A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor-π-Acceptor Phosphonium Fluorophores

Article abstract of DOI:10.1002/anie.201906929

The D-π-A type phosphonium salts in which electron acceptor (A=-⁺PR₃) and donor (D=-NPh₂) groups are linked by polarizable π-conjugated spacers show intense fluorescence that is classically ascribed to excited-state intramolecular charge transfer (ICT). Unexpectedly, salts with π=-(C₆H₄)_n- and -(C₁₀H₆C₆H₄)- exhibit an unusual dual emission (F₁ and F₂ bands) in weakly polar or nonpolar solvents. Time-resolved fluorescence studies show a successive temporal evolution from the F₁ to F₂ emission, which can be rationalized by an ICT-driven counterion migration. Upon optically induced ICT, the counterions move from -⁺PR₃ to -NPh₂ and back in the ground state, thus achieving an ion-transfer cycle. Increasing the solvent polarity makes the solvent stabilization dominant, and virtually stops the ion migration. Providing that either D or A has ionic character (by static ion-pair stabilization), the ICT-induced counterion migration should not be uncommon in weakly polar to nonpolar media, thereby providing a facile avenue for mimicking a photoinduced molecular machine-like motion.

Full text of DOI:10.1002/anie.201906929

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Accepted Article
Title: A Facile Molecular Machine: Optically Triggered Counterion
Migration via Charge Transfer of Linear D-π-A Phosphonium
Fluorophores
Authors: Andrey Belyaev, Yu-Hsuan Cheng, Zong-Ying Liu, Antti J.
Karttunen, Pi-Tai Chou, and Igor O. Koshevoy
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906929
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10.1002/anie.201906929
Angewandte Chemie International Edition
RESEARCH ARTICLE
A Facile Molecular Machine: Optically Triggered Counterion
Migration via Charge Transfer of Linear D-π-A Phosphonium
Fluorophores
†
†
†
Andrey Belyaev,[a] Yu-Hsuan Cheng,[b] Zong-Ying Liu,[b] Antti J. Karttunen,[c]* Pi-Tai Chou,[b]* Igor
O. Koshevoy[a]*
Abstract: The D–π–A type phosphonium salts 1–9, in which electron
acceptor (A = –+PR3) and donor (D = –NPh2) groups are linked by
polarizable π-conjugated spacers, show intense fluorescence
classically ascribed to the excited-state intramolecular charge transfer
(ICT). Unexpectedly, salts 1–3 (π = –(C6H4)n–) and 7 (π = –
(C10H6C6H4)–) exhibit unusual dual emission (F1 and F2 bands) in
weakly or nonpolar solvents. The time-resolved fluorescence studies
show a successive temporal evolution from F1 to F2 emission region.
The results most plausibly can be rationalized by an ICT-driven
counterion migration, which is evidenced by the strongly anion-
dependent emission spectra and relaxation dynamics. Upon optically
induced ICT, the counterions move from –+PR3 to –NPh2 and return
back to –+PR3 in the ground state, achieving an ion transfer cycle.
Increasing the solvent polarity makes the solvent stabilization
dominant, and virtually ceases the ion migration. As long as either D
or A is endowed with ionic character, due to the static ion-pair
stabilization, the ICT induced counterion migration should not be
uncommon in weakly to nonpolar media, paving a new and facile
avenue for mimicking a photoinduced molecular machine-like motion.
