Phosphine-based push-pull AIE fluorophores: Synthesis, photophysical properties, and TD-DFT studies

Article abstract of DOI:10.1016/j.dyepig.2021.109485

Herein, we report the design and characterization of a novel series of four push-pull fluorophores using diphenylphosphino as an electron-donating terminal group (P-chromophores). The spectroscopic properties in solution, the aggregation-induced emission

Full text of DOI:10.1016/j.dyepig.2021.109485

Dyes and Pigments 193 (2021) 109485
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Phosphine-based push-pull AIE fluorophores: Synthesis, photophysical
properties, and TD-DFT studies
a
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b
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Maxime Remond , Pauline Colinet , Erwan Jeanneau , Tangui Le Bahers , Chantal Andraud ,
a,*
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Yann Bretonniere
a
b
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Universite de Lyon, Ecole Normale Superieure de Lyon, Universite Lyon 1, CNRS UMR 5182, Laboratoire de Chimie, 46 Allee D’Italie, 69364, Lyon, France
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Universite de Lyon, Centre de Diffractometrie Henri Longchambon, Universite Lyon I, 43 Boulevard Du 11 Novembre 1918, 69622, Villeurbanne, Cedex, France
A R T I C L E I N F O
A B S T R A C T
Keywords:
Herein, we report the design and characterization of a novel series of four push-pull fluorophores using diphe-
nylphosphino as an electron-donating terminal group (P-chromophores). The spectroscopic properties in solu-
tion, the aggregation-induced emission (AIE) properties, as well as the emission properties in the solid state were
studied and compared with those of the diphenylamino-analogues (N-chromophores). Comparative analysis of
the crystalline structures coupled with density functional theory calculations revealed that the diphenylphos-
phino group adopts a pyramidal geometry, whereas a trigonal planar configuration around the N-center is
observed for the diphenylamino group. Consequently, the phosphorous lone pair is localized on the upper side of
the average chromophore plan and considerable geometrical change of the P-center can occur between the
ground state and the first excited state, explaining the much larger Stokes shift observed (10 000 cmꢀ 1) for the P-
chromophores compared to the N-analogues.
Phosphine based dyes
Push-pull dyes
DFT
AIE
Large Stokes shift
1. Introduction
either a five-membered phosphole ring [14–23] or six-membered ring
variants[24–37], showed great potential for the construction of inter-
Organic synthesis offers virtually endless possibilities to fine-tune the
optical properties of organic fluorophores. To this end, the incorporation
esting far-red emissive
π-conjugated structures. The obtained
cyclic-organophosphorous derivatives differ from their widely used
sulphur or nitrogen analogues by a substantial bathochromic shift of
absorption and emission up to the NIR[25,30,34]. This can be ascribed
to the lower aromaticity bestowed by specific orbital interactions be-
of heteroatoms into the chromophore
π -backbone is an effective
structure-modification strategy inducing significant electronic pertur-
bations in the parent molecules and, usually, a large wavelength shift.
While light main-group heteroatoms such as boron (B), nitrogen (N), or
oxygen (O) have been extensively used[1–3], heavier second-row ele-
ments such as phosphorous (P) have been rather neglected, especially in
small push-pull molecules. However, straightforward chemical trans-
formations of the P-center through oxidation/reduction, complexation
with metal ion or quaternarization provides numerous options to
–
tween the exocyclic σ*(P-R) or σ*(P O) and π* (butadiene) orbitals,
–
together with the pyramidal configuration of the phosphorus atom[14,
17,18].
Besides phosphole derivatives, there are few examples of chromo-
phores containing a λ3σ3 P-center in general and phosphine particularly.
The lesser interest for P(III) can be explained by its sensitivity towards
oxidation, particularly in solution, which limits synthetic yields, spec-
troscopic studies and final applications. However, P(III) to P(V) oxida-
modulate the photophysical and electrochemical properties of π-elec-
tron frameworks in which the P-center is embedded or grafted. λ5σ4
(phosphorus oxide or sulfide) and λ4σ4 phosphorus groups (phospho-
nium cations) behave as strong electron-withdrawing groups [4–6] but
has only seldom been used as such in simple combination with donor
groups to construct dipolar rod-shaped [7–12] or three-dimensional
star-shaped octupolar chromophores[10,12,13].
tion can be prevented by delocalization of the lone pair in a π-system.
Thus, conjugated phosphines, such as triphenylphosphine or BINAP, are
quite stable towards oxidation. Using this strategy, stable primary or
tertiary phosphines linked to a BODIPY derivative were synthesized by
Higham et al. for metal sensing[38,39]. However, this approach can be
limited by potential fluorophore quenching due to a photoinduced
By contrast, λ5σ4 and λ4σ4 P-heterocyclic compounds, containing
* Corresponding author.
