Syntheses, X-ray crystal structures, and optical, fluorescence, and nonlinear optical characterizations of diphenylphosphino-substituted bithiophenes

Article abstract of DOI:10.1021/ic201309k

A series of bithiophene derivatives that are either symmetrically disubstituted with two Ph₂(X)P groups (X = O, S, Se) or monosubstituted with one Ph₂(X)P group (X = O, S, Se) and an organic functional group (H, CHO, CH₂OH, CO₂Me) have been synthesized. The X-ray crystal structures of Ph₂(Se)P(C ₄H₂S)₂P(Se)Ph₂, Ph₂ (O)P(C₄H₂S)₂H, Ph₂(S)P- (C ₄H₂S)₂H, and Ph₂(O)P(C ₄H₂S)2CH₂OH exhibit very different solid-state structures depending on the type of intermolecular π-π interactions that occur. The compounds have been characterized by electronic absorption and fluorescence studies. Of particular interest is that the quantum yields of Ph₂(O)P(C₄H₂S)₂H, Ph ₂(O)P(C₄H₂S)₂P(O)Ph₂, Ph₂(O)P(C₄H₂S)2CO₂Me, and Ph ₂(O)P(C₄H₂S)2CH₂OH are significantly larger than that of bithiophene (factors of 13, 14, 14, and 22, respectively). This behavior is quite different from that of analogously substituted terthiophenes in which substitution results in only modest increases in the quantum yields over that of terthiophene (factors of 0.94, 2.7, 1.3, and 1.5, respectively). DFT studies of the emission process suggest that modifying the Ph₂(X)P group affects both the fluorescence and nonradiative rate constants while modifications of the organic substituents primarily affect the nonradiative rate constants. The higher quantum yields of the substituted bithiophenes make them promising for application in organic light-emitting devices (OLED). The optical power limiting (OPL) performances of these Ph ₂(X)P-substituted bithiophenes were evaluated by nonlinear transmission measurements in the violet-blue spectral region (430-480 nm) with picosecond laser pulses. The OPL performances are enhanced by heavier X groups and when by higher solubilities. Saturated chloroform solutions of Ph ₂(O)P(C₄H₂S)₂H and Ph ₂(S)P(C₄H₂S)₂H exhibit significantly stronger nonlinear absorption than any previously reported compounds and are promising candidates for use in broadband optical power limiters.

Full text of DOI:10.1021/ic201309k

Article
pubs.acs.org/IC
Syntheses, X-ray Crystal Structures, and Optical, Fluorescence, and
Nonlinear Optical Characterizations of
Diphenylphosphino-Substituted Bithiophenes
Qun Zhao,*,† Jason L. Freeman,† Jianwei Wang,‡ Yuanli Zhang,‡ Tracy P. Hamilton,†
Christopher M. Lawson,‡ and Gary M. Gray*,†
†Department of Chemistry, University of Alabama, 201 Chemistry Building, 901 14th Street South, Birmingham,
Alabama 35294-1240, United States
‡Department of Physics, University of Alabama, CH 310, 1530 Third Avenue South, Birmingham, Alabama, 35294-1170
S
* Supporting Information
ABSTRACT: A series of bithiophene derivatives that are either
symmetrically disubstituted with two Ph2(X)P groups (X = O,
S, Se) or monosubstituted with one Ph2(X)P group (X = O,
S, Se) and an organic functional group (H, CHO, CH2OH,
CO2Me) have been synthesized. The X-ray crystal structures of
Ph2(Se)P(C4H2S)2P(Se)Ph2, Ph2(O)P(C4H2S)2H, Ph2(S)P-
(C4H2S)2H, and Ph2(O)P(C4H2S)2CH2OH exhibit very different
solid-state structures depending on the type of intermolecular
π−π interactions that occur. The compounds have been characterized by electronic absorption and fluorescence studies. Of
particular interest is that the quantum yields of Ph2(O)P(C4H2S)2H, Ph2(O)P(C4H2S)2P(O)Ph2, Ph2(O)P(C4H2S)2CO2Me,
and Ph2(O)P(C4H2S)2CH2OH are significantly larger than that of bithiophene (factors of 13, 14, 14, and 22, respectively). This
behavior is quite different from that of analogously substituted terthiophenes in which substitution results in only modest
increases in the quantum yields over that of terthiophene (factors of 0.94, 2.7, 1.3, and 1.5, respectively). DFT studies of
the emission process suggest that modifying the Ph2(X)P group affects both the fluorescence and nonradiative rate constants
while modifications of the organic substituents primarily affect the nonradiative rate constants. The higher quantum yields of
the substituted bithiophenes make them promising for application in organic light-emitting devices (OLED). The optical
power limiting (OPL) performances of these Ph2(X)P-substituted bithiophenes were evaluated by nonlinear transmission
measurements in the violet-blue spectral region (430−480 nm) with picosecond laser pulses. The OPL performances are
enhanced by heavier X groups and when by higher solubilities. Saturated chloroform solutions of Ph2(O)P(C4H2S)2H and
Ph2(S)P(C4H2S)2H exhibit significantly stronger nonlinear absorption than any previously reported compounds and are
promising candidates for use in broadband optical power limiters.
the nonlinear absorption was in the blue spectral region, and
both the intensity and wavelength of the NLO absorption
could be tuned by changing either the X group or other
substituent on the terthiophene. Nonetheless, because of the
significant linear absorptions of these compounds in the blue
spectral region, rather dilute dichloromethane solutions (3.5 ×
10−3 mol/L) had to be used, which in turn resulted in relatively
weak NLO absorption.
To avoid the linear absorption issues encountered with the
Ph2(X)P-substituted terthiophenes, we investigated closely re-
lated Ph2P(X)-substituted bithiophenes because these com-
pounds should have higher linear transmittances in the violet-
blue region of interest.25 We recently reported that saturated
dichloromethane solutions of two of these compounds, Ph2(X)-
P(C4H2S)2P(X)Ph2 (X = O (2), S (3)) (chart 1) exhibited
INTRODUCTION
■
Materials exhibiting strong third-order nonlinear optical
(NLO) absorptions have attracted considerable interest be-
cause of their potential applications in optical switching,1,2
three-dimensional (3D) fluorescence imaging,3 3D optical
data storage,4 3D lithographic microfabrication,5,6 and optical
power limiting.7−12 Of particular interest for many of these
applications are materials exhibiting multiphoton absorption,
and the development of such materials has been recently
reviewed by Marder,13−15 Belfield,16,17 and He.18
For power limiting applications, compounds with strong
NLO absorption in all areas of the visible spectrum19−22 are
needed, and until very recently, few NLO absorbers in the
violet and blue spectral regions were known.23 In early 2011,
we reported that dichloromethane solutions of Ph2(X)P-
substituted terthiophenes (X = O, S, Me+I−) exhibited
promising NLO absorption ranging from 450 to 532 nm with
picosecond laser pulses.24 The results were promising because
Received: June 17, 2011
Published: February 9, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
unless otherwise indicated. Urea hydrogen peroxide (1 g tablets) was
purchased from Acros Organics. Tetrahydrofuran (THF) was dried
over MgSO4, refluxed over sodium/benzophenone, and then distilled
prior to use. Compound 1 was made by a modification of a literature
procedure.28 Compounds 2 and 3 were prepared using our previously
reported procedures.25
Chart 1. Chemical Structures of the Ph2(X)P-Substituted
Bithiophenes
One-dimensional multinuclear 1H NMR, 31P{1H} NMR, and
13C{1H} NMR spectra were recorded on a Bruker DRX-400 NMR
spectrometer. The 31P{1H} NMR spectra were referenced to external
85% phosphoric acid (H3PO4) in a coaxial tube that also contained
1
chloroform-d. The H NMR and 13C{1H} NMR spectra were ref-
erenced to internal tetramethylsilane (TMS). Two-dimensional COSY
(correlated spectroscopy), HSQC (heteronuclear single quantum
coherence spectroscopy), and HMBC (heteronuclear multiple bond
correlation) were recorded on a Bruker DPX-300 NMR spectro-
meter. Microanalyses for % C, H, and N were performed by Atlantic
Microlab, Inc.
stronger NLO absorption than the analogously substituted ter-
thiophenes between 420 and 480 nm.26 Moreover, the X group
has a significant effect on the nonlinear absorption with 3
exhibiting stronger NLO absorption than 2 at all wavelengths.
These preliminary studies suggest that NLO absorptions may
be due to two-photon absorption with ps laser pulses.
