1,3-Dipolar cyclisation reactions of nitriles with sterically encumbered cyclic triphosphanes: synthesis and electronic structure of phosphorus-rich heterocycles with tunable colour

We describe the synthesis, solid state and electronic structures of a series of tunable five-membered cationic and charge-neutral inorganic heterocycles featuring a P3CN core. 1-Aza-2,3,4-triphospholenium cations [(PR)3N(H)CR′]+, [1R]+ (R′ = Me, Ph, 4-MeOC6H4, 4-CF3C6H4) were formed as triflate salts by the formal [3 + 2]-cyclisation reactions of strained cyclic triphosphanes (PR)3 (R = tBu, 2,4,6-Me3C6H2 (Mes), 2,6-iPr2C6H3 (Dipp), 2,4,6-iPr3C6H2 (Tipp)) with nitriles R′CN in the presence of triflic acid. The corresponding neutral free bases (PR)3NCR′ (2R) were readily obtained by subsequent deprotonation with NEt3. The P3CN cores in 2R show an envelope conformation typical for cyclopentenes and present as yellow to orange compounds in the solid state as well as in solution depending on both substituents R and R′ in (PR)3NCR′. The P3CN cores in [1R]+ show a significant deviation from planarity with increasing steric bulk of the R groups at phosphorus, which results in a decrease in the HOMO–LUMO gap and distinct low-energy UV-Visible absorption bands. This allows access to colours spanning red, blue, indigo, and magenta. TD-DFT calculations provide valuable insight into this phenomenon and indicate an intramolecular charge-transfer from the HOMO located on the P3 framework to the N 
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 C–R′-based LUMO in the cationic species. The cations [1R]+ represent rare examples of phosphorus-rich heterocycles with tunable colour, which can be incorporated into polymers by post-polymerization modification to afford coloured polymers, which demonstrate utility as both proton and ammonia sensors.


Introduction
8][9][10] In main group chemistry reductive coupling reactions or salt metatheses continue to represent the state of the art, but oen involve aggressive reaction conditions that restrict the introduction of functional groups and lead to the formation of unwanted byproducts.][13][14][15] A common atom-economic route to organic heterocycles is the Huisgen cycloaddition of 1,3-dipoles with dipolarophiles.Huisgen rst demonstrated the thermal [3 + 2]-cycloaddition of organic azides with alkynes, which proceeds without regioselectivity. 16Through the use of Cu-catalysts, Meldal, 17 Sharpless and Fokin, 18 concurrently and independently developed a regioselective process.Accordingly, a wide range of 4-triazoles can be easily accessed at room temperature (Scheme 1, i).In terms of using main-group multiply bonded systems as dipolarophiles, the formal [3 + 2]-cycloadditions with organic azides have been extended to phosphaalkynes (Scheme 1, iii), [19][20][21][22] cyaphides, 23,24 arsadiazoniums and arsaalkynes 25 to give 5membered heterocycles selectively with 100% atom-economy.Vicinal donor-acceptor cyclopropanes (DACs) represent a class of "disguised" or "masked" 1,3-dipoles.Electron donating and withdrawing groups in vicinal position polarise the C-C bond and stabilise partial positive and negative charges (Scheme 1, ii), respectively, thereby introducing signicant 1,3-dipolar character. 26,27These DACs react with dipolarophiles to yield a plethora of organic heterocycles akin to the Huisgen-Sharpless cycloaddition.][30][31][32][33][34] Triphosphiranes, cyclic phosphanes of the general formula (PR) 3 , can be considered as heavier cyclopropane analogs by isolobal replacement of CR 2 for PR. 35Accordingly, triphosphiranes have a long history as starting materials for the synthesis of inorganic ring systems and the eld was recently reviewed. 36ecent examples include phosphenium ion insertion and Lewis acid activation reactions. 37,38In general, the insertion of organic molecules into P-P bonds is a valuable tool for the construction of new phosphorus compounds. 39 analogy to DACs, the triphosphirane ( t BuP) 3 can be activated by either Brønsted (HOTf, OTf = [SO 3 CF 3 ] − ) or Lewis acids such as (Ph 3 Sb(Cl)OTf), through polarization of one P-P bond and subsequently react with nitriles R 0 CN to give ve-membered P 3 CN-species. 40In the absence of HOTf or MeOTf no reaction with nitriles is observed.This methodology provides an operationally simple and rapid route to 1-aza-2,3,4-triphospholenium salts [P 3 t Bu 3 N(H)CR 0 ]OTf and their neutral 1-aza-2,3,4triphospholene P 3 t Bu 3 NCR 0 congeners, which can be cycled between neutral and cationic states by addition of acid or base, respectively.The neutral rings were previously only accessible through formal [3 +2]-cyclisations of W(CO) 5 -coordinated diphosphenes and nitrilium phosphanylides, respectively (Scheme 1, iv). 41However, both the cationic [P 3 t Bu 3 N(H)CR 0 ] + as well as the neutral P 3 t Bu 3 NCR 0 rings are either colourless or pale yellow and show no evidence for low-energy electronic transitions.
In this contribution we show that the colour of 1-aza-2,3,4triphospholenium salts can be effectively tuned through modication of the steric demand of the P-substituents, and through the electronic properties of the nitrile coupling partner (Fig. 1b).This is achieved by using aryl-substituted triphosphiranes (PAr) 3 (Ar = Mes, 2,4,6-Me 3 C 6 H 2 ; Dipp; Tipp, 2,4,6-i Pr 3 C 6 H 2 ) as starting materials. 79The colouration can be traced to intramolecular charge transfer processes, and we also show switchable colour when going from the cationic to the neutral state through addition of external base.This feature was used to prepare a Brønsted acid-base responsive polymer.