the development of sensing and signaling systems.[7] Furthermore,
tunable photophysical behavior in combination with
biocompatibility of small (element)-organic dyes makes them
suitable candidates for numerous fluorescence bio-imaging
applications.[8]
Among structurally versatile multipolar chromophores, which are
generally based on (D–π)n–A or (A–π)n–D (n = 1–4) architectures,
the simplest design relies on the dipolar D–π–A arrangement
where π is an unsaturated spacer. Control over physical
properties of these push–pull chromophores can be achieved by
changing the strengths of donor/acceptor groups and aromaticity
of
a π–linker. Such manifold modifications provide rich
opportunities to harness the energies of the frontier orbitals, and
the efficiency of electronic transitions between them.[9] As a result,
modulation of the ICT allows to reach low energy emission,[8c, 10]
11]
positive and negative solvatochromism,[10a,
persistent
phosphorescence,[12] stimuli-responsive and phase-dependent
luminescence.[11d,
Moreover, the dipolar chromophores with
13]
two non-equilibrated close lying excited states, can demonstrate
dual emission. This behavior is manifested by high energy and
red-shifted low energy bands, typically assigned to locally excited
(LE) and ICT states. Albeit dual luminescence has been actively
studied for the D–π–A systems[14] since 1960s, when it was
observed for 4-dimethylaminobenzonitrile,[15] there is still an open
debate concerning the mechanism of the LE↔ICT
population/equilibrium and its relationship with molecular
structure-geometry.[1, 16]
Despite there is a considerable number of organic electron
withdrawing motifs A (containing cyano, nitro, carboxy and
carbonyl groups, quaternized N-heterocycles),[9] strong acceptors,
which can be tailored to the range of conjugated backbones in a
synthetically facile way in order to decrease the HOMO–LUMO
Introduction
Conjugated molecules comprising electron-rich donor (D) and
electron-deficient acceptor (A) often demonstrate a fundamental
phenomenon of intramolecular charge transfer (ICT), which plays
a key role in many practically important processes related to the
energy conversion.[1] Thus, it is not surprising that donor–acceptor
compounds hold a prominent position in materials science[2] and
have been extensively used in organic electronics, in particular for
the development of organic electroluminescent devices,[3] solar
cells,[4] field effect transistors,[5] non-linear optical materials,[6] Due
to a pronounced dependence of the energy of electronic
transitions on the stereochemistry and the polarity of the donor–
acceptor pair, these CT molecules have been widely exploited in
gap, are rather limited. In this context, the main group element
10d, 17]
acceptors based on boron,[10a,
sulfur[18] and especially
phosphorus[19] offer appealing opportunities to yield D–π–A
molecular systems with strong ICT character.
A simple modification of λ3σ3 phosphorus center in tertiary
phosphines produces λ5σ4 (by chalcogenation) and λ4σ4
derivatives (by alkylation/arylation, boronation, complexation to
the metal ions). Notably, the phosphonium cations, being
noticeably stronger acceptors compared to the neutral
oxide/sulfide species, lead to substantial bathochromic shifts of
absorption/emission.[20] Surprisingly, notwithstanding such
distinct electron deficient properties of a quaternized –R3P+ motif,
it has been seldom combined with extended π–D conjugated
fragments to generate push-pull chromophores in a systematic
manner.[21] Most of the relevant reports concern cationic P-
heterocyclic compounds, containing 5-membered phosphole
moiety[20a, 22] and the variations of 6-membered rings (phosphinine,
phosphaacenes).[23]
[a]
[b]
[c]
A. Belyaev, Prof. I. O. Koshevoy
Department of Chemistry, University of Eastern Finland
Yliopistokatu 7, 80101 Joensuu (Finland)
E-mail: igor.koshevoy@uef.fi
Y.-H. Cheng, Z.-Y. Liu, Prof. P.-T. Chou
Department of Chemistry, National Taiwan University
Taipei 106 (Taiwan)
E-mail: chop@ntu.edu.tw
Prof. Antti J. Karttunen
Department of Chemistry and Materials Science, Aalto-University
FI-00076 Aalto (Finland)
E-mail: antti.j.karttunen@iki.fi
†A. Belyaev, Y.-H. Cheng and Z.-Y. Liu contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201906929
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RESEARCH ARTICLE
Prompted by poor utilization of the –R3P+ cationic group as a
pendant strong electron acceptor, we have developed a series of
D–π–A chromophore molecules where the polarizable π-
conjugated spacers comprise linear oligophenylene/acene-
phenylene blocks, and diphenylamino –NPh2 fragment serves as
a typical donor. The photophysical studies of theese ionic dyes
reveal efficient photoemission in a wide range of the visible
spectrum, good two-photon absorption properties, and interesting
solvatochromism. Unexpectedly, in weakly and nonpolar solvents,
certain salts demonstrate dual emissions, which are considered
uncommon for ionic species.[24] This remarkable behavior, which
depends on a delicate interplay of counterion and π–spacer, in
addition to the polarity of the medium, illustrates a fundamentally
novel approach to molecular systems, producing panchromatic
emission as well as molecular machine-like motion.