5,5′-Bis(diphenylselenidophosphino)-2, 2′-bithiophene, Ph2(Se)-
P(C4H2S)2P(Se)Ph2, 4. Excess gray selenium (0.22 g, 2.8 mmol)
was added to a solution of compound 1 (0.50 g, 0.94 mmol) in dry
CH2Cl2 (30 mL), and the mixture was stirred at room temperature
overnight. The resulting solution was evaporated under reduced pres-
sure to give a residue, which was purified by column chromatography
on silica gel (1:1 CHCl3/hexanes) to yield analytically pure 4 as a pale
yellow crystalline solid (0.58 g, 89%). 31P{1H} NMR (CDCl3): 23.250
(s), 23.251 (d, |1JPSe| 739 Hz). 1H NMR (CDCl3): 7.80−7.74 (m, 8H,
In addition to their promising nonlinear optical properties,
the fluorescence spectra of the Ph2P(X)-substituted terthio-
phenes and bithiophenes are also of great interest. Our
previous studies showed that the Ph2P(O)-substituted
terthiophenes exhibit strong fluorescence emission and wide
band gaps, making them promising candidates as materials for
blue organic light emitting devices (OLED).24 The parent
compounds, Ph2P-substitued bithiophenes and terthiophenes,
were reported to have quite different ground and first singlet
excited states,25 but it is unknown whether the Ph2P(X)-
substitution could have different effects on the fluorescent
properties in these two classes of compounds. In our preli-
minary study the substituted bithiophene 2 exhibits strong
fluorescence in the violet-blue spectral region,26 which was
surprising because the quantum yield of bithiophene is only
20% of that of terthiophene. However, a systematic investi-
gation of the influence of variation of the Ph2P(X) groups
and other α-substituents on the fluorescent properties of the
substituted bithiophenes is necessary to develop compounds
with optimal fluorescent properties.
The NLO studies described above suggest that the optical
limiting performances of the NLO absorbers could be further
improved either by attaching even heavier chalcogens to the
Ph2P(X) substituent or by increasing the solubilities of the
molecules. To test these possibilities and to gain more insight
into the factors that might affect the fluorescent properties of
the Ph2P(X)-substituted bithiophenes, we have synthesized and
characterized a bithiophene derivative that is symmetrically
disubstituted with two Ph2(Se)P groups, 4, and bithiophene
derivatives that are substituted with one Ph2(X)P group (X = O
(6), S (7), Se (8)) or with one Ph2(O)P group and an organic
group (CHO (9), CH2OH (10), CO2Me (11)) at the 5- and
5′-positions (Chart 1). Absorption and fluorescence spectra,
NLO absorptions of solutions of these compounds in the
violet-blue spectral region and structure−property relations
for the linear and NLO properties of these compounds are
reported. To gain additional insight into these structure−
property relationships, density functional computational studies
have been carried out, and the X-ray crystal structures of the
compounds, 4, 6, 7, and 10 have also been determined.
HPh‑o), 7.54−7.44 (m, 12H, HPh‑p and HPh‑m), 7.31 (dd, 2H, |3JPH
|
8.3 Hz, |3JHH| 3.8 Hz, HTh‑4), 7.24 (dd, 2H, |4JPH| 1.8 Hz, |3JHH| 3.8 Hz,
HTh‑3). 13C{1H} NMR (CDCl3): δ 144.98 (d, |3JCP| 7 Hz, CTh‑2),
138.20 (d, |2JCP| 8 Hz, CTh‑4), 134.74 (d, |1JCP| 82 Hz, CTh‑5), 132.20 (d,
|1JCP| 81 Hz, CPh‑i), 132.28 (d, |2JCP| 11 Hz, CPh‑o and CPh‑p), 128.83 (d,
|3JCP| 14 Hz, CPh‑m), 126.07 (d, |3JCP| 13 Hz, CTh‑3). UV−vis (CHCl3):
λmax/nm (ε/dm3·mol−1·cm−1) = 348 (2.25 × 104). Anal. Calcd for
[C32H24P2S2Se2]: C, 55.50; H, 3.49. Found: C, 55.11; H, 3.45.
5-(Diphenylphosphino)-2,2′-bithiophene, Ph2P(C4H2S)2H, 5.
Although the synthesis of 5 was reported in the literature,28 we pre-
pared 5 by a different method. A solution of 2,2′-bithiophene (2.0 g,
12 mmol) in dry THF (50 mL) was cooled in an acetone/dry ice slush
bath (−78 °C), and n-BuLi (1.6 M in hexane, 7.5 mL, 12 mmol)
was added dropwise to the solution. The addition was completed in
∼20 min, during which time the color of the solution darkened to a
deep green. The resulting solution was warmed to room temperature
for 0.5 h and then cooled again in an ice bath (0 °C). Then PPh2Cl
(2.5 mL, 13 mmol) was added dropwise with a syringe. The reaction
mixture was gradually brought to room temperature and stirred for
2 h. A few drops of degassed water were added to decompose any
remaining lithium salt, and then the solvent was removed in vacuo,
yielding a viscous oily residue. This residue was purified by column
chromatography under a nitrogen atmosphere by degassing the eluent
(hexanes−diethyl ether, 10:1) and the silica slurry. The resulting crude
product was further purified by multiple precipitations from CH2Cl2
with hexanes yielding a colorless oil (3.2 g, 76%). 31P{1H} NMR
(CDCl3): −17.62 (s).
5-Diphenyloxophosphino-2,2′-bithiophene, Ph2(O)P(C4H2S)2H,
6. A 30% aqueous solution of hydrogen peroxide (1.5 mL) was
added to a solution of 5 (1.00 g, 2.85 mmol) in acetone/CH2Cl2
(1:1, 50 mL). The reaction mixture was stirred at room tem-
perature overnight and then was evaporated to dryness under re-
duced pressure. The residue was purified by column chromatography
on silica gel (1:1 hexanes-ethyl acetate) to yield analytically pure 6 as a
white crystalline solid (0.83 g, 81%). 31P{1H} NMR (CDCl3): 22.64
(s). 1H NMR (CDCl3): 7.80−7.74 (m, 4H, HPh‑o), 7.58−7.46 (m, 6H,
HPh‑p and HPh‑m), 7.34 (dd, 1H, |3JPH| 7.5 Hz, |3JHH| 3.7 Hz, HTh‑4), 7.25
(d, 1H, |3JHH| 5.1 Hz, HTh‑5′), 7.22−7.20 (m, 2H, HTh‑3 and HTh‑3′),
7.00 (dd, 1H, |3JHH| 5.1 Hz, |3JHH| 3.7 Hz, HTh‑4′). 13C{1H} NMR
(CDCl3): δ 146.16 (d, |3JCP| 5 Hz, CTh‑2), 137.73 (d, |2JCP| 9 Hz, CTh‑4),
135.88 (d, |4JCP| 2 Hz, CTh‑2′), 132.77 (d, |1JCP| 110 Hz, CPh‑i), 132.12
(d, |1JCP| 111 Hz, CTh‑5), 132.37 (d, |4JCP| 3 Hz, CPh‑p), 131.87 (d, |2JCP
|
EXPERIMENTAL SECTION
Materials and Spectroscopic Characterization. All reagents
were reagent grade quality, purchased commercially, and used as such
■
11 Hz, CPh‑o), 128.66 (d, |3JCP| 13 Hz, CPh‑m), 128.17 (s, CTh‑4′), 126.08
(s, CTh‑5′), 125.32 (s, CTh‑3′), 124.59 (d, |3JCP| 13 Hz, CTh‑3). UV−vis
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Inorganic Chemistry
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Table 1. Data Collection Parameters for X-ray Structure Determination
4
6
7
10
CCDC number
empirical formula
formula weight
wavelength (Å)
crystal system
space group
a (Å)
823524
823525
823522
C20H15PS3
382.47
823523
C21H17O2PS2
396.44
C32H24P2S2Se2
692.49
C20H15OPS2
366.41
0.71073
0.71073
triclinic
0.71073
triclinic
0.71073
monoclinic
P2(1)/n
9.4366(19)
11.474(2)
17.698(4)
90°
102.12(3)
90°
1873.6(6)
4
triclinic
P1
̅
P1
̅
P1
̅
9.565(2)
9.6172(19)
16.930(3)
85.68(3)
74.46(3)
78.10(3)
1467.9(5)
2
6.2348(12)
8.5970(17)
17.503(4)
79.99(3)
83.97(3)
70.75(3)
871.1(3)
2
9.4687(19)
9.6133(19)
21.395(4)
82.00(3)
80.68(3)
78.17(3)
1869.6(6)
4
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
volume (Å3)
Z
density (calculated) (mg/m3)
1.567
1.397
1.359
1.405
−1
abs coeff (mm )
2.791
0.401
0.480
0.382
F(000)
692
380
792
824
cryst size (mm)
0.9 × 0.6 × 0.3
0.6 × 0.3 × 0.2
2.37−24.97
−7 ≤ h ≤ 1
−10 ≤ k ≤ 10
−20 ≤ l ≤ 20
3988
0.5 × 0.4 × 0.3
2.18−24.97
−11 ≤ h ≤ 11
−1 ≤ k ≤ 11
−25 ≤ l ≤ 25
7872
0.6 × 0.4 × 0.4
2.13−24.99
−11 ≤ h ≤ 11
−1 ≤ k ≤ 13
−1 ≤ l ≤ 21
4036
θ range for data collection (deg) 2.16−24.97
index ranges
−11 ≤ h ≤ 0
−11 ≤ k ≤ 11
−20 ≤ l ≤ 19
5517
reflns collected
independent reflns
5176
3069
6591
3299
[R(int) = 0.0206]
100.0%
[R(int) = 0.0350]
100.0%
[R(int) = 0.0402]
100.0%
[R(int) = 0.0242]
99.9%
completeness to θ = 22.48°
abs correction
ψ-scan
none
none
none
max. and min transmission
refinement method
1.0000 and 0.3596
full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2
data/restraints/params
GOF on F2
5176/0/344
1.060
3069/0/218
1.053
6591/150/470
1.059
3299/0/240
1.091
final R indices [I > 2σ(I)]
R1 = 0.0458
wR2 = 0.1199
R1 = 0.0700
wR2 = 0.1335
0.694 and −0.892
R1 = 0.0537
wR2 = 0.1578
R1 = 0.0658
wR2 = 0.1690
0.682 and −0.522
R1 = 0.0480
wR2 = 0.1291
R1 = 0.0698
wR2 = 0.1410
0.329 and −0.494
R1 = 0.0410
wR2 = 0.1106
R1 = 0.0557
wR2 = 0.1233
0.467 and −0.279
R indices (all data)
largest diff. peak/hole (eÅ−3)
(CHCl3): λmax/nm (ε/dm3·mol−1·cm−1) = 321 (1.75 × 104). Anal.
