Luminescent Diazaborolyl-Functionalized Polystyrene - ACS Macro

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Luminescent Diazaborolyl-Functionalized Polystyrene Henry Kuhtz,† Fei Cheng,‡ Stefanie Schwedler,† Lena Böhling,† Andreas Brockhinke,† Lothar Weber,*,† Kshitij Parab,‡ and Frieder Jak̈ le*,‡ †

Universität Bielefeld, Universitätsstraße 25, D-33615, Germany Department of Chemistry, Rutgers UniversityNewark, 73 Warren Street, Newark, New Jersey 07102, United States

‡

S Supporting Information *

ABSTRACT: We present two different procedures for the synthesis of poly[4-(1′,3′-diethyl-1′,3′,2′-benzodiazaborolyl)styrene] (3a) and poly[4-(1′,3′-diphenyl-1′,3′,2′-benzodiazaborolyl)styrene] (3b). The new polymers were fully characterized by GPC, multinuclear NMR, and elemental analysis. The thermal properties and stability were studied by DSC and TGA, and the optical characteristics were examined by absorption and time-resolved fluorescence spectroscopy. Remarkably high quantum yields of up to 77% were measured. In comparison to molecular species we found significantly shorter lifetimes, likely as a result of incorporation of the chromophores into the polymer structure. rganic π-systems containing three-coordinate boron atoms continue to be the focus of diverse research efforts.1,2 The intense current interest is mostly related to the efficient overlap of the empty p-orbital on boron with attached organic π-systems, which gives rise to unusual and desirable electronic and photophysical properties.3 Organoboron compounds of this type have been studied for several fields of application, for example, in linear and nonlinear optics, as emitting and electron conduction layers in organic light emitting devices (OLEDs) and as luminescent probes for anions.2 Polymers are particularly interesting because of the possible use of solution processing techniques for device fabrication, the observation of unusual electronic effects as a result of extended conjugation via the empty p-orbitals on the borane moieties, and the potential for signal amplification effects in the case of sensory materials.4 Synthetic methods to prepare new polymeric materials have been pursued extensively, and a broad range of different techniques is available nowadays to integrate borane moieties into the main chain or side chain of conjugated polymers. The resultant polymers frequently show novel and, in some cases, tunable photophysical properties.5 An attractive alternative is to pursue boron-modified polyolefins, in which borane chromophores are linked to the side chains.6−10 A distinct advantage is that chain growth polymerization methods more readily lead to high molecular weight polymers, and even the synthesis of copolymers containing different functionalities is easily possible.11 A versatile polymer side-chain modification procedure recently introduced by the Jäkle group involves polymerization of 4trimethylsilyl styrene by controlled free radical polymerization, followed by exchange of the silyl for BBr2 groups with BBr3.12 Subsequent replacement of the Br substituents with conjugated organic or organometallic chromophores leads to a wide range of polymeric materials that contain highly electron-deficient

organoborane pendant groups (Chart 1A: R1 = fluorenyl, bithienyl, carbazolyl, ferrocenyl; R2 = 2,4,6-trimethylphenyl,

O

© 2012 American Chemical Society

Chart 1

2,4,6-triisopropylphenyl).7−9 The resultant polymers exhibit a broad range of different structural features as well as optical and electronic properties. In this work, we discuss the attachment of an unusual electron-donating boron-containing unit to polystyrene, namely the 1,3,2-benzodiazaborolyl group (Chart 1B: R3 = H, Et, Ph, R4 = aryl, heteroaryl, arylethynyl).13−17 Benzannulated systems that feature this structural motif generally show strong fluorescence and, interestingly, the boron-containing heterocycle can act as a donor rather than an acceptor, in particular, when combined with other acceptor-type functionalities such as Received: February 10, 2012 Accepted: April 4, 2012 Published: April 12, 2012 555

