TICT based fluorescence “turn-on” hydrazine probes

Sensors and Actuators B 199 (2014) 93–100

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

TICT based fluorescence “turn-on” hydrazine probes Bin Chen a , Xi Sun a , Xin Li b , Hans Ågren b , Yongshu Xie a,∗ a b

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China Department of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 9 December 2013 Received in revised form 17 March 2014 Accepted 21 March 2014 Available online 1 April 2014 Keywords: Hydrazine Fluorescence probes DFT Crystal structure

a b s t r a c t Fluorescence “turn-on” probes PC3 and C3 with weak background emissions were developed for hydrazine sensing. The aldehyde and dicyanovinyl groups were used as the recognition units for PC3 and C3, respectively. Because of low reactivity of the aldehyde group, the fluorescence of PC3 was enhanced by only ca. 93 folds upon addition of a large amount of 1646 eq. hydrazine. In contrast, C3 exhibited fluorescence enhancement by ca. 239 folds upon addition of only 1.3 eq. hydrazine, and thus it showed high sensitivity towards hydrazine, with the detection limit of 7 ppb. In aqueous systems, it also works well with improved selectivity for hydrazine over CN− . The weak fluorescence of PC3 and C3 can be ascribed to twisted intramolecular charge transfer (TICT) processes by the combination of the bulky diphenylamino and 9-anthryl units, which were well demonstrated by theoretical calculations, viscosity dependent fluorescence, and fluorescence decay behaviour. Addition of hydrazine induced the disappearance of the TICT deactivation pathway, resulting in the observed fluorescence enhancement. It can be concluded that the combination of the bulky diphenylamino and 9-anthryl units is an effective approach for developing fluorescence turn-on hydrazine probes based on the TICT mechanism. © 2014 Elsevier B.V. All rights reserved.

1. Introduction As a significant industrial material, hydrazine is widely applied in chemical, pharmaceutical and agricultural areas [1–4]. And it is well known as high-energy fuel for rocket-propulsion systems [1–4]. However, it is also a neurotoxin. And chronic exposure to high concentration hydrazine may damage people’s visceral organ, induce respiratory tract infection and destroy the central nervous system [3–5]. Thus, many governments and international organizations set up the standards for the safe concentration of hydrazine in drinking water [6,7]. Because of its widespread applications and biotoxicity, it is urgently needed to develop effective techniques for hydrazine detection [4a,8,9]. In this respect, fluorescent probes have demonstrated the advantages of high sensitivity, easy operation and low cost. Compared with fluorescence quenching probes, fluorescence “turn-on” probes are preferred due to their larger signal-to-noise ratio and less interference from the nonspecific quenching by factors other than the analytes [10–13]. For ideal fluorescence “turn-on” probes, the fluorescence of the host should be as weak as possible. In a previous communication, we have proposed a strategy for designing fluorescence “turn-on” cyanide

∗ Corresponding author. Tel.: +86 21 6425 0772; fax: +86 21 6425 2758. E-mail address: [email protected] (Y. Xie). http://dx.doi.org/10.1016/j.snb.2014.03.087 0925-4005/© 2014 Elsevier B.V. All rights reserved.

probes based on the dicyanovinyl (DCV) recognition unit, which exhibited extremely weak fluorescence due to the twisted intramolecular charge transfer (TICT) mechanism [14]. Meanwhile, we noticed that DCV is also a good recognition unit for hydrazine [7a]. Thus, these compounds also may be developed as promising hydrazine probes. On the other hand, the aldehyde group also can be used as the recognition unit for hydrazine [7e]. In this work, based on the above mentioned considerations, we employed compounds PC3 and C3 as hydrazine probes, utilizing the carbonyl and DCV groups as the recognition units, respectively. To our delight, C3 shows good sensitivity and selectivity towards hydrazine.

