Structure-property studies toward the stimuli-responsive behavior of benzyl-phospholium acenes

Article abstract of DOI:10.1021/ic2026335

A series of new phospholium acenes, quaternized with benzyl groups, was synthesized. Both different π-conjugated backbones and electron-donating/- withdrawing benzyl groups were systematically studied to reveal details on the nature of their structural dynamics. Extensive NMR studies (including variable concentration/temperature and 2D)suggested that the systems undergo intramolecular conformation changes in solution that are strongly affected by the electronic nature of the benzyl group, and thereby significantly affecting the phosphole-typical σ ^*-π^* interaction. This class of "smart" phosphole system exhibits enhanced emission in the solid state and at low temperature in solution, due to aggregation-induced enhanced emission (AIEE). The dynamic features of these smart phospholes also endow the systems with external-stimuli (thermal and mechanical force)responsive photophysical properties. Crystallographic studies and theoretical calculations confirmed that the thermal response of the phospholium system is mainly due to the conformation changes in solution, while the mechanical response of the system can be attributed to both the intramolecular conformation and the intermolecular organization changes in the solid state.

Full text of DOI:10.1021/ic2026335

Article
pubs.acs.org/IC
Structure−Property Studies Toward the Stimuli-Responsive Behavior
of Benzyl−Phospholium Acenes
Yi Ren and Thomas Baumgartner*
Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada
S
* Supporting Information
ABSTRACT: A series of new phospholium acenes, quater-
nized with benzyl groups, was synthesized. Both different π-
conjugated backbones and electron-donating/-withdrawing
benzyl groups were systematically studied to reveal details on
the nature of their structural dynamics. Extensive NMR studies
(including variable concentration/temperature and 2D) sug-
gested that the systems undergo intramolecular conformation
changes in solution that are strongly affected by the electronic
nature of the benzyl group, and thereby significantly affecting
the phosphole-typical σ*−π* interaction. This class of “smart”
phosphole system exhibits enhanced emission in the solid state
and at low temperature in solution, due to aggregation-induced
enhanced emission (AIEE). The dynamic features of these smart phospholes also endow the systems with external-stimuli
(thermal and mechanical force) responsive photophysical properties. Crystallographic studies and theoretical calculations
confirmed that the thermal response of the phospholium system is mainly due to the conformation changes in solution, while the
mechanical response of the system can be attributed to both the intramolecular conformation and the intermolecular organization
changes in the solid state.
can efficiently tune the properties of the systems (Chart 1,
left).7,9
INTRODUCTION
■
Heteroatoms such as boron,1 silicon,2 phosphorus,3 and sulfur4
provide a variety of new and unique opportunities for tailoring
the properties of π-conjugated systems including photophysics,
solid organization, and electronics. Electronic doping using B,5
Si,6 P,7 and/or S8 heteroatoms (Chart 1) has thus been
The most intriguing electronic features of phospholes are the
hyperconjugation between the phosphorus lone pair with the π
system (Chart 1A) as well as the coupling of the σ* orbital of
the exocyclic P−C bond with the endocyclic π* system of the
conjugated backbone (Chart 1B) that play a very important
role for the electronic properties of such systems.3,9 Instead of
chemical modification of the phosphorus center, we envisioned
that conformation changes of the phosphorus center may also
be utilized to change the σ*−π* orbital coupling. Moreover,
the potential for addressing the photophysical properties of the
systems via external stimuli (temperature and mechanical force)
that trigger conformation changes may also exist. Recently, we
were able to design a family of novel phosphole−lipids with
dynamic features capable of inducing a variety of stimuli-
responsive photophysical features and intriguing self-assembly
properties (Chart 2, I).10 In this contribution, we now report a
series of model systems for these smart phospholes with both
different π-conjugated backbones and contrasting electron-rich
and electron-poor benzyl functionalization (Chart 2, II) to
reveal more details on the recently discovered external-stimuli
responsive behavior of the system. Our current systematic
structural, photophysical, and theoretical studies unequivocally
confirm that the electronic features of the benzyl substituents
Chart 1. (Left) Phosphole-Based π-Conjugated Systems;
(right, A) n−π Orbital Coupling and (B) σ*−π* Orbital
Coupling of the Phosphole Systems
extensively studied in the context of organic functional
materials for applications such as organic light-emitting diodes
(OLEDs) and organic field-effect transistors (OFETs). The
phosphole ring system in particular provides an intriguing
perspective for organic electronics due to its intrinsically diverse
and switchable chemistry involving both modification of the
functional group E′ at phosphorus, via oxidation (S, O),
borylation (BH3), methylation (Me+), and metalation (e.g., Au,
Pd, Pt), as well as the electronic nature of the R groups3 that
Received: December 7, 2011
Published: January 27, 2012
© 2012 American Chemical Society
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Chart 2. Previously Reported Phosphole−Lipid Systems (I) and New Phospholium Model Species (II)
Table 1. Crystal Data and Structure Refinement for 1c, 3a, 3b, and 3c
1c
3a
3b
C29H20BrPS2
3c
empirical formula
fw
C21H11F5PS2Br·CHCl3
652.67
C32H26O3PS2Br·4CHCl3
1111.01
C29H15F5PS2Br·CHCl3
543.45
752.79
temp. (K)
wavelength (Å)
cryst syst
space group
a (Å)
173(2)
173(2)
173(2)
173(2)
0.71073
0.71073
0.71073
0.71073
triclinic
triclinic
monoclinic
monoclinic
P−1
P−1
C2/c
P21/c
10.0170(2)
11.6590(3)
12.7350(4)
68.464(2)
13.9910(4)
14.0780(4)
14.3410(4)
67.8960(10)
62.2360(10)
84.393010)
2305.39(11)
3
25.0070(7)
13.0230(4)
b (Å)
9.9490(3)
13.9240(5)
c (Å)
19.5790(6)
20.6650(6)
α (deg)
90
90
β (deg)
70.6040 (2)
67.185(2)
93.640(2)
126.676(2)
γ (deg)
90
90
vol. (Å)
1243.60(6)
2
4861.3(2)
3005.37(17)
Z
8
5
density, calcd (Mg/m3)
1.743
1.600
1.485
1.664
−1
abs coeff (mm )
2.256
1.747
1.945
1.879
F (000)
644
1112
2208
1496
cryst size (mm−3)
0.12 × 0.11 × 0.10
1.76−27.55
10 688
0.17 × 0.16 × 0.15
1.57−27.55
19 437
0.11 × 0.10 × 0.10
2.46−27.52
10 490
0.15 × 0.11 × 0.10
1.91−27.48
10 454
θ range (deg)
reflns collected
independent reflns
data/restraints/params
GoF on F2
5686 (R(int) = 0.0285)
5686/0/307
1.090
10 525 (R(int) = 0.0352)
10 525/0/496
1.103
5538 (R(int) = 0.0485)
5538/0/298
6828 (R(int) = 0.0437)
6828/0/379
1.146
1.085
final R indices [ I > 2σ(I)]
R indices (all data)
R1 = 0.0416, wR2 = 0.0828
R1 = 0.0552, wR2 = 0.0947
R1 = 0.0619, wR2 = 0.1348
R1 = 0.0865, wR2 = 0.1574
R1 = 0.0546, wR2 = 0.1317
R1 = 0.0777, wR2 = 0.1459
R1 = 0.0777, wR2 = 0.1820
R1 = 0.1122, wR2 = 0.2146
spectrofluorometer and UV−vis−NIR Cary 5000 spectrophotometer.
