Article pubs.acs.org/JPCA
Protonation-Induced Room-Temperature Phosphorescence in Fluorescent Polyurethane Wei Sun,† Zhaowu Wang,‡,§ Tao Wang,† Li Yang,‡,§ Jun Jiang,*,‡,§ Xingyuan Zhang,*,† Yi Luo,§ and Guoqing Zhang*,‡ †
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026 Anhui, P. R. China ‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui, 230026, P. R. China § Innovation Center of Chemistry for Energy Materials, Department of Chemical Physics, University of Science and Technology of China, Hefei, 230026 Anhui, P. R. China S Supporting Information *
ABSTRACT: Room-temperature phosphorescence (RTP) from purely organic systems is of practical importance in biological imaging, oxygen sensing and displaying technologies. The key step to obtaining RTP from organic molecules is efficient intersystem crossing (ISC), which is usually low compared to inorganic materials. Here we show that protonation of a dye molecule, a thioflavin derivative, in strongly polar polyurethane can be used to effectively harness RTP. Prior to protonation, the predominant transition is π−π* for the polymer, which has nearly undetectable RTP due to the large singlet−triplet energy splitting (0.87 eV); when Brønsted acids are gradually added to the system, increasingly strong RTP is observed due to the presence of a new intramolecular charge-transfer state (ICT). The ICT state serves to lower the singlet−triplet energy gap (0.46 eV). The smaller gap results in more efficient ISC and thus strong RTP under deoxygenated conditions. The thioflavin−polyurethane system can be tuned via proton concentration and counterions and opens new doors for RTP-based polymeric sensors and stimuli-responsive materials.
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aldehyde group of the fluorophore and halogen atoms from the crystal matrix to promote ISC. Another strategy to achieve RTP from purely organic molecules is by orthogonalizing the origin and destination orbitals involved in the lowest transition state and producing nearly degenerate singlet and triplet excited states. Numerous examples of organic RTP materials have been reported from Adachi and others22−24 based on this molecular design. In previous studies, we also explored various strategies, including carbonyl chelation, polymerization-enhanced intersystem crossing, and ordered tethered charge-transfer complexes, to generate RTP.25−27 Nevertheless, small-molecule-based organic materials suffer from potential problems such as low thermal stability, propensity to crystallization, intolerance toward organic solvents etc., and therefore polymeric alternatives are also frequently sought after. Lately, polymers that are incorporated with organic RTP dyes21,26 are studied because they are more easily processed into a wide variety of morphologies such as
INTRODUCTION Optical materials that exhibit room-temperature phosphorescence (RTP)1,2 are widely used as light-emitting diodes,3,4 oxygen sensors5,6 and background-free bioimaging agents.7,8 Recently, RTP from purely organic molecules is gaining tremendous interests as a result of their easy preparation, biocompatibility, low cost and low toxicity. However, the most prevalent emitting states in organic molecules are usually from π−π* transitions, which typically possess relatively large singlet−triplet energy splitting (ΔEST). Consequently, the rate of intersystem crossing (ISC) is limited because of this large energy separation between 1(π−π*) and 3(π−π*) and RTP is in general absent or very weak in these systems. To enhance the rate of ISC, a commonly used strategy is to introduce a substituent group that can increase spin−orbit coupling, such as carbonyls,7,9,10 nitro compounds,11 or heavy atoms (e.g., metal12,13 or halogen2,14). For instance, Tang et al.10,15,16 recently reported a series of RTP crystalline materials that take advantage of both aggregation-induced emission (AIE)17−19 and triplet-generating carbonyl compounds.20 Kim et al.2,14,21 described a type of highly emissive RTP crystal by external heavy-atom effect, using halogen bonding between the © 2017 American Chemical Society
Received: February 21, 2017 Revised: May 16, 2017 Published: May 22, 2017 4225
DOI: 10.1021/acs.jpca.7b01711 J. Phys. Chem. A 2017, 121, 4225−4232
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The Journal of Physical Chemistry A