2. Experimental 2.1. General Commercially available solvents and reagents were used as received. Water was used after redistillation. Deuterated solvents for NMR measurements were available from Aldrich. UV–vis absorption spectra were recorded on a Varian Cary 100 spectrophotometer, with a quartz cuvette (path length = 1 cm). Fluorescence lifetime measurements were performed by using the Time Correlated Single Photon Counting (TCSPC) technique following excitation by a nanosecond flash lamp (Edinburgh instruments

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FL920), and the errors (2 ) for all the measurements presented were below 1.2. 1 H NMR and 13 C NMR spectra were obtained using a Bruker AM 400 spectrometer with tetramethylsilane (TMS) as the internal standard. HRMS were performed using a Waters LCT Premier XE spectrometer. Column chromatography was carried out using silica gel (200–300 mesh) purchased from Qingdao Haiyang Chemical Co., Ltd (China). Reactions were monitored by thin-layer chromatography. PCH was prepared according to adaptations of the reported procedure [15,16]. And the preparation of C3 has been communicated by us [14]. The detection limits [14] and the fluorescence quantum yields [17,18] of PC3 and C3 were obtained according to the literature method.

were refined against Fo2 (all data), final wR2 = 0.1558 (all data), S = 1.020, R1 (I > 2(I)) = 0.0648, largest final difference peak/hole = +0.184/−0.225 e A˚ −3 . Structure solution by direct methods and full-matrix least-squares refinement against F2 (all data) using SHELXTL. CCDC-971576 (PC3) contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif.

2.2. Determination of the fluorescence quantum yields

Time-dependent density functional theory (TDDFT) calculations were employed to study the fluorescent processes of PC3, C3 and TP. The hybrid MPW1B95 functional [19] containing 31% of Hartree-Fock exchange was used in TDDFT calculations as it provides better description than the widely used B3LYP functional [20] for these three compounds, PC3, C3 and TP, which show charge-transfer character upon photoexcitation. To capture the nature of fluorescence in these two compounds, the geometries of their singlet first excited state (S1 ) were optimized at the TDMPW1B95/6-31G* level of theory with solvent effect of acetonitrile taken into account by the polarizable continuum model [21]. All theoretical calculations were performed using the Gaussian 09 program package. The corresponding data are shown in Tables S1–S6.

Fluorescence quantum yield [17] was determined using optically matching solution of Rhodamine 6G (˚r = 0.95 in ethanol) [18] as standard and the quantum yield was calculated using the following equation: ˚s = ˚r (Ar Fs /As Fr ) (ns /nr )2 , where As and Ar are the absorbance of the sample and the reference, respectively, at the excitation wavelength. Fs and Fr are the corresponding relative integrated fluorescence intensities, and n is the refractive index of the solvent. 2.3. Fluorescence spectra measurements for hydrazine probing The fluorescence emission spectral changes of PC3 and C3 during the titrations were measured at 25 ◦ C in the specified solutions, with the excitation wavelengths fixed at one of the corresponding isosbestic points. The slit width was 5 nm and PMT voltage was 600 V for both excitation and emission. Tested ions Cu2+ , Zn2+ , Cd2+ , Hg2+ , Ni2+ , Mn2+ , Ni2+ , Pb2+ , and Ca2+ were added as acetates dissolved in the corresponding solvents. Fe3+ , Fe2+ , Na+ , K+ , Mg2+ , and Co2+ were added as chlorides. Anions such as CN− , F− , Cl− , Br− , I− , AcO− and H2 PO4 − were added as TBA salts dissolved in the corresponding solvents, and N3 − was added as the sodium salt. 2.4. Detection limits The detection limit of probe C3 in CH3 CN was obtained according to the literature method [14]. C3 (1 ␮M) was dissolved in CH3 CN. Fluorescence changes during the titration of C3 (1 ␮M) with hydrazine (0–5.5 ␮M) in CH3 CN is shown in Fig. S5a. The enhancement of fluorescence intensity is clearly resolved and has a good signal to noise ratio. Fig. S5b shows the plot of I versus [NH2 NH2 ]. A linear regression curve was fitted to the twelve intermediate values (0–5.5 ␮M hydrazine) as shown in Fig. S5b. The Standard Deviation ( = 0.0718) was obtained by fluorescence responses (10times of consecutive scanning on the Cary Edipse fluorescence spectrophotometer). Thus, the detection limit of C3 to hydrazine was calculated by the formula (3/k) and gave a result as 0.2 ␮M (7 ppb) in CH3 CN. The detection limit in CH3 CN-H2 O was obtained by a similar procedure. 2.5. Crystallography Single crystals of PC3 suitable for X-ray analysis were obtained by slow evaporation of its solution in a mixture of CH3 CN and H2 O at room temperature. Crystal data for PC3: C35 H35 NO, Mw = 485.64 g/mol, ˚ b= 0.45 × 0.36 × 0.11 mm3 , Triclinic, P-1, a = 8.3617(7) A, ˚ c = 18.2895(13) A, ˚ ˛ = 79.9710(10)◦ , ˇ = 83.098(2)◦ , 9.6976(8) A, V = 1422.3(2) A˚ 3 , F(000) = 520, calcd =  = 77.8530(10)◦ , 1.134 Mg/m3 , (Mo-K␣) = 0.067 mm−1 , T = 298(2) K, 7228 data were measured on a Bruker SMART Apex diffractometer, of which 4941 were unique (Rint = 0.0440); 412 parameters