Low-temperature fluorescence, fluorescence quantum yield, and
lifetime were measured in dichloromethane solution using an
Edinburgh Instruments Ltd. FLS920P fluorescence spectrometer
equipped with an integrating sphere.
can effectively be used to manipulate the systems’ properties
without further chemical modification of the phosphorus
center.
EXPERIMENTAL SECTION
■
General Synthesis. Equimolar mixtures of the trivalent phospholes
(1, 2, 3) and the corresponding benzyl bromide derivatives (a, b, c) in
toluene/THF mixture were refluxed overnight. Subsequently, all
solvents were removed under vacuum. The crude products were then
washed with pentane and ethyl ether. The pure compounds were
obtained by subsequent recrystallization from CHCl3, CH2Cl2, or
acetone. Note that the crystallized products commonly also capture
solvents molecules. Dry products can be obtained after drying under
vacuum.
General. All manipulations were carried out under a dry nitrogen
atmosphere employing standard Schlenk techniques. Solvents were
dried using an MBraun Solvent Purification System. Unless noted
otherwise, starting materials were used as received. All trivalent
phosphole compounds were synthesized according to our procedures
reported before.10 31P{1H} NMR, H NMR, and 13C{1H} NMR, and
1
2D NMR and variable-temperature 1H NMR were recorded on Bruker
DRX400 and Avance (-II,-III) 400 MHz spectrometers. Chemical
shifts were referenced to external 85% H3PO4 (31P) and external TMS
1
(13C, H). Elemental analyses were performed in the Department of
1
Compound 1a. H NMR (600 MHz, CDCl3): δ = 8.48−8.44 (m,
Chemistry at the University of Calgary. Mass spectra were run on a
Finnigan SSQ 7000 spectrometer or a Bruker Daltonics AutoFlex III
system. Thermal analyses were performed using a TA-Q200 DSC
instrument. Crystal data and details of data collection are provided in
Table 1 and the enclosed cif files, Supporting Information. Diffraction
data were collected on a Nonius Kappa CCD diffractometer using Mo
Kα radiation (λ) 0.71073 Å (graphite monochromator). Structures
were solved by direct methods (SHELXTL) and refined on F2 by full-
matrix least-squares techniques. All photophysical experiments were
recorded in dichloromethane solution on a Jasco FP-6600
2H), 8.12 (dd, J = 5.04 Hz, J = 1.20 Hz, 2H), 7.73−7.69 (m, 1H),
7.64−7.62 (m, 2H), 7.53 (dd, J = 5.04 Hz, J = 4.14 Hz, 2H), 6.47 (d, J
= 3.12 Hz, 2H), 5.22 (d, J = 14.94, 2H), 3.75 (s, 3H), 3.62 (s, 6H)
ppm. 31P {1H} NMR (162 MHz, CDCl3): 18.4 ppm. 13C{1H} NMR
(100 MHz, CDCl3): δ = 152.9 (d, JC−P = 4.4 Hz), 149.4 (d, JC−P
=
18.8 Hz), 137.7 (d, JC−P = 5.5 Hz), 134.9 (d, JC−P = 3.2 Hz), 134.1 (d,
JC−P = 11.8 Hz), 130.4 (d, JC−P = 15.4 Hz), 130.0 (d, JC−P = 13.7 Hz),
129.2 (d, JC−P = 14.3 Hz), 126.1 (d, JC−P = 97.0 Hz), 121.1 (d, JC−P
=
10.0 Hz), 115.7 (d, JC−P = 83.5 Hz), 107.5 (d, JC−P = 6.2 Hz), 60.8 (s),
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Figure 1. Plausible conformation changes in CDCl3 at 298 K (low concentration, ∼10−4 M; high concentration, ∼10−2 M; arrow indicates Ar group
leaving away from conjugated cores).
60.8 (s), 31.2 (d, JC−P = 42.1 Hz) ppm. Anal. Calcd for C21H11BrF5PS2
JC−P = 3.9 Hz), 128.3 (d, JC−P = 4.5 Hz), 127.4 (S), 126.8 (s), 126.8
(d, JC−P = 11.5 Hz),126.3 (d, JC−P = 91.2 Hz), 124.3 (s), 123.1 (s),
123.0 (d, JC−P = 8.3 Hz), 115.8 (d, JC−P = 82.1 Hz), 114.6 (d, JC−P =
(533.31): C, 54.04; H, 4.16. Found: C, 53.63; H, 3.99.
