthesis of 5e−5i was carried out by (1) preparation of donor
units and (2) sequential Pd cross-coupling reaction with two
kinds of acceptor units. The synthetic method is the same as the
one used in the synthesis of 5d (details, Supporting
Information). Absorption and emission spectra of the synthe-
sized 5e−5i were measured in CHCl3 solution. As a result, 5h, in
which the 4-cyanophenyl group is introduced to the para-
position of two benzene rings in the terminal NPh2 group of 5d,
was found to emit the desired intense red fluorescence (log ε =
4.56, Φf = 0.76, λem = 611 nm), though λem was slightly shifted to
a shorter wavelength than that of 5d (λem = 639 nm) (details in
the Supporting Information).
As described in the section of molecular design, we reported in
the previous paper88 that the optical transition energy (νem:
cm−1) from the S1 state to the S0 state correlates linearly with the
HOMO(D)−LUMO(A) difference in the D/A-OPE system.
Thus, the bathochromic shift of λem in WRF (1−5) can be
explained by the degree of HOMO(D)−LUMO(A) difference.
As shown in Figure 11, the relevant optical energy gap (νem) from
S1 to S0 correlates linearly with the HOMO(D)−LUMO(A)
difference (details, Supporting Information). Therefore, the
fluorescence of WRF (1−5) is considered to have the high
charge-transfer (CT) nature in the S1 state.
Electronic and Spectral Features of WRF. The relation-
ship between the emission maxima (λem) and quamtum yield
(Φf) for WRF (1−5) was examined for each donor group (Figure
10). Interestingly, the positive linear relationships between Φf
and λem were found for WRF (1−5) having weak electron-
donating groups (OMe, a series; SMe, b series), whereas the
negative linear relationships between Φf and λem were found for
WRF (1−5) having strong electron-donating groups (NMe2, c
series; NPh2, d series).
The observed linear relationship is closely connected with the
optical energy gap law, which represents the exponential
dependence of the radiationless rate constant (knr, Supporting
Information)91,92 on the energy gap between the S1 state and the
S0 state for the fluorophores.93−96 That is to say, the positive
linear relationship between ln knr and the relevant optical energy
gap (νem) in a and b series demonstrates that they do not follow
the optical energy gap law, whereas the negative linear
relationship between ln knr and νem in c and d series clearly
Figure 11. Relationship between νem and HOMO(D)−LUMO(A)
difference calculated by DFT method (B3LYP/6-311G(d) level) in
WRF (1−5).
F
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Figure 12. Molecular orbitals of 1a, 2c, 3c, 4b, and 5d by the DFT method (B3LYP/6-311G(d) level). The upper ones represent the LUMOs, and the
lower ones represent the HOMOs.
Figure 13. Correlation between absolute fluorescence quantum yield (Φf) and magnitude (Aπ) of π conjugation length in the S1 state of WRF (1−5).
Solid line: theoretical line based on eq 1.
The high CT nature in the electronic transition of WRF (1−5)
is also suggested by the quantum chemical calculations with DFT
method (B3LYP/6-311G(d) level). The molecular orbitals
calculated by the DFT method using the Gaussian 03 package97
display a remarkable difference in the distribution of HOMO and
LUMO in 1a, 2c, 3c, 4b, and 5d (Figure 12).
As shown in Figure 12, the CT transition from the HOMO to
the LUMO is strongly suggested to occur in 1a, 2c, 3c, 4b, and
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Figure 14. Observed values (Φf(expt), shaded bars) and predicted values (Φf(calcd), open bars) of Φf for WRF (1−5).
a
Table 1. Φf and λem of 1a−5i in CHCl3 and 1a−5i Dispersed in PS or PMMA Film
b
b
Φf
λem (nm)
Φf
λem (nm)
compd
solution
PS
PMMA
solution
PS
PMMA
compd
solution
PS
PMMA
solution
PS
PMMA
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
0.86
0.82
0.78
0.86
0.90
0.85
0.60
0.77
0.94
0.85
0.58
0.75
0.81
0.83
0.64
0.74
0.83
0.83
0.68
0.78
0.83
0.73
0.68
0.76
0.75
0.79
0.66
0.71
0.81
0.79
0.65
0.72
0.83
0.73
0.69
0.74
409
429
506
518
444
460
569
565
449
468
579
586
416
430
488
488
429
449
515
520
443
459
530
536
415
431
506
500
431
448
543
525
448
467
561
552
4a
4b
4c
4d
5a
5b
5c
5d
5e
5f
0.93
0.92
0.50
0.73
0.91
0.88
0.32
0.56
1.00
0.01
0.26
0.76
0.03
0.73
0.79
0.65
0.80
0.71
0.77
0.66
0.83
0.61
0.57
0.80
0.71
0.71
0.72
0.80
0.70
0.77
0.72
0.76
0.53
0.79
0.61
0.32
0.72
0.71
0.54
459
488
594
609
473
506
624
639
525
721
671
611
732
447
467
553
553
459
481
556
577
503
630
596
558
621
452
474
574
567
460
485
580
583
521
628
605
580
614
5g
5h
5i
a
b
All spectra were measured at 295 K. Absolute quantum yield (Φf) values were determined with a Hamamatsu C9920-01 calibrated integrating
sphere system.