dx.doi.org/10.1021/mz300071f | ACS Macro Lett. 2012, 1, 555−559

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dimesitylboryl or cyano groups.13−17 Indeed, computational studies on 2-arylethynyl-1,3,2-diazaboroles (Chart 1B: R4 = arylethynyl) indicate a strong localization of the HOMO on the diazaborolyl groups.17 1,3-Diethyl- and 1,3-dihydro-1,3,2benzodiazaborolyl groups (Chart 1B: R3 = H, Et) are most commonly employed, for synthetic reasons and because the Baryl derivatives are quite stable in air. Their structural and photophysical properties are increasingly developed, but so far only in molecular compounds (e.g., Chart 1B,C) and a rare example of a conjugated polymer (Chart 1D: R5 = Ph, p-tBu-Ph, p-MeO-Ph).18 Earlier work on triarylborane-functionalized polymers showed that not only the pendent chromophores, but also the π-systems of the phenyl groups in the polystyrene chain effectively overlaps with the empty p orbital on boron.9 This effect resulted in materials that emit blue light with high quantum yields when irradiated with UV light. Hence, we hypothesized that direct attachment of benzodiazaborolyl moieties to polystyrene might lead to new materials with interesting luminescent properties. As a result of the thermodynamic stabilization due to integration of the boron atom into an aromatic heterocycle, the benzodiazaborolyl group, we also anticipated that these types of polymers should be reasonably stable in air, which is important for practical applications. The desired polymers, poly[4-(1′,3′-diorganyl-1′,3′,2′benzodiazaborolyl)styrene] (3), were synthesized according to two different procedures (Scheme 1). Selective boron−

formation of triethylammonium bromide as a byproduct, which proved to be rather difficult to separate. Thus, the isolated yield of polymer 3a, after extensive purification, was only about 35%. In Procedure 2, the protonated N,N′-diethyl-1,2-diaminobenzene precipitated from the reaction mixture in dichloromethane and could be removed readily by filtration.19 The phenyl derivative 3b was prepared similarly using Procedure 2 in 83% yield. The products were obtained as off-white powders that can be stored under argon and even in air for several months without any sign of decomposition according to 1H NMR analysis. This is consistent with thermogravimetric results, which for both polymers showed no signs of degradation up to over 400 °C (see Figure S1). Polymers 3 are well soluble in common organic solvents such as THF, toluene, benzene, CHCl3, and CH2Cl2 but insoluble in aliphatic hydrocarbon solvents such as n-pentane and n-hexane. While the materials are perfectly air-stable in the solid state, in solution gradual decomposition occurs in the presence of air, as evident by a gradual darkening. The polymers 3 were fully characterized by 1H, 13C, and 11B NMR spectroscopy.20 The 1H NMR spectra showed broad overlapping signals, which were assigned based on the characteristic integrals and chemical shifts (Figure 1). Due to

Scheme 1. Synthesis of Diazaborolyl-Functionalized Polymers 3

Figure 1. 1H and 11B NMR spectra of polymer 3b in CDCl3.

the broad nature of the signals, the expected coupling between different 1H-nuclei could only be shown through a H,H−COSY NMR experiment (3a). The signals of the aromatic protons of the benzodiazaborolyl unit were assigned based on an HMQCNMR experiment. The 13C NMR spectra correlate well with those of comparable molecular diazaboroles and borylated polymers, respectively.10,21 As expected, the signals of the styrene units showed much more pronouced line-broadening than those of the pendent benzodiazaborolyl moieties, further confirming the structure of the polymer. The 11B NMR spectra displayed a single broad peak at 25−26 ppm (Figure 1), which agrees reasonably well with NMR data of other known organodiazaboroles.22 Nevertheless, the position of the signal is shifted by roughly 3 ppm to higher field compared to several monomeric phenyl-substituted benzodiazaboroles.16 This kind of upfield shift compared to monomeric reference compounds was also observed for other boron-containing styrene polymers and was attributed to shielding effects due to neighboring groups on the polymer chain.7,9 The molecular weights of the polymers were determined by gel permeation chromatography analysis in THF relative to narrow PS standards to be Mn = 58000, Mw = 81900 for 3a,20 and Mn = 66500, Mw = 83000 for 3b, which are slightly higher

silicon exchange in poly(4-trimethylsilylstyrene) (1) with 1.1 equiv of boron tribromide in dichloromethane over 24 h led to essentially quantitative formation of poly(4-dibromoborylstyrene) (2). The product was reacted in situ with a mixture of N,N′-diethyl-1,2-diaminobenzene and 2 equiv of triethylamine (Procedure 1) or, alternatively, with 2 equiv of N,N′-diethyl-1,2diaminobenzene, one of which is incorporated in the product while the other serves as a base toward HBr that is liberated (Procedure 2). Procedure 1 leads to the target compound with good functional group conversion, but it also results in the 556