1
Compound 1c. H NMR (400 MHz, CDCl3): δ = 8.52−8.46 (m,
2H), 8.04 (dd, J = 4.80 Hz, J = 2.00 Hz, 2H), 7.81−7.76 (m, 2H),
7.71−7.65 (m, 1H), 7.58 (dd, J = 4.40 Hz, J = 5.20 Hz, 2H), 5.65 (d, J
= 16.00 Hz, 2H) ppm. 31P {1H} NMR (162 MHz, CDCl3): 15.0 (s br)
ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 149.7 (d, JC−P = 20.4
Hz), 145.1 (dm, JC−F = 258.4 Hz), 137.5 (dm, JC−F = 257.6 Hz), 135.9
(d, JC−P = 3.3 Hz), 134.0 (d, JC−P = 12.4 Hz), 131.4 (d, JC−P = 16.0
Hz), 130.4 (d, JC−P = 14.2 Hz), 128.9 (d, JC−P = 14.8 Hz), 125.0 (d,
JC−P = 97.4 Hz), 119.2 (dm, JC−F = 208.8 Hz), 114.6 (d, JC−P = 85.9
Hz) ppm. Anal. Calcd for C21H11BrF5PS2·CHCl3 (652.69): C, 40.48;
H, 1.85. Found: C, 40.86; H, 1.34.
95.3 Hz), 30.9 (d, JC−P = 41.9 Hz) ppm. Anal. Calcd for C27H20BrPS
(487.39): C, 66.54; H, 4.14. Found: C, 66.17; H, 3.97.
1
Compound 2c. H NMR (400 MHz, CDCl3): δ = 9.82−9.23 (m,
1H), 8.64−8.58 (m, 2H), 8.03−8.58 (m, 1H), 8.03−8.00 (m, 2H),
7.74−7.68 (m, 4H), 7.57−7.48 (m, 2H), 7.40 (d bro, J = 8.0 Hz, 1H),
6.02 (t bro, J = 15.6 Hz, 1H), 5.74 (t bro, J = 15.6 Hz, 1H) ppm. 31P
{1H} NMR (162 MHz, CDCl3): 22.3 (s, br) ppm. 13C{1H} NMR
(100 MHz, CDCl3): δ = 161.0 (d, JC−P = 23.2 Hz), 145.4 (d bro, JC−P
= 258.1 Hz), 142.9 (d, JC−P = 13.2 Hz), 137.9 (d, JC−P = 16.1 Hz),
136.2 (d, JC−P = 3.4 Hz), 136.0 (d, JC−P = 2.2 Hz), 135.5 (d, JC−P
=
1
Compound 2a. H NMR (400 MHz, CDCl3): δ = 9.46−9.41 (m,
11.5 Hz), 134.7 (d, JC−P = 12.7 Hz), 134.2 (d, JC−P = 12.6 Hz), 131.9
(d, JC−P = 12.5 Hz), 130.8 (d, JC−P = 13.9 Hz), 127.6 (s), 127.2 (s),
125.7 (d, JC−P = 91.7 Hz), 124.5 (s), 123.4 (d, JC−P = 8.7 Hz), 122.4
(s), 114.6 (d, JC−P = 83.5 Hz), 20.8 (d, JC−P = 47.3 Hz) ppm. HRMS
(MALDI-TOF): m/z = 497.0544 ([M − Br]+, calcd 497.0547).
1H), 8.61−8.55 (m, 2H), 8.02−7.99 (m, 1H), 7.75−7.70 (m, 1H),
7.69−7.60 (m, 5H), 7.59−7.51 (m, 2H), 6.35 (d, J = 3.2 Hz, 2H), 5.83
(d, J = 15.2 Hz, 2H), 5.32 (d, J = 14.4 Hz), 3.67 (s, 3H), 3.28 (s, 3H)
ppm. 31P {1H} NMR (162 MHz, CDCl3): 25.9 ppm. 13C{1H} NMR
1
(100 MHz, CDCl3): δ = 160.4 (d, JC−P = 21.7 Hz), 152.9 (d, JC−P
4.4 Hz), 142.9 (d, JC−P = 13.1 Hz), 138.4 (d, JC−P = 14.5 Hz), 137.6
(d, JC−P = 5.0 Hz), 135.4 (d, JC−P = 3.5 Hz), 135.3 (s), 135.2 (d, JC−P
=
Compound 3a. H NMR (400 MHz, CDCl3): δ = 8.74−8.68 (m,
2H), 8.30 (d bro, J = 8.0 Hz, 2H), 7.96 (d br, J = 8.4 Hz, 2H), 7.81−
7.77 (m, 1H), 7.73 − 7.65 (m, 4H), 7.54−7.50 (m, 2H), 6.33 (d, J =
3.2 Hz, 2H), 5.73 (d, J = 13.2 Hz, 2H), 3.67 (s, 3H), 3.18 (s, 6H)
ppm. 31P {1H} NMR (162 MHz, CDCl3): 21.4 ppm. 13C{1H} NMR
=
2.0 Hz), 134.0 (d, JC−P = 11.7 Hz), 131.2 (d, JC−P = 12.3 Hz), 130.5
(d, JC−P = 13.1 Hz), 127.5 (S), 126.8 (s), 126.1 (d, JC−P = 91.8 Hz),
124.3 (s), 123.3 (s), 123.2 (d, JC−P = 7.9 Hz), 116.1 (d, JC−P = 81.7
Hz), 121.9 (d, JC−P = 96.2 Hz), 107.4 (d, JC−P = 5.8 Hz), 60.7 (s), 60.7
(s), 55.8 (s), 30.8 (d, JC−P = 41.5 Hz) ppm. HRMS (MALDI-TOF):
m/z = 463.0724 ([M − Br]+, calcd 463.0739).
