Vapochromic films of π-conjugated polymers based on coordination and desorption at hypervalent tin(IV)-fused azobenzene compounds

Abstract

We report the synthesis and vapochromic behaviors of film materials consisting of hypervalent tin-containing π-conjugated polymers. We prepared copolymers with brominated tin-fused azobenzenes and modified fluorene having tetraethylene glycol as a side chain. The synthesized polymers showed good film-formability and high affinity with coordinating solvent molecules such as dimethyl sulfoxide (DMSO). In particular, we discovered distinct color changes from blue to purple when exposed to DMSO vapor. It was revealed that color changes should originate from reversible alteration of the coordination-number between five and six of hypervalent tin(IV) in the azobenzene compounds involved in the main-chain conjugation. Moreover, we also observed that binding constants between tin and coordinating solvents could be influenced by two substitutions on the tin atom and subsequently modulated responsivity of vapochromism in films by altering the type of substituent. Furthermore, the color-change behaviors can be estimated by quantum calculations with density functional theory. We demonstrate not only that hypervalent tin can work as a switching unit for modulating the electronic structures of π-conjugated polymers triggered by solvent coordination but also that vapochromic behaviors in films can be predicted by estimating the affinity between hypervalent tin and solvent molecules with theoretical calculations.

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Introduction

Molecular detection without expensive instruments is a fundamental technology for constructing facile environmental assessments and daily health checking systems. To meet the demands for manufacturing practical sensors, organic materials are a versatile platform because of flexibility in material design which enables us to realize selective and sensitive detection for the targets by adjusting molecular structures.^1–3 In particular, π-conjugated polymers have attracted attention because of various advantages such as not only superior light-absorption and emission properties⁴ but also good film-formability and printability in addition to designability.^5,6 Owing to such advantages, so far, various chemical sensors based on π-conjugated polymers have been developed.^7–10

Vapochromism is known as the phenomenon in which color changes can be reversibly induced by vapor absorption and subsequently desorption.^11–13 In the absence of a direct connection between a chromophore and a target, vapochromic behaviors can be obtained. In the case of some kinds of crystalline samples, crystal–crystal transition can be induced by vapor fuming with specific solvents, followed by chromic behaviors.^14–23 Alterations of intramolecular structures and/or intermolecular distributions of chromophores including π-conjugated systems or transition metals should be responsible for optical changes in these materials. Optical changes can be recently realized in films consisting of π-conjugated polymers.^24,25 By vapor fuming, annealing processes should proceed. As a result, apparent and emission color changes that originated from morphology alterations of π-conjugated polymers can be observed. In these materials, dissolving into solvents and film-formation should be required for restoring the initial state.

If a connection can be formed between chromophores and the targets, sensitivity and selectivity can be improved. Indeed, in the case of metal complexes with organic ligands, chromic behaviors are also inducible by the direct coordination of solvent molecules to the metal centers of complexes with good sensitivity.^26–32 As a practical application, complex-doped polymer films have been also prepared.^33–38 Selectivity can be tuned by choosing the type of metal center that can coordinate with the target. However, vapochromism from film materials with main-chain-type π-conjugated polymers consisting of metal complexes could be hardly accomplished. Although organoboron complex-containing polymers can exhibit optical changes by reacting with fluoride anions, reversible responses were principally impossible due to the extremely high stability of the B–F bond.^39–43 Furthermore, anion sensors can work only in solution.⁴⁴ Besides anions, the partially reversible sensors for neutral donor molecules such as pyridine in solution were also prepared.⁴⁵ Although the vapochromic film was developed with the interaction between poly(9-borafluorene) and NH₃ vapor,⁴⁶ the number of examples is limited. Therefore, the development of vapochromic film materials composed of main-chain-type π-conjugated polymers consisting of metal complexes is a next goal not only for demonstrating a new modulation method of electronic properties of highly expanded π-conjugated systems but also for obtaining property-adjustable film sensors in which various properties can be tuned according to the preprogrammed design.

