Ladder-Type Dye with Large Transition Dipole Moment for

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A Ladder-type Dye with Large Transition Dipole Moment for Solvatochromism and Microphase Visualization Jian Cao, Qi-Ming Liu, Si-Jie Bai, Hua-Chun Wang, Xiancheng Ren, and Yun-Xiang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07677 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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ACS Applied Materials & Interfaces

A Ladder-type Dye with Large Transition Dipole Moment for Solvatochromism and Microphase Visualization Jian Cao, Qi-Ming Liu, Si-Jie Bai, Hua-Chun Wang, Xiancheng Ren and Yun-Xiang Xu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China. ABSTRACT: Large transition dipole moment is usually pursued by strategies of twisted intramolecular charge transfer (TICT) or planar intramolecular charge transfer (PICT) to obtain obvious Stokes shifts and dramatic color changes with tuning of polarities. However, both strategies have their drawbacks and suffered from fluorescence quenching in solid states. Herein, a ladder-type molecule ISOAA-H with intramolecular hydrogen bond is designed which undergoes intramolecular charge transfer and proton shift to harvest large transition dipole moment under light irradiation. Thanks to its out-of-plane side chains, intermolecular 0.0 stacking of backbones is prohibited and solid emission is generated. ISOAA-H exhibits outstanding solvatochromic behaviors with polarity changes of solvents or polymer matrixes and is successfully used to detect microphase separation of polymer blends. These results indicate that a strategy combining the advantages of TICT and PICT is established for environment-sensitive dyes used in both solution and solid state. KEYWORDS: solvatochromic, transition dipole moment, ladder-type, solid emission, microphase visualization

INTRODUCTION Environment-sensitive dyes are able to change emission colors or intensities responding to the change of surroundings such as polarity, pH and viscosity (also called solvatochromic dyes).1-4 This feature makes them widely used in fluorescent probes2, 5, chemical sensors6, 7, micro-environmental changes detection8, 9, biological imaging2, 10 and so on11. However, there are still some limitations of current strategies for molecular design, especially few of solvatochromic dyes could show responsive fluorescence in solid state. Although these dyes have potential application in solid environmental detection and microphase visualization12. Generally speaking, the requirements for solvatochromic dyes are as followings: (1) large Stokes shift to avoid self-absorption effect; (2) dramatic color changes of the fluorescence emission with the change of environment; (3) high photoluminescence quantum yield (PLQY)5. To achieve these criteria, a twisted intramolecular charge transfer (TICT) state (Scheme S1, Supporting Information) is usually pursued in which intramolecular charge transfer (ICT) proceeds with drastic conformational change, thus resulting in large transition dipole moments13, 14. This kind of molecules are highly environment-sensitive and their donor and acceptor units are linked to each other by single bonds. Up to now, a series of solvatochromic dyes with TICT emission have been developed, such as DMABN15, 16, 6DMN17, Nile Red18, 19, Dapoxyl20 and their derivatives21 (Scheme S1, Supporting Information). However, the emissions of these dyes are usually not

Scheme 1. Molecular structure of ISOAA-H (The dashed line represents H-bonding). C6H13 C6H13 O Br

O

Se O

H

C6H13 C6H13

ISOAA-H

strong due to limited absorption abilities22-26. And the emission of TICT molecules will become even weaker because strong intermolecular 0.0 stacking and nonradiative decay27 quench fluorescence in aggregation state. As another approach, molecular emission based on planar intramolecular charge transfer (PICT) usually exhibited high PLQY irrespective of solvent polarity. For example, Prodan and its derivatives exhibited high PLQY in common solvents including highly polar ones such as methanol (Scheme S1, Supporting Information) 18, 28-31. After proper modification of molecular structures, wide color changes could be harvested for PICT dyes such as Prodan derivatives29, 31. However, these PICT dyes also suffer from serious fluorescence quenching in aggregation state. Therefore, an alternative design strategy to overcome the current limitations of TICT and PICT mechanisms could be very useful.

