Solvatochromic Probes Displaying Unprecedented Organic Liquids

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Solvatochromic Probes Displaying Unprecedented Organic Liquids Discriminating Characteristics Huijing Liu, Xiaojie Xu, Zijun Shi, Kaiqiang Liu, and Yu Fang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02721 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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Analytical Chemistry

Solvatochromic Probes Displaying Unprecedented Organic Liquids Discriminating Characteristics Huijing Liu,1,2 Xiaojie Xu,1,3 Zijun Shi,3 Kaiqiang Liu,1,3 Yu Fang1,3,∗ 1

Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), 2

3

School of Materials Science and Engineering,

School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’ an 710062, P. R. China ∗

Corresponding author: [email protected]

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ABSTRACT: Four highly fluorescent derivatives of bis(phenyl-ethynyl-)-2-naphthyl (BPEN) with push-pull structures were designed and synthesized, of which azetidine was adopted as an electron-donating unit. For the electron withdrawing moiety, it varies from hydrogen, to formyl, then to 2-ethoxyethyl derivative of dicyanovinyl and finally to dicyanovinyl itself, and the corresponding fluorophores are denoted as A1, A2, A3 and A4, respectively. To enhance the solubility of the compounds, two n-hexadecyl residues were grafted onto the side positions of BPEN. Interestingly, introduction of azetidine not only improves the fluorescence quantum yield and enlarges the Stoke’s shift of the parent compound, but also endows them, in particular A2 and A4, exceptional capability to distinguish structurally relevant organic liquids, such as ethylbenzene and its isomers (o-xylenes, m-xylenes and p-xylenes), mono-alkyl-substituted benzene derivatives, gasolines of different grades and other organic liquids. Theoretical calculation and Lippert-Mataga equation-based tests revealed the intra-molecular charge transfer (ICT) nature of the solvatochromic properties of the compounds. Further quantitative analysis of the data obtained from studies of the probes/n-hexane-dioxane systems revealed the big differences in the dipole moments between the excited- and ground states of A1, A2, A3 and A4, which are about 23, 29, 43 and 38 D, respectively. Moreover, the four novel fluorophores possess exceptional photochemical stability as demonstrated by the fact that more than 2 hours UV light illumination did not result in detectable reduction in the fluorescence emission of the fluorophores. It is the long wavelength absorption (>380, ≈400, >410 and >430 nm), large molar absorption coefficient (>59 000, >52 000, >39 000 and >34 000 cm-1 M-1), great color change (400~620 nm), and good solubility in common organic liquids that makes the as developed compounds, in particular A2 and A4, very competitive solvatochromic probes.

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INTRODUCTION Organic liquids are largely produced every day and widely used throughout the world due to their importance for industry and scientific research. However, as solvents, organic liquids affect chemical reactions and supramolecular interactions through influencing the conformations and alternating the solubility of reactants via general solvation effect and specific interactions originated from H-bonding, π-π stacking, dipole-dipole interaction, etc.1-3 Moreover, organic liquids are generally toxic, and hard to be degraded in nature, and thereby they are need to be treated and re-used or turned into harmless products before releasing. This is why accurate discrimination of organic liquids is recognized as a pre-requirement for proper use and efficient treatment of them. Actually, discrimination of chemically and structurally similar organic liquids has never been a simple problem, and it is inherently challenging indeed. In fact, gas/high-performance liquid/ion chromatography, mass spectrometry, UV spectrometry, and FTIR spectrometry, etc. have been used for such applications.4-6 However, utilization of them usually suffers from disadvantages of sophisticated instrumentation, multi-step procedures and time-consuming, etc. Moreover, they are hard to be used on-site and at real time. Therefore, there has been a recent upsurge of interest toward development of facile and low-cost techniques for discriminating organic liquids.7-12 To address the issue, one of the choices is to develop novel solvatochromic probes with superior performances, in particular with the ability to distinguish organic liquids with similar structures.13 Due to simplicity, celerity and sensitivity, solvatochromic probes based on either absorption and/or fluorescence changes have become efficient tools for discriminating organic liquids.14-19 For most of the probes, the positions of their absorptions and/or fluorescence emissions are solvent-dependent, and related to the polarity and structures of the liquids, which is the basis for the discrimination.20,21 Recently, Xu and co-workers developed a novel naphthalimide-based fluorescent sensor, AMN, which can selectively differentiate CH2Cl2, CHCl3, CCl4 and CBr4 from other 3 / 22



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halogenated

organic

liquids.22

Brummond

et al.