5d, because the HOMOs are almost localized on the donor
groups, whereas the LUMOs are wholly localized on acceptor
units. Therefore, WRF (1−5) having both the donor and
acceptor moiety appears to have the dipolar structure on the basis
of the charge transfer98 in the S1 state. Consequently, the increase
in interaction between WRF (1−5) and solvent (particularly,
polar solvent) results in the increase in knr value providing a
decrease in Φf values. Because a and b series having weak
electron-donating groups (OMe and SMe) do not show such
remarkable change, the CT nature stabilizes the S1 state leading
to increase in Φf values. However, the CT nature in c and d series
having a strong electron-donating group (NMe2 and NPh2)
increases with an increase in electron-withdrawing ability in the
acceptor unit (the structural change from Type I to Type V).
Therefore, although the S1 state in c and d series is stabilized, the
interaction between substrates and solvent also increases,
resulting in the decrease in Φf values.
and radiationless rate constant (knr) [Aπ = ln(kr/knr)]. Relation-
ship between Φf and Aπ is expressed as eq 1.
Φ = 1/(exp(−Aπ) + 1)
(1)
f
To see the generality of this concept, we examined the
correlation between Φf and Aπ for WRF (1−5) (details in the
Supporting Information). As shown in Figure 13, it is of
particular interest that the plot of Φf against Aπ for WRF (1−5)
falls on the theoretical line (solid line) based on eq 1.
For the prediction of Φf from a structural model, a Φf-
independent measure is necessary, because Aπ is derived from the
Φf-dependent kr and knr. This could be achieved by replacing Aπ
̃ ̃
in eq 1 by (νa − νf)
1/2 × a3/2 as described below: The dipole in the
S1 state is formed by light absorption (transition dipole
moment). Thus, the difference (Δμ) between the S1 state dipole
moment (μ1) and S0 state dipole moment (μ0) would give a
measure of the distance between the dipole, assuming that the
charge in the S1 state is close to unity, and the charge in the S0
state is negligibly small.
In our previous work,99 the effect of π conjugation length on
the fluorescence emission efficiency was elucidated by
examination of theoretical and experimental relationship
between Φf and magnitude (Aπ) of π conjugation length in the
S1 state. The Aπ is derived from the radiative rate constant (kr)
The Lippert−Mataga equation is100−102
H
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Figure 15. Emission color of 1a−5i dispersed in PS film under irradiation at 254 nm.
(μ − μ )2
As shown in Table 1, emission maxima (λem) of D/A-BPBs
(1a−5i) in both polymer films are significantly blue-shifted from
those in CHCl3 solution. Because the concentration of D/A-
BPBs (1a−5i) in films (ca. 10−2 M) is higher than that in solution
(ca. 10−6 M), the association of compounds is expected to occur
in films. D/A-type compounds are known to form a H-type or J-
type association in high concentrated state to result in a blue shift
or red shift of λabs. Thus, the significant blue shift of λem in both
polymer films indicates a H-type coupling of D/A-BPBs (1a−5i)
in both films, because the Stokes shift does not change much in
both solution and polymer. As shown in Table 1 and Figure 15, it
is demonstrated that 1a−5i dispersed in polymer film emit light
at the whole visible region (from 416 nm (1a) to 630 nm (5f) in
PS, from 415 nm (1a) to 628 nm (5f) in PMMA) and have the
small Δλem values (<20 nm, except for Δλem (between 5g and 5i)
= 25 nm in PS and Δλem (between 5d and 5g) = 22 nm in
PMMA). Furthermore, 1a−5i dispersed in polymer film is seen
to have high Φf values (>0.6) in almost all cases (except for 5f in
PS and 5c, 5f, 5i in PMMA) and log ε values (>4.5) in all cases
regardless of the kind of polymer used. Therefore, D/A-BPBs
(1a−5i) can be mentioned to be the desired WRF in doped
polymer film also.
