Multiple Emission of Phosphonium Fluorophores Harnessed by the Pathways of Photoinduced Counterion Migration

Abstract

In the emerging field of intramolecular charge transfer induced counterion migration, we report the new insights into photophysical features of luminescent donor–acceptor phosphonium dyes (D−π−) _n A⁺[X⁻] (π=−(C₆H₄) _x −). The unique connectivity of the phosphorus atom affords multipolar molecules with a variable number of arms and the electronic properties of the acceptor group. In the ion‐paired form, the transition from dipolar to quadrupolar configuration enhances the low energy migration‐induced band by providing the additional pathways for anion motion. The multipolar architecture, adjustable lengths of the π‐spacers and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with quantum yields around 30 %. The methyl substituent at the phosphorus atom reduces the rate of ion migration and suppresses the red shifted bands, simultaneously improving total emission intensity. The results unveil the harnessing of the multiple emission of phosphonium fluorophores by anion migration via structure and branching of donor–acceptor arms.

The photophysical properties of a series of multipolar donor–acceptor phosphonium fluorophores (D−π−) _n A⁺[X⁻] have been investigated. These dyes exhibit steady‐state panchromatic luminescence attributed to the photoinduced counterion migration. The intra‐ion‐pair dynamics and the fluorescence response can be rationally tuned on the molecular level that might broaden the scope of functionalities and stimulate the development of new ionic systems.

Introduction

Light‐driven molecular motion is a fascinating property, which forms a basis for the development of unconventional materials, ^[1] molecular switches, motors and machines. ^[2] A major strategy to photo‐controlled movement of molecular components relies on the use of isomerizable organic units that ensures robustness of such photodynamic systems and simultaneously defines their functional behavior. ^[3] Alternatively, the change of the electronic structure that occurs upon photoexcitation can perturb non‐covalent contacts between the constituents of supramolecular species and affect their mutual displacement. ^[4] Thus, switching hydrogen‐bonding ability or electrostatic interactions by means of photoinduced electron transfer allowed one to realize fast (ca. 1–100 μs) translational motion in a rotaxane type of molecular shuttles. ^[5] The employment of a photoacidic moiety capable of intermolecular proton transfer in a rotaxane structure allowed one to increase the rate of translocation on a distance of ca. 1 nm nearly by an order of magnitude. ^[6] Even faster movement occurring on a picosecond time scale driven by excited‐state intramolecular charge transfer (ESICT) has been proposed for pyridinium donor–acceptor dyes embedded in the cucurbit[7]uril host. ^[7] However, the relocations of the composite elements in these photo‐driven (pseudo)rotaxane shuttles typically generate very limited alterations of optical characteristics, which hardly can serve as convenient read‐out signals.

The sensitivity of CT state of organic chromophores to the polarity of the local environment nevertheless potentially offers an attractive opportunity to design molecular systems, demonstrating motion‐produced optical function at high speed and in a reversible manner. For instance, it has been shown that photoexcitation of a donor–acceptor (D‐A) dye combined with a calixarene receptor leads to very fast translocation of a coordinated potassium ion within the host molecule, which is clearly reflected in time‐resolved absorption spectra. ^[8] In the case of ionic D−π−A⁺ fluorophores, the CT‐modulated interactions within non‐dissociated ion pairs have been noticed to exhibit a more pronounced photophysical response. ^[9] Recently, we reported structurally simple linear D−π−A⁺ fluorophores with phosphonium and pyridinium acceptor A⁺ units (D=−NR₂ group, π=phenylene spacer), ^[10] which reveal regular photoinduced CT and depletion of electron density from the donor motif. The redistribution of electron density and the formation of the −N^δ+R₂ site ^[8] is stabilized in polar solvents by dipole–dipole interactions with a rapidly reorganized nearest shell of the medium. On the contrary, in solvents with low polarity, the photophysical behavior of these dyes is strongly influenced by the ion‐pairing effect. The latter is manifested by ESICT‐driven migration (or translocation) of a small counter anion from the cationic A⁺ area (ground state location) in the direction of the transient −N^δ+R₂ group (excited state target) that decreases the energy of the emissive state and leads to the evolution of a continuum of red‐shifted fluorescence bands (Scheme 1). The resulting dual‐band‐like steady‐state luminescence was shown to be dependent on the length of the π‐spacer, the size of migrating anion, and the solvent viscosity. Comparison of phosphonium ^[10a] and pyridinium ^[10b] congeners also indicate the important role of the cationic group and, consequently, polarization of the spacer in the process of light‐triggered anion migration and corresponding optical response.

Scheme 1.

Schematic representation of ESICT‐induced anion migration in donor–acceptor ionic dyes in the absence of solvent relaxation (low polarity solvents).