`
E-mail address: yann.bretonniere@ens-lyon.fr (Y. Bretonniere).
https://doi.org/10.1016/j.dyepig.2021.109485
Received 24 February 2021; Received in revised form 17 May 2021; Accepted 17 May 2021
Available online 28 May 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

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M. Remond et al.
Dyes and Pigments 193 (2021) 109485
electron transfer to the nearby phosphine[40,41]. Fluorescent λ3σ3
those of the triphenylamine analogues (N1–N4) that we reported pre-
viously[50–52]. The influence of the donor atom on the optical prop-
erties was rationalized by comparison with the X-ray diffraction
structures and by theoretical calculations. In particular, we showed that
conformational changes around the P-atom between the ground state
and the first excited state led to higher fluorescence turn-on coefficients
upon aggregation (170-fold increase emission for P2) than their amine
analogues, as well as a drastically increased Stokes shift (up to 10 000
cmꢀ 1 on average).
phosphines were nevertheless obtained by either including the phos-
phorus atom in a fused
π system [42] or with three extended π-systems
around the phosphorus atom[43]. It is also possible to use the electron
donating capacities of phosphines to design intramolecular charge
transfer dyes. For example, Madrigal et al. studied the absorption
properties of quadripolar and octupolar triphenylamine and triphenyl-
phosphine stilbene derivatives[44–46]. The absorption and non-linear
optical properties of a dipolar dye containing a phosphine donor and a
nitro acceptor, as well as a few donor/acceptor structures, based on an
aryl phosphine coupled with a boron acceptor, were also investigated
[47,48]. Finally, Baumgartner et al. studied 2,7-diketophosphepin as
2. Materials and methods
fluorescent
π-conjugated building blocks[49]. Interestingly, some of
2.1. General
those compounds showed solid-state emission, but quantum yields were
not reported.
Commercially available materials were used as received. Analytical
thin-layer chromatography (TLC) was carried out on Merck 60 F254
precoated silica gel plate (0.2 mm thickness). Visualization was per-
formed using a UV lamp (254 nm or 365 nm). Anhydrous THF was
distilled over sodium and benzophenone. Microwave synthesis were
conducted in 10 mL sealed tube on a Biotage Initiator 2.5 single-mode
reactor using external IR temperature control. Column chromatog-
Hence, only a few examples of fluorescent phosphine derivatives
have been studied, even fewer in a push-pull configuration, and none of
them emit in the biological window (650 nm–1000 nm) for imaging
applications. It appears that more studies of these derivatives are called
for. Indeed, thanks to the intramolecular charge transfer from the donor
to the acceptor end, these chromophores have unique optical properties
such as a large Stokes shift that any variation of the donor or the
acceptor end will modify.
raphy was performed on Merck silica gel Si-60 (40–63 μm). NMR spectra
were recorded at ambient temperature on a standard spectrometer
operating at 400 MHz or 300 MHz for 1H NMR, 125 MHz or 101 MHz for
13C NMR and 202 MHz or 162 MHz for 31P NMR. Chemical shifts are
given in parts per million (ppm) and are reported relative to tetrame-
thylsilane (1H, 13C) using the residual solvent peaks as internal standard.
Herein, we used P-λ3σ3 triphenylphosphine as the donor group in a
series of small push-pull fluorophores (P1–P4, Fig. 1) incorporating
various potent electron acceptor groups 1–4. The optical properties in
solution but also in the solid state were measured and compared with
Fig. 1. Structures of the P-λ3σ3 chromophores P1–P4 and nitrogen analogues N1–N4.
2

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M. Remond et al.
Dyes and Pigments 193 (2021) 109485
1H NMR splitting patterns are designated as singlet (s), doublet (d),
triplet (t), quartet (q), dd (doublet of doublets), q (quadruplet), quint
(quintuplet) or m (multiplet). Low resolution mass spectra were
measured at 298 K by direct injection of dilute sample into the mass
analyzer of an Agilent Technologies 1260 Infinity LC-MS instrument.
High-resolution mass spectrometry (HRMS) measurements were per-
formed by ESI-QTOF mass spectrometry (Bruker Daltonics MicroTOF-Q
2.2.4. Synthesis of compounds P1–P4
In a 10 mL microwave tube, P (200 mg, 0.69 mmol) and the corre-
sponding acceptor group (0.65 mmol) were suspended in absolute
ethanol (4 mL). One drop of piperidine was added before sealing of the
microwave tube. The reaction was heated to 100 ◦C by microwave
irradiation during 30 min. After cooling down to RT, the precipitated
dye was filtered and washed with cold ethanol. The dyes were purified
by column chromatography over silica gel using the eluent indicated.