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In this article, a new strategy based on excited-state charge transfer and excited-state proton shift of ladder type dye was proposed. As shown in Scheme 1, solvatochromic dye ISOAA-H possesses an intramolecular hydrogen bond in its acceptor part. After excitation under light irradiation, intramolecular charge transfer and proton shift occurs with the generation of a large transition dipole moment. As a result, the molecule became highly sensitive to the environment especially the polarity. In the meantime, solid emission was achieved by isolating chromophores through out-of-plane side chains of ladder-type structures. ISOAA-H offered the following advantages: (1) ISOAA-H exhibited unusual PICT emissions with large Stokes shifts and high PLQYs in polar solvents (a Stokes shift of 139 nm and PLQY of 0.1 in acetonitrile solution); (2) The large steric alkylphenyl sidechains and corresponding intermolecular interactions isolated conjugated backbones, leading to efficient solid emission; (3) The solid emission is sensitive to polarity change of polymer matrixes and used to detect microphase separation of polymer blends. EXPERIMENTAL SECTION Measurements. The 1H and 13C NMR spectra were collected on a Bruker AVANCE III HD 400 spectrometer in deuterated chloroform solution with TMS as reference, respectively. High-resolution mass spectroscopy (HRMS) measurements were performed on a Shimadzu LCMS-IT-TOF. UV-vis spectra were measured using a Shimadzu UV-3600 spectrophotometer. Photoluminescence spectra were measured using a HORIBA FluoroMax-4 spectrophotometer. The photoluminescence quantum yields (PLQYs) in solutions were determined by comparing to quinine sulfate in 0.10 M H2SO4 (EPL=0.546 at room temperature) as standard. The PLQYs in solid states were obtained on HORIBA Fluorolog-3 spectrophotometer equipped with an integrating sphere. The powder X-ray diffraction (XRD) patterns were collected using a Rigaku ultima iv diffractometer with Cu KR radiation. Data collection from single crystals was conducted using an Xcalibur Eos diffractometer at 293.15 K. Using Olex2, the structure was solved with the Superflip structure solution program using Charge Flipping and refined with the ShelXL refinement package using Least Squares minimisation. Hydrogen atoms of disordered solvents were not introduced, but are taken into account in the compound formula. The images of ISOAA-H-doped polymer blends were acquired from a Nikon Eclipse Ti-U fluorescent microscope excitated by ultraviolet light. All calculations of geometry optimization and excitation properties are performed using density functional theory (DFT) in the electronic structure package DMol3, implementing the generalized gradient approximation (GGA) method. The Perdew-Burke-Ernzerhof (PBE) functional is used to take into account of the exchange-correlation effects

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Scheme 2. Synthetic route of ISOAA-H and ISOAAM. R COOEt

R

Br

Se Se

AcOH , H2SO4

Se

Se

octane,reflux

BuLi , THF , -78°C

EtOOC 1

R R IDS-H: R=C6H13 IDS-M: R=CH3 R

NBS chloroform ,R.T

oxone

R

Se

Br

O O

NaHCO3 , H2O , DCM , acetone

R

HO

R ISOAA-H: R=C6H13 ISOAA-M: R=CH3

with the double numerical polarization (DNP) basis set. The convergence criteria for the self-consistent field (SCF) was set to 10-5 Ha. The model structure was optimized until the maximum force is below 0.002 Ha/Å. X-ray crystallography. Crystal data for ISOAA-M: C44H33BrO3Se, monoclinic, P21/n, T = 293.15 K, a = 14.6254(9) Å, b =16.0784(8) Å, c = 16.8019(11) Å, J = 90°, = 102.263(7)°, L = 90°,V = 3860.9(4) Å3, Z = 4, = 2.050 mm-1, calc = 1.341 g/cm3, F(000) = 1517.0, 20681 reflections measured, 8949 independent reflections (Rint = 0.0414, Rsigma = 0.0929), Goodness-of-fit on F2 = 1.008, R1 = 0.0591, wR2 = 0.1559 (all data). The data can be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC number: 1884639). Materials and Synthetic procedures. All chemicals, unless otherwise noticed, were purchased from Adamas or other commercial resources and used as received. Tetrahydrofuran (THF) was distilled from sodium benzophenone under Argon prior to use. Other reagents and solvents were used as received. Compound 1 and IDS-H were synthesized according to previously reported procedure 32, 33. Synthesis of IDS-M: n-BuLi (4.2 mL, 2.5 M in hexane, 10.4 mmol) was added dropwise to a solution of 4Bromotoluene (1.77 g, 10.4 mol) in anhydrous THF (30 mL) at -78 °C under argon. The mixture was kept at -78 °C for 1 h and compound 1 (1.0 g, 2.07 mmol) in THF (10 mL) was added dropwise. The reaction was kept at -78 °C for 1 h, then slowly warmed to room temperature and allowed to stir overnight. Then the reaction was quenched with water and extracted with CH2Cl2. The organic layer was dried over anhydrous Na2SO4. The solvent was removed under vacuum to obtain yellow solid, which was used in the next step without purification. The above crude product was added into the mixture of octane (100 mL) and acetic acid (10 mL). Then 2 drops of concentrated H2SO4 was added under argon. The reaction was allowed to reflux for 3 h and