In this emerging field, herein, we aim at exploring the factors affecting the ESICT‐induced ion motion and photophysical features of emissive ion‐paired D−π−A⁺ salts (π=−(C₆H₄) _x −) via a new series of phosphonium dyes with various branching CT chromophores. The strategy lies in the unique connectivity and accessible quaternization of the phosphorus atom, ^[11] which allow us to synthesize novel multipolar molecules (D−π−) _n A⁺ with variable number of and length of linear donor–acceptor arms, ⁺PPh_4‐n (−π−D) _n (n=1–4) and ⁺PPh₂(−π−D)(−π′−D). To probe the cationic effect, we also chemically alter the acceptor group by introducing the methyl group along the aromatic substituents in ⁺PMePh_3‐m (−π−D) _m (m=1, 3). The results elaborated below, for the first time, unveil the key role played by cation structure and donor–acceptor arms to the ESICT‐driven counterion migration and hence to the harnessing of the multiple emission.

Results and Discussion

Synthesis and Characterization

The preparation of (D−π−) _n A⁺[X⁻] phosphonium dyes is outlined in Scheme 2, where the experimental details are given in the Supporting Information. Following the general NiBr₂‐catalyzed protocol, ^[12] the reactions of appropriate donor‐functionalized phosphines PPh_3‐n [−(C₆H₄) _x −NPh₂] _n (x=2, n=1 L2₁ , ^[13] 3 L2₃ ; ^[14] x=3, n=1 L3₁ , 3 L3₃ ^[14] ) with the corresponding bromo‐N,N‐diphenyl‐aryl amines, Br−(C₆H₄) _y −NPh₂ (y=1–3), ^[15] or bromobenzene (for 2₃ [Br]) afford tetraaryl bromide salts, which were isolated as amorphous solids. These quaternized cationic species are designated as Y _n Z _{n ′} [Br], where Y and Z correspond to the length of phenylene linkers and the n, n′ subscripts indicate the number of the respective donor‐containing substituents. Compounds 2₂ [Br] and 2₃ [Br] are obtained in good yields (77 and 80 %), and for the rest of the series the target compounds are generated with visibly smaller efficiency (34–52 %). The Pd‐catalyzed formation of phosphonium ions ^[16] in this case is found to be less selective; for instance, the reactions of equimolar amounts of L2₁ and Br−(C₆H₄) _y −NPh₂ give mixtures of several P‐cationic products (Figure S1), which are difficult to separate. Whereas selectivity of the process can be significantly improved by lowering the temperature from 140 to ca. 115 °C, the overall conversion of the starting materials does not exceed 30 %. A phenol‐promoted metal‐free approach ^[17] also delivers moderate to low yields (13 % for 2₂ [Br] and 10 % for 2₃ [Br]) because of limited solubility of the oligophenylene phosphine precursors L2₁ and L2₃ . Salts 2₁ [X] and 3₁ [Br] have been studied by us previously. ^[10a] The metathesis of the counterion Br⁻→OTf⁻ was performed with silver(I) triflate in a nearly quantitative manner. Methylation of phosphines L2₁ , L2₃ and L3₃ was readily carried out with methyl triflate, giving salts 2₁Me[OTf], 2₃Me[OTf] and 3₃Me[OTf] in 83–93 % yield.

Scheme 2.

General synthetic routes (A) and structures of donor–acceptor phosphonium salts (B) studied in this work (named Y _n Z _{n ′} [X], where Y and Z correspond to the length of phenylene linkers and the n, n′ subscripts indicate the number of the respective donor‐containing substituents).

The composition of the products was confirmed by spectroscopic methods. The ESI⁺ mass spectra of all salts show one dominating signal that in each case corresponds to the molecular cation (Figure S2). The ³¹P, ¹³C and ¹H NMR spectra (see the Supporting Information) are also compatible with the proposed structures (Scheme 2) and the presence of single species for each compound. Thus, the phosphorus resonances in CD₂Cl₂ appear in the ranges 22.3–23.4 ppm (aryl‐substituted ions) and 20.6–21.5 (methylated ions), which belong to a typical region for other acyclic phosphonium compounds.[ ^10a , ¹⁸ ] The exchange of the anion shows virtually no effect on the chemical shift of the ³¹P signal in dichloromethane, indicating weak ion pair association in this solvent. In toluene‐d₈, the NMR spectra are substantially more anion‐dependent, which can be seen in a non‐negligible difference in the δ(³¹P) for triflate and bromide salts (e.g. 21.6 and 22.4 ppm for 2₁Me[OTf] and 2₁Me[Br]; 21.1 and 20.7 ppm for 1₁2₁ [OTf] and 1₁2₁ [Br]). The corresponding proton spectra for the selected compounds (Supporting Information) also confirm a non‐innocent role of anion in ion pairing in low polarity medium, which has been demonstrated for the family 2₁ [X]. ^[10a]