Results and Discussion
Synthesis and characterization. In accordance to chosen
molecular design Ph2N–π–+PR3, bromoarylamines Ph2N–π–Br (π
= –(C6H4)n–, n = 1–4; 1,4-naphthalene; 9,10-anthracene) were
used as key precursors (Schemes S1 and S2, ESI).[25] For the P–
C bond formation, we followed the Ni-catalyzed method,[26] which
delivered the target chromophores 1–5 and 2Cbz in moderate to
high yields (34–92%, Schemes 1 and S2, method I). The
efficiency of this reaction showed a perceptible decrease with
increasing the length of oligophenylene spacer and with
extending the conjugated system. This yield variation might be
due to (i) a lower solubility of the longer bromoaminoaryl
precursors, and (ii) steric hindrance and electronic factors, which
likely suppress oxidative addition of polyaromatic bromides to the
reactive nickel center.
Scheme 1. Synthetic procedures of phosphonium salts 1–8. (a) PPh3, NiBr2,
ethylene glycol, 180–200 oC, 5h; (b) i. n-BuLi 2.5 M, THF, –78 oC, 1h; ii. PPh2Cl,
–78 oC → rt, 2h; (c): MeI, CH2Cl2, 2h.
Consequently, no phosphonium salt could be identified in the
case of 10-bromo-9-(N,N-diphenylamino)-anthracene (Scheme
S3), including the modified Charette (Scheme S3, condition b),[26d]
Migita[26b] and Heck[27] palladium catalyzed syntheses (Scheme
S3). Because of these failures, we explored an alternative two-
step approach. It involved the preparation of tertiary aryl
phosphines, which were treated with methyl iodide to give salts
6–8 (Scheme 1, method II). Diphosphonium salt 9 with two
different D–π–A fragments (Scheme 2) was synthesized via
sequential alkylation procedure shown in Scheme S4 (overall
27% yield) starting from the donor-functionalized phosphines[28]
and diiodopropane.
Scheme 2. Structures of 2Cbz and 9.
The 31P{1H} NMR singlet resonances of the salts 1–8, 2Cbz are
found in the range 17.8–23.7 ppm, typical for other cationic
phosphorus derivatives.[29] The dication 9 shows two 31P signals
(δ = 23.4 and 22.0 ppm) due to the presence of two inequivalent
P
nuclei, which demonstrate distinct long-range magnetic
coupling (4JP-P = 9.9 Hz).
The ESI+ MS of the phosphonium salts revealed the signals of the
mono- (1–8 and 2Cbz) and dications (9, Figure S1), which
correspond to the proposed compositions.
The structures of salts 1 and 2Cbz in the solid state (Figure 1; the
crystal data and selected structural parameters are given in
Tables S1 and S2) confirm that quaternized phosphorus
Figure 1. Molecular views of phosphonium salts 1 (left) and 2Cbz (right, one of
two independent molecules found in the unit cell is depicted) Thermal ellipsoids
are shown at the 50% probability level.
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10.1002/anie.201906929
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electron acceptor ability of –+PPh3 group vs –PPh2O. The values
of λabs for all studied species 1–9 are little dependent on solvent
polarity as a solvation sphere remains virtually unrelaxed upon
fast vertical excitation.[10a, 11b, 11e, 17e] Nevertheless, for all title salts
the lowest-lying absorptions in water systematically appear in a
blue region with respect to DCM solutions (hypsochromic shifts of
90 cm-1 for 1 to 1183 cm-1 for 6) that can be termed as negative
solvatochromism, described for other ionic bipolar dyes.[32]
As for the emission in polar solvents such as DCM, salts 1–4 and
7–9 exhibit one emission band each with high quantum yields of
0.71–0.95 under ambient conditions (Table S3 and Figure 2).
