Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

Abstract Tunable electron‐accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π‐conjugated motifs, define the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π‐extended molecules functionalized with P‐cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation‐anion interactions, which can drastically change physical behavior of these two‐component systems.


Introduction
Charged organic molecular chromophores demonstrate remarkably diverse photophysical and electrochemical properties, which have been utilized in aw ide range of photonic applications. Due to the ionic nature,m any fluorophores (e.g. cyanines,squaraines, rhodamines etc.) have appreciable solubility in protic solvents, and therefore serve as bioimaging agents and chemosensors in physiologic medium. [1] Cationicc ompounds,w hich typically incorporate quaternized pyridyl motifs (i.e. viologen-type species), undergo reversible redox processes and play ak ey role in the development of electro(fluoro)chromic devices. [2] Furthermore, involvement of charged p-conjugated molecules in non-covalent (ion-p,e lectrostatic) interactions can substantially affect optical properties of the system and has been activelye mployed in the designo fs timuli-responsivea nd smart molecular materials. [3] For instance, intraand intermolecular cation(p + )-p stacking comprisingp yridinium units has provent ob ea ne fficient approacht oe nhance fluorescencei ns olution and in solid, representing ap aradigm for switchable luminescence signaling. [4] Ionic dyes, as twocomponent salts, are capable of undergoing non-innocent cation-anion interactions in aggregated or non-dissociated (close or contact ion pair) states. Alreadyi nt he 80'si tw as noticed that the formation of ion pairs in the solvents of low polarity influences the absorption spectra of ionicc hromophores, [5] pointingt ot he ground state aggregation.I ntuitively, similar association effects might be expected in the excited state, thus modulatingt he emission characteristics of ionic fluorophores. Despite of this intriguing feature, very limited experimental and theoretical data have been reportedi nr elation to the excited state dynamics of ion pairs during the last decade. [6] The field of ionic dyes has been dominated by nitrogen-and oxygen-containing organic compounds. The heavierp nictogen homologue, phosphorus, has received considerably less systematica ttention as positivelyc harged electron deficient center( l 4 s 4 or l 4 s 3 )w ithin the p-conjugated chromophore motif, albeit cationic organophosphorus species (mostly phosphonium salts) have long been investigated and appliedi no rganic synthesis, catalysis, and biomedicine. [7] In contrast to Pcationic photofunctional organic compounds, the P-neutral congeners (primarily l 5 s 4 chalcogenided erivatives of tertiary phosphines and P-heterocycles) have been as ubjecto ft horough investigation, [8] particularly as promisingm aterials for opto-electronic devices (light-emitting diodes and solar cells) that was ac entral topic of an umber of detailed reviews. [9] In this respect, herein we mainly focus on the preparation and the properties of the chromophorem olecules with the phosphorus-containing cationic groups attached to or integrated in a p-conjugated scaffold, where they can act as electronaccepting componentsh aving ad istinct impacto nt he photophysicalo re lectrochemical behavior.T hus, compounds bearing remote phosphonium groups linked via aliphatic spacers (e.g. mitochondrial probes [10] ), or ionic liquidsw ith non-chromophoricp hosphonium cations [11] are out of the scope of this work.

Donor-acceptors ystems
The pronounced electron withdrawing character of the pendant l 4 s 4 -+ PR 3 group makes it an attractive unit to tune the energies of frontier orbitals of the chromophore p-system. In particular, the electron poor nature of -+ PR 3 substituents can be used for the constructiono fd onor-acceptor (or push-pull) ionic compounds with large charge separation, which demonstrate distinct photoinduced intramolecular charget ransfer (ICT). The electronic properties of the donor (D), acceptor (A) motifs, and the degree of communication between them by means of a p-spacer,c ontrol the HOMO-LUMO gap and the ICT,w hich define the photophysical behavior.I nt his respect it is essential that the electronic features of the phosphonium motif and the degree of the electron delocalization can be tuned through the modification of Rg roups. Despite the accessibles yntheses described above, such D-p-A + chromophores are not excessive. Thev ersatile and facile connectivity of the phosphorus atom is also important for the development of multipolar architectures, as it allows for the stepwises ynthesis of heterosubstituted compounds of [PR 4Àx R' x ] + type.