(100 MHz, CDCl3): δ = 153.0 (d, JC−P = 4.4 Hz), 150.8 (d, JC−P
=
18.3 Hz), 143.0 (d, JC−P = 13.8 Hz), 137.7 (d, JC−P = 2.4 Hz), 135.9
(d, JC−P = 3.1 Hz), 135.0 (d, JC−P 13.5 Hz), 134.2 (d, JC−P = 12.4 Hz),
130.9 (d, JC−P = 13.7 Hz), 128.2 (s), 127.0 (s), 124.3 (s), 123.9 (s),
1
Compound 2b. H NMR (400 MHz, CDCl3): δ = 9.44−9.73 (m,
121.5 (d, JC−P = 95.4 Hz), 121.5 (d, JC−P = 9.9 Hz), 114.4 (d, JC−P =
1H), 8.62−8.57 (m, 2H), 8.01−7.99 (m, 1H), 7.77−7.72 (m, 1H),
7.70−7.63 (m, 4H), 7.61−7.45 (m, 3H), 7.26 (d, J = 7.60 Hz, 1H,),
7.15−7.09 (m, 3H), 7.00 (t, J = 7.6 Hz, 2H), 5.58 (t, J = 15.2 Hz, 1H),
5.52 (t, J = 15.2 Hz, 1H) ppm. 31P {1H} NMR (162 MHz, CDCl3):
81.4 Hz), 107.4 (d, JC−P = 6.2 Hz), 60.7 (s), 60.7 (s), 55.6 (s), 29.7 (d,
JC−P = 41.3 Hz) ppm. Anal. Calcd for C32H26BrO3PS2 (633.55): C,
60.66; H, 4.14. Found: C, 60.21; H, 4.24.
1
Compound 3b. H NMR (400 MHz, CDCl3): δ = 8.72−8.66 (m,
25.8 ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 160.5 (d, JC−P
=
2H), 8.10 (d br, J = 7.6 Hz, 2H), 7.96 (d br, J = 8.4 Hz, 2H), 7.81−
7.76 (m, 1H), 7.73−7.68 (m, 2H), 7.64−7.60 (m, 2H), 7.54−7.50 (m,
2H), 7.16−7.08 (m, 3H), 7.01 (m, 2H), 5.76 (d, J = 14.0 Hz, 2H)
ppm. 31P {1H} NMR (162 MHz, CDCl3): 21.4 ppm. 13C{1H} NMR
22.5 Hz), 142.9 (d, JC−P = 13.2 Hz), 138.0 (d, JC−P = 15.1 Hz), 135.5
(d, JC−P = 3.3 Hz), 135.4 (d, JC−P = 11.3 Hz), 135.3 (d, JC−P = 12.8
Hz), 135.1 (d, JC−P = 2.0 Hz), 134.2 (d, JC−P = 12.0 Hz), 131.5 (d, JC−P
= 12.4 Hz), 130.7 (s), 130.5 (d, JC−P = 1.2 Hz), 130.5 (s), 128.8 (d,
(100 MHz, CDCl3): δ = 143.0 (d, JC−P = 14.2 Hz), 135.9 (d, JC−P
=
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3.2 Hz), 135.0 (d, JC−P = 13.3 Hz), 134.3 (d, JC−P = 12.5 Hz), 131.0
(d, JC−P = 13.7 Hz), 130.5 (d, JC−P = 6.3 Hz), 128.9 (d, JC−P = 4.0 Hz),
128.5 (d, JC−P = 13.7 Hz), 128.2 (s), 127.1 (s), 126.5 (d, JC−P = 9.6
Hz), 124.1 (s), 123.9 (s), 29.8 (d, JC−P = 41.5 Hz) ppm. HRMS
(MALDI-TOF): m/z = 463.0724 ([M − Br]+, calcd 463.0739).
In addition, we were able to crystallize 1a,c, 2b,c, and 3a−c
from different organic solvents (Figures 2−5).12 The new
1
Compound 3c. H NMR (400 MHz, CDCl3): δ = 8.72−8.65 (m,
2H), 8.25 (d br, J = 8.0 Hz, 2H), 8.01−7.98 (m, 2H), 7.86−7.81 (m,
1H), 7.77−7.61 (m, 2H), 7.69−7.65 (m, 2H), 7.58−7.54 (m, 2H),
6.18 (d, J = 14.8 Hz, 2H) ppm. 31P {1H} NMR (162 MHz, CDCl3):
16.8 (s, br) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 151.3 (d,
JC−P = 19.6 Hz), 143.1 (d, JC−P = 14.3 Hz), 136.6 (d, JC−P = 3.2 Hz),
134.6 (d, JC−P = 14.2 Hz), 134.2 (d, JC−P = 13.0 Hz), 131.2 (d, JC−P
14.3 Hz), 128.4 (s), 127.4 (s), 124.1 (s), 123.7 (s), 119.9 (d, JC−P
=
=
95.7 Hz), 113.2 (d, JC−P = 84.2 Hz), 19.6 (d, JC−P = 46.3 Hz) ppm.
Anal. Calcd. for C30H76BrCl3F5PS2·CHCl3 (752.81): C, 47.86; H, 2.14.
Found: C, 48.22; H, 2.15.
RESULTS AND DISCUSSION
■
Synthesis and Conformation Studies. Synthesis of the
new phospholium compounds was adopted from our earlier
studies involving efficient quaterization of the trivalent
phosphorus center in the respective conjugated ring-fused
phospholes with the corresponding benzyl bromides.10 The
different conjugated backbones and substituted benzyl groups
were used to systematically investigate their effects on the
geometric parameters and electronic features of the systems.
The identity of all new phospholium acenes was confirmed by
multinuclear (1H, 13C, and 31P) NMR spectroscopy, high-
resolution mass spectrometry, and elemental analysis.
In analogy to our initial work on the phosphole−lipid system,
variable-concentration (1H, 31P, and 2D) NMR studies (10−4−
10−2 M, Figure 1 and Supporting Information) of the
compounds with the trimethoxybenzyl and benzyl groups
(1a,b 2a,b, and 3a) revealed a noticeable intramolecular
conformation change with the benzyl group being pushed
away from the conjugated backbone through intermolecular
interactions (ionic, H−π, and π−π; see Supporting Informa-
tion).11 As seen in Figure 1, Hd of 2a shows a significant
downfield shift (ca. 0.3 ppm) supporting the deshielding effect
of the leaving benzyl group upon increasing the concentration
from 3.46 × 10−4 to 4.81 × 10−2 M. In addition, the chemical
shift changes of Hh and Hh′ also support rotation of the P−
Figure 2. (a) Molecular structure of 1c in the solid state (50%
probability level; hydrogen atoms omitted for clarity). Selected bond
lengths [Angstroms] and angles [degrees]: P1−C3, 1.778(3); P1−C6,
1.776(3); P1−C10, 1.792(3); P1−C20, 1.804(3); C20−C21,
1.506(4); C3−C4, 1.384(4); C4−C5: 1.454(4); C5−C6, 1.386(4);
C3−P1−C6, 94.50(14); C3−P1−C10, 113.16(12); C6−P1−C10,
110.98(15); C21−C20−P1, 112.5(12). (b) Molecular packing of 1c
(50% probability level; solvent molecule and hydrogen atoms omitted
for clarity).