fluorescence and phosphorescence were acquired with a NanoLED and a SpectraLED laser with the excitation peak at 420 nm, respectively. Lifetime data were analyzed with DataStation v6.6 (Horiba Scientific). Absolute fluorescence quantum yields were measured on a Hamamatsu QuantaurusQY spectrometer. The molecular structures were optimized by the density function theory (DFT) at the hybrid functional with long-range corrected cam-B3LYP and 6-31G(d) basis level. Time-dependent density functional theory (TDDFT) at the same level were carried out to calculate the energy level structures and excited states. All calculations are performed by the Gaussian09 program. Synthesis of F1 and Covalent Dye−Polymer Conjugate F1-PUs. 2-(4-Chlorophenyl)benzothiazole. A threeneck flask was charged with 2-aminothiophenol (2.5 g, 20 mmol), 4-chlorobenzaldehyde (2.8 g, 20 mmol), and dimethyl sulfoxide (10 mL). The flask was heated at 160 °C for 0.5 h. After cooling to room temperature, water (30 mL) was poured into the reaction mixture. The insoluble solid thus formed was filtered out and then recrystallized by ethyl acetate, yielding a pale-yellow powder (4.1 g, 83.75%).1H NMR (300 MHz, CDCl3): δ 8.06 (dd, J = 11.3, 8.5 Hz, 3H), 7.91 (d, J = 8.0 Hz, 1H), 7.56−7.36 (m, 4H). 2-(4-(Di(hydroxyethyl)amino)phenyl)benzothiazole (F1). 2-(4-Chlorophenyl)benzothiazole (1.2 g, 5 mmol), diethanolamine (5.25 g, 50 mmol), and KOH (0.56 g, 10 mmol) were added to a 50 mL three-neck flask equipped with a magnetic stir bar. The reaction was completed after heating and stirring for 24 h at 130 °C. After the reaction was cooled to room temperature, distilled water (30 mL) was poured into the reaction mixture. The insoluble solid thus formed was filtered out. The crude product was purified by column chromatography (ethyl acetate/petroleum ether = 4:1, v/v), yielding a pale-yellow powder (0.28 g, 17.8%). 1H NMR (300 MHz, DMSO): δ 8.03 (d, J = 7.8 Hz, 1H), 7.92 (d, J = 8.0 Hz, 1H), 7.85 (d, J = 8.8 Hz, 2H), 7.47 (t, J = 7.3 Hz, 1H), 7.35 (t, J = 7.3 Hz, 1H), 6.83 (d, J = 8.9 Hz, 2H), 4.84 (t, J = 5.2 Hz, 2H), 3.59 (t, J = 5.2 Hz, 4H), 3.53 (d, J = 5.2 Hz, 4H). 13C NMR (75 MHz, DMSO): δ 168.26, 154.42, 150.95, 134.24, 129.07 (2C), 126.71, 124.80, 122.30 (2C), 120.05, 111.85 (2C), 58.54 (2C), 53.61 (2C). HRMS (ESI): m/z [M + H]+ calcd for C17H19O2N2S, 315.11618; found, 315.11578. Synthesis of Polyurethane F1-PU. A 100 mL round-bottom, three-necked flask with a mechanical stirrer, thermometer, condenser was used as the reactor. The reactants IPDI and PTMG (Mn = 2,000) were added into the reactor and the molar ratio was specified in Table S1 for different samples. The flask was heated to 90 °C and the NCO content was determined using a standard dibutylamine titration method. Subsequently, BDO and F1 were put into the flask and reacted at 80 °C for 1 h. Afterward a trace amount of catalyst DBDTL (0.05−0.1 wt %) was needed and kept at 70 °C for 3 h. After cooling to room temperature, MDEA was then added into the mixture after 1 h at 40 °C and then heat to 60 °C for 3 h. During the prepolymerization, a moderate amount of acetone was required to reduce the viscosity. As a neutralization agent, the acetone solution of acetic acid was then added into the mixture and reacted with the amino group on the side chain of the polyurethane prepolymer for 5 min to form a NCOterminated prepolymer. Finally, the high shearing speed of stirrer (2500 r/min) was used to emulsify the solution for 30 min after suitable water was poured into the mixture. A paleyellow aqueous dispersion was obtained after acetone was
nanoparticles,28 thin films,27 microfibers,29 and vesicles. They have been used in practical applications such as flexible electronics,30 oxygen nanosensors5,31 etc. Fraser et al.6,7,28 described boron dual emissive materials of a biocompatible polymer used on tumor hypoxia imaging and wound healing. We previously developed RTP waterborne polyurethanes (WPU), where WPUs containing amino-substituted benzophenone were prepared which result in fluorescence and RTP single-component dual-emissive materials (SDMs).27 Here in the current study, we show that when a fluorescent derivative of thioflavin (F1, Scheme 1) is covalently incorporated into the Scheme 1. Schematic Illustration of the Electronic Transition States for F1 before and after Protonation (FL, Fluorescence; RTP, Room-Temperature Phosphorescence)
WPU matrix, the otherwise undetectable RTP could be activated via the addition of various Brønsted acids. The design is based on a more recent study, where we utilized mediating charge-transfer (CT) states to bridge the well-separated 1 (π−π*) and 3(π−π*) states for a class of naphthalimides.32 Here we show that molecular RTP is “switched on” when a new CT state is generated via external stimuli such as the addition of protic acids, indicating the generality of the strategy. The reason PU is suitable for conducting the current experiment is that the strongly polar matrix can be used to stabilize the protonated dye molecules without causing significant aggregation/phase separation of protonated dyes.