The 3D octupolar chromophores 8a/b and 9 ( Figure 5) demonstrate non-linear optical( NLO) behaviori ns olution and crystalline powder,r espectively. [28] Azo-dyes 8 were expectedly non-fluorescent. The phosphorus connecting centerb rings together several virtually non-interacting chromophore arms that additively increased hyperpolarizability of 8a and 8b compared to their dipolar relative 8c.A ccording to quantum chemicalc alculations, the phosphonium group -+ PPh 2 Me of 8c does not serve as at rue electron acceptor.I nstead, it strongly polarizes the adjacent phenylene ring and therefore stabilizes the negative charget ransfer from the electron rich amino-phenylene partu pon photoexcitation. This ICT resultsi n the inversion of the direction of the dipole moment in the excited state, which is destabilized in more polar solvents leading to the negative solvatochromism (e.g. l abs for 8a shift from 511nmi nC HCl 3 to 499 nm in MeCN). The symmetrical anisolederived salt 9 wass hown to produce moderate second-harmonic generation and thus illustrates aw ay to NLO materials by means of crystal engineering of simple ionic compounds.
The zwitterionic betained yes 10 [29a] and 11 [29b] ( Figure 6) reveal more drastic absorption changes in different solvents than 8.T hus, 10 (R = R' = Ph) is red in methanol( l abs = 498 nm), purple in dichloromethane (l abs = 596 nm) and green in toluene( l abs = 686 nm), giving ah ypsochromics hift of the lowest energy (charget ransfer) absorption band of = 5500 cm À1 (= 188 nm). The substituents at the phosphorus atom in 10 have ad istinct influence on the absorption maxima, which appear at 20-30 nm longerw avelengths for R = Ph vs. R = Bu. The alterationo fR ' groups( Ph!Bu) shows an opposite effect of approximately the same magnitude; however,t he largest hypsochromic shift (ca. 90 nm) of the absorptionw ithin this familyw as achieved when the butyl motif was replaced by the more polar chloride at the R' positions. Unfortunately,n oi nformation was providedc oncerning the emission properties of compounds 10 and 11.
The negative solvatochromic behavior of cationicd onor-acceptor salts 12 (l abs = 422 nm in MeOH and 442 nm dichloromethane for n = 2) is quite moderate compared to that of betaines 10 and 11, [29c] which can be attributed to al ess polar ground state of the ionicd yes. Compounds 12 were reported to be fluorescent in solution and are evidently prone to photoinduced ICT leading to an appreciable charge redistribution ( Figure 6), which in turn might affect the anion-cation interaction (see section4below).
The modulation of the ICT in simple phosphonium salts 13, 14 containing chemically active arylborane functions suits for highly sensitivef luorescent and colorimetric detection of the fluoride anion ( Figure 7). [30] Theionic character of the phosphonium group allows to perform the analysisi na queous medium. Importantly,t he Lewis acidity of the borane unit is readily regulated by the substituents at the Pa tom and reaches its maximum forc ompound 13 d (R = Ph), which showst he highest affinity for the fluoride. This efficient complexation of the F À by the borane combinedw ith cell penetrating ability of  Figure 6. Negativelysolvatochromic phosphonium betaine dyes 10, 11 and the dipolardonor-acceptor salts 12;the proposed photoinduced transitions to less polar states for 10 and 12 are shown. [29] the phosphonium group has been lately appliedf or the selective transport of this anion across phospholipid membrane by 13 b and similar compounds. [31] Another way to enhance both the acidity of the boron atom and the electrostatic effect for anion sensing is exemplified by the arylboranes (Mes) 3Àx B(Ar P + ) x (Ar P + = 4-(MePh 2 P + )-2,6-Me 2 -C 6 H 2 )b earing one (15 a, x = 1), two (15 b, x = 2) and three (15 c, x = 3, Figure 7) phosphonium substituents. [32] Amongt hese species, the tricationic borane 15 b is capable of most efficient binding cyanidei ons in water at pH of 7( K = 1.7 10 7 m À1 ), accompanied by clearly detectable changes in the absorption spectra.
Noteworthy,i nb orane-phosphonium compounds, the -BMes 2 group, despite being an electron deficientm otif, can be the main contributor to HOMO according to the DFT analysis, [30c] and therefore during the photoinduced charget ransfer is considered as aw eak donor with respect to the cationic highly polarizing -+ PR 3 fragment. The electron richer 1,3,2-benzodiazaborole function leads to am ore distinct push-pull character of salts 16 (Figure 8), [33] whichr eveals visibly smaller HOMO-LUMO gaps (predicted 3.075-3.099 eV) in comparison to its phosphine-chalcogenider elatives (predicted 4.093-4.394 eV) due to the lower lying LUMO levels.