Cbenzyl bond. It is worth mentioning that Hi of 2a and Ha of 3a
1
exhibit interactions with Hd and He in the H−1H NOESY
spectrum (see Supporting Information) at both lower (10−3 M)
and higher concentrations (10−5 M), which is different from the
related tridodecyloxy−phospholium derivative reported be-
fore.10 Such difference could be rationalized by the less bulky
trimethoxybenzyl group of 2a being located closer to the
conjugated backbone. In contrast to the electron-donating
benzyl group, 1c and 2c with the electron-poor pentafluor-
phospholium compounds have similar structures to the
corresponding methyl phospholium derivatives reported before
(1d, 2d, and 3d).9a,e,f Due to the expected correlation between
the environment of the phosphorus center and the σ*−π*
orbital coupling, the geometry around phosphorus was
considered to be an excellent indicator for this purpose. The
sum of the angles around phosphorus (based on the two
endocyclic phosphole P−C bonds and the exocyclic P−CPh
bond) was found to be highly dependent on the electronic
nature of the benzyl groups (3a, 316.8°; 3b, 317.7°; 3c, 322.9°;
cf. 3d, 317.4° and 1c, 318.6°, cf. 1d, 317.7°), indicating that the
benzyl group has a comparatively strong effect on the geometry
of the phosphorus center.9e,f As reported before in this context,
we could obtain two different isomers of 1b (“open form” and
“closed form”) from slow evaporation of the different solvents
that support the structural flexibility of the benzyl group in
1
obenzyl group only exhibit a small change in their H and 31P
chemical shifts (see Supporting Information),11 thereby
supporting a more rigid structure in the latter case, which can
be attributed to different electrostatic interactions between the
conjugated cores and the various benzyl moieties (electron
donating or withdrawing); variable-temperature NMR experi-
ments further support the differences in the dynamic features of
these compounds (see Supporting Information). Clearly,
systematic NMR studies confirm that the dynamic structural
features of the new phospholium systems are effectively
controlled by the electronic nature of substituted benzyl
group and its electrostatic interactions with the conjugated
acene scaffold.
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Figure 4. (a) Molecular structure of 3b in the solid state (50%
probability level; hydrogen atoms omitted for clarity). Selected bond
lengths [Angstroms] and angles [degrees]: P1−C7, 1.791(4); P1−
C10, 1.788(4); P1−C21, 1.797(4); P1−C31, 1.794(4); C31−C32,
1.514(5); C7−C8, 1.378(6); C8−C9, 1.456(6); C9−C10, 1.364(5);
C7−P1−C10, 93.59(18); C7−P1−C21, 113.85(18); C10−P1−C21,
110.28(18); C32−C31−P1, 116.81(3). (b) Molecular packing of 3b
(50% probability level; hydrogen atoms omitted for clarity).
Figure 3. (a) Molecular structure of 3a in the solid state (50%
probability level; solvent molecules and hydrogen atoms omitted for
clarity). Selected bond lengths [Angstroms] and angles [degrees]: P1−
C7, 1.788(4); P1−C10, 1.794(4); P1−C20, 1.783(4); P1−C30,
1.799(4); C30−C31, 1.509(6); C7−C8, 1.372(6); C8−C9,
1.451(6); C9−C10, 1.373(6); C7−P1−C10, 93.8(2); C7−P1−C20,
111.7(2); C10−P1−C20, 111.3(1); C31−C30−P1, 114.7(3). (b)
Molecular packing of 3a (50% probability level; solvent molecules and
hydrogen atoms omitted for clarity).
Photophysical Properties. The photophysical properties
of the new systems that were also found to be highly dependent
on the electronic nature of the benzyl groups are summarized in
Table 2. Generally, the electronic nature of the benzyl groups
does essentially not affect the absorption wavelengths of the
compounds in solution within each of the series 1, 2, or 3
(Figure 7a). However, the emission properties indeed show a
dependence on the electronic nature of the benzyl groups with
the electron-poor perfluorinated benzyl group inducing the
strongest red shift in CH2Cl2 with the general trend λem c > b >
a (Figure 7a). These observations suggest that the phospholium
systems with the electron-poor pentafluorobenzyl group have a
more polar excited state compared to the respective ground
state, which likely also involves a considerably changed σ*−π*
interaction.3 It should be mentioned that 1c exhibits a blue-
shifted emission compared with its nonfluorinated congener 1b.
Although the exact reason for this observation is not clear at
this stage, this feature can likely be attributed to a different
excited state of 1c, induced by the electron-withdrawing nature
of the perfluorinated benzyl group.
Compared to the known methyl phospholium derivatives
(1d, 0.53; 2d, 0.36; 3d, 0.31),9a,e,f the benzyl-substituted
phospholium species generally show lower fluorescence
quantum yields in solution (Table 2) that can be attributed
to the flexibility of the benzyl groups as confirmed with the
(variable-concentration/temperature) NMR studies (vide
supra). Compounds 1a, 2a, and 3a with the electron-rich
trimethoxybenzyl group exhibit relatively low quantum yields in
general (Figure 6).10 Importantly, the sum of the angles around
phosphorus is also a direct result of these conformations of 1b
(1b-open, 321.0°; 1b-closed, 316.0°). Besides, the 1b-open
isomer exhibits better π−π-stacking interactions (Figure 6b).
Compared to the methyl phospholium derivatives 2d and 3d,9a,f
dimer-like structures could only be observed in 3a, 3b, and 3c,
however, with less efficient π−π stacking, which is likely due to
the bulkier benzyl group reducing the long-range intermolec-
ular interactions (Figures 3b, 4b, and 5b).
Compound 3c with the perfluorinated benzyl group shows
better π−π stacking (3.49 Å, Figure 5b) with the benzannelated
thieno moieties of two adjacent molecules partially overlapping.
