Pluripotent Features of Doubly Thiophene-Fused Benzodiphospholes as Organic Functional Materials

A Journal of

Accepted Article

Title: Pluripotent Features of Doubly Thiophene-fused

Benzodiphospholes as Organic Functional Materials

Authors: Tomohiro Higashino, Keiichi Ishida, Tsuneaki Sakurai, Shu

Seki, Tatsuki Konishi, Kenji Kamada, and Hiroshi Imahori

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To be cited as: Chem. Eur. J. 10.1002/chem.201900661

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Pluripotent Features of Doubly Thiophene-fused

Benzodiphospholes as Organic Functional Materials

[

a]

[a]

[b,c]

Tomohiro Higashino,* Keiichi Ishida, Tsuneaki Sakurai, Shu Seki, Tatsuki Konishi,

Kenji

[

b,c]

[a,d]

Kamada,

Hiroshi Imahori*

Abstract: Linear ladder-type π-conjugated molecules have attracted

much interest because of their intriguing physicochemical properties.

To modulate their electronic structures, an effective strategy is to

incorporate main group elements into ladder-type π-conjugated

molecules. In line with this strategy, a variety of ladder-type π-

conjugated molecules with main group elements have been

synthesized to explore their potential utility as organic functional

materials. In this context, phosphole-based π-conjugated molecules

are highly attractive owing to their unique optical and

desired physicochemical properties for practical applications,

numerous efforts have been directed toward synthesis and

functionalizations of π-conjugated molecules. Among them,

linear ladder-type π-conjugated molecules are one of the

promising

candidates

because

of

their

intriguing

physicochemical properties such as intense fluorescence and

high charge mobility, which are attributed to their rigid and

planar structures, efficient π-conjugation, and effective

intermolecular π-π interactions in the solid state. Oligoacenes

electrochemical properties arising from the phosphorus atom. Herein, and

carbon-bridged

oligo-p-phenylenevinylenes

are

To

[

5,6]

we report the synthesis and physicochemical properties of doubly

thiophene-fused benzodiphospholes as a new class of phosphole-

based ladder-type π-conjugated molecules. Systematic investigation

on the physicochemical properties of doubly thiophene-fused

benzodiphospholes revealed their pluripotent features: intense near

infra-red fluorescence, excellent two-photon absorption property,

and remarkably high electron-transporting ability. This study

demonstrates the potential utility of doubly thiophene-fused

benzodiphospholes as organic functional materials for biological

imaging, nonlinear optics, organic transistors, and organic

photovoltaics.

representative ladder-type π-conjugated molecules.

modulate their electronic structures, an effective strategy is to

incorporate main group elements into ladder-type π-conjugated

molecules. In line with this strategy, a variety of ladder-type π-

[

7]

conjugated molecules with main group elements, i.e., boron,

[

8,9]

[10–12]

[13–15]

silicon,

phosphorus,

and sulfur

have been

synthesized to explore their potential utility as organic functional

materials. Typical examples are sulfur-containing π-conjugated

molecules

(i.e.,

thiophene

derivatives)

as

organic

semiconductors because of facile synthetic access by

modification of thiophene units. The intermolecular S–S

interactions as well as π-π interactions afford two-dimensional

(

2D) structures, e.g., herringbone motifs in the solid state, which

in turn leads to increases in electronic coupling between the

Introduction

[16]

neighboring molecules and charge mobility.

For instance,

[

1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives are

The development of novel π-conjugated molecules has attracted

well-known thiophene-based ladder-type molecules, which

much attention owing to their tremendous potential in various

reveal their excellent hole mobility in organic field-effect

[

1]

[2]

fields including energy conversion,

organic electronics,

[14]

transistors (OFETs).

Recently, thiophene-based ladder-type

[

3]

[4]

fluorescence imaging, and nonlinear optics. To achieve

molecules have been utilized as a donor unit of acceptor-donor-

acceptor (A-D-A) structure in non-fullerene acceptors for organic

photovoltaics (OPVs), surpassing the photovoltaic performance

[

a]

b]

Dr. T. Higashino, K. Ishida, Dr. T. Sakurai, Prof. Dr. S. Seki, Prof.

Dr. H. Imahori

[

15]

of OPVs using conventional fullerene acceptors.

Department of Molecular Engineering

Graduate School of Engineering, Kyoto University

Nishikyo-ku, Kyoto 615-8510 (Japan)

Meanwhile, phosphorus-containing π-conjugated molecules

(

i.e., phosphole derivatives) are also highly attractive owing to

their unique optical and electrochemical properties arising from

the phosphole unit. In contrast to thiophene, phosphole adopts a

trigonal pyramid geometry at the phosphorus center and

behaves as a highly conjugative cis-1,3-diene-like p-system,

which would have a large impact on electronic structures of

E-mail: t-higa@scl.kyoto-u.ac.jp

R-mail: imahori@scl.kyoto-u.ac.jp

T. Konishi, Prof. Dr. K. Kamada

Inorganic Functional Materials Research Institute

National Institute of Advanced Industrial Science and Technology

(

1

AIST)

-8-31 Midorigaoka, Ikeda, Osaka 563-8577 (Japan)

1

0

phosphole derivatives. To date, dibenzo[b,d]phospholes,

[

11]

[

c]

d]

T. Konishi, Prof. Dr. K. Kamada

Department of Chemistry

dithieno[3,2-b:2’,3’-d]phospholes,

and phosphorus-bridged

[12]

trans-stilbenes

have been actively studied. Since several

School of Science and Technology, Kwansei Gakuin University

Sanda, Hyogo 669-1337 (Japan)

Prof. Dr. Imahori

phosphorus(V) derivatives exhibit high electron mobility because

of their excellent electron-accepting ability, they are expected to

Institute for Integrated Cell-Material Sciences (WPI-iCeMS)

Kyoto University

Sakyo-ku, Kyoto 606-8501 (Japan)

be used as n-type organic semiconducting materials that are

[11g,h]

applicable to OPV

and organic light-emitting diode

[10f]

(

OLED).

In addition, a zwitterionic ladder-type phosphole is

Supporting information for this article is given via a link at the end of

the document.

known to possess a large two-photon absorption (TPA) cross

[12c]

section (ca. 780 GM at 1200 nm).

Furthermore, phosphorus-

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bridged stilbene derivatives with a high fluorescence quantum

yield were found to be highly photostable fluorescent dyes for

thiophene-fused benzodiphospholes as organic functional

materials, i.e., intense near infrared (NIR) emission, excellent

TPA cross-section, and remarkably high electron-transporting

ability.

[

12i]

fluorescence microscopy.

Despite the extensive effort to

develop phosphole-based ladder-type π-conjugated molecules,

examples of benzodiphospholes are still rare. The first synthesis

of phenyl-substituted benzodiphosphole was reported by Tanaka

[

17a]

and co-workers in 2008.

Then, Yamaguchi et al. also

presented the synthesis and optical and electrochemical

[

17b]

properties of phenyl-substituted benzodiphospholes.

benzodiphospholes have been synthesized by intramolecular

cyclization reaction using 1,4-bis(phenylethynyl)benzene

derivatives. It is notable that the benzodiphospholes were

These

[

17c]

applied to OPV as a cathode buffer layer.

Besides, Liu and

co-workers reported the synthesis of aromatic-fused

[

18a]

benzodiphospholes by radical phosphanylation reaction.

the assistance of synthetic methodologies

dibenzophospholes, doubly benzo-fused

With

for

Figure 1. Structures of doubly thiophene-fused benzodiphospholes.

[

18b,c]

and naphto-fused

have also been prepared.

[

18d]

benzodiphospholes

However, to the best our knowledge, no other example of

benzodiphospholes has appeared despite their potential utility

as a leading platform to explore various phosphole-based

ladder-type π-conjugated molecules. The synthetic difficulties,

i.e., direct chemical modifications of the terminal phenyl rings

and fused benzene rings, prevent further π-extension and

Results and Discussion

Synthesis and characterizations of doubly thiophene-fused

benzodiphospholes.