2.6. Theoretical calculations

2.7. Synthesis of PC3 POCl3 (0.64 mL, 7 mmol) was slowly added to dimethylformamide (5 mL) and stirred for 20 min at 0 ◦ C. The solution of PCH (0.46 g, 1 mmol) in 1,2-dichloroethane (25 mL) was added to the above solution, warmed up to room temperature, left for 1 h, and stirred at 50 ◦ C for 12 h. The solvent was removed under reduced pressure. The product was isolated by silica gel columns and recrystallized from CH2 Cl2 /CH3 OH. Weight: 0.29 g, Yield: 60%. 1 H NMR (CDCl3 , Bruker 400 MHz, 298 K): ı 1.25 (s, 18H, t-butyl-CH3 ), 6.95 (d, J = 8.8 Hz, 4H, Ph ), 7.16 (d, J = 8.8 Hz, 4H, Ph ), 7.43 (m, 2H, anthryl), 7.63 (m, 2H, anthryl), 8.24 (d, J = 8.8 Hz, 2H, anthryl), 9.00 (d, J = 8.8 Hz, 2H, anthryl), 11.55 (s, 1H, CHO). 13 C NMR (CDCl3 , Bruker 100 MHz, 298 K): ı 31.37, 34.14, 119.88, 124.11, 124.91, 125.82, 126.10, 126.71, 128.85, 130.54, 133.44, 144.21, 145.06, 145.18, 192.93. HRMS (ESI, m/z): [M + H]+ calcd for C35 H36 NO, 486.2797; found, 486.2802.

2.8. Synthesis of TP C3 (107 mg, 0.2 mmol) and hydrazine hydrate (100 mg, 2 mmol) were dissolved in CH2 Cl2 (40 mL), and the mixture was stirred at room temperature for 12 h. Then the solvent was removed under reduced pressure and the residue was dissolved in CH2 Cl2 , washed with water, and dried over anhydrous sodium sulfate. The product was isolated by chromatography over silica gel followed by recrystallization from CH2 Cl2 /CH3 OH. Weight: 50 mg, Yield: 50%. 1 H NMR (CDCl , Bruker 400 MHz, 298 K): ı 1.24 (s, 18H, t-butyl3 CH3 ), 5.99 (s, 2H, NH2 ), 6.97 (d, J = 8.8 Hz, 4H, Ph ), 7.13 (d, J = 8.4 Hz, 4H, Ph ), 7.38 (t, J = 7.4 Hz, 2H, anthryl), 7.48 (t, J = 7.2 Hz, 2H, anthryl), 8.17 (d, J = 8.8 Hz, 2H, anthryl), 8.51 (d, J = 8.8 Hz, 2H, anthryl), 8.86 (s, 1H, CH C ). 13 C NMR (CDCl3 , Bruker 100 MHz, 298 K): ı 31.42, 34.09, 119.52, 125.19, 125.78, 125.88, 126.23, 126.41, 127.20, 130.72, 131.22, 138.79, 140.71, 143.51, 145.13. HRMS (ESI, m/z): [M + H]+ calcd for C35 H38 N3 , 500.3066; found, 500.3068.