Photophysical Properties

To avoid complication, we segregate the dyes under study into three groups–i) compounds 2 _n [X] where we probe the effect of branching the multipolar cations bearing identical substituents; ii) quadrupolar compounds containing two elongated π‐spacers 3₂ [X], and two different spacers Y ₁ Z ₁ [X] with competing migration pathways; iii) methylated species 2₁Me[OTf] and Y ₃ Me[OTf] with sterically and electronically different quaternized phosphorus center.

Polar Solvents

In polar media such as CH₂Cl₂ and CH₃CN one can neglect ion pairing effect due to efficient dissociation. In these solvents, optical properties of all titled salts are not exceptional and arise from intramolecular charge transfer (ICT) transitions expected from the D‐A architecture of these cations, being nearly independent on the counterion (Figures S3–S5, Table S1). The absorption profiles of 2 _n [X] (X⁻=Br⁻, OTf⁻, Figure S3) are close to that of 2₁ [X] ^[10a] and display the lowest energy bands around 390–394 nm in CH₂Cl₂ (373–378 nm in CH₃CN), where the molar attenuation coefficient additively grows with the increase of the number n of the −π−D arms. The structureless emission bands are almost identical for the entire series 2 _n [X]. In addition, they all show positive solvatochromism, for which the peak wavelength shifts from 522–528 nm (Φ_em=0.73–0.81) in CH₂Cl₂ to 547–551 nm (Φ_em=0.25–0.39) in CH₃CN with moderate to high emission quantum yields. Note that a small hypsochromic shift of the emission maxima for ca. 5 nm is identified within the series of 2 _n ⁺ (n=1→4) cations, which likely arises from a combination of slight decrease of electron‐accepting strength of the phosphonium unit and an additive effect of the donor‐containing substituents. This correlation is further supported by gradually decreasing Stokes shifts (from 6900 cm⁻¹ for 2₁ [Br] to 6288 cm⁻¹ for 2₄ [Br] in CH₂Cl₂, Table S1), and the chemical shifts in the ³¹P NMR spectra; the latter indicate slight shielding of the phosphorus nuclei from n=1 to n=4 (23.5 ppm for 2₁ , 23.2 ppm for 2₂ , 22.9 ppm for 2₃ , 22.6 ppm for 2₄ ). Very close fluorescence spectral properties of 2 _n [X] in dichloromethane and acetonitrile suggest that the corresponding electronic transitions mostly occur within one {⁺P−(C₆H₄)₂−NPh₂} push‐pull arm, i.e. there is no appreciable electronic coupling between A⁺−D subchromophores in A⁺(−D) _n (n=2–4) multipolar ions, which can be approximated in polar solvents as chromophores with dipolar symmetry‐broken excited state. ^[19] Optimization of the S₀ geometry for the cation 2₂ ⁺ with density functional theory (DFT) leads in a geometry where both biphenylene spacers are somewhat twisted with estimated torsion angle of about 30°. The lowest energy absorption S₀(optimized)→S₁ in 2₂ ⁺ involves two equivalent chromophore arms and results in substantial charge transfer from the donor −NPh₂ motifs as well as changing polarization of the π‐spacer (Figure 1). At the optimized S₁ geometry, one −(C₆H₄)₂−NPh₂ fragment becomes flattened and is responsible for the emission that makes 2₂ ⁺ behaving in a dipolar manner.

Figure 1.

Lowest energy excitation S₀→S₁ and emission S₁→S₀ electron density difference plots for cation 2₂ ⁺ (isovalue 0.002 a.u., DFT‐LRC‐ωPBEh method, optimized S₀ and S₁ geometries). During the electronic transition, the electron density increases in the blue areas and decreases in the red areas. Hydrogen atoms and counter anions are omitted for clarity.