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prepared

a

number

of

PRODAN-related, naphthalene-containing solvatochromic fluorophores and studied their photophysical properties.14 It was found that the fluorescence colors of the fluorophores are largely dependent upon the solvents used, laying foundation for them to be used to discriminate a variety of organic liquids of different structures. Very recently, Itami and co-workers reported two unique cycloparaphenylenes with inserted acceptors in the rings.23 The compounds as produced exhibit solvatochromic behavior, but unfortunately, the fluorescent quantum yields of them are quite low. As is well known, the majority of solvatochromic probes inevitably display broad overlaps in their absorption and emission bands in different solvents. Therefore, it is extremely difficult to differentiate organic liquids of similar polarity, especially a group of closely related liquids with conventional solvatochromic probes. To address this problem, Kitagawa and co-workers prepared 1,4,5,8-naphthalenediimide (NDI)-based porous coordination polymers, and realized, for the first time, visual discrimination of aromatic volatile organic compounds (VOCs) which are benzene derivatives with different substituents on the phenyl ring.24 A recent breakthrough was achieved by Warner and co-workers.25 They realized the discrimination of a group of structurally similar alcohols via combined use of a single drop of ionic liquid, which is insoluble in the alcohols under test, and a known solvatochromic probe. The principle behind is that the partition of the probe in the ionic liquid phase and alcohol phase changes along with changing the alcohol, resulting in fluorescence change. This is a smart strategy to deal with the problem as described but it cannot be taken as a general strategy. Very recently, we designed two fluorescent conjugates, PNBD and BPNBD, with a push-pull structure, and they exhibit unprecedented discrimination capability among solvents of similar structures, such as (CH2Cl2, CHCl3, CCl4), (ethyl ether, THF, dioxane), or (methanol, ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-decanol), allowing rapid and selective visual identification.26 However, more challenging solvents such as mono-substituted benzenes, ethyl-benzene and its isomers, as well as gasolines of different grades can’t simply be discriminated by known solvatochromic probes. Therefore, visual and 4 / 22


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accurate differentiating the liquids as mentioned is still a challenging task. For most known solvatochromic probes, they are generally characterized by two features, of which one is a flat and rigid framework system with conjugated structure,27 and the other is a push-pull system based on a donor unit and an acceptor residue.28,29 Although thousands of solvatochromic probes are extensive, most of them present a number of limitations in the practical applications such as lower fluorescence quantum yields, poor photochemical stability and small molar absorption coefficients.30,31 Recently, Lavis et al. reported an effective strategy to improve the fluorescence property of a variety of fluorophores.32,33 It was found that replacement of general electron donor of N,N-dialkyl groups with azetidine not only improves the fluorescence quantum yields of classical fluorophores, but also enhances the photochemical stability of them. This observation was further confirmed by Xu, Liu, and co-workers.34 To address the problems as described and inspired by the work conducted by Lavis and co-workers, in the present work, four new derivatives of bis(phenyl-ethynyl-)2-naphthyl (BPEN) with a push-pull structure, A1, A2, A3 and A4, were designed and synthesized, of which azetidine was adopted as an electron-donating unit, and the pristine hydrogen, or a substitute of either formyl, a 2-ethoxyethyl derivative of dicyanovinyl or dicyanovinyl itself was chosen as an electron withdrawing group (c.f. Scheme 1). To improve the solubility of the compounds, two n-hexadecyl residues were grafted onto the side positions of the middle phenyl ring of BPEN. Fluorescence and UV-vis absorption studies revealed that introduction of azetidine functionality leads to superior solvatochromic properties as demonstrated by successful discrimination of structurally very relevant organic liquids through fluorescence color change. This paper reports the details.