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suggests that efficient energy transfer occurs from the Ph groups in 3b to the emitting benzodiazaborole moiety.23 Worth noting is that, for polymer D (R5 = phenyl), the absorption and emission maxima are detected at significantly longer wavelength (λAmax = 360 nm, λEmax = 407 nm), most likely due to the larger π-system of the polymer main chain. When cast into a thin film from CH2Cl2 solution, 3b shows absorption and emission maxima of λAmax = 297 nm and λEmax = 364 nm, respectively; a quantum yield of ϕF ≈ 0.10 was measured using an integrating sphere. The lower quantum yield for the thin film is likely due to bimolecular quenching as a result of the planar structure of the chromophores although reabsorption effects may also play a role due to slight overlap of the excitation and emission bands (Figure S5). In the course of our investigations we noticed a decrease of fluorescence intensity upon irradiation of the sample. To explore the possibility that this fading of the luminescence occurs because of photoinduced reactions with atmospheric oxygen or moisture, two identical samples of 3a were prepared under the same conditions. One sample was then stirred for 2 h under UV radiation, while the other one was stirred in the dark for the same period of time. A comparison of the fluorescence intensities measured after this pretreatment is shown in Figure 3. The results show clear evidence of bleaching after irradiation,

than the molecular weight of the silylated polystyrene precursor (Mn = 56300; Mw = 65200). While a larger molecular weight increase may have been expected upon modification with the borane side groups, GPC measures the relative hydrodynamic volume of the polymer chain, which does not change significantly because the number of polymer repeat units remains unchanged. Indeed, a triple-detection GPC measurement on polymer 3b confirmed that the absolute molecular weight is significantly higher. The GPC traces showed polydispersity indices (3a: PDI = Mw/Mn = 1.41; 3b: PDI = 1.25) that are generally similar to that of the silylated precursor polymer (PDI = 1.16). The slightly higher PDI especially for 3a appears to be due to tailing as a result of (modest) interactions with the column material. Overall, the close similarity of the GPC profiles (Figure S2) is consistent with highly selective postpolymerization modification, without significant polymer cross-linking or degradation. We further investigated the optical properties of polymers 3a and 3b in CH2Cl2 solution (Figure 2). The absorption and

Figure 3. Photobleaching studies on polymer 3a: Fading curves with and without prior irradiation.

indicating a photoinduced degradation process. The fading curves can all be fitted to second order exponentials, which suggests a mechanism that involves at least one more parameter in addition to the irradiation, for example, oxygen or moisture are possible reaction partners considering that the measurements could not be preformed under Schlenk conditions, although degassed solvents were. To reduce the influence of photobleaching on our results we tried to minimize the irradiation time of the samples prior to the further photophysical measurements. Quantum yields of ϕF = 0.77 (3a) and 0.51 (3b) were determined, which are remarkable for these types of polymeric compounds (c.f. for D: ϕF = 0.32−0.48). The fluorescence decay curves with fits, instrument response function (IRF), and residuals are shown in Figure 4. All decay curves show exponential dependencies to the third order. This is quite high in comparison to molecular 1,3-ethyl-substituted benzodiazaborole compounds, for which only first or second order exponential fits are necessary.14−17,24 The measured lifetimes for polymers 3a (780 ps) and 3b (820 ps) in CH2Cl2 are short in comparison to monomeric 1,3-ethyl-

Figure 2. (a) Absorption and emission spectra of 3a and 3b in CH2Cl2 and photographs of CDCl3 solutions of the polymers under UV irradiation. (b) Absorption and emission spectra of 3b as a drop-cast thin film and photograph under UV irradiation with a hand-held UV lamp at 254 nm.