The decrease of the opticalg ap does not necessarily require donor-acceptor architecturea nd can be achieved by decorating the p-system with terminal phosphonium groups;f or example,t he emission maximaf or compounds 17 a-c are bathochromically shifted relativelyt ot heir non-substituted analogues for 33-65 nm (894-3338 cm À1 ). [34] This strategy has been successfully employed in the fabrication of phosphonium-linked fluorescent polyelectrolytes. [35] Nevertheless,t he combinationo ft he phosphonium group with as trong electron donor gives D-p-A + dyes with wide tunability of the opticalb and gap (compounds 18,F igure 8). [36] For example, the oligophenylene-based salts 18 a-d are brightly fluorescent in dichloromethane solutions (F em = 0.71-0.95) with emissionw avelength changing from blue (487 nm) to orange (619 nm) upon increasing the number of the phenylene rings in the p-spacer.R emarkably,t he pyridinium donoracceptord yes, including the direct congenerso ft he phosphonium salts 18 a-c,e xhibit much lower quantum efficiencies (typicallyl ess than 0.01). [37] The use of the polyaromatic (acene) p-system ultimately results in deep-red luminescence (l em = 696 nm for 18 g in dichloromethane), though with am uch lower quantum yield of 0.02 only.T hese compounds demonstrate good two-photon absorption (TPA) cross-section up to 321 GM (Goeppert-Mayer GM = 1 10 À50 cm 4 sphoton À1 molecule À1 )f or phenylene series (18 d), while derivative 18 f reaches the value of 977 GM at the excitation of 800 nm.

Phosphonium-modified dyes
Appending terminal phosphonium motifst oh ydrophobic organic dyes is as uitable methodt oi ncrease their biocompatibility.I th as been actively used for the development of various  probest ot arget mitochondria,a lthough in most of the cases the P-cationic group is electronically innocent as it is isolated from the chromophore scaffold by an aliphatic spacer. [10] The examples of ad irectb onding of the phosphonium fragment to the dye molecules are rare,w hich,f or instance, comprise coumarin and BODIPYd erivatives 19 [38] and 20 [39] (Figure 9), showings pecificm itochondrion localization.T he quantum yield of 19 reaches 0.91 in dichloromethane (l em = 441 nm), whereas for unsubstituted 7-methoxycoumarin the emission intensity is lower (F = 0.53 in buffer and 0.03 in methanol) and the energy is substantially higher( l em = 394 nm), [40] indicating an on-innocent effect of -+ PMeTol 2 electron acceptoro nt he photophysics of the parentdye.
The reversiblef ormation of the phosphonium adductsa sa result of nucleophilic addition of phosphines to cyanine or squaraine dyes offers unconventional stimuli-responsive systems. [41] For instance. the equilibrium between intensely blue colored squaraine and the bleached adduct is regulated by nucleophilicity of the PR' 3 reagent as well as temperature ( Figure 10). The adducts 21 also have ap otentialt os erve as chemodosimeters for the metal ions and complexes that have high affinity to phosphine ligands (e.g. Rh, Pd, Ir,A u). [41c] Decoration of the naphthalenediimide with phosphonium groups generates stable ultra-electron deficient species 22 a (Figure 11 A) having exceptionally low LUMO energy of À4.90 eV and the first reduction peak at À0.199 V( vs. Fc + /Fc, R = Ph). [42] The corresponding radical ion 22 b,w hich can be efficiently prepared from the diimide dibromide and the phosphine under solvent-free conditions, [43] appearst ob eh ighly stable and tolerates conventionalw orkup operations on air. This feature arises to al arge extent from hypervalent Lewis acid-base ( + P ! O) interactions, which are known for other phosphonium salts with hard donors. [44] An easy electron transfer to 22 a that is accompanied by ad ramatic color change (Figure 11 B), and extraordinary stabilityo fr adical 22 b form an attractive platform fors witchable electrochromic materials. Moreover,t he judicious choice of the Ra nd R' groups made possible the isolationo fh ighly electron rich doubly reduced derivatives 22 c with diverse color palette. [45] Twos tep reversible reduction with waves at À0.28 and À0.90 V( vs. Fc + /Fc) has been also realized in the a-phosphonio-acridinium dication 23 a that produced air-stable phosphonio-radical, and ultimately an easy to decompose antiaromatic phosphorus ylide 23 c (Figure 11 A). [46] On the other side, the use of pyromellitic diimide motif allows to obtain highly stable Figure 9. Phosphonium-modified coumarin (19) [38] and BODIPY (20) [39] chromophores(inset showsconfocal microscopy photo of the H9c2 cell line stained with 20).   [42,[45][46][47] Adaptedf romr eference [42] with permission from American Chemical Society (2020). chromophoric ylide 24,w hich does not show typical Wittig reactivity.I ts persistence has been attributed to O:/C: ! + P(n O/C ! s* P-C )i nteractions indicated by the natural bond orbital calculations. [47] 3. Cyclic Organophosphorus Cationic Chromophores

Synthetic approaches to ionic phospha-annulated systems
The methods of merging the phosphonium functionR 2 P + < into the p-conjugated scaffold yet have not been as ubject of systematic studies, and therefore the number of general protocols are still quite limited. The reported approaches can be roughlyc lassified into three main categories depictedi n Scheme6.