By contrast, the trimethoxybenzyl (3a, Figure 3b) and benzyl
(3b, Figure 4b) varieties only show a small overlap of their
thieno moieties. The main reason for this observation seems to
be the steric bulk of the trimethoxybenzyl groups that prevents
strong π−π stacking, while the electron-withdrawing penta-
fluorobenzyl group decreases the electron density of the
conjugated backbones that consequently induces stronger π−π-
stacking interactions. This hypothesis is further supported by
1c with the smaller conjugated backbone, displaying even better
π−π overlap between the dimers (Figure 2b, 3.67 Å) than the
extended relatives such as 3a and 3b, which is also supported by
a more red-shifted solid-state fluorescence emission (powder
and crystal) of 1c compared to its solution (vide infra).
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the a-, to the b, to the c series (with 3b and 3c showing values
as high as ϕ = 0.38 and 0.44 in solution, respectively), reflecting
the donor capability (or lack thereof) of the benzyl group that
is necessary to induce the PET. It is important to note that all
compounds exhibit higher fluorescence quantum yields in the
solid state (1a, 4.7 times; 1b, 10 times; 1c, 1.2 times; 2a, 1.5
times; 2b, 2 times; 2c, 1.3 times; 3a, 2.2 times; 3b, 2.2 times;
1a, 1.6 times) due to aggregation-induced enhanced emission
(AIEE) likely resulting from the restricted intramolecular
rotation (IMR) of the benzyl groups (Figure 7b).9a,e,f,13
Remarkably, 3b and 3c with a larger conjugated core show
surprisingly high quantum yields (ca. 80%) in the solid state by
taking advantage of the bulky phospholium center that prevents
fluorescence quenching through aggregation (vide supra).14
The dynamics of the benzylated phospholium center endow
the systems with very interesting photophysics that are
distinctly different from the known dithienophosphole
derivatives (Figure 8a and 8b).9 First, compounds with the
electron-rich trimethoxyphenyl group exhibit a more red-shifted
solid-state emission (powder and crystal) compared to their
solutions (1a, Δλem = 17 nm; 2a, Δλem = 22 nm; 3a, Δλem = 29
nm). On the basis of the solid-state structures and theoretical
calculations using π-stacked dimers of 1a and 3a (vide infra)
this stronger red-shifted solid-state emission can be attributed
to an intermolecular charge transfer from the electron-rich
trimethoxybenzyl group (HOMO) to the π-conjugated cores of
a neighboring molecule (LUMO). In stark contrast, 2c and 3c
with the electron-poor pentafluorobenzyl ring exhibit a blue-
shifted emission in their powder state compared to their
solutions. Due to the steric bulk around the phosphorus center,
H-aggregation can be ruled out for these phospholium systems.
Considering that J-aggregation usually induces red-shifted
emission features in the solid state, such blue-shifted emission
can only be explained by different molecular conformations in
the solid state and solution, respectively. As already mentioned,
1c with the small conjugated backbone shows a significant red-
shifted emission compared to its solution, likely due to excimer
formation, which correlates well with effective intermolecular π-
stacking interactions observed in the X-ray diffraction study of
1c. Second, the solution emission spectra of the 1, 2, and 3
series are generally much broader than those of the
corresponding solids, suggesting the existence of different
isomers in solution as a result of the flexible structures (Figure
8c and Supporting Information). Finally, the emissions of the
corresponding single crystals (1a−c, 2b,c, and 3a−c) are also
different from those in solution and the solid powders (Figure
8a, and Supporting Information). More importantly, as
observed previously, the 1b-open isomer exhibits a 9 nm red-
shifted solid-state emission compared with the 1b-closed isomer
(Figure 8b), which further supports our hypothesis of
conformation-dependent emission features.
Figure 5. (a) Molecular structure of 3c in the solid state (50%
probability level; hydrogen atoms omitted for clarity). Selected bond
lengths [Angstroms] and angles [degrees]: P1−C7, 1.787(6); P1−
C10, 1.787(6); P1−C21, 1.790(6); P1−C31, 1.803(6); C31−C32,
1.508(7); C7−C8, 1.367(8); C8−C9, 1.458(8); C9−C10, 1.376(8);
C7−P1−C10, 94.1(3); C7−P1−C21, 112.1(3); C10−P1−C21,
116.7(3); C32−C31−P1, 113.5(4). (b) Molecular packing of 3c
(50% probability level; solvent molecule and hydrogen atoms omitted
for clarity).
Stimuli-Responsive Studies. On the basis of our initial
study and further solidifed by this work, the dynamic structural
features offer the system with intriguing thermally responsive
photophysics that are highly dependent on both the electronic
nature of the benzyl ring and the conjugated backbones. As
shown in Figure 9a, compounds 1a and 3a with the electron-
rich trimethoxyphenyl group show the most significant
hyperchromic shift (1a, 24 times; 3a, 8 times), and 1a, 2a,
and 3a exhibit the most red-shifted emission (1a, Δλem = 6 nm;
2a, Δλem = 13 nm; 3a, Δλem = 16 nm) in CH2Cl2 (10−5 M,
Figure 8a and 8b) compared with the other compounds in the
same series upon decreasing the temperature from room
Figure 6. Molecular packing of 1b: (a) 1b-closed and (b) 1b-open (a
solvent molecule, counteranion, and hydrogen atoms omitted for
clarity).10
solution (1a, ϕ = 0.03; 2a, ϕ = 0.04; 3a, ϕ = 0.06), indicating
that a photoinduced electron transfer (PET) from the
trimethoxyphenyl donor moiety to the conjugated phospho-
lium acceptor (Figure 7b) could be an additional nonradiative
channel next to intramolecular rotation (IMR), similar to the
properties of the related phosphole−lipids.10 Notably, the
photoluminescence quantum yields commonly increase from
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Table 2. Photophysical Properties of the 1, 2, and 3 Series
a
c
c
ε
[L·mol
λabs [nm]
(CH2Cl2)
λem [nm]
(CH2Cl2)
λem [nm]
(solid)
λem [nm]
(crystal)
λem [nm]
b
ϕPL
ϕPL (solid
compd
−1·cm−1]
(AS )
(CH2Cl2)
powder)
d
1a
372
372
8300
450
463
467
466
461 (CHCl3)
NA
469
453
462
474
NA
441
442
521
515
522
0.03
0.14
0.60
e
472 (EA)
d
1b
6384
456 (DCM)
0.060
d
465 (CHCl3)
476 (CHCl3)
NA
d
1c
2a
2b
2c
3a
3b
3c
376
337
337
340
417
419
425
8987
453
424
442
452
500
516
469
446
446
443
529
516
525
0.12
0.04
0.18
0.18
0.06
0.36
0.49
0.14
0.06
0.36
0.23
0.13
0.80
7853
10 134
9642
445
444
d
d
11 285
17 065
523 (CHCl3)
526
16 554
528
533 (CHCl3)
0.77
a
b
c
d
ε: molar absorption coefficient. After shearing. Fluorescence quantum yield was determined by a calibrated integrating sphere system. Solvents
e
that cocrystallized with the compounds. Crystal obtained from slow evaporation of ethyl acetate (EA).