In CH₂Cl₂ and CH₃CN, the behavior of salts 3₂ [X], 1₁3₁ [X] and 2₁3₁ [X] (X⁻=Br⁻, OTf⁻) is dominated by the −π−D fragment with terphenylene spacer (Figure S4 and Table S1). The non‐symmetric cations 1₁3₁ ⁺ and 2₁3₁ ⁺ reveal minor hypsochromic shifts of yellow‐orange emission, and substantially higher quantum yields (Φ_em=0.77/0.66 in CH₂Cl₂, 0.32/0.15 in CH₃CN) than 3₂ ⁺ congener (Φ_em=0.30 in CH₂Cl₂, 0.06 in CH₃CN). The lack of dual fluorescence for 1₁3₁ ⁺ and 2₁3₁ ⁺ in polar media is in line with Kasha's rule, i.e. the emission is associated with the dipolar lowest excited state, which corresponds to the ICT occurring on the arm with the longest terphenylene spacer. The DFT‐calculated electronic transitions S₀→S₁/S₀←S₁ in cation 2₁3₁ ⁺ obtained for symmetry unconstrained geometries (C ₁) primarily involve the shorter chromophore component, which becomes flattened during the geometry optimization of the S₁ state (Figure S5). However, considering an average conformation (C_s ) with flattened phenylene spacers on both chromophores shifts the localization of electronic transitions related to the excitation/emission mainly to the terphenylene fragment. The C_s structure is energetically higher than the C ₁ local minimum by only 10 kJ mol⁻¹ per phenylene group, making the C_s structure an energetically feasible idealized description of the molecular conformation in solution.

The absorption and emission signals of the methylated dyes 2₁Me[OTf], 2₃Me[OTf] and 3₃Me[OTf] in CH₂Cl₂ and CH₃CN demonstrate modest hypsochromic shifts with respect to their tetraarylated relatives (Figure S6 and Table S1). Apparently, this energy increase, which is more pronounced in more polar acetonitrile, is caused by non‐electron accepting methyl substituent and therefore less polarizing effect of the phosphonium group.

Non‐polar Solvents

i) Multipolar dyes 2 _n [X] (n=2–4; X ⁻ =Br⁻, OTf⁻) In the lower polarity solvent such as toluene, where ion pairing is anticipated, branching the molecular structure of the phosphonium cations has a distinct influence on the photoluminescence characteristics. In accordance with earlier studies, ^[10a] new fluorophores 2 _n [X] demonstrate high‐energy (HiE, F₁) and low‐energy (LoE, F₂) emission bands of variable ratio in their steady‐state spectra (Table 1, Figures 2 and S7). The excitation spectra recorded for F₁ and F₂ emission bands for all 2 _n [X] (Figure S8) are identical with the absorption profiles, confirming that they both share common ground‐state species. By analogy with 2₁ ⁺, the F₁ band is assigned to an anion‐destabilized ICT state S₁, whereas the red‐shifted F₂ band emerges upon photoinduced anion migration and consequent stabilization of the initial charge separated state (S₁′, Scheme 1). The emission spectra of 2₂ [OTf] in frozen toluene and dichloromethane solutions at 77 K display only the blue shifted HiE signals (Figure S9). The results imply completely suppressed anion migration in frozen ion pair, which is electrostatically stabilized in the ground state but destabilized in the CT excited state.

Table 1.

Photophysical data of the studied phosphonium salts in toluene at 298 K.