Quaternization of l 3 s 3 P-heterocycles
Arguablyt he most straightforward way to obtain ionic phosphacyclic chromophores is based on the preparation of parent l 3 s 3 -P cyclic chromophores and their subsequentS N 2q uaternization (pathway I, Scheme 6), which has been exploited for a wide selection of alkylating agents RX having different leaving groups (X = halides, mesylate, tosylate, or triflate).
Ring-fused phosphole systems serve as as ource for phospholium acenes (Figure12). [48] The six-membered organophosphorusc ations also can be prepared by alkylation of cyclic l 3 s 3 -predecessors. [49] In comparison to the tertiaryphosphines, lower nucleophilicity of the l 3 s 3 Pa tom in phospholes evidently makesi tl ess reactive towarda rylation; so far there has been only one exam-ple of direct Pd-catalyzed P-arylation of the benzo[b]phosphole. [48g]

Intramolecular phosphacyclization
Fusingp hosphacyclic ionic scaffolds with conjugated hydrocarbon systemsc an be achieved by meanso fv ariousi ntramolecular reactions, which imply the incorporation of PÀCb ond formation (pathway II, Scheme 6). Following this methodology, the preparation of arylphospholium salts often involves activation of the alkyne motif that is in ortho-position to the phosphoruscenter (Scheme 7).
An efficient metal-free synthesis to phospholium-borate zwitterionic ladders tilbenes 25 was disclosed by Yamaguchi group (route A). [50] The electron-donating substituents at the phosphorus atoms (R = tBu and Cy) result in immediate nucleophilic cascade cyclization of the intermediate phosphine at room temperature, whereas for R = Ph thermalo rp hotochemical initiation is required. [51] The closure of 5-membered phospha-ring occurs in the courseo fp hosphinoauration of alkynes (route B, Scheme 7), the gold centerc an be subsequently removed with strong acid to afford cations of af amily 26. [52] Thiss ort of reactionc an be carried out withoutm etal reagent as it is promoted by excess of protic acid. [53] The dialkynyl precursors of suitable stereochemistry undergo cascade process, where the first step of stoichiometric phosphinoauration is followed by ag old-catalyzed cyclization to afford fused p-extended salts 27 in up to quantitative yields (Scheme 8). [54] Importantly,r eadily obtainable phosphine oxides can also be transformed into phospholiumd erivatives in good yields. Their treatment with oxalyl chloride gives electrophilic l 5 s 5 -chlorophosphonium intermediates, which then transform into the heterocycle at moderately elevated temperature (70 8C, route C, Scheme 7). [55] Scheme6.Generalroutes to embed P-quaternized group into aromatic cyclicscaffold. Figure 12. Examples of neutral l 3 s 3 heterocyclic precursors for P-quaternization reactions.

Phospholium-based dyes
Photophysical behavior of P-heterocyclic dyes has been predominantly investigated for the systems containing 5-membered rings, along with ag rowingn umber of the reports on the 6-membered congeners. The electron-accepting properties of l 5 s 4 -a nd l 4 s 4 -phosphole derivatives, assigned to the low energy of the LUMO, have been explained by the phenomen-on of negative hyperconjugation highlighted in an umber of reviews. [8c, 9c] The electrophilicity of phosphole-containing species evidently dependso nt he substituents on the phosphorus atom, and is particularly enhanced for l 4 s 4 cationic compounds. [64] The incorporation of strongly electron deficient Pionic motif into the p-conjugated scaffold dramatically affects frontier molecular orbitals of the latter.C learly,t he optoelectronic characteristics of such phospha-annulated systems are primarily dictated by the structure and the composition of the organic framework, but they also can be modulated by means of the ancillary substituents at the phosphorus atom and the counterioncomponent.
One of the simplest motifs used for the construction of phosphacyclic chromophores is benzophospholium unit (Figure 13). Ther esulting phenyl-substituted salts 26 a-c with moderate ICT are intenseb lue emittersi ns olution( l em = 446 nm R = H, 476 nm R = OMe, CH 2 Cl 2 )w ith quantum yields ranging from 0.75 to 0.89 for PF 6 À or OTf À anions. [48g, 53] In the case of iodide, the intensity of fluorescencei ss ystematically lower (F em = 0.36-0.80) that might be ar esult of anion-p charge transfer interaction. [65] The bulkier groups at the phosphorusa tom lead to ac ertain improvemento fq uantum efficiency by suppressing non-radiatived ecay rate;f or example, the methylated derivative 26 a shows twice larger k nr (3 10 7 s À1 )t han ethylated and phenylatedc ongeners 26 b,c (k nr = 1.4 10 7 s À1 ). [48g] The decrease of the opticalg ap hasb een also realized in phosphoniafluorenes 37 and 38 by decorating the dibenzophosphospholiumm otif with diethylamino styryl donors. [66] Salt 38,arare example of spiro-phosphonium compounds, shows deep-red to near-IR fluorescencew ith l em = 695 nm (F em = 0.1) in toluene and 715 nm in CH 2 Cl 2 (F em = 0.04). The essential impact of the spiro-structure is reflectedn ot only by the red shift of the absorption and emission bands, but also by almost3 -fold increase of the TPAc ross-section,w hich amountst o1 022 GM (at 932 nm) for 38 and 356 GM for 37.