Figure 7. (a) Absorption and emission wavelengths of the 1, 2, and 3
series. (b) Quantum yields of the compounds in solution and the solid
state (PET = photoinduced electron transfer; IMR = intramolecular
Figure 8. (a) Emission properties of the 1, 2. and 3 series. (b)
Normalized emission spectra of 3c under different conditions.
rotation).
temperature to −60 °C. These observations suggest that the
stronger electon-donating character of the trimethoxybenzyl
group leads to a more flexible structure than in the other
systems (vide supra) due to both the steric demand and the
electronic repulsion of the trimethoxybenzyl group. The weak
hyperchromic shift of 2a is likely due to different conformations
that change the distance between the trimethoxybenzyl group
and conjugated cores and the underlying flexibity of the
structure that affects the nonradiative decay processes (PET or
IMR) and the emission intensity. Besides, the 1 series with the
smaller conjugated backbone exhibits a more enhanced
emission at low temperature than the 2 and 3 series with
extended conjugated backbones. It is likely that the smaller
conjugated backbone, with decreased potential of intra-
molecular benzyl/π-scaffold interaction, endows the 1 series
with a generally more flexible structure. Remarkably, the
thermally responsive hyperchromic shift and red-shifted
emission are comparable to the chemical modification of the
phosphorus center of the corresponding systems reported
before.9 Variable-temperature (VT) 1H NMR (CD2Cl2)
suggests that the benzyl rings approach the conjugated cores
more closely upon decreasing the temperature (see Supporting
Information). The VT 31P NMR experiments further support
the change of the phosphorus environment in the systems.
Therefore, it is believed that the thermally responsive features
of these compounds are due to conformation changes upon
altering the temperature of the solution and thus determine the
communication between the phosphorus center and the
conjugated backbones (i.e., the σ*−π* interaction).
In addition to the thermally responsive features in solution,
the crystals of the new phospholium compounds also exhibit
interesting mechanically responsive features (Figure 10a and
Supporting Information). As general observation, the resulting
powders from mechanical shearing of the crystals between two
glass slides display a blue-shifted and broadened emission (1a,
Δλem = −3 nm; 1b, Δλem = −3 nm; 1c, Δλem = −2 nm; 2b,
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Figure 9. (a) Emission of 1, 2, and 3 series in CH2Cl2 (10−5 M) at room temperature (RT) and −60 °C (LT). (b) Emission wavelengths of the 1, 2,
and 3 series in CH2Cl2 (10−5 M) at room temperature (blue circle) and −60 °C (red circle). (c and d) Emission spectra of 1a and 3a in CH2Cl2
(10−5 M) at room temperature (blue) and −60 °C (red).
Figure 10. (a) Emission of the crystal phase of the 1, 2, and 3 series (BS, before shearing; AS, after shearing). (b, c, and d) Emission spectra of 1a,
2c, and 3c under different conditions.
Δλem = −4 nm; 2c, Δλem = −2 nm; 3a, Δλem = −2 nm; 3b,
Δλem = −9 nm; 3c, Δλem = −11 nm). This mechanically
responsive emission shift is believed to be the result of
formation of different conformational isomers upon the
mechanical force. In particular, crystals and sheared crystals
of 2c display blue-shifted emission compared with solution
emission (Figure 10c), which further supports that comforma-
tion changes play an important role in this mechanical
response. However, from the available data the intermolecular
interaction changes, particularly for 1c and 3c with the
observed π−π stacking in the solid state, can also not fully be
ruled out.
Interestingly, the sum of the emissions of the crystal and
sheared crystal phases of 2b, 3b, and 3c is very similar to the
broad emission of these compounds in solution (Figure 10d
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and Supporting Information), which further points to the
altered photophysics being based on conformation changes.
Theoretical Studies. The experimental results obtained in
the context of this structure−property study suggest that the
communication between the phosphorus center and the π-
conjugated backbones (σ*−π* coupling and inductive effect) is
a crucial element of the dynamic structural features. Therefore,
we evaluated the Mulliken charges on the phosphorus center
for different conformations in order to further solidify the
nature of the stimuli-responsive photophysical properites. Both
“open” and “closed” conformations (Figure 11) of the 1, 2, and
of 1b-open (dimer-open, ELUMO = 3.86 eV) shows a smaller
HOMO−LUMO gap compared to the dimer of 1b-closed
(dimer-closed, EHOMO−LUMO = 3.96 eV), which is consistent
with the stronger π−π stacking observed in the crystal of 1b-
closed.10 Moreover, the HOMO and LUMO energy levels of the
dimer of 3b (EHOMO = −6.61 eV, ELUMO = −3.38 eV) are very
similar to its monomer (EHOMO = −6.65 eV, ELUMO = −3.21
eV) and consistent with the low degree of π−π stacking
observed in the solid state. By contrast, the HOMO and
LUMO energy levels of the dimer structures of 1a (EHOMO
=
−6.13 eV, ELUMO = −2.61 eV), 1c (EHOMO = −6.99 eV, ELUMO
= −3.22 eV), 3a (EHOMO = −6.69 eV, ELUMO = −3.43 eV), and
3c (EHOMO = −6.73 eV, ELUMO = −3.38 eV) are very different
from their corresponding monomers (1a EHOMO = −6.28 eV
and ELUMO = −2.74 eV; 1c EHOMO = −6.71 eV and ELUMO
=
−2.78 eV; 3a EHOMO = −6.50 eV and ELUMO = −3.24 eV; 3b
EHOMO = −6.65 eV and ELUMO = −3.21 eV; 3c EHOMO = −6.48
eV and ELUMO = −3.04 eV). Moreover, electronic coupling was
exclusively observed between the π-conjugated scaffolds in the
1c dimer (Figure 12b). Furthermore, TD-DFT calculations
cleary support the potential for intermolecular charge transfer
between the electron-rich trimethoxybenzyl group (HOMO)
and the dimeric cores (LUMO) in 1a and 3a (Figure 12a, see
Supporting Information). On the basis of the theoretical
calculations, it is clear that both intermolecular charge transfer
and dimerization in the excited state can contribute the red-
shifted emission in the crystals.