R

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ACS Applied Materials & Interfaces quenched with water. The mixture was extracted with CH2Cl2 and the combined organic layer was washed with water three times and dried over anhydrous Na2SO4. After removing the solvent, the crude product was purified by silica gel chromatography (petroleum ether/CH2Cl2 = 10:1) to afford a yellow solid (398 mg, 26.5%, IDS-M was not stable in solution). 1H NMR (400 MHz, CDCl3, N* 7.87 (d, J = 5.4 Hz, 2H), 7.37 (s, 2H), 7.18 (d, J = 5.4 Hz, 2H), 7.13 (d, J = 8.2 Hz, 8H), 7.05 (d, J = 8.2 Hz, 8H), 2.30 (s, 12H). 13C NMR (100 MHz, CDCl3, N* 157.37, 152.83, 144.20, 141.77, 137.60, 136.33, 132.03, 129.02, 127.95, 125.48, 117.52, 63.78, 21.00. HRMS (ESI) m/z: [M+Na]+, calcd for C44H34Se2, 745.0883; found, 745.0858. Synthesis of ISOAA-H: IDS-H (500 mg, 0.500 mmol) and NBS (196 mg, 1.10 mmol) were dissolved in chloroform (5 mL). And the reaction mixture was stirred in dark for 2 h at room temperature. Then the mixture was poured into water and extracted with CH2Cl2. The organic layer was dried over anhydrous Na2SO4. The solvent was then removed under vacuum to obtain a yellow solid, which was used in the next step without purification. The above crude product was added into the mixture of CH2Cl2 (9 mL), water (10 mL) and acetone (8 mL). Then sodium bicarbonate (600 mg, 0.71 M in water) was added into the mixture. Oxone (2KHSO5·KHSO4·K2SO4, 444 mg, 0.722 mmol) was

added to the resulting mixture carefully. The reaction mixture was stirred for 2 h at room temperature. Then water was added and extracted with CH2Cl2 three times. The combined organic layer was dried over anhydrous Na2SO4 and evaporated under vacuum. The crude product was purified by silica gel chromatography using CH2Cl2 as the eluent to give ISOAA-H (63 mg, 12.5%). 1H NMR (400 MHz, CDCl3, N* 14.62 (s, 1H), 7.85 (s, 1H), 7.26 (s, 1H), 7.24 (s, 1H), 7.16 – 7.02 (m, 16H), 6.36 (s, 1H), 2.58 (dd, J = 16.0, 8.8 Hz, 8H), 1.59 (dd, J = 14.9, 7.1 Hz, 8H), 1.28 (d, J = 17.4 Hz, 24H), 0.91 – 0.84 (m, 12H). 13C NMR (100 MHz, CDCl3, N* N 195.24, 165.12, 161.93, 159.86, 155.75, 153.83, 150.12, 143.42, 142.76, 142.59, 141.16, 139.81, 133.95, 133.04, 129.66, 128.94, 128.86, 128.65, 127.73, 123.45, 122.44, 117.32, 64.70, 61.59, 35.68, 31.84, 31.40, 29.85, 29.23, 22.73, 14.24. HRMS (ESI) m/z: [M+H]+, calcd for C64H73BrO3Se, 1049.3981; found, 1049.3964. Synthesis of ISOAA-M: The compound was prepared according to the same procedure as ISOAA-H using IDS-M as the starting material (43 mg, 18.2%). 1H NMR (400 MHz, CDCl3, N* 14.55 (s, 1H), 7.84 (s, 1H), 7.24 (d, J = 2.7 Hz, 2H), 7.15 – 7.02 (m, 16H), 6.34 (s, 1H), 2.34 (s, 6H), 2.32 (s, 6H). HRMS (ESI) m/z: [M+H]+, calcd for C44H33BrO3Se, 769.0851; found, 769.0899.