Modification of the phenyl-benzophospholium 26 a with the secondary heteroatom (sulfur) virtuallyd oes not change the fluorescencew avelength (39 a: l em = 445 nm, F em = 0.36 in CH 2 Cl 2 ). [67] The electronic state of sulfur however affects the conjugation within the heterocyclicc ore, and the electronic behavior of the phosphorus. Oxidation of the S-atom converts it into an electron accepting group (39 b), diminishes the band gap and drastically suppressest he non-radiativer elaxation of the excited state (k nr = 4.3 10 À7 s À1 for 39 a and 0.1 10 À7 s À1 for 39 b), giving the quantume fficiency closet ou nity (39 b: l em = 483 nm, F em = 0.99 in CH 2 Cl 2 ). Several other thienophospholium derivatives, as well studied by the group of Baumgartner, [8b, 68] were elegantly engineered via the modification of both the conjugated backbone and the auxiliary substituents at the phosphorus atom. These chromophores demonstrate intriguing stimuli-responsive fluorochromisma nd aggregationinduced emission (AIE),which have been discussed earlier.
The symmetricald icationic stilbenes 40 a,b (Figure 13) are brightlyf luorescent in both solution and solid state. [69] Remarkably,c hloride 40 b gives the quantumy ield of 0.74 in water (l em = 518 nm) with significant Stokes shift of 5780 cm À1 that makes this motif an attractive paradigmf or bioimagingp urposes. On the other hand, relativelyl ow-lying LUMOs( CV E LUMO = À4.19 and À4.04 eV for 40 a,b)f avor electron injection, suggesting that theseelectron-accepting speciescan be employed for the development of n-type semiconducting materials.
High electron affinity has been also demonstrated by the dyes with extended polyaromatic system.D iacenaphtho phospholium salt 41 ( Figure 14) shows lowr eductionp otential of only À1.00 V( vs. Fc + /Fc), which is less negative than that of congener neutral phospholes apparently due to the electron deficient nature of the P-cationic center. [48b] Photoluminescent properties of the phosphacenes are distinctly dictated by the size and stereochemistry of the p-backbone. If 41 is aw eakly deep-red fluorophore (l em = 684 nm, F em < 0.01 in CH 2 Cl 2 ), the binaphthyl derivative 42 has been described as bright green-yellow emitter in crystalline state. [70] Planarized PAHf ramework in dibenzophosphapentaphenes 43 provided highly delocalized p MO-s ( Figure 14). [48c] The cationic character of 43 a,b is manifested by the significant contribution of the Pa tom to the LUMO andh igherr eduction potential vs. their chalcogenide analogues. The methylated derivative 43 a reveals moderately intense orange-red fluorescence (l em = 599 nm, F em = 0.19 in CH 2 Cl 2 ). Importantly,a ir-and moisture-stable thio-phospholium compound 43 b,w hich was obtained by treating the corresponding sulfide with MeOTf, shows spectacular bathochromic shifts of both the lowest  Chemistry-A European Journal energy absorption (from 554 to ca. 610 nm) and emission (to 669 nm) bands compared to 43 a.C onsidering the enhanced electron-accepting ability of R 3 P + -SMe motif, its stabilitya nd the easinesso fp reparation, 43 b illustrates av ery attractive yet undeveloped way to decreaseo pticalb and gap of donor-acceptor organophosphorus chromophores. ICT has been successfully tuned in zwitterionic stilbenes 25 (Scheme 7) andt heir extended polycyclic derivatives that allowed to vary the emission from green to red-orange (l em = 517-623nmi nt etrahydrofuran). [50,51] The concept of embedding cationic phospholium( acceptor)a nd anionic borate (donor)u nits in the same molecular scaffold wasf urther accomplished in 44,w hich was readily obtainedf rom the orthoalkynyl triphenyl phosphine and B(C 6 F 5 ) 3 as an electrophile in a nearly quantitative yield ( Figure 15). [53] The presence of the borate substituent in 44 has am inor influence on the emission parameters (l em = 471 nm, F em = 0.76 in CH 2 Cl 2 )w ith respectt oi ts predecessor 26 b (l em = 476 nm, F em = 0.88), butc onsiderably increases the Stokes shift from 4633 cm À1 (26 b)t o8709 cm À1 (44).