Results
Synthesis and NMR spectroscopic characterisation of cationic and neutral P 3 CN heterocycles [1 R ] + and 2 R Our investigation into the effect of steric hindrance on the Brønsted acid-mediated [3 + 2]-cyclisation of cyclic triphosphanes began with the reaction of (PTipp) 3 with an excess of acetonitrile (MeCN) in the presence of HOTf (Scheme 2).(PTipp) 3 was dissolved in a 1 : 1 v/v mixture of toluene/MeCN, which, aer addition of one equivalent of neat HOTf at 20 °C, resulted in a deep red homogeneous solution.This colouration was unexpected, given that analogous reactions with (P t Bu) 3 yield yellow solutions and pale-yellow solid products (Fig. 2).An aliquot of the red reaction mixture analysed by 31 P{ 1 H} NMR spectroscopy showed conversion to a single phosphorus containing species with an AMX spin system consistent with formation of the 1-aza-2,3,4-triphospholenium ring   Scheme 2 Synthesis of 1-aza-2,3,4-triphospholenium triflate salts and their corresponding free bases in this work with a labelling notation, where R describes the substitution at phosphorus and R 0 describes the nitrile substituent.Given that (PTipp) 3 demonstrated facile MeCN insertion chemistry, we sought to extend the synthetic protocol to (PDipp) 3 and (PMes) 3 .[1 Dipp ] + (R 0 = Me) was prepared similarly and was found to be virtually identical to [1 Tipp ] + (R 0 = Me) in colour, with similar 31 P, 1 H and 19 F NMR spectroscopic characteristics (ESI Fig. S-41 to S-43 †); in contrast, the attempted synthesis of [1 Mes ] + (R 0 = Me) was less straightforward.Aer conducting a similar work-up to that used for the isolation of [1 Tipp ] + (R 0 = Me) and [1 Dipp ] + (R 0 = Me), a yellow powder was obtained.Recrystallisation of the powder from PhF/n-pentane afforded crystals which, when dissolved in CDCl 3 and subsequently analysed by 31 P{ 1 H} NMR spectroscopy, showed the anticipated AMX spin system, alongside impurities characterized by resonances at −59.5 ppm (singlet, ca.7%) and −18.9 (two peaks, ca.2%) (ESI Fig.