´
II) at the Centre Commun de Spectrometrie de Masse (UCBL, Villeurbanne,
France). Compound 1[53], 2 and 3[52], and 4 [50] were obtained ac-
cording to reported procedure.
2.2.5. P1
From P and 1 (130 mg). Eluent: CH2Cl2. Red solid (200 mg, 65%). 1H
NMR (400 MHz, CD2Cl2, δ/ppm) 7.66–7.59 (m, 3H), 7.42–7.32 (m,
11H), 7.08 (d, J = 16.5 Hz, 2H), 1.78 (s, 6H); 13C{1H}-NMR (125 MHz,
CD2Cl2, δ/ppm) 175.8, 174.3, 147.0, 145.0 (d, JP-C = 15.4 Hz), 136.5 (d,
JP-C = 11.1 Hz, 2C), 134.4 (d, JP-C = 20.2 Hz), 134.3 (d, JP-C = 18.3 Hz),
129.7 (2C), 129.2 (d, JP-C = 7.3 Hz), 129.0 (d, JP-C = 6.2 Hz), 115.9,
112.1, 111.5, 110.6, 100.8, 98.4, 26.6; 31P{1H}-NMR (202 MHz, CD2Cl2,
δ/ppm) ꢀ 4.12; HR-MS (ESI-QTOF, pos) m/z calcd for C30H23N3OP:
472.1573 [M+H]+, found: 472.1558.
2.2. Synthetic procedures and characterization data
2.2.1. 2-(4-Bromophenyl)-1,3-dioxolane [54–56]
4-Bromobenzaldehyde (2 g, 10.8 mmol) and p-toluenesulfonic acid
(0.2 g, 1 mmol) were dissolved in toluene (50 mL), and an excess of
ethylene glycol (2 mL, 35 mmol) was added. The mixture was then
heated to reflux overnight with azeotropic removal of water by a
Dean–Stark trap. After cooling, the solution was washed three times
with saturated aqueous Na2CO3, dried over Na2SO4, filtrated and
concentrated. Purification by column chromatography on silica eluting
with CH2Cl2/petroleum ether (v/v 1:1) gave a white solid (2.3 g, 93%).
1H NMR (300 MHz, CDCl3, δ/ppm) 7.51 (d, J = 8.4 Hz, 2H), 7.35 (d, J =
8.4 Hz, 2H), 5.77 (s, 1H), 4.15–3.98 (m, 4H); 13C{1H}-NMR (100 MHz,
CDCl3, δ/ppm) 137.2, 131.7, 128.3, 123.4, 103.2, 65.5; HR-MS (ESI-
QTOF, pos) m/z calcd for C9H9BrO2: 227.9780 [M]+, found: 227.9751.
2.2.6. P2
From P and 2 (173 mg). Eluent: CH2Cl2/petroleum ether (v/v 1:1) to
CH2Cl2. Yellow solid (250 mg, 71%). 1H NMR (400 MHz, CD2Cl2,
δ/ppm) 8.32 (d, J = 16.9 Hz, 1H), 8.08–8.03 (m, 2H), 7.70–7.62 (m,
3H), 7.59–7.54 (m, 2H), 7.41–7.33 (m, 12H), 7.26 (d, J = 16.9 Hz, 1H),
1.70 (s, 6H); 13C{1H}-NMR (125 MHz, CD2Cl2, δ/ppm) 171.1, 165.0,
144.4, 142.9 (d, JP-C = 14.3 Hz), 140.2, 136.9 (d, JP-C = 11.3 Hz), 135.5,
134.6, 134.4 (d, JP-C = 18.7 Hz), 134.3 (d, JP-C = 20.1 Hz), 129.6, 129.5,
129.1 (d, JP-C = 7.2 Hz), 128.9, 128.7 (d, JP-C = 6.4 Hz), 124.4, 116.8,
85.7, 27.4; 31P{1H}-NMR (202 MHz, CD2Cl2, δ/ppm) ꢀ 4.86; HR-MS
(ESI-QTOF, pos) m/z calcd for C32H28O4PS: 539.1440 [M+H]+, found:
539.1425.
2.2.2. 2-(Diphenylphosphino)-1,3-dioxolane [57–59]
2-(4-Bromophenyl)-1,3-dioxolane (2.2 g, 9.6 mmol) was dissolved in
anhydrous THF under argon and cooled to ꢀ 78 ◦C. nBuLi (2.5 M in
hexanes, 4.25 mL, 10.6 mmol) was added dropwise and the mixture was
stirred for 1 h at ꢀ 78 ◦C. Chlorodiphenylphosphine (2.2 g, 10 mmol) in
anhydrous THF (5 mL) was then added dropwise. The temperature was
let heat up to RT and the solution was stirred overnight. The reaction
was quenched with saturated NH4Cl (4 mL) and the layers were sepa-
rated. The organic layer was washed with water and brine, dried over
Na2SO4, filtered and concentrated under vacuum to give a yellow oil.