Recently, we proposed the design concept of heteroatom-containing polymers by employing “element-blocks”, which are a minimum functional unit containing heteroatoms, to obtain novel functions originating from loaded elements.^47–49 Based on this concept, we have developed main-chain-type π-conjugated molecules and polymers with boron-fused azo (N=N) or azomethine (C=N) groups.^25,50–57 They showed various functions such as film-state visible to NIR emission,^{25,51–54,57} stimuli-responsiveness,²⁵ aggregation-induced emission (AIE),^53,54,57,58 and crystallized-induced emission enhancement (CIEE).^{51,52,59–63} Hypervalent compounds are a class of molecules in which over eight electrons are formally assigned around a valence shell of a main group element beyond the limit of the Lewis octet rule.^64–66 More recently, we reported that a series of hypervalent tin-fused azobenzene (TAz) compounds with five-coordinated distorted trigonal bipyramidal geometries.^67,68 They exhibited absorption and emission bands from the orange to the NIR region despite the fact that the small π-conjugated systems are involved in these molecules.⁶⁹ Some hypervalent compounds can change the coordination numbers of their atom centers upon the interaction between the Lewis acidic atom centers and Lewis basic substrates.^70–73 In our case, hypsochromic shifts of the spectra can be induced by solvent coordination to the tin center with the coordination-number change from five to six.⁶⁷ In the crystalline state, vapochromic behaviors were observed by encapsulation of dimethyl sulfoxide (DMSO) vapor with crystal–crystal transition. However, it is still difficult to apply the vapochromism of these compounds to practical sensing materials due to fragility of crystal powders. Moreover, crystal–crystal transitions followed by collapse of regular structures limited the reversibility of chromic behaviors.

Herein, we show the synthesis and unique properties of π-conjugated copolymers with TAz and fluorene moieties which can be a film-type vapochromic sensor. Two types of the TAz derivatives having methyl or phenyl groups at tin were prepared. In the fluorene units, tetraethylene glycol (TEG) side chains were introduced for improving the affinity of the hydrophobic π-conjugated polymers to highly polar coordinating solvents. Accordingly, the distinct color change from blue to purple was observed in films when exposed to DMSO vapor. We demonstrate here that molecular coordination at the hypervalent element in polymers can be a trigger for drastically changing the electronic structure of main-chain π-conjugation. Moreover, we were able to explain chromic behaviors with theoretical calculations. In particular, we proved that binding constants of solvent molecules can be estimated by the length of dative bonds between tin and coordinated atoms. We also showed that vapochromic behaviors are correlated with the affinity which can be evaluated with calculations.

Results and discussion

Synthesis and characterization

Scheme 1 shows the synthesis of the TAz compounds. The tridentate ligand of azobenzene (1) was prepared according to the literature.⁵⁷ Dehydration condensation was performed with 1 and diphenyltin(IV) oxide in acetone under reflux conditions to afford TAzPh in 59% isolated yield. TAzMe was obtained through the reaction with 1 and dimethyltin(IV) dichloride in the presence of sodium hydroxide in 74% isolated yield. The bromine groups in these TAz compounds were intended to be used for polymerization.

Scheme 1 Synthesis of TAz monomers.

Next, we synthesized π-conjugated copolymers including the TAz moieties (Scheme 2). The modified fluorene compound having two TEG groups as side chains was designed as a comonomer to improve solubility in polar solvents, such as dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), methanol (MeOH) and acetonitrile (MeCN), which have the potential to coordinate to the tin center. Trimethylstannyl groups were introduced through lithiation with 2 and subsequently addition of trimethyltin chloride, and the comonomer 3 was obtained in 95% isolated yield. Polymerization for preparing P-TAzPh and P-TAzMe was conducted via the Migita–Kosugi–Stille cross coupling reaction^74,75 with TAzPh or TAzMe and 3 in the presence of Pd₂(dba)₃ (dba = dibenzylideneacetone) and XPhos (2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl), respectively. The products were purified and fractionated by high performance liquid chromatography (HPLC). The molecular weights were determined using gel permeation chromatography (GPC) with polystyrene standards. Consequently, we isolated P-TAzPh (17%, M_n = 5,200 (n = 6.9), PDI = 1.3, PDI: poly dispersity index) and P-TAzMe (25%, M_n = 6,200 (n = 9.7), PDI = 1.3). All products were characterized by ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopy, high-resolution mass spectrometry (HRMS) and elemental analyses (see the ESI†). These characterization data enabled us to investigate the properties of the TAz derivatives.

Scheme 2 Synthesis of TAz polymers.