Short lifetimes of several ns (τobs = 2.5–14.2 ns in DCM) and
insensitivity of the intensity to molecular oxygen indicate prompt
fluorescence. Dyes
5
and
6
possessing polyaromatic
(naphthalene and anthracene) cores demonstrate substantially
weaker emission (0.29 and 0.02, respectively) due to visibly
smaller radiative rate constants and markedly larger non-radiative
rates (knr) in comparison to other compounds within this series
(Table S3). The anomalously large knr for 6 can be rationalized by
the steric strain due to the repulsion between pendant –
+PPh2Me/–NPh2 groups and the π-C-H of the anthracene moiety,
inducing dense but low-frequency torsional motions that
deactivate the excited state and hence quench the emission.
The distinct push–pull nature of these compounds results in the
ICT character of their emissive excited states. Consequently, 1–9
show structureless spectral profiles and large Stokes shifts
ranging from 4900 cm-1 (6) to 11300 cm-1 (4) in DCM. The latter
value for quaterphenylene dye 4 arises from its higher absorption
energy compared to shorter homologues
2 and 3, and
simultaneously from the longest excited state charge separation
(Figure S6), for which the solvent relaxation presumably leads to
a particularly strong stabilization.
Figure 2. Absorption (dashed lines) and emission (solid lines) spectra of 1–8 in
DCM solution (298 K, the photo shows solutions of 1–4 under UV light).
Variation of the π–spacer allows to alter the emission from blue
to red-orange retaining high quantum yield in DCM solution.
Increasing the number of phenylene rings from one to four in the
family of homologues 1–4 changes the fluorescence wavelength
in the order 487 nm (1) < 528 nm (2) < 577 nm (3) < 619 nm (4),
Figure 2. In line with the absorption data, a stronger phosphonium
acceptor induces appreciably larger emission red shift than P-
oxide and SF5 groups.[18c],[31],[33] Expectedly, using a weaker
carbazole donor in 2Cbz increases the energy of the excited CT
state and hypsochromically shifts the emission maximum to 478
nm (DCM). The extension of the π–system from phenylene to the
polycyclic naphthalene (5) and anthracene (6) motifs also causes
gradual growth of λem to 560 nm (5) and 679 nm (6) in DCM, which
is, however, accompanied by a drop of the quantum yield (vide
supra). Remarkably, tailoring phenylene (7) and ethynyl-
phenylene (8) moieties to the naphthalene core raises the
emission intensity along with successive decrease of its energy
(Φem = 0.95, λem = 579 nm for 7; Φem = 0.81, λem = 626 nm for 8).
The fluorescence wavelengths for the studied dyes reveal
bathochromic shifts with the increase of solvent polarity from DCM
to MeCN (Table S3). Compounds 1 and 2 are reasonably
resistant to the emission quenching in polar medium; Φem in
acetonitrile is 0.35 (1) and 0.51 (2), although in water quantum
efficiency is lowered to 0.12 (1) and 0.07 (2) primarily due to
elevated non-radiative rate constants upon lowering the emission
gap. Surprisingly, for compounds with longer π-spacers (3, 4, 7
and 8) an unexpected blue shift of the emission in water was
atoms adopt expected tetrahedral geometry, characteristic for the
phosphonium groups.[30]
The title compounds demonstrate high thermal stability according
to TGA analysis (Figure S2); the selected salts (2, 4, 5, 8) showed
minimum weight loss up to 300 oC.
Photophysical properties. In medium and high polar solvents.
The absorption and emission spectra for compounds 1–9 were
recorded in dichloromethane, methanol, acetonitrile and water
(Figures 2, 3 and S3, S4). The pertinent photophysical data are
tabulated in Table S3. In dichloromethane (DCM) solution, the
lowest-lying absorption bands can be assigned to the CT
transitions, which arise from their donor-acceptor nature and are
confirmed by the computational analysis (see Figures S5-S8).
The peak wavelengths (λabs) are gradually red-shifted along with
the extension of the π-spacer, 1 (phenylene, 333 nm) < 2
(biphenylene, 387 nm) < 5 (naphthalenylene, 407 nm) < 6
(anthrylene, 519 nm); additionally, similar trend can be found in
other solvents (Table S3 and Figure S3). Note that further
elongation of the phenylene chain (2-4) does not lead to a
decrease in the absorption energy (see Figure 2) that is also
confirmed theoretically (Table S4).