2
⎛
⎜
⎞
⎟
2
ε − 1
n − 1
e
g
v − v =
−
+ constant
(2)
̃
̃
a
f
hc 2ε + 1
⎝
2n2 + 1 ⎠
a3
̃ ̃
where νa and νf
are the wavenumber (cm−1) of the absorption
and fluorescence peaks, respectively, h is Planck’s constant, c is
the speed of light, ε is the dielectric constant of the solvent, n is
the refractive index of the solvent, and a is the molecular radius.
From eq 2,
Δμ ∝ (v − v )1/2 × a3/2in given solvent
Upon replacing Aπ in eq 1 by (νa − νf)
̃ ̃
1/2 × a3/2, we get eq 3.
̃
̃
a
f
1
Φ =
f
(−(v −v )1/2 ×a3/2
)
̃
̃
f
a
(3)
e
+ 1
̃ ̃
In eq 3, νa and νf are observed values, and a is obtained by
MM2 calculation, for example. The predicted values (Φf(calcd))
calculated with eq 3 and observed values (Φf(expt)) of Φf for
WRF (1−5) are shown in Figure 14 (details in the Supporting
Information).
Although some deviation [ΔΦf = Φf(expt) − Φf(calcd)] is
seen between the calculated values (Φf(calcd)) and the
experimental values (Φf(expt)) (Figure 14), the values calculated
with eq 3 agree very closely with the experimental values except
for 2c (ΔΦf = −0.23), 3c (ΔΦf = −0.24), 4c (ΔΦf = −0.29), 5c
(ΔΦf = −0.50), and 5d (ΔΦf = −0.26), where both the strong
electron-donating groups (NMe2 and NPh2) and the strong
electron-withdrawing acceptor units (Type IV and Type V) exist.
Light-Emitting Properties of Doped Polymer Films.
From the viewpoint of application of WRF to various functional
materials, the elucidation of light-emitting characteristics in
solution alone is not satisfactory; accordingly, light-emitting
characteristics of 1a−5i in doped polymer films was examined as
the fundamental experiment for application. 1a−5i dispersed in
polymer film were prepared by mixing each compound and
polystyrene (PS) or poly(methyl methacrylate) (PMMA) in
THF, followed by spin-coating. The light-emitting characteristics
for 1a−5i dispersed in polymer film were measured. The
obtained results together with the relating data in CHCl3
solution are summarized in Table 1. The emission photographs
of 1a−5i dispersed in PS film under irradiation at 254 nm are
shown in Figure 15.
Although the λem values of WRF (1a−5i) dispersed in polymer
film were smaller than those observed in CHCl3 solution, it is
worth noting that the Φf values of 5c, 5f, 5g, and 5i dispersed in
polymer film increase dramatically compared with those in
CHCl3 solution. Thus, the superior light-emitting properties of
WRF (1a−5i) dispersed in polymer film would enable the
various application to functional materials.
CONCLUSION
■
In conclusion, we synthesized 20 D/A-BPBs (1−5), which are
expected to emit light in the full visible region on the basis of the
positive linear relationship between νem and the HOMO(D)−
LUMO(A) difference calculated by DFT method, by devising
the favorable reaction conditions, and disclosed their fluo-
rescence emission characteristics in CHCl3 solution and in doped
polymer film. Consequently, it has been demonstrated that D/A-
BPBs (1−5) are the desired WRF (whole-rainbow-fluoro-
phores) that emit light in full visible region (400 nm < λem
>
650 nm) and have the small Δλem (the difference of λem between
a given fluorophore and nearest neighboring fluorophore having
I
DOI: 10.1021/acs.jpca.5b05077
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
the same core skeleton) values (<20 nm), the high log ε values
(>4.5), and the high Φf values (>0.6).
for the Strategic Research Foundation at Private Universities,
2014−2018).
We synthesized five new compounds (5e−5i) to get the
intense red fluorophore (Φf > 0.7, λem > 610 nm) and elucidated
their photophysical proterties in CHCl3 solution. As a result, 5h,
in which the 4-cyanophenyl group is introduced to the para-
position of two benzene rings in the terminal NPh2 group of 5d,
was found to be the desired intense red fluorophore (log ε = 4.56,
Φf = 0.76, λem = 611 nm).
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AUTHOR INFORMATION
Corresponding Authors
*Y. Yamaguchi. E-mail: yamaguch@chem.kindai.ac.jp.
*Z. Yoshida. E-mail: yoshida@chem.kindai.ac.jp.
■
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Ministry of Education, Culture,
Sports, Science and Technology (MEXT)−Supported Program
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