Calcd for [C20H15OPS2]: C, 65.55; H, 4.13. Found: C, 65.62; H, 3.95.
5-Diphenylsulfidophosphino-2,2′-bithiophene, Ph2(S)P-
(C4H2S)2H, 7. Elemental sulfur (0.16 g, 5.0 mmol) was added to a
solution of 5 (1.0 g, 2.9 mmol) in CH2Cl2 (50 mL). The reaction
mixture was stirred at room temperature overnight and then was eva-
porated under reduced pressure. The residue was purified by column
chromatography on silica gel. First, the excess sulfur was removed by
elution with hexanes, and then elution with 10:1 hexanes/Et2O yielded
analytically pure 7 as a white crystalline solid (0.95 g, 87%). 31P{1H}
chromatography on silica gel (1:1 hexanes/Et2O) yielding analytically
pure 8 as a white crystalline solid (0.45 g, 82%). 31P{1H} NMR
1
(CDCl3): 23.28 (s), 23.24 (d, |1JPSe| 735 Hz). H NMR (CDCl3):
7.83−7.77 (m, 4H, HPh‑o), 7.54−7.44 (m, 6H, HPh‑p and HPh‑m), 7.33
(dd, 1H, |3JPH| 8.6 Hz, |3JHH| 3.8 Hz, HTh‑4), 7.27 (d, 1H, |3JHH| 5.1 Hz,
HTh‑5′), 7.22−7.20 (m, 2H, HTh‑3 and HTh‑3′), 7.01 (dd, 1H, |3JHH| 5.1
Hz, |3JHH| 3.8 Hz, HTh‑4′). 13C{1H} NMR (CDCl3): δ 146.96 (d,
|3JCP| 5 Hz, CTh‑2), 138.41 (d, |2JCP| 9 Hz, CTh‑4), 136.08 (s, CTh‑2′),
132.62 (d, |1JCP| 82 Hz, CPh‑i), 132.36 (d, |1JCP| 84 Hz, CTh‑5), 132.36
(d, |2JCP| 12 Hz, CPh‑o), 132.06 (d, |4JCP| 3 Hz, CPh‑p), 128.78 (d, |3JCP
|
1
NMR (CDCl3): 34.68 (s). H NMR (CDCl3): 7.83−7.77 (m, 4H,
13 Hz, CPh‑m), 128.28 (s, CTh‑4′), 126.20 (s, CTh‑5′), 125.40 (s, CTh‑3′),
124.74 (d, |3JCP| 13 Hz, CTh‑3). UV−vis (CHCl3): λmax/nm
(ε/dm3·mol−1·cm−1) = 329 (1.92 × 104). Anal. Calcd for
[C20H15PS2Se]: C, 55.94; H, 3.52. Found: C, 55.93; H, 3.42.
HPh‑o), 7.54−7.44 (m, 6H, HPh‑p and HPh‑m), 7.322 (dd, 1H, |3JPH
|
8.3 Hz, |3JHH| 3.8 Hz, HTh‑4), 7.26 (d, 1H, |3JHH| 5.1 Hz, HTh‑5′), 7.21−
7.17 (m, 2H, HTh‑3 and HTh‑3′), 7.01−6.99 (m, 1H, HTh‑4′). 13C{1H}
NMR (CDCl3): δ 146.68 (d, |3JCP| 5 Hz, CTh‑2), 137.90 (d, |2JCP| 9 Hz,
CTh‑4), 136.10 (s, CTh‑2′), 133.62 (d, |1JCP| 89 Hz, CTh‑5), 133.61 (d,
|1JCP| 90 Hz, CPh‑i), 132.09 (s, CPh‑p), 131.99 (d, |2JCP| 11 Hz, CPh‑o),
128.80 (d, |3JCP| 13 Hz, CPh‑m), 128.30 (s, CTh‑4′), 126.18 (s, CTh‑5′),
125.38 (s, CTh‑3′), 124.70 (d, |3JCP| 13 Hz, CTh‑3). UV−vis (CHCl3):
λmax/nm (ε/dm3·mol−1·cm−1) = 328 (2.01 × 104). Anal. Calcd for
[C20H15PS5]: C, 62.80; H, 3.95. Found: C, 62.96; H, 3.78.
5-Diphenyloxophosphino-2, 2′-bithiophene-5′-carbaldehyde,
Ph2(O)P(C4H2S)2CHO, 9. A solution of 6 (1.00 g, 2.73 mmol)
in anhydrous DMF (2.00 mL) stirred as POCl3 (1.25 mL,
13.6 mmol) was added dropwise. The reaction mixture was heated
at 80 °C under nitrogen for 36 h and then was poured slowly into
100 mL of saturated aqueous sodium acetate solution. This mixture
was stirred for 2 h, and then the organic layer was diluted with
CH2Cl2, separated, and washed with water. After evaporation of
the solvents, the solid residue was purified with silica gel flash
chromatography (ethyl acetate) to yield analytically pure 9 as a yellow
5-Diphenylselenidophosphino-2,2′-bithiophene, Ph2(Se)P-
(C4H2S)2H, 8. Gray selenium (0.12 g, 1.5 mmol) was added to a
solution of 5 (0.45 g, 1.3 mmol) in CH2Cl2 (30 mL). The reaction
mixture was stirred at room temperature overnight and then was eva-
porated under reduced pressure. The residue was purified by column
1
product (0.92 g, 83%). 31P{1H} NMR (CDCl3): 22.53 (s). H NMR
(CDCl3): 9.87 (s, 1H, HCHO), 7.80−7.74 (m, 4H, HPh‑o), 7.68 (d, 1H,
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Inorganic Chemistry
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|3JHH| 3.9 Hz, HTh‑3′ or HTh‑4′), 7.61−7.57 (m, 2H, HPh‑p), 7.53−7.48
(m, 4H, HPh‑m), 7.40−7.38 (m, 2H, HTh‑3 or HTh‑4), 7.29 (d, 1H,
|3JHH| 3.9 Hz, HTh‑3′ or HTh‑4′). 13C{1H} NMR (CDCl3): δ 182.69
(s, CCHO), 145.17 (s, CTh‑2′ or CTh‑5′), 144.24 (d, |3JCP| 6 Hz, CTh‑2),
143.19 (s, CTh‑2′ or CTh‑5′), 137.72 (d, |2JCP| 9 Hz, CTh‑4 or CTh‑3),
137.19 (s, CTh‑3′ or CTh‑4′), 135.72 (d, |1JCP| 108 Hz, CTh‑5), 132.39
(d, |1JCP| 111 Hz, CPh‑i), 132.65 (d, |4JCP| 3 Hz, CPh‑p), 131.91 (d,
polarization effects. The data for 4 was corrected for absorption using
a ψ-scan with four reflections with χ ≥ 80°.
Crystallographic calculations were performed with the Siemens
SHELXTL-PC program package.30 All heavy atom positions were
located using direct methods. Full-matrix refinements of the positional
and anisotropic thermal parameters for all non-hydrogen atoms versus
F2 were carried out. All hydrogen atoms were placed in calculated
positions with the appropriate molecular geometry and δ(C−H) =
0.96 Å. The isotropic thermal parameter of each hydrogen atom was
fixed equal to 1.2 times the Ueq value of the atom to which it was
bound. The unsubstituted thienyl group of 7 shows positional disorder
with the one thienyl group being related to another via a 180° rotation
about the C−C bond between the two groups. The disorder was
modeled with the help of similarity restraints and rigid bond con-
straints. The occupancy was refined to a free-variable value of 0.610.
Crystallographic data for all crystals have been deposited with the
Cambridge Crystallographic Database (4, CCDC 823524; 6, CCDC
823525; 7, CCDC 823522; 10, CCDC 823523).
|2JCP| 11 Hz, CPh‑o), 128.83 (d, |3JCP| 13 Hz, CPh‑m), 126.86 (d, |2JCP
|
13 Hz, CTh‑4 or CTh‑3), 125.84 (s, CTh‑3′ or CTh‑4′). UV−vis (CHCl3):
λmax/nm (ε/dm3·mol−1·cm−1) = 356 (2.63 × 104). Anal. Calcd for
[C21H15O2PS2]: C, 63.94; H, 3.83. Found: C, 63.79; H, 3.67.