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EXPERIMENTAL AND THEORETICAL METHODS Materials. Isophorone diisocyanate (IPDI) was purchased from Bayer Co. Polytetramethylene ether glycol (PTMG, Mn = 2000 Da) was supplied by Mitsubishi Co. and thoroughly dehydrated at 110 °C prior to use. 2-Aminothiophenol, 4chlorobenzaldehyde, and methyldiethanolamine (MDEA) were purchased form Aladdin Reagent Co.. 1,4-Butanediol (BDO), dibutyltin dilaurate (DBTDL), acetic acid, acetone, diethanolamine, and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Water was deionized with a Milli-Q SP reagent water system (Millipore) to a specific resistivity of 18.2 MΩ·cm. All other solvents and reagents were purchased from Aladdin Reagent and were used as received. Methods. 1H and 13C NMR spectra were obtained on a Bruker Avance 300-MHz NMR spectrometers using CDCl3 or DMSO-d6 as the solvent and tetramethylsilane (TMS) as the internal standard. Mass spectral data (ESI/MS) were obtained on a Micromass auto spectrometer. UV/vis absorption spectra were recorded on a Beijing Persee TU-1901 UV−vis spectrometer. The spectra of photoluminescence excitation and emission were determined at room temperature on a Horiba Fluorolog-4 spectrofluorometer. The lifetimes of 4226
DOI: 10.1021/acs.jpca.7b01711 J. Phys. Chem. A 2017, 121, 4225−4232
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The Journal of Physical Chemistry A Scheme 2. Protocol for the Preparation of F1-PU Polymer Films
Figure 1. Normalized steady-state excitation (a) and emission (b) spectra of F1-PU without and with the presence of various molar ratios of benzenesulfonic acid in dichloromethane ([F1] = 10−4 M; λex = 365 nm).
Figure 2. a) 1H NMR spectra of F1 (red line) and F1 in the presence of various molar ratios of benzenesulfonic acid (lines of other colors) in DMSO-d6. (b) Peak assignments of 1H NMR spectra as indicated in alphabetical letters. (c) Schematic illustration of fast protonation− deprotonation process of F1 at two possible nitrogen sites, which are circled in red.
Preparation of Protonated F1-PUs. The F1-PU film and proton acid was add into DCM with different molar ratio of BA and F1 (F1:5 wt %/wt in F1-PU). After complete dissolution, the solution can dilute into suitable concentration or evaporate
removed in vacuo. The solid content of the obtained fluorescent dispersion was 25% (wt). The F1-PU films can be obtained by complete evaporation of acetic acid and water in vacuo on a cast substrate such as Teflon (Scheme 2). 4227
DOI: 10.1021/acs.jpca.7b01711 J. Phys. Chem. A 2017, 121, 4225−4232
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The Journal of Physical Chemistry A
Figure 3. Steady-state emission spectra of F1-PU films in the presence of different ratios of BA/F1 at room temperature in air (a) and under vacuum (b); (c) delayed emission spectra (Δt = 10 ms); (d) steady-state emission spectra at 77 K. (λex = 365 nm).