The product was purified over silica gel chromatography eluting with
CH2Cl2 and a white solid was obtained (1.5 g, 47%). An attempt to
recrystallize the product in EtOH only led to partial oxidation of the phos-
phine. 1H NMR (300 MHz, CDCl3, δ/ppm) 7.48–7.41 (m, 2H), 7.39–7.26
(m, 12H), 5.80 (s, 1H), 4.15–4.00 (m, 4H); 13C{1H} NMR (100 MHz,
CDCl3, δ/ppm) 138.7 (d, Jp-c = 11.5 Hz), 138.5, 137.1 (d, JP-C = 10.7
Hz), 133.9 (d, JP-C = 19.5 Hz), 128.9, 128.7 (d, JP-C = 7.0 Hz), 126.7 (d,
JP-C = 7.0 Hz), 103.6, 65.5; 31P{1H}-NMR (162 MHz, CDCl3, δ/ppm)
ꢀ 5.6; HR-MS (ESI-QTOF, pos) m/z calcd for C21H20O2P: 335.1195
[M+H]+, found: 335.1201.
2.2.7. P3
From P and 3 (100 mg). Eluent: CH2Cl2. Red solid (140 mg, 51%). 1H
NMR (400 MHz, CD2Cl2, δ/ppm) 7.68 (d, J = 16.5 Hz, 1H), 7.62–7.57
(m, 2H), 7.42–7.30 (m, 12H), 6.95 (d, J = 16.4 Hz, 1H), 1.67 (s, 6H); 13
C
{1H}-NMR (125 MHz, CD2Cl2, δ/ppm) 176.4, 166.3, 145.3, 143.4 (d, JP-
C = 14.9 Hz), 136.7 (d, JP-C = 11.1 Hz), 134.7, 134.3 (d, JP-C = 20.2 Hz),
134.3 (d, JP-C = 18.8 Hz), 129.6, 129.1 (d, JP-C = 7.2 Hz), 128.6 (d, JP-C
= 6.2 Hz), 116.0, 112.5, 99.6, 87.4, 26.1; 31P{1H}-NMR (202 MHz,
CD2Cl2, δ/ppm) ꢀ 4.67; HR-MS (ESI-QTOF, pos) m/z calcd for
C
27H23NO2P: 424.1461 [M+H]+, found: 424.1448.
2.2.8. P4
From P and 4 (121 mg). Eluent: CH2Cl2. Red solid (70 mg, 23%).1H
NMR (400 MHz, CD2Cl2, δ/ppm) 7.51–7.47 (m, 2H), 7.40–7.28 (m,
12H), 7.07 (s, 2H), 6.85 (s, 1H), 2.61 (s, 2H), 1.48 (s, 2H), 1.07 (s, 6H);
2.2.3. 4-(Diphenylphosphino)benzaldehyde P [57–59]
13C{1H}-NMR (125 MHz, CD2Cl2, δ/ppm) 169.7, 154.2, 140.2 (d, JP-C
=
2-(Diphenylphosphino)-1,3-dioxolane (1.5 g, 4.5 mmol) was dis-
solved in a toluene (10 mL)/water (1 mL) mixture before addition of p-
toluenesulfonic acid (20 mg, 0.1 mmol). The mixture was refluxed and
the reaction monitored by TLC. After completion, the mixture was let
cool down to RT and more water (5 mL) was added. The mixture was
extracted 3 times with ethyl acetate. The combined organic phases were
washed 3 times with saturated Na2CO3, dried over Na2SO4, filtered and
concentrated to give a yellow oil. The crude was purified over silica gel
chromatography with CH2Cl2 as eluent to obtain P as a white solid (1.05
g, 81%). 1H NMR (300 MHz, CDCl3, δ/ppm) 10.0 (s, 1H), 7.85–7.77 (m,
2H), 7.46–7.29 (m, 12H); 13C{1H}-NMR (100 MHz, CDCl3, δ/ppm)
192.1, 146.6 (d, JP-C = 15.6 Hz), 136.1, 135.9 (d, JP-C = 10.5 Hz), 134.2
(d, JP-C = 20.2 Hz), 133.7 (d, JP-C = 18.5 Hz), 129.5, 128.9 (d, JP-C = 7.4
Hz); 31P{1H}-NMR (162 MHz, CDCl3, δ/ppm) ꢀ 4.3; HR-MS (ESI-QTOF,
pos) m/z calcd for C19H16OP: 291.0933 [M+H]+, found: 291.0926.