5′-Hydroxymethyl-5-diphenyloxophosphino-2,2′-bithio-
phene, Ph2(O)P(C4H2S)2CH2OH, 10. Sodium borohydride (0.080 g,
2.1 mmol) was added to a solution of 9 (0.40 g, 1.01 mmol) in dry
THF (10 mL) under nitrogen at ambient temperature. The reaction
mixture was stirred overnight, and then, the THF was evaporated. The
resultant residue was recrystallized from ethyl acetate affording 10 as
an analytically pure white crystalline powder (0.32 g, 80%). 31P{1H}
1
Linear Absorption and Solution Phase Fluorescence
Measurements. Linear absorption spectra for all compounds in
3.0 × 10−4 mol/L chloroform solutions were recorded on a Varian
Cary-100 UV−visible spectrophotometer in a 1-mm cuvette. Room-
temperature emission spectra were measured on a Cary Eclipse Fluorom-
eter. Each solution, in 10-mm path length quartz cell, was excited at
the absorption wavelength maximum, and all the fluorescence spectra
were recorded with a constant slit width of 5 nm. The optical den-
sities at the excitation wavelengths were kept below 0.1 to avoid any
inner-filter effects. All solutions were prepared using Optima grade
chloroform, and the solutions were deoxygenated by bubbling N2
through them for approximately five minutes. The emission quantum
yields of the samples were determined by a comparative method31
using eq 1
NMR (CDCl3): 23.24 (s). H NMR (CDCl3): 7.76−7.71 (m, 4H,
HPh‑o), 7.59−7.55 (m, 2H, HPh‑p), 7.50−7.46 (m, 4H, HPh‑m), 7.23 (dd,
1H, |3JPH| 7.6 Hz, |3JHH| 3.8 Hz, HTh‑4), 7.10−7.08 (m, 1H, HTh‑3), 6.99
(d, 1H, |3JHH| 3.6 Hz, HTh‑3′), 6.82 (d, 1H, |3JHH| 3.6 Hz, HTh‑4′), 4.78
(s, 2H, HCH2), 3.68 (br, 1H, HOH). 13C{1H} NMR (CDCl3): δ 146.53
(d, |3JCP| 5 Hz, CTh‑2), 146.42 (s, CTh‑2′ or CTh‑5′), 137.74 (d, |2JCP
|
10 Hz, CTh‑4), 135.17 (s, CTh‑2′ or CTh‑5′), 133.17 (d, |1JCP| Hz, CPh‑i),
132.33 (d, |4JCP| 2 Hz, CPh‑p), 131.79 (d, |2JCP| 11 Hz, CPh‑o), 131.26 (d,
|1JCP| 112 Hz, CTh‑5), 128.59 (d, |3JCP| 13 Hz, CPh‑m), 125.51 (s, CTh‑3′
or CTh‑4′), 124.87 (s, CTh‑3′ or CTh‑4′), 124.14 (d, |3JCP| 13 Hz, CTh‑3),
59.81 (s, CCH2). UV−vis (CHCl3): λmax/nm (ε/dm3·mol−1·cm−1) =
329 (1.98 × 104). Anal. Calcd for [C21H17O2PS2]: C, 63.62; H, 4.32.
Found: C, 63.85; H, 4.22.
Methyl 5-(Diphenyloxophosphino)-2,2′-bithiophene-5′-car-
boxylate, Ph2(O)P(C4H2S)2CO2Me, 11. Molecular iodine I2 (3 equiv)
and K2CO3 (3 equiv) were added to a solution of 10 (0.21 mg,
0.54 mmol) in dry CH3OH (0.5 mL) and dry ClCH2CH2Cl (2 mL)
under a nitrogen atmosphere. The reaction mixture was refluxed at
70 °C for 20 h before it was quenched with saturated aquaous Na2SO3
at 0 °C. This mixture was extracted with CH2Cl2; the organic layer was
dried and evaporated to dryness, and the crude product was purified
with flash chromatography on silica gel (ethyl acetate) to yield analy-
tically pure product as a white crystalline powder (0.18 mg, 80%).
31P{1H} NMR (CDCl3): 22.55 (s). 1H NMR (CDCl3): 7.80−7.74 (m,
2
I OD
n
n
S
R
S
Φ = Φ
S
R
2
I
OD
S
R
(1)
R
where the subscripts S and R refer to the sample and reference solutions
respectively, I is the integrated emission intensity, OD is the optical
density at the excitation wavelength, and n is the refractive index of the
solvent. A degassed quinine sulfate solution in 0.1 M H2SO4 (Φ = 0.53 at
350 nm excitation)32 was the reference.
Linear and Nonlinear Optical Transmission Measurements.
The linear transmission measurements of 1.66 mol/L chloroform
solutions of compounds 6−8 in 1-mm cuvettes were performed on a
Shimadzu UV-3101PC Spectrophotometer. The NLO transmission
measurements were performed at 430, 450, and 480 nm, respectively,
on the same solutions in the same cuvettes. NLO transmission meas-
urements on 2−4, which are considerably less soluble, were performed
at the same wavelengths using 0.067 mol/L CH2Cl2 solutions. A
wavelength tunable laser system, consisting of an EKSPLA PG401
optical parametric generator (OPG) pumped by a PL2143A/ss model-
locked Nd:YAG laser with a pulse width of 27 ps and a repetition rate
of 10 Hz, was used as the light source. Laser radiation energy
was gradually changed by using a motorized attenuator based on a
combination of a half-wave plate and a Glan-Taylor prism. Part of the
laser beam was then reflected by a beam splitter to a reference detector
to monitor the incident energy. The remaining beam was focused by a
500 mm lens to the center of a 1-mm sample cell (Spectrosil 21-Q-1,
Starna Cells, Inc.). The diameters of the focused beams were 94, 98,
and 98 μm at 430, 450, and 480 nm, respectively. The transmit-
ted signal energy and the reference energy were monitored by two
Molectron J4−09 pyroelectric joule meters.
4H, HPh‑o), 7.68 (d, 1H, |3JHH| 4.0 Hz, HTh‑4′), 7.59−7.56 (m, 2H,
HPh‑p), 7.52−7.48 (m, 4H, HPh‑m), 7.38 (dd, 1H, |3JPH| 7.3 Hz, |3JHH
|
3.7 Hz, HTh‑4), 7.32 (dd, 1H, |3JHH| 3.8 Hz, |4JPH| 1.9 Hz, HTh‑3), 7.18
(d, 1H, |3JHH| 4.0 Hz, HTh‑3′), 3.88 (s, 3H, HCH3). 13C{1H} NMR
(CDCl3): δ 162.32 (s, CTh‑2′ or CTh‑5′), 144.68 (d, |4JCP| 6 Hz, CTh‑2),
142.4 (s, CTh‑2′ or CTh‑5′), 137.69 (d, |2JCP| 9 Hz, CTh‑4), 134.54 (d,
|1JCP| 109 Hz, CTh‑5), 134.33 (s, CTh‑4′), 133.18 (d, |1JCP| 15 Hz, CPh‑i),
133.55 (d, |4JCP| 3 Hz, CPh‑p), 131.90 (d, |2JCP| 11 Hz, CPh‑o), 128.77 (d,
|3JCP| 13 Hz, CPh‑m), 126.05 (d, |3JCP| 13 Hz, CTh‑3), 125.47 (s, CTh‑3′),
52.44 (s, CCH3). UV−vis (CHCl3): λmax/nm (ε/dm3·mol−1·cm−1) =
340 (2.77 × 104). Anal. Calcd for [C22H17O3PS2]: C, 62.25; H, 4.04.
Found: C, 62.55; H, 4.22.
X-ray Data Collection and Solution. Suitable single crystals of 4
(slow diffusion of Et2O into a concentrated CH2Cl2 solution), 6 (slow
diffusion of hexanes into a concentrated CH2Cl2 solution), 7 (slow
diffusion of hexanes into a concentrated Et2O solution), and 10 (slow
evaporation of an ethyl acetate solution) were attached to glass fibers
with epoxy cement and then were aligned upon an Enraf-Nonius
CAD4 single-crystal diffractometer under aerobic conditions. Standard
peak search and automatic indexing routines followed by least-squares
fits of 25 accurately centered reflections resulted in accurate unit cell
parameters. The space groups of the crystals were assigned on the
basis of systematic absences and intensity statistics. All data collections
were carried out using the CAD4-PC software,29 and details of the
data collections are given in Table 1. The analytical scattering factors
of the complex were corrected for both Δf′ and iΔf″ components
of anomalous dispersion. All data were corrected for Lorentz and
Computational Methods. The ground state geometries of bithio-
phene (T2), terthiophene (T3), 2−3, 6−7, and 9−11 were optimized
at the B3LYP/6-31G(d) level of theory using the Spartan ’08 software
package.33 Spartan ’08 was used to plot the highest occupied molec-
ular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO). The starting geometries for 2, 3, 6, 7, and 10 were the
experimental X-ray diffraction structures.34 Optimized geometries
of 9 and 11 were global minima when the sulfur atoms were anti
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Scheme 1. Syntheses of Derivatives of 5,5′-Bis(Diphenylphosphino)-2,2′-bithiophene (1)
in the bithiophene moiety. Emission spectra were computed
using Gaussian ’03 for Windows35 by the following procedure,
which gave very good agreement between theory and experiment
for closely related compounds.36,37 The geometry of the first
excited singlet state was optimized using the CIS/6-31G(d) level of
theory. Time dependent DFT (based on B3LYP/6-31G(d)) was
then used to calculate the emission spectra at the CIS/6-31G(d)
geometry.
iodine and K2CO3 yielded the corresponding methyl ester, 11.