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3. Results and discussion

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probe C3 may be further developed for the detection of hydrazine in practical applications.

3.1. Synthesis of PC3 and C3 3.3. Hydrazine sensing selectivity in CH3 CN

3.2. Titration study in CH3 CN Consistent with our expectations, PC3 and C3 exhibited very weak fluorescence, with the quantum yields of 0.05% and 0.03%, respectively. Thus, these weakly fluorescent compounds are promising candidates for the design of fluorescence “turnon” probes with weak background fluorescence. To check the sensing behaviour, we firstly investigated the performance of PC3 in acetonitrile (Figs. 1 and 2). With the addition of hydrazine, slight UV–vis absorption spectral changes were observed (Fig. 2a). Meanwhile, approximately 93-fold fluorescence enhancement was observed when 1646 eq. of hydrazine was added (Fig. 2b). For the purpose of practical applications, it is important to sense analytes at low concentrations. However, the detection limit of PC3 was found to be 9228.8 ␮M (Fig. S4), which lies far above the safe value of 10 ppb recommended by ACGIH [6,7]. This result may be related to the insufficient reactivity of the aldehyde group. To improve the hydrazine probing performance, DCV was selected as the binding moiety, which has stronger reactivity than the aldehyde group. Thus, DCV was introduced to the 9-position of anthracene to afford C3. As we expected, C3 shows excellent performance with remarkable fluorescence “turn-on” behaviour (Figs. 1 and 3). When hydrazine was added to the acetonitrile solution of C3, the peaks centred at 422 and 490 nm in the UV–vis spectra gradually decreased, and new peaks developed at 387 and 445 nm, with concurrent appearance of three clear isosbestic points at 416, 426, and 465 nm, respectively (Fig. 3a). It is noteworthy that only 1.3 eq. of hydrazine was required to saturate the UV–vis and fluorescence spectral changes, and the fluorescence was enhanced by as large as 239-folds (Fig. 3b). To our pleasure, the detection limit of C3 for hydrazine was found to be 0.2 ␮M (7 ppb) (Fig. S5), which lies below the safe hydrazine level (10 ppb) [6,7], indicating that

Selectivity is a major issue for probes. The selectivity of PC3 and C3 in CH3 CN was checked by using environmentally and biologically important cations and anions as the interfering species. When added separately, only CN− caused obvious fluorescence enhancement. All other species induced only neglectable fluorescence changes (Figs. S6 and S7), which is in sharp contrast to the drastic fluorescence enhancement observed for hydrazine. These results indicated that only CN− had slight interference with hydrazine sensing. In addition, potentially interfering neutral analytes ammonia and formaldehyde were also checked, which revealed that ammonia had slight interference with the detection of hydrazine, and formaldehyde had neglectable interference (Figs. S6 and S7). To further elucidate the selectivity of probes PC3 and C3 towards hydrazine, competition experiments were investigated in CH3 CN. And the addition of competing ions did not interfere significantly with the sensing of hydrazine (Figs. S6 and S7), which explicitly elucidated that probes PC3 and C3 can be established as a promising type of highly selective fluorescence “turn-on” hydrazine probes. 3.4. Solvent effect and the application of C3 in aqueous systems C3 was previously demonstrated to be a promising fluorescence turn-on CN− probe in DCM [14]. However, as mentioned above, it exhibits better response to hydrazine than CN− in CH3 CN. In fact, the selectivity between hydrazine and CN− is highly dependent on the solvent polarity (Fig. 4). In the less polar solvents such as CH2 Cl2

a) Absorbance

The syntheses of PC3 and C3 have been communicated [14], and the synthetic route is shown in Scheme 1. PC3 was prepared by the Vilsmeier reaction in a moderate yield of 60% (Figs. S1–S3). And the Knoevenagel condensation between PC3 and malononitrile afforded C3. The compounds were fully characterized by 1 H NMR, 13 C NMR and HRMS [14].