At the first stage, we aimed to establish the synthetic method

of doubly thiophene-fused benzodiphosphole via halogen-lithium

exchange of 5 (Scheme 1). When we performed Suzuki-Miyaura

coupling reaction of 1,4-dibromo-2,5-diiodobenzene with 3-

functionalization. Nevertheless, to create

phosphole-based ladder-type π-conjugated molecules, we need

breakthrough in their π-extension and functionalization.

a new class of

a

bromo-2-thienylboronic acid in the presence of Pd(PPh

3 4

) or

Considering the synthetic advantages of thiophene over

benzene and phosphole, i.e., the high chemical stability and

facile chemical modification at the α-position, we focus on

doubly thiophene-fused benzodiphosphole 1 as the key leading

platform (Figure 1). Fusion of electron-donating thiophene to

electron-withdrawing benzodiphosphole would stabilize the

electronic structure, creating the new platform depending on the

substituents. Another important benefit of the thiophene-fused

structure is plausible enhancement of intermolecular interactions

involving the heteroatoms in the solid state. Unique three-

dimensional (3D) packing structures of 1 in the solid state would

be formed by combination of intermolecular hydrogen bondings

and S–S interactions as well as π-π interactions derived from its

rigid and planar structure, considering that phosphole-based

molecules are known to exhibit intermolecular hydrogen bonds

Pd(OAc) /SPhos as catalysts, the starting materials were

2

recovered and the key precursor 5 was not obtained. On the

other hand, Suzuki-Miyaura coupling reaction of 1,4-dibromo-

2

,5-bis(pinacolatoboryl)benzene with 2,3-dibromothiophene in

the presence of Pd(PPh smoothly proceeded to afford 5 in

3 4

)

acceptable yield (49%). In the next step, when a solution of n-

BuLi was added to a THF solution of 5 at –78 °C, the solution

immediately turned black and gave a complicated mixture.

Conversely, the addition of a THF solution of 5 into a solution of

n-BuLi afforded a clear solution containing tetralithiated species.

2 2 2

Treatment of the lithiated species with PhPCl and H O

provided trans-1 and cis-1 as a diastereomeric mixture. These

diastereomers were successfully separated by silica-gel column

chromatography to yield trans-1 in 17% and cis-1 in 18%. The

[

12d,19]

doubly

thiophene-fused

benzodiphospholes

1

were

between P=O and C–H moieties in the solid state.

1

13

31

characterized by H, C, and P NMR spectroscopies and high-

Therefore, we envisioned that 1 would be an attractive platform

for π-extension and functionalization to explore diverse potential

applications. Herein, we report the synthesis and

physicochemical properties of 1–4, and its π-extension by

conventional palladium-catalyzed coupling reactions (Figure 1).

First, we established the general synthetic route to doubly

thiophene-fused benzodiphospholes trans-1 and cis-1 as the

core unit and elucidated their unusual properties depending on

the isomers. Then, we introduced aryl, arylethenyl, and

arylethynyl groups with electron-donating or electron-

withdrawing substituents to the α-positions of the synthetic

accessible fused-thiophene moieties of trans-1, yielding the π-

extend derivatives 2–4. The systematic exploration was found to

be effective to unveil their pluripotent features of doubly

3

1

resolution mass spectrometry. The P NMR spectra of trans-1

and cis-1 exhibit the peaks at 22.9 and 22.7 ppm, respectively

(

Figures S20 and S21). The upfield shifts relative to the doubly

[

18a]

phenyl-substituted benzodiphosphole (33.7 ppm)

reflect their

thiophene-fused structures. Fortunately, the structures with

trans/cis-configurations were revealed by X-ray diffraction

[

20a,b]

analysis.

The crystal structure of trans-1 shows a highly

planar structure (Figure 2a). On the other hand, cis-1 reveals a

slightly warped structure, probably originated from the packing

forces (Figure 2b). The C–C and C–P bond lengths on the

benzodiphosphole unit are similar to those of reported

[

17a,b,18b,c]

benzodiphosholes,

indicating that the fusion of

thiophene to the benzodiphosphole has little impact on their

structural parameters (Figures S1 and S2). The trans and cis

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isomers reveal unusual contrast in the packing modes. The

packing structure of trans-1 displays a one-dimensional (1D)

slip π-stacked alignment (Figure S1). The separation distance

between two π-planes is ca. 3.5 Å, which implies effective

intermolecular π-π interaction. In contrast, cis-1 forms a face-to-

face dimeric structure. The intermolecular separation distance is

approximately 3.5 Å with effective overlapping with a small

rotation angle between the two planes (ca. 7°) (Figure S2).

Additionally, each dimeric structure is isolated by the phenyl

groups on the phosphorus atoms in the molecular packing.

chose trans-1 as the platform for the π-extension and

functionalization. First, we attempted halogenation of trans-1.

However, the reaction of trans-1 with 2 equivalent of N-

bromosuccinimide (NBS) resulted in the complete recovery of

trans-1. After trial-and-error optimization of reaction conditions,

we succeeded to obtain dibrominated benzodiphosphole 6 as

the important intermediate quantitatively by treatment of trans-1

with 10 equivalent of NBS in a mixture of chloroform and acetic

acid (Scheme 2). It is noteworthy that the α-positions of the

thiophene rings were brominated with complete selectivity

irrespective the large excess of NBS. Then, we carried out

palladium-catalyzed cross-coupling reactions of 6 to obtain

versatile

π-extended

doubly

thiophene-fused

benzodiphospholes (Scheme 2). The Suzuki-Miyaura coupling

reaction of 6 with arylboronates afforded aryl-substituted 2 in

high yields (67–88%). The Mizoroki-Heck reaction of 6 with

styrene also proceeded to give 3a, but the yield was low (27%).

On the other hand, the Suzuki-Miyaura coupling reaction with

alkenylboronates afforded arylethenyl-substituted 3a and 3b in

high yields (65 and 78%, respectively). In addition, the

Sonogashira coupling reaction with arylacetylenes gave

arylethynyl-substituted 4a,c,d in moderate to high yields (32–

8

6%), although the reaction with dimethylaminophenylacetylene

resulted in the complete recovery of 6. These results

unambiguously corroborate that various π-extended doubly

thiopene-fused

benzodiphospholes

are

accessible

by

conventional palladium-catalyzed cross-coupling reactions of 6

converted from trans-1 as the platform.

Scheme 1. Synthesis of doubly thiophene-fused benzodiphospholes.

Scheme 2. Coupling reactions of 6 to produce versatile π-extended doubly

thiophene-fused benzodiphospholes 2–4.

Fortunately, we obtained single crystals of 2a (Figure 3a)

Figure 2. X-ray crystal structures of (a) trans-1 and (b) cis-1: top view (left)

and side view (right). Thermal ellipsoids represent 50% probability. Solvent

molecules are omitted for clarity. For cis-1, one of the two independent

molecules in the unsymmetric unit cell is shown.

and 4a (Figure 3b) suitable for X-ray crystallographic

[20c,d]

analysis.

The C–C and C–P bond lengths on the

benzodiphosphole unit of 2a (Figure S3) and 4a (Figure S4) are

comparable to those of the unsubstituted trans-1. The torsion

angles of the terminal phenyl rings relative to the

benzodiphosphole core are small (11~22°), implying effective π-

extension through the substituents. The packing structure of 4a

signifies a 1D slip π-stacked alignment, which is similar to the

Taking into account the more extensive intermolecular π-π

interactions in trans-1 than that in cis-1 in the solid state, we

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packing structure of trans-1. The separation distance between

two π-planes is enlarged (ca. 3.7 Å), indicating the weakened π-

π interaction by the introduction of the phenylethynyl group

Optical properties.

We examined the optical properties of doubly thiophene-

fused benzodiphospholes 1–4 (Table 1). UV/Vis absorption and

fluorescence spectra of trans-1 and cis-1 were measured in

(

Figure S4). Meanwhile, the packing structure of 2a is unique. It

displays an ununiform 1D slip π-stacked alignment with

independent molecules A and B (Figure S3). The molecule A

shows high planarity with a mean plane deviation (MPD) of the

thiophene-fused benzodiphosphole core (18 atoms, 0.039 Å),

while the molecule B reveals a slightly distorted structure with a

MPD of 0.060 Å. The separation distances between the terminal

phenyl group and thiophene-fused benzodiphosphole core are

2 2

CH Cl . The absorption spectra of trans-1 and cis-1 display the

lowest energy absorption at 404 nm (Figure 4a), which are red-

shifted compared to those of aromatic-fused benzodiphospholes

[

18a]

(ca. 380 nm).

These red-shifts can be attributed to

intramolecular charge-transfer (ICT) interaction as a result of

fusion of the electron-rich thiophene rings to the electron-

deficient benzodiphosphole core. We measured the absorption

spectra in various solvents and found their solvatochromism,

which supports their ICT interactions (Figure S5). The

benzodiphospholes trans-1 and cis-1 exhibit blue fluorescence

with similar emission maxima at 481 and 478 nm, respectively

(Figure 4b). The fluorescence peaks are remarkably shifted to

3

.6–3.7 Å, suggesting the reduced intermolecular π-π

interactions by the direct introduction of the phenyl groups.

Importantly, the doubly thiophene-fused benzodiphosphole core

of 2a exhibits

a

2D herringbone structure with

a

short

[

16]

intermolecular S-S contact of 3.64 Å (Figure 3c).