To obtain 11 in high yield, it is important to use only a small
amount of CH3OH (0.5 mL).38
All the new compounds have been characterized by
1
31P{1H}, 13C{1H}, and H NMR spectroscopy. The 13C{1H}
and 1H NMR spectra were assigned using a combination
of two-dimensional COSY, HSQC, and HMBC techniques.
The 31P{1H} NMR resonance of 1 and 5 in CDCl3 are singlets
(−17.60 and −17.62 ppm, respectively) that are typical for
diphenyl-phosphines with 2-thienyl substituents.28 The
31P{1H} NMR resonances for 6, 7, and 8 at δ 22.64, 34.68,
and 23.28 ppm, respectively, are downfield of that of 5, while
the 31P{1H} NMR resonances of 2, 3, and 4 at δ 22.54, 34.64,
and 23.25 ppm, respectively, are downfield of that of 1.
The significantly downfield chemical shifts of the 31P{1H}
NMR resonances of the Ph2(X)P (X = O, S, Se) substituted
bithiophenes relative to that of the parent compounds indicate
that coordination of the lone pair of phosphorus causes a
significant decrease in the electron density at the phospho-
rus and that each substituent has a significantly different effect.
The similarity of the chemical shifts of 31P{1H} NMR reso-
nances for the disubstituted bithiophenes and the monosub-
stituted bithiophenes with the same Ph2(X)P groups (X = O,
S, Se), indicates that the introduction of a second Ph2(X)P
group at the 5′-position has no significant effect on the local
magnetic field at the phosphorus atom.
RESULTS AND DISCUSSION
■
Syntheses and NMR Spectroscopic Characterization.
The syntheses of the two starting materials, 1 and 5, had quite
different levels of difficulty. A modification of a literature pro-
cedure was used to prepare 1.28 In contrast, the synthesis of
5 was a challenging undertaking. Previous reports claimed
that the selective monolithiation of bithiophene was impossible
despite using a wide range of stoichiometries and reaction
conditions.28 We have found that this is not the case that 5 can
be prepared by first lithiating bithiophene with n-butyllithium
and then quenching with chlorodiphenylphosphine. The crude
5 was purified by column chromatography on silica gel under a
nitrogen atmosphere and then by multiple precipitations from
CH2Cl2 with hexanes until a colorless oil with a single 31P{1H}
NMR resonance was obtained.
Compound 4 was synthesized by treating 1 with selenium
powder and purified by column chromatography. Compounds
6, 7, and 8 were synthesized by treating 5 with hydrogen
peroxide, elemental sulfur, and selenium powder, respectively,
as shown in Schemes 1 and 2. Compound 6 could be readily
functionalized with a formyl, 9, or a hydroxymethyl group, 10,
at the 5′-position by modifications of the procedures that
we previously reported for the syntheses of the terthiophene
analogues.11 The oxidative conversion of 10 with molecular
The |1JPSe| coupling constants for 4 (739 Hz) and 8 (735 Hz)
are quite similar indicating the presence of a second Ph2(Se)P
group does not affect the 31P−77Se coupling. It has been pre-
viously reported that |1JPSe| coupling constants correlate linearly
with the s-character of the phosphorus lone pair of electrons,
which in turn is related to the electronegativity of the phos-
phorus substituents.39,40 The |1JPSe| coupling constants of 4
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Scheme 2. Syntheses of Derivatives of 5-Diphenylphosphino-2,2′-bithiophene (5)
and 8 are consistent with trend because they are smaller
than that of (2-furyl)3PSe (793 Hz), which has more electron with-
drawing 2-furyl groups, and larger than that of (p-MeOC6H4)3-
PSe (708 Hz), which has more electron-donating p-MeOC6H4
groups.41
X-ray Structure of 4. Compound 4, which is symmetrically
disubstituted with two Ph2(Se)P groups, crystallizes with a
center of symmetry at the midpoint of the bond bridging
the two thienyl rings, and thus the two thienyl rings in 4 are
coplanar and arranged in an anticonfiguration as is the case for
3 but not for either of the structures of 2.25 The P−Se bonds in
4 show two different rotations out of the plane of the adjacent
thienyl rings as indicated by the S−C1−P−Se torsion angle of
−35.2(2)° and the S′−C1′−P′−Se′ torsion angle of 129.4(2)°.
These angles are quite similar to the S−C1−P−S torsion angles
in 3 (36.6(3)° and −130.7(3)°).25
The similar molecular conformations of 3 and 4 result in
similar packing of the molecules in the solid state with the
bithiophenes of the two half molecules in the asymmetric
unit oriented perpendicular to one another (Figure 1b). Offset-
slipped π−π stacking occurs between the phenyl rings on
adjacent molecules, and point-to-face interactions occur both
between the perpendicular thienyl rings and between the
phenyl and thienyl rings of adjacent molecules (Table 2).
X-ray Structure of 6. Unlike the disubstituted phosphine
oxide, 2, the monosubstituted analogue, 6, cannot crystallize
on a center of symmetry. The two thienyl rings in 6 are twisted
relative to one another with the dihedral angle between
the mean planes through the two thienyl rings being
15.2°.The P−O bond is nearly in the plane of the adjacent
Single Crystal X-ray Diffraction Analysis. We have previ-
ously reported the crystal structures for the bithiophene deri-
vatives, Ph2(X)P(C4H2S)2P(X)Ph2 (X = O (2), S (3), and
25
CH3 I−). For each of the structures, the most interesting
feature was the extensive π−π stacking interactions that de-
pended on the nature of the Ph2(X)P group and the crystal-
lization method. It is important to note that π−π
interactions lead not only to various solid state architectures
but also may generate interesting electrical, optical and
magnetic properties in the solid state.42−44 Because it is of
interest to investigate how the molecular conformation and
unit cell packing type could be affected by varying the
X group on the phosphorus, the number of Ph2(X)P
group, and the substitution on the bithiophene, the X-ray
crystal structures of compounds 4, 6, 7, and 10 have been
determined. The molecular structures and the correspond-
ing unit cell diagrams of 4, 6, 7, and 10 are shown in Figures
1a and b, Figures 2a and b, Figures 3a and b, and Figures 4a
and b, respectively. Intermolecular π−π interactions and
torsion angles are given in Table 2.
+
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Figure 3. (a) Molecular structure of 7. Hydrogen atoms are omitted
and the atomic displacement ellipsoids are drawn at 25% probability.
(b) The bc face of the unit cell of 7. Hydrogen atoms are omitted and
thermal ellipsoids are shown at 15% probability.
Figure 1. (a) Molecular structure of 4. Hydrogen atoms are omitted
and the atomic displacement ellipsoids are drawn at 25% probability.
(b) The ac face of the unit cell of 4. Hydrogen atoms are omitted and
thermal ellipsoids are shown at 15% probability.
Figure 4. (a) Molecular structure of 10. Hydrogen atoms are omitted
and the atomic displacement ellipsoids are drawn at 25% probability.
(b) Packing diagram of 10 showing hydrogen bonding. Hydrogen
atoms are omitted and thermal ellipsoids are shown at 15% probability.
Both offset-slipped and point-to-face π−π stacking inter-
actions are observed in 6 (Figure 2b). The offset-slipped
interactions occur both between the thienyl rings on adjacent
molecules and between the phenyl rings. The point-to-
face interactions occur both between the perpendicular phenyl
rings and between the perpendicular thienyl and phenyl rings.
These interactions allow all of the bithiophene groups in the
crystal to have similar orientations, and the least-squares
planes through all of the bithiophenes in the crystal to be
parallel.
Figure 2. (a) Molecular structure of 6. Hydrogen atoms are omitted
and the atomic displacement ellipsoids are drawn at 25% probability.
(b) The bc face of the unit cell of 6. Hydrogen atoms are omitted and
thermal ellipsoids are shown at 15% probability.
thienyl ring as indicated by the C2−C1−P−O torsion angle of
174.6(3)°.
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Table 2. Intermolecular π−π Interactions and Torsion Angles in the Crystal Structures of 4, 6, 7, and 10
crystal
offset-slipped
interplanar distance (Å)
3.776
point-to-face
atom-centroid distance (Å)
torsion angles (deg)
a
b
4
phenyl−phenyl
thienyl−thienyl
phenyl−thienyl
phenyl−phenyl
thienyl−phenyl
phenyl−phenyl
3.691
3.758
3.665
3.736
3.690
S−C1−P−Se
S′−C1′−P′−Se′
C2−C1−P−O
−35.2(2)
129.4(2)
174.6(3)
b
b
b
a
6
thienyl−thienyl
phenyl−phenyl
3.681
4.058
a
b
7
C2−C1−P−S1
C2′−C1′−P′−S1′
C2−C1−P−O1
45.5(3)
147.3(3)
74.9(3)
10
a
b
The offset-slipped π−π interactions occur between the two parallel but displaced rings on adjacent molecules. The point-to-face π−π interactions
occur between the two perpendicular rings on adjacent molecules.
Figure 5. UV−vis absorption spectra of 3.0 × 10−4 mol/L chloroform solutions of bithiophene derivatives of (a) 2−4; (b) 6−8; and (c) 6, 9, 10 and
11 in a 1-mm cell.
X-ray Structure of 7. In spite of the very similar chemical
structures of 6 and 7, the molecular conformations and packing
of these two molecules are quite different. In compound 7,
the two thienyl rings are nearly coplanar (dihedral angle 7.0°).