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Scheme 1 Molecular structures of A1, A2, A3 and A4

EXPERIMENTAL SECTION Synthetic procedure. All reagents were purchased from commercial suppliers and used as received unless otherwise noted. Prior to use, CH2Cl2, toluene, and diisopropylamine were purified by distillation from CaH2. Reactions were conducted under a dry N2 atmosphere using standard vacuum line techniques. Column chromatographic purification was performed with silica gel (300-400 mesh) as the stationary phase. 1H NMR and 13C NMR spectra were recorded on Bruker 600 MHz spectrometer in CD2Cl2 and CDCl3 with tetramethylsilane (TMS) as an internal standard. Mass spectrometry measurements were performed on an AXIMA-CFR in MALDI-TOF mode by using a-cyano-4-hydroxycinnamic acid (CCA) as the matrixes. Photophysical characterization. UV-visible absorption spectra were recorded on an UV-visible spectrophotometer (U-3900, Hitachi), and the molar absorption coefficients for the four compounds were calculated using the Lambert-beer law, A =

εbc, where A is the absorbance, ε the molar absorption co-efficient, c the concentration of the compound under study, and b the length of the cell, which is 1 cm in general. Steady state fluorescence measurements were performed at room temperature on a time correlated single photon counting Edinburgh Instruments FLS 920 fluorescence spectrometer. All the solvents for fluorescence measurements were freshly distilled prior to use. Absolute fluorescence quantum yields were determined on the same system equipped with an integral sphere at a concentration of 1.0×10-6 mol/L. With this instrument, the measured Φ f values may be subject to an error of + 5%. Fluorescence lifetimes for the four compounds in different solvents were measured on the same system using picosecond pulsed light emitting diode 6 / 22


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(EPLED-340, EPLED-405 and EPLED-445) as excitation source at a concentration of 5.0×10-6 mol/L. RESULTS AND DISCUSSION UV-vis absorption. The absorption spectra of A1, A2, A3 and A4 in toluene are shown in Figure 1. With reference to the spectra, it can be seen that the spectra for solutions of different compounds are characterized by two bands, which could be originated from the S1←S0 and S2←S0 transitions. With further inspection of the spectra, it is also seen that from A1 to A4, the absorptions significantly red-shifted as evidenced by the change of the maximum absorption from 391 to 403, then to 426, and finally to 446 nm. Moreover, for the S1←S0 transition, the maximum absorptions of A1 and A2 at the concentration and solvent tested are around 0.32, but A3 and A4 are only ~0.20, and 0.19, respectively, suggesting the differences between the dipole moments of the Franck-Condon (FC) excited states, which are the so-called local excited states (LE), and those of the corresponding ground states of the compounds. Moreover, the molar extinction coefficients (ε) of A1 and A2 in toluene at their long-wavelength absorption maximums are ~59201 M-1 cm-1 and ~62000 M-1 cm-1, respectively. For A3 and A4, however, the values are only ~40602 M-1 cm-1 and ~37765 M-1 cm-1, respectively.

Figure 1 UV-vis absorption (a) and fluorescence emission (b) spectra of the toluene solution of A1, A2, A3 and A4 recorded at a concentration of 5.0×10-6 mol/L. (A1: λEx = 365 nm, A2: λEx = 400 nm, A3: λEx = 420 nm, A4: λEx = 450 nm.)

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The UV-vis absorption spectra of A1, A2, A3 and A4 were further recorded in other solvents of different polarities. The results are provided in Figure S-1 and the relevant parameters from the absorptions are given in Table 1 and Table S-1. It is seen that the positions of the absorption maximums of the four compounds in different solvents vary from 384 to 456 nm depending on the exact structure of the substitute group at the other terminal position of the fluorophores. Moreover, the positions of the maximum absorptions of the compounds at longer wavelengths slightly change with increasing solvent polarity, but the change can’t be correlated with the ET(30) values of the solvents, which is a parameter describing the polarity of a liquid (solvent).35 Further examination of the data shown in the tables, it can also be noticed that the spectra of the compounds in bromobenzene and toluene are slightly red-shifted if compared to the absorptions of their neighboring solvents (c.f. Table 1 and Table S-1), which may be a result of stabilization of the π∗ energy level of the fluorophore due to interaction of the solvent π system with that of the compounds.36 The molar extinction coefficients at the long-wavelength absorption maximums of the compounds in different solvents are all large, and even the minimum is greater than 34000 M-1cm-1. The long wavelength absorption, large absorption co-efficient and the inherent push-pull structures of the fluorophores as prepared suggest their potentials in the use as solvatochromic probes.37,38 Fluorescence properties. To interrogate the potential of solvatochromic properties of the compounds as produced, steady-state and time-resolved fluorescence measurements of the compounds were conducted in different solvents. The results are depicted in Table 1 and Table S-1. It is to be noted that the radiative decay rate constants were calculated by kr = Φf/τf and the non-radiative decay rate constants by knr = (1/Φf-1)kr, where Φf and τf are the fluorescence quantum yield and fluorescence lifetime, respectively, and kr and knr are the radiative decay rate constant, and non-radiative decay rate constant, respectively. Similar with that observed in the absorption measurements, the emission maximum shows more than 160 nm red-shift in toluene from A1 to A4. Specifically, the emission of A1 appears around 436, A2 8 / 22