Moreover,

there are short distances between the oxygen atoms of the P=O

moieties and the hydrogen atoms of the thienyl and the terminal

phenyl groups (2.3–2.5 Å), indicating the presence of multiple

hydrogen bondings. Thus, the terminal phenyl rings attached

directly to the fused-thiophene moieties play an important role in

the formation of 2D herringbone structure of 2a with the

assistance of the versatile intermolecular interactions involving

the heteroatoms, unlike trans-1, cis-1, and 4a. This unique

packing structure encouraged us to further examine the charge-

long

dibenzophosphole (420 nm).

trans-1 and cis-1 (4000 and 3800 cm ) are much larger than

wavelengths

relative

to

the

benzene-fused

[

18a]

Namely, the Stokes shifts of

–

1

–

1

[18a]

that of the benzene-fused dibenzophosphole (2300 cm ).

Notably, trans-1 and cis-1 are highly fluorescent. The

fluorescence quantum yields (Φ ) of trans-1 and cis-1 are 0.92

and 0.91, which are higher than that of the benzene-fused

F

[

18a]

dibenzophosphole (0.81).

were measured by the time-correlated single-photon counting

(TCSPC) technique. The τ values in CH Cl were determined to

F

The fluorescence lifetimes (τ )

transporting

ability

of

the

doubly

thiophene-fused

F

2

benzodiphospholes in the solid state (vide infra).

be 11.9 ns and 12.0 ns, respectively. Although no fluorescence

lifetimes of benzodiphospholes have been reported, these

values are similar to those of phosphole-based ladder-type

[

12a,e]

molecules.

The radiative (k

r

) and nonradiative (knr) rate

and Φ values. The k and

nr values are 7.6–7.7 ×10 and 6.7–7.5 ×10 s , respectively.

The knr values are one order of magnitude smaller than those of

constants are calculated from the τ

F

r

7

6

–1

k

7

–1

typical benzo[b]phospholes (~10 s ). These small knr values

relative to k values are responsible for the high F values in

r

F

spite of the large Stokes shifts. Although the large Stokes shifts

and rather strong fluorescence are counterintuitive, the unusual

fluorescence

properties

of

doubly

thiophene-fused

benzodiphospholes are beneficial for biological imaging to avoid

background fluorescence (vide infra).

Table 1. Optical properties of doubly thiophene-fused benzodiphospholes in

2 2

CH Cl .

[

a]

[b]

[e]

λ

abs / nm

λ

em / nm

E

opt

[d]

/

τ

F

/

k

r

–1

/

k

nr

–1

/

Δν

–

1

–1

[c]

–1

(

ε / M cm

)

(Φ

F

)

eV

ns

s

/cm

tran

404 (8000)

404 (7600)

481

2.76

2.50

2.14

11.9

7.7

6.7

4000

3800

3500

4400

7

8

6

8

s-1

(0.92)

×10

cis-

478

12.0

3.6

7.6

7.5

Figure 3. X-Ray crystal structures of (a) 2a and (b) 4a: top view (left) and side

view (right). Thermal ellipsoids represent 50% probability. Solvent molecules

are omitted for clarity. For 2a, one of the two independent molecules in the

unsymmetric unit cell is shown. (c) The packing structure of 2a along with c-

axis (left) and b-axis (right). S-S contacts and hydrogen bonds are indicated

1

(0.91)

×10

2a

2b

443 (19000)

505 (33000)

525

1.4

(0.50)

×10

(

dashed green lines) with the distances between S···S and O···H (Å).

650

4.6

1.1

(

0.51)

×10

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spectra in the solid state are red-shifted considerably in

comparison with those in solution. Remarkably, the red-shift of

cis-1 (55 nm) is much larger than that of trans-1 (10 nm). The

fluorescence quantum yields of trans-1 and cis-1 in the solid

state were determined to be 0.16 and 0.10, respectively. The

remarkable red-shift of fluorescence in the solid state may result

from the isolated unique dimeric structure of cis-1, as observed

in the packing structure, which reveals an excimer emission

solely in the solid state. In accordance with this interpretation, no

red-shift of fluorescence was observed in the solution even at

high concentrations (Figure S6). Although the solid-state

excimer emission is often observed in polyaromatic

2

3

4

c

a

b

a

c

d

434 (27000)

470 (15000)

520 (37000)

445 (31000)

439 (31000)

445 (42000)

496, 514

0.28)

2.58

2.33

2.05

2.49

2.53

2.50

1.6

1.7

1.8

1.4

0.9

1.8

4.5

2900

3300

4900

3200

2800

2700

8

(

×10

556

0.27)

1.7

4.6

(

×10

699

0.22)

1.3

4.6

(

×10

520

0.37)

2.1

3.5

(

×10

501, 520

0.34)

2.4

4.7

(

×10

[

21]

[22]

hydrocarbons (i.e., anthracene

of excimer emission of phosphole derivatives have been limited

and pyrene ), the examples

505, 532

0.30)

3.3

7.8

(

×10

[

19,23]

to dithienophosphole derivatives.

Thus, the subtle change,

[

a] Absorption maxima at the longest wavelength. [b] The samples were

i.e., the mutual orientation of the phenyl groups and oxygen

atoms on the phosphorous atoms, has a large influence on the

packing structure in the solid state, altering the photophysics in

the excited states of trans-1 and cis-1 in the solid state.

excited at 415 nm. [c] Absolute fluorescence quantum yields. [d] Optical

HOMO–LUMO gaps from the intersection of normalized absorption and

fluorescence spectra. [e] Stokes shift.

The

phenyl-substituted

doubly

thiophene-fused

benzodiphospholes 2a-c display red-shifted absorption and

fluorescence spectra compared to the unsubstituted trans-1

(

Figure 5). The thiophene-fused structure is responsible for the

red-shifted absorption of 2a (443 nm) relative to those of the

[

17a,b]

phenyl-substituted benzodiphospholes (ca. 425 nm).

The

dimethylaminophenyl-substituted 2b reveals red-shifted

a

intense absorption at 505 nm in comparison with 2a, indicating

effective ICT interaction between the electron-donating

dimethylamino

benzodiphosphole unit. Strikingly, 2b shows

fluorescence with a peak at 650 nm and wavelengths exceeding

moiety

and

electron-withdrawing

a

moderate

9

00 nm (Figure 5b). Therefore, the push-pull D-A-D structure of

2

b is an effective molecular design to achieve the small HOMO–

LUMO gap and NIR fluorescence. On the other hand, the

bis(trifluoromethyl)phenyl-substituted 2c exhibits rather blue-

shifted absorption and fluorescence relative to those of 2a.

There may be a trade-off between the electron-withdrawing

nature of the trifluoromethyl group and electron-donating nature

of the fused-thiophene ring, eliminating the effect of ICT

interaction in 2c. The direct introduction of the aryl groups into

trans-1 lowers the Φ

F

values (2a: 0.50, 2b: 0.51, 2c: 0.28). In

8

–1

fact, the knr values of 2a-c (1.1–4.5 ×10 s ) become two orders

6

–1

of magnitude larger than that of trans-1 (6.7 ×10 s ), whereas

8

–1

the k

that of trans-1 (7.7 ×10 s ). The significant enhancement in the

nr values of 2a-c is interpreted by the increased vibrational and

r

values of 2a-c (1.1–1.7 ×10 s ) are rather comparable to

7

–1

k

rotational degrees of freedom owing to the introduced aryl

substituents.

Figure 4. a) UV/Vis absorption and b) normalized fluorescence spectra of

doubly thiophene-fused benzodiphospholes trans-1 (black) and cis-1 (red) in

2 2

CH Cl (solid lines) and in the solid state (dashed lines). For fluorescence

measurements, the samples were excited at 415 nm.

Fluorescence spectra of trans-1 and cis-1 were also

measured in the solid state (Figure 4b). The fluorescence

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[

24c]

emission at 684 nm (Φ

F

= 0.19).

Importantly, the large Stokes

–

1

shift (4900 cm , Δλ = 179 nm) can avoid interference by

[

25]

excitation source, self-absorption, and autofluorescence. The

rather high Φ

F

and large Stokes shift of 3b demonstrate that

push-pull

D-A-D type, doubly thiophene-fused

benzodiphospholes are excellent candidates as an efficient NIR

fluorescent organic dye for biological imaging. In the cases of

arylethynyl-substituted 4c and 4d with electron-withdrawing

substituents, blue-shifted absorption and fluorescence relative to

those of 4a are observable, which is the same behavior as

described in 2a and 2c (Figure S7).

Figure 5. a) UV/Vis absorption and b) normalized fluorescence spectra of

aryl-substituted doubly thiophene-fused benzodiphospholes 2a (blue), 2b

(

2 2

yellow), and 2c (green) in CH Cl . For fluorescence measurements, the

samples were excited at 415 nm. To obtain the whole fluorescence spectra,

we used two detectors, one for wavelengths less than 850 nm, and the other

one for wavelengths more than 850 nm.