Unlike the P−O bonds in 6, which are nearly in the plane of
the bithiophene, the P−S bonds in 7 are rotated out of the
plane of the adjacent thienyl ring as indicated by the C2−C1−
P−S1 torsion angle of 45.5(3)° and the C(2′)−C(1′)−P′−S(1′)
torsion angle of 147.3(3)°.
In contrast to 6, only point-to-face π−π stacking interactions
are observed between the perpendicular phenyl rings on
adjacent molecules in 7 (Figure 3b). This results in the
bithiophenes having two different orientations with the least-
squares planes through the bithiophenes in one of the
molecule nearly perpendicular to the least-squares planes
through the bithiophenes in the other molecule.
the plane of the adjacent thienyl ring, as indicated by the
C2−C1−P−O1 torsion angle of 74.9(3)°.
The introduction of a hydroxymethyl group at the 5′-position
of 6 results in a very different solid state structure of 10
in which no π−π interactions between the thienyl and
phenyl rings are observed. Instead, an intermolecular hydrogen
bonding interaction between the hydroxyl hydrogen on one
molecule and the phosphoryl oxygen on the other is observed.
This seems to be relatively strong based on the O2 to O1″
distance of 2.744(3) Å and the H−O2 to O1″ distance of
1.79(5) Å.
Electronic Absorption Spectra. Figure 5 shows the linear
absorption spectra of 3.0 × 10−4 M solutions of 2−4 and 6−11
in chloroform. The absorption band maxima and the molar
absorptivities are given in Table 3. Each compound exhibits a
broad absorption band between 300 and 400 nm. Regardless of
the number and type of phosphine substitution and the nature
of the 5′-substituent, these compounds are essentially trans-
parent above 430 nm, providing a broad optical window in the
violet to blue region in which optical power limiting may occur.
X-ray Structure of 10. Compound 10 does not have a
center of symmetry, and the thiophene rings are not coplanar
(dihedral angle of 12.8°). The P−O bonds are rotated out of
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molar absorptivity. In more conjugated compounds the molar
absorptivity increases in the order 6 ≈ 10 < 11 ≈ 9.
Table 3. Emission Data for Compounds 2−11 in Chloroform
Solution
Emission Spectra. Our previous study of the solution phase
fluorescence spectra of Ph2(X)P-substituted terthiophenes
demonstrated that both the emission maxima and fluorescence
quantum yields could be tuned by varying the number of
Ph2(X)P groups, the X group and the 5″-substituent on the
terthiophene.24 To determine how replacing a terthiophene
with a bithiophene could affect the fluorescence properties,
the emission spectra of compounds 2−4 and 6−11 have
been measured. The normalized emission spectra are shown in
Figure 6. The quantum yields for these compounds have been
determined using quinine sulfate solution in 0.1 M H2SO4 as
the reference and are summarized in Table 3. All the com-
pounds exhibit maximum emission bands between 380 and
430 nm, and these maxima are blue-shifted relative to those
of their terthiophene analogues (which have emission band
maxima between 430 and 480 nm).24
The number of Ph2(X)P groups attached to the bithiophene
has only a slight effect on either the emission wavelengths or
the quantum yields. In contrast, the nature of the X substituent
of the Ph2(X)P group has a significant effect on the emission
quantum yields of both the disubstituted and monosubstituted
bithiophenes but has only a small effect on the emission wave-
lengths. For the disubstituted bithiophenes, the quantum yield
of 2 is significantly higher than that of 3, and no fluorescence is
observed for 4. The trend in quantum yields for the mono-
substituted bithiophenes is quite similar with 6 exhibiting a
significantly higher quantum yield than either 7 or 8. The lack
of fluorescence for 4, and the much weaker fluorescence for 8,
both of which contain Se as the X substituent, may be due to
the heavy-atom effect in which spin and orbital interactions
increase the probability for intersystem crossing to the triplet
state and decrease the fluorescence.45
a
b
c
d
compound λabs/nm (ε/dm3·mol−1·cm−1)
λem/nm
Δλ (nm)
Φ
e
T2
2
315 (1.24 × 104)
326 (2.86 × 104)
342 (2.86 × 104)
321 (1.75 × 104)
328 (2.01 × 104)
329 (1.92 × 104)
356 (2.63 × 104)
329 (1.98 × 104)
340 (2.77 × 104)
369
390
400
384
387
385
424
393
400
54
64
58
63
59
56
68
64
60
0.012
0.17
3
0.0056
0.15
6
7
0.0052
0.0037
0.014
0.26
8
9
10
11
0.17
a
Linear absorption band maximum and molar absorptivity in CHCl3
solution. Emission band maximum in CHCl3 solution. Stokes shift.
Emission quantum yield using quinine sulfate solution in 0.1 M
H2SO4 as the reference. Bithiophene.
b
c
d
e
The linear absorption maxima of these compounds are
blue-shifted relative to those of the corresponding terthio-
phenes,24 and those for the disubstituted bithiophenes are
at longer wavelengths than are those for the monosubsti-
tuted bithiophenes with the same X groups. The linear ab-
sorption maxima for the disubstituted bithiophenes are slightly
red-shifted as the atomic number of X increases (2 (X = O),
λabs = 326 nm; 3 (X = S), λabs = 342 nm; 4 (X = Se), λabs = 348
nm). In contrast, changing the X group has a smaller effect on
the linear absorption maxima of the monosubstituted
bithiophenes (6 (X = O), λabs = 321 nm; 7 (X = S), λabs
=
328 nm; 8 (X = Se), λabs = 329 nm). This means that the dif-
ference between the maximum wavelengths for the absorption
maxima of the disubstituted and monosubstituted bithiophenes
also increases as the atomic number of X increases. This
result is in agreement with our density functional theory (DFT)
calculations that show smaller HOMO−LUMO energy differ-
ences for the disubstituted bithiophenes than for their mono-
substituted analogues (see Supporting Information).
As expected, the linear absorption maxima of the mono-
substituted bithiophenes are very sensitive to the nature of
the R substituent at the 5′-position, and a red shift of the linear
absorption maximum is observed as a more highly conjugated
group is introduced at this position (6 (R = H): λabs = 321 nm;
The nature of the organic (R) substituent in the mono-
substituted bithiophenes also has a significant effect on both
the emission wavelength and emission quantum yields. As
is the case for the electronic absorption spectra, a red shift
of the emission maximum is observed as the R substituent
is varied in the order 6 (R = H), 10 (R = CH2OH), 11 (R =
CO2Me), 9 (R = CHO) consistent with the increasing
conjugation of the organic substitutent. The effects of the
organic substituents on the quantum yields are more complex,
and DFT calculations, discussed in the next section, have
been carried out to gain insight into these structure−property
relationships.
10 (R = CH2OH): λabs = 329 nm; 11 (R = CO2Me): λabs
=
340 nm; 9 (R = CHO): λabs = 356 nm). This redshift is
consistent with our DFT calculations that the HOMO−LUMO
energy difference for the monosubstituted bithiophenes de-
creases in the order 6 > 10 > 11 > 9 (see Supporting
Information). The substituent at the 5′-position also affects the
Figure 6. Normalized emission spectra of 3.0 × 10−6 M chloroform solutions of (a) 2−3, (b) 6, and 9−11 in a 10-mm cell.
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The effects of variations in the substituents of the Ph2(O)P-
substituted bithiophenes on the quantum yields are quite dif-
ferent from those previously reported for Ph2(O)P-substituted
terthiophenes as shown in Table 4.24 Adding one Ph2(O)P
phosphorus group contributes more to the HOMO and LUMO
in the bithiophene than in the terthiophenes.27 Further, a recent
study of oligoarylfluorenes has shown that many important
molecular photophysical parameters can be directly calculated
by the time-dependent density functional theory (TDDFT)
and the calculated lifetimes are in good agreement with experi-
mental data.36,37
(a) Frontier Molecular Orbitals. To gain qualitative insight
into how the various substituents of the Ph2(X)P-substituted
bithiophenes affect excitation and fluorescence, the frontier
orbitals (ground state HOMOs and LUMOs at the B3LYP/
6-31G(d)// B3LYP/6-31G(d) level of theory) of 2−3, 6−7,
and 9−11 are shown in Figure 7.
Table 4. Comparison of the Emission Data of the
Bithiophene Derivatives and Those of Analogous
Terthiophene Derivatives
n = 3
n = 2
a
b
c
d
e
compound
H(C4H2S)nH
Ph2(O)P(C4H2S)nH
Φ1
Φ1/ΦT3
Φ2
Φ2/ΦT2 Φ2/Φ1
f
0.059
0.055
1.0
0.94
2.7
1.3
1.5
0.012
0.15
0.17
0.17
0.26
1.0
13
0.20
2.7
1.1
2.2
2.9
Ph2(O)P(C4H2S)nP(O)Ph2 0.16
Ph2(O)P(C4H2S)nCO2Me 0.077
Ph2(O)P(C4H2S)nCH2OH 0.089
14
14
22
a
Quantum yields of the terthiophene derivatives in CH2Cl2 solutions
b
using terthiophene as the reference. The ratio of the quantum yield
relative to terthiophene. Quantum yields of the bithiophene deriva-
c
tives in CHCl3 solutions using quinine sulfate solution in 0.1 M H2SO4
d
as the reference. The ratio of the quantum yield relative to bithio-
e
phene. The ratio of the quantum yield between the bithiophenes
derivatives and the terthiophene analogues with the same substituent.
f
Data is from ref 46 and is for a CH2Cl2 solution of terthiophene.
group to terthiophene to form Ph2(O)P(C4H2S)3H has
little effect on the quantum yield, while adding a second
Ph2(O)P group to form Ph2(O)P(C4H2S)3P(O)Ph2 increases
the quantum yield by a factor of 2.7 times relative to that of
terthiophene.24 In contrast, adding one Ph2(O)P group to bi-
thiophene to form Ph2(O)P(C4H2S)2H (6) increases the quan-
tum yield by a factor of 13 relative to that of bithiophene while
adding a second Ph2(O)P group to form Ph2(O)P(C4H2S)2P-
(O)Ph2 (2) causes only a slight increase in the quantum yield.