13.3 Hz), 137.2 (d, JP-C = 11.3 Hz), 136.6, 136.5, 134.32 (d, JP-C = 18.4
Hz), 134.2 (d, JP-C = 19.8 Hz), 130.2, 129.4, 129.0 (d, JP-C = 7.1 Hz),
127.8 (d, JP-C = 6.6 Hz), 124.3, 113.9, 113.2, 79.3, 43.3, 39.5, 32.3,
28.1; 31P{1H}-NMR (202 MHz, CD2Cl2, δ/ppm) ꢀ 5.29; HR-MS (ESI-
QTOF, pos) m/z calcd for C31H28N2P: 459.1985 [M+H]+, found:
459.1975.
2.3. Spectroscopy
Absorption spectra were recorded on a JASCO V670 spectropho-
tometer. Excitation and fluorescence emission spectra were recorded
using a Horiba-Jobin Yvon Fluorolog-3 spectrofluorimeter equipped
with a Hamamatsu R928 or water-cooled R2658 photomultiplier tubes.
Spectra were corrected for the intensity variations of both the excitation
light source (lamp and grating) and the emission spectral response
(detector and grating). All solvents were of spectrophotometric grade.
Solid state measurements were performed using a calibrated integrating
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M. Remond et al.
Dyes and Pigments 193 (2021) 109485
sphere (model F-3018 from Horiba Jobin Yvon) collecting the full
3. Results and discussion
emission (2π steradians covered with spectralon®), and quantum yields
were measured as described previously[53].
3.1. Synthesis
Absorption and fluorescence spectra in acetone and in acetone/water
mixtures were measured for 10
μ
M solutions. Suspensions were made
Synthesis of compounds P1–P4 from 4-bromobenzaldehyde is
depicted in Scheme 1. First, the aldehyde function was protected by
formation of an acetal with ethylene glycol[54–56]. Then, the P-center
was formed by a lithium-halogen exchange using nBuLi at ꢀ 78 ◦C fol-
lowed by nucleophilic substitution on chlorodiphenylphosphine.
Finally, deprotection in acidic conditions gave 4-(diphenylphosphino)
benzaldehyde P[57–59]. Note that, in our hands, this four steps meth-
odology offered a much better overall yield of 4-(diphenylphosphino)
benzaldehyde (35%) than direct palladium catalyzed phosphination of
4-bromobenzaldehyde using triphenylphosphine[64]. Compounds
P1–P4 were obtained in moderate to good yield (23 %–71%) by a
Knoevenagel reaction with the active exocyclic methyl group of the
corresponding electron-acceptors derivatives 1–4 [52] in ethanol using
a catalytic amount of piperidine. The later step was performed under
microwave irradiation, giving better yields than traditional heating.
The stability of the powder dyes to oxidation was verified by 1H and
31P NMR. After two years of storage at room temperature (15–30 ◦C)
under air, the NMR of the redissolved powder showed no P(V) peak in
31P NMR, nor any change of the protons’ chemical shift. On the other
hand, colorless solutions of P2 or P3 in acetone turned slightly yellow
after one week.
directly in the quartz fluorescence cells. Water and acetone were mixed
in the cell in the proportion of the given water fraction (fw) to give a
final volume of 2.5 mL 25 μL of stock dye solution (1 mM) in acetone
were then quickly injected into the solvent mixture. The mixture was
then shaken several times by hand to obtain the final suspension with a
10
μM dye concentration. α values were calculated by dividing the
emission in acetone/water 10:90 mixture by the emission in acetone
solution. Solvatochromism was measured on 8
μM (P1) and 4 μM
(P2–P4) solutions of the dyes in toluene, CH2Cl2 or acetone.
2.3.1. Crystallography
Single crystals of P1 suitable for X-ray diffraction were grown by
slow diffusion of a non-solvent (EtOH) in a concentrated solution of dye
(CHCl3) at room temperature. CCDC 1563373 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via www.cc
dc.cam.ac.uk/data_request/cif.
2.3.2. Computational details
All calculations were performed using Gaussian package[60]. At the
ground state, the structure of all the compounds was optimized at the
DFT/6-31G(d) level of theory using the M06-2X hybrid functional[61].
Bulk solvent effects (acetone) were considered using a continuum sol-
vation model (C-PCM)[62]. For each compound, 10 vertical excitation
energies and the associated excited state densities of interest were
computed at the TD-DFT level with the same basis set and with the
M06-2X functional. The charge transfers transitions were characterized
thanks to the DC,T index developed by Le Bahers et al. [63] using the
cubegen utility provided by the Gaussian package.
3.2. Spectroscopic properties in solution
The optical properties of the dyes were studied in dilute acetone
solutions. Spectra are shown in Fig. 2 and the optical properties are
summarized in Table 1. The broad absorption of the dyes in the UV or
visible range showed typical charge transfer transitions. For a given
acceptor group, the absorption maxima were considerably blue-shifted
for the P-dye compared to the amine dye. Whereas the absorption
maxima of N1–N4 were located between 460 nm and 540 nm, the ab-
sorption maxima of P1–P4 ranged between 370 nm and 420 nm, with
only P1 in the visible range. This blue-shift, of 5000 cmꢀ 1 on average,
Scheme 1. Synthetic route to the target compounds P1–P4.