The series of zwitterionic carboranes 45 are also bright blue emitters( l em = 446-469nm, F em = 0.34-0.99i nC H 2 Cl 2 ). [71] In contrastt ot he donor-functionalized 26 d with red fluorescence, [48f] compound 45 with the same diphenyl-aniline substituent does not exhibit low energy ICT due to the localization of the HOMO on the carborane fragment. The peculiar feature of compounds 45 lies in their remarkable easinesso ft he reduction, which occurs at the potential E red ranging from À1.02 to just À0.4 V( vs.Fc + /Fc)a ccording to electrochemical data.

Chromophores containing six-membered P-cationic heterocycles
The development of photofunctional molecular materials based on six-membered P-heterocycles has been considerably delayedi nc omparison to the corresponding phosphole chemistry.
[8d] Nevertheless,s uch phospha-annulateds ystems apparently offer richo pportunities forr ing modification, and can introduce new chemical and electronic properties,w hich might be difficult to attain with other cyclic motifs. The so far very limitede xamples of compounds, incorporating six-membered P-cationic rings, are classified as l 5 s 2 -endo,s 2 -exo species. Their opticalb ehavior is determined by the p-conjugated backbone, akin to phosphole derivatives. The fluorescence has been detected for compounds with acene backbones, while very weak or no emission was reportedf or structurally simpler cations (e.g. dibenzophosphonioborine, [30a, 72] dithienodihydrophosphinine, [49a] acridophosphine [49c] derivatives).
The naphthalene-based fluorophores 29 a, 47 and 48 a-g ( Figure 16) reveal strong impact of non-acene fragment of the backboneo nt he photophysical performance.R elatively weak deep blue emission of thienyl compound 47 (l em = 418 nm, F em = 0.07 in CH 2 Cl 2 ) [49b] is significantly improved in its phenylene congener 29 a (l em = 418 nm, F em = 0.3 in CH 2 Cl 2 ). [57b] Salts 48,o btained via alkyne insertion into phospharuthenacycles followed by P-C reductive elimination, are luminescentf or aromatic Rs ubstituents only. [74] The emissioni st uned from l em = 531 (48 e, F em = 0.03 in CH 2 Cl 2 )t o5 88 nm (48 g, F em = 0.46) primarily by changing the energy of the HOMO, to which the pendant aryl Rg roups make the dominant contribution, whereas the LUMO localized on the phosphaphenalenium core remains rather unaffected. The modulation of the emission wavelength by moving from R = Ph (48 c)t op-C 6 H 4 OMe (48 g, Figure 15. Recent examples of zwitterionic borate-phospholiumfluorophores. [53,71]  Dl = 54 nm, 1720 cm À1 )i sc onsiderably more efficient than that fort he analogouss tructuralv ariation of the abovementioned phospholium dyes 26 b and 26 c (Dl = 23 nm, 1067 cm À1 ). It is worth noting that compounds 48 c-g are intensely fluorescent in the solid with quantum yields up to 0.77 (48 g)a nd hypsochromic shift of the emission maxima (l em = 422-507nm). These features are attributed to the absence of p-stacking interactions prevented by the bulky counterion BAr F 4 À . The employment of different acene cores togetherw ith donor-acceptor architecture in 29 a-h allowst oa lter the fluorescencet hrought he entire visible range (l em = 418-780nmi n CH 2 Cl 2 ,F igure 16 B) with good quantum yield even in the near-IR region (29 h, l em = 780 nm, F em = 0.18 in CH 2 Cl 2 ). [57b] Due to the ionic nature,m osto fs alts 29 are moderately soluble in water.I mportantly,s ome of these species retain intense emission in aqueous medium (F em = 1f or 29 c-e in H 2 O) and demonstrate good TPAp roperties( the corresponding cross-section for 29 c-f ranges from 310 to 637 GM measured at 800 nm in water). [57] It has been shown that these photophysical properties, together with high photostability and low toxicity, make anthracene-based dyes 29 c-h applicable in one-and twophoton cell imaging.
The involvement of the quaternized cycle-embedded phosphorusa tom into the LUMO means that increasing the number of the cationic centers connected via p-conjugated skeleton necessarilyi nfluences frontier orbitals, optical band gap and the electrochemical properties of the system. In line with dicationic nature, the diphosphahexaarene 49 ( Figure 17) presentsaset of reduction potentials with the lowest value of À1.1 V, while no oxidation was observed. [49d] This points to high stability of the cation, also confirmed by the absence of photodegradation under prolonged irradiation. The vibronicstructured emission of 49 in solution (l em = 422 nm, F em = 0.8 in CH 2 Cl 2 )r emains nearly the same in water (l em = 423 nm, F em = 0.67) providing the opportunities for biovisualization.