Figure 11. Mulliken charge of the phosphorus atom in the 1, 2, and 3
series for the different conformations.
3 series compounds were chosen as qualitative model isomers
and subsequently optimized using DFT calculations (B3LYP/6-
31G(d), PCM; solvent = CH2Cl2). Where applicable, X-ray
diffraction data were used as input for the calculations. Indeed,
the different conformations result in a noticeable change in the
Mulliken charges at the P center (Figure 11), which clearly
supports an altered electronic communication between the
phosphorus center and the π-conjugated backbones. Besides,
the theoretical calculations also show that the different
molecular conformations can effectively change both LUMO
and HOMO energy levels (Supporting Information) and can
thus explain the thermally responsive emission in the solutions.
Futhermore, the intermolecular effects on the stimuli-
responsive behavior of the system were evaluated based on
DFT calculations (B3LYP/6-31G(d), PCM; solvent = CH2Cl2)
using the X-ray diffraction data for the dimeric structures of
1a−c and 3a−c (see Supporting Information). Although the
two isomers of 1b show very similar HOMO and LUMO
energies (1b-closed, EHOMO = −6.61 eV and ELUMO = −2.63 eV;
1b-open, EHOMO = −6.60 eV and ELUMO = −2.64 eV), the dimer
CONCLUSION
■
We reported a series of new phospholium compounds whose
dynamic structural features can be controlled via the different
electronic nature of the substituted benzyl groups at the
phosphorus center. Unlike in known phosphole/phospholium
derivatives, this unique feature induces aggregation-induced
enhanced emission (AIEE) due to restricted intramolecular
rotation in the solid state. As a new molecular design concept,
the structural dynamics can also endow this system with
external-stimuli (temperature and mechanical force) responsive
photophysics. The experimental data show that the σ*−π*
interaction can offer about 10 nm emission shift at low
concentrations in solutions, which is quite efficient considering
the small contribution of the phosphorus center. In the solid
state, only the 3 series with a larger conjugated head exhibits a
relatively strong mechanically responsive emission, indicating
that the intermolecular interactions also play an important role
due to the restricted conformation changes. Theoretical studies
Figure 12. Molecular orbitals of 1a (a) and 1c (b) using X-ray diffraction data as input.
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2010, 16, 7101−7105. (f) Durben, S.; Baumgartner, T. Inorg. Chem.
2011, 50, 6823−6836. (g) Durben, S.; Baumgartner, T. Angew. Chem.,
Int. Ed. 2011, 50, 7948−7952.
further confirmed that both intramolecular conformation
changes and intermolecular interactions (excimer and charge
transfer) changes play an important role in this responsive
emission as they are integral for the phosphole-typical σ*−π*
interaction that determines the overall photophysics of the
system. More importantly, the thermally responsive hyper-
chromic shift and red-shifted emission are comparable to
chemical modification of the phosphorus center of the
corresponding systems reported before. These external-
responsive features thus make the new phospholium materials
promising candidates for a variety of sensing/biosensing
applications.
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ASSOCIATED CONTENT
* Supporting Information
■
Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P. Chem. Rev. 2010, 110,
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S
Additional 1H, 31P, VC/VT, and 2D NMR data, TD-DFT data,
photophysical data, CIF files of all structures determined by X-
ray crystallography, and intermolecular packing interactions of
all crystallographically characterized compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
The CIF files were also deposited with the Cambridge
Crystallographic Data center (CCDC), and CCDC codes
847303, 847304, 847305, and 847306 were allocated. These
data are available without cost at www.ccdc.cam.ac.uk/conts/
retrieving.html or from the CCDC, 12 Union Road, Cambridge
CB2 IEZ, United Kingdom; fax: +44(0)1223−336033; e-mail:
deposit@ccdc.cam.ac.uk.
́
(6) (a) Chu, T.-Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J.-R.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: thomas.baumgartner@ucalgary.ca.
■
Wu, C.-C.; Rea
́
u, R. J. Am. Chem. Soc. 2006, 128, 983−995. (f) Fave,
C.; Cho, T.-Y.; Hissler, M.; Chen, C.-W.; Luh, T.-Y.; Wu, C.-C.; Rea
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R. J. Am. Chem. Soc. 2003, 125, 9254−9255.
Notes
(8) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang, F.; Gao, J.; Qiu, W.;
Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D. J.
Am. Chem. Soc. 2005, 127, 13281−13286.
The authors declare no competing financial interest.
(9) (a) Ren, Y.; Baumgartner, T. J. Am. Chem. Soc. 2011, 133, 1328−
ACKNOWLEDGMENTS
■
́ ́
1340. (b) Dienes, Y.; Durben, S.; Karpati, T.; Neumann, T.; Englert,
Financial support by NSERC of Canada and the Canada
Foundation for Innovation (CFI) is gratefully acknowledged.
We also thank Alberta Ingenuity now part of Alberta Innovates-
Technology Futures for a New Faculty Award (T.B.) and a
graduate scholarship (Y.R.) as well as Talisman Energy for a
graduate scholarship (Y.R.). The authors thank Prof. C. P.
Berlinguette for access to the fluorescence instrument.
U.; Nyulas
́
zi, L.; Baumgartner, T. Chem.Eur. J. 2007, 13, 7487−
7500. (c) Ren, Y.; Baumgartner, T. Chem.Asian J. 2010, 5, 1918−
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