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M. Remond et al.
Dyes and Pigments 193 (2021) 109485
chromophores P1–P3 were not emissive in any solvent. Only P4 showed
a weak orange fluorescence (centered at 600 nm) in toluene (Φ not
measured, Fig. SI 5).
3.3. Spectroscopic properties in the solid-state
3.3.1. Aggregation-induced emission (AIE) properties
Interestingly, things considerably changed when studying the optical
properties in the solid state. We have previously shown that the N-
chromophores exhibited aggregation induced emission properties (AIE):
the corresponding data are presented in Table
1
[51,52].
Nano-precipitation experiments were therefore performed to access the
eventual AIE properties of the P-based chromophores. Absorption and
emission spectra were recorded from different acetone/water mixtures
of P2 (10 μM) with increased volume fractions of water (fw). As shown in
Fig. SI 6, increasing the water fraction of the acetone/water mixtures
above a critical water fraction (fw > 0.65 here) resulted in the appear-
ance of an orange fluorescence (λem = 630 nm) as well as a severe
broadening of the absorption spectra, which is characteristic of AIE ef-
fect. A 170-fold increase of αAIE (defined as the ratio in the fluorescence
intensity between the aggregate and the solution in pure acetone, fw = 0)
was attained for fw = 0.9. This is 17-fold more than for N2 in the same
conditions.
The other P-based chromophores P1, P3 and P4 also showed similar
AIE properties in the orange to far-red region, and only the data for an
acetone/water ratio fw = 0.9 are given in Table 1 and Fig. 3. The αAIE
observed for the P-dyes are 7- to 70-folds higher than for the corre-
sponding N-dyes. As observed for the dilute solution, the absorption
spectra of the P-chromophores aggregates were strongly blue-shifted
compared to their N analogues (around 5000 cmꢀ 1). Hypochromic
shifts were also observed in emission, albeit much less pronounced
(around 1000 cmꢀ 1). Indeed, the emission maxima obtained for the P-
dyes ranged from 635 to 705 nm (against 655–750 nm for the N-dyes).
Note the particularly long emission wavelength for compound P1 (705
nm), which is particularly interesting for the design of turn-on probe for
analytical or biological applications and is remarkable for a fluorophore
with such a small molecular weight. This is the consequence of a
considerably larger Stokes shift for the four P-based dye aggregates
(9000-11000 cmꢀ 1) than for their N-based analogues (5000-6000
cmꢀ 1). This can be attributed to consequent geometrical changes be-
tween the ground state and the first excited state, as quantum chemical
calculations revealed (vide infra).
Fig. 2. Absorption spectra of P1–P4 (plain lines) and N1–N4 (dotted lines) in
dilute solution in acetone.
was likely due to the lower donating capacity of phosphorus compared
to nitrogen caused by the tetrahedral geometry of the former[14]. As
anticipated, for a given donor group, increasing the strength of the
electron accepting part led to an increase in the charge transfer, visible
by the red-shift of the absorption. Finally, molar absorption coefficients
ε
were in the range of 30 000 to 50 000 Mꢀ 1cmꢀ 1, which is very similar
to what we previously reported for such small charge-transfer chromo-
phores[51,52]. Note that for both P- and N- compounds the donor group
does not seem to have a strong influence on the molar absorption co-
efficients values. A small solvatochromism is observed in absorption for
all the dyes (Fig. SI 1 to SI 4, Table SI 1), and is higher for P-dyes
compared to N-dyes. The highest effect is observed for P2 with a 1446
cmꢀ 1 difference between the maximum in acetone and in
dichloromethane.
In a dilute solution at room temperature, N-based chromophores
N1–N4 showed a moderate emission intensity that is strongly dependent
on the solvent with a large red-shift, along with decrease in the quantum
yield emission when the solvent polarity is increased[52]. A measurable
emission was still obtained in polar acetone. On the contrary, P-based
3.3.2. Emission in the crystalline state
As shown in Fig. 4, P-based chromophores P1–P4 are also emissive in
crystalline powder. Emission maxima ranged from 620 nm (P4) to 650
nm (P1) for measured quantum yield of a few percent (Table 1). In
comparison, only N2 and N3, bearing the weaker acceptors, are emissive
Table 1
abs
em
abs
Absorption maxima (λ ), molar absorptivity (ε), emission maximum (λ ) for each dye in solution in acetone; Absorption and emission maxima (λ ), αAIE and
max max
em
max
Stokes Shift (Δν) for nanoaggregates obtained in acetone/water mixture (10:90); Emission maximum (λ ) and quantum yield (ϕF) in solid powder.