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Table 1 Photophysical data for A1, A2, A3 and A4 in selected solvents: solvent polarity parameter ET(30), molar absorption coefficient (ε), maximum absorption wavelength (λabs) and emission wavelength (λem), Stokes shifts (∆ν), fluorescence quantum yield (Φf), lifetimes (τf), radiative decay rate constant (kr) and non-radiative decay rate constant (knr) for the four compounds recorded in different solvents. For fluorescence quantum yield measurements, the probe’s concentration was kept at 1.0 µM. For other fluorescence measurements, the concentrations were set at 5.0 µM. Fluorescent Probes

Solvent

ET(30)

ε

λabs

λem

△ν

(M-1 cm-1)

(nm)

(nm)

(cm-1)

Φf

τf

kr

knr

(ns)

(MHz)

(MHz)

Hexane

31.0

62591

384

413

1829

87

1.19

731

109

CCl4

32.4

52378

381

417

2266

89

1.36

654

189

Toluene

33.9

59201

391

436

2640

99

1.23

805

8.0

1,4-Dioxane

36.0

60123

388

441

3097

95

1.56

609

32

Hexane

31.0

64776

395

441

2641

75

1.20

625

208

A1

CCl4

32.4

47068

398

436

2190

34

1.24

274

532

Toluene

33.9

62000

403

481

4024

91

1.53

595

59

1,4-Dioxane

36.0

67653

401

499

4898

99

1.90

521

5.0

Hexane

31.0

46990

419

492

3541

99

1.74

569

5.7

CCl4

32.4

41214

425

521

4336

78

2.02

386

109

A2

A3 Toluene

33.9

40602

426

566

5806

89

2.27

392

48

1,4-Dioxane

36.0

45051

418

590

6974

65

1.28

266

516

Hexane

31.0

42369

445

516

3092

99

2.08

476

4.8

CCl4

32.4

34847

456

548

3682

84

2.62

321

61

Toluene

33.9

37765

446

600

5755

41

2.47

166

239

1,4-Dioxane

36.0

41306

434

623

6990

13

0.82

159

1061

Hexane

31.0

41738

430

480

2369

87

1.97

442

66

CCl4

32.4

42303

436

500

2936

81

1.94

418

98

Toluene

33.9

34130

429

530

4174

82

2.53

352

43

1,4-Dioxane

36.0

37102

420

554

6104

50

3.10

161

161

A4

PNBD*

* Note: The data for the reference compound, PNBD, were reported in an earlier submitted paper.26

around 481, A3 and A4 around 566 and 600 nm, respectively (Figure 1). However, the fluorescence quantum yields of them in the solvent decrease from 0.99 to 0.91, then to 0.89, and finally to 0.41 with increasing the polarity of the substitute. Further examination of the data shown in the table reveals that the fluorescence quantum yields of the four compounds are generally high in less polar solvents, and low in polar solvents. Moreover, in the studies, it was found that the probes as developed are 9 / 22



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dissolvable in common organic solvents, and as examples Table S-2 provides the solubility of them in some typical solvents. It was also demonstrated that the Stokes shifts of the fluorophores are highly solvent dependent. Based on these findings, it is anticipated that the fluorophores as created may find important applications in the identification and discrimination of solvents of similar structures. The photo-chemical stability of the four compounds was also studied as it is often limiting the practical applications of fluorescent probes.39 The toluene solution of compound A1 and those of the others were employed for conducting the test. For the four compounds, two hours’ photo-illumination did not result in any significant deduction of their initial emission, indicating their exceptional photochemical stability, at least in solution state. Figure S-2 depict four example results from the tests.