Cation	Anion A−	λ abs [nm]	λ em [nm]	F 1/F 2 [a]	Φem	τ F1 [ps] (prefactor)[b]	τ F2 [ps] (prefactor)[b]	CIE
21 [10a]	Br−	379	468, 592sh	1/0.41	0.17	67 (0.63), 450 (0.37)	93 (−0.49), 792 (0.36), 2439 (0.15)	0.29, 0.34
	OTf−	383	468, 589	0.39/1	0.47	150 (0.73), 660 (0.27)	298 (−0.45), 3807 (0.55)	0.42, 0.43
22	Br−	380	468, 586	0.57/1	0.15	70 (0.95), 350 (0.05)	95 (−0.51), 648 (0.36), 2058 (0.13)	0.39, 0.41
	OTf−	385	468, 586	0.29/1	0.38	111 (0.99), 2741 (0.01)	185 (−0.51), 3306 (0.49)	0.42, 0.45
23	Br−	382	468, 584	0.60/1	0.15	71 (0.99), 605 (0.01)	97 (−0.48), 610 (0.33), 1983 (0.19)	0.39, 0.41
	OTf−	386	468, 584	0.30/1	0.34	90 (0.99), 2362 (0.01)	163 (−0.51), 2904 (0.49)	0.43, 0.45
24	Br−	383	468, 582	0.69/1	0.10	65 (0.98), 1037 (0.02)	112 (−0.50), 550 (0.31), 1669 (0.19)	0.38, 0.40
	OTf−	386	468, 580	0.35/1	0.30	79 (0.99), 2285 (0.01)	171 (−0.49), 2627 (0.51)	0.42, 0.44
31	Br−[10a]	373	480, 660	1/0.18	0.17	120 (0.39), 780 (0.61)	120 (−0.39), 2540 (0.61)	0.21, 0.29
	OTf−	372	482, 661	1/0.64	0.42	80 (0.52), 782 (0.48)	405 (−0.38), 3600 (0.62)	0.31, 0.34
32	Br−	375	480, 665	1/0.29	0.19	118 (0.36), 470 (0.64)	118 (−0.46), 1971 (0.64)	0.24, 0.30
	OTf−	375	481, 664	1/0.84	0.29	93 (0.37), 466 (0.63)	290 (−0.46), 3433 (0.54)	0.34, 0.34
1121	Br−	370	470, 570	1/0.80	0.17	86 (0.74), 500 (0.25), 2798 (0.01)	105(−0.51), 938 (0.27), 3385(0.22)	0.32, 0.35
	OTf−	371	470, 576	0.60/1	0.53	93 (0.55), 637 (0.43), 3381 (0.02)	424 (−0.47), 4043 (0.53)	0.38, 0.41
1131	Br−	361	473, 645	1/0.30	0.21	115 (0.73), 761 (0.27)	90 (−0.50), 1520 (0.27), 2694 (0.23)	0.25, 0.28
	OTf−	361	471, 642	1/0.55	0.49	85 (0.53), 1012 (0.47)	520 (−0.44), 3908(0.56)	0.30, 0.31
2131	Br−	377	477, 615	1/0.48	0.09	65 (0.55), 237 (0.44), 1536 (0.01)	121 (−0.49), 769 (0.27), 1806 (0.24)	0.29, 0.32
	OTf−	378	480, 613	0.66/1	0.23	65 (0.45), 248 (0.54), 2707 (0.01)	197 (−0.50), 3262 (0.50)	0.40, 0.40
21Me	OTf−	378	465, 583	1/052	0.53	604 (0.87), 3477 (0.13)	657 (−0.49), 3602 (0.51)	0.29, 0.31
23Me	OTf−	380	461, 573	1/0.55	0.62	439 (0.89), 2700 (0.11)	643 (−0.49), 3225 (0.51)	0.29, 0.31
33Me	OTf−	371	473, 650	1/0.12	0.79	1088 (0.95), 2350 (0.05)	1496 (−0.44), 3264 (0.56)	0.20, 0.27

[a] The F₁/F₂ ratio was calculated using the intensities of the emission spectra at the maxima of the F₁ and F₂ bands. [b] Measured at the 440 (τ _F1) and 640/660 nm (τ _F2) for 2/3 containing cations.

Figure 2.

A) Normalized emission spectra of 2 _n [Br] (filled graph corresponds to 2₁ [Br] ^[10a] ). B), C) Time‐resolved emission spectra of 2₂ [OTf] in toluene at 298 K.

Due to higher stability, triflate salts were used for most of time‐resolved studies. Transient emission spectra monitored in the 0.1–1.0 ns time range for 2 _n [OTf] (n=2–4; Figures 2 and S7) reveal that the initial HiE F₁ band, recorded at 0.1 ns delay after the excitation pulse, undergoes continuous transformation into the LoE F₂ fluorescence, which is completed within 0.8–1.0 ns. The spectral temporal evolution of red‐shifted emission correlates well with the kinetics of the F₂ band, for which a rise component of ca. 170 ps can be resolved by time‐correlated single photon counting (TCSPC) measurements, followed by the relaxation of the excited state with τ _obs=2.6–3.3 ns (Table 1, Figure S9).

Triflate salts demonstrate systematically higher quantum yields (Φ_em=0.47–0.30) than the bromide congeners (Φ_em=0.17–0.10), while the multipolar dyes show somewhat lower intensity than dipolar compounds. The transition from dipolar (D−A⁺) 2₁ ⁺ to quadrupolar (D−A⁺−D) 2₂ ⁺ architecture in the presence of Br⁻ anion dramatically increases the intensity of F₂ band (λ=586 nm) relative to F₁ band (λ=468 nm) that changes the F₁/F₂ ratio from 1/0.41 to 0.57/1 (Figure 2 and Table 1) but retains both the emission wavelengths and the total quantum yields (Φ_em=0.17 and 0.15 for 2₁ [Br] and 2₂ [Br]). Qualitatively, the same trend is observed for OTf⁻ salts, for which the LoE signal dominates and grows for 2₂ ⁺ vs 2₁ ⁺ cation (Figure S7). Schematically, it can be illustrated by the symmetrically branched molecular structure of 2₂ ⁺ that increases the probability of photoinduced anion translocation by providing two accessible migration pathways and minimizing steric restrictions, i.e. eliminating migration‐unfavorable locations for relatively small counterions around the cationic motif (Figure 3). An additional enhancement of migration can be offered by the quadrupolar excited state, which might have an appreciable contribution in non‐polar solvents. ^[19d]

Figure 3.