(a)

(b)

(c)

(d)

-5

Figure 1. (a) UV-Vis absorption and (b) PL spectra of ISOAA-H in various solvents. Excitation wavelength: 365 nm. c = 10 -1 2 M. (c) The Lippert–Mataga plot for ISOAA-H in various solvents. The slope for the fit is 8992 cm (R = 0.95). (d) Photos of ISOAA-H in hexane, toluene, tetrahydrofuran (top, from left to right), chloroform, acetone and acetonitrile (bottom, from left to right) taken under 365 nm UV lamp.



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RESULTS AND DISCUSSION The synthetic route of ISOAA-H was shown in Scheme 2. ISOAA-H was readily synthesized from indacenoselenophene by bromination and oxidation reactions. The structure of ISOAA-H was confirmed by 1H-NMR, 13C-NMR and mass spectrometry. The UV-vis absorption spectra of ISOAA-H in various solvents were measured and shown in Figure 1(a). The maximum absorption wavelengths 'Sabs) at lower energy and the molar absorption coefficients 'T* were summarized in Table S1. As shown in Figure 1(a), ISOAA-H exhibited two typical absorption bands in the region of 250 nm 500 nm. The absorption band at the short wavelength with higher absorbance represents the 0.0U transition. While another band at the long wavelength with lower absorbance represents ICT process. As shown in Figure 2, the highest occupied molecular orbital (HOMO) of ISOAA-H was located at donor units (benzene and selenophene rings) whereas the lowest unoccupied molecular orbital (LUMO) predominantly resided on acceptor units (the ring containing carboxyl and carbonyl groups). The distinct HOMO and LUMO distribution in ISOAA-H indicated ICT characteristics. The positions of 0.0U transition peaks didn’t change much with the increase of solvent polarities. And the ICT absorption peaks showed only subtle changes in the ground state. For example, the ICT absorption peak of ISOAA-H was located a 427 nm in acetonitrile and at 440 nm in chloroform with a change of 13 nm, suggesting that ISOAA-H has less polar character without light excitation.

HOMO

LUMO

Figure 2. Molecular orbital diagrams (HOMO and LUMO) of ISOAA-H.

solvent polarities, which were ascribed to excited ICT states (Figure 1b). The parameters were summarized in Table 1. For example, only locally excited emission was observed in the nonpolar solvent hexane because ICT emission was quenched. In toluene, the absorption and emission peaks of ICT state were observed at 425 nm and 482 nm respectively, with a Stokes shift of 57 nm. With the increase of solvent polarities, the ICT emission peaks of ISOAA-H were shifted significantly in a bathochromic way, leading to large Stokes shifts in polar solvents. Especially in acetonitrile, the ICT emission peak reached up to 566 nm, with a large Stokes shift of 139 nm. As a result, the fluorescent colors changed from blue to orange by tuning the solvents from hexane to acetonitrile, covering most visible region (Figure 1d). The highly planar conformations of ISOAA-H backbone exhibited in the ground (S0) and excited

However, under the light irradiation, the board emission bands in the region of 450-700 nm showed obvious bathochromic shifts with the increase of

Table 1. Photophysical properties of ISOAA-H in various solvents.

Solvents

X

Sabs b (nm)

Hexane

-

431

Toluene

2.38

Chloroform

T

Sem c (nm)

XS d (nm)

EPLe

2.99

446

-

0.18

425

2.63

486

61

0.55

4.90

440

2.90

525

85

0.62

Tetrahydrofuran

7.58

427

1.19

525

98

0.34

Acetone

20.7

424

1.50

554

130

0.10

Acetonitrile

37.5

427

1.70

566

139

0.10

(104

M-1 cm-1)

a

orientation polarizability. b Longest wavelength absorption maximum. c CT emission maximum 'S 1 = 365nm). Sem - Sabs. e Photoluminescence quantum yield.