S-70 †).
A second recrystallisation of [1 Mes ] + (R 0 = Me) by allowing a PhF/Et 2 O solution layered with n-pentane to stand for a month at −30 °C resulted in a mixture of yellow prisms of [1 Mes ] + (R 0 = Me) and a small amount of yellow plates that were crystallographically identied as the Et 2 O solvate of (PMes) 6 (ESI Section 5.18 †).(PMes) 6 has been previously observed as a side-product of the reaction between Na 2 [P 4 Mes 4 ] with [Rh(COD)Cl] 2 by Hey-Hawkins and coworkers by means of single-crystal X-ray crystallography. 81The mechanism of the formation of (PMes) 6 is currently unknown.
Next, we synthesized the benzonitrile-derived series In this case (PTipp) 3 was dissolved in a 2 : 1 v/v solution of toluene/PhCN, giving a colourless solution.Addition of one equivalent of neat triic acid at 20 °C resulted in a deep blue solution.Aer removal of the volatiles, the dark blue residue of [1 Tipp ] + (R 0 = Ph) was washed with n-hexane to yield a blue powder, whose 31 P{ 1 H} NMR spectrum in CDCl 3 consists of an AMX spin system, which is virtually identical to that of [1 Tipp ] + (R 0 = Me) (Fig. 2, bottom right).The analogous reaction was repeated with (PDipp) 3 , again yielding a blue powder of [1 Dipp ] + (R 0 = Ph) possessing the characteristic AMX spin system in the 31 P{ 1 H} NMR spectrum (ESI Fig. S-56 †), likewise virtually identical to [1 Dipp ] + (R 0 = Me).In the case of [1 Mes ] + (R 0 = Ph), we obtained an orange powder, as opposed to the yellow [1 Mes ] + (R 0 = Me), that gave rise to an AXY spin system with second order effects in the 31 P{ 1 H} NMR spectrum in CDCl 3 , similar to that observed in [1 Mes ] + (R 0 = Me).We note that [1 Mes ] + (R 0 = Ph) has greater stability in solution than [1 Mes ] + (R 0 = Me) and that the only impurities that form upon dissolution correlate to (PMes) 4 as evidenced by 31 P NMR spectroscopy.Finally, [1 tBu ] + (R 0 = Ph), was synthesized and isolated as a yellow powder with an AMX 31 P{ 1 H} NMR spectrum featuring broad signals (Fig. 2, bottom le).
To probe the effect of electron-donating and electron withdrawing substituents on [1 Tipp ] + (R 0 = Ph), p-MeOC 6 H 4 CN and p-CF 3 C 6 H 4 CN were used in place of PhCN to synthesize formed a magenta solution with 31 P { 1 H} NMR spectra bearing an AMX spin system in which the M and X nuclei are more shielded relative to [1 Tipp ] + (R 0 = Ph).
[1 Tipp ] + (R 0 = p-CF 3 C 6 H 4 ), contrarily, afforded royal blue solutions, with a similar AMX spin system in the 31 P{ 1 H} NMR spectrum, in which the M and X nuclei are shied downeld relative to [1 Tipp ] + (R 0 = Ph).As these compounds could be potentially used as tunable P-based dyes, the photo-stability of a prototypical species was investigated.Irradiation of [1 Tipp ] + (R 0 = Ph) with a broad band solar irradiation LED (480 mW, see ESI Fig. S-115 and S-116 † for lamp and 31 P NMR spectra) for 72 h revealed no discernible decomposition as assayed by 31 P{ 1 H} NMR spectrometry in CDCl 3 , demonstrating the photostability of these 1-aza-2,3,4-triphospholenium salts.
Subsequently, the formation and properties of the corresponding free bases, 1-aza-2,3,4-triphospholenes 2 R (R 0 = Me and Ph) were explored, to determine if this unexpected and intriguing colouration was unique to the protonated heterocycles.The free base 2 Tipp (R 0 = Me) was synthesized through the addition of NEt 3 to a slurry of [1 Tipp ] + (R 0 = Me) in n-hexane, and was isolated as a yellow solid aer removal of [Et 3 NH]OTf by ltration and of volatiles in vacuo.The 1 H NMR spectrum lacks the N-H signal, consistent with quantitative deprotonation.Loss of triate, and thus formation of [HNEt 3 ]OTf was indicated by the absence of detectable 19 F NMR signals in isolated 2 Tipp (R 0 = Me).The 31 P{ 1 H} NMR spectrum of 2 Tipp (R 0 = Me) also consists of an AMX spin system, however, with notably shielded signals for the A and M nuclei compared to previously reported 2 tBu (R 0 = Me). 402 Dipp (R 0 = Me) had similar colouration to 2 Tipp (R 0 = Me), while the 31 P{ 1 H} NMR spectrum exhibits a second order AXY spin system with the A nucleus shiing downeld to 98.6 ppm compared to the chemical shi of 82.3 ppm observed for the A nucleus of 2 Tipp (R 0 = Me). 2 Mes (R 0 = Me) is colourless in solution and exhibits an AMX spin system distinct from that of 2 Dipp (R 0 = Me) and 2 Tipp (R 0 = Me). 2 Mes (R 0 = Me) and also proved to be stable in solution, in contrast to its protonated analogue [1 Mes ] + (R 0 = Me), as NMR spectra collected on the solution aer several weeks showed no signicant change.In the case of the 2 R (R 0 = Ph) series, no signicant changes were observed in the 31 P{ 1 H} NMR chemical shis compared to the 2 R (R 0 = Me) series.When single crystals of 2 Mes (R 0 = Ph) were dissolved in CDCl 3 , partial decomposition was noted, as evidenced by formation of an unknown impurity which appears as a singlet in the 31 P{ 1 H} NMR spectrum at 62.4 ppm in CDCl 3 .
Not only are the neutral 1-aza-2,3,4-triphospholenes readily synthesized through deprotonation with NEt 3 , these free bases are near quantitatively re-protonated with triic acid to again form the 1-aza-2,3,4-triphospholenium salts [1 R ]OTf, which may again be reversibly deprotonated.This ability to reversibly convert between cationic and free base rings provides a second route to forming these heterocycles in high purity, and also provides a useful colorimetric switch (vide infra).