Schematic representation of the effect of dipolar and quadrupolar architectures on the possibility of anion migration.

Further addition of the subchromophores to the phosphorus atom in 2₃ ⁺ and 2₄ ⁺ slightly raises both the emission energy of the F₂ band, and the intensity of the F₁ band (Figures 2A and S7A, Table 1). The subsequent increase of the number of equivalent −π−D arms in C ₃ and T symmetric 2₃ ⁺ and 2₄ ⁺ is not expected to improve significantly steric conditions for anion migration compared to those in 2₂ ⁺. This supposition correlates with close rise times for the F₂ band among multipolar 2 _n [OTf] species (n=2–4, with a rise component of τ_F2 of 163–185 ps). Possible delocalization of the CT excited state over three (2₃ ⁺) and four (2₄ ⁺) sites might somewhat decrease the depletion of electron density from the NPh₂ groups with respect to dipolar compounds. This should disfavor the Coulomb attraction X⁻⋅⋅⋅ ^δ+NPh₂ in the charge‐separated states (Scheme 1 and Figure 3), and therefore slightly diminish the efficiency of anion migration and the intensity of F₂ band.

ii) Quadrupolar dyes 3 ₂ [A], 1 ₁ 2 ₁ [A], 1 ₁ 3 ₁ [A], 2 ₁ 3 ₁ [A] (A ⁻ =Br⁻, OTf⁻) Taking into account that major alterations of luminescence for 2 _n ⁺ series are observed between the a) dipolar and quadrupolar ions, and b) different counterions, in the next stage we explore the tunability of the emission among the quadrupolar D−A⁺−D and D−A⁺−D′ type of dyes. As we have shown for salts 2₁ [Br] and 3₁ [Br], ^[10a] elongation of the π‐spacer from biphenylene to terphenylene insignificantly red shifts the F₁ band (from 468 to 480 nm), but has a more drastic effect on the F₂ signal, increasing its wavelength from 592 to 660 nm that is accompanied by a drop of its intensity to produce sky‐blue fluorescence. Quadrupolar configuration of 3₂ [Br] enhances the F₂ band (the F₁/F₂ ratio varies from 1/0.18 for 3₁ [Br] to 1/0.29, Figure 4). In line with the behavior of 2 _n [X], the intensity of the LE band for 3₂ ⁺ demonstrates further growth upon exchange of the Br⁻ for the OTf⁻ counterion (F₁/F₂=1/0.84 for 3₂ [OTf]). This results in nearly pure white light emission demonstrated by 3₂ [OTf], Commission Internationale de l'Eclairage (CIE) coordinates (0.34, 0.34), with good quantum yield of 0.29 (Table 1).

Figure 4.

A) Normalized emission spectra of 3 _n [X] (n=1, 2; A⁻=Br⁻, OTf⁻). B), C) Time‐resolved emission spectra of 3₂ [OTf] in toluene at 298 K.

The hypothesis of photoinduced anion migration that is responsible for temporal growth of the LoE band, initially elaborated for 2 _n ⁺ series, is applicable to the dyes with longer terphenylene π‐spacers. Similar to 2 _n ⁺, for the 3 _n ⁺ series, the identity of the excitation spectra monitored in the range of the emission from 450 to 700 nm for salts containing −(C₆H₄)₃−NPh₂ fragment (Figure S10) points to the common origin of the F₁ and F₂ fluorescence bands. Time‐resolved data (Figures 4 and S11) also fit well the proposed concept and confirm the rise of red shifted F₂ signal that follows the initially developed HiE band.

Combination of two different chromophores in cations 1 ₁ 2₁ ⁺, 1₁3₁ ⁺ and 2₁3₁ ⁺ has little influence on their emission in polar solvents compared to that of individual species 2₁ ⁺ and 3₁ ⁺ (see above, Figures S3 and S4), which is dominated by the longest −π−D fragment. Contrarily, in toluene these asymmetric dyes exhibit the contribution of both components into the total steady state luminescence (Figures S12–S14). For detailed analyses we have chosen salt 2₁3₁ [OTf], the emission spectrum of which is shown in Figure 5. Its spectral profile suggests the presence of two active migration pathways along two different −π−D arms and may be seen as a superposition of those of dipolar dyes 2₁ [OTf] and 3₁ [OTf]. Due to similarity of HiE bands of the latter compounds (468 and 482 nm), the experimental spectrum of 2₁3₁ [OTf] can be fitted with three components, which correspond to the average F₁ and two migration‐induced LoE bands (F₂ and F₂′) assigned to the −biphenylene−NPh₂ (2) and −terphenylene−NPh₂ (3) motifs (Figure 5B). Time‐dependent emission spectra of 2₁3₁ [OTf] (Figure 5C) also support this process of competing migration, and clearly display the appearance of the intermediate F₂ signal around 575 nm, which occurs in a faster time scale than F₂′and is absent for 3 _n [OTf] congeners (Figure 4).