max
Solution
Nanoaggregates
Solid
λ
(nm)
ε
(L.molꢀ 1cmꢀ 1
)
λ
(nm)
λ
(nm)
λ
(nm)
αAIE
Δν
(cmꢀ 1
)
λ
(nm)
ϕF (%) b
abs
em
abs
em
a
em
max
max
max
max
max
P1
P2
P3
P4
N1
N2
N3
N4
420
365
370
400
540
465
465
480
39 500
34 100
29 000
53 700
52 100
42 300
41 000
35 300
–
–
–
–
745
670
656
689
425
370
380
405
535
470
485
490
705
630
625
635
750
665
655
700
90
170
50
35
8
9350
11 150
10 300
8950
5350
6250
5350
6100
650
635
655
620
–
635
645
–
1
5
4
7
–
34
9
–
10
7
0.5
a
Defined as the fluorescence intensity ratio between acetone/water mixture (10/90) and pure acetone solution.
Measured using a calibrated integrating sphere.
b
5

´
M. Remond et al.
Dyes and Pigments 193 (2021) 109485
Fig. 3. a) Absorption and b) normalized emission spectra of dyes aggregates obtained in a 10:90 acetone/water mixture (10
μ
M)). The phosphorus based dyes P1–P4
are represented using plain lines and the nitrogen based dyes N1–N4 with dotted lines. λexc = 410 nm for P-dyes and λexc = 490 nm, for N-dyes except for N1, λexc
=
540 nm.
3.4.1. Structural variations between the ground- and the first excited-state
For the P-dyes P1–P3, considerable structural variations can be
observed between the optimized structures of the ground state and first
excited state in acetone, especially around the P-center. In the ground
state, the P-centers adopt a trigonal pyramidal geometry, with an
average height (h) of 0.79 Å for the pyramidal structure. Upon excita-
tion, in the first excited state, the planarity of the compounds increases
characterized by a decrease of the pyramid height of 0.20 Å on average.
This increase of the planarity of the phosphorus donor and the conju-
gation of the
π system is not observed for the P4 compound which
maintains the pyramidal geometry of its ground state (Table 2). Inter-
estingly, this order of geometry reorganization according to the electron
acceptor (0.00 for P4, 0.13 for P1 and 0.22 for both P2 and P3) perfectly
matches with the order of Stokes shift values obtained experimentally
(P4 < P1 < P3 ≈ P2) and the fact that P4 is the only P-dye fluorescent in
(apolar) solution.
By contrast, for the N-dyes N1–N4, the geometry around the N donor
atom is trigonal planar, both in the ground states and in the first excited
states, with an average height of 0.01 Å. The comparison between the
ground states and excited states of P1 and N1 is given in Fig. 5a.
3.4.2. Electronic density variations
The electronic density variations were also calculated and are rep-
resented in Fig. 5b for N1 and P1 and in SI for the other dyes. The red
(blue) area represents a decrease (respectively enhancement) of the
electronic density during the transition. In both N-dyes and P-dyes, the
Fig. 4. Crystal state fluorescence spectra of dyes P1–P4 (plain lines) and
N2–N3 (dotted lines). λexc = 390 or 400 nm for all dyes.
in crystalline powder, with quantum yields of 34% at 635 nm and 8% at
645 nm, respectively[51,52]. Here again, the hypsochromic shift
observed between P-based dyes and their N-analogues is small.
Table 2
Heights for the pyramidal structure of the different compounds from DRX ex-
periments (hRX) or optimized structures of both ground and excited states from
DFT/M062X calculations (respectively hDFTGS and hDFTES).
hRX
hDFTGS (Å)
hDFT
(Å)
3.4. Theoretical studies and comparison with crystal structures
(Å)GS
ES
To rationalize the optical properties observed, quantum chemical
calculations were performed. A benchmark of exchange correlation
functionals on the calculation of the S0–S1 transition energies compared
to experimental values identifies M062X as the best functional for our
study (Table SI 2). This functional was used in the rest of the study.
N1
N2
N3
N4
P1
P2
P3
P4
–
0.01
0.01
0.02
0.01
0.79
0.79
0.80
0.79
0.00
0.00
0.01
0.01
0.66
0.57
0.57
0.78
0.07
0.05
0.06
0.82
–
–
–
6

´
M. Remond et al.
Dyes and Pigments 193 (2021) 109485
Fig. 5. a) Structural variations between the optimized structures of the ground state and the first excited state in acetone for P1 (top) and N1 (bottom) compounds.