X-ray structural characterisation of the P 3 NC heterocycles [1 R ] + and 2
Characterisation of each of the aforementioned heterocycles [1 R ] + and 2 R in the solid state was performed by single crystal Xray diffraction (SC-XRD) experiments to investigate a potential structural rationale for the unexpected colouration.Structures, key bond lengths and angles for all species can be found in Section 5 of the ESI, † while representative structures of [1 R ] + (R 0 = Ph) and of [2 R ] (R 0 = Ph) for R = Tipp, Dipp, and Mes are shown in Fig. 3.For this discussion, we will refer to the phosphorus atoms of the central P 3 CN framework as P C (phosphorus bound to the nitrile-derived carbon atom), P N (phosphorus bound to the nitrile nitrogen atom), and P P for the phosphorus atom between P C and P N .First, the down, down, up arrangement of the aryl substituents with respect to the P 3 unit is maintained in all species when compared to (PAr) 3 .Additionally, there is a close O1-H1 contact (d(O-H) ca.1.8 Å) between the nitrilium unit in [1 R ] + and the triate counter anion, indicative of H-bonding in the solid state, while the 19  Secondly, we observed a substantially increased bending in both [1 Tipp ] + and [1 Dipp ] + (R 0 = Me and Ph) heterocycles compared to their [1 tBu ] + and [1 Mes ] + congeners (Fig. 4).The angle of bending, q, is measured from the horizontal P C -C]N-P N plane downward to the plane formed by the three catenated phosphorus atoms (see top of Fig. 4).
Using this parameter, [1 tBu ] + (R 0 = Me and Ph) are minimally bent, with the angles between the P N -P P -P C plane and the P N -N]C-P C plane being 18.58°and 17.25°, typical of cyclopentanes, while these angles are substantially increased to 55.73°and 48.46°in [1 Tipp ] + (R 0 = Me and Ph), respectively.Comparing the complete set of metrical parameters for [1 R ] + (R 0  View was chosen such that the bent-out-of-plane P P atom sits to the right, and all q values are measured from the horizontal P C -C-N-P N plane (represented as a dotted line parallel to the bottom of the page) downward to the plane formed by the three phosphorus atoms.Thermal ellipsoids are drawn at the 50% probability level.
= Ph) and 2 R (R 0 = Ph), it can be seen that the change in bending angle as steric bulk increases is greater for the [1 R ] + $(R 0 = Ph) series than in their neutral analogues.Similar structural distortions are also observed in the electronically modied compounds [1 Tipp ] + (R 0 = p-MeOC 6 H 4 , p-CF 3 C 6 H 4 ), which display bending angles of 50.65°and 57.17°, respectively, the latter being the most bent of the series.This substantial bending is also linked to changes in the P N -P C distance in these species, with [1 Tipp ] + (R 0 = p-MeOC 6 H 4 CN) displaying a 4.86% shortening and [1 Tipp ] + (R 0 = p-CF 3 C 6 H 4 CN) displaying a 6.76% shortening (ESI Section 5.19 †).