In comparison to the congener phenylene-based phosphine
oxides (Ph2N(C6H4)n)3P=O (n = 1–3),[31] the absorption bands for
the corresponding salts 1–3 show bathochromic shifts of 365 cm-
1
(1), 1709 cm-1 (2) and 1943 cm-1 (3), which reflect a higher
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Figure 4. UV-vis absorption and normalized emission spectra of 1–3 (bromide
salts) in toluene (the photo shows the corresponding solutions under UV light.
Figure 3. Absorption (dashed lines) and emission (solid lines) spectra of 3 in
different solvents (298 K). The photo shows (from left to right) water, DCM,
methanol and acetonitrile solutions of 3 under UV light).
of the emission profile probably implies the emergence of a weak
F2 shoulder.
detected (Δν = 1737 to 3759 cm-1 with respect to acetonitrile,
Figures 3, S3 and S4). Simultaneously, the decay curves for
aqueous solutions of the latter species were treated with
biexponential components (Table S3). This observation suggests
that for 3, 4, 7 and 8 two distinct excited species of comparable
energies exist in water, which might be ascribed to different
conformations regulated by the dynamics of a relatively viscous
environment.[34] Significantly higher emission energies for the
given salts in the polar protic solvent might be tentatively
explained by the particularly effective stabilization of the ground
state due to HOH…NPh2 hydrogen bonding and electrostatic
interaction H-Oδ–…+PR4 favoring large charge separation.
Salt 9 containing two chromophores (as in 1 and in 8) shows
fluorescence that is alike to that of 8 (Table S3). The single
emission, together with nearly twice larger molar absorptivity of 9
vs 8 around 330 nm, indicate complete excitation energy transfer
from phenylene- to naphthalene-based donor-acceptor fragment.
Taking advantage of the intense emission, we have also
investigated the two-photon absorption (TPA) properties of the
selected compounds 2–5 and 8 in water solutions by the Z-scan
method (Figure S9). Under the 800 nm excitation, oligophenylene
salts show moderately good[8a, 35] TPA cross section (δ) values of
263±15 GM (2), 299±85 GM (3) and 321±7 GM (4), which
progressively but slightly grow with the elongation of the π-spacer.
Dye 5 with naphthalene core features nearly the same TPA as
quaterphenyl compound 4 (δ = 313±28 GM), but its derivative 8
unveils a dramatic improvement of δ reaching 977±40 GM. The
given results show that such phosphonium salts can be potentially
applied in the bio-imaging field.
In the case of 2[Br] with biphenyl spacer, the total Φem in toluene
is increased to 0.17, whereas the F2 band gets more intense and
is shifted to ca. 600 nm, generating white light (Figure 4). For the
terphenyl-based dye 3[Br], the F2 signal experiences an additional
decrease in energy reaching 674 nm simultaneously with the drop
of F1/F2 intensities ratio, but the overall quantum efficiency
improves to 0.37. Similar dual emission properties for 2 are
observed in nonpolar solvents such as 1,4-dioxane and CCl4
(Figure S11 and Table 1). Note that the title compounds are
insoluble in alkane solvents such as cyclohexane.
More importantly, the emission properties strongly depend on the
nature of counter anions. We performed anion exchange for the
parent dye 2[Br] to give the series of 2[X] derivatives (X = Cl–, I–,
–
–
–
–
NO3 , ClO4 , PF6 , CF3SO3 , CF3COO–, tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (BARF–)). As shown in Figures 5
and S11, the substitution of the Br– for a bulkier triflate (OTf–)
significantly quenches the F1 band and enhances the F2 band,
resulting in a spectacular change of the emission color of salt 2 in
toluene and 1,4-dioxane from white (2[Br]) to yellow (2[OTf]). As
for the even larger BARF– anion, the emission seems to merge to
one band maximized at ~500 nm. The results manifest the
importance of counterion effect, elaborated below with time-
resolved measurements.