Figure 2 Normalized fluorescence emission spectra of A1, A2, A3 and A4 dissolved in different solvents. The spectra were recorded at a concentration of 5.0×10-6 mol/L with 365, 400, 420 and 450 nm as the excitation wavelength of the respective compounds. To be clear, the spectra of A1, A2, A3 and A4 were depicted in a, b, c and d, respectively. Note: HEX: hexane, TCM: carbon tetrachloride, TOL: toluene, DEE: diethyl ether, DIO: dioxane, BBNZ: bromobenzene, THF: tetrahydrofuran, DCM: dichloromethane, and OOH: o-octanol. 10 / 22


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Solvatochromic properties. The fluorescence emission spectra of A1, A2, A3 and A4 recorded in different solvents are shown in Figure 2. Inspection of the spectra shows that the emission spectra of the four compounds exhibit two emissions in hexane, but become less structured and even structure-less with the emission moving to longer wavelengths owing to, possibly, increases in the solvent polarities, a result different from that observed in absorption studies, which can be taken as an indication of presence of a twisted intra-molecular charge transfer state, the so called TICT state, in the excited states of the compounds.40 As for A1, the maxima of its emission spectra in the solvents studied extend from 413 nm in n-hexane to 515 nm in acetone. With introduction of an electron-withdrawing formyl substituent in the terminal position, the emission maximum changes from 436 nm in CCl4 to 597 nm in THF. These remarkable shifts could be observed by naked eyes when the samples were illuminated with UV light (365 nm, c.f. Figure S-3). Theoretical calculations. To understand the UV-vis absorption properties, the DFT calculations were performed by using the Gaussian 09 package. The geometries of A1 to A4 were optimized at the CAM-B3LYP/6-31g* level (Figure 3). Based on the calculation, it is known that the HOMO-LUMO gaps of the four compounds decrease gradually with increasing the electron-withdrawing ability of the substitutes attached on the core structure of them. The TDDFT calculations were also performed at the long-range corrected CAM-B3LYP function. The calculated λmax, the main orbital transition, and oscillator strength (ƒ) values are presented in Table 2. The calculated values of λmax are in good agreement with the experimental results considering the fact that solvent effect was not considered during the calculations. Molecular orbital diagrams of the fluorophores obtained from TDDFT calculations further support our findings (c.f. Figure 3). This is because the electron densities of the HOMO orbitals of the four compounds are localized on the azetidine-(naphthalene ethynyl)-benzene part of the molecules. However, with increasing the electron-withdrawing ability of the substitutes connected at another terminal, the electron densities of the LUMOs of the compounds move to the acceptor part. The as described theoretical calculations also 11 / 22



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A1

A2

A3

A4

-0.44 eV

-0.92 eV

-1.47 eV

-1.78 eV

-6.01 eV

-6.06 eV

-6.09 eV

-6.16 eV

Optimized Structures

LUMO

HOMO

Figure 3 Molecular orbital energies and iso-density surface plots of HOMO and LUMO for A1, A2, A3 and A4 depicted in different columns.

Table 2 TDDFT calculations at the long-range corrected CAM-B3LYP function and experimentally obtained absorption properties.

Compound

λmaxa

Main orbital

(nm)

transition

S1←S0

346

HOMO→LUMO

2.12

391

S2←S0

303

HOMO→LUMO+1

0.58

305

S1←S0

370

HOMO→LUMO

2.17

403

S2←S0

309

HOMO→LUMO+1

0.08

311

S1←S0

397

HOMO→LUMO

2.47

426

S2←S0

318

HOMO→LUMO+1

0.06

337

S1←S0

410

HOMO→LUMO

2.24

446

S2←S0

322

HOMO→LUMO+1

0.06

348

Transition

ƒ

λmaxb (nm)

5.6

A1

9.7

A2

15.5

A3

20.7

A4

a

µe-µg(Debye)