the DCM to form homogeneous film to test the spectra property.
spectra (in DCM) suggests only one protonation scenario, the difference of which could be due to changes in the acid pKa in different solvents. Correspondingly, the steady-state emission maximum at 411 nm (1.57 ns, ΦF = 0.77) is shifted to 462 nm (1.96 ns, ΦF = 0.38) for F1H+ (Figure 1b). Protonation at either nitrogen site of F1 adds a positive charge and is likely to induce a charge-transfer (CT) state, which is usually red-shifted compared to the vertical π−π* state, particularly in a polar environment. Despite the presence of excess BA (e.g., at 12 equiv), the shoulder peak at 411 nm cannot be completely eliminated possibly because the excited state pKa may be much larger than that in the ground state and may cause adiabatic dissociation of the excited-state protonated dye (F1H+). As such, formation of excited neutral dye (F1) may account for the residual emission intensity. Fluorescence lifetimes at different BA/F1 ratios were also measured; it is found that the values are slightly increased for F1H+ (1.51−2.00 ns) despite visually dimmed emission intensity and decreased fluorescence quantum yield (e.g., ΦF = 0.38 at 3.9 equiv of BA) since F1H+ fluorescence contains more CT character (Table S2). The intrinsic fluorescence lifetime for F1H+ at 3.9 equiv of BA is calculated to be 5.16 ns, a value significantly larger than that for F1 (2.04 ns). Usually, increased intrinsic fluorescence lifetime is an indication of poorer overlaps between origin and destination orbitals involving electronic excited states for fluorescent molecules. Given that the only difference between F1 and F1H+ is the proton, it is reasonable to assume that the poorer orbital overlap is related to larger charge separation of the ground and/or excited state, or more pronounced ICT state. Shown in Figure 2, the thioflavin derivative F1 has two possible protonation sites: in molecular plane nonbonding lone pair on the thiazole nitrogen (labeled in red) and the amino lone pair (labeled in blue) participating the conjugation. We expect that the orthogonal, nonbonding lone pair is a better
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RESULTS AND DISCUSSION The thioflavin-derivative-containing polyurethane (F1-PU) was synthesized according to our previous report (Supporting Information).27 The excitation and emission spectra of F1-PU (F1: 5 wt %/wt) before and after protonation with benzenesulfonic acid (BA) are recorded in dicholoromethane (DCM), a solvent that does not show significant binding to protic acids. As can be seen from the excitation spectrum, F1PU has an excitation maximum at 355 nm in DCM without the presence of BA. At a molar ration of 1:0.6 (F1: BA), the excitation spectrum remains largely unchanged (Figure 1a). This is likely due to the competitive binding of BA with other heteroatoms in PU, indicating that F1 is a rather weak base. However, the excitation spectrum is severely altered when the BA ratio is increased to 1.7 equiv, where the major peak is shifted to 429 nm. The new red-shifted peak is assigned as the protonated form of F1, F1H+. The shoulder peak around 355 nm is still observable but vanishes beyond 3.9 equiv of BA. The UV−vis absorption spectra, which are very similar to those of the excitation spectra, are shown in Figure S7 (Supporting Information). For unprotonated F1-PU, the maximal absorption peak is at 354 nm and no absorption could be observed beyond 400 nm. When 0.6 equiv of BA was added to the solution, a new peak at 429 nm appeared while the one at 354 nm was decreased. At 1.7 equiv of BA, the new peak at 429 nm became the new absorption maximum. The original main peak at 354 nm was almost indistinguishable from the baseline beyond 6.7 equiv of BA. At the same time, an isosbestic point in the figure suggests that two species coexist in solution, presumably from F1 and F1H+. It is curious that 1H NMR spectra (in DMSO-d6 Figure 2) seem to indicate two possible protonation sites while the isosbestic point in the absorption 4228
DOI: 10.1021/acs.jpca.7b01711 J. Phys. Chem. A 2017, 121, 4225−4232
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The Journal of Physical Chemistry A
Table 1. Luminescence Characterization of F1-PU Thin Films with Different Molar Ratios of Benzenesulfonic Acid (BA) and F1 in Air and in a Vacuum at Room Temperature ratioa
λem(air)b (nm)
lifetime(air)c (ns)
ΦFd
λem(vacuum) (nm)b
lifetime(vacuum) (ms)e
0 0.68:1 2.07:1 4.39:1 6.70:1 9.02:1 11.34:1 13.66:1
412 466 481 489 495 494 492 490
1.52 2.32 1.65 1.43 1.79 1.62 1.38 1.34
0.79 0.092 0.066 0.048 0.042 0.040 0.035 0.034
412 458 506 522 517 508 493 492
1.59 × 10−6 45.6 32.7 32.8 23.1 21.2 13.4 6.66
Molar ratio between BA and F1. bEmission maxima excited at 365 nm. cPre-exponent weight-averaged fluorescence lifetime. dAbsolute fluorescence quantum yield in air. ePre-exponent weight-averaged RTP lifetime.