Because the biphenyl-containing salt 2 displays higher intensity
of the F2 signal and better solubility among species 1–3, 7, it was
chosen as a model for gaining detailed insights into the
mechanism. The excitation spectra of 2[Br] monitored at F1 and
F2 bands in toluene, shown in Figure S12, are identical, indicating
that the two emission bands originate from the same ground-state
species.
Therefore, the relationship between F1 and F2 bands is essentially
established during the lifespan of the excited state. The intensity
ratio for F1 versus F2 bands was found to be independent of 2[Br]
concentration (see Figure S13) that eliminates the possibility of
the excimer formation. Likewise, other concentration dependent
reactions, such as reversible micelle-like aggregation, could be
also excluded. In fact, 2[Br] and other salts exhibit only one
emission band (the F1 band) in the solid (Figure S14).
In weakly polar and nonpolar solvents. Unlike solely ICT
emission band in medium-to-strong polar solvents elaborated
above, 1–3 and 7 fluorophores reveal prominent dual emission
character in weakly and nonpolar solvents, qualitatively defined
as F1 and F2 for high energy and low energy bands, respectively.
Using 1–3 in toluene as an example (Figures 4 and S10), the ratio
of F1 and F2 bands evidently depends on the length of –(C6H4)n–
π-linker. For 1[Br] in toluene, it shows the lowest quantum yield
(Φem = 0.066); moreover, the visible broadening and asymmetry
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resolved measurements of salts 2 with different conterions in
toluene are shown in Figures 6, S15 and are listed in Tables 1,
S5. When monitoring at the emission wavelength of 430 nm,
being ascribed to the F1 band (Figure 4), two decay components
can be resolved, the time constants of which are fitted to be 67 ps
and 450 ps with an uncertainty error of ±5 ps. Upon monitoring
at the F2 band region of e.g., 650 nm, the overall kinetics can be
fitted by an obvious rise component of 93 ps and two decay
components of 792 ps and 2.4 ns (see Table 1). Note that the two
decay components are the minimum requirement for a good fit of
the F2 band. More importantly, depending on the selected
emission wavelength, upon best fitting, the resulting rise and
decay times are varied (see Figure 6 and Tables S6, S7). Having
acquired the relaxation dynamics at different wavelengths, we
were able to integrate each wavelength-dependent intensity at a
specific delay time and then generate the spectral temporal
evolution of 2[Br] in toluene (Figure 6A, C).
The results show the continuous spectral evolution rather than
two distinct emission bands. This viewpoint is also evidenced by
the lack of isoemissive point, indicating the existence or multiple
emitting species during the temporal evolution. Similar results
were observed for other representative compound 2[OTf] having
triflate as the counter anion, which also shows continuous spectral
evolution (Figure 6B, D). Note that their temporal spectral profiles
are drastically different, resulting in much difference in the steady-
state emission (cf. Figures 4 and 5).
The continuous relaxation dynamics are reminiscent of the polar
solvent relaxation that stabilizes the ICT state,[37] dubbed
solvatochromism (see Scheme 3A). In fact, in the polar solvents,
where the fraction of ion pairs of the title compounds is marginal,
the energy stabilization of the excited charge transfer state is
mainly governed by the dipole–dipole interactions between the
excited species and the nearest shell of solvent molecules,
Figure 5. Normalized emission spectra of 2 with different counterions in toluene
and 1,4-dioxane.
Therefore, the dual emission in weakly polar and nonpolar solvent
resulting from any aggregation effect can be discarded and
evidently originates from an intramolecular process. The time-
Table 1. Photophysical properties of 2[X] in toluene, CCl4 and 1,4-dioxane solutions at 298 K.