Calculated absorption maximum, b Observed absorption maximum in toluene

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reveal the differences between the excited state dipole moments and those of the ground state dipole moments (∆µ), which are also provided in the table. It is seen that the values for the four compounds are 5.6, 9.7, 15.5 and 20.7 D, respectively, which may explain why A2 shows superior solvatochromic property than that of A1 (c.f. Figure S-3), and for A4 and A3, they identify the solvents with similar ability (c.f. Figures 2c and 2d). Rationalization based on experimental interrogation. In order to know if the solvatochromic properties of the fluorophores are related to the polarities of the solvents under test, a widely-used empirical solvent polarity scale, ET(30), was employed. As is known, ET(30) is defined as the transition energy for the longest wavelength absorption band of the dissolved pyridinium-N-phenolate betaine dye measured in kal⋅mol-1.35 Figure S-4 shows the plots of the emission maxima of A1, A2, A3 and A4 vs. the polarity parameter, ET(30), of the relevant solvents under study. It is found that a relatively good correlation between the position of the emission maximum and the empirical polarity parameter ET(30) for each of the four compounds in aprotic solvents. Thus, the compounds, as expected, could be used as solvatochromic probes. For protic solvents, however, such as n-octanol, the data points deviate from the correlations, suggesting possible specific interactions between the compounds and the solvents. To explore the nature of the solvatochromic characteristics of the four fluorophores as created, Lippert-Mataga theory was employed to conduct further experimental studies.41 Specifically, n-hexane-dioxane was chosen as a class of typical solvent mixtures to adjust the orientation polarizabilities (△f) of the mixture solvents through regular variation of their compositions, of which △f is a parameter from the Lippert-Mataga equation.

′

̅ − ̅ = ′ − ∆# =

$

$

−

+ constant (2)

∆%

∆̅ = &' − &( + constant (3) 13 / 22


(1)


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Where ̅ − ̅ and &' − &( are the frequency shift (in cm-1) between absorption and emission maxima, and the dipole moment difference (in Debye) between the excited and ground states of the fluorophore, respectively. The symbols n and ξ΄ are the refractive index and dielectric constant of the solvent, respectively. The other terms, ), + and -, are Planck’s constant, the speed of light and the radius of the cavity (for a given probe, the exact value can only be estimated), in which the fluorophore resides, respectively.

Figure 4 (a) Fluorescence emission spectra of A3 in n-hexane-DIO of different rations at a concentration of 5.0×10-6 mol/L, of which excitation wavelength is 420 nm. (b) Lippert plots for A3 in the set of mixture solvents.

Figure 5 (a) Fluorescence emission spectra of A4 in n-hexane-DIO of different rations at a concentration of 5.0×10-6 mol/L, of which excitation wavelength is 450 nm. (b) Lippert plots for A4 in the set of mixture solvents.

The emission spectra of A1, A2, A3 and A4 in the solvent mixtures were recorded. The spectra as obtained for A1 and A2 are shown in Figure S-5a and Figure S-6a, respectively, but those for A3 and A4 are shown in Figure 4a and Figure 5a, 14 / 22


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respectively. It is to be noted that corresponding each of the figures as mentioned, there is another closely related figure (b), which shows the plot of the relevant compound’s Stokes shift ̅ − ̅ vs. △f. Moreover, the equations depicted in the insets of Figure 4b and Figure 5b are mathematical fittings of the data, which are the exact expressions of equation 3 for the systems under study. With further analysis of the equations shown in the figures, the dipole moment changes (∆µ) of the corresponding fluorophores upon excitation can be calculated by using equations 1-3. As it is well known, the Lippert equation is a result of derivation based on approximation and many assumptions. In particular, a fluorophore under concern is considered to be a dipole in a continuous medium of uniform di-electric constant. In other words, the molecule of the fluorophore is supposed to exist within a cavity with an ideal spherical shape. But actually, for the systems under study, the four fluorophores are all characterized by linear structures that makes their medium quite complicated. Accordingly, a modified model was adopted, of which the cavities fitted by the molecules of the fluorophores are supposed to take an ellipsoidal shape.38,42 Based on this modified model, and taking into account of the radii of the four fluorophores (aA1 = 6.14 Å, aA2 = 6.22 Å, aA3 = 6.73 Å, aA4 = 6.35 Å), which are the results from the calculation by employing the CAM-B3LYP/6-31g* method, the dipole moment changes of the four probes on excitation were calculated, and the results are ∼23, ∼29, ∼43 and ∼38 D, respectively. As expected from theoretical calculations, A2 does possess a larger ∆µ than that of A1, and the value for A3 is close to that for A4. As for why there is a big difference between the results from theoretical calculations and those from experimental determinations, the reason behind might be probe-solvent interaction, which was not considered during the theoretical calculations. Anyway, the calculation does reveal that the sensitivity of the probes to solvent polarity is most likely due to charge shift from the azetidine naphthalene end to the electro-withdrawing structures of the compounds, and furthermore, the difference in the electro-withdrawing ability of the groups explains their difference in solvatochromic characteristics. 15 / 22