We also compared the absorption and fluorescence spectra

of arylethenyl-substituted

3

and arylethynyl-substituted

4

(

Figures 6 and S7) together with those of 2a. The absorption

maximum of phenylethynyl-substituted 4a (445 nm) is almost

identical to that of 2a (443 nm), whereas phenylethenyl-

substituted 3a exhibits significant red-shifted absorption (470

nm). The corresponding fluorescences show the same trend (2a:

Figure 6. a) UV/Vis absorption and b) normalized fluorescence spectra of

arylethenyl-

and

arylethynyl-substituted

doubly

thiophene-fused

Cl . For

5

25 nm, 3a: 556 nm, 4a: 520 nm). More effective π-conjugation

benzodiphospholes 3a, (purple), 3b (cyan), and 4a (light green) in CH

2

through the vinylene linker than the ethynylene linker causes the

fluorescence measurements, the samples were excited at 415 nm. To obtain

the whole fluorescence spectra, we used two detectors, one for wavelengths

less than 850 nm, and the other one for wavelengths more than 850 nm.

smaller HOMO–LUMO gap of 3a than 4a, as seen in the

F

significant red-shifts in 3a. The Φ value decreases in the order

of 2a (0.50) > 4a (0.37) > 3a (0.27). This trend reflects that of the

vibrational and rotational degrees of freedom. Indeed, the knr

8

–1

Electrochemical properties.

value increases in the same order of 2a (1.4 ×10 s ) < 4a (3.5

8

–1

8

–1

The electrochemical properties of doubly thiophene-fused

benzodiphospholes 1–4 were assessed by cyclic voltammetry

×

r

10 s ) < 3a (4.6 ×10 s ), while the k value remains similar.

As compared to 3a and even 2b, 3b exhibits more enhanced

red-shifted absorption (520 nm) and fluorescence (699 nm).

Surprisingly, the fluorescence of 3b is extended into the NIR

region deeply (~1200 nm). NIR emissive compounds have

drawn much attention because of their potential applications in

(

CV) and differential pulse voltammetry (DPV) technique in

CH Cl with tetrabutylammonium hexafluorophosphate

NPF ) as an electrolyte versus ferrocene/ferrocenium ion

Fc/Fc ) (Table 2 and Figure S8). Although trans-1 and cis-1

2

(

Bu

4

6

+

[

3c–f]

exhibit the reversible reduction peaks at –1.90 V, they reveal the

irreversible oxidation peaks at +1.22 and +1.29 V, respectively.

The reversible reduction peaks of 1 substantiate the excellent

electron-accepting character of doubly thiophene-fused

biological imaging.

dyes with intense visible fluorescence, the NIR fluorescent

However, compared to numerous organic

[

24]

F F

organic dyes with a high Φ are still scarce. Although the Φ

value of 3b (0.22) has not been optimized yet, it is comparable

to that of a representative BODIPY-based molecule with an

Chemistry - A European Journal

10.1002/chem.201900661

FULL PAPER

phospholes. The trans/cis-configurations have little impact on

the oxidation potentials (Eox) and reduction potentials (Ered).

and use of the ethynylene linkers are effective to enhance the

high electron-accepting ability of doubly thiophene-fused

benzodiphospholes. This information would be very useful for

the exploitation of electron-transporting materials.

+

Table 2. Electrochemical oxidation and reduction potentials (versus Fc/Fc )

and calculated energy levels of doubly thiophene-fused benzodiphospholes.

Theoretical calculations.

To obtain the insight into the structural and electronic

properties of doubly thiophene-fused benzodiphospholes 1–4,

we performed density functional theory (DFT) calculations at the

B3LYP/6-31G(d,p) level. The optimized structures of trans-1, 2a,

E

ox2

E

ox1

E

red1

/

E

red2

/

E

el

/

E

HOMO

[c]

E

LUMO

[c]

E

DFT

[d]

/

[

b]

/

V

/ V

V

eV

/ eV

/eV

eV

tran

–

1.22

–1.90

–

3.12

3.19

–5.78

–2.27

3.51

s-1

3

a, and 4a exhibit highly coplanar structure of the doubly

thiophene-fused benzodiphosphole unit (Figure S9). The phenyl

group of 2a is slightly tilted (28°), whereas the styryl group of 3a

and phenylethynyl group of 4a adopt completely coplanar

structures. The orbital distributions of HOMO and LUMO of 1 are

well delocalized over the whole π-conjugated system (Figure

S10). The HOMO possesses almost no distribution on the

phosphorus atom. In contrast, the LUMO possesses a large

distribution on the phosphorus atom, indicating effective σ*–π*

interaction that is unique in phospholes. To support the excimer

emission of cis-1 in the solid state, we carried out calculations

on the dimeric structure taken from its crystal structure (Figure

S11). Although the HOMO shows no interaction between the two

doubly thiophene-fused benzodiphosphole units, the LUMO

visualizes the effective interaction through the space. The

calculated HOMO–LUMO gap of the dimer (3.28 eV) is smaller

than that of the monomer (3.52 eV), which is consistent with the

red-shifted fluorescence stemming from the dimeric structure of

cis-1 in the solid state.

cis-

–

1.29

–5.79

–2.27

3.52

1

2

3

4

a

b

c

a

b

a

c

d

0.94

0.28

1.20

0.76

0.18

1.01

1.19

–1.81

–1.88

−1.67

–1.75

–1.82

–1.71

–1.63

–2.15

2.75

2.16

2.87

2.51

2.00

2.72

2.82

–5.44

–4.75

–5.94

–5.17

–4.58

–5.36

–5.80

–5.86

–2.33

–2.00

–2.79

–2.44

–2.10

–2.52

–2.91

–3.14

3.11

2.75

3.15

2.73

2.48

2.84

2.89

2.72

0.87

–

–2.03

–

–2.03

–

[

[a]

n.d.

a]

[

a] Not determined due to low solubility. [b] Electrochemical HOMO–LUMO

gap. [c] Calculated at the B3LYP/6-31G(d,p) level. [d] Calculated HOMO–

LUMO gap.

For

the

π-extended

doubly

thiophene-fused

benzodiphospholes, the HOMOs of 2a, 3a, and 4a are

delocalized over the whole π-system, while their LUMOs are

mainly

localized

on

the

doubly

thiophene-fused

benzodiphosphole unit (Figures S12−S14). In addition, their

LUMOs possess large distributions on the phosphorus atoms

due to the effective σ*–π* interaction. Since the LUMO level of

Meanwhile, the Eox values of 2a, 3a, and 4a are shifted to a

negative direction relative to that of trans-1, while the Ered values

are moved to a positive direction. Consequently, the smaller

electrochemical HOMO–LUMO gaps of 2a (2.74 eV), 3a (2.51

eV), and 4a (2.72 eV) than that of trans-1 (3.12 eV) agree with

the corresponding optical HOMO-LUMO gaps (Table 1). For

dimethylamino-substituted 2b and 3b, the quasi-reversible

oxidation peaks are observed at +0.28 and +0.18 V, respectively,

which are assigned to oxidations of the dimethylaminophenyl

moiety. The Ered value becomes more positive in the order of 2a

2

a estimated by CV and DPV measurements is slightly higher

[

17b]

than those of the phenyl-substituted benzodiphospholes,

the

fused thiophene rings can act as an electron-donating group.

Nevertheless, the effective σ*–π* interactions indicate the

considerable contribution of phosphole-based LUMOs, which

supports the electron-accepting ability of doubly thiophene-fused

benzodiphospholes. The energy levels of LUMO are lowered in

the following order: 2a: –2.33 eV > 3a: –2.44 eV > 4a: –2.52 eV,

(

–1.81 V) < 3a (–1.75 V) < 4a (–1.71 V), which reflects the

which matches with the trend on the

Ered values. The

degree of the electron-donating effect of the aryl moiety through

the linkers. As expected, introduction of the electron-withdrawing

trifluoromethyl moieties on the aryl groups leads to more positive

introduction of the electron-donating dimethylamino moieties

decreases the orbital distributions of LUMO on the

dimethylaminophenyl groups in 2b and 3b, while those of

HOMO on the dimethylaminophenyl groups are increased.

Consequently, these orbital distributions support efficient ICT

interactions in 2b and 3b (Figures S12 and S13). In marked

contrast, the HOMOs and LUMOs of 2c and 4c,d with electron-

withdrawing substituents on the aryl groups are similar to those

of 2a and 4a to a great extent. The comparable orbital

distributions are consistent with the their rather similar optical

properties irrespective of the electron-withdrawing substituents

E

red values (–1.67 V for 2c and –1.63 V for 4c) than those of 2a

+

and 4a. Given the energy level of Fc/Fc is –4.8 eV under

[

26]

vacuum, the LUMO level of 2a is estimated to be –2.99 eV.

This level is slightly higher than those of the phenyl-substituted

[

17b]

benzodiphospholes (ca. –3.05 eV),

which is used as a

The LUMO levels of 2c, 4a, and

c are also calculated to be –3.13, –3.09, and –3.17 eV,

[

17c]

cathode buffer layer in OPV.