0.10

0 eq NH2NH2 1646 eq

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b) FL Intensity

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1646 eq NH2NH2

30 20

0 eq

10 0 500

600

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Wavelength/nm Fig. 1. Images of PC3 and C3 (40 ␮M) in CH3 CN taken under a portable UV lamp. (a) PC3, (b) C3, (c) PC3 + 100 eq. hydrazine, and (d) C3 + 1.3 eq. hydrazine.

Fig. 2. a). Electronic absorption spectral changes during the titration of PC3 (20 ␮M) with hydrazine (0–1646 eq.) in CH3 CN. (b) Corresponding fluorescence spectral changes with ex fixed at 455 nm (one of the isosbestic points).

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Scheme 1. Synthesis of PC3 and C3.

Absorbance

a)

and THF, CN− induced higher response than hydrazine. And the selectivity is reversed in more polar solvents such as CH3 CN and DMSO. To further elucidate the possibility of practical applications, we investigated the performance of C3 in aqueous systems. Interestingly, the selectivity of C3 between hydrazine and CN− was improved in the mixtures of organic solvents and water, compared with that observed in pure organic solvents (Fig. 4), and the selectivity was enhanced with the increasing polarity of the organic solvents. To be more specific, in H2 O-CH3 CN (v/v, 1/4), both of the absorption and emission spectra displayed changes similar to those observed in acetonitrile (Fig. S8). Excellent selectivity for hydrazine with neglectable interference was observed (Fig. 5). And the detection limit for hydrazine was found to be 1.1 ␮M (35 ppb) (Fig. S9). These results indicated that the sensitivity of C3 for hydrazine

0.15

0 eq NH2 NH2

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relative intensity

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Imax,NH NH /Imax,CN

b) relative intensity

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6 3 1

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0.8 0.6 0.4 0.2 0.0

Fig. 3. (a) Electronic absorption spectral changes during the titration of C3 (20 ␮M) with hydrazine (0–1.3 eq.) in CH3 CN. (b). Corresponding fluorescence spectral changes with ex fixed at 465 nm (one of the isosbestic points).

0

1.0

7 −

Fig. 4. The ratios of fluorescence intensities upon addition of hydrazine and CN in various solvents. Imax represent the fluorescence intensity of C3 (20 ␮M) after addition of 5 eq. of analytes (hydrazine or cyanide). The sequential numbers represent various solvents. (1) CH2 Cl2 , (2) THF, (3) CH3 CN, (4) DMSO, (5) THF-H2 O (v/v, 4/1), (6) CH3 CN H2 O (v/v, 4/1), (7) DMSO H2 O (v/v, 4/1). The horizontal red line indicates the height for the ratio of 1:1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1.0 0.8 0.6 0.4 0.2 0.0

1

2

3

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5

6

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9 10 11

Fig. 5. Relative fluorescence intensity of C3 (20 ␮M) in H2 O CH3 CN (v/v, 1/4) upon excitation at 465 nm (one of the isosbestic points): Red bars represent the addition of 10.0 eq. of ions. Black bars represent the addition of 5 eq. of hydrazine mixed with 10.0 eq. of indicated ions. (a) metal ions, the numbers represent C3, Zn2+ , Cd2+ , Hg2+ , Cu2+ , Fe3+ , Fe2+ , Na+ , K+ , Mg2+ , Ca2+ , Co2+ , Ni2+ , and Pb2+ , respectively; (b) anions and neutral analytes, the sequential numbers represent C3, F− , Cl− , Br− , I− , OAc− , H2 PO4 − , N3 − , CN− , ammonia and formaldehyde, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. X-ray crystal structure of probe PC3.