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Example applications of the solvatochromic probes. Spectroscopy and theoretical studies demonstrated that the as created fluorophores should be good solvatochromic probes. To demonstrate the discriminating ability of the compounds, A4 was chosen as an example, and firstly used to distinguish two sets of structurally very relevant organic solvents, of which the first set includes ethylbenzene and its isomers, o-xylene, m-xylene and p-xylene, and the second is composed of different mono-alkyl-substituted benzene derivatives, of which the alkyl groups change from methyl to ethyl, then to n-propyl, then to n-butyl, then to n-amyl, and finally to n-dodecyl. The results are presented in Figures 6a and 6b, respectively. Interestingly, the color of A4 in the two sets of solvents under UV light (365 nm) are different from each other. To the best of our knowledge, this is first report that ethylbenzene and its isomers, and the mono-substituted benzene derivatives bearing different straight alkyl structures could be discriminated by using a solvatochromic probe.

Figure 6 The fluorescence images of A4 in ethylbenzene and its isomers (o-xylene, m-xylene and p-xylene) (a), and the fluorescence images of A4 in toluene and its homologous (b). All photons were

recorded under UV light (365 nm) illumination.

The superior discrimination ability of A4 to organic solvents is further demonstrated by its ability to identify commercial gasolines of different grades. It is known that gasolines are divided into 92# and 95# in China according to the content of isooctane in them, and using different kinds of gasoline has a significant effect upon a car’s power and its engine’s maintenance. Therefore, accurate identification of gasoline is of practical importance. Figure 7a depict the images of the samples of 16 / 22


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gasoline of 92# and 95# grades under sun light and UV light (365 nm), respectively. It is seen that in both cases, the two samples can’t be discriminated by naked eyes. However, with addition of 30 µL of the CH2Cl2 solution of A4 (5.0×10-4 mol/L) into 3 mL of each of the gasoline samples, the difference between them are clearly observed especially under UV light (c.f. Figure 7b).

Figure 7 (a) The images of 92# and 95# gasoline under sun light and UV light (365 nm); (b) The images of 92# and 95# gasoline with addition of 30 µL of the CH2Cl2 solution of A4 (5.0×10-4 mol/L) into 3 mL of each of the liquids under sun light and UV light (365 nm).

CONCLUSIONS Four fluorescent conjugates with bis(phenyl-ethynyl-)-2-naphthyl (BPEN) as the core structure were designed and synthesized. Azetidine was introduced as a donor group at the 6th position of the naphthyl end of BPEN, and two long alkyl chains were grafted onto the side positions of the middle phenyl ring of the conjugate. Moreover, formyl, dicyanovinyl or a derivative of dicyanovinyl, of which one cyano-unit was replaced by 2-ethoxyethyl, was introduced at the other terminal position of the conjugate. Through the design, the compounds, in particular A4, are endowed with superior solvatochromic property. As demonstrated already, A4 discriminates ethylbenzene and its three isomers, o-xylenes, m-xylenes and p-xylenes, with no difficult. Similarly, mono-substituted benzenes with straight alkyls as the substituents could also be identified by the probe even if only one methylene unit is added or removed from the alkyls, in particular when the chain is not very long. Further examination demonstrates that A4 can also be used to discriminate gasolines of different grades. The extraordinary solvatochromic properties of the as created probes were rationalized by theoretical calculations and by the Lippert-Mataga principle17 / 22



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based experimental studies. It was revealed that the solvatochromic properties of the compounds may be originated from the excitation related intra-molecular charge transfer (ICT) of their BPEN-based push-pull structures. Considering the long wavelength absorption (>380, ≈400, >410 and >430 nm for A1, A2, A3 and A4, respectively), large molar absorption coefficient (>59 000, >52 000, >39 000 and >34 000 cm-1 M-1 for A1, A2, A3 and A4, respectively), great color change (400~620 nm), exceptional photochemical stability, good solubility in common organic solvents and extraordinary performance in organic liquids discrimination, it is believed that the work as described represents a substantial progress in the studies of the solvatochromic probes. ASSOCIATED CONTENT Supporting Information Detail synthesis procedures, characterizations of compounds, supplementary schemes, figures and tables are described in the supporting information. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Tel: +86-29-81530786. Fax: +86-29-81530787.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by NSF of China (21273141, 21527802, 21673133), the 111 project (B14041), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1070).

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for TOC only

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Solvatochromic Probes Displaying Unprecedented Organic Liquids

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