4

respectively, which are comparable to or even lower than those

of the phenyl-substituted benzodiphospholes. Accordingly,

introduction of electron-withdrawing groups on the aryl moiety

(

Figures S12 and S14). Importantly, the introduction of electron-

Chemistry - A European Journal

10.1002/chem.201900661

FULL PAPER

withdrawing groups on the aryl groups stabilizes the LUMO

levels and enhances their electron-accepting ability.

We also carried out time-dependent DFT (TD-DFT)

calculations to evaluate the absorption (Table S3). For all the

doubly thiophene-fused benzodiphospholes, the lowest

excitations are derived from HOMO/LUMO transitions. The

lowest excitation energies largely match with the absorption

spectra. It should be noted here that the HOMO/LUMO

transitions with large oscillator strengths for 2b (f = 0.9596) and

3

b (f = 1.7015) agree with the efficient ICT interactions.

Two-photon absorption spectrometry of D-A-D type doubly

thiophene-fused benzodiphosphole.

Motivated by intense absorption of 3b at 520 nm as a result

of the enhanced ICT interaction originated from the push-pull D-

A-D structure with the electron-donating substituents, we

selected 3b to evaluate the two-photon absorption (TPA)

properties. We expected that the large π-conjugated system and

(

2)

D-A-D quadrupole would afford large TPA cross sections (σ ).

The TPA measurements of 3b in CH Cl were conducted by

2

using the open-aperture Z-scan method (Figure S15). The TPA

spectrum exhibits two broad peaks at around 900 and 650 nm

(

2)

(

Figure 7a). The σ values at 855 and 654 nm were determined

to be 970 ± 190 and 2200 ± 420 GM, respectively, which are

significantly larger than those of other phosphole-based

[

11c,12c]

molecules (~800 GM).

To understand the origin of the TPA

transitions, we performed the DFT calculations for the TPA

spectral simulation of 3b (See Supporting Information). The

simulated one-photon absorption (OPA) and TPA spectra are

displayed in Figure 7b. Although the transition energies are

overestimated (i.e. the peaks are blue-shifted) at the CAM-

B3LYP/6-31+G(d) level of calculations, the simulation largely

reproduces the experimental spectral shapes. The lowest-

energy TPA peak is located at the different transition energy

from the OPA peaks in both simulation and experiment,

reflecting alternating selection rule of symmetry between OPA

and TPA for centrosymmetric molecules. From these

comparisons with the simulated spectra, the TPA peaks at 880

and 650 nm in the experiment are assigned to the transitions,

2 2

Figure 7. a) TPA spectrum (bottom and left axes) of 3b in CH Cl measured

at an excitation power of 0.4 mW (black dots, with solid curve of eye guide)

and by changing the power (orange dots with error bars). The OPA spectrum

(

top and right axes, purple curve) is also overlapped with the half scale in the

horizontal axis so that the same transition energies of OPA and TPA fall at the

same position. b) Simulated TPA (red) and OPA (blue) spectra calculated at

the CAM-B3LYP/6-31+G(d) level in CH

manner.

2 2

Cl with PCM plotted in the same

Charge-transporting properties of π-extended doubly

thiophene-fused benzodiphosphole.

2 4

respectively, to S and S excited states both having gerade

symmetry. These results corroborate that the introduction of

electron-donating arylethenyl groups into doubly thiophene-

fused benzodiphosphole is an effective means to create novel

TPA materials.

Finally, we evaluated the potential utility of the π-extended

doubly thiophene-fused benzodiphosphole as electron-

transporting materials. We measured the photoconductivity of

trans-1, 2a, 3a, and 4a using the flash-photolysis time-resolved

[

27]

microwave conductivity (FP-TRMC) method.

The FP-TRMC

profiles for drop-cast films of trans-1, 2a, 3a, and 4a upon

irradiation at λ = 355 nm are displayed in Figure 8. The

maximum transient conductivity (φΣµ)max for trans-1 was

–

9

2

–1 –1

determined to be 1.2 × 10 m V s , where φ and Σµ represent

the charge carrier generation efficiency and the sum of the

charge carrier mobilities, respectively. The (φΣµ)max values for 2a

–

9

2

–1 –1

–9

2

–1 –1

(

13 × 10 m V s ), 3a (3.1 × 10 m V s ), and 4a (6.2 ×

–

9

2

–1 –1

1

0

m V s ) are larger than that for trans-1, suggesting that

the π-extension can enhance the intrinsic charge-transporting

ability greatly. The powder X-ray diffraction measurements of

drop-cast films of trans-1, 2a, and 4a show sharp diffraction

peaks, which are largely consistent with their unit cells (Figure

Chemistry - A European Journal

10.1002/chem.201900661

FULL PAPER

S16). Importantly, the highest (φΣµ)max value for 2a is

rationalized by effective electronic communication in the packing

derived from the 2D herringbone structure, which is realized by

the multiple intermolecular interactions including S–S contacts

and hydrogen bonds between P=O and C–H groups (Figure

In this paper, we established the synthetic methodology of

doubly thiophene-fused benzodiphosphole and their π-extended

derivatives as a new class of linear ladder-type π-conjugated

molecules. Systematic investigation on the physicochemical

properties of doubly thiophene-fused benzodiphospholes

revealed that their pluripotent features for various potential

applications could be extracted by rational π-extension with the

introduction of aryl, arylethenyl, and arylethynyl groups with

electron-donating or electron-withdrawing substituents to the α-

positions of the synthetic accessible fused-thiophene moieties.

We summarize the highlight on their pluripotent features as

follows: The 1D slip π-stacked arrangement of trans-1 in the

[

16g,28]

2

c),

in addition to the 1D slip π-stacked interactions (Figure

S3). Such S–S interactions are known to play an important role

in providing an alternative charge-transporting pathway in the

[

16]

organic semiconductors.

Since the FP-TRMC measurements

under air provide the sum of the electron and hole mobilities, we

sought to determine the main contributor by the measurements

under SF

Moreover, the (φΣµ)max value under air is recovered to the initial

value even after exposure to SF . These results indicate that the

6

, electron scavenger gas. The (φΣµ)max value for 2a

6

is decreased to ca. 20% of that under air (Figure S17). solid state suggested effective π-π interaction through the 1D

chain. On the other hand, cis-1 exhibited the isolated face-to-

6

face dimeric structure with effective overlapping of the doubly

observed electrical conductivity of 2a is largely contributed from

thiophene-fused

thiophene-fused benzodiphosphole trans-1 and cis-1 were

highly florescent (Φ = 0.92 and 0.91). Notably, cis-1 exhibited

benzodiphosphole

cores.

The

doubly

electron transport. We also measured the photoconductivity of

4

c,d. Their FP-TRMC profiles are displayed in Figure S18. Since

F

–

9

2

the observed (φΣµ)max value for nitro-substituted 4d (10 ×10

m

solid-state excimer fluorescence owing to its unique isolated

dimeric structure. More importantly, the packing structure of the

directly phenyl-substituted 2a in the solid state showed the

unique 2D herringbone structure created from the fusion of the

phosphole and thiophene moieties. Namely, the structure is

generated by multiple intermolecular interactions including S–S

contacts and hydrogen bondings between P=O and C–H groups,

which is beneficial for enhancing the electron-transporting

properties. As expected, the π-extended doubly thiophene-fused

benzodiphospholes exhibited smaller optical HOMO-LUMO gaps

than that of trans-1. It is noteworthy that dimethylamino-

substituted 2b and 3b showed enhanced red-shifted absorption

and fluorescence extending into the NIR region (700–1200 nm)

with rather high quantum yields. Especially, the NIR

fluorescence of 3b (up to 1200 nm) and its large Stokes shift (Δλ

–

1

–1

–9

2

–1 –1

V

s ) is larger than those of 4a and 4c (6.7 × 10 m V s ),

introduction of suitable electron-withdrawing groups on the

phenylethenyl-substituted 4a can also enhance

photoconductivity as well as electron-accepting ability.

Remarkably, the (φΣµ)max values for 2a and 4d surpass those of

previously reported materials based on perylenediimide

–

9

2

–1 –1 [27c,d]

derivatives (4–5 × 10 m V s ),

a representative n-type

organic semiconducting motif. Therefore, we verify that π-

extended doubly thiophene-fused benzodiphospholes with

suitable substituents possess the potential utility as excellent

electron-transporting materials.

=

179 nm) are suitable for efficient NIR fluorescent organic dye

for biological imaging. Furthermore, 3b revealed large TPA

cross section (970 GM) at 855 nm, which demonstrated the

potential for novel TPA materials. Meanwhile, phenyl-substituted

–

9

2

–1 –1

s

2

a displayed the highest (φΣµ)max value of 13 ×10 m V

in

the FP-TRMC measurements. The 2D herringbone and 1D slip

π-stacked structures in the solid state may be responsible for

the highest (φΣµ)max value of 2a. It is notable that the observed

conductivity of 2a was largely contributed from electron transport.