Fig. 6. (a) The proposed detection mechanism for probe C3; (b) The 1 H NMR spectra of probe C3 in CDCl3 at 298 K; (c) The 1 H NMR spectra of adduct TP in CDCl3 at 298 K. (* denotes solvent).

was decreased in polar solvents, especially in water containing solvents. Meanwhile, the selectivity for hydrazine over CN− was improved. These observations may be rationalized by the fact that polar solvents, especially water may decrease the nucleophilicity of hydrazine, and this effect is even more serious for CN− . 3.5. The hydrazine sensing mechanism In order to understand the hydrazine sensing mechanism, we tried to separate the adducts of hydrazine with PC3 and C3, respectively. It turned out that the same adduct TP was isolated for both of the probes, and it was fully characterized (Figs. S10–S13). In the 1 H NMR spectrum of TP, a characteristic peak of the amino protons are observed at 5.99 ppm (Fig. 6). And the adduct structure was further confirmed by HRMS (calculated for [M + H]+ , 500.3066; found, 500.3068) (Fig. S13). These results are in agreement with the sensing mechanism proposed for C3 (Fig. 6a). And PC3 also reacts with hydrazine by a similar mechanism. 3.6. Single crystal structure and theoretical calculations As previously communicated, the weak fluorescence of C3 may be ascribed to its distorted molecular structure [14]. In this work, the single crystal of PC3 was successfully obtained and analyzed by X-ray diffraction, which clearly revealed the nonplanar structure of PC3 (Fig. 7). The dihedral angles between the anthryl unit and the phenyl rings are 79.7◦ and 83.7◦ , respectively. The severe distortion of PC3 may be ascribed to the steric hindrance effect. Thus, the nonplanar structure may facilitate the intramolecular rotation, leading to the nonradiative energy loss of the excited singlet state through a TICT mechanism, resulting in very weak fluorescence [14]. To further elucidate the existence of TICT states of probes PC3 and C3, time-dependent density functional theory (TDDFT) calculations were employed, and the results are shown in Fig. 8. Both PC3 and C3 have donor-␲ bridge-acceptor (D-␲–A) frameworks,

with the diphenylamino group acting as the donor, and the aldehyde and dicyanovinyl groups acting as the acceptors in PC3 and C3, respectively. In the ground state, the electron densities in the HOMOs of PC3 and C3 are mainly distributed over the donor, and the LUMOs were delocalized to the anthryl unit and the acceptor. The interplane angles between the anthryl and the phenyl rings lie within the range of 64.7–68.2◦ . However, in the lowest singlet excited states, the anthryl group is almost perpendicular to the phenyl rings for both of PC3 and C3, with the interplane angles lying between 83.9◦ and 85.3◦ . The HOMO is localized within the diphenylamine group, and the LUMO is localized over the anthryl unit and the acceptor (Fig. 8 and Table S1), with the formation of charge-separated species. These results further supported the existence of the TICT state associated with the distortion between the donor and the anthryl group, and thus the probes showed rather weak fluorescence [22]. After reacting with hydrazine, the dihedral angles between the phenyl rings and the anthryl unit in the first excited state are sharply decreased to 64.8◦ (Fig. 8 and Table S1), and the electron densities in HOMO and LUMO have a certain degree of overlap, and thus the charge-separated state was not observed, which resulted in the strong fluorescence associated with intramolecular charge transfer effect. Moreover, we followed Jacquemin and co-workers to describe the charge transfer process by means of the charge transfer parameter fCT = qD − qA , where qD and qA denote the changes in the charge of the donor and the acceptor upon excitation from the ground state to the lowest excited state [23,24]. We can see from Table S4 that all the three compounds have similar fCT around 1.0 at the ground state geometry. However, at the relaxed excited state geometry, both C3 and PC3 show large fCT around 1.3, while TP has a much smaller fCT around 0.7. This indicates that the excited states of C3 and PC3 are associated with charge-separated species with TICT character (see Fig. 8 and Table S1), and that such a TICT state is suppressed by formation of TP upon addition of hydrazine. The overall agreement between the theoretical TD-DFT results and the experimental measurements is satisfactory albeit a global-hybrid functional (MPW1B95) and a moderate-sized basis set (6-31G*) were used in the calculations, owing to a fortunate cancellation of the errors [25]. It has also been demonstrated that state-specific polarizable continuum model (SSPCM) could be combined with the range-separated CAM-B3LYP functional to afford TD-DFT results with higher accuracy [26]. We thus further refined the TD-DFT results at the CAM-B3LYP/6-311++G**/SSPCM level of theory, as shown in Table S5 and S6. The computed absorption wavelengths at the ground state geometries have nicely