Figure 5.

A) Comparison of normalized emission spectra of 2₁ [OTf], 3₁ [OTf] and 2₁3₁ [OTf]. B) Simulated deconvolution of the emission spectrum of 2₁3₁ [OTf] into three components (one HiE F₁ and two migration‐induced F₂ and F₂′ bands). C) Time‐resolved emission spectra of 2₁3₁ [OTf]; toluene, 298 K.

The possibility of such anion motion in two non‐equivalent directions in the ion pair of 2₁3₁ [OTf] is tentatively ascribed to the excitation of both constituting chromophores having close energies (Figure S5). Possible delocalization of the S₁ state over the entire cation or the contribution of two dipolar excited configurations (i.e. 2₁*3₁ ⁺ and 2₁3₁* ⁺) should provide two opportunities for anion migration and therefore lead to the evolution of two red shifted bands, F₂ and F₂′. Two other non‐symmetrical salts, 1 ₁ 2₁ [OTf] and 1₁3₁ [OTf], generally operate pursuant to 2₁3₁ [OTf]. Their LoE bands are slightly blue shifted relative to 2₁ [OTf] and 3₁ [OTf], respectively (Figure S12). The short −phenylene−NPh₂ (1) group, the emission of which is poorly responsive to anion migration, ^[10a] evidently causes the growth of the F₁ signal in 1 ₁ 2₁ [OTf] and 1₁3₁ [OTf]. On the other hand, in contrast to 2₂ [OTf] (Φ_em=0.38), 2₁3₁ [OTf] (Φ_em=0.23) and 3₂ [OTf] (Φ_em=0.29), branching the dipolar phosphonium ions with chromophore 1 does not have a detrimental effect on the quantum yields, which in the case of 1 ₁ 2₁ [OTf] (Φ_em=0.53) and 1₁3₁ [OTf] (Φ_em=0.49) slightly exceed those of 2₁ [OTf] (Φ_em=0.47) and 3₁ [OTf] (Φ_em=0.42, Table 1).

iii) Methylated dyes 2 ₁ Me[OTf], 2 ₃ Me[OTf] and 3 ₃ Me[OTf]. Tuning the electron deficient nature of the phosphonium group and its polarizing ability toward the −π−D motifs envisages the variation of the environment around the phosphorus atom. In this approach, the methylated derivatives 2₁Me[OTf], 2₃Me[OTf] and 3₃Me[OTf] were selected to probe the effect of electronic and steric features of the −⁺PR₃ moiety on their optical properties. Accordingly, the electron deficiency of the phosphonium group can be directly identified by the chemical shift from the ³¹P NMR spectrum (see above). The replacement of the phenyl in cations 2₁ ⁺ and 2₃ ⁺ for the electron‐donating methyl substituent leads to upfield shift of the phosphorus resonances from 22.3–23.4 ppm (tetraaryl‐substituted ions) to 20.6–21.5 ppm (methylated ions). Surprisingly, this simple modification of the phosphonium group transforms yellowish emission of the tetraaryl triflates 2₁ [OTf] and 2₃ [OTf] with dominating LoE band (F₁/F₂=0.39/1 and 0.30/1, CIE 0.42,0.43 and 0.43,0.45) into cold white‐blueish luminescence of the methylated congeners 2₁Me[OTf] and 2₃Me[OTf] with predominant HiE band (F₁/F₂=1/0.52 and 1/0.55, CIE 0.29,0.31) (Figure 6). Further, the largest quantum yield (Φ_em=0.62) is achieved for multipolar configuration of 2₃Me[OTf], which is opposite to arylated cations exhibiting the highest intensities among the dipolar compounds.

Figure 6.

A) Normalized emission spectra of 2₁Me[OTf], 2₃Me[OTf] and 3₃Me[OTf] (filled graph corresponds to 2₁ [OTf] ^[10a] ). B) Time‐resolved emission spectra of 2₁Me[OTf]. C) CIE 1931 coordinates for 2₁ [OTf], 2₁Me[OTf], 3₂ [OTf] and 3₃Me[OTf]; toluene, 298 K.