a
and more predominant at ∼490−500 nm depending on the concentration of BA. The red-shifted emission is presumably the contribution from the pronated fluorophore, F1H+. Compared to the emission spectra of protonated F1-PU measured in solution (Figure 1b), the spectra of protonated F1PU (F1H+-PU) in the solid state shows a significant red-shift of almost 30 nm (Figure 3a). Unlike the solution emission spectra where neutral F1 fluorescence is still persistent even at a BA ratio of 12.6 (Figure 1b), the solid state spectra exhibit complete F1H+ fluorescence at a BA ratio of ∼2. Such complete disappearance of F1 emission is ascribed, either to F1 → F1H+ resonance energy transfer (Figure S9), which is generally very efficient in the solid state, or to the much less mobile medium of PU that can lead to permanently pronated F1H+ species due to entrapment of BA and F1 in a small local area. In addition, the emission maximum of F1H+ is independent of BA concentration in solution; in the solid state film, however, the maximum first shows a slight red shift (481 → 495 nm) when the BA/F1 ratio increases but is eventually blue-shifted (495 → 490 nm) at a much higher BA/ F1 ratio. From Figure 3a, it is apparent that the emission spectra are broader when the acid/F1 ratio increases beyond 2.07. Compared to the spectra in Figure 3b, the appearance of the broad emission bands in Figure 3a is likely owing to the contribution from residual emission at the longer wavelengths (possibly surviving RTP in air). The quenching of emission at the longer wavelengths in high acid concentration environment can be supported by their reduced lifetimes presented in Table 1. The measured fluorescence lifetimes change from 2.32 ns at 0.68 equiv of BA to 1.34 ns at 13.66 equiv of BA. Combined with the absolute quantum yield data in Table 1, it is easy to obtain the intrinsic fluorescence lifetime change from F1 to F1H+ at different BA ratios: 1.94 ns (0 equiv), 25.21 ns (0.68 equiv), 25.00 ns (2.07 equiv), 29.79 ns (4.39 equiv), 42.6 ns (6.7 equiv), 40.5 ns (9.02 equiv) 39.42 ns (11.34 equiv) and 39.41 ns (13.66 equiv). These significantly lengthened intrinsic lifetimes are clear evidence of emission with strong ICT character due to less orbital overlapping during electronic transitions. Under vacuum, when the F1-PU film was either neutral or overprotonated, the spectra appear almost identical vs the ones obtained in air, except that the maxima exhibit slight red shifts. However, the spectral shapes vary greatly when the BA/F1 ratio is within ∼ 2−9 eq under vacuum, presumably because the RTP contribution is so conspicuous that the maxima have been shifted beyond 500 nm (the RTP peak becomes the new emission maximum). Shown in Figure 3c are the delayed emission spectra; all protonated films exhibited visible RTP
binding site for BA. To test our assumption, we performed 1H NMR measurements for F1 with increasing amounts of BA (Figure 2). The peak assignment is indicated by alphabets in the chemical structure of F1 and in the spectra. First of all, we did not observe two sets of NMR peaks for F1 and F1H+ as anticipated; instead, all protons on F1 exhibit a gradual shift, but to different extents, to the downfield position as an increasing amount of BA is added. Second, the protons adjacent to the two possible protonation sites register more downfield shift (Δppm >0.12) vs the further-away protons (Δppm