UV-Vis spectroscopic studies of [1 R ] + and 2 R
UV-Visible spectra were obtained in CH 2 Cl 2 solution for each of the cyclic species to assess whether the observed data would be correlated to the steric proles of the aryl substituents, or the characteristics of the nitrile used.Given that we were unable to successfully crystallise sufficiently pure 2 Mes (R 0 = Me), and that [1 Mes ] + (R 0 = Me) and 2 Mes $(R 0 = Ph) always undergo partial decomposition in solution, even when using single crystals, UV-Vis spectra were not acquired for these species.
All compounds studied by UV-Vis spectroscopy show broad absorption bands in the 300-400 nm region typically associated with the aryl substituents at phosphorus (Table 1, l 2 ).However, the spectra of all cationic rings (except [1 tBu ] + (R 0 = Me)) also exhibit a comparatively low-energy absorption band (Table 1, l 1 ) which follows Beer's Law.The low-energy absorption band observed in [1 R ] + (R 0 = Ph) is, on average, bathochromically shied by 59 nm compared to the absorption band of the analogous [1 R ] + (R 0 = Me) species (Fig. 5, top), which is attributed to increased electronic conjugation involving the p-system of the aromatic Ph substituent (vide infra).When the steric bulk is increased at phosphorus (where t Bu < Mes < Dipp z Tipp), the bending angle of the P 3 CN-rings increases signicantly, resulting in a progressive red-shi in the value of the low-energy absorption bands.
Unlike [1 R ] + (R 0 = Me), the spectra of 2 R (R 0 = Me) showed no evidence of low-energy absorption bands between 300-800 nm, while spectra of 2 R (R 0 = Ph) species showed low-energy absorption bands that are hypsochromically shied relative to those observed in 1) and manifest in a change of observed colouration to colourless, yellow, or orange in all species 2 R (Fig. 5, bottom).In comparing