Optical properties in diluted solution

First, we investigated the fundamental optical properties of the TAz monomers and polymers by UV–vis absorption and photoluminescence (PL) measurements in toluene (1.0 × 10⁻⁵ M) (Fig. 1 and Table 1). TAzPh and TAzMe showed absorption bands at around 550 nm and emission with the peaks around 670 nm. These long absorption and emission bands were unique characteristics of the TAz derivatives originating from distorted trigonal bipyramidal geometry consisting of a three center–four electron (3c-4e) bond at apical positions and a sp² hybrid orbital at equatorial positions.⁶⁷ Both spectra showed bathochromic shifts after copolymerization with the fluorene units. The peak tops of absorption and emission bands reached 600 and 680 nm, respectively. In addition, the band shapes of the emission spectra were sharpened, and the absolute PL quantum yields (Φ_PLs) were improved. The enhancement of rigidity by the expansion of π-conjugation through the polymer main-chain could be responsible for emission improvement. The TAzMe derivatives had more efficient emission than TAzPh ones in the deep red region, implying that small alkyl chains on the tin center might contribute to the construction of planar π-conjugation system through polymer main-chains especially in the excited state.

Fig. 1 (A) UV–vis absorption and (B) PL spectra of TAzMe, TAzPh, P-TAzMe and P-TAzPh (1.0 × 10⁻⁵ M for monomers and 1.0 × 10⁻⁵ M per repeating unit for polymers) in toluene (blue line) and toluene/DMSO = 1/99 v/v (orange line). (C) Photographs under room light (upper) and irradiated by 365 nm (below).

Table 1 Spectroscopic data of TAz derivatives in diluted solution (1.0 × 10⁻⁵ M)

	Solvent	λ abs/nm	λ PL /nm	Φ PL /%
a In mixed solvent, toluene/DMSO = 1/99 v/v. b Excited at λabs. c Absolute PL quantum yield.
TAzMe	Toluene	552	671	18.0
	DMSO ^a	534	618	1.3
TAzPh	Toluene	547	665	15.6
	DMSO ^a	533	605	3.7
P-TAzMe	Toluene	614	682	24.4
	DMSO ^a	593	680	1.0
P-TAzPh	Toluene	606	677	19.2
	DMSO ^a	583	640	2.2

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We previously reported that solvent coordination to the tin center induced the coordination-number change from five to six followed by hypsochromic shifts of the absorption and emission bands.⁶⁷ To examine the optical properties of the TAz derivatives in the polymer main-chain, we compared the solvent effects on the optical properties of monomers and polymers and estimated the coordination-number change (Fig. 1 and Fig. S1, ESI,†Table 1). In the case of non-coordinating solvents such as toluene and chloroform (CHCl₃), spectrum shifts were hardly observed, while clear hypsochromic shifts were observed in coordinating solvents such as DMSO, DMF, MeOH and MeCN. Since similar changes in optical properties were induced both in monomers and polymers, we concluded that the solvent coordination was able to proceed at the hypervalent tin compounds in the polymer main-chains. Moreover, the degree of the peak shift, which represents strength of binding, was significantly dependent on the type of solvents. Fig. 2 shows relationships with wavelengths of maximum absorption by increasing the ratio of the coordinating solvents from 0 to 20 vol% in toluene. Full titration data from 0 to 99 vol% are shown in Fig. S2–S7 and Tables S1–S4 (ESI†). Accordingly, the solvent coordination to the tin center was stronger in the following order: DMSO > DMF > MeOH > MeCN. Additionally, it was shown that the TAzPh derivatives showed larger shifts than TAzMe ones judging from width of the hypsochromic shifts. To get more insight into the coordination, we estimated a binding constant (K) with a curve fitting method based on 1 to 1 coordination according to our previous work (Table 2 and Fig. S8, ESI†).⁶⁷ It was shown that the K values were larger in the following order; DMSO > DMF ≫ (MeOH and MeCN, too weak to obtain reliable values). Furthermore, the K value of TAzPh was larger than that of TAzMe. This should be because that a methyl group has stronger electron-donating ability than a phenyl group and it reduces the Lewis acidity of tin. In addition, lower Φ_PL values were observed from the stronger-coordinated compounds (Fig. S6 and S7, ESI†). Structural distortion followed by enhancement of nonradiative decay should be caused by the solvent coordination.⁶⁷ It should be noted that the polymers showed similar solvent effects. In other words, solvent coordination to the TAz moieties should occur in the polymers. In P-TAzMe, a complete hypsochromic shift in the PL spectrum was not observed in toluene/DMSO = 1/99 v/v (Fig. 1B). This indicates that the solvent coordination can be controlled by the substituents on the tin center.