In addition, electron-withdrawing nitro-substituted 4d also

–

9

2

–1 –1

showed the higher (φΣµ)max value (10 ×10 m V s ) than that

of unsubstituted reference 4a. The (φΣµ)max values for 2a and 4d

exceed those of previously reported representative electron-

transporting materials based on perylenediimide derivatives.

Thus, π-extended doubly thiophene-fused benzodiphospholes

with suitable substituents possess the potential utility as

excellent electron-transporting materials. Overall, these doubly

thiophene-fused benzodiphospholes as the platform were found

to be highly attractive ladder-type π-conjugated molecules as

NIR fluorescent organic dyes, TPA materials, and electron-

transporting materials. We believe that this report unveils the

Figure 8. Flash-photolysis time-resolved microwave conductivity (FP-TRMC)

profiles for drop-cast films of trans-1 (black), 2a (red), 3a (green), and 4a

1

5

(

blue) from CHCl solution upon 355 nm photoexcitation at 4.6 × 10 photons

3

–2 –1

cm pulse under air.

pluripotent

features

of

doubly

thiophene-fused

Conclusions

benzodiphospholes as promising organic functional materials in

Chemistry - A European Journal

10.1002/chem.201900661

FULL PAPER

various fields, e.g. biological imaging, nonlinear optics, organic

transistors, and organic photovoltaics.

light yellow solid (7.7 mg, 15.8 µmol, 18% yield). Single crystals

suitable for X-ray crystallographic analysis were obtained by

vapor diffusion of n-hexane into CHCl

3

solutions of trans-1 or

cis-1.

1

trans-1: H NMR (395.88 MHz, CDCl

3

, 25 °C): δ = 7.77 (q, J =

Experimental Section

7

.2 Hz, 4H), 7.68 (dd, J = 2.8, 11.2 Hz, 2H), 7.54 (m, 2H), 7.44

1

3

(

m, 6H) and 7.20 (q, J = 2.8 Hz, 2H) ppm; C NMR (100.40

MHz, CDCl , 25 °C): δ = 152.8 (d, J = 26.0 Hz), 142.1 (d, J =

07 Hz), 137.8 (t, J = 15.5 Hz), 136.8, 133.0, 132.0 (d, J = 742

Hz), 131.2 (d, J = 12.4 Hz), 131.0 (d, J = 15.5 Hz), 129.3 (d, J =

Instrumentation and Materials. Commercially available

solvents and reagents were used without further purification

unless otherwise mentioned. Silica-gel column chromatography

was performed with UltraPure Silica Gel (230-400 mesh,

SiliCycle) unless otherwise noted. Thin-layer chromatography

3

1

5.4 Hz), 126.2 (d, J = 15.4 Hz), and 121.9 (t, J = 9.6 Hz) ppm;

3

1

P NMR (161.7 MHz, CDCl

3

, 25 °C): δ = 22.9 ppm. UV/Vis

(

TLC) was performed with Silica gel 60 F254 (Merck). UV/Vis/NIR

absorption spectra were measured with a Perkin-Elmer Lambda

00 UV/vis/NIR spectrometer. Steady-state fluorescence spectra

–

1

–1

(

CH

17600), and 404 (8000) nm. Fluorescence (CH

= 0.92. FT-IR (ATR): ν = 1242 cm

P=O). HRMS (APCI, positive) calcd. for C26 [M+H]

2 2

Cl ): λ (ε, M cm ) = 271 (26300), 331 (12700), 346

Cl

2

, λex = 415

9

2

–

1

nm): λmax = 481 nm; Φ

F

were obtained by a HORIBA Nanolog spectrometer. An absolute

fluorescence quantum yield was obtained by Hamamatsu

+

(

16 2 2 2

H O P S

1

13

31

4

87.0140; found 487.0131. m.p.: >280 °C.

Photonics Quantaurus-QY spectrometer. H, C, and P NMR

spectra were recorded with a JEOL EX-400 spectrometer

1

3

cis-1: H NMR (395.88 MHz, CDCl , 25 °C): d = 7.70 (m, 6H),

1

13

7

.56 (t, J = 7.2 Hz, 2H), 7.45 (m, 6H) and 7.22 (dd, J = 2.4, 4.8

(

operating at 399.65 MHz for H, 100.40 MHz for C, and 161.7

1

3

1

Hz, 2H) ppm; C NMR (100.40 MHz, CDCl

3

, 25 °C): δ = 153.0

MHz for P) by using the residual solvent as the internal

1

(

d, J = 27.9 Hz), 142.1 (d, J = 105 Hz), 137.4 (m), 135.7 (d, J =

reference for H (CDCl Cl

3

: δ = 7.26 ppm, CD

2

: δ = 5.32 ppm, or

1

3

1

10 Hz), 133.0, 131.2 (d, J = 11.6 Hz), 131.0 (d, J = 15.4 Hz),

29.2 (d, J = 107 Hz), 129.2 (d, J = 12.5 Hz), 126.3 (d, J = 14.5

C

2

D

2

Cl : δ = 6.00 ppm), C (CDCl : δ = 77.16 ppm), and

4

3

1

trimethylphosphite as the external reference for P (δ = 140.0

3

1

3

Hz), and 121.8 (t, J = 9.6 Hz) ppm; P NMR (161.7 MHz, CDCl ,

ppm). High-resolution mass spectra (HRMS) were measured on

–

1

–1

2

5 °C): δ = 22.7 ppm. UV/Vis (CH

25000), 331 (12000), 346 (17000) and 404 (7600) nm.

Fluorescence (CH Cl , λex = 415 nm): λmax = 478 nm; Φ = 0.91.

FT-IR (ATR): ν = 1172 (P=O) cm . HRMS (APCI, positive) calcd.

2 2

Cl ): λ (ε, M cm ) = 273

a

Thermo Fischer Scientific EXACTIVE Fourier-transform

(

orbitrap mass spectrometer (APCI and ESI). Single-crystal X-ray

diffraction analysis data for compounds trans-1 and cis-1 were

collected at –150 °C with a Rigaku Saturn70 CCD diffractometer

with graphite monochromated Mo-Kα radiation (0.71069 Å).

Single-crystal X-ray diffraction analysis data for compounds 2a

and 4a were collected at –180 ºC with a Rigaku XtaLAB P200

apparatus using two-dimensional detector PILATUS 100K/R with

Cu-Kα radiation (1.54187 Å). The structures were solved by

direct method (SHELXS-2014). Redox potentials were

measured by cyclic voltammetry and differential pulse

voltammetry method on an ALS electrochemical analyzer model

2

F

–

1

+

16 2 2 2

for C26H O P S [M+H] 487.0140; found 487.0141. m.p.: 191–

1

92 °C.

Dibrominated benzodiphosphole 6: To a solution of trans-1

100 mg, 0.206 mmol) in CHCl /AcOH (v/v = 3/2, 3.43 mL) was

added N-bromosuccinimide (366 mg, 2.06 mmol) and stirred for

4 h at ambient temperature. Then, the reaction was quenched

with acetone and extracted with chloroform (10 mL × 3). The

organic layer was washed with sat. Na CO aq., dried over

Na SO and concentrated in vacuo. The crude was purified by

(

3

2

6

60A. XRD measurements of powders on a quartz plate were

performed on Rigaku MiniFlex600 X-ray diffractometer

2

3

a

2

4

column chromatography (eluent: chloroform) to give

equipped with a X-ray tube using Cu Kα radiation beam with a

benzodiphosphole 6 as a yellow solid quantitatively.

wavelength of 1.54 Å. Diffraction intensities were recorded with

1

–

1

6

4

7

: H NMR (395.88 MHz, CDCl

3

, 25 °C): δ = 7.70 (q, J = 6.9 Hz,

a 0.02° step in θ at scan rate of 0.1º min .

H), 7.61 (t, J = 7.6 Hz, 2H), 7.55 (dd, J = 2.8, 10.4 Hz, 2H),

.50 (td, J = 3.2, 7.6 Hz, 4H), and 7.16 (d, J = 2.4 Hz, 2H) ppm;

Synthesis

1

3

C NMR (100.40 MHz, CDCl , 25 °C): δ = 152.4 (d, J = 25.1 Hz),

Doubly thiophene-fused benzodiphospholes (trans-1 and

cis-1): A solution of thienylbenzene 5 (50.0 mg, 0.0896 mmol) in

THF (2 mL) was added dropwisely to a solution of n-BuLi (1.55

M in hexane, 0.243 mL, 0.376 mmol) in THF (1.8 mL) at –78 ºC.

After stirring for 1 h at –78 ºC, dichlorophenylphosphine (26.7

mL, 0.197 mmol) was added slowly and stirred for 1 h at –78 ºC.

The mixture was allowed to warm to ambient temperature and

then aqueous solution of hydrogen peroxide (30% in water, 1

mL) was added. After stirring for 30 min, the reaction was

quenched with water and extracted with chloroform (10 mL × 3).