DFT studies of [1 R ] + and 2 R
To shed further light on the unusual nature of the UV-visible absorption bands and their seemingly strong correlation to 57.17 583 the bending angle in cations [1 R ] + , the electronic structures of [1 Tipp ] + and [1 tBu ] + (R 0 = Me), 2 Tipp and 2 tBu (R 0 = Me), [1 R ] + (R 0 = Ph), and 2 R (R 0 = Ph) were investigated by DFT calculations on the BP86-D3/def2SVP level of theory, while taking the molecular structures from SC-XRD experiments as a structural basis.In all cationic species [1 R ] + the triate ion was explicitly retained.Time-dependent density functional theory (TD-DFT) calculations were performed at the B3LYP/cc-pVTZ (in both the gas phase and using the polarizable continuum solvation model (PCM) for CH 2 Cl 2 (smd = DCM), for comparison) level of density functional theory using the respective BP86-D3/def2-SVP optimized gas-phase S 0 geometries, which were found to be in excellent agreement with the experimentally determined solid state structures (for complete computational details, including a comparison of the structural parameters, please refer to the ESI p. S-150 ff.†).The UV-Vis spectra of all species were reproduced well by the calculations (cf.compare ESI p. S-155 ff.†) in the gas phase.There is no signicant difference between the data calculated in the gas phase and those using a PCM for solvation.The calculated lowest energy transitions (l 1 ) of [1 R ] + correspond to a HOMO-LUMO transition, with all other transitions associated with transitions from the HOMO−1 and HOMO−2 orbitals to the LUMO or higher unoccupied molecular orbitals (ESI Section 6 †).To better understand the origin of colour in these species, it is useful to assess the differences in the HOMO and LUMO orbitals of [1 tBu ] + (blue-shied absorption), and those in [1 Tipp ] + which absorbs at longer wavelengths (Fig. 6), and shows a considerably larger bending angle q.In the case of [1 Tipp ] + (R 0 = Ph), the HOMO is best described mainly (ca.74%) as linear combination of the n(P) orbitals (e.g.non-bonding combination, P lone pairs), with minimal delocalization into the C]N (ca.5%) and Tipp (ca.7%) p-systems.The LUMO, by comparison, is mainly delocalized over the NCPh unit and has p* character, with small contributions at P C , P N and P P .The HOMO in [1 tBu ] + (R 0 = Ph) is similar to that of [1 Tipp ] + (R 0 = Ph), while the LUMO again has mainly NCPh p* character, however, it lacks the contribution from P P observed in [1 Tipp ] + (R 0 = Ph).
While the electronic differences in [1 tBu ] + and [1 Tipp ] + (R 0 = Ph) are seemingly small, the structural effects imposed by the bulky Tipp substituents on the molecule have a large inuence on the HOMO-LUMO gap in these two species.For example, the ring bending in [1 Tipp ] + (R 0 = Ph) substantially destabilises the HOMO compared to that in [1 tBu ] + (R 0 = Ph) while leaving the energy level of the LUMO virtually identical, resulting in a smaller HOMO-LUMO gap that allows a lower energy absorption in [1 Tipp ] + (R 0 = Ph).In the case of 2 R , it should rst be noted that deprotonation of both [1 Tipp ] + and [1 tBu ] + (R 0 = Ph) results in destabilisation of both the HOMO and LUMO, resulting in a greater delocalisation of the HOMO across the entire P N -P P -P C framework and onto the PhCN moiety.However, Tipp substitution in 2 Tipp (R 0 = Ph) results in destabilisation of the HOMO but stabilisation of the LUMO relative to 2 tBu (R 0 = Ph), again resulting in a smaller HOMO-LUMO gap and allowing for orange colouration to occur compared to the pale yellow 2 tBu (R 0 = Ph).To shed light on the nature of the HOMO-LUMO transition and the resultant unexpected range of colouration, we calculated the D indices of each species.
The D index is used to qualitatively assess the distance of donor-acceptor electron regions within a molecule by calculating the positive and negative barycenters of the orbitals involved in an electronic transitionin this case, the HOMO and LUMOand then subsequently calculating the distance between these two barycenters.For values greater than 1.6 Å it can be determined that charge is owing between the two barycenters, especially in cases molecules are not symmetric and where orbitals involved in the electronic transition have minimal spatial overlap, as is the case in [1 R ] + .In this way, the D index qualitatively supports the existence of charge transfer across a molecule. 83,84A comparison across the series of [1 R ] + (R 0 = Ph), reveals D indices of $2.3 Å, in line with a HOMO-LUMO charge transfer event from the non-bonding linear combination of n(P) orbitals to the N(H)CPh moiety in these species.This intramolecular charge transfer is nicely illustrated by plotting the charge density difference between S 0 and S 1 state (Fig. 7).We also observe large D values for the free bases 2 Tipp (R 0 = Ph) and 2 tBu (R 0 = Ph), while [1 Tipp ] + (R 0 = Me), 2 Tipp (R 0 = Me), and 2 tBu (R 0 = Me) all showed D values of less than 1.4 Å.This is in line with the colouration of benzonitrile-derived free bases, while acetonitrile-derived free bases appear weakly coloured, and computationally bear no signicant HOMO-LUMO chargetransfer phenomena.However, there is no clear relation between the D indices and the bending angles of the P 3 CN-rings (Fig. S-189 †).