The time‐resolved spectra (Figures 6B and S15) unequivocally indicate considerably slower evolution of F₂ band for 2₁Me[OTf] and 2₃Me[OTf] that correlates with much longer rise times of this LoE signal (657 and 643 ps, Figure S16 and Table 1) in comparison with those of 2₁ [OTf] and 2₃ [OTf] (298 and 163 ps, Table 1). Within the framework of our hypothesis, such spectroscopic features propound less efficient anion migration for methyl‐containing cations. Tentatively, we attribute the phenomenon to the combination of steric and polarization effects induced by the methyl group. In the S₀ state (no solvation model applied), the dipole moment of 2₁Me ⁺ (13.8 D) is visibly larger than that of 2₁ ⁺ (11.9 D). This is in line with ca. 30 kJ mol⁻¹ higher ion pairing energy predicted for the optimized structures of 2₁Me[OTf] and 2₂ [OTf]. In the excited S₁ state, the dipole moment of 2₁ ⁺ and 2₁Me ⁺ decreases to 1.7 D and 4.3 D, respectively. This implies a somewhat smaller charge redistribution for 2₁Me ⁺ compared to that for 2₁ ⁺ in the S₁ state and could result in a less favourable anion migration along the 2₁Me ⁺ cation. Consequently, the system undergoes radiative relaxation primarily from charge destabilized S₁ state (Scheme 1) producing predominant HiE fluorescence. The smaller bromide anion in 2₁Me[Br] reveals further decrease in the intensity of the F₂ band (Figure S19), which generally supports the hypothesis. We have optimized the S₀ geometry for salts 2₁Me[Br] and 2₁ [Br], see Supporting Information. In both cases the bromide anion shows attractive interactions with the H atoms of the phenyl or methyl fragments, which seem to be stronger for H(Me)⋅⋅⋅Br contact thanks to higher polarization of the Me. These secondary interactions remain in the CT state and further hinder the movement of the anion for methylated cations. The terphenylene‐based compound 3₃Me[OTf] adheres the same tendency, manifested by a substantially decreased intensity of the F₂ band (F₁/F₂=1/0.12, CIE 0.20, 0.27) and additional growth of its rise time as long as 1496 ps.

Conclusion

In summary, via utilizing the unique organophosphonium motif, which chemically combines versatile connectivity of the quaternized phosphorus atom with pronounced electron‐deficient properties, we have prepared a series of multipolar donor–acceptor ionic dyes (D−π−) _n A⁺[X⁻] (D=NPh₂, A⁺=⁺PR_4−n derivative) to investigate the effect of molecular structure on the photophysical behavior in relation to the counterion migration proposed for non‐dissociated ion pairs in solvents of low polarity. For this purpose, we varied the amount of the constituting chromophores around the P center ⁺PPh_4−n (−π−D) _n (n=1–4, π=−(C₆H₄) _x −), combined two different donor–acceptor arms ⁺PPh₂(−π−D)(−π′−D) in the same cation, and changed the steric and electronic features of the phosphonium fragment in methylated species ⁺PMePh_3−m (−π−D) _m (m=1, 3). In polar solvents (dichloromethane, acetonitrile), the new compounds reveal regular fluorescence with positive solvatochromism, virtually independent on the branching of the dye cation and assigned to dipolar‐like intramolecular charge transfer (ICT) excited state, which originates primarily from the −π−D component with the largest π‐spacer. In lower polarity medium (toluene), all salts are expected to exist as ion pairs and exhibit spectral temporal evolution of multiple emissions shifting from blue to red region. The resulting steady‐state panchromatic luminescence is associated with continuous electrostatic stabilization of the excited state upon anion migration from the ground‐state location in the direction opposite to that of ICT. In the ion paired form, the transition from dipolar to quadrupolar molecular structure substantially enhances the low energy band due to stereochemical reasons by providing the additional migration pathways. The quadrupolar architecture together with the adjustable lengths of the π‐spacers, and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with appreciable quantum yields. On the other hand, introducing the methyl substituent to the phosphorus atom dramatically reduces the rate of ion migration and suppresses the red shifted bands simultaneously improving total emission intensity. The presented results thus point to a quite general character of fast photoinduced anion motion that occurs in ion‐paired dyes and is accompanied by the optical response. This multiple intra‐ion‐pair dynamics and the corresponding fluorescence response can be rationally fine‐tuned on the molecular level that might considerably broaden the scope of functionalities and stimulate the development of new ionic systems for molecular machinery.

Experimental Section

Synthetic, other experimental and computational details (PDF) are given in the Supporting Information.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Supporting Information

A. Belyaev, B.-K. Su, Y.-H. Cheng, Z.-Y. Liu, N. M. Khan, A. J. Karttunen, P.-T. Chou, I. O. Koshevoy, Angew. Chem. Int. Ed. 2022, 61, e202115690; Angew. Chem. 2022, 134, e202115690.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.