1

2

41.0 (d, J = 107 Hz), 136.5 (d, J = 107 Hz), 133.4, 131.2 (d, J =

1.6 Hz), 129.5 (d, J = 13.5 Hz), 128.6 (d, J = 13.5 Hz), 128.5,

27.1 (d, J = 67.4 Hz), 121.9 (t, J = 10.1 Hz), and 117.9 (d, J =

3

1

3

0.2 Hz) ppm; P NMR (161.7 MHz, CDCl , 25 °C): δ = 24.6

–

1

ppm. FT-IR (ATR): ν = 1191 cm (P=O). HRMS (ESI, positive)

+

calcd. for C26

2 2 2 2

H14Br O P S [M+H] 643.8343; found 643.8350.

m.p.: >280 °C.

Phenyl-substituted

benzodiphosphole 6 (18.9 mg, 0.0293 mmol), phenyl boronate

S1a (15.0 mg, 0.0733 mmol), Na CO (12.4 mg, 0.117 mmol)

and Pd(PPh (3.4 mg, 2.93 µmol) was flushed argon and then

THF (0.66 mL) and water (0.33 mL) were added. After stirring for

2a:

A

Schlenk flask containing

2 4

The combined organic layer was dried over Na SO and

concentrated in vacuo. The crude product was purified by

column chromatography (eluent: chloroform) to give trans-1 as

a light yellow solid (7.2 mg, 14.8 µmol, 17% yield) and cis-1 as a

2

3

3 4

)

Chemistry - A European Journal

10.1002/chem.201900661

FULL PAPER

1

4

h at 70 ºC, the mixture was cooled to room temperature and

extracted with CHCl (5 mL × 3). The organic layer was washed

with brine, dried over Na SO and concentrated in vacuo. The

crude product was purified by silica-gel column chromatography

eluent: CHCl ) to give 2a as an orange solid (17.3 mg, 27.1

2c: H NMR (395.88 MHz, CDCl

3

, 25 °C): δ = 7.96 (bs, 4H), 7.83

3

(bs, 2H), 7.77 (m, 4H), 7.71 (m, 2H), 7.63 (m, 2H), and 7.55–

3

1

2

4

7.49 (m, 6H) ppm; P NMR (161.7 MHz, CDCl

3

, 25 °C): δ = 23.1

): λ (ε, M cm ) = 297 (35000), 361

(29000), 373 (34000), 434 (27000), and 456(sh) (22000) nm.

Fluorescence (CH Cl , λex = 415 nm): λmax = 496, 514 nm; Φ

0.28. FT-IR (ATR): ν = 1276 (P=O), 1135 (C–F) cm . HRMS

–

1

–1

2 2

ppm. UV/Vis (CH Cl

(

3

µmol, 88%). Single crystals suitable for X-ray crystallographic

2

F

=

–

1

analysis were obtained by vapor diffusion of acetonitrile into a

+

CHCl

3

solution of 2a.

a: H NMR (395.88 MHz, CDCl

H), 7.66 (dd, J = 3.2, 10.0 Hz, 2H), 7.57 (m, 6H), 7.50 (td, J =

(ESI, positive) calcd. for C42

20 12 2 2 2

H F O P S [M+H] 911.0261;

1

2

4

3

, 25 °C): δ = 7.78 (q, J = 7.2 Hz, found 911.0248. m.p.: >300 °C. Due to low solubility, we could

1

3

not obtain the C NMR spectrum with a sufficient S/N ratio.

1

3

.2, 8.0 Hz, 6H) and 7.38 (m, 6H) ppm; C NMR (100.40 MHz,

, 25 °C): δ = 151.1, 150.8, 137.8, 133.3, 133.1, 131.3,

31.2, 129.4, 129.35, 129.30, 129.0, 128.3, 126.1, and 121.6 (t,

CDCl

3

Styryl-substituted benzodiphosphole 3a:

1

Mizoroki-Heck Reaction: A Schlenk flask containing Pd(OAc)

2

3

1

J = 14.4 Hz) ppm; P NMR (161.7 MHz, CDCl

3

, 25 °C): δ = 23.6

): λ (ε, M cm ) = 286 (27000), 295

29000), 360 (24500), 374 (27000) and 442 (19000) nm.

(0.7 mg, 3.1 µmol) and XPhos (3.0 mg, 6.2 µmol) was flushed

argon and DMF (1.0 mL) was added. After stirring 30 min at

ambient temperature, benzodiphosphole 6 (20.0 mg, 0.031

–

1

–1

ppm. UV/Vis (CH

2

Cl

2

(

Fluorescence (CH

2

Cl

2

, λex = 415 nm): λmax = 525 nm; Φ

F

= 0.50.

3

mmol), styrene (14.2 µL, 0.124 mmol) and NEt (10.8 µL, 0.0775

–

1

FT-IR (ATR): ν = 1207 cm (P=O). HRMS (ESI, positive) calcd.

mmol) were added. After stirring for 20 h at 110 ºC, the mixture

was cooled to room temperature. Water was added to the

+

for

38 24 2 2 2

C H O P S [M+H] 639.0766; found 639.0762. m.p.:

>

280 °C.

mixture and the mixture was extracted with CHCl

The organic layer was washed with brine, dried over Na

and concentrated in vacuo. The crude product was purified by

silica-gel column chromatography (eluent: CHCl ) to give styryl-

substituted 3a as a red solid (5.7 mg, 8.25 µmol, 27%).

Suzuki-Miyaura coupling: Schlenk flask containing

benzodiphosphole 6 (21.2 mg, 0.0329 mmol), styryl boronate

S2a (18.9 mg, 0.0823 mmol), Na CO (13.9 mg, 0.132 mmol)

and Pd(PPh (3.8 mg, 3.29 µmol) was flushed argon and then

3

(5 mL × 3).

2

SO

4

-Dimethylaminophenyl-substituted 2b:

A Schlenk flask

containing benzodiphosphole 6 (30.8 mg, 0.0478 mmol), phenyl

boronate S1b (29.5 mg, 0.119 mmol), Na CO (20.2 mg, 0.191

mmol) and Pd(PPh (5.5 mg, 4.78 µmol) was flushed argon

3

2

3

)

4

A

and then THF (1.2 mL) and water (0.60 mL) were added. After

stirring for 23 h at 70 ºC, the mixture was cooled to room

2

3

temperature and extracted with CHCl

layer was washed with brine, dried over Na

concentrated in vacuo. The crude product was purified by silica-

3

(5 mL × 3). The organic

3 4

)

2

SO and

4

THF (0.66 mL) and water (0.33 mL) were added. After stirring for

3 h at 70 ºC, the mixture was cooled to room temperature and

gel column chromatography (eluent: CHCl

3

) to give 2b as a dark

extracted with CHCl

with brine, dried over Na

crude product was purified by silica-gel column chromatography

(eluent: CHCl ) to give styryl-substituted 3a as a red solid (14.6

3

(5 mL × 3). The organic layer was washed

red solid (23.1 mg, 31.9 µmol, 67%).

2

SO and concentrated in vacuo. The

4

1

2

7

1

b: H NMR (395.88 MHz, C

2

D

2

Cl

4

, 25 °C): δ = 7.76 (m, 4H),

.62–7.57 (m, 4H), 7.52–7.48 (m, 4H), 7.45 (d, J = 8.6 Hz, 4H),

.22 (d, J = 1.8 Hz, 2H), 6.70 (d, J = 9.1 Hz, 4H) and 3.00 (s,

3

mg, 21.1 µmol, 65%).

1

3

1

2H) ppm; C NMR (100.40 MHz, C

2

D

2

Cl

4

, 25 °C): δ = 150.4,

3a: H NMR (395.88 MHz, CD

2

Cl

2

, 25 °C): δ = 7.74 (q, J = 6.4

40.7, 137.4, 130.9, 130.8, 129.1, 129.0, 128.2 (d, J = 29.6 Hz),

Hz, 4H), 7.65 (dd, J = 7.3, 2.7 Hz, 2H), 7.60 (t, J = 6.4 Hz, 2H),

7.5 (m, 8H), 7.3 (m, 6H), 7.21 (d, J = 16.0 Hz, 2H), 7.15 (d, J =

3

1

26.8, 120.7, 120.5, 120.2, 112.1, 99.4, and 40.1 ppm; P NMR

, 25 °C): δ = 23.7 ppm. UV/Vis (CH Cl ): λ (ε,

M cm ) = 264 (36000), 311 (37000), 401 (43000), and 505

33000) nm. Fluorescence (CH Cl , λex = 415 nm): λmax = 650

nm; Φ = 0.51. FT-IR (ATR): ν = 1180 (P=O), 1113 (N–C) cm .