Fig. 2 Titration data of TAzMe, TAzPh, P-TAzMe and P-TAzPh. Hypsochromic shifts of wavelengths of maximum absorption depending on solvent vol% in toluene (1.0 × 10⁻⁵ M for monomers and 1.0 × 10⁻⁵ M per repeating unit for polymers). Full spectrum data are shown in the ESI.†

Table 2 Binding constants of TAz monomers in various solvents^a

	K DMSO/M^−1	K DMF/M^−1
a Determined in toluene at room temperature (25 °C).
TAzMe	2.7	0.59
TAzPh	11	1.3

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Optical properties in polymer thin films

Next, we explored vapochromism in polymer films. We prepared thin films of P-TAzPh and P-TAzMe on quartz substrates (0.9 cm × 5.0 cm) using a spin-coating method (1000 rpm, 30 s, 100 μL of CHCl₃ solution (2 mg/300 μL)) and subsequently dried in vacuo for 12 h. We evaluated color changes and especially focused on their reversibility with the films by monitoring UV–vis absorption measurements with the exposure to DMSO, DMF, MeOH and CH₃CN vapors for 30 min and following dry with a hair dryer for 30 min at room temperature (Fig. 3 and Fig. S9, S10, ESI†). Accordingly, the most drastic color change and the recovery were found in the combination of P-TAzPh and DMSO vapor (Fig. 3B and D), and almost all other pairs were inactive (Fig. 3A and Fig. S10, ESI†). It should be noted that P-TAzPh showed repeatability of at least two cycles (Fig. 3C). If color changes are induced by morphology alteration and/or degradation of hypervalent tin compounds, it should be impossible to observe reversible behaviors. Thus, it can be said that coordination and desorption of solvent vapor at the hypervalent tin should be responsible for the vapochromic properties in the polymer film. Since the vapochromic behaviors should be dependent on the magnitude of the binding constant, it is likely that the pair of TAzPh and DMSO showed the largest degree of optical changes.⁶⁷ Crystal–crystal transition was essential for presenting color changes with small molecules according to the previous work.⁶⁷ It should be emphasized that we succeeded in realizing vapochromism with polymer films which are easily applied to stimuli-responsive materials manufactured by a printing method. It should be noted that reversible color change was also detectable using DMF vapor, which was not observed from the crystalline sample in which it was dissolved by DMF vapor.⁶⁷ Interestingly, the P-TAzMe was inactive by exposure to DMSO vapor despite the fact that K_DMSO of TAzMe is larger than K_DMF of TAzPh. It is implied that bulky substituents around tin might make cavities which are capable of capturing solvent molecules. As a result, P-TAzPh can show higher sensitivity toward vapor fuming. In contrast to absorption spectra, the changes in PL spectra were hardly observed upon exposure to various solvent vapors regardless of non-coordinating or coordinating ones. (Fig. S11, ESI†). It might be because of intra- and intermolecular energy migration from DMSO coordinated sites to luminescent sites in film. By combining these results of absorption and emission behaviors in film, it is supported that the color change in absorption should be caused not by the solvent-vapor annealing process but by the solvent coordination to the tin center. Basically, strong concentration quenching occurred (Φ_PL = 3.4% for P-TAzMe and Φ_PL = 0.3% for P-TAzPh), and the value was almost similar before and after DMSO addition (Φ_PL = 3.2% for P-TAzMe and Φ_PL = 0.3% for P-TAzPh with DMSO vapor). It was hard to observe the luminescence behavior by the naked eye because the emission band reached the NIR area.

Fig. 3 UV–vis absorption spectra of (A) P-TAzMe and (B) P-TAzPh in the film before and after solvent annealing for 30 min, and recovery test by drying the film for 30 min at room temperature. (C) Reversible controls of the UV–vis absorption properties of P-TAzPh with exposure to DMSO vapor (30 min) and dry (30 min) processes at room temperature. (D) Illustration of plausible mechanism of vapochromism in the film with photographs under room light.