emission maxima of 3a (λAmax = 295 nm, λEmax = 360 nm, ε = 8290 [M−1 cm−1] per polymer repeat unit, ϕF = 0.77) and 3b (λAmax = 297 nm, λEmax = 360 nm, ε = 7840 [M−1 cm−1] per polymer repeat unit, ϕF = 0.51) are similar and comparable to the respective data of compound C (R2 = Et; λAmax = 296 nm, λEmax = 364 nm).15 The only apparent difference is that polymer 3b absorbs significantly more strongly in the range of 225−275 nm than 3a, which is attributed to the presence of the Ph groups on nitrogen. The fact that both polymers emit at the same wavelength, independent of the excitation wavelength, 557


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PLEDs in conjunction with well-established transition metalbased triplet emitters.27

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the National Science Foundation (CHE-0809642, CHE-1112195, and MRI 0116066) for support of this research. F.J. thanks the Alexander von Humboldt Foundation for a Friedrich Wilhelm Bessel Research Award. We are grateful to Dr. Yang Qin for the synthesis of polymer 1 and to Dr. Patrick Shipman for acquisition of triple-detection GPC data on polymer 3b. H.K. thanks the entire Jäkle group for their warm and supportive welcome during his research visit at Rutgers University.

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Figure 4. Decay curves with fitting, instrument response function (IRF), and residuals for polymers 3a and 3b in CH2Cl2.

REFERENCES

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substituted benzodiazaborole compounds,14−17,24 all of which show lifetimes of >2 ns. This relatively short lifetime may be due to interactions of the chromophore units within the polymer chain or with chromophores in other chains.7,9,25,26 It is also important to remember that the upfield shifted 11B NMR signal noted above is reflective of a different chemical environment of the indivdual chromophores, as a result of incorporation into the polymer chain. In conclusion, we have demonstrated the successful synthesis of benzodiazaborole-functionalized luminescent polymers using a series of highly efficient polymer modification procedures. The reaction of N,N′-diorganyl-1,2-diaminobenzene derivatives with dibromoborylated polystyrene leads to the generation of the new high molecular weight polymers 3. To the best of our knowledge, 3a and 3b represent the first polymers containing benzodiazaborolyl chromophores as side chains attached to a polyolefin. Functionalized with relatively small individual chromophore units, in CH2Cl2 solution, the polymers show remarkably high quantum yields of 0.77 and 0.51, respectively. The lower quantum yield in the thin film state and the relatively poor photostability could be a potential disadvantage with respect to applications as nonlinear optical (NLO) or polymer light emitting device (PLED) materials. Nonetheless, the high modularity of our synthetic approach should allow us to readily address these issues. Thus, future work will focus on introducing larger substituents at the nitrogen atoms of the benzodiazaborole heterocycle, to investigate if steric hindrance at the boron chromophores leads to higher solid state quantum yields and improved photostability. We will also explore the tunability of the optical properties by judicious introduction of electron donating and electron withdrawing groups to the diazaborolyl moiety. Moreover, given the relatively high energy emission, the new polymers could serve as efficient hosts for 558


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(23) For related observations, see: Nagata, Y.; Otaka, H.; Chujo, Y. Macromolecules 2008, 41, 737−740. Nagai, A.; Miyake, J.; Kokado, K.; Nagata, Y.; Chujo, Y. J. Am. Chem. Soc. 2008, 130, 15276−15278. Thivierge, C.; Loudet, A.; Burgess, K. Macromolecules 2011, 44, 4012− 4015. (24) Chrostowska, A.; Maciejczyk, M.; Dargelos, A.; Baylere, P.; Weber, L.; Werner, V.; Eickhoff, D.; Stammler, H.-G.; Neumann, B. Organometallics 2010, 29, 5192−5198. (25) The lifetimes could be affected by the discussed photobleaching effects, considering the much higher power output of the laser compared to the Xenon lamp used in the photobleaching experiments. The signal intensity was consequently relatively weak. (26) The lifetime for the thin film of 3b was measured in a timecorrelated single-photon counting (TCSPC) experiment, because our time-resolved-LIF setup was not available. Although the relatively short lifetime of the polymer (see Figure S6) proved to be at the limit of the measurable range, we can estimate the lifetime to be between 0.93 and 1.40 ns, which is similar to the data obtained for polymer 3b in CH2Cl2 solution (0.82 ns). (27) Zhang, X.; Jiang, C.; Mo, Y.; Xu, Y.; Shi, H.; Cao, Y. Appl. Phys. Lett. 2006, 88, 051116.

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