The net result is that the quantum yields of Ph2(O)P(C4H2S)3-
P(O)Ph2 and 2 are essentially the same.
The most significant difference between the bithiophene and
terthiophene derivatives is that adding one Ph2(O)P and one
organic substituent to bithiophene causes a much greater in-
crease in the quantum yield than does adding the same groups
to terthiophene. The quantum yields of Ph2(O)P(C4H2S)2-
H (6), Ph2(O)P(C4H2S)2P(O)Ph2 (2), Ph2(O)P(C4H2S)2-
CO2Me (11) and Ph2(O)P(C4H2S)2CH2OH (10) are signifi-
cantly larger than that of bithiophene (factors of 13, 14, 14,
and 22 respectively), whereas similar substitutions in terthio-
phenes only cause a modest increase in the quantum yields
over that of terthiophene (factors of 0.94, 2.7, 1.3, and 1.5
respectively). The differences in the increases are such that
the substituted bithiophenes generally have larger quantum
yields than do the terthiophenes with the same substituents
even though the quantum yield of terthiophene is five times
larger than that of bithiophene. For example, the quantum
yield of Ph2(O)P(C4H2S)2CH2OH (10) is 2.9 times higher
than that of the terthiophene analogue, Ph2(O)P(C4H2S)3-
CH2OH.
Theoretical Study of Fluorescent Properties for
Bithiophene Derivatives. Insight into the different effects
of organic substituents on the fluorescent properties of the
Ph2(X)P-substituted bithiophenes and terthiophenes may
be provided by DFT calculations. For example, Stott and
Wolf have reported that DFT calculations on the ground and
first excited singlet states of the parent phosphines of the two
classes, Ph2P(C4H2S)nPPh2 (n = 2 (1), 3) demonstrate that the
Figure 7. Frontier molecular orbitals for compounds 2−3, 6−7, and
9−11 calculated by the B3LYP/6-31G* method.
As shown in Figure 7, the HOMO of 3 has significant
contribution from the two P−S bonds by bonding interaction
with the bithienyl π-system, whereas the LUMO is mainly
localized on the bithienyl group. In contrast, the HOMO
of 2 exhibits a moderate O lone-pair character. Similarly
the HOMO of 7 exhibits a significant P−S bond character
while the HOMO of 6 shows just a moderate O lone-pair
character. According to a simple picture of oscillator strength
being roughly proportional to spatial extent of the HOMO and
LUMO, the oscillator strength of 3 and 7 are expected to be
smaller, i.e. for X = S rather than X = O.
Both the HOMOs and the LUMOs of the Ph2(O)P-substituted
bithiophenes 6 and 9−11 have large contributions from the
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Table 5. Theoretical Study of Emission Spectra by TD-B3LYP/6-31G(d) Method at CIS/6-31G(d) Optimized Geometry
calculated
estimated
experimental
a
b
c
d
e
f
g
compound
Eflu (eV)
λem (nm)/ error (%)
f
τn (ns)
kf (109/s)
τs (ns)
knr (109/s)
λem (nm)
τs (ns)
Φ
h
T2
T3
2
3.57
2.95
3.26
2.88
3.41
3.11
3.17
3.33
347/−5.9
420/−2.3
380/−2.5
431/7.7
0.44
0.84
0.98
0.31
0.67
0.21
0.84
0.76
4.11
3.15
2.21
8.88
2.95
11.06
2.72
2.74
2.89
0.243
0.317
0.452
0.113
0.339
0.0904
0.368
0.365
0.049
0.19
20.0
5.06
2.21
20.1
1.92
17.3
25.9
1.04
1.69
369
430
390
400
384
387
424
393
0.051
0.012
0.059
0.17
h
0.18
0.38
3
0.050
0.44
0.0056
0.15
6
364/−5.2
398/2.8
7
0.058
0.038
0.71
0.0052
0.014
0.26
9
391/−7.7
373/−5.1
10
11
3.16
392/−2.0
0.80
c
0.346
0.49
400
0.17
a
b
d
e
calcd
Fluorescence energy. Error = (λ
− λeemxptl)/λeemxptl. Oscillator strength. Intrinsic natural fluorescence lifetime. Fluorescence rate constant kf =
em
τn−1. Excited state lifetime τs = τnΦ . Nonradiative rate constant knr = kisc + kic + kec. τs values for T2 and T3 are from reference46 and converted to
f
g
h
chloroform solution.
5′ organic substituents: the carbonyl oxygen in the formyl
group in 9, the hydroxyl oxygen in the hydroxymethyl group
in 10, and the carbonyl oxygen and ester oxygen in
the methyl ester group in 11. The largest interaction is be-
tween the thiophene ring and the carbonyls in 9 and 11, with
a nonbonding π-interaction at the carbonyl carbon in the
HOMOs, and a carbonyl π* in the LUMOs. The delocalization
of the HOMO and LUMO in 9 and 11 is expected to lower
the HOMO−LUMO gap and hence give longer transition
wavelengths.
(b) Emission Spectra. For applications in OLEDs and in
sensor protection, the excited state lifetimes of the com-
pounds in this study are important parameters. The fluores-
cence energies, maximum emission wavelengths and oscil-
lator strengths of the compounds in this study have been
calculated using the same TDDFT computational methods
and are listed in Table 5. The intrinsic natural fluorescence
lifetimes for spontaneous emission, τn, were then calculated
from these parameters using the Einstein transition proba-
bilities according to eq 236,47
9−11 in this study, are also quite similar to the experimental
values.36,37
Both the fluorescence rate constant, kf, which is the reci-
procal of intrinsic lifetime, τn, and nonradiative rate constant,
knr, which is defined as knr = kisc + kic + kec where kisc is
intersystem crossing rate constant, kic and kec are the respec-
tive rate constants for internal and external conversion decay,
affect the fluorescence quantum yield, Φ, as shown by eq 345,46
kf
Φ =
kf + knr
(3)
Because both Φ (Table 4) and kf (Table 5) are available for
each of the Ph2(X)P-substituted bithiophenes in this study, eq
3 can be used to determine knr.
From the data in Table 5, it is quite clear that changes in
the number of Ph2(X)P groups and the nature of the X
substituent in the Ph2(X)P-substituted bithiophenes affect
both kf and knr values. The higher Φ values of 2 and 6 relative
to those of 3 and 7 and T2 are because of combination of
larger kf values and the smaller knr values for 2 and 6. By
comparison with T2, 2 and 6 have 10-fold smaller knr values
than T2, whereas 3 and 7 have approximately the same large
knr values as T2. On the other hand, the O substituent in 2
and 6 results in larger kf values while the S substituent in 3
and 7 makes kf values smaller relative to T2. The larger kf
values for 2 and 6 are consistent with the greater overlaps
between the LUMO and the HOMO in 2 and 6 shown in
Figure 7.
Contrary to the effects of the Ph2(X)P groups, the organic
substituents in the Ph2(O)P-substituted bithiophenes 6 and
9−11 primarily affect the knr values. All four compounds have
kf values that differ by no more than 10% suggesting that the
organic substituent has little effect on the kf values. In contrast,
the knr value of 9 (R = CHO) is larger than those of 6 (R = H),
10 (R = CH2OH) and 11 (R = CO2Me) by an order of
magnitude, suggesting that the primary effect of the organic
substituent is to alter the nonradiative decay pathways. The
large knr value for 9 is presumably due to the deactivation of an
excited electronic state resulting from interaction and energy
transfer between the excited molecule and the solvent or other
solutes.48
3
c
τ =
n
2
2(E ) f
(2)
flu
where c is the velocity of light, Eflu is the fluorescence energy,
and f is the oscillator strength.
The calculated maximum emission wavelengths in Table 5
are in good agreement with experimental data, with errors in
calculated maximum emission wavelengths relative to the ex-
perimental emission wavelengths ranging from −7.7% to 7.7%.
Except that the calculations for the sulfur containing 3 and
7 overestimate the wavelengths, all other calculated fluores-
cence wavelengths are slightly shorter than experiment. These
errors are comparable to those found for oligoarylfluorene
derivatives.36,37
The excited state lifetime, τs, was estimated from the cal-
culated intrinsic natural lifetime, τn, and the experimental
quantum yield, Φ, and using the equation of τs = τnΦ. The
lifetime, τs, will only be accurate if the computation method is
able to accurately calculate the τn. Comparisons of estimated
and experimental lifetimes for closely related compounds sug-
gest that this is the case. The estimated excited state lifetimes
for both T2 and T3 differ from the experimental excited state
lifetimes by 3.9% and 5.5%, respectively as shown in Table 5.