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Fig. 8. Frontier molecular orbitals of PC3, C3 and TP.

reproduced the experimental data, which implies necessity of employing the CAM-B3LYP/SSPCM methodology in computing the absorption spectra of the charge-transfer species. However, the emission energies at the optimized excited state geometries are overestimated by the CAM-B3LYP functional, resulting in a blueshifted emission compared with experimental observations. The MPW1B95 functional has nicely reproduced the experimentally observed emission wavelengths; moreover, it also predicts that the emission of compound C3 is in very low energy (<1.5 eV) and that the emission of compound PC3 has a small oscillator strength, in agreement with experimentally observed weak fluorescence of C3 and PC3.

4. Conclusion

3.7. Solvent viscosity effect and fluorescence lifetime As described above, the nonplanar structures of the probes may facilitate the nonradiative energy loss of the excited singlet state through intramolecular rotation, resulting in very weak fluorescence of the probes. If this is true, intramolecular rotation will be suppressed in viscous solvents, and thus the probes will demonstrate solvent viscosity dependent fluorescence, i.e., the higher viscosity, the stronger fluorescence [14,27]. Consistent with this

polyetheramine D2000 (V/%)

60

FL Intensity

100 40

expectation, significant fluorescence enhancement was observed with increasing solvent viscosity (Fig. 9, Fig. S14). On the other hand, the fluorescence decay curves for PC3 and C3 were observed to be biexponential with the slow decay component corresponding to the TICT state (Figs. S15 and Table S7) [28]. However, upon addition of hydrazine, the curves became monoexponential (Figs. S15 and Table S7), concurrent with the disappearance of the TICT deactivation pathway. Thus, the fluorescence was recovered. These results are consistent with theoretical calculations and solvent viscosity dependent fluorescence.

0

In this work, we employed PC3 and C3 as fluorescence “turn-on” hydrazine probes. Because of the higher reactivity of the dicyanovinyl group, C3 showed higher sensitivity towards hydrazine, with the detection limit of 7 ppb in CH3 CN. In aqueous systems, it also works well with improved selectivity for hydrazine over CN− . The weak fluorescence of PC3 and C3 can be ascribed to the distortion between the bulky diphenylamino and the 9-anthryl groups, and the resulting TICT states disappeared upon addition of hydrazine, accompanied with dramatic fluorescence enhancement. It can be concluded that the combination of the bulky diphenylamino and 9-anthryl units is an effective approach for developing weakly fluorescent compounds as fluorescence turn-on type hydrazine probes based on the TICT mechanism. These results provide further insights into the design of high performance hydrazine probes.

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Wavelength/nm Fig. 9. Emission of PC3 (20 ␮M) in dichloromethane/polyetheramine D2000 with increasing viscosity (ex = 446 nm).

This work was supported by NSFC/China (21072060, 91227201), NCET-11-0638, SRFDP (20100074110015), the Oriental Scholarship, and the Fundamental Research Funds for the Central Universities (WK1013002).

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Biographies Bin Chen is a Ph.D. Candidate in Key Laboratory for Advanced Materials and Institute of Fine Chemicals at East China University of Science and Technology. His current research interests are molecular recognition, organic light-emitting materials and dye-sensitized solar cells.

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Xi Sun is a master student in Key Laboratory for Advanced Materials and Institute of Fine Chemicals at East China University of Science and Technology. His current research interests are molecular recognition and dye-sensitized solar cell. Xin Li is a postdoctoral researcher in Division of Theoretical Chemistry and Biology at KTH Royal Institute of Technology, Sweden. His current research interest is theoretical simulations of optical properties of supramolecular systems. Hans Ågren is a professor and head of Division of Theoretical Chemistry and Biology at KTH Royal Institute of Technology, Sweden. His current research interest is

predictive multiscale modelling that combines the accuracy of quantum mechanics and the applicability of classical physics. Yongshu Xie is a professor in Key Laboratory for Advanced Materials and Institute of Fine Chemicals at East China University of Science and Technology. His current research interests include supramolecular chemistry, optoelectronic materials and porphyrin chemistry.