Preparation of a polymer-based Brønsted acid-base sensor
Having shown reversible H + -and base-induced switching between the neutral triphosphaazolene and cationic triphosphaazolenium states, we were intrigued by the idea of  85 Post-polymerization functionalization to synthesise a co-polymer was effected by suspending the assynthesised poly(4-cyanostyrene) in CH 2 Cl 2 and stepwise addition of 0.5 eq. of both (PTipp) 3 and HOTf, which immediately afforded a purple suspension.Aer stirring for 12 h and precipitation with n-hexane P4CS-co-([1 Tipp H][OTf] R 0 = Ph) was afforded in 74% yield as purple beads.This insoluble purple polymer was suspended in toluene and an excess of neat NEt 3 was added, resulting in the purple beads developing an orange colouration; continued stirring for 24 h gave an orange solution.The neutral, soluble polymer P4CS-co-(2 Tipp R 0 = Ph) was afforded in good yields as an orange powder aer workup (Scheme 3, top), which according to 31 P NMR spectroscopy contained, in addition to the expected 1 : 1 : 1 resonances of the 1-aza-2,3,4-triphospholene, minimal amounts of an unidenti-ed impurity (d( 31 P) = 59.6 ppm), which could not be removed by further purication and therefore seems likely to be trapped in the polymer matrix (cf.ESI p. S-15 ff.†).
Compared to the starting poly(4-cyanostyrene) the dispersity (Đ = 3.8) of P4CS-co-(2 Tipp R 0 = Ph) is considerably higher, which may be a result of phosphorus lone pairs in the polymer interacting with the GPC column.To immobilise P4CS-co-(2 Tipp R 0 = Ph), oven-dried glass wool was dipped into a toluene solution of the neutral polymer and aer drying for 24 h pale orange glass bers were obtained.Sensing was tested by placing the bundled bers in the headspace of a beaker containing concentrated aqueous (35%) HCl, which resulted in a noticeable colour change to purple (Scheme 3, bottom right), characteristic of P4CS-co-([1 Tipp H][Cl] R 0 = Ph).When these purple bers were exposed to NH 3 vapours by suspending the sample above saturated aqueous ammonium hydroxide, the purple colour vanished and the characteristic orange of P4CS-co-(2 Tipp R 0 = Ph) was again observed (Scheme 3, bottom le).This procedure could be repeated at least ve times, without noticeable build-up of [NH 4 ]Cl on the glass bers.Future studies will focus on obtaining P 3 CN-containing polymers that can be more easily processed to harness the protonresponsiveness in simple sensors more effectively.

Discussion
A rare example of tunable colouration in phosphorus-rich main group species is described via intramolecular charge transfer, as demonstrated prototypically in compound [1 Tipp ] + .While colour in poly-phosphorus species is not uncommon and can even be tuned electronically, colour typically arises from p-p* transitions, and this, to the best of our knowledge, is the rst report on sterically-induced tuning of the HOMO energy, to enable facile modication of the colour in P 3 CN ring systems.The lowenergy charge transfer arises from perturbation of the HOMO-LUMO energy levels, which can be related to the structural bending (q) upon N-protonation of the P 3 CN heterocycle and is further enhanced by increased steric demand of the phosphorus substituents.This is corroborated by a strong correlation between the wavelength of charge-transfer (l CT , previously labelled as l 1 ) as a function of the bending angle as observed in Table 1 and    bending angles are similar in solution and the solid-state.Additionally, modication of the R 0 substituent affects the l CT as shown in Fig. 5, presumably by modulation of the LUMO energy level.In silico investigations into this charge transfer phenomenon reveal that the sterically-induced bending found in [1 R ] + (R = Dipp, Tipp) raises the energy of the HOMO to allow for intramolecular charge transfer to occur in the visible region (ESI Section 6 †).Further, these systems exhibit notable photostability in the solid state (vide supra), and further tuning of the HOMO-LUMO gap can be readily effected by varying the nature of the nitrile used in the synthesis.