d

XS =

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singlet-state (S1) (Figure S1, Supporting Information) suggest a PICT emission and thence high PLQY. The PLQYs of ISOAA-H increased from hexane to chloroform with the increase of solvent polarities due to enhanced generation and stabilization of ICT state. ISOAA-H showed a maximum PLQY of 0.62 in chloroform solution. Then the PLQYs decreased to 0.34 in THF solution and 0.10 in acetonitrile since the non-radiative deactivation channels (mainly solvent relaxation) become active with the increase of solvent polarities. Even the PLQY values dropped, they are still much higher than those TICT dyes in the same solvents. Then the Lippert-Mataga equation34 (see the Supporting Information) was adopted to further understand the origin of these solvatochromic behaviors. The Stokes shifts (cm-1) of ISOAA- H in various solvents with a range of Xf (orientation polarizability) values were listed in Table S1 (Supporting Information). And the Lippert-Mataga plot was displayed in Figure 1c, which showed a strong linear correlation between Stokes shifts and the Lippert-Magata polarity parameter. This result demonstrated that dipole-dipole interactions between ISOAA-H and solvents in the excited state are the main reason for the solvent-dependent fluorescent shifts. The transition dipole moment 'X = e - g) of ISOAA-H was estimated to be 19.3D via the slope of the fit curve (8992 cm-1)35. The large transition dipole moment was the main reason that ISOAA-H exhibited large Stokes shifts and dramatic color changes in different solvents. To rationalize the origin of large transition dipole moment of ISOAA-H, density functional theory (DFT) calculation was carried out to probe the charge distribution of ISOAA-H in ground state (S0) and excited state (S1). The distinct distribution of electron cloud in HOMO and LUMO energy levels shown in Figure 2 indicated large transition dipole moment

(a)

during S0-S1 transition. Furthermore, the optimized geometries of ISOAA-H in S0 and S1 (Figure S1, Supporting Information) showed a proton shift happened during the transition process. The distance of intramolecular hydrogen bond (COO-H···O) in ground state of ISOAA-H is 1.570 Å. When the molecule is excited by light irradiation, the length of hydrogen bond becomes as short as 1.278 Å. In the meantime, the O-H bond is lengthened from 1.017 Å to 1.139 Å during S0-S1 transition. The above results reveal that proton slightly transfer from carboxyl to carbonyl in the excited state thus the proton is shared by these two groups. As a result, the transition dipole moment is further enhanced due to the increased distance between opposite charges. It is also shown that the backbone of ISOAA-H exhibited planar conformations in both ground and excited states (Figure S2, Supporting Information). In addition, the proposed photophysical process of ISOAA-H based on above results was shown in Scheme S2 (Supporting Information). Notably the calculation is done without involvement of solvents, therefore the results only revealed the preliminary mechanism. Moreover, the methyl ester of ISOAA-H (ISOAM-H, Scheme S2, Supporting Information) was prepared as control compound to further study the effects of excited proton shift on solvatochromic behaviors. As shown in Figure S3 and Table S2 (Supporting Information), ISOAM-H showed narrower color ranges of fluorescence emission in various solvents compared to those of ISOAA-H. These experimental results confirmed that the excited proton shift indeed elevated transition dipole moment. Intriguingly, ISOAA-H showed tunable fluorescence emission in the solid state. The PL spectra of ISOAA-H as film and powder were measured and showed in Figure S4 (Supporting Information). The PL emission peaks of ISOAA-H as film located at 540 nm and 599 nm, showing bright orange color under UV lamp.

(c)

(b)

Figure 3. (a) Molecular structure of ISOAA-M. (b) Detailed packing mode and intermolecular interactions in the crystal structure. (c) Top view of the dashed white box in b and corresponding centroid-centroid distance of 0.0 stacking. C (grey), H (white), O (red), Se (orange). H-bonding, Se···O, C-H···O, C-H···H-C and 0.0 distances are marked. The unit of distance is Å.




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ACS Applied Materials & Interfaces large Stokes shifts, high PLQYs and unusual environment-sensitive solid emissions. Theoretical calculation indicated that charge transfer and proton shift played a key role on the fluorescence behavior by enhancing the charge separation and thus harvesting large transition dipole moment. The out-of-plane side chains of ladder-type structure and specific mode of intermolecular interaction are both responsible for the occurrence of solid emission. This strategy is significantly different from traditional TICT and PICT models. Considering the generality and ready modification of this kind of ladder-type dyes, it is expected that more useful molecules could be developed for fluorescent probe, biological imaging and so on.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The Lippert-Mataga equation, XRD curves, additional photoluminescence spectra, fluorescent images, and experimental spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This study is supported by the National Natural Science Foundation of China (51403135) and Sichuan Science and Technology Program (Grant No. 2019YJ0123). The authors thank Zhan-Ting Li from Fudan University and Jianbin Lin from Xiamen University for insightful discussion. The authors would like to thanks the ceshigo, “www.ceshigo.com” for providing the theoretical calculation.

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Table of Contents

Solvatochromic fluorescence C6H13 C6H13 O Br

Se O

O H

Large transition dipole moment C6H13 C6H13

ISOAA-H

Increasing solvent polarities

PS/PIP blends


9

Ladder-Type Dye with Large Transition Dipole Moment for

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