Protonation-Induced Room-Temperature Phosphorescence in

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Protonation-Induced Room-Temperature Phosphorescence in Fluorescent Polyurethane Wei Sun, Zhaowu Wang, Tao Wang, Li Yang, Jun Jiang, Xing Yuan Zhang, Yi Luo, and Guoqing Zhang J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Protonation-Induced Room-Temperature Phosphorescence in Fluorescent Polyurethane Wei Sun,1 Zhaowu Wang,2,3 Tao Wang,1 Li Yang,2,3 Jun Jiang,*2,3 Xingyuan Zhang, *1 Yi Luo,3 and Guoqing Zhang *2 1

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 2

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 3

Innovation Center of Chemistry for Energy Materials, Department of Chemical Physics,

University of Science and Technology of China, Hefei, 230026 Anhui, P.R.China.

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

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

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. INTRODUCTION Optical materials that exhibit room-temperature phosphorescence (RTP) 1, 2 are widely used as light-emitting diodes3, 4, oxygen sensors5, 6 and background-free bioimaging agents7, 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 (∆EST). 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 carbonyls7,9,10, nitro compounds11, 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. Kim et al.2,14,21 described a type of highly emissive RTP crystal by external heavy-atom effect, using halogen bonding between the aldehyde group of the fluorophore and halogen atoms from the crystal matrix to promote ISC. Another strategy to achieve RTP from purely organic molecules


2

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 towards 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 nanoparticles28, thin films27, microfibers29, and vesicles. They have been used in practical applications such as flexible electronics30, 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 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 wellseparated 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


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

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.

Scheme 1. Schematic illustration of the electronic transition states for F1 before and after protonation (FL: fluorescence; RTP: room-temperature phosphorescence). EXPERIMENTAL AND THEORETICAL METHODS Materials. Isophorone diisocyanate (IPDI) was purchased from Bayer Co.. Polytetramethylene ether glycol (PTMG, Mn = 2,000 Da) was supplied by Mitsubishi Co. and thoroughly dehydrated at 110 °C prior to use. 2-Aminothiophenol, 4-Chlorobenzaldehyde 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

13

C 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.


4

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 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 Quantaurus-QY 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 three-neck flask was charged with 2-Aminothiophenol (2.5 g, 20 mmol) and 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 oC. After cooling to room temperature, distilled water (30 mL) was poured into the reaction mixture. The insoluble solid thus formed was filtered out. The


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

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).

13

C 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 NCO-terminated 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 pale-yellow aqueous dispersion was obtained after acetone was 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).


6

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. Protocol for the preparation of F1-PU polymer films. 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 the DCM to form homogeneous film to test the spectra property. RESULTS AND DISCUSSION The thioflavin-derivative-containing polyurethane (F1-PU) was synthesized according to our previous report (Supporting Information, SI).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, F1-PU 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


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

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 eq., 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 eq. of BA. The UV-Vis absorption spectra, which are very similar to those of the excitation spectra, are shown in Figure S7 (SI). For unprotonated F1-PU, the maximal absorption peak is at 354 nm and no absorption could be observed beyond 400 nm. When 0.6 eq. 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 eq. 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 eq. of BA. At the same time, an isosbestic point in the figure suggests that two species co-exist 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 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 eq.), 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


8

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ns) despite visually dimmed emission intensity and decreased fluorescence quantum yield (e.g.,

ΦF = 0.38 at 3.9 eq. BA) since F1H+ fluorescence contains more CT character (Table S2). The intrinsic fluorescence lifetime for F1H+ at 3.9 eq. 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.

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] = 104

M; λex = 365 nm). Shown in Figure 2, the thioflavin derivative F1 has two possible protonation sites: in molecular

plane non-bonding lone pair on the thiazole nitrogen (labelled in red) and the amino lone pair

(labelled in blue) participating the conjugation. We expect that the orthogonal, non-bonding lone pair is a better binding site for BA. To test our assumption, we performed

1

H-NMR


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

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. Secondly, the protons adjacent to the two possible protonation sites register more downfield shift (∆ppm > 0.12) vs. the further-away protons (∆ppm

Protonation-Induced Room-Temperature Phosphorescence in

Recommend Documents