Estimated excited state lifetimes of oligoarylfluorene derivatives,
which are closely related to the compounds 2−3, 6−7, and
Nonlinear Optical Characterization. To optimize optical
limiting performance in a class of materials, it is necessary to
maximize the nonlinear optical absorption (or reverse satur-
able absorption) of those materials, which is both wavelength
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Figure 8. Comparison of the nonlinear transmittances of CH2Cl2 solutions of 2−4 at the same concentration of 0.067 M with 27 ps laser pulses at
(a) 430, (b) 450, and (c) 480 nm. The NLO absorption for 4 shows stronger than that for 2 and 3 at the same concentration and the same
wavelength.
and concentration dependent.49 Our preliminary study of
two Ph2(X)P-substituted bithiophenes demonstrated that the
NLO absorption of a saturated CH2Cl2 solution of Ph2(S)P-
(C4H2S)2P(S)Ph2 (3) was stronger than that of a saturated
CH2Cl2 solution of Ph2(O)P(C4H2S)2P(O)Ph2 (2) at wave-
lengths of 420, 430, and 440 nm for 27 ps laser pulses.26 This
suggests that the nonlinear absorption is very sensitive to the
chalcogen attached to the nonbonding pair of electrons on the
diphenylphosphino group, and thus that the violet-blue NLO
absorbers could be further improved by attaching an even
heavier chalcogen to the Ph2P substituent.
The fact that saturated solutions of the bithiophene deriva-
tives can be used without suffering from high linear transmission
losses means that increasing the solubility of the molecules could
also give a stronger NLO absorption. A limitation of the bithio-
phenes that are symmetrically disubstituted with two Ph2(X)P
groups (X = O (2), S (3), Se (4)) is that they all have relatively low
solubilities (Table 5). In light of this, the analogous bithiophenes
substituted with a single Ph2(X)P group (X = O (6), S (7), Se (8))
were prepared. As shown by the data in Table 6, the bithiophenes
Table 6. Saturation Concentration of Ph2(X)P-Substituted
To test this possibility, Ph2(Se)P(C4H2S)2P(Se)Ph2 (4) was
synthesized. To compare the NLO transmittances of 2, 3, and 4
on a per molecule basis, the NLO transmittances of 0.067 mol/
L CH2Cl2 solutions of the compounds were measured at 430,
450, and 480 nm (Figure 8). At 430 nm, when the incident
fluence reaches 0.9 J/cm2, the transmittance drops to 60% for
2, 41% for 3, and 25% for 4. The threshold for NLO absorp-
tion, defined as the incident fluence at which the transmittance
falls to 50%, is 1.2 J/cm2 for 2, 0.55 J/cm2 for 3, and 0.27 J/cm2
for 4. These results demonstrate that the NLO absorption
increases with the increasing of atomic weight of X (Se > S > O).
The same trend is seen at 450 and 480 nm, but all of the
chromophores show a decrease in nonlinear absorption at these
wavelengths.
Bithiophene Derivatives 2−10
compound
saturation conc. (mol/L)/solvent
2
0.184/CH2Cl2
0.217/CH2Cl2
0.103/CH2Cl2
2.11/CHCl3
2.66/CHCl3
1.66/CHCl3
0.851/CHCl3
0.909/CHCl3
3
4
6
7
8
9
10
with a single Ph2(X)P group are approximately ten times more
soluble than are those with two Ph2(X)P groups. Saturated
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Figure 9. (a) Linear transmittances of saturated CHCl3 solutions of 6−8 with all solutions having linear transmittances greater than 80% from 430 to
500 nm. Comparison of the nonlinear transmittances of (b) saturated solutions for 2 and 6 and (c) saturated solutions for 3 and 7, with 27 ps laser
pulses at 430 nm.
solutions of these compounds are still sufficiently transparent in
the violet and blue regions of the visible spectrum that they have
linear transmittances above 80% from 430 to 500 nm (Figure 9a).
The high linear transmittances of saturated solutions of
bithiophenes substituted with a single Ph2(X)P group allow
the nonlinear absorptions of these solutions to be evaluated at
all wavelengths in the violet and blue regions. As shown in
Figure 9b and c, the nonlinear absorption of the saturated
solution for the asymmetric compound 6 is significantly
larger than that of the symmetric compound 2 at 430 nm.
A comparison of the nonlinear absorption of the saturated
solution for 7 and 3 shows a similar trend with 7 exhibiting
much stronger nonlinear absorption than 3 at 430 nm. When
the incident fluence reaches 0.6 J/cm2, the transmittance
drops to 49% for 2, 42% for 3, 23% for 6, and 30% for 7. The
optical limiting threshold when the transmittance falls to 50%
is 0.60 J/cm2 for 2, 0.40 J/cm2 for 3, 0.10 J/cm2 for 6, and
0.18 J/cm2 for 7. The saturated chloroform solutions of the 6
and 7, exhibit the best NLO absorption of picosecond laser
pulses that has been reported.
bithiophenes to be made on a per molecule basis, the NLO
transmittances of 1.66 mol/L CHCl3 solutions of 6, 7, and 8
were measured at 430, 450, and 480 nm. As shown
in Figures 10a−c, the solutions of these compounds exhibit
strong NLO absorptions from 430 to 480 nm with the
strongest NLO absorption observed at 430 nm for all com-
pounds. At 430 nm, when the incident fluence reaches 0.25 J/cm2,
the transmittance is 63% for 6, and 54% for 7 and 8. In
contrast to the disubstituted bithiophenes, 2, 3, and 4, in which
the NLO absorption clearly increases in the order Se > S > O,
only small differences are observed in the nonlinear trans-
mittances of the monosubstituted bithiophenes with different
X groups (6, 7 and 8). The saturated solutions of com-
pounds 6 and 7 showed some decomposition beginning at
an incident fluence of approximately 0.6 J/cm2. This may be
due to the strong absorption of laser by the highly con-
centrated solutions resulting in rapid heating even though the
laser repetition rate is only 10 Hz. Measurement of the power
limiting performance at higher fluences will require the use of
single laser shots.
To eliminate the effect of solution concentrations and allow
comparison of the NLO absorbances of the monosubstituted
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Figure 10. Comparison of the nonlinear transmittances of CHCl3 solutions of 6−8 at the same concentration of 1.66 M in a 1-mm cell with 27 ps
laser pulses at (a) 430, (b) 450, and (c) 480 nm.
substituted bithiophenes. The bithiophene derived materials
have the added advantages that they can be prepared in high
yields from inexpensive and commercially available bithio-
phene, and they are more soluble and more easily processed
than are those derived from higher oligothiophenes.
Third-order NLO studies demonstrate that members of this
family of compounds exhibit high linear transmission but strong
nonlinear absorption in the violet-blue spectral region from 430
to 480 nm, and that both extent of the nonlinear absorption
and the maximum wavelength at which the nonlinear
absorption occurs can be tuned by varying the groups attached
to the phosphorus and the substituents on the bithiophene.
Saturated chloroform solutions of the monosubstituted
bithiophenes, Ph2(O)P(C4H2S)2H and Ph2(S)P(C4H2S)2H,
exhibit the best NLO absorption of picosecond laser pulses
that has been reported, and thus these compounds are promis-
ing candidates for use in broadband optical power limiters.
CONCLUSIONS
■
A series of bithiophene derivatives that are either symmetrically
disubstituted or monosubstituted with Ph2(X)P (X = O, S, Se)
groups have been synthesized, and the X-ray crystal struc-
tures of four of the compounds have been determined. Their
linear optical properties have been evaluated using electronic
absorption and fluorescence spectroscopy, and their third-order
NLO properties have been evaluated using nonlinear trans-
mission measurements at wavelengths between 430 and
480 nm with picosecond laser pulses.
The fluorescence studies demonstrate that the emission maxima
and quantum yields of the Ph2(O)P-substituted bithiophenes
are much higher than that of bithiophene with the quantum
yields of Ph2(O)P(C4H2S)2H, Ph2(O)P(C4H2S)2P(O)Ph2,
Ph2(O)P(C4H2S)2CO2Me, and Ph2(O)P(C4H2S)2CH2OH in-
creased by factors of 13, 14, 14, and 22, respectively, relative
to that of bithiophene. The quantum yields of the Ph2P(X)-
substituted bithiophenes are generally higher than are those
of the terthiophene analogues with the same substituents
even though the quantum yield of bithiophene is much lower
than that of terthiophene (0.012 versus 0.059). The DFT
studies provide insight into the manner in which changes in the
structures affect the fluorescence and suggest that fluorescent
materials with even higher quantum efficiencies could be deve-
loped by judicious choices of substituents on the Ph2P(O)-
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystals structure data for 4, 6, 7, and 10, including tables
of atomic coordinates, anisotropic thermal parameters, all bond
lengths and angles, and hydrogen isotropic displacement param-
eters using the crystallographic information file (CIF) format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Madison, WI, 2001.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: (205) 934-8294. Fax: (205) 934-2543. E-mail: gmgray@
uab.edu (G.M.G.); qzhao551@gmail.com (Q.Z.).
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge an Army Research Labo-
ratories Cooperative Agreement (W011NF-06-2-0033) and NSF
Cooperative Agreements (EPS-0814103 and EPS-0903787) for
support of this research.
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