An interesting set of dicationic molecules (selectede xamples 50-52 are given in Figure 17) has been recently described by Bouit and co-workers,p repared from tertiary diphosphinesa nd diphenylacetylene (method A, Scheme 11). [60] Decoration of the naphthalene with phosphininium rings produces 1,8-bisphosphapyrenium 50 scaffold with orange fluorescence( l em = 606 nm, F em = 0.19 in CH 2 Cl 2 ). The extension of the acene framework from naphthalene to chrysene( 51)h owever induces blue shift of the emission (l em = 560 nm, F em = 0.06 in CH 2 Cl 2 ). Similarly,t he binaphthyl dication 28 [56] reveals further blue shift and emits comparably to 49 (l em = 453 nm, F em = 0.11i nw ater). [57b] On the downside, 52 possessing anthracene motif shows deep-red fluorescence( l em = 672 nm, F em = 0.04 in CH 2 Cl 2 )e ven withouti nvolving strong donor groups, which confirms the importance of P + -PAH connectivity but not only the size of the aromatic framework. It is essential that salts 50-52 are also bright emittersi ns olid (F em = 0.11-0.39) including 52,t he fluorescenceo fw hich is almost unaltered (l em = 670 nm). Along with the opticald ata, 52 is the easiest ion to reduce in this series (E red = À0.64 Vv s. Fc + /Fc), all dications displaying two separated "viologen-like"r eduction waves in accordancew ith their structures.

Cation-Anion Interactions in P-Cationic Chromophores
The electrostatic noncovalent interactions between the chargedc omponents are known to have as ubstantial effect on the photophysical properties of the ionic dyes.
[6d-f] Thus, it has been shownt hat the intensityo ft he pyridinium-p exciplex emission depends on the nature of the counterion, and is enhanced in the presence of PF 6 À but quenched with halides.
[6b] The relatedp henomena of anion-responsive absorption and/or emissionb ehaviorh aveb een also encountered for the P-cationic chromophores. Thus, the fluorescenceo ft he dicationic compound 17 b in moderately polar dichloromethane can be regulated by the anion and the concentration (Scheme 12). [34c] At high concentration (which corresponds to the opticald ensity (O.D.) of 1.0) the iodide salt of 17 b is weakly emissive (F em = 0.07), and the fitting of the decay curves gives three different lifetimes (t varies from 0.04 to 2.7 ns). In diluted solution (O.D. = 0.1) the quantum yield increases to 0.24, whereas the change of the anion (for PF 6 À )o rt he solvent( e.g.,m ethanol) leads to as ingle exponentiald ecay (t = 2.8 ns) and more dra-matic improvement of the luminescencee fficiency to 0.86 and 0.77, respectively,a tt he O.D. of 1.0. These observations were rationalized by the equilibrium between the contact and solvent separated ion pairs. As proposed by the authors, the nondissociated state is favored by the iodide and higherc oncentration in the less polar medium (CH 2 Cl 2 ), resulting in (i)the presenceo fs everal emissive species, and (ii)quenching of luminescence due to the heavy atom effect. The same group further extended the concept of ion pairs on phospholiumfluorophores 26 a,b (Scheme 12). These species also reveal as ystematic increaseo ft he emission intensity in lower polarity solvent for the hexafluorophosphates compared to the iodide analogues,m ainly due to the suppression of non-radiatived ecay rate. [48g] Although anions have as ignificant impact on thee fficiency of fluorescencef or contact ion pairs, the energieso ft he radiativet ransitions remain virtually unchanged.I ns tark contrast, the linear D-p-A + oligophenylene salts 18 a-c (Figure 8) feature unconventional dual emission in non-polar solvents (toluene, dioxane,c arbon tetrachloride). [36] The ratio of two energetically distinct emission bands (e.g. l em = 468 and 592 nm for 18 b in toluene, F em = 0.17), which appear as ar esult of a continuouss pectralt emporal evolution (Figure 18), strongly dependso nt he nature of the counteranion,t he length of the p-spacer,s olventv iscositya nd the temperature. Such relaxation dynamics has been attributedt ot he hypothesized anion migration that occurs in donor-acceptor contact ion pairs upon photoinduced ICT in the absence of solventr elaxation (Figure 18 C), and ultimately produces al ower energy emissive excited state. It should be mentioned, in support of the mechanism,t hat the anion migration in the excited ion pair has been also proposed for some N-heterocyclic (acridinium, naphthoquinolizinium) cationic dyes. [6a, d, 75] No less important can be the role of the counterion in the solid-state emission of P-cationicluminophores. It has been noticed for liquid crystalline thienyl phospholiumf luorophores that intermolecular arrangement and,a saconsequence, the photophysical properties of the bulk material, can be adjusted