[a]
Solvent
λ
, nm
F1/F2

F1, ps (mon. 430 nm)

F2, ps (mon. 650 nm)
Φem
em
2[Cl] / 1.81 Å [b]
Toluene
1,4-dioxane
CCl4
468, 592
477
1.00/0.54
–
42 (0.67), 392 (0.33)
65 (0.57), 586 (0.43)
–
89 (-0.50), 785 (0.36), 2419 (0.14)
0.17
0.16
0.12
65 (-0.49), 658 (0.37), 1992 (0.24)
450, 606
1.00/0.12
–
2[Br] / 1.96 Å
67 (0.63), 450 (0.37)
73 (0.56), 600 (0.44)
–
Toluene
1,4-dioxane
CCl4
468, 592
477
1.00/0.41
–
93 (-0.49), 792 (0.36), 2439 (0.15)
0.17
0.15
0.14
73 (-0.51), 652 (0.37), 2066 (0.22)
450, 606
Column 3
–
2[OTf] / 2.48
150 (0.73), 660 (0.27)
265 (0.70), 917 (0.30)
–
Toluene
1,4-dioxane
CCl4
468 sh, 589
482 sh, 584
450, 590
0.39/1.00
0.69/1.00
1.00/0.44
298 (-0.45), 3807 (0.55)
0.47
0.35
0.26
431 (-0.46), 3063 (0.54)
–
[a] F1/F2 ratio was calculated using the intensities of the emission spectra at 470 nm for F1 band and 600 nm for F2 band. [b]
Calculated thermochemical radii of the anions.[36]
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Figure 6. Time-resolved emission spectra of 2[Br] (A, C) and 2[OTf] (B, D) in toluene at 298 K.
Scheme 3. Proposed energy level diagram for 2[X] in solvents with contrasting polarity
fast reorganization of which (~10–12 s) converts Franck–Condon
state into the relaxed emissive state (Scheme 3A). The
wavelength of fluorescence in this case is virtually independent
on the counterion[38] but is determined by the solvent polarity and
change of the dipole moment in the excited state (vs. ground
state).
In weakly polar and nonpolar solvents, continuous relaxation
dynamics, together with the counter anion dependent emission
and spectral evolution, leads us to propose a mechanism that
incorporates a successive type of counter anion migration to
stabilize the charge transfer state, which is akin to the above ICT
induces polar solvent relaxation (Scheme 3A). Using salt 2 as a
prototype, Scheme 3B depicts the general concept. In a low
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polarity medium, the ion pairs of 2 have appreciable electrostatic
interaction but poor dipole interaction with solvation sphere.
Prompt excitation conceivably produces the S1 state with CT
character, in which –+PR3 (acceptor) and –NPh2 (donor) become
more neutral and positively charged, respectively, assuming the
complete charge transfer (see also Figure S5 showing electron
density difference for S0→S1 transition). The net result is the
formation of unstable charge redistributed state due to the lack of
stabilization by weakly or nonpolar solvent. Therefore, driven by
the Coulomb attraction, the counter anion starts to migrate toward
the partially positively charged =+NPh2 site. Amid the anion
migration to the S´1 state of appreciably lower energy (see
Scheme 3B), each corresponding minimum potential energy
surface (PES) may have non-negligible emission transition
moment, resulting in a successive type of emission relaxation
dynamics. Moreover, due to the accompanied destabilization of
the “hot” ground state S´0, large Stokes shifted emission is then
expected. Notably, the F1 emission band can be attributed to the
S1 state prior to the anion migration (Scheme 3B).
These experimental observations support the mechanism of
counterion migration where the larger size anion should result in
slower relaxation dynamics. Remarkably, while small-medium
anions can modulate the dual luminescence and alter the rate as
well as probability of migration, too large anion size such as bulky
BARF–, which is proportionate to the cation 2+, hinders the
migration during the excited-state lifespan and thus leads to the
single emission band (see Figures 5 and S16).
Notwithstanding the similarity between anion migration and
solvent relaxation, the anion migration can be only observed in
weakly or nonpolar solvents due to the absence of discernible
solvent relaxation. Also, in view of fundamental bases, several
other distinct remarks can be pointed out. First, the migration of
counterion is more like a charge-induced translational motion,[39]
which is different from the type of solvent reorientation in
solvatochromism. Therefore, in comparison to the solvent
reorientation of few picoseconds in low viscous solvents, the rate
of counterion motion should be much slower in time scale, as
evidenced by the initial decay of several tens to hundred
picoseconds in toluene for 2[X] salts (Table 1 and Tables S5-S7).