Theoretical calculations

We carried out quantum calculations with density functional theory (DFT) and time-dependent (TD)-DFT to simulate optical behaviors of the TAz derivatives with DMSO coordination. M-TAzMe and M-TAzPh which have fluorene units at both sides of TAzMe and TAzPh, respectively, were used as model compounds of the polymers for saving calculation costs. Accordingly, TAzMe exhibited a slightly longer absorption band (509 nm) than TAzPh (501 nm), and that behavior was qualitatively in good agreement with experimental data (Fig. S12B, ESI†). It was observed that the methyl groups can have an electron-donating ability because the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels of TAzMe were elevated compared to those of TAzPh (Fig. S12A, ESI†). Their DMSO adducts, TAzMe-DMSO and TAzPh-DMSO, showed larger energy gaps (475 and 469 nm, respectively) due to the larger degree of elevation of the energy levels of the LUMO than the HOMO (Fig. S12, ESI†) with keeping molecular orbital (MO) shapes (Fig. S13, ESI†). Our previous research suggested that the hypsochromic shift and different effects on MO energy levels can be caused by coordination-number change of the hypervalent compound from five to six.⁶⁷ Concretely, elevation of LUMO and HOMO energy levels should be induced by the coordination of the oxygen lone pair in DMSO, which weakened the Sn–N coordination and enhanced electron-donating ability of oxygens at the apical positions of the ligand in the 3c-4e bond.

The hypsochromic shift induced by DMSO coordination was also effective in M-TAzMe and M-TAzPh (Fig. 4 and Fig. S12, ESI†). Similarly to the monomers, the shapes of MOs were preserved before and after DMSO coordination. These results are of significance for presenting distinct chromic behaviors because slight shape changes of MOs can secure large oscillator strength (f) associated with the value of a molar extinction coefficient (Fig. S14, ESI†). We previously reported that the binding constants of TAzPh derivatives can be estimated by the equation of d = −0.0049 × (In K_DMSO) + 2.3564 (d: Sn–O (DMSO) bond length (Å) from an optimized structure).⁶⁷ According to this equation, we can estimate the binding constants as K_DMSO = 11.6 for TAzPh and 2.8 for M-TAzPh (Fig. S15, ESI†). The former value was well matched to the experimental data in this study (K_DMSO = 11 for TAzPh). Although M-TAzPh was not synthesized and the K value of P-TAzPh was difficult to experimentally estimate due to weak affinity, we can assume that the affinity to DMSO of M-TAzPh should be weaker than that of TAzPh. These calculation results support that DMSO coordination can induce hypsochromic color changes in the polymer thin film.

Fig. 4 Calculation results of M-TAzPh and M-TAzPh-DMSO as a model compound of P-TAzPh and its DMSO adduct, respectively, with DFT. (A) Chemical structures. (B) Calculated HOMO and LUMO energy levels, and (C) calculated S₀ → S₁ transition bands and oscillator strengths (f). (D) Side view of optimized structures, and selected Kohn–Sham orbitals (isovalue = 0.02). Calculation details are shown in the ESI.†

Conclusions

π-Conjugated copolymers composed of TAz and fluorene moieties with TEG side chains were synthesized. The substituents at the tin center play a critical role in solvent coordination. Indeed, phenyl groups had larger binding constants than methyl groups. As a result, we achieved observing clear vapochromism in thin films of the copolymer including TAzPh by fuming DMSO vapor, which was the pair having the largest binding constant. The hypsochromic shifts induced by solvent coordination both in solution and film were attributed to the coordination-number change of tin from five to six. This fact clearly indicates that the hypervalent tin atom can work as a switch module for modulating the electronic properties of π-conjugated polymers by employing the azobenzene ligand. Furthermore, the spectrum shift was supported by theoretical calculation. In particular, the binding constant was well predicted by the Sn–O (DMSO) bond length. This means that the binding constant and subsequently sensitivity in vapochromic behaviors in films could be estimated with the theoretical calculations on the basis of our estimation protocols described here. The studies are expected to be used for developing stimuli-responsive materials with inherent element properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the Fujimori Science and Technology Foundation (for M. G.), and a Grant-in-Aid for Early-Career Scientists (for M. G.) (JP20K15334), for Scientific Research (B) (for M. G.) (JP22H02130), for Scientific Research (B) (for K. T.) (21H02001), for Exploratory Research (for K. T.) (JP21K19002), and for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (JP24102013).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qm01295b

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