by steric and electronic effects imposed by counterions. [48a] Hence, in the studied series (Br À ,B F 4 À ,B Ph 4 À ,O Tf À )t he anions of larger size diminish the probability of p-stacking that decreasest he difference between the solutiona nd solid state emission, and might increase the quantum yield. Moreover,t he substitution of small bromide for amphiphilic dodecylsulfate combined with cationic lipid 53 clearly decreases the crystallinity of the solid and limits the AIE property (Figure 19), highlighting an opportunity to control the morphologya nd optical characteristics of the ionic material. [76] On the other hand, the anions prone to form strong non-covalenti nteractions are capable to facilitate AIE behavior. AIEactive phospholium 54 crystallized with polyoxometalates [M 6 O 19 ] 2À (M = Mo, W), shown in Figure 20, are involved in the extensive network of anion-p interactions and CÀH···O hydrogen bonding, while the p-p stacking is prevented upon incorporating the bulky anions as spacers. These multiple contacts rigidify the fluorescent cation that raises the quantum yield almost3 -fold (F em = 0.43) with respectt oi ts triflate salt (F em = 0.15) and ca. 90-foldc ompared to that in the solution. [77] Although phosphoniumg roups in salts 55 and 56 are separated from the polyaromatic p-systems by the methylene Figure 19. Di(benzothieno)phospholium cation 53 (left), andpolarized optical imagesof53 salt with Br À and dodecylsulfate anions. [76]  (C) proposed charge transfer-induced anion migration resulting in dual emission of 18 b in non-polar solvents. [36] Adaptedf rom reference [ 36] with the per-missionofW iley-VCH GmbH (2020). spacer (Figure 21), the non-innocent interactions between cations and anions give rise to interesting mechanofluorochromic properties. [78] In particular,v ariation of counterions from Br À to NTf 2 À for 56 changes the emission of crystalline materials from green (525 nm) to red-orange (622 nm). Upon grinding, salts [56 + ](X À )e xhibit nearly identical fluorescence( ca. 560 nm) due to the formation of amorphousp hase, that is, the direction of mechanochromic shifto ft he emission is triggered from red (Br À )t ob lue (NTf 2 À )s imply by meansoft he anion. The anion-cation interactions (anion = Cl À ,B r À ,I À ,N O 3 À , BF 4 À ,P F 6 À ,B Ph 4 À )i nvolving phosphonio-diimide radicali ons of the type 22 b (Figure 11 A) as well have shown an ability to amend the photophysical and magnetic properties of the cationic p-radical systems. In crystals, the radicalcations form various supramolecular assemblies utilizing charge-assisted hydrogen bonding and anion-p contacts. Ion pairing effect appears to be non-innocent in solution too, especially in the solvento f lower polarity.F or example, the UV-vis measurements supported by theoretical calculations suggest that nitrate radical salts undergo unusual "reverse" p!anion photoinduced electron transfer in contrast to otheranions. [79]

Concluding Remarks
In this minireview we tried to highlight the reported data on the organophosphorus ionic chromophores, in which the Pcationic groupsp lay an active role in composing the electronic structures, and therefore show ac lear influence on the optical and/or electrochemical properties. These dyes stillc an be considered as an emerging class of compounds as most of the relevant works have been published during the last decade. The diverses ynthetic pathways to p-conjugated P-cations, particularly to P-heterocyclics pecies, go far beyond simple quaternization of l 3 s 3 tertiary phosphines, and can produce intriguing molecular scaffolds, many of which very probably are stillu ndisclosed. Stronge lectron deficient character of l 4 s 4 (and l 4 s 3 ) phosphorus centers, which can be tuned with the help of the substituents, is immensely useful in the constructiono ff ascinating donor-acceptor systems exhibiting versatile charge transfer and redox behavior.Inaddition, the unique connectivity of the pyramidal phosphorus atom allows the designa nd accessible preparation of branched multipolar and multichro-mophore architectures, although this option has been exploited yet to av ery limited extent.T aking into account the chargedn ature of the described chromophores, the effect of cation-anion interactions and ion pair formation both in the solid and in solution cannot be neglected. Albeit ion pairing brings an additional degree of freedom, which is often difficult to evaluatep recisely and to control in reproducible andp redictable manner,s imultaneously it can greatly diversify physical and chemicalp roperties of two-component aggregates, leadingt ou nconventional phenomena andf unctionalities that may be far-reaching in both fundamental and applications.