The optimized structures of both the ground and the excited states are shown on the left, while the pyramidal geometry around the P or N center is shown on the
right. P and N-centers are represented in cyan and pink respectively, connected carbon atoms in green. b) Electronic density variations of P1 (top) and N1 (bottom)
compounds. Red (blue) area represents a decrease (respectively enhancement) of the electronic density during the transition.
electronic density of the donor is depleted and that of the acceptor is
enriched, which is consistent with the character of the strong charge
transfer. However, the depletion of the donor side is quite different
depending on the donor atom. In the case of the (C6H5)2P- group, the
charge transfer mainly originates from the lone pair of the phosphorus
atom, highly localized on the upper side of the chromophore plane as
shown in Fig. 5b. In the case of a (C6H5)2N- donor, there is a stronger
contribution of the orbitals of the phenyls rings and the lone pair is
localized on both sides of the chromophore plane. Quantitatively, we
estimated the strength of the charge transfer associated to the force
transition by computing the DCT index (Table SI 4), a parameter defining
the spatial extent associated to the transition as well as the associated
fraction of electron transferred[63]. From P- to N-dyes, the average DCT
increases by around 10%, from 4 Å to 4.4 Å, consistent with the stronger
charge transfer within the amine dyes, as well as the implication of the
orbitals of the phenyls.
pyramid height of 0.06 Å (Fig. 6b, Fig. SI 14). Note that N2 and N3 show
very similar planarity in their crystal structures (Figure SI-11 to SI-13).
The P1
π
system is very similar to that of N4, with a small 12◦ angle
between the acceptor and donor mean planes. On the other hand, Fig. 6
clearly shows the pyramidal configuration of phosphorus in P1. The
measured pyramid height is 0.82 Å (Fig. 6a, Fig. SI 15), very close to the
calculated value (0.79 Å) obtained by optimization of the ground state
structure in acetone. Those geometry differences have consequences for
the position of the lone pair, perpendicular to the
π system for N-dyes
and tilted to the back and to the side for P-dyes, thus, on the conjugation
of the donor with the rest of the chromophore. All computationally
obtained pyramid heights are given in Table 2, along with available DRX
data. Finally, for both chromophores, neighbors are arranged in a
dimeric head-to-tail configuration with an intermolecular distance of
3.2 Å for N4 and 3.1 Å for P1 (Fig. SI 16). Despite the enhanced steric
hindrance of the out-of-plane phenyls of P1 compared to N4, neigh-
boring dyes are actually closer in the crystal arrangement of compound
P1. On a larger scale, the herringbone arrangement of P1 dyes, as
viewed down the crystallographic a axis (Fig. SI 17), is usually consid-
ered as beneficial for solid state fluorescence.
3.4.3. Comparison with X-ray crystal structures
To probe the phenomenon more deeply, we studied the crystalline
structures. Crystals suitable for X-ray diffraction analysis were obtained
for P1, by slow diffusion of ethanol in chloroform. We previously re-
ported the crystal structures of N-dyes N2[51], N3 [52] and N4 [50].
Unfortunately, all attempts to obtain crystals for N1 and other P-dyes
P2–P4 only produced thin needles, unsuitable for X-ray diffraction. So,
despite the acceptor difference, we chose to compare the solid-state
structures of P1 and N4, selected as a model of N-dyes (Fig. 6). The
4. Conclusion
The synthesis of four novel dyes based on the diphenylphosphino
donor-group enabled us to study their optical properties in solution and
in the solid state and compare them with the diphenylamino-analogues.
Despite being quenched in solution, probably due to the intramolecular
planarization movement of the phosphine donor, they light up in solid
state (amorphous aggregates or crystalline powder) as typical AIE was
N4
π
system is quasi-planar, with only a small 10◦ angle between the
acceptor and donor mean planes. In agreement with the calculations, the
triphenylamino-moiety adopts a trigonal planar configuration, with a
7

´
M. Remond et al.
Dyes and Pigments 193 (2021) 109485
Fig. 6. Comparative views of the X-ray crystal structures of a) P1 and b) N4, showing the pyramidal configuration of the phosphorus and the trigonal planar
configuration of the nitrogen atom (ORTEP views with 50% probability displacement ellipsoids. H atoms are omitted for clarity. Compound N4 crystallized with an
acetonitrile molecule).
demonstrated by nanoprecipitation procedure involving a solvent
shifting process. Phosphine aggregates showed higher fluorescence turn-
on coefficients upon aggregation (170-fold increase emission for P2)
than their amine analogues, but above all much larger Stokes shift (10
000 cmꢀ 1 on average), that can be attributed to large conformational
changes around the P-atom between the ground state and the first
excited state. In fine, we believe that the diphenylphosphino-can be an
interesting donor group for the design of large Stokes shift fluorophores
and could be used to control intramolecular packing in the solid-state, or
could even be used in molecular machines as they are capable of pro-
ducing small but reproducible planarity change upon light absorption.
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