This slow time scale for anion migration also explains the dual
emission, or multiple emission in a more general way, in toluene
rather than solely a large Stokes-shifted emission in polar
solvents for the given species. The time-resolved spectra (Figure
6) clearly show that the initially detected high energy band and
progressive charge transfer emission have strong kinetic
relationship indicating that the excited-state processes are
temporally evolving. The role of solvent-separated pairs in toluene
as well as intermolecular migration are thought to be insignificant
as the equilibrium should strongly favor contact ion pair formation
to account for the time-resolved studies. This is supported by the
literature data,[40] according to which the conductivity of ionic salts
in toluene is ca. 108 times smaller than that in water, and also by
the 1H NMR spectra in toluene-d8, which significantly depend on
the nature of counterions (see Experimental in the ESI). As a
result, the emission associated with ion separation must be too
small to be resolved in this study.
Figure 7. The fast decay time component of the F1 band for 2[X] in toluene as
a function of thermochemical radius of the counter anions.[36]
The proposed mechanism also infers that the length of the –
(C6H4)n– spacer should affect the population of the lower lying S´1
state. In the family of 1–3, the longer oligophenylene chain in 3 (n
= 3) makes the migration of Br– less likely and the migration rate
becomes slower with respect to the biphenyl compound 2 (rise
times for F2 band are 93 ps for 2 and 120 ps for 3, Figure S17).
For 4 the F2 signal was not observed putatively because of too
long quaterphenylene linker. On the other hand, too short
separation between the D and A groups diminishes the energetic
difference between the S´1 and S1 states, manifested by the
emission profile of 1 (Figure 4) and the absence of F2 band for 5
and 6. Thus, to realize the migration process, one has to consider
the D–A distance and also the degree of photoinduced charge
separation, which is more pronounced for –+PPh3 vs. –+PMePh2
cations, i.e. in 2–4 compared to 7, 8 (Figures S6–S8). To confirm
the essense of ICT, we measured the steady-state spectra of the
2[Br] congener without a donor moiety, [PPh3biphenyl][Br][26c]
,
Figure S18, for which virtually no F2 band is observed. This can
be rationalized by the marginal ICT property for
[PPh3biphenyl][Br] (cf. 2[Br]) due to the lack of electron rich –NPh2
group, resulting in negligible ion migration.
In addition to the anion radius and the length of the –(C6H4)n–
spacer, the viscosity of a solvent is also supposed to play an
important role in anion migration dynamics. To examine this
influence, we chose a nonpolar 1,4-dioxane (1.19 cp at 298 K),[41]
which has a higher viscosity than toluene (0.56 cp at 298 K).[42]
The results shown in Figure 5 and S10 demonstrate that the
intensity ratio of F1/F2 drastically increases, and the time-resolved
Secondly, assuming that the given ionic species would primarily
exist in a form of non-dissociated ion pairs in toluene and other
nonpolar solvents, it is reasonable to expect a significant influence
of counterions on the excited state dynamics of ionic
chromophores if the mechanism of ICT induced counterion
migration is operative. Figure 7 shows the time constant of the
initial decay component of the F1 band versus the anion X radius
for salts 2[X] in toluene. Apparently, the decay time systematically
increases with the growth of thermochemical radius of the counter
anions[36] (see also Tables 1, S5 and Figure S15).
This article is protected by copyright. All rights reserved.

10.1002/anie.201906929
Angewandte Chemie International Edition
RESEARCH ARTICLE
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Keywords: charge transfer • donor-acceptor systems • dual
emission • fluorescence • ion migration
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10.1002/anie.201906929
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Andrey Belyaev, Yu-Hsuan Cheng,
Zong-Ying Liu, Antti J. Karttunen,* Pi-Tai
Chou,* Igor O. Koshevoy*
Page No. – Page No.
A Facile Molecular Machine: Optically
Triggered Counterion Migration via
Charge Transfer of Linear D-π-A
Phosphonium Fluorophores
Pair at work: Optically driven charge transfer in non-dissociated donor-acceptor ion
pairs induces a counterion migration, reminiscent of a molecular machine. This
motion leads to the relocation of the cation and anion, and results in unusual dual
fluorescence due to the previously unrecognized feature of ionic D–π–A systems in
nonpolar media.
This article is protected by copyright. All rights reserved.