HRMS (ESI, positive) calcd. for

3

1

(

161.7 MHz, CDCl

3

2

1.8 Hz, 2H) and 7.01 (d, J = 16.0 Hz, 2H) ppm; P NMR (161.7

–

1

–1

–

MHz, CD

2

Cl

cm ) = 295 (15000), 306 (19000), 380 (17600), 396 (18000)

and 470 (15000) nm. Fluorescence (CH Cl , λex = 415 nm): λmax

= 558 nm; Φ = 0.27. FT-IR (ATR): ν = 1169 (P=O) cm . HRMS

(ESI, positive) calcd. for C42

91.1072. m.p.: >280 °C. Due to low solubility, we could not

2 2 2

, 25 °C): δ = 22.0 ppm. UV/Vis (CH Cl ): λ (ε, M

1

–1

(

2

–

1

F

2

+

–1

C

42

H

34

N

2

O

2

P

2

S

2

[M+H]

F

+

7

25.1610; found 725.1603. m.p.: >300 °C.

28 2 2 2

H O P S [M+H] 691.1079; found

6

1

3

,5-Bis(trifluoromethyl)phenyl-substituted 2c:

A

Schlenk

obtain the C NMR spectrum with a sufficient S/N ratio.

flask containing benzodiphosphole 6 (19.0 mg, 0.0295 mmol),

phenyl boronate S1c (25.0 mg, 0.0737 mmol), Na CO (12.5 mg, 4-Dimethylaminostyryl-substituted benzodiphosphole 3b: A

.118 mmol) and Pd(PPh (3.4 mg, 2.95 µmol) was flushed

2

3

0

3

)

4

Schlenk flask containing benzodiphosphole 6 (23.4 mg, 0.0363

mmol), 4-dimethylaminostyryl boronate S2b (24.8 mg, 0.0908

argon and then THF (1.2 mL) and water (0.60 mL) were added.

After stirring for 18 h at 70 ºC, the mixture was cooled to room

2 3 3 4

mmol), Na CO (15.4 mg, 0.145 mmol) and Pd(PPh ) (4.2 mg,

temperature and extracted with CHCl

layer was washed with brine, dried over Na

concentrated in vacuo. The crude product was purified by silica-

3

(5 mL × 3). The organic

3.63 µmol) was flushed argon and then THF (0.81 mL) and

water (0.40 mL) were added. After stirring for 3 h at 70 ºC, the

mixture was cooled to room temperature and extracted with

2

SO and

4

gel column chromatography (eluent: CHCl

3

) to give 2c as a dark

CHCl

dried over Na

product was purified by silica-gel column chromatography

3

(5 mL × 3). The organic layer was washed with brine,

red solid (18.6 mg, 20.4 µmol, 69%).

2

SO and concentrated in vacuo. The crude

4

Chemistry - A European Journal

10.1002/chem.201900661

FULL PAPER

(

eluent: CHCl

3

) to give 4-dimethylaminostyryl-substituted 3b as a

dark red solid (22.0 mg, 28.3 µmol, 78%).

4-Nitrophenylethynyl-substituted benzodiphosphole (4d):

1

3

b: H NMR (395.88 MHz, CDCl

3

, 25 °C): δ = 7.75 (m, 4H), 7.52

An orange solid (8.3 mg, 10.7 µmol, 35%).

1

(

m, 8H), 7.34 (d, J = 9.2 Hz, 4H), 7.01 (d, J = 2.4 Hz, 2H), 6.91

4d: H NMR (395.88 MHz, CDCl

3

, 25 °C): δ = 8.22 (d, J = 9.2 Hz,

3

1

d, J = 6.0 Hz, 4H) and 6.68 (d, J = 8.4 Hz, 4H) ppm; P NMR

161.7 MHz, CDCl , 25 °C): δ = 23.5 ppm. UV/Vis (CH Cl ): λ (ε,

4H), 7.75-7.60 (m, 12H), 7.51 (m, 4H) and 7.39 (d, J = 2.3 Hz,

3

1

3

2

2H) ppm; P NMR (161.7 MHz, CDCl

3

, 25 °C): δ = 22.8 ppm.

): λ (ε, M cm ) = 296 (29500), 378(sh) (29500),

396 (34000), 447 (42000) and 469(sh) (36000) nm.

Fluorescence (CH Cl , λex = 415 nm): λmax = 505, 532 nm; Φ

0.30. FT-IR (ATR): ν = 1177 (P=O), 1515 (NO

(APCI, positive) calcd. for C42 [M+H] 777.0467;

found 777.0470. m.p.: >280 °C. Due to low solubility, we could

–

1

–1

M cm ) = 330 (24000), 420 (40000) and 520 (37000) nm.

Fluorescence (CH Cl , λex = 415 nm): λmax = 699 nm; Φ = 0.22.

FT-IR (ATR): ν = 1258 (P=O), 1165 (NMe ) cm . HRMS (APCI,

positive) calcd. for C46 [M+H] 777.1923; found

77.1922. m.p.: >280 °C. Due to low solubility, we could not

2 2

UV/Vis (CH Cl

2

F

–

1

2

F

=

+

–1

H

38

N

2

O

2

P

2

S

2

) cm . HRMS

+

7

22 2 6 2 2

H N O P S

1

3

obtain the C NMR spectrum with a sufficient S/N ratio.

1

3

not obtain the C NMR spectrum with a sufficient S/N ratio.

Typical procedure of Sonogashira coupling with

arylacetylenes: A Schlenk flask containing benzodiphosphole 6

(

19.7 mg, 0.0306 mmol), CuI (0.3 mg, 1.5 µmol) and Pd(PPh

3 4

)

Acknowledgements

1.8 mg, 1.5 µmol) was flushed argon and then toluene (1.1 mL),

triethylamine (0.5 mL) and arylacetylene S3 (0.0673 mmol, 2.2

equiv.) were added. After stirring for 30 min at ambient

temperature, the mixture was stirred for 4 h at 60 ºC. The

mixture was cooled to room temperature, and water was added

This work was supported by the JSPS (KAKENHI Grant

Numbers

JP18K14198(T.H.),

JP18H01943

(K.K.)

and

JP18H03898 (H.I.)) and by MEXT (Grants-in-Aid for Scientific

Research of the Innovative Areas “Photosynergetics”

JP26107004 (K.K.)). The authors thank Prof. Yoshihiro Matano

and Ms. Ayana Wakatsuki for measurement of absolute

fluorescence quantum yields. The authors thank Dr. Takayuki

Tanaka and Prof. Atsuhiro Osuka for assistance with X-ray

crystal structure analysis.

to the mixture. Then the mixture was extracted with CHCl

mL × 3). The organic layer was washed with brine, dried over

Na SO and concentrated in vacuo. The crude product was

3

(10

2

4

3

purified by silica-gel column chromatography (eluent: CHCl ) to

give ethynyl-substituted 4.

Phenylethynyl-substituted benzodiphosphole (4a): A orange

solid (18.9 mg, 26.3 µmol, 86%). Single crystals suitable for X-

ray crystallographic analysis were obtained by vapor diffusion of

Keywords: phosphole • thiophene • near-infrared • fluorescence

•

two-photon absorption

acetonitrile into a CHCl

3

solution of 4a.

a: H NMR (395.88 MHz, CDCl , 25 °C): δ = 7.73 (m, 4H), 7.62

m, 4H), 7.5 (m, 8H), 7.36 (m, 6H) and 7.31 (d, J = 2.8 Hz, 2H)

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(

2 2

CH Cl

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F

–

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C

26

H

16

O

2

P

2

S

2

·2(water),

r

M =

5

6

4

0

1

2

1

8

22.48, orthorhombic, space group Pca2

.7969(14), c = 13.384(3) Å, V = 2371.4(8) Å , ρcalcd = 1.463 g cm , Z =

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Crystallographic data for cis-1:

·3(water), M = 1140.26, triclinic, space group P-

1

(No.29), a = 26.069(5), b =

3

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(No.2), a = 11.9560(19), b = 12.4019(18), c = 19.522(4) Å, α =

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38.63, monoclinic, space group P2

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3

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data for 4a:

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3.983(2), γ = 102.168(2)°, V = 1026.89(5) Å , ρcalcd = 1.496 g cm , Z =

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1

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Cambridge Crystallographic Data Centre.

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We report the synthesis and

T. Higashino*, K. Ishida, T. Sakurai, S.

Seki, T. Konishi, K. Kamada, H. Imahori*

physicochemical properties of doubly

thiophene-fused benzodiphospholes

as a new class of phosphole-based

ladder-type π-conjugated molecules.

Systematic investigation on the

physicochemical properties of doubly

thiophene-fused benzodiphospholes

revealed their pluripotent features:

intense near infra-red fluorescence,

excellent two-photon absorption

property, and remarkably high

Page No. – Page No.

Pluripotent Features of Doubly

Thiophene-fused Benzodiphospholes

as Organic Functional Materials

electron-transporting ability.

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Pluripotent Features of Doubly Thiophene-Fused Benzodiphospholes as Organic Functional Materials

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