Conclusions
This contribution presents a thorough investigation of the structural and photophysical properties of 1-aza-2,3,4triphospholenes and related 1-aza-2,3,4-triphospholenium cations, some of which exhibit strong colouration due to a hitherto unrealized charge transfer phenomenon, resulting in an unprecedented potential for control over colouration in phosphorus-rich molecules.Complexes [1 R ] + show colouration associated with low-energy absorption bands that become increasingly red-shied as steric bulk at the cyclic phosphane substituent is increased.Use of benzonitrile as a substrate allows access to yellow, red, and blue colours as steric bulk at phosphorus is increased in [1 R ] + , and colouration may be further tuned by the variation of the aryl nitrile substrate, for example use of para-substituted phenyl nitriles, as was shown for selected examples.UV-Visible spectroscopy revealed that [1 Tipp ] + and [1 Dipp ] + have broad, red-shied absorption bands characteristic of intramolecular charge transfers.Single crystal X-ray crystallography revealed increasing ring bending of [1 R ] + as steric encumbrance at the phosphorus centres are increased, while 2 R species conversely remain closer to planarity.A linear correlation was found between the angle of ring bending and the wavelength of the low-energy absorption bands across all benzonitrile-derived species (R 0 = Ph), indicating a relationship between ground-state structure and the photophysical properties in this family of compounds.TD-DFT calculations support an intramolecular HOMO-LUMO (n(P) / p*) charge-transfer to which the observed colouration is ascribed.This unusual structure-property relationship provides insight for the rational design of pnictogen-rich charge transfer materials, while the facile and modular synthesis, along with photostability, tunable colour and proton responsiveness of 1-aza-2,3,4triphospholenes, suggest these heterocycles may have considerable utility in numerous applications.As proof-of-concept, covalent incorporation of these moieties into a 4-cyanostyrene polymer backbone through post-polymerization modication allowed for expedient generation of an air-stable polymer capable of reversible Brønsted acid-base colorimetric chemosensing.
[1 Tipp ] + (R 0 = Me) (Fig. 2, top right).Compared to previously synthesized [1 tBu ] + (R 0 = Me) the most downeld signal in species [1 Tipp ] + (R 0 = Me) is shielded by approximately 52 ppm, while both J PP coupling constants are smaller than in [1 tBu ] + (R 0 = Me) by approximately 120 Hz and 70 Hz, respectively (Fig. 2, top le).Removal of the volatiles followed by washing of the tacky residue with n-hexane yielded a red powder. 1 H NMR spectra in CDCl 3 (ESI Fig. S-1 †) revealed inequivalent methine environments for the Tipp substituents, as well as a characteristic doublet of doublets (J HP = 10.2Hz, J HH = 4.6 Hz) corresponding to the methyl resonance of the incorporated acetonitrile-derived moiety.A broad singlet at 12.61 ppm is assigned to the H-N resonance of the protonated skeletal nitrogen atom.In the 19 F{ 1 H} NMR spectrum in CDCl 3 (ESI Fig. S-2 †) a singlet at d F = −78.7 ppm, indicated a noninteracting triate anion (cf.[Me 3 SiN(C 6 H 10 )P(C 6 H 10 )][O 3 SCF 3 ] d F = −78.8ppm).

Fig. 1
Fig. 1 Previously reported coloured (poly-)phosphorus species (a) and coloured phosphorus species reported in this work (b), shown in line with their visible colours.

Fig. 4
Fig.4Side-on view of the central P 3 CN rings in the molecular structures as determined by SC-XRD experiments of [1 R ] + and 2 R demonstrating the angle of bending of each species (P = orange, C = grey, N = blue).View was chosen such that the bent-out-of-plane P P atom sits to the right, and all q values are measured from the horizontal P C -C-N-P N plane (represented as a dotted line parallel to the bottom of the page) downward to the plane formed by the three phosphorus atoms.Thermal ellipsoids are drawn at the 50% probability level.

6 H 4 )
bears a hypsochromically shied l 1 compared to [1 Tipp ] + (R 0 = Ph), while the l 1 associated with [1 Tipp ] + (R 0 = p-CF 3 C 6 H 4 ) is bathochromically shied by 20 nm compared to [1 Tipp ] + (R 0 = Ph).When considering the Hammett parameter of the para-substituent at R 0 a correlation is found between an increasing Hammett parameter s p , and an increase in the wavelength of the low-energy absorption band l 1 (p-MeO-C 6 H 4 < p-H-C 6 H 4 < p-CF 3 -C 6 H 4 ).

Fig. 8 .
Furthermore, solid-state UV-Vis spectra of a representative set of samples show near-identical l CT values to that of the analogous solution UV-Vis spectra (ESI Section 3, cf.[1 Tipp ] + (R 0 = Me) Fig. S-95 and S-96 ff.†), indicating that the

Fig. 8
Fig. 8 Graph depicting wavelength of charge transfer as a function of crystallographic ring-bending angle in [1 R ] + species both experimentally (blue) and with calculated absorption maxima by TD-DFT (orange).

Table 1
Comparison of angle of ring bending and charge-transfer absorption band for crystallized [1 R ] + and 